| Abstract|| |
A 12 year old boy presented with ankle sprain. The physical examination revealed mild weakness of ankle dorsiflexion. An ultrasound was done for the soft tissues of the ankle. In addition to relative atrophy of the peroneus longus muscle, a compressive common peroneal nerve (CPN) lesion with a synovial cyst at the level of the proximal tibiofibular joint (PTFJ) was accidently found. Since there were features of CPN compression by the synovial cyst, total excision of the cyst was performed. After the operation, muscle strength improved and the neurological deficit subsided. Therefore, the early diagnosis of PTFJ synovial cyst with nerve injury was crucial in order to achieve a better result. The obscure nature of clinical presentations can delay the diagnosis, which may potentially lead to a poor prognosis after treatment in such cases. This report highlights the fact that ankle sprain do need a thorough clinical work up in some cases.
Keywords: Ankle sprain, proximal tibiofibular joint, synovial cyst, ultrasound
|How to cite this article:|
Huang SW, Wang WT. Early detection of peroneal neuropathy by ultrasound. Indian J Orthop 2014;48:104-6
| Introduction|| |
A synovial cyst is defined as a ganglion or soft tissue tumor of a joint, which arises from the deep serous bursa and leads to leakage of synovial fluid from the joint to the surrounding tissues. It is usually seen at the dorsum of wrist and the popliteal fossa.  The proximal tibiofibular joint (PTFJ) is a rare location for the development of synovial cysts and accounts for only 0.03% of the symptomatic knee lesions which have been screened by magnetic resonance imaging (MRI) studies. , The clinical presentation of synovial cysts of PTFJ are variable. It may be asymptomatic or cause neurological symptoms by the compression of the common peroneal nerve. Pain, sensory loss radiating from the knee to the dorsum of the foot, ankle dorsiflexion weakness and foot drop are commonly seen in symptomatic cases.  Most PTFJ synovial cysts are noticed following a palpable mass or swelling on the anterolateral aspect of the knee or when a significant neurological defect occurs. The early detection of common peroneal nerve injury is important to diminish the effects of neurological deficit and to help ensure that a better prognosis is possible after effective management is implemented. We report a case of synovial cyst of the PTFJ presenting as compressive neuropathy of the common peroneal nerve in a 12 year old boy. The case is presented for its rarity but more importantly to emphasize early diagnosis to improve chances for neurological recovery.
| Case Report|| |
A 12 year old boy presented with left ankle swelling and pain. He sustained left ankle sprain while playing at school. On examination, active ankle sprain dorsiflexion toe dorsiflexion and eversion were normal and there was no sensory impairment as well. Only left toe extension impairment was noted. The ultrasound was done to evaluate the soft tissue injury of the ankle. Initially, we focused on the ankle area for a sprain. When comparing the bilateral peroneus longus muscles, we found the loss of normal muscular echogenic appearance over the entire left side. Tracing this back to the origin of peroneus muscle, we found a hypoechoic cyst with a well demarcated margin neighboring the left peroneal nerve just posterior to the fibula head [Figure 1]. Lower limb nerve conduction and electromyography study for peroneal nerve were done. The left common peroneal nerve injury over the fibular head area was found (conduction velocity: 37 ms, normal value: More than 41 ms; peak amplitude: 1.7 mV, normal value: More than 2.0 mV in nerve conduction velocity (NCV) study; only increased polyphasic motor unit action potential was found in electromyography study). In order to more thoroughly evaluate the origin of the mass lesion and adjacent structure, a knee MRI was done. The MRI showed a 33 mm synovial cyst as originating from the proximal tibiofibular articulation with neither rupture nor regional infiltration [Figure 2]. A left PTFJ synovial cyst with extraneural compression over the common peroneal nerve was diagnosed.
|Figure 1: Ultrasonograph showing (a) Comparing bilateral peroneus muscles at ankle (transverse view of posterior lateral side): Less apparent muscle fibril pattern, which is infiltrated by fat tissue and unclear perimyosium margin of muscles at lesion side (PL: Peroneus longus; PB: Peroneus brevis, x: muscle bulk thickness of lesion side, +: muscle bulk thickness of sound side). (b) Ultrasound revealed hypoechoic cyst (1.14 cm × 1.35 cm in area) adjacent to left peroneal nerve (arrow) just adjacent to the fibula head (F: Fibula head; C: Cyst)|
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|Figure 2: (a) T2 weighted MRI coronal view of left knee showing a hyper intense signal (arrows) by the synovial cyst proximal to proximal tibiofibular joint and with extension to the fibular head. (b) T2 weighted MRI axial view showing hyper intense signal around the fibular head (arrow)|
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Surgery was performed and the synovial cyst was exposed in the PTFJ area. Compression of the common peroneal nerve by the synovial cyst at fibular head and neck was found during surgical exploration. The compressed nerve was thin by about 60% in diameter when compared to the noncompressed area. Total excision of the synovial cyst was performed and a synovial cyst with a fibrous wall was confirmed on histopathology. After 3 months of the surgery, the physical examination revealed normal muscle power of the peroneal muscle and normal gait pattern was obtained. He returned to sports postoperatively. One year later, the subsequent conduction velocity study of the common peroneal nerve revealed the parameters had returned to normal range (NCV: 46 ms; peak amplitude: 3.7 mV in NCV study; no polyphasic motor unit action potential was found in electromyography study). There was no recurrence of the cyst at 3 years of followup.
