DOI: 10.1148/radiol.2422051993
(Radiology 2007;242:509-517.)
© RSNA, 2007
Anatomic Variants Associated with Peroneal Tendon Disorders: MR Imaging Findings in Volunteers with Asymptomatic Ankles1
Nadja Saupe, MD,
Bernard Mengiardi, MD,
Christian W. A. Pfirrmann, MD,
Patrick Vienne, MD,
Burkhardt Seifert, PhD and
Marco Zanetti, MD
1 From the Departments of Radiology (N.S., B.M., C.W.A.P., M.Z.) and Orthopedic Surgery (P.V.), Orthopedic University Hospital Balgrist, Forchstrasse 340, CH-8008 Zurich, Switzerland; and Institute of Biostatistics, University of Zurich, Zurich, Switzerland (B.S.). Received December 8, 2005; revision requested January 20, 2006; revision received February 8; accepted March 3; final version accepted June 5.
Address correspondence to N.S. (e-mail: nadja.saupe{at}balgrist.ch).
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ABSTRACT
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Purpose: To evaluate prospectively, on magnetic resonance (MR) images in volunteers with asymptomatic ankles, various features of anatomic variants that are potentially associated with peroneal tendon disorders.
Materials and Methods: The study had institutional review board approval; informed consent was obtained from each volunteer. The prevalence of accessory peroneus quartus muscles, the location of the muscle-tendon junction of the peroneus brevis muscle, the prevalence and size of the peroneal tubercle and the retrotrochlear eminence, and the shape of the retromalleolar fibular groove were evaluated on MR images in 65 volunteers with asymptomatic ankles (35 women, 30 men; age range, 23–70 years; median age, 45 years). MR images were analyzed by two radiologists in consensus. The relationship between anatomic features and age and sex was analyzed by using Spearman rank correlation and the Wilcoxon rank sum test.
Results: A peroneus quartus muscle was identified in 11 (17%) ankles. Ninety percent of the musculotendinous junctions of the peroneus brevis muscle were located in a range between 27 mm proximal to and 13 mm distal to the fibular tip (median, 0 mm). A peroneal tubercle was identified in 36 (55%) ankles. Ninety percent of all peroneal tubercles were 4.6 mm or smaller (median height, 2.9 mm). A retrotrochlear eminence was seen in all ankles (median, 3.0 mm; 90% were 4.6 mm or smaller). The retromalleolar groove was concave in 18 (28%), flat in 28 (43%), convex in 12 (18%), and irregular in seven (11%) volunteers. A significant difference (P = .04) for the height of the retrotrochlear eminence was found between men (median, 3.4 mm) and women (median, 2.5 mm). All other P values were greater than .05.
Conclusion: Anatomic variants thought to predispose individuals to peroneal tendon disorders can be seen in volunteers with asymptomatic ankles.
© RSNA, 2007
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INTRODUCTION
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Most disorders of the peroneal tendons are caused by trauma, overuse, degeneration, and chronic inflammation resulting in tendinosis, tenosynovitis, and tearing (1,2). Some anatomic variants have been reported to predispose individuals to peroneal tendon lesions (3–7). The presence of peroneus quartus muscles with various insertions is usually asymptomatic; however, such muscles may cause crowding in the retromalleolar fibular groove, predisposing persons to peroneus brevis tendon dislocation and tear (3,8). The presence of low-lying peroneal muscle bellies may also predispose individuals to superior peroneal retinaculum injuries (3,5). Enlarged peroneal tubercles or retrotrochlear eminences may irritate the peroneus longus tendon sheath, leading to tenosynovitis and tearing (7). Morphologic abnormalities of the retromalleolar fibular groove predispose persons to lateral subluxation, longitudinal tears, and irritation of the peroneal tendons (9). The association of these anatomic variants with peroneal tendon disorders has been described in small case series (4–7) and in cadaveric specimens (8,10). However, the prevalence and extent of these predisposing factors to peroneal tendon disorders in the asymptomatic population is largely unknown.
