The technological advancements of high-resolution helical CT and SPECT (single-photon emission computed tomography) have improved CT for the purpose of assessing OCLs [19, 36]. Although CT does not have the capability to evaluate articular cartilage, it has been shown to effectively evaluate the size, location, and degree of bony injury in lesions involving subchondral bone [8] (Fig. 12.2). High-resolution helical CT has been compared to both MRI and arthroscopy with results showing that there is no significant difference between modalities in their ability to detect the presence of an OCL. Helical CT was shown to have high specificity (0.99) in accurately grading a lesion and correctly identifies the presence of 81 % of OCLs [36]. Regarding SPECT, it is a three-dimensional scintigraphy bone scan superimposed on a CT scan in order to localize scintigraphic osteoblastic activity and present biological information regarding a lesion [15, 16, 19, 27]. SPECT has also been directly compared to MRI for OCL evaluation and was shown to provide supplemental information that can affect decision making with respect to treatment choice. However, poor inter-rater reliability revealed that the techniques are subject to errors in interpretation [19].

MRI signal patterns in the talus resulting from pathologies such as bone edema have been suggested to lead to an overestimation of the extent of bony injury involved in an OCL. Because of this, CT may be a useful addition to MRI at follow up [19, 26, 32]. However, it is important to note that in a comparison study by Verhagen and co-workers assessing MRI, arthroscopy, and helical CT, MRI was noted as the more sensitive modality and identified four OCLs that helical CT did not. Additionally, CT imaging resulted in five false negatives [36]. With regard to follow-up imaging, CT is most pertinent in the presence of subchondral lesions, subchondral cysts, and bone edema, in order to assess the true extent of bone involvement [7].

MRI has been thoroughly studied as a method for evaluating cartilage and has the capacity to distinguish between normal native cartilage, repair cartilage (including fibrocartilage), and synovial tissue [30]. This modality can characterize cartilage morphology, biochemistry, and function and is even sensitive enough to determine collagen orientation and changes associated with degradation [21, 30] (Fig. 12.3). MRI has been used to assess cartilage repair after procedures including bone marrow stimulation techniques, fixation with biodegradable pins, autologous chondrocyte implantation (ACI), and osteochondral autograft and allograft techniques [6, 13]. MRI following these surgical procedures provides evaluation of subchondral bone, three-dimensional geometry of the joint, percent fill of lesion, and signal morphology of repair tissue [13]. Thus, it is an informative objective measure for preoperative diagnosis, surgical planning, and postoperative assessment at follow-up as well as for retrospective and prospective studies [11, 13, 30].

There is also some comparative evidence indicating that MRI is an effective follow-up tool. Magnetic resonance observation of cartilage repair tissue (MOCART) scores have been correlated with American Orthopaedic Foot & Ankle Society (AOFAS) clinical outcome scores at both 5 ± 1 year and 10 years postoperatively following ACI in the talus. MOCART scores were shown to have a direct correlation with AOFAS clinical outcome scores [3, 10]. Additionally, the appearance of cartilage on MRI shows strong correlation with the findings of second-look arthroscopy [16, 18, 24, 38]. Henderson and co-workers compared MRI at 12 months with both second-look arthroscopy and histological evaluation of biopsies in the knee and reported that MRI findings generally agreed with arthroscopic evaluation [14]. The authors concluded that MRI may be as accurate as arthroscopic visual scoring and histological evaluation, when used to assess the state of cartilage [14].

