Accuracy of Open MRI for Guiding Injection of the Equine Deep Digital Flexor Tendon within the Hoof

Lesions of the distal deep digital flexor tendon (DDFT) are frequently diagnosed using magnetic resonance imaging (MRI) in horses with foot pain. The prognosis for horses with DDFT lesions to return to previous levels of performance is poor. Treatment options are limited; consisting of conservative therapy, desmotomy of the accessory ligament of the deep digital flexor tendon, injection of the digital sheath or navicular bursa, navicular bursoscopy or intralesional injection. Intralesional injection of biologic therapeutics shows promise in tendon healing, with increased number of experimental and clinical studies finding positive results. However, accurate injection of DDFT lesions within the hoof is difficult and requires general anesthesia. The Hallmarq open, low-field MRI unit was used to develop an MRI-guided technique to inject structures within the hoof. This procedure has been previously reported for injecting the collateral ligaments of the distal interphalangeal joint. Four clinical cases of deep digital flexor tendinopathy have been treated with MRI-guided injections using a similar technique.

The aim of this study was to evaluate accuracy of a technique for injection of the deep digital flexor tendon within the hoof using MRI-guidance, which could be performed in standing patients. We hypothesized that injection of the DDFT within the hoof could be accurately guided using open low-field MRI to target either the lateral or medial lobe at a specific location. Ten cadaver limbs were positioned in an open, low-field MRI unit to mimic a standing horse. Each DDFT lobe was assigned to have a proximal (adjacent to the proximal aspect of the navicular bursa) or distal (adjacent to the navicular bone) injection. A titanium needle was inserted into each tendon lobe, guided by T1-weighted transverse images acquired simultaneously during injection. Oil-based colored dye was injected as a marker. Post-injection MRI and gross sections were assessed by three blinded investigators experienced in equine MRI. The success of injection as evaluated on gross section was 85% (70% proximal, 100% distal). The success of injection as evaluated by MRI was 65% (60% proximal, 70% distal). There was no significant difference between the success of injecting the medial versus lateral lobe. The major limitation of this study was the use of cadaver limbs with normal tendons. The authors concluded that injection of the DDFT within the hoof is possible using MRI guidance. Future work should be focused on using the techniqe in live horses with tendon lesions, and more clinical studies are needed to determine the most efficacious biologic therapeutic for tendon healing.