Imaging 15:242-269 (2003)
© 2003 The British Institute of Radiology
Imaging the ankle and foot
S Ostlere, FRCR
Nuffield Orthopaedic Centre and Oxford Radcliffe Hospital, Oxford, UK
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Summary
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- Plain films are useful as an initial screen test for diffuse ankle and foot pain.
- MRI is the most suitable technique for global assessment of bone and soft tissues for persistent pain following injury.
- Ultrasound should be reserved for specific focal soft tissue conditions and when dynamic information is required.
- Ultrasound is an excellent technique for guiding injections.
Imaging plays a major role in the management of ankle and foot problems. Most conditions are assessed by plain films alone. MRI is an excellent technique for those cases where the diagnosis is uncertain as it can exclude most clinically relevant pathologies. Ultrasound is an excellent tool for imaging focal soft tissue abnormalities. CT is occasionally useful when bony detail is required. Bone scintigraphy has a limited role and has been largely replaced by MRI in many centres. The main reasons for referral are swelling and pain. Many conditions of the ankle and foot are related acute or repetitive trauma.
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Diffuse and focal swelling ankle and foot
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Diffuse ankle and foot swelling is usually dependent on or secondary to venous insufficiency or cardiac failure. The diagnosis is usually clear and imaging is not required. However, if unexplained swelling is isolated to the ankle or foot then imaging is often warranted. MRI is the technique of choice as the joints, bones and soft tissues can all be assessed. Plain films are usually performed as they are readily available and may provide more specific information. Inflammatory arthropathy, infection and reflex sympathetic dystrophy can all result in diffuse swelling. MRI will determine whether the primary pathology is related to joint, bone or soft tissue and will provide an accurate assessment of the extent of disease.
Focal swellings of the ankle and foot usually require further investigation. The differential diagnosis is similar to other periarticular sites. Tumours that have a propensity for the ankle and foot are ganglia, lipomas, haemangiomas, soft tissue chondromas, plantar fibromas and synovial sarcomas. Ultrasound is the most effective initial test as it can quickly differentiate cystic from solid masses. Most masses turn out to be ganglia and usually no further imaging is required. Ultrasound will also detect tendon pathology such as tenosynovitis or focal degeneration that may be associated with ill-defined swelling. Non-specific solid tumours require MRI to obtain further diagnostic information and staging. Synovial sarcomas often contain flecks of calcification which may be evident on the plain film before there is a palpable lump (Figure 1
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Figure 1. Synovial sarcomas. (a) Plain film and (b) CT showing a calcified soft tissue mass in the mid foot.
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A mass that is specific to the foot is the plantar fibroma (plantar fibromatosis). This tumour of the plantar fascia presents as a painful nodule in the sole of the foot. They may be single or multiple and are bilateral in 25% of cases. Although difficult to treat they are benign unlike the locally aggressive fibromatosis of soft tissue. The condition is related to Dupuytren's contracture (palmar fibromatosis) and the two conditions may coexist. On imaging the lesions are seen to be arising from the plantar fascia usually its mid portion. On ultrasound the reflectivity is variable but the lesion is usually hypoechoic. Doppler is negative in 90% of cases [1]. On MRI plantar fibromas usually return intermediate/low signal on all sequences reflecting the fibrous nature of the lesion (Figure 2). More aggressive cases may show some increase signal on T2 weighted images and some enhancement following intravenous gadolinium [2].
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Figure 2. Plantar fibroma. (a) Ultrasound shows a small well definded hypoechoic mass (arrow) related to the plantar fascia (open arrow). (b) T1 weighted sagittal image and (c) transverse T2 weighted image of a larger lesion showing a mass related to the plantar fascia. The mass returns intermediate signal intensity on T1 and low signal intensity on T2 weighted images.
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Masses arising from the bone are less common. Osteochondromas may present in children as a bony lump arising from the tibial metaphyses. The ankle is a typical site for the rare Trevor's disease (dysplasia epiphysealis hemimelica). This dysplasia is essentially one or more osteochondroma-like lesions arising from the tibial epiphysis and not infrequently, the talus. Plain film findings are typical (Figure 3).
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Figure 3. Trevor's disease. (a) Typical ossified mass in the medial portion of the epiphysis (arrow). (b) With time the ossification merges with the ossification centre forming a bony protuberance.
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Ankle pain following injury
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The outcome following fracture of the ankle depends on the degree of articular cartilage damage and the degree of anatomical reduction. Secondary osteoarthritis is a common cause of ankle pain following some intra-articular fractures. Complex imaging is rarely required although the radiologist may be asked to inject joints with local anaesthetic and/or steroid for diagnostic or therapeutic purposes.
Sprains
Acute injuries to the lateral ligaments of the ankle (ankle sprain) have a good outcome and in the majority of cases heal without long term consequences. In a small percentage of cases where symptoms persist beyond the expected timeframe of around 6 weeks, imaging may be helpful. Residual instability, osteochondral defects, missed fractures and secondary osteoarthritis are the most common causes of persistent pain. Fractures may be invisible on the original radiographs and may delay the expected period of recovery of an injury treated as a sprain. Osteochondral fractures of the lateral aspect of the talar dome occur with inversion injuries (either ankle sprains or malleoli fractures) and may be invisible or overlooked on the original films. The defect may be painful if it does not heal or form a loose body.
