Determining the ideal position of the longitudinal centre of rotation for a Motec® total wrist arthroplasty.

Introduction: The centre of rotation (COR) of the Motec^® ball-and-socket total wrist arthroplasty (TWA) lies at the centre of the head, and is determined by the position of the cup and thus the radial implant. The ideal position for the COR is not known.

Methods: The effect of the longitudinal position of the COR on performance of the wrist was studied using custom-made articular components in a cadaveric wrist model controlled by an active wrist motion simulator. Analysis of the standard surgical technique and clinical corroboration of postoperative radiographs were also performed.

Results: The ideal position for the COR is 8 mm distal to the point where the long axis of the radius meets the distal articular surface (the RCS) and 95% of optimal movement and efficiency can be achieved within 3.3 mm of that point. The standard surgical technique results in the COR being positioned7mm distal to the RCS. Clinical corroboration, in 40 well-functioning Motec^® wrists, confirmed the COR was within the range where the implant could be expected to achieve at least 95% of preoperative movement and efficiency.

Conclusion: Moving the COR of a Motec^® TWA affects movement and efficiency. Flexion and extension are more affected than other movements. Performance of the wrist deteriorates with more proximal and distal positions; however excellent results can be obtained over a relatively wide area.The standard surgical technique reliable places the COR of the Motec^® TWA in a position where optimal movement and efficiency can be expected.