(Radiology. 2001;218:415-419.)
© RSNA, 2001
Sonographic Appearance of the Peroneus Quartus Muscle: Correlation with MR Imaging Appearance in Seven Patients1
Neeraj B. Chepuri, MD,
Jon A. Jacobson, MD,
David P. Fessell, MD and
Curtis W. Hayes, MD
1 From the Department of Radiology, University of Michigan Medical Center, 1500 E Medical Center Dr, Taubman Center, Rm 2910-G, Ann Arbor, MI 48109-0326. Received March 28, 2000; revision requested May 2; revision received June 7; accepted July 14. Address correspondence to J.A.J. (e-mail: jjacobsn@umich.edu).
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ABSTRACT
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PURPOSE: To evaluate the sonographic appearance of the peroneus quartus muscle.
MATERIALS AND METHODS: Thirty-two consecutive patients who underwent both sonography and magnetic resonance (MR) imaging of the lateral ankle were retrospectively identified during 42 months. Seven of these patients demonstrated a peroneus quartus muscle, with MR imaging as the standard of reference. The peroneus quartus muscle was characterized at retrospective review of sonographic images with regard to origin, insertion, echogenicity, and location.
RESULTS: Of the seven peroneus quartus muscles, six originated from the peroneus brevis muscle, and all seven inserted onto the calcaneus. The muscle portion of the peroneus quartus muscle was hypoechoic, while the tendon portion was hyperechoic and fibrillar. The location of the peroneus quartus musculotendinous junction was variable. Hence, the appearance of the peroneus quartus muscle in the region of the distal fibula ranged from 100% muscle to 100% tendon.
CONCLUSION: Variability in the appearance of the peroneus quartus muscle is common with sonography due to variation in the location of the musculotendinous junction. Recognition of these variations will allow correct diagnosis of a peroneus quartus muscle, and more important, it will allow differentiation from adjacent tendon abnormality.
Index terms: Magnetic resonance (MR), comparative studies • Muscles, 462.13, 462.92 • Muscles, MR, 462.121411, 46.121415 • Muscles, peroneus quartus, 462.92 • Muscles, US, 462.1298 • Ultrasound (US), comparative studies
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INTRODUCTION
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The peroneus quartus muscle is an accessory muscle of the lateral part of the ankle that has been reported to be present in up to 22% of human cadavers (1). It is reported to be unique to humans, and it is believed to represent an evolutionary step in the development of upright posture (2,3). The peroneus quartus muscle lifts the lateral edge of the foot and stabilizes hind-foot pronation (2,3).
There has been recent interest in imaging of the peroneus quartus muscle for several reasons. First, a peroneus quartus muscle can be misinterpreted as a longitudinal tear of the peroneus brevis tendon (4). Second, the presence of a peroneus quartus muscle can cause crowding of the lateral ankle compartment and subsequent longitudinal tearing of the peroneus brevis tendon (5). Third, the peroneus quartus muscle has been used successfully in the reconstruction of the peroneal retinaculum for the treatment of recurrent anterior subluxations of the peroneal tendons (1,6).
Because clinical interest in the peroneus quartus muscle has increased, it is important to correctly identify this structure with imaging. The appearance of the peroneus quartus muscle has been described with magnetic resonance (MR) imaging (2,4). However, to our knowledge, the sonographic appearance of the peroneus quartus muscle has not been previously described in the medical literature. Understanding the sonographic appearance of the poroneus quartus muscle is important because sonography is becoming a common modality for the assessment of ankle tendon abnormalities. The purpose of our study was to determine the sonographic appearance of the peroneus quartus muscle.
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MATERIALS AND METHODS
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Institutional review board approval was obtained for this study. By using a computerized database and a departmental log, 265 consecutive patients who underwent either MR imaging or sonography of the ankle during 42 months (January 1996 to June 1999) were identified. Forty of these patients underwent both MR imaging and sonography of the same ankle. Thirty two patients (17 male, 15 female; age range, 8–64 years; mean age, 32 years) who had sonographic images of the lateral ankle and transverse T1-weighted MR images of the ankle were included. In 14 (44%) of 32 patients, sonography was performed prior to MR imaging (range, 0–64 weeks; mean, 10 weeks). In 18 (56%) of 32 patients, MR imaging was performed prior to sonography (range, 0–76 weeks; mean, 14 weeks).
