The Journal of Arthroplasty Vol. 22 No

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1 The Journal of Arthroplasty Vol. 22 No Interobserver and Intra-observer Errors in Obtaining Visually Selected Anatomical Landmarks During Registration Process in Non Image-Based Navigation-Assisted Total Knee Arthroplasty W.P. Yau, MBBS, FRCSE, FHKCOS,* Anthony Leung, BSc, MSc (Phys),y K.G. Liu, MD,z C.H. Yan, MBBS,* Lisa L.S. Wong, MBBS, FRCR, FHKCR, FHKAM (Radiology), and K.Y. Chiu, MBBS, FRCSE, FHKCOS* Abstract: This study investigated the errors of obtaining visually selected anatomic landmarks for use in the registration process in a passive optical non image-based computer-assisted total knee arthroplasty system in 5 fresh frozen cadavers. The projected maximum errors in the femoral mechanical axis (due to registration errors of the center of the distal femur) were 0.78 in the coronal and 1.48 in the sagittal plane. The projected maximum errors in the tibial mechanical axis arising from registration errors of the center of the proximal tibia were 1.38 in the coronal and 28 in the sagittal plane. The projected maximum errors in the transepicondylar axis were 9.18 (registration errors of the medial femoral epicondyle) and 7.28 (registration errors of the lateral femoral epicondyle). It should be noted that the results may be partly related to the use of the particular system in this experiment. Key words: interobserver and intra-observer errors, registration, computer assisted orthopedic surgery, navigation, total knee arthroplasty. n 2007 Elsevier Inc. All rights reserved. Long-term survivorship of total knee arthroplasty is related to the accuracy of bone cuts with respect to the mechanical axes of the femur and tibia [1-3]. The cut should be within 38 of the optimal alignment to minimize the chance of early prosthesis loosening [1]. Recent literature suggests that From the *Department of Orthopaedic and Traumatology, Queen Mary Hospital, The University of Hong Kong, Pokfulam, Hong Kong; ybrainlab, Germany; zyan Tai Shan Hospital, Yan Tai, Shan Dong, China; and Department of Radiology, Queen Mary Hospital, Hong Kong. Submitted January 22, 2006; accepted October 7, No benefits or funds were received in support of the study. Reprint requests: W.P. Yau, MBBS, FRCSE, FHKCOS, Department of Orthopaedic and Traumatology, Queen Mary Hospital, The University of Hong Kong, Pokfulam, No. 102, Pokfulam Road, Hong Kong. n 2007 Elsevier Inc. All rights reserved /07/ $32.00/0 doi: /j.arth radiographic alignment of the implanted prosthesis in total knee arthroplasty performed with the aid of computer-assisted technology is superior to that by conventional technique (namely intramedullary and extramedullary alignment systems) [4-23]. However, in spite of the apparently high degree of accuracy obtained by using computer navigation in total knee arthroplasty, errors are still unavoidable. One of the major errors lies in the selection of visually identified anatomic landmarks during registration. This leads to errors in the initial mechanical axes and transepicondylar axis formulated by the computer navigation system. It has been reported by various authors that the accuracy in the selection of medial and lateral femoral epicondyles in computer-navigated total knee arthroplasty is far from satisfactory [24-28]. 1150

2 Registration Errors in Computer Assisted TKA! Yau et al 1151 in the identification of individual landmarks was compared, and inter-observer error studied. Methods Fig. 1. Fresh frozen cadaveric knee model (with markers implanted). Yau et al [24] reported the registration errors of 6 visually selected anatomic landmarks in a piece of cadaveric bone (namely, the center of the distal femur, and the center of the proximal tibia, medial and lateral femoral epicondyle, and medial and lateral malleoli). The error reported was in the range of 0.1 to 8.2 mm. However, as pointed out in that paper, the major pitfall was the absence of baseline data for comparison. The reported error was only a reflection of the repeated selection of the same anatomic landmark. Potential bias was not addressed. Furthermore, the specimen studied in that experiment was a piece of cadaveric bone denuded of all soft tissue. The reported error was likely an underestimation of this feature. The current study was an attempt to repeat the same experiment in fresh frozen cadaveric lower limbs with intact knee and ankle joints. A computed tomography (CT) was