Abstract:
A humeral prosthesis including a shaft adapted to be anchored in a medullary canal of humerus, a metaphyseal portion extending upwardly and endwardly from said shaft and being joined at its outer end to a flange which is adapted to support a dome to cooperate with a glenoidal cavity of a shoulder wherein the metaphyseal portion includes an inner part having at least one antero-posterior rib which is structured to provide an anatomical support for humeral tuberosities and which the at least one rib is oriented at an angle of between 45° to 135° with respect to a frontal plane of the prosthesis.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a humeral prosthesis. 
     BACKGROUND OF THE INVENTION 
     When there is a fracture of the upper end of a humerus, the latter generally breaks into several pieces, namely the diaphysis, the humeral tuberosities and, finally, the head of the humerus which cooperates with the glenoidal cavity of the shoulder. 
     When such a fracture is reduced, only the humeral head has to be replaced by a substantially hemispherical dome, while the other fractured pieces may be reconstituted around a prosthesis. 
     It is known that such a prosthesis generally comprises a shaft, or diaphyseal portion, introduced in the humeral canal of the diaphysis of the bone. This shaft is extended by a metaphyseal portion which, with reference to a prosthesis borne by a patient standing up, is curved upwardly and inwardly, and around which the tuberosities constituting the metaphysis of the bone are grouped together, with a view to reconstitution thereof. The metaphyseal portion of the prosthesis is joined to a flange for receiving the dome replacing the humeral head. 
     However, this type of prosthesis presents a drawback, in that it does not allow optimum positioning of the large and small tuberosities with a view to suture thereof. 
     In order to overcome this drawback, WO-A-98/15241 provides a humeral prosthesis presenting means for positioning these humeral tuberosities. To that end, the zone of connection between the metaphyseal portion of the prosthesis and the flange is provided, in its outer part, with at least one rib extending, with respect to the plane of symmetry of this prosthesis, at an angle included between 20 and 40°. Such a rib, which may be divided into two, extends in the direction of the bicipital groove, in the vicinity of which the large and small tuberosities must be joined. However, such a solution is not completely satisfactory, from the anatomical standpoint, as it is not certain that the humeral tuberosities can be positioned in their natural configuration. 
     In order to overcome all the drawbacks of the prior art set forth hereinabove, the invention proposes to provide a humeral prosthesis allowing a precise and anatomical positioning of the large and small tuberosities to permit the suturing thereof. 
     SUMMARY OF THE INVENTION 
     To that end, the invention relates to a humeral prosthesis comprising a shaft intended to be anchored in the medullary canal of a humerus, a metaphyseal portion extending this shaft upwardly and inwardly, this metaphyseal portion being joined to a flange supporting a dome adapted to cooperate with the glenoidal cavity of the shoulder, characterized in that the metaphyseal portion comprises, in its outer part, at least one antero-posterior rib for support of the humeral tuberosities, the or each rib extending in a plane oriented at an angle included between 45 and 135° with respect to the frontal plane of the prosthesis. 
     Thanks to the invention, and in particular to the orientation and dimensions of the or each antero-posterior rib, the humeral tuberosities can be reconstituted around the outer part of the metaphyseal portion of the prosthesis in a satisfactory configuration from the anatomical standpoint. In particular, the angle of orientation of the plane of the or each antero-posterior rib with respect to the frontal plane of the prosthesis may be provided to be close to 90°. This configuration proves particularly advantageous for reconstituting the humeral metaphysis. 
     According to advantageous aspects of the invention, the prosthesis incorporates one or more of the following characteristics: 
     The or each antero-posterior rib extends, from the frontal plane, with a maximum width such that its projection on a plane perpendicular to the frontal plane corresponds substantially to the distance separating the posterior edge of the bicipital groove of the humeral metaphysis with respect to a central axis of the humeral head. The width of the or each antero-posterior rib is thus adapted to the anatomy of the humeral metaphysis. 
     The metaphyseal portion further comprises, in its outer part, an outer median rib extending substantially along said frontal plane, the or each antero-posterior rib extending from the inner edge of said outer median rib. This configuration allows a particularly satisfactory bearing for the humeral tuberosities. 
     The metaphyseal portion of the prosthesis comprises two antero-posterior ribs, substantially symmetrical to each other with respect to the frontal plane. These ribs make it possible to use the prosthesis of the invention for a right or left shoulder and give this metaphyseal portion a good dimensional stability, particularly in torsion. 
     The metaphyseal portion comprises two branches, outer and inner respectively, defining therebetween a recess, the outer branch being provided with the or each antero-posterior rib. This recess allows osseous fusion through the metaphyseal portion of the prosthesis. 
     The outer branch is formed by said outer median rib and said two antero-posterior ribs. This embodiment is advantageous since it is of simple construction and it allows the prosthesis to present a wide outer surface for contact with the humeral tuberosities. In that case, the outer branch advantageously presents a T-shaped cross-section. 
     The free edge of the or each rib is inclined with respect to the frontal plane, the width of the or each rib increasing in the direction of the flange. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more readily understood on reading the following description given by way of non-limiting example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic plan view showing the upper end of a humerus. 
     FIGS. 2 and 3 are respectively front and side views of a humeral prosthesis according to the invention. 
     FIG. 4 is a section along line IV—IV in FIG.  3 . 
     FIG. 5 is a view in perspective illustrating the prosthesis of FIGS. 2 and 3 implanted in the medullary canal of a humerus, and humeral tuberosities arranged in the vicinity of this prosthesis, and 
     FIG. 6 is a plan view, similar to FIG. 1, illustrating the prosthesis once implanted, the dome for bearing on the glenoidal cavity having been omitted. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring now to the drawings, FIG. 1 shows the upper end of a humerus  10  which comprises a metaphyseal portion surmounted by a humeral head  12  cooperating with the glenoidal cavity of the shoulder (not shown). On the metaphyseal portion are formed a large tuberosity  14  and a small tuberosity  16  which define therebetween a so-called bicipital groove  18 . 
