Bead head for locking bone screws

Abstract

The invention relates to a bead to be inserted between a locking bone screw head (2) and a receiving hole (34) in a bone plate (10), and to an implant construction comprising such a bead.

Description

Technical Field

[0001] The present invention relates to a bead that is to be inserted between the head of a locking bone screw and a receiving hole in a bone plate, and to an implant construction including such a bead. Background Art

[0002] A defining feature of the present invention is the slightly conical or tapered head of the screw. Commonly known as a "Morse taper", this mechanical solution for the precise, reliable, and reversible coupling of mechanical components, invented in 1854 by Samuel Colt (famous for Colt pistols), was adopted by Stephen Ambrose Morse, who also invented a similarly lucrative auger and founded the Morse Twist Drill and Machine Co. in New Bedford, MA in 1864, one of the unique success stories of 19th-century American industrial manufacturing that continues to this day.

[0003] The original and still most commonly used Morse taper in machinery is defined as 1:20 (e.g., a 1 mm change in diameter over an axial length of 20 mm), which corresponds to a total taper angle of 2.86 degrees. In orthopedic devices, such as for coupling a head to a stem in a modular hip prosthesis (P. Hernigou, S. Queinnec, C.H.F. Lachaniette: One hundred and fifty years of history of the Morse taper: from Stephen A. Morse in 1864 to complications related to modularity in hip arthroplasty, International Orthopaedics (SICOT) (2013) 37:2081 - 2088 (P. Hernigou, S. Queinnec, C.H.F. Lachaniette: Un siècle et demi d'histoire du tréfil Morse : de Stephen A. Morse en 1864 aux complications liées à la modularité en chirurgie de la hanche, Orthopédie Traumatologie Chirurgie Vertebrale (OTCV) (2013) 37:2081 - 2088)), the taper has been changed to 1:10 (5.72 degrees), with many geometric and fit variations. An important unifying feature of all Morse-type tapers is the self-holding or self-locking property of the connection - once the parts are assembled together, they will not separate unless a large force is used to separate them. The smaller the taper angle, the tighter the connection. However, the smaller the angle, the more stringent the manufacturing tolerances required. The final decision is a compromise between requirements, the risk of separation and jamming, the expected coefficient of friction in use, and manufacturing costs.

[0004] The first known and documented use of the Morse taper locking screw for internal fixation plates was in a research and development project led by the present inventor at the AO Research Institute, Davos, Switzerland in 1986, with the code name FIXIN (from FIXator INtern) (see for example USP 5,151,103 by Tepic et al.). The tapered head of the screw is locked in the plate via an interposed, slotted and thus expandable ball. These plates and screws were only used for osteosynthesis in experimental sheep to study the effect on the periosteal blood supply. Whether the design was strong enough to stabilize fractured bones was not tested because the first discovery of osteosynthesis in this project had already demonstrated the damage to endosteal blood perfusion by the conventional use of bicortical screws. The decision to switch to only single cortical screws made the expandable ball unnecessary, and the project adopted a new name (PC-Fix for Point Contact Fixator), and the new élan at the Davos Institute continued to prove the many advantages of keeping the blood perfusion of fractured bones as intact as possible through extensive preclinical and clinical tests over the next decade (The biomechanics of the PC-Fix internal fixator, Tepic, S. et al., Injury, Volume 26, B5-B10). Locking the screw head in the plate was necessary, but maintaining blood perfusion through the periosteum required more features in the plate design, mainly on the bone-facing side of the plate. At that time, the risk of the screw head getting stuck in the plate was reduced by the unique geometry of the coupler. The Morse taper coupler used in the machine was made of a solid, hardened material and ground to a high precision and smooth surface finish. The screws and plates of PC-Fix were machined from relatively soft c.p. titanium, with grooves for the screwdriver on the head. Preventing jamming due to so-called cold welding of the screw head in the plate required some careful observation and design, including finite element analysis. The remaining risk caused by over-tightening of the screw was considered (including the surgeons involved in the clinical study of nearly 2,000 forearm fracture patients) to be acceptable. These results were published when the number of registered follow-up patients reached approximately 1,200, Eijer H, Hauke C, Arens S, Printzen G, Schlegel U, Perren SM. Local anti-infective effects, clinical and experimental results of PC-Fix and implant design on the development of postoperative infections.Injury. September 2001; 32 Suppl 2: B38-43 (Eijer H, Hauke C, Arens S, Printzen G, Schlegel U, Perren SM. PC-Fix and local infection resistance - influence of implant design on postoperative infection development, clinical and experimental results. Injury. 2001 Sep; 32 Suppl 2: B38-43).

