Comparison of the reliability of leg length and offset data generated by three hip replacement CAOS systems using EOS™ imaging

Computer-assisted orthopaedic surgery (CAOS) theoretically will help to optimise total hip arthroplasty (THA) results. Although CAOS systems have become more sophisticated, they are not widely used, partially because of their suspect reliability. To assess reliability of these systems, we carried out a study with multiple objectives in mind: (1) establish and compare the accuracy of the leg length (LL) measurement from three CAOS systems; (2) analyse the correlation of LL and offset data generated by these CAOS systems with those of the EOS™ imaging system; (3) determine if the goals of leg length restoration with ±2 and ±5 mm were achieved; (4) evaluate why certain cases fails.The three CAOS systems have the same accuracy for LL, and their error is less than or equal to 0.6mm.We retrospectively studied 106 cases of primary THA where preoperative and postoperative measures of leg length had been performed with an EOS™ imaging system. The cases were placed in three groups, depending on which CAOS system had been used: group A (Amplivision™, amplitude), group B (Hip Express™, Brainlab), group P (THS™, Praxim). The accuracy of the leg length data was calculated by finding the difference between the data from each CAOS system and the gold-standard EOS measurements.The leg length accuracy was -0.846 [-5 to 9], -0.675 [-9 to 18] and 0.542 mm [-5 to 13], respectively for groups A, B and P. The accuracy was significantly lower in group A than B (P=0.044) and group P (P=0.038). The Pearson correlation coefficient for CAOS and EOS measurements was 0.189, 0.701 and 0.891 for leg length and 0.668, 0.202 and 0.680 for offset, for groups A, B and P, respectively. No difference between groups were observed relative to the leg length objectives being achieved within ±2 mm (P=0.61) and ±5 mm (P=0.314). There were no differences in terms of the number of CAOS failures: three in group A, one in group B and three in group P (P=0.06).The Praxim™ and Brainlab™ CAOS systems had similar accuracy for leg length measurements, and both were better than the Amplitude™ system. Only the Praxim™ had an error of less than 0.6mm. All the CAOS systems had values less than 1mm, which is considered excellent.Level IV, retrospective study.