The use of CT is superior in the detection of OCDs as compared to conventional radiography [12, 13, 16]. Imaging of OCDs in the ankle by multi-detector computed tomography (CT) has several other benefits.

An advantage of CT is that additional bony pathologies which could influence treatment, such as (undercalled) fractures, osteophytes, loose bodies, ossicles, osteoarthritis, bony coalitions, transient osteoporosis, or osteonecrosis, can be detected, especially when two sides are compared. Verhagen also showed that a CT scan provides better visibility of cortical outlines and lower risk for overestimation of the OCD in comparison with MRI which often overcalls the extent of the defect due to the clearly visible bone marrow edema [13] on MRI.

As compared to MRI in particular, CT scans have the advantage that the ankle can be placed in various positions. As no coil is needed to image the ankle, a CT scan of the ankle can also be performed in plantar flexion. This is beneficial as this can aid the surgeon in deciding which operative approach should be chosen. This position is comparable to the X-ray of the ankle in plantar flexion, but with more detail and in three dimensions. With the aid of a preoperative CT scan in plantar flexion, the surgeon can make a reliable and accurate assessment preoperatively of the arthroscopic location of the defects. Bergen et al. concluded in a prospective blinded study that there is an excellent correlation between the CT and arthroscopic location of the OCDs [1]. This can be used to determine the method of surgery, whether an anterior approach is feasible.

Next, compared to MRI, CT scans are performed very fast and at submillimeter resolution. A standard MRI scan of the ankle lasts approximately 30 min with at most 2 mm resolution, whereas a CT scan of the ankle is performed within 1 min while providing very detailed, often submillimeter, images. Fast imaging reduces motion artifacts. Mainly due to the shorter scan time, less manpower is needed per patient. Consequently a CT scan is cheaper than an MRI scan of the ankle. Furthermore CT scans can be used for the imaging of OCDs of patients with contraindications for MRI, i.e., claustrophobia and metal implants (e.g., ICDs and neurostimulators). CT can easily be used for follow-up of OCDs treated both conservatively as well as surgically. After surgery the boney healing response can be monitored well by CT. The formations of callus, the progressive sclerosis of a defect, and periosteal reaction are depicted well by CT.

Another advantage of CT above MRI is that if desired both ankles can be imaged at once. Scanning both ankles at the same time is beneficial. It does not hamper image quality or significantly increase radiation burden yet provides the opportunity to compare both bony and soft tissues of both ankles. Imaging the other ankle provides an anatomical comparison in the same scan time as one ankle.

A new technique that is explored is a weight-bearing cone beam CT of the ankle. This new device allows the assessment of a small FOV, of only one ankle, yet adds weight bearing as a potential important tool in analysis of chronic ankle pain. Its use in patients with an OCD needs to be studied.

CT scans can only depict cartilage indirectly, as it mainly visualizes bone. However, cartilage can be depicted more accurately with CT arthrography. For CT arthrography, negative or positive contrast could be applied with, respectively, water or iodinated contrast material in a single or double method, with or without additional air. Iodinated contrast material provides better contrast than water in respect to cartilage, thereby achieving more reliable delineation of the cartilage pathology. Therefore, preferably positive contrast material is used for CT arthrography. A single contrast method, without the additional injection of air, is most often used.

For this procedure, iodinated contrast is injected intra-articularly in the tibiotalar joint, with fluoroscopic guidance (Fig. 4.3a). The preferred approach of the joint is anterior, placing the needle between the extensor hallucis longus tendon and the extensor digitorum tendon while avoiding the dorsalis pedis artery. Contrast injected intra-articularly will quickly spread throughout the joint. More contrast can be added if there is communication with the posterior subtalar joint or the flexor hallucis tendon. If the patient reports a sensation of tension in the joint, the injection is terminated. For the ankle this most often occurs after approximately 5 ml.

CT arthrography has been reported to be just as good or even better than MR arthrography for the detection of cartilage pathology [4, 10]. The intrinsic combination of high-resolution CT imaging and indirect cartilage mapping with detailed imaging of the cartilaginous defects makes CT arthrography powerful (Fig. 4.3b, c). The disadvantage of CT arthrography is that it is an invasive procedure, which as any invasive procedure can cause complications and side effects such as hemorrhage and infection.

In recent years several OCD classification systems have been designed in an attempt to aid in prognosis and therapeutic planning of the defects. Two of these often-used radiologic staging systems are mentioned below. As various clinicians may use different classification systems, it is advised for the radiologist to describe the appearance of the OCD as well, to prevent possible misunderstandings. In general, it is important for radiologists, surgeons, and other clinicians to use the same terminology so that each person knows what is meant by a certain description or stage of a disease. The sole use of a classification system in radiology reports should be discouraged as this leads to loss of information which could be important to the surgeon.

More than 50 years ago, in 1959, Berndt and Harty designed a classification system for transchondral fractures (OCDs) in the talus based on conventional radiographs [2]. For this classification, refer to Chap.​ 1.

In stages 1 and 2, a cystic defect with an intact roof or a minor disruption of the talar roof or tibial plafond can be noted (Fig. 4.4). These stages are difficult to detect by conventional radiographic imaging. However, these stages can also be overlooked on CT imaging, as the bony changes can be very subtle. CT imaging has the highest sensitivity and specificity for stage 3 and 4 defects. CT imaging plays an important role in the delineation of defects that may present with loose fragments (Fig. 4.5).