Bone plate fixation

Abstract

An implant system comprises a bone plate (1) with a plurality of plate holes (2) having an internal thread-like structure, for example internal thread. The bone plate system further includes at least one bone screw (3) having a screw shaft and a screw head, the screw shaft having a first external thread equipped to engage with bone tissue, and the screw head having a second external thread shaped to engage with the internal thread-like structure. Moreover, the implant system comprises at least one anchoring pin (5), the anchoring pin having a pin shaft and a pin head, the pin shaft comprising a thermoplastic material liquefiable by mechanical energy and the pin head comprising a thermoplastic material liquefiable by mechanical energy, wherein the pin shaft is dimensioned to be inserted through one of the plate holes, and the pin head is dimensioned for not being able to be moved through this plate hole. Thereby, the anchoring pin can be implanted by an input of mechanical energy that causes thermoplastic material of the anchoring pin both, to flow into structures of the bone tissue and to flow into structures of the internal thread-like structure of the plate hole.

Description

[0001] BONE PLATE FIXATION

[0002] FIELD OF THE INVENTION

[0003] Invention is in the field of surgical bone plate systems for use in surgery of humans and in veterinary medicine, i.e. for surgery of non-human animals.

[0004] BACKGROUND OF THE INVENTION Bone plate systems for use in surgery are well-known. They are especially used for the fixation of bone fractures of human or non-human animal patients.

[0005] Traditional bone plates of bone plate systems are made of stainless steel or titanium and comprise a plurality of plate holes that receive surgical screws. The surgical screws are inserted through the plate holes and are threaded into the bone tissue to press the bone plate against the bone. There exist both, systems in which the angle between the bone plate of the surgical screw relative to the bone plate is variable - for example by the screw having a screw head with a spherically shaped surface portion - and systems in which the angle between the bone plate and the surgical screw is fixed. The former allow more flexibility during surgery. However, for sufficient stability they require that the bone plate is tightly pressed against the bone surface. This has disadvantages tough. To avoid that the screw gests loose as a consequence of cyclic loads, it is necessary that the screw is anchored in the bone tissue bicortically. Systems with a fixed angle between the bone plate and the surgical screw comprise a thread connection between the surgical plate and the head of the surgical screw. Thereby the angle between the surgical plate and the surgical screw is fixed, and the connection between them remains stable independent of the dynamics of postoperative physiological loads. However, the screw needs to be inserted coaxially with the plate hole, and this limits the flexibility during surgery. Especially, it is difficult to press the bone fragments against each other using bone plate systems with screws fixed in the bone plate (sometimes also called “locking screws”).

[0006] For these reasons, industry has suggested combinations and hybrids of the principles angle-fixed bone screws and bone screws without a fixed angle.

[0007] While these combinations and hybrids may provide good solutions for some indications, they do not do so for others. Especially, the implantation of screws, in particular of the bone screws without a fixed angle, affect a lot of bone tissue and have their limits when the bone plate is to stabilize a fracture of relatively thin or delicate bone tissue that nevertheless is load-bearing such as bone tissue of the mandible or maxilla, bones of the feet or hand, or, for veterinary use in animals with small and thin bones, etc.

[0008] US 2015 / 196253 Al concerns an implantable device that is equipped for material or signal delivery. One embodiment (Fig. 19) concerns an osteosynthesis plate of a resorbable polymeric material that can be fixed to bone with the aid of a screw or a thermoplastic pin reaching through a through opening of the plate. The head is welded to the plate into the through opening to yield a seal for material delivered on a distal side of the plate. The teaching of US 2015 / 196253 Al focuses on sealing body regions from each other despite the presence of the implantable device and does not provide any solutions for the challenges brought about by the mentioned limits.

[0009] US 2020 / 0222089 Al relates to an implant system to be fastened to a posterior side of a spinal column. US 2020 / 0222089 Al does not provide any solutions for the challenges brought about by the mentioned limits, either.

[0010] SUMMARY OF THE INVENTION

[0011] It would be desirable to have a surgical bone plate implant system for use in surgery of humans or in veterinary medicine, thus surgery of non-human animals, that overcomes drawbacks of the prior art and that is especially advantageous for stabilizing fractures of thin, small and / or delicate bone. It would further be desirable to have a surgical bone plate implant system that that requires less extensive surgery if an implant removal is required.

[0012] The implant system comprises a (surgical) bone plate, for example made of titanium or of stainless steel. The bone plate is equipped to be placed relative to bone tissue, for example by extending along the surface of bone tissue, especially across a fracture, so as to stabilize fragments of the human or non-human animal bone. It has a plurality of plate holes for fasteners that fix the bone plate relative to the bone. The plate holes - or at least some of the plate holes - have an internal thread-like structure. They are thus suitable for so-called ‘locking screws’ - bone screws with a first thread that engages with the bone tissue and a second thread, provided in the screw head, engaging with the internal thread-like structure - for an angularly stable connection. The surgical bone plate system accordingly comprises at least one bone screw of this kind, the bone screw having a screw shaft and a screw head, with the screw shaft having the first external thread equipped to engage with bone tissue, and the screw head having the second external thread shaped to engage with the internal thread-like structure of the plate hole. In addition to comprising the bone screw, the implant system also comprises at least one anchoring pin. The anchoring pin has a pin shaft and a pin head. Both, the pin shaft and the pin head comprise a thermoplastic material. The thermoplastic material(s) of the pin shaft and of the pin head is liquefiable by mechanical energy, especially ultrasonic energy. Thereby, the anchoring pin can be implanted by an input of ultrasonic energy that causes thermoplastic material of the anchoring pin both, to flow into structures of the bone tissue and to flow into structures of the internal threadlike structure of the plate hole, thereby creating a fixation of the anchoring pin in the bone tissue and in the bone plate after re-solidification.

[0013] In internal thread-like structure is a structure along the wall around the plate hole which structure in section perpendicular to a plane defined by the plate has a sequence of grooves and ridges between the grooves, thus which has a plurality of groove sections extending around the plate hole. The internal thread-like structure may especially be an internal thread, whereby the groove sections are sections of one helically running groove (or more than one helically running grooves). Alternatively, it may be another structure that has a sequence of grooves and ridges between the grooves, for example a structure with a plurality of discrete grooves extending parallel to the plane defined by the plate. When the screw head of bone screw gets into contact with the internal thread-like structure, the second external thread eats into the internal thread-like structure and causes an angularly stable engagement.

