How to Succeed in Repairing Medial Patellar Luxation in Dogs and Cats

Brian S. Beale, DVM, Diplomate ACVS

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Introduction

Patella luxation is a problem in all breeds and sizes of dogs, but the condition is most common in small breed dogs. Commonly affected breeds include the Yorkshire terrier, maltese, toy poodle, miniature poodle, pomeranian, pekingese and chihuahua. Medial patellar luxation predominates in both small and large breeds, although past literature suggests lateral luxation is much more common in large breeds. Patellar luxation occurs less frequently in cats and medial luxation is most common. Patellar luxation is generally graded from 1-4 based on increasing severity. Grade 1 patellar luxations are generally not repaired, but surgical repair is recommended for grades 2-4, depending on the age and clinical presentation of the patient. Treatment of medial patella luxation may be conservative (small breeds only) or surgical. The decision as to which method is applicable for a patient is dependent upon the clinical history, physical findings and the age of the patient. An older patient in which patella luxation is noted as an incidental finding on physical examination and in which the client reports nonclinical lameness does not warrant surgical intervention. Rather, the client should be informed as to the clinical signs associated with patella luxation. Surgery is advised in the young adult patient even though no clinical problem is apparent since intermittent luxation may prematurely wear the articular cartilage of the patella. Surgery is indicated in any aged patient exhibiting lameness and is strongly advised in a patient with active growth plates since skeletal deformity may worsen rapidly. However surgical techniques used in actively growing animals should be those that will not adversely affect skeletal growth. Surgical options include trochleoplasty, trochlear wedge recession, trochlear block recession, tibial tuberosity transposition, tibial tuberosity transposition, rectus femoris transposition, retinacular imbrication, derotational suture, retinacular releasing incision and corrective osteotomy in cases of femoral or tibial deformity. In severe cases that do not respond to the above treatments, patellectomy and stifle arthrodesis are a possibility; these techniques are fortunately rarely needed (these techniques will not be presented).

Clinical Findings

Pet owners typically report a skipping lameness in affected pets. Typically the pet uses the affected leg normally between skipping episodes. Some owners do not recognize any lameness or gait abnormality in affected patients. Patellar luxation frequently occurs bilaterally, but may one stifle may be more severely affected than the other. Owners often report a slow progression in severity of clinical lameness. The lameness may appear to resolve in some patients over time, but this may be due to the progression of patellar luxation from grade 2 to grade 3. The skipping gait may disappear because the patella is no longer displacing into and out of the trochlear groove. It the patella remains in a luxated position, the patient may not exhibit obvious lameness, but may have a bowlegged gait. Lameness that acutely worsens in patients with patellar luxation may be associated with a concomitant tear of the cranial cruciate ligament. Cranial cruciate ligament injury occurs in approximately 25% of patients with patellar luxation.

Patellar luxation is generally graded from 1-4 based on increasing severity. Grade 1 luxation is not associated with clinical lameness. The patella can be displaced out of the trochlear groove by applying digital pressure, but spontaneous luxation does not occur. Grade 2 luxation typically presents with an intermittent non-weightbearing lameness, the typical “skipping-gait”. Digital displacement of the patella is possible during examination, but the patella moves back into the trochlear groove when pressure is released or when the stifle is extended. Grade 3 luxation may present with intermittent non-weightbearing lameness or persistent weightbearing lameness. Many of these patients do not have an obvious lameness, but rather display a bowlegged posture when walking. The patella is typically luxated at the time of examination, but can be replaced into the trochlear groove with digital pressure. The patella usually quickly luxates again once pressure is released or the stifle is moved through a range of motion. Grade 4 luxation presents as a persistent weightbearing lameness or bowlegged gait. The patella is fixed in a luxated position and can not be reduced with digital pressure, even in the anesthetized patient.

Radiographic Findings

Figure 1. This grade 4 MPL patient has varus deformity of the distal femur and valgus deformity of the proximal tibia. Slight internal rotation of the bones is also present.