| Discussion|| |
The correlation between ankle sprain and peroneal neuropathy has been mentioned. Brief et al., found that ankle injury could predispose injury from a fall, leading to subsequent peroneal nerve palsy.  A foot drop from a peroneal nerve palsy could predispose one to a fall and a secondary ankle sprain. Besides, the peroneal nerve injuries can be caused by torsional injury at the ankle because of the translational force along the interossesous membrane of the leg. The transmitting tension force is transmitted to the common peroneal nerve subsequently at the fibular neck and results in a nerve injury.  They also stated that the ankle torsional injury may cause the development of fibular ganglion cysts which can cause peroneal neuropathy.  Therefore, peroneal neuropathy can be a causative factor in the mechanism of an ankle sprain and the ankle sprain torsion force could cause peroneal neuropathy or ganglion cyst.
As of differential diagnosis of peroneal neuropathy caused by a mass lesion, in a previous case series by Kim et al., they found forty patients (13%) had tumors in a group of 318 operative peroneal neuropathy cases. They found that the most common of these was an intraneural ganglion cyst. Other tumors included schwannoma, neurofibroma, osteochondroma, neurogenic sarcoma, focal hypertrophic neuropathy, desmoid tumor and glomus tumor in order of decreasing frequency.  How to establish the early detection of mass lesion on PTFJ with common peroneal neuropathy is an important issue for differential diagnosis in clinical practice. Ultrasound can be used as useful evidence for common peroneal neuropathy.  When foot drop or tibialis anterior and peroneus muscle wasting is noted, the common peroneal nerve can be traced from a distal to a proximal site of the fibula. Furthermore, echogenecity and margin of mass lesion, which are revealed by ultrasound, can be used in getting a differential diagnosis and also for further evaluation by MRI. On a T2 weighted image, a synovial cyst appears enhanced. In addition to the nature of the mass lesion, MRI can also reveal adjacent structures such as nerves, muscles and joints and thus provide anatomical information before surgical intervention that can be planned. However, for our patient, given that he complained of ankle sprain, MRI was not the first choice of investigations for initial evaluation because of high cost and difficult applicability. There is a size discrepancy of synovial cyst detected by ultrasound and subsequently on an MRI. It can be explained that ultrasound is operator dependent and the image obtained depends on the experience of the sonographer. In contrast, MRI is more objective for mass lesion evaluation. However, before MRI examination, ultrasound provided a basic evaluation and gave a differential diagnosis.
Early detection of muscle atrophy, can be a clue for further nerve studies and even uncovering of the etiology of nerve injury (PTFJ synovial cyst). While there are many methods for muscle atrophy evaluation by ultrasound, directly comparing the affected and unaffected side (or normal range of values) muscle thickness or cross sectional area by ultrasound has generally been the adopted method. , The reliability of muscle atrophy evaluation by sonographic muscle bulk thickness has been found to well correlate with MRI and computed tomography evaluations in previous studies. , However, as ultrasound has the advantage of being performed as a bedside examination and taking less time than an MRI, early muscle atrophy detection by ultrasound is appropriate in clinics. A previous study has indicated that the peroneal nerve can be traced by ultrasound in order to visualize neural and extraneural pathologies and further that ultrasound can be used to define the exact level and extent of lesions; it was also pointed out that ultrasound is a valuable factor in decision making vis-a-vis need of intervention. 
To conclude, when physical examination reveals strength impairment or suspicious muscle atrophy after an ankle sprain, common peroneal nerve injury evaluation by ultrasound is recommended. It can help in early detection of possible etiology for a nerve lesion and give a differential diagnosis before an MRI.
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Department of Physical Medicine and Rehabilitation, Changhua Christian Hospital, #135 Nan-Hsiao Street, Changhua 500
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2]