Plain radiographs and MRI are the techniques of choice in investigating post-traumatic pain. X-rays may show osteoarthritis, evidence of missed bony or osteochondral fractures and loose bodies but are less sensitive than MRI. Ultrasound is useful in assessing the tendons and ligaments but is not indicated if there is suspected intrarticular pathology. CT provides excellent bony detail but is rarely required providing there is sufficient access to MRI.
Fractures
MRI will identify missed undisplaced fractures, which are seen as linear defects surrounded by oedema (Figure 4). The acute talar osteochondral fracture is due to a shearing force as the lateral corner as the talar dome impacts against the fibula with the ankle in the inverted position. The lesion is usually clearly identified on plain film if displaced or, as is often the case, inverted. However plain films may be normal or show subtle abnormality which may be overlooked (Figure 5). MRI will reveal the defect which will be seen as a linear low signal line surrounded by oedema (Figure 6). Trabecular microfracture or bone bruising may occur at the sites of bony impaction following injury. On MRI these lesions appear as subchondral oedema. The mechanism of injury can often be determined from the relative position of these lesions (Figure 7) [3].
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Figure 4. Missed fracture. (a) T1 weighted and (b) short tau inversion recovery image showing an undisplaced fracture of the distal fibula (arrows) that was not apparent on the plain radiograph.
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Figure 5. Subtle osteochondral fracture of the lateral corner of the talar dome. (a) The lesion cannot be diagnosed on the frontal view. (b) The fracture is seen on the oblique view (arrow).
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Figure 6. Persistent pain following inversion injury. Short tau inversion recovery coronal image. There is an undisplaced fracture of the lateral corner of talar dome (arrows) surrounded by oedema.
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Figure 7. Subchondral bone bruise. There is a chondral defect (arrow) and oedema affecting the medial aspect of the talar dome and the mid-portion of the tibial plateau (open arrow). The pattern of bony oedema indicates recent inversion injury with the medial corner of the tilted talus impacting the tibial plafond.
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Ankle ligaments
The term "ankle sprain" refers to a tear one or more of the lateral ligaments. The most common lesion is a tear of the anterior talofibular ligament. In more severe injuries the calcanofibular ligament will also rupture. The third lateral ligament, the posterior talofibular ligament, is a strong stabilizer that is rarely torn. The decision to X-ray depends on the clinical findings. The main purpose of the plain film to exclude malleolar fractures. Usually the ligaments heal satisfactorily and stability is restored but chronic instability occurs in the minority of patients.
The integrity of the ligaments can be assessed indirectly by performing plain radiograph stress views or directly by ultrasound or MRI. Often MRI has the advantage of being sensitive for associated abnormalities such as talar osteochondral fractures. The normal ligaments are approximately aligned in the axial plane with the foot in the plantar flexed position and therefore can be assessed satisfactorily in this plane. Coronal and axial oblique images have also been advocated. On ultrasound the anterior and calcaneofibular ligaments are seen as echogenic bands. The normal ligaments are consistently identified on MRI and ultrasound. The posterior talofibular ligament is difficult to see on ultrasound on account of its coronal orientation, but imaging of this structure is rarely required.
There is little to be gained from imaging acute tears as sprains are treated conservatively. They are readily diagnosed on MRI and ultrasound. Imaging is more useful in chronic instability to assess the need for stabilization surgery. Although stress views (forced inversion for calcaneofibular ligament and forced anterior draw for anterior talofibular ligament) are still used to determine if the ligaments are incompetent, there is a large variability in measurements in both the injured and uninjured ankle [4]. Ultrasound and MRI can accurately diagnose chronic rupture of the ligaments [5, 6]. If there is evidence of previous healed tear the ligaments are often seen to be thickened (Figure 8). Incompetent ligaments may appear attenuated or are simply absent (Figure 9). Another ligament that is torn in twisting injuries is the anterior tibiofibular ligament. This ligament which forms the anterior part of the syndesmosis can be accurately assessed by MRI or ultrasound [5, 7] although the usefulness of these techniques over clinical assessment and plain films is uncertain. Sprains of the syndesmosis take longer to recover than the more common lateral ligament injury [8].
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Figure 8. Healing tear of the anterior talofibular ligament. T2 weighted image showing an abnormally wide ligament (arrows).
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Figure 9. Chronic tear of the calcanofibular ligament. Fluid is seen in the expected position of the ligament (arrows) deep to the peroneal tendons.
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Sinus tarsi syndrome
The sinus tarsi syndrome is another reported cause of post traumatic pain. The sinus tarsi is situated on the lateral aspect of the ankle between the talus and calcaneus anterior to the posterior subtalar joint. Patients typically have tenderness laterally over the sinus tarsi and a history of inversion injury. The condition is not well-understood but is thought to represent pain associated with fibrosis, inflammatory tissue or synovitis within the sinus tarsi. The condition is often related to tears of one or other of the two ligaments that occupy the sinus: the cervical and intertarsal talocalcaneal ligaments. On MRI variable abnormal signal intensity representing these changes is seen to replace the normal fat within the sins tarsi (Figure 10). Tears of the sinus ligaments can sometimes be detected. Injection of the area with local anaesthetic and steroid may help alleviate symptoms [9].
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Figure 10. Persistent pain following trauma due to sinus tarsi syndrome. (a) T1 weighted sagittal image showing normal sinus tarsi containing the intertarsal ligament (arrow) and fat. (b) T1 weighted sagittal image in a patient with sinus tarsi syndrome. The sinus is filled with abnormal tissue returning low signal intensity material (arrows).