Patients having medial patellar luxation should be evaluated with appropriately positioned orthogonal survey radiographic views of the stifle. Orthogonal views of the entire femur and tibia should also be evaluated if limb deformity is present in small breed dogs and in all medium and large breed dogs with patellar luxation. The patient should be assessed for patella position, distension of the joint capsule, presence of tibial translation, tibial tuberosity position, axial alignment of the femur and tibia, torsional alignment of the femur and tibia, and osteoarthritis. CT imaging is recommended, if available; to more accurately assess hind limb alignment.

Radiographic changes vary from no obvious change to severe limb deformity and marked patellar displacement depending on the grade of luxation, age at onset of patellar luxation and duration of the condition. Minimal radiographic changes are seen in adult patients with uncomplicated grade 1 or 2 medial patellar luxation. Some patients have no abnormal radiographic changes. Radiographic changes that may be seen include patellar displacement, tibial tuberosity displacement, and rarely mild osteoarthritis and mild joint effusion. Grade 3 and grade 4 patellar luxations are more likely to have radiographic patellar displacement, tibial tuberosity displacement, joint effusion and osteoarthritis. These patients are also more commonly affected with axial or torsional abnormalities of the femur or tibia. Patients with severe medial patellar luxation and abnormal limb alignment usually have distal femoral varus, proximal tibial valgus, internal femoral torsion or internal tibial torsion. Radiographic assessment of the depth of the trochlear groove is usually best evaluated by palpation or gross observation, but severely shallow trochlear grooves can be seen radiographically.

Radiographic changes are most severe in puppies where the onset of patellar luxation occurs at an early age when the physis is undergoing rapid growth. Medial luxation of the patella in these dogs causes compression on one side of the distal femoral and proximal tibial physes and compression on the opposite side. As a consequence, the medial aspect of the femoral physis has retarded growth and the lateral aspect has accelerated growth resulting in distal femoral varus. The lateral aspect of the tibial physis has retarded growth and the medial aspect has accelerated growth resulting in proximal tibial valgus. Torsional deformity of the femur and tibia can also occur simultaneously. Correction of the deformity is usually based on comparison of the degree of angulation and torsion found on radiographic examination of the affected patient in comparison to normal reference values. The surgeon should be cautious when interpreting the measured angle of axial deformity as torsional deformity can artificially raise or lower the actual amount of axial malalignment. A CT scan is likely to give the most accurate measurement of axial and torsional deformity.

Figure 2. Tears of the cranial cruciate ligament is seen in approximately 25% of dogs with MPL

Patients with medial patellar luxation should also be evaluated for the potential for concomitant cranial cruciate injury. Typical radiographic changes include joint distension and cranial tibial displacement. Osteoarthritic changes are more likely with cranial cruciate ligament injury. If cranial cruciate ligament injury is suspected, measurement of the slope of the tibial plateau may be helpful when deciding on a surgical plan.

Complications associated with medial patellar luxation (MPL) repair can be categorized as intraoperative or postoperative. Complications are fairly common, but fortunately many are easy to resolve or prevent. Most complications can be avoided by better preoperative planning, meticulous surgical technique and appropriate postoperative care.

Decision-Making for Patellar Luxation Repair

Many surgical options are available when considering repair of the luxating patella. It is important to consider the underlying problems associated with the particular luxation when choosing a surgical plan. Factors to consider include, depth of the trochlear groove, alignment of the quadriceps mechanism (quadriceps, patella, patellar tendon), and the presence of excessive laxity or tension of the joint capsule and retinacular tissues medially and laterally. The surgical options chosen should alleviate the underlying factor contributing to the luxation. For example, if a dog has good alignment of the quadriceps mechanism, but a shallow trochlear groove- the surgical plan should include a technique to deepen the femoral trochlea, but not a tibial tuberosity transposition.

