Medial Patellar Luxation

Medial patellar luxation (MPL) is a common orthopedic condition in dogs and cats characterized by displacement of the patella from the trochlear groove of the femur in a medial direction. This luxation can occur as a congenital/developmental abnormality, often associated with skeletal deformities such as coxa vara, medial displacement of the quadriceps mechanism, and femoral varus, or as a result of trauma. The condition is graded from I to IV based on the severity of luxation and the ability to manually reduce the patella. Grade I involves intermittent luxation with manual reduction possible; Grade II shows frequent luxation with spontaneous reduction; Grade III involves persistent luxation that can be manually reduced but recurs; Grade IV is permanent luxation that cannot be manually reduced. Surgical correction is often indicated for symptomatic grades II-IV, especially in young animals to prevent progressive degenerative joint disease and skeletal deformity.

The etiology of medial patellar luxation is multifactorial. In congenital/developmental cases, there is a genetic predisposition, particularly in small and toy breeds, with a polygenic mode of inheritance. The primary anatomical abnormality is a malalignment of the quadriceps muscle group, which pulls the patella medially. This is often accompanied by a shallow trochlear groove, hypoplasia of the medial femoral condyle, and lateral bowing of the distal femur (femoral varus). Additionally, there may be torsion of the femur and tibia, and medial displacement of the tibial tuberosity. Traumatic causes include direct blows to the stifle, rupture of the medial retinaculum, or avulsion of the patellar ligament. In some cases, iatrogenic factors such as incorrect surgical techniques (e.g., over-tightening of the lateral retinaculum) can lead to iatrogenic medial luxation. Less commonly, neoplasia or severe osteoarthritis can alter joint biomechanics and contribute to luxation.

Medial patellar luxation is most prevalent in dogs, particularly in small and toy breeds such as the Chihuahua, Pomeranian, Yorkshire Terrier, and Boston Terrier. It also occurs in larger breeds, including the Labrador Retriever and Golden Retriever, where it may be associated with other orthopedic conditions like cranial cruciate ligament rupture. The condition is often bilateral, with a higher incidence in females. Age of onset is variable; congenital cases may be detected in puppies as early as 6-8 weeks of age, while traumatic cases can occur at any age. In cats, MPL is less common but can be seen in breeds like the Devon Rex and Abyssinian. The overall incidence in dogs is estimated at 1-2% of the population, but it is significantly higher in predisposed breeds. Working and athletic dogs may be more prone to traumatic MPL due to high-impact activities.

The pathophysiological cascade of medial patellar luxation involves a combination of skeletal and soft tissue abnormalities. The primary biomechanical defect is a medial displacement of the quadriceps mechanism, which includes the quadriceps muscle, patella, and patellar ligament. This displacement results in an abnormal pull on the patella, causing it to luxate medially. The trochlear groove may be shallow or absent, providing inadequate bony constraint. As the patella luxates, it articulates with the medial femoral condyle, leading to cartilage erosion and degenerative joint disease. Chronic luxation causes remodeling of the distal femur and proximal tibia, with progressive femoral varus and tibial torsion. The medial retinaculum and joint capsule become stretched, while the lateral retinaculum becomes contracted. In growing animals, the abnormal forces can lead to growth deformities, such as a valgus deformity of the distal femur and internal rotation of the tibia. Over time, the patella may become permanently luxated, and the quadriceps muscle may atrophy. In severe cases, the patella can cause erosion of the medial femoral condyle, leading to pain and lameness.

Predisposing factors for medial patellar luxation include genetic predisposition, with certain breeds having a higher risk due to inherited conformational traits. Conformational factors such as a shallow trochlear groove, hypoplastic medial femoral condyle, and femoral varus are intrinsic risk factors. Age is a factor, as developmental MPL often manifests in young animals, and trauma is more common in active adults. Body weight and obesity can exacerbate the condition by increasing stress on the stifle joint. Extrinsic factors include trauma, such as falls or vehicular accidents, and iatrogenic causes from previous surgeries. Nutritional factors, such as over-supplementation of calcium or vitamin D during growth, may contribute to skeletal deformities. Management factors, such as excessive exercise on hard surfaces, can also predispose to traumatic luxation.

Clinical signs of medial patellar luxation vary with the grade of luxation. In Grade I, animals may show intermittent lameness, often with a 'skipping' gait, where the limb is carried briefly after a luxation episode. Palpation may reveal a patella that can be manually luxated but reduces spontaneously. Grade II animals exhibit frequent lameness, with the patella luxating during normal activity and reducing spontaneously. There may be crepitus and mild joint effusion. Grade III animals have persistent luxation, with the patella remaining luxated most of the time but manually reducible. Lameness is more consistent, and there may be visible deformity of the stifle, with the tibial tuberosity deviated medially. Grade IV animals have permanent luxation, with the patella not reducible. Severe lameness, muscle atrophy, and angular limb deformities are evident. On physical examination, palpation of the stifle reveals the patella displaced medially, and there may be pain on extension and flexion. In chronic cases, signs of degenerative joint disease, such as joint thickening and decreased range of motion, are present.

