Glenoid prosthesis and a modular system with glenoid prostheses

[0002] 1. Field of the Invention

[0003] The invention relates to a glenoid prosthesis comprising a plastic bearing shell with a plurality of integral anchoring pins, which are arranged parallel to one another at its reverse side.

[0004] 2. Description of the Prior Art

[0005] The operation technique and the material used are described in detail in a brochure: “Schulter-Endoprothese zementiert”, 1998 edition of Sulzer Orthopedics Ltd., CH-6340 Baar. A glenoid prosthesis with four pins on the reverse side is shown which is cemented in after the milling and boring of the natural glenoid.

[0006] The object of the present invention is to be able to insert a glenoid prosthesis of this kind in a wider spectrum of applications. This object is achieved by the present invention in that at least one anchoring pin has a thread as a coupling element, and in that a metal sleeve with protruding longitudinal ribs is solidly screwable to that one anchoring pin for a mechanical hammering in of the sleeve into an undersized bone hole.

[0007] This design of the glenoid prosthesis brings about a plurality of advantages. It permits the prosthesis to be selectively cemented in or to be anchored directly in the bore without cement after the securing of sleeves with anchoring structures. In the cementing in, the coupling structure at the pins has the advantage that projections and recessions are present for the cement jacket and that no overloading takes place since cement can flow back into depressions. In the hammering in without cement a sleeve material which is suitable for the growing in and an anchoring structure on the sleeve which is suitable for the hammering in can be used.

[0008] Other embodiments of the present invention provide advantages. It is advantageous if the pins have an outer thread as a coupling element and the sleeves have an inner thread as a fitting coupling element. Bayonet locks are likewise possible. In addition it is advantageous if all pins are provided with equally large thread diameters independently of the size of the bearing shells since only one additional part, the sleeve, need be taken up into the bearing holder. Furthermore, a bearing shell with pins of a material which has good sliding properties can be chosen without it being necessary for the anchoring to suffer thereby. A construction of plastic in the form of a bearing shell of polyethylene combined with metallic sleeves of a titanium ally brings about a good anchoring. This can be improved if knife-like ribs in the longitudinal direction are distributed over the periphery of the sleeve. On the one hand, the support surface is enlarged and, on the other hand, the bone material is encouraged to grow in. Furthermore, the growing in of bone matter can be improved if the reverse side of the bearing shell is provided with a metal mesh which for example consists of a titanium alloy.

[0009] A further advantage results if the orthopedist can still make the decision, after the boring of the bores for the anchoring pins, in accordance with the state of the bone found, whether he wishes to cement or to hammer in without cement. This is achieved in that the sleeve has such a low wall thickness that the bore diameter, which corresponds to a desired thickness of the cement jacket during the cementing, at the same time also forms the desired bore diameter for the direct hammering in of sleeves with an anchoring structure. That is, the anchoring structure is designed in such a manner relative to the pin that the bore diameter is less than the outer diameter over the anchoring structure and lies in the vicinity of the outer diameter of the actual sleeve.

[0010] FIG. 1 schematically illustrates the view of a bearing shell with threaded pins and a sleeve with longitudinal ribs which can be screwed on;

[0011] FIG. 2 schematically illustrates a view of the bearing shell of FIG. 1 from the reverse side;

[0012] FIG. 3 schematically illustrates a side view of the bearing shell of FIG. 1;

[0013] FIG. 4 schematically illustrates the rear view of a bearing shell which is additionally provided at its reverse side with a metal mesh;

[0014] FIG. 5 schematically illustrates a side view of the bearing shell of FIG 4;

[0015] FIG. 6 schematically illustrates a side view of a sleeve with anchoring ribs;and

[0016] FIG. 7 schematically illustrates a front view showing the threaded bore of the sleeve in FIG. 6.

[0017] The figures show a glenoid prosthesis with a bearing shell 1, the reverse side 2 of which has a plurality of anchoring pins 3, 4, 5, 6 which are arranged parallel to one another. At least one anchoring pin has a coupling element 7 and at least one sleeve 8 with a fitting securable coupling element 9 and an outer anchoring structure 10 is provided in order to selectively enable a cementing in of the anchoring pin or a mechanical hammering in of the pin together with the sleeve which is fixed to it.