Methods to Deepen the Trochlea

Three methods are commonly used to deepen a shallow trochlear groove. These methods are described below. A head-to-head comparison as not been performed to document superior efficacy of one technique compared to the others. Usually trochleoplasty is reserved for toy-breed dogs and cats. Trochlear wedge recession and trochlear block recession are preferred for small, medium and large breed dogs, but also can be performed effectively in toy-breed dos and cats with a slight increase in technical difficulty.

Trochleoplasty

Trochleoplasty is a traditional technique that involves removal of articular cartilage and subchondral bone from the trochlear sulcus, thereby deepening the sulcus. Fibrocartilage repair is generally seen. This technique is considered less desirable to cartilage-sparing techniques described below, although it is sometimes used in toy breeds very successfully. Trochleoplasty is technically easy to perform. A deepened groove can be quickly formed using appropriate sized rongeurs. Attention should be paid to ensuring adequate depth of the groove proximally.

Trochlear Wedge Recession

Figure 3. A shallow trochlear groove should be deepened using a trochlear wedge or trochlear block recession.

Trochlear wedge recession provides a means of adequately deepening the trochlear sulcus, while preserving most of the articular cartilage. This technique is described elsewhere, but basically involves removal of a v-shaped wedge of bone and cartilage from the trochlear sulcus, removal of underlying bone, followed by replacement of the original wedge in a recessed position. This is an excellent technique, but technically more demanding than trochleoplasty. The technique is performed using a fine-tooth hand saw-blade. Care should be taken when beginning the saw cut, not to excoriate the adjacent cartilage due to slippage. The cut is initiated perpendicular to the cartilage surface adjacent to the peak of the trochlear ridge. Once the saw blade has engaged the subchondral bone, the blade is gradually redirected in the proper direction, parallel to the v-shaped trochlear groove. A cut is made from the lateral and medial ridge, meeting deep to the central sulcus of the groove. The wedge is removed and carefully stored to avoid accidental discard. The groove is further deepened by removing a block of bone from one side of the groove by making a parallel cut with the handsaw. A modification of this technique is to broaden and deepen the proximal aspect of the new, deepened groove by performing a partial trochleoplasty in the proximal aspect of the groove only, as described above using rongeurs. A portion of bone can also be removed from the underside of the trochlear wedge to further deepen the groove. The wedge is replaced and the adequate depth of the groove is documented. Fixation of the wedge is usually not needed due to pressure applied from the patella lying above and the congruency between the groove and wedge geometry.

Trochlear Block Recession

Figure 4. Saw-blade cut for trochlear block recession

Trochlear block recession is similar to trochlear wedge recession except that a block-shaped wedge is removed from the trochlear sulcus rather than a v-shaped wedge. This technique allows a deeper sulcus proximally, which may provide better biomechanical stability of the patella when the stifle is in an extended position. This is an excellent technique, but technically more demanding than trochleoplasty. The technique is performed using a fine-tooth hand saw-blade, a small osteotome and mallet. Care should be taken when beginning the saw cut, not to excoriate the adjacent cartilage due to slippage. The cut is initiated perpendicular to the cartilage surface adjacent to the peak of the trochlear ridge. Once the saw blade has engaged the subchondral bone, the blade is gradually redirected in the proper direction, perpendicular to the long axis of the bone. A cut is made from the lateral and medial ridge and each cut is carried to an adequate depth deep to the central sulcus of the groove. The block of cartilage and bone is removed gently using an osteotome and mallet. The osteotome is positioned just proximal to the intercondylar notch beginning at the depth of the trochlear cuts. The osteotome is directed towards the proximal extent to the trochlear groove. Gentle raps with the mallet will advance the osteotome, dislodging the trochlear block. The trochlear block is removed and carefully stored to avoid accidental discard. The groove is further deepened by removing a complimentary block of bone from the deep portion of the groove by making a parallel cut with the osteotome or by deepening with a rongeur. A portion of bone can also be removed from the underside of the trochlear block to further deepen the groove. The block is replaced and the adequate depth of the groove is documented. Fixation of the block is not needed due to pressure applied from the patella lying above and the congruency between the groove and block geometry.

