Subluxation of Patella

Subluxation of the patella, more commonly known as patellar luxation or luxating patella, is an orthopedic condition in which the kneecap (patella) slips out of its normal anatomical position within the trochlear groove of the femur. The patella is a small, flat, roughly triangular bone embedded within the tendon of the quadriceps muscle group. It glides smoothly within the trochlear groove during normal flexion and extension of the stifle joint, serving as a fulcrum that improves the mechanical efficiency of the quadriceps.

When the patella subluxates, it displaces either medially (toward the inside of the leg) or laterally (toward the outside of the leg), disrupting the normal mechanics of the knee. Medial patellar luxation is by far the more common presentation, accounting for approximately eighty percent of cases, and is especially prevalent in small and toy breed dogs. Lateral luxation occurs more frequently in large and giant breed dogs, though it can affect dogs of any size.

The condition ranges from a mild, intermittent displacement that spontaneously returns to its normal position to a severe, permanent dislocation that cannot be manually reduced. This spectrum of severity is classified using a standardized grading system from Grade I through Grade IV, which guides treatment decisions and helps predict long-term outcomes. Patellar subluxation is one of the most frequently diagnosed orthopedic conditions in dogs and is a significant consideration in breeding programs for predisposed breeds.

Patellar subluxation can affect one or both knees. Bilateral involvement is common, particularly in dogs with a congenital predisposition, though one knee may be more severely affected than the other. The condition can develop at any age, but signs often become apparent during the first year of life in dogs with developmental causes, or later in life when the condition is associated with trauma or degenerative joint changes.

The most common cause of patellar subluxation is congenital or developmental skeletal abnormalities that alter the alignment of the extensor mechanism of the stifle joint. These structural abnormalities can include a shallow or absent trochlear groove, rotational deformities of the tibia, malalignment of the tibial tuberosity where the patellar ligament attaches, bowing of the femur or tibia, and abnormalities in the angulation of the hip joint. These anatomical variations create a mechanical environment in which the patella is predisposed to displacement.

Genetic factors play a major role in the development of patellar subluxation. The condition is strongly heritable, and selective breeding of affected individuals has contributed to its high prevalence in certain breeds. Small and toy breeds are disproportionately affected, with the Chihuahua, Pomeranian, Yorkshire Terrier, Toy and Miniature Poodle, Maltese, and Pekingese among the most commonly diagnosed. However, the condition is increasingly recognized in larger breeds as well, including the Labrador Retriever, Golden Retriever, and Boxer.

Traumatic injury can also cause patellar subluxation in dogs that do not have a congenital predisposition. A direct blow to the stifle, a sudden twisting injury during vigorous activity, or a fall from a significant height can damage the soft tissue structures that hold the patella in place, including the joint capsule, retinacular ligaments, and quadriceps tendon. Traumatic luxation can occur in any breed and is typically unilateral, affecting only the injured knee.

Obesity is a significant contributing factor that can both precipitate the initial subluxation and accelerate the progression of joint damage over time. Excess body weight increases the mechanical load on the stifle joint and amplifies the forces that tend to displace the patella from its groove. Maintaining a lean body condition is one of the most effective measures owners can take to reduce the severity of patellar subluxation and slow the development of secondary osteoarthritis.

Veterinary orthopedic specialists use a standardized four-grade classification system to assess the severity of patellar subluxation. This grading system is essential for guiding treatment decisions, as the grade directly influences whether conservative management or surgical intervention is recommended. The grading is determined through physical examination, with the veterinarian manually manipulating the patella to assess its stability and relationship to the trochlear groove.

Grade I represents the mildest form of the condition. In Grade I luxation, the patella can be manually displaced from the trochlear groove during examination, but it spontaneously returns to its normal position when the manipulating pressure is released. Dogs with Grade I luxation are often asymptomatic or show only occasional, brief episodes of lameness characterized by a skipping gait that resolves on its own within a few steps.

Grade II luxation is characterized by a patella that can be manually displaced or that spontaneously luxates during normal flexion of the stifle, but can be manually reduced or returns to its normal position when the joint is extended. Dogs with Grade II luxation typically show intermittent lameness that becomes more frequent over time. The repeated displacement and reduction of the patella causes progressive erosion of the articular cartilage on both the patella and the trochlear groove, leading to increasing discomfort and the development of osteoarthritis.