To our knowledge, no quantitative data about the extension of the muscle bellies, the size of the peroneal tubercle, and the size of the retrotrochlear eminence in asymptomatic ankles are available. Thus, the purpose of our study was to evaluate prospectively, on magnetic resonance (MR) images in volunteers with asymptomatic ankles, various features of anatomic variants that are potentially associated with peroneal tendon disorders.
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MATERIALS AND METHODS
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A large number of asymptomatic volunteers (age range, 20–70 years) was recruited for this study. Patient ages were broken down by decades. For an equal distribution of volunteers per decade, 14 patients were included in each decade. Thus, a total of 70 patients (14 in each decade between the ages of 20 and 70 years) were enrolled in this study to ensure an equal age distribution. The power analysis indicated that 63 patients would be sufficient to enable us to detect even small correlations (r = 0.4) between the size of anatomic variants and age (power, 90%). A questionnaire was used to guarantee absence of symptoms in the ankle. The following items were addressed in the questionnaire: (a) no ankle or foot pain, (b) never seen a physician for ankle or foot complaints, (c) no trauma to the ankle or foot in the past 2 years, (d) no prior ankle or foot surgery, and (e) no systemic inflammatory disease. The examined ankle (left or right) was randomly selected. Five volunteers were excluded on the basis of abnormal findings within the peroneal tendons (two tendinopathies, one tendon splitting, two partial tears) to ensure a study group with more truly normal ankles.
The final study group consisted of 65 volunteers (35 women, 30 men; age range, 23–70 years; median age, 45 years; 32 left and 33 right ankles). The study was approved by the institutional review board of Orthopedic University Hospital Balgrist. Informed consent was obtained from each volunteer.
MR Imaging Protocol
MR imaging was performed with a 1.5-T system (Symphony; Siemens Medical Solutions, Erlangen, Germany). Subjects were examined in the supine position with the ankle in a neutral position. A dedicated circularly polarized send-receive extremity coil was used. T1-weighted spin-echo images were obtained in the coronal plane (repetition time msec/echo time msec, 572/14; section thickness, 3.5 mm; field of view, 96 x 170 mm; matrix size, 288 x 512; two acquisitions), in the transverse oblique plane (45° between the coronal and transverse planes) (572/14; section thickness, 4 mm; field of view, 113 x 150 mm; matrix size, 284 x 512; two acquisitions), and in the sagittal plane (330/14; section thickness, 3 mm; field of view, 190 x 190 mm; matrix size, 512 x 512; one acquisition). T2-weighted fast spin-echo images were obtained in the coronal plane (4130/86; section thickness, 3.5 mm; field of view, 104 x 170 mm; matrix size, 336 x 512; two acquisitions; echo train length, seven) and the transverse plane (3900/85; section thickness, 4 mm; field of view, 124 x 180 mm; matrix size, 352 x 512; two acquisitions; echo train length, nine). A fast spin-echo short inversion time inversion-recovery sequence (4000/89; inversion time, 170 msec; section thickness, 3 mm; field of view, 190 x 190 mm; matrix size, 256 x 256; two acquisitions; echo train length, 21) was performed in the sagittal plane.
Analysis of MR Images
Qualitative evaluation.—MR images were read in consensus by an experienced musculoskeletal radiologist (M.Z., with 10 years of experience in musculoskeletal MR imaging) and a fellowship-trained musculoskeletal radiologist (N.S., with 12 months of experience in musculoskeletal MR imaging). So that the presence of an accessory peroneus quartus muscle could be diagnosed unequivocally, each study was additionally evaluated by another musculoskeletal radiologist (B.M., with 3 years of experience in musculoskeletal MR imaging), and a consensus was reached with the two other readers (M.Z., N.S.). The following items were assessed:
1. The presence of an accessory peroneus quartus muscle was determined. The study was defined as positive when the accessory muscle and tendon were seen unequivocally on the images as a separate structure beside the peroneus brevis and longus tendons. The attachment site of the accessory tendon was noted—that is, whether the insertion was into the calcaneus (peroneocalcaneus externum), the cuboid bone (peroneocuboideus), or the peroneus longus tendon (peroneoperoneolongus) (11).
