Two common causes of limping in the hind leg are:
1) Cranial cruciate ligament rupture
2) Medial patella Luxation
During flexion and extension of the knee joint (stifle), the patella (knee cap) should slide smoothly along the bone groove (called the trochlear groove) which lies at the end of the femur (thigh bone). Medial patella luxation occurs when the patella dislocates (or luxates) out of the trochlear groove. This condition affects many breeds of dog and one or both hind legs can be affected. Medial Patella luxation can cause a range of signs in your dog from mild skipping when running (where they will lift up the affected leg for a few steps and skip and then return back to normal gait) to permanently lifting the affected the leg and not using it at all. The condition can lead to arthritis in the knee joint and also increase the likelihood of cranial cruciate ligament rupture.
Medial patella luxation is a common condition in small breed dogs (less than 10kg), but can also occur in larger breeds. The causes of medial patella luxation can generally be divided into two groups: congenital or traumatic. Congenital or developmental causes involve multiple factors such as abnormalities of the bone and overall alignment of the stifle joint and are most commonly seen in small breed dogs. Traumatic causes are more common in larger dogs. This condition does occur in cats but only rarely.
The severity of the condition can range from mild to severe and not all animals require surgery. After a thorough physical examination along with the history you provide, our veterinarians will classify the medial patella luxation from grade I to IV (where IV is the most severe) and make the appropriate recommendations. In Grade I luxations, the Patella occasionally pops in and out of the trochlear groove but does not cause any lameness and generally will not require surgery. In Grade IV luxation the patella is always outside the trochlear groove and can’t be returned to its normal place, causing severe lameness. In this instance surgery is always indicated.
In order to evaluate the patella and stifle joint for surgery, the patient is anesthetised and radiographs (x-rays) are taken. The corrective surgery involves a combination of procedures to realign the ligament of the patella and may include remodelling of the bony groove so that it is deeper, making the patella less likely to luxate. These procedures involve cutting and repositioning of bone and insertion of surgical implants to aid with healing.