Outcome Following Surgical Correction of Grade 4 Medial Patellar Luxation in Dogs: 47 Stifles (2001–2012)
ABSTRACT
Grade 4/4 medial patellar luxation (MPL) is a complex disease of the canine stifle that often requires surgical realignment of the patella to resolve clinical lameness. Outcome following surgery remains poorly described. Medical records were retrospectively reviewed for surgical correction of grade 4 MPL. Signalment and exam findings, surgical procedures performed, complications, and clinical outcome were reported. Data was statistically analyzed for association with major complication occurrence and unacceptable function following surgery. Forty-seven stifles from 41 dogs were included. The surgical procedures most frequently utilized for patellar realignment were the combination of femoral trochleoplasty, tibial tuberosity transposition, and joint capsule modification. Median in-hospital veterinary examination was performed at 69 days (range 30–179 days) following surgery. Full function was reported for 42.6% of cases (n=20). Acceptable function was reported for 40.4% of cases (n=19). Unacceptable function was reported for 17% of cases (n=8). The overall complication rate was 25.5% (n=12), with revision surgery for major complications required in 12.8% of cases (n=6). Corrective osteotomies were associated with major complications (P < 0.001). In general, pelvic limb function improves following surgical correction of grade 4 MPL; however, a return to full function should be considered guarded.
Introduction
Medial patellar luxation (MPL) is a commonly diagnosed orthopedic disease of the stifle in small and large breed dogs.1–3 This disease is often developmental in origin, but can be acquired following trauma or surgery.2,4 The etiopathogenesis for MPL is not fully understood; however, quadriceps displacement, a shallow femoral trochlear groove, torsion of the tibial tuberosity, and patella alta are common features.5–8 Dogs diagnosed with MPL often display signs of stifle discomfort and intermittent or persistent lameness. Surgical realignment of the patella within the femoral trochlear groove is the recommended treatment to resolve clinical signs and improve pelvic limb function.9
The severity of MPL is characterized by a grading scheme (1–4) first devised by Putman, and later adapted to veterinary publication by Singleton and others.9–11 A grade 4/4 luxation is the most severe form of MPL, and is defined as a permanent luxation that the clinician is unable to manually reduce. Internal rotation of the tibial tuberosity, a shallow or absent trochlea of the femur, and displacement of the quadriceps muscle group have also been included in some grade 4 definitions.5,11 Grade 4 MPL is also associated with femoral and tibial angular limb deformity.12,13 The culmination of these anatomic abnormalities impair stifle extension, often resulting in a bow legged posture and severe pelvic limb lameness. Erosion of the articular cartilage occurs secondary to MPL resulting in stifle osteoarthritis.14,15 It is also theorized that MPL increases the strain placed on the cranial cruciate ligament (CrCL). Stifles with grade 4 MPL are at greater risk for concomitant rupture of the CrCL based on the results of one study.16
Currently, in comparison to less severe grades of MPL, there is limited information in the veterinary literature describing the clinical outcome following surgical correction of grade 4 MPL in dogs. Previous studies describing surgical outcome have reported poor to excellent results in very small cohorts (1 to 16 cases).1,17–19 Details pertaining to signalment, body weight, type and combination of surgical procedures performed, and the occurrence of postoperative complications were not specifically reported. It has been suggested that surgical correction at a young age (≤ 1 year) is essential for a successful outcome.17 Recently, multiple studies have emphasized using corrective osteotomies, notably distal femoral osteotomy (DFO) for correction of femoral varus, as part of patellar realignment, particularly in large breed dogs.20–22 These reports have also generally lacked inclusion of stifles diagnosed with grade 4 MPL. Following surgery, complications were demonstrated in one study to occur significantly more frequently after correction of grade 4 MPL.23 The clinical outcome for those cases with postoperative complications remains unknown. Ultimately, due to the limited published reports, small cohorts lacking sufficient outcome data, and increased incidence of postoperative complications, the prognosis associated with surgical correction of grade 4 MPL is considered guarded.2,5,9,24
The purpose of this case series is to detail the clinical outcome following surgical correction of grade 4 MPL in a large cohort of dogs, and determine if any clinical variables may be associated with major complications or unacceptable function following surgery. We hypothesize that surgical correction of grade 4 MPL will achieve full or acceptable function in the majority of cases, including those cases that require revision surgery. We also hypothesize that no clinical variables will be associated with either the occurrence of major complications or unacceptable function following surgery.
