DOI: 10.1148/radiol.2371041067
(Radiology 2005;237:235-241.)
© RSNA, 2005
Radiography and US of Os Peroneum Fractures and Associated Peroneal Tendon Injuries: Initial Experience1
Monica K. Brigido, MD,
David P. Fessell, MD,
Jon A. Jacobson, MD,
David S. Widman, MD,
Joseph G. Craig, MB, ChB,
David A. Jamadar, MB, BS and
Marnix T. van Holsbeeck, MD
1 From the Department of Radiology, University of Michigan Medical Center, 1500 E Medical Center Dr, TC-2808, Ann Arbor, MI 48109-0326 (M.K.B., J.A.J., D.A.J.); Department of Radiology, William Beaumont Hospital, Royal Oak, Mich (D.P.F.); and Department of Radiology, Henry Ford Hospital, Detroit, Mich (D.S.W., J.G.C., M.T.v.H.). Received June 16, 2004; revision requested August 25; revision received November 24; accepted December 24.
Address correspondence to M.K.B. (e-mail: mbrigido{at}umich.edu).
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ABSTRACT
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PURPOSE: To retrospectively evaluate the imaging features of os peroneum fractures and associated peroneus longus tendon injuries at radiography and ultrasonography (US) and to retrospectively compare these imaging features with those of multipartite os peroneum.
MATERIALS AND METHODS: Institutional review board approval was obtained and informed consent was waived for this HIPAA-compliant study. Retrospective review of findings in nine patients (five men, four women; age range, 35–59 years) with os peroneum fracture at radiography and lateral foot pain after injury who had undergone US of the foot was performed. Three patients underwent magnetic resonance (MR) imaging, and two underwent surgery. Os peroneum fragment separation and displacement relative to the calcaneocuboid joint were measured on radiographs. Os peroneum fracture and peroneus longus tendon injuries were characterized with US and MR imaging. Review of 43 foot radiographs obtained in 36 control subjects (eight men, 28 women; age range, 18–84 years) who were found to have an os peroneum at radiography but were asymptomatic in that area was completed to measure os peroneum distance from the calcaneocuboid joint and bipartite os peroneum fragment distraction.
RESULTS: Os peroneum fragment separation of 6 mm or more or displacement of the proximal fragment by 10 mm or more on a lateral radiograph or 20 mm or more on an oblique radiograph was associated with full-thickness peroneus longus tendon tear in seven of seven patients (100%). Os peroneum fragment separation of 2 mm or less or proximal displacement of 8 mm or less was associated with normal tendons, partial-thickness tears, or tendinosis. In the control subjects, os peroneum location ranged from 7 mm proximal to 8 mm distal to the calcaneocuboid joint on lateral radiographs and from 9 mm proximal to 8 mm distal to the joint on oblique radiographs. Bipartite os peroneum fragment separation was 2 mm or less.
CONCLUSION: Os peroneum fragment separation of 6 mm or more suggests os peroneum fracture and associated full-thickness peroneus longus tendon tear. Separation of 2 mm or less may be seen with nondisplaced os peroneum fractures and bipartite os peroneum.
© RSNA, 2005
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INTRODUCTION
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The os peroneum is a sesamoid bone located within the peroneus longus tendon in the region of the cuboid tunnel (Fig 1) (1,2). It is present in its fully ossified form in up to 20% of adults and is bilateral in approximately 60% of cases (1,3,4). A bipartite appearance is a frequent finding, occurring in approximately 30% of adults with an os peroneum (4).
A fracture of an os peroneum can occur after trauma and may be associated with a peroneus longus tendon tear (3–5). Recognition of an os peroneum fracture and associated peroneus longus tendon injuries is important because misdiagnosis can result in meaningful sequelae, including ankle instability and peroneal compartment syndrome (5–7). The initial diagnosis of os peroneum fracture can be overlooked or delayed owing to a low index of suspicion, nonspecific symptoms, or confusion with a bipartite os peroneum (5,7).
In our clinical practice, we have noted cases of displaced os peroneum fracture fragments at radiography that have simulated other types of fractures, intraarticular bodies, and other accessory ossicles. In addition, we have identified unexpected os peroneum fractures at ultrasonography (US) in patients with lateral foot pain—fractures that, to our knowledge, have not been previously described. Therefore, the purpose of our study was to retrospectively evaluate the imaging features of os peroneum fractures and associated peroneus longus tendon injuries at radiography and US and to retrospectively compare these imaging features with those of multipartite os peroneum.
