Introduction: Computer-assisted TKA improves alignment accuracy; however few articles cite any clinical benefit over conventional TKA. The author’s experience and outcomes with CAS for TKA including ligament balancing with a spring loaded tensioning device is reported.

Methods: This is a retrospective review of prospectively collected data on 1005 TKAs (975 had OA) with 464 cases using Depuy^® LCS^® CompleteTM Rotating Platform and 474 cases using Depuy^® P.F.C^® SigmaTM Rotating Platform. Seventy-six were conventional TKAs and 929 were CAS TKAs. Average follow up was 17 months. Outcome variables included radiographic alignment, Knee Society Scores, and complications.

Results: Eighty-eight percent of CAS TKAs were placed within three degrees of neutral mechanical axis. Eighty-one percent were placed within three degrees of optimal sagittal tibial component angle. Ninety-two percent were placed within three degrees of optimal coronal tibial component angle.

Mean pain score improved 39.4 points, Knee Score improved 47.8 points, and the functional component improved 17.1 points. The pain score improvement for CAS was 39.2 compared to 33.0 for conventional knees (p< 0.002). The Knee Score improvement for CAS was 48.3 compared to 41.4 for conventional knees (p< 0.013). The functional component improvement was not significant between CAS and conventional TKA. When CAS is utilized along with the spring loaded tensioning device for ligament balancing, manipulation rates dropped to 7% (p< 0.01). There were a total of thirteen infections, three deep infections (0.3%) and ten superficial infections (1%). There were no fractures from the pin sites, and no patients were revised for instability.

Conclusion: Our series showed a statistically significant improvement in pain and Knee Society Scores compared with conventional TKA. In addition, CAS resulted in excellent radiographic alignment and well balanced knees with the spring loaded tensioning device. Furthermore, improved radiographic alignment is likely to increase implant survivorship and provide further cost savings. With continued use of CAS, long term studies may show significant beneficial clinical effects.

Minimally invasive total hip replacement surgery not only decreases the number of visual cues necessary for proper acetabular component position, the small incision makes it technically more difficult to use traditional mechanical alignment guides. Furthermore, traditional mechanical guides have been shown to be unable to accurately predict component position as determined by intraoperative computer measurements.[ 1,2 ] Computer assisted intraoperative navigation can enable minimally invasive surgery by giving the surgeon immediate intra-operative feedback of actual component position. We wished to compare the intraoperative computer determined measurement of acetabular inclination with the postoperative radiographic measurement of inclination in order to validate the results of the computer assisted measurements in the clinical setting. To determine whether computer assisted navigation of the acetabular component allows the surgeon to accurately place the prosthesis in minimally invasive hip replacement and to compare the results of intraoperative navigation with the postoperative radiograph.

42 consecutive patients underwent a minimally invasive posterior approach for total hip arthroplasty with the assistance of CT based intraoperative navigation with the BrainLAB VectorVision software. Preoperative surgical planning was performed after acquisition of a CT scan. All components were templated to be placed in 45 degrees of inclination and 25 degrees of anteversion. Intraoperatively, cementless acetabular components were aligned with the computer navigation at these values prior to implant impaction. Because of the press fit nature and limited soft tissue exposure, many components would shift during impaction. Final component position was then verified and values recorded by detecting points on the acetabular surface. If the prosthesis was felt to be in an acceptable position, no attempt was made to modify component position to the predetermined values in order to avoid potentially compromising component fixation. Postoperative supine AP pelvis radiography was then used to determine final inclination. Measurements were made by drawing a line perpendicular to the acetabular teardrop and parallel to the acetabular component and measured with a standard goniometer. These data were then placed in an SPSS database and analyzed by an independent statistician.

Assessing acetabular component position in routine total hip arthroplasty has been shown to be unreliable even with experienced surgeons with mechanical alignment guides. [1,3] In minimally invasive total hip arthroplasty, routine visual cues are limited and mechanical instruments are difficult to place in the small operative wounds making an already difficult task even more difficult. CT based image guided surgery can has been shown to improve the acetabular component position intraoperatively 2. However, postoperative validation studies comparing the intraoperative computer assessment with the postoperative radiographic measurement are scarce. [ 2 ] In this consecutive series, which represents the author’s first experience with this technology, several conclusions can be made. First, the act of impacting a solid, porous coated, hemispherical cementless acetabular component in minimally invasive hip surgery often leads to a final component position different from the intended position. Second, computer generated determination of implant position is reliable but care must be taken to make sure the reference arrays do not lose fixation during the procedure or spurious results can occur. Third, routine AP pelvis radiographic measurements are not accurate enough to determine whether the computer determined values are accurate. In spite of these measurement inaccuracies, the computer determined results and the radiographic results were within 10 degress 95 % of the time which is far more accurate than results obtained with mechanical alignment tools 3. Finally, further validation studies need to be done with postoperative CT scanning to determine the accuracy of the intraoperative computerized measurements and determine the measurement errors inherent in the clinical setting. Given these limitations, computer assisted navigation improves the accuracy and reliability of acetabular component position over traditional mechanical instruments and can be utilized in minimally invasive hip surgery to assist in the appropriate placement of the acetabular prosthesis.