Materials and Methods
Medical records were retrospectively reviewed for dogs that underwent surgical correction of grade 4 MPL at two veterinary specialty surgical centers (2001–2012). Dogs were included in this study with the following criteria: (1) a diagnosis of grade 4 MPL confirmed intra-operatively; (2) complete medical record and surgery report; (3) follow-up in-hospital examination by a veterinarian >4 weeks from surgery; (4) complete surgery report for any revision surgeries (if performed) and follow-up in-hospital examination >4 week from the date of revision surgery; and (5) radiographic evaluation at follow-up when indicated. Grade 4 MPL was defined in this report as: “a patella that is continually luxated, and cannot be manually reduced” as published by Roush, adapted from Singleton.9,11 Data obtained from the medical record included: signalment, body weight, history, general physical and orthopedic exam findings, complete surgical report, all follow-up exam findings, any recorded postoperative complications, surgical report for revision surgeries (if performed), and imaging reports/findings.
Surgical Technique
Either a board-certified veterinary surgeon or a resident under the direct supervision of a board-certified veterinary surgeon performed all surgeries. All techniques utilized for MPL correction were at the discretion of the operating surgeon. Dogs were anesthetized using hospital specific protocols, maintained on inhalant gas (isoflurane or sevoflurane) throughout surgery, and administered IV cefazolina (22mg/kg q 90 min) intra-operatively. Epidural analgesia was provided in some cases. Either a medial or lateral parapatellar approach to the stifle was made, followed by stifle arthrotomy. Stifles identified as having concomitant CrCL rupture were stabilized using a lateral fabello-tibial suture technique or a bone anchor suture systemb.25,26 Femoral trochlear depth was assessed and deepened when appropriate using either a wedge or block recession technique, or a trochlear sulcoplasty.24 Tibial tuberosity transposition (TTT) was performed in select cases by cutting the proximal aspect of the tibial tuberosity using either a molar splitter, motorized saw, or osteotome and then laterally repositioned and stabilized with either Kirschner wires (K-wire) or intramedullary pins.26 A tension band was added in select cases. Soft tissue joint capsule modification included capsulectomy, imbrication, and desmotomy, and were performed as needed. Additionally, a rectus femoris release at the origin of the rectus muscles on the lateral surface of the body of the ilium was performed in select cases. Prior to closure, the stifle was put through range of motion to ensure normal patellar alignment and tracking within the femoral trochlear groove. If angular limb deformity of the femur or tibia were identified on preoperative imaging and considered clinically significant by the surgeon, then a corrective osteotomy was performed.13,27
Perioperative Patient Management
Postoperatively, all dogs were administered hydromorphonec (0.05–0.1mg/kg q 4–6 hr IV) analgesia for less than 24 hr following surgery. Dogs were transitioned to per os (PO) medications 24 hr following surgery. These consisted of a nonsteroidal anti-inflammatory (carprofend [2.2mg/kg q 12 hr PO], meloxicame [0.1mg/kg q 24 hr PO], or firocoxibf [5mg/kg q 24 hr PO]) and/or tramadolg (3–5mg/kg q 8–12 hrs PO). Exercise restriction (cage confinement with leash walks only) was recommended until the time of follow-up in-hospital examination. Oral pain medications were typically discontinued at 2 wk postoperatively.