Grade III luxation describes a patella that remains displaced most of the time but can be manually reduced to its normal position, only to re-luxate as soon as the manipulating pressure is removed. Grade IV is the most severe, in which the patella is permanently luxated and cannot be manually reduced to the trochlear groove. Dogs with Grade III and IV luxations typically have significant skeletal deformities, chronic lameness, and may adopt a crouched posture with the affected limbs held in abnormal positions. These higher grades almost always require surgical correction.

The clinical signs of patellar subluxation vary widely depending on the grade of the condition, the duration of the problem, and whether one or both knees are affected. The most recognizable sign, particularly in Grade I and II cases, is an intermittent skipping or hopping gait. The dog will be walking or trotting normally and then suddenly lift one hind leg for a few strides, carrying it in a flexed position before placing it back down and resuming normal locomotion. This occurs when the patella slips out of the groove and then pops back in.

As the condition progresses or in dogs with higher-grade luxations, lameness becomes more persistent and pronounced. Dogs with Grade III or IV luxation may walk with a crouched, bowlegged stance, with the hind limbs held in a semi-flexed position. The range of motion in the stifle joint may be noticeably reduced, and the dog may be reluctant to jump onto furniture, climb stairs, or engage in vigorous play. In bilateral cases, both hind limbs are affected, which can make the gait abnormality appear more generalized and may be mistaken for neurological disease.

Pain associated with patellar subluxation can range from minimal in early, low-grade cases to significant in advanced cases with secondary osteoarthritis. Dogs may vocalize briefly at the moment the patella luxates, or they may show more chronic signs of discomfort such as decreased activity, difficulty rising from a resting position, and reluctance to bear full weight on the affected limb. Muscle atrophy of the quadriceps group on the affected side is common in chronic cases as the dog favors the limb.

Crepitus, a grinding or crackling sensation felt or heard when the joint is manipulated, indicates cartilage erosion and early osteoarthritic changes within the stifle. Swelling of the stifle joint may be palpable in cases with significant inflammation or effusion. In severe, longstanding cases, particularly those with Grade IV luxation, the skeletal deformities associated with the condition may be visibly apparent, with noticeable angular limb deformities and rotational abnormalities of the tibia.

Diagnosis of patellar subluxation is primarily made through physical examination by a veterinarian experienced in orthopedic assessment. The examiner palpates the patella with the dog in lateral recumbency, flexing and extending the stifle while applying medial and lateral pressure to assess the stability of the kneecap. The ease with which the patella can be displaced, whether it returns spontaneously, and whether it can be manually reduced all determine the grade of luxation.

Radiographic imaging is an important complement to the physical examination. Standard views of the stifle, including craniocaudal and mediolateral projections, help evaluate the depth of the trochlear groove, the position of the tibial tuberosity, the presence of femoral or tibial rotational deformities, and the degree of osteoarthritic changes. Skyline views of the trochlear groove provide additional detail about groove depth and patellar tracking. Radiographs are essential for surgical planning, as they reveal the specific skeletal abnormalities that need to be addressed.

Advanced imaging modalities such as computed tomography may be recommended in complex cases, particularly those involving significant skeletal deformities or when surgical planning requires precise three-dimensional assessment of bone alignment. CT scanning provides detailed cross-sectional images that allow the surgeon to measure torsional deformities of the femur and tibia with greater accuracy than standard radiographs.

A thorough orthopedic examination should also evaluate the integrity of the cranial cruciate ligament, as patellar luxation and cranial cruciate ligament disease frequently coexist. The chronic instability created by patellar subluxation places abnormal stress on the cruciate ligament, predisposing it to partial or complete rupture. Concurrent cranial cruciate ligament disease significantly affects the surgical approach and prognosis, making its identification an essential part of the diagnostic workup.

