Looking Closer at Luxating Patellas in Small Animals

A Yorkshire terrier presents for its annual examination at the general veterinary practice, and the owner has questions about occasional “skipping” of the pelvic limbs while walking. Across town, an emergency clinic admits a medium-sized mixed-breed dog that was roughhousing with its housemate and is now non–weight-bearing on its right pelvic limb. Next door, a family has an appointment at a specialty surgical department with their newly adopted dog, which has an unusual, crippling pelvic-limb stance and walk.

All of these dogs have the same diagnosis, but with varying severity: luxating patella. Luxating patella is among the most common causes of pelvic lameness in small animal veterinary medicine,¹ and veterinary nurses play a crucial role in its successful treatment and management. Skill with client communication and goals, diagnostic tools, and assisting and facilitating treatment is the groundwork for success and requires a thorough understanding of the condition.

Stifle Joint Anatomy and the Role of the Patella

The femorotibial joint, typically referred to as the stifle joint, is a complex joint composed of bones, menisci, and ligaments (Figure 1),² including the distal and proximal femoral condyles, the medial and lateral menisci, and the cranial and caudal cruciate ligaments. The stifle joint also includes the largest sesamoid bone in the body, the patella.³

The patella articulates with the femoral trochlea and is stabilized by the patellar tendon and patellar ligament. The patellar tendon is located proximal to the patella and serves as the insertion of the quadriceps. The patellar ligament connects the patella to the tibial tuberosity. The anatomy from the insertion of the patellar tendon in the quadriceps to the insertion of the patellar ligament in the tibial tuberosity is collectively referred to as the patellar mechanism. The alignment of the patellar mechanism is a major contributing factor to the stability of the patella within the trochlear groove, a depression between the femoral condyles. Encompassing the joint is the joint capsule, a fibrous tissue that contains the synovium. Surrounding the joint capsule is a collection of soft tissues collectively referred to as the retinaculum or retinacular tissues, which serve to further stabilize the joint.

The patella plays a crucial role in hind limb ambulation. It acts as a fulcrum for the stifle joint, enabling increased momentum and force during movement.⁴

Figure 1. Canine stifle joint anatomy.

Definition of Patellar Luxation

A deviation in the patellar mechanism can lead to dislocation, or luxation, of the patella from the trochlear groove of the femur. A common image to use with clients is a pulley system, where the rope needs to reside within the groove of the wheel to work properly. Luxation occurs when the rope (patellar mechanism) leaves the groove (trochlear groove). This can happen intermittently, or it can be a permanent condition. The patella can luxate medially, laterally, or bidirectionally.

Medial patellar luxation is more common than lateral or bidirectional luxation. While historically thought to be a “small dog problem,” medial patellar luxation is increasingly seen in larger breeds.⁵ Lateral or bidirectional luxation tends to be seen in larger breeds more often than in smaller breeds.⁴

Causes and Risk Factors for Patellar Luxation

Patellar luxations can be caused by skeletal deformities or trauma or be secondary to other stifle conditions such as a cranial cruciate rupture. They can also be iatrogenic following a surgical intervention in this area.¹ Skeletal deformities that can lead to patellar luxation include femoral deformities that affect the alignment of the quadriceps, tibial deformities that affect the position of the tibial tuberosity, and trochlear deformities that can lead to a shallow groove or misshapen ridges.⁴

Traumatic injuries to the stifle joint can also lead to luxating patella. Care must be taken when evaluating the patient and interpreting diagnostic tests to ensure that the primary injury and any secondary injuries are found.

A cranial cruciate ligament tear can exacerbate a previously low-grade patellar luxation, and patellar luxation has been reported as a complication following treatment for cranial cruciate ligament rupture or fracture repair of the distal femur or proximal tibia.¹ These surgeries can result in patellar luxation by altering the alignment of the patellar mechanism.

Grading Patellar Luxation

A grading scale is used to classify the severity of patellar luxation (Figure 2).⁴ Grade 1 luxations are the least severe and are typically incidental findings on examination, with no lameness observed in the patient. The patella can be manually luxated while the stifle joint is in extension and will spontaneously return to the trochlear groove (reduce).

