Canine Patellar Luxation Study Guide

Overview and Clinical Importance

Patellar luxation is one of the most common orthopedic conditions in dogs, characterized by displacement of the patella from the femoral trochlear groove. This condition represents a significant category of musculoskeletal disease on the NAVLE and is frequently encountered in clinical practice.

Patellar luxation can be medial (toward midline), lateral (away from midline), or bidirectional. Medial patellar luxation is approximately 12 times more common in small breed dogs compared to large breeds, while lateral luxation is more commonly seen in large and giant breeds.

Epidemiology and Breed Predisposition

Demographics

Sex predisposition: Females are 1.5 times more likely to be affected than males across all breeds.

Neuter status: Neutered dogs have approximately 3 times the odds of developing patellar luxation compared to intact dogs.

Age of diagnosis: Most luxations are diagnosed in young adults, typically within 3 years of age, though the condition is congenital or developmental in nature.

Bilateral occurrence: Reported in 20 to 52 percent of affected dogs. Bilateral luxation occurs in 46 percent of small dogs and 36 percent of large dogs.

Breed-Specific Patterns

Pathophysiology and Etiology

Primary Etiology

The vast majority of patellar luxations are congenital and developmental in origin, with a strong hereditary component. Although the exact mode of inheritance is not fully understood, genomic studies have identified chromosomal anomalies in specific breeds such as the Dutch Flat-Coated Retriever and Pomeranian.

Traumatic patellar luxation can occur following blunt trauma to the stifle joint, causing tearing or stretching of the joint capsule and retinacular structures. This is less common and typically has acute onset associated with a specific injury.

Quadriceps Mechanism Malalignment

The fundamental problem in patellar luxation is malalignment of the quadriceps extensor mechanism. This mechanism consists of the quadriceps muscle group, patella, trochlear groove, patellar ligament, and tibial tuberosity. Current theory suggests the pathology begins at the hip joint level, with abnormalities cascading distally.

The quadriceps angle (Q-angle) is formed between the axis of the rectus femoris muscle and the patellar tendon. Small increases in the Q-angle produce dramatic increases in medially directed vector forces, triggering progressive limb deformities during growth.

Associated Skeletal Deformities

Multiple skeletal abnormalities develop secondarily to the abnormal forces exerted during growth:

Exam Focus: The NAVLE loves to test the concept that skeletal deformities in patellar luxation develop SECONDARILY to the malalignment, not as the primary cause. The abnormal forces during growth remodel the developing bones. Remember: it's not that the shallow groove causes luxation; rather, the luxation prevents normal groove development.

Clinical Grading System

Patellar luxation is graded on a four-point scale based on physical examination findings. The grading system is crucial for determining appropriate treatment and prognosis.

Diagnostic Approach

Physical Examination Technique

Gait evaluation: Observe patient at walk and trot. Classic finding is intermittent 'skipping' lameness where the dog holds the affected limb up for a few steps. May see dog extend leg backward in attempt to reduce patella. Assess for bowlegged (genu varum) or knock-kneed (genu valgum) conformation.

Standing examination: Begin with patient standing. Palpate for joint effusion and periarticular soft tissue thickening. Locate tibial tuberosity first, then follow patellar ligament proximally to identify patella. If patella is already luxated in standing position, assign minimum grade III.

Manipulation technique: Best performed with patient in lateral recumbency to reduce muscle tension. Isolate patella between thumb and index finger of one hand. With other hand, grasp hock or foot and manipulate leg. For medial luxation: extend stifle and twist leg internally while pushing patella medially. For lateral luxation: extend stifle and twist leg externally while pushing patella laterally. Assess ease of luxation, ability to reduce, crepitus, range of motion, and presence of drawer sign (to rule out concurrent cranial cruciate ligament disease).

Additional palpation: Palpate trochlear groove depth if possible. Assess for tibial tuberosity deviation and limb torsion or angulation. Note muscle atrophy, especially of quadriceps.

Diagnostic Imaging

Radiographic evaluation is essential for confirming diagnosis, assessing skeletal deformities, and evaluating degenerative joint disease. A minimum database includes:

• Mediolateral and craniocaudal projections of the stifle
• Ventrodorsal view of the pelvis
• For high-grade luxations: orthogonal views of entire femur and tibia to assess rotational and angular deformities

Radiographic findings:

Advanced imaging: Computed tomography (CT) is increasingly used for preoperative planning, especially in high-grade luxations requiring corrective osteotomies. CT allows accurate measurement of femoral and tibial torsion, femoral varus angle, and other deformities. Three-dimensional reconstructions can be created for surgical planning.

