Medial patella luxation (MPL) is common in young, small-breed dogs. A luxating patella can occur due to congenital, developmental, or traumatic causes. It is most commonly a developmental disease that occurs when dogs are born with an anatomic conformation that predisposes the limb to patellar luxation over time.1
Clinical signs of patellar luxation vary based on the severity of luxation but include gait abnormalities and intermittent progressive lameness.1 Not all affected dogs show clinical signs that are noticed by owners, and MPL is often an incidental finding later in life. Surgery is controversial when patients do not have a consistent or obvious lameness at the time of consultation. The decision to pursue surgery in an attempt to prevent clinical progression and to improve lameness is straightforward for animals that have obvious lameness. The decision is not clear cut for dogs that are intermittently lame, do not appear to be affected by their lameness, or whose lameness is not progressing. With increased age and luxation grade, joint pathology will continue to progress despite surgical intervention.2–4
The prevalence of luxating patellas in dogs has been reported in the United States to be approximately 5.4%.5 Surgical correction commonly involves a combination of procedures that deepen the trochlear groove via a wedge or block recession, transpose the tibial tuberosity, and imbricate or release periarticular soft tissues.
It has been reported that for every occult grade 2 medially luxating patella that was presented to a clinic and surgically corrected, 1 dog would have had an unnecessary surgery to prevent lameness within 4 years.6
Reported postoperative complication rates vary between 13% and 48%.7 A common complication is reluxation, which has been reported in 8% and 48% of operated dogs and is more likely to occur in large breed dogs.1 Not all reluxations required a second surgery with 68% of reluxation cases resulting in a grade 1 MPL. Overall, 92% of dogs were considered sound at follow-up.1 In a paper by Cashmore et al,7 11.6% of cases had a luxation after initial patellar stabilization and only 6.4% required a surgical revision. Historically, patellar luxation correction has been recommended for animals that are showing obvious lameness. To our knowledge, there are no reports of reliable, objective measurement of lameness in dogs with patellar luxation following surgery. Data obtained via stance analysis can provide an objective measurement of lameness before or after surgery on clinically sound animals with a luxating patella. An objective measurement of lameness could help the clinician advise owners in making the decision if surgical correction is the best option.
Both subjective and objective measures of lameness in dogs have been evaluated, but there are no reports of objective measures of lameness due to patellar luxation. Subjective lameness analysis methods such as the visual analog scale (VAS) and the numeric rating scale (NRS) have been validated in lame animals.8,9 These subjective assessments are most useful for animals on either extreme of the scale (very lame or sound dogs) but neither is reliable for the dogs that are subtly lame.8,9
Force plate analysis has been used as an objective measure to evaluate lameness not easily observed subjectively. Force plate measurements have been compared and validated compared to the VAS or NRS showing that the force plate was a more sensitive objective measurement.10 Stance analysis was first reported when Seibbert and colleagues11 found that measuring the percentage of body weight (BW) by placing a bathroom scale under each limb was a valid measurement for differentiating which limb is most affected due to signs of coxofemoral joint pain. The percentage of BW is a measure of the percentage of weight an animal is placing on each limb during weight bearing. It has been used to obtain objective measurements after various surgical procedures such as cranial cruciate ligament repair, total hip replacement, and osteoarthritis models as well as a method to evaluate a patient’s response to analgesia.12–15
The PetSafe Stance Analyzer13 (LiteCure, Companion Animal Health) is an objective tool to diagnose lameness in dogs by measuring the percentage of BW. The dog is required to stand with a single foot in 4 different quadrants to assess the weight distribution in each limb at a given time point. This device is less expensive than more advanced gait analysis techniques such as pressure platform force collection or joint kinematics, is a quick and easy tool for most hospital staff to learn and understand, and provides an objective measure to evaluate lameness.16 The objective of the study reported here was to document and compare preoperative and postoperative stance analysis measurements in animals with naturally occurring patellar luxation.