[0014] The thermoplastic materials of the pin shaft and of the pin head may be the same or different. Especially, the anchoring pin may be one-piece. The pin shaft is dimensioned to be inserted through one of the plate holes, and the pin head is dimensioned for the internal thread-like structure of this plate hole to interfere with the pin head when the anchoring pin is moved further, after insertion of the pin shaft through the plate hole. This means that the pin head has a larger cross section (perpendicular to a proximodistal axis) than the pin shaft and is dimensioned so that it cannot be moved through the plate hole.

[0015] Thereby, if ultrasonic energy impinges on the pin when the pin is moved into the plate hole, the interference between the pin head and the internal thread-like structure of the plate hole will cause material of the pin head to become flowable and to flow into the grooves of the internal thread-like structure. After solidification, the - for example metallic - bone plate and the anchoring pin having the thermoplastic - especially polymeric - material form an angular stable fixation.

[0016] Both, the bone screw, which may be metallic, and the anchoring pin may serve as fasteners. In the present text, an “anchoring pin” is a fastener that has a pin shaft comprising thermoplastic material liquefiable by mechanical energy and a pin head also comprising thermoplastic material liquefiable by mechanical energy. Anchoring pins in accordance with the terminology used herein are thereby equipped to be anchored in bone tissue by a process that comprises impinging the anchoring pin with mechanical energy, especially ultrasonic vibration energy, while the anchoring pin is held against tissue. In this, especially, the anchoring pin may be pressed against bone tissue, for example by being pressed into a pre-drilled hole that may be slightly undersized with respect to the largest diameter of the pin shaft, while the mechanical (ultrasonic) energy impinges. The process will comprise causing at least a flow portion of the thermoplastic material to become flowable and flow into structures of the bone tissue to yield, after re-solidification, a positive-fit between the anchoring pin and the tissue wherein the anchoring pin is subject-to a root-like anchoring in the bone tissue. It has been found that anchoring of an anchoring pin of this kind requires only little bone tissue, whereby the anchoring pin can be relatively short - for example shorter than the bone screw. Nevertheless, the connection between the anchoring pin on the one hand and the bone tissue on the other hand is very intimate and stable.

[0017] The material of the bone plate may be a material of a kind known for bone plates. For example, the bone plate may be metallic. The material of the bone plate will especially be such that it does not become flowable under the conditions under which the thermoplastic material of the anchoring pin becomes flowable. This also implies that the connection between the material of the pin head and the bone plate is not a weld but a positive-fit connection between the re-solidified material of the flow portion on the one hand and the structures of the inner thread on the other hand.

[0018] The approach according to the invention in embodiments also has advantages relating to implant removal. Implant removal in prior art systems requires to turn all screws out an requires a second surgery. To avoid this second surgery, industry has developed bioresorbable plates and screw system that are made of polylactic materials. However, these polymeric plate systems can withstand only limited loads and are therefore only indicated for non-load bearing fracture treatment, although the polymeric pins, when inserted and fused into the bone with ultrasonic energy, provide a very good anchorage of the plate in the bone, even when the bone is very thin, such as bone tissue of the mandible or maxilla, bones of the feet or hand, or, for veterinary use in small animals like dogs and cats with thin bones. The approach according to the present invention, in contrast, allows the surgeon to use anchoring pins of a resorbable material together with the - usually metallic - bone plate. Depending on the indication, more or fewer of the bone screws may be used. It is even possible for the surgeon to choose only anchoring pins for the anchoring, without any screw. If compared to the prior art a reduced number of screws is used, removal of the system is accordingly less complex. Due to the approach described herein, the surgeon using the implant systems has two options to stabilize the bone plate relative to bone tissue at the location of a particular plate hole. According to a first option, she / he can use a (for example metallic) bone screw of the mentioned kind, with a first and second external thread. According to a second option, she / he can use an anchoring pin. The anchoring pin is implanted by inserting the anchoring pin, pin shaft ahead, through the plate opening when the bone plate is placed relative to bone tissue. When the pin shaft is in contact with bone tissue, for example by extending into a pre-drilled hole in the bone, then it may be subject to a pressing force and a mechanical (especially ultrasonic) vibration energy input by a sonotrode that for example presses against the proximal end face of the pin head. This is done until, due to heating caused by friction between the vibrating anchoring pin and the bone tissue, at least a flow portion of the thermoplastic material of the pin shaft becomes flowable and flows into structures of the bone tissue. At least after some movement of the anchoring pin into the distal direction, due to the pressing force and material of the pin shaft being displaced into the bone tissue, the periphery of the pin head comes into contact with structures around the plate hole, especially with the internal thread-like structure. Friction between the internal thread-like structure and the pin head, that is subject to mechanical vibration energy input, then will cause thermoplastic material of the pin head to be heated and become flowable, too and to flow into the grooves of the internal thread-like structure. When the energy input stops, thermoplastic material will re-solidify. This results in a positive-fit connection both, between the anchoring pin and the bone tissue, and between the anchoring pin and the bone plate.

[0019] The approach according to the present invention thus makes it possible that the surgeon may decide any time, even during operation, for example even after for example having locally removed bone tissue, for example by drilling, for the fastener (bone screw or anchoring pin) which kind of fastener to use. Especially, she / he may use both, bone screws and anchoring pins, for the same plate. For example, anchoring pins may be used at positions close to a fracture where utmost care to not damage any bone tissue is required and where it may, depending on the situation, be beneficial if the fastener can be introduced at an angle to the plate opening axis, for example for generating a slight compression force between bone fragments. For positions more remote from a fracture, especially extremal positions, bone screws may be used to enhance the overall mechanical stability. In an example, if a fracture of a mandible or maxilla is to be stabilized, at positions where a dental root is underneath a plate hole, a (short) anchoring pin may be implanted, whereas at other positions a bone screw may be used.

[0020] The internal thread-like structure around the plate hole may be tapering, i.e., on an outer side it is wider than on a bone side (the bone side of the plate being defined to be the side that after implantation faces the bone to which the bone plate is attached, and the outer side being the opposite side). More precisely, an outer side mouth of the plate opening, being the mouth in the outer surface of the bone plate, has a larger area than a bone side mouth of the plate opening, which latter is the mouth in the bone side surface of the bone plate. The combination of a tapering thread and an anchoring pin with a head comprising thermoplastic material is especially advantageous, because when thermoplastic material is liquefied at the interface between portions of the pin head where the thermoplastic material is still solid, the anchoring pin may be continuously moved forward and liquefaction continues at the thread-solid-material interface, causing a liquefaction zone extending along an axial extension of the plate hole and further supporting the efficient and complete interpenetration of the structures of the thread by the thermoplastic material.

