Canine patellar luxation is a complex inherited condition which involves at least 15 chromosomal regions (Wangdee et al., 2017). Prevalence is increasing rapidly due to the growing popularity of dog breeds with known conformational defects.
The commonest form, medial patellar luxation (MPL), typically affects dogs with short bowed hindlimbs. Bulldogs and Bull Terriers are notable examples; however, not all affected dogs have visible conformational deformities. For instance, Pomeranians have less obvious hindlimb bowing compared with Bulldogs, but they have the highest reported patellar luxation risk, with a prevalence as high as 75 percent (Soontornvipart et al., 2013).
The consequence of selective breeding for bowed hindlimbs is that MPL is more common overall, regardless of dog size (Hayes et al., 1994). However, on the rare occasions when lateral patellar luxation (LPL) is diagnosed, it is typically associated with a “knock-kneed” conformation in a large- or giant-breed dog.
What causes patellar luxation?
Although the exact pathogenesis of canine patellar luxation is uncertain, a compelling argument suggests that its pathology begins near the hip joint (Roush, 1993). In normal dogs, the origin of the rectus femoris muscle, which forms part of the quadriceps, lies immediately adjacent to the hip. The quadriceps (Q) angle is the angle formed between the axis of this muscle and the patellar tendon. Small increases in Q-angle produce dramatic increases in medially directed vector forces (Yoo et al., 2020). These forces, in turn, trigger a cascade of progressive limb deformities (Figure 1).
The earlier luxation occurs, the greater the degree of deformity. Femoral and tibial bowing and torsion significantly impact stifle biomechanics; however, deformity is not limited to long bones. Normal patellar groove development relies on a centrally positioned patella (Figure 2A).
In puppies, mitigation of severe deformities easily justifies urgent surgical intervention. Where recurrent patellar dislocation is associated with fewer deformities, surgical management is also considered the gold standard in children (Herdea et al., 2021).
Patellar luxation is usually graded with a standard 0 to 4 grading system via physical assessment (Figure 3) using the below criteria:
Physical assessment begins with gentle palpation of the standing patient. If periarticular soft tissue thickening makes patellar palpation difficult, the tibial tuberosity should be identified first. The fingers should follow the patellar tendon proximally towards the femoral trochlear groove. If the finger identifies the trochlear groove, the patella is luxated.
A minimum grade of 3 is assigned if luxation is identified in the standing patient before the stifle joint is moved. Manipulation is required to determine which specific grade to assign. Luxation grade is influenced by experience and chance. For example, a patella might be luxated 90 percent of the time (grade 3) but spontaneously reduces immediately before the exam (grade 2). This explains why experienced clinicians are very accurate at assigning a binary grade (94 percent agreement for normal versusMPL) but disagree 35 percent of the time when using the five-point classification (Hansen et al., 2022).
Skeletal maturity occurs when the size, shape and mineralisation of the bones can no longer change. Claims that dogs do not reach skeletal maturity until 18 months are becoming popular; however, these claims are not supported by scientific evidence. Many studies show growth ceases between 8 and 14 months of age, and in most breeds, residual growth after eight months is under 10 percent. Importantly, the magnitude of progressive deformity depends on residual growth after the patella luxates. Thus, luxation occurring after eight months cannot trigger a cascade of severe deformities.
Consequently, surgery performed in adults does not have the potential to mitigate severe limb deformities, and contrary to popular opinion, surgery does not reduce the risk of clinically important osteoarthritis (Roy et al., 1992). Nevertheless, lameness can progress in dogs with high-grade luxation or cartilage ulceration, and in dogs affected by concurrent cranial cruciate ligament (CCL) injury.
Only grade 4 MPL significantly increases the risk of subsequent CCL injury (Campbell et al., 2010). But it is important to remember that the relationship between MPL and a CCL injury is two-way. Concurrent CCL disease is especially common in large-breed dogs, with a reported incidence as high as 42 percent (Brower et al., 2017). An increased risk of MPL subsequent to CCL injury is thought to involve rotational tibial instability coupled with stifle effusion.
Surgery is indicated for adult dogs when it is the best option to treat a clinically significant problem at the time of diagnosis. A relationship between grade and clinical importance is well accepted. In grade 1 MPL, the patella does not spontaneously dislocate, so surgery is not indicated. In contrast, grades 3 and 4 MPL consistently cause lameness, so surgical treatment is justifiable.
