Abstract
This prospective study used radiographic techniques to evaluate the inclination, Norberg, quadriceps and femoral varus angles, and the patellar position in dogs with developmental medial patellar luxation. The study also compared these measurements pre- and post-operatively to determine how they were affected by surgical procedures. Eighteen dogs with unilateral or bilateral luxation were used, and 4 groups were established according to luxation grade. Except for the quadriceps angle, there was no difference among the groups preoperatively. Significant differences for the quadriceps angle were observed between Grades 1 and 3, and between Grades 2 and 3. The Norberg and femoral varus angles and the patellar position showed no changes between pre- and postoperative groups. However, measurements of the preoperative quadriceps angle and postoperative inclination angle may be useful for evaluating dogs with grade-3 luxation. Further studies using other imaging techniques are required in dogs with severe grade-4 luxation due to the difficulty in obtaining a good radiographic position.
Résumé
Utilisation des mesures radiographiques dans l’évaluation des chiens atteints de luxation interne patellaire. Cette étude prospective a utilisé des techniques radiographiques pour évaluer les angles d’inclinaison, de Norberg, des quadriceps et de l’arc fémoral chez les chiens avec une luxation médiale patellaire de croissance. L’étude a aussi comparé ces mesures avant et après l’opération afin de déterminer comment elles étaient affectées par les interventions chirurgicales. Dix-huit chiens avec une luxation unilatérale ou bilatérale ont été utilisés et 4 groupes ont été établis en fonction du degré de luxation. Sauf pour l’angle des quadriceps, il n’y avait aucune différence parmi les groupes avant l’opération. Des différences importantes pour l’angle des quadriceps ont été observées entre les degrés 1 et 3 et entre les degrés 2 et 3. Aucun changement n’a été détecté pour les angles de Norberg et de l’arc fémoral et pour la position patellaire entre les groupes avant et après l’opération. Cependant, des mesures de l’angle des quadriceps avant l’opération et de l’angle d’inclinaison après l’opération peuvent être utiles pour l’évaluation des chiens présentant une luxation du degré 3. De nouvelles études à l’aide d’autres techniques d’imagerie sont requises chez les chiens avec une luxation grave du degré 4 en raison de la difficulté d’obtenir une bonne position radiographique.
(Traduit par Isabelle Vallières)
Introduction
Developmental medial patellar luxation (MPL) is a common disease especially in toy dogs and other small breeds, such as the pomeranian, chihuahua, Boston terrier, poodle, and Yorkshire terrier (1–4). A large number of dogs present primarily bilateral MPL, which strongly suggests a hereditary disorder (2). However, familial anamneses and heredity studies are required in order to construct a genetic background (5).
According to MPL grade, dogs may show several musculoskeletal abnormalities, such as the displacement of the quadriceps muscle, coxa vara, lateral torsion of the distal femur, lateral bowing of the distal 3rd of the femur, dysplasia of the distal femoral epiphysis, shallow femoral trochlear groove, and rotational instability of the stifle joint and proximal tibial deformity (6–8). Angular skeletal deformities are hypothesized to be secondary to abnormal torsional and pressure forces exerted on the distal femoral or proximal tibial growth plate, associated with the displacement of the quadriceps muscle during the growth phase (6,9). However, the cause and sequence of events have not been fully established (7,10). Another contributing factor may be hip conformation (11).
Radiographic, tomographic, and magnetic resonance imaging studies have been used to evaluate the hip joint conformation, quadriceps alignment, patellar position, femoral varus and torsion in both normal dogs and dogs with patellar luxation (11–13). Failure to correct femoral and tibial malalignment has been proposed as a cause of MPL recurrence following surgical repair (14). Therefore, this prospective study primarily aimed at evaluating, by means of radiographic techniques, the inclination, Norberg, quadriceps and femoral varus angles as well as the patellar position in dogs with developmental MPL. Secondly, it aimed at comparing such measurements pre- and post-operatively, in order to determine the influence of surgical procedures on their values.
Materials and methods
Eighteen dogs from small to medium-sized breeds, weighing 2 to 13 kg, with naturally occurring unilateral or bilateral MPL were selected. The dogs were evaluated and numbered from 1 to 18, and 4 groups were established according to MPL grade: G1 (grade 1) = 8 hind limbs, G2 (grade 2) = 8 hind limbs, G3 (grade 3) = 9 hind limbs, G4 (grade 4) = 6 hind limbs (Table 1). The MPL clinical grading system was based on the Singleton (15) and Roush (7) classification, and grading was performed by 1 surgeon.