[0005] However, three commercial partners of the AO Foundation, which were later incorporated into Synthes and subsequently acquired by Johnson & Johnson and are now part of DePuy-Synthes, did not commercialize the PC-Fix, and no other company incorporated all its key features into clinical use until the Advanced Locking Plate System by Kyon in 2007 was used in veterinary applications (USP 8,968,368, Tepic).

[0006] The locking mechanism of... is different from the one used in PC-Fix. The screw head is conical but has a larger angle and is thus not self-retaining. A similar design is used for the long flat head bolts of automobile wheels - The threads of the screw do engage with the (partial) threads in the plate holes when the screw is screwed into the bone. This design offers new opportunities - each plate hole can accept common screws of reduced size, with the same angular freedom as the original DCP (Dynamic Compression Plate of AO / Synthes) plates, and allows the fracture compression function of the DCP.

[0007] During the patent protection of PC-Fix (the AO Foundation transferred the patent rights to its commercial partners), the Morse taper screw head was only used in Kyon's "Zurich Cementless" THR (USP 5,458,654, Tepic) and the FIXIN TM plate system of Intrauma in Rivoli TO, Italy. The Intrauma locking system relies on screws similar to those of PC-Fix, but a machine-threaded ring is inserted between the plate and the screw head. These screw heads are always stuck in the ring, but the ring can be removed from the plate if needed. Summary of the Invention

[0008] The present invention discloses a solution for angling a bone screw having a conical self-locking head in a bone plate with a conical hole. By adding a bead head, especially a spherical bead head interposed between the screw head and the plate hole, an angle can be formed. To allow the screw to be inserted at an angle, the hole in the bone is drilled in the bone with the aid of a special drill sleeve. Then, the bead with the conical hole is inserted into the plate hole in a suitable orientation, for example with the aid of a bead retainer. Finally, a screw reduced in size compared to a nominal locking screw is inserted through the hole of the bead into the bone. This locks the head of the angled screw within the bead and locks the bead within the plate hole. In addition, the locking screws can be augmented by the bead to allow them to be used as dynamic compression screws.

[0009] In a first aspect, the present invention relates to a bead, especially a spherical bead, having a conical hole for interposition between a locking bone screw head and a receiving hole in a bone plate. In medical applications, the bead can be used for bone fixation, typically in combination with a mating bone screw and bone plate.

[0010] In certain embodiments, the conical hole of the bead tapers at a self-locking angle that can be between about 2.7° and about 8°, especially between about 4° and about 6.5°, more especially about 5.7°, corresponding to a screw taper of 1:10.

[0011] In a second aspect, the present invention relates to an implant construction comprising a bead as described above, especially a spherical bead, and a locking screw, and to an implant construction comprising a bead as described above, especially a spherical bead, and a locking screw, and a bone plate. The bead includes a conical hole, and the bone screw is adapted to be inserted through the conical hole of the bead into the bone. The screw can include a self-locking head for fixation. The bone plate includes at least one receiving hole adapted for bead insertion.

[0012] In certain embodiments, the screw includes a conical head, wherein the total angle of the conical screw head is greater than the angle of the hole in the bead, especially by about 0.2° to about 0.4°, more especially by about 0.3°.

[0013] In certain embodiments, the total angle of the conical hole in the bone plate is substantially the same as the total angle of the conical hole in the spherical bead.