Materials and Methods
One hundred forty-seven dogs were evaluated at the Kansas State University Veterinary Health Center Orthopedic Surgery Service from March 2015 to February 2020 for naturally occurring unilateral or bilateral patella luxation. One hundred thirty-one dogs were included in the study after the exclusion of dogs that had no clinically visible lameness during physical examination. All included dogs were subjected to stance analysis (PetSafe Stance Analyzer; LiteCure LLC, Companion Animal Health) preoperatively and during the first and second postoperative recheck examination (V1 and V2, respectively) when one occurred. The V1 was recommended to occur at 4 or 8 weeks for all animals having a tibial tuberosity transposition performed to evaluate bone healing on radiography. Animals were returned for a follow-up examination based on the client’s decision to schedule a recheck examination. Records were reviewed for V1 and V2 and included stance data that were routinely collected at Kansas State University for lame patients presented to the orthopedic service.
Stance analysis was performed according to a protocol for clinical research approved by the Institutional Animal Care and Use Committee of Kansas State University. Individual owner consent was not required because the stance analyzer was routinely used for the clinical evaluation of every lame orthopedic patient under the approved protocol.
Data were collected from the medical records of client-owned dogs with a naturally occurring luxating patella. Data collected included breed, age, sex, hind limb affected (right, left, or bilateral), laterality of lameness (right, left, or bilateral), direction of luxation, grade of patellar luxation, surgery performed (right, left, or bilateral), and surgical method of stabilization. Surgical corrections related to the luxating patella included a trochlear wedge or block recession, tibial tuberosity transposition, joint capsule imbrication, or release as a single procedure or as a combination of procedures. Animals that had a concurrent procedure for a cruciate ligament rupture such as a lateral suture or tibial plateau leveling osteotomy on the surgical limb were classified under procedure as “other.” Dogs were excluded if the luxating patella was secondary to vehicular or major trauma or previous surgery. Dogs in the no MPL surgery group were categorized as animals with a MPL but no surgical correction for the MPL. Data were collected for the initial visit (V0), V1, and V2, if one occurred.
The device for analyzing stance (PetSafe Stance Analyzer; LiteCure LLC, Companion Animal Health) was calibrated by the use of a registered known weight in accordance with the manufacturer’s instructions before use each day. The analyzer was placed in the center of a dedicated gait analysis laboratory, and leashed dogs were walked onto the device from behind the analyzer and encouraged to stand in a relaxed position with each foot placed in its respective quadrant, and the percentage of BW was recorded. A minimum of 5 valid measurements were collected over 30 seconds and averaged for a single percentage of BW value. Data were collected on the day of the initial examination or at the time of reevaluation before the administration of any sedation used to facilitate other diagnostic procedures.
The contralateral hind limb (CHL) was recorded in comparison to the lame hind limb (LHL). The LHL was determined from the medical record according to the presenting complaint from the owner, or in patients with bilateral hind limb lameness, the hind limb with the lowest percentage of BW was recorded as the LHL for analysis.
Luxation was classified during physical examination into 4 standard grades and recorded as follows: grade 1, easily luxated but immediately reduces when released; grade 2, easily luxated and will reduce when the joint is placed through a range of motion; grade 3, permanently luxated but easily reducible with manipulation; and grade 4, permanently luxated and unable to be reduced.17 Any dog that did not cooperate with being placed on the stance analyzer or did not have valid measurements recorded was excluded.
Patients were divided into the following groups as having unilateral patella luxation, bilateral patella luxation, unilateral patella luxation with a cruciate tear, or bilateral patella luxation with a cruciate tear for analysis.
Statistical analysis
The percentage of BW of LHL was compared to MPL grade and animal sex/neuter status by multiple ANOVA with a Newman-Keuls post hoc. Initial age and weight were assessed for correlation with each other and with the percentage of BW of the LHL by Pearson correlation coefficient. Weight change over time by assessed by repeated-measures ANOVA with Newman-Keuls post hoc. Changes in the percentage of BW between V0, V1, and V2 were compared by repeated-measures ANOVA with Newman-Keuls post hoc. A Student’s t-test was used to analyze the relationship between the lame side and V0 lamenesss, surgery side and V0 lameness, and surgery side and age. The correlation of the last postoperative recheck day, percentage of BW of the LHL at the last recheck, and change in lameness of the LHL from the initial presentation to last recheck were assessed by Pearson correlation coefficient analysis. Descriptive statistics were collected on age, sex, body weight, patella luxation grade, and initial lameness grade. Comparisons and correlations were considered significant at P < .05. All statistical analysis was performed via standard statistical software (WINKS Kwikstat SDA 7 version 7.0.9; Texasoft Inc).
