Role of CT and radiography in diagnostics
Conventional radiography remains the first-line diagnostic imaging modality, but it has a significant drawback: superposition of three-dimensional structures onto a two-dimensional plane. While radiographs are often sufficient to identify avulsion fractures (avulsion fractures) indicating ligamentous injury, multiplanar CT is required to stage complex ankle injuries.
Important nuances of ankle diagnostic imaging:
- Joint space narrowing and angular deformities (planovalgus — planovalgus, cavovarus — cavovarus) can only be assessed on radiographs with weight-bearing.
- Ankle joint effusion and tendon status are well visualized on lateral radiographs due to the density difference between soft tissues and surrounding adipose tissue.
- Soft tissue calcification is clearly visible on radiography, but virtually indistinguishable on MRI.
- To reliably identify a focus of destruction in aggressive bone lesions, local bone mass loss must be at least 50%. Tomography is indicated for earlier and more accurate diagnosis.
Trauma and specific fractures
Computed tomography is the gold standard for surgical planning as it details the anatomy of intra-articular fragments.
Specific ankle fractures:
- Tillaux fracture (Tillaux fracture): avulsion of the anterolateral tibial epiphysis. Radiography shows only a fragment, whereas CT allows precise assessment of its typical location and displacement.
- Triplane fracture (Triplane fracture): involves both the sagittal and coronal planes, with fracture lines passing through the growth plate.
Calcaneal injuries: CT is always used for intra-articular calcaneal injuries. The most important radiographic criterion is the Böhler angle (Bohler angle), formed by lines drawn through the highest points of the bone. Normally, it is 30 degrees. A decrease in this angle indicates depression of the articular surface and requires surgical reconstruction to restore vertical height.
An impacted fracture manifests as an abnormal zone of sclerosis (trabecular compaction). In fracture-dislocations, CT reveals a loss of normal congruence between the talus (talus) and calcaneus (calcaneus). It is extremely important to differentiate a fracture of the anterior process of the calcaneus from an accessory ossification center — Os calcaneus secundarius (the fragment has an angular shape, whereas the accessory bone is smooth). Failure to diagnose this fracture leads to nonunion and degenerative changes in the Chopart joint.
Diagnosis of osteoid osteoma
The search for an osteoid osteoma in the ankle region can be extremely challenging. The tumor often masks itself within the cancellous bone of the hindfoot and is hidden on standard radiographs. The classic location is the subperiosteal region of the talar neck. Periosteal reaction may be absent or, conversely, pronounced (solid), for example, on the fibula.
MRI reveals reactive, edema-like bone marrow changes limited to a single bone, but the tumor nidus (nidus) may be too small (approximately 35% false-negative rate on non-contrast MRI). Only CT reliably visualizes the nidus, which is critical for confirming the diagnosis. CT guidance is also used during minimally invasive radiofrequency ablation. Complete ossification of the nidus on follow-up CT scans is considered the criterion for cure.
Hemophilic arthropathy and pseudotumors
Recurrent hemarthrosis in hemophilia leads to musculoskeletal damage. In hemophilic ankle arthropathy, flattening of the talar dome, asymmetric joint space narrowing, subchondral cysts, and marginal osteophytes are identified.
A rare but dangerous complication is hemophilic pseudotumor. It results from progressive intraosseous or subperiosteal hemorrhages independent of trauma and causes pressure-induced bone erosion. On CT, such a pseudotumor appears as a lesion with a thick, hyperdense pseudocapsule and a hypodense center. Radiographs reveal geographic lytic destruction and cortical bone thickening.
For differential diagnosis: anterior process fracture is acute (angular fragment), whereas the accessory bone (Os calcaneus secundarius) is round and smooth.
Class and exam questions
Why is MRI sometimes inferior to radiography in ankle ligament injuries?
Avulsion fractures at ligament attachment sites appear as thin bone crescents. They are well visualized on radiography, but easily missed on MRI due to partial volume effects and mild bone marrow edema.
Why are ankle radiographs performed with weight-bearing?
Weight-bearing is necessary for an objective assessment of joint space narrowing degree and accurate angular measurements in foot deformities.
Which imaging study is required to confirm a talar osteoid osteoma?
CT is mandatory. MRI only shows reactive edema, whereas definitive diagnosis and treatment planning require clear visualization of the nidus itself.
What else to study on this topic
- MRI diagnosis of osteoid osteoma: dynamic contrast enhancement and enhancement patterns.
- Radiological features of osteoma: medullary and parosteal forms, and Nora's disease.
- MRI semiotics of hemophilic arthropathy (T1, STIR, and T2* gradient echo).
- Evaluation of the ligamentous apparatus using radiographic stress tests.
- Limitations of conventional radiography and radiation dose justifications.
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