LIMITATIONS OF COMPUTER-ASSISTED INTRAOPERATIVE MEASUREMENT OF THE MECHANICAL AXIS OF THE LOWER LIMB

Abstract

Purpose of the study: Computer assisted surgery for total knee arthroplasty is widely used in Europe. The reliability of these systems appears to be very good with bone cuts within 3° of the planned mechanical axis. Nevertheless, the relationship between intraoperative measurements provided by the navigation system and the postoperative gonometry can be used to assess the quality of realignment. The purpose of this work was to determine this relationship.

Material and methods: Thirty-three knees operated on with the Navitrack® system were assessed. Two senior operaters performed all procedures. An Omnia® cemented prosthesis with an ultracongruent fixed plateau was used in all cases. Intraoperative measurements wer made with the definitive prosthesis after cement solidification without stress on the knee. Postoperative gonometry was undertaken when the intraoperative flexion had disappeared six weeks to six months after the operation. Gonometric measurements were made by an independent operator using a computerized system operating on digitalized x-rays. The gonometric protocol had been standardized previously and only the gonometric measurements in compliance with this protocol were retained for analysis. The difference between measurements was analyzed with the t test for paired variables. Search for correlations was also performed.

Results: On average the intraoperative deviation was 0.8±0.8° (3° valgus to 2.4° varus). The postoperative gonometry showed 1.7±1.1° (3.4° valgus to 4.3° varus). The mean difference between the intraoperative axis and the measured postoperative axis was significant (p< 0.0001). There was no significant correlation between intraoperative and postoperative measurements.

Discussion: Computer-assisted navigation systems have their limitations which should be measured. The present findings would demonstrate a significant difference of minimal amplitude between the intraoperative measurement and the postoperative gonometry. Although the clinical pertinence of this difference remains to be demonstrated, it must be kept in mind for safe use of these navigation systems.