Concurrent Conditions

Cranial cruciate ligament (CrCL) disease: Concurrent CrCL rupture has been reported in 13 to 41 percent of dogs with medial patellar luxation. Risk factors include age, higher grade of luxation, and advanced degenerative joint disease. The CrCL should be systematically assessed during orthopedic examination using cranial drawer and tibial compression tests.

The mechanism linking MPL and CrCL disease is thought to be increased strain on the ligament due to anatomic abnormalities associated with patellar luxation, leading to progressive degeneration.

Treatment Options

Conservative Management

Indications: Grade I patellar luxation in asymptomatic dogs or dogs with infrequent, mild lameness. Some Grade II cases with minimal clinical signs.

Prognosis with conservative management: Many Grade I dogs remain comfortable without surgery. Grade II may progress to Grade III due to progressive erosion of trochlear ridge and chronic degenerative changes. Monitor closely for progression.

Surgical Management

Indications: Grades II, III, and IV with clinical lameness or functional impairment. Some Grade II cases may be managed conservatively if infrequently symptomatic. Young dogs benefit from early surgery to prevent progressive skeletal deformity and osteoarthritis. The decision to operate on Grade II should be based on frequency and severity of clinical signs.

Soft Tissue Procedures

Soft tissue techniques address imbalanced forces on the patella but are rarely sufficient alone. Typically used as adjuncts to osseous procedures.

Osseous Procedures

Osseous techniques form the cornerstone of surgical correction. Most commonly combined procedures are used.

Tibial Tuberosity Transposition (TTT):

TTT is the single most important procedure for correcting patellar luxation. Failure to perform TTT is the most common cause of surgical failure. Performed in 71 percent of small breed cases and 86 percent of large breed cases.

Technique: Osteotomy of tibial tuberosity performed with osteotome or oscillating saw, leaving distal periosteal attachment intact. Tuberosity is transposed laterally (for MPL) or medially (for LPL) until quadriceps-patella-patellar tendon forms straight line. Fixed with Kirschner wires (0.035 to 0.062 inch diameter) and figure-of-eight tension band wire (18 to 22 gauge). Two pins provide superior stability. Pins are bent proximally and cut 3 mm beyond bend for later removal.

Critical considerations: In large, active dogs, lag screw or supplemental tension band wire is critical to prevent fixation breakdown. Tuberosity may also be moved distally to create appropriate patellar tendon tension. Adequate fragment size essential to accept implants.

Trochleoplasty (Sulcoplasty):

Techniques to deepen trochlear groove. Goal is groove deep enough to embed approximately 50 percent of patella above trochlear ridges. All techniques involve some degree of cartilage damage.

Evidence for trochleoplasty: Studies show significantly lower reluxation rates when trochleoplasty is combined with TTT compared to TTT alone. Reluxation rates: 6 to 8 percent with trochleoplasty versus 19.8 percent without. Performing trochleoplasty provides a 5.1-fold reduction in reluxation risk.

Corrective Osteotomies:

Required for significant femoral or tibial malalignment, particularly in large breed dogs and high-grade luxations. Common procedures include distal femoral osteotomy (for femoral varus) and tibial derotational osteotomy (for excessive tibial torsion). Require advanced imaging (CT) for accurate preoperative planning and measurement of deformities. Fixed with plates and screws.

Postoperative Management

Complications and Prognosis

Surgical Complications

Overall complication rate: 18.5 to 29 percent. Most common complications are implant-associated problems and patellar reluxation.

Prognosis

Overall prognosis: Favorable for grades I, II, and III. Excellent to good outcome in 94 percent of surgically treated cases. Fair to poor outcome in 6 percent.

Grade-specific outcomes:

• Grade I: Excellent; most dogs remain asymptomatic with or without surgery
• Grade II: Excellent; 100 percent success rate in some studies when surgery performed
• Grade III: Good to excellent; approximately 11 percent recurrence rate
• Grade IV: Guarded, especially in young dogs; 36 percent reluxation rate due to severe skeletal deformities

Factors affecting prognosis:

• Body weight: Increasing weight associated with higher complication risk
• Age at surgery: Earlier intervention prevents progressive deformity
• Completeness of correction: Addressing all components of malalignment improves outcome
• Concurrent CrCL disease: Worsens prognosis; requires simultaneous or staged treatment
• Large breed dogs: May have less favorable prognosis due to additional orthopedic conditions like hip dysplasia

Remember: P.A.T.E.L.L.A. for Patellar Luxation Prognosis - Progression worsens without surgery, Age at surgery matters (earlier is better), Trochleoplasty reduces reluxation significantly, Early osteoarthritis develops without treatment, Large breeds face more challenges, Luxation grade correlates with outcome, Address ALL components of malalignment for success.