[0021] One finding underlying the present invention is, therefore, that the combination of such a tapering internal thread-like structure of the bone plate and of an anchoring pin with a pin head of material liquefiable by energy input is especially advantageous. Due to this construction, when the anchoring pin is anchored and the pin head gets into contact with the bone plate while vibrating, no energy or at most little energy is absorbed on the outer plate surface, but energy absorption takes place along a substantial axial extension of the plate hole, whereby the thermoplastic material of the pin head is liquefied primarily along the surface of the internal thread-like structure, and this is observed to cause an efficient and complete interpenetration of the structures of the thread by the thermoplastic material.

[0022] With the tapering thread, the pin shaft may be dimensioned to be movable through the entire plate opening, and the pin head may be dimensioned to be movable through the outer side mouth but not through the bone side mouth.

[0023] An axial extension of the pin head may be similar to or essentially identical with the thickness of the bone plate. It may in particular correspond to at least 80% and at most 120% of a thickness of the bone plate around the plate opening. It has been found that given these parameters, and if the diameters are chosen appropriately, the volume of the plate hole and the volume of the pin head are adapted to each other so that the head during the process melts well into the internal thread-like structure around the plate hole. More in general, the volume of the pin head may be determined, depending on materials, experimentally. While a too high pin head volume may result in a backflow of polymer material, causing an imperfect healing, a too low volume may lead to the positive-fit connection between the pin head and the bone plate to be not ideal.

[0024] The pin shaft may be generally cylindrical, especially circularly cylindrical, however with a structured outer surface. Especially, the outer surface of the pin shaft may comprise one of: ridges, for example axial ridges, or humps, protruding radially outwardly; at least one step. Such ridges and / or humps and / or steps serve as energy directors during implantation and promote liquefaction not only at the distal end of the pin shaft, but along further positions along its length. The distal end of the shaft portion may comprise one or more tips or other structure causing it to have a cross section that gradually reduces towards the distal end. Also this may have energy directing properties and promote liquefaction.

[0025] A diameter (average diameter) of the pin shaft may be smaller than a diameter of the plate hole by for example between 0.1 mm and 0.5 mm.

[0026] Between the pin shaft and the pin head, the anchoring pin may comprise a transition portion, where its diameter for example gradually increases from the pin shaft to the pin head. Alternatively, or in addition, it may comprise a step between the pin shaft and the pin head.

[0027] The pin head may have an overall cylindrical shape - especially the shape of a circular cylinder, optionally with energy directing structures along its outer periphery, for example ribs. The diameter D of this circular cylinder may be smaller than the diameter H of the outer side mouth of the plate opening and larger than the diameter h of the inner side mouth. It may be larger than the average between the diameters of the outer side mouth and the inner side mouth, thus for example, the inequality (h+H) / 2<D<H may be hold, especially (h+2H) / 3<D<H.

[0028] The anchoring pin may have a proximal guiding hole reaching from a proximal end face of the pin head distally into the anchoring pin. Such guiding hole may for example have the shape of a truncated cone or other suitable shape. It may serve for a guiding protrusion of a sonotrode to engage therewith. The anchoring pin then may be provisionally stuck to the sonotrode for being placed relative to the bone tissue and the bone plate for the anchoring process. As an alternative to comprising a proximal guiding hole, the anchoring pin may comprise a conical coupling hole for a conical coupling structure of a sonotrode to engage. Then, the mechanical energy (ultrasonic energy) is coupled into the anchoring pin for the anchoring process via the conical wall around the coupling hole. In this, the ultrasonic vibration may be longitudinal vibration, i.e., vibration along the axis of the anchoring pin, which may essentially coincide with the axis of the plate opening and the insertion axis when the anchoring pin is inserted.

[0029] The thermoplastic materials of the pin shaft and of the pin head may be different or may be the same. Especially, in embodiments the anchoring pin is one-piece of a (single) thermoplastic material, for example an injection molded part.

[0030] In embodiments, the pin head comprises a proximal stop structure, for example constituted by a proximal flange-like protrusion, thus an outwardly protruding circumferential ridge. The stop structure defines the insertion depth at the end of the process and prevents a less experienced surgeon from pressing the sonotrode and the anchoring pin too deep into the plate hole and tissue.

[0031] A thickness (axial extension) of this stop structure may be limited, for example by being less than 1 mm, especially less than 0.8 mm or less than 0.6 mm so that the pin head after anchoring does not wear to much.

[0032] The anchoring pins are especially suited for unicortical anchoring, i.e., it is not necessary that a hole traversing the bone tissue is made. Preparing the bone tissue for implantation of the anchoring pin may just consist of locally removing the cortical bone tissue next to which the bone plate is to be positioned, and possibly some cancellous bone tissue, without any impact on the cortical bone tissue on the other side of the bone. Also for the bone screw implantation, unicortical anchoring is an option, since because of the angular stabilization it is not necessary that the bone screw is secured against tilting moments or the like. Accordingly, the anchoring pin(s) and / or the bone screws may be configured and dimensioned for unicortical anchoring.

[0033] The anchoring pin may especially consist of a thermoplastic material of which both, the pin shaft and the pin head are made, and the anchoring pin with pin head and pin shaft may be in one piece.

[0034] The anchoring pin may, in a group of embodiments, be resorbable. For example if resorbable anchoring pins are placed near the fracture, after their resorption, the bone plate may be free to undergo more pronounced plastic deformation compared to a situation in which also near the fracture bone screws are used. This possibility for the bone plate to undergo plastic deformation dynamizes the bone fragments, and this in turn promotes callus formation.

[0035] However, the invention can also be carried out using non-resorbable anchoring pins.

[0036] For explanting the bone plate, when the anchoring pin(s) is / are resorbable, only the bone screws need to be unscrewed, for which a small puncture incision may be sufficient, through which incision also the plate may be removed. If the anchoring pin(s) is / are not resorbable, or if resorption has not been completed, a drill may be used for removal.