Conservative, non-surgical management may be appropriate for dogs with Grade I patellar luxation and some dogs with Grade II luxation that experience only infrequent, self-resolving episodes of lameness. The goal of conservative management is to maintain joint function, minimize discomfort, strengthen the supporting musculature, and slow the progression of osteoarthritis. It is important to understand that conservative management does not correct the underlying anatomical abnormality and that progression to a higher grade may eventually necessitate surgery.

Weight management is the single most impactful non-surgical intervention. Keeping the dog at a lean, ideal body condition reduces the mechanical stress on the stifle joint and decreases the force driving the patella out of its groove. A structured weight management program involving caloric restriction and portion control should be implemented for any overweight dog with patellar subluxation. The veterinarian can help determine the dog's ideal weight and develop an appropriate feeding plan.

Physical rehabilitation and controlled exercise play important roles in conservative management. Targeted exercises that strengthen the quadriceps and hamstring muscle groups help stabilize the stifle joint and improve patellar tracking. Underwater treadmill therapy and swimming provide low-impact cardiovascular exercise that builds muscle strength without placing excessive stress on the joint. A certified canine rehabilitation therapist can design a customized exercise program tailored to the individual dog's needs and limitations.

Pharmaceutical management may include non-steroidal anti-inflammatory drugs for dogs experiencing pain or inflammation associated with their patellar subluxation. Joint-supportive nutraceuticals such as glucosamine, chondroitin sulfate, and omega-3 fatty acids may be recommended to support cartilage health. Adequan (polysulfated glycosaminoglycan) injections are another option that may help protect articular cartilage and reduce the progression of osteoarthritis. Regular veterinary reassessment, typically every six to twelve months, is necessary to monitor for progression and determine if surgical intervention becomes warranted.

Surgical correction is recommended for dogs with Grade II luxation that shows progressive lameness, and is generally considered necessary for all dogs with Grade III and Grade IV patellar luxation. The specific surgical techniques employed depend on the underlying anatomical abnormalities identified during the diagnostic workup. Most surgical corrections involve a combination of soft tissue and bony procedures designed to restore normal patellar tracking and prevent re-luxation.

Trochleoplasty is a procedure that deepens the trochlear groove to provide a more stable channel for the patella. Several techniques exist, including trochlear block recession, trochlear wedge recession, and trochlear chondroplasty. Block and wedge recession techniques preserve the articular cartilage lining of the groove, which promotes better long-term joint health compared to older techniques that removed the cartilage entirely. The choice of trochleoplasty technique depends on the degree of groove shallowness and the surgeon's preference and experience.

Tibial tuberosity transposition involves repositioning the bony prominence on the tibia where the patellar ligament attaches. By moving this attachment point to a more anatomically correct position, the line of pull of the quadriceps mechanism is realigned, reducing the tendency for the patella to displace. The transposed tuberosity is secured in its new position with pins or a combination of pins and tension band wiring. This procedure is often performed in conjunction with trochleoplasty for optimal results.

Soft tissue reconstruction addresses the imbalances in the joint capsule and retinacular tissues that contribute to patellar instability. On the side toward which the patella luxates, the stretched and redundant joint capsule and fascia are tightened through imbrication or overlap techniques. On the opposite side, tight or contracted tissues that pull the patella out of alignment may be released. In severe cases with significant angular limb deformities, corrective osteotomy of the femur or tibia may be necessary to restore normal limb alignment before the patellar luxation can be effectively addressed.

Recovery from patellar luxation surgery requires a structured rehabilitation program that typically spans eight to twelve weeks. During the initial two to three weeks, strict rest and confinement are essential to allow the surgical sites to heal. The dog should be restricted to a crate or small pen, with leash walks limited to brief outings for elimination only. An Elizabethan collar or recovery suit should be used to prevent the dog from licking or chewing at the surgical incision.

Physical rehabilitation begins early in the recovery process, often within the first week after surgery, with gentle passive range-of-motion exercises performed on the operated joint. These exercises help prevent the formation of adhesions, maintain joint flexibility, and promote cartilage nutrition. As healing progresses, controlled leash walking is gradually increased in duration and distance, with the veterinarian or rehabilitation therapist providing specific guidelines for each stage of recovery.