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MATERIALS AND METHODS
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Institutional review board approval was obtained and informed consent was waived for patients and control subjects at the institutions participating in this study, which was compliant with the Health Insurance Portability and Accountability Act.
Patients and Control Subjects
Nine patients with a history of acute injury and lateral foot pain who had undergone US evaluation of the lateral foot and who had radiographic evidence of os peroneum fracture were identified between July 1998 and April 2000 through the clinical experience of one of the authors (D.P.F., who had 2 years of musculoskeletal radiology experience). The patients included five men and four women who ranged in age from 35 to 59 years (mean age, 47 years). Two of the patients had diabetes mellitus, and one patient had rheumatoid arthritis. The criterion for os peroneum fracture at radiography was cortical discontinuity with nonsclerotic margins and a "pieces of a puzzle" appearance (4–6,8). Lateral and oblique views were available for seven of the nine patients. Oblique views were not available for two patients. Comparison radiographs for the evaluation of the progression of diastasis were not available. Medical records were also reviewed by two of the authors (D.P.F. and M.K.B.) to determine the history of surgical or magnetic resonance (MR) imaging evaluation of the foot and ankle.
An additional group of 36 consecutive control subjects (eight men and 28 women; age range, 18–84 years; mean age, 47 years) was identified through the clinical experience of one of the authors (D.A.J., who had 6 years of musculoskeletal radiology experience) between September 1999 and April 2004. All 43 radiographs obtained in this group (seven of the subjects had undergone radiography of both feet) revealed a unipartite or bipartite os peroneum. Lateral and oblique views of the foot were available for all control subjects. The criterion for os peroneum was the presence of a bone ossicle or ossicles with smooth, rounded, and sclerotic margins at the lateral margin of the cuboid or calcaneocuboid joint. The subjects had no history of injury or pain in the lateral foot. Foot images were ordered at the discretion of referring physicians to rule out, among other conditions, osteoarthritis, erosive changes, and the presence of foreign bodies or to evaluate heel and medial foot pain. Medical records were reviewed by one of the authors (M.K.B.) to determine if any of these subjects had undergone US or MR imaging of the foot and ankle.
Imaging Technique
Radiography consisted of routine digital and film-screen examinations of the foot and ankle. US was performed with 7.5–10.0-MHz linear-array transducers (Model 5200; Acoustic Imaging Technologies, Phoenix, Ariz) or 10.5–12.5-MHz linear-array transducers (HDI 3000 or HDI 5000; Advanced Technology Laboratories, Bothell, Wash) by one of five radiologists (including D.P.F. and M.T.v.H.) who had 2–15 years of musculoskeletal US experience.
MR imaging was performed with a 1.5-T MR unit (Signa; GE Medical Systems, Milwaukee, Wis) and an extremity coil. The typical field of view was 12–14 cm, the section thickness was 3–4 mm, the intersection gap was 0.5–1.5 mm, the matrix was 256 x 192–256, and the echo train length was one or two. All patients underwent T2-weighted fast spin-echo fat saturation imaging in the transverse, sagittal, and coronal planes (repetition time msec/echo time msec, 3000–3500/80–90). An additional MR imaging protocol for one patient included the acquisition of (a) transverse, coronal, and sagittal T1-weighted spin-echo images (500–700/14–16); (b) transverse and sagittal intermediate-weighted fast spin-echo fat saturation images (3000–3500/30–40); and (c) oblique sagittal T1-weighted gradient-echo images. Additional MR imaging for the other patients included transverse, sagittal, and coronal T2-weighted fast spin-echo fat saturation imaging and coronal intermediate-weighted fast spin-echo imaging. The radiographic, US, and MR imaging examinations were ordered by the referring physicians as part of the diagnostic evaluation for ankle and foot pain.