2. The presence of prominences on the lateral aspect of the calcaneus was evaluated. Two osseous prominences may be present. The anterior prominence is the peroneal tubercle (also known as the trochlear process) described by Pfitzner in 1892 (12) and Hyrtl in 1860 (13), and the posterior prominence is termed the retrotrochlear eminence, as described by Laidlaw in 1904 (14).
3. The retromalleolar fibular groove was characterized according to Wang et al (3) as concave, flat, convex, or with an irregular contour. The evaluation was performed 1 cm proximal to the fibular tip (3).
4. The presence of a low-lying peroneal brevis muscle belly was determined. The musculotendinous junction was defined as the location where the muscle belly was no longer visible when followed from its proximal to its distal point. A study was defined as positive when the musculotendinous junction was distal to the fibular tip (3,5).
5. The position of the peroneal tendons was determined as centered, medially situated (ie, the peroneus brevis tendon was partially medial to the medial edge of the retromalleolar groove), or laterally situated (ie, the peroneus longus tendon was partially lateral to the lateral edge of the retromalleolar fibular groove). The evaluation was performed 1 cm proximal to the fibular tip (3).
Quantitative evaluation.—Measurements were performed in consensus by the same readers (M.Z., N.S.) at a picture archiving and communication system workstation (Cedara 1; Cerner, Idstein, Germany). Measurements were obtained to the nearest 10th of a millimeter.
The sizes (heights) of the peroneal tubercle and the retrotrochlear eminence were measured as the maximal distance between the apex and the reference line along the lateral cortex of the calcaneus on transverse T2-weighted and transverse oblique T1-weighted MR images (Fig 1). Subsequently, the average of the T1-weighted and the T2-weighted measurements was calculated.
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Figure 1: Measurement of peroneal tubercle and retrotrochlear eminence. Left: Size (height) of peroneal tubercle (1) and retrotrochlear eminence (2) was measured as maximal distance between apex and reference line along lateral cortex of calcaneus. Right: Measurement on transverse T2-weighted MR image (3900/85). pb = peroneus brevis tendon, pl = peroneus longus tendon.
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The site of the musculotendinous junction was noted for the peroneal muscles and the accessory muscles in relationship to the fibular tip. The distance from the musculotendinous junction to the fibular tip was measured. Positive values indicated that the musculotendinous junction was distal to the fibular tip; negative values indicated that the musculotendinous junction was proximal to the fibular tip. Results of a previous study (15) indicated that the musculotendinous junction extends more distally when the foot is more dorsiflexed. To evaluate such influence of the foot position, the angle between the plantar fascia and the longitudinal axis of the tibia was also measured.
Statistical Analysis
The frequency and size of the peroneal tubercle and retrotrochlear eminence and the position of the peroneus brevis muscle belly were noted and shown as histograms. Medians, ranges, and 10th and 90th percentiles were calculated for the quantitative data. Relationships between the size of the peroneal tubercle and retrotrochlear eminence and the location of the musculotendinous junction of the peroneus brevis muscle and age and sex were analyzed by using Spearman rank correlation (age) and the Wilcoxon rank sum test (sex). Spearman rank correlation was also used to assess the correlation between the foot position angle and the distance from the musculotendinous junction of the peroneus brevis muscle to the fibular tip. The Wilcoxon rank sum test was used to assess the relationship of the size of the peroneal tubercles and retrotrochlear eminences to the presence of a peroneus quartus muscle. This analysis was performed because results of previous studies (8,16) have suggested a positive relationship between enlargement of the retrotrochlear eminence and the presence of a peroneus quartus muscle. Intersequence reliability between the measurements of the peroneal tubercle and retrotrochlear eminence on T1- and T2-weighted images was computed by using restricted maximum-likelihood estimation of variance components in a two-way analysis of variance with volunteer and sequence as random factors. The intersequence reliability was the proportion of variability not influenced by the sequence—that is, the variance component of volunteer divided by the sum of variance components.
