MEDULLARY ROD OF A JOINT ENDOPROSTHESIS

Description

[0001] The present invention relates to a medullary rod of a joint endoprosthesis, intended to be placed in the medullary canal of a long bone which comprises the joint concerned.

[0002] It is well known to restore a joint including a long bone by means of a joint endoprosthesis. This endoprosthesis comprises a tapered medullary implant intended to be placed on the long bone by its insertion into the medullary canal of this bone, duly resected, and a conjugate implant intended to be placed on the other bone constituting the joint. In particular, a shoulder endoprosthesis comprises a humeral implant, intended to be placed on the long bone constituting the humerus, and a glenoid implant intended to be placed on the scapula. The humeral implant comprises a tapered medullary rod, intended to be inserted into the medullary canal of the humerus, after milling and reaming the proximal part of this bone forming the medullary canal; the humeral implant also comprises a proximal part intended to be mounted on this tapered rod, which forms an articular surface.This proximal part may comprise, in the case of a so-called "anatomical" shoulder endoprosthesis, a prosthetic head forming a convex articular surface, capable of cooperating with a concave conjugate articular surface; in the case of a so-called "inverted" shoulder endoprosthesis, the proximal part comprises a cavity forming a concave articular surface, which is intended to cooperate with a conjugate convex articular surface formed by the glenoid implant.

[0003] In a hip endoprosthesis, the medullary implant is a femoral stem and the conjugate implant is an acetabulum placed in the acetabulum of the patient's pelvis.

[0004] A conventional spinal rod is formed by a solid, solid piece made of a metallic material, generally titanium alloy. Such a rod is capable of withstanding the numerous and repeated stresses experienced by the endoprosthesis due to the patient's movements.

[0005] However, it appears that cases of detachment of a medullary rod from the bone receiving this rod are frequent. It is known that this detachment is caused by the fact that the rod has a significant rigidity, significantly greater than that of the bone, which is a living material that is in perpetual regeneration. In addition, it appears that the bone does not regenerate at the level of certain bone parts having received the endoprosthesis because these bone parts are no longer stressed by the forces exerted on the endoprosthesis. Bone lysis can then be observed along the endoprosthesis, as well as melting of the tuberosities ensuring the anchoring of this endoprosthesis.

[0006] The invention aims to remedy this problem.

[0007] Document US 2012 / 0064288 A1 discloses a spinal rod according to the preamble of claim 1.

[0008] For this purpose, the invention relates to a medullary rod according to claim 1.

[0009] It will be understood that the internal and external edges of the long bone in question are the longitudinal portions of that bone which lie respectively, when the bone is viewed in the frontal plane, on the internal side of the patient and on the external side of that patient.

[0010] The medullary rod according to the invention thus does not have a monolytic, solid and massive structure, as is the case with a conventional medullary rod made of metallic material, but has a structure with an envelope wall and a plurality of partitions inside this envelope wall, spaced from each other and forming a body with said envelope wall. This structure proves to be particularly interesting in that it is capable of supporting the load exerted on the medullary rod, due to the plurality of partitions and the fact that these partitions form a body with the envelope wall, while having a slight deformation capacity close to that of a bone.

[0011] The envelope wall may have a thickness ranging from approximately 0.5 to 2 mm. This thickness varies depending on the size of the spinal rod, such a rod being available in different sizes adapted to different patient sizes. It may be around 0.5 mm for a smaller rod in a range of spinal rods, and around 2 mm for a larger rod in this range.

[0012] Each septum can be about 0.5 to 1.5 mm thick. The space between the septa can be about 1 to 2 mm. These dimensions also vary depending on the size of the spinal rod, as such rods come in different sizes to suit different patient sizes.

[0013] The number of septa can range from four to six septa on a spinal rod intended for a small patient and from six to ten septa on a spinal rod intended for a large patient.

[0014] The partitions can be flat or have a wavy profile.

[0015] The medullary stem may also comprise a plurality of second partitions, generally parallel to each other, having an orientation different from that of said first partitions and thus intersecting these first partitions, each of these second partitions forming a body with the envelope wall of the medullary stem and forming a body with said first partitions.

[0016] The set of first and second partitions thus presents, seen in cross section, a cross-braced structure.

[0017] Said first partitions may have a purely internal-external orientation, that is to say extend parallel to a frontal plane of the patient; these first partitions, and second partitions as mentioned above, may also extend at non-right angles relative to this frontal plane; the angles formed by the first partitions relative to this frontal plane may be equal to the angles formed by the second partitions relative to this same frontal plane, so that the first and second partitions are arranged in a St. Andrew's cross relative to this frontal plane.

[0018] Said second partitions allow the optimisation of the resistance of the endoprosthesis to the repeated stresses that such an endoprosthesis is intended to undergo, while having a slight deformation capacity corresponding to the Young's modulus of the bone, thus limiting the risk of separation of the endoprosthesis from this bone.

[0019] The cells delimited by said first and second partitions may have a substantially circular, octagonal, triangular or square cross-section, depending on the shape and / or orientation of said partitions relative to each other.

