Our results confirmed the good reproducibility of the 3D-CT based postoperative evaluation system in measuring TKA component positioning. We were able to demonstrate a small but significant difference between measurements based on the traditional mechanical axis and those based on the implant-based axis.
Appropriate limb alignment after TKA has been discussed for many years. The optimal position to obtain satisfactory outcomes both in the short and long term is still debated[8, 17]. Nevertheless, regardless of the underlying theory, surgeons try to position the implant in the manner they believe is “optimal" as the accuracy of implant positioning is essential to obtain positive surgical outcomes. Evaluation methods applied in previous studies to assess TKA positioning after surgery typically include plain anteroposterior and lateral radiographs or two-dimensional CT images[12, 17–19]. Many studies do not report the accuracy of the evaluation technique itself [20], yet the accuracy and reproducibility of the measuring technique are critical factors that may bias the results. 3D-CT-based imaging techniques can be expected to provide better accuracy in the evaluation. Franceschi et al. evaluated the reproduction of 3D-CT preoperative planning by comparing it to the postoperative CT image by superimposing pre- and postoperative CT scans and reported good reproducibility[21]. However, no validation of the intra- and inter-observer reliability of the postoperative evaluation method was reported.
A limited number of studies report the use of 3D-CT analysis for component position measurements after TKA[22, 23]. These reports used the method reported by Hirschmann et al.,[24] which has a high inter- and intra-observer reliability for component position evaluation[25, 26]. However, the method to determine the component position was not described in detail. The accuracy and reproducibility of evaluation could be influenced not only by the imaging methods[17], but also by how the reference points and the axis were determined by the raters. Validation is required for each technique. In the present study, we validated the intra- and inter-observer reliability in evaluating TKA component positioning after surgery by a 3D-CT evaluation system. The system we used in this study is one of the most common validated methods used to perform preoperative planning[11], and also can be used for postoperative evaluation. Our results demonstrate the good reproducibility of the system, both for intra- and inter-observer reliability, confirming its usefulness in postoperative assessment.
The position of the implant is mainly determined by how the bone is resected during surgery. Conventional intra- and extra-medullary guides determine the resection plane by setting the cutting block to a preoperatively planned angle against the rod, which is either inserted in the medullary canal or aligned outside the canal according to anatomical landmarks. Therefore, the precision of the surgery reflected by the postoperative alignment of TKA components, ideally should be evaluated by measuring the angle against the mechanical axis. However, it is sometimes difficult to evaluate the traditional mechanical axis due to difficulty in identifying the anatomical landmarks on the postoperative CT. By minimizing the halation of the implant itself, the sharp edges of the component provide easier reference points to digitize and possibly provide a more accurate measurement. Our results show that both measurement methods, referring to the traditional mechanical axis or the implant-based axis, provide reliable and reproducible evaluation. Thus, the implant-based axis may be utilized in situations in which the anatomical landmarks of the traditional mechanical axis are difficult to identify. Still, the slight difference between the two axes should be considered in interpretation of the results.
This study has several limitations. First, the results we obtained cannot be generalized to all 3D-CT based systems as we did not compare the results from different platforms. However, it was not possible to test all systems. The system used in this study is a commonly used system in this region and the measurements were proved to be reliable and reproducible based on standard anatomical reference points. Thus, we believe that these results could be applied to other similar systems. Second, we did not assess the rotational alignment of the implanted components. The ability to evaluate the rotational alignment is a major advantage of 3D-CT imaging systems against two-dimensional imaging modalities. However, the primary purpose of this study was to analyze the accuracy of implant positioning in the coronal and sagittal planes. Furthermore, unlike the implant position in the coronal and sagittal planes, there is currently no standard reference axis to evaluate the rotational alignment of both femoral and tibial components after TKA[27].