Os peroneum is one of several accessory ossicles of the foot and ankle, located lateral to the cuboid within the distal peroneus longus tendon. Like most accessory ossicles, os peroneum is generally asymptomatic and detected incidentally on imaging, though its intratendinous location can result in its involvement in pathologies affecting the peroneus longus tendon. Painful os peroneum syndrome is a general term referring to conditions presenting as lateral midfoot pain associated with the os peroneum such as acute fracture, stress fracture associated with chronic repetitive trauma, contusion, avascular necrosis, or tendinosis, tenosynovitis, tendon tears and subluxation involving the peroneal tendons. Other causes of lateral midfoot pain also include ligamentous injury (talofibular or calcaneofibular), fractures of the base of the fifth metatarsal bone, anterior calcaneus or cuboid. Routine foot radiographs are a common first step in the assessment of lateral midfoot pain. Abnormalities in the morphology and location of the os peroneum can easily be identified on radiographs, specifically the oblique view, thereby elevating clinical suspicion for a peroneus longus tendon injury.
Accessory ossicles are common, generally asymptomatic, normal variants seen throughout the musculoskeletal system with several examples within the foot and ankle. Awareness of their presence and location is essential to limit misdiagnosis of these normal variants for pathology. The origin of many of these ossicles is debated in the literature. One proposed mechanism is from non-union of secondary ossification centers. However, a recent cadaveric study suggests that a precursor of the os peroneum is present in the fetal period in some patients. Additionally, experts have proposed that the os peroneum develops as a response to local stresses within the tendon. A fibrocartilaginous node is a fibrous, non-ossified structure analogous to the os peroneum appreciated on MRI as a focal T1/T2 hypointense oval structure within the peroneus longus tendon.
Ossicles and sesamoids are known to be associated with painful syndromes such as the os navicular, os trigonum, and hallux sesamoids. Likewise, the os peroneum can be symptomatic or associated with pathology. Painful Os Peroneum syndrome is described as lateral foot pain which may be attributed to several causes, including acute os peroneum fracture, peroneal tendon rupture, and chronic tendinopathy. Fractures of the os peroneum may be associated with peroneus longus tendon rupture in the acute setting or may be associated with chronic degeneration and tendinopathy. Acute fracture of the os peroneum is most commonly caused by strong contraction of the peroneus longus muscle with associated inversion or supination.
Among the numerous foot and ankle ossicles, os peroneum is relatively common, with prevalence figures ranging from 5% to 30%. A bilateral presentation may be noted in 60% of adults with os peroneum. Bipartite/multipartite appearance has been reported in up to 30% of cases. The peroneus quartus is a common accessory muscle seen along the posterolateral foot with an incidence ranging from 6% to 21%, also implicated in lateral foot/ankle pain in some patients.
Inversion injuries to the foot and ankle are a common cause for patient presentation to the emergency department. The ankle joint is the most commonly injured joint in the extremities, and lateral ankle sprains the most common traumatic joint disorder. In the absence of a fracture, most patients respond to conservative treatment; however, 10% to 20% will have continued pain and instability. When patients do not respond to conservative treatment and ligamentous instability is not evident on exam findings, peroneus tendon and os peroneum should be considered. Failure to consider alternative less common etiologies can lead to a delayed diagnosis.
Os peroneum is typically asymptomatic but can be fractured or displaced in the setting of a peroneus longus tendon tear, either from direct injury or indirectly from abrupt foot dorsiflexion or inversion. Os peroneum fractures may present in the setting of an acute traumatic injury or with chronic symptoms, occasionally without a known history of trauma. Presenting symptoms often include lateral foot/ankle pain, instability, swelling, and tenderness to palpation.
Physical examination findings related to peroneus longus tear include edema, tenderness, and weakness on eversion and plantar flexion. Evaluation for cavovarus alignment on standing examination of the patient may also be beneficial given a known association of peroneal tendon abnormalities with hindfoot varus.
