2005 Papers - Higashigawa


Kevin H. Higashigawa, MD, University of Hawaii Orthopaedic Residency Program

Objective: One of the primary functions of the rotator cuff muscles is to stabilize the shoulder by compressing the humeral head into the glenoid concavity. This concept of concavity compression has been well described in the literature. The contribution of the rotator cuff musculature to shoulder stability can be expressed in terms of the stability ratio, which is the amount of translation force necessary to dislocate the humeral head divided by the compressive force provided by the rotator cuff. The coracoacromial arch, created by the acromion, CA ligament, and coracoid, forms a functional concavity. We hypothesize, therefore, that the deltoid muscle can serve as another stabilizer of the humeral head to anterior translation by providing a compressive force into the coracoacromial arch. This effect of the deltoid can also be expressed as a stability ratio.

Methods: A total of ten fresh-frozen cadaveric shoulders will be utilized for this study. The shoulders will be fully dissected and disarticulated, leaving only the rotator cuff tendon insertions attached to the humeral head and the labrum on the glenoid. The scapula will be mounted on a stationary device and the humeral head will be loaded into the glenoid using various weights to simulate the rotator cuff compression. Another weight will be attached at the deltoid insertion to simulate deltoid compression into the CA arch. The humeral head will be positioned in 0 degrees of abduction and 5 degrees of internal rotation. The scapula will be positioned with the CA arch parallel to the line of dislocation and the glenoid in 0 degrees of version and 0 degrees of vertical tilt. Finally, an anteriorly directed translation force will be applied to dislocate the shoulder. The dislocation force will be measured at varying deltoid loads. The stability ratio for the deltoid can then be calculated.

Results: Without any deltoid load, a 13.5 N rotator cuff load was able to stabilize the humeral head against an average dislocation force of 9.3 N, providing a stability ratio of .69. As deltoid loads are added to the humeral head, the total stability of the shoulder construct increases. With deltoid loads of 13.5 N and above, the average contribution of the deltoid alone to stability was .85. A magnetic spatial sensor device placed on the humeral head tracked the path of dislocation, revealing the concavity nature of the coracoacromial arch.

Conclusion: The deltoid is an important stabilizer of the shoulder against anterior dislocation. The mechanism of stability appears to be concavity compression into the coracoacromial arch. Future experiments can be designed to better characterize this phenomenon.