Image Review
Radiographic and US images obtained in the nine patients were retrospectively reviewed at the same time in an unblinded fashion by three fellowship-trained musculoskeletal radiologists (M.K.B., D.P.F., and J.A.J.; range of experience, 1–6 years), and opinions were reached by consensus. Three of the nine patients had also undergone MR imaging, and these images were also reviewed in an unblinded fashion by the same authors, with opinions reached by consensus. Radiographs were reviewed for the presence of os peroneum fracture by using criteria similar to those described above. The maximum distance of fragment separation was measured to the closest millimeter by using a ruler. The maximum distance from the most posterior aspect of the proximal fragment to the calcaneocuboid joint was also measured on both lateral and oblique radiographs. Oblique radiographs were not obtained in two patients.
US images were reviewed for the presence of ossific fragments in the region of the peroneus longus tendon, as well as peroneus longus and brevis tendon abnormalities. The US findings of os peroneum were characterized with regard to location, contour, echogenicity, and shadowing. At US, a normal tendon was defined as one that had a uniform hyperechoic fibrillar appearance; tendinosis and partial-thickness tear were characterized by hypoechoic swelling and a hypoechoic or anechoic cleft, respectively; and a full-thickness tear was defined as complete tendon disruption with retraction (9).
MR images were reviewed for the presence of bone fragments in the region of the peroneus longus tendon, peroneus longus tendon abnormality, and the signal intensity of the fracture fragments. At MR imaging, a normal tendon was defined as one that had uniform thickness and low signal intensity on images obtained with all sequences, while tendinosis was defined as a situation in which the tendon was enlarged and had abnormally increased signal intensity that was similar to that of muscle on images obtained with all sequences, partial-thickness tear was characterized by an incomplete cleft that had the signal intensity of fluid on T2-weighted images, and full-thickness tear was defined as complete tendon disruption with retraction (10).
The 43 foot radiographs obtained in the control subjects were retrospectively reviewed by two of the authors (M.K.B. and D.A.J., with 3 and 11 years of experience in musculoskeletal radiology, respectively), and agreement was reached by consensus. The maximum distance from the most posterior aspect of the os peroneum to the calcaneocuboid joint was measured on both lateral and oblique views. The os peroneum was classified as either unipartite or multipartite, with the latter kind characterized by smooth, rounded, and sclerotic margins (4–6,8). If a multipartite os peroneum was present, the maximum distance of ossicle fragment separation was measured to the closest millimeter by using a ruler. The os peroneum was also characterized on US and MR images with regard to its location, contour, echogenicity, shadowing, and signal intensity.
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RESULTS
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Patients
The time interval between radiography and US in the nine patients ranged from 1 to 147 days (mean, 37 days). The histories of the patients included inversion injury in six, direct impact injury in two, and unspecified injury in one. All nine patients underwent foot US. Three patients underwent MR imaging, and two underwent surgery of the lateral foot.
In the nine patients, radiographs showed that os peroneum fracture fragments were separated by an average of 20 mm (range, 1–80 mm) (Figs 2–4, Tables 1 and 2). The average distance from the proximal os peroneum fragment to the calcaneocuboid joint was 23.3 mm (range, 8–75 mm) on lateral radiographs. At US, the os peroneum fragments were identified in nine (100%) of nine patients and appeared hyperechoic, with irregular contours within the expected location of the peroneus longus tendon and variable degrees of distraction (Figs 2–4). The large fragments also demonstrated posterior acoustic shadowing, while smaller fragments did not (Fig 3b). Full-thickness tear of the peroneus longus tendon was seen in seven (78%) of nine patients (Figs 2–4), partial-thickness tear or tendinosis was seen in one (11%) patient, and a normal tendon was seen in one (11%) patient. Tears were seen at the site of the os peroneum fracture. The peroneus brevis tendon was normal in all nine patients (100%).
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Figure 2a. Images in 56-year-old woman with fracture of os peroneum and full-thickness tear of peroneus longus tendon. (a) Oblique radiograph of foot and (b) US image obtained longitudinally to the peroneus longus tendon show diastasis of os peroneum fragments (arrows). The fragments were separated by 10 mm, and the distance from the most posterior fragment to the calcaneocuboid joint was 22 mm. (c) Sagittal T2-weighted fat-saturated MR image (3500/90) shows the two most proximal os peroneum fracture fragments (arrows). The small distal fragment was not well visualized. C = calcaneus, Cu = cuboid bone, T = tibia. Note the loss of normal fibrillar tendon appearance and the abnormal hypoechogenicity at the site of the tendon tear (*) on b and the fluid signal intensity at the site of the tear (*) and in the proximal fracture fragments on c.