[0018] In FIGS. 1, 2 and 3 a bearing shell 1 is shown, the bearing surface 14 of which and the reverse side 2 of which are sections of spherical surfaces R₁, R₂with a common center. The material of the bearing shell 1 is a UHMW polyethylene and the wall thickness 13, which represents the radius difference R₂-R₁, corresponds to an amount of about 5.5 mm. Wall thicknesses with values between 3 and 7 mm are conceivable with other materials, with the goal therein consisting in achieving as low a wall thickness as possible while retaining a sufficient bearing function and bearing stability since the natural glenoid must be milled in by about this amount if the ligaments are to be tensioned to the same extent after the insertion of the artificial bearing shell.

[0019] Four anchoring pins 3, 4, 5, 6 project on the reverse side 2 of the bearing shell and are in each case provided with an outer thread 7 with a thread diameter 12. Pins 3 and 4 are horizontally arranged and pins 5 and 6 vertically arranged corresponding to the securing possibilities at a natural glenoid, with it still being necessary to rotate the bearing shell by 180° for the securing at a person standing erect. The threaded pins 3, 4, 5, 6 are in this case connected in a single piece to the bearing shells and are arranged parallel to one another in order to be able to insert them together in suitably pre-bored bores. The outer thread diameter 5 is the same for all pins. Corresponding to the thickness of a natural glenoid the middle pin 5 projects further forward for the anchoring. The bearing surface 14 of the bearing shell 1 is provided with sufficient surface quality and precision of shape for artificial joints.

[0020] A bearing shell of this kind can be pressed directly in with its pins into bores at a natural glenoid which are filled with bone cement. The bone cement which wells forth over the bearing shell is brushed off and the bearing shell is held in place until the bone cement has hardened. During the pressing in of the pins the threads which are placed therein help in the ventilation of enclosed air bubbles since the bore cement does not wet immediately down to the thread base. After the hardening the thread surfaces form clearly defined support surfaces for the transmission of force in the direction of the pins.

[0021] Should the operator after the boring of the reception holes in the natural glenoid be of the opinion that the bone substance is stable enough for a direct anchoring, then he can screw the metallic sleeves 8 which belong to the modular system onto the threaded pins, 3, 4, 5, 6 and hammer them in with the bearing shell without bone cement. The wall thickness of the sleeve 8 is executed to be small in order that the bores can be used in the natural glenoid (not shown here) selectively for a direct anchoring of the sleeves 8 or for an anchoring of the pins with bone cement. The sleeves 8 are closed at their front ends and have knife-like longitudinal ribs distributed over the periphery which dig into the bone tissue during the hammering in. The primary securing arises in this case through the frictional forces at the ribs 10 and the sleeves 8. Pure titanium or body compatible titanium alloys are advantageous as a metal for the sleeves 8.

[0022] In the example of FIGS. 4 and 5 a bearing shell 1 of polyethylene is provided with a metallic mesh 11 on its reverse side 2. A mesh which is rounded at its outer edge is heated and pressed on at the reverse side 2 of the bearing shell 1. The metal mesh 11 penetrates partly into the polyethylene and individual wires of the mesh are enclosed by the melted polyethylene in order to achieve an anchoring over the entire surface. If the bearing pin is not to be damaged in this procedure, one possibility consists in providing the metal mesh 11 with cut-outs and securing the pins 3, 4, 5, 6 of polyethylene later at the reverse side 2 by friction welding at the bearing shell 1. Another possibility consists in placing a protective sleeve of a poor heat conductor on the pins for the duration of the joining of the bearing shell 1 and the metallic mesh 11. In addition to the selectively insertable sleeves 8 a bearing shell of this kind has the advantage that a structure which is suitable both for securing with bone cement and also for a direct growing in in the bone is attached to the reverse side 2. FIGS. 6 and 7 show a sleeve 8 in a side view from the rear. The longitudinal ribs 10, which are formed in the shape of knives, are uniformly distributed over the periphery. Bore gauges are provided for the bearing shells. The bores which are produced in the bone with bore gauges have a diameter D₂>d≧0.9D₁, with the diameter D₂being measured over the ribs 10 and the diameter D₁corresponding to the outer diameter of the sleeve body. At the same time this diameter d produces a layer thickness of bone cement between the bore and the pins 3, 4, 5, 6 which is aimed for during the cementing in. The front side of the sleeve 8 is rounded in the shape of a bullet.