Standard two-dimensional multi-slice turbo or fast spin-echo (FSE) proton density and fat-suppressed proton density sequences acquired in multiple planes are widely accepted as standard MRI cartilage protocol. These are able to evaluate postoperative cartilage healing and morphology. Moreover, technological advancements have produced three-dimensional techniques that can generate models of the joint surface, repair fill, and thickness and volume measurements [28, 29]. Newer, quantitative matrix assessment techniques, including T2 mapping, T1 rho, T1-weighted three-dimensional fat-suppressed fast spoiled gradient echo (FSPGR), and delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) offer information regarding the histological and biochemical status of repair cartilage [12, 28, 29]. For example, FSPGR MRI is thought to be more sensitive than conventional MRI in detecting talar OCLs and can measure glycosaminoglycan content [12, 26]. T1 rho has been shown to correlate with proteoglycan content [28]. The International Cartilage Repair Society (ICRS) recommends intermediate-weighted FSE and 3D fat-suppressed T1-weighted gradient-echo (GRE) sequences, which are the most commonly used for repair cartilage imaging [6]. With regard to T2 mapping MRI, calculated relaxation times have been related to changes in articular cartilage with respect to collagen presence and orientation [1, 25, 37]. High spatial resolution is another valuable feature that can be attained with 1.5 or 3 Tesla scanners. These scanners allow surface congruity, osseous incorporation, and graft morphology and integration to be evaluated following replacement procedures such as autologous osteochondral transplantation [33]. Specifically, high-resolution MRI is advocated for analysis of articular cartilage defects of the talus because of its ability to reveal clinically relevant features that can impact treatment decisions [7].

It is recommended that MRI follow-up studies take place at 3–6 months after a cartilage repair procedure and again before the end of the first postoperative year [6]. The first follow-up at 3–6 months is for the purpose of evaluating integration of repair tissue and volume of the cartilage. The next round of follow-up imaging, administered within the first year after surgery, allows for assessment of cartilage maturation or graft maturation, in the case of autograft or allograft procedures [33]. Analysis of the imaging requires expertise and familiarity with repair procedures, characteristic MRI features of repair tissue at postoperative intervals, and image acquisition protocols and techniques. The information gained from MRI is vital to patient follow-up after surgical treatment of OCLs for both research and clinical purposes and has become the primary method of noninvasive follow-up imaging.

Second-look arthroscopy has the advantage of allowing direct visualization of the articular surface and the ability to probe for softening, ballotability, and fissuring of the cartilage (Fig. 12.4). However, it requires a second operation and invasion of the joint. Therefore, the procedure is rarely performed, and few studies have compared second-look arthroscopy to other cartilage assessment modalities. Arthroscopy is known to provide information that is complimentary to MRI and several scoring systems have been devised on this basis [6]. Ferkel and Cheng proposed a 6-stage arthroscopic grading system that ranges from smooth and intact cartilage to a displaced cartilage fragment [5]. The ICRS has also designed a postoperative arthroscopic assessment system based on the degree to which a defect is filled with repair tissue, the degree of integration of repair tissue with the surrounding cartilage, and the macroscopic appearance of the articular surface [16, 34]. Second-look arthroscopy can be performed using previously created portals, typically anterolateral, anteromedial, and posterolateral, and is most informative around 1 year postoperatively because cartilage integration and maturation may be assessed [6, 9, 16].

Arthroscopy has been directly compared to other follow-up imaging modalities. Lee and co-workers reported a good correlation between second-look arthroscopy and AOFAS scores 12 months after microfracture treatment, using both the ICRS and Ferkel and Cheng arthroscopic grading systems [16, 23]. With regard to imaging comparison, Lee and co-workers reported that scores for degree of defect repair and filling using second-look arthroscopy and MOCART demonstrated significant agreement and an intraclass correlation coefficient indicating good reliability 1 year following ACI in the talus. However, scores for integration of repair tissue with adjacent cartilage showed poor reliability [17]. A separate study stated that correlations between clinical outcomes, MOCART scores, and second-look arthroscopy were not significantly different, and, thus, second-look arthroscopy was not necessary for follow-up [18]. In another study 12 months following ACI, the paper concluded that due to a moderate correlation between second-look arthroscopy and MRI, MRI seemed to score cartilage maturation less favorably. It was also concluded that surgeon bias may contribute to favor arthroscopic scores, and that MRI may be equally effective as second-look arthroscopy and histology [14].