Entrapment neuropathy of common peroneal nerve by fabella: A case report

Abstract

BACKGROUND

Neuropathy of the common peroneal nerve caused by compression by a fabella is an extremely rare form of compression neuropathy. Involving both the superficial and deep peroneal nerves, it usually manifests as either impaired sensation from the lower lateral leg to the top of the foot or drop foot, or as a combination of both.

CASE SUMMARY

We report the case of a 58-year-old Asian female who presented with inversion of the right foot during the stance phase of gait without sensory complaints related to the lower leg. Electrodiagnostic testing revealed the neuropathy of the common peroneal nerve at the level of the knee, exclusively affecting the muscular branch of the superficial peroneal nerve. A neuromuscular ultrasound disclosed swelling of the right common peroneal nerve just before it crossed over a large fabella as well as atrophy and fatty infiltration of the right peroneus longus and peroneus brevis muscles. Surgical excision of the fabella and neurolysis were performed. Subsequently, the strength of the right foot evertors improved, but the unsteady gait with occasional falls persisted for nine months after the surgery. Therefore, another procedure was performed to transfer the split posterior tibialis tendon to the peroneus brevis in order to correct the gait.

CONCLUSION

This is the first case of neuropathy of the common peroneal nerve caused by compression by a fabella affecting exclusively the muscular branch of the superficial peroneal nerve. Clinicians should be aware of this unusual peripheral neuropathy while evaluating and treating patients with gait disturbance.

Core Tip: This is the first case of common peroneal nerve entrapment neuropathy caused by compression by a fabella affecting exclusively the muscular branch of the superficial peroneal nerve. Instead of features typically found in neuropathy of the common peroneal nerve, such as drop foot, steppage gait, and numbness in the anterolateral leg and the dorsum of the foot, the case reported here presented with inversion of the right foot during the stance phase of gait for every step taken without any sensory problems. Therefore, clinicians should be aware of this unusual peripheral neuropathy while evaluating and treating patients with gait disturbance.

INTRODUCTION

The most common type of peripheral neuropathy in the lower extremities is neuropathy of the common peroneal nerve[1,2]. The usual etiology includes compression due to either prolonged immobilization or a mass lesion like a ganglion cyst or a nerve sheath tumor, a repetitive strain injury, and a direct blow to the knee[1,2]. On the other hand, neuropathy of the common peroneal nerve caused by compression by a fabella is extremely rare[3]. Only 11 cases have been reported in the literature to date, all affecting either the superficial branch of the peroneal nerve alone or both its deep and superficial branches. We report the first case of neuropathy of the common peroneal nerve caused by compression by a fabella which preferentially affected the muscular branch of the superficial peroneal nerve.

CASE PRESENTATION

Chief complaints

A 58-year-old Asian female presented with a gait disturbance for six months.

History of present illness

The patient had progressive weakness in the eversion of the right foot and a gait disturbance which had lasted six months. There was no history of trauma. She denied having numbness, pain, or impaired sensation in the right lower extremity.

History of past illness

The patient had undergone pacemaker implantation surgery for sick sinus syndrome several years ago.

Personal and family history

The patient denied any family history of peroneal neuropathy.

Physical examination

A physical examination disclosed severe atrophy of the lateral calf muscles and decreased evertor strength in the right foot, graded 1/5 on the Medical Research Council scale. It was noted that she walked with a wide-based gait and with inversion of the right foot during the stance phase of the gait cycle for every step taken. A sensory examination which tested for light touch, pinprick and proprioception sensation showed that sensation in the lower extremities was intact, and the patellar reflex and ankle jerk reflex tests for both legs were normal.

Laboratory examinations

Routine blood analysis revealed the following: White blood cell count, 6.8 × 10³/μL [reference range: (3.3-9.9) × 10³/μL]; red blood cell count, 2.69 × 10⁶/μL [reference range: (3.78-5.11) × 10⁶/μL]; hemoglobin, 9.9 g/dL (reference range: 11.0-15.0 g/dL); and platelet count, 324 × 103/μL [reference range: (157-392) × 10³/μL]. Liver function and kidney function were normal (serum creatinine, 0.77 mg/dL; epidermal growth factor receptor, 77; aspartate transaminase, 29 U/L; alanine aminotransferase, 26 U/L).