Results
Of the 131 dogs, sex was recorded as 61 males (56 neutered; 5 intact) and 70 females (65 neutered; 5 intact). Age ranged from 5.0 months to 13.0 years (mean, 4.62 years) and body weight ranged from 1.66 to 61.18 kg (mean 13.03 kg). Dogs were most commonly reported as Yorkshire Terrier (17) followed by 10 each of Chihuahua and English Bulldog; 7 Maltese; 6 Poodle mixed-breed dogs; 5 other mix-breed dogs; 4 each of Boston Terrier, Cavalier King Charles Spaniel, Miniature Poodle, and terrier-type dogs; 3 each of Boxer, Labrador Retriever, Pomeranian, and Wheaten Terrier; 2 each of Akita, Beagle, Border Collie, Bichon Friese, Great Pyrenees, Golden Retriever, Jack Russell Terrier, Miniature Australian Shepherd, Miniature Schnauzer, Pit Bull–type dog, and Toy Poodle; and 1 each of Australian Shepherd, Australian Blue Heeler, Brittany Spaniel, Belgian Malinois, Bullmastiff, Chow Chow, Cocker Spaniel-Poodle cross, Doberman Pinscher, English Mastiff, English Setter, Golden Retriever-Poodle cross, German Shepherd Dog, German Shorthaired Pointer, Havanese, Italian Greyhound, Labrador Retriever-Poodle cross, Miniature Pinscher, Parson Russell Terrier-Jack Russell Terrier cross, Pekingese, Papillon, Rhodesian Ridgeback, Red Heeler, Shetland Sheepdog, Shiba Inu, Spitz, and Whippet.
Eighteen dogs had bilateral lameness, 57 dogs were lame on the left hind limb, and 56 were lame on the right hind limb (Table 1). Surgery was performed bilaterally in 15 dogs, on the left in 61 dogs, and on the right in 55 dogs (Table 2).
Mean ± SD age, body weight, and results for stance analyses in terms of percentage of body weight (% of BW) for 131 dogs undergoing surgery for naturally occurring unilateral or bilateral patella luxation between March 30, 2015, and February 12, 2020, grouped on the basis of whether dogs showed clinical signs of bilateral, left, or right hind limb lameness on initial examination (V0).
| Recorded hind limb lameness | Age (y) | Contralateral front limb (% of BW) | Ipsilateral front limb (% of BW) | Contralateral hind limb (% of BW) | Lame hind limb (% of BW) | Body weight (kg) |
|---|---|---|---|---|---|---|
| Bilateral (n = 18) | 2.0 ± 1.8 | 29.44 ± 7.67 | 33.50 ± 5.92 | 23.83 ± 5.48 | 13.22 ± 6.58 | 9.6 ± 9.1 |
| Left (n = 57) | 4.4 ± 3.5 | 30.32 ± 7.70 | 31.88 ± 9.39 | 24.43 ± 8.59 | 13.96 ± 7.82 | 12.2 ± 9.0 |
| Right (n = 56) | 5.7 ± 10.9 | 32.04 ± 7.56 | 32.45 ± 7.50 | 23.88 ± 7.07 | 11.64 ± 7.69 | 14.9 ± 12.8 |
Mean ± SD age, body weight, and results for stance analyses (% of BW) at V0 for the dogs described in Table 1, grouped by which hind limbs were surgically treated.
| Hind limb operated | Age (y) | Contralateral front limb (% of BW) | Ipsilateral front limb (% of BW) | Contralateral hind limb (% of BW) | Lame hind limb (% of BW) | Body weight (kg) |
|---|---|---|---|---|---|---|
| Bilateral (n = 15) | 2.1 ± 1.9 | 30.33 ± 7.68 | 33.40 ± 5.91 | 23.53 ± 5.28 | 12.73 ± 6.37 | 7.9 ± 7.8 |
| Left (n = 61) | 4.2 ± 3.5 | 30.08 ± 7.65 | 32.03 ± 9.19 | 24.40 ± 8.43 | 14.03 ± 7.73 | 12.4 ± 9.2 |
| Right (n = 55) | 5.8 ± 11.0 | 32.04 ± 7.63 | 32.40 ± 7.56 | 23.95 ± 7.11 | 11.62 ± 7.76 | 15.1 ± 12.8 |
A total of 71 dogs had tibial tuberosity transposition and wedge recession at the same time, 15 dogs received only a tibial tuberosity transposition, and 29 received a trochlear wedge recession only. Ten dogs did not have a surgical procedure and 6 were classified as “other” (Table 3). A total of 66 had a unilateral luxating patella, 33 dogs had a bilaterally luxating patella, 30 had a cruciate with a unilateral luxating patella, and 2 had a cruciate tear with bilateral luxating patella (Table 4).