Postoperative Complications
Complications following surgery were classified as catastrophic, major, or minor based on previous definitions published by Cook et al.28 A catastrophic complication was defined as a complication that resulted in the death or euthanasia of the dog. A major complication was defined as a complication that required revision surgery or aggressive in-hospital treatment to resolve. A minor complication was defined as a complication that could be resolved on an outpatient basis, or necessitated a simple procedure under sedation from which the patient recovered with minimal morbidity and was discharged the same day. Continued patellar luxation on follow-up examination was also reported separately. If patellar luxation at follow-up examination was considered a source of pain or lameness, and surgical revision recommended, then this was defined as a clinically significant continued patellar luxation and classified as a major complication as well. Each stifle was evaluated individually for post-operative complications.
Clinical Outcome
Clinical outcome following surgery was determined by critical comparison of the pre-operative orthopedic exam findings to follow-up orthopedic exam findings (lameness description, normal or abnormal range of motion, absence or presence of stifle pain), as well as owner reported activity pre- and postoperatively. Each stifle and associated limb function was evaluated individually. If a dog was diagnosed with bilateral grade 4 MPL, and had unilateral surgical correction of each stifle in staged procedures (e.g., 6–12 wk apart), then clinical outcome for the initial limb was assigned at the final follow-up visit prior to surgical repair of the contralateral limb. Generally, surgeons would not approve surgery for the contralateral stifle until full or near full healing was determined for the initial stifle. If a dog had bilateral surgical correction of grade 4 MPL in a single anesthetic session, then outcome was assigned for each limb at the final follow-up examination. If revision surgery was performed to treat a major complication, then follow-up examination was required >4 wk from the date of the revision surgery for clinical outcome assessment. For all dogs, outcome was considered short-term, obtained ≤6 months from the date of surgery.28 Clinical outcome classification for this report was “full function,” “acceptable function,” or “unacceptable function” as defined in Cook et al. (Table 1).28 If functional outcome for a particular limb was questionable between two outcome groups, then the worse clinical outcome was assigned.
Statistical Analysis
Descriptive statistics were given as median and range. Continuous independent variables (age, body weight, and time of follow-up examination) were compared across outcome groups using Student's t-test. Categorical variables (body weight, concomitant CrCL rupture, corrective osteotomy, bilateral grade 4 MPL, and major complication) were compared across outcome groups using Pearson's chi-square analysis. Multiple logistic regression was used to determine which variables were predictive of the outcome measures. The magnitude of predictive capability for individual variables was measured using Odds Ratios. For all tests, a P value < 0.05 is considered statistically significant. Adjustment of the P values was not made when multiple tests were required. Analyses were performed using a statistical software programh.
Results
Dogs
Forty-one dogs with 47 stifles surgically treated for grade 4 MPL were included. Breeds included Yorkshire terriers (n=6), Chihuahuas (n=5), Pomeranians (n=5), toy poodles (n=3), Cavalier King Charles spaniels (n=3), Labrador retrievers (n=2), bulldogs (n=2), Jack Russell terriers (n=2), miniature pinscher (n=2), as well as one each of the following: boxer, Maltese, shih tzu, pug, tick hound, Boston terrier, and Shetland sheepdog. All remaining dogs were mixed-breed dogs (n=4). Twenty-one dogs were female (18 spayed, 3 intact) and 20 dogs were male (16 neutered, 4 intact). Median age at the time of surgery was 36 mo (range 6–192 mo). Median body weight at the time of surgery was 5 kg (range 1.2–44.9 kg). Origin of grade 4 MPL was considered developmental for 38 dogs. Acquired grade 4 MPL was suspected for three dogs based on historical pelvic limb trauma. Reported trauma consisted of femur fracture (n=2) and a tibial fracture. Thirty-four dogs were diagnosed with bilateral MPL. Fourteen of those dogs were specifically diagnosed with bilateral grade 4 MPL. Seven dogs were unilaterally affected with grade 4 MPL. The presenting complaint for all dogs was pelvic limb lameness, with a median duration of 3 mo (range 0.5–48 mo). Five stifles were additionally suspected to have concomitant rupture of the CrCL based on orthopedic exam findings of stifle instability.