[0020] Alternatively, said first partitions and second partitions delimit between them cells having a honeycomb structure.

[0021] Such a medullary rod proves to present optimal performances for obtaining the desired result, namely providing an endoprosthesis which is both resistant and presents a slight degree of elastic deformability, close to that of a bone.

[0022] The medullary stem has a longitudinal internal structure, with a solid cross-section, forming a body with the envelope wall and said first partitions and second partitions when these second partitions are present.

[0023] This structure allows for longitudinal reinforcement of the stem.

[0024] This internal structure is in the form of a central internal bar extending longitudinally between the distal end of the rod, or an area of this rod close to this distal end, and the proximal end of the rod, or an area close to this proximal end. This bar may be straight between these ends or these areas, or may be curved at its proximal portion so as to follow a curved shape at the proximal level that the medullary rod may have.

[0025] Said internal structure could also, according to an example not covered by the invention, be in the form of a solid proximal portion connected to a longitudinal portion extending along the internal edge of the rod. This longitudinal portion may be of constant cross-section or may have a cross-section which reduces in the distal direction so that the longitudinal portion has a tapered shape when viewed in the frontal plane.

[0026] The invention will be better understood, and other characteristics and advantages thereof will appear, with reference to the attached schematic drawing; this drawing represents, by way of non-limiting examples, several possible embodiments of the endoprosthesis concerned. In this drawing: [ Fig. 1 ] is a view of the humeral implant of a shoulder prosthesis, in the frontal plane, according to a first embodiment; three components of this implant being shown exploded; [ Fig. 2 ] is a view of the implant similar to the figure 1 , the three components being in a state of assembly; [ Fig. 3 ] is a view of the implant in the sagittal plane, from the external side of this implant; [ Fig. 4 ] is a view of a medullary rod that the implant includes, in section along plane IV-IV of the figure 2 ; [ Fig. 5 ] is a view of the medullary stem in section along plane VV of the figure 2 ; [ Fig. 6 ] is a view of the medullary stem of the implant according to a second embodiment, in the sagittal plane, from the internal side of this stem; [ Fig. 7 ] is a view of the stem in section according to plane VII-VII of the figure 6 ; [ Fig. 8 ] is a view of the stem similar to the figure 7 , according to the invention; and [ Fig. 9 ] a view of the stem similar to the figure 7 , according to an example not covered by the invention. Description détaillée

[0027] THE figures 1 à 3 represent the humeral implant 1 of a shoulder joint endoprosthesis, comprising a medullary rod 2, a joint head 3 and a neck 4 for joining this head to this rod. The endoprosthesis also comprises a glenoid implant intended to be placed on the patient's scapula, not described because it is not in itself part of the invention.

[0028] The articulation head 3 is, in the example shown, of the so-called "anatomical" type, that is to say forming a convex articular surface, capable of cooperating with a conjugate concave articular surface formed by the glenoid implant. This head is of the conventional type, in particular made of metallic material, and comprises a slightly conical receiving cavity 5 with wedging of a corresponding slightly conical tip formed by the neck 4. Also in the example shown, and as visible on the figure 3 , the cavity 5 is off-centered relative to the head 3 so as to allow the position of the head 3 to be varied relative to the rod 2 according to the angular position of placement of this head on the tip 4, this to allow optimal adaptation of the humeral implant 1 to the specific anatomy of the patient. To allow identification of the angular position of the head 3 relative to the rod 2, the face of this head facing this rod is graduated at the periphery of this face.

[0029] The neck 4 is of a conventional type, in particular made of metallic material; in addition to the aforementioned end piece for assembly to the head 3, it comprises, opposite this first end piece, a matching end piece for its assembly to the rod 2, the latter comprising a cavity 6 of a shape adapted to receive this second end piece with wedging.

[0030] The medullary rod 2 has a shape suitable for insertion into the medullary canal of a humerus, after cutting the proximal part of this humerus and appropriate reaming of the medullary canal of the bone.

[0031] As visible on the figures 4 And 5 , the rod 2 has an envelope wall 10, defining the external shape that this rod has, and a plurality of partitions 11 defining a honeycomb structure, these partitions 11 filling the entirety of the hollow space delimited internally by this wall 10, with the exception of a proximal solid part delimiting the cavity 6.

[0032] The partitions 11 form a body with the wall 10, the entire rod 2 being produced by a manufacturing process called "3D printing" or "additive", known in itself, by which the object to be manufactured is produced by the constitution of successive layers of hardenable material; this material is hardened in specific locations of each layer, intended to constitute a layer of the object to be obtained, and is in particular polymerized by means of a laser.

[0033] Some of the partitions 11 which delimit the honeycomb cells are oriented in an internal-external direction of the rod 2, that is to say in a left-right direction on the figure 5 while the other partitions 11 are oriented in an anterior-posterior direction, that is to say in an up-down direction on the figure 5 The longitudinal axes of the alveoli delimited by the partitions 11 are moreover, in the embodiment shown, parallel to the longitudinal axis of the medullary stem 2.