performed to provide a baseline for comparison and hence made the study of potential bias possible. The magnitude of errors The model studied was a piece of fresh frozen cadaveric lower limb with intact knee and ankle joints. The original cadaveric femur was osteotomized at a point 20 cm proximal to the knee joint. The cadaveric femur was rigidly fixed to a piece of commercially available plastic femur (model 1146, Sawbones, Pacific Research Laboratories, Inc, Vashon, Wash) by 2 pieces of 7 holed broad DCP plate (Fig. 1). This allowed pivoting to be carried out to determine the center of the femoral head during the registration process. Plastic markers were implanted into the shafts of the femur and tibia. These markers were identified in both the navigation system and CT. This allowed direct comparison of the data obtained from the non image-based computer navigation system and the data obtained from the computed tomogram. Five markers were placed in the shaft of the femur and 4 in the shaft of the tibia. A total of 5 cadaveric lower limbs were studied in this experiment. The specimen was mounted on a frame. The cadaveric knee was positioned in 908 flexion as would be in a standard total knee arthroplasty. Medial parapatellar arthrotomy was performed. The surgeons approached the distal end of the femur and proximal end of tibia anteriorly as in a total knee arthroplasty (Fig. 2). Registration in Computer Navigation The navigation system software used in this experiment was Vector Vision CT Free Knee 1.1 Fig. 2. Approach and exposure. Fig. 3. Distribution of registration errors of the center of the proximal tibia (including specimen 3).

3 1152 The Journal of Arthroplasty Vol. 22 No. 8 December 2007 Table 1. Errors in Registration of Visually Selected Anatomical Landmarks Min (mm) Max (mm) Range (mm) Mean (mm) SD (mm) Center of the distal femur Center of the proximal tibia Center of the talus Medial femoral epicondyle Lateral femoral epicondyle ANOVA, P b (BrainLAB, Munich, Germany). The hardware included a computer working station, an infrared camera with passive optical sensor, optical arrays (one on the femur and one on the tibia) constructed of 3 disposable markers mounted on a rigid body for passive reflection of the infra-red beam, and a pointer with a 2-mm diameter ball tip for registration. A standard registration process was performed according to the manufacturer s protocol. The system allowed the investigators to input additional digitalized points into the computer. The positions of the tip of the 9 plastic markers were input into the computer as additional digitalized points. Their coordinates were noted. The repeatability and reproducibility in identification of the visually selected anatomical landmarks were studied. The landmarks under investigation were the center of the distal femur, the center of the proximal tibia, medial and lateral malleolus, and the medial and lateral femoral epicondyles. The coordinates of these landmarks were used to construct the mechanical axis of the femur, and the mechanical axis of the tibia and the transepicondylar axis, respectively. Repeated selection of these 6 anatomic landmarks was performed 25 times by each of 2 orthopedic surgeons. The coordinates of these landmarks were input into the computer as additional digitalized points. The same experiment was repeated 1 week later. The coordinates of these visually selected anatomical landmarks were retrieved from the computer for mathematical analysis. The coordinates of the medial and lateral malleolus were combined to give the coordinate of the center of the talus. This was performed according to the assumption of the system: that is, the coordinate of the center of the talus = the coordinate of the medial malleolus the coordinate of lateral malleolus The error in the registration of the landmarks was studied in the plane of interest, which was a plane perpendicular to the corresponding axes. Computed Tomography Computed tomography of the entire limb was performed with a 16-detector CT scanner (Lightspeed-16, General Electric, Milwaukee, Wis). The cadaveric lower limb was placed with its long axis along the length of the scanning crouch. The scanning parameters used were 120 kv, 9 ma, 0.6-mm slice collimation, matrix, and a bone algorithm with no gantry tilting. The images Fig. 4. Distributions of error of registration of the center of the distal femur, the center of the proximal tibia, and the center of the talus.

