A positive response to blocking the stifle joint does not rule outlameness in the foot, recent research has shown.
Diagnostic analgesia is a common technique for locating the site of pain in lameness examinations. An anaesthetic agent is injected into a joint, or around nerves, with the aim of desensitising a specific area. If the lameness improves, that supports the site of pain being in that area.
However, things are not always as simple as they might seem. Anaesthetics don’t always stay where they have been injected and can diffuse out of joints. Their action may spread to affect neighbouring structures.
The sensory nerves supplying the lower hind limb pass close to the stifle joint, and may be blocked if the anaesthetic diffuses out of the joint.
Researchers at Cornell University College of Veterinary Medicine investigated whether anaesthetic injected into the stifle joint would influence lameness originating in the lower leg.
The study used nine adult horses and ponies of various breeds. Horses were excluded from the investigation if clinical examination or gait analysis found either a moderate bilateral hindlimb lameness or a severe unilateral hindlimb lameness.
The research team induced temporary hindlimb lameness by applying a clamp around one hind foot.
They injected local anaesthetic into the three compartments of the stifle joint, and then monitored the response with gait analysis every ten minutes for an hour and a half. At that point the foot was anaesthetised at the level of the fetlock joint (abaxial sesamoid block) to confirm that any remaining lameness originated in the foot.
In large joints like the stifle, it may take up to 30 minutes for anaesthesia to take effect. Sometimes if the lameness improves after that time it can be due to the stifle but may be due to anaesthetic affecting nerves passing close to the stifle joint..
The researchers found that intra-articular anaesthesia of the stifle produced reduction in degree of the induced foot lameness by up to 50% in a third of the horses within 30 minutes.
They suggest that clinicians should bear these results in mind before concluding that the stifle joint is the seat of the lameness. It may be that further tests need to be performed to rule out lower limb pain.
For more details, see:
Intra-articular anaesthesia of the equine stifle improves foot lameness
A Radtke, LA Fortier, S Regan, S Kraus, ML Delco.
Equine Vet Journal (2020) 52 p 314-319