MR images were used as the standard of reference to determine the presence of a peroneus quartus muscle. The images were retrospectively reviewed by one fellowship-trained musculoskeletal radiologist (J.A.J.) and one radiology resident (N.B.C.), with consensus agreement. Criteria for diagnosing a peroneus quartus muscle on MR images included identification of muscle and/or tendon coursing parallel to but separated from the peroneus brevis muscle by a fat plane at the level of the distal fibula. On MR images, the peroneus quartus muscle was identified as a structure that was isointense to the adjacent peroneus brevis musculature. The peroneus quartus tendon was identified as a hypointense structure.
MR images were also retrospectively reviewed in a similar fashion to characterize the peroneus quartus muscle. The peroneus quartus muscle was assessed at two levels identified on transverse MR images: proximal (11–20 mm proximal to the fibular tip) and distal (5–10 mm proximal to the fibular tip). At these two transverse levels, MR images were assessed for the following: peroneus quartus muscle diameter (two perpendicular dimensions), peroneus quartus tendon diameter (two perpendicular dimensions), peroneus quartus tendon-to-muscle ratio (0%, 1%–5%, 6%–25%, 26%–50%, 51%–75%, 76%–100%), peroneus brevis muscle diameter (two perpendicular dimensions), and peroneus quartus muscle location with respect to the peroneus brevis muscle. The peroneus quartus tendon-to-muscle ratios were estimates based on cross-sectional area.
In addition, other imaging findings, such as peroneus quartus muscle origin and insertion, associated lateral ankle abnormality, and a low-lying peroneus brevis muscle (peroneus brevis muscle identified beyond the tip of the fibula), were recorded. Prospective MR imaging reports and available surgical reports were also reviewed.
The presence of a peroneus quartus muscle on sonographic images was determined by means of correlation with MR images. In addition, the peroneus quartus muscle was characterized with regard to echogenicity, origin, insertion, and location. Prospective sonographic reports were also reviewed.
MR imaging was performed with a 1.5-T magnet (Signa; GE Medical Systems, Milwaukee, Wis). Imaging protocols varied due to the retrospective nature of this study. Transverse T1-weighted spin-echo MR images (repetition time msec/echo time msec, 450–700/14; matrix, 256 x 192–256; number of signals acquired, one to two; section thickness, 3–5 mm; section gap, 0.5–2.0 mm) were used to characterize the peroneus quartus muscle. Transverse intermediate-weighted fast spin-echo MR images with fat saturation (2,383–4,500/14-16; echo train length, eight; matrix, 256 x 128–256; number of signals acquired, two to four; section thickness, 3–4 mm; section gap, 1 mm) were used to identify associated abnormalities in the lateral part of the ankle. MR images were interpreted by one of six fellowship-trained musculoskeletal radiologists (including J.A.J., D.P.F., C.W.H.) whose experience with musculoskeletal MR imaging ranged from 4 to 15 years.
One of the six fellowship-trained musculoskeletal radiologists performed ankle sonography by using 7–12-MHz linear transducers (HDI 3000 and 5000; Advanced Technology Laboratories, Bothell, Wash); their experience with musculoskeletal sonography ranged from 2 to 8 years. Transmission gel was used liberally in place of a standoff pad.
MR imaging was completed prior to sonography in four of the seven patients with a peroneus quartus muscle. It is not known whether correlative images were reviewed at the time of MR imaging and sonography.
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RESULTS
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Of the 32 patients with both sonographic and MR images of the lateral part of the ankle who met the inclusion criteria, seven (22%) patients had a peroneus quartus muscle, with MR images as the standard of reference. The patients consisted of five men (71%) and two women (29%), with a mean age of 37 years (range, 27–47 years). In three of the seven patients, presence of a peroneus quartus muscle had been confirmed at surgery. The remaining four did not undergo surgery.
