When the cuff deteriorates, the humeral head can translate superiorly because of the loss of this interpositional effect. This can result in an arthritic humeroacromial articulation.
However when the humeral head is superiorly decentered preoperatively as shown below
Support of the interpositional arthroplasty mechanism is provided by a paper presented at the recent ASES meeting (Functional and MRI outcomes of Superior Capsule Reconstruction with Acellular Dermal Matrix by Raffy Mirzayan et al.) Followup MRI revealed that 38% had a completely intact graft, 33% had a tear from the glenoid, 12% had a mid-substance tear, 14% tear from the tuberosity, and 2% had complete graft absence. There was no difference between pre-op and post-op acromiohumeral distance (7.3 mm vs 6.9mm, P=0.57). There was no difference in post-operative ASES score when the graft was completely intact or torn from the glenoid (P=0.39), but graft tear from the tuberosity resulted in a significantly lower ASES score (P=0.013). The authors interpreted these results as demonstrating that the graft functioned as an interposition between the proximal humerus and acromion.
In another ASES paper (Short Term Comparative Imaging and Clinical Analysis of Superior Capsular Reconstruction by Brian L. Badman et al), postoperative MRI's showed graft failure at the glenoid or mid substance) in 40% of the cases. As in the prior report, as long as the graft remained intact to the tuberosity, the results were not different for those with and without graft failure - a result again supporting the interpositional arthroplasty mechanism.
Comment: While there is much yet to be learned about the role that superior capsular reconstruction plays in the management of irreparable cuff tears, we interpret the evidence available so far as showing (1) fascial autografts appear to have a higher healing rate than acellular dermal matrix, (2) superior capsular reconstruction is more likely to be successful if the head is not superiorly displaced relative to the glenoid before surgery, (3) clinical improvement can be realized in cases of graft failure as long as the graft remains intact to the tuberosity, and (4) the most likely mechanism for the successes of superior capsular reconstruction is through the creation of a fascial arthroplasty between the proximal humerus and glenoid. While other mechanisms have been proposed (see this link), at this point the evidence supporting them does not seem compelling.
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A video of our approach to shoulders with irreparable cuff tears can be seen by clicking this link.
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