Imaging examinations

Electrodiagnostic (EDX) testing was performed with findings compatible with subacute neuropathy of the right common peroneal nerve at the level of the fibular head, preferentially affecting the muscular branch of the superficial peroneal nerve (Tables 1 and 2). A neuromuscular ultrasound revealed atrophy of the right peroneal longus and peroneal brevis muscles, graded 3/4 on the modified Heckmatt scale[4] (Figures 1A and B). Swelling of the right common peroneal nerve just proximal to an incidentally found fabella was also identified by means of ultrasound imaging (Figures 1C and D). Atrophy of the right peroneus longus and peroneus brevis muscles was also disclosed by means of a computed tomography scan of the lower extremities, with decreased muscle cross-sectional area and decreased radiodensity at the level of the mid-calf compared to the asymptomatic contralateral side (Figure 2).

Table 1.

Nerve conduction studies for both lower extremities

Nerve stimulated	Stimulation site	Recording site	Onset latency (ms)		Amplitude motor (mV), sensory (μV)		Velocity (m/s)
			Right	Left	Right	Left	Right	Left
Motor studies
Tibia	Ankle	AHB	4.3	4.7	13.6	14.2
	Popliteal fossa		12.7	13.6	8.9	9.4	47	44
Peroneal	Ankle	EDB	4.9	5.2	3.5	3.4
	Fibular head		11.3	11.7	2.9	2.9	46	45
	Popliteal fossa		13.4	13.8	2.4	2.7	49	47
Peroneal	Fibular head	TA	3.4	3.2	4.2	4.3
	Popliteal fossa		4.6	4.5	4.0	3.9	54	50
Peroneal	Fibular head	PL	2.6	2.7	3.11	6.5
	Popliteal fossa		6.3	4.4	1.11	5.9	221	47
Sensory studies
Sural	Calf	Posterior ankle	3.4	3.5	14.7	15.6	42	40
Superficial peroneal	Lateral calf	Ankle	3.3	3.4	15.7	16.1	42	41

¹Indicates abnormal data based on our reference values.

AHB: Abductor hallucis brevis; EDB: Extensor digitorum brevis; TA: Tibialis anterior; PL: Peroneus longus.

Table 2.

Electromyography for both lower extremities

Muscle	Spontaneous activity			Voluntary motor unit action potentials
	Insertional activity	Fibrillation potentials	Positive sharp waves	Amplitude	Duration	Polyphasia	Recruitment	Interference pattern
Right
Vastus lateralis	NL	0	0	NL	NL	NL	NL	Full
Tibialis anterior	NL	0	0	NL	NL	NL	NL	Full
Peroneus longus	Increased	2+	2+	NL	NL	NL	Discrete	Incomplete
Peroneus brevis	Increased	2+	2+	NL	NL	NL	Discrete	Incomplete
Gastrocnemius	NL	0	0	NL	NL	NL	NL	Full
Left
Tibialis anterior	NL	0	0	NL	NL	NL	NL	Full
Peroneus longus	NL	0	0	NL	NL	NL	NL	Full
Peroneus brevis	NL	0	0	NL	NL	NL	NL	Full

The sampling of the right peroneus longus and bilateral peroneus brevis muscles was performed under ultrasound guidance. NL: Normal.

Figure 1.

Results of the neuromuscular ultrasound. A: The atrophic peroneus longus and peroneus brevis muscles on a short axis scan of the symptomatic side, graded 3/4 on the modified Heckmatt scale; B: The peroneus longus and peroneus brevis muscles of the contralateral asymptomatic side; C: The common peroneal nerve in the popliteal fossa (arrowhead); D: The common peroneal nerve adjacent to the fabella (arrow). The nerve was enlarged when it lied closer to the fabella. PL: Peroneus longus; PB: Peroneus brevis.

Figure 2.

Results of the computed tomography scan of both lower extremities. Cross sectional area of the right peroneal muscles: 209.55 mm²; average Hounsfield unit for the right peroneal muscles: -50.16; cross sectional area of the left peroneal muscles: 332.22 mm²; Hounsfield unit for the left peroneal muscles: 34.28.