[0037] The bone screw may be of a kind known per se, with a screw shaft and a screw head. In this, the screw shaft has the first external thread. The second external thread extends around a periphery of the screw head. The first external thread has a higher lead (and pitch) than the second external thread, whereby the first external thread is configured to drive the bone screw into bone tissue, and the second thread engages with the internal thread-like structure of the bone plate towards the end of the screwing-in movement, whereby the bone plate is biased, to some degree, against the bone surface due to the different leads between the first and second external threads.

[0038] In embodiments, the bone plate may be equipped for being secured to bone tissue also by so-called nonlocking screws, i.e., screws with a partly spherical screw head (spherical on a distally facing side) by having plate holes with a corresponding structure having a concave portion along a periphery of the respective plate hole. Such dynamic compression plate holes may be present in addition to plate holes with the internal thread-like structure. Also combined dynamic compression-locking plate holes are possible. Such combined plate holes are known in the art.

[0039] The bone plate may in embodiments in addition to the plate holes with the internal thread-like structure at least one smaller hole for a K-wire and / or at least one smaller hole for sutures.

[0040] In addition or as an alternative to having this property, the shape of the bone plate may be adapted to the shape of the bone to which is to be attached, as known in the art.

[0041] In embodiments, the implant system may comprise a plurality of bone plates of different thicknesses but with plate holes having same coupling portions. In this, the coupling portions may comprise an internal thread-like structure of the kind discussed hereinbefore. Especially, the bone plate may have a shoulder proximally of the coupling portion, which shoulder serves as a stop for a distally facing stop of a proximal stop structure of the anchoring pin. The shoulder may be formed by the proximal surface of the bone plate itself, in which the plate openings have a mouth, i.e. the shoulder may (especially for a thinner bone plate of the plurality of bone plates) be flush with the proximal surface. In another, thicker bone plate of the plurality of bone plates, the shoulder may be countersunk with respect to the proximal surface, but in both, the thinner and the thicker bone plate the shoulder may be at a same distance to the distal surface of the bone plate. The coupling portion is constituted by that part of the plate hole that extends between the distal mouth of the plate hole and the shoulder.

[0042] If the shoulder is at a same distance to the distal surface of the bone plate, and especially the coupling portion for the thicker and thinner bone plates are the same, the same anchoring pin may be used for both, the thinner and the thicker bone plate.

[0043] This concept an implant system with bone plates of different thicknesses, the bone plates having plate holes for anchoring pins, and the plate holes of the different bone plates having same coupling portions, may be realized independent of the concept according to the claimed invention, i.e. it may also be realized in implant systems the plate holes of which do not necessarily have an internal thread-like structure and / or that do not necessarily comprise a bone screw.

[0044] Thus, in general, the implant system may be an implant system comprising a plurality of bone plates with a plurality of plate holes each, at least two of the bone plates having different thicknesses, the implant system further comprising at least one anchoring pin, the anchoring pin having a pin shaft and a pin head, the pin shaft comprising a thermoplastic material liquefiable by mechanical energy and the pin head comprising a thermoplastic material liquefiable by mechanical energy, wherein the pin shaft is dimensioned to be inserted through one of the plate holes, and the pin head is dimensioned for not being able to be moved through this plate hole. The plate holes have coupling portions extending between a distal mouth of the respective plate hole and a proximally facing shoulder, and the coupling portions of plates of different thicknesses having coupling portions that have same dimensions. Such system may further comprise at least one of an anchoring pin and of a bone screw.

[0045] In this, optionally at least some of the plate openings may have an internal thread-like structure, and the implant system may comprise at least one bone screw having a screw shaft and a screw head, the screw shaft having a first external thread equipped to engage with bone tissue, and the screw head having a second external thread shaped to engage with the internal thread-like structure.

[0046] The implant system may optionally comprise, in addition to the bone plate, the bone screw, and the anchoring pin, at least one further item, for example a sonotrode for coupling mechanical vibration energy as well as a pressing force into the anchoring pin. To this end, the sonotrode has a distal outcoupling face mating with the proximal surface of the pin head, so as to achieve a good mechanical coupling. In addition or as an alternative to comprising a sonotrode, the implant system may comprise a screwing tool for cooperating with the bone screw to screw the same into the bone tissue. If helpful, the screwing tool may have a torque defining or torque limiting functionality.

[0047] The implant system in addition or as an alternative may contain a drilling template for allowing the surgeon to appropriately position the locations where the bone tissue is prepared for implantation. Such implant may be shaped like the bone plate, with holes at the positions of the plate holes, however, possibly with different diameters.

[0048] The present invention also concerns a method of implanting a surgical bone plate, using an implant system as described and defined in the present text. The method firstly comprises providing an implant system with a bone plate, a surgical screw and an anchoring pin. The method further comprises preparing the bone tissue for implantation of the surgical screw and the anchoring pin, especially by locally removing at least cortical bone tissue, and of positioning the bone plate relative to the bone. In this, the step of preparing is for example - but not necessarily - carried out before the step of positioning. If this is the case, a template may be used before or during the step of preparing, so that the locations of the spots prepared for subsequent implantation are defined by the template and can be marked or alternatively the drilling tool can be guided by the appropriately positioned holes of the template.

[0049] The method then comprises implanting the anchoring pin by inserting the pin shaft through a first one of the plate holes and bringing it into contact with a prepared location of the bone tissue and using a vibrating tool to press the anchoring pin against the bone tissue while mechanical vibration energy is coupled into the anchoring pin until thermoplastic material of the pin shaft is flowable and flows into structures of the bone tissue and thermoplastic material of the pin head is flowable and flows into grooves of the internal thread-like structure. The method further comprises implanting, before or after implanting the anchoring pin, the bone screw by screwing the bone screw through a second one of the plate holes into the bone tissue until its second external thread engages with the internal thread -like structure of the second plate hole.

[0050] In embodiments, the method comprises implanting a plurality of anchoring pins at positions close to a bone fracture and implanting bone screws at positions more remote from the bone fracture.

[0051] Mechanical vibration suitable for implantation of the anchoring pin, which includes liquefaction of a thermoplastic material, especially a polymer, by friction heat created through the mechanical vibration has preferably a frequency between 2 and 200 kHz (even more preferably between 10 and 100 kHz, or between 20 and 40 kHz) and a vibration energy of 0.2 to 20 W per square millimeter of active surface. The vibrating element (sonotrode) is e.g. designed such that its contact face oscillates predominantly in the direction of the element axis (longitudinal vibration) and with an amplitude of between 1 and 100pm, preferably around 20 to 90 pm. Rotational or radial oscillation is possible also.