In a grade 2 luxation, the patella can be spontaneously luxated and reduced on examination.¹ Typically, the patella luxates when the tibia is rotated in the direction of the luxation (e.g., rotated medially in a medial luxation) and reduces when rotated in the opposite direction. “Skipping” lameness can be observed with a grade 2 luxation; in this gait pattern, the patient holds the affected limb up for several steps and returns to a normal gait when the patella spontaneously returns to the trochlear groove.⁴

Grade 3 luxation is associated with a continuously luxated patella that can be manually reduced. However, once pressure to reduce the patella is released, the patella immediately reluxates.¹ Lameness with a grade 3 luxation can vary in severity but will be present due to the continuous luxation. At this grade of luxation, skeletal deformities of the femur and tibia are usually evident.

Grade 4 luxation is the most severe, with a permanent luxation that cannot be reduced manually. Severe skeletal deformities are typically present, and a severely abnormal gait and stance are apparent on examination.⁴

Figure 2. Grades of patellar luxation.

Clinical Consequences of Patellar Luxation

Patellar luxation results in decreased stifle range of motion, limited mobility, and pain. While low-grade patellar luxation is typically an incidental finding, it can be aggravated by poor joint health and maintenance. Obesity in pets is very common and can lead to a higher risk of joint disease and injury, including patellar luxation.⁴

Prolonged luxation can lead to cartilage loss on the underside of the patella and the femoral condyles, resulting in development and progression of degenerative joint disease.⁴ Patients with moderate or high-grade patellar luxations or luxations with cartilage loss require long-term management solutions. As pets age, those with degenerative joint disease can see further progression of the condition,⁴ warranting more advanced treatment.

Clinical Presentation of Patellar Luxation

Clinical signs of patellar luxation vary depending on the severity of the luxation. Dogs and cats with grade 1 patellar luxation often do not show any lameness. Those with grade 2 luxation commonly present with skipping lameness as described above.

Patients with grade 3 luxating patella have a consistent lameness that can range from mild to severe. If the condition is bilateral, the patient may have an aversion to activity due to discomfort when ambulating. Due to the patella consistently luxating, the condition can progress to cartilage loss and degenerative joint disease if no early treatment is provided.⁴ As the condition progresses, the lameness will worsen.

With grade 4 patellar luxation, patients show abnormal stances and gaits secondary to typically noticeable skeletal deformities (Figure 3). Ambulation is greatly affected, and the lameness is usually severe.

Figure 3. Patient with grade 4 bilateral patellar luxation.

Diagnosis of Patellar Luxation

Diagnosing a patellar luxation requires careful analysis of the patient’s gait, stance, and physical examination findings. In determining the grade of luxation, concurrent injuries should be ruled out. For example, a cranial cruciate tear can compound a patellar luxation to a more severe grade, and the treatment options will need to address the primary injury as well as the patellar luxation.

A physical examination to determine the grade and direction of the luxation is performed first. Sometimes, deformities of the tibia or femur are observed during the examination. Radiographs can further determine whether skeletal deformities are a contributing factor and what degree of degenerative joint disease is present (Figure 4). A recent study showed evidence of increased stifle soft tissue opacity in cases of patellar luxation, which can be a sign of stifle pathology.⁶ In severe cases of skeletal deformities, further imaging, such as computed tomography (CT), is indicated to evaluate the deformities and create a plan for surgical correction.

Figure 4. Radiograph showing medial patellar luxation (arrow).

Treatment of Patellar Luxation

Once assessment is complete, decision-making about treatment can begin. For patients with an asymptomatic grade 1 patellar luxation, conservative management is typically chosen over surgical treatment. In patients with a grade 2 patellar luxation, treatment is determined by severity of lameness. Mild lameness may not warrant surgical correction unless it worsens. Moderate or severe lameness would be an indication for surgical correction. Owners should be informed that, without treatment, chronic luxation might result in cartilage loss and development of cranial cruciate ligament rupture; thus, preemptive treatment should be discussed. Patients with grade 3 or 4 patellar luxation benefit from early surgical intervention to help prevent progression of skeletal deformities and subsequent development of osteoarthritis.⁴

Conservative Treatment

Regardless of grade, any patient with patellar luxation can benefit to some degree from conservative management. Conservative management for a luxating patella is similar to conservative management for other joint conditions and is focused on maintaining a healthy joint and reducing stress on the joint. Since obesity is a major contributing factor to a joint stress, weight management is a main focus of conservative management.

