New radiographic monitoring indices for total trapeziometacarpal prostheses

The objective of this study is to propose and evaluate a method of monitoring implants via the calculation of a trapezial and metacarpal index from radiological measurements and to describe an initial patient analysis.

This retrospective study describes the trapezial index which reflects the trapezial bone stock not occupied by the trapezial cup, while the metacarpal index reflects the rate of metacarpal occupation by the prosthetic stem. Those indexes were used on a series of 20 patients with a Maïa™ prosthesis with a minimum follow-up of seven years. The indexes were measured immediately postoperatively and at the various annual check-ups. Four observers measured each index on two occasions, to obtain an inter- and intra-observer correlation coefficient.

The average intra-observer correlation coefficient for the trapezium index was 0.94, for the metacarpal index 0.98. The inter-observer correlation coefficient was 0.93 for the trapezium index, 0.94 for the metacarpal index on average. The post-hoc calculated power was 0.98 as the number of subjects required was not usable. The mean immediate postoperative trapezial index was 45.74%, compared with a value at longest follow-up of 41.74%, reflecting a highly significant loss of height of 8,74%. The mean immediate postoperative metacarpal index was 77.69% compared with a mean value at longest follow-up of 78.99% indicating a non-significant increase in the index of 1,67%.

Proposed indexes had excellent inter- and intra-observer correlation, the metacarpal one is stable over time whereas the trapezial one reveals changes in some patients, requiring further investigations. These simple and reproducible indexes allow precise monitoring of trapeziometacarpal prostheses and identify radiographic changes that should lead to additional examinations to improve survival of implants.

III, retrospective single cohort study.