     When the humerus  10  is fractured, the posterior edge  20  of the groove  18 , adjacent the large tuberosity  14 , often constitutes the starting point of such a fracture. A denotes the central axis of the humeral head  12 , and d the distance separating this axis A and the posterior edge  20  of the groove  18 . Axis A is determined, in known manner, in accordance with a method described in the article “Anatomic determination of humeral head retroversion: the relationship of the central axis of the humeral head to the bicipital groove”, extracted from the publication “Journal of Shoulder and Elbow Surgery”, in the September-October 1993 number. 
     FIGS. 2 to  4  show a prosthesis according to the invention, generally designed by reference  22 . The terms lower, upper, inner, outer, anterior and posterior used hereinafter must be understood as referring to a prosthesis borne by a patient standing up. 
     The prosthesis  22  is intended to support a substantially hemispherical dome  24  adapted to cooperate with the glenoidal cavity of a patient&#39;s shoulder. 
     This prosthesis comprises a shaft  26  of substantially circular cross-section, intended to engage in the humeral canal  28  as shown in particular in FIG.  5 . This shaft  26  is extended by a metaphyseal portion  30  to which is joined a flange  32  adapted to be removably connected to the dome  24  thanks to a lug  32   a.    
     The frontal plane P of the prosthesis is the plane containing the central axis Q of the shaft  26  and a median axis or diameter D of the flange  32 . Plane P is perpendicular to the plane of FIG.  2  and its trace merges with that of axes D and Q in this Figure. Plane P is parallel to the plane of FIG.  3 . In practice, plane P is a plane of symmetry of the prosthesis  22 . 
     The metaphyseal portion  30  comprises outer and inner parts, namely an outer branch  34  and an inner branch  36  which are each joined to the lower face of the flange  32 , by respective connection zones  34   a  and  36   a . These latter are located substantially along axis D. Once the prosthesis is implanted, axis D of the flange is superposed on the central axis A of the humeral head  12 . 
     Between the branches  34  and  36  there is defined a recess  38  constituting a free volume for bringing together and fusion of the fragments of bone of the metaphysis. The existence of this free volume allows the formation of an osseous bridge ensuring efficient anchoring of this prosthesis in the humerus. 
     The inner branch  36  presents a rectangular cross-section which is substantially constant over the whole length of this branch. 
     The outer branch  34  comprises an outwardly projecting median rib  40  whose thickness is clearly less than that of the inner branch  36 . This outer median rib  40  extends substantially along the frontal plane P. It is pierced with orifices  42  intended for the passage of the suture threads. 
     Two antero-posterior ribs  44 , namely a rear lateral rib  44   a  and a front lateral rib  44   b  each extend, from the inner edge  40   a  of the median rib  40 , along a plane π of which α denotes the angle of orientation with respect to the frontal plane P. Angle α is about 90°, i.e. the antero-posterior ribs  44  are substantially perpendicular to the median rib  40 . As is visible in FIG. 4, the outer branch  34  therefore presents a T-shaped cross section. 
     The antero-posterior ribs  44  allow an anatomic repositioning of the tuberosities  14  and  16  and of the bicipital groove  18  thanks to bearing on at least one of the ribs, as represented by arrows F in FIG.  5 . 
     Although the value of 90° is particularly advantageous, satisfactory results were able to be obtained with antero-posterior ribs extending in a plane-π of which the angle with respect to the frontal plane is included between 45 and 135°. 
     Each of the antero-posterior ribs  44   a  and  44   b  is pierced with orifices  46  allowing passage of suture threads. 
     As is illustrated in particular in FIG. 2, the width of the antero-posterior ribs  44  increases towards the flange  32 . The respective free edges  48   a  and  48   b  of the ribs  44   a  and  44   b  diverge from the outside towards the inside of the shoulder, i.e. are inclined with respect to the frontal plane P. The maximum width l of the ribs  44  is equal to the distance d separating the central axis A of the humeral head  12  and the posterior edge  20  of the bicipital groove  18 . 
     In effect, in order to effect synthesis of the humeral tuberosities, the surgeon has a reference in the vertical direction, as the top of the greater tuberosity must lie slightly below the head of the prosthesis. Now, the greater tuberosity may rotate about the axis of the dome. The invention provides the surgeon with a reference to enable him to position the greater tuberosity correctly. This reference is the theoretical position of the bicipital groove  18 , determined by the maximum width l of the antero-posterior ribs. This width corresponds in effect to the average anatomical distance measured between the central axis A and the posterior edge  20  of the bicipital groove  18 . 
     Once the prosthesis  22  has been positioned and the humeral tuberosities  14  and  16  joined in the vicinity of this prosthesis, as shown in FIG. 6, osseous fusion can take place, such fusion resulting in an anatomical reconstitution of the bone. 
     The width of the ribs, and in particular their maximum width l may be adapted as a function of the size of the prosthesis, i.e., in particular, of the position of the bicipital groove. In practice, this width l is included between 5.5 and 13 mm and three sizes of prostheses can be provided, of which the antero-posterior ribs present a maximum width l equal to 6, 9 or 12 mm respectively. 
     In the case of the ribs  44   a  and  44   b  not being perpendicular to the frontal plane, their maximum width l is chosen so that the projection thereof on a plane perpendicular to this frontal plane corresponds to the distance d. In the case shown in the Figures, the width l is equal to their projection on the plane π.