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Original Research |
1 All authors: Department of Radiology, New York University (NYU) School of Medicine and NYU Hospital for Joint Diseases, 301 E 17th St., New York, NY 10003.
Received July 27, 2006;
accepted after revision April 19, 2007.
Address correspondence to R. La Rocca Vieira.
Abstract
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MATERIALS AND METHODS. One hundred consecutive 1.5-T knee MR studies of 97 asymptomatic patients were retrospectively reviewed by two observers in consensus for, first, normal anatomy of the distal biceps femoris muscle; second, anatomic variations of the muscle; and, third, the relationship of the muscle to the common peroneal nerve. Measurements of the distal and posterior extents of the short and long heads of the biceps femoris were performed. An MR study of a symptomatic patient with clinical evidence of common peroneal neuropathy associated with a surgically proven anatomic variation of the distal biceps femoris was reviewed.
RESULTS. Two MR anatomic patterns were seen in the asymptomatic patient group: First, in 77 knees (77%), the common peroneal nerve was located within abundant fat posterolateral to the biceps femoris; and, second, in 23 cases (23%), the common peroneal nerve traversed within a narrow fatty tunnel between the biceps femoris and lateral head of the gastrocnemius muscles. There was a positive correlation between the distal and posterior extents of the short head of the biceps femoris muscle and the presence of the tunnel.
CONCLUSION. Variations in the posterior and distal extents of the biceps femoris muscle can produce a tunnel in which the common peroneal nerve travels. We also described a case of common peroneal neuropathy secondary to tunnel formation.
Keywords: anatomy • biceps femoris muscle • common peroneal nerve • MRI • neuropathy
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Recently we encountered a surgically proven case of common peroneal entrapment neuropathy secondary to an anatomic variation of the biceps femoris and an atypical relationship between the biceps femoris muscle and the nerve. The anatomic variation contributed to the formation of a muscular tunnel between the lateral head of the gastrocnemius muscle and the distal biceps femoris. In this case, the common peroneal nerve traversed within this narrow fatty tunnel. To our knowledge, this anatomic relationship has not been previously reported as a cause of common peroneal neuropathy. Furthermore, there is no detailed anatomic description in the literature of the distal biceps femoris muscle and its exact relationship with the common peroneal nerve in the posterior aspect of the knee (Fig. 1). All of these issues motivated us to study the normal MRI anatomy of the distal biceps femoris muscle and the relationship of the muscle with the common peroneal nerve.
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Asymptomatic Population
One hundred consecutive 1.5-T knee MR studies of 97 patients (41 males and
56 females; age range, 14-84 years; mean age, 52.8 years) imaged for internal
knee derangements were retrospectively reviewed by two musculoskeletal
radiologists in consensus. The radiologists had 20 and 5 years' experience in
musculoskeletal radiology, respectively. Exclusion criteria included lesion of
the posterolateral corner of the knee defined by signal abnormality or
deformity of the joint capsule and ligaments of the posterolateral knee, and
history or imaging findings compatible with common peroneal neuropathy. None
of these criteria were met, so no examinations were excluded. The MR studies
were obtained from July 2005 to January 2006.
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The following parameters were recorded regarding the anatomic relationship of the common peroneal nerve to the distal biceps femoris muscle and the lateral head of the gastrocnemius muscle: the presence of denervation edema of the muscles of the anterior and lateral compartments of the upper leg and signal abnormality of the common peroneal nerve in the popliteal fossa; and variations in the distal biceps femoris muscle. The variations that were noted included the posterior extent of the muscle belly of the short head of the biceps femoris muscle, the distal extent of the muscle belly (distal extent of the distal myotendinous junction) of the short head of the biceps femoris from the joint space, and the presence and distal extent of the muscle belly (distal extent of the distal myotendinous junction) of the long head of the biceps femoris.
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The posterior extent of the short head of the biceps femoris was measured in the following manner using an axial T1 or PD image (Fig. 2): A line was drawn posterior to the femoral condyles, and a second line was drawn to measure the posterior extent of the short head of the biceps femoris muscle relative to the first line. The axial image of the knee that showed the deepest femoral notch (resembling a roman arch) was arbitrarily chosen to perform this measurement.