4 Registration Errors in Computer Assisted TKA! Yau et al 1153 obtained were loaded into a computer workstation (Advantage Workstation 3.1, General Electric, Milwaukee, Wis). Three-dimensional reconstruction images of the distal femur, proximal tibia, and ankle region were created. The center of the distal femur, center of the proximal tibia, center of the talus, and the 2 femoral epicondyles were identified jointly by an orthopedic surgeon and a radiologist. The medial femoral epicondyle was defined as the sulcus where the deep part of the medial collateral ligament attached. The lateral femoral epicondyle was defined as the most prominent point over the region. The tips of the 9 plastic markers were identified in the axial CT images. The CT coordinates were recorded. Neither interobserver nor intra-observer testing was performed in the process of the identification of the landmarks in the computed tomogram. Computed tomography coordinates of the tips of the 9 markers were matched with the coordinates of the markers obtained with the computer navigation system. This allowed direct comparison of 2 sets of data. The CT data of the studied anatomical landmarks were used as the baseline. The errors in the registration of the visually identified anatomical landmarks in the computer navigation total knee arthroplasty system were expressed as a deviation from the baseline along the plane of interest. Null hypothesis was assumed. It was found that there was no difference in the errors in the registration of the 6 studied anatomic landmarks. This was examined by using analysis of variance (ANOVA) test. Interobserver difference in the registration of landmarks was compared by using paired t test. Statistical significance was assumed if P b.05. The potential errors in the axes that resulted were calculated. Results A total of 5 cadaveric specimens were used in this study. Four were right and 1 was left. One of the tibial markers in specimen 3 was found to have loosened during the experiment. The average error in the registration of the center of the proximal tibia in specimen 3 was found to be approximately 2 cm by both observers (Fig. 3). It was believed that the loosening of the marker contributed to this error. The results concerning the tibia portion of specimen 3 were excluded from the analysis. The errors in the registration of the studied anatomical landmarks are listed in Table 1. The magnitude of error was significantly different in terms of the registration of different anatomic landmarks ( P b.001, ANOVA). It is noteworthy that the magnitude of error was significantly higher in the registration of the medial and lateral femoral epicondyle (7.1 F 2.4 and 11.1 F 5.6 mm, respectively). The errors in the registration of landmarks were studied along the direction of the mechanical axes/ transepicondylar axis of interest. For instance, for the center of the distal femur, the error was studied along the medial-lateral direction (which influenced the accuracy of mechanical axis of the femur in the coronal plane) and anterior-posterior direction (which influenced the accuracy of the mechanical axis of the femur in the sagittal plane). The average registration errors were 0.8 F 1.9 mm (medial-lateral direction) and 2.3 F 4.6 mm (anterior-posterior direction) for the center of the distal femur, 0.9 F 1.8 mm (medial-lateral direction) and 3.2 F 4.8 mm (anterior-posterior direction) for the center of the proximal tibia, 0.4 F 3.7 mm (medial-lateral direction) and 0.2 F 4.6 mm Fig. 5. Distributions of registration errors of the medial femoral epicondyle and the lateral femoral epicondyle.

