Q&A: Locking stifle joint

Q. I have a three-year-old ID X TB mare who, after an injury to her hind leg last August (and subsequent loss of condition), started to display fixation of the patella. She has now recovered from the injury and has put weight and condition back on, but still experiences what I can only describe as a ‘collapsing’ of her rear end.

While it is not obvious all the time, and doesn’t appear to cause her pain, it is very worrying. Should I ask a horse physio to have a look at her? My vet says that if the fixating doesn’t disappear, a minor operation can be performed – what are the implications of this?

Vet Jo Holmes answers: “It appears that your mare has developed upward fixation of the patella (kneecap) due to muscle wastage of her quadriceps muscles while recovering from her injury last year.

“The ‘quads’ unlock the stifle joint when the horse moves forward – otherwise, the stifle can lock as part of the ‘stay apparatus’ of the hind limb, preventing stifle flexion.

“When the quads contract, they should pull the middle and medial patellar ligaments off the end of the femur to free-off the patella itself, allowing full-range joint movement. If this does not happen, the stifle remains locked in extension and the leg is dragged behind the horse. Over time, this will damage joint ligaments and cartilage irreparably.

“In some cases, there is only intermittent catching of the joint, especially in young horses who are not yet musculo-skeletally mature. It is advisable to delay surgery to cut the medial patellar ligament in these cases, since they are likely to mature out of the problem. In time, your horse may also develop enough muscle to overcome the condition.

“Surgery is fairy straightforward and can be carried out on the sedated horse using local anaesthetic. There are some reports of premature stifle arthritis in horses who underwent this operation in their youth, but the risk has to be weighed against the risks of chronic stifle joint inflammation if the operation is not carried out.

“A physio may be able to help you build up the quads, but correct exercise should work as well. I would suggest building your horse up carefully using uphill work in walk and trot.”