P < .05 was considered to indicate a statistically significant difference. A computer software package (SPSS, version 11.5.0; SPSS, Chicago, Ill) was used to perform all statistical calculations.
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RESULTS
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Qualitative Results
Eleven (17%) volunteers had a peroneus quartus muscle (Fig 2). In 10 (91%) of these 11 volunteers, the tendon inserted at the calcaneus as a peroneocalcaneus externum muscle. In one volunteer (9%), the tendon inserted at the peroneus longus tendon as a peroneoperoneolongus muscle. None of the accessory muscles inserted at the cuboid bone. The peroneus quartus muscle was present in seven men (64%) and four women (36%).
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Figure 2a: Peroneus quartus muscle. (a) Left to right: Sequential transverse T2-weighted MR images (3900/85) from cranial to caudal in 25-year-old male volunteer show peroneus quartus tendon (open arrow). (b) Left to right: Sequential coronal T1-weighted MR images (572/14) from posterior to anterior show peroneus quartus tendon (arrowheads). fc = calcaneofibular ligament, pb = peroneus brevis tendon, pl = peroneus longus tendon.
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Figure 2b: Peroneus quartus muscle. (a) Left to right: Sequential transverse T2-weighted MR images (3900/85) from cranial to caudal in 25-year-old male volunteer show peroneus quartus tendon (open arrow). (b) Left to right: Sequential coronal T1-weighted MR images (572/14) from posterior to anterior show peroneus quartus tendon (arrowheads). fc = calcaneofibular ligament, pb = peroneus brevis tendon, pl = peroneus longus tendon.
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A peroneal tubercle (Fig 3) was identified in 36 (55%) volunteers, while a retrotrochlear eminence was identified in all 65 (100%) volunteers. The retromalleolar fibular groove was concave in 18 (28%), flat in 28 (43%), convex in 12 (18%), and irregular in seven (11%) volunteers (Fig 4). In 25 (38%) volunteers, the musculotendinous junction of the peroneus brevis muscle was distal to the fibular tip (Fig 5).
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Figure 3a: Enlarged peroneal tubercle. T1-weighted MR images in (a) transverse oblique (45° between coronal and transverse planes, 572/14) and (b) coronal (572/14) planes show enlarged peroneal tubercle (*) in 60-year-old male asymptomatic volunteer. pb = peroneus brevis tendon, pl = peroneus longus tendon.
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Figure 3b: Enlarged peroneal tubercle. T1-weighted MR images in (a) transverse oblique (45° between coronal and transverse planes, 572/14) and (b) coronal (572/14) planes show enlarged peroneal tubercle (*) in 60-year-old male asymptomatic volunteer. pb = peroneus brevis tendon, pl = peroneus longus tendon.
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Figure 4a: Retromalleolar fibular groove. (a, b, d) Transverse T2-weighted (3900/85) and (c) transverse oblique T1-weighted (45° between coronal and transverse planes, 572/14) MR images in asymptomatic volunteers (24-year-old woman in a, 48-year-old man in b, 52-year-old woman in c, 32-year-old man in d). Images depict (a) concave, (b) convex, (c) irregular, and (d) flat retromalleolar fibular groove (arrowheads). A fibrous ridge is seen at origin of superior peroneal retinaculum in d (arrow). pb = peroneus brevis tendon, pl = peroneus longus tendon.
