Acetabular component malpositioning during total hip arthroplasty (THA) increases the risk of dislocation, reduces the range of motion, and can be the cause of early wear and loosening. There have been numerous reports on the optimal orientation of the acetabular component in THA. Lewinnek et al. recommended an abduction angle of 40 ± 10° and an anteversion angle of 15 ± 10° for cup alignment in THA. In order to prevent malpostioned hip implants and improve the reproducibility of implant alignment in THA, numerous computer-assisted orthopedic systems have been described, using computed tomography (CT)-based or imageless navigation. Among the imageless systems available, one is based on Bone Morphing^® technology, initially described by Stindel for computer-assisted knee arthroplasty and adapted for THA. The purpose of this study was to compare computer-assisted acetabular component insertion versus freehand placement.

A controlled randomized matched prospective study was conducted in two groups of 30 patients. The study was approved by the French Ethics Committee. In the first group, cup positioning was assisted by an imageless computer-assisted orthopedics system based on Bone Morphing^® (CAOS+ group). In the control group, cup placement was freehand (CAOS group). The same cementless cup was used in both groups. The same surgeon performed all procedures using an anterolateral approach. Cup anteversion and abduction angles were measured on 3D CT scan reconstructions obtained postoperatively for each patient by an independent observer using special cup evaluation software.

There were 16 males and 14 females in each group, the mean age was 62 years (range, 24-80) years, and the mean body mass index was 25 in each group. The mean additional time for the CAOS procedure was 12 min (range, 8-20 min). Intra-operative subjective agreement of the surgeon with the computer guidance system demonstrated a high correlation in 23 cases, a moderate correlation in six cases, and poor correlation in one case. There were no statistical differences between the CAOS+ and the CAOS- groups regarding the means of the abduction and anteversion angles, but a significant range of variance, with the lowest variations observed in the CAOS+ group.

This study has shown the accuracy of cup positioning using a CT-free navigation system in a prospective randomized controlled protocol.