Surgical Correction
Forty-seven stifles underwent surgical correction of grade 4 MPL. Five dogs underwent staged unilateral surgical correction of grade 4 MPL. A single dog underwent single session bilateral surgical correction of grade 4 MPL. The following surgical procedures were performed as part of surgical correction: wedge recession trochleoplasty (n=23), block recession trochleoplasty (n=14), trochlear sulcoplasty (n=8), TTT (n=42), joint capsulectomy/imbrication (n=45), desmotomy (n=38), and rectus femoris release (n=8). An anti-rotational suture was placed in 10 stifles. Additionally, in five stifles, concomitant CrCL rupture was corrected via lateral fabello-tibial suture technique (n=3) or bone anchor suture system (n=2). A medial meniscal tear was identified in one stifle with CrCL rupture, and a caudal pole menisectomy was performed. A corrective osteotomy was performed in eight pelvic limbs and included: DFO (n=6), proximal tibial osteotomy, and both a DFO and tibial osteotomy in the same limb. Stabilization of DFO was accomplished with a 2.7mm DFO platei combined with an intramedullary pin (n=3), 3.5mm DFO plate alone (n=3), and 3.5mm DCP plate combined with an intramedullary pin. Each tibial corrective osteotomy were stabilized using two stacked 2.7mm T-plates (n=2). Overall, the most common combination of procedures performed for patellar realignment included femoral trochleoplasty, TTT, and joint capsule modification (n=28; 59.5%). Surgical procedures performed with associated complications and clinical outcomes are summarized in Table 2.
Postoperative Complications
Postoperative complications were recorded for 12 stifles (25.5%) following surgery. None of these complications were classified as catastrophic. Complications were classified as minor for six stifles (12.8%). Minor complications included: removal of superficially migrated TTT fixation pin at follow-up (n=4), seroma, and incisional irritation. Complications were classified as major for six stifles (12.8%). Major complications included: clinically significant persistent patellar luxation (n=2), incisional dehiscence, implant failure/migration, incorrect osteotomy alignment, and a tibial crest avulsion fracture. Revision surgery for correction of a major complication was performed at a median of 49 days (range 13–74 days) following the initial surgery.
Continued patellar luxation at follow-up examination was documented in five stifles (10.6%). Continued patellar luxation in two of these stifles (4.3%) was considered clinically significant as the dogs displayed unacceptable pain and lameness on examination. Revision surgery was recommended in these cases. The remaining dogs were comfortable on examination and continued patellar luxation had no apparent effect on clinical outcome.
Major complication occurrence was not significantly associated with age, body weight, bilateral grade 4 MPL, or concomitant CrCL rupture at the time of surgery. A corrective osteotomy as part of surgical correction was significantly (P < 0.001) associated with major complication occurrence and revision surgery (Table 3). A major complication occurred in four of the eight cases (50%) in which a corrective osteotomy was performed. Those complications included: implant failure/migration, incisional dehiscence, tibial crest avulsion fracture, and incorrect osteotomy alignment. The odds of a major complication occurring was 157.1 times more likely in those cases where a corrective osteotomy was performed (P = 0.008; 95% confidence interval, 3.58 to >999.99).
Clinical Outcome
Median in-hospital follow-up veterinary examination occurred at 69 days (range 30–179 days) for all stifles. Clinical outcome was classified as full pelvic limb function for 20 stifles (42.6%) following surgical correction, acceptable pelvic limb function for 19 stifles (40.4%) following surgical correction, and unacceptable pelvic limb function for eight stifles (17%) following surgical correction. Cumulatively, surgical correction of 39 stifles (83%) for grade 4 MPL resulted in full or acceptable pelvic limb function at final follow-up exam. Median follow-up examination for those cases classified with either full or acceptable function was 66.5 days (range 30–127). Median follow-up examination for those cases classified with unacceptable function was 69 days (range 33–179). There was no significant difference between these outcome groupings with regard to follow-up. Unacceptable function was not associated with age, body weight, bilateral grade 4 MPL, concomitant CrCL rupture, or a corrective osteotomy at the time of surgery (Table 4).