Figure 5. Osteotome cut begins above the intercondylar notch Figure 6. Osteotome is used to elevate the trochlear block Figure 7. Osteotome is used to elevate the trochlear block

Alignment of the Quadriceps Mechanism

Tibial Tuberosity Transposition

Figure 8. The tibial tuberosity is moved laterally an appropriate distance to align the patellar mechanism such that the patella lies in the trochlear groove during flexion and extension.

Tibial tuberosity transposition is an excellent method of improving alignment of the patellar mechanism in patients having an abaxially displaced tibial tuberosity. If the tuberosity is displaced medially, luxation occurs medially; therefore, the tuberosity must be transposed laterally and secured. Lateral luxations require medial tibial tuberosity transposition. An osteotomy is performed as previously described; the tuberosity is transposed then secured with a single or multiple k-wires. An attempt is made when performing the osteotomy to leave the distal cortical bone intact to act as a tension band against the pull of the quadriceps mechanism. If the tuberosity is freed completely, it is prudent to secure the transposed bone with either a pin and tension band or a lag screw. The tuberosity should be transposed to a position that restores axial alignment to the quadriceps mechanism.

Rectus Femoris Transposition

This is a technique described by Dr. Barclay Slocum for use in bow-legged dogs having medial patellar luxation. This technique is done in combination with a medial releasing incision. A trochlear deepening technique should also be performed as needed. The rectus femoris is transected from its pelvic origin with a small piece of attached bone, then laterally transposed by tunneling under the vastus lateralis and reattaching it to the cervical tubercle or third trochanter of the proximal femur with wire or heavy suture. This realigns the quadriceps mechanism, restoring a straight-line pull.

Corrective Osteotomy of the Femur

Varus deformity of the distal femur is a contributing factor to medial patellar luxation particularly in large breed dogs. Accurate radiographic assessment of the distal femur is needed to measure angulation. If the distal femur has a varus deviation of greater than 10° a varus corrective osteotomy may be needed. A closing wedge osteotomy using a bone plate is commonly used for this procedure.

Corrective Osteotomy of the Tibia

Valgus deformity of the proximal tibia may require corrective osteotomy using a closing wedge osteotomy. This typically is only needed in dogs having severe medial patellar luxation when they were puppies. Unequal pressure on the growth plate leads to incongruent growth and angulation of the proximal tibia.

Retinacular Imbrication

Lateral imbrication is usually performed with correction of a medial patellar luxation as a means of creating lateral restraint. The stretching of the lateral joint capsule and retinaculum occurs chronically with longstanding patellar luxation. Occasionally a traumatic luxation may result in rupture of these tissues; imbrication is also a good technique for repair in this case. Imbrication is usually performed using heavy, absorbable, monofilament suture placed in a vest-over-pants- or horizontal mattress pattern. Care must be taken not to tighten the retinaculum excessively (especially if a retinacular releasing incision has been performed on the opposite side), because it is possible to create an iatrogenic luxation in the opposite direction. An alternative method of supplying lateral restraint is placement of a lateral derotational suture from the lateral fabella to a bone tunnel in the tibial tuberosity.

Retinacular Releasing Incision

A medial releasing incision is performed if fibrous hyperplasia has occurred medially following prolonged or severe medial patellar luxation. An incision is made through the retinacular tissues in a medial parapatellar location. The incision should extend proximally beside the medial edge of the quadriceps tendon. Placement of the incision in this location will release the insertion of the sartorius muscle, decreasing pull on the patella. The incision occasionally has to be carried deeper to include the joint capsule if marked joint capsular fibrosis has occurred creating excessive medial restraint. The incision is left open and not sutured. Arthroscopic medial releasing incisions can be performed. This technique is quick, easy to perform and has low morbidity. Long-term follow-up is presently unavailable. In addition, the clinical indications with this technique are presently unknown.