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Ankle pain related to repetitive trauma
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There are a variety of traumatic conditions not clearly related to a specific traumatic event that may cause pain around the ankle. Plain films often provide a diagnosis such as osteochondritis dissecans, or stress fracture. MRI is the preferred second line investigation if the plain film is normal or does not provide sufficient information. CT is occasionally useful if fine bony detail is required.
Stress fractures
Stress and insufficiency fractures at the ankle typically occur at the distal fibula just proximal to the syndesmosis, the medial malleolus, and the calcaneus [10, 11]. These lesions may be invisible or difficult to appreciate on plain film. MRI is highly sensitive and quite specific. The typical appearance is a linear low signal line surround oedema [12]. CT is also specific and will show a linear sclerotic lesion and periosteal reaction (Figures 11–13).
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Figure 11. Typical stress fracture of the distal fibula in a long-distance runner. Periosteal reaction is seen at the site of the fracture (arrow).
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Figure 12. Typical stress fracture of the medial malleolus. (a) Short tau inversion recovery coronal image showing focal increase signal at the base of the medial malleolus surrounding a small focal low signal lesion extending to the articular surface (arrow). (b) Axial T1 weighted image showing the linear fracture line (arrows). (c) Corresponding CT slice confirms the sclerotic fracture line (arrows).
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Figure 13. Stress fracture of the calcaneus. (a) T1 weighted image shows short linear defect (arrow). (b) Corresponding short tau inversion recovery image shows the focal oedema around the lesion.
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Osteochondritis dissecans
Osteochondritis dissecans is a term to describe subchondral osteonecrosis of the medial corner of the talar dome. The overlying cartilage is usually involved and loose bodies may develop. The aetiology is unclear but the condition is thought to due to repetitive trauma. Subchondral lucency or focal sclerosis may be seen on plain film but abnormalities may be subtle. Appearances on MRI will vary depending on the age of the lesion. A focal subchondral fragment of necrotic bone may be seen. MRI can help to differentiate unstable from stable lesions. Unstable lesions will usually have at least one of the following MRI features: a high signal intensity line or cystic area beneath the lesion, a high signal intensity line through the articular cartilage, or a focal articular defect (Figures 14, 15) [13]. Focal subchondral cystic change, often surrounding bony oedema, is a common finding in chronic cases (Figure 16). Although detail of the articular surface and detection of loose bodies may be assessed on standard MRI, CT arthrography or MR arthrography is more sensitive [14]. In practice this is rarely required as symptomatic lesions usually require arthroscopy.
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Figure 14. Osteochondritis dissecans of the talar dome. (a) T1 weighted sagittal image shows low signal intensity lesion at the medial talar dome (arrow). (b) Short tau inversion recovery sagittal image shows high signal line under the lesion (arrows) indicating instability.
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Figure 15. Osteochondritis dissecans of the talar dome. (a) T1 weighted coronal image shows a focus of low signal at the medial talar dome (arrow). (b) T2 gradient echo sagittal image shows a defect in the cartilage and small cysts under the lesion (arrow) suggesting that this is an unstable lesion.
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Figure 16. Osteochondritis dissecans. (a) Coronal T1 weighted image showing subchondral low signal at the medial corner of the talar dome (arrow). (b) Coronal short tau inversion recovery shows small subchondral cysts surrounded by oedema. The overlying cartilage appears to be intact.
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Impingement syndromes
Impingement syndromes are quite common around the ankle. They are chronic traumatic injuries where soft tissue is repeatedly trapped between the bones of the ankle joint during ankle movement. Anterolateral impingement is usually seen following inversion injury with tear of the anterior talofibular ligament. On arthroscopy a mass of hypertrophied synovium and fibrosis is seen in the anterolateral gutter. In chronic cases there may be hyalinization resulting in a meniscoid type lesion. If imaging is deemed necessary then MR arthrography is the technique of choice. The features of impingement are nodularity or irregularity of the contour of the anterolateral capsule representing the hypertrophied tissue seen on arthroscopy. Although there is excellent correlation between MRI and arthroscopic findings, abnormalities are frequently found with both techniques in patients without clinical symptoms of anterolateral impingement. The usefulness of imaging is also limited by the fact that 90% of cases of impingement diagnosed on clinical criteria alone will have abnormality found at arthroscopy [15, 16].
Anteriomedial impingement is less common and not well documented in the literature. Similar findings to those seen in anteriolateral impingement may be found in the anteromedial gutter on MR arthroscopy.
Anterior impingement is commonly seen in soccer players and is thought to be due to a chronic traction injury secondary to repetitive plantar flexion and/or repetitive impingement in dorsiflexion. On plain films a bony spur is seen arising from the anterior aspect of the distal tibial articular cartilage sometimes with an accompanying spur on the dorsal aspect of the talus. These radiographic signs are also frequently seen in asymptomatic athletes. MRI, which is rarely required, will show additional synovitis and bony oedema adjacent to the site of impingement (Figure 17). Posterior impingement is due to repetitive plantar flexion and is seen in soccer players and ballet dancers. The impingement may be purely soft tissue or involve a prominent posterolateral talar process or os trigonum. Plain films are of limited value. MRI features include chronic synovitis of the posterior joint recess, oedema with or without fragmentation of the os trigonum or posterolateral process and tenosynovitis of the adjacent flexor hallucis longus tendon [17]. Physiotherapy, local injection of steroid and arthroscopic debridement are effective therapies (Figure 18).