Formal physical rehabilitation can help increase muscle mass and patellar mechanism alignment. Joint supplements, as the major source of marine-based omega-3 fatty acids, have been shown effective in management of osteoarthritis.⁷ If inflammation and pain are present, anti-inflammatories or analgesics can help. Conservative management as primary treatment is only indicated in grade 1 and asymptomatic grade 2 luxations.⁸

Surgical Treatment

All patients facing surgical treatment for patellar luxation should be fully evaluated to ensure they are a good candidate for anesthesia. Preanesthetic workups vary based on surgeon and practice preference but typically include, at minimum, a complete general physical examination and blood analysis. Surgical treatment is best performed by a veterinary surgeon with experience in orthopedic surgery who is a diplomate of the American or European College of Veterinary Surgeons or holds comparable qualifications.

Patient Preparation

After induction of anesthesia, the limb to be treated is surgically prepared. Shaving should extend distally past the tibiotarsal joint, proximally to the hip, and circumferentially around the limb. If a distal femoral corrective osteotomy is expected, the shaved portion should extend proximally to the body’s midline.⁸ An initial preparation of either chlorhexidine scrub or povidone-iodine to remove dirt and debris is performed in the treatment area.

The patient is then moved to the operating room and positioned in dorsal recumbency. Sterile presurgical preparation is performed and the patient is draped using aseptic technique. As part of the presurgical checklist recommended for any surgical procedure, the surgeon performs another evaluation of the joint to ensure that the correct limb is prepared, the diagnosis is confirmed, and there are no concurrent injuries that would change the treatment plan. Thereafter, the joint is evaluated, typically as an arthrotomy, but an arthroscopic evaluation can also be performed. This entails assessment of the trochlear groove, underside of the patella, cranial and caudal cruciate ligaments, lateral and medial menisci, distal femoral anatomy and proximal tibial anatomy, and retinacular tissues.

Trochleoplasty

Surgical treatment typically starts with a deepening of the trochlear groove, called trochleoplasty. Several methods for this procedure can be used. The most common techniques are wedge recession and block recession trochleoplasty.⁴ The main difference between a wedge recession and a block recession is the shape of the recession. For both techniques, cuts are made into the trochlear groove to temporarily remove a portion of the trochlea. While removed, this section must be handled with utmost care to preserve the cartilage surface and prevent damage. The subchondral bone of the distal femur is then cut deeper, creating a larger bed. When the removed portion of the trochlea is placed back into the recipient bed of subchondral bone, the effect is a deeper groove for the patella.

Following trochleoplasty, the limb is evaluated for evidence of patellar relaxation to assess if the initial procedure resulted in adequate stabilization. Flexion and extension with medial and lateral rotation are performed. For low-grade patellar luxation caused by an extremely shallow trochlear groove, a trochleoplasty can sometimes be all that is required for treatment. If the patella continues to luxate, additional treatment steps are required.

Tibial Tuberosity Transposition

For a patellar luxation with a displaced tibial tuberosity (the tibial tubercle is located either too medial or too lateral), a tibial tuberosity transposition is indicated. This can create a straighter alignment of the patellar mechanism by correcting the insertion of the patellar ligament.

In tibial tuberosity transposition, an osteotomy is made to separate the proximal portion of tibial tuberosity from the tibia, leaving the distal portion intact. The proximal portion is transposed either laterally to treat medial patellar luxation or medially to treat lateral patellar luxation and secured with Kirschner wires. In larger patients, a tension band wire is needed for further stabilization due to the increased force on the tibial tuberosity from the larger muscles (Figure 5).

Figure 5. Postoperative radiograph showing the lateral view of a tibial tuberosity transposition using Kirschner wires and a tension band for stabilization (red arrow). The osteotomy from a block trochleoplasty is also visible (white arrow).