Six revision surgeries were required for correction of a major complication. Clinical outcome following revision surgery was full pelvic limb function for two stifles (33.3%), acceptable pelvic limb function for two stifles (33.3%), and unacceptable pelvic limb function for two stifles (33.3%). Cumulatively, four stifles (66.7%) that required multiple surgical procedures had full or acceptable pelvic limb function at final follow-up examination. Revision surgery due to a major complication was not associated with unacceptable function (Table 4).
Discussion
To date, this study represents the largest cohort of stifles surgically treated for grade 4 MPL. Surgical correction of grade 4 MPL improved pelvic limb function in 83% of cases (n=39), but only 42.6% (n=20) were considered to have full function at the time of final follow-up exam. Limb function was considered unacceptable in 17% of cases (n=8). A variety of surgical procedures were utilized to achieve patellar realignment. The most frequently utilized combination of surgical procedures included: femoral trochleoplasty, TTT, and soft tissue modification to the joint capsule. Postoperatively, complications were noted following 25.5% of stifle surgeries (n=12), with revision surgery required for 12.8% of stifles (n=6). The majority of stifles undergoing multiple corrective surgeries were still evaluated to have full or acceptable pelvic limb function (n=4; 66.6%) at final follow-up.
Prior studies focused on MPL have included a limited number of stifles diagnosed with grade 4 MPL.1,3,17–20 Direct comparison of these prior studies to our outcome data is challenging since inherent differences exist in regard to study design, clinical outcome definitions, and time and method of follow-up. Despite these differences, our outcome data appears to agree with reports by Alam et al. (2007) and Gibbons et al. (2006), which contained 16 and 8 grade 4 MPL stifles, respectively. In those reports, the majority of dogs (86% and 87.5%) were described as having either “good” or “excellent” clinical outcome following surgical correction.1,19 In contrast, Remedios et al. (1992) described a poor outcome for a single stifle surgically treated for grade 4 MPL.18 Various other reports have described surgical correction of grade 4 MPL, yet failed to adequately define clinical outcome enough for comparison to our results.3,17,24 Based on the limited information previously available, a guarded prognosis was recommended following surgical correction of grade 4 MPL.9,17,24 The results of the present study would clarify that limb function typically improves following surgery; however, a return to full function should still be considered guarded.
None of the clinical variables evaluated in this report were associated with unacceptable pelvic limb function. This indicates surgeons should offer a similar prognosis in all cases of grade 4 MPL. The notion that older patients or larger breeds warrant a more guarded prognosis was not supported by our results. It is important to remember that despite the overall positive outcomes reported here, full function alone is still not as common following grade 4 MPL correction when compared to less severe grades of MPL (i.e., grade 2).1,18,19,29 Consistent with previous reports, the grade of MPL is likely the most important factor to consider when advising clients regarding prognosis following surgery.
As previously stated, the major complication rate in this study was 12.8% (n=6). This is decreased in comparison to the major complication rate (32%) previously published for a cohort of 22 stifles with grade 4 patellar luxation (predominately MPL).23 The only variable associated with major complication occurrence was performing a corrective osteotomy as part of surgical correction. The authors' recommend interpreting this result with caution as the sample size of corrective osteotomies was small, and only two of the complications were directly related to the osteotomy itself: improper alignment and implant migration/failure. Each of these complications occurred following a DFO. The other major complications reported following osteotomy could be attributed to improper TTT stabilization and lack of owner compliance. Interestingly, body weight was not found to be associated with major complication occurrence when evaluated as either a continuous variable or categorically, although two prior studies have demonstrated an association between major complication occurrence and larger dogs.2,4
Corrective osteotomies are technically difficult procedures as pre-operative imaging, osteotomy positioning, implant selection, and surgeon experience can all affect outcome. Performing a corrective osteotomy as part of MPL correction was not widely described until 2006.20,22 Considering the period of time (2001–2012) over which cases were included in this report, surgeon experience was likely variable depending on the date of surgery. Implant technology has also evolved during this time, such as DFO specific plates and wider use of locking screw-plate constructs, which may minimize implant failure related complications. Prospective evaluation of outcome for grade 4 MPL cases requiring corrective osteotomies in a large cohort performed by an experienced surgeon may yield different results. As such, we regularly advocate and perform corrective osteotomies in cases of canine grade 4 MPL when indicated, but clients and surgeons should be aware of the increased risk for complication and revision surgery.
