Upward Fixation (Locking) of the Patella
The patella is located in the horse's stifle precariously perched on the lower (distal) end of the femur, which can be equated to the knee in a human. Many a good reference text will tell you that this condition can temporarily or permanently fixate the horse's stifle in an extended (straightened) position and thus leave the hind leg dragging out behind.
This condition is bought about by the action of the patella in the femoropatellar joint (F-P joint) sliding too far up the femur's trochlear groove. Now if that doesn't sound bad enough, fixation occurs through the medial patellar ligament catching over the medial condyle of the femur and thereby temporarily or permanently snagging itself in this position. This painful episode can stretch both the medial and middle patellar ligaments and destabilise the joint. Known contributing factors can vary from conformation (genetic predisposition), to nutrition, poor muscle tone and even neuromuscular disorders, all of which incorporate many possibilities.(1) My experience with this condition had been limited until early 1999 when a discerning miniature breeder bought it to my attention. Knowing my specific interest in the musculo-skeletal system of horses, I was invited to look at a horse that literally "clunked" as it walked and showed marked weight loss over its back and hindquarters.
Upon visual and, I might add, audible inspection, the so-called "clunk" was not only very loud, but the stifle joint had a two-phase action as it extended. After gathering myself back from disbelief, I then examined through palpation those regions of concern. Notably the back and hindquarters which exhibited severe muscle wastage and the stifles (where the "clunking" noise came from. Both stifles displayed severe instability and I can still recall the horror I experienced when the two bones passed over each other under my hands. In other words, when I held the stifle firmly with my hand the femur and tibia moved beyond the normal range of motion within the joint and clunked as they passed over each other. Stretched ligaments associated with the joint were the likely cause of instability, which explained why this horse's natural gait was so peculiar. It had learned to compensate over a period of time with an action that limits upward fixation of the patella and leads to muscle atrophy.(2)
The conformation of this horse was one aspect that undoubtedly led to this problem - a steep angle between the femur and tibia (a straight hind limb), with what appeared to be a long tibia. This had been a noted problem in Shetland ponies(3) and as my examination progressed over other suspect miniature horses on the stud, I could see that it occurred in varying degrees, much to the alarm of the unsuspecting breeder.
Unfortunately, this problem also appeared in a very attractive yearling that was sired by the same horse previously mentioned. This yearling had actually displayed intermittent locking of the stifle as observed by the owner, who was notably concerned. Upon an in depth discussion, the prognoses of various treatments were explained and included: firstly no treatment whatsoever, secondly a medial patellar desmotomy, or thirdly a devised therapy program.
With the first point of discussion, there was no guarantee that this problem would just disappear, and no treatment whatsoever could possibly lead to the same chronic condition that appeared in the older horse previously mentioned. The second option was the medial patellar desmotomy, which involves cutting the medial patellar ligament followed by a course of antibiotics and specific rehabilitation. However, a problem in ponies has occurred whereby the remaining two ligaments may cause patella luxation (dislocation), if they have an abnormally shallow trochlear groove.(1) The third alternative was a devised therapy program involving a specified training regime to strengthen and tone those muscles, tendons and ligaments involved with the joint.(2) This also incorporated designated stretches, joint mobilisation and massage therapy with specified essential oils. There was no certainty that the devised therapy program would work either, but as this horse was young and the condition minor, with only two observed locked patella episodes, the owner opted for the therapy program.
The initial step was for the owner to learn palpation techniques to ascertain whether or not significant changes had occurred. After one week a discussion took place between the owner and myself, and it was agreed that the yearling should come to me to continue with the program. Therefore, I had a unique opportunity to examine and work with this horse on a daily basis and write up the following case study.
Upon initial examination, the horse's stride fell short (the hind footprint did not reach the footprint from the foreleg), particularly the near side. Upon palpation the back and hindquarters were tight, with both stifle joints displaying minor instability, but notably the near side. Previous history involved a paddock by day, a stable by night and intermittent lunging as controlled exercise.
Once the yearling had arrived and settled in, the first week began with massage to release those muscles in spasm, going for walks through very tall grass and joint mobilisation. The horse tired quickly and by the beginning of week two displayed upward fixation of the patella on several occasions. Like in any athlete, the muscles will tire first before toning can begin and although it was disheartening to see the obvious locked position of the hind leg, I persisted with the regime.
By the beginning of week three, there had been no 'lock up' for several days and we slowly extended the work to incorporate small hills. The logs and fallen branches in this paddock intrigued the yearling and by the end of week three, I decided to incorporate the low logs into the routine, as upward fixation had not been seen for ten days. This proved to be very stimulating for the youngster, who turned out to be a natural over the logs and was now thoroughly enjoying its daily routine.
At the beginning of week four I decreased stretching, joint mobilisation and massage to twice a week and had its feet done. At no time while the hooves were being trimmed did there appear to be any sign of discomfort. The next step I took involved increasing the workload, which incorporated a steeper gradient in hill work and an increase in log size. It was also at this time that I contacted the owner and suggested a visit.
By the end of week four, the owner arrived to examine our progress. Upon palpation the back and hindquarters were supple, and the stifles showed no sign of instability. The next step was to show how the horse was working in hand. When we ventured to the hill paddock, the yearling, in an excited state and with a zest for life, gave for the very first time an upright and full thrust 'REAR'. As much as this was naughty and chastised accordingly, my smile was broad and extended from ear to ear. I knew that this maneuvre could not have been performed with such symmetrical grace and agility had those stifles been unstable and the back and hindquarters stiff.
This day ended with a graceful display of in-hand jumping, trotting with extension and a walk that displayed an over stride of three inches (7.5 cm)! We all left the hill paddock with a feeling of relief and hope for this young one's future.
In conclusion, the devised therapy program had been successful up to this point. But, it must be remembered that it has only been four weeks, and for a favourable prognosis to be complete, the conditioning program must eventually cease and the horse maintain joint stability for itself.
NOTE: As previously mentioned there are many factors that lead to upward fixation of the patella, and it is always best to have the condition correctly diagnosed before making a decision. I always recommend that a full inspection by a veterinarian take place prior to purchase, so that the unsuspecting buyer is not caught with an unsound horse.