Other Procedures

If a patient has a concurrent cranial cruciate ligament injury, patellar luxation treatment is included in the procedure used to treat the cruciate ligament rupture. Options include osteotomy-based treatment, such as a tibial plateau leveling osteotomy, or a suture-based stabilization of the cruciate ligament injury.⁴

Soft tissue augmentation is typically also performed as part of any surgical treatment for a luxating patella. A capsulotomy or capsulectomy (removal or opening of the tight joint capsule on the side of the patellar luxation) is performed to release pressure on the affected side of the joint. If indicated, a release of the retinacular tissues can further reduce tension on the luxating side. Additionally, imbrication—incision, overlapping, and suturing—of retinacular tissues on the side opposite the luxation can be performed to tighten them.

In higher-grade luxating patella cases with a femoral deformity that contributes to the luxation, a distal femoral osteotomy is performed. Radiography or CT is used to create a surgical plan to achieve best alignment. An osteotomy is performed to correct the deformity, and a bone plate secured with screws or an interlocking nail is typically used to provide adequate stabilization. In severe cases of skeletal deformity, patient-specific cutting guides are created based on presurgical CT images for more concise reduction and increased success.

Postoperative Care

Postoperative care is crucial for a successful outcome. During the immediate postoperative period, appropriate analgesics and anti-inflammatory medications should be given. Some patients may require sedation to help keep them quiet and prevent overactivity. The use of antibiotics depends on the patient, surgeon, and practice. Some surgeons apply a bandage to reduce motion and aid with initial scar tissue formation to potentially limit the risk of relaxation. Self-trauma to the surgical site should be prevented with the use of an Elizabethan collar and/or anti-lick device.⁹

During the first couple of weeks, activity restriction is mandatory to allow for soft tissue healing. After the first few weeks, short leash walks can be started; their duration should be increased slowly to allow muscle rebuilding. Running, jumping, and off-leash activity are directly related to development of complications and must be avoided until a full assessment, typically including radiographs taken at 8 to 12 weeks postoperatively, is performed and shows adequate bone healing with no evidence of complications.

Formal physical rehabilitation can be carefully applied to help build and support the soft tissues and muscles surrounding the joint to prevent reinjury.⁷ However, it is important to understand that too much activity and too-aggressive rehabilitation too early can disrupt scar tissue formation and lead to reluxation of the patella, which—in the worst case—might require a revision surgery. Hence, it might be more prudent to reserve formal physical therapy as part of additional therapy as needed, based on the 8- to 12-week follow-up examination.

Complications

Surgery always involves anesthetic risk, and proper patient workup to ensure safe anesthesia must be performed. Risks inherent in any surgery include self-trauma to the surgical site, surgical site infection, and failure of the applied technique (including implant failure) resulting in reluxation of the patella and typically requiring another surgery.⁹ Complication rates range widely depending on surgical treatment, surgeon experience, and client compliance with postoperative instructions. Complications lead to additional expenses; therefore, it is critical that owners understand the risks prior to any surgery.

Prognosis

The prognosis of low-grade patellar luxation is good with treatment. High-grade patellar luxation typically carries a higher risk for complications but, with early treatment, can have a favorable prognosis.⁴ Degenerative joint disease will progress as the patient ages, and monitoring for signs of discomfort is necessary. Symptomatic treatment is indicated to ensure patient comfort. For more advanced joint disease, other surgical treatment options may be necessary. Patellar groove replacement can be indicated when cartilage loss in the trochlear grooves and ridges is severe. For end-stage osteoarthritis, total knee replacement can be considered as long as it is performed by a qualified surgeon after detailed communication about this procedure with the owners.¹⁰

Summary

Patellar luxation is a common cause of lameness in dogs and cats. The luxation can be caused by developmental factors or trauma and can be medial, lateral, or bidirectional. A grading scale (1 to 4) is used to classify luxations and, together with assessment of clinical signs and imaging, aids in selecting the best treatment. Early detection and treatment of patella luxation can enable good joint stability, function, and health. Overall, as long as treatment is adequate, the prognosis is good and the risk for complications is low.