Your diagnosis? The case: n radiologic case study. For answer see page 807

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1 Enhance your diagnostic skills with this test yourself monthly column, which features a radiograph and challenges you to make a diagnosis. The case: 45-year-old man presented with acute lateral hindfoot pain and swelling and the A inability to bear weight after an inversion injury that occurred while he was descending stairs. Figure: Internal oblique (A) and lateral (B) radiographs of the left ankle. A B Your diagnosis? For answer see page 807 OCTOBER 2013 Volume 36 Number

2 Section Editor: Terrence C. Demos, MD & Laurie M. Lomasney, MD Diagnosis: Migration of the Os Peroneum Associated With Rupture of the Peroneus Longus Tendon Talentshia Vethanayagamony, MD; Himanshu Patel, MD; Laurie M. Lomasney, MD; Terrence C. Demos, MD; Francis J. Rottier, DPM Answer to Radiologic Case Study Case facts appear on page xxx year-old man presented A with acute lateral hindfoot pain and swelling and the inability to bear weight after an inversion injury that occurred while he was descending stairs. Initial internal oblique and lateral radiographs of the left ankle revealed a bony fragment lateral to the distal/plantar aspect of the calcaneus, which was possibly an avulsion fracture from an unknown donor site (Figure 1). Subsequently, computed tomography (CT) images showed proximal migration of an intact os peroneum due to a full-thickness peroneus longus tendon tear (Figure 2), which was confirmed by magnetic resonance imaging showing a complete tear of the peroneus longus with retraction of the intact os peroneum (Figure 3). Acute lateral plantar foot pain has various etiologies, including painful os peroneum syndrome, a term coined by Sobel et al. 1 Pain with this syndrome can be either acute, following an os peroneum fracture or diastasis of a multipartite os peroneum, or chronic, due to a healed or healing os peroneum fracture, diastasis of a multipartite os, or regional mechanical pathology. Clinical manifestations related to the os peroneum often coincide with associated peroneus longus tendinopathy. Delayed diagnosis of os peroneum pathology and associated The authors are from the Department of Radiology (TV, HP, LML, TCD) and the Department of Orthopedics (FJR), Loyola University Medical Center, Maywood, Illinois. The authors have no relevant financial relationships to disclose. Correspondence should be addressed to: Terrence C. Demos, MD, Department of Radiology, Loyola University Medical Center, 2160 S First Ave, Maywood, IL (tdemos@lumc.edu). doi: / A Figure 1: Internal oblique (A) and lateral (B) radiographs of the left ankle showing an ossicle (arrow) lateral to the distal/plantar calcaneus with surrounding soft tissue swelling. peroneus longus tendon injuries can result in progressive deterioration, including peroneal compartment syndrome and ankle instability. 1 1B Anatomy The os peroneum is one of the most common accessory ossicles of the foot. This round or oval-shaped sesamoid is within the substance of the distal peroneus longus tendon at the level of the calcaneocuboid joint, 1,2 has varying degrees of ossification, and can be bipartite. The mature ossicle is seen on radiographs in 5% to 20% of normal patients, with a slight predominance in men. 3 Four tethers anchor the sesamoid to the adjacent anatomical structures: the peroneus longus tendon, the fifth metatarsal tubercle, the plantar fascia, and the cuboid bone. 4 The peroneus longus tendon passes along the lateral wall of the calcaneus to the cuboid bone. There, the tendon courses plantar in the bony cuboid tunnel to insert onto the lateral plantar surface of the first cuneiform and the base of the first OCTOBER 2013 Volume 36 Number