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Figure 17. Anterior impingement. (a) Lateral plain film showing spurs arising from the anterior lip of the distal talus and the dorsal aspect of the talus (arrows). (b) Sagittal short tau inversion recovery image shows a spur arising from the anterior aspect of the distal tibia. Note the typical intra-articular position of the spur (arrow). There is also posterior subluxation of the talus indicating instability with incompetent anterior talofibular ligament.
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Figure 18. Posterior impingement (os trigonum syndrome). (a) T1 weighted sagittal image showing low signal intensity within the os trigonum representing sclerosis and oedema (arrow). (b) Corresponding short tau inversion recovery image showing oedema in the os trigonum (arrow) and adjacent posterior talus.
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Non-traumatic ankle pain
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The ankle is commonly involved in generalized arthropathies. The diagnosis is usually not in doubt and imaging other than the plain film is rarely required.
As with other joints unexplained persisting or atypical pain requires further investigation. When the presentation is acute or subacute the most important diagnosis to consider is infection. Plain films will usually reveal an effusion but the bone will appear normal early in the disease. Periarticular osteopenia, joint space loss, bony erosion and periosteal reaction are all typical late features. Ultrasound is useful to confirm the presence of an effusion and to guide aspiration. MRI is useful to assess the extent of bony involvement and to detect intraosseous abscess that may need draining.
Other forms of monoarthropathy are rare. Pigmented vilonodular synovitis and synovial osteochondromatosis are occasionally seen in the ankle. The joint or tendon sheaths may be involved. MRI is the technique of choice for diagnosing these conditions and for planning treatment (Figure 19). The imaging features are identical to that seen in the knee (see Imaging the knee).
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Figure 19. Pigmented villonodular synovitis. (a) T1 weighted and (b) short tau inversion recovery (STIR) sagittal images showing mass associated to the flexor hallucis longus tendon. The periphery of the lesion returns low signal intensity on both T1 and STIR images indicating haemosiderin deposition (arrows).
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The distal tibial metaphysis is a relatively common site for osteomyelitis particularly in children. Plain films are often normal and early MRI is advised if there is any suspicion of this diagnosis (Figure 20).
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Figure 20. Osteomylelitis of the tibia. (a) Normal plain film (b) T1 weighted and (c) short tau inversion recovery coronal images showing focal metaphyseal lesion with some surrounding oedema.
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Malignant bony tumours are rarely seen at the ankle. Osteoid osteoma (see Evaluation of focal bone lesions) should always be considered as a cause of persisting pain. Lesions may be seen in any bone but the talar neck is a favoured site. Most are visible on plain film as a well-defined lucency with or without calcification. Perilesional sclerosis is usually not a prominent feature in intra-articular lesions. Lesions that are difficult to appreciate on plain film are initially diagnosed on MRI. The most prominent MRI feature is often the perilesional oedema and reactive effusion. CT can be used to confirm the diagnosis and guide percutaneous treatment (Figure 21).
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Figure 21. Osteoid osteoma. (a) T1 weighted and (b) short tau inversion recovery sagittal images showing focal lesion (arrows) on the dorsum of the talar neck with surrounding oedema and an effusion. (c) CT shows characteristic features of a well defined lytic lesion with a sclerotic rim and central calcification (arrow). (d) CT showing radiofrequency ablation.
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Tendon disease
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Imaging is useful in identifying degeneration (tendinosis), tenosynovitis, partial and full thickness tears. Tendon pathology is usually diagnosed clinically without the aid of imaging. In patients with chronic pain imaging is used to confirm the diagnosis of tendinosis or tenosynovitis. In acute trauma imaging can confirm complete rupture and provide information concerning site and extent of the tear. The common tendons to be imaged are the Achilles, tibialis posterior and the peroneal tendons.
Achilles tendon
The most common indications for imaging the Achilles tendon are pain or suspected rupture. Both ultrasound and MRI are excellent methods of demonstrating Achilles tendon pathology. Ultrasound has the advantage of being cheap, quick, patient friendly, and dynamic. MRI is a good alternative, but offers no advantages if musculoskeletal ultrasound expertise are available. Ultrasound can be used to guided paratendinous or bursal injections.
Achilles pain (achillodynia)
The diaganosis of tendinosis of the Achilles tendon is usually based on clinical criteria, imaging being reserved for when the diagnosis is in doubt. Although there is no good evidence that imaging for achillodynia is beneficial, the extent of the pathology may influence the treatment and sports training programmes of athletes. If imaging is normal then the prognosis is good. An abnormal tendon on imaging is more likely to be associated with chronic problems including future rupture.
The Achilles tendon is formed as tendons of the two heads of gastrocnemius and soleus merge. In the sagittal plane the normal tendon has a uniform thickness throughout its length and in the axial plane the anterior border is seen to be concave or flat distal to the insertion of soleus muscle. The tendon is surrounded by a fibrous paratenon. There is constant bursa lying between the distal tendon and the calcaneus. The slender plantaris muscle tendon origin has close to the lateral head of gastrocnemius. The tendon courses obliquely between the gastrocnemius and the soleus in the calf to lie on the medial side of the Achilles tendon within in the paratendon. The blood supply to the Achilles tendon is poor, particularly in its mid portion.
