What are the options if you have a shoulder replacement that is unstable?

Postoperative instability of an unconstrained "anatomic" total shoulder is usually related to one (or more) of 3 factors:  the prosthesis (alignment ,size)the capsule, and the rotator cuff.

Problems with the prosthesis having to do with the humeral side include excessive "retroversion" which can cause posterior instability and excessive "anteversion" which can cause anterior instability. Too small a prosthetic head  can cause global instability.
Problems with the glenoid side include failure to correct glenoid wear which can cause posterior instability. These prosthetic causes typically require revision arthroplasty---and  the extent of the instability and the status of the rotator cuff will impact on whether revision to a reverse shoulder design will be necessary.

Capsular problems leading to instability are generally related to failure to tighten the posterior capsule at the time of the original arthroplasty in a patient with posterior glenoid wear and a chronically distended posterior capsule--resulting in posterior instability. Treatment may be limited to a posterior capsulorraphy, but conversion to a reverse design is more predictable.

Rotator cuff problems can cause both static and dynamic instability. If an anatomic shoulder arthroplasty has been performed in a rotator cuff deficient shoulder, static instability results in 'anterosuperior escape", at the very least. Such patients are best treated with a reverse design at the outset. After an anatomic shoulder replacement, loss of the space beneath the acromion reflects either inadequate cuff function, a new massive cuff tear or improper prosthetic placement---ie the glenoid implant may be placed to high on the native socket.
Dynamic instability usually occurs because of failure of the subscapularis repair, which results in anterior instability. When this is diagnosed early (within 4-6 weeks) an attempt at repair is warranted. If the tendon  is retracted or of poor quality, the option of a pectoralis major tendon transfer exists, but the functional results are commonly poor, which makes revision to a reverse design more reliable. When addressed early, however, it's important to make sure that the humeral head is not too large and is of the appropriate "offset", as too large a head or a mismatched offset can jeopardize the success of the subscapularis repair.

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