In this article our vet David explains all about Luxating Patellas.
The kneecap (patella) is located in the tendon of insertion of the quadriceps muscle group. The quadriceps tendon inserts at the top of the shinbone (tibia) on a bony prominence called the tibial tuberosity. The patella runs in a groove (trochlear groove) on the end of the thighbone (femur.) The trochlear groove and the under side of the patella are covered in cartilage. All are within the stifle joint and the area is bathed in joint fluid.
A patella luxation is when the patella no longer runs in the trochlear groove. Patella luxation may lateral or medial i.e. the patella moves to the outside or inside of the joint respectively. Medial patella luxation is far more common than lateral luxation.
Fig 1. The diagram on the left shows normal position of the patella. In the diagram on the right the tibial tuberosity is located more medially consequently the patella is luxated medially and no longer runs in the trochlear grove
Medial patella luxation is graded 1-4 depending on severity.
Grade 1 the patella can be manually pushed out of the trochlear groove medially but returns to the correct position when the manipulator lets go. These dogs are not lame and do not require surgery.
Grade 2 the patella can be pushed out of the groove medially but does not return once the manipulator lets go.
Grade 3 the patella is out of the trochlear groove medially but can be returned to the trochlear groove by manipulation.
Grade 4 the patella is permanently luxated medially and cannot be returned to the trochlear groove.
Grade 2 may respond to an exercise program aimed at building quadriceps muscle mass and inwardly rotating the hip. If progress is slow with conservative management then surgery is indicated.
Grades 3&4 need surgery as soon as possible.
Any dog can suffer a traumatic patella luxation that may be medial or lateral. The dog suffers damage to the structures medial or lateral to the patella resulting in patella luxation. These usually need surgical repair.
Certain breeds present more commonly with medial patella luxation. These are usually the toy breeds such as the Yorkshire Terrier, Pug, and Toy Poodle. Larger breeds commonly presented are the Labrador and Staffordshire Bull Terrier. Often the smaller breeds seem to live with the condition and do not show lameness in early life despite the patellae being luxated. They often present later in life with lameness due to other pathology in the joint such as a ruptured cruciate ligament or as a result of arthritis in the joint.
Grade 1 and some Grade 2 patella luxations benefit from building up the quadriceps muscle mass. Exercises such as sit to stand and Up Hill Walking will help. The application of a tensor bandage can produce inward rotation of the hip that helps prevent medial patella luxation.
A number of techniques have been described for the surgical management of a medial luxating patella. Below are the common operations used either singly or more often in combination.
Tibial Tuberosity Transplant (TTT)
The tibial tuberosity with the quadriceps tendon attached is cut and moved laterally. The tibial tuberosity is then re-attached to the tibia using pins and wire or a screw.
Fig 2. Post op lateral x-ray of a tibial tuberosity transplant. The tibial tuberosity has been cut from the shaft of the tibia and moved laterally. It has then been reattached with a pin and wire.
The cut (osteotomy) site takes 6-12 weeks to heal.
In this technique the trochlear groove is deepened. The favoured technique is to remove a wedge from the groove and then take another slice from one of the cut side. The wedge is then replaced but now sits much lower creating a deeper groove. The advantage of this technique is that it preserves the cartilage in the trochlear groove. The wedge will reattach to the underlying bone in about 4-6 weeks.
With this technique a tuck is taken in the lateral joint capsule. This has the effect of holding the patella more firmly in the trochlear groove. This technique is usually done in conjunction with other techniques.
Originally posted in April 2015 of The Agility Voice.
By David Prydie BVMS, Cert SAO, CCRT, MRCVS