Table 1.
Description of dogs with medial patellar luxation according to grades and hind limb affected
| Case number | Dog description | Weight (kg) | Right hind limb | Left hind limb |
|---|---|---|---|---|
| 1 | 7-month-old intact male poodle | 4.5 | Grade 2 | Grade 3 |
| 2 | 5-year-old spayed female poodle | 6.4 | Grade 3 | Grade 3 |
| 3 | 1-year-old intact male poodle | 4.5 | Normal | Grade 1 |
| 4 | 1-year-old intact male poodle | 4.3 | Normal | Grade 1 |
| 5 | 2-year-old neutered male poodle | 9.0 | Grade 2 | Grade 1 |
| 6 | 7-month-old intact male poodle | 5.5 | Grade 1 | Grade 2 |
| 7 | 2-year-old spayed female poodle | 3.8 | Grade 3 | Grade 3 |
| 8 | 3-year-old intact male pinscher | 3.6 | Normal | Grade 2 |
| 9 | 2.5-year-old spayed female pinscher | 2.2 | Grade 2 | Grade 2 |
| 10 | 8-month-old intact female pinscher | 1.0 | Grade 4 | Grade 4 |
| 11 | 2-year-old intact female poodle | 5.0 | Grade 1 | Grade 1 |
| 12 | 4-year-old spayed female pinscher | 2.0 | Grade 2 | Grade 2 |
| 13 | 1-year-old spayed female pinscher | 1.8 | Grade 4 | Not evaluated |
| 14 | 1-year-old intact female crossbred | 6.3 | Grade 1 | Normal |
| 15 | 3.5-month-old intact male crossbred | 2.5 | Grade 4 | Grade 4 |
| 16 | 1-year-old intact male Yorkshire | 2.0 | Grade 3 | Grade 3 |
| 17 | 5-year-old spayed female cocker spaniel | 9.5 | Grade 1 | Grade 3 |
| 18 | 7-month-old intact female Australian cattle dog | 9.8 | Grade 3 | Grade 4 |
After premedication with acepromazine (Acepran 0.2%; Univet S.A., São Paulo, Brazil), 0.05 mg/kg bodyweight (BW), IM, and morphine (Dimorf; Cristália, Itapira, Brazil), 0.5 mg/kg BW, IM, general anesthesia was induced with propofol (Propovan; Cristália) 4 mg/kg BW, IV, and maintained with isoflurane (Isoforine; Cristália). Epidural anesthesia was performed with lidocaine 2% (Xylestesin; Cristália) at 1 mL per 4.5 kg BW, and morphine (Dimorf; Cristália) at 0.1 mg/kg BW.
The surgical procedures were performed by 2 experienced surgeons and included combined techniques according to MPL grade severity observed intraoperatively. A lateral retinacular overlap was performed on all G1 dogs, and a wedge recession sulcoplasty was accomplished in cases 3 and 14. A lateral retinacular overlap and wedge recession sulcoplasty were performed on all G2 dogs, except for the right hind limb in dog no. 1, which received only a lateral retinacular overlap. Medial desmotomy was also performed in cases 8 and 12. Lateral retinacular overlap, wedge recession sulcoplasty, release of the quadriceps muscle, which was generally dissected free to the midfemoral level, and the tibial tuberosity transposition that was fixed by screw (no. 1 — left hind limb, no. 7 — left hind limb, no. 16 — right hind limb, no. 17 — left hind limb, no. 18 — right hind limb) or by Kirschner wire (no. 7 — right hind limb, no. 16 — left hind limb) were accomplished in dogs of G3. An exception was made for dog no. 2, which did not receive tibial tuberosity transposition. In G4, wedge recession sulcoplasty, lateral retinacular overlap, medial desmotomy, release of the quadriceps muscle, tibial tuberosity transposition using screw (no. 15) or Kirschner wire (no. 10 — right and left hind limbs, 13 — right hind limb, 18 — left hind limb) were performed as well as femoral osteotomy in dog no. 15. In case 15, the screw was positioned to avoid the tibial physis.