[0034] It will be understood that the expression "inter-external direction" designates a direction going from the edge of this medullary rod intended to be located after implantation along the internal edge of the humerus to the opposite edge intended to be located after implantation along the external edge of the humerus; similarly, the expression "anterior-posterior direction" designates the direction going from the edge of the rod 2 intended to be located after implantation along the anterior edge of the humerus to the edge of this rod located after implantation along the posterior edge of the humerus.

[0035] The medullary rod 2 thus has a structure with an envelope wall 10 and a plurality of partitions 11 having internal-external and anterior-posterior orientations, spaced from each other and forming a body with said envelope wall 10. This structure proves to be particularly interesting in that it is capable of supporting the load exerted on the medullary rod 2, due to the plurality of partitions 11 and the fact that these partitions form a body with the envelope wall 10, while having a slight capacity for deformation, close to that of a humerus.

[0036] THE figures 6 And 7represent a medullary stem 2 according to a second embodiment, in which said internal-external and anterior-posterior partitions 11 do not have undulations like those delimiting the alveoli according to the first embodiment, but are on the contrary flat and intersect at right angles to each other, the internal structure formed by these partitions thus being "cross-shaped".

[0037] There figure 8 represents a medullary rod 2 according to the invention, in which the rod 2 has a solid internal bar 15, extending between the distal end wall of the rod 2 and at least one wall delimiting the cavity 6. This bar 15 forms a body with the envelope wall 10 and the partitions 11 and is made of the same material as these walls 10 and partitions 11; it can have any suitable cross-section, in particular a circular section; its dimension in the internal-external direction can in particular represent, as visible on the figure 8 , approximately one-third of the internal-external dimension of the medullary stem at its tapered distal portion.

[0038] The bar 15 allows localized longitudinal reinforcement of the rod 2.

[0039] There figure 9 represents a medullary rod 2 according to an example not covered by the invention, in which said hollow space occupied by the partitions 11 does not extend over the entire volume delimited by the envelope wall 10. Indeed, the rod 2 has a solid part, made of the same material as the wall 10 and the partitions 11, located along at least one edge of the envelope wall 10. In the example shown, this solid part comprises a portion 20a extending over a proximal part of the rod, around the cavity 6 and a portion 20b extending over a proximal part of the longitudinal edge of the rod 2 intended to be located after implantation along the internal edge of the humerus, this portion 20b reducing in cross-section in the distal direction so that the portion 20 has, seen in the frontal plane as according to the figure 9 , a tapered shape.

[0040] This solid part, like the bar 15, also allows localized longitudinal reinforcement of the rod 2.

Claims

1. Medullary stem (2) for an endoprosthesis for a joint, intended to be inserted into the medullary canal of a long bone comprising the joint concerned, comprising: - a casing wall (10), defining the external shape of the medullary stem (2) and delimiting a hollow space internally; and - a plurality of first partitions (11), generally parallel to each other, extending between the inner side of this medullary stem (2), i.e. the side intended to be located after implantation along the inner edge of the long bone, and the outer side of the medullary stem (2), i.e. the side intended to be located after implantation along the outer edge of the long bone, each of these first partitions (11) forming a single piece with the casing wall (10) at these inner and outer sides of the medullary shaft, - a longitudinal internal structure (15; 20a, 20b) for longitudinal reinforcement of the medullary stem, with a solid cross-section, forming a single piece with the casing wall (10) and the said first partitions (11) and second partitions (11) when these second partitions are present, characterized in that said internal structure is in the form of a central internal bar (15) extending longitudinally between a distal end of the stem (2), or an area of said stem close to said distal end, and a proximal end of the stem (2), or an area close to said proximal end.

2. Medullary stem (2) according to claim 1, characterized in that the casing wall (10) has a thickness ranging from 0.5 to 2 mm.

3. Medullary stem (2) according to claim 1 or claim 2, characterized in that each partition (11) has a thickness of 0.5 to 1.5 mm.

4. Medullary stem (2) according to one of claims 1 to 3, characterized in that the space between the partitions (11) is between 1 and 2 mm.

5. Medullary stem (2) according to one of claims 1 to 4, characterized in that the number of partitions (11) ranges from four to six partitions (11) on a medullary rod (2) intended for a small patient and from six to ten partitions (11) on a medullary rod (2) intended for a large patient.

6. Medullary stem (2) according to one of claims 1 to 5, characterized in that the partitions (11) are flat.

7. Medullary stem (2) according to one of claims 1 to 5, characterized in that the partitions (11) have a wavy profile.

8. Medullary stem (2) according to one of claims 1 to 7, characterized in that it comprises a plurality of second partitions (11), generally parallel to each other, having a different orientation from that of the first partitions (11) and thus intersecting these first partitions, each of these second partitions (11) forming a single piece with the casing wall (10) of the medullary stem (2) and forming a single piece with the said first partitions (2).

9. Medullary stem (2) according to claim 8, characterized in that the first partitions (11) and second partitions (11) delimit between them cells having a honeycomb structure.