Complications after corrective surgery for lateral patellar luxation in dogs: 36 cases (2000–2011)

Stephanie L. Shaver Departments of Surgical and Radiological Sciences.

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Kelli N. Mayhew Departments of Surgical and Radiological Sciences.

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Jessie S. Sutton Departments of Surgical and Radiological Sciences.

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Philipp D. Mayhew Departments of Surgical and Radiological Sciences.

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Jeffrey J. Runge School of Veterinary Medicine, University of California-Davis, Davis, CA 95616; and the Department of Clinical Studies–Philadelphia, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104.

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Dorothy C. Brown School of Veterinary Medicine, University of California-Davis, Davis, CA 95616; and the Department of Clinical Studies–Philadelphia, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104.

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Philip H. Kass Departments of Population Health and Reproduction.

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 DVM, MPVM, PhD

Abstract

Objective—To determine the frequency and severity of complications after corrective surgery in dogs with lateral patellar luxation (LPL) and identify risk factors for reluxation.

Design—Retrospective case series.

Animals—36 client-owned dogs with 47 affected stifle joints.

Procedures—Medical records of dogs that underwent surgical correction of LPL at 1 of 2 veterinary teaching hospitals between 2000 and 2011 were reviewed. Data analyzed included signalment, grade of luxation, orthopedic comorbidities, surgical procedures performed, frequency and type of complications, and whether a second surgery was performed.

Results—A total of 36 dogs with 47 affected stifle joints met the inclusion criteria. Complications were recorded for 24 of 47 (51.1%) stifle joints; there were major complications for 18 of 47 (38.3%) stifle joints. All complications were confirmed through examination by a veterinarian. The most frequent complication was reluxation, which was detected in 10 of 47 (21.3%) stifle joints. Dogs that underwent bilateral surgical repair during a single anesthetic episode had odds of reluxation that were 12.5 times the odds of reluxation for dogs that underwent unilateral surgical repair.

Conclusions and Clinical Relevance—Complication rate after corrective surgery for LPL was high, with reluxation being the most common complication in this population of dogs. Performing staged bilateral surgeries may decrease the risk of reluxation.

Abstract

Objective—To determine the frequency and severity of complications after corrective surgery in dogs with lateral patellar luxation (LPL) and identify risk factors for reluxation.

Design—Retrospective case series.

Animals—36 client-owned dogs with 47 affected stifle joints.

Procedures—Medical records of dogs that underwent surgical correction of LPL at 1 of 2 veterinary teaching hospitals between 2000 and 2011 were reviewed. Data analyzed included signalment, grade of luxation, orthopedic comorbidities, surgical procedures performed, frequency and type of complications, and whether a second surgery was performed.

Results—A total of 36 dogs with 47 affected stifle joints met the inclusion criteria. Complications were recorded for 24 of 47 (51.1%) stifle joints; there were major complications for 18 of 47 (38.3%) stifle joints. All complications were confirmed through examination by a veterinarian. The most frequent complication was reluxation, which was detected in 10 of 47 (21.3%) stifle joints. Dogs that underwent bilateral surgical repair during a single anesthetic episode had odds of reluxation that were 12.5 times the odds of reluxation for dogs that underwent unilateral surgical repair.

Conclusions and Clinical Relevance—Complication rate after corrective surgery for LPL was high, with reluxation being the most common complication in this population of dogs. Performing staged bilateral surgeries may decrease the risk of reluxation.

In dogs, LPL occurs less frequently than does MPL. Lateral patellar luxation constitutes approximately 5% to 12% of dogs with patellar luxation.1,2 Medial patellar luxation is found in dogs of all sizes, but LPL is most common in large- and giant-breed dogs, comprising 17% and 33% of patellar luxations in those dogs, respectively.3 Because LPL is a relatively infrequent condition, investigators have only included small numbers of dogs with LPL as a component of patellar luxation.1,2,4 To the authors' knowledge, there have been no studies conducted to specifically examine complications after corrective surgery in a cohort of dogs with LPL.

