The utility of diagnostic and interventional ultrasound in identifying common fibular (peroneal) neuropathy secondary to fabellae 048 Alper Cesmebasi, MD1,2, Robert J Spinner, MD1, Jay Smith, MD3, Stephen M Bannar, MD4, Jonathan T Finnoff, DO3 1Department of Neurosurgery, Mayo Clinic, Rochester, MN, USA; 2Department of Anatomic Sciences, St George’s University School of Medicine, Grenada, West Indies 3Department of Physical Medicine and Rehabilitation, Mayo Clinic, Rochester, MN, USA; 4Tahoe Orthopedics and Sports Medicine, South Lake Tahoe, CA , USA Abstract Objectives Table 1 Figure 2 Discussion Background: The fabella is a sesamoid bone in the posterolateral knee which may present itself as an etiologic cause of pain. In close proximity to the common fibular (peroneal) nerve (CFN), the fabella may impinge upon the nerve to cause fibular nerve pain and palsies. Objective: While anatomical studies have established a relationship between the fabella and CFN, we present cases where sonography was able to determine the source of the pain secondary to CFN compression by an adjacent fabella in posterolateral knee pain that radiates into the anterolateral leg pain Methods: Four patients presented with complaints of posterolateral knee pain radiating into the anterolateral leg pain. All patients underwent diagnostic and interventional ultrasound (US) in the evaluation of posterolateral knee pain. Results: Two female and two male patients presented with complaints of posterolateral knee pain radiating into the anterolateral leg pain. Diagnostic and interventional ultrasound was able to determine the source of the pain was secondary to CFN compression by an adjacent fabella. In two cases, pain symptoms were resolved with US-guided CFN block. One patient had temporarily relief with CFN block and required surgical treatment with fabellectomy for complete resolution of symptoms. Another patient was managed with oral medications. Conclusions: Dynamic sonography plays a role in providing convincing in vivo evidence to establish a causal relationship between fabella and fibular neuropathy in posterolateral knee pain Dynamic imaging with sonography may play a role in providing convincing in vivo evidence to establish support a causal relationship between fabellae and CFN neuropathy. We examine 4 cases of patients in whom the CFN neuropathy resulted from the fabella compressing the CFN proximal to the level of the fibular tunnel. The fabella is a sesamoid bone embedded within the tendinous portion of the lateral head of the gastrocnemius muscle; forming an articulation with the posterior surface of the lateral femoral condyle. While asymptomatic, in rare instances, the fabella has been associated with posterolateral knee pain. The CFN passes superficially or immediately lateral to the fabella in almost all patients, leaving it prone to irritation or compression by the sesamoid bone. Sonography is valuable in assessing the soft tissue and bony structures of the knee and can detect fabellae, which are commonly missed on plain radiography. Dynamic sonography can confirm the presence of fabella and provide guidance for interventional management of CFN compression and irritation in patients. Table 1. Case Demographics and Treatment Case Sex Age at Onset (yrs) Time from Symptom Onset to Diagnosis (months) Pain Scale (0-10) Treatment 1 F 56 12 10 CFN Block; Surgical excision 2 M 22 6 0-7 CFN Block; Lateral Heel Wedge 3 5-6 Peri-fabellar Injection; Oral medications 4 54 4+ (24++) N/A + Peri-fabellar Injection ++CFN block Results Demographics: There were two male and two female patients (age range: 22-56 yrs) whom presented with posterolateral knee pain with radiation of pain and paresthesias along the anterolateral leg. (See Table 1) Clinical Presentation: The left side was the affected side in all patients with achy pain occurring upon weightbearing (Cases 1-3) and knee extension (Cases 1-4). Radiation of pain and paresthesias from the posterolateral knee along the anterolateral leg was described in all patients. No history of trauma was reported in any patient. Time from symptom onset to diagnosis ranged from 3 months to 1 year. Physical Findings: Localized tenderness over the posterolateral aspect of the knee was noted in all patients. Extension of the knee in 2 patients (Cases 1 and 3) reproduced