3 2A 2B 2C 2D Figure 2: Cephalad axial computed tomography image showing a well-corticated triangular bony fragment lateral to the calcaneus (arrow) without fracture and the proximally retracted os peroneum (A). Caudal computed tomography images showing triangular fluid collection (white arrow pointing to the fluid-filled empty sheath) posterior to the cuboid along the expected course of the peroneus longus tendon sheath (B) with amorphous soft tissue anteromedial to the os peroneum (arrow head) at the stump of peroneus longus tendon (C). Lateral 3-dimensional computed tomography surface reconstruction showing the retracted os peroneum lateral to the calcaneus with well-corticated margins (D). 3 Figure 3: Coronal proton-density weighted magnetic resonance image of the left ankle showing the retracted os peroneus (arrow). metatarsal. 5 The primary action of the peroneus longus muscle/ tendon group is plantar flexion and eversion of the first ray of the foot with simultaneous plantar flexion and eversion of the ankle. It is also an important stabilizer of the ankle joint. 5 Pathology Diseases of the peroneal tendons, including tendonitis, tenosynovitis, partial rupture, and complete rupture, are frequently underdiagnosed causes of lateral ankle pain and instability. Although uncommon, an os peroneum may predispose the lateral ankle to inflammation and peroneus longus pathology. 5 Peacock et al 6 suggested that the presence of an os peroneum may predispose the tendon to wear at the distal junction of the ossicle and tendon, where the tendon slides into the groove of the cuboid. Mechanical stress on the tendon at the site of the os peroneum, in combination with the oblique course of the tendon in the cuboid groove, can lead to underlying tendon malfunction and chronic dysfunction. 6,7 Furthermore, other studies have shown that a fractured os peroneum is associated with a complete or partial peroneus longus tendon tear. A direct blow or indirect stress, such as violent contraction of the peroneus longus muscle in response to a sudden inversion or supination motion, are the most common mechanisms of acute injury Such a contraction can compress the os peroneum against the cuboid bone, resulting in subsequent sesamoid fracture and acute tearing of the peroneus longus tendon. 6,9 Brigido et al 2 repoted that 7 of 9 patients with an os peroneum fracture had a complete peroneus longus tendon tear and 1 patient had a partial tear at the site of the fractured os peroneum. Imaging Radiographs Three views of the foot (anteroposterior, lateral, and internal oblique) and 3 views of the ankle (anteroposterior, lateral, and mortise) are used for initial evaluation of foot and ankle pain. In addition to common fracture patterns, radiographs can be used to evaluate bone density, alignment, and morphology, as well as variants including accessory ossicles. Altered position of the os peroneum provides an important clue to soft tissue pathology. In cases of complete peroneus longus ruptures distal to the ossicle, radiographs show displacement and migration of an intact os peroneum proximal to the calcaneocuboid joint, 8,13 similar to the findings in the current patient. The proximal tendon segment is under continued muscular tension and causes retraction of the os peroneum. 13,14 The retracted ossicle may be relocated up to several centimeters proximal to the calcaneocuboid joint. Fracture of the ossicle can also be seen on radiographs and may be associated with significant tendon pathology. In a study of os peroneum fracture reported by Brigido et al, 2 separation of fracture fragments 6 mm or larger was highly suggestive of a full-thickness tear of the peroneus longus tendon (Figure 4). They also observed that separation of fracture fragments less than 2 mm was found with both nondisplaced os peroneum fractures and bipartite os peroneum (Figure 5) ORTHOPEDICS Healio.com/Orthopedics