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Figure 2b. Images in 56-year-old woman with fracture of os peroneum and full-thickness tear of peroneus longus tendon. (a) Oblique radiograph of foot and (b) US image obtained longitudinally to the peroneus longus tendon show diastasis of os peroneum fragments (arrows). The fragments were separated by 10 mm, and the distance from the most posterior fragment to the calcaneocuboid joint was 22 mm. (c) Sagittal T2-weighted fat-saturated MR image (3500/90) shows the two most proximal os peroneum fracture fragments (arrows). The small distal fragment was not well visualized. C = calcaneus, Cu = cuboid bone, T = tibia. Note the loss of normal fibrillar tendon appearance and the abnormal hypoechogenicity at the site of the tendon tear (*) on b and the fluid signal intensity at the site of the tear (*) and in the proximal fracture fragments on c.
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Figure 2c. Images in 56-year-old woman with fracture of os peroneum and full-thickness tear of peroneus longus tendon. (a) Oblique radiograph of foot and (b) US image obtained longitudinally to the peroneus longus tendon show diastasis of os peroneum fragments (arrows). The fragments were separated by 10 mm, and the distance from the most posterior fragment to the calcaneocuboid joint was 22 mm. (c) Sagittal T2-weighted fat-saturated MR image (3500/90) shows the two most proximal os peroneum fracture fragments (arrows). The small distal fragment was not well visualized. C = calcaneus, Cu = cuboid bone, T = tibia. Note the loss of normal fibrillar tendon appearance and the abnormal hypoechogenicity at the site of the tendon tear (*) on b and the fluid signal intensity at the site of the tear (*) and in the proximal fracture fragments on c.
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Figure 3a. Images in 35-year-old man with os peroneum fracture and full-thickness peroneus longus tendon tear. (a) Oblique radiograph of foot and (b) US image obtained longitudinally to the peroneus longus tendon show diastasis of os peroneum fragments (arrows). The greatest degree of fragment separation was 25 mm, and the distance from the most posterior fragment to the calcaneocuboid joint was 45 mm. (c) Oblique sagittal T1-weighted gradient-echo MR image (500/14) shows the most proximal os peroneum fracture fragment (arrow). The small distal fragments were not well visualized. B = peroneus brevis tendon, F = fibula, L = peroneus longus tendon. Note the loss of normal fibrillar tendon appearance and the abnormal hypoechogenicity at the site of the tendon tear (*) on b and the fluid signal intensity at the site of the tear (*) and the marrow replacement of the proximal fracture fragment on c.
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Figure 3b. Images in 35-year-old man with os peroneum fracture and full-thickness peroneus longus tendon tear. (a) Oblique radiograph of foot and (b) US image obtained longitudinally to the peroneus longus tendon show diastasis of os peroneum fragments (arrows). The greatest degree of fragment separation was 25 mm, and the distance from the most posterior fragment to the calcaneocuboid joint was 45 mm. (c) Oblique sagittal T1-weighted gradient-echo MR image (500/14) shows the most proximal os peroneum fracture fragment (arrow). The small distal fragments were not well visualized. B = peroneus brevis tendon, F = fibula, L = peroneus longus tendon. Note the loss of normal fibrillar tendon appearance and the abnormal hypoechogenicity at the site of the tendon tear (*) on b and the fluid signal intensity at the site of the tear (*) and the marrow replacement of the proximal fracture fragment on c.
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Figure 3c. Images in 35-year-old man with os peroneum fracture and full-thickness peroneus longus tendon tear. (a) Oblique radiograph of foot and (b) US image obtained longitudinally to the peroneus longus tendon show diastasis of os peroneum fragments (arrows). The greatest degree of fragment separation was 25 mm, and the distance from the most posterior fragment to the calcaneocuboid joint was 45 mm. (c) Oblique sagittal T1-weighted gradient-echo MR image (500/14) shows the most proximal os peroneum fracture fragment (arrow). The small distal fragments were not well visualized. B = peroneus brevis tendon, F = fibula, L = peroneus longus tendon. Note the loss of normal fibrillar tendon appearance and the abnormal hypoechogenicity at the site of the tendon tear (*) on b and the fluid signal intensity at the site of the tear (*) and the marrow replacement of the proximal fracture fragment on c.
