Medially luxating patella (MLP) refers to the tendency for the kneecap to slip out of its normal groove and to become trapped on the inside of the knee or stifle joint. It is a common cause of hind limb lameness in small breed dogs but will occur in dogs of all ages, sizes and breeds.
During growth and development the quadriceps muscles in front of the thigh tends to pull the kneecap to the inside of the leg rather than pulling the kneecap up and down the trochlear groove. Over time the inside ridge of the trochlear groove erodes and the groove fails to deepen because of the absence of normal wear.
Lameness occurs because the kneecap does not have a normal groove to slide in. With time joint restriction and osteoarthritis may develop. There is an increased risk of rupturing the cruciate ligaments.
Severity of MLP is determined by palpation of the knee joint and is classified from 0 (normal) to 4 (severely affected) as follows;
the patella can be pushed out of the groove but spends most of time in the groove
the patella spends half the time in the groove and half the time out of the groove. It can easily be manipulated in or out but has no tendency to stay in either position.
the patella spends almost all the time outside the groove and with pressure can be pushed back into the groove.
the patella spends all the time outside the groove and even with pressure the patella cannot be pushed back into the groove.
Grade 1 and early grade 2 MLP may only require medical treatment for pain and minor swelling within the knee joint. Mid-stage grade 2, grade 3 and grade 4 are best corrected with surgery. The three common surgical procedures are described as follows;
Tibial Crest Transposition (TCT). The patella tendon attaches to the top of the tibia. This area of bone is surgically separated from the shaft of the tibia and repositioned on the outside of the tibia. The bone is secured by pins and wires or a surgical screw. Recovery time to normal walking is 6-8 weeks although most dogs are weight bearing within 1 to 2 days.
Wedge Recession Trochleoplasty (WRT). The groove in the femur where the patella lies is surgically deepened by cutting out a wedge shaped section. The resected wedge can be rotated 180° to increase the height of the medial ridge.
Lateral imbrication and medial release. This involves overlapping the sheaths on the outside of the knee to reinforce their strength and cutting and stretching the tendon sheaths on the inside of the knee to reduce their contraction effect.
Any, a combination of or all of the above techniques may be used to repair MLP. This decision is made during surgery and depends on the size of the dog, degree of luxation, conformation of the knee joint and the presence of osteoarthritis.
The prognosis following surgery is good to excellent in at least 95% of cases. Those cases that have a poorer prognosis are those with poor knee conformation, grade 3 and 4 luxations and those knees with osteoarthritis.