Anomalies in the extensor tendons of the hand are well-documented, ranging from variations in muscle bellies and tendons to connections between tendons [1]. Türker et al. [2] classified extensor pollicis longus (EPL) abnormalities based on their origin and connections. While these anomalies may exist, they are often asymptomatic, making the discovery of accessory EPL tendons a rare occurrence typically found incidentally during cadaveric studies or emergency surgeries [3]. Physical examination alone does not guarantee the diagnosis of tendon variability. On the other hand, finger tendon injuries are commonplace in both occupational and sports-related activities. Physical examination serves as a crucial aspect in evaluating such injuries, aiding in the assessment of damage to anatomical structures. However, variations in anatomy can sometimes lead to misinterpretation of structural damage [4]. Appreciation of these anatomical variations is vital to achieve a comprehensive understanding of thumb extension and to navigate the complexities of tendon injuries accurately [5].

A 54-year-old male presented at the emergency clinic with left thumb pain. The patient reported an injury from a saw blade 3 days prior. Although the wound had healed without medical intervention, the patient experienced pain during thumb extension. During the examination, there was a partial restriction in the range of motion, with the interphalangeal joint capable of extension (Fig. 1). Given the limited but present extension of the distal phalanx and the spontaneous healing of the laceration, tendon diseases and an inflammatory condition were suspected.

To further investigate, magnetic resonance imaging (MRI) was performed revealing a suspected rupture of the EPL tendon (Fig. 2). The MRI report mentioned only the ruptured tendon, and we did not detect any accessory tendon before surgery. Surgical intervention was performed for diagnostic purposes due to discrepancies between the physical examination and imaging results. During surgery, a complete rupture of the EPL tendon was confirmed (Fig. 3).

Interestingly, an intact tendon was found running parallel to the EPL tendon on the radial side (Fig. 4). This tendon was half the thickness of the normal EPL tendon. Although thinner, this accessory tendon prompted extension of the interphalangeal joint when subjected to forceful traction, without affecting the other fingers. Given its location on the radial side of the EPL tendon and its effect on interphalangeal joint extension, it was identified as an accessory EPL tendon, rather than the extensor pollicis brevis. This suggested that, despite the total rupture of the EPL tendon, limited thumb extension was possible due to the presence of this supporting accessory structure. The surgery was performed between the proximal phalanx and metacarpal level of the thumb. There was no unnecessary wrist-level incision, so we could not confirm the origin or compartment of the accessory tendon.

Repair of the ruptured tendon was performed using central and peripheral suturing techniques. Postoperatively, the thumb was maintained in an extended position with immobilization of the interphalangeal joint for one month. Six months later, the patient regained nearly normal thumb function, with mobility and strength comparable to the contralateral side.

Written informed consent was obtained from the patient for the publication of this report including all clinical images.

The extensor tendon system in the hand exhibits a wide range of anatomical variations, including the presence of an accessory EPL tendon. However, these variations can present challenges in diagnosis, particularly when determining the exact location and anatomical position of tendon damage. When evaluating dorsal injuries involving the thumb with suspected extensor involvement, understanding variations in tendon size, strength, and insertion sites becomes crucial [6]. This understanding helps assess their impact on thumb extensor movements.

Most variations are asymptomatic and are discovered incidentally [7]. Türker et al. [2] classified EPL tendon variations, and understanding these variations is important, as previous literature often discusses their potential impact on surgical outcomes. In the Türker classification, tendon anomalies are divided into two categories: separate tendons (type 1) and tendon interconnections (type 2). Type 1 is further divided into six subtypes, based on the origin of the supernumerary tendon, specifically the compartment from which it exits, its path, and its attachment point. In type 1a, the tendon runs through the fourth compartment and attaches to the base of the distal phalanx of the thumb. In type 1b, the tendon runs parallel to the EPL in the third compartment and it has its own attachment at the base of the distal phalanx. In type 1c, the tendon exits from the fourth compartment, runs parallel to the EPL, and attaches to the distal portion of the original EPL tendon. In type 1d, the tendon passes through the fourth compartment, crosses over the EPL, and attaches to the proximal phalanx of the thumb. In type 1e, the tendon exits from the fourth compartment, runs parallel to the EPL towards the first web space, and then splits into two branches: one attaching to the radial side of the extensor hood of the index finger and the other to the distal phalanx of the thumb. In type 1f, the EPL tendon divides into two slips from the same muscle belly, with the radial slip passing through an abnormal tunnel between the first and second compartments and the ulnar slip running over the extensor retinaculum. Type 2 focuses on the relationships with the extensor indicis proprius and extensor digitorum communis, as well as accessory slips originating from the EPL that attach to the extensor hood of the index finger, comprising a total of four subtypes [2].

Munn and Gillis [5] further classified cases with two anomalous tendons as multiple accessory tendons (type 3), referring to situations involving an anomalous EPL in conjunction with an anomalous tendon to the index finger. Applying this classification to the patient’s accessory EPL tendon, we can conclude that it corresponds to type 1a, 1b, or 1f based on its parallel course to the EPL and its attachment to the distal phalanx, as well as the fact that traction does not affect the index finger. Although the origin of the accessory tendon could not be identified during the surgery, it can be classified as type 1f based on its radial position.

As Chiu [8] reported, a complete rupture of the EPL may be mistaken for a partial tear when an accessory extensor tendon to the thumb is present. In this case, limited extension was observed, and MRI imaging subsequently confirmed the total rupture. In addition to clinical examination, imaging techniques such as ultrasound can be valuable for assessing the extensor tendon. Ultrasound is a reliable, quick, and cost-effective method for visualizing the tendon. In cases where a tendon anomaly is not clearly observed using ultrasound, MRI may be considered for a more detailed evaluation.

In conclusion, this case underscores the significance of a comprehensive understanding of anatomical variations in the extensor tendon system, utilizing both clinical and imaging assessments to improve diagnostic accuracy and guide appropriate treatment strategies.

Given that tendon anomalies are more commonly found on the ulnar side, the presence of a radial-side anomaly in this case is particularly noteworthy and invites further exploration of its relationship with the extensor pollicis brevis.