Joint-line medialization after anatomical total shoulder replacement requires more rotator cuff activity to preserve joint stability

Abstract

Background The biomechanical effects of joint-line medialization during shoulder surgery are poorly understood. It was therefore the purpose of this study to investigate whether medialization of the joint line especially associated with total shoulder arthroplasty leads to changes in the rotator cuff muscle forces required to stabilize the arm in space. Methods A validated computational 3-D rigid body simulation model was used to calculate generated muscle forces, instability ratios, muscle-tendon lengths and moment arms during scapular plane elevation. Measurements took place with the anatomical and a 2 mm and 6 mm lateralized or medialized joint line. Results When the joint line was medialized, increased deltoid muscle activity was recorded throughout glenohumeral joint elevation. The rotator cuff muscle forces increased with medialization of the joint line in the early phases of elevation. Lateralization of the joint line led to higher rotator cuff muscle forces after 52° of glenohumeral elevation and to higher absolute values in muscle activity. A maximum instability ratio of >0.6 was recorded with 6 mm of joint-line medialization. Conclusion In this biomechanical study, medialization and lateralization of the normal joint line during total shoulder arthroplasty led to substantial load changes on the shoulder muscles used for stabilizing the arm in space. Specifically, medialization does not only lead to muscular shortening but also to increased load on the supraspinatus tendon during early arm elevation, the position which is already most loaded in the native joint.

Restoration of glenoid joint line: a three-dimensional analysis of scapular landmarks

“…This decreases the length of the muscles tendon complex around the GH joint, negatively affecting the length-tension relationship of the deltoid and rotator cuff muscles resulting in altered joint reaction forces. 11 , 16 , 38 The long-term aim of surgical management of glenoid bone loss is to find the optimal balance between maintaining joint stability and restoring the functional motion of the joint while maximizing prosthesis longevity. 5 Thus, understanding the GH pathoanatomy and assessing the glenoid bone loss is extremely important for optimal treatment.…”