Study population
The present study was approved by the Institutional Review Board (IRB). Patients were included who underwent THA via the Anterior Minimally Invasive Surgery (AMIS) technique using a traction table (Medacta International SA, Castel San Pietro, Switzerland) between 11/2018 and 03/2019 at a single institution. During the study period, a total number of 390 patients underwent DAA THA, with 71 cases performed by the principal investigator. Exclusion criteria were defined as unavailable data, an age at surgery < 18 years, revision total hip arthroplasty, unavailable preoperative standing antero-posterior plain radiographs of the pelvis with calibration markers, as well as a refusal to participate in research studies.
Data collection
Data were collected on age, gender, height, weight, body mass index (BMI), and several preoperative radiographic parameters. Additionally, data were gathered on the patient’s preoperative pelvic position (a) in the supine position on the operating table, and (b) after positioning on the traction table with the foot mounted in the foot boot. Preoperative standing antero-posterior pelvic radiopraphs were analyzed for the caput-collum-diaphyseal (CCD) angle, the femoral offset, the distance between the centers of both femoral heads, the distance between the most lateral edges of both greater trochanters, the hip axis length [13], the tilt ratio calculation and sagittal pelvic tilt estimation according to Schwarz et al. [14] The CCD angle is defined as the femoral neck-shaft angle, and was measured between the longitudinal axes of the femoral neck and the femoral shaft. The femoral offset was measured as the perpendicular distance between the longitudinal femoral shaft axis and the center of the femoral head. Furthermore, an offset ratio was calculated by dividing the femoral offset by the distance between the centers of both femoral heads. All plain radiographic measurements were performed after calibration using a preoperative planning software (mediCAD® Classic v4.5, mediCAD Hectec Gmbh, Germany). (Fig. 2).
Measurement of pelvic position
The bilateral anterior superior iliac spines (ASIS) were defined as the reference for measuring the axial pelvic orientation relative to the horizontal plane. (Fig. 1) A digital inclinometer (Stanley Black & Decker, New Britain, CT) was routinely used to assess (a) the pelvic position on the regular operating table in the supine position, and (b) the pelvic position after the leg has been mounted in the foot boot of the traction table, with the difference then calculated. The patient was positioned on the traction table, with the table in neutral position (no traction, no abduction, no adduction), with the foot boot fixed at the height of the patient’s pelvis. Two identical magnetic pedestals were used for accurate palpation of the ASIS, as illustrated in Fig. 3. Each measurement was performed twice with the average calculated. An aPT towards the side of operation (operated hip joint lower relative to the contralateral side) is reported with a negative value, whereas an aPT towards the contralateral hip joint (operated hip joint higher relative to the contralateral side) is reported with a positive value. In case of aPT, the patient’s position is preoperatively corrected by the surgeon to a neutral position. All measurements were performed by the same consultant surgeon as a pilot project as opposed to an institutional standard (non-consecutive patient inclusion).
Statistical analysis
Categorical variables are reported as frequencies and percentages, whereas continuous variables are reported as mean ± standard deviation (SD) for parametric data and as median with interquartile range (IQR) for non-parametric data. The Pearson’s chi squared test, or Fisher’s exact test if the expected cell count in any cell was < 5, were used for the comparison of proportions between groups. In order to test for a normal distribution of data, the Kolmogorov Smirnov test was used. In case of a parametric distribution of continuous data, the Student’s t-test was applied (unpaired/paired), whereas the Mann-Whitney U test (unpaired) was used for the comparison of continuous variables in case of a non-parametric distribution. A p-value of < 0.05 was considered statistically significant. Data analysis was performed using IBM SPSS Statistics, Version 23.0 (IBM Corp., Armonk, NY).