A 38-year-old male military officer presented to the orthopaedics department for evaluation of a left ankle mass. According to the history given, the swelling was first noticed 4 weeks ago after a 10 km marching exercise and the patient did not recall any major traumatic injury.
Physical examination showed a painless, hard, immobile soft tissue mass located anteroinferiorly to the left fibular tip (Fig. 1). Range of motion was severely restricted in all planes. No clinical or laboratory evidence of inflammation or infection was identified. An ultrasound examination showed a superficial, complex, multilocular cystic mass (Fig. 2). Furthermore ultrasound depicted loss of the fibrillar echotexture of the peroneus brevis tendon (PBT), hypoechogenicity compared to the peroneus longus tendon (PLT) and a longitudinal tear of the PBT (Fig. 2). MR imaging verified the presence of a split peroneal tendon syndrome and demonstrated also the complex cystic mass (Fig. 3). The diagnosis of a chronic split peroneal brevis tendon tear associated with a ganglion cyst was thus established and the patient was referred to a specialised podiatric clinic for surgical therapy. The one year postoperative evaluation demonstrated a good cosmetic and functional outcome (Fig. 4).
The primary role of the peroneal tendons is plantar flexion and eversion of the foot at the ankle [1-3]. Furthermore, they act as lateral stabilisers of the ankle joint. The peroneus brevis tendon (PBT) lies between the bony retromalleolar groove and the peroneus longus tendon (PLT) and thus is susceptible to degeneration, tendinopathy and rupture [1-4]. Tears of the PBT at the level of the retromalleolar groove are often associated with predisposing factors: a) traumatic rupture and insufficiency of the superior peroneal retinaculum, b) preexisting chronic rupture of the calcaneofibular ligament and ankle instability, c) normal anatomic variants (e.g. peroneus quartus muscle) and d) retromalleolar groove irregularities. The normal retromalleolar groove demonstrates a smooth and concave shape whereas convexity, straight configuration or irregularity may predispose to PBT tear [1-4]. In our case the irregularity and straight orientation of the retromaleolar groove, in the absence of any major traumatic injury, may have led to split PBT syndrome. MRI is considered the imaging modality of choice for evaluation of possible peroneal pathology [1-3]. The PBT is depicted with a characteristic crescent like configuration between the retromaleolar groove and the PLT. Loss of this normal characteristic appearance and a resulting bifid configuration with the PLT intervening between the two parts of PBT are the MRI findings of the syndrome [1-4]. Although the MR signs are well documented in the literature, to the best of our knowledge the ultrasound identification of this syndrome is an underreported entity. Recent developments in sonographic technology have enhanced the diagnostic capacity of musculoskeletal ultrasound [5-6]. Excellent details of the superficial structures can be depicted and additional information is gained while performing dynamic examinations [5-6]. In our patient the tear was further complicated by a ganglion cyst, probably due to the chronic nature of the split syndrome. The strenuous activity (marching exercise) was considered retrospectively the triggering factor. Although the association of ganglion cysts with tendon pathology is well documented, the actual aetiology of ganglion cyst formation remains obscure [7-9]. The repetitive-overuse injury to the tendon or tendon sheath with consequent cystic or mucoid degeneration of the tendon’s collagen fibres, cellular hyperplasia and tendon tear with tenosynovitis are believed to be the main pathophysiologic mechanism that precedes ganglion cyst development [7-9]. In our case the tendon tear and the associated ganglion cyst were not suspected clinically but ultrasound suggested the accurate diagnosis and MRI confirmed the findings.
Split peroneal tendon syndrome with an associated ganglion cyst formation
1. A noticeable protrusion or mass-like change is visible on the lateral side of the patient’s left ankle joint, clinically palpable as soft tissue swelling.
2. Ultrasound and MRI indicate that at the posterior aspect of the lateral malleolus (near the posterior border of the fibula), the peroneal tendon (mainly the peroneus brevis tendon) exhibits splitting, abnormal layering, or a “fissure,” presenting a “double-split” or “bifurcation” sign.
3. In the area around the tendon or near the tendon sheath, cystic structures are observed, appearing round or oval with clear boundaries. The signal or echo suggests a fluid-filled lesion, considered likely to be a tenosynovial or synovial cyst.
4. No obvious major fracture is noted. The localized fluid lesion within the soft tissue is closely related to the tendon abnormality, consistent with chronic overuse or tendon degeneration combined with cyst formation.
Considering the patient is a 38-year-old male soldier who developed lateral left ankle swelling after high-intensity marching (10 kilometers), and imaging findings suggest a split in the peroneus brevis tendon accompanied by a cystic lesion, the most likely final diagnosis is: Split Peroneus Brevis Tendon (Split PBT) Syndrome with Tenosynovial Cyst Formation. This conclusion aligns with the patient’s clinical symptoms and ultrasound/MRI findings, making it the most credible diagnosis at present.
According to the FITT-VP principle, phased and gradual return to exercise is recommended.
Throughout the rehabilitation process, it is important to closely monitor ankle swelling, pain, or other discomfort. Should recurrent swelling or pain intensify, seek medical consultation promptly.
This report is based on current clinical and imaging information and is intended to provide reference opinions. It does not replace an in-person evaluation or professional medical diagnosis and treatment. If you have any questions or if symptoms worsen, please seek medical attention promptly for further examination and treatment recommendations.
Split peroneal tendon syndrome with an associated ganglion cyst formation