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Figure 4b: Retromalleolar fibular groove. (a, b, d) Transverse T2-weighted (3900/85) and (c) transverse oblique T1-weighted (45° between coronal and transverse planes, 572/14) MR images in asymptomatic volunteers (24-year-old woman in a, 48-year-old man in b, 52-year-old woman in c, 32-year-old man in d). Images depict (a) concave, (b) convex, (c) irregular, and (d) flat retromalleolar fibular groove (arrowheads). A fibrous ridge is seen at origin of superior peroneal retinaculum in d (arrow). pb = peroneus brevis tendon, pl = peroneus longus tendon.
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Figure 4c: Retromalleolar fibular groove. (a, b, d) Transverse T2-weighted (3900/85) and (c) transverse oblique T1-weighted (45° between coronal and transverse planes, 572/14) MR images in asymptomatic volunteers (24-year-old woman in a, 48-year-old man in b, 52-year-old woman in c, 32-year-old man in d). Images depict (a) concave, (b) convex, (c) irregular, and (d) flat retromalleolar fibular groove (arrowheads). A fibrous ridge is seen at origin of superior peroneal retinaculum in d (arrow). pb = peroneus brevis tendon, pl = peroneus longus tendon.
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Figure 4d: Retromalleolar fibular groove. (a, b, d) Transverse T2-weighted (3900/85) and (c) transverse oblique T1-weighted (45° between coronal and transverse planes, 572/14) MR images in asymptomatic volunteers (24-year-old woman in a, 48-year-old man in b, 52-year-old woman in c, 32-year-old man in d). Images depict (a) concave, (b) convex, (c) irregular, and (d) flat retromalleolar fibular groove (arrowheads). A fibrous ridge is seen at origin of superior peroneal retinaculum in d (arrow). pb = peroneus brevis tendon, pl = peroneus longus tendon.
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Figure 5a: Low-lying peroneus brevis muscle belly in left ankle of 23-year-old male volunteer. (a) Left to right: Sequential transverse T2-weighted MR images (3900/85) from cranial to caudal. Fibula (F) is visible only on most cranial image. Caudal extent of peroneus brevis muscle is demonstrated (open arrow) beneath peroneus brevis tendon (pb), peroneus longus tendon (pl), and calcaneofibular ligament (fc). (b) Coronal T1-weighted MR image (572/14) shows low-lying peroneus brevis muscle (open arrow) beneath peroneus brevis tendon, peroneus longus tendon, and calcaneofibular ligament.
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Figure 5b: Low-lying peroneus brevis muscle belly in left ankle of 23-year-old male volunteer. (a) Left to right: Sequential transverse T2-weighted MR images (3900/85) from cranial to caudal. Fibula (F) is visible only on most cranial image. Caudal extent of peroneus brevis muscle is demonstrated (open arrow) beneath peroneus brevis tendon (pb), peroneus longus tendon (pl), and calcaneofibular ligament (fc). (b) Coronal T1-weighted MR image (572/14) shows low-lying peroneus brevis muscle (open arrow) beneath peroneus brevis tendon, peroneus longus tendon, and calcaneofibular ligament.
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In 55 (85%) asymptomatic volunteers, the peroneal tendons were centered, in nine (14%), they were medially situated, and in one (2%), they were laterally situated.
Quantitative Results
The frequency and size of the peroneal tubercle and retrotrochlear eminence and the positions of peroneus brevis muscle bellies are shown in histogram format in Figure 6. The median size of the 36 peroneal tubercles was 2.6 mm (range, 0.5–7.1 mm) on transverse oblique T1-weighted images and 2.9 mm (range, 1.0–5.6 mm) on transverse T2-weighted images. The median of the average of both transverse oblique T1- and transverse T2-weighted measurements of the size of the peroneal tubercle was 2.9 mm (range, 0.8–6.4 mm). The median size of the retrotrochlear eminence was 2.8 mm (range, 1.0–5.8 mm) on transverse oblique T1-weighted images and 3.1 mm (range, 0.9–6.2 mm) on transverse T2-weighted images. The median of the average of both measurements of the size of the retrotrochlear eminence was 3.0 mm (range, 1.1–5.9 mm).
