By Dr Ross Elliot
Medial patella luxation (MPL) is a common developmental orthopaedic condition of the canine patient. Small breed dogs are more commonly affected by the condition than large breed dogs. Patella luxation also occurs in large breeds dogs with medial luxation occurring more commonly than lateral luxation. Medial patella luxation can also be seen in cats, the surgical management is similar to that of dogs.
Medial patella luxation is easily detected clinically by palpating the patella with the leg in extension and feeling the patella luxating medially to the trochlear groove. These patients are usually young, under a year of age.
Radiographs are generally used to rule out any other conditions which may be present. It is essential to radiograph these patients for 2 reasons. Firstly, in middle age patients it is less likely for the patella to be clinically significant and the clinician should make sure there is no other condition such as cranial cruciate ligament disease. The other reason for radiographs is that the patella luxation may be the result of more complicated conformational conditions. These patients will usually have associated musculoskeletal abnormalities, including medial displacement of the quadriceps muscle group, distal femoral varus (Fig 1), hypoplasia of the medial condyle and rotation tibial deformity.
Clinically these patients have been graded as to the degree of patella luxation through the full range of motion (Table 1). This grading system takes into account the effects of the other components of the condition by their effect on the patella. The grading system does not however quantify the underlying components of the condition which may have lead to the patella luxation. The big concern is that a grade 2 or 3 luxation can be present in a patient with a marked femoral varus and in another patient with only a mild femoral varus. This grading does not thus allow us to accurately determine the best way forward in regards to surgical treatment of each individual patient. Grade 4 patients mostly have moderate to severe bony deformities that need correcting.
Table 1. Grading Patella luxation in the dog
|Grade 1||Patella can be luxated but spontaneous luxation seldom occurs. Once the patella has been luxated during clinical exam, the patella spontaneously reduces when the examiner releases pressure. The patella is stable in the trochlear in full range of motion|
|Grade 2||Angular and rotation deformities of the femur may be present. The patella can be luxated and remains luxated through full range of motion unless reduced by the examiner. The animal can clinically reduce the patella and this is seen with the skipping gait. Once reduced the patella remains in the trochlear groove for the full range of stifle motion.|
|Grade 3||Angular and rotational deformities are often present. The patella remains luxated most of the time, The patella can be reduced by the examiner however normal range of motion of the stifle lead to luxation of the patella.|
|Grade 4||Moderate to severe angular limb deformities are often present. The patella is permanently luxated and cannot be reduced to the trochlear groove at any point in the normal rang of motion of the stifle. The trochlear groove itself is often shallow or convex.|
There are many suggestions put forward to explain how MPL occurs. Some authors suggest that the pathogenesis differs in small breed and large breed dogs. A reasonable suggestion is that MPL develops from a decreased angle of inclination of the femoral neck, coxa vara. This leads to marked angular deformities of the distal femur from bowing of the distal femur, genu varum. This causes a relative tibial varus and internal rotation of the tibia on the femur. The patella is then forced medially due to the pull of the medial thigh muscles and hypoplasia of the medial condyle. Other studies have shown an increased angle of inclination of the femoral neck is associated with patella luxation in small breed dogs. Large breed dogs seem to have a relatively normal conformation of the femur when compared with small breed dogs. However large breed dogs with MPL tend to have a relative degree of patella alta (high-riding patella – structurally found more proximal in the trochlear groove) when compared to the conformation of normal large breed dogs, this may be cause or consequence of MPL in large breed dogs.
Surgical repair of patella luxation should be performed on animals showing clinical signs of lameness associated with the patella luxation or in young animals to prevent the long-term complications later on in life from the patella luxation. The main goal of surgical repair is re-alignment of the patella-quadriceps mechanism leading to normal sliding of the patella in the patella groove. Surgical repair has two categories, a release or an augmentation of the soft tissue components and corrective distal femoral osteotomy (CDFO) or tibial crest transplant (TCT).