Cephalexin (Keflex; Eli Lilly, São Paulo, Brazil), 30 mg/kg BW, PO, q8h, was administered prior to the surgical procedure and 3 to 5 d postoperatively. Tramadol chlorhydrate (Anangor; Biosintética, São Paulo, Brazil), at a dose of 0.1 mg/kg BW, SC, q12h, and carprofen (Rimadyl; Lab. Pfizer, Guarulhos, Brazil), 2.2 mg/kg BW, PO, q12h, were used after surgery for 4 and 7 d, respectively. Chlorhexidine acetate solution (Riohex; Rioquimica, São José do Rio Preto, Brazil) was used daily for topical application on the wound, and the skin sutures were removed 10 d after surgery.
Radiographs were obtained preoperatively and 3 mo after surgery under dissociative anesthesia. After premedication with acepromazine (Acepran 0.2%; Univet S.A., São Paulo, Brazil), 0.05 mg/kg BW, and morphine, 0.5 mg/kg BW, IM, anesthesia was induced and maintained with ketamine (Vetaset; Fort Dodge, Campinas, Brazil), 15 mg/kg BW and midazolam (Dormire; Cristália), 0.5 mg/kg BW, IM. Traditional OFA-like and mediolateral radiographs including pelvis and stifle were taken. Mediolateral radiographs were considered acceptable when the femoral condyles were superimposed. Radiographic measurements included the angle of inclination, Norberg angle, quadriceps angle, and the length ratio of the patellar tendon to the patella (L:P), as reported by Towle et al (11). The femoral varus angle was measured according to Dudley et al (13). One experienced radiologist blinded to the surgical grade and outcome, made all of the measurements manually. A transparent plastic goniometer was used to measure the joint angles directly on the radiographs.
The preoperative radiographic data concerning the mean values of G1, G2, and G3 were submitted to analysis of variance (ANOVA) followed by the Tukey test using the GraphPad InStat software package. Pre- and post-operative radiographic data among the same groups were compared using Student’s t-test for paired samples. Differences were considered statistically significant for P < 0.05. In G4, the preoperative measures of only 4 hind limbs were taken.
Results
The 18 dogs, aged between 3.5 mo and 5 y, were made up of poodles (n = 8), pinschers (n = 5), crossbreeds (n = 2), a Yorkshire terrier (n = 1), a cocker spaniel (n = 1), and an Australian cattle dog (n = 1) (Table 1). There were 8 males and 10 females, 14 of which were sexually intact; of the remainder, 1 was neutered and 3 were spayed.
Except for the dogs in G4 that were too deformed, the others showed good preoperative weight bearing on the operated limb with the patella positioned in the trochlear groove during the evaluation conducted 3 mo after the surgery, thus allowing radiographic measurements to be taken. Preoperative statistical analyses (mean, standard deviation, and P-values) of the inclination angle, Norberg angle, quadriceps angle, femoral varus angle, and the length ratio between the patellar tendon and the patella of G1, G2, and G3 are reported in Table 2. Except for the quadriceps angle, no difference was identified among the 3 groups. Statistical differences in the quadriceps angle were observed between G1 and G3 (P = 0.001) and between G2 and G3 (P = 0.042). Three dogs (numbers 11, 17, 18) showed a Norberg angle of < 105° and radiographic signs of hip dysplasia.
Table 2.
Statistical analysis of measurements for the inclination angle (IA), Norberg angle (NA), quadriceps angle (QA), femoral varus angle (VA), and the length ratio between the patellar tendon and the patella (L:P) in the preoperative evaluation by groups
| G1 | G2 | G3 | G4 | ||
|---|---|---|---|---|---|
| Groups
|
n = 8
|
n = 8
|
n = 9
|
n = 4
|
|
| Variable | Mean (s)a | Mean (s)a | Mean (s)a | Mean (s)a | P-valueb |
| IA | 131.2 (5.3) | 130.4 (9.5) | 133.8 (12) | 136.7 (4.3) | 0.65 |
| NA | 105.7 (8.8) | 109.4 (5.7) | 104.0 (12.1) | 111.5 (1.7) | 0.471 |
| QA | 14.9 (7) | 22.1 (6.4) | 34.4 (13.7) | 34.0 (9.4) | 0.0016c |
| VA | 13.0 (7.8) | 10.3 (5.2) | 17.8 (6.9) | 18.2 (2.8) | 0.78 |
| L:P | 2.0 (0.2) | 1.8 (0.1) | 1.9 (0.2) | 1.8 (0.3) | 0.20 |
s = standard deviation.