Achilles tendinosis is a repetitive strain injury thought to result from imperfect repair of microtears within the tendon. The pathology shows a mixture ischaemia and collagen degeneration [18]. On imaging the most common appearance of tendinosis is a fusiform, or less commonly focal nodular, swelling of the tendon affecting predominately its hypovascular middle third. Neovascularity is frequently present. In some cases the pathology is predominately at the tendon insertion. Focal mucoid degeneration results in cyst-like areas on imaging. Larger intratendinous fluid-like lesions seen on imaging are often referred to as partial tears [19] but this term is probably best reserved for acute traumatic injuries. The paratendon may be thickened and inflamed and there may be an associated pre-achilles and/or retro-achilles bursitis. These pathological features are reflected in the imaging.
On ultrasound the normal tendon is uniformly echogenic. In tendinosis the echotexture becomes heterogeneous accompanied by diffuse fusiform or focal nodular swelling. In most cases Doppler will show hypervascularity with vessel seen to penetrating the anterior surface of the tendon (Figure 22). These vascular channels can be quite prominent. The significance of hypervascularity is not known although injecting sclerosing agent into the larger vessels has been reported to improve symptoms [20]. On MRI there is similar diffuse swelling with or without focal or diffuse increase signal. The focal high signal areas represent disrupted collagen fibres on histology [21]. Areas of mucoid degeneration are seen as intratendinous hypoechoic lesions on ultrasound and fluid signal on MRI (Figure 23a). Paratenonitis is seen as a hypoechoic rim involving mainly the posterior portion of the tendon on ultrasound and as a high signal intensity rim on T2 weighted MRI. On MRI the anterior surface of the tendon may appear indistinct and oedema may be seen in the adjacent pre-achilles fat [21]. Fluid within the pre-achilles bursa often accompanies tendon pathology and is easily detected on both techniques (Figure 23b).
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Figure 22. Achilles tendinosis. (a) Extended field of view ultrasound image showing diffuse fusiform swelling of the tendon (arrows) (C, calcaneus). (b) Sagittal scan with Doppler showing fusiform widening of the tendon. The tendon is hyopechoic and has a heterogeneous internal structure. Power Doppler demonstrates the typical pattern of hypervascularity with vascular channels penetrating the anterior surface of the tendon (arrows).
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Figure 23. Achilles tendinosis. (a) Sagittal T2 weighted image showing focal area of high signal in keeping with mucoid degeneration. (b) Sagittal T1 weighted image of a different case showing diffuse swelling of the tendon with an ill-defined anterior border indicating tendinosis with associated paratenonitis. There is evidence of pre achilles bursitis (arrow).
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Abnormality of the distal portion of the tendon involving the insertion may be due to a chronic traction injury, impingement against a prominent posterior superior corner of the calcaneus (Haglund's deformity) or as part of a generalized arthropathy, particularly of the seronegative variety (Figure 24 and 25). The retrocalcaneal bursa is usually involved. Plain films may show traction spurs or typical fluffy erosion of seronegative arthropathy. The soft tissue abnormality and any bony erosion can be assessed accurately by both ultrasound and MRI.
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Figure 24. Insertional tendinopathy. (a) Ultrasound showing widening of the tendon and a partial tear (arrow). (b) T1 weighted image of a different case. (c) Corresponding short tau inversion recovery image (note prominent posterosuperior corner of the calcaneus (arrow) which is impinging on the tendon) showing oedema in the calcaneus at the insertion and bursitis of the pre-Achilles (arrow) and retrocalcaneal (open arrow) bursae.
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Figure 25. Tendinopathy secondary to seronegative arthropathy. (a) Ultrasound shows marked widening of the distal portion of the tendon which is diffusely hypoechoic (arrows) (C, calcaneus). (b) Short tau inversion recovery sagittal MRI in a different case showing fluid in the pre-achilles bursa (arrow). Note also inflammatory change in the plantar fascia (open arrow) and associated oedema in the calcaneus (arrow heads).
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Acute Achilles rupture
Achilles tendon rupture will only occur in an abnormal tendon and commonly affects the middle aged recreational athlete. The tears usually occur in the mid-portion of the tendon or at the musculotendinous junction. The diagnosis is rarely in doubt clinically and imaging is primarily used to characterize the tear. Management protocols may be inconsistent from centre to centre and from year to year and the information required from imaging will need to vary accordingly. Currently many centres will use imaging to determine whether to manage the injury in a cast or to repair the tendon by open surgery or percutaneous suture. Tears at the musculotendinous junction heal well and are technically difficult to repair and are therefore treated conservatively. Tears of the mid portion of the tendon will be repaired surgically unless it can be demonstrated that the ends of the tendon are opposed with the foot in plantar flexion. Conservative management of tears associated with a large gap results in an increased risk of repeat tear.
On ultrasound the diagnosis of a full thickness tear is easy. The free ends of an acute tear of the tendon will be clearly identified if the haematoma is liquefied and hyporeflective (Figure 26). Although the tendon gap is sometimes harder to resolve, passive plantar and dorsiflexion of the foot will reveal paradoxical motion of the tendon ends. The tendon gap should be measured with the foot in plantar flexion as this is the casting position (Figure 27). A precise measurement of the gap is often difficult as the free ends of the tendon are ragged. Acute partial tears are unusual but sometimes an intact plantaris tendon will be seen coursing along the medial aspect of the tendon (Figure 28). Full thickness tears are also well demonstrated on MRI. Discontinuity of the tendon and fluid-like signal in the gap are seen.
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Figure 26. Acute achilles tendon rupture. Longitudinal ultrasound demonstrating tear with a large gap (arrows).
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Figure 27. Complete tear achilles tendon. (a) There is a gap with the foot in the dorsiflexed position. (b) The gap narrows on plantar flexion.
