Accuracy of acetabular cup positioning using imageless navigation
1 Musculoskeletal Research Unit, CQ University, Yaamba Road, Rockhampton 4700, Australia
2 Department of Orthopaedic Surgery, Rockhampton Hospital, Canning Street, Rockhampton QLD 4700, Australia
3 Centre for Health, Exercise and Sports Medicine, Faculty of Medicine, The University of Melbourne, 200 Berkeley Street, Melbourne VIC 3010, Australia
4 Department of Orthopaedic Surgery, Royal Brisbane Hospital, Butterfield Street, Herston QLD 4029, Australia
5 CONROD Professor of Orthopaedic Trauma Surgery, Division of Surgery, University of Queensland Medical School, Butterfield Street, Herston QLD 4029
Journal of Orthopaedic Surgery and Research 2011, 6:40 doi:10.1186/1749-799X-6-40Published: 10 August 2011
Correct placement of the acetabular cup is a crucial step in total hip replacement to achieve a satisfactory result and remains a challenge with free-hand techniques. Imageless navigation may provide a viable alternative to free-hand technique and improve placement significantly. The purpose of this project was to assess and validate intra-operative placement values for both inclination and anteversion as displayed by an imageless navigation system to post-operative measurement of cup position using high resolution CT scans.
Thirty-two subjects who underwent primary hip joint arthroplasty using imageless navigation were included. The average age was 66.5 years (range 32-87). 23 non-cemented and 9 cemented acetabular cups were implanted. The desired position for the cup was 45 degrees of inversion and 15 degrees of anteversion. A pelvic CT scan using a multi-slice CT was used to assess the position of the cup radiographically.
Two subjects were excluded because of dislodgement of the tracking pin. Pearson correlation revealed a strong and significant correlation (r = 0.68; p < 0.006) for cup inclination and a moderate non-significant correlation (r = 0.53; p = 0.45) between intra-operative readings and cup placement for anteversion.
These findings can be explained with the possible introduction of systematic error. Even though the acquisition of anatomic landmarks is simple, they must be acquired with great precision. An error of 1 cm can result in a mean anteversion error of 6 degrees and inclination error of 2.5 degrees. Whilst computer assisted surgery results in highly accurate cup placements for inclination, anteversion of the cup cannot be determined accurately.