Because of the paucity of dogs with LPL described in the veterinary literature, many of the pathophysiologic processes of the condition have been extrapolated from dogs with MPL. Lateral patellar luxation can be the result of trauma or may have an iatrogenic cause. However, most cases are believed to be a combination of congenital and developmental abnormalities, with multiple musculoskeletal abnormalities soon after birth contributing to malalignment of the affected pelvic limb. The cause and effect of these changes is poorly understood, but possible deformities of the pelvic limbs that may contribute to LPL include anteversion of the femur, coxa valga, genu valgum, torsional deformities of the femur, dysplasia of the lateral condyle, a shallow trochlear groove, lateral rotation of the tibia and displacement of the tibial tuberosity, and pes varus.5 In humans, LPL is the most common type of patellar luxation, and it has been hypothesized that hypoplasia of the vastus medialis muscle may be responsible for initiating the lateral luxation.6 This hypothesis has not been investigated in dogs.

Outcome after surgical correction of LPL is often described anecdotally as being worse than the outcome after surgical correction of MPL; however, this has not been scientifically evaluated or confirmed. Currently, the largest data set comprised 20 stifle joints with LPL and 143 stifle joints with MPL in a study2 on outcome after surgical correction; a greater number of complications were reported for dogs after correction of LPL, but the difference was not significantly different from the number of complications for dogs after correction of MPL.2

Increasing body weight has been associated with a higher incidence of complications after surgery for correction of patellar luxation.4 That study4 included dogs with MPL or LPL (or both) that weighed > 15 kg (> 33 lb), and 29% had postoperative complications; body weight was significantly associated with the development of the complications. In another study2 with a similarly mixed population of 109 dogs with patellar luxation, the frequency of dogs with complications and major complications as well as the frequency of reluxation was greater among dogs that weighed > 20 kg (> 44 lb). Because larger dogs more commonly have LPL, these dogs may be at greater risk for postoperative complications owing to greater body weight. The objective of the study reported here was to identify the frequency of postoperative complications and to determine risk factors for reluxation after corrective surgery in dogs with LPL.

Materials and Methods

Case selection—Dogs with LPL that were examined at the veterinary medical teaching hospital of the University of California-Davis or the University of Pennsylvania between 2000 and 2011 were identified. Dogs were excluded if the LPL was the result of trauma or if a previous surgical correction had been attempted.

Medical records review—Information gathered from the medical records included breed, sex, age, body weight, grade of patellar luxation, orthopedic comorbidities, radiographic findings (when available), surgical procedures, timing of surgery (single session or staged procedures) for dogs with bilateral LPL, and frequency and type of postoperative complications. Patellar luxation was graded on a scale of 1 to 4 (1 = patella can be manually luxated but spontaneously reduces, 2 = patella luxates on flexion of the stifle joint or manipulation of the patella and remains luxated until extension of the stifle joint or manual reduction of the patella, 3 = patella is luxated continuously but can be manually reduced, and 4 = patella is permanently luxated and cannot be manually reduced).5 Major complications were defined as those that required further medical or surgical treatment to achieve resolution, whereas minor complications (eg, incisional inflammation or a seroma) did not require additional treatment.7 All postoperative complications were confirmed through examination by a veterinarian.

Statistical analysis—Descriptive statistics were calculated. Continuous data were expressed as mean and SD for data that were normally distributed; median and range were reported for continuous data that were not normally distributed. Categorical data were expressed as frequencies. Logistic regression analysis was performed to evaluate the association of factors extracted from the medical records on reluxation (yes/no). The analysis accounted for dependency in the replicate (ie, > 1 limb) data by use of a robust variance estimator for clustered data.8 Univariate analysis was performed initially, and all factors with a value of P < 0.20 were included in a multivariate model; age and body weight were included in the multivariate model as confounders. Model fit was assessed with the Hosmer-Lemeshow test. All analyses were performed with statistical software.a For all tests, significance was defined as P < 0.05.

