The clinical decision to obtain an MRI or CT to evaluate for possible OCD after a thorough physical examination and baseline radiographs is determined by a number of factors. However, there is debate regarding the optimal imaging modality to evaluate an OCD. Verhagen and co-workers investigated the utility of MRI, CT, and arthroscopy in the diagnosis of OCDs [29]. Although all three modalities were found to be superior to physical examination and radiographs alone, there was no statistical significance between MRI, CT, and diagnostic arthroscopy in detecting or excluding an OCD. Sensitivity and specificity for detecting an OCD with arthroscopy in this study were 100 and 97 %, respectively. The sensitivity and specificity values for MRI were 96 and 96 % and 81 and 99 % for CT.

The capability of MRI and arthroscopy to identify and exclude chondral defects of the talus has been compared previously [13]. However, when MRI findings did not correlate with arthroscopic findings, it was found that MRI tended to overgrade the lesion severity, especially with subchondral edema [13, 16]. Moreover, MRI’s predilection to detect subchondral changes as opposed to superficial lesions might result in missing surface defects [24].

As opposed to MRI, arthroscopy has the advantage of being able to directly visualize and identify a surface OCD. However, one drawback to arthroscopy is its inability to potentially identify a subchondral lesion with intact surface cartilage [15].

O’Neill and co-workers assessed the accuracy of the radiologist and orthopedic surgeon readings of MRI in patients with ankle instability [18]. The physician’s preoperative readings were compared to intraoperative findings. Interestingly, the radiologist and orthopedic surgeon only identified 39 and 45 % of chondral lesions, respectively. In a separate study, 38 % of chondral lesions were missed by MRI [24].

These articles question the accuracy of preoperative MRI for evaluating for chondral defects. O’Neill and co-workers indicated that almost all of the unidentified chondral defects were full thickness that warranted microfracture and were not necessarily large or deep lesions. This again indicates the difficulty of identifying superficial lesions in a region known for a thin layer of cartilage compared to other joints such as the knee [23]. The difficulties in detecting these defects were attributed to studies with low-powered magnets [12, 17, 25], differences in patient positioning [22], variability in the radiologist skills [18], and differences in imaging sequences [9, 19, 21]. These problems can be commonly encountered in the general orthopedic community who may not have access to a musculoskeletal radiologist or 1.5 or 3.0 T MRI. As a result, many OCDs can be missed.

In the past, the value of diagnostic ankle arthroscopy in the setting of a patient with no definitive diagnosis has been questioned [27, 28]. However, the study by O’Neill, Van Aman, and Guyton suggests the difficulty in identifying OCDs with MRI alone. Their study suggests a more common scenario for community orthopedic surgeons without access to a musculoskeletal radiologist or a high-powered magnet with various sequences to identify an OCD. In the setting of a patient with a high clinical suspicion for an OCD, especially if considering a separate procedure such as a modified Brostrom to treat ankle instability, a diagnostic ankle arthroscopy may be warranted to accurately diagnose and treat patients.

As written by Drs. Ferkel and Hommen, “arthroscopic examination of the ankle and foot provides the opportunity to directly visualize and evaluate articular cartilage and soft tissue pathology [6].” If the index of suspicion for an OCD is high in the setting of negative imaging studies, and surgery is already planned to treat a separate pathology, a diagnostic ankle arthroscopy may be warranted to evaluate and treat a possible OCD. There are several possible etiologies for OCDs to include macrotrauma, repetitive microtrauma, ankle instability, and idiopathic avascular necrosis of the talus. As indicated above by O’Neill and co-workers, only 39 % of chondral lesions were identified by MRI in the setting of ankle instability [18].

Contraindications for a diagnostic ankle arthroscopy include localized soft tissue infection which could potentially cause intra-articular dissemination and severe degenerative joint disease where adequate range of motion and joint distraction cannot be achieved for joint visualization [6].

Arthroscopic evaluation of the articular surface should be done in a systematic manner to carefully evaluate the cartilage defect. This allows one to document the arthroscopic findings in a reproducible fashion, to accurately diagnose any potential intra-articular pathology, and to improve the quality of future clinical studies of the ankle arthroscopy patient population. A systematic 21-point ankle arthroscopic examination is used to ensure no pathology is missed [5]. The 21-point examination consists of three phases: the eight-point anterior examination, the six-point central examination, and the seven-point posterior examination (Table 5.1). The eight-point anterior examination includes the deltoid ligament, medial gutter, medial talus, central talus, lateral talus, talofibular articulation (trifurcation of the talus, tibia, and fibula), lateral gutter, and anterior gutter. The six-point central examination is performed by maneuvering the arthroscope through the notch of Harty. The notch of Harty is an anatomic elevation of the anteromedial distal tibia. The central examination includes the medial central tibiotalus, middle tibiotalus, lateral tibiotalus, capsular reflection of the FHL tendon, transverse tibiofibular ligament, and posterior inferior tibiofibular ligament. The seven-point posterior examination includes the medial gutter, medial talus, central talus, lateral talus, talofibular articulation, lateral gutter, and posterior gutter. Generally, the combination of the anteromedial, anterolateral, and posterolateral portals allows excellent visualization of the entire joint.