Comparison of analog and digital 3D preoperative templating in total knee arthroplasty

Introduction: Either the oversizing or undersizing of the components can cause pain and/or a functional impairment, therefore accurate preoperative templating to predict the implant size and position is important for obtaining successful outcomes in total knee arthroplasty (TKA). A templating system has therefore been recommended for joint arthroplasty and it is now routinely used with most designs. Several studies have shown that computer-assisted navigation is more accurate than conventional instrumentation and CT scans are useful for preoperative planning. The aim of this study was to compare the accuracy of preoperative templating in TKA between conventional twodimensional (2D) and computed tomography (CT)-based 3D procedures in order to confirm the efficacy of using 3D evaluations for preoperative planning. METHOS: This study prospectively analyzed 100 primary TKAs performed. The mean age of the patients was 73.3 years. There were 16 male patients and 72 female patients, with twelve bilateral cases. All patients received LCS knee implants (Depuy Orthopaedics, Warsaw, IN, USA). The LCS component was available in 6 sizes. Preoperative templating was performed for each TKA using both conventional 2D radiographs (both anteroposterior and lateral views) which were analyzed by a single senior surgeon. Preoperative CT scans of the knee were performed and a CT-based 3D image model (superimposing the computer aided design model of the implant) was generated using KneeCAS (Knee CAS, LEXI, Inc., Tokyo, Japan) and then was analyzed by a radiology technologist without any knowledge of the 2D procedure. More detailed information about this system has been previously published. 1,2 The accuracy and reliability were assessed for all measurements of the two different templating procedures (2D and CT-based 3D procedures). The Chi-square test for independence for paired observations was used to analyze the accuracy. The Wilcoxon signedranks test was used for the differences between the 2D procedure and the CT-based 3D procedure regarding the mean absolute differences between the planned and actual implant size. In all tests, p<0.05 was considered significant. The weighted kappa test was used to analyze the reliability. RESULTS: Since the templated and implanted sizes between the femoral and tibial components closely coincided in all 100 cases, the evaluations of both components were thus performed simultaneously. The results are shown in (Table 1). Only 56% (56/100) of the 2D procedures were found to be an exact match. This increased to 98% (98/100) for the templates within one size above or below that used and 2% (2/100) were two sizes or more adrift. Otherwise, 59% (59/100) of the CT-based 3D procedures were an exact match; 98% (98/100) were within one size and 2% (2/100) were two sizes or more adrift. The CTbased 3D procedure was slightly more accurate than the 2D procedure. However, the difference was not statistically significant (p = 0.67). The CT-based 3D procedure was on average slightly more accurate than the 2D procedure with a mean absolute error of 0.43 versus 0.47. However, the difference was not statistically significant (p = 0.59). Further analysis of the data shows that templating with the 2D procedure was below one size in 28% (28/100) and above one size in 14% (14/100), and CT-based 3D procedure was below one size in 36% (36/100) and above one size in 3% (3/100). There was a tendency to underestimate the size in the CT-based 3D procedure and that tendency was statistically significant. (P=0.033) The weighted kappa coefficient of the 2D procedure was 0.49 (which indicates a moderate agreement), while that of the CT-based 3D procedure was also 0.49. The results of the weighted kappa coefficients were not statistically significant (p = 0.65). DISCUSSION: Recent research suggests that preoperative analog templating in TKA may provide inaccurate results. Heal et al found preoperative radiologic templating in TKA to be accurate in 57% of all cases and thus questioned its benefit in preoperative management. Arora et al reported that templating in TKA was only accurate for both tibial and femoral components in 53.2%, and concluded that preoperative templating is neither accurate nor reproducible. Aslam et al reported that the exact size of the prosthesis of has been predicted for 49% of femoral and 67% of tibial components, and the statistical agreement between the templated size and the actual implant size was only fair to moderate using the weighted kappa test and the acetate templating for TKAs is prone to error and can only be used as an approximate guide. The current study found that the 2D procedure showed similar results, with an accuracy of 56% and the statistical agreement was moderate, using the weighted kappa test. The current series evaluated whether preoperative computed tomography (CT)-based 3D templating, which used now as one of the functions of computer assisted surgery systems, improve the accuracy of component size in comparison to 2D templating. The accuracy of the 2D procedure was 56% in the current series and of CT-based 3D procedure is 59%. The weighted kappa test statistical showed the agreement between the templated size and the actual implant size to be moderate in both procedures. The use of different magnifications according to preoperative flexion contracture of each patient in the 2D procedure might be the main cause for the “moderate” results obtained in this study. Observer error in interpreting the radiographs, the rotation of the radiographs, and magnification errors caused by a fixed flexion deformity, have all been identified as a possible reasons for the inaccuracy of the templates in TKAs. The observer in the 3D procedure, who had no previous experience of performing TKA surgery, might thus have paid special attention to the oversizing of implants, because the femoral components were fixed without cement (which is a requirement of the press-fit procedure) and the tibial components need to avoid soft tissue friction. Therefore, there was a tendency to underestimate the size in the CT-based 3D procedure and that tendency was found to be statistically significant in this study. Conversely, the accuracy increased to 98% for templated sizes within one size above or below that used both in 2D procedure and CT-based 3D procedure. These results indicate that neither the 2D procedure nor the CT-based 3D procedure for TKAs is reliable and accurate and can only be used as an approximate guide. However, predicting implant sizes to within one size provides a tool for stock control. Thereby the number of surgical instrument that must be prepared for an operation can be reduced, and the surgical time and the risk of infection can also be reduced as a result. We recognized that further study to address the specific questions of reproducibility and interobserver reliability of the 3D procedure by various experienced surgeons may therefore be needed in order to fully demonstrate the advantages of the 3D procedure. Computer-assisted surgery systems are often used for preoperative planning in TKA. However, the present results do not support the superiority of 3D preoperative templating over 2D conventional evaluation in predicting the implant size. Therefore, 3D templating may not be necessary for the preoperative planning for TKA and it can only be used as an approximate guide. REFERENSES: 1. Sato T et al. J Arthroplasty 2004; 19: 620. 2. Sato T et al. J Arthroplasty 2007; 22: 560. 3. Heal J et al. J Arthroplasty 2002; 17:90. 4. Arora J et al. Knee Surg Sports Traumatol Arthrosc 2005;13:187. 5. Aslam N et al. Acta Orthop Belg 2004;70:560.