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Figure 6a: Histograms show prevalence and size of (a) peroneal tubercle, (b) retrotrochlear eminence, and (c) position of peroneus brevis muscle bellies in volunteers with asymptomatic ankles. (a) Majority of peroneal tubercles in volunteers with asymptomatic ankles were smaller than 5 mm. X-axis shows size in millimeters. (b) Majority of retrotrochlear eminences were smaller than 5 mm. X-axis shows size in millimeters. (c) Musculotendinous junctions of peroneus brevis muscle were commonly distal to the fibular tip. X-axis shows distance (in millimeters) of musculotendinous junctions from fibular tip. Positive values on x-axis indicate a position distal to fibular tip.
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Figure 6b: Histograms show prevalence and size of (a) peroneal tubercle, (b) retrotrochlear eminence, and (c) position of peroneus brevis muscle bellies in volunteers with asymptomatic ankles. (a) Majority of peroneal tubercles in volunteers with asymptomatic ankles were smaller than 5 mm. X-axis shows size in millimeters. (b) Majority of retrotrochlear eminences were smaller than 5 mm. X-axis shows size in millimeters. (c) Musculotendinous junctions of peroneus brevis muscle were commonly distal to the fibular tip. X-axis shows distance (in millimeters) of musculotendinous junctions from fibular tip. Positive values on x-axis indicate a position distal to fibular tip.
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Figure 6c: Histograms show prevalence and size of (a) peroneal tubercle, (b) retrotrochlear eminence, and (c) position of peroneus brevis muscle bellies in volunteers with asymptomatic ankles. (a) Majority of peroneal tubercles in volunteers with asymptomatic ankles were smaller than 5 mm. X-axis shows size in millimeters. (b) Majority of retrotrochlear eminences were smaller than 5 mm. X-axis shows size in millimeters. (c) Musculotendinous junctions of peroneus brevis muscle were commonly distal to the fibular tip. X-axis shows distance (in millimeters) of musculotendinous junctions from fibular tip. Positive values on x-axis indicate a position distal to fibular tip.
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The musculotendinous junctions of the peroneus brevis muscle were located in a range between 27 mm proximal to and 24 mm distal to the fibular tip (median, 0 mm). The mean angle between the plantar fascia and the longitudinal axis of the tibia was 98° (range, 81°–109°).
Statistical Analysis
Medians, ranges, and 10th and 90th percentiles are presented in Table 1. Results of statistical analysis (Spearman rank correlation and Wilcoxon rank sum test) (Tables 2 and 3) for the relationship between the size of the peroneal tubercle and the retrotrochlear eminence and the location of the musculotendinous junction of peroneus brevis muscle and age and sex revealed a statistically significant difference (P = .04) only for the height of the retrotrochlear eminence between men (median, 3.4 mm) and women (median, 2.5 mm). All other P values were greater than .05. No significant correlation (Spearman rank correlation) was found between foot position angle and the location of the musculotendinous junction of the peroneus brevis muscle. The size of the peroneal tubercle and the size of the retrotrochlear eminence were also not significantly related (P > .05, Wilcoxon rank sum test) to the presence of a peroneus quartus muscle. Intersequence reliability (according to analysis of variance for repeated measurements) was 0.912 for peroneal tubercle size and 0.833 for retrotrochlear eminence size (Table 4).
