Reversed palmaris longus muscle: Anatomical variant – case report and literature review

Abstract

The palmaris longus, a slender fusiform muscle, is especially prone to exhibiting anatomical variance relative to other muscles in the upper extremity. The most frequent anatomical variation is the completely absent palmaris longus, followed by the reversed, duplicated, bifid or hypertrophied palmaris longus muscles. The reversed palmaris longus muscle represents a structure that is tendinous proximally and muscular distally (opposite of the normal palmaris longus). The present report describes a case of reversed palmaris longus muscle, followed by a literature review to illustrate the wide spectrum of anatomical variations in the palmaris longus muscle and their clinical and surgical relevance.

CASE PRESENTATION

Routine dissection of the left upper extremity of an 82-year-old Caucasian female cadaver, performed in the Department of Anatomy at the University of Saskatchewan (Saskatoon, Saskatchewan), revealed a rare variation in the anatomy of the palmaris longus (PL) muscle.

On dissection and examination, it was discovered that the PL of the left upper extremity of this cadaver was ‘reversed’ – with the proximal portion being tendinous and the distal portion being muscular.

Examination of the upper extremity showed no signs of atrophy or hypertrophy, nor median or ulnar nerve entrapment in the left upper extremity. The cadaver’s medical history, occupation and handedness were unknown. No anatomical variations were noted in the right upper extremity of this cadaver.

In the present case, the PL originated at the common flexor origin from the medial epicondyle. However, the origin was tendinous – a long thin tendon, instead of the normally observed PL muscle fibres.

Two-thirds down the forearm, the PL structure became more muscular in nature, with a muscle belly that spanned the distal one-third of the forearm. At the distal insertion of the PL, at the level of the wrist, the muscle fibres became tendinomembranous and continued into the palmar aponeurosis where they ultimately terminated (Figure 1).

Figure 1).

Anomalous reversed palmaris longus muscle found during cadaveric dissection, illustrating the palmaris longus origin, palmaris longus insertion and palmaris aponeurosis, as well as the palmaris tendinous portion (proximal) and the palmaris muscle belly (distal)

DISCUSSION

The standard anatomical origin of the PL is the common flexor at the medial epicondyle, shared by other superficial flexors such as the flexor digitorum superficialis – located beneath the PL – and the flexor carpi radialis and flexor carpi ulnaris – between which the PL is located. The standard anatomical insertion is the palmar aponeurosis, volar to the flexor retinaculum. Although the palmar aponeurosis joins the PL tendon, the two are distinct entities, as shown by their different origin and development (1). The vascularity is most commonly supplied by branches of the ulnar artery, followed by branches of the brachial artery. Branches of the median nerve supply innervation (2).

Several authors have described the PL muscle as a mere phylogenetically degenerate metacarpophalangeal flexor because of its clinical insignificance and its numerous anatomical variations (3–5). However, the less common variants can lead to pathological conditions due to the narrow topographical relationship between the PL and the median nerve, causing median nerve compression (6).

Agenesis of PL has been described as the most common anatomical variation, observed in approximately 2% to 25% of the population, with a higher prevalence in Caucasians (7–9).

All other anatomical variants combined account for those found in approximately 9% of the population (8). These include duplicated or bifid PL (depending on whether they share a common origin and muscle belly); triple-headed PL muscles; accessory PL; palmaris profundus (distal tendon passes deep to the flexor retinaculum); and reversed PL (9).

In the present case, the PL had a long tendinous origin proximally and a muscular portion distally. Since the initial description of a reversed PL in 1916 (10), several cases have been reported in the literature together with the clinical implications (11–14).

The reversed muscle belly can divide into separate muscular slips, which can insert into the flexor retinaculum, or onto the flexor carpi radialis tendon and volar carpal ligament (15). Thus, patients with a reversed PL may present with development of edema and inflammation accompanied by a bluish discolouration leading to numbness or pain in the volar aspect of the distal forearm. This results in a reduction of hand function usually exacerbated by wrist flexion (9).

Additionally, repetitive motions can cause hypertrophy of the muscle belly of a reversed PL and, due to the unyielding nature of the antebrachial fascia, may lead to an effort-related compartment syndrome (16).

Compression of the ulnar nerve (Guyon’s syndrome) associated with the reversed PL variation, although less common, has also been reported (17).

Understanding the anatomical variations of the PL is important because it often plays a crucial role during reconstructive surgeries due to its suitable length and diameter, ease of harvest and lack of donor site morbidity (18).

It is essential to clinically examine reconstructive patients for potential PL anatomical variations to avoid inappropriate surgical procedures and prevent disappointing surgical results.

KEY POINTS

• The PL is one of the anatomical structures most prone to exhibiting variance in the human body.

• Understanding the anatomical variations of the PL is essential due to its crucial role during surgery as a landmark guide and as a reconstructive tool.