[0014] In a third aspect, the present invention relates to a method for fixing a bone plate to a bone, comprising the steps of:

[0015] (I) drilling at least one hole in the bone,

[0016] (ii) providing a bone plate including at least one conical hole,

[0017] (iii) Providing beads, especially beads having conical holes, wherein the beads are adapted to fit into the conical holes of the bone plate,

[0018] (iv) Inserting the beads into the conical holes of the bone plate, and

[0019] (v) Inserting a bone screw having a conical self-locking head through the conical hole of the bead into the bone.

[0020] The beads, bone screws and bone plates of the present invention can be made of any suitable material such as metals or metal alloys. In a particular embodiment, the beads, bone screws and / or bone plates are made of titanium or a titanium-containing alloy.

[0021] There is sufficient evidence that titanium and its alloys can provide all the required mechanical requirements, but have improved biocompatibility compared to stainless steel (such as the most commonly used 316L or EN 1.4404).

[0022] However, the human orthopedic industry has been reluctant to abandon its dependence on stainless steel, especially in trauma devices. Thus, in certain embodiments, the beads, bone screws and / or bone plates are made of stainless steel.

[0023] In addition to the screw holes, the bone plates of the present invention can have an external shape that is almost the same as the original . Minor adjustments are made to increase the overall strength by taking advantage of the now smaller holes. The preferred material for the plate is c.p. titanium grade 4, but for plates that may require additional strength, a titanium alloy (Ti6Al4V), so-called titanium grade 5, can also be selected. Titanium grade 4 is slightly weaker than the stainless steel 316L most commonly used for plates and screws. However, the shape and size of the implant can be easily adjusted to compensate for this difference. For example, the original size 10 plate having the same external dimensions as the reference plate (Synthes' DCP 3.5 made of stainless steel 316L) for comparison has a bending strength 20% higher. Under typical conditions of heat treatment and cold working for implants, titanium grade 5 has a strength approximately 50% higher than grade 4 or stainless steel 316L. The disadvantage of using grade 5 is lower ductility, so the plate cannot be bent as easily as grade 4 or stainless steel.

[0024] The bone screws of the present invention are preferably made of a titanium-aluminum-niobium alloy (Ti6Al7Nb or TAN). Decades after its invention, TAN has attracted some attention in the industry. The mechanical properties are the same as those of TAV (Ti6Al4V), but due to the replacement of highly toxic vanadium with very inert niobium, the biocompatibility of TAN is comparable to that of pure titanium. The adhesion of bone to TAN is so excellent that the removal of integrated TAN implants may be more difficult than that of any other metal implant.

[0025] The plate of the present invention is preferably subjected to micro-shot peening to improve the fatigue strength.

[0026] Locking screws with Morse taper type heads, such as in PC-Fix, can be used alone in almost all cases in their locking configuration at a 90-degree angle to the plate, but occasionally angled screws relative to the plate and compression applied in the fracture or osteotomy plane are required.

[0027] To provide the same screw angle amplitude as in the original (which is the same as in DCP), a new solution disclosed herein is achieved by adding beads. The outside of the beads can be spherical and provided with a conical hole to receive the locking head of the screw. The use of beads goes beyond the obvious conversion from a conical head to a spherical head. If a screw with a spherical head is inserted into the conical hole of the plate, the plate cannot maintain its axial position on the bone - a screw with a spherical head (or augmented with a bead head) inserted at an angle will hit the top of the receiving hole before the head can be seated in the plate hole. If only one such screw is used per plate, this axial slip is tolerable, but the angled screw must be inserted as the first screw, which is usually an unacceptable limitation for fixing the plate to a fractured bone. A special drill sleeve can be used to drill the hole for the angled screw. Then, with the aid of a bead retainer, the bead is inserted into the screw hole in the plate. Once the bead is firmly seated in the plate hole, a locking screw, such as a screw with a reduced size compared to a normal screw, can be screwed through the bead into the bone until it is securely locked in the bead. Then, the bead is also locked in the plate, although not as firmly as a conical head screw. The use of beads in combination with locking screws is only rarely shown, for example for screws very close to the joint or for screws that may be used as compression screws. It should be noted that the dynamic compression principle cannot be used unless the fracture (osteotomy) is close to transverse. In veterinary (but also in human) trauma surgery, the vast majority of fractures are oblique, spiral or comminuted and are therefore not suitable for treatment by interfragmentary compression. Description of the Drawings

[0028] Figure 1 Shows a locking bone screw with a Morse taper type head.

