Interposition grafting material frequently is used to treat various disorders or ligamentous injuries of the hand. Given reports of duplicated tendons in the first dorsal compartment of the hand, we explored the possibility of using the AAPL tendon as grafting material. Specifically we (1) described the number of tendons and muscle bellies and innervation in the first dorsal compartment of the hand, (2) described their insertion site, (3) recorded the tendon dimensions and (4) established side-to-side differences in the dimensions of the APL and AAPL.
Our study has several limitations. First, the specimens available for dissection were Caucasian, and direct extrapolation of the data presented in this study may not be applicable to other racial groups. In a study to determine the ethnic variability of palmaris longus agenesis, Sebastin et al. revealed a low prevalence of absence in Asian, black, and Native American populations and a much higher prevalence of absence in Caucasian populations [29
]. Second, we used x2.5 loupe magnification to dissect and describe the innervation of the muscle and found a specific nervous branch in 80% of the cases in which there was an independent muscle belly. Dos Remedios et al. reported finding a specific nervous branch in all their cases using x4 loupe magnification [9
]. Thus, the lower magnification might influence the rate at which innervation can be found.
An APL tendon duplicity has been widely documented appearing in 56% to 98.5% of hands [38
]. In our study, it appeared in 85%, which is close to the results presented by other authors [20
] with 92% and 89%, respectively. When planning a reconstructive procedure of the hand, the use of preoperative ultrasound can provide useful information regarding the number of tendons and approximate dimensions.
In APL muscle dissection, we observed some AAPL tendons have their own muscle belly. It appears as an independent muscular entity, fusiform and proximal and radial to the proper APL muscle. van Oudenaarde and Oostendorp [35
] and Zancolli and Cozzi [38
] reported the existence of two muscular components, one deep and one superficial. The deep one is more proximal and has multiple insertions sites around the trapeziometacarpal joint. This distribution would match our AAPL muscle. The superficial portion is more distal and inserts at the base of the first metacarpal and would match what we considered the APL muscle. However, some authors consider the differentiation of these muscles difficult even with the use of vascular studies [28
In the majority of cases (13 of 16) in which an independent muscle belly for the AAPL appears, we identified two nerve branches: one descending long branch that enters the muscle in the ulnar and distal portion of the APL muscle and continues toward the EPB muscle and a shorter one that inserts in the more proximal and radial side of the APL muscle. There is some discrepancy in the literature regarding the number of forearms with two muscle bellies and forearms with two specific nervous branches. Two studies [9
] suggest a similar pattern of nervous distribution with an independent nervous branch for each muscle belly. Others, however, have found only one nervous branch [31
The distal insertion sites are in accordance with published data [2
]. The insertion of the APL in the thenar muscle instead of the base of the first metacarpal is rare and we found it in one of 78 hands [27
We found no difference in tendon dimensions between the APL and the AAPL. Others have reported the existence of multiple slips but have not analyzed the tendons’ dimensions [8
]. Our findings favor the use of the term accessory tendon instead of tendon slips.
The dimensions of the AAPL are appropriate for its use as a graft donor in hand surgery. The recommended length of tendon used in TMC arthroplasty for treatment of osteoarthritis was 5 cm [1
]. The average length of the AAPL in our cadaveric specimens was almost 7 cm, thus allowing the use of the AAPL for that specific technique. The use of the AAPL for extensor tendon reconstruction after chronic EPL ruptures also has been reported with good functional scores and patient satisfaction [6
]. Loss of abduction with the use of the AAPL compares favorably to the use of the APL [6
]. The APL has been considered more relevant for thumb mobility and the AAPL was presumed more important for TMC stability. TMC stability is not compromised with the sacrifice of the AAPL tendon [5
]. This may be the cause of the diminished morbidity with the use of the AAPL [34
]. Some authors have credited the APL with a complex function in movement of the thumb [4
]. However, others reported minimal functional impairment of the thumb with absence of the APL, probably owing to the coordinated action of the rest of the musculotendinous units acting in the thumb [5
]. Additional studies are required to determine the morbidity of using the AAPL for grafting material procedures.
The absence of differences in tendon dimensions between the APL and the AAPL, along with the findings of a separate muscle belly and proper innervation, suggest the AAPL can be considered a true tendon and not just an extra slip of a present tendon. The information regarding tendon dimensions may prove useful to the surgeon performing reconstructive surgery of the hand.