Rupture or sprain of the medial or lateral collateral ligaments of the stifle is usually the result of an acute traumatic episode in which the distal limb is forced medially or laterally, thereby stressing the ligaments. Rupture or sprain is more common in the medial collateral ligament than in the lateral collateral ligament. Concurrent injury of the menisci or cruciate ligaments is common, particularly in severe injuries. Lameness depends on the severity of the injury but is usually quite severe initially. Localized edema and joint effusion is seen, particularly in the acute stages. Flexion tests of the stifle usually exacerbate lameness. Clinical signs usually improve within a few days unless there is significant joint instability. Intra-articular anesthesia of the ipsiaxial femorotibial joint will not always alleviate lameness. If the ligament rupture is complete, stressed caudocranial radiographs of the stifle may demonstrate joint widening on the affected side. Enthesiophyte formation at the origin or insertion of the ligament may be evident radiographically in chronic cases. Diagnosis is usually confirmed by ultrasonographic examination.
Horses with mild sprains may be treated conservatively with stable rest and anti-inflammatory medication for 6–8 wk, followed by a controlled, ascending exercise rehabilitation program for a further 6–8 wk. Horses with mild sprains and no joint instability have a fair prognosis for return to athletic use. The prognosis for horses with severe injury is poor.