Introduction: Various variants of the extensor indicis (EI) have be described in the literature. We wanted to detect whether there exist any variants of the EI that may cause restricted mobility of the thumb following EI transposition to the extensor pollicis longus (EPL).

Patients & method: Intraoperatively the function of the extensor tendons of 168 hands (98 right / 70 left) of 159 patients (96 female / 63 male) were examined. The function of the muscles was simulated using a tendon-hook. For ethical reasons the approach was not extended for the study.

Results: In 34 of 168 hands 39 accessory tendons were found: 8 were localized between EPL and EI (1 from the EPL to the index; 3 extensor pollicis et indicis; 1 from the EI-muscle to the thumb; 3 to the radial extensor hood of the index). 31 accessory tendon were found ulnar to the EI (2 to the ulnar extensor hood of the index; 25 to the middle finger; 3 to the ring finger; 1 to the little finger). The EI was missing in only one hand, were a strong extensor anularis-tendon was found, which would have been suitable for EPL-reconstruction. 8 of these variants would hinder the thumb from isolated extension following EPL-reconstruction with the EI-tendon.

Conclusion: The extensor tendons should be inspected carefully through EI-transposition for reconstruction of EPL to ensure a free function of the thumb postoperatively. Small accessory tendons that may cause trouble should be cut, strong tendons should be transposed together with the EI-tendon.