4 Figure 4: Internal oblique radiograph showing distracted fragments of the multipartite os peroneum (arrow) indicative of tendon injury. 4 Figure 5: Internal oblique radiograph of the left foot showing a normal multipartite os peroneum with juxtaposed fragments in a middle-aged adult. 5 Figure 6: Longitudinal ultrasound of the right foot of a young adult showing rounded hyper-reflective cortex of the os peroneum (arrow) within the peroneus longus tendon (PL) lateral to the calcaneal (Calc)-cuboid (Cub) joint. 6 Ultrasound Many studies have demonstrated the value of ultrasound for evaluation of lateral foot pain in patents with a suspected pathology of an os peroneum or an injury of the peroneal tendons. Ultrasound can be used to identify an incompletely mineralized accessory ossicle or a fractured ossicle. Sofka et al 15 noted a higher detection rate of os peroneum with ultrasound compared with radiographs. This was thought to be secondary to variable ossification of the accessory ossicle, with different degrees of cartilage and bone allowing ultrasound to be more sensitive in detecting structures with differing acoustic penetration properties. As for the peroneal tendons, ultrasound evaluates the location, morphology, and echogenicity of the peroneal tendons throughout the entire course of the tendons. The normal tendon has a uniform hyperechoic fibrillar appearance. This contrasts with tendinosis and tears where hypoechogenic swelling, heterogeneous echotexture with hyperreflective foci (calcification), or cleft or discontinuity of the tendon may exist. 15,16 Tendon retraction also indicates a complete tear. Larger rounded intratendinous hyper-reflective foci with shadowing at the lateral wall of the distal calcaneus indicate the presence of an os peroneum (Figure 6). Magnetic Resonance Imaging Magnetic resonance imaging is a valuable modality for evaluation of tendon pathology, including pathologies related to an os peroneum. Normal tendons show consistent diameter with signal void on all pulse sequences. Abnormal findings in peroneus longus tendon tears include increased intrasubstance signal on proton density and T2- weighted images (Figure 7) and morphologic inconsistencies of the tendon, in some cases with frank tendon discontinuity. 17 Secondary findings, such as bone marrow edema of the lateral calcaneal wall or cuboid wall, can be seen at sites where friction can affect the tendon or 7A Figure 7: Axial proton density weighted magnetic resonance image at the talar dome level showing thickening of the peroneus longus tendon (arrow) with intrasubstance intermediate signal and intrasubstance tear (A). Sagittal T2-weighted magnetic resonance image confirming focal thickening and intrasubstance signal within the peroneus longus tendon (arrow) and an intrasubstance tear (B). adjacent to the site of the tendon tear (Figure 8). 18 An os peroneum may be difficult to identify if it is composed of fibrocartilage, resulting in it sharing signal void characteristics with the peroneus longus tendon. However, if sufficient marrow content is available, the ossicle will be clearly distinguished due to high T1- weighted signal characteristics of fat (Figure 9). Alternatively, high T2-weighted signal abnormalities of osteitis or fracture may enhance visualization of the ossicle (Figure 10). 7B Treatment Peroneus longus tendon rupture with a fractured or displaced os peroneum can be conservatively treated with nonsteroidal anti-inflammatory drugs and immobilization by casting followed by physical therapy. 6 Surgical treatment is indicated if conservative therapy fails or if dysfunction or debilitating pain affect the patient s quality of life. Surgical management includes tendon debridement and tenosynovectomy with primary repair of a torn peroneus longus tendon. 1,17 Excision of an os peroneum can also be helpful. Surgical treatment of peroneal tendon injuries has been shown to maintain dynamic ankle function and mobility 18 and can potentially reduce hallux OCTOBER 2013 Volume 36 Number