Results

Records review identified 36 dogs with 47 stifle joints that had LPL. Median body weight was 26.0 kg (57.2 lb), with a range of 2.1 to 71.5 kg (4.6 to 157.3 lb). Median age was 16 months (range, 2 to 168 months). Twenty-two breeds were represented; the most common breeds were Newfoundland (n = 6 stifles), Cocker Spaniel (5), and mixed breed (5). Classification of luxation was grade 1 for 1 stifle joint, grade 2 for 11 stifle joints, grade 3 for 22 stifle joints, and grade 4 for 11 stifle joints; luxation grade was not recorded for 2 stifle joints.

Concurrent CCL injury was detected during physical examination for 6 (12.8%) stifle joints, and various severities of hip dysplasia were confirmed radiographically for 20 (42.6%) stifle joints. Twenty-five of the 36 (69.4%) dogs had unilateral surgical repair. Eleven (30.6%) dogs had bilateral surgical repair. For dogs that underwent bilateral surgical repair, both surgical procedures were performed during a single session (performed during 1 anesthetic episode) in 4 dogs, whereas the surgeries were performed as staged procedures (performed during 2 anesthetic episodes on different days) in 7 dogs. Median interval between the bilateral staged procedures was 5 months (range, 1.5 to 26.0 months).

The most common surgical procedure used for correction of LPL was trochleoplasty (n = 42 stifle joints). Procedures used to treat patellar luxation for each grade classification were determined (Table 1). Abrasion (n = 17 stifle joints), wedge (9), modified block (4), and block (12) trochleoplasties were performed. Other procedures used to treat LPL included medial imbrication (n = 39 stifle joints), lateral joint capsule release (28), TTT (26), and osteotomy of the distal portion of the femur (3). Tibial tuberosity transpositions were secured with pins (n = 9 stifle joints), pins and tension band wires (11), pins and screws (3), screws (2), and nylon sutures (1). In stifle joints in which there was concurrent injury of the CCL, TPLO (n = 1 stifle joint) or suturing of the medial and lateral fabellae (1) was performed; no specific stabilization procedure for the injured CCL was performed in 4 stifle joints.

Table 1—

Surgical procedures used to treat LPL in 47 stifle joints of 36 dogs.

Grade of luxationTrochleoplastyTTTLateral joint capsule releaseMedial imbricationFemoral osteotomy
1 (n = 1)10110
2 (n = 11)934110
3 (n = 22)211318170
4 (n = 11)1010583
Unknown (n = 2)10020
Total (n = 47)422628393

Patellar luxation was graded on a scale of 1 to 4 (1 = patella can be manually luxated but spontaneously reduces, 2 = patella luxates on flexion of the stifle joint or manipulation of the patella and remains luxated until extension of the stifle joint or manual reduction of the patella, 3 = patella is luxated continuously but can be manually reduced, and 4 = patella is permanently luxated and cannot be manually reduced).5

Median duration of follow-up monitoring after surgery was 60.5 months (range, 3 to 141 months). Complications were recorded for 24 of 47 (51.1%) stifle joints. Major complications were recorded for 18 (38.3%) stifle joints. Complications encountered were reluxation (n = 8 stifle joints), TTT implant removal (4), seroma (2), TTT implant removal and seroma (2), seroma and incisional inflammation (2), seroma and reluxation (1), infection and reluxation (1), patellar tendon laceration (1), tibial tuberosity osteopenia (1), tibial tuberosity avulsion not requiring treatment (1), and implant failure (1). Major complications were reluxation (n = 10), TTT implant removal (6), infection (1), patellar tendon laceration (1), and implant failure (1); 1 stifle had 2 major complications (reluxation and infection). Eight (17.0%) stifle joints underwent a subsequent surgery to correct postoperative complications (6 for correction of reluxation, 1 for stabilization of a femoral osteotomy after implant failure, and 1 for repair of a lacerated patellar tendon).