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Figure 4a. Images in 42-year-old man with fracture of os peroneum and full-thickness peroneus longus tendon tear. (a) Lateral radiograph of foot shows diastasis of os peroneum fragments (arrows). The fragment separation was 80 mm, and the distance from the most posterior fragment to the calcaneocuboid joint was 75 mm. US images obtained (b) longitudinally and (c) transversely to the peroneus longus tendon show the two distal small fracture fragments (arrows in b) and a proximal fracture fragment (arrow in c) at the level of the lateral malleolus (LM). (d) Sagittal T1-weighted MR image (500/14) and (e) sagittal T2-weighted MR image (3500/90) show the most proximal fracture fragment (arrow) and the site of the tendon tear (*). Note the loss of normal fibrillar tendon appearance and the abnormal hypoechogenicity at the site of the tendon tear (*) in b and the fluid signal intensity at the site of the tear and the marrow replacement of the proximal fracture fragment, which has retracted to the level of the tibiotalar joint, in e.
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Figure 4b. Images in 42-year-old man with fracture of os peroneum and full-thickness peroneus longus tendon tear. (a) Lateral radiograph of foot shows diastasis of os peroneum fragments (arrows). The fragment separation was 80 mm, and the distance from the most posterior fragment to the calcaneocuboid joint was 75 mm. US images obtained (b) longitudinally and (c) transversely to the peroneus longus tendon show the two distal small fracture fragments (arrows in b) and a proximal fracture fragment (arrow in c) at the level of the lateral malleolus (LM). (d) Sagittal T1-weighted MR image (500/14) and (e) sagittal T2-weighted MR image (3500/90) show the most proximal fracture fragment (arrow) and the site of the tendon tear (*). Note the loss of normal fibrillar tendon appearance and the abnormal hypoechogenicity at the site of the tendon tear (*) in b and the fluid signal intensity at the site of the tear and the marrow replacement of the proximal fracture fragment, which has retracted to the level of the tibiotalar joint, in e.
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Figure 4c. Images in 42-year-old man with fracture of os peroneum and full-thickness peroneus longus tendon tear. (a) Lateral radiograph of foot shows diastasis of os peroneum fragments (arrows). The fragment separation was 80 mm, and the distance from the most posterior fragment to the calcaneocuboid joint was 75 mm. US images obtained (b) longitudinally and (c) transversely to the peroneus longus tendon show the two distal small fracture fragments (arrows in b) and a proximal fracture fragment (arrow in c) at the level of the lateral malleolus (LM). (d) Sagittal T1-weighted MR image (500/14) and (e) sagittal T2-weighted MR image (3500/90) show the most proximal fracture fragment (arrow) and the site of the tendon tear (*). Note the loss of normal fibrillar tendon appearance and the abnormal hypoechogenicity at the site of the tendon tear (*) in b and the fluid signal intensity at the site of the tear and the marrow replacement of the proximal fracture fragment, which has retracted to the level of the tibiotalar joint, in e.
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Figure 4d. Images in 42-year-old man with fracture of os peroneum and full-thickness peroneus longus tendon tear. (a) Lateral radiograph of foot shows diastasis of os peroneum fragments (arrows). The fragment separation was 80 mm, and the distance from the most posterior fragment to the calcaneocuboid joint was 75 mm. US images obtained (b) longitudinally and (c) transversely to the peroneus longus tendon show the two distal small fracture fragments (arrows in b) and a proximal fracture fragment (arrow in c) at the level of the lateral malleolus (LM). (d) Sagittal T1-weighted MR image (500/14) and (e) sagittal T2-weighted MR image (3500/90) show the most proximal fracture fragment (arrow) and the site of the tendon tear (*). Note the loss of normal fibrillar tendon appearance and the abnormal hypoechogenicity at the site of the tendon tear (*) in b and the fluid signal intensity at the site of the tear and the marrow replacement of the proximal fracture fragment, which has retracted to the level of the tibiotalar joint, in e.
