Relationship between cup position and neck type in the TOUCH^® trapeziometacarpal prosthesis: a CAD simulation and...

Posteroanterior cup angulation relative to the proximal articular surface of the trapezium (PAST) and neck type (15° offset or 0° straight) may influence the risk of intraprosthetic impingement in the TOUCH^® prosthesis. We assessed this relationship using a computer-aided design (CAD) simulation and a cadaver experiment.

In the CAD simulation, cup orientation was varied anteriorly and posteriorly from 0 to 30°. For each cup placement, both necks were tested by simulating flexion and extension of the thumb until impingement occurred between the neck and the cup's rim. In the cadaver study, which included 24 hands, 14 cups were tilted between 0 and 15° anteriorly, eight cups were tilted 15° or more anteriorly and two cups were tilted between 0 and 15° posteriorly. For all specimens, and both neck types, the occurrence of impingement was determined when the thumb was flexed to 10°, then extended to 20 and 40°.

For both necks and with the cup in a neutral position, there was no impingement. An incorrect cup orientation caused mechanical impingement when the thumb was extended. For extensions of 30° (CAD) and 40° (cadaver), impingement occurred when the cup was tilted 15° anteriorly with the offset neck, whereas it occurred with a cup tilted 30° anteriorly with the straight neck. The neck length did not influence the risk of impingement.

Posteroanterior cup angulation influences the choice of neck type. To minimize the risk of impingement, cup placement parallel to the PAST is optimal. When the cup is tilted 15° or more anteriorly, we recommend using a straight neck to reduce the risk of impingement during thumb extension.