Friday, December 8, 2017

Dislocation after reverse total shoulder

Classification of instability after reverse shoulder arthroplasty guides surgical management and outcomes

Recognizing that dislocation is the leading mechanical complication of reverse total shoulder, these authors sought to develop a classification for instability after RSA, to describe the clinical outcomes of patients stabilized operatively, and to identify those patients at higher risk of recurrence.

Their classification system is shown below.

They reviewed 43 revision cases in 34 patients. In the revisions the authors usually upsized the glenosphere to 40 or 44 neutral, often with +6 or +8 mm humeral offset and semiconstrained liners.

Persistent instability most commonly occurred in persistent deltoid dysfunction and postoperative acromial fractures but also in 1 case of soft tissue impingement. Twenty-one patients remained stable at minimum 2 years of follow-up and had significant improvement of clinical outcome scores and range of motion.

Comment: Instability after a reverse total shoulder remains a substantial problem. Additional insight into the factors affecting stability can be gained from this article:

Hierarchy of Stability Factors in Reverse Shoulder Arthroplasty

These authors asked: (1) what is the hierarchy of importance of joint compressive force, prosthetic socket depth, and glenosphere size in relation to stability, and (2) is this hierarchy defined by underlying and theoretically predictable joint contact characteristics? They examined the intrinsic stability in terms of the force required to dislocate the humerosocket from the glenosphere of eight commercially available reverse shoulder arthroplasty devices. The hierarchy of factors was led by compressive force followed by socket depth; glenosphere size played a much lesser role in stability of the reverse shoulder arthroplasty device. Similar results were predicted by a mathematical model, suggesting the stability was determined primarily by compressive forces generated by muscles.
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