Background: The positioning of the acetabular component is of critical importance in total hip arthroplasty. Due to the orientation of the acetabulum and limitations of observation imposed at the operative site mal-positioning is common. We believe that by utilising the transverse acetabular ligament (TAL) and acetabular labrum, we are able to anatomically position our cup. In this study, we evaluate the correlation between placement of the acetabular component by reference to the TAL and the acetabular labrum with the taught safe zones for cup placement.

Method: 7 embalmed hips were studied. Following disarticulation the labrum and TAL were digitised and their plane was calculated. Orientation of cup placement in this plane was calculated from a pre-dissection pelvic CT.

Results: The plane of the labrum/TAL varied between 5–26° of anteversion and 32–59° of inclination. Interob-server differences in acetabular cup placement based on the TAL/labral plane indicate reasonable coherence. Almost all components were inside the documented “safe zone” 0–40° of anteversion and 30–55° of inclination of placement.

Conclusion: The acetabular labrum and TAL form a plane that reflects the documented “safe zones” for acetabular component placement. We feel that this plane allows a surgeon to determine optimal patient specific acetabular component placement, irrespective of patient position.

Introduction and Aims: A method has been designed to accurately measure post-operative alignment of hip (acetabular) and knee (femoral and tibial) prosthetic components relative to the pre-operative plan. Conventional methods involve 2D measurements; this new method uses 2D-3D registration to align both the prosthesis and the pre-operative CT volume to the post-operative x-ray.

Method: The method uses an automatic approach to align a CAD model of the prosthesis to the post-operative x-ray. A rendering of the prosthesis is produced and overlaid onto the post-operative x-ray image. The prosthesis can be rotated and translated in 3D to match the outline of the rendering shown on the post-operative x-ray. An initial manual procedure is used to align the rendering of the bone surface from pre-operative CT to the bony anatomy on the post-operative x-ray. This manual registration position is then used as a starting position for an automated intensity-based registration algorithm.

Results: The automated intensity-based registration algorithm allowed 3D verification of the prosthesis position. A number of digitally reconstructed radiographs (DRRs) were produced by casting rays through the pre-operative CT volume. The DRRs were then compared with the post-operative X-ray image using a similarity measure. This similarity measurement was optimised using gradient decent-type search strategy to alter the rotation and translation parameters. If the Hounsfield numbers of the voxels, which the casting rays passed through, were integrated along the ray and projected onto an imaging plane, a radiograph-like image was produced. To concentrate the area of registration and thus quicken registration algorithm, the user also manually defined a region of registration interest. Hence, DRRs were only produced within the region of interest. Due to the large size of the pelvis and tube-like nature of the femur and tibia, a total of 10 starting positions were used for this algorithm. These starting positions were found by adding random Gaussian noise to the parameters found using the manual process. The registration position was defined as the final position that produced the best similarity measurement value.

Conclusion: Validation has demonstrated this method’s accuracy in calculating the post-operative position of acetabular and knee prostheses with respect to the pre-operative plan. The results are repeatable, robust and enable pre- and post-operative 3D implant position comparison. The inaccuracies observed with conventional methods due to incorrect alignment on x-ray are reduced.

Correct alignment is important for success in total knee replacement. Currently this is achieved by a combination of intramedullary and extramedullary alignment using jigs and cutting blocks. This multicentre study evaluates the use of computer assisted planning and the interactive guidance of instruments for total knee replacement.

Prior to surgery computer scans of the hip, knee and ankle were performed of patients enrolled in the study. Pre operative planning of the position and size of the knee components was performed by the surgeon using a CT based Vector Vision Navigation System (Brain LAB AG, Heimstetten, Germany). P.F.C.x (De Puy Leeds UK) knee replacements were then implanted in 38 patients. Surgery was carried out according to the standard surgical technique using traditional instruments. Information of the planned and intraoperatively recorded position of the cutting blocks were analysed to check varus/valgus alignment, flexion/extension alignment, the amount of planned resection from both the femoral and tibial bones and the size of the components. Information from all the separate centres was sent to a central data processing base for analysis.

Results were calculated comparing the differences between the planned and performed cuts for each of the different variables studied. Graphs demonstrate the differences in the alignment between that planned by the surgical navigation system and what was actually carried out by the instrumented cuts.

Based on the data obtained from the multicentre study we have concluded that the planned position of the implants using the standard instruments was similar to that using the Vector Vision Navigation System. We believe that it is safe to proceed with surgical navigation total knee arthroplasty using the P.F.C.x total knee prosthesis with Image Guided Surgery and a further multicentre study is currently underway evaluating this.