Patients
110 patients with a diagnosis of knee osteoarthritis (Kellgren-Lawrence classification III or IV) from January 2010 to December 2015 were enrolled in the research. These patients accepted unilateral primary cemented TKA by the same surgeon (Professor Xu) approximately 1 week after admission in the Department of Joint Surgery of the First Affiliated Hospital of Sun Yat-Sen University. A posterior-stabilized prosthesis (DePuy Synthes, P.F.C. Sigma, Warsaw, IN, USA) was implanted for TKA. These patients were followed up for up to 5 years, and they returned to the hospital 1 week, 1 year, 3 years, and 5 years after the surgery for full-length X-rays of both lower limbs, also to evaluate clinical function score (WOMAC score and KSS score) and physical activity level score (UCLA activity score). X-rays were used to evaluate postoperative prosthesis position and measure rBMD based on X-ray. This study was approved by the Medical Ethics Committee of The First Affiliated Hospital of Sun Yat-Sen University (code number [2011] 57), and all the procedures followed the principles of the Helsinki Declaration.
rBMD (calibrated grayscale value) measurements
rBMD was measured with reference to previous literature[2, 10]. The X-ray images at the different time points were collected with the patient in the same standing position and the tibia neutral. Knee flexion was minimized by fixing the tibial tubercle at the lower end of the knee. Rotation was controlled by fixing the heel and the first and second toes. Standard anteroposterior (AP) X-rays obtained during follow-up (one week, 3 months, 1 year, 3 years, and 5 years postoperatively) were saved as JPG files with 255 (8-bit) grayscale and 300 dpi resolution. Ten regions of interest (ROIs) were chosen as the measured regions in tibia: four lateral regions (L1, L2, L3, L4), four medial regions (M1, M2, M3, M4), and two distal regions (D1, D2) (Fig 1). ImageJ, a public domain Java-based scientific image processing and analysis package, was used to measure the mean grayscale value in the established regions of the radiographs. For each designated region, the grayscale value of regions ‘a’ and ‘f’, representing the surrounding air (assigned ‘a’) and the femoral component (‘f’), could be regarded as the minimum and maximum grayscale measurements in each radiograph for inter-film comparison. To account for variability between follow-up radiographs, the measured grayscale value of each designated region was calibrated by software using the formula:, where GC,R is the calibrated grayscale value, also representing the rBMD in a given region, GR is the mean grayscale value within an ROI, Ga is the value of air within the radiograph, and Gf is the mean grayscale value of the femoral component[2]. rBMD is the relative ratio, it has no units.
We defined the medial tibial rBMD as the mean values of M1, M2 and M3, the lateral tibial rBMD as the mean values of L1, L2 and L3, and the distal tibial plateau as the mean values of D1 and D2, and the tibial rBMD as the mean values of 10 ROIs. The baseline rBMD was measured within 1 week after TKA. The percent change in rBMD (rBMD%) was calculated by the following equation: rBMD% =100%*[rBMD (baseline)-rBMD (each time)]/ rBMD (baseline).
Clinical outcome evaluation
All patients were clinically evaluated with respect to knee function using the Knee Society Score (KSS)[14, 15], the Western Ontario and McMaster University Osteoarthritis Index (WOMAC)[15, 16] and visual analogue scale (VAS) score preoperatively and at each follow-up time point (3 months, 1 year, 3 years, and 5 years postoperatively). The level of activity was evaluated using the University of California Los Angeles (UCLA) Activity Rating Scale[17-19]which has ten points: 1) wholly inactive, dependent on others; 2) mostly inactive, restricted to minimal activities of daily living; 3) sometimes participate in mild activity such as walking, limited housework or shopping; 4) regularly participate in mild activities; 5) sometimes participate in moderate activity such as swimming and unlimited housework or shopping; 6) regularly participate in moderate activities; 7) regularly participate in active events such as bicycling; 8) regularly participate in very active events such as bowling or golf; 9) sometimes participate in impact sport such as jogging, tennis, skiing, heavy labour; 10) regularly participate in impact sport.
Statistical analysis
The Shapiro-Wilk test was used to confirm that the data were normally distributed. One-way ANOVA was used to compare clinical scores and rBMD at different time points. Pearson’s chi-squared test was performed to clarify the relationship between sex, age, hip-knee-ankle (HKA) angle, BMI and rBMD% at 1year, 3 years and 5 years. Curve fitting and nonlinear regression were performed to clarify the relationship between the UCLA activity rating and rBMD% at different time point. Significance was defined as P<0.05. All statistical analyses were performed using SPSS 21.0.