The normal origin of the thenar motor branch is distal to the flexor retinaculum (i.e., “extraligamentous ”) and from the volar/central or volar/radial aspect of the median nerve [10]. Variations in the origin and course of the thenar motor branch relative to the transverse carpal ligament were first described by Poisel [11]. In this original description, two anomalies were described: “subligamentous ” (Fig. 1.1, 1A) in which the thenar motor branch originates beneath the transverse carpal ligament and “transligamentous ” (Fig. 1.1, 1B) in which the thenar motor branch passes through the transverse carpal ligament. In his series of 100 specimens, Poisel reported that 46% were extraligamentous, 31% subligamentous, and 23% transligamentous [11].

This distribution of extraligamentous , subligamentous , and transligamentous branching patterns reported has been corroborated by other authors in both clinical and cadaveric series. Hurwitz published a series of 80 carpal tunnel releases in which the thenar motor branch was extraligamentous in 55%, subligamentous in 29%, and transligamentous in 16% [4]. In addition, 9% had an anomaly in which motor branch originated from the anterior aspect of the median nerve, coursed in an ulnar direction distally before coursing toward the thenar muscles superficial to the flexor retinaculum [4]. This anomaly was first described by Mannerfelt and Hybbinette and is associated with transversely oriented muscle fibers overlying the distal flexor retinaculum (most likely flexor pollicis brevis or abductor pollicis brevis) [4]. This “supraligamentous” course was included by Lanz in his classification (Fig. 1.1, 1D). In a cadaveric study of 60 specimens, Alizadeh et al. reported 47% of thenar motor branches were extraligamentous, 28% subligamentous, and 12% transligamentous [5]. Interestingly, the thenar branch originated from the ulnar aspect of the median nerve in 12% of specimens [5]. Lanz also included this anatomic variant in his classification (Fig. 1.1, 1C).

However, there has been debate in the literature regarding the incidence of these abnormalities—particularly the transligamentous and ulnar origin variants. The incidence of transligamentous branching has varied widely in the literature. Falconer and Spinner reported that 60% of ten cadaveric specimens had a transligamentous branching pattern [9]. On the other hand, Kozin reported that only 7% of 101 cadaveric specimens had a transligamentous branching pattern and concluded that previous studies had overestimated the incidence of transligamentous branching as a result of the close proximity of obliquely oriented fascia distally with the transverse carpal ligament [10]. Of note, Kozin did not observe any thenar motor branches originating from the ulnar aspect of the median nerve, and only 4% had multiple motor branches [10]. Similarly, in Lindley and Kleinert’s clinical series of 526 carpal tunnel releases , only one patient had a thenar motor branch with an ulnar origin. Despite conflicting data on the incidence of the transligamentous and ulnar origin variants, awareness of all possible thenar motor branch anomalies is critical to prevent iatrogenic injury during carpal tunnel release.

Terminal branches of the median nerve include common digital nerves to the second and third web spaces. Engineer et al. described three variations of the third common digital nerve based on dissection of 20 cadaveric specimens (Fig. 1.2) [12, 13]. Type I originates proximal to the distal edge of the transverse carpal ligament and found in 15% of specimens. Type II originates distal to transverse carpal ligament, but proximal to the superficial palmar arch, and was found in 70% of specimens. Type III originates distal to the transverse carpal ligament and at (or distal to) the superficial palmar arch and was found in 15% of specimens [12]. Knowledge of type I third common digital nerve anatomy is critical to prevent iatrogenic injury during carpal tunnel release [12].