[0029] Figure 2 Shows transverse and longitudinal sectional views of a bone plate with a conical hole.

[0030] Figure 3 Shows a perspective view of a bone plate segment with a conical hole.

[0031] Figure 4 Shows transverse and longitudinal sectional views of a locking screw inserted into the plate.

[0032] Figure 5 Shows a bead for the head of a conical locking screw.

[0033] Figure 6 Shows a bone screw inserted through a bead angled in a transverse plane.

[0034] Figure 7 Shows a bone screw inserted through a bead angled in a longitudinal plane.

[0035] Figure 8 Shows a drill bit sleeve placed in a conical hole angled in the longitudinal direction before drilling into bone.

[0036] Figure 9 Shows the use of beads to allow angling of the bone screw.

[0037] Figure 10 Shows the placement of a bias screw for interfragmentary compression of fracture fragments using a locking screw augmented by beads. DETAILED DESCRIPTION

[0038] Figure 1 Shows the bone screw 1 of the present invention having a conical head 2 that tapers at a total taper angle 3. The angle 3 is less than the angle required for the Morse taper self-locking function. The range of the angle 3 is typically limited by machining tolerances at the lower end to approximately 2.9 degrees, corresponding to a 1:20 taper. At the upper end, an 8-degree angle requires a high coefficient of friction, which is not easily provided in vivo as lipids cover all implant surfaces. A compromise value of 5.7 degrees corresponding to a 1:10 taper has proven satisfactory. However, if the receiving hole in the plate has this angle, the compression between the head and the plate will be concentrated at the lower end of the screw head with a solid core, and thus have a higher stiffness. To reduce this stress concentration, the angle of the screw head should be greater than the angle of the receiving hole - a value of 6 degrees has been found to provide a satisfactory stress distribution for using a hexalobular (Torx TM type) screwdriver recess 4. When the screw is inserted into a hole in the plate with an angle of 5.7 degrees, the first contact occurs at the top of the hole, and as the head deforms on the screwdriver recess with higher compliance, the contact extends to the bottom of the hole and the lower end of the screw head. This reduces the risk of cold welding. The threads 5 of the screw engage the bone along the threads cut in the bone by the cutting grooves 6 of the screw.

[0039] Figure 2Shows transverse (a) and longitudinal (b) sectional views of bone plate 10, which has holes 14 for receiving the conical heads of bone screws. The plate has a width 12 and a height 13, and has facets 11 along the upper edge to facilitate soft tissue coverage. The holes 14 are tapered towards the upper surface, with the total angle 15 being slightly less than the angle of the screw head. For a preferred self-locking combination with a nominal taper of 1:10, the angle of the hole is approximately 5.7 degrees and the head angle is approximately 6 degrees. On the bone-facing surface, the holes 14 are surrounded by grooves 16, and the combination of the grooves 16 and transverse incisions 17 reduces the potential contact with the bone to a small area 18. On the underside of the plate, the holes 14 are provided with longitudinally oriented cylindrical undercuts 19, which allow the screws to be angled.

[0040] Figure 3 Shows a perspective view of a section of plate 10 with tapered holes 14. View (a) of the top 20 of plate 10 shows the facets 11 and undercuts 19. View (b) of the underside of the plate shows the grooves 16 surrounding the holes 14, the transverse incisions 17, the potential bone contact area 18, and the cylindrical undercuts 19.