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Table 1. Tenth, 50th, and 90th Percentiles and Range for Size of Peroneal Tubercle and Retrotrochlear Eminence and Position of Musculotendinous Junction of Peroneus Brevis Muscle
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Table 2. Relationship between Size of Peroneal Tubercle and Retrotrochlear Eminence and Location of Musculotendinous Junction of Peroneus Brevis Muscle and Volunteer Age
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Table 3. Relationship between Size of Peroneal Tubercle and Retrotrochlear Eminence and Location of Musculotendinous Junction of Peroneus Brevis Muscle and Volunteer Sex
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Table 4. Variance Components and Intersequence Reliability between Measurements of Peroneal Tubercle and Retrotrochlear Eminence on T1- and T2-weighted MR Images
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DISCUSSION
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Our study results demonstrate that many anatomic variants that are potentially associated with peroneal tendon disorders are commonly seen on MR images in volunteers with asymptomatic ankles. The prevalence of a peroneus quartus muscle was 17%, which is in the same range as at cadaveric dissection (6.6%–22% [8,10,17]) and in an earlier MR study (10% [16]). The terminology for accessory peroneal muscles is controversial. One group (17) considered all lateral compartment muscles to be peroneal variants and defined a peroneus quartus muscle as a muscle arising from the lower leg and inserting onto the calcaneus. In our study, we used the terminology described by Hecker (11). Hecker classified peroneus quartus muscles into three main types on the basis of their insertions—into the calcaneus (peroneocalcaneus externum), the cuboid bone (peroneocuboideus), or the peroneus longus tendon (peroneoperoneolongus). We agree with Cheung et al (16) that by far the most common variant peroneus quartus muscle is the peroneocalcaneus externum (91% in our study; 79% in the study of Cheung et al). In our study, the peroneus quartus muscle was more common in male volunteers (64%); this is in agreement with the prevalence of 55% in the study of Sobel et al (10) and that of 70% in the study of Cheung et al (16).
Differentiation between the calcaneofibular ligament and an accessory peroneus quartus muscle may sometimes be difficult. The calcaneofibular ligament arises from the deep aspect of the fibular malleolus and courses downward and backward to its attachment at the lateral aspect of the calcaneus. The peroneal tendons lie immediately superficial to the calcaneofibular ligament, and the peroneal tendon sheath is attached to this ligament. If a peroneus quartus muscle is suspected, it is necessary to follow the ligament from its origin to its attachment to differentiate both structures. To distinguish the peroneus quartus muscle from a low-lying peroneus brevis muscle belly, the peroneus quartus muscle has to be posteromedial to the peroneus brevis muscle and separated from the peroneus brevis muscle by a fat plane. The low-lying peroneus brevis muscle belly usually stops abruptly, while the peroneus quartus muscle tapers into a long thin tendon. Knowledge of the presence of a peroneus quartus muscle is important to the clinician to avoid confusing this muscle variant with soft-tissue tumors that may arise in this area (18).
Along the lateral aspect of the calcaneus, two osseous prominences are usually visible. At the anterolateral aspect of the calcaneus, the peroneal tubercle (also known as the trochlear process or peroneal process) was present in 55% of volunteers in our study; this is in the same range as in previous studies by Agarwal et al (19) and Sarrafian (20), in which the prevalence ranged from 32% to 97%. The retrotrochlear eminence, which is located just posterior to the peroneal tubercle, was present in our study in 100% of volunteers; this is also close to the results of Wang et al (3,16), in whose study the retrotrochlear eminence was present in 98% of consecutive patients.
Using the 90th percentile as a cutoff, we believe the terms enlarged peroneal tubercle and enlarged retrotrochlear eminence should be used only when these structures are higher than 5 mm. In our study, the size of both the peroneal tubercle and the retrotrochlear eminence was independent of the presence of the peroneus quartus muscle. This is in contrast to the results of Sobel et al (10) and Cheung et al (16). Cheung et al (16) acknowledge that the position of the ankle and calcaneal pitch alter the apparent shape and size of the osseous prominences at the lateral wall of the calcaneus. To compensate for individual variation and to obtain the true size of the peroneal tubercle and the retrotrochlear eminence, in our study, we therefore performed measurements in two planes (transverse and transverse oblique). We found a high reliability between both planes (analysis of variance for repeated measurements) for measurements of both the peroneal tubercle (0.912) and the retrotrochlear eminence (0.833). These high proportions indicate that variations depend predominantly on the size of osseous prominences and not on the imaging plane.
