Constrained arthroplasty of the shoulder.

Constrained replacement, like all prosthetic replacements, is constantly undergoing change and will improve as the current state of the art changes. It is not a standard, usual operation like unconstrained arthroplasty, and it should be reserved for the patient who requires arthroplasty and does not have a functional rotator cuff mechanism. If, in addition, the acromial fulcrum and loss of deltoid is present, then there is a greater reason for constrained replacement; it is a salvage procedure that is not as durable as the unconstrained device. Constrained arthroplasty has much merit if the risks and possible complications are well understood; however, pain relief and improved function will be lost if the device fails. The dislocation feature of the MRTS is desirable in order to minimize the risk of scapular fracture. Its disadvantage is that, with this complication, open operation is needed to reduce the assembly and insert new polyethylene components. The method of surface mounting of the metal glenoid component without removing the glenoid subchondral plate or cancellous bone of the vault is believed to lessen the risk of glenoid component loosening or pull-out. Finally, active function always depends upon residual deltoid muscle power. Thus, a poor or absent deltoid will only permit passive motion.