Peroneus Tertius muscle and tendon Injury: A lesser known entity

A 21-year male patient presented with pain and swelling right ankle after getting injured during football play. According to the patient the mechanism of injury was combination of plantar flexion and inversion. On examination the lateral side of the ankle was swollen, red and tender with pain on passive movements.

T1 axial and coronal images (figure 1a and 1b) show intermediate to hyperintense signals in the peroneus tertius (PT) muscle.T2 axial and coronal images (figure 2a and 2b) show long-segment hyperintense signals in the peroneus tertius muscle.STIR sagittal image (figure 3) show hyperintense signals within a muscle.PD coronal and sagittal images (figure 4a and 4b) showing long segment hyperintense signals within the muscle and tendon of PT.

The Fibularis tertius is the most superficial and largest muscle of the anterior group of leg muscles and it usually arises from the distal third/half of the fibula and of the intermuscular septum[2]. This muscle is also known as peroneus tertius (PT). It’s tendon runs obliquely and laterally to the most lateral tendon of the extensor digitorum longus. It inserts in the lateral and medio-dorsal, proximal aspect of the fifth metatarsal and often onto the fascia covering the fourth inter osseous space taking a broad fan shape [3]. An additional frequent projection is only visualised after lateral mobilisation of the muscle-tendon towards the base of the fourth metatarsal [4]. Another additional slip towards the dorsal aponeurosis of the fifth toe might be sometimes observed [5]. The function of PT is eversion and dorsiflexion of the foot [6]. It is also used clinically as a muscle flap to cover soft tissue defects [7].

The Prevalence of PT muscle is about 49% to 94% in anatomical studies by Ramirez and Roorkee [8]. It is well-known muscle and it’s injury is rare with virtually no case reported in the literature [9]. The only case reported was a short-segment injury with a longitudinal split tear near the insertion of the PT tendon and its tenosynovitis. These authors also reported the presence of peroneus quadratus and longitudinal split tears of both the peroneus longus and brevis tendons [8].

MRI is a Diagnostic modality of choice demonstrating hyperintense T2 and STIR signals within the muscle and tendon. Radiograph and CT may depict an additional fracture of the base of the 5th metatarsal. Ultrasonography (USG) may be useful to depict injury owing to its superficial location in the foot.

The location and injury to muscle and its tendon are usually classic with virtually no differential.

There is little information available about the therapeutic options.

Take home message

Knowledge of this lesser-known anatomical variant and its pathologies should be kept in mind while reporting cases of lateral foot pain, which is critical for management purpose.