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Figure 4e. Images in 42-year-old man with fracture of os peroneum and full-thickness peroneus longus tendon tear. (a) Lateral radiograph of foot shows diastasis of os peroneum fragments (arrows). The fragment separation was 80 mm, and the distance from the most posterior fragment to the calcaneocuboid joint was 75 mm. US images obtained (b) longitudinally and (c) transversely to the peroneus longus tendon show the two distal small fracture fragments (arrows in b) and a proximal fracture fragment (arrow in c) at the level of the lateral malleolus (LM). (d) Sagittal T1-weighted MR image (500/14) and (e) sagittal T2-weighted MR image (3500/90) show the most proximal fracture fragment (arrow) and the site of the tendon tear (*). Note the loss of normal fibrillar tendon appearance and the abnormal hypoechogenicity at the site of the tendon tear (*) in b and the fluid signal intensity at the site of the tear and the marrow replacement of the proximal fracture fragment, which has retracted to the level of the tibiotalar joint, in e.
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In the three patients who had undergone MR imaging, MR imaging also revealed the os peroneum fracture and complete tears of the peroneus longus tendon. The larger fracture fragments showed internal fluid signal intensity, while smaller fragments showed low signal intensity on images obtained with all sequences. In one of these three patients, a partial-thickness tear of the peroneus brevis tendon consisting of a longitudinal split was identified at MR imaging but not at retrospective review of the US images. In two patients who underwent surgery, surgical findings confirmed the os peroneum fracture and the full-thickness tear of the peroneus longus tendon that were seen at US.
Control Subjects
All control subjects had undergone foot radiography. Only two had undergone foot US, and one of these two subjects had also undergone MR imaging. Twenty-nine (67%) of the 43 radiographs obtained in the control subjects showed a unipartite os peroneum (Fig 5), and 14 (33%) showed a bipartite or multipartite os peroneum (Fig 6). The unipartite os peroneum typically appeared as a smooth well-corticated ossicle at the level of the calcaneocuboid joint and adjacent cuboid bone. When the os peroneum was bipartite, its fragments were separated by an average of 1.1 mm (range, 0.2–2.0 mm) (Table 2) (Fig 6).
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Figure 5a. Unipartite os peroneum in 47-year-old man. (a) Oblique radiograph, (b) sagittal T1-weighted MR image (500/14), and (c) US image obtained longitudinally to the peroneus longus tendon show the normal os peroneum (curved arrow) at the level of the calcaneocuboid joint (arrowheads) within the substance of the peroneus longus tendon (*). Note the posterior acoustic shadowing deep to the os peroneum (straight arrows) (proximal is to left and distal is to right) in c. The straight arrow in a indicates the peroneal tubercle of the calcaneus. C = calcaneus, Cu = cuboid bone, F = fibula, PL = peroneus longus tendon.
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Figure 5b. Unipartite os peroneum in 47-year-old man. (a) Oblique radiograph, (b) sagittal T1-weighted MR image (500/14), and (c) US image obtained longitudinally to the peroneus longus tendon show the normal os peroneum (curved arrow) at the level of the calcaneocuboid joint (arrowheads) within the substance of the peroneus longus tendon (*). Note the posterior acoustic shadowing deep to the os peroneum (straight arrows) (proximal is to left and distal is to right) in c. The straight arrow in a indicates the peroneal tubercle of the calcaneus. C = calcaneus, Cu = cuboid bone, F = fibula, PL = peroneus longus tendon.
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Figure 5c. Unipartite os peroneum in 47-year-old man. (a) Oblique radiograph, (b) sagittal T1-weighted MR image (500/14), and (c) US image obtained longitudinally to the peroneus longus tendon show the normal os peroneum (curved arrow) at the level of the calcaneocuboid joint (arrowheads) within the substance of the peroneus longus tendon (*). Note the posterior acoustic shadowing deep to the os peroneum (straight arrows) (proximal is to left and distal is to right) in c. The straight arrow in a indicates the peroneal tubercle of the calcaneus. C = calcaneus, Cu = cuboid bone, F = fibula, PL = peroneus longus tendon.
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Figure 6a. Bipartite os peroneum. (a) Oblique radiograph of foot in 38-year-old woman shows that the os peroneum (arrow) has smooth and sclerotic margins. The distance between the os peroneum fragments was 1.7 mm. (b) US image obtained longitudinally to the peroneus longus tendon in a 33-year-old man shows a bipartite os peroneum (arrow) in which each fragment has a smooth and hyperechoic surface.