We defined a short head of the biceps femoris as hypertrophied if its posterior extent measurement was greater than the mean value of the posterior extent in the asymptomatic population plus 2 SDs.
We defined a tunnel as an instance in which the common peroneal nerve, rather than traveling within abundant fat lateral to the biceps femoris and posterior to the lateral head of the gastrocnemius muscle (Figs. 3A, 3B and 4A, 4B, 4C, 4D), was noted in a narrow passage between the two muscles. The fat quantification was subjective. A passage shorter than 1 cm in its superior-to-inferior extent was not included in the definition of a tunnel. The longitudinal extent of the tunnel was measured.
Statistical Analysis
Patients with and those without a tunnel were compared with respect to,
first, the distal extent of the muscle's bellies of the long head (when
present) and short head of the biceps femoris from the joint space; and,
second, the posterior extent of the short head of the biceps femoris using the
Mann-Whitney test. All statistical computations were performed using SAS
software (SAS version 9.0, SAS Institute) for Windows (Microsoft).
Case Presentation
One MRI study, obtained from our teaching file, of a 14-year-old boy with
clinical evidence of peroneal entrapment neuropathy and MRI evidence of
aberration of the insertion of the distal biceps femoris muscle was
retrospectively reviewed in consensus by the same radiologists. The MRI
protocol consisted of axial T1 (466/9) and T2 fat-saturated (3,800/119),
coronal PD (2,300/12) and T2 fat-saturated (2,400/17), and sagittal PD
(2,900/21) and PD fat-saturated (2,750/56) images of the knee and upper leg.
All MRI sections were obtained with a slice thickness of 4 mm, a field of view
of 16 cm, and a matrix of 256 in a 1.5-T magnet (Avanto, Siemens Medical
Solutions). The same analysis performed in the asymptomatic population was
also performed in this patient. The patient's medical records and
electromyography (EMG) studies were reviewed.
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In the remaining 23 cases (23%), the common peroneal nerve was situated within a narrow tunnel between the lateral head of the gastrocnemius muscle and the short head of the biceps femoris muscle (Fig. 3A, 3B). The tunnel averaged 2.4 cm in length (range, 1.5-4.0 cm). Note in Table 1 that almost 50% of the asymptomatic population had a tunnel length of 2 cm or less and only 4.2% had a tunnel length of 4 cm or greater.
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Variations in the Distal Biceps Femoris Muscle
In the entire series of 100 knees, there was no accessory tendon of the
biceps femoris.
Posterior extent of the short head of the biceps femoris muscle—The posterior extent of the short head of the biceps femoris muscle at the level of the femoral condyles in all cases averaged 1.19 cm (range, 0.0-3.0 cm) (Table 2). In cases without tunnel, the posterior extent of the short head of the biceps femoris muscle averaged 1.02 cm (range, 0-3.0 cm). In cases with tunnel, the posterior extent of the short head of the biceps femoris averaged 1.50 cm (range, 0.0-3.0 cm). The short head of the biceps femoris met the criteria for muscle hypertrophy in two cases—that is, the posterior extent of the muscle was greater than 2.59 cm, which corresponds to the mean value of the posterior extent of the short head of the biceps femoris in the asymptomatic population plus 2 SDs.
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In 50% of the asymptomatic population, the posterior extent of the short head of the biceps femoris was 1 cm or less (Table 3). Only 16% of the cases had posterior extension of the muscle 2 cm or greater.
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Distal extent of the muscle belly of the short head of the biceps femoris from the joint space—In all the asymptomatic knees, the distance between the short head of the biceps femoris muscle and the joint space averaged 0.05 cm below the level of the joint space (range, from 1.5 cm above the joint to 2.0 cm below the joint) (Table 4).
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In knees with a tunnel, the distance between the short head of the biceps femoris muscle and the joint space averaged 0.6 cm below the level of the joint space (range, from 0.7 cm above the joint to 2.0 cm below the joint space).
In knees without a tunnel, the distance between the short head of the biceps femoris muscle and the joint space averaged 0.1 cm above the level of the joint space (range, from 1.5 cm above the joint to 1.0 cm below the joint).
