Upward fixation of the patella is a common condition of the stifle joint. It occurs when the medial patellar ligament hooks over the upper end of the medial trochlea of the femur (one of the two the bony ridges on which the patella slides).
In the most severe cases the patella becomes fixed and the horse is unable to flex the leg. The horse stands with the affected hind leg extended behind it. Less severe cases show partial, intermittent upward fixation, which produces visible, and sometimes audible, clicking as the patella frees itself.
The condition is most common in younger horses, and is often associated with poor muscle conditioning. Horses with straight hind limb conformation are more susceptible than horses with normal conformation. Both hind legs may be affected to some extent. When upward fixation of the patella occurs, the ligament stretches making a recurrence of the condition likely.
In less serious cases increased exercise, to improve muscle tone, may be all that is required to reduce recurrence of the condition. However, persistent cases may require surgical intervention.
For many years the recommended treatment involved cutting the medial patellar ligament ("medial patellar desmotomy"). Although this is effective at preventing the patella becoming fixed, recent work has shown that it is not without adverse effects. Fragmentation of the patella occurs after the operation in a significant number of cases.
Now a new technique has been developed which is effective and does not appear to be associated with side effects. Dr Aziz Tnibar, working in France at the Equine Clinic of the Veterinary School of Alfort, has devised a procedure in which the upper third of the medial patellar ligament is split.
The horse is anaesthetised for the procedure. Dr Tnibar uses ultrasound guidance to ensure the correct placing of the cuts, making sure that they do not extend into the femoro-patella joint or into the fibrocartilage extension of the patella
The aim of the operation is to cause thickening in the ligament to prevent it becoming hooked over the end of the femur.
"The rationale for percutaneous splitting of the upper third of the medial patellar ligament is to induce a localized desmitis, which subsequently leads to a localised thickening of the ligament. This will make it more difficult for the upper part of the ligament to hook over the medial ridge of the femoral trochlea" says Dr Tnibar.
Dr Tnibar has now performed this operation on seven horses and ponies. All seven were affected with upward fixation of the patella in both back legs. After medial patellar ligament splitting, all seven became sound and returned to work. In three horses the upward fixation of the patella resolved completely within 24 hours. In the other four cases, the condition resolved within 24 hours in one leg. The other leg improved gradually over a period of 4 - 12 days.
Dr Tnibar recommends daily light exercise (walking in hand for 15 minutes three times day) for 2 weeks after surgery. Normal activity is gradually introduced after two weeks.
Follow-up examinations show that the upper third of the medial patellar ligament becomes thickened. There is a progressive increase in medial patellar ligament size during the first four weeks after surgery. Radiographic examinations have not shown any sign of damage. In particular, no horses have developed fragmentation of the patella .
Dr Tnibar concludes that this is a successful technique for the treatment of upward fixation of the patella. "In each case all evidence of upward fixation of the patella has disappeared, and the horse (or pony) has regained its normal activity".
For more details see: M Aziz Tnibar Medial patellar ligament splitting for the treatment of upward fixation of the patella in the horse.Proc Am Assoc Equine Pract (2001) 47, 491 - 493