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Figure 6b. Bipartite os peroneum. (a) Oblique radiograph of foot in 38-year-old woman shows that the os peroneum (arrow) has smooth and sclerotic margins. The distance between the os peroneum fragments was 1.7 mm. (b) US image obtained longitudinally to the peroneus longus tendon in a 33-year-old man shows a bipartite os peroneum (arrow) in which each fragment has a smooth and hyperechoic surface.
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The average distance from the unipartite os peroneum or from the proximal fragment of the bipartite os peroneum to the calcaneocuboid joint was 0.26 mm proximal to the joint (range, 7 mm proximal to 8 mm distal) on lateral radiographs and 0.08 mm distal to the joint (range, 9 mm proximal to 8 mm distal) on oblique radiographs. The same average calculated only in the control subjects with a bipartite os peroneum was 1.3 mm proximal to the joint (range, 6 mm proximal to 5 mm distal) on lateral radiographs and 0.9 mm proximal to the joint (range, 8 mm proximal to 7 mm distal) on oblique radiographs. At US, the unipartite os peroneum was echogenic and showed shadowing, while the bipartite os peroneum consisted of separate hyperechoic and shadowing bone fragments with well-defined margins that were in close proximity to each other (Fig 6). The one subject who had undergone MR imaging had a unipartite os peroneum that appeared as a well-defined fat-containing ossicle within the peroneus longus tendon (Fig 5b).
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DISCUSSION
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The results of this study show that os peroneum fragment separation of 6 mm or more or displacement of the proximal os peroneum fragment by 10 mm or more proximal to the calcaneocuboid joint on a lateral radiograph or by 20 mm or more proximal to the calcaneocuboid joint on an oblique radiograph is associated with full-thickness tear of the peroneus longus tendon, as it was in 100% (seven of seven) of our patients. Conversely, os peroneum fragment separation of 2 mm or less or proximal fragment displacement relative to the calcaneocuboid joint of 8 mm or less was not associated with a full-thickness peroneus longus tendon tear (in two of two patients) but was instead associated with a normal tendon, a partial-thickness tear, or tendinosis. In the control subjects, the location of the os peroneum ranged from 7 mm proximal to 8 mm distal to the calcaneocuboid joint (on lateral radiographs) and from 9 mm proximal to 8 mm distal to the joint (on oblique radiographs). Separation of the fragments of a bipartite os peroneum was 2 mm or less.
Fracture of the os peroneum can be complicated by an associated peroneus longus tendon tear. Violent contraction of the peroneus longus muscle in response to a sudden inversion or supination motion is the most common mechanism (3,5). Such a contraction can compress the os peroneum against the cuboid bone, resulting in fracture and tearing of the peroneus longus tendon (3,5). Peacock et al (3) have suggested that the presence of an os peroneum may predispose the tendon to an attritional tear at the distal junction of the ossicle and tendon. Mechanical stress at this site, in combination with the oblique course of the tendon in the cuboid groove, may lead to tendon failure (3,10). In our series, seven of nine patients with an os peroneum fracture had a complete peroneus longus tendon tear, and one patient had a partial tear at the site of the fractured os peroneum.
Acute tears, however, have been reported to occur in the presence of an os peroneum without the bone being involved in the tear (11). In those cases, tears were seen in the lateral calcaneus distally and in the cuboid groove (11). Underlying systemic diseases such as diabetes mellitus and rheumatoid arthritis, which were noted in three of our patients, as well as tendon degeneration secondary to advanced age and a prominent peroneal tubercle of the calcaneus, can also contribute to tendon injuries (5,6,11,12). In these situations, tears can also occur without an associated os peroneum fracture (12). Additionally, in our series, no tendon tear was seen in one of the nine patients with os peroneum fracture.
Radiographic differentiation between a fractured os peroneum and a multipartite os peroneum can be problematic. In an acute setting, the fracture margins appear relatively nonsclerotic and the bone fragments typically fit together like "pieces of a puzzle." Conversely, a rounded, smooth, sclerotic margin is consistent with multipartite os peroneum (4–6,8). It is conceivable that remodeling of fracture fragments over time could give the appearance of a multipartite os peroneum. Because of this potential difficulty, the importance of os peroneum fragment displacement should be emphasized. The presence of fragment separation of 6 mm or more was associated with full-thickness peroneus longus tendon tear. In our study, individuals with separation of 2 mm or less were either in the control group or had a normal peroneus longus tendon, partial-thickness tear, or tendinosis; none of these individuals required surgery. If uncertainty remains in the differentiation between os peroneum fracture and normal variation, US or MR imaging can be used for further evaluation.
