|ESSR 2017 / P-0242|
|Staging of osteochondral lesions of the talus: Comparison of Cone Beam CT arthrography with MR Imaging|
Osteochondral lesions (OCL) of the talus are defined as any damage involving both hyaline cartilage and subchondral bone of the talar dome. This term covers a wide spectrum of pathologies including (sub)chondral contusion, osteochondritis dissecans, osteochondral fracture and osteoarthritis resulting from longstanding disease. Subchondral bone involvement can be manifested by bone marrow edema, fracture, sclerosis and/or cyst formation. Cartilage damage may have a variable imaging appearance ranging from a small fissure, a distinct defect, flap formation or delamination. The majority of those lesions occur in active patients and are related to trauma. Table 1 summarizes the most common occurring predisposing factors.
Osteonecrosis can develop when the lesion's vascularity is disrupted.
The articular surface of the talus is large and its blood supply is critical in the so-called watershed areas  explaining an impaired healing process and predisposition to posttraumatic necrosis in those vulnerable areas.
Accurate staging of cartilage lesions is of utmost importance, as this will have a major impact on the treatment strategy and ultimate prognosis. Unstable lesions -if left untreated- predispose for early osteoarthritis. Arthroscopic evaluation of the cartilage is regarded as the gold standard , but due to its invasiveness and the need for anesthesia, it should be reserved for preoperatively well-documented cases and combined with surgical treatment procedures. This underscores the value of preoperative imaging.
Fig. 3 illustrates the normal anatomy of the talocrural joint on a schematic drawing with correlation on Conventional Radiography (CR) Fig. 4, MRI Fig. 5 and Cone Beam Computed Tomography arthrography (CBCT-A) Fig. 6. On plain films, the subchondral bone is seen as a thin layer of compact bone with a smooth surface with uniform adjacent trabecular bone. MRI allows to distinguish normal cartilage from subchondral bone as well as to evaluate the adjacent bone marrow, ligaments and other surrounding soft tissues. Compared to the articular cartilage of the knee, cartilage of the ankle joint is very thin and the spatial resolution of MRI may be insufficient for detection of small lesions. Therefore, for more accurate evaluation of cartilage covering of articular surfaces of talar dome and distal tibia and fibula, direct arthrographic techniques combined with CT and MRI may be useful.
Recently, CBCT of small joints has been introduced as an alternative technique for Multi Detector CT, combining a very high spatial resolution, low radiation dose and low cost.
For the staging of OCL of the talus several grading systems have been proposed. The first system of classification has been reported by Berndt and Harty in 1959 , including four stages based on their radiological appearance (see Table 2). Although CR is still initial diagnostic modality used for evaluation of ankle pain, later studies showed that 43 % of talar OCL diagnosed on MRI were invisible on CR .
Later on, this grading system has been modified to computed tomographic evaluation and correlated with arthroscopy, distinguishing cystic lesion of talar dome seen in primary stages with or without communication to the articular surface and detached fragment in more advanced lesions . With the advent of MRI, this grading system was further revised including evaluation of structures invisible on conventional radiology, such as the integrity of the cartilage and presence of bone marrow edema. An example of MR modified grading system is illustrated in Table 3  and correlated with arthroscopic appearance by Dipaola et al. .
According to this grading system, stage 1 refers to a lesion restricted to the cartilage. In daily clinical practice however, isolated cartilage lesion without bone marrow changes are rarely seen. Due to the widespread use of fluid-sensitive sequences on MRI, subtle bone marrow changes, even subtle foci of Bone Marrow Edema (BME) may be seen adjacent to a cartilage defect, particularly in acute or subacute OCL lesions (Fig. 7 and Fig. 8). On the other hand, although MRI is very useful and sensitive technique for evaluation of the subchondral compartment (showing either BME, sclerosis or cyst formation), the precise depth and extent of the overlying cartilage lesion is often misstaged. In adult patients, the depth of the cartilage lesions is often understaged.
Because the plasticity of the cartilage in children and adolescents is higher than in adults, OCL lesions in young patients are often characterized by isolated subchondral bony changes without overlying cartilage disruption (Fig. 9). This implies that for precise evaluation of the articular cartilage compartment, MRI may be inaccurate. Similar to the Outerbridge classification widely used in staging of cartilage lesions of the knee, a modified staging system for evaluation of the depth of cartilage defects with correlation to arthroscopy may be used in the ankle (see Table 4). Furthermore, cartilage lesions may be isolated (one defect), complex (one lesion with variable depth of the lesion) or multifocal (involving multiple areas of the talus or tibia). Direct arthrographic techniques such as CBCT arthrography are superior in the evaluation of the cartilaginous compartment.
An alternative MRI staging system has been proposed by Mintz  et al. in 2003.
Nowadays MR staging of OCL on MRI is usually done by the Anderson classification , which is another modification of the initial staging system based on plain film evaluation by Berndt and Harty (Table 5).
Stage 1 lesions are due to bone marrow contusion. MRI is the most sensitive method to depict this stage without any correlating signs on CR or CBCT with injection of intra-articular contrast. Additionally bone scintigraphy can be used, but due its low specificity, this technique is helpful in more accurate grading. The hyaline cartilage lining remains homogenous without any signal changes (Fig. 10 and Fig. 11).
Stage 2 refers to partial detachment of OCL with subchondral cyst formation or fissure incompletely separating the lesion from the talar dome. In stage 3 an undisplaced completely separated fragment can be seen on MRI with adjacent bone marrow oedema (Fig. 12). On CBCT arthrographic images, the contrast separating the OCL fragment from the talar dome can be evaluated with more confidence (Fig. 13).
Useful MR scoring parameters include lesion location, lesion size in 3 planes (Fig. 14 , Fig. 15 and Fig. 16), subchondral bone marrow edema, subchondral cyst formation and/or sclerosis, status of the overlying cartilage, contour depression of the articular bone plate. Despite the combination of these MR parameters, accurate cartilage evaluation remains often illusive.
The main reason for that is the fact that we need images with high spatial resolution to detect early changes of articular cartilage of the ankle joint. MRI sensitivity in detection of OCL of the talus, correlated with arthroscopic correlation, varies according to different studies and has been reported as high as 81% . The accuracy also depends of the strength of the field and is lower on 1.5 Tesla magnets in comparison to 3T . Studies on cadavers performed on Computed Tomography arthrography  showed more accurate cartilage thickness measurements in comparison to standard MRI, which is in line with a superior evaluation of OCL with CT arthrographic techniques . Besides limitations of MRI in this field, it is still considered as the most comprehensive imaging modality of the ankle because of its capability to assess soft tissue and bone marrow abnormalities on a single examination.
Cone Beam Computed Tomography (CBCT) which was first introduced for preoperative evaluation of dental implants, is currently also used for musculoskeletal applications. It uses the conical X-ray beam and flat panel detector collecting all volumetric data in one rotation of the gantry. It combines high spatial resolution, relatively low radiation dose and low cost of the equipment and is useful for evaluation of trauma of small bones and joints, particularly when there is clinical suspicion for a fracture despite negative plain radiographs . The equipment is designed to perform exams in sitting or supine position and is relatively compact, allowing installation in many radiology departments and private practices (Fig. 17).
CBCT following intra-articular injection of Iodine contrast (CBCT-Arthrography) may render exquisite detail of the articular cartilage using very thin slices and multiplanar reformation. In addition, the trabecular architecture of subchondral bone is far better visualized on CBCT than on CR.
In this regard, CBCT-Arthrography (CBCT-A) may be very promising technique for precise staging of cartilage lesions of the ankle as an alternative for Multi Detector Computed Tomography (MDCT).
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