Continued patellar luxation following surgery was reported separately from other complications, and only classified as a major complication if revision surgery was recommended. The overall rate of continued patellar luxation at final follow-up exam was 10.3% (n=5). The authors caution that continued patellar luxation may be under-represented here, as clinically insignificant continued patellar luxation (i.e., grade 1) may have gone unreported in some cases. Prior studies have reported continued patellar luxation in 6 to 48% of stifles following surgery; however, no distinctions were made in those studies between luxation that significantly affected clinical function and those considered insignificant.1,17,19,29 Arthurs and Langley-Hobbs (2006) made such a distinction, and specifically reported revision surgery for continued patellar luxation following 14% (n=3) of surgeries for grade 4 patellar luxation.23 Two stifles (4.3%) in our cohort were diagnosed with clinically significant continued patellar luxation. The authors consider reporting clinically significant continued patellar luxation more relevant to clinical outcome, and more likely to be accurately represented within a retrospective study. We attribute the decreased incidence of major complications and clinically significant continued patellar luxation in our report to use of combined aggressive corrective procedures and potentially improved assessment of musculoskeletal abnormalities by surgeons.
Limitations to this study include: retrospective design, subjective outcome evaluation, and lack of long-term outcome data. The outcome and complication definitions used in this report are as recommended by Cook et al. (2010) for retrospective orthopedic studies.28 However, the application of these guidelines is still dependent on the completeness and accuracy of medical records. The authors also acknowledge the potential introduction of outcome bias whenever subjective assessment is performed. The short-term follow-up also limits outcome classification as pelvic limb function may continue to change over time. That stated, the median follow-up examination for dogs in this study was 69 days (approximately 9–10 wk following surgery). It has been suggested that pelvic limb function should be normal or near normal between 8–12 wk following surgery.30 Currently, long-term (>1 year) outcome evaluation following surgical correction for grade 4 MPL is limited to only a few cases, and all by subjective assessment.1,18 Future studies including objective long-term outcome data (i.e., force platform gait analysis) would help better define pelvic limb function following surgical correction, but may be difficult to obtain as many of the breeds overrepresented with MPL are toy breeds.2,3,19
The authors were also limited in statistical analysis of the study cohort. The authors ideally would have evaluated for associations within each of the three outcome groups, but that was not statistically feasible. As a result, statistical analysis focused on major complications and unacceptable function alone. In some instances, the clinical variables investigated were limited in number so a type II error cannot be ruled out. There were also clinical variables that could not be evaluated due to the multi-institutional retrospective design. Patella alta has previously been associated with MPL.6 Measuring the proximodistal patellar alignment of each stifle following correction and comparing postoperative alignment between those stifles with and without continued luxation may have been useful in elucidating the cause of continued luxation; however, this was not performed due to the small number of stifles with continued luxation, lack of standardized radiographic projections, and the difficulty in obtaining radiographs over a ten year period from multiple institutions. Prospective evaluation would be more ideal for assessing patellar stability and standardizing radiographic projections. Furthermore, prospective studies can be designed to evaluate variables not commonly available to retrospective studies, such as the quadriceps deviation angle (Q-angle) or the effects of rehabilitation therapy, and potentially correlate these variables with clinical outcome.7
Conclusion
Grade 4 MPL is a complex orthopedic disease in dogs that requires surgical patellar realignment to improve limb function. The short-term clinical outcome following surgery is often positive; however, the prognosis for full function should be considered guarded. Major complications following surgery are generally limited, with those cases that include a corrective osteotomy having the greatest risk for requiring surgical revision. Fortunately, surgical revision does not appear to be associated with unacceptable function, and, when necessary, often still results in improved pelvic limb function.
Contributor Notes
