Jeremy Genter (Winterthur / CH; Basel / CH), Eleonora Croci (Basel / CH), Prof. Dr. Andreas M. Müller (Basel / CH), Prof. Annegret Mündermann (Basel / CH), Prof. Dr. Daniel Baumgartner (Winterthur / CH)
Abstract-Text (inkl. Referenzen und Bildunterschriften)
Introduction
Glenohumeral (GH) biomechanics after rotator cuff (RC) tears are not fully understood. The critical shoulder angle (CSA) is related to RC tear risk1, but its impact on shoulder biomechanics (GH translation, center of force (CoF), muscle and joint reaction force (JRF)) in RC-tear and weight-bearing shoulders is unclear. Here, we investigate the effect of the CSA on joint biomechanics.
Methods
A GH simulator (Fig.1) with muscle-mimicking cables2 was used to simulate motion of 0–30° scaption. An artificial humerus and five sets of scapular anthropometries of cadaver specimens were used to test the effect of six RC tear types, weight-bearing loads (0–2kg), and CSA by changing the deltoid origin sites of each anthropometric set (CSA range: 28°–45°). Linear mixed-effects models (CSA, RC tear type, and weight bearing) with random effects were used to assess differences in GH biomechanics.
Results
GH translation increased by 0.1mm per 1° CSA increase; JRF decreased by 2.1N per 1° CSA increase, and CoF shifted slightly inferiorly with CSA increase (Fig.2). Compared to the healthy condition, RC tears did not significantly affect GH translations but shifted the CoF up to 5.3mm more superiorly and increased RC and deltoid forces by up to 61.6N. Weight bearing significantly increased GH translation by 0.9mm per 1kg additional weight.
Discussion
Greater CSAs increased GH translation and decreased JRF, potentially making RC-tear shoulders more susceptible to subluxation. Surprisingly, RC tears themselves were not linked to greater GH translation, probably because of the compensation by the remaining synergistic muscles (greater force) to maintain translations comparable to the healthy condition. The superior shift of the CoF concords with areas of glenoid erosion (superior) in RC arthropathy patients.3
Funding; SNF 189082
1Moor et al., J Bone Joint Surg Am, 2013
2Genter et al., at-automatiserungstechnik, 2023
3Ozel et al., BMC Musculoskelet. Disord., 2020