The retromalleolar fibular groove underlies morphologic variations. A convex, flat, or irregular fibular groove has been considered to predispose individuals to peroneal tendon dislocation and tendon irritation (21). In cases of peroneal tendon dislocation, a fibular groove–deepening procedure may be performed to prevent recurrent peroneal tendon instability (22,23). Interestingly, 72% of our volunteers exhibited such convex, flat, or irregular fibular grooves that are considered to predispose individuals to peroneal tendon dislocation and tendon irritation. In contrast to Wang et al (3), who saw in 82% of individuals a smooth and concave groove, in our study, only 28% of ankles showed a smooth concave fibular groove.
A low-lying peroneus brevis muscle belly is defined as an extension of the muscle tissue distal to the fibular tip (1,24). Rademaker et al (15) described that with dorsiflexion, extension of the muscle belly into the fibular groove or beyond reflects a normal excursion of the muscle in most people. In 11 of 12 volunteers in their study, the muscle belly was found to be distal to the fibular tip in dorsiflexion, but was found to be distal in only three volunteers in plantarflexion. In our investigation, foot position was almost neutral, with a mean angle between the plantar fascia and the longitudinal axis of the tibia of 97°. Despite the neutral position of the feet in our study, we observed a musculotendinous junction distal to the fibular tip in 25 of 65 ankles. Considering that in our study, 90% of the musculotendinous junctions of the peroneus brevis were located in a range between 27 mm proximal to 13 mm distal to the fibular tip (mean, 0 mm) we believe that the definition of a low-lying peroneus brevis muscle should be adapted. A low-lying peroneus brevis muscle belly should probably only be diagnosed in cases with muscle extension more than 15 mm distal to the fibular tip. However, the definition of cutoff values ideally includes the analysis of diseased peroneal tendons, which was not the purpose of this study.
As a limitation of our study we acknowledge that the number of 65 asymptomatic ankles—although to our knowledge the largest series to date—is still modest for defining normal standards. The lack of inclusion of a group with symptomatic peroneal tendon disorders precludes a confident conclusion about pathologic cutpoints for structures such as the peroneal tubercle and the retrotrochlear eminence. Additionally, the recruitment of asymptomatic volunteers with a questionnaire but without clinical examination may have introduced recall bias.
In summary, our study results demonstrate that many anatomic variants that are potentially associated with peroneal tendon disorders are commonly seen on MR images in volunteers with asymptomatic ankles.
A peroneus quartus muscle can be found in approximately one-sixth of asymptomatic ankles. Seventy-two percent of our asymptomatic volunteers showed fibular grooves commonly considered to be a predisposing factor for peroneal tendon dislocation and tendon irritation. The musculotendinous junction of the peroneus brevis muscle frequently lies distal to the fibular tip, and a height of peroneal tubercles and retrotrochlear eminences of up to 5 mm is commonly seen in asymptomatic ankles.
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ADVANCES IN KNOWLEDGE
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- Convex, flat, or irregular fibular grooves considered to predispose individuals to peroneal tendon dislocation and tendon irritation can be found in more than two-thirds (72%) of asymptomatic ankles.
- Peroneal tubercles and retrotrochlear eminences are commonly seen in asymptomatic ankles but are infrequently larger than 5 mm (90th percentile = 4.6 mm for both prominences).
- The musculotendinous junction of the peroneus brevis muscle extended distal to the fibular tip in 38% of ankles studied, but the musculotendinous junction infrequently lies more than 13 mm (90th percentile) distal to the fibular tip.
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FOOTNOTES
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Authors stated no financial relationship to disclose.
Author contributions: Guarantors of integrity of entire study, N.S., M.Z.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, N.S., P.V., M.Z.; clinical studies, N.S., B.M., C.W.A.P., M.Z.; statistical analysis, N.S., B.S., M.Z.; and manuscript editing, C.W.A.P., M.Z.
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ABSTRACT
INTRODUCTION