Mean ± SD age, body weight, and results for stance analyses (% of BW) at V0 for the dogs described in Table 1, grouped by the type of surgical treatment performed.
| Surgery | Age (y) | Contralateral front limb (% of BW) | Ipsilateral front limb (% of BW) | Contralateral hind limb (% of BW) | Lame hind limb (% of BW) | Body weight (kg) |
|---|---|---|---|---|---|---|
| Tibial tuberosity transposition and wedge recession together (n = 71) | 5.0 ± 9.9 | 31.66 ± 6.87 | 32.58 ± 7.26 | 23.80 ± 7.94 | 12.42 ± 7.60 | 12.5 ± 10.4 |
| Trochlear wedge recession (n = 29) | 4.0 ± 3.2 | 30.38 ± 6.26 | 30.28 ± 8.56 | 25.66 ± 6.99 | 12.69 ± 8.32 | 10.7 ± 10.6 |
| Tibial tuberosity transposition (n = 15) | 2.3 ± 2.2 | 29.20 ± 11.79 | 32.93 ± 12.20 | 23.60 ± 6.33 | 14.27 ± 7.68 | 17.8 ± 10.9 |
| Other procedure (n = 6) | 6.8 ± 3.3 | 30.83 ± 5.95 | 34.50 ± 3.73 | 22.67 ± 7.42 | 12.00 ± 5.93 | 11.3 ± 10.3 |
| No MPL surgery (n = 10) | 5.8 ± 2.4 | 30.00 ± 10.36 | 34.50 ± 7.95 | 23.40 ± 8.67 | 12.10 ± 7.53 | 17.4 ± 14.1 |
Mean ± SD age, body weight, and results for stance analyses (% of BW) at V0 for the dogs described in Table 1, grouped by whether their patella luxation was unilateral (S), bilateral (B), unilateral in combination with a cruciate tear (C), or bilateral in combination with a unilateral or bilateral cruciate tear (CB).
| Surgery | Age (y) | Contralateral front limb (% of BW) | Ipsilateral front limb (% of BW) | Contralateral hind limb (% of BW) | Lame hind limb (% of BW) | Body weight (kg) |
|---|---|---|---|---|---|---|
| S (n = 66) | 4.8 ± 10.1 | 31.26 ± 7.33 | 31.74 ± 8.43 | 24.35 ± 7.83 | 12.15 ± 7.88 | 13.8 ± 11.1 |
| B (n = 33) | 2.4 ± 2.5 | 30.03 ± 7.48 | 32.39 ± 7.35 | 22.91 ± 5.80 | 14.67 ± 7.49 | 11.5 ± 10.1 |
| C (n = 30) | 6.6 ± 3.2 | 30.37 ± 7.39 | 34.57 ± 6.32 | 24.70 ± 8.48 | 10.37 ± 6.87 | 13.6 ± 1.5 |
| CB (n = 2) | 7.4 ± 2.3 | 43.50 ± 19.09 | 18.00 ± 24.04 | 27.00 ± 12.73 | 11.50 ± 7.78 | 4.1 ± 0.6 |
Only dogs that had surgery had their recheck examination data included in the statistical analysis. Sixty-six dogs returned for a V1, and 21 dogs returned for a V2 (Table 5). Animals returned for a recheck examination at a mean of 96.5 ± 124.9 days (range, 14 to 672 days) for V1 and 278.25 ± 267 days (range, 56 to 1,034 days) for V2.
