(A) CONGENITAL ANATOMICAL PREDISPOSITION to medial patella luxation is the most common cause of the dislocation.There are two main issues involved in a patient with a medially luxating patella due to congenital abnormal anatomy
- The curved bones of both the femur (upper hind leg / thigh) and the tibia (shin) bones, which causes the pull of the tendon in which the patella is embedded to be medial, that is it constantly pulling the kneecap to the middle side of the knee.
- The shallow patella groove. Often this is so shallow that it is actualy non-existant! In such cases, the kneecap ALWAYS ends up dislocating.
(B) TRAUMATIC MEDIAL PATELLA LUXATION is the other, less common cause.
The severity of the dislocation varies from Grade 1 to 4.
A normal stifle / knee without any dislocation is a grade 0! Humans rarely suffer this medial dislocation, they are much MUCH more likely to have a lateral dislocation of the kneecap!
- GRADE 1: The kneecap slips in and out of its groove easily and will sit in its normal position when not being manipulated and when the patient is standing normally
- GRADE 2: Same as grade 1 except that the kneecap does not move back to its normal position when the manipulator lets go. Also there is often some degree of crepitus present (crepitus is the grating sensation we feel on manipulating the knee cap and is a result of the dislocating kneecap eroding away the joint cartilage as it pops in and out of its groove). Early to moderate arthritis usually present.
- GRADE 3: The patella is out of place, dislocated, all the time but can be manipulated back into its normal position manually (though it will not stay there). May be less painful than a grade 2 because the kneecap is no longer popping in and out, but the permanent dislocation causes the leg to be rotated or held outwards to some degree. Arthritis present.
- GRADE 4:The patella is not only out of place all the time but cannot even be manipulated back into place by the vet - even when under an anaesthetic. It is permanently stuck on the medial side. Such a dog has extreme difficulty extending his knees and walks with his knees bent virtually all the time and often with the whole leg angled and rotated out so it can walk.
What is the management advised for each of these stages of the patella luxation?
- GRADE 1: No surgery required. Advise ZYDAX (Pentosan polysufate) treatments (an injection course that stimulates the repair of worn or eroded joint cartilage, stimulates joint fluid production and inhibits a potent inflammatory enzyne that initiates arthritis - This course of 4 injections is repeated EVERY 6 months). MONITOR for deterioration to a grade 2 or 3. Concetrated Green Lip Mussel extract supplement also likely hel
- GRADE 2: Once crepitus is present or arthritis is noted on x-rays, surgery is advised. Surgery is a TWO STEP procedure involving BOTH the deepeining of the patella groove whilst maintaining its cartilage ( called a Wedge Trochleoplasty) AND ALSO transplanting the the patella tendon and its bone insertion point on the tibia to a more lateral, outside position (called lateral transposition of the Tibial Tubercle). It is strongly advised to commence ZYDAX courses, if not already having them
- GRADE 3: Surgery is always advised and necessary for the well being (pain management) and mobility of the patient. Both the procedures above (wedge trochleoplasty and lateral transposition) are performed PLUS a partial medial release of the scarred and contracted medial ligament and tissues on the medial side of the knee.of the Advise ZYDAX courses.
- GRADE 4: Surgery is always advised. The surgery is more advanced, requiring extensive medial-release surgery on the inside of the knee to free-up the kneecap from the permanent position on the medial / inside of the knee that it has effectively 'fused' into. Then a wedge trochleoplasty and lateral transpoistion surgery performed. Advise ZYDAX courses.
- TRAUMATIC MEDIAL PATELLA LUXATION: Surgery is advised in almost ALL cases - very few will heal without surgery, BUT it can occur. If there is some degree of curvature of the leg bones and/or some degree of shallowness to the patella groove, then the above procedures are required. If the legs are straight and the groove is normal in depth, then only the ruptured lateral patella ligament needs surgery plus the tightening of the lateral side of the joint capsule. Because the lateral joint capsule is stretched, then and now flacid, it provides no support, to the excess joint capsule is excised and the joint capsule repaired such that it provides the normal tension to guide the patella through its journey up and down in the groove. The tough, thick fibrous band of tissue above this is called the tensor fascia lata and is doubled (thereby tightening it) and a suture pattern called imbrication is used and this provides the lateral strength that replaces the function of the torn lateral patella ligament, and keeps the patella in place through out its normal range of movement.
What happens if I do not have surgery on my dog's knee?
Some grade 1 luxations do not worsen with time and never require surgery and keeping up with Zydax courses every 6 months minimises arthritis in this knee. Grade 2 knees develop arthritis over time and although Zydax will help, surgery is needed. Grade 3 knees already have arthritis after having gone through the grade 2 stage where the slipping kneecap shears off/ erodes the cartilage causing inflammation in the joint and degenerative changes that are arthritis.
Grade 2 knees may be the most painful also because of the constant stretching of the ligaments, attached to the kneecap, every time the kneecap pops in and out of its position in its groove.
*** The patella also adds structural strength to the stifle / knee joint. The patella tendon attaches the large quadriceps muscles to the tibial crest at the top-front of the shin bone. The strength of the muscle 'pull' on the patella helps prevent forward thrust forces within the knee and the reduced strength caused by a medial patella luxation INCREASES the risk of the anterior cruciate ligament tearing or rupturing.