The temporomandibular joint (TMJ) is comprised of two independent joint compartments separated by a biconcave fibrocartilagenous intra-articular disc. The discotemporal joint (DTJ) compartment lies above the disc and, in the horse, is composed of the articular tubercle, the mandibular fossa and retroarticular process of the temporal bone. The discomandibular joint (DMJ), located below the disc, contains the condylar process of the mandible and is the smaller of the two joint compartments. TMJ abnormalities occur commonly in humans and domestic animals. Intra- and extra-capsular conditions are recognized in humans, whereas in animals, the focus has been primarily on intra-capsular disease – specifically osteoarthritis. Advanced imaging forms an integral part of the diagnostic process when investigating human TMJ pain. Computed tomography (CT), either conventional or cone-beam, is considered the superior modality to examine osseous changes, whereas magnetic resonance imaging (MRI) is best for examining the intra-articular disc (1, 2). MRI generates static images, so contrast-enhanced arthrography using fluoroscopy is used for dynamic assessment of the joint to investigate possible disc adhesions or perforations (3, 4). Double-contrast enhancement of the human TMJ was developed in the 1960s and has since been superseded by MRI (5–7).

Diagnostic imaging of the equine TMJ has been reported to include radiography, ultrasound, CT and MRI (8–12). Specialized radiographic projections have been established to view the TMJ and limit the overlap of other cranial structures (13–15). Relative to the bone margins of the joint, the intra-articular disc is radiolucent (soft tissue radiodensity), unless there is dystrophic mineralization associated with the structure. Contrast CT has been used to assess the integrity of the joint compartments and the intra-articular disc, but this modality is not readily accessible to a significant portion of the horse-owning population (16). Single- and double-contrast arthrography has been used to examine the joints of the appendicular skeleton and may be beneficial in the investigation of the equine TMJ.

The objectives of this study were two-fold. Firstly, to develop a technique of radiographic contrast arthrography for both compartments of the equine TMJ in cadavers mimicking that already published using computed tomography, and secondly to perform the refined technique in three living horses as a proof-of-principle.

The hypotheses were that contrast arthrography of the equine TMJ would result in delineation of the intra-articular disc and detection of disc perforation (if present in the study population); and that the margins of the joint and extent of the synovial cavities would be readily appreciable using this technique.

The results of our study confirmed the hypotheses that the intra-articular disc could be successfully delineated, and the extent of the joint margins and synovial pouches could be readily determined. Indeed, the images obtained were similar to the computed tomographic images (axial projections) previously reported by the research group (16).

The rostral pouch of the DTJ is expansive and extends below (ventral to) the rostro-dorsal surface of the intra-articular disc. Therefore, when using the R45°V30°L-CdDLO projection, contrast agent overlies the dorsal aspect of the intra-articular disc and articular tubercle, obscuring them from view. The present study found that even though contrast material also pooled in the rostro-ventral pouch of the DMJ, the small volume used (3 mL) settled below the joint surface leaving a clear demarcation between the condylar process of the mandible and the ventral aspect of the intra-articular disc. These findings indicated that the DMJ should be subjected to contrast injection prior to evaluating the DTJ.

Another important finding was that the needle used to inject contrast agent could be inadvertently passed through the intra-articular disc. In both cases, this occurred while attempting to enter the DMJ and the error resulted in contrast material being present in both joint compartments simultaneously. This diagnostic finding mimics that associated with pathological tears of the disc and could potentially lead to erroneous diagnoses or further, unnecessary, diagnostic tests such as arthroscopy (18–20). The condylar process of the mandible slopes medioventrally away from the skin with the intra-articular disc in close apposition. Consequently, the 22G 1″ needle can impact the underside of the curving disc if it is directed straight into the DMJ, 90° to the skin surface. Possible solutions were to avoid deeper penetration into the joint, bend the needle to approximate the radius of curvature, or use a shorter needle. More care was subsequently used when inserting the 22G 1″ needles in the cadavers, and the 21G butterfly catheters in live horses, and no further anomalies were seen.

Discomandibular joint injection was challenging, despite the operator having extensive experience injecting both TMJ compartments. A recent report concluded that ultrasound guidance did not improve the accuracy of TMJ arthrocentesis in the horse (21). However, this paper only focused on the DTJ and it is possible that arthrocentesis of the DMJ is facilitated by using this modality. Alternatively, more practice or a different approach to the joint could improve the success rate of first time injection.

Overall, double contrast arthrography of the equine TMJ delivers more information than non-contrast techniques specifically pertaining to demarcation of the joint and the surfaces of the intra-articular disc. Despite these additional advantages, the resolution and multiplanar reconstruction techniques available with CT (especially when contrast enhanced), still make the latter technique preferable.

Single and double contrast arthrography of the equine TMJ allows delineation of the joint and intra-articular disc margins as well as determining the integrity of the disc. Inadvertent disc penetration during contrast administration may result in a false positive indication of disc compromise. Injection technique is not easy and must be performed carefully. Clinically, the clear presence of contrast agent in both joint compartments after single compartment injection is highly suggestive of an abnormality in the integrity of the intra-articular disc. That said, the absence of contrast in both compartments does not preclude pathology.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

The animal study was approved by University of Saskatchewan Animal Research Ethics Board. The study was conducted in accordance with the local legislation and institutional requirements.

CK: Investigation, Methodology, Writing – original draft, Writing – review & editing. NR: Conceptualization, Investigation, Methodology, Project administration, Supervision, Writing – original draft, Writing – review & editing. JC: Conceptualization, Investigation, Methodology, Supervision, Writing – original draft, Writing – review & editing.