Figure 1: Grade 3 medial patella luxation on the right in a French bulldog. The right pelvic limb demonstrates abduction of the hock, internal rotation of the tibia and internal rotation of the foot. Photos courtesy Jessica McCarthy Canine medial patella luxation is a common cause of lameness. While it most often affects small and toy breed dogs,1 medium and large breed dogs can also be impacted.2 Clinical presentation varies depending on the severity (i.e. grade) of luxation. Most patients with patella luxation will present at a young age, some as early as three months, but many are around 12 months to four years old.2 Surgical treatment techniques vary depending on the grade, underlying anatomical derangements, and surgeon preference. Case selection for surgical candidates depends on several factors, grade included. What follows is a detailed discussion of the different grades of medial patella luxation, along with an exploration of a subclassification of grade 2. The latter may help in decision-making when considering patients which may require surgical treatment. Grade 1 (No clinical signs associated) During orthopedic examination, as you fully extend the stifle, if you place slight medial pressure on the patella, it will luxate medially. Slight internal rotation will also elicit luxation. Typically, it is most easily elicited if both the hip and stifle are extended, and the foot is internally rotated slightly. Once the pressure and/or rotation is released, the patella will spontaneously relocate into the trochlea groove. During normal flexion and extension of the stifle, the patella will tend to stay in the groove. Grade 2 Figure 2: Grade 4 medial patella luxation on the right and grade 3 medial patella luxation on the left in a Shih Tzu. The patient displays a hunched rear end posture with increased stifle flexion that is often seen in high grade patella luxation. The patient will skip on the affected leg. It does not appear to be very painful, and there is no noticeable lameness in between skipping episodes. Patients with cranial cruciate ligament (CCL) disease will also sometimes have a skipping type of component to their lameness; however, when not skipping, a patient with a CCL rupture will remain mild to moderately lame on the affected limb. Try to have the patient as relaxed as possible during your orthopedic examination as any tension in the quadriceps muscles may alter your findings. I recommend performing your conscious examination both standing and then repeating it in lateral recumbency, if tolerated by the patient. Flexion of the stifle in a weight bearing position with internal rotation of the foot will usually cause the patella to spontaneously luxate in a grade 2 medial patella luxation. Release of the rotation and stifle extension will allow the patella to spontaneously reduce into the trochlea groove on the femur. Grade 2 covers a wide range of severity of the condition. Technically, any luxation that spontaneously reduces into the groove during flexion and extension should be categorized as a grade 2. In some cases, the patella may remain out of the groove for many steps during the patient’s gait and in other cases it may only luxate once or twice over 25 yards, yet this category spans them both. To allow a clearer classification, I suggest sub-categorization as follows: Grade 2A: The patient skips infrequently during gait and during stance the patella is consistently within the groove. Grade 2B: The patient skips frequently but during stance the patella is consistently within the groove. Grade 2C: The patient skips frequently or walks with a consistent lameness, suggestive that the patella is out of the groove during most of the gait. On palpation, there are times when the patella is not within the groove. If the limb is flexed and extended the patella spontaneously returns to the trochlea groove. Grade 3 Figure 3: Computed tomography scan showing a grade 4 medial patella luxation on the right with femoral varus, internal rotation of the tibia, and foot. The patient will walk with an altered gait due to an internal rotation of the tibia and altered quadriceps biomechanics. The hock will be abducted and the foot internally rotated (Figure 1). The limb, and whole rear end in the case of bilateral disease, will appear crouched or hyper flexed compared to normal (Figure 2). Many cases of grade 3 patella luxation are bilaterally affected, although the grade on the other pelvic limb can be less, the same, or worse. The patella will not be within the groove, but you will be able to manually manipulate it