[0052] The amplitude depends on various parameters, such as the thermoplastic material and the shape of the thermoplastic material.

[0053] In particular, the above-mentioned amplitudes are preferred, i.e. optimized, in terms of short times for local liquefaction, minimal or no heating of tissue and other parts of the implant system, and minimal mechanical load on the thermoplastic material and hence on the tissue. In particular, the time for local liquefaction can be below 5 s, for example below 2 s, such as between 1 and 2 s.

[0054] In this text, the expression "thermoplastic material being liquefiable e.g. by mechanical vibration" or in short “liquefiable thermoplastic material” or “liquefiable material” is used for describing a material comprising at least one thermoplastic component, which material becomes liquid (flowable) when heated, in particular when heated through friction i.e. when arranged at one of a pair of surfaces (contact faces) being in contact with each other and vibrationally or rotationally moved relative to each other, wherein the frequency of the vibration is between 2 kHz and 200 kHz, preferably 20 to 40 kHz and the amplitude between 1 pm and 100 pm, preferably around 20-90 or 20-70 pm. Such vibrations are e.g. produced by ultrasonic devices as e.g. known for dental applications. In embodiments, suitable liquefaction connected with an acceptable thermal loading of the tissue and giving suitable mechanical properties to the positive fit connections is achievable by using materials with thermoplastic properties having an initial modulus of elasticity of at least 0.5 GPa and a melting temperature of up to about 350°C.

[0055] Suitable resorbable polymers are e.g. based on lactic acid and / or glycolic acid (PLA, PLLA, PGA, PLGA etc.) or polyhydroxyalkanoates (PHA), polycaprolactones (PCL), polysaccharides, polydioxanones (PD), polyanhydrides, polypeptides or corresponding copolymers or blended polymers or composite materials containing the mentioned polymers as components are suitable as resorbable liquefiable materials.

[0056] Thermoplastics such as for example polyacrylates, polymetacrylates, polycarbonates, polyamides, polyesters, polysulphones, polyaryl ketones, polyimides, polyphenyl sulphides or liquid crystal polymers (LCPS), polyacetals, halogenated polymers, in particular halogenated polyoelefins, polyphenylene sulphides, polysulphones, polyethers, polypropylene (PP), or corresponding copolymers or blended polymers or composite materials containing the mentioned polymers as components are suitable as non-resorbable polymers. Examples of suited thermoplastic material include any one of the polylactide products LR708 (amorphous Poly-L-DL lactide 70 / 30), L209 or L210S by Bbhringer Ingelheim, or a poly ether etherketone such as PEEK 450G from Invibio, UK.

[0057] Specific embodiments of non-degradable materials are: Polyetherketone (PEEK Optima, Grades 450 and 150, Invibio Ltd), Polyetherimide, Polyamide 12, Polyamide 11, Polyamide 6, Polyamide 66, Polycarbonate, Polymethylmethacrylate, Polyoxymethylene, or polycarbonateurethane (in particular Bionate® by DSM, especially Bionate 75D and Bionate 65D; according information is available on datasheets publicly accessible for example via www.matweb.com by Automation Creations, Inc.). An overview table of polymers and applications is listed in Wintermantel, page 150; specific examples can be found in Wintermantel page 161 ff (PE, Hostalen Gur 812, Hbchst AG), pages 164 ff. (PET) 169ff (PA, namely PA 6 and PA 66), 171 ff. (PTFE), 173 ff. (PMMA), 180 (PUR, see table), 186 ff. (PEEK), 189 ff. (PSU), 191 ff. (POM - Polyacetal, tradenames Delrin, Tenac, has also been used in endoprostheses by Protec).

[0058] The liquefiable material having thermoplastic properties may contain foreign phases or compounds serving further functions. In particular, the hard thermoplastic material may be strengthened by admixed fillers, for example fibers, e.g. carbon fibers that may increase strength and wear resistance.

[0059] If the liquefiable material is to be liquefied not with the aid of vibrational energy but with the aid of electromagnetic radiation, it may locally contain compounds (particulate or molecular) which are capable of absorbing such radiation of a specific frequency range (in particular of the visible or infrared frequency range), e.g. calcium phosphates, calcium carbonates, sodium phosphates, titanium oxide, mica, saturated fatty acids, polysaccharides, glucose or mixtures thereof.

[0060] The bone screw and the bone plate are of a non-liquefiable material such as, for example, a titanium alloy. An example of a material is titanium grade5. Alternative materials are other metals like other titanium alloys, stainless steel, or hard plastics such as PEEK etc.

[0061] In this text, generally a “non-liquefiable” material is a material that does not liquefy at temperatures reached during the process, thus especially at temperatures at which the thermoplastic material of the anchoring pin is liquefied. This does not exclude the possibility that the non-liquefiable material would be capable of liquefying at temperatures that are not reached during the process, generally far (for example by at least 80°C) above a liquefaction temperature of the thermoplastic material or thermoplastic materials liquefied during the process. The liquefaction temperature is the melting temperature for crystalline polymers. For amorphous thermoplastics the liquefaction temperature is a temperature above the glass transition temperature at which the becomes sufficiently flowable, sometimes referred to as the ‘flow temperature’ (sometimes defined as the lowest temperature at which extrusion is possible), for example the temperature at which the viscosity drops to below 104Pa*s (in embodiments, especially with polymers substantially without fiber reinforcement, to below 103Pa*s)), of the thermoplastic material.

[0062] For example, a non-liquefiable material may be a metal, or ceramic, or a hard plastic, for example a reinforced or not reinforced thermosetting polymer or a reinforced or not reinforced thermoplastic with liquefaction temperature considerably higher than the liquefaction temperature of the liquefiable material, for example with a melting temperature and / or glass transition temperature higher by at least 50°C or 80°C or 100°C.

