Canine Patellar Luxation: Diagnosis and Treatment Options for General Practitioners

The patella is the largest sesamoid bone in the body and lies within the tendon of insertion of the quadriceps femoris muscle.^1,2 The main function of the patella is to alter the direction of pull of the quadriceps muscle over the femoral trochlea, increasing the moment arm of the extensor mechanism and allowing for effective stifle extension.^1-3 The extensor mechanism includes the quadriceps muscle and tendon, patella, and patellar tendon. Normal patellar tracking depends on extensor mechanism alignment over the femur, femoral trochlea, and tibial tuberosity.¹ Additional stabilizers of the patella include trochlear groove conformation and balanced tension in the joint capsule, patellofemoral ligaments, and retinaculum.^3,4 Patellar luxation (PL) is malalignment of the extensor mechanism relative to the underlying femur, resulting in displacement of the patella from the trochlear groove.

PL is among the most common orthopedic conditions in the dog and has been described in the veterinary literature since the 1960s.^5-9 Most cases are considered developmental, but congenital and traumatic luxation are also possible.^10-12 Traumatic luxations are poorly described in the veterinary literature and are secondary to tearing of the joint capsule and retinaculum.¹³

Breeds with a predisposition to PL are miniature and toy breeds, including but not limited to Pomeranians, Yorkshire terriers, Chihuahuas, miniature and toy poodles, Pekingese, French bulldogs, Maltese, and Lhasa apsos.^5,6,12,14,15 Because of the overrepresentation of certain breeds, heritability is suspected along with environmental factors thought to contribute to the condition.^12,14 PL commonly affects both hind limbs (30% to 93% of cases), and most cases are diagnosed in dogs younger than 3 years of age.^5,14-18 PL may also be more common in male large-breed dogs and female small-breed dogs.⁶

PL can be medial, lateral, or bidirectional.

• Medial: The most common type of luxation among all patients is medial PL; incidence is 12 times higher among small-breed than large-breed dogs.^16,19 Large-breed dogs that are overrepresented include Labrador retrievers, German shepherds, Staffordshire bull terriers, and Great Pyrenees.^14,16,17
• Lateral: Lateral PL is more commonly diagnosed in large-breed dogs, and the probability increases with breed size.^{5,6,10,12,16,18,20}
• Bidirectional: PL both medially and laterally is poorly described in the veterinary literature, but a recent study reported that toy poodles may be overrepresented.²¹

The pathogenesis of PL is not completely understood. A leading theory for medial PL is that changes arise at the level of the hip (e.g., coxa vara, decreased femoral neck anteversion) and lead to varying degrees of skeletal deformities, resulting in PL.^7,10,11 Potential skeletal deformities include coxa vara, distal femoral varus, femoral torsion, a shallow trochlear groove, a poorly developed or absent medial trochlear ridge, a hypoplastic medial femoral condyle, medial displacement (torsion) of the tibial tuberosity, internal rotation of the tibia at the stifle joint, proximal tibial varus, external tibial torsion, and hypoplasia of the patella (FIGURES 1 AND 2).^1,5,10,14,22 With worsening severity of PL (i.e., higher grade), more skeletal deformities may be present in addition to severe soft tissue alterations, some of which are more amenable than others to surgical correction.^1,22

Figure 1. Skeletal abnormalities associated with medial patellar luxation (PL). (A) Normal left pelvic limb, cranial view. The quadriceps mechanism and patella are aligned with the anatomic axis of the femur, the trochlear groove, and the dashed line connecting the center of the proximal femur to the center of the distal tibia. (B) Deformities accompanying medial PL. Note the malalignment of the quadriceps mechanism. 1, Coxa vara; 2, distal femoral varus and and external femoral torsion; 3, shallow trochlear groove with poorly developed or absent medial ridge; 4, hypoplastic medial femoral condyle; 5, medial displacement (torsion) of the tibial tuberosity associated with internal rotation of the tibia at the stifle joint; 6, proximal tibial varus; 7, internal rotation of stifle resulting in an internal orientation of the pes. With severe cases this can lead to eventual compensatory external tibial torsion. Illustration: Kip Carter

Figure 2. Position of the tibia relative to the femur and shape of the femoral trochlea in dogs with medial patellar luxation grades 1 through 4. The dashed line represents the distal femur in the frontal plane, while the solid line represents the proximal tibia in the frontal plane. As the grades increase, progressive internal rotation of the tibia at the stifle joint and deformity of the medial trochlear ridge are noted. Illustration: Kip Carter

Orthopedic Examination

Patients with PL may primarily exhibit lameness associated with their PL (symptomatic PL), or the luxation may be an incidental finding during routine physical examination for an otherwise sound patient (occult PL).²³

Gait Evaluation

Gait evaluation should consist of observing the patient walking toward and away from the clinician and grading any observed lameness by using a numerical rating system or a visual analog scale.²⁴

A patient with lower-grade PL may appear sound during gait examination or may demonstrate only a mild, intermittent skip of the affected hind limb. As the grade worsens and osteoarthritis and/or cartilage wear develops, more severe lameness may be apparent.