The myotendinous junctions were situated at the level of the joint space in 51% of the asymptomatic population, as shown in Table 5. The rest of the cases were almost equally divided into those in which the myotendinous extended above the joint (21.4%) and those in which the myotendinous extended below the joint (27.5%).
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Presence and distal extent of the muscle belly of the long head of the biceps femoris— The muscle belly of the long head of the biceps femoris was identified in 40 of the 100 knees (40%). The distance between the long head myotendinous junction to the joint space averaged 5 cm (range, 1.5-10.0 cm above the joint) (Table 6). In only one case (1%), the low-lying muscle belly of the long head of the biceps femoris contributed to the formation of a tunnel (Fig. 5A, 5B).
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There was no evidence of denervation edema of the muscles of the anterior and lateral compartments of the upper leg or signal abnormality of the common peroneal nerve in the popliteal fossa in any of the cases in the asymptomatic population.
Statistical Results
The presence of a tunnel was significantly predicted by the posterior
extent of the short head of the biceps femoris (p = 0.005) and the
distal extent of the muscle belly of the short head of the biceps femoris from
the joint space (p < 0.001). However, there was greater
correlation between the posterior extent compared with the distal extent of
the short head of the biceps femoris and the presence of a tunnel.
Symptomatic Case
The MR studies of a 14-year-old boy with clinical and EMG evidence of
common peroneal neuropathy were studied. The patient presented with
nonprogressive weakness of the right foot that had begun 3 months earlier. He
had difficulty lifting the foot off the ground. One month after the onset of
his symptoms, he twisted his right ankle. He was also tripping frequently, but
he had not fallen. He denied previous trauma-related surgery or systemic
disease.
Physical examination revealed dorsiflexion, eversion weakness, and diminished touch sensation over the distal part of the lateral aspect of the right leg and dorsum of the right foot.
The EMG and nerve conduction studies revealed severe right common peroneal nerve dysfunction at the fibular head.
MRI was performed (Fig. 6A, 6B, 6C, 6D), and the short head of the biceps femoris was subjectively interpreted as hypertrophied. In retrospect, we analyzed the posterior extent of the short head, which measured 1.8 cm and contributed to the formation of a tunnel. The short head of the biceps femoris extended 1.0 cm below the level of the joint space.
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Denervation edema and minimal atrophy of the anterior and lateral compartments of the upper third of the leg were characterized by high signal intensity on T2-weighted images and iso-signal on T1-weighted sequence. The degree of muscle atrophy was subjectively estimated in comparison with the muscle of the posterior compartment of the calf. The involved muscles were the anterior tibial, extensor digitorum, peroneus longus, and peroneus brevis.
The MRI findings showed that a variation in the distal length of the short head of the biceps femoris muscle was producing entrapment neuropathy of the common peroneal nerve and secondary subacute denervation of the anterior and lateral compartments.
The patient underwent decompressive surgery with removal of the short head of the biceps femoris. The patient described significant daily improvement of symptoms 2 weeks after surgery. The neurologic examination performed 1 month after surgery was normal except for a mild Tinel sign over the fibular head. Repeat EMG and nerve conduction studies depicted a significant functional improvement with only minimal residual neurogenic dysfunction of the right common peroneal nerve at the fibular head.
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The common peroneal nerve arises from the sciatic nerve at the upper level of the popliteal fossa. It descends obliquely along the lateral side of the popliteal fossa, posterior to the short head of the biceps femoris muscle, and then lateral and superficial to the lateral head of the gastrocnemius muscle. More inferiorly, it winds around the neck of the fibula, enters the anterior and lateral muscle compartments of the leg, and divides into the deep and superficial peroneal branches [4] (Fig. 1). The common peroneal nerve and its branches provide motor innervation to the anterior compartment of the leg (tibialis anterior, extensor hallucis longus, extensor digitorum longus, and peroneus tertius muscles) and lateral compartment of the leg (peroneus longus and brevis muscles) and sensory innervation to the distal two thirds of the leg [3].
The most frequent mononeuropathy in the lower extremity involves the common peroneal nerve around the fibular head and neck area [1]. There are many causes of common peroneal neuropathy [1-5], and no report in the literature, to our knowledge, describes common peroneal entrapment neuropathy secondary to an anomalous relationship between the biceps femoris muscle and the nerve.