It has been shown that soft tissue procedures performed without correction of the bone deformities have a high failure rate and should never be used as a sole method of repair. Surgical complications using the current techniques are reported as high as 85-48%. Surgical complications using the TCT in combination with soft tissue repair techniques are reported to be as high as 20%. These complication rates should be considered to high for a condition that is relatively common. (Fig 2a, 2b)
Surgical repair most often utilizes a combination of soft tissue and bone repair to reposition the patella in the trochlear groove. The trochlear groove can then be deepened using one of the many described techniques for a trochlearplasty. Only techniques that salvage the hyaline cartilage in the trochlear groove should be used.
Newer methods of creating a deeper trochlear groove have been developed in recent times. These consist of RidgeStop™ developed by Orthomed UK.(Fig 3) This uses a high-density polyurethane implant placed on the medial trochlear ridge to aid in the treatment of patella luxation. The current recommendation is that is should only be used alone in cases of mild patella luxations with no marked bone deformity of the leg. It can be used together with the bone corrective techniques to augment repair. The RidgeStop™ offers a less invasive method to deepen the patella groove to aid in movement of the patella in the normal alignment. It does not require the removal of a cartilage wedge to deepen the groove but does require accurate placement of 3 bone screws in the medial condyle to secure the implant. Kyron have developed an entire groove replacement made from titanium that the patella slides in. The author has no experience with this technique.
The disadvantage to both these techniques is the increased cost to the client for the surgery. The implant is significantly more than expensive than a trochlearplasty. This should be discussed with the client on a case to case basis. Our recent experience with Ridgestop™ is that is provides an adequate method for deepening the trochlear groove with less damage to the cartilage.
A corrective distal femoral osteotomy was until recent times only used for severe cases of patella luxation with a severe femoral varus and severe patella luxation. In these cases a guarded prognosis was given even though the animals did improve clinically but were never normal. However recently CDFO has gained popularity in correction of patella luxations with a moderate femoral varus.(Fig 4a,b) The challenge comes in patient selection for a TCT or CDFO. Most patients presenting with a patella luxation will have plain film radiographs as an initial step after the clinical exam. Plain film radiographs were shown to be 96% accurate in ruling a patient out of having a CDFO but only 76% accurate in patient selection for a CDFO. This was performed measuring the R-aLDFA of the femur, which was found to be an acceptable measurement to assess the varus deformity. R-aLDFA is a measurement made on radiographs to determine the center of rotation of the distal femur, CORA in order to perform a corrective osteotomy. It stands for the anatomic lateral distal femoral angle. The technique for measurement can be found in most surgical texts.
It is recommended, given the extra cost to the client, that patients which are radiographically selected for a CDFO should have a computed tomography scan performed of the femur and the R-aLDFA should be measured on the CT images. This gives the most accurate measurement of the femoral deformity in all planes. This allows the surgeon accurate planning for the surgery and gives the best possible outcome.
The reality of patella luxation surgery is that we don’t understand the cause of the disease in these patients hence the conflicting literature on how to fix it. Current evidence, be it only a few small studies, is pointing towards a lower complication rate with CDFO than TCT. This needs to be further evaluated to help us provide the best options for our patients. The reality is that the TCT with associated soft tissue procedures is an excellent surgical procedure in 80% of cases. However in the 20% of cases we see major complications it can be a nightmare to repair. Major complications require a corrective surgery with additional cost to the owner and morbidity to the animal. These complications can be symptomatic return of the patella luxation, tibial crest fracture, infection or tibia fracture. Implant migration is often seen and even though it requires another surgical procedure to remove it will often not affect the outcome once clinical union has occurred.
The 60% of dogs that develop a recurrent patella luxation at 4-6 weeks post TCT often are asymptomatic. Most of these will not require corrective surgery unless symptomatic as previously stated.
The underlying question is what harm is this recurrent patella luxation doing to the articular cartilage down the line?
The big challenge and hope of this lecture is that the surgeon will no longer apply 1 surgical technique to all patients with patella luxations. Instead then we should be assessing the underlying anatomical deformities of the patient and plan the surgery from there. The author suspects the high failure rate for TCT, up to 50%, is caused by this technique being used in all patients presenting with medial patella luxation without thought to the underlying anatomical deformities leading to or exacerbating the patella luxation.
Hopefully with further studies on the initial cause of patella luxation in young dogs and the long term outcomes of CDFO we can develop a treatment modality that gives us an excellent outcome in surgically correcting patella luxations.