Cases of canine patellar luxation can be some of the most challenging orthopedic cases encountered in veterinary practice. Patients can exhibit an array of clinical signs with varying degrees of soft tissue distortions and skeletal deformities. As with any surgical case, fundamental knowledge of the disease process and its possible progression is required for diagnosis and management. When surgery is considered, appropriate case selection and decision-making will optimize outcomes.<\/p>\n
Take-Home Points<\/strong><\/p>\n <\/div><\/div><\/p>\n The patella is the largest sesamoid bone in the body and lies within the tendon of insertion of the quadriceps femoris muscle.1,2<\/sup> 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<\/sup> 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<\/sup> Additional stabilizers of the patella include trochlear groove conformation and balanced tension in the joint capsule, patellofemoral ligaments, and retinaculum.3,4<\/sup> Patellar luxation (PL) is malalignment of the extensor mechanism relative to the underlying femur, resulting in displacement of the patella from the trochlear groove. <\/span><\/p>\n PL is among the most common orthopedic conditions in the dog and has been described in the veterinary literature since the 1960s.5-9<\/sup> Most cases are considered developmental, but congenital and traumatic luxation are also possible.10-12<\/sup> Traumatic luxations are poorly described in the veterinary literature and are secondary to tearing of the joint capsule and retinaculum.13<\/sup><\/span><\/p>\n 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<\/sup> Because of the overrepresentation of certain breeds, heritability is suspected along with environmental factors thought to contribute to the condition.12,14<\/sup> 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 <\/sup> PL may also be more common in male large-breed dogs and female small-breed dogs.6<\/sup> <\/span><\/p>\n PL can be medial, lateral, or bidirectional. <\/span><\/p>\n 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<\/sup> 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 (<\/span>FIGURES 1 AND 2<\/b><\/span>).1,5,10,14,22<\/sup> 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<\/sup><\/span><\/p>\n 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<\/p><\/div>\n 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<\/p><\/div>\n 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<\/sup><\/span><\/p>\n 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<\/sup> <\/span><\/p>\n 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. <\/span><\/p>\n 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. <\/span><\/p>\n 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 (<\/span>FIGURE\u00a03<\/b><\/span>).24,25<\/sup> 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.<\/span><\/p>\n Figure 3. Patient with bilateral grade III medial patellar luxation. Noted the bowlegged stance.<\/p><\/div>\n 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.<\/span><\/p>\n 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. <\/span><\/p>\n 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. <\/span><\/p>\n To induce PL, the patient\u2019s 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<\/sup> 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<\/a>.6,26<\/sup><\/span><\/p>\n The practitioner should grade the PL and describe the direction of displacement, which dictates treatment recommendations (<\/span>TABLE 1<\/b><\/span>). The most commonly utilized grading scale for PL is based on Putnam\u2019s 1968\u00a0thesis,7<\/sup> which was later adapted by Roush.5,23\u00a0 <\/span><\/sup>Similar grading scales have been proposed that also include the degree of skeletal deformities.1,9<\/sup> 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 \u201cclunk\u201d 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.<\/span><\/p>\n .<\/span><\/p><\/div>\n 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<\/sup> 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<\/sup> 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<\/sup><\/span><\/p>\n To screen for cases of excessive distal femoral varus that may require corrective femoral osteotomies, well-positioned cranial\u2013caudal femoral radiographs (femoral alignment views) can be helpful.30<\/sup> 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<\/sup> After this view is obtained, frontal plane alignment can be assessed by measuring the anatomic lateral distal femoral angle (aLDFA) (<\/span>FIGURE\u00a04<\/b><\/span>).32-34<\/sup> 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 ><\/span>\u2009<\/span>102\u00b0), which may necessitate a corrective femoral osteotomy.11,30,33,34<\/sup><\/span><\/p>\n\n
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<\/a><\/a>Orthopedic Examination<\/h2>\n
Gait Evaluation<\/h3>\n
<\/a>Orthopedic Examination<\/h3>\n
<\/a>Preoperative Diagnostic Testing<\/h2>\n