Reluxation after surgical correction of LPL was detected in 10 of 47 (21.3%) stifle joints. Characteristics for the dogs were stratified on the basis of postoperative reluxation (Table 2). Univariate logistic regression analysis revealed that factors associated with reluxation (P < 0.2) included the absence of trochleoplasty (P = 0.084) and bilateral surgery during a single session (P = 0.034). Multivariate analysis (controlling for age and body weight) revealed that bilateral surgery during a single session was the only variable significantly associated with reluxation. Dogs that had corrective surgery performed on both stifle joints during a single session (ie, the same anesthetic episode) had odds of reluxation that were 12.5 times the odds of reluxation for dogs that had unilateral surgery (P = 0.026; 95% confidence interval, 1.3 to 116.1). The multivariate model fit was satisfactory (P = 0.20).

Table 2—

Characteristics of 47 stifle joints with LPL that underwent surgical correction.

VariableReluxation (n = 10)No reluxation (n = 37)P value*
Body weight (kg [lb])34.9 ± 15.9 (76.8 ± 35.0)23.2 ± 14.7 (51.0 ± 32.3)0.15
Sex
 Castrated male412Ref
 Sexually intact female140.84
 Spayed female3110.86
 Sexually intact male2100.69
Age (mo)10.5 (5–72)17.7 (2–168)0.19
Bilateral single-session surgery
 No435Ref
 Yes620.034
Concurrent CCL injury
 No1031Ref
 Yes06NE
Concurrent hip dysplasia
 No423Ref
 Yes6140.33
Luxation grade§3 (2–3)3 (1–4)0.30
Trochleoplasty
 No32Ref
 Yes7350.084
TTT
 No615Ref
 Yes4220.35
Lateral joint capsule release
 No217Ref
 Yes8200.19

Represents results for univariate logistic regression analyses with robust variance estimation to account for dogs with > 1 stifle joint with LPL; values were considered significant at P < 0.05.

Value reported is mean ± SD.

Value reported is median (range).

Patellar luxation was graded on a scale of 1 to 4.5

NE = Not estimable. Ref = Reference category.

See Table 1 for remainder of key.

Discussion

Characteristics of the population of dogs in the study reported here were consistent with those described for dogs with LPL in other reports.1–3 Dogs of all sizes were represented in the present study, but large-breed dogs such as Newfoundlands had the greatest number of stifle joints with LPL. Cocker Spaniels, which are a breed predisposed to LPL,2 were also prevalent. There were several older dogs with a history of chronic lameness prior to LPL correction; however, most dogs were young at the time of examination, which is consistent with previous reports.2,3

Reluxation is the most common complication after surgical correction and was prevalent in the present study (10/47 [21.3%]). In studies1,2,4 published in 2006 and 2007, 6% to 9% of stifle joints with patellar luxation (both MPL and LPL) reluxated. In one of those studies,2 investigators found that 17% of stifle joints with LPL reluxated, which is comparable to the percentage of dogs with reluxation reported here (21.3%). Historically, much higher rates of patellar reluxation have been reported. In a 1987 study,9 48% of dogs with MPL had persistent luxation when evaluated ≥ 1 year after surgical correction. The apparent decrease in rate of reluxation during the past couple of decades may be related to improved or more aggressive surgical treatment in more recent cohorts of dogs.