ANOVA (α = 0.05).
G1 < G3 (P = 0.001), G2 < G3 (P = 0.042).
Statistical analyses comparing pre- and postoperative data for groups 1 to 3 are reported in Table 3. The postoperative values for the inclination angle were statistically lower than the preoperative values for G2 (P = 0.046) and G3 (P = 0.046). Although not statistically significant, the quadriceps angle decreased 24.13% in G3 (P = 0.084). The pre- and postoperative measurements for the quadriceps angle in a G3 dog are illustrated in Figure 1. No difference was identified for the other measurements among the 3 groups.
Table 3.
Statistical analysis of measurements for the inclination angle (IA), Norberg angle (NA), quadriceps angle (QA), femoral varus angle (VA), and the length ratio between the patellar tendon and the patella (L:P) at pre- and postoperative evaluation by groups
| Evaluation
|
||||
|---|---|---|---|---|
| Groups | Variable | Preoperative mean (s)a | Postoperative mean (s)a | P-valueb |
| G1 | IA | 131.2 (5.3) | 131.7 (6.8) | 0.887 |
| NA | 105.7 (8.8) | 104.2 (11.7) | 0.589 | |
| QA | 14.9 (7.0) | 17.4 (8.5) | 0.498 | |
| VA | 13.0 (7.8) | 10.9 (5.6) | 0.258 | |
| L:P | 1.0 (0.2) | 2.1 (0.4) | 0.610 | |
| G2 | IA | 130.4 (9.5) | 128.2 (4.2) | 0.046c |
| NA | 109.4 (5.7) | 110.2 (3.7) | 0.708 | |
| QA | 22.1 (7.0) | 21.7 (5.3) | 0.847 | |
| VA | 10.3 (5.2) | 10.0 (4.6) | 0.786 | |
| L:P | 1.7 (0.1) | 1.9 (0.3) | 0.430 | |
| G3 | IA | 133.8 (11.9) | 125.4 (9.2) | 0.046c |
| NA | 104.0 (12.1) | 102.2 (13.0) | 0.514 | |
| QA | 34.4 (13.7) | 26.1 (9.2) | 0.084 | |
| VA | 17.8 (6.9) | 14.8 (7.0) | 0.397 | |
| L:P | 1.9 (0.2) | 1.8 (0.5) | 0.430 | |
s = standard deviation.
Student’s t-test for paired samples. (α = 0.05)
Preoperative mean > postoperative mean.
Figure 1.
Pre- (a) and postoperative (b) ventrodorsal radiographic views of dog number 16 (G3) showing a representative schematic example of the measurements taken. Although not statistically significant, note the decrease in the quadriceps angle in the postoperative evaluation (b).
The length ratio between the patellar tendon and the patella, for all groups, averaged 1.9 preoperatively and 1.9 postoperatively and there was no difference among the groups.
Due to a severe deformity in dog 15 (Grade 4), it was impossible to obtain an adequate radiographic position to perform the pre- and postoperative measurements, and statistical analysis of G4 was not carried out. However, the mean values of the preoperative measurements for dogs numbers 10 (both hind limbs), 13 (right hind limb), 18 (left hind limb) were: inclination angle of 136.7° [standard deviation (s) = 4.3], Norberg angle of 111.5° (s = 1.7), quadriceps angle of 34° (s = 9.4), femoral varus angle of 18.2° (s = 2.9°), and the length ratio between the patellar tendon and the patella of 1.84 (s = 0.3).
Discussion
Preoperative planning in deformities requires adequate radiographic positioning (16–19). However, in severe MPL cases, such as those observed in G4 and some G3, the ventrodorsal radiographic position may be difficult to achieve, since the full extension of the stifle joint is compromised. Some authors recommend centering the fabella to obtain the correct positioning of the femur, as the patella cannot be centralized in such cases (17). Despite this, the severe deformity of both hind limbs in dog 15 preoperatively and poor surgical results in dogs 10, 13, and 15 postoperatively the radiographic measurements could not be taken and, consequently, the statistical analysis was compromised due to the small sample number. Orthogonal segmental radiographic techniques, 3-D CT reconstruction, and stereo-lithography, as used to localize and quantify the rotational center of angulation in a dog with severe antebrachial limb deformity (18), may be necessary in cases of severe-grade MPL (20).