In patients with higher-grade luxations (grades III and IV), soft tissue alterations can be more severe and hind limb conformational changes may be present, resulting from displacement of the quadriceps/patellar tendon mechanism over time.

For patients with higher-grade III and IV medial PL, the changes can result in outward bowing of the stifles (genu varum) and a bowlegged conformation (FIGURE 3).^24,25 For patients with higher-grade III and IV lateral PLs, inward bowing of the stifles (genu valgum) may be apparent with a knock-kneed conformation.

Figure 3. Patient with bilateral grade III medial patellar luxation. Noted the bowlegged stance.

Patients with the most severe PL can lose the ability to extend the stifle due to severe malalignment of the extensor mechanism and development of irreversible soft tissue changes around the stifle joint. Thus, early intervention is key.

Orthopedic Examination

After gait observation, a full orthopedic examination should be performed and may reveal muscle atrophy, stifle joint effusion, and pain through stifle range of motion. For patients with lower-grade PL, very mild to no abnormalities of the stifle aside from the luxation may be present and pain can be challenging to appreciate.

When identifying the patella, which can be very small in many patients, first palpate the tibial tuberosity and move proximally. The patellar tendon will guide the practitioner to the patella, which may or may not be located in the trochlear groove. Then assess the quadriceps/patellar tendon (extensor) mechanism relative to the underlying femur. Examining standing patients is beneficial because standing produces moderate muscle tone within the quadriceps femoris muscle group. Evaluating standing patients also enables simultaneous symmetric palpation of both hind limbs, which may be useful for detecting abnormalities.

To induce PL, the patient’s weight can be shifted off of the affected hind limb while pressure is applied to the patella in both a medial and lateral direction. Some movement within the trochlear groove is normal.¹¹ If the patella remains stable, luxation can be further encouraged by gently extending the stifle as well as the hip and repeating the maneuver. Slight hip extension should be maintained as this creates additional tonus in the rectus femoris muscle, the only muscle of the quadriceps femoris group that traverses the hip. The additional tonus creates a bowstring effect, straightening out the quadriceps muscle group over the top of the underlying femur. In addition, the distal tibia can be rotated internally or externally, which rotates the stifle internally or externally, further encouraging PL. Excessive internal or external stifle rotation should also be assessed. The same maneuvers can be performed in lateral recumbency. Additional stifle maneuvers (e.g., cranial drawer, tibial thrust) should be completed as 13% to 25% of patients with medial PL may have concurrent cranial cruciate ligament injury.^6,26

The practitioner should grade the PL and describe the direction of displacement, which dictates treatment recommendations (TABLE 1). The most commonly utilized grading scale for PL is based on Putnam’s 1968 thesis,⁷ which was later adapted by Roush.^5,23  Similar grading scales have been proposed that also include the degree of skeletal deformities.^1,9 The feel of the patella luxating in and out of the trochlear groove also provides information about the quality of the trochlear ridge in addition to presence of cartilage wear. For example, if a grade II medial PL is very distinct and a large “clunk” is palpable as it luxates and reduces, then the medial trochlear ridge is likely robust. Alternatively, if the patella slides medially very easily, the medial trochlear ridge may be hypoplastic or worn down. If patellofemoral crepitus (bone-on-bone grinding) is palpable during luxation, then cartilage wear along the patellar subsurface and trochlear ridge is likely developing.

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Preoperative Diagnostic Testing

Preoperative radiography remains a fundamental component of the PL workup and aids in surgical planning. After diagnosis and grading have been completed, orthogonal radiographs of the stifle may reveal luxation of the patella medially or laterally, joint effusion, osteoarthritis, and/or concurrent femoral deformities. They also help screen for concurrent cranial cruciate ligament disease as caudal femoral subluxation may be apparent. For patients with low-grade or occult luxation, radiographic abnormalities may be very mild. Skyline radiographs of the trochlear groove to assess depth and determine if a trochleoplasty is indicated can be considered; however, this view is challenging to obtain. A better way to determine trochlear groove depth may be computed tomography (CT).²⁷ For grade II PL, orthogonal radiographs centered on the stifle are often sufficient. For patients with grade III PL, the clinician must screen for skeletal deformities. For patients with grade III medial PL, medial angulation to the distal femur (femoral varus) is believed to be the most common skeletal deformity and has been reported to contribute to PL.^11,28 Distal femoral varus shifts the long axis of the extensor mechanism medial to the underlying femur, which then encourages medial PL during quadriceps contraction.²⁹