Simultaneous bilateral surgical repair was identified as a significant risk factor for reluxation. Dogs with bilateral LPL that underwent staged surgical repairs were not significantly more likely to have reluxation than were unilaterally affected dogs. Problems associated with bilateral correction of patellar luxation have not been described in the veterinary literature; however, there is controversy regarding other orthopedic procedures (eg, TPLO) that sometimes are performed bilaterally during a single session. Increased incidence of complications,10 specifically tibial tuberosity fracture,11,12 have been associated with bilateral single-session TPLO. In contrast, it was suggested in 2 retrospective studies13,14 that outcomes after bilateral single-session TPLO are reasonable. In one of those studies,14 outcomes after bilateral single-session TPLO did not differ significantly from outcomes after unilateral TPLO. We hypothesize that dogs with LPL that underwent bilateral surgical repair during a single session may have placed excessive stress on implants and healing tissues, which led to subsequent reluxation.

Six (12.8%) stifle joints in the present study were affected concurrently with LPL and CCL injury. This is consistent with results of studies1,9 on patellar luxation (primarily MPL), which have reported concomitant CCL injury in 12% to 15% of affected stifles. Of the 6 stifle joints with CCL injury, only 1 had a major complication (implant failure after femoral osteotomy). None of the dogs with LPL and concurrent CCL injury had reluxation after surgical correction.

Femoral osteotomy was performed in 3 stifle joints. All 3 dogs had coxofemoral incongruity or subluxation and genu valgus; however, specific femoral angles were not recorded, and diagnostic images were not available for review. Results of studies15–17 conducted to evaluate the use of radiographic measurements for corrective osteotomies in MPL suggest that the correlation between radiographic and cadaveric measurements for determining varus of the distal aspect of the femur is unacceptably low. In addition, changes in position also significantly affect the measured lateral angle of the distal aspect of the femur. In 1 study,17 any increase in elevation of > 5° significantly altered assessment of femoral varus. Femoral valgus associated with LPL has not been as critically assessed as has varus deformity; however, similar obstacles exist with regard to obtaining standardized, repeatable measurements of angular deformity.

The 3 dogs in the present study that underwent femoral osteotomy were all large (23 to 49 kg [50.6 to 107.8 lb]), young (< 3 years old) dogs with grade 4 LPL. One of these 3 dogs had a major complication related to implant failure, but none of these 3 dogs had reluxation. This group of 3 dogs is an extremely limited subset, and another study18 provides more representative information regarding outcome after corrective femoral osteotomy. A femoral osteotomy was not performed in 8 dogs with grade 4 LPL, and there were no major complications or reluxations for these stifle joints, although direct comparison of anatomic deformities in these dogs was not possible with the available data.

Limitations of the present study included the highly variable nature of surgical correction of LPL, which can make it impossible to draw conclusions about the effect of specific treatments. Despite the fact that we evaluated the largest cohort of stifle joints with LPL ever reported, we may have lacked sufficient power to detect significant differences that truly existed. Confounding data may have also prohibited detection of significant risk factors for complications. For example, multivariate analysis did not detect increasing body weight as a significant risk factor for complications, which is in contrast to results of previous studies2,4 that have detected a correlation between body weight and a higher risk of complications and reluxation after corrective surgery.

Further limitations of the present study involved its retrospective nature. In particular, diagnostic images that would have allowed accurate assessment of skeletal deformities commonly associated with LPL were not available. Untreated angular limb deformities may have contributed to complications and persistent luxation. Nevertheless, a high number of complications and reluxations associated with LPL corrective surgery were identified in this study, with bilateral single-session surgery as a risk factor for reluxation. A prospective study should be conducted to corroborate these findings as well as to evaluate the effect of skeletal deformities and limb angulation on the prevalence of complications and reluxation.

ABBREVIATIONS

CCL

Cranial cruciate ligament

LPL

Lateral patellar luxation

MPL

Medial patellar luxation

TPLO

Tibial plateau leveling osteotomy

TTT

Tibial tuberosity transposition

a.

Stata/IC 12.1, StataCorp LP, College Station, Tex.

