Ultrasound Examination in a Patient With Foot Drop: Answering a Missed Call

A 26-year-old male tennis player/trainer was seen for a sudden-onset, right-sided foot drop that began 7 days ago (treated with ibuprofen 400 mg/day). He denied any trauma or accompanying low back symptoms. Current physical examination revealed an ankle/toe dorsiflexion strength of 4/5 (initially reported as 2/5). The rest of his neuromusculoskeletal examination was unremarkable. Electrophysiological studies were not considered due to timing. In addition to bilateral fabellae, ultrasound (US) examination of the common peroneal nerves (CPNs) showed mild edema on the symptomatic side exactly where sono-Tinel was also positive (Figure 1A,B, Video S1). Accordingly, the patient was diagnosed to have neuropraxia due possibly to fabellar compression/irritation during sports activities. He was called for a control visit on the third week when electrodiagnostic evaluations or a local injection would be planned—in case of insufficient recovery. Common peroneal neuropathy secondary to fabella compression is an uncommon and underrecognized entity, with only a few anecdotal cases described in the literature [1]. Anatomically, the fabella lies in the posterolateral aspect of the knee (often within the lateral head of the gastrocnemius muscle) and may form ligamentous connections with surrounding structures such as the fabellofibular ligament and the arcuate complex [2]. The peroneal nerve can pass medial, lateral, or posterior to the fabella, with the bone occupying approximately a quarter of the nerve's course in the popliteal fossa [2]. While direct causal evidence remains limited, this close anatomical relationship suggests a potential mechanical impact that could contribute to nerve compression under certain anatomical or postural conditions [2]. Such dynamic interactions may remain clinically silent until increased training load or subtle anatomical variations trigger symptomatic entrapment. Notably, although the symptoms were unilateral in our case, routine/comparative US examination demonstrated a similar scenario of vulnerability also on the contralateral side. To this end, in addition to more prevalent causes (e.g., habitual leg crossing or recent major/rapid weight loss), the presence of fabella and its possible role in the causation of a peroneal nerve injury needs to be considered—especially in subjects with overloading knee activities. Once again, a substantial US examination would be prompt and convenient to uncover not only the morphological changes in the entrapped peroneal nerve but also the underlying etiology during an interactive, dynamic and patient—as well as physician—friendly approach. Tuğba Urlu: investigation, writing – original draft. Berkay Yalçınkaya: investigation, writing-original draft. Ahmet Furkan Çolak: investigation, writing-original draft. Hilmi Berkan Abacıoğlu: investigation, writing-original draft. Gamze Kılıç: writing – review and editing, supervision. Levent Özçakar: conceptualization, writing – review and editing, supervision. The authors have nothing to report. The authors have nothing to report. The authors have nothing to report. Informed consent was obtained from the patient. The authors declare no conflicts of interest. The authors have nothing to report. Video S1: Dynamic short-axis ultrasonographic tracking of the right common peroneal nerve (arrow) from proximal to distal demonstrates its close anatomical relationship with the adjacent fabella. b: biceps femoris muscle, g: lateral head of the gastrocnemius muscle. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.