The extensor mechanism of the stifle composed of the quadriceps muscle group, patella, trochlear groove, patellar tendon, and tibial tuberosity must be in a straight line to maintain patellar stability (6,10). The malalignment of 1 or more of these components induces deviation of quadriceps muscle force direction and consequently of the quadriceps angle (8,12). In the present study, the preoperative mean values of the quadriceps angles were 14.9° (G1), 22.1° (G2), and 34.4° (G3). These values are comparable to the mean values of 12.2° (grade 1), 24.3° (grade 2) and 36.6° (grade 3) observed in a study using magnetic resonance imaging to measure the quadriceps angle in dogs (30 stifle joints) with congenital medial patellar luxation (12). In addition, statistical differences between normal limb group values (mean of 10.5°) compared with G2 or G3 groups, and between G1 and G3 were observed (12). In our study, the preoperative differences were observed not only between G1 and G3, but also between G2 and G3, which was probably associated with differences in breeds and severity of skeletal deformities. Although not significant, in a study using 6 dogs, the surgical procedures used to treat MPL corrected the quadriceps angle by 33%, as measured by radiographic evaluation (11). In our study, although not statistically significant, G3 showed 24.13% postoperative difference which was probably related to tibial tuberosity transposition and medial desmotomy.
Alterations of the normal inclination angle (135° to 145° mean) between the femoral head-neck and femur shaft induce coxa-valgus or coxa-vara deformities (8). In a study performed in dogs with G2 and G3 MPL, the mean inclination angle measured in ventrodorsal radiographic views was 128.5° (± 5.3) (11). The data were similar to those observed preoperatively in the present study (G1 — 131.2°; G2 — 130.4°; G3 — 133.8°). The mean values of G3, however, showed a reduction of this angle (125.4°) postoperatively, which was probably associated with better radiographic positioning due to patellar correction.
Hip dysplasia may occur in dogs with patellar luxation (8,11). According to Orthopedic Foundation for Animals (OFA), a normal Norberg angle score is ≥ 105° (21), hence the mean values for the groups G1 (105.7°) and G2 (109.4°) in the present study were close to normal. Two G3 dogs (numbers 7 and 17) and one G4 dog (number 18) showed values less than 105°, associated with radiographic signs of hip dysplasia. A mean of 93.0° (± 11.3) was reported for dogs with grades 2 and 3 MPL (11).
The length ratio between the patellar tendon and the patella, for all groups, averaged 1.9 pre- and postoperatively. An overall mean of 1.8 was previously described for small to medium-sized dogs with MPL (11). For large-breed dogs with clinically normal stifles, a mean of 1.71 was reported, suggesting that the alta patella was absent in such dogs (22).
Alteration of the femoral varus angle may contribute to the pathogenesis of MPL (13,23). The measurements of this angle may be accentuated by external rotation and diminished by internal rotation of the femur during radiographic positioning (15). In the present study, means of 13° and 10.3° were observed preoperatively in G1 and G2, respectively, which are close to the mean value of 9.4° (± 2.3) radiographically observed in another study using 9 normal canine cadavers (13). On the other hand, G3 and G4 dogs showed preoperatively means of 17.7° and 18.2°, respectively. These findings may be associated with femoral deformity in severe grades of MPL. Although no statistical differences were observed, the surgical procedures decreased mean femoral varus angles to 10.9° (G1), 10° (G2) and 14.8° (G3).
Since most of the dogs in the present study were over 7 mo of age, the type of surgical repair and measurements were probably not influenced by the patient’s age. However, it may occur in younger dogs with potential for growth, and further studies are required.
In conclusion, only the quadriceps angle preoperatively showed a difference between dogs with grades 1, 2 and 3 MPL. The Norberg and femoral varus angles and the patellar position did not change from pre- to postoperative measurements. However, the measurents of the preoperative quadriceps angle and postoperative inclination angle may be useful for evaluating dogs with G3 luxation. Further studies using other imaging techniques are required in dogs with severe G4 luxation due to the difficulty in obtaining a good radiographic position.
Acknowledgments
The authors thank FAPESP and CNPq for financial support of the research. CVJ
Footnotes
Use of this article is limited to a single copy for personal study. Anyone interested in obtaining reprints should contact the CVMA office ( hbroughton@cvma-acmv.org) for additional copies or permission to use this material elsewhere.
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