Differential diagnoses for medial patellar luxation include: 1) Cranial cruciate ligament rupture - presents with acute lameness, positive cranial drawer sign, and joint effusion; radiographs may show osteoarthritis but not patellar luxation. 2) Lateral patellar luxation - patella displaced laterally, often associated with coxa valga and lateral bowing of the femur. 3) Fracture of the patella - acute lameness, swelling, and crepitus; radiographs show fracture line. 4) Osteochondritis dissecans of the stifle - lameness, joint effusion, and pain on full flexion; radiographs may show subchondral bone defect. 5) Septic arthritis - severe lameness, joint swelling, fever, and systemic signs; synovial fluid analysis shows septic inflammation. 6) Immune-mediated polyarthritis - multiple joint involvement, stiffness, and fever; synovial fluid analysis shows non-septic inflammation. 7) Neoplasia of the stifle - progressive lameness, palpable mass, and radiographic bone lysis. 8) Patellar ligament rupture - acute lameness, inability to extend the stifle, and patella alta on radiographs. 9) Quadriceps contracture - chronic lameness, stiff gait, and inability to flex the stifle; often a sequela of femoral fracture. 10) Hip dysplasia - lameness may be referred to the stifle, but orthopedic examination reveals hip pain and positive Ortolani sign.

The diagnostic algorithm for medial patellar luxation begins with a thorough history and physical examination, including orthopedic and neurological assessment. Palpation of the stifle is performed to assess patellar stability, with the animal in lateral recumbency. The patella is manually luxated medially and laterally to determine the grade. If MPL is suspected, radiographs of the stifle are obtained in craniocaudal and mediolateral views to evaluate the depth of the trochlear groove, position of the patella, and presence of degenerative changes. In cases with suspected angular deformities, full-limb radiographs are taken to measure femoral varus and tibial torsion. Advanced imaging such as CT or MRI may be indicated for complex cases, especially to assess the trochlear groove depth and rotational deformities. Diagnostic arthroscopy can be used to evaluate the articular cartilage and confirm the diagnosis. In traumatic cases, a thorough neurological examination is essential to rule out nerve damage. Synovial fluid analysis may be performed if septic arthritis is suspected. The diagnostic algorithm ensures a systematic approach to confirm MPL and identify any concurrent orthopedic conditions.

Laboratory findings in medial patellar luxation are typically unremarkable unless there is concurrent disease. Synovial fluid analysis may show mild inflammation in chronic cases, with increased protein and cell count, but it is usually non-septic. Hematology and biochemistry are within normal limits. Coagulation panel (PT/aPTT) is recommended before surgery to assess bleeding risk. In traumatic cases, blood gas analysis may be indicated to evaluate for metabolic acidosis. Inflammatory biomarkers such as C-reactive protein (CRP) and serum amyloid A (SAA) may be elevated in cases with significant joint inflammation. Urinalysis is part of the routine preoperative workup. These tests help rule out systemic disease and ensure the patient is a suitable surgical candidate.

Radiography is the primary imaging modality for medial patellar luxation. Craniocaudal and mediolateral views of the stifle are obtained. In MPL, the patella is displaced medially, and the trochlear groove may appear shallow. In chronic cases, signs of osteoarthritis, such as osteophytes and subchondral sclerosis, are evident. Full-limb radiographs are essential to assess femoral varus and tibial torsion, which are common in MPL. The femoral varus angle is measured on a craniocaudal view of the femur, with a normal angle of 0-5 degrees. In MPL, the angle is often increased. Tibial torsion can be assessed on a tibial plateau view. Ultrasonography may be used to evaluate the quadriceps muscle and patellar ligament, but it is not routinely performed. CT is valuable for precise measurement of femoral varus and trochlear depth, and 3D reconstructions aid in surgical planning. MRI is useful for assessing cartilage damage and soft tissue structures, but it is rarely needed. Arthroscopy allows direct visualization of the articular cartilage and can confirm the diagnosis and assess the severity of cartilage damage.

Cytology of synovial fluid in medial patellar luxation typically shows a mild increase in nucleated cell count, predominantly mononuclear cells, with good mucin clot quality. In cases with concurrent osteoarthritis, there may be a mild increase in neutrophils. Histopathology is not routinely performed for MPL, but if a biopsy of the joint capsule or synovium is taken during surgery, it may show mild synovial hyperplasia and fibrosis. In cases with severe cartilage damage, histopathology of the articular cartilage may show fibrillation and erosion. If a mass is present, fine-needle aspiration and histopathology are indicated to rule out neoplasia.