To screen for cases of excessive distal femoral varus that may require corrective femoral osteotomies, well-positioned cranial–caudal femoral radiographs (femoral alignment views) can be helpful.³⁰ The femoral alignment view is obtained by placing the patient in dorsal recumbency with the femur parallel to the table and the beam centered on the mid-diaphysis without internally or externally rotating the hip. Appropriate frontal plane alignment is confirmed by ensuring that the femur is parallel to the long axis of the pelvis, the patella is centered within the trochlear groove (if possible), the fabellae are bisected by their respective femoral cortices, and the corticocancellous tip of the lesser trochanter is protruding from the medial aspect of the femur.^31,32 After this view is obtained, frontal plane alignment can be assessed by measuring the anatomic lateral distal femoral angle (aLDFA) (FIGURE 4).^32-34 Heavy sedation, if not brief anesthesia, is recommended. A lateral femur view is also obtained to evaluate the bone in the sagittal plane. Again, the femoral alignment view is helpful for screening for excessive femoral varus (aLDFA > 102°), which may necessitate a corrective femoral osteotomy.^11,30,33,34

Figure 4A. Cranial-caudal (femoral alignment) view of a patient with grade II medial patellar luxation (PL). The anatomic lateral distal femoral angle (aLDFA) is formed by the intersection of the distal joint reference line and anatomic axis of the proximal femur as described by Tomlinson et al.34 The patient has an aLDFA of 97° (no distal femoral varus).

Figure 4B. In contrast, another patient with a grade III medial PL with excessive femoral varus (aLDFA 108°). In this patient, the femoral varus displaces the quadriceps muscle group medially, contributing to medial PL. A computed tomography scan should be considered to screen for additional deformities and accurately measure the femoral varus.

Femoral alignment views are helpful when performed correctly, but they have limitations. They can be challenging to obtain; even 5° of femur elevation can significantly alter radiographic measurements of the aLDFA.²⁹ Also, external rotation of the femur can artifactually increase femoral varus, and slight internal rotation can decrease it.^11,31 Thus, scrutinizing the positioning of these views is paramount when using them for surgical decision-making. Although normal aLDFA reference ranges have been published for some breeds of dog, the threshold at which a corrective femoral osteotomy should be performed (i.e., an “excessive” aLDFA) is often based on clinical experience rather than on objective measurement.^34,35 However, it is widely accepted that when excessive femoral varus is identified radiographically, cross-sectional imaging in the form of CT should be pursued for accurate surgical planning and to identify any additional deformities (FIGURE 5).^30-32

Figure 5A. Patient with a grade IV medial patellar luxation (PL) and excessive femoral varus (anatomic lateral distal femoral angle of 110°). The patella is shown in yellow.

Figure 5B. Evaluation of the patient’s entire hindlimb. The internal rotation of the stifle is significant, but the patient’s toes are still pointing forward, which is suggestive of compensatory external tibial torsion. The patient has a multilevel deformity (i.e., femoral varus, femoral torsion, hypoplastic trochlea, significant stifle rotation, tibial torsion), which is common among patients with grade IV medial PL. 3D models produced with Materialise software (materialise.com).

Surgical Options

Procedures

The traditional surgical procedures considered for grade II and III PL include a trochleoplasty, tibial tuberosity transposition (TTT) retinacular/capsular release, and contralateral retinacular/capsular imbrication.^{6,11,16,35,36} Detailed descriptions of the surgical technique for these various procedures are beyond the scope of this article. For the general practitioner who is interested in surgically addressing canine PL, patients that respond well to the traditional procedures are those with symptomatic grade II medial PL and those with early or mild grade III medial PL.

Two patient groups that are challenging for surgeons include those with occult (asymptomatic) grade II or grade III medial PL. For patients with occult PL, the decision to pursue surgery is influenced by the risks and costs of surgery weighed against the potential risks and costs of not pursuing surgery. According to a recent study, 50% of dogs with occult grade II medial PL that were followed up for 4 years experienced chronic lameness or required surgery.²³ For these patients, client education and counseling are major components of the surgical decision-making, as is active surveillance (e.g., serial evaluations to monitor for progression, asking the client to look for clinical signs at home).

Given that studies have shown increased complication rates with increasing grade of PL, grade III luxations should be approached with caution.^15,16,36,37 Many mild grade III medial PLs can be effectively treated with traditional procedures; however, as previously noted, skeletal deformities and the severity of soft tissue changes that could lead to suboptimal outcomes are more likely and may necessitate corrective osteotomies.^{1,11,16,22,36} Osteotomies require knowledge of angular limb deformity planning and familiarity with bone plating principles.