Mean ± SD body weight, results for stance analyses (% of BW), and timing of the first or second recheck examination for the dogs described in Table 1, grouped by visit (initial examination [V0], first recheck examination [V1], or second recheck examination [V2]).
| Visit | Duration (d) | Contralateral front limb (% of BW) | Ipsilateral front limb (% of BW) | Contralateral hind limb (% of BW) | Lame hind limb (% of BW) | Body weight (kg) |
|---|---|---|---|---|---|---|
| V0 (n = 131) | — | 30.93 ± 7.64 | 32.34 ± 8.17 | 23.92 ± 7.8 | 11.75 ± 7.63 | 13.0 ± 10.9 |
| V1(n = 66) | 96.5 ± 124.94 | 31.91 ± 6.67 | 31.98 ± 7.99 | 22.83 ± 7.4 | 13.27 ± 8.75 | 14.6 ± 12.3 |
| V2 (n = 21) | 278.25 ± 267.03 | 30.5 ± 5.36 | 34.2 ± 8.46 | 17.65 ± 7.0 | 17.65 ± 5.77 | 12.5 ± 8.7 |
The mean body weight of all dogs was 13.03 kg ± 10.9 kg (median, 8.44 kg; range, 1.66 to 61.18 kg). There was a significant statistical correlation between age and body weight at the initial visit (P = .025). This difference showed an inverse correlation with younger dogs presenting with a higher body weight. The average weight did not change significantly from V0 to V1 (P = .19) or from V0 to V2 (P = .74). There was no statistically significant correlation between body weight and the V0 percentage of BW of LHL (P = .882) or age and V0 percentage of BW of LHL (P = .062). As age increased, the V0 percentage of BW of LHL tended to be less severe compared to younger dogs but was not statistically significant (P = .062). There was no statistical significance between sex and the V0 percentage of BW of LHL (P = .306) or if the percentage of BW of LHL was recorded as bilateral, right limb, left limb, or not visibly lame and V0 percentage of BW of LHL (P = .322). There was no statistical significance in whether surgery was performed on the right limb, left limb, or bilaterally and V0 percentage of BW of LHL (P = .168) or whether surgery was performed on the right limb, left limb, or bilaterally and age (P = .317).
No significant change in the percentage of BW of LHL was noted when comparing V0 (11.75% of BW) to V1 (13.27% of BW, P = .67). The percentage of BW of LHL did significantly improve from V0 (11.75% of BW) to V2 (17.65% of BW, P = .008). There was no significant difference in the CHL comparing V0 (22.35% of BW) to V1 (13.27% of BW, P = 0.157); however, a significant decrease was noted when comparing V0 (22.35% of BW) to V2 (17.65% of BW, P = 0.032). Postoperative recheck examination day was significantly correlated with the percentage of BW of the LHL at the last recheck examination (P < .001, r = 0.4398) and to the increase in the percentage of BW from initial presentation to last recheck examination (P < .001, r = 0.503) (Figure 1).
Scatterplot depicting results for change in signs of hind limb lameness (as a percentage of body weight [% BW]) from initial examination to last postoperative recheck examination for 131 dogs undergoing surgery for naturally-occurring unilateral or bilateral patella luxation between March 30, 2015, and February 12, 2020. Each circle represents results for 1 dog, and the dotted line represents the line of best fit (Pearson correlation coefficient r = 0.503; P < .001).
Citation: American Journal of Veterinary Research 84, 3; 10.2460/ajvr.22.10.0186
The groups did not influence the degree of the percentage of BW of LHL at V1 (P = .72) or V2 (P = .63). There was no effect on the improvement of the percentage of BW of LHL when comparing groups at V1 (P = .79) or V2 (P = .95). Initial patella luxation grade had no effect on lameness as measured by the percentage of BW at V2 (P = .62).
Discussion
Dogs with naturally occurring patellar luxation presented initially with a mean 11.75% of BW on the lame limb compared to the expected normal of nearly 20% of BW on each hind limb. This study showed a significant improvement in dogs that underwent a surgical correction for their luxating patella when they returned for a second follow-up examination with a mean 17.65% of BW regardless of the type of surgery recommended and performed by the surgeon.
