Abstract
Introduction One of the most difficult aspects of shoulder arthroplasty is retroversion. The ideal angle is about 30 of posterior rotation of humeral head with regard to the frontal plane so that the humeral head squarely faces the glenoid surface in the resting position. The axis, lateral epicondyle- medial epicondyle is often taken as reference and serves as landmark in many arthroplasty instrumentation. [1,2]
Clinical experience has shown that estimating a 30 angle in space is definitely not easy even with the help of diverse goniometers.
Methods Each operator has to put 3 prostheses with a 30 degrees retroversion according to the position of the forearm so we had proceeded to 52 putting of prostheses .
The measures were made by taking into account of the humerus axis, the plan of condyles and angle of inclination of the collar, given by the angle of cutting. Three barycentres of the three humeral sections have determined the humeral axis. The condylar axis is determined from the 2 barycentres of the digitalized points on the anterior articular condylar surfaces. These 2 axis determine the frontal plane on which a reference mark R(x, y, z) is attached with Z lined up with the humeral shaft and X lined up on the condyles. Different angles could then be determined.
In the sagittal plan (perpendicular in the humeral axis), the retroversion angles of the prosthesis and the angle of cutting are calculated.
Results The standard deviation of the retroversion angle of the prosthesis is 14,22 which is really too high. In fact, 4 prostheses were inserted with poor retroversion (17°, 17°, 18°, 4,4°) and 20 with excessive retroversion (max =65°). This retroversion angle is not dependant on the other factors (cut angle, inclination angle...) The implant height was not taken into account
Conclusions Only 28 of the prostheses were placed in the right orientation within 20° to 40° of retroversion angle. It shows the difficulties to place a shoulder prosthesis in good position.even in standard conditions and with the standard marks.
Theses abstracts were prepared by Professor Roger Lemaire. Correspondence should be addressed to EFORT Central Office, Freihofstrasse 22, CH-8700 Küsnacht, Switzerland.
References
[1] Boileau F., Walch G.,: The three dimension and geometry of the proximal humerus . Implications for surgical technique and prothetic design J. Bone Joint Surgery (Br)1997B: 857–59. Google Scholar
[2] Pearl ML, Volk AG. Retroversion of the proximal humerus in relationship to prosthetic replacement arthroplasty. J Shoulder Elbow Surg1985, Volume 4, Number 4, p. 286–9. Google Scholar