5 Center of the distal femur Medial-lateral Anterior-posterior Table 2. Errors in Registration of Visually Selected Anatomical Landmarks in Specimen 1 to Specimen 5 Specimen 1 Specimen 2 Specimen 3 Specimen 4 Specimen 5 Average 0.2 F 2.0 mm ( 7.4 to 3.3 mm) 5.6 F 2.7 mm ( 10.9 to 1.1 mm) Center of the proximal tibia Medial-lateral 0.2 F 1.3 mm ( 3.5 to 2.3 mm) Anterior-posterior 1.8 F 2.1 mm ( 1.3 to 8.2 mm) Center of the talus Medial-lateral Anterior-posterior 1.5 F 1.4 mm ( 4.5 to 1 mm) 3.5 F 1.5 mm ( 7.7 to 0.6 mm) 1.2 F 0.8 mm ( 4.1 to 0.7 mm) 3.7 F 3.3 mm ( 1.3 to 10.4 mm) 1.3 F 2.1 mm ( 3.5 to 4.3 mm) 7.1 F 2.8 mm (0.9 to 10.9 mm) 2.0 F 1.4 mm ( 2.2 to 3.1 mm) 1.9 F 3.6 mm ( 8.2 to 4.9 mm) 2.4 F 1.3 mm ( 0.5 to 5.1 mm) 6.6 F 1.7 mm ( 11.2 to 2.9 mm) 17.7 F 4.2 mm (Excluded) 9.1 F 3.4 mm (Excluded) 8.9 F 1.2 mm (Excluded) 3.3 F 3.8 mm (Excluded) 0.6 F 1.4 mm ( 1.4 to 2.8 mm) 3.0 F 3.3 mm ( 7.3 to 6.2 mm) 1.2 F 1.2 mm ( 1.9 to 5.6 mm) 2.6 F 1.5 mm ( mm) 2.6 F 1.5 mm ( 7.2 to 0.4 mm) 1.8 F 2.4 mm ( 3 to 6.4 mm) 2.3 F 0.7 mm (0.7 to 3.8 mm) 3.0 F 2.0 mm ( 5.1 to 4.6 mm) 1.4 F 1.9 mm ( 5 to 4.6 mm) 6.5 F 3.8 mm ( 1.4 to 11.5 mm) 5.5 F 3.0 mm ( mm) 2.7 F 6.2 mm ( 5.8 to 18.6 mm) 0.8 F 1.9 mm 2.3 F 4.6 mm (Specimen 3 excluded) 0.9 F 1.8 mm 3.2 F 4.8 mm (Specimen 3 excluded) 0.4 F 3.7 mm 0.2 F 4.6 mm 1154 The Journal of Arthroplasty Vol. 22 No. 8 December 2007 Medial femoral epicondyle Anterior-posterior 2.5 F 2.4 mm ( 7.2 to 2.3 mm) 0.1 F 5.0 mm ( 7.8 to 8.3 mm) 6.5 F 3.4 mm ( 0.5 to 12.7 mm) 3.3F 2.4 mm ( 1.3 to 8 mm) 5.6 F 5.5 mm ( 11.1 to 0.7 mm) 0.3 F 5.4 mm Lateral femoral epicondyle Anterior-posterior 4.7 F 1.2 mm ( 6.8 to 2.2 mm) 5.4 F 1.3 mm ( 8.9 to 2.1 mm) 18.9 F 3.3 mm ( 24.2 to 10.6 mm) 6.7 F 2.8 mm ( 12.2 to 1.8 mm) 2.0 F 1.8 mm ( 5.5 to 3.4 mm) 7.5 F 6.3 mm Frontal plane: medial, positive; lateral, negative. Sagittal plane: anterior, positive; posterior, negative.

6 Table 3. Range of Errors in Registration of Visually Selected Anatomical Landmarks and the Projected Errors in the Corresponding Axes Specimen 1 Specimen 2 Specimen 3 Specimen 4 Specimen 5 Average Center of the distal femur Medial-lateral 10.7 mm 4.9 mm 4.7 mm 4.2 mm 3.1 mm 5.5 mm (Mechanical axis of femur: varus-valgus) Anterior posterior 9.7 mm 11.7 mm 8.3 mm 13.5 mm 9.7 mm 10.6 mm (Mechanical axis of femur: extension-flexion) Medial femoral epicondyle Anterior-posterior 9.4 mm 16.1 mm 13.2 mm 9.3 mm 11.8 mm 12 mm (Transepicondylar axis: ER vs IR) Lateral femoral epicondyle Anterior-posterior 4.6 mm 6.7 mm 13.6 mm 10.4 mm 8