pain. Normal motor strength was noted in all patients. Sonographic Studies: Diagnostic ultrasound determined the source of the pain in all patients was secondary to CFN compression by an adjacent fabella. Treatment: In one patient (Case 3), bilateral fabellae were noted on contralateral views (Figure 1). In three cases (Cases 1, 2, and 4), US- guided CFN blocks were done to treat pain. Pain symptoms were resolved with US-guided CFN block in two patients (Cases 2 and 4). US-guided peri-fabellar injections were also done in two patients (Cases 3 and 4). One patient (Case 1) required surgical treatment with fabellectomy for complete resolution of symptoms (Figure 2), while another patient (Case 3) was treated with oral medications. Outcomes: Patients had complete resolution of symptoms after conservative treatment in three cases and surgical excision in one case. *Case 4 presented and didn’t rate pain on either visit. (1st Visit= +, 2nd Visit =++) Conclusions Figure 1 A fabella can serve as a focus of CFN compression in the posterolateral knee. Patients present with posterolateral knee pain with paresthesias radiating into the anterolateral leg. Sonography is a valuable tool for the evaluation of fabellar compression of the CFN, manifesting with posterolateral knee and anterolateral leg pain. It provides a quick realtime evaluation of all structures within the knee and can localize the nerve lesion, providing confirmation of the fabellar presence associated with CFN pain. Background References The fabella is a sesamoid bone which may be present in 10-30% of individuals, occurring bilaterally in up to 80% of patients and having a higher propensity in the Asian population. The fabella is found embedded within the tendon of the lateral head of the gastrocnemius muscle in the posterolateral knee and is often mistaken for an intra-articular loose body, fracture, or osteophyte. While it may be mostly asymptomatic, in rare instances the fabella has been associated with posterolateral knee pain in younger athlete individuals with chondromalacia patella and older patients with osteoarthritis. Due to its proximity to the CFN, the fabella may also act as an etiology in peroneal nerve palsies, presenting as posterolateral knee and leg pain, leg weakness, and foot drop. 1. Duncan W, Dahm DL. Clinical anatomy of the fabella. Clin Anat 2003;16: 448-449. 2. Zeng SX, Dong XL, Dang RS, et al. Anatomic study of fabella and its surrounding structures in a Chinese population. Surg Radiol Anat 2012; 34:65-71. 3. Tabira Y, Saga T, Takahashi N, Watanabe K, Nakamura M, Yamaki K. Influence of a fabella in the gastrocnemius muscle on the common fibular nerve in Japanese subjects. Clin Anat 2013; 26:893-902. 4. Mangieri JV. Peroneal-nerve injury from an enlarged fabella. A case report. J Bone Joint Surg Am 1973; 55:395-397. 5. Takebe K, Hirohata K. Peroneal nerve palsy due to fabella. Arch Orthop Trauma Surg 1981; 99:91-95. 6. Sekiya JK, Jacobson JA, Wojtys EM. Sonographic imaging of the posterolateral structures of the knee: findings in human cadavers. Arthroscopy 2002; 18:872-881. Figure 1 (Case 3) Bilateral short-axis (transverse) views of the posterior knees in a 56 year old woman with complaint of left posterolateral knee pain. Figure 1A- Transverse view of the right knee posterior to the lateral femoral epicondyle, which demonstrates the presence of an asymptomatic fabella (white star). Figure 1B- Transverse view of the left knee posterior to the lateral femoral epicondyle, which demonstrates an enlarged, pointed fabella (white star) in close proximity to the CFN (arrow). LAT= lateral, ASYM= asymptomatic (right knee), SYM- symptomatic (left knee) Figure 2 (Case 1) Figure 2A- Short-axis (transverse) view of the CFN (white oval) located superficial to the fabella (white star). The CFN maintains a normal appearance, though tenderness in the CFN distribution is elicited on palpation. Left/right = medial/ lateral, Figure 2B- Intraoperative image demonstrating the CFN crossing over the fabella. The left leg was placed in the prone position, with the lateral gastrocnemius (LG) retracted laterally. The CFN was noted to be compressed by the fabella (Fb). Left/right- distal/proximal, top/bottom= lateral/medial. © 2015 Mayo Foundation for Medical Education and Research

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