5 Figure 8: Sagittal T2-weighted magnetic resonance image of the ankle of a middle-aged adult showing bone marrow edema (arrow) at the lateral border of the calcaneus in a case of peroneal tendinopathy with os peroneum (not shown). Figure 9: Oblique axial proton density weighted magnetic resonance image of the right hindfoot of an older adult showing fatty marrow signal of the os peroneum (yellow arrow) and abnormal peroneus longus tendon (white arrow) with intrasubstance intermediate signal due to tendinopathy. dysfunction, including development of elevatus. 19 Management of these injuries needs to be tailored to individual patients based on chronicity vs acuity of the lesion, as well as each patient s desired level of activity Figure 10: Coronal fat-saturated, T2- weighted magnetic resonance image of the hindfoot in an older man showing marked bone marrow edema of the os peroneum (arrow) with mild marginal soft tissue edema, supporting inflammatory/mechanical changes of symptomatic os peroneum. Conclusion The os peroneum is a common accessory ossicle of the foot. On occasion, the ossicle may contribute to or share in the pathology of the host tendon: the peroneus longus tendon. Radiographs can provide valuable diagnostic clues for peroneus longus tendon injuries when the os peroneum is displaced or fractured. Ultrasound and magnetic resonance imaging are secondary modalities that provide more in-depth interrogation of equivocal or recalcitrant cases. Although most cases are managed conservatively, surgical intervention can enhance the outcome in some patients. References 1. Sobel M, Pavlov H, Geppert MJ, Thompson FM, DiCarlo EF, Davis WH. Painful os peroneum syndrome: a spectrum of conditions responsible for plantar lateral foot pain. Foot Ankle Int. 1994; 15: Brigido MK, Fessell DP, Jacobson JA, et al. Radiography and US of os peroneum fractures and associated peroneal tendon injuries: initial experience. Radiology. 2005; 237: Hadley HG. Unusual fracture of sesamoid peroneum. Radiology. 1942; 38: Manners-Smith T. A study of the cuboid and os peroneum in the primate foot. J Anat Physiol. 1908; 42: Wang, X, Rosenberg ZS, Mechlin MB, Schweitzer, ME. Normal variants and diseases of the peroneal tendons and superior peroneal retinaculum: MR imaging features. Radiographics. 2005; 25: Peacock KC, Resnick EJ, Thoeder JJ. Fracture of the os peroneum with rupture of the peroneus longus tendon. Clin Orthop Relat Res. 1986; (202): Rademaker J, Rosenberg ZS, Delfaut EM, Cheung YY, Schweitzer ME. Tear of the peroneus longus tendon: MR imaging features in nine patients. Radiology. 2000; 214: Thompson F, Patterson A. Rupture of the peroneus longus tendon. Report of three cases. J Bone Joint Surg Am. 1989; 71: Cachia VV, Grumbine NA, Santoro JP, Sullivan JD. Spontaneous rupture of the peroneus longus tendon with fracture of the os peroneum. J Foot Surg. 1988; 27: Bessette BJ, Hodge JC. Diagnosis of the acute os peroneum fracture. Singap Med J. 1998; 39: Mains DB, Sullivan RC. Fracture of the os peroneum. A case report. J Bone Joint Surg Am. 1973; 55: Pessina R. Os peroneum fracture. A case report. Clin Orthop Relat Res. 1988; (227): Bashir WA, Lewis S, Cullen N, Connell DA. Os peroneum friction syndrome complicated by sesamoid fatigue fracture: a new radiological diagnosis? Skelet Radiol. 2009; 38: Tehranzadeh J, Stoll DA, Gabriele OM. Case report 271. Posterior migration of the os peroneum of the left foot, indicating a tear of the peroneal tendon. Skelet Radiol. 1984; 12: Sofka CM, Adler RS, Saboeiro GR, Pavlov HP. Sonographic evaluation and sonographicguided therapeutic options of lateral ankle pain: peroneal tendon pathology associated with the presence of an os peroneum. HSS J. 2010: 6: Waitches GM, Rockett M, Brage M, Sudakoff G. Ultrasonographic-surgical correlation of ankle tendon tears. J Ultrasound Med. 1998; 17: Jeong SO, Kim YH, Kim SK, Kim MW. Painful os peroneum syndrome presenting as lateral plantar foot pain. Ann Rehabil Med. 2012; 36: Saxena A, Wolf SK. Peroneal tendon abnormalities: a review of 40 surgical cases. J Am Podiatr Med Assoc. 2003: 93: Bohne WH, Lee KT, Peterson MG. Action of peroneus longus tendon on the first metatarsal against metatarsus primus varus force. Foot Ankle Int. 1997; 18: ORTHOPEDICS Healio.com/Orthopedics