Os peroneum is identified radiographically as an oval or round well-corticated accessory ossicle adjacent to the cuboid, near the calcaneocuboid joint, and is best seen on oblique views of the foot. A bipartite or multipartite os peroneum will demonstrate a fragmented appearance, but its well-corticated, smooth borders can help distinguish it from a fracture, which will demonstrate sharp, irregular, jagged margins. Fragment separation of greater than 6 mm is suggestive of os peroneum fracture with an associated tendon tear, while a separation of 2 mm or less can be seen in non-displaced fractures or normal variant bipartite os peroneum. Chronic stress injury to the os peroneum can manifest as an enlarged, sclerotic ossicle. An intact os peroneum which has changed in position relative to a comparison radiograph, proximal to the calcaneocuboid joint, may be seen in peroneal tendon rupture with retraction.
Distinguishing between a bipartite or multipartite os peroneum and a fracture can be difficult on radiographs, specifically in the absence of a prior comparison study. Since os peroneum is commonly bilateral radiographs of the contralateral foot can sometimes be enlightening. In challenging cases, CT can provide a more detailed evaluation of osseous margins allowing for the distinction between a non-displaced fracture versus a bipartite or multipartite os peroneum.
On ultrasound, the os peroneum, if present, may be seen as a hyperechoic structure with posterior shadowing, with only its outer cortex visible; it is best evaluated by following the peroneus longus tendon distal to the peroneal tubercle of the calcaneus. A fractured os peroneum may sonographically demonstrate an elongated appearance or irregular cortical contours. Associated pathology of the peroneus longus tendon may demonstrate fluid in the tendon sheath as well as a hypoechoic enlargement in tendinosis or a hypoechoic/anechoic cleft within the tendon in the case of a partial tear. A complete peroneus longus tear may show a disrupted tendon and/or retraction. Comparison with the contralateral tendon side can aid in the diagnosis of tendinosis. Additionally, peroneal subluxation associated with injury to the superior peroneal retinaculum can be demonstrated on ultrasound with provocative maneuvers.
Magnetic resonance imaging of the os peroneus will demonstrate marrow characteristics similar to the cuboid, with a hyperintense signal on T1-weighted and a hypointense signal on T2-weighted fat-suppressed sequences. An unossified fibrocartilaginous node in place of a fully-formed os peroneus is a deceptive mimic of a peroneus longus tendon tear, due to the node’s intermediate signal on all sequences. Injury of the os peroneus can result in edematous marrow changes, hypointense on T1 sequences and hyperintense on T2 fat-suppressed sequences, though the os peroneus itself may be difficult to visualize. Assessment of the cortex is best performed using radiography and/or CT. MR is useful in the assessment of associated peroneus longus tendon injury and may show hyperintense intratendinous signal on T1 and T2 fat-suppressed sequences, fluid within the tendon sheath, abnormal morphology (tendon enlargement, flattening, or chevron shape), partial/complete tendon discontinuity may also be noted.
A classification scheme has been developed to describe peroneus longus tendon injuries and helps describe/explain the radiographic appearance of the os peroneum. The tears have been divided into three subtypes: proximal to the os peroneum (type 1), at the level of the os peroneum (type 2), or distal to the os peroneum (type 3). Isolated tears of the peroneus longus tendon proximal to the os peroneum generally present as an os peroneum with normal morphology and position. Type 2 tears occurring at the os peroneum are normally associated with fractures or distraction of bi- or multipartite fragments. Distraction of the os peroneum fragments of greater than 6 mm is highly suspicious for complete disruption of the peroneus longus tendon. If comparison radiographs are available, new or increasing distraction of the fragments is concerning for tendon injury (partial or complete depending on the extent of distraction). When the tendon injury is distal to the os peroneum, it can be displaced proximally. The ossicle is rarely displaced proximally to the peroneus tubercle (< 2 cm) along the lateral margin of the calcaneus unless there is a strong contraction force of the tendon. It should be noted that in the acute setting the os peroneum may not be displaced or only minimally displace, however short interval follow-up may show new or increasing displacement (Figure) as the torn tendon continues to retract.
