Optimizing the Intrinsic Stability of the Glenoid Fossa with Non-prosthetic Glenoidarthroplasty

Background: In shoulders requiring arthroplasty, the extant glenoid may provide insufficient intrinsic stability for the humeral head – for example if the glenoid is flat or biconcave. In such cases the surgeon can restore the desired intrinsic stability using a glenoid prosthesis with a known surface geometry. Alternatively, the surgeon can modify the surface of the glenoid to a geometry that provides the desired intrinsic stability. This study explores the feasibility, reliability and predictability with which glenoid intrinsic stability can be restored through spherical reaming of the glenoid bone around the glenoid centerline. It tests the hypothesis that the intrinsic stability of reamed glenoids can match that of clinically used polyethylene glenoid components. Method: The intrinsic stability provided by the glenoid in a given direction can be characterized by the maximal angle the humeral joint reaction force can make with the glenoid centerline before the humeral head dislocates; this quantity is defined as the balance stability angle (BSA) in the specified direction (Fig 1). The BSA can be calculated by analysis of the shape of the glenoid surface (Fig.2). The BSA can also be measured directly by placing an unconstrained humeral head loaded only by gravity within the glenoid oriented with the centerline vertical and then tipping the glenoid until the humeral head dislocates (see fig. 1). In this study, the balance stability angles were both calculated and measured in 8 different directions for 3 unused polyethylene glenoid components and 11 cadaveric glenoids in 4 different states: 1) native without capsule or rotator cuff 2) denuded of cartilage and labrum 3) after reaming the glenoid surface around the glenoid centerline using a spherical reamer with a radius of 25.0 millimeters and 4) after reaming around the glenoid centerline using a spherical reamer with a radius of 22.5 millimeters (Fig. 3). We also attempted to predict the BSAs achievable with reaming to each of the two different radii from measurement of the width of the glenoid before it was reamed. Results: Denuding the glenoids of their articular cartilage reduced the intrinsic stability, especially in the posterior direction (Table 1). Reaming the glenoid restored the intrinsic stability back to values comparable to those of the normal glenoid. For example, the average calculated BSA in the posterior direction for all eleven glenoids was: native = 24° ± 9°, denuded = 14° ± 6°, reamed to radius 25.0 millimeter = 25° ± 5°, reamed to radius 22.5 millimeter = 33° ± 6°. The BSAs for the polyethylene glenoids, which had a manufactured radius of curvature of 25.0 millimeters, were not significantly different from those of the cadaveric glenoids following reaming to a 25.0 millimeter of curvature (p = .165) (see Table 1). They were significantly less than the BSAs of the cadaveric glenoids following reaming to a 22.5 millimeter radius of curvature (p < .0001). There was a high degree of correlation between the measured and the calculated BSAs for each of the different preparations. The correlation coefficients ranged from .75 to .94. All correlations were significant (p<.0001). These strong correlations between results obtained with completely different techniques support the validity of the methods and indicates that factors other than glenoid surface geometry, such as friction, where not important in this model system. The BSAs predicted prior to reaming were similar to those after the glenoid had been reamed. Considering the BSAs in the anterior and posterior directions (90° and 270°), the average absolute difference between the measured and predicted BSAs was 5.3° ± 3.6° with reaming to 25.0 millimeters and 3.4° ± 2.4° with reaming to 22.5 millimeters. Conclusion: The intrinsic stability of human glenoids was decreased by removal of the cartilage and increased reliably and predictably through spherical reaming of its surface. Denudation of the glenoid surface had the most profound reduction of intrinsic stability in the posterior direction: a common direction of humeral head displacement in glenohumeral arthritis. Non-prosthetic glenoid arthroplasty through spherical reaming can achieve surface geometry and intrinsic stability similar to that of normal glenoids and that provided by a polyethylene glenoid. Figure 1-balance stability angle, theta.