Implant position calculation for acetabular cup placement considering pelvic lateral tilt and inclination

Objective: The purpose of this study was to investigate the relationship of cup position versus pelvic lateral tilt and inclination. While computer assisted navigation systems use only the pelvic frontal plane for cup placement, it is important to realize the effect of leg length differences or increased pelvic inclination, i.e., caused by contracted hip flexors. Materials and Methods: Using a sawbone model of the pelvis, cup position measurements were performed with different pelvic inclination and lateral tilt angles. The measured values were compared with outcome variables from a mathematical model. A computer program was developed to perform a reverse calculation to verify the mathematical model. Results: The mathematical model proved correct in the sawbone pelvis experiment. The cup position changed from 15° anteversion in 0° pelvic inclination to 0.5° retroversion in 15° pelvic inclination. Regardless of pelvic inclination, the vertical cup projection stayed in a safe range of 37-47°. Leg length differences greater than 3 cm have significant effects on the positioning of acetabular cups in the frontal plane. Conclusions: Using computer navigation, it is possible to determine pelvic inclination and lateral tilt during an operation by calculating the angular difference between the anatomic frontal plane and the "real world" frontal plane (i.e., the OR table). This method may be helpful in increasing the accuracy of positioning of acetabular cups.