Medial Patellar Ligament Splitting

Upward fixation of the patella (UFP) occurs when the medial patellar ligament (MPL), with its parapatellar fibrocartilage, fails to disengage the notch of the medial ridge of the femoral trochlea at the commencement of limb flexion. This condition is more common in young horses and ponies, and Shetland ponies are most commonly affected.

Clinical signs of UFP are variable both in severity and frequency. The severe form is when the catching of the patella is complete so the leg cannot extend and does this repeatedly. The mild form is when there is a partial and intermittent locking of the patella and a palpable and sometimes audible click as the patella is released. The mildest form manifests as a subtle delayed release of the patella, which appears to move in a jerky fashion, especially as the horse decelerates. UFP is often bilateral and may affect one limb more than the other.

Medial patellar desmotomy (MPD) is advocated as the treatment of choice for surgical correction of UFP in cases unresponsive to conservative management and in severe cases.1 MPD was found to have detrimental effects on the femoropatellar joint of normal horses. Therefore it is no longer recommended as the treatment of choice for UFP.2

The purpose of this article is to describe MPL splitting for the treatment of UFP in the horse. The results and postoperative course of treating 7 cases are given.

Materials and Methods

Four horses and three ponies were admitted for surgery because of UFP. There were one Frieser (case 1), one Portuguese (case 2), 3 Shetland ponies (cases 3, 4, 5), one Hispano-Arabian (case 6) and one Anglo- Arabian (case 7). The ages ranged from 18 months to 10 years. A history was taken and a lameness examination performed and recorded on videotape before surgery and at each postoperative control. Lateromedial and caudocranial radiographs of both stifles were made before surgery and at every postoperative control.

The author in each case performed ultrasonography of the MPL with a real-time B-mode scanner using a 7.5 MHz transducer. Prior to surgery and at every postoperative control, the craniocaudal diameter of the proximal part of the MPL and its ultrasonographic appearance were evaluated. For surgery, the horses and ponies were placed under general anesthesia in dorsal recumbency and both hindlimbs were suspended under complete extension.

Each case underwent an ultrasound-guided percutaneous splitting of the proximal third of both MPL using a surgical knife with a No. 15 blade. The ultrasonographic transducer was placed transversally over the proximal part of the MPL and the blade was introduced longitudinally into the ligament in a craniocaudal direction. The blade did not proceed through the femoropatellar synovial pouch, which lies immediately under the ligament, nor through the articular cartilage of the medial ridge of the femoral trochlea. The blade was then fanned 45° proximally, then distally, laterally, and medially. The procedure was repeated at approximately 5 mm increments until the entire length of the proximal third of the MPL, as determined by intraoperative ultrasonography, had been split. Splitting was not performed on the parapatellar fibrocartilage of the MPL.