Further diagnostic work-up

Surgical excision of the fabella was performed through a dorsolateral incision between the biceps femoris muscle and the iliotibial band. The common peroneal nerve was isolated. The fabella was removed and measured 25 mm × 19 mm × 27 mm.

FINAL DIAGNOSIS

Combined with the patient’s medical history, the final diagnosis was common peroneal entrapment neuropathy by fabella which exclusively affects the muscular branch of superficial peroneal nerve.

TREATMENT

Afterwards, a nine-month rehabilitation program was prescribed consisting of exercises for strengthening the right foot evertors and therapeutic electrical stimulation of the foot evertors. The evertor muscles in the right foot were partially strengthened, graded 3/5 on the Medical Research Council scale. However, the unsteady gait persisted with occasional falls while walking, presumably due to the inversion of the right ankle. Thus, another surgical procedure was carried out to transfer the split posterior tibialis tendon to the peroneus brevis.

OUTCOME AND FOLLOW-UP

Following a one-month postoperative rehabilitation program, the severity of the right ankle inversion while walking decreased, with no more falls reported on the last clinical visit two months after the surgery.

DISCUSSION

The fabella is a sesamoid bone or fibrocartilage embedded in the proximal tendon of the lateral head of the gastrocnemius muscle[5]. Although its actual function remains unclear, it has been suggested that it reinforces the structure of the posterior part of the knee, in particular the juncture of the fabellofibular ligament and oblique popliteal ligament, or that it lengthens the lever arm of the tendon of the lateral gastrocnemius muscle[3,5]. The fabella prevalence in humans is approximately 10%-30% with higher rates among Asian populations[6]. Overall, it occurs bilaterally in 70%-80% of cases[6].

Despite such a high prevalence, the presence of a fabella rarely causes compression neuropathy of the peroneal nerve[3]. In fact, with stringent diagnostic criteria which included EDX testing, only 11 cases have so far been reported in the literature (Table 3), with all cases accompanied by sensory impairment. Those whose motor functions were also affected had in common symptoms such as drop foot and active denervation potentials observed in the tibialis anterior by means of electromyography tests. Tinel’s sign could be elicited in around half of the cases. On the other hand, we report the first case of neuropathy of the common peroneal nerve caused by compression by a fabella which preferentially affected the muscular branch of superficial peroneal nerve.

Table 3.

Reported cases of neuropathy of the common peroneal nerve due to compression by the fabella

Ref.	Age	Gender	Symptom	Tinel sign	Fabella size	NCV	EMG	Treatment	Outcome
This case	46	Female	Inverted foot with evertors atrophy	+	25 mm × 19 mm × 27 mm	Conduction block	AD and R-MUAP in PL, PB	Surgery	Partially resolved
Mangieri[7]	46	Female	Sensory impairment	+	22 mm × 14 mm	Focal slowing	Normal	Surgery	Completely resolved
Takebe and Hirohata[6]	49	Male	Drop foot with sensory impairment	+	9 mm × 7 mm × 6 mm	Focal slowing	AD and R-MUAP in TA, PL	Surgery	Slight functional disturbance
	34	Male	Drop foot with sensory impairment	+	11 mm × 9 mm × 7 mm	Focal slowing	AD and R-MUAP in TA, PL	Surgery	Completely resolved
	36	Male	Drop foot with sensory impairment	-	10 mm × 10 mm × 8 mm	Focal slowing	AD, R-MUAP and polyphasia in TA, PL	Surgery	Completely resolved
	53	Male	Drop foot with sensory	-	11 mm × 11 mm × 8 mm	Conduction block	R-MUAP in TA, PL	Conservative	Satisfactory improvement
	40	Male	Drop foot with sensory impairment	+	8 mm × 8 mm × 6 mm	Focal slowing	AD in PL, R-MUAP in TA, PL	Conservative	Completely resolved
	22	Male	Drop foot with sensory impairment	-	9 mm × 9 mm × 7 mm	Focal slowing	R-MUAP in TA, PL	Conservative	Completely resolved
	25	Male	Drop foot with sensory impairment	-	7 mm × 5 mm × 4 mm	Focal slowing	AD in TA	Conservative	Satisfactory improvement
Patel et al[3]	67	Male	Drop foot with sensory impairment	-	23 mm × 20 mm	Focal slowing	Normal	Surgery	Completely resolved
Lee et al[8]	79	Female	Drop foot with sensory impairment	+	16 mm × 20 mm × 15 mm	Conduction block	AD in TA	Surgery	Completely resolved
Kubota et al[9]	50	Female	Drop foot with sensory	-	5 mm × 5 mm	Focal slowing	AD in TA, PL, EDL	Conservative	Partially resolved

