Introduction

• The medial extension of the patella and the medial patellar ligament are caught on the medial trochlear ridge of the femur either persistently or intermittently.
• Cause: straight hindlimb conformation   Musculoskeletal: conformation  , abnormal structure of the stifle joint, inflammation and pain of the stifle joints associated with conditions such as osteochondrosis or traumatic cartilage injuries, poor condition, unfitness, sudden changes in management and exercise.
• Signs: jerky gait during transition from stifle extension to flexion. Hindlimb held in caudal extension with fetlock joint flexed in persistent cases, leg releases either spontaneously or after help with sudden jolt or snap in the leg. Delayed release is intermittent and can be confused with Stringhalt, usually most obvious as the horse starts to move.
• Diagnosis: history and clinical signs, manual palpation of patella   Musculoskeletal: manipulative tests  , lameness examination, radiography   Hindlimb: radiography  , ultrasonography.
• Treatment: conservative - controlled exercise regimens, controlled turnout, improved health and conditioning, manipulation of patella or leg in acute cases, raised heel shoes     ; surgery - remove section    Medial patellar ligament: desmotomy  or split medial patella ligament   Medial patellar ligament: splitting  .
• Prognosis: good to fair with early treatment.

Pathogenesis

Etiology

• 'Straight' hindlimb conformations   Musculoskeletal: conformation   where the normal stifle angle of about 135° is extended to nearer 140° so that less extension movement of the stifle joint is necessary to allow upward fixation to occur. The straight limb conformation is common in some breeds, including many European Warmbloods, and is probably hereditary.
• Some clinicians believe that the condition of upward fixation is inheritable in some breeds.
• Occasional horses and ponies can present with anatomical abnormalities within their stifle joints that may predispose to upward fixation.
• Young animals, especially in poor condition.
• Undertraining.
• Sudden loss of condition.
• Animals being rested in stables for other reasons, particularly where there is a sudden change in management.
• Injury and/or pain in the stifle joints, especially involving the femoropatellar joint can lead to upward fixation as a secondary condition.
• Foot conformation may play a part with one study suggesting that a long toe/low heel and/or high inside wall hind foot may predispose to this condition.
• Persistent upward fixation can occur rarely secondary to luxation/subluxation of the coxofemoral joint of the same leg.

Predisposing factors

• Break in training.

Pathophysiology

• When the stifle joint extends to an angle of approximately 145° the medial cartilaginous extension of the patella (and associated medial patellar ligament) can catch over the enlargement that is present on the proximal medial trochlear ridge of the distal femur. This locks the stifle joint and, via the reciprocal apparatus, the rest of the hindlimb in extension allowing the horse to bear weight on the limb without muscular effort. In a normal horse this position is unlocked by the horse contracting the quadriceps and tensor fasciae latae muscles of the thigh region and pulling the patella proximally, and slightly laterally, off the medial trochlear ridge.
• In upward fixation the horse or pony either locks the patella more easily than normal, or more commonly, is unable to unlock the patella once it is in the locked position. The exact cause in each case varies, but in many cases appears to be related to inadequate function of the quadriceps and other thigh muscles. Some clinicians believe that persistent locking of the patella can lead to damage and changes in the patellar ligaments that then predispose to further locking episodes.
• In the condition of delayed release the patella catches on the medial trochlear ridge intermittently as the stifle joint is flexed and extended, causing the patella to move abnormally and the animal to flex.
• In long-standing and severe cases of upward fixation damage to the femoropatellar joint, including the cartilage on the underside of the patella and synovitis, may lead to lameness and joint distension increasing the likelihood of further episodes.

Diagnosis

Treatment

Prevention

Outcomes