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

Overview

• This study investigated how injecting botulinum toxin into the deep digital flexor (DDF) muscle of healthy horses affects the biomechanics of their feet.
• The researchers measured changes in ground reaction forces and center of pressure to determine the impact of this treatment on foot mechanics during standing and walking.

Background and Objective

• The deep digital flexor (DDF) muscle plays a crucial role in equine foot movement and load distribution.
• Botulinum toxin, known for temporarily paralyzing muscles, was hypothesized to reduce the force generated by the DDF muscle.
• The goal was to understand the biomechanical effects of this reduced muscle activity on foot loading and positioning, which could have clinical implications for conditions like laminitis.

Methodology

• Six healthy horses were selected for the study.
• Each horse received a botulinum toxin injection into the DDF muscle of one forelimb; the other forelimb remained untreated as a control.
• Researchers measured ground reaction forces (GRF) on specific regions of the hoof, particularly focusing on the dorsal hoof wall (toe region), using pressure sensors.
• The center of pressure (COP) location was tracked relative to the dorsal hoof wall during static standing and peak stance phase of walking.
• Measurements were taken before treatment and then repeated at multiple time points post-injection: Days 3, 7, 14, 28, and 42.
• Statistical comparisons were performed using Friedman tests with Dunn’s multiple comparisons to evaluate changes over time.

Key Findings

• At 7 days post-injection:
  • There was a median reduction of 122 newtons (range 66 to 254 N) in peak ground reaction force at the toe of the injected limb.
  • The center of pressure shifted palmarly (towards the rear of the hoof) by 10 mm (range 8 to 16 mm) during static standing.
  • During ambulation, the COP also shifted palmarly by 6 mm (range 3 to 7 mm) compared to baseline.
• These biomechanical changes were transient, with values returning to non-significant differences by day 14 post-injection.
• No significant changes in GRF or COP were observed in the untreated control limbs throughout the study period.

Interpretation and Clinical Relevance

• Botulinum toxin injection into the DDF muscle temporarily reduces the force exerted by this muscle on the toe of the foot.
• The palmar shift in the center of pressure indicates a redistribution of load away from the dorsal hoof wall and toe region during standing and walking.
• This shift and reduction in force could reduce mechanical stress on the dorsal lamellae, which are the sensitive tissues connecting the hoof wall to the underlying bone.
• Such biomechanical effects suggest a potential therapeutic use for botulinum toxin in acute laminitis cases, where reducing distal phalangeal rotation and mechanical load on the hoof structures is desirable.
• The transient nature of the effects also informs treatment timelines and expectations for recovery and intervention planning.

Conclusion

• Injection of botulinum toxin into the deep digital flexor muscle of healthy horses produces a short-term reduction in toe loading and shifts pressure rearwards in the hoof.
• These effects may be harnessed clinically to mitigate mechanical stresses involved in laminitis and support hoof health during acute episodes.
• Further research may explore optimal dosing, duration, and clinical efficacy in diseased horses.