Sensory impairment indicating sensory disturbance of anterolateral region of the lower leg and dorsal region of the foot; focal slowing of conduction velocity and conduction block indicating NCV test results across fibular neck; Surgery indicating resection of fabella and neurolysis of common peroneal nerve. AD: Active denervation; R-MUAP: Reduced motor unit action potential; TA: Tibialis anterior; PL: Peroneus longus; PB: Peroneus brevis; EDL: Extensor digitorum longus.

Our case had a unique clinical presentation. Instead of features typically found in neuropathy of the common peroneal nerve, such as drop foot, steppage gait, and numbness in the anterolateral leg and the dorsum of the foot, the case reported here presented with inversion of the right foot during the stance phase of gait for every step taken without any sensory problems. The results of a physical examination, a radiological study, and EDX testing all pointed to a diagnosis of neuropathy of the common peroneal nerve caused by compression by a fabella which preferentially affected the muscular branch of the superficial peroneal nerve. Eliciting Tinel’s sign and identifying swelling of the proximal end of the right common peroneal nerve adjacent to a fabella in an ultrasound were helpful in establishing the underlying cause of the problem.

Previous studies have suggested such predisposing factors of peroneal neuropathy caused by a fabella as an enlarged fabella, a low body mass index, and an external force such as trauma[5,6]. Our case subject had a normal bodily shape without any history of trauma, but had a relatively large fabella (25 mm × 19 mm × 27 mm) compared with the average size reported in the literature (7.9 mm × 4.2 mm)[10]. It has been shown that the internal fascicular anatomy of the common peroneal nerve is arranged in such a way that the fibers destined for the superficial peroneal nerve lie more laterally[1]. Therefore, we hypothesized that the sizeable fabella in our case somehow caused the lateral portion of the common peroneal nerve to be stretched and injured, which in turn resulted in peroneal muscular atrophy. The resulting weakness of the peroneal muscles led to the inversion of the foot while walking, which then extended the stretching of the common peroneal nerve, thus exacerbating the damage to the already injured nerve.

The treatment of choice for compression neuropathy by a fabella is surgical removal of the fabella and neurolysis[3]. In past research, the symptoms completely resolved in almost all of the cases who had undergone surgery, with a single exception which the author attributed to delayed surgical intervention (Table 3). For patients with results of EDX testing which are suggestive of neuropraxia, conservative treatment is an alternative approach which has also provided satisfying results[6]. However, it’s important to note that unsteady gait caused by peroneal neuropathy can lead to an increased risk of falls[11] and potentially higher medical costs[12]. Therefore, it is necessary to modify the home environment to reduce the risk of falls[13]. Our case subject underwent surgical excision of the fabella and neurolysis. Following the surgery, she experienced a significant increase in peroneal muscle strength. However, full recovery from her symptoms was not achieved, which may be due to the prolonged duration of the condition and severe axonal loss revealed through both EDX and sonographic testing. The remaining weakness led to a persistent unsteady gait and multiple falls. Therefore, another surgical procedure was performed to transfer the split posterior tibialis tendon to the peroneus brevis in order to help the patient regain a steady gait.

CONCLUSION

Neuropathy of the common peroneal nerve caused by compression by a fabella is an extremely rare form of compression neuropathy. While drop foot and sensory impairment in the anterolateral lower leg and the dorsum of the foot are the most common clinical features, atypical presentations such as weakness of the lateral calf muscle are also possible. Clinicians should remain vigilant for the possible causes of peroneal neuropathy and tailor treatments accordingly for each individual patient.

ACKNOWLEDGEMENTS

This research was supported in part by Higher Education Sprout Project, Ministry of Education to the Headquarters of University Advancement at National Cheng Kung University.