With regard to US of os peroneum fracture, little has been described in the peer-reviewed literature (9). In the present study, the os peroneum fragments were hyperechoic at US in all cases. Posterior shadowing was present in large fragments; if the fracture fragments were extremely small, posterior shadowing was absent. A fractured os peroneum shows irregular margins, as opposed to the smooth, well-defined margins of a bipartite ossicle. Although separate bone fragments can be seen with both bipartite and fractured os peroneum, the combination of a large gap with irregular margins, as well as pain or discomfort with transducer pressure, suggest os peroneum fracture. Additionally, the presence of an associated peroneus longus tendon abnormality can improve diagnostic accuracy.
With regard to MR imaging, although several reports have described the imaging of peroneal tendon injuries, to our knowledge, none have described the appearance of os peroneum fractures (6,9,11). Peterson and Bancroft (13) reported a case of a surgically confirmed comminuted os peroneum fracture with an associated partial tear of the peroneus longus tendon. The peroneus longus tendon was assessed at MR imaging, but the appearance of the os peroneum fracture was not described (13). In the three patients in our study who had undergone MR imaging, the proximally displaced fracture fragment was easily identified given the large size of the fragment. However, the visualization of small distal bone fragments was difficult at MR imaging. The small size and low signal intensity of the fragments can cause difficulty in visualization. Bone marrow edema in the os peroneum can also complicate assessment of os peroneum fracture because the edema can be difficult to distinguish from the abnormally high signal intensity in adjacent abnormal peroneal tendons. In the three patients in our study, the os peroneum fracture was subjectively better evaluated with radiography and US.
It is important to recognize that the distracted proximal fragment of an os peroneum fracture may locate several centimeters (range, 8–75 mm) proximal to the calcaneocuboid joint. Knowledge of this displacement pattern is important not only in identifying the os peroneum fracture as the cause but also in avoiding confusing these fragments with unrelated bone fragments. For example, the location of the fracture fragment at the level of the tibiotalar joint in one patient may simulate an os trigonum. Similar displaced os peroneum fracture fragments could also be confused with fragments generated by other avulsion fractures of the foot and ankle.
There were several limitations to this retrospective study. These include a small sample size and potential selection bias (patients were identified through the clinical experience of one of the authors, and patients who seek medical attention and subsequently undergo imaging are often those with more marked symptoms). In addition, MR imaging and surgical results were not available for four (44%) of nine patients. In these patients, the diagnosis of os peroneum fracture versus multipartite os peroneum relied on radiographic image interpretation and patient history and symptoms. However, a consensus agreement by three radiologists was used to address this limitation. Given the consensus interpretation of the imaging findings, no interobserver variability could be calculated. Last, the accuracies of all of the imaging modalities used in this study for diagnosis of os peroneum fracture and peroneus longus tendon abnormality were not assessed.
In conclusion, os peroneum fragment separation of 6 mm or more suggests os peroneum fracture and an associated full-thickness peroneus longus tendon tear. Although separation of 2 mm or less may be seen with nondisplaced os peroneum fractures and bipartite os peroneum, nonsclerotic margins with a "piece of a puzzle" configuration suggest fracture. If there is uncertainty in the diagnosis, US or MR imaging may be considered for further evaluation, especially given a possible associated peroneus longus tendon abnormality.
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FOOTNOTES
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Authors stated no financial relationship to disclose.
Author contributions: Guarantors of integrity of entire study, all authors; study concepts and design, all authors; literature research, M.K.B., D.P.F.; clinical studies, M.K.B., D.P.F., J.A.J., D.A.J., J.G.C., M.T.v.H.; data acquisition and analysis/interpretation, all authors; statistical analysis, M.K.B., D.P.F., J.A.J.; manuscript preparation, definition of intellectual content, revision/review, and final version approval, all authors; manuscript editing, M.K.B., D.P.F., J.A.J., D.A.J.
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References
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ABSTRACT
INTRODUCTION
