The prevalence of the extensor indicis tendon and its variants: a systematic review and meta-analysis

The tendon of the extensor indicis (EI) is frequently used to restore the loss of function in other digits. However, it shows many variations which include splitting of the extensor indicis proprius (EIP) into two or three distal slips, attachment to fingers other than the index such as the extensor medii proprius (EMP), attachment onto the index and the third finger such as the extensor indicis et medii communis, or attachment to both the index and the thumb such as the extensor pollicis et indicis (EPI). This systematic review gathers the available data on the prevalence of EI tendon and its variation in the hand. Twenty-nine cadaveric studies met the inclusion criteria with a total of 3858 hands. Meta-analysis results yielded an overall pooled prevalence estimate (PPE) of EI of 96.5 % and PPEs of 92.6, 7.2 and 0.3 % for the single-, double- and triple-slip EIP, respectively. The single-slip EIP is frequently inserted on the ulnar side of the extensor digitorum communis of the index (EDC-index) in 98.3 %. The double-slip EIP is located on the ulnar side of the EDC-index in 53.5 %, on its radial side in 17 % and on both sides in 28.7 %. Indian populations showed the highest rate of single-slip EIP and the lowest rate of double-slip EIP when compared to Japanese, Europeans and North Americans. The pooled prevalence of EMP, EMIC and EPI were 3.7, 1.6 and 0.75 %, respectively. Knowledge of the variants of the EI tendon and their prevalence should help surgeons in correctly choosing the tendon to transfer in hand surgery.