Hydrotherapy, including underwater treadmill sessions and supervised swimming, is typically introduced at approximately four to six weeks post-surgery, once the incision has fully healed and the bone work is sufficiently stable. The buoyancy of water reduces weight bearing on the healing joint while allowing active muscle contraction and joint movement. This form of exercise is particularly valuable for rebuilding quadriceps muscle mass, which is critical for long-term patellar stability.

Full return to normal activity is typically permitted at ten to twelve weeks post-surgery, contingent upon satisfactory healing confirmed by follow-up radiographs and physical examination. Some dogs recover to full function within this timeframe, while others may require several additional weeks of graduated exercise before achieving their best outcome. The overall success rate for patellar luxation surgery is high, with studies reporting good to excellent outcomes in approximately ninety percent of cases. However, re-luxation rates of approximately ten to fifteen percent have been reported, particularly in dogs with high-grade luxation, significant skeletal deformities, or bilateral disease.

While the majority of dogs that undergo patellar luxation surgery have excellent outcomes, complications can occur. The most common complication is re-luxation of the patella, which may result from inadequate surgical correction of all contributing factors, failure of implant fixation, or progression of skeletal deformities in young, growing dogs. Re-luxation may necessitate revision surgery, which is generally more technically challenging and carries a somewhat lower success rate than primary correction.

Implant-related complications include migration or breakage of pins used to secure the transposed tibial tuberosity, and irritation of surrounding soft tissues by prominent hardware. In some cases, implant removal is required after the bone has healed to resolve ongoing discomfort. Seroma formation, surgical site infection, and patellar fracture are less common complications that may occur in the early postoperative period and typically respond to appropriate medical management.

Osteoarthritis is a long-term consequence of patellar subluxation that develops regardless of whether the condition is managed surgically or conservatively. The degree of arthritic change depends on the severity and duration of the luxation, with higher-grade and longer-standing cases experiencing more significant cartilage damage. Surgical correction performed early in the course of the disease, before extensive cartilage erosion has occurred, is associated with less severe arthritis and better long-term joint function.

The long-term outlook for dogs with patellar subluxation is generally favorable, particularly when appropriate treatment is provided in a timely manner. Dogs that undergo successful surgical correction typically return to normal or near-normal activity levels and enjoy a good quality of life. Those managed conservatively for low-grade luxation can also do well with consistent weight management, exercise modification, and ongoing monitoring. Regular veterinary follow-up throughout the dog's life ensures that any progression or complications are detected and addressed early, maximizing the chances of a positive long-term outcome.

Prevention of patellar subluxation centers on responsible breeding practices, as the condition has a strong genetic component in most affected breeds. Dogs diagnosed with patellar luxation of any grade should not be used for breeding, as they are likely to pass the predisposition to their offspring. Breeding stock should undergo orthopedic evaluation, including patellar palpation by a veterinarian, prior to being included in a breeding program. Organizations such as the Orthopedic Foundation for Animals maintain registries for patellar evaluation results, providing breeders with a tool for making informed mating decisions.

Early screening of puppies from predisposed breeds allows for early detection and intervention before significant joint damage occurs. Veterinarians routinely check patellar stability as part of puppy wellness examinations, and any abnormality should prompt further evaluation and monitoring. Puppies identified with early-stage patellar instability may benefit from weight management, controlled exercise, and strengthening activities that support joint stability during the critical growth period.

For owners of breeds predisposed to patellar subluxation, maintaining a lean body condition from puppyhood through adulthood is one of the most effective preventive measures. Avoiding activities that place excessive rotational stress on the stifle joints, such as repetitive jumping from heights or sharp turning at high speeds on slippery surfaces, may help reduce the risk of both congenital luxation becoming symptomatic and traumatic luxation occurring.

Breed clubs and kennel organizations have an important role in reducing the prevalence of patellar subluxation through education, health testing requirements, and promotion of health-focused breeding objectives. Some breed clubs have implemented mandatory patellar evaluation as a prerequisite for breeding certification or registry listing. These collective efforts, combined with advances in genetic research that may eventually identify the specific genes involved, offer the best hope for reducing the burden of this common orthopedic condition in future generations of dogs.