The experimental protocol was approved by the local Animal Care and Use Committee.
Fifty 1-month-old female New Zealand healthy white rabbits, weighing between 320 and 420 g (provided by the local Animal Center) were included in the study. The control group consisted of 50 right knees, which underwent no surgical procedures. The two experimental groups comprised 50 left knees, which were performed patella instability surgery (medial soft tissue restraint release). In order to stabilize the patella, the reduction group (N = 25 knees) underwent medial soft tissue sutura 2 months postoperative. The non-reduced group (N = 25 knees) underwent no suture surgery. All procedures executed involving animals were under the Western University’s Animal Care and Use Guidelines (London, Ontario, Canada) .
The surgical protocol for making patella instability models of growing rabbits have been proved by previous studies [12, 13]. First, the rabbits were intravenous anesthetized with ketamine (20 mg/kg) and xylazine (5 mg/kg) through the ear vein. The knee was shaved and prepped in a sterile following standard procedure pre-operation. Next, a 4-cm longitudinal incision was preferred on the knee joint in the two experimental group, the soft tissue of medial retinaculum and the joint capsule were dissected and exposed. Then, a 2.5-cm longitudinal incision was made along the medial border of the patella to incise the medial retinaculum and joint capsule. The state of patella instability was found intraoperatively (Fig. 1). The patella was in dislocated state when the knee was flexed, and the patella returned to the relatively normal location when the knee was straighted. Finally, the incision was irrigated and sutured without reconstruction of medial retinaculum. 2 months after medial soft tissue restraint release surgery, the reduction group underwent patellar reduction via the original longitudinal incision. The skin and soft tissue were dissected and the medial patellar retinaculum were exposed. Then, the medial patellar retinaculum was sutured, keeping patella in the normal trochlear groove. Finally, the incision was irrigated and sutured in layers. Prudence was performed to avoid damage the articular cartilage. All rabbits were raised under the same conditions (food, water and individual steel cage (310 × 550 × 320 mm) ) and allowed free activity in cages. Skeletal maturation of rabbits is complete at 6 months of age , so all the rabbits were euthanized (by intravenous injection of pentobarbital, 100 mg/kg) at 5 months after soft tissue release surgery.
CT scans of the rabbits were performed immediately after the release operation, 2 and 5 months post-operatively using a 16-slice CT scanner (SOMATOM Sensation 16; Siemens Medical Solutions, Erlangen, Germany). All CT images were captured on the axial plane, which is the optimal position to observe and measure the patellofemoral joint. The measurements were performed by RadiAnt-DICOM software (Medixant Ltd, Poznan, Poland) which has a 0.1°and 0.01mm measuring accuracy (Fig. 2). The CT slice image with the widest diameter of the patella was acquired for the indice measurements in the transverse plane. The transverse diameter (AB) of the patella was described by Stäubli , which was defined as the length between the most medial edge (A) and the most lateral edge (B) of the patella. The posterior patellar edge farthest from the baseline (AB) was defined as point D. The thickness of the patella was measured between the most anterior point (C) of the patella and the most posterior point (D) of the patellar edge, and line CD vertical to the baseline (AB). The insertion between line AB and line CD was defined as point E. The Wiberg index was defined as the ratio of the length of BE to the length of AB. For the Wiberg angle (∠D) measurement, as described by Fucentese , which was defined as between the slopes of the medial patella and the lateral patella. All measurements were taken blindly by two independent researchers. To determine the intra-observer variation, one researcher repeated the observations at 7 days after the first measurement.
CT scans were performed 5 months post-surgery. Subsequently, the rabbits were euthanized, The skin and soft tissue were carefully removed and the patella was dissected. Morphological differences were taken to observed and recorded.
SPSS statistical software (version 22.0; SPSS, IL, USA) was performed for data analyse. The data were compared with Student’s t test between the control and experimental groups immediately after release operation. Dunnett’s multiple-comparisons was used to evaluate the transvers diameter, thickness, Wiberg index, and Wiberg angle of patella among the three groups at each time point. P < 0.05 were defined as the threshold for statistical significance. The results values were expressed by Mean ± SD. The inter-and intraobserver reliabilities were then determined by calculating intra-class correlation coefficients (ICC).