MPL is the most common orthopaedic problem in small breed (<10kg) dogs. It affects the stifle (knee) joints of the hindlegs and results in chronic intermittent luxation of the patella (knee cap).
The essential anatomical abnormality is that the Tibial Crest is medially deviated, i.e. it is positioned too far towards the inside of the leg. Because the tibial crest is the point of attachment for the patella ligament this medial displacement pulls the patella medially, towards the inside of the leg as the dog walks. The patella constantly being dragged across the medial ridge of the trochlea results in the medial ridge being worn down and reducing it’s resistance to the patella luxating out of the trochlea groove. Eventually the constant wear and tear results in significant arthritic changes in the stifle and pain and decreased function of the hind leg.
Some dogs may have no apparent clinical signs and even dogs with severe MPL seem to run around just fine, particularly if they are younger. That being said the most common clinical sign is “skipping” or the dog holding the leg up for 2 or 3 strides before going back to a normal gait. Usually this is intermittent and is caused by the patella locking in the dislocated position until the dog manages to manipulate the leg to get it back into a normal position. Other less frequent signs include a reluctance to jump or in severe cases a bandy legged appearance of the hind legs.
It is important to note that the disease is typically progressive i.e. a dog that has a mild MPL at a checkup will almost always get worse. This can occur quite quickly, over a period of 6 months, or it can be many years. Some dogs have low grade MPL their entire life without having clinical signs or needing treatment.
Patella luxation is graded according to an internationally recognised grading system.
Grade 0: Normal. Patella cannot be manipulated out of the trochlea groove.
Grade 1: IN/IN. Patella can be manipulated out of the trochlea groove but returns immediately once released.
Grade 2: IN/OUT. Patella can be manipulated out of the trochlea groove and remains out once released. Movement or manipulation of the leg will eventually cause the patella to return to the trochlea groove.
Grade 3: OUT/IN. Patella is out of the trochlea groove but can be pushed in to the trochlear groove. Movement or manipulation of the leg will eventually cause the patella to move out of the trochlear groove again.
Grade 4: OUT/OUT. Patella’s out of the trochlear groove and cannot be manipulated in to the trochlea groove. In many cases like this the trochlea groove is so worn that it barely exists.
As the underlying problem is anatomical treatment involves surgical realignment of the patella mechanism. Occasionally in very old animals (>14yrs) we may suggest medical management of the arthritis rather than surgery however most animals initially present with clinical signs requiring treatment at under 7 yrs of age and early treatment maximises long term outcome.
The current recommendations of the American, European, Australian and New Zealand Colleges of Veterinary Surgery for dogs with MPL are:
We have been trialling a new implant for use in cases of patella luxation. The implant is a medical grade high density polyethylene and is attached with surgical screws to the trochlear ridge on the side the patella luxates to thus replacing the need for a TGR. This creates increases the height of the ridge thus holding the patella in place. The big advantage of this implant is that it has MUCH lower post operative pain and morbidity than a TGR and a much faster return to function in the knee. Whereas with a TGR most dogs would take at least 3-4 weeks before they started toe touching and many would be 5-6 months before they regain preoperative function in the leg, with the Ridgestop most dogs are toe touching the next day and by 4 weeks post op have greater than 90% athletic function in the leg. In some cases it can even alleviate the necessity for a TCT, though if the tibial crest is displaced then realignment will still usually be required for long term benefit.