back in, only for it to luxate as soon as manual pressure is released. In small dogs, it can sometimes be tricky to discern the patella. Palpating the lateral femur and following it down to the stifle, it is then possible to move your fingers across the cranial and distal end of the femur where you will palpate the trochlea groove. Normally, the groove is not palpable because the patella is within it. Finding the tibial tuberosity and then following proximally up the patella tendon will aid in location of the patella. Typically, these dogs will have an internally rotated tibia with a medially positioned tibial tuberosity and, therefore, the tuberosity will be positioned very medially in comparison to the femur. A craniocaudal radiograph of the femur can confirm the diagnosis and direction of luxation. Grade 4 The patient’s gait will be similar to that which is described for grade 3, with abduction of the hock, internal rotation of the foot, and increased flexion of the stifle, giving a crouched and “bow-legged” appearance. Intermittent non-weight bearing can be seen, likely due to some discomfort as the patella moves over the medial aspect of the femur. Figure 4: Computed tomography scan of the right femur from the patient in Figure 3 with lines drawn to demonstrate excessive femoral varus of 103 degrees. The tibia will be internally rotated, sometimes with up to 90 degrees internal rotation (Figure 3). It will not be possible to move the patella towards or into the groove and attempting to do so will often cause discomfort to the patient. The quadriceps muscles will be shortened and tight due to the medial position of the patella and most cases will have femoral varus/medial bowing (Figure 4), which will enhance the “bow-legged” appearance of these cases. Surgical intervention Grades 3 and 4 are considered surgical candidates, as patients are unable to walk with a normal gait. Early surgical intervention aims to reduce secondary effects, such as progressive osteoarthritis and cranial cruciate ligament rupture.3 Grade 2 patella luxation cases can also benefit from surgical intervention; however, the risks of surgery must be weighed against the benefits in each individual case. Grade 2C cases will usually be good surgical candidates with a noticeable improvement following surgical correction. Grade 2B patients should be considered on an individual case-by-case basis. Grade 2A and grade 1 cases can often be monitored for progression of disease and surgical intervention reconsidered if this occurs. The effect on a patient’s quality of life is usually minimal when disease is classified as low grade. Cases can progress from a lower to a higher grade over time. As such, ongoing monitoring is recommended. Patients with medial patella luxation can also rupture their cranial cruciate ligament, which can lead to an acute progression (e.g. from a grade 2A to a grade 3, due to the increased internal rotation of the tibia without the cranial cruciate ligament intact). While no grading system can truly encompass the complexities of a disease such as patella luxation, hopefully this description and additional sub-categorization can help guide decision-making and client discussion. Jessica McCarthy, BVSc, ECVS, works as a clinical instructor in orthopedic surgery at the University of Wisconsin. She became a diplomat of the European College of Veterinary Surgeons in 2020 after completing her residency training at the University of Edinburgh. Dr. McCarthy attended Bristol University for her veterinary degree and spent two years in general practice in England after graduating. She has a particular interest in elbow disease, both developmental and traumatic. McCarthy is passionate about ensuring everyone in the veterinary profession can work in a diverse and inclusive atmosphere. References Bosio F, Bufalari A, Peirone B, Petazzoni M, Vezzoni A. Prevalence, treatment and outcome of patellar luxation in dogs in Italy. Vet Comp Orthop Traumatol. 2017;30(05):364-70. https://pubmed.ncbi.nlm.nih.gov/28763525/ O’Neill DG, Meeson RL, Sheridan A, Church DB, Brodbelt DC. The epidemiology of patellar luxation in dogs attending primary-care veterinary practices in England. Canine genetics and epidemiology. 2016 Dec;3(1):1-2. https://pubmed.ncbi.nlm.nih.gov/27280025/ Tobias KM, Johnston SA. Veterinary surgery: small animal-E-BOOK: 2-volume set. Elsevier Health Sciences; 2013 Dec 26. Grade classification modified from Singleton WB: The surgical correction of stifle deformities in the dog. J Small Anim Pract 10:59, 1969 https://pubmed.ncbi.nlm.nih.gov/5812819/
