The Rubis II prosthesis has provided good short-term and medium-term results and survival, but prosthetic dislocation occurred in about 9% of cases in our previous series. The objective of this study was to find risk factors for Rubis II dislocation.

The Rubis II is a uncemented reverse total trapeziometacarpal prosthesis made of a screwed circular plate topped by a sphere which articulates with the metacarpal stem. Sixteen never reoperated prostheses with at least one dislocation were reviewed clinically and radiologically according to a standardized and detailed protocol. Forty-eight controls paired on age and sex and which never have been dislocated were assessed with the same protocol. Implant positioning, thumb length and adjacent joint arthritis were assessed.

Forty percent of cases lacked bearing of the plate on the trapezium bone vs. 4% in the control group (P=0.0015). Cases were painful in 80% of case vs. 37.5% of controls (P=0.0067). Prostheses were more frequently on left hands (81% vs 54% P=0.077) although it was not significative. Other clinical and radiographic parameters did not differ between groups.

A technical error could have been involved in some of our prostheses, but implant positioning did not seem in cause for the other implants. The Rubis II prosthesis has a high rate of dislocation but a low rate of loosening, possibly related to its unconstrained design. Changes of the prosthetic neck and head could be studied to improve the implant stability while keeping its longevity.