[0041] Figure 4 Shows transverse (a) and longitudinal (b) sectional views of bone plate 10, in which locking screw 1 is inserted into the plate hole at 90 degrees. The head 2 of the screw 1 is fully seated and is thus locked in plate 10 in all degrees of freedom, forming a construction capable of transferring all loads between the plate and the bone. The plate and the screw essentially become a single implant unit. This is important not only for mechanical reasons of load transfer, but also for avoiding any movement between the plate and the screw, which would cause fretting corrosion and thus release very fine metal particles and ions. The tissue response to such debris can produce serious medical sequelae, both local and potentially systemic. The cylindrical undercuts 19 do reduce the contact area between the screw head and the plate hole, but there is still sufficient contact at the sides of the hole to provide a secure lock.

[0042] Figure 5 Shows the central item of the present invention, the bead head 30. The external shape of the bead is a segment of a sphere with a diameter 31. The height 32 of the bead is sufficient to cover most of the conical head of the screw inserted into the conical hole 34 at a total angle 33. For a nominal taper of 1:10, the angle 33 is approximately 5.7 degrees, i.e., approximately the same as the angle in the bone plate hole.

[0043] Figure 6A cross-sectional view of a bone screw 40 inserted through a bead 30 into a bone plate 10 is shown. The diameter 31 of the bead is selected and machined to precise tolerances such that the depth 35 by which the bead is recessed in the hole 14 is approximately equal to about half of the hole height, e.g., about 40% to about 60%, especially about 50%. In this cross-sectional view, the screw is slightly inclined relative to the plate - the typically required range is + / - 5 degrees. The diameter of the screw 40 is less than the diameter of a nominal locking screw that would fit in the hole 14 when inserted at 90 degrees. For practical reasons of bead production and bone screw selection, the difference in the two screw diameters is about 0.9 mm to about 1.1 mm, especially about 1 mm. The total angle 42 of the conical head 41 of the screw 40 is the same as Figure 1 the total angle 3 of the screw head 2 of the screw 1 in

[0044] Figure 7 A longitudinal cross-sectional view of the screw 40 and the plate 10 is shown, where the screw is inclined to a maximum angle of about 30 degrees. This can be achieved by a cylindrical undercut 19 in the plate 10. The bead 30 still engages fully with the conical hole in the plate 10 to lock, but this contact is only a line contact, and thus the resistance to bending loads is lower. However, compared to a conventional screw inserted into a conventional non-locking plate, the screw is less likely to become loose from the bone due to unscrewing, and the possibility of fretting is also greatly reduced.

[0045] Figure 8 A drill sleeve 50 with a spherical tip 52 is shown, the diameter of the spherical tip 52 being the same as the diameter of the bead to be used in this hole of the plate 10. The sleeve is placed straight down into the hole and then inclined as required. The nose 53 of the sleeve matches the outer diameter of the bone screw inserted at an angle. The drill 51 matches the core diameter of the screw. Once a hole is drilled in the bone, the drill is retracted; the sleeve is rotated about 90 degrees relative to the plate and removed from the plate. A careful examination of the geometry shows that the sleeve cannot be removed from the plate hole if the plate does not slide longitudinally on the bone while the drill is still in the bone.

[0046] Figure 9 The insertion sequence of the angled screw is shown. The bead 30 is held on a bead holder 54, cross-sectional view (a), and the bead holder 54 can be inserted in the direction of the pre-drilled hole 56 in the bone 55. By applying a slight pressure on the bead 30 via the bead holder 54, the bead can be positioned in the plate hole and will remain in the plate hole when the bead holder is removed. The bead holder is made of plastic and its tip is slotted just enough to hold the bead for manipulation. Once the bead 30 is in place, cross-sectional view (b), the screw 40 can be inserted through the bead and screwed into the bone 55, locking its head in the bead and locking the bead in the plate.

[0047] Figure 10 Shows the use of a screw 40 amplified by beads 30 to produce so-called dynamic compression. Some of the holes in the bone plate 10 are made into compression holes by extending the conical hole 14 into the hole 61, and the hole 61 is offset from the axis of the hole 14 by a distance 60. If the hole in the bone is drilled with a special sleeve, the sleeve can be centered in the hole 61, and the screw 40 and the beads 30 pre-placed on its head are inserted through the plate 10 and screwed down into the bone 55, and the plate will be longitudinally displaced as shown by the arrow 62.