[0063] BRIEF DESCRIPTION OF THE DRAWINGS

[0064] In the following, ways to carry out the invention and embodiments are described referring to drawings. The drawings mostly are schematical. In the drawings, same reference numerals refer to same or analogous elements. The drawings show:

[0065] Fig. 1 a fractured bone with a bone plate secured to the bone by bone screws and anchoring pins; Fig. 2 a bone screw;

[0066] Fig. 3 a bone plate;

[0067] Fig. 4 an anchoring pin;

[0068] Fig. 5 a further embodiment of an anchoring pin;

[0069] Fig. 6 a schematical section through a bone plate and an implanted anchoring pin;

[0070] Fig. 7 a section through a bone plate and an implanted anchoring pin, wherein the insertion direction of the anchoring pin is different from coaxial with the plate hole;

[0071] Fig. 8 a section through a bone plate and an anchoring pin prior to implantation;

[0072] Fig. 9 a view of an alternative bone plate;

[0073] Fig. 10 elements of an implant system in a set;

[0074] Fig. 11 a vibration generating apparatus;

[0075] Fig. 12 an even further embodiment of an anchoring pin; and

[0076] Fig. 13 a section through a portion of two bone plates having different thicknesses but identical coupling portions.

[0077] DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0078] Figure 1 depicts a fractured bone, namely a pet’s mandible 100 having a fracture 101. The bone fragments on both sides of the fracture are held together or at least stabilized by a bone plate system that comprises a bone plate 1 with a plurality of plate holes 2 in addition to bone screws 3 and anchoring pins 5. The bone screws 3 and the anchoring pins 5 are implanted in bone tissue. For implantation, they have been inserted through the plate holes 2 and anchored in bone tissue. To this end, the bone tissue has locally been removed, for example by a drill that has drilled a hole at least in the cortical bone immediately underneath the bone plate 1, at the location of the respective plate hole.

[0079] Figure 2 shows a bone screw 3. The bone screw 3 may be of a type that is per se known in the field. It has a first external thread 12 that extends along a lateral surface of the screw shaft 11 and that may be self-tapping. Further, it comprises a second external thread 14 extending along a lateral surface of the screw head 13. The lead of the first external thread 12 (corresponding to its pitch if the thread is a single thread) is much larger than the lead of the second external thread 14. The screw head 13 is tapering towards distally. The proximal end of the screw head 13 has an engagement structure 16 for a tool (screw driver) to engage.

[0080] Figure 3 depicts a bone plate 1 in a basic version, with four plate holes 2. Each plate hole has an internal thread-like structure, namely an internal thread 7. The plate holes 2 are tapering towards distally. The shape and dimension of the plate holes 2 and the internal threads 7 are adapted to the shape and dimension of the screw head 13 and the second external thread 14 of the bone screw 3. Thereby, the bone screw 3, when inserted through one of the plate holes 2 and screwed into bone tissue, is fixed relative to the bone plate in an angularly stable manner - as is known from systems with so- called “locking plates”.

[0081] Figure 4 shows an anchoring pin 5. The anchoring pin 5 comprises a pin shaft 21 and a pin head 23. It consists of a thermoplastic material and is one-piece. The shaft has a structure comprising a plurality of steps 24. In the shown embodiments, the steps 24 are steps between polygonal (hexagonal in the depicted embodiment) sections of the pin shaft, with orientations rotated with respect to each other. At least the distally facing steps have energy directing properties and thereby assist the anchoring process described hereinafter.

[0082] Towards the distal end, the

[0083] In the embodiment of Fig. 4, the pin head 23 is tapered, for example with a taper adapted to the taper of the plate holes 2 (see below). Between the pin head 23 and the pin shaft 21, the anchoring pin 21 has a transition portion 26.

[0084] Figure 5 depicts an alternative anchoring pin 5. The pin shaft 21 of the anchoring pin 5 of Fig. 5 has a smaller diameter than the pin shaft 21 of the anchoring pin 5 of Fig. 4. In addition, the pin head 23 is cylindrical instead of conical. The anchoring pin of Fig. 5 is designed to be used together with a bone plate with smaller plate holes compared to bone plates to be used together with the anchoring pins of the kind shown in Fig. 4. Generally, for more delicate bone tissue, smaller plates with smaller plate holes and smaller anchoring pins (and smaller bone screws) are used, whereas for heavier bone tissue larger bone plates with larger plate holes and larger anchoring pins (and accordingly larger bone screws) are utilized. Also, the implant system may comprise anchoring pins of different shaft lengths per pin shaft diameter.

[0085] Generally, for anchoring pins with comparably larger diameters (with a shaft diameter of for example at least 1.8 mm), pin heads that are conical have proven to be advantageous in view of the process described in more detail hereinafter, whereas for anchoring pins of smaller diameters, cylindrical pin heads may be uses. For anchoring a bone screw 3 or an anchoring pin 5, the bone tissue may be pre-treated by locally removing at least the cortical bone tissue at an anchoring location. To this end, optionally the bone plate 1 or a template may be positioned relative to the bone tissue to define the anchoring location, and a tool may be used to mark the position. Then, a bone drill or other tool may be used to locally remove bone tissue, optionally with the template still in position. The depth of a drilled hole may be shorter than the difference between the length of the bone screw 3 or anchoring pin and the thickness of the bone plate - especially the drilled hole may in embodiments be restricted to cortical bone.

[0086] Figure 6 illustrates the principle of using an anchoring pin 5 for fastening the bone plate 1 to bone tissue. The bone tissue comprises cortical bone tissue 111 and cancellous bone tissue 112. The anchoring pin is implanted by inserting the anchoring pin, pin shaft ahead, through the plate opening into the drilled hole and then subjecting the anchoring pin to a pressing force and a mechanical vibration energy input by a sonotrode. In the depicted embodiment, the sonotrode is of a kind that presses against the proximal end face of the pin head. Alternatively, as shown hereinbelow, the sonotrode may have a cone-shaped portion engaging with a cone-shaped indentation (coupling hole) in the proximal and face of the anchoring pin to couple the vibration energy into the anchoring pin via the according cone-shaped interface. The energy input and the pressing force are maintained until, due to heating caused by friction between the vibrating anchoring pin 5 and the bone tissue, at least a flow portion of the thermoplastic material of the pin shaft becomes flowable and flows into structures of the bone tissue. Also, when the pin head is moved forward, its periphery comes into contact with structures around the plate hole, especially with the internal thread 7. The friction caused thereby will cause thermoplastic material of the pin head to become flowable, too and to flow into the structures of the internal thread 7. When the energy input stops, thermoplastic material will be caused to re-solidify. This results in a connection both, between the anchoring pin 5 and the bone tissue 111, 112, and between the anchoring pin 5 and the bone plate 1. Both, the connection between the anchoring pin and the bone tissue, and the connection between the anchoring pin and the bone plate are positive-fit connections. Fig. 6 depicts a - resolidified - first flow portion 32 interpenetrating the cancellous bone tissue 112 as well as - to a lesser extent because of the enhanced density - of the cortical bone tissue 111. It also shows a resolidified second flow portion 33 that has flown into the structures of the internal thread 7 of the bone plate. The conical shape of the plate hole filled by the material of the pin head additionally secures the bone plate 1 against the bone tissue.

