Biomechanical comparison of anatomic humeral head resurfacing and hemiarthroplasty in functional glenohumeral positions.

BACKGROUND Resurfacing of the humeral head has gained interest as an alternative to traditional hemiarthroplasty because it preserves bone stock and respects the native geometry of the glenohumeral articulation. The purpose of this study was to compare the biomechanics of the intact glenohumeral joint with those following humeral head resurfacing and following hemiarthroplasty. METHODS Seven fresh-frozen cadaveric shoulders were tested with the rotator cuff, pectoralis major, and latissimus dorsi musculature loaded with 20 N and the deltoid muscle loaded with 40 N in a custom shoulder testing system. Each specimen was tested in 20°, 40°, 60°, and 80° of vertical abduction. The articular surfaces of the humeral head and the glenoid were digitized to calculate the positions of the geometric center and apex of the humeral head relative to the geometric center of the glenoid at each testing position. The contact area and contact pressures were also measured with use of a Tekscan pressure sensor. RESULTS The geometric center of the humeral head shifted by a mean (and standard error) of 2.2 ± 0.3 mm following humeral resurfacing and 4.7 ± 0.3 mm following hemiarthroplasty (p < 0.0002). The apex of the humeral head was shifted superiorly at all abduction angles following hemiarthroplasty (p < 0.03). Both humeral resurfacing and hemiarthroplasty decreased the glenohumeral contact area and increased the peak pressure. CONCLUSIONS Resurfacing more closely restored the geometric center of the humeral head than hemiarthroplasty did, with less eccentric loading of the glenoid. CLINICAL RELEVANCE Compared with hemiarthroplasty, humeral resurfacing may limit eccentric glenoid wear and permit better function because the glenohumeral joint biomechanics and the moment arms of the rotator cuff and the deltoid muscle are restored more closely to those of the intact condition.