Jane van Lennep answers:
At 17hh, she is a big girl and probably still growing! Stifles are likely to lock if the conformation of the hindleg is on the straight side. If the hocks are straight, the stifle will be straight and the patella less securely located in the correct position – it gets slightly out of alignment and fixes the joint. To click it back into position, push the horse’s weight onto the other hindlimb.
The hindleg may be conformed straight or it may be a phase the horse is going through as she grows. Usually, horses grow back end first and ‘go up behind’ when they have stifle problems, but this should resolve once they’ve stopped growing.
If your mare’s conformation is such that she will always have straight hindlegs, the situation may improve once she’s in work, and the muscles, tendons and ligaments tighten and strengthen as she becomes fit. Until she is mature enough to start proper work and be fittened up, as much turn out as possible will help, so she has a chance to gain as much natural fitness as possible. And keep her in good condition, as thin or weak horses seem more prone to this, but if in any doubt, consult your vet.
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