[0048] The invention of the screw head beads disclosed herein makes it possible to use smaller diameter inclined screws in bone plates with conical holes in addition to the vertical, nominally sized locking screws with Morse taper heads. If desired, beads-amplified screws can also be used to produce dynamic compression. In fact, only one type of screw - the locking screw with a tapered head - can meet all the applications of the bone plate. In most cases, the use of beads is optional - only in cases of periprosthetic fractures and very few cases where dynamic compression may be required. When needed, an important advantage of using beads is also the elimination of micromotion between the screw and the plate, which is a major risk for harmful tissue responses to the implant.

[0049] The following items of the specification further describe the features of the present invention:

[0050] 1. An implant structure, comprising a bone locking plate (10), a bone locking screw (40), and a spherical bead (30), wherein the bead (30) includes a conical hole (34) and is to be interposed between the tapered head (41) of the bone locking screw (40) and the receiving hole (14) in the bone locking plate (10), characterized in that the receiving hole (14) in the bone locking plate (10) is conical.

[0051] 2. An implant structure, comprising a bone locking plate (10), a bone locking screw (40), and a spherical bead (30); wherein the bead (30) includes a conical hole (34) and is to be interposed between the tapered head (41) of the bone locking screw (40) and the receiving hole (14) in the bone locking plate (10), characterized in that the receiving hole (14) in the bone locking plate (10) is conical, and wherein the total angle (15) of the conical receiving hole (14) in the plate (10) is approximately the same as the total angle (33) of the conical hole (34) in the bead (30).

[0052] 3. The implant structure according to item 1 or 2, wherein the total angle (42) of the conical screw head (41) is approximately 0.2 to approximately 0.4 degrees larger than the total angle (33) of the conical hole (34) in the bead (30), preferably approximately 0.3 degrees larger.

[0053] 4. The implant structure according to any one of Items 1-3, wherein the conical bead hole (34) tapers at a self-locking angle (33).

[0054] 5. The implant structure according to any one of Items 1-4, wherein the conical bead hole angle (33) is between approximately 2.7 degrees and approximately 8 degrees, preferably approximately 5.7 degrees, corresponding to a taper of 1:10.

Claims

1. An implant structure, the implant structure comprising a bone locking plate (10), bone locking screws (40) and spherical beads (30), wherein, The spherical bead (30) includes a conical hole (34) and is to be inserted between the conical head (41) of the bone locking screw (40) and the receiving hole (14) in the bone locking plate (10), characterized in that the receiving hole (14) in the bone locking plate (10) is conical, and wherein, on the lower side of the bone locking plate, the receiving hole is provided with a longitudinally oriented cylindrical undercut to allow angulation of the bone locking screw.

2. The implant structure according to claim 1, wherein, The total angle (15) of the conical receiving hole (14) in the bone locking plate (10) is the same as the total angle (33) of the conical hole (34) in the spherical bead (30).

3. The implant structure according to claim 1 or 2, wherein, The total angle (42) of the conical head (41) of the bone locking screw is 0.2 degrees to 0.4 degrees greater than the total angle (33) of the conical hole (34) in the spherical bead (30).

4. The implant structure according to claim 3, wherein, The total angle (42) of the conical head (41) of the bone locking screw is 0.3 degrees greater than the total angle (33) of the conical hole (34) in the spherical bead (30).

5. The implant structure according to claim 1 or 2, wherein, The conical hole (34) of the spherical bead tapers at a self-locking angle.

6. The implant structure according to claim 1 or 2, wherein, The total angle (33) of the conical hole of the spherical bead is between 2.7 degrees and 8 degrees.

7. The implant structure according to claim 6, wherein, The total angle (33) of the conical hole of the spherical bead is approximately 5.7 degrees, corresponding to a taper of 1:10.

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