MR Imaging Features
The peroneus quartus muscle was identified on MR images as a structure that coursed parallel to the peroneus brevis muscle and that was separated from it by a fat plane (Figs 1–3). In all seven patients, the peroneus quartus muscle was located either posteromedial or medial to the peroneus brevis muscle. In regard to the proximal location (11–20 mm proximal to the fibular tip), two peroneus quartus muscles were located directly medial to the peroneus brevis muscle, and five were located posteromedially. Distally, one peroneus quartus muscle was located medial to the peroneus brevis muscle, and six were posteromedial. The muscle component of the peroneus quartus muscle was isointense to the adjacent peroneus brevis muscle, while its tendon component was hypointense. A low-lying peroneus brevis muscle was differentiated from a peroneus quartus muscle by the presence of a fat plane between the peroneus quartus muscle and peroneus brevis muscles, and the identification of calcaneal insertion (if present) of the peroneus quartus muscle.
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Figure 1a. Peroneus quartus muscle composed primarily of muscle in a 28-year-old woman (patient 6). (a) Transverse T1-weighted MR image (650/14) of the left ankle at the level of the distal peroneus quartus muscle demonstrates the intermediate signal intensity of the muscle (arrowhead), which is separate from the peroneus brevis muscle (open arrow) and tendon (straight solid arrow). Note the peroneus longus tendon (wavy solid arrow). (b) Corresponding transverse sonographic image demonstrates the hypoechoic muscle component of the peroneus quartus muscle (arrowheads). Note peroneus brevis muscle (open arrows), peroneus brevis tendon (straight solid arrows), and peroneus longus tendon (wavy solid arrow). Left side of the image is anterior; right, posterior. F = fibula. (c) Sonographic image longitudinal to distal peroneus quartus muscle demonstrates the predominantly hypoechoic muscle component of the peroneus quartus muscle (arrowheads). Note the surrounding hyperechoic fatty tissue. Note that the muscle belly enlarges proximal to its musculotendinous junction. Left side of the image is proximal.
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Figure 1b. Peroneus quartus muscle composed primarily of muscle in a 28-year-old woman (patient 6). (a) Transverse T1-weighted MR image (650/14) of the left ankle at the level of the distal peroneus quartus muscle demonstrates the intermediate signal intensity of the muscle (arrowhead), which is separate from the peroneus brevis muscle (open arrow) and tendon (straight solid arrow). Note the peroneus longus tendon (wavy solid arrow). (b) Corresponding transverse sonographic image demonstrates the hypoechoic muscle component of the peroneus quartus muscle (arrowheads). Note peroneus brevis muscle (open arrows), peroneus brevis tendon (straight solid arrows), and peroneus longus tendon (wavy solid arrow). Left side of the image is anterior; right, posterior. F = fibula. (c) Sonographic image longitudinal to distal peroneus quartus muscle demonstrates the predominantly hypoechoic muscle component of the peroneus quartus muscle (arrowheads). Note the surrounding hyperechoic fatty tissue. Note that the muscle belly enlarges proximal to its musculotendinous junction. Left side of the image is proximal.
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Figure 1c. Peroneus quartus muscle composed primarily of muscle in a 28-year-old woman (patient 6). (a) Transverse T1-weighted MR image (650/14) of the left ankle at the level of the distal peroneus quartus muscle demonstrates the intermediate signal intensity of the muscle (arrowhead), which is separate from the peroneus brevis muscle (open arrow) and tendon (straight solid arrow). Note the peroneus longus tendon (wavy solid arrow). (b) Corresponding transverse sonographic image demonstrates the hypoechoic muscle component of the peroneus quartus muscle (arrowheads). Note peroneus brevis muscle (open arrows), peroneus brevis tendon (straight solid arrows), and peroneus longus tendon (wavy solid arrow). Left side of the image is anterior; right, posterior. F = fibula. (c) Sonographic image longitudinal to distal peroneus quartus muscle demonstrates the predominantly hypoechoic muscle component of the peroneus quartus muscle (arrowheads). Note the surrounding hyperechoic fatty tissue. Note that the muscle belly enlarges proximal to its musculotendinous junction. Left side of the image is proximal.