Complications

The complication rates for surgery to treat PL vary across studies but are generally in the range of 13% to 48%.^{6,11,15-17,35,36,38} Major complications associated with the traditional patellar procedures (e.g., TTT, trochleoplasty, soft tissue balancing) include K-wire migration, avulsion and/or fracture of the tibial tuberosity, dislodgment of the trochleoplasty wedge, tibial fracture, lateral trochlear ridge fracture, injury to the patellar ligament, wound dehiscence, peroneal neuropraxia, dehiscence, infection, and progressive osteoarthritis.^11,17 The most commonly encountered major postoperative complications are implant-associated morbidity and reluxation of the patella.³⁹

Referral

Many cases of medial PL can be addressed by general practitioners (FIGURE 6). Cases that the general practitioner should consider referring include PL in juvenile patients, grade IV luxations, grade III luxations with skeletal deformities present, grade III luxations in older patients, luxations with concurrent cranial cruciate ligament injury, lateral luxations in large- and giant-breed dogs, and bidirectional luxations. Treatment of PL in juvenile patients is complex; often, multiple procedures that consider the patient’s growth potential are required. Concurrent cruciate injury requires stabilizing the stifle, often using modifications of current techniques.^38,41-43 Reluxation risk may also be higher for large- and giant-breed dogs.^6,10   

Postoperative Recovery Plan

A formal postoperative recovery plan should be implemented for all surgical patients. The author’s recovery plan after traditional patella surgery is as follows:

• 24 to 36 hours: Opioid analgesia and fluid support
• 72 hours: Cryotherapy (cold compression)
• Days 4 to 7: Warm compress therapy
• Weeks 1 to 2:
  • Multimodal analgesia to promote weight bearing (i.e., nonsteroidal anti-inflammatory drugs with or without the addition of gabapentin)
  • Formal physical therapy with a certified canine rehabilitationist, involving passive range of motion, massage, and postural exercises (hip sway)
• Weeks 0 to 8:
  • Restricted activity consisting of short, leashed walks
  • Orthopedic recheck examination and radiography at 8 weeks to evaluate patellar stability, bone healing of TTT osteotomy, and implants, followed by a gradual return to activity
• Weeks 2 to 8: Strength, postural, and flexibility exercises as tolerated by the patient

Throughout the postoperative recovery process, additional short courses of pain medications and multimodal pain management should be considered to encourage weight bearing.

Role of the Veterinary Surgical Nurse in the Care of Orthopedic Patients

Conservative Management

Nonsurgical management is typically elected for patients with grade I PL and subclinical cases (asymptomatic grade II). All cases that are to be conservatively managed require thorough client education and an active surveillance plan. Recommendations for conservative management are as follows:

• Provide pain management, if indicated, such as nonsteroidal anti-inflammatory drugs with or without neuropathic pain medications (e.g., gabapentin, amantadine given preemptively to avoid windup).
• Achieve and maintain a lean body weight.
• Develop a lifestyle of regular but moderate activity.
• Provide physical therapy to strengthen the quadriceps muscle and lateral thigh musculature. Targeted strengthening exercises can include sit-to-stand exercises, step ups, hill walking, physio ball core stability, and swimming/underwater treadmill.

The PL grade (i.e., severity) is a single snapshot in time for each patient. For example, a patient with grade II medial PL may be asymptomatic at a visit, but as the patella luxates medially, it can wear down the medial trochlear ridge, which can lead to cartilage loss and clinical signs. As the luxation continues, the same patient can exhibit a grade III medial PL, conformational changes, and more advanced soft tissue alterations months to years later. Intervention for that patient as soon as clinical signs develop and before more advanced changes develop is preferred. Thus, serial evaluations and client education, which allow for prompt surgical intervention when indicated, are recommended. The timeline for when to reevaluate patients should be decided on a case-by-case basis.

A rough outline of the author’s recheck preferences for nonsurgical PL cases is as follows:

• Incidental grade I: Educate clients on signs of lameness that may develop and encourage them to take videos for evaluation by the veterinary team. After the patient’s first diagnosis, schedule a recheck visit in 6 months, or sooner if clinical signs develop. If static in 6 months, then recheck at annual wellness examinations, documenting PL grade, direction, and feel (smooth or crepitant).
• Occult grade II: Educate clients and recheck the patient in 3 to 6 months, or sooner if signs of lameness develop. If clinical signs are static after 6 months, then recheck at annual wellness examinations, documenting PL grade, direction, and feel (smooth or crepitant).

Summary

PL is a common orthopedic condition in dogs, especially those of miniature and toy breeds. Appropriate screening of cases requires accurate diagnosis of PL grade. Patients with grade II and mild grade III PL without underlying skeletal deformities are good surgical candidates for traditional patellar surgical procedures. Screening for some of the underlying skeletal deformities (e.g., femoral varus) can be performed in general practice with well-positioned femoral alignment radiographs. Client education is paramount for patients managed surgically or conservatively.