Source of lateralization in reverse total shoulder arthroplasty matters: a comparison of glenoid and humeral lateralization on rotator cuff biomechanics.

BACKGROUND

Compared with the traditional Grammont design, modern reverse total shoulder arthroplasty (rTSA) implant designs often introduce lateralization of the glenoid and/or humeral components. This study aimed to evaluate the impact of different strategies for achieving lateralization (ie, humeral or glenoid lateralization) in rTSA implant design on rotator cuff biomechanics.

METHODS

Computed tomography scans from 16 nonosteoarthritic subjects were used to build customized computational 3-dimensional shoulder models based on the Newcastle Shoulder Model. Four rTSA implant constructs were created: (1) medialized glenoid-medialized humerus (MG-MH); (2) medialized glenoid-lateralized humerus (MG-LH); (3) lateralized glenoid-medialized humerus (LG-MH); and (4) lateralized glenoid-lateralized humerus (LG-LH). All constructs used a humeral stem with 135° neck-shaft angle where the diameter of the glenosphere was 36mm. Simulated rTSA constructs included a subscapularis tendon repaired to its native attachment on the lesser tuberosity. For each design construct, moment arms for both the subscapularis and infraspinatus were calculated for 4 motions: humeral elevation in frontal and scapular plane, internal/external rotation at 20° and 90° of abduction. Moment arms for each construct were also compared to those in a native shoulder.

RESULTS

All rTSA constructs influenced the moment arms of the rotator cuff muscles. During humeral elevation, both the subscapularis and infraspinatus exhibited increased adductive moment arms compared to the native shoulder, particularly at lower angles of elevation (0-80° in abduction and 0-50° in the scapular plane). Glenoid lateralization did not significantly affect these changes; however, humeral lateralization enhanced the adductive moment arms of both muscles. Additionally, all rTSA constructs altered the internal and external rotation moment arms of the RC muscles relative to the native shoulder. The subscapularis showed increased internal rotation moment arms that got larger than the native shoulder only after 40° of internal rotation, while the infraspinatus demonstrated increased external rotation moment arms during all external rotation range of motion. Again, glenoid lateralization did not significantly impact these rotational moment arms, whereas humeral lateralization led to an increase in both internal (subscapularis) and external (infraspinatus) rotation moment arms.

CONCLUSIONS

While glenoid lateralization of an rTSA implant construct does not substantially alter rotator cuff moment arms, humeral lateralization may have a dual effect: potentially introducing an antagonistic adductive moment relative to the deltoid during early abduction, while also augmenting beneficial rotational moment arms-namely, increased internal rotation from the subscapularis and increased external rotation from the infraspinatus.