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Figure 2a. Peroneus quartus transition from muscle to tendon in a 42-year-old man (patient 2). (a) Transverse T1-weighted MR image (700/14) of the right ankle at the level of the distal peroneus quartus muscle demonstrates the intermediate signal intensity of the muscle (solid arrowhead) and a small hypointense tendon (black arrow). Note peroneus brevis tendon (straight solid white arrow), peroneus brevis muscle (open arrowhead), and peroneus longus tendon (wavy solid white arrow). (b) Corresponding transverse sonographic image demonstrates the hypoechoic muscle component of the peroneus quartus muscle (arrowheads). Note the peroneus brevis muscle (open arrows), peroneus brevis tendon (straight solid arrows), and peroneus longus tendon (wavy solid arrow). Left side of the image is posterior; right, anterior. F = fibula (c) Sonographic image longitudinal to the distal peroneus quartus muscle demonstrates the predominantly hypoechoic muscle component of the peroneus quartus muscle (arrowheads), with transition to predominantly hyperechoic fibrillar tendon (arrows) at its distal insertion on the calcaneus (C). Left side of the image is proximal.
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Figure 2b. Peroneus quartus transition from muscle to tendon in a 42-year-old man (patient 2). (a) Transverse T1-weighted MR image (700/14) of the right ankle at the level of the distal peroneus quartus muscle demonstrates the intermediate signal intensity of the muscle (solid arrowhead) and a small hypointense tendon (black arrow). Note peroneus brevis tendon (straight solid white arrow), peroneus brevis muscle (open arrowhead), and peroneus longus tendon (wavy solid white arrow). (b) Corresponding transverse sonographic image demonstrates the hypoechoic muscle component of the peroneus quartus muscle (arrowheads). Note the peroneus brevis muscle (open arrows), peroneus brevis tendon (straight solid arrows), and peroneus longus tendon (wavy solid arrow). Left side of the image is posterior; right, anterior. F = fibula (c) Sonographic image longitudinal to the distal peroneus quartus muscle demonstrates the predominantly hypoechoic muscle component of the peroneus quartus muscle (arrowheads), with transition to predominantly hyperechoic fibrillar tendon (arrows) at its distal insertion on the calcaneus (C). Left side of the image is proximal.
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Figure 2c. Peroneus quartus transition from muscle to tendon in a 42-year-old man (patient 2). (a) Transverse T1-weighted MR image (700/14) of the right ankle at the level of the distal peroneus quartus muscle demonstrates the intermediate signal intensity of the muscle (solid arrowhead) and a small hypointense tendon (black arrow). Note peroneus brevis tendon (straight solid white arrow), peroneus brevis muscle (open arrowhead), and peroneus longus tendon (wavy solid white arrow). (b) Corresponding transverse sonographic image demonstrates the hypoechoic muscle component of the peroneus quartus muscle (arrowheads). Note the peroneus brevis muscle (open arrows), peroneus brevis tendon (straight solid arrows), and peroneus longus tendon (wavy solid arrow). Left side of the image is posterior; right, anterior. F = fibula (c) Sonographic image longitudinal to the distal peroneus quartus muscle demonstrates the predominantly hypoechoic muscle component of the peroneus quartus muscle (arrowheads), with transition to predominantly hyperechoic fibrillar tendon (arrows) at its distal insertion on the calcaneus (C). Left side of the image is proximal.
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Figure 3a. Peroneus quartus muscle consisting of muscle and prominent central tendon in a 30-year-old man (patient 4). (a) Transverse T1-weighted MR image (600/14) of the right ankle at the level of distal peroneus quartus demonstrates the intermediate signal intensity of the muscle (solid arrowhead), with a central hypointense tendon (solid black arrow). Note the peroneus brevis muscle (open arrowhead), peroneus brevis tendon (straight solid white arrow), and peroneus longus tendon (wavy solid white arrow). (b) Transverse sonographic image demonstrates the hypoechoic muscle component of peroneus quartus muscle (arrowheads) and the central hyperechoic tendon (curved arrow). Note the peroneus brevis tendon (straight arrows) and peroneus longus tendon (wavy arrow). Left side of the image is posterior; right, anterior. F = fibula. (c) Sonographic image longitudinal to the distal peroneus quartus muscle demonstrates the hypoechoic muscle component of the peroneus quartus muscle (arrowheads); the central hyperechoic fibrillar tendon (arrows) transitions to tendon at its calcaneal insertion (C). Left side of the image is proximal.
