How does subscapularis repair affect joint loads? Effect of humeral tray thickness in Reverse Total Shoulder Arthroplasty (rTSA).

Background: This study aimed to evaluate how subscapularis tendon repair influences joint loads in relation to humeral offset and arm position.

Patients and methods: Two fresh-frozen, whole-body cadaveric shoulders underwent a reverse total shoulder arthroplasty (rTSA) on the humeral side using an internal proprietary load-sensing system (LSS) (Goldilocks, Statera Medical, Montreal, Canada). In addition to three "complex" Activity Daily Life positions ("behind the back", "overhead reach", and "across the chest"), four standard postures (external rotation, extension, abduction, and flexion) were used to record the glenohumeral loads (Newtons) and their locations applied to the implant. The humeral system's adjustability function was used to gradually raise the thickness for each setting from 0 to 6 mm. The first part of the tests consisted of taking these measurements with the repaired subscapularis tendon. Once all the measurements had been taken, the tendon was detached, and the same measurements were repeated in the same order. Load and contact point were measured using the load centroid radial distance (|r|).

Results: The measurements showed good repeatability with an intra-class correlation coefficient (ICC) greater than 0.9 for all positions. Increasing the thickness of the humeral implant from 0 to 6 mm, without subscapularis, resulted in a refocusing of loads on the humeral cup for several positions, except for abduction at 60° (|r|_0mm = 1 mm VS |r|_6mm = 0.9 mm; p = 0.570), cross body (|r|_0mm = 0.7 mm VS |r|_6mm = 0.5 mm; p = 0.413) and overhead reach (|r|_0mm = 1 mm VS |r|_6mm = 0.9 mm; p = 0.284). Subscapularis repair enhances this recentering effect for all positions, with a particularly significant influence on abduction at 60°, "behind back" and "overhead reach" (|r|_6mm = 0.2 mm VS 0.9 mm, p < 0.001). Subscapularis repair also significantly increased joint loads for these positions at 6 mm humeral lateralization.

Discussion: The findings of this biomechanical study provide substantial evidence to support the hypothesis that the subscapularis muscle plays a pivotal role in the process of refocusing joint loads in rTSA. Consequently, subscapularis repair may exert a biomechanical effect on rTSA stabilisation.

Level of evidence: V; Biomechanical study.