Treatment of medial patellar luxation depends on the grade and clinical signs. Conservative management is reserved for Grade I and asymptomatic Grade II cases, including weight management, exercise restriction, and non-steroidal anti-inflammatory drugs (NSAIDs) for pain. Surgical correction is indicated for symptomatic Grade II, Grade III, and Grade IV cases. The surgical approach involves a lateral parapatellar arthrotomy. The surgical technique includes: 1) Lateral retinacular release: an incision is made in the lateral joint capsule and retinaculum to allow lateralization of the patella. 2) Trochleoplasty: deepening of the trochlear groove, either by recession wedge trochleoplasty or trochlear chondroplasty, to provide better bony constraint. 3) Tibial tuberosity transposition: the tibial tuberosity is osteotomized and moved laterally, then fixed with a Kirschner wire or a small bone screw. 4) Medial retinacular imbrication: the medial joint capsule and retinaculum are tightened to prevent medial luxation. In cases with severe femoral varus, a distal femoral osteotomy may be necessary to correct the angular deformity. Postoperative management includes pain control with opioids and NSAIDs, cold therapy, and restricted activity for 4-6 weeks. Physical rehabilitation, including passive range of motion exercises and controlled leash walks, is initiated early. The choice of suture materials includes absorbable monofilament (e.g., polydioxanone) for capsular closure, and non-absorbable (e.g., nylon) for retinacular imbrication. Implants for tibial tuberosity transposition include Kirschner wires (0.045-0.062 inch) or 2.0-2.7 mm screws. Intraoperative complications include patellar fracture, implant loosening, and iatrogenic damage to the articular cartilage.

The prognosis for medial patellar luxation is generally good to excellent, especially for grades I-III. Surgical correction has a success rate of over 90% in restoring normal limb function. Factors that negatively affect prognosis include severe osteoarthritis, grade IV luxation, concurrent orthopedic conditions, and surgical complications such as infection or implant failure. In young animals, early surgical intervention can prevent the development of skeletal deformities and osteoarthritis. Long-term follow-up studies show that most dogs return to normal activity, but some may develop mild lameness or progressive osteoarthritis. The recurrence rate is low, around 5-10%, but may be higher in cases with severe deformities.

Postoperative follow-up for medial patellar luxation includes suture removal at 10-14 days. Radiographs are taken at 4, 8, and 12 weeks to assess bone healing and implant position. Restricted activity is recommended for 4-6 weeks, with gradual return to normal activity over 8-12 weeks. Physical therapy, including passive range of motion exercises, swimming, and controlled leash walks, is initiated at 2 weeks postoperatively. Long-term monitoring includes annual orthopedic examinations and radiographs to assess for the development of osteoarthritis. In cases with tibial tuberosity transposition, radiographs are taken at 8 weeks to confirm healing. If complications such as implant loosening or infection occur, additional follow-up is required.

Clinical pearls: 1) Always assess the entire limb for angular deformities, as femoral varus is common in MPL. 2) In young animals, consider early surgical intervention to prevent progressive deformity. 3) Use a recession wedge trochleoplasty in young animals to preserve articular cartilage. 4) When performing tibial tuberosity transposition, ensure the patella tracks centrally in the trochlear groove throughout the full range of motion. 5) Perform a lateral retinacular release before assessing patellar tracking to avoid over-tightening. Pitfalls: 1) Failure to correct femoral varus can lead to recurrence. 2) Over-tightening of the lateral retinaculum can cause lateral patellar luxation. 3) Inadequate trochlear deepening can result in persistent instability. 4) Damage to the articular cartilage during trochleoplasty can lead to osteoarthritis. 5) Improper implant placement can cause patellar fracture or implant loosening.

Perioperative drug protocols for medial patellar luxation surgery are based on Plumb's Veterinary Drug Handbook. Prophylactic antimicrobials: Cefazolin 22 mg/kg IV at induction and every 90 minutes during surgery. Postoperative analgesics: Opioids such as morphine 0.5-1 mg/kg IM or IV q4-6h, or fentanyl CRI at 2-5 mcg/kg/h for 24 hours. NSAIDs: Carprofen 2.2 mg/kg PO q12h for 3-5 days, or meloxicam 0.1 mg/kg PO q24h. Local anesthetic blocks: Bupivacaine 1-2 mg/kg intra-articularly or as a femoral nerve block. Muscle relaxants: Methocarbamol 15-20 mg/kg PO q8h for 3-5 days. Chondroprotectants: Polysulfated glycosaminoglycan 4.4 mg/kg IM or SC twice weekly for 4 weeks, or oral glucosamine/chondroitin supplements. For pain management, a multimodal approach is recommended, including opioids, NSAIDs, and local anesthetics. In cases with severe inflammation, corticosteroids may be used, but they are generally avoided due to potential adverse effects on cartilage.

Landmark studies on medial patellar luxation include: 1) A study by Roush (1993) evaluating the results of surgical correction in 100 dogs, reporting a 92% success rate. 2) A study by Arthurs and Langley-Hobbs (2006) comparing trochleoplasty techniques, showing that recession wedge trochleoplasty provides better long-term outcomes. 3) A study by Fitzpatrick et al. (2012) on the use of distal femoral osteotomy in combination with MPL correction, demonstrating improved outcomes in cases with femoral varus. 4) A consensus statement by the American College of Veterinary Surgeons (ACVS) on the management of patellar luxation, recommending surgical correction for grades II-IV. 5) A meta-analysis by Wangdee et al. (2018) on the prevalence and risk factors for MPL, confirming breed predispositions. These studies support the current surgical techniques and emphasize the importance of addressing concurrent skeletal deformities.