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Figure 28. Complete tear of the achilles tendon with intact plantaris tendon. (a) Axial image through the region of an acute tear. The intact plantaris tendon is seen lying on the medial side of the tear (arrow) within the paratenon (open arrows) (AT, torn achilles tendon). (b) Longitudinal scan demonstrates the intact plantaris (arrows).
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Tibialis posterior tendon
The second most common tendon requiring imaging is tibialis posterior. It is important to recognise pathology of this tendon as a neglected tear leads to a flat foot and secondary osteoarthritis. The tendon is vulnerable as it courses around the medial malleolus. The tendon is normally elliptical in shape. The normal transverse diameter of the tendon should measure less than 4 mm [22]. Tendon degeneration (tendinosis) is typically seen in middle-aged females. The patient complains of pain at the site of pathology which is usually behind the medial malleolus. On imaging the tendon is seen to be widened over a variable distance. On ultrasound the tendon will be heterogeneous containing areas of reduced reflectivity. There is usually surrounding low reflective rim representing synovitis. On Doppler hypervascularity is frequently seen both around and within the tendon (Figure 29). On MRI the tendon is usually seen to be widened with or without diffuse of focal loss of the normal signal void (Figure 30). There may be fluid like signal within the tendon when there is focal mucoid change or a longitudinal split (Figure 31). A small secondary bony spur is often seen arising from the posterior lip of the shallow groove in the posterior surface of the malleolus that contains the tendon (Figure 32). In chronic cases the tendon can become atrophic. Complete rupture usually results form chronic attrition and can be readily diagnosed on ultrasound or MRI (Figure 33).
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Figure 29. Tibialis posterior tendinosis. (a) Axial ultrasound showing a minor widening of the tendon (arrow) with a hypoechoic rim representing tenosynovitis (open arrow). (b) Longitudinal section shows a widened tendon with fluid in the tendon sheath (arrow) and synovial hypertrophy (open arrow). (c) Power Doppler shows hypervascularity in the tendon and surrounding synovium.
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Figure 30. Tibialis posterior tendinosis. Axial T2 weighted image distal to the malleolus showing a widened tendon (arrow) and fluid in the tendon sheath. There is no abnormal intratendinous signal.
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Figure 31. Longitudinal split of tibialis posterior tendon. Sagittal T2 weighted image showing linear high signal within the tendon (arrow) (FD, flexor digitorum longus).
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Figure 32. Tibialis posterior tendinosis. T1 weighted axial image showing increased signal within the tendon (arrow) and surrounding synovitis. Note the secondary bony spur arising from the tibia just medial to the tendon (open arrow).
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Figure 33. Complete chronic tear of the tibialis posterior tendon. (a) T1 weighted axial image showing a small amount of scar tissue at the expected site of the tendon (arrow) (FD, flexor digitorum longus). (b) Sagittal image showing the widened retracted proximal free end of the tendon (arrow).
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Posterolateral ankle pain may be due to peroneal tendon dysfunction. One of the more common conditions is a longitudinal split in the peroneal brevis tendon, thought to be caused by chronic abrasion on the fibula. The condition is associated with laxity of the peroneal retinaculum and lateral instability [23]. The split tendon can be diagnosed on ultrasound or MRI and is best appreciated in the axial plane (Figure 34). Incompetence of the retinaculum may also result in chronic subluxation or dislocation of the tendons. Ultrasound or MRI will demonstrate the position of the tendons. If the subluxation is intermittent then dynamic ultrasound is the method of choice (Figure 35).
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Figure 34. Longitudinal split of the peroneus brevis. Axial scan showing peroneus longus (arrow) lying within the split peroneus brevis (open arrows).
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Figure 35. Peroneal dislocation. (a) Axial ultrasound showing peroneus longus tendon lying on the anterolateral aspect of the fibula (arrow). Peroneus brevis is lying in the normal position behind the fibula (open arrow). The stripped reinaculum can be seen (arrowheads) (F, fibula).
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The foot
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Pain
In the vast majority of patients with foot pain plain films are the only investigation required. Suspected soft tissue problems can initially be imaged by ultrasound, MRI being reserved for difficult cases. Occasionally bone scans and CT are useful in specific situations.
The foot is commonly involved in polyarthropathies, particularly osteoarthritis, rheumatoid and seronegative arthropathies (see Imaging in rheumatology).
Hind foot pain
Plantar fascitis is a common cause of heel pain. It occurs in the middle age and is thought to be a chronic traction type injury. It is also seen as a manifestation of seronegative arthropathy. The diagnosis is usually made on clinical grounds and injections of steroid are often performed in outpatient clinics without image guidance. Plain films are generally unhelpful. A plantar spur is a non-specific sign. Ultrasound is the easiest way of making a definite diagnosis. The condition primarily affects the medial portion of the tendon at its insertion to the calcaneus. On ultrasound focal thickening and a reduction in reflectivity is seen in this portion of the tendon (Figure 36) [24]. Most studies indicate that a fascial thickness of 4 mm is the upper limit of normal [25]. MRI shows a widened fascia with increased signal often with some reactive oedema in the adjacent bone. Injection of long acting steroid is often effective and ultrasound guidance is a good way of ensuring optimal placement of the needle tip. Rupture of the plantar fascia is rare but previous steroid injection appears to increase this risk [26].
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Figure 36. Plantar fascitiis. Longitudinal ultrasound image showing focal widening of the fascia at its origin (arrows) (C, calcaneus).
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Other specific causes of heel pain are insertional Achilles tendon pathology and insufficiency fractures (see above).
