Patellar Dislocation

All pigs underwent monitored general anesthesia with a combination of ketamine, xylazine, and isoflourane. Surgical exposure of the femoral trochlea of the pig stifle joint was performed by the same surgeons (MB & JF) using either a traditional medial parapatellar approach with retinaculum incision and luxation of the patella (n = 5) or the MIS approach which spared the patellar retinaculum (n = 5). Four full thickness, 4 mm circular cartilage defects were created in the trochlea. All pigs were allowed full weight-bearing and full range of motion immediately postoperatively and were euthanized between 1 and 3 weeks.
Prior to surgery, pigs were placed in a supine position and the skin over the stifle joint was shaved with an electric shaver. The hind limb to be operated on was suspended by the foot. No tourniquet was applied, and limbs were aseptically prepared for surgery. The foot was covered with a sterile glove and sealed with adhesive tape, and an extremity drape was applied and fixed with adhesive tape. It is important to keep the hip and stifle joint extended during the wrapping to avoid later shifting of the drapes with mobilization of the limb during surgery.
After the pigs were euthanized, two blinded raters performed gross evaluation of the trochlea cartilage excluding the defects, according to the modified ICRS cartilage injury classification (Goebel et al. 2012 (link)). In this classification, a 0 represents intact cartilage, and a 4 represents full thickness wear.

Owing to the fact that the Lysholm knee score was initially designed for patients with ACL injuries [8 (link), 15 (link), 21 (link)], we also recruited patients with ACL injuries to minimize deviations. Cases involved in the present study were mainly recruited among patients with ACL injuries scheduled for arthroscopic ACL reconstruction at our hospital [22 (link)]. Our inclusion criteria were: Aged over 16 years old with independent signing authority; Chinese as the first language with adequate capability to read and complete a questionnaire; and a definitive diagnosis of ACL injury as determined by arthroscopy [21 (link)]. Patients with other complicated knee injuries, such as meniscal injuries or patellar dislocation; with a history of lower limb or spine surgery; patients who had surgery within a month of the study; and patients with a history of systemic disease and/or malignancy, were excluded from this study. Our study met the quality criteria proposed by Terwee and associates [23 (link)] for measurement properties of health status questionnaires, which required the results from at least 100 patients to perform internal consistency analysis and from at least 50 patients for floor or ceiling effects, reliability, and validity analyses. All patients involved in the study had thoroughly read and signed the informed consent. This study was approved by the ethics committee in the local hospital (No. CHEC2013-199).
The patients provided demographic information, such as sex, age and weight, on the first day of enrollment and independently completed the four questionnaires, C-LKS, WOMAC, IKDC, and Medical Outcomes Study Short-Form 36 (SF-36) in a quiet meeting room, followed by a C-LKS again 1 week later before receiving reconstruction surgery, in order to evaluate the test-retest reliability of the questionnaire. Patients were also reached via mail or telephone 6 months postoperatively to complete the C-LKS a time to help evaluate the responsiveness of the questionnaire.

Inclusion criteria for patients with patellar dislocation:

History of patellar dislocation (at least 3 times),

Closure of epiphysis confirmed by preoperative CT scans,

Preoperative lateral radiographs of knee joint and CT scan of hip, knee and ankle

Range of normal femoral anteversion: 9°–17°. (It is generally accepted that the normal range of femoral anteversion is between 10°and 15°. Due to possible measurement differences, we prioritized measurement on the control group and found that the femoral anteversion ranged from 9° to 17°).

Statistical analysis software of SPSS 21.0 (SPSS, USA) was used for data analysis. The parameters were measured by two orthopedists and the interclass correlation coefficient (ICC) was calculated to determine the reliability of each parameter, with a value greater than 0.75 indicating excellent agreement. Among the four anatomical parameters, three (FAA, DFA and TT–TG distance) were continuous variables while the Patella Alta was the categorical variable. Monofactor analysis was first conducted. Chi-square test was used to compare categorical variables between groups; independent T-Test was used to compare continuous variables between groups when the homogeneity of variance and normal distribution was satisfied. Otherwise, Mann–Whitney U Test would be used for test. Then Binary Logistic regression analysis was used to analyze the correlation between anatomical parameters and patellar dislocation. An alpha of 0.05 was used on both sides. The Pearson correlation coefficient was also used to analyze the correlation among the FAA, DFA and TT–TG. A P value less than 0.05 was considered statistically significant. All values were expressed as mean ± standard deviation.