Contrast tenography in the equine digital flexor tendon sheath

The digital flexor tendon sheath can be a significant source of lameness in the hind limb or, less commonly, the forelimb of both athletic and pleasure horses. Injury is most commonly associated with the manica flexoria, deep digital flexor tendon, palmar annular ligament, and superficial digital flexor tendon, or a combination of multiple structures. Following localization with regional or intrathecal analgesia, multiple imaging modalities can be used to help identify the specific injury and help guide clinical decisions.

Ultrasound is the most common first-line imaging modality because it allows comprehensive evaluation of soft tissue structures within the tendon sheath. However, the skill level in acquiring and interpreting images is dependent on a clinician’s prior experience and training. Regardless of one’s ultrasonographic proficiency, the anatomical topography of the palmar/plantar fetlock can interfere with obtaining an accurate diagnosis, particularly involving the manica flexoria and the deep digital flexor tendon in this region. Non–weight-bearing and dynamic methods have been described to improve the sensitivity and specificity of these exams.^1,2

MRI is also a useful option, but it represents a larger financial investment for the client. High-field MRI requires general anesthesia, and standing low-field acquisition can miss longitudinal tears that are not severely disrupted when weight-bearing.

CT contrast tenography can be performed as a standing, non–weight-bearing procedure and provides high-quality diagnostic images.³ The cost incurred and need for referral may prohibit this from being a first-line diagnostic, unless such technology and financial resources are readily available.

Radiography with contrast

A readily available imaging modality for general practitioners is radiography with contrast. This technique can easily be performed in a field or hospital setting. Following aseptic preparation, 1 of 3 proposed injection sites for the digital flexor tendon sheath can be used to instill a combination of iodinated contrast with either saline or local anesthetic. The preferred location is the basi sesamoid approach between the proximal and distal digital annular ligaments in case there is severe palmar annular ligament constriction that would prevent adequate contrast distribution if the proximal lateral approach were to be used. The palmar/plantar pastern is a third option. However, it is difficult in the hind limb when the leg is picked up to be flexed, and incidental subcutaneous or intertendinous distribution of contrast is more likely to occur in this location.

A total of 20 mL is generally used, with a 50% dilution of the contrast solution in either saline or 2% mepivacaine, depending on whether your goal is simply to acquire imaging or to block the region as well. Radiographs using a standard lateral-medial projection of the fetlock are then acquired in succession, first immediately post injection and then several over the next 2 to 3 minutes after either walking the horse a few steps or lifting and flexing the leg (Figure 1).

Literature authors described this technique with a well-designed flowchart to assist with interpretation.⁴ The first question is determining whether there is an intact manica flexoria, which is indicated by 2 parallel lines converging at the proximal aspect of the proximal sesamoid bones (Figure 2).

The next step is to determine whether the deep digital flexor tendon is normal or whether contrast is present within its outline (Figure 3). Lastly, contrast distribution and the palmar soft tissue contour of the fetlock should be closely evaluated to see whether any constriction related to palmar annular ligament desmitis is present (Figure 4).

Takeaway

Practitioners with limited referral options or financial constraints should be encouraged to use this technique to help provide more accurate diagnoses and recommendations for their clients. This technique is also useful in the referral setting to determine whether further advanced diagnostic imaging modalities are necessary or whether tenoscopy of the digital flexor tendon sheath is the next reasonable step in a patient’s treatment protocol.

References

1. Garcia da Fonseca RM, Evrard L, Rabba S, Salciccia A, Busoni V. Dynamic flexion/extension and non‐weight bearing ultrasonography is helpful for identifying manica flexoria tears in horses. Vet Radiol Ultrasound. 2019;60(1):65-74. doi:10.1111/vru.12675
2. Hibner‐Szaltys M, Cavallier F, Cantatore F, Withers JM, Marcatili M. Ultrasonography can be used to predict the location of manica flexoria tears in horses. Equine Vet Educ. 2022;35(3):e200-e207. doi:10.1111/eve.13687
3. Shanklin AJ, Baldwin CM, Ellesmere L, Stack JD. Computed tomographic contrast tenography aids pre‐operative diagnosis in clinical conditions of the digital flexor tendon sheath. Equine Vet Educ. 2024;36(4):197-205. doi:10.1111/eve.13928
4. Kent AV, Chesworth MJ, Wells G, et al. Improved diagnostic criteria for digital flexor tendon sheath pathology using contrast tenography. Equine Vet J. 2020;52(2):205-212. doi:10.1111/evj.13166