Mid-foot pain
In the absence of an athropathy several specific causes of mid-foot pain should be considered.
Tarsal coalition is usually seen in children and adolescence. Patients present with a rigid often flat foot with pain in the tarsal region. Spasm of the peroneal muscles (peroneal spastic foot) is characteristic but only seen in the minority of cases [27]. The diagnosis is often suspected on clinical grounds but imaging is needed to make the diagnosis and classify the lesion. The two common types are the talocalcaneal (involving the middle subtalar joint) and calcaneonavicular coalitions. The union between the two bones may be bony, fibrous or cartilaginous. In the talocalcaneal type standard plain films may be normal or show only subtle abnormality. The most useful sign is the talar beak on the dorsum of the talus which represents a stress response at the talonavicular ligament due to the abnormal dynamics. The enlarged sustentaculum tali contributes to the C sign (Figure 37). On MRI or CT the bony and non bony coalitions are easily identified on coronal or sagittal images [28]. On MRI the coronal scans should be orientated perpendicular to the middle subtalar joint (Figures 38 and 39).
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Figure 37. Talocalcaneal coalition. Typical plain film appearances of talocalcaneal coalition with a talar beak (arrow) and a "C" sign (open arrows).
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Figure 38. Talocalcaneal coalition. (a) T1 weighted coronal showing a non-bony coalition. There is a wide irregular middle subtalar joint (arrow). (b) Marked bony oedema is seen on the short tau inversion recovery sequence.
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Figure 39. Talocalcaneal coalition, T1 coronal image showing complete fusion of the middle subtalar joint (arrow).
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The calcaneonavicular coalition is well seen on an oblique view of the foot. Both MRI and CT will confirm the diagnosis and exclude an associated talocalcaneal lesion. In the non-bony types of coalition oedema is often seen in the adjacent bones on MRI.
The medial accessory ossification of the navicular is usually an incidental finding but may present with focal mid-foot pain [29]. Plain radiographs reveal a synchondrosis between the ossicle and main body of the navicular (Figure 40). The diagnosis is mainly clinical. On MRI oedema is seen both sides of the synchondrosis. Bone scintigraphy is of limited value as positive results are frequently seen in asymptomatic accessory ossicles [30]. The lesion should not be confused with the os tibiale externum which is a normal sesamoid bone in the tibialis posterior tendon close to the navicular. Excision of the ossicle is effective in most cases.
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Figure 40. Painful os naviculare. The accessory ossicle (O) is forming a joint with the main body of the navicula (N). (T, talus.)
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Osteonecrosis of the lateral aspect of the navicular in the adult may result in deformity and pain. The diagnosis can be made on a frontal view of the foot. Sclerosis and bony collapse of the lateral portion of the navicular and medial migration of the bone are seen (Figure 41). On MRI the necrotic portion of bone returns low signal intensity on most sequences, usually with some increased signal on short tau inversion recovery (STIR) or fat suppression T2 weighted images [31].
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Figure 41. Osteonecrosis of the lateral part of the navicular. (a) Frontal and (b) lateral views show the typical appearances with a small triangular shaped residual medial portion (arrows) displaced medially and dorsally. More laterally the talus articulates with the cuneiforms (open arrow).
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Forefoot pain
Forefoot pain is common and often a definite diagnosis is not reached. Specific conditions should be considered. Stress fractures are common and usually involve the neck of the 2nd or, less commonly, the 3rd metatarsus. A history of pain on exercise, relieved by rest is suggestive of the diagnosis. The plain films are usually abnormal showing a periosteal reaction with or with out a linear cortical lucency. If plain films are normal the diagnosis can be made on MRI which will reveal focal intramedullary and parosteal oedema. Bone scintigraphy is invariably positive.
Morton's neuroma is a fibrous thickening of the interdigital nerve usually at the interspace between the 3rd and 4th metatarsal heads. The condition is very common and occurs mainly in females, resulting in focal pain on walking and paraesthesia in the toes. Ultrasound and MRI can detect these neuromas which are seen as round or disc-like lesions between the plantar portion of metatarsal heads. On ultrasound scanning from the plantar side in the sagittal plane is preferred to get a good view of the interdigital space by avoiding edge artefacts from the metatarsal heads. The neuromas appear as hypoechoic mass without internal vascularity. A neuroma may be confused with inflammation in the adjacent metatarsal bursa. The two conditions often coexist (Figure 42) [32]. The metatarsal bursa lies dorsal to interdigital nerve between the metatarsal heads. The fluid nature of the distended metatarsal bursa can usually be demonstrated on compressing the lesion. Injection of Morton's neuroma with steroid is beneficial and can be done under ultrasound control. On MRI Morton's neuroma returns intermediate signal on T1 weighted images and usually low signal on T2 weighted fat suppression and STIR sequences. The lesion is therefore best seen on axial T1 weighted images (Figure 42). Post intravenous gadolinium scans are not helpful as the lesions usually enhance poorly [33]. Intermetatarsal bursitis is seen as a high signal between the metatarsal heads on T2 weighted or STIR images.
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Figure 42. Morton's neuroma. (a) Sagittal ultrasound showing typical rounded hypoechoic lesion (arrow). Deep to the lesion there is some fluid seen in the intermetatarsal bursa (open arrow). (b) Axial T1 weighted image showing small intermediate signal intensity mass between the 3rd and 4th metatarsals (arrow).