References

  • 1. Alam MR, Lee JI, Kang HS, et al. Frequency and distribution of patellar luxation in dogs: 134 cases (2000–2005). Vet Comp Orthop Traumatol 2007; 20: 5964.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 2. Arthurs GI, Langley-Hobbs SI. Complications associated with corrective surgery for patellar luxation in 109 dogs. Vet Surg 2006; 35: 559566.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 3. Hayes AG, Boudrieau RJ, Hungerford LL. Frequency and distribution of medial and lateral patellar luxation in dogs: 124 cases (1982–1992). J Am Vet Med Assoc 1994; 205: 716720.

    • Search Google Scholar
    • Export Citation
  • 4. Gibbons SE, Macias C, Tonzing MA, et al. Patellar luxation in 70 large breed dogs. J Small Anim Pract 2006; 47: 39.

  • 5. Hayashi K, Lansdowne J, Déjardin L. Patellar luxation in dogs. In: Bojrab MJ, Monnet E, eds. Mechanisms of disease in small animal surgery. 3rd ed. Jackson, Wyo: Teton NewMedia, 2010; 655661.

    • Search Google Scholar
    • Export Citation
  • 6. Colvin AC, West RV. Patellar instability. J Bone Joint Surg Am 2008; 90: 27512762.

  • 7. Cook JL, Evans R, Conzemius MG, et al. Proposed definitions and criteria for reporting time frame, outcome, and complications for clinical orthopedic studies in veterinary medicine. Vet Surg 2010; 39: 905908.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 8. Rogers WH. Regression standard errors in clustered samples. Stata Tech Bull 1993; 13: 1923.

  • 9. Willauer CC, Vasseur PB. Clinical results of surgical correction of medial luxation of the patella in dogs. Vet Surg 1987; 16: 3136.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 10. Priddy NH II, Tomlinson JL, Dodam JR, et al. Complications with and owner assessment of the outcome of tibial plateau leveling osteotomy for treatment of cranial cruciate ligament rupture in dogs: 193 cases (1997–2001). J Am Vet Med Assoc 2003; 222: 17261732.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 11. Kergosien DH, Barnhart MD, Kees CE, et al. Radiographic and clinical changes of the tibial tuberosity after tibial plateau leveling osteotomy. Vet Surg 2004; 33: 468474.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 12. Bergh MS, Rajala-Schultz P, Johnson KA. Risk factors for tibial tuberosity fracture after tibial plateau leveling osteotomy in dogs. Vet Surg 2008; 37: 374382.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 13. Barnhart MD. Results of single-session bilateral tibial plateau leveling osteotomies as a treatment for bilaterally ruptured cranial cruciate ligaments in dogs: 25 cases (2000–2001). J Am Anim Hosp Assoc 2003; 39: 573578.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 14. Fitzpatrick N, Solano MA. Predictive variables for complications after TPLO with stifle inspection by arthrotomy in 1,000 consecutive dogs. Vet Surg 2010; 39: 460474.

    • Search Google Scholar
    • Export Citation
  • 15. Swiderski JK, Radecki SV, Park RD, et al. Comparison of radiographic and anatomic femoral varus angle measurements in normal dogs. Vet Surg 2008; 37: 4348.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 16. Palmer RH, Ikuta CL, Cadmus JM. Comparison of femoral angulation measurement between radiographs and anatomic specimens across a broad range of varus conformations. Vet Surg 2011; 40: 10231028.

    • Search Google Scholar
    • Export Citation
  • 17. Jackson GM, Wendelburg KL. Evaluation of the effect of distal femoral elevation on radiographic measurement of the anatomic lateral distal femoral angle. Vet Surg 2012; 41: 9941001.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 18. Swiderski JK, Palmer RH. Long-term outcome of distal femoral osteotomy for treatment of combined distal femoral varus and medial patellar luxation: 12 cases (1999–2004). J Am Vet Med Assoc 2007; 231: 10701075.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 1. Alam MR, Lee JI, Kang HS, et al. Frequency and distribution of patellar luxation in dogs: 134 cases (2000–2005). Vet Comp Orthop Traumatol 2007; 20: 5964.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 2. Arthurs GI, Langley-Hobbs SI. Complications associated with corrective surgery for patellar luxation in 109 dogs. Vet Surg 2006; 35: 559566.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 3. Hayes AG, Boudrieau RJ, Hungerford LL. Frequency and distribution of medial and lateral patellar luxation in dogs: 124 cases (1982–1992). J Am Vet Med Assoc 1994; 205: 716720.