The follow-up for V1 was a mean of 96.5 days (range, 14 to 672 days) for a total of 66 dogs. The V2 follow-up occurred at a mean of 278.25 days (range, 56 to 1,034 days) for 21 dogs. A longer outcome would be preferable as some dogs diagnosed with bilateral luxating patella during the study only had surgical correction of a single side. A longer follow-up would allow evaluation of the nonsurgical side for a worsening lameness and if the grade of luxating patella had any effect on weight bearing according to stance analysis. Results indicated that lameness had improved for the surgical and contralateral limb by the second visit and that improvement in lameness was significantly linearly correlated with increased postoperative time.
Sixty-five animals were not returned for the first visit. One hundred and ten animals were not returned for a second visit. Patient return for recheck examination was subject to the owner’s willingness to return and limited the follow-up data in this study. Our reported results are thus subject to nonresponse bias.18 If animals are showing no signs of lameness at 8 weeks, owners may elect to not return for a recheck examination, and thus the average improvement was underestimated in this study, or alternatively, if owners are disappointed in the clinical response to surgery, they may not have returned to this hospital for recheck examination, which would bias results in the opposite direction.
Statistical analysis showed an inverse correlation between age and body weight at the initial visit. This may suggest that larger breed dogs were referred by veterinarians at an earlier age due to a lower prevalence of luxating patella in large breed dogs.19 It was not recorded if the patient was referred for a hind limb lameness or for luxating patella specifically. Large breed dogs with static or progressive hind limb lameness could potentially be referred sooner due to the concern for other conditions such as osteochondrosis, hip dysplasia, or a cranial cruciate ligament tear.
Despite the correlation between age and initial lameness not being significant, a trend was noted (P = .062). As age increased, the initial lameness decreased. This could be due to older animals having a lower luxation grade resulting in less secondary arthritic change and less secondary inflammation and pain. It is also possible that younger animals with a more significant lameness would be brought to the hospital sooner resulting in surgery earlier versus younger animals with mild lameness may receive medical management and therefore delay the need for surgery and referral. The purpose of this study was not to evaluate arthritis progression.
Despite surgical intervention, several papers have shown that dogs with patellar luxation often develop arthritis, with no difference in the amount of arthritis present between animals with successful surgical intervention and those that failed surgical intervention.2–4 However, surgical intervention has subjectively been shown to significantly improve limb use in dogs with lameness due to MPL based on a standardized questionnaire from the owner.4 The objective of this paper was not to evaluate the amount of arthritis present but to objectively assess lameness in animals with a patellar luxation. Based on this paper, there was a significant improvement when comparing V0 to V2 (P = .008). This objective measurement provides a basis for future studies to compare the outcome of other surgical interventions in dogs with patella luxation.
The ability to assess occult or minimal lameness may aid in guiding owner decisions regarding the advisability of surgery. A major factor in the management of nonlame luxating patella cases is the concern for developing severe arthritis in the future. For patients with an occult lameness, it is possible that arthritis is delayed or diminished by performing surgery sooner.3 However, it is difficult to justify performing surgery on a clinically sound dog. Having a reliable way to objectively assess lameness in dogs with a luxating patella can help guide clinician recommendations and owner decisions to pursue surgery at an earlier date.
This paper did not indicate a specific surgical procedure that resulted in an improved outcome. This lack of a statistical difference between procedures is possibly due to nonresponse bias. Postoperative animals without clinical lameness might be less likely to return for a follow-up examination and may be lost unequally from more successful surgical groups.
The initial patellar luxation grade did not significantly correlate with lameness at the second visit. There have been no studies comparing luxation grade and long-term outcomes. It has been reported that dogs with grade IV luxation were more likely to have a concurrent cranial cruciate tear.20 It is possible to have long-term follow-up for animals with a luxating patella and gain objective information about lameness.
This study determined that patella luxation presents with a detectable clinical lameness by stance analysis and that there was no effect of luxation grade, surgical procedure, age, or sex on initial lameness due to patellar luxation. Lameness on the operated limb did improve after surgery at the second follow-up, regardless of the surgical procedure. This study also demonstrated a statistically significant inverse relationship between age and body weight at presentation for patella luxation, indicating that older animals may be able to compensate for a prolonged time or that it is more concerning for owners that have a young, large breed dog with a luxating patella.
Acknowledgment
No external funding was used in this study. The authors declare that there were no conflicts of interest.
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