On the basis of the MRI findings in our asymptomatic population, we believe that the relationship between the common peroneal nerve and the distal biceps femoris muscle can be categorized into two major patterns. In the most common pattern, noted in 77% of knees in our study, the common peroneal nerve is situated within abundant fat superficial to the lateral head of the gastrocnemius muscle and posterior to the short head of the biceps femoris muscle. In the remaining 23% of knees, the common peroneal nerve is situated within a narrow tunnel between the lateral head of the gastrocnemius muscle and the short head of the biceps femoris muscle. A third pattern, which was seen in one asymptomatic patient, was distal extension of the long head of the biceps femoris muscle that contributed to the formation of the tunnel. The long head was depicted in 40 knees, and in only one case did a low-lying long head of the biceps femoris muscle contribute to the formation of a tunnel. To our knowledge, the presence of the tunnel and the different patterns of anatomic relationships between the common peroneal nerve and the short head of the biceps femoris have not yet been described in the English-language literature.
We found that there is no description in the literature of the distal extent of the short head of the biceps femoris relative to the joint level. In Gray's Anatomy [7], the only information available is "the short head arises from the lateral lip of the linea aspera, between the adductor magnus and vastus lateralis, extending up almost as high as the insertion of the gluteus maximus; from the lateral prolongation of the linea aspera to within 5 cm of the lateral condyle." We found that the distal myotendinous junction of the short head of the biceps femoris ended at the level of the joint space in 51% of the cases and ranged from 1.5 cm above to 2.0 cm below the level of the joint. In our study, subjects with a tunnel had significantly greater distal extent of the muscle belly of the short head of the biceps femoris relative to the joint space than subjects without a tunnel (p < 0.001).
We also measured the distance between the distal myotendinous junction of the long head of the biceps femoris from the joint space. In our study, the muscle belly of the long head of the biceps femoris was identified in 40 knees (40%). The distance between the long head myotendinous junction to the joint space was 5 cm on average (range, 1.5-10.0 cm above the joint space). Our findings are different from the previously reported data (7-10 cm above the knee joint) [6].
Another interesting issue, which motivated us to start this study, was the case of common peroneal neuropathy associated with the presence of a tunnel. Most interestingly, the length of the tunnel in the symptomatic patient (4.5 cm) was almost twice the mean length of the tunnel in the asymptomatic group (2.4 cm). Moreover, only 4.2% of the asymptomatic population had a tunnel length of 4 cm or greater.
Although we believe that the length of the tunnel in our symptomatic patient contributed to his neuropathy, we think that the presence of the tunnel is a normal variation that is not necessarily associated with but that can pre-dispose to common peroneal nerve entrapment. In our 100 consecutive cases, we did not find any signal abnormality compatible with common peroneal neuropathy, such as denervation edema of the anterior and posterior compartments of the proximal leg or signal abnormality of the common peroneal nerve in the popliteal fossa.
It is important to highlight that a passage shorter than 1 cm in its superior-to-inferior extent was not included in the definition of a tunnel because we believe that a tunnel shorter than 1 cm could not represent a real tunnel anatomically.
Our study has several limitations, such as the retrospective design, lack of cadaveric correlation, and only one symptomatic case of common peroneal neuropathy associated with abnormal distal biceps femoris. Also, we did not study the contribution of the lateral head of the gastrocnemius muscle to the formation of the tunnel. On the other hand, we believe that in our population the variations in the gastrocnemius muscle did not contribute significantly to tunnel formation because we did not see many anatomic variations in the gastrocnemius muscle.
In conclusion, we describe the normal MRI anatomy of the distal biceps femoris and the relationship of this muscle with the common peroneal nerve. The common peroneal nerve typically courses downward within abundant fat posterior to the short head of the biceps femoris muscle and superficial to the lateral head of the gastrocnemius muscle, but it can also descend in a tunnel between the two muscles (23%). The presence of this tunnel, previously undescribed in the literature, correlates with the posterior and distal extents of the short head of the biceps femoris. We also describe a case of common peroneal neuropathy secondary to an unusual relationship between the common peroneal nerve and the distal biceps femoris.
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