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An unusual cause of focal forefoot pain is osteonecrosis of one of the sesamoid bones at the first metatarsophalangeal joint which is often referred to as "sesamoiditis" [34]. MRI findings are quite specific showing low signal on T1 and T2 weighted images reflecting necrotic bone. Some relative high SI may be seen on STIR and T2 weighted fat suppression images [35] (Figure 43). A more common site of osteonecrosis is the head of the 2nd metatarsus (Freiberg's disease). Collapse of the articular surface and sclerosis is initially seen. In chronic cases the metatarsal head is widened and flattened.
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Figure 43. Osteonecrosis of the sesamoid. (a) T1 weighted axial scan showing diffuse low signal intensity in the lateral sesamoid (arrow). (b) Short tau inversion recovery axial image showing low signal laterally in the sesamoid (arrow) and oedema related to secondary osteoarthritis more medially (open arrow).
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Pain and paraesthesia on the medial and plantar aspects of the foot may be due to compression of the tibial nerve as it passes through the fascial tarsal tunnel which lie on the medial side of the ankle just distal to the medial malleous (tarsal tunnel syndrome). A variety of tumours, trauma and foot deformities may be responsible for the neuropathy. Ultrasound and MRI are useful in identifying and characterizing mass lesions (Figure 44).
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Figure 44. Tarsal tunnel syndrome. Sagittal short tau inversion recovery image showing ganglion in the tarsal tunnel (arrow).
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Bone oedema
On MRI, bone oedema in the bones of the foot is often encountered. There may be a specific cause such as infection or trauma but the aetiology may be obscure. There are three probably related conditions that should be considered: reflex sympathetic dystrophy, transient (or migratory) osteoporosis and "bone marrow oedema syndrome of the foot". In all these conditions oedema is seen in one or more bones and usually to some extent the surrounding soft tissues (Figure 45). In sympathetic dystrophy the oedema tends to be periarticular [36]. In the so called bone marrow oedema syndrome of the foot follow-up scans may show resolution of oedema in one bone but reveal new areas of oedema in another [37]. Radiographs often show osteopenia. As the label given to this condition implies, the aetiology is not known.
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Figure 45. Bone marrow oedema syndrome. (a) T1 weighted and (b) short tau inversion recovery sagittal images showing diffuse oedema within the talus with an ankle joint effusion.
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Diabetes
Abnormalities of the foot related to diabetes warrants special mention. As a result of the associated neuropathy and arteriopathy the diabetic foot is susceptible to neurogenic arthropathy, ulceration and infection. The role of imaging in management of the condition is not fully established. The main problem with imaging is the difficulty in differentiating infection, reactive oedema and neuropathic changes. Often cases are managed on the basis of clinical judgement and plain films. Neuropathy involves the mid tarsal joints and is seen as fragmentation, disorganization and sclerosis on plain film. MRI will show these features with the addition of intraosseous and soft tissue oedema in the more acute stages (Figure 46). Although it is difficult to exclude infection on a background of such abnormalities, infection is rare in the absence of a communicating ulcer. Ulcer formation, and therefore infection, is more commonly seen in the forefoot or at the heel (Figure 47) [38]. In the presence of an ulcer plain films and MRI are useful in determining whether there is osteomyelitis in the adjacent bones. Plain films may show localized bone erosion. MRI is much more sensitive and will show oedema within the medulla with both infection and reactive change [39]. Infection should be suspected if the signal intensity on T2 weighted images is intense. The degree of uptake of gadolinium is of limited value in practice [39] although lack of enhancement may favour reactive oedema [38]. Infection can also be confidently diagnosed if the cortex is eroded or fluid type signal representing an abscess is present.
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Figure 46. Diabetic neuropathy. (a) T1 weighted sagittal image showing disorganization of the midcarpal bones and extensive low signal intensity. (b) There is high signal on the short tau inversion recovery image indicating that the process is still active.
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Figure 47. Diabetic osteomyelitis. (a) T1 and (b) short tau inversion recovery sagittal images showing focal erosion and oedema in the posterior aspect of the calcaneus which is communicating with an ulcer (arrow). There is also some inflammation of the Achilles tendon and pre-Achilles bursa.
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In general MRI is useful in difficult cases as it can exclude osteomyelitis and diagnose soft tissue and bony abscess formation which may be suitable for drainage. MRI may also help plan the level of amputation so that all the pathological tissue is removed.
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References
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- Griffith JF, Wong TY, et al. Sonography of plantar fibromatosis. AJR Am J Roentgenol 2002;179:1167–72.[Abstract/Free Full Text]
- Theodorou DJ, Theodorou SJ, et al. Disorders of the plantar aponeurosis: a spectrum of MR imaging findings. AJR Am J Roentgenol 2001;176:97–104.[Free Full Text]
- Sijbrandij ES, van Gils AP, et al. Posttraumatic subchondral bone contusions and fractures of the talotibial joint: occurrence of "kissing" lesions. AJR Am J Roentgenol 2000;175:1707–10.[Abstract/Free Full Text]
- Frost SC, Amendola A. Is stress radiography necessary in the diagnosis of acute or chronic ankle instability? Clin J Sport Med 1999;9:40–5.[Medline]
- Milz P, Milz S, et al. Lateral ankle ligaments and tibiofibular syndesmosis. 13-MHz high-frequency sonography and MRI compared in 20 patients. Acta Orthop Scand 1998;69:51–5.[Medline]
- Kreitner KF, Ferber A, et al. Injuries of
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Summary
Diffuse and focal swelling...