The ideal management of grade 2 MPL is controversial. This grade covers a broad spectrum of clinical relevance, with some dogs demonstrating frequent or constant lameness while others have infrequent lameness or none at all. Consequently, for adult dogs with grade 2 MPL, the decision to operate is usually made based on the frequency of lameness episodes. For example, lameness occurring once every year would be managed non-surgically, while constant lameness would be managed surgically.
Rationalising surgical treatment is difficult if patellar luxation is not clinically important at the time of diagnosis. In this scenario, performing prophylactic surgery means accepting the morbidity, risk and cost of an operation which might be unnecessary. General practitioners regularly face this dilemma because prophylactic neutering operations are routine practice. For example, ovariectomy is a low-morbidity operation performed to eliminate or reduce the risk of potentially life-threatening conditions, including pyometra and mammary cancer.
One study assessed the long-term outcome of surgery for dogs with bilateral MPL and unilateral lameness (Hamilton et al., 2020). Thirty-eight dogs underwent unilateral surgery and long-term monitoring for contralateral lameness. After a minimum follow-up time of four years, 17 dogs remained normal, and two had infrequent lameness (Figure 4). Nineteen dogs developed contralateral lameness, which was treated surgically in 17 cases. Based on the results of this study, owners of dogs with unilateral lameness and contralateral grade 2 MPL should be asked, “How much future risk is worth your pet enduring bilateral surgery instead of unilateral surgery?” The 50 percent long-term risk shown in this study must be carefully weighed against increased short-term risks.
Although differences are not statistically significant, overall complication rates are higher in dogs having simultaneous versus staged bilateral surgery. Major complications triggering surgical revision are reported in around one in six dogs that undergo simultaneous bilateral surgery (Fullagar et al., 2017).
In humans, patellar maltracking and instability commonly occur without patellar dislocation. Lateral maltracking is treated with physical therapy that prioritises vastus medial oblique (VMO) strengthening exercises.
It is difficult to prove the existence of an equivalent canine condition; however, the increasingly popular French Bulldog is predisposed to a condition which appears very similar, albeit with medial maltracking (Figure 5). Affected individuals have a characteristic intermittent skip, but veterinary exams reveal apparently normal patellar tracking. In these dogs, digital pressure applied to the patella often induces a pain response which might signify cartilage ulceration.
Although there are similarities between canine patellar subluxation and human patellar maltracking, there are also important differences. The Q-angle is very large in small-breed dogs. This anatomical feature distinguishes canine patellar maltracking from the human equivalent.
In humans, small Q-angle deviations are relatively easy to overcome by selectively strengthening the VMO. In contrast, it is difficult to see how selective strengthening of the canine vastus lateralis could be achieved, knowing that unwanted strengthening of the rectus femoris would worsen patellar maltracking. Consequently, skipping French Bulldogs and terriers with idiopathic intermittent hindlimb lameness are ideal candidates for specialist surgical referral.
It’s noteworthy that the 38 dogs in the 2020 study of non-clinical MPL did not receive long-term physiotherapy (Hamilton et al., 2020). We know that therapeutic exercise optimises core strength and fitness, regardless of the species being treated, but in human healthcare, physiotherapy programmes for patellar dislocation are consistently outperformed by surgical treatment. For example, in a 2020 meta-analysis of 654 patients, surgery significantly reduced the odds of persistent patellar instability and re-dislocation (Migliorini et al., 2020). Physiotherapists are rightly treated as key decision-makers: in human healthcare, they often decide when to refer patients with recurrent patellar dislocation for a surgical opinion.
In a 2022 survey, registered physiotherapists based in the UK were three times more likely to delay a surgical opinion compared to orthopaedic surgeons (Irving et al., 2022). We do not know if a similar mismatch exists in the advice offered by veterinary physiotherapists and orthopaedic surgeons. However, based on the results of the 2020 study of 38 dogs with non-clinical grade 2 MPL, any physical therapy programme, including hydrotherapy, would need to significantly exceed a 50 percent success rate to prove a prophylactic benefit for occult MPL.
The ideal way to avoid compromised care caused by disparate opinions is to adopt a transparent multidisciplinary approach. In other words, patient-centred decisions require regular three-way communication between owners, veterinary physiotherapists and orthopaedic surgeons.
| Further information and animations of the figures can be found here. |