Anterior instability after total shoulder replacement is a rare but potentially devastating complication. The reported incidence ranges from 0-29%, and it has been estimated that instability  represents 38% of all complications related to total shoulder arthroplasty. Anterior instability is the most common type, and determining the cause can be difficult. When evaluating a patient with an unstable TSA, potential causes include soft-tissue deficiencies, bone loss, and component positioning. Often, the problem is multi-factorial in nature. If the patient has pain and/or poor function as a result of instability, revision surgery may be indicated.

In cases where component positioning is adequate---that is there is no excessive anteversion of either the humeral or glenoid componenets,  and when the
the components are stable, anterior instability is usually associated with subscapularis tendon disruption.

Most patients are symptomatic, reporting weakness and/or pain, even though they may not experience a complete
shoulder dislocation. Static anterior subluxation may also be evident on plain radiographs.
                                                                                         
                                                                                           

In these cases, surgical treatment may be indicated. Reported options for treatment have included subscapularis repair or reconstruction, pectoralis major tendon transfer, and revision to a reverse prosthesis.Unlike cases of subscapularis failure in nonarthroplasty cases where repair or transfer may be possible, re-repair or pectoralis transfer are usually not succesful in re-centering  the humeral head following arthroplasty  when  static anterior subluxation is present.

Reverse shoulder prosthesis may be the most  reliable salvage procedure. If this complication is noted early, removal of press fit  total shoulder components may be possible with little risk of bony damage, but once fixation is mature or when components have been cemented, the complexity of the revision increases, and one needs to be prepared to provide bony augmentation with either whole limb or femoral head allograft.
 

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