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Figure 3b. Peroneus quartus muscle consisting of muscle and prominent central tendon in a 30-year-old man (patient 4). (a) Transverse T1-weighted MR image (600/14) of the right ankle at the level of distal peroneus quartus demonstrates the intermediate signal intensity of the muscle (solid arrowhead), with a central hypointense tendon (solid black arrow). Note the peroneus brevis muscle (open arrowhead), peroneus brevis tendon (straight solid white arrow), and peroneus longus tendon (wavy solid white arrow). (b) Transverse sonographic image demonstrates the hypoechoic muscle component of peroneus quartus muscle (arrowheads) and the central hyperechoic tendon (curved arrow). Note the peroneus brevis tendon (straight arrows) and peroneus longus tendon (wavy arrow). Left side of the image is posterior; right, anterior. F = fibula. (c) Sonographic image longitudinal to the distal peroneus quartus muscle demonstrates the hypoechoic muscle component of the peroneus quartus muscle (arrowheads); the central hyperechoic fibrillar tendon (arrows) transitions to tendon at its calcaneal insertion (C). Left side of the image is proximal.
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Figure 3c. Peroneus quartus muscle consisting of muscle and prominent central tendon in a 30-year-old man (patient 4). (a) Transverse T1-weighted MR image (600/14) of the right ankle at the level of distal peroneus quartus demonstrates the intermediate signal intensity of the muscle (solid arrowhead), with a central hypointense tendon (solid black arrow). Note the peroneus brevis muscle (open arrowhead), peroneus brevis tendon (straight solid white arrow), and peroneus longus tendon (wavy solid white arrow). (b) Transverse sonographic image demonstrates the hypoechoic muscle component of peroneus quartus muscle (arrowheads) and the central hyperechoic tendon (curved arrow). Note the peroneus brevis tendon (straight arrows) and peroneus longus tendon (wavy arrow). Left side of the image is posterior; right, anterior. F = fibula. (c) Sonographic image longitudinal to the distal peroneus quartus muscle demonstrates the hypoechoic muscle component of the peroneus quartus muscle (arrowheads); the central hyperechoic fibrillar tendon (arrows) transitions to tendon at its calcaneal insertion (C). Left side of the image is proximal.
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Peroneus quartus tendon-to-muscle ratios estimated from MR images are listed in the Table. In regard to proximal peroneus quartus tendon-to-muscle ratios (Table), two patients had a tendon-to-muscle ratio of 6%–25%, three had a ratio of 1%–5%, and two had no demonstrable tendon. At a distal level, one peroneus quartus muscle was 100% tendon, one was 51%–75% tendon, two were 6%–25% tendon, two were 1%–5% tendon, and one was 0% tendon.
Sonographic Features
Retrospective review of sonographic images demonstrated a peroneus quartus muscle in all seven patients, as correlated with MR images (Figs 1–3). At sonography, the peroneus quartus muscle appeared as a discrete structure separate from (and posteromedial or medial to) the adjacent peroneus brevis and peroneus longus muscles (Figs 1–3). The muscle component of the peroneus quartus muscle was hypoechoic relative to adjacent fat and had an echotexture similar to that of the adjacent peroneus brevis muscle. The tendon component of the peroneus quartus muscle was relatively hyperechoic, similar to the echogenicity of other tendons and fat. However, the fibrillar architecture of the hyperechoic tendon on longitudinal images allowed its differentiation from hyperechoic fat.
Identification of a calcaneal insertion allowed differentiation of a peroneus quartus muscle from the peroneal brevis or longus muscles. Of the seven peroneus quartus muscles, six originated from the peroneus brevis muscle, while the seventh origin could not be determined. Six inserted onto the calcaneus (five onto the retrotrochlear eminence), while the seventh inserted onto either the peroneus brevis tendon and/or calcaneus.
Follow-up
Of the seven patients with a peroneus quartus muscle, MR images depicted a peroneus brevis tendon tear in one (confirmed at surgery), peroneus brevis and longus tendon subluxation anterolateral to the retromalleolar groove in one, and a low-lying peroneus brevis muscle (presence of peroneus brevis muscle beyond the fibular tip) in one. Because tendon abnormality is ideally diagnosed at real-time sonography, no attempt was made to assess associated tendon abnormality by using static sonographic images. Of the three surgically proved peroneus quartus muscles, no additional abnormality was demonstrated at surgery.
