Inclusion and Exclusion Criteria
Inclusion Criteria included: (ⅰ) Patients who underwent unilateral primary THA in the Department of Orthopaedics at our institution between June 2013 and July 2019; (ⅱ) meet the diagnostic criteria for osteoarthritis，osteonecrosis or femoral neck fracture,and have definite indications for surgery; (ⅲ) the related outcomes of patients were completely recorded.
Exclusion criteria included: (ⅰ) Acetabular dysplasia or defect; (ⅱ) contralateral hip space stenosis or fusion; (ⅲ) history of hip surgery; (ⅳ) proximal femoral deformity; (ⅴ) preoperative LLD was greater than 2 cm in patients with osteoarthritis and osteonecrosis;
Two groups of patients were included in this study: 107 consecutive patients with LLD no more than 3 mm, who underwent THA during June 2013 and July 2019 were defined as leg length equality group; whereas 71 consecutive patients with LLD of 15 mm or grater were defined as severe LLD group, who underwent THA during the same time period. Our cohort was subdivided by LLD: ≥15 mm were severe LLD: ≤3 mm were leg length equality.
Preoperative template measurements were taken for all patients, and all operations were completed by the senior surgeons. In each operation, the rotation center and the lower limb length were restored anatomically as much as possible. Implants used were the M/L Taper (Zimmer Biomet, Warsaw, Indiana) and the LCU (Link, Hanburg, Germany).
All measurements were taken with Picture Archiving and Communication Systems (PACS). FCI was measured on the basis of anteroposterior hip radiographs taken before surgery, and other radiological data were taken at least four weeks after operation. We only measured intra-observer reliability (not inter-observer reliability), and the intra-observer reliability of LLD, VCR, HCR, stem height, femoral neck osteotomy, FCI level were 0.905, 0.865, 0.872, 0.870, 0.885, 0.875.
Leg Length Discrepancy
The leg length discrepancy (LLD) refers to the difference of bilateral lower limb length after THA. LLD was defined as the perpendicular distance between the lesser trochanters and a horizontal reference line connecting the inferioraspect of the acetabular teardrops (Figure 1).
Vertical Centre of Rotation
Vertical centre of rotation (VCR) refers to the height at which the rotational center moves up or down after THA. After surgery, VCR was measured as the distance from the rotational center to the teardrop line (Figure 1).
Horizontal Centre of Rotation
Horizontal centre of rotation (HCR) is the distance that the acetabular cup moves horizontally inward. It was defined as the horizontal distance between a perpendicular line through the teardrop and the hip rotational center (Figure 1).
Stem height is the height of the femur stem relative to the femur after surgery. Stem height was measured as the distance from the rotational center to the lesser trochanters (Figure 2).
Femoral Neck Osteotomy Level
Femoral neck osteotomy level refers to the osteotomy height at the neck of the femur. Scholars usually define the distance between the medial side of the femoral neck osteotomy and the superior level of the lesser trochanter as femoral neck osteotomy level (Figure 2).
Femoral Cortical Index
Femoral cortical index (FCI) refers to the proximal morphology of the femur, which was defined as the ratio of cortical width minus endosteal width to the cortical width at a level of 10 cm below the tip of the lesser trochanter on the AP radiograph (Figure 3).
Statistical analyses were performed using SPSS Version 21 (IBM Corp., Armonk, New York, USA). All continuous variables are reported as mean and standard deviation, and an independent t-test was used to detect differences between two groups. All categorical variables are reported as number and percent, and the chi-squared test or Fisher’s exact test was used. All significance levels were set at P =0.05.