The progressive palmar rotation of the distal phalanx is a common sequelae of laminitis. A horse’s body weight and ground reaction forces put significant radial strain on the dorsal lamellae. This enables the deep digital flexor tendon (DDFT) to overcome the lamellae’s resistance and cause them to lose integrity, leading to coffin bone rotation.
During the 2025 American Association of Equine Practitioners (AAEP) Convention, in Denver, Colorado, Kali Slavik, DVM, DACVIM (LAIM), from the University of Pennsylvania, explained the importance of reducing the pull of the DDFT peak dorsal radial stress to reduce the severity of coffin bone rotation. Historically, veterinarians and farriers have used heel wedges to reduce tension exerted by the DDFT. However, wedges change the orientation of forces acting on the dorsal lamellae and may predispose to further damage. They can also perform a DDFT tenotomy, but this salvage procedure is undesirable when there is hope for a horse’s return to athletic potential.
Study on Botulinum Neurotoxin
In their study, Slavik and colleagues from UPenn investigated the use of botulinum neurotoxin to weaken the DDFT’s pull. At a dose less than 1,500 IU/horse, there are no expected systemic effects of botulinum toxin (botulism). They evaluated the effect of botulinum toxin (Botox) on ground reaction force distribution compared to untreated using orthotic devices.
The study involved six healthy, unshod, light breed study horses previously vaccinated against botulism with no active orthopedic diagnosis or history of laminitis. Slavik’s team recorded them standing and walking straight on a flat surface and took measurements of the whole foot, dorsal sole, palmar sole, toe, and heels. One forelimb was injected with 200 IU botulinum toxin A using ultrasound guidance; consecutive horses’ injected limb alternated between left and right, with the opposite forelimb serving as a control. The neurotoxin was diluted to 100 IU/2½ ml, with five 1 ml aliquots injected into the superficial (1), middle (2), and deep (2) muscle bellies of the deep digital flexor muscle.
At seven days following botulinum toxin injection, peak toe pressure reduced significantly by 20%, although the breakover time remained the same. The toe peak force at Day 7 then gradually returned to baseline.
Also at seven days, peak heel pressures increased significantly (63%), which also occurred at Weeks 4 and 6 but initially declined at two weeks. This biphasic effect has been documented in humans: The initial neuromuscular blockade results in decreased muscular tone with a transient rebound and secondary peak of muscle weakness due to muscle atrophy from chemical denervation. A biphasic effect did not occur in equine peak toe pressure results.
There were no changes in the control limbs at any time point.
In addition, Slavik reported, static stance center of pressure of the foot shifted by 10 mm palmarly by Day 7 with gradual return to baseline. Ambulatory peak stance center of pressure also had a significant palmar shift of 5.5 mm by Day 7. Moving the center of pressure toward the caudal heel reduced flexor moment and dorsal lamellar tensile strain. Offloading of the dorsal lamellae has a greater effect in static stance; however, the distal limb remains in its normal orientation with normal forces acting on the lamellae.
The botulinum toxin injection was more effective in shifting the foot’s center of pressure than external heel wedging (e.g., a 20-degree heel wedge). However, orthotics did outperform botulinum toxin in reducing peak toe pressure. Slavik suggested combining orthotics with Botox for additive effects, but more research is needed.
Take-Home Message
Treatment with botulinum toxin significantly reduced peak ground reaction force at the toe during breakover and transferred load to the palmar regions of the foot.
The use of botulinum toxin in the deep digital flexor muscle was not associated with any adverse effects, lameness, or systemic effects. The impact on the deep digital flexor muscle is transient. Therefore, therapeutic use is likely limited to short-term, acute laminitis flares such as seen with acute systemic inflammatory response syndrome or endocrine dysregulation that typically resolve within one to two weeks with medication. This treatment is not appropriate for support-limb laminitis or chronic endocrine dysfunction where lamellar insults last beyond a couple of weeks.
Slavik also noted that at this time, a 200 IU vial to treat one limb costs $1,260 USD, plus other supplies, skilled ultrasound guidance, and procedure costs. In addition, botulinum toxin is a category A bioterrorism agent, so it can only be used by personnel with appropriate training.
Related Reading
- Disease Du Jour: Managing Equine Laminitis
- A Comparison of Cryotherapy Techniques to Treat Laminitis
- Recognizing Early Lamellar Structural Failure in Performance Horses
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