[0087] In the shown embodiment, the process is carried out until the proximal end 31 of the anchoring pin is approximately flush with the proximal surface of the bone plate 1. The proximal end 31 is illustrated to have some deformation, due to the impact of the energy input, causing plastification of the most of the thermoplastic material of the anchoring pin, and pressing force.

[0088] Because of the positive-fit connection between the anchored anchoring pin and the bone plate, the connection between the anchoring pin and the bone plate is angularly stable and is stable also if / when the bone plate is not substantially pressed against the bone tissue and is therefore long-time stable, much like the connection between “locking plates” and bone screws with a head having an external thread. Also, if the bone plate is not substantially and permanently pressed against the bone surface, there is, compared to nonlocking screws with a partly spherical screw head, the advantage of less interference with blood supply to the bone tissue.

[0089] In the embodiments of Figs. 3 and 7, the inner surface of the plate hole, which inner surface has the internal thread 7, serves as a coupling portion for coupling with the anchoring pin. In contrast to locking screws of conventional locking plate systems, however, the anchoring also works if the insertion direction of the anchoring pin is not strictly parallel to the axis 10 of the plate opening. This is illustrated schematically in Figure 7, where the insertion axis 30 - which may be the axis along which the vibrations act, i.e. the axis of the sonotrode - is illustrated at an acute angle to the axis 10 of the plate opening. The fact that this is possible is advantageous at least for two reasons. Firstly, the surgeon has more flexibility in situations where access to the bone plate is difficult or where the bone plate opening is close to the fracture. Secondly, the surgeon may use the possibility to apply a non-perpendicular force for pressing the bone fragments against each other.

[0090] Fig. 7 also illustrates the possibility to dimension the pin head relative to the bone plate hole in a manner that a residual head 34 of the anchoring pin 5 remains, protruding proximally from the surface of the bone plate and additionally secures the bone plate 1 to the bone tissue. This possibility is independent of whether or not the insertion axis 30 coincides with the axis 10 of the plate opening.

[0091] Figure 8 illustrates an embodiment of an anchoring pin 5 together with a bone plate 1. In the embodiment of Fig. 8, the anchoring pin has a proximal conical coupling hole 28 for a coupling structure of the sonotrode (not shown in Fig. 8) to engage. The coupling hole 28 is an option that applies generally for anchoring pins, independent of the principles explained hereinafter referring to Fig. 8, which in turn are independent of the existence of the coupling hole 28. Especially, as an alternative to a conical coupling structure engaging a conical coupling hole, the sonotrode may comprise an essentially flat distal outcoupling face that during the process is pressed against a proximal end face of the anchoring pin. As is shown in Fig. 8, the plate hole with the internal thread 7 tapers, with the plate hole getting narrower towards to bone side (the distal side, being the lower side in Fig. 8). Thus, the outer side mouth of the plate opening (the mouth on the outer plane 37) has a larger diameter and larger area than the bone side mouth (the mouth on the distal side / bone side plane 38).

[0092] The external thread 14 of the bone screw 3 (Fig. 2) tapers accordingly. The opening angle A is different from 0° and may for example be between 3° and 35°, especially between 5° and 25°.

[0093] A first design principle is that the largest diameter d of the pin shaft 21 may be smaller than or equal to the smallest diameter h of the plate hole. More in general, the pin shaft 21 is designed for being insertable through the plate hole.

[0094] The diameter D of the pin head 23 - here illustrated to be cylindrical -, at least on its distal side, is smaller than the largest diameter H of the plate hole so that the pin head at least on its lower side can be inserted through the proximal mouth of the plate hole. The diameter D of the pin head 23 at least on its distal side is, however, larger than the smallest diameter h of the plate hole.

[0095] The axial extension t of the pin head may be approximately equal to the thickness T of the bone plate at the position of the plate hole. For example, the inequality 0.8T<t<l ,2T may hold, in particular 0.9T<t<l.lT. The volume of the plate hole and the volume of the pin head are adapted to each other so that the head during the process melts entirely into the internal thread 7 around the plate hole. A too high pin head volume results in a backflow of polymer material, causing a protrusion protruding proximally from the proximal end face of the bone plate. If the head volume is too low, the positive-fit connection between the pin head 23 and the bone plate 2 is not ideal, whereby the mechanical strength of the connection suffers. Die optimum parameters, within the limits mentioned herein, may be material dependent and may be determined experimentally.

[0096] Figure 9 illustrates a bone plate 1 with plate holes 2 that have a first portion with an internal thread 7 and a second portion with a spherical surface 8 for accommodating a screw with a spherical head portion that permits insertion at different angles (sometimes referred to as “non-locking screw”). Combination hole bone plates 1 of this kind are known in the art. The approach according to the present invention stipulates that an anchoring pin may, as an alternative to a locking screw, be inserted through the first portion of one of the plate holes and cooperate with the internal thread 7 in the manner described hereinbefore.

[0097] Figure 10 schematically depicts a container 41 for storing elements of a surgical plate implant system. The system comprises a plurality of bone plates 1, for example of different shapes and configurations, as only schematically illustrated in Fig. 10. The bone plates 1 - or at least some of the bone plates - are of the kind having plate holes with internal threads, as described in this text hereinbefore. In addition, the surgical plate system comprises a plurality of bone screws 3 with a first external thread extending along the screw shaft and a second external thread extending along the screw head. In addition, the surgical plate system comprises a plurality of the anchoring pins 5. In Fig. 10, also drill tools 42, a screw driver 43 for the surgical screws 3 and a sonotrode 46 are illustrated. The sonotrode 46 is illustrated to comprise a coupling protrusion 49 for cooperating with the coupling hole 28 of the anchoring pins 5.

