Traumatic loss of thenar muscle function is a rare injury. Thus, it often leads to severe impairment of hand function.
The abductor pollicis brevis (APB) muscle is, besides the opponens pollicis (OP) muscle, the prime muscle of thumb opposition. Therefore, damage to the thenar branch or the muscles itself leads to impairment of opposition rather than a complete loss (14).
Opposition movement of the thumb is the crucial function which makes the human hand unique. Accordingly, impairment is not only disastrous for accomplishing daily life activities and occupational tasks, sometimes leading to psychological burden, but also has socioeconomic consequences (15).
To reconstruct opposition, standardly tendon transfers are used. In 1917, Steindler performed the first opponensplasty using a radial strip of the flexor pollicis longus tendon (FPL) (2). Subsequently, Huber described the transfer of the abductor digiti minimi in 1921 (16). Further on, Bunnel (1924) and Camitz (1929) used the palmaris longus muscle with a split tendon transfer to reconstruct opposition (17). Another viable tendon transfer was described by Caplan et al. in 1956 using the extensor indices muscle and thereby also addressing combined median and ulnar nerve injuries (18).
To broaden the reconstructive repertoire, the first free functional muscle transfer was described in a canine model in 1970 by Tamai et al. (19) and was first successfully used in clinical routine in 1976 (20). With emerging free flap routine and advancements in microsurgery further viable functional flaps were described.
Accordingly, Dellon et al. were first to describe the PQ as muscle flap of the forearm (5). Primarily described as a pedicled flap, Lee and Idler (6) and Tzou and Aszmann (7) published case reports using the PQ as a free functional muscle transfer.
Without the need for a second incision for flap raising and therefore minimal donor site morbidity, we used the PQ as a FFMT in the largest series of cases so far. Because the original thenar motor branch was used to innervate the flap, no cortical reeducation was necessary for the patients to oppose their thumbs, which was easily feasible for all patients. No impairment in forearm pronation was observed, due to intact pronator teres-function. Other FFMT found in the literature used the serratus flap or a gracilis flap for functional thenar reconstruction (3, 4). Hereby, the donor site morbidity should be considered higher as with the PQ flap due to the need of a second incision on a different donor site on the body and potential intraoperative relocation of the patient. Furthermore, some of these FFMTs are bulky for delicate hand reconstruction. (Fig. 4)
We found a regeneration of conduction velocity of 53.35 ± 1.15 m/s on the uninjured side compared to 46.9 ± 3.1 m/s to the healthy side with a corresponding recovery of grip strength of 26.2 ± 1.2 kg. Although the force almost doubled compared to preoperative grip force of 14 ± 2.2 kg, there is still a discrepancy to the 34 ± 2 kg on the healthy side. A recent study from Thora et al. (21) showed a mean postoperative grip force of 16kg after tendon transfer opposition-reconstruction. Although limited patient numbers, this data indicates a superior grip strength after FFMT compared to tendon transfers.
This might be explained with studies from Brand et al. who demonstrated that the PQ has better muscle properties compared to the original thenar muscles considering relative tension and excursion than all described tendon transfers (8, 9).
Bertelli et al. (10) found 649 ± 237 axons in the thenar branch which is almost equivalent to 600 ± 250 axons found in the AIN by Schenk et al. (22). Furthermore, the number of fascicles (2.3 ± 0.6 for the thenar branch vs. 2.3 ± 1.5 for the AIN) was found to be equal, providing ideal preconditions for regeneration after reinnervation of the thenar branch to the AIN of the PQ.
Considering functional aspects, we found a clinically significant improvement of thumb opposition indicated by a postoperative Kapandji Score of 8.7 ± 0.47 (preoperative 4.3 ± 0.94) with corresponding decrease of Bourrel’s angle to 36.96 ± 3.68 degrees (preoperative 75.75 ± 3.45 degrees).
In comparison, Al-Quattan (23) and Thora et al. (21) described their functional results for tendon transfers as “good” and “excellent” according to the score of Sundararaj et al. (24). This classification uses the term “excellent” for an opposition of the thumb to the tip of the fourth or fifth digit (23, 24) which is equivalent to a Kapandji score of 5 respectively 6 (12). Therefore, our results found after FFMT are, due to limited comparability, at least equal to those found after tendon transfer.
Injuries to the thenar muscle mass or motor entry point combined with high demanding patients are excellent indications for a FFPQ. Furthermore, thenar motor branch injuries and prolonged diagnosis with consecutive irreversible muscle damage can be reconstructed with a FFPQ. Due to the single incision procedure, intraoperative exploration of the thenar can then be extended by preparation and transfer of the FFPQ without the need of additional operation sites. Hereby compliance of the patient is as important as free joints to provide sufficient range of motion after regeneration. Still tendon transfers also require compliance and consecutive physiotherapy with the addition of cortical reeducation.
The FFPQ should not be performed in high median nerve injuries with potential damage to the AIN or PQ. Also combined median and ulnar nerve injuries cannot be reconstructed using the FFPQ due to the absence of a donor nerve. In this scenario, or with ligamental instability of the thumb, a CMC 1 arthrodesis might be a better solution.