The effect of botulinum toxin injection into the deep digital flexor muscle on foot biomechanics in healthy horses

Objective: To examine the effects of botulinum toxin inoculation into the deep digital flexor (DDF) muscle on foot biomechanics. Methods: 6 healthy horses were injected with botulinum toxin in the DDF of 1 forelimb. The opposite forelimb was untreated. Ground reaction forces (GRF) were measured using a pressure sensor in regions of interest, including the dorsal hoof wall (toe). The location of the center of pressure (COP) was measured relative to the dorsal hoof wall during static standing and at peak stance during walking. Repeat measurements 3, 7, 14, 28, and 42 days after injection were compared to baseline (pretreatment) using Friedman tests with Dunn multiple comparisons. Results: There was a median 122-N (IQR, 66 to 254) reduction in the peak GRF at the toe at 7 days after Botox injection. The COP was shifted palmarly by 10 mm (IQR, 8 to 16) during stationary stance and by 6 mm (IQR, 3 to 7) during ambulation at day 7 compared to baseline. These changes were no longer significantly different to control by 14 days. No significant changes were seen in untreated limbs. Conclusions: Treatment with botulinum toxin transiently reduced peak GRF at the toe and caused a palmar shift in COP at rest and during ambulation. Clinical Relevance: These biomechanical effects of DDF botulinum toxin inoculation would be expected to reduce mechanical stress on the dorsal lamellae and could be useful for treatment of acute laminitis to help reduce distal phalangeal rotation.