Review of the prospective imaging reports of the seven patients with a peroneus quartus muscle demonstrated that the muscle was described by reviewing MR images in six patients and sonographic images in four. Also, the two patients with peroneal tendon subluxation and a low-lying peroneus brevis muscle were not described in either MR imaging or sonography reports. The surgically confirmed case of a longitudinal tear of a peroneus brevis tendon was prospectively dignosed with sonographic but not MR images.
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DISCUSSION
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The peroneus quartus muscle has been reported to be present in up to 22% of human cadavers (1), and our data show a similar frequency. The identification of a peroneus quartus muscle on images is made difficult by its anatomic variability. The peroneus quartus muscle commonly originates from the muscular portion of the peroneus brevis muscle in the distal one-third of the leg (63% of cadaveric specimens) and inserts onto the retrotrochlear eminence of the calcaneus (1,2,7,8). Other variations include but are not limited to insertion into the peroneus longus tendon, insertion into the fifth metatarsal bone, and origination from the peroneus longus muscle in the upper leg (1,7,8). The findings in our study group correlate well with the distribution of origin and insertion quoted in the literature (1) based on autopsy series.
We found considerable variability in the appearance of the distal peroneus quartus musculotendinous junction (Figs 1–3). In a comparison of the tendon-to-muscle ratios at the two separate transverse levels on MR images, there was a tendency for the ratio to be greater (increasing percentage of tendon) in the more distal aspect of the muscle, as would be expected in a muscle at its tendinous insertion. This finding corresponded to the appearance of the peroneus quartus muscle on longitudinal sonographic images, where it was hypoechoic (primarily composed of muscle) until its more distal aspect where it became hyperechoic (composed of an increasing percentage of tendon).
At the levels analyzed in our study, the tendon-to-muscle ratio was highly variable (0%–100%) at both levels proximal to the fibular tip. Therefore, the sonographic appearance of the distal peroneus quartus muscle on transverse images can range from 100% hypoechoic (composed of muscle) to 100% hyperechoic (composed of tendon) (Figs 1–3). Similar findings were present on correlative MR images on which the appearance of peroneus quartus muscle ranged from 100% isointensity due to muscle to 100% hypointensity due to tendon (Figs 1–3). This variation at the distal musculotendinous junction explains the variable imaging appearances of the peroneus quartus muscle demonstrated in our clinical practice.
Limitations of this study include its retrospective nature. Retrospective review of sonographic images may cause problems given the inherent dependence of sonography on operator skill. However, this limitation was minimized with the identification of reproducible landmarks on sonographic images (distal fibula and peroneus longus and/or brevis tendons) and with MR imaging correlation. The use of these reproducible landmarks assisted us in identifying the expected location of the peroneus quartus muscle. In addition, the use of MR imaging as the standard of reference for sonographic images is a limitation. However, a peroneus quartus muscle was surgically confirmed in three of the seven patients. Measurement error also exists with MR images, given the small size of the distal peroneus quartus muscle. Finally, our small sample of seven patients with a peroneus quartus muscle is a relative limitation.
In summary, the peroneus quartus muscle has characteristic features at sonography. It courses parallel to and is located posteromedial or medial to the peroneus brevis muscle. The peroneus quartus muscle can be differentiated from the peroneus brevis and longus muscles with the presence of a fat plane separating these structures and with its common insertion onto the calcaneus (unlike the peroneus brevis and peroneus longus tendons). Because of variation in the location of the musculotendinous junction, the appearance of the peroneus quartus muscle can range from 100% muscle (hypoechoic on sonographic images) to 100% tendon (hyperechoic and fibrillar on sonographic images). Knowledge of this appearance will allow its correct identification and distinction from other normal structures and adjacent abnormalities.
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FOOTNOTES
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Author contributions: Guarantors of integrity of entire study, N.B.C., J.A.J.; study concepts and design, N.B.C., J.A.J.; definition of intellectual content, N.B.C., J.A.J., D.P.F.; literature research, N.B.C., J.A.J.; clinical studies, N.B.C., J.A.J.; data acquisition, N.B.C.; data analysis, N.B.C., J.A.J.; manuscript preparation, editing, review, and final version approval, all authors.
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ABSTRACT
INTRODUCTION