    • Search Google Scholar
    • Export Citation
  • 4. Gibbons SE, Macias C, Tonzing MA, et al. Patellar luxation in 70 large breed dogs. J Small Anim Pract 2006; 47: 39.

  • 5. Hayashi K, Lansdowne J, Déjardin L. Patellar luxation in dogs. In: Bojrab MJ, Monnet E, eds. Mechanisms of disease in small animal surgery. 3rd ed. Jackson, Wyo: Teton NewMedia, 2010; 655661.

    • Search Google Scholar
    • Export Citation
  • 6. Colvin AC, West RV. Patellar instability. J Bone Joint Surg Am 2008; 90: 27512762.

  • 7. Cook JL, Evans R, Conzemius MG, et al. Proposed definitions and criteria for reporting time frame, outcome, and complications for clinical orthopedic studies in veterinary medicine. Vet Surg 2010; 39: 905908.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 8. Rogers WH. Regression standard errors in clustered samples. Stata Tech Bull 1993; 13: 1923.

  • 9. Willauer CC, Vasseur PB. Clinical results of surgical correction of medial luxation of the patella in dogs. Vet Surg 1987; 16: 3136.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 10. Priddy NH II, Tomlinson JL, Dodam JR, et al. Complications with and owner assessment of the outcome of tibial plateau leveling osteotomy for treatment of cranial cruciate ligament rupture in dogs: 193 cases (1997–2001). J Am Vet Med Assoc 2003; 222: 17261732.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 11. Kergosien DH, Barnhart MD, Kees CE, et al. Radiographic and clinical changes of the tibial tuberosity after tibial plateau leveling osteotomy. Vet Surg 2004; 33: 468474.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 12. Bergh MS, Rajala-Schultz P, Johnson KA. Risk factors for tibial tuberosity fracture after tibial plateau leveling osteotomy in dogs. Vet Surg 2008; 37: 374382.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 13. Barnhart MD. Results of single-session bilateral tibial plateau leveling osteotomies as a treatment for bilaterally ruptured cranial cruciate ligaments in dogs: 25 cases (2000–2001). J Am Anim Hosp Assoc 2003; 39: 573578.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 14. Fitzpatrick N, Solano MA. Predictive variables for complications after TPLO with stifle inspection by arthrotomy in 1,000 consecutive dogs. Vet Surg 2010; 39: 460474.

    • Search Google Scholar
    • Export Citation
  • 15. Swiderski JK, Radecki SV, Park RD, et al. Comparison of radiographic and anatomic femoral varus angle measurements in normal dogs. Vet Surg 2008; 37: 4348.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 16. Palmer RH, Ikuta CL, Cadmus JM. Comparison of femoral angulation measurement between radiographs and anatomic specimens across a broad range of varus conformations. Vet Surg 2011; 40: 10231028.

    • Search Google Scholar
    • Export Citation
  • 17. Jackson GM, Wendelburg KL. Evaluation of the effect of distal femoral elevation on radiographic measurement of the anatomic lateral distal femoral angle. Vet Surg 2012; 41: 9941001.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 18. Swiderski JK, Palmer RH. Long-term outcome of distal femoral osteotomy for treatment of combined distal femoral varus and medial patellar luxation: 12 cases (1999–2004). J Am Vet Med Assoc 2007; 231: 10701075.

    • Crossref
    • Search Google Scholar
    • Export Citation

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