Variations in forearm tendons – A clinical-anatomical guide

Anatomical variations of the muscles and tendons of the forearm and hand are both common and significant in daily clinical practice. The flexor muscles, specifically the palmaris longus (PL) and flexor carpi radialis (FCR), are frequently utilized in reconstructive procedures for ruptured tendons and torn ligaments.¹ Examples of such procedures include the reconstruction of flexor tendons, the ulnar collateral ligament, and the dorsal scapholunate ligament in cases of scapholunate dissociation.^2–4

The presence of different tendon variations can complicate tendon harvesting and potentially compromise the success of the procedure. Additionally, the variability in the anatomical structures around the PL and FCR tendons can pose risks to neurovascular structures during surgery, thereby necessitating careful preoperative planning and precise surgical techniques to avoid complications.

The palmaris longus (PL) muscle, in particular, exhibits the most frequently described anomalies. Variations include duplication, bifurcation, reversal, central location, division with an ulnar slip, and hypertrophy. Some reports have even depicted an anomalous PL with three heads or as three distinct muscles.⁵ Equally variable are the insertion sites of the PL, ranging from various muscular and osseous insertions to a doubled distal insertion, also known as the palmaris longus bicaudatus of Gruber.⁶ Reimann et al. reported that the PL is absent in 12,8% of the donors,⁷ more commonly in females and on the left side in both sexes.⁶

The flexor carpi radialis (FCR) is another vital forearm muscle, used for tendon grafts in hand surgery and serving as a consistent landmark for surgical approaches to the forearm and wrist. Yeager et al. observed a consistent insertion of the FCR onto the trapezial ridge in all donors,⁸ whereas Akita et. al. described the FCR insertion as more variable, most commonly partially or totally attached to the trapezium, but also, to the annular ligament, scaphoid, and the bases of the third and fourth metacarpals. The FCR can sometimes be duplicated or absent, with various muscular variations reported.⁶

Understanding the anatomical complexity of the forearm’s flexor tendons, specifically the PL and FCR, is crucial, as they are often used for tendon transplants in hand surgery.¹ Given the numerous anatomical variations, this study aimed to provide a comprehensive overview of these variations, highlighting their clinical significance and implications for surgical planning in orthopedic and reconstructive procedures. The PL and FCR muscles and tendons were examined to outline the length and width of the tendons, the variability of the muscles relative to other muscles and neurovascular structures, and their relationship to the flexor retinaculum.

Results

The PL was present in 74% of all donors, with 64% having a bilateral PL, 10% a unilateral PL and 26% showing agenesis of the PL. When comparing between the sexes, 12 females (63% of all female and 39% of all donors) and 8 males (67% of all male/26% of all donors) showed a bilateral PL, 3 females (16% of all female/ 10% of all donors) and 3 males (25% of all male/10% of all donors) had a unilateral PL. The tendon length, which could have been harvested (between the most distal muscle fibers and the insertion in the palmar aponeurosis) was approximately 13cm (SD = 3cm, range = 4–19cm).

The normal PL muscle origin was the medial epicondyle (ME) and the insertion was the palmar aponeurosis and the transverse carpal ligament. The PL tendon had a round structure in 44 cases (96% of all PL/71% of all donors) and a flat, aponeurotic structure (width more than 1cm) in 2 cases (4% of all PL/3% of all donors). The PL inserted radial in 45 cases (98% of all PL/ 73% of all donors), only in one case (2% of all PL/2% of all donors) the PL inserted ulnar into the transverse carpal ligament. Furthermore, we assessed the course/location of the PL insertion with respect to the thenar muscles, and described three types – volar to thenar, into thenar and deep to thenar. In the majority of 30 cases (65% of all PL/48% of all donors) the PL inserted volar to the thenar muscles, in 12 specimens (26% of all PL/19% of all donors) the PL inserted into the thenar and in 4 cases (9% of all PL/6% of all donors) the insertion of PL was located deep to thenar.

The PL tendon showed a bifid variation in 6 cases (13% of all PL/10% of all donors), splitting between 1 to 4,5cm before inserting in the palmar aponeurosis, a trifid and multifid variation were not seen.

One case (2% of all PL/2% of all donors) showed a central muscle belly, a proximal tendon and a distal bifurcated tendon forming the PL.

In one case (2% of all PL/2% of all donors) the ulnar artery (UA) overcrossed the PL, resulting in a possibly more difficult and hazardous procedure of tendon harvesting.

FCR was present in all specimens. The distance between the most distal muscle fibers and the transverse carpal ligament was 8cm (SD= 2cm, range = 5,5–11,5cm). There were antebrachial connections in 35 cases (56%), which often consisted of muscle strands to PT and/or to FDS in the proximal third of the forearm. In 10 cases (16%) the FCR had a muscular connection to the FDS, in 15 cases (24%) to FDS and pronator teres (PT) and in 10 specimens (16%) only to PT in the proximal third of the forearm.

Discussion and conclusion

Different tendon variations can compromise the optimal length or width for tendon harvesting, result in poor tendon structure, or even endanger neurovascular structures due to anatomical differences around the flexor muscles. The literature reports a prevalence of PL in 50% to 90% of the population, with bilateral PL observed in 62,2% and unilateral PL in 15%.^6,9–11

Furthermore, a reversed PL can potentially compress the median nerve (MN) at the wrist, as described by Meyer et al., although this variation was not observed in our study.¹³ A rare and critical variant to note is the superficial ulnar artery (SUA), which courses over the PT, FCR, and PL muscles before entering the Guyon’s canal.¹⁴

In our case, the UA ran deep to the FCR muscle, becoming superficial radially and crossing over the PL. According to Hanada et al., the SUA can be damaged during surgery, and the broad tendon of the PL might not be suitable as a tendon graft.¹⁵ Preoperative ultrasound visualization can help identify variations in either muscles or neurovascular structures. These findings can guide surgeons in understanding anatomical variations of the flexor tendons and the associated potential risks during tendon harvesting or other reconstructive procedures in this area.

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