[0098] The surgical plate implant system may further comprise an apparatus for generating the mechanical vibration needed for implanting the anchoring pins. Figure 11 schematically depicts such an apparatus that generates mechanical vibration energy. The apparatus comprises a control device 51 and a handpiece 52 connected to the control device. The handpiece 52 comprises a vibration generator equipped to set a sonotrode 46 into mechanical oscillation. The electrical signal for activating the vibration generator may be supplied by the control device 51 via a connecting line 7. It may however also be possible that the control device merely transmits electrical power and possibly a command, and that the handpiece itself comprises a control for generating the electrical signal - or even that the apparatus consists of the handpiece only, the handpiece comprising an appropriate power supply, for example comprising a battery or a connection to mains.

[0099] Figure 12 shows an anchoring pin 5. The anchoring pin 5 with a pin shaft 21 and an essentially cylindrical pin head 23. In contrast to the embodiments of Figs. 4 and 5, the pin head has a proximal stop structure 61 defining a distally facing stop 62 that defines the axial position of the anchoring pin 5 at the end of the anchoring process. While Fig. 12 illustrates the anchoring pin with pin head 23 that is essentially (with the exception of the stop structure 61) cylindrical, such stop structure is an option also for anchoring pins with a conical pin head. The proximal stop structure 61 with the distally facing stop 62 at the end of the anchoring process will abut against the shoulder formed by the proximal surface of the bone plate in embodiments of the kind shown in Figs. 3 and 6 but may also abut against a countersunk shoulder in other embodiments; the coupling portion being constituted by that portion surface of the plate hole that extends between the shoulder and the distal mouth of the plate opening. Instead of being constituted by a flange-like, circumferential ridge, the stop structure could also comprise discrete protrusions or have any other suitable shape.

[0100] Figure 13 illustrates the principle that an implant system may comprise bone plates 1 of different thicknesses t but with plate holes having same coupling portions. In Fig. 13, the thickness t of the plate on the right is larger than the thickness t of the left plate, whereas the depths d (axial extension) of the coupling portions between the distal mouths 71 of the plate openings and the proximally facing shoulders 72 are the same. Also, for the bone plate on the left, t=d holds, whereas for the bone plate on the right t>d. The proximally facing shoulders 72 serve, together with the stop structures 61, for defining the axial positions of the anchoring pins 5 at the end of the anchoring process.

[0101] The anchoring pins 5 for the different plate holes may, therefore, be identical with each other so that no specialized anchoring pins are required for the different bone plates. Also, the same bone screws may be used for both kinds (all kinds) of bone plates of different thicknesses.

Claims

WHAT IS CLAIMED IS:

1. An implant system, comprising a bone plate with a plurality of plate holes having an internal thread-like structure, and at least one bone screw having a screw shaft and a screw head, the screw shaft having a first external thread equipped to engage with bone tissue, and the screw head having a second external thread shaped to engage with the internal thread-like structure, the implant system further comprising at least one anchoring pin, the anchoring pin having a pin shaft and a pin head, the pin shaft comprising a thermoplastic material liquefiable by mechanical energy and the pin head comprising a thermoplastic material liquefiable by mechanical energy, wherein the pin shaft is dimensioned to be inserted through one of the plate holes, and the pin head is dimensioned for not being able to be moved through this plate hole.

2. The implant system according to claim 1, wherein the internal thread-like structure is tapering, whereby an outer side mouth of the plate opening has a larger area than a bone side mouth of the plate opening.

3. The implant system according to claim 2, wherein the pin shaft is dimensioned to be movable through the entire plate opening, and wherein the pin head is dimensioned to be movable through the outer side mouth but not through the bone side mouth.

4. The implant system according to claim 3, wherein a diameter of the pin shaft is smaller than a diameter of the plate hole by between 0. 1 mm and 0.5 mm.

5. The implant system according to any one of the previous claims, wherein an axial extension of the pin head corresponds to at least 80% and at most 120% of a thickness of the bone plate around the plate opening.

6. The implant system according to any one of the previous claims, wherein the pin shaft essentially has a shape of a circular cylinder, with at least one step.

7. The implant system according to any one of the previous claims, wherein the pin head has a shape of a circular cylinder or cone, optionally with energy directing structures on a peripheral surface.

8. The implant system according to any one of the previous claims, wherein the anchoring pin has a proximal coupling hole reaching from a proximal end face of the pin head distally into the anchoring pin.

9. The implant system according to any one of the previous claims, wherein the anchoring pin consists of the thermoplastic material.

10. The implant system according to any one of the previous claims, wherein the thermoplastic material is resorbable.

11. The implant system according to any one of the previous claims, further comprising a sonotrode equipped to couple mechanical vibration energy, especially ultrasonic energy, and a pressing force into the anchoring pin.

12. The implant system according to any one of the previous claims, further comprising a vibration generating apparatus equipped to generate mechanical vibration energy for being coupled into the anchoring pin via a sonotrode.

13. The implant system according to any one of the previous claims, further comprising a screwing tool adapted to the bone screw of screwing the bone screw into bone tissue.

14. The implant system according to any one of the previous claims, comprising a plurality of the bone plates, at least two of the bone plates having different thicknesses, but having plate openings with identically dimensioned coupling portions.

15. A method of implanting a surgical bone plate, the method comprising the steps of:• providing the bone plate, the bone plate having a plurality of plate openings with an internal thread-like structure,• preparing the bone tissue for implantation by locally removing at least cortical bone tissue at locations corresponding to locations of at least some of the plate openings to yield prepared locations;• providing at least one anchoring pin, the anchoring pin having a pin shaft and a pin head, the pin shaft comprising a thermoplastic material liquefiable by mechanical energy and the pin head comprising a thermoplastic material liquefiable by mechanical energy;• implanting the anchoring pin by inserting the pin shaft through a first one of the plate holes and bringing it into contact with one of the prepared locations and using a vibrating tool to press the anchoring pin against the bone tissuewhile mechanical vibration energy is coupled into the anchoring pin until thermoplastic material of the pin shaft is flowable and flows into structures of the bone tissue and thermoplastic material of the pin head is flowable and flows into structures of the internal thread-like structure of the first one of the plate holes;• providing a surgical screw having a first external thread equipped to engage with bone tissue, and the screw head having a second external thread shaped to engage with the internal thread-like structure;• implanting, before or after implanting the anchoring pin, the bone screw by screwing the bone screw through a second one of the plate holes into the bone tissue until the second external thread engages with the internal threadlike structure of the second one of the plate holes.

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