Treatment options are focused on the pathology of the peroneus longus tendon which is typically managed conservatively via immobilization, NSAIDs, and/or steroid injections and may be followed with a course of physical therapy. Surgical treatment may be performed after failure of conservative methods or in the event of intractable pain or impaired function. Early surgical intervention may also be considered in high-level athletes. Surgical options include primary tendon repair or grafting, excision of the fractured os peroneum if present with tendon repair or tenodesis, internal fixation of the fractured os.peroneum, or tendon debridement/tenosynovectomy  The type of surgical intervention will depend on the location and extent of tendon injury. One proposed algorithm for surgical management of peroneal tendon injuries classifies patients into three categories: type 1 both tendons intact; type 2 one tendon is torn, the other intact; type 3 both tendons are torn. In this algorithm, type 1 patients undergo primary repair, type 2 tenodesis, and type 3 tendon transfer.
A presenting symptom of lateral foot pain and/or ankle instability elicits a broad range of possible etiologies to include peroneal tendinopathy or subluxation, lateral ankle ligamentous injury (including the anterior and posterior talofibular and the calcaneofibular ligaments), sinus tarsi syndrome, osseous fractures, and peroneal neuropathy, among numerous other considerations. Peroneal tendon injury may be overlooked in favor of a lateral ankle ligament injury and should be considered in a patient presenting with persistent pain. Magnetic resonance imaging provides an optimal anatomic evaluation of the lateral ankle structures and thus can be particularly helpful in elucidating the etiology.
An os peroneum fracture may be confused with a bipartite/multipartite os peroneum as previously explained. An os vesalianum and os cuboideum secundarium are other accessory ossicles which can be found near the expected location of an os peroneum. In the setting of a distal peroneus longus tendon rupture the os peroneum with extreme proximal displacement it can be confused with an os trigonum. A fracture of the adjacent cuboid or an avulsion fracture from the base of the fifth metatarsal is additional considerations.
Insufficient high-quality evidence exists for determining the efficacy of conservative and surgical therapies of peroneal tendon injuries, though select studies of post-surgical outcomes showed figures such as 87% of patients returning to sporting activities within 3.5 months of surgery, as well as 91%, achieving normal/moderate peroneal strength.
Patients with suspected or diagnosed peroneal tendon injuries may experience chronic lateral foot/ankle pain and instability with lack of treatment, or after failure of conservative therapies. Delays in diagnosis are not uncommon, with one study reporting the duration of symptoms persisting from 7 to 48 months before the correct diagnosis was made.
An untreated os peroneus fracture can result in degeneration and tearing of the peroneus longus tendon related to chronic frictional forces. In patients with repeat ankle injuries in the setting of a peroneal tendon rupture, serial radiographs may demonstrate the migration of the os peroneum if intact, or if fractured, diastasis of the fracture fragments.
Intraoperative findings during repair of a primary peroneus longus tendon injury and associated os peroneum abnormality often include the discovery of additional tendinous and/or osseous pathologies, such as peroneus brevis tear, hypertrophied peroneal tubercle of the calcaneus, and subluxation or dislocating peroneal tendons.
Insufficient data exist regarding the efficacy of surgical treatments. However, reported postoperative complications of repair of peroneal tendon rupture include superficial wound infection, wound dehiscence, repair failure, sural neuritis, adhesive tendinitis, and chronic regional pain syndrome.
Clinical or radiologic suspicion for a peroneal tendon injury with or without an os peroneus fracture may warrant consultation with an Orthopedic surgeon and/or advanced imaging (MRI or ultrasound), specifically in the setting of patients that failed conservative therapy. Operative outcomes are based on level-IV and level-V studies,, and thus, an interprofessional approach individualized to the patient’s case and involving a primary care provider, an orthopedic surgeon, and a radiologist may provide the most optimal course of treatment.
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