Intermitten Upward Fixation of the patella in a horse (aka Sticky stifles)

The pony in the above video presented for a history intermittent dragging of his right hind leg at the walk. Occasionally, the leg would "lock" in the extended position and the pony would drag his limb for several strides until it "unlocked". On presentation, the pony tended to have a delay in flexion of the right hind when walked in a straight line. However, when walked in a figure 8, the right hind leg would stay in the extended position for a second or two and then suddenly jerk into the flexed position. When the pony was trotted in a straight line he was completely sound however during the downward transition his right hind wound "drop out" at the moment the leg went from the extended position to the flexed position.

This pony is experiencing a condition known as intermittent upward fixation of the patella. The patella (knee cap) is controlled by three patellar ligaments (medial, middle, and lateral patellar ligaments) which originate within the quadriceps muscle. As such, the contracture and relaxation of the quadriceps muscle allows for engagement of the patella to keep the leg in extension and ALSO allow for disengagement of the patella to allow flexion of the limb.

Intermittent upward fixation of the patella is common in young horses when they are first introduced to work under saddle. It is also more common in fast growing horses that tend to have a "straight" or upright  conformation through their stifle. In my experience, this condition is more common in female versus male horses and is most often resolved by increasing the intensity of exercise, especially at the canter, uphill. However, some horses experience this condition regardless of increased exercise and require medical/surgical intervention. Years ago it was common place to transect the medial patellar ligament however long term evaluation of these horses suggested a predisposition to arthritic conditions of the stifle joint. 

More commonly, these horse are treated with hormonal therapy (estrogen) , iodine blister of the medial patellar ligament, and scarring of the medial patellar ligament via surgical intervention. Mild cases often improve with increased work plus estrogen treatment. However, horses that are "locking" their stifles most likely will need surgical intervention.

Prior to recommending medical or surgical treatment, these horses should be evaluated for soundness and the stifle joint should be radiographed. Horses that have pain associated with OCD conditions of the stifle joint may present in a similar fashion and require a different approach in managing this condition. If your horses feels like he/she is "dropping out behind" when you are at the trot, especially during the downward transition from the trot to the walk or canter to the trot, it is likely that intermittent upward fixation of the patella is playing a role in their poor performance!!

Fractured Patella in a Horse

A teenage gelding presented for sudden non-weight bearing lameness in the right hind limb. There was a history of a "kick" from another horse but the exact location of the kick was not witnessed. On presentation, there was a basketball-size swelling centered on the stifle and the horse was very painful. He was not willing to walk on the limb. The initial radiographs were not conclusive due to the severe swelling and a follow-up exam was performed 10 days later. On presentation, the gelding was walking on the limb but was very resistant to have the limb flexed at the stifle/hock. There was minimal swelling compared to previous exam.
Figure 1 is a lateral radiograph of the patella. The yellow lines corresponds to bone fragments that are noted along the edges of the patella.

Figure 1

In order to better assess the patella, a special "sky line" projection is required and was especially difficult in this horse because he was resistant to having the stifle flexed! However, it proved to be the most important radiographic view. In  Figure 2, the patella is imaged and a distinct line can be noted traveling through the body of the patella. In Figure 3, the "line" is highlighted in red and is consistent with a complete or near-complete fracture of the patella. In Figure 4, a normal patella is imaged for comparison to the fractured patella.

Figure 2

Figure 3

The degree of fracture appears complete or near-complete which significantly worsens the prognosis. There are cases of patella fracture in horses that are described in the literature that have healed after several months of stall rest. However, there is minimal discussion regarding the future of these horses with regards to as equine athletes. The prognosis for this horse to return to work is poor and his outcome is yet to be determined.

Stifle OCD!!

A yearling thoroughbred presented for sudden lameness and swelling around the stifle joint. The filly had been purchased several months prior from a select thoroughbred sale and repository radiographs were performed at the time of purchase. At the time of purchase, the filly was sound, there was no effusion of the stifle joint and there were no obvious radiographic changes noted in either stifle joint. When the filly presented for lameness, the stifle radiographs were repeated. Close inspection of the lateral trochlear ridge suggested irregular contours (yellow arrows inside blue box, Figure 2) of the trochlear ridge along the mid body region (Figure 1 and 2, blue box). Based on the radiographs alone, the filly was treated with intra-articular Hylartin and forced stall rest for 30 days.

Figure 1

Figure 2

Unfortunately, the filly remained lame after 30 days of stall rest and a follow-up exam included an ultrasound evaluation of the stifle joint. The ultrasound exam revealed significant changes along the lateral trochlear ridge which were not fully appreciated on the radiographic exam. In Figure 3, the lateral and medial trochlear ridge are displayed. The bony surfaces of the trochlear ridges appear as a bright white lines and the cartilage as  thin black lines that follow the contour of the bony surface. Any disruption of the bony surface/cartilage will appear as a mix of disrupted white lines and black pockets. In Figure 3, the image on the left is of the lateral trochlear ridge and there is clearly disrupted bone and cartilage along the bony surface.

Figure 3

In Figure 4, the lateral trochlear ridge is imaged in a longitudinal plane to assess the length of the defect. Irregular bone/cartilage appears to extend for several centimeters along the length of the trochlear ridge (yellow lines). The filly was referred for arthroscopic exam and a large bone/cartilage defect was identified on the lateral trochlear ridge consistent with a osteochondritis dissecans or OCD lesion.

Figure 4

Osteochondritis dissecans is defined as a disorder that results in cracks in the articular cartilage and underlying subchondral bone. The "cracks" and cartilage separation result due to lack of blood flow to the region resulting in avascular necrosis of the tissues involved.  This condition is considered in part a hereditary disorder AND nutritional.  Unfortunately OCD lesions are common in young horses that were bred for sport such as racing thoroughbreds, performance quarter horses, and warm blood breeds.  Some cases of OCD can be managed with surgical intervention however the OCD lesion in this filly's stifle was so large that there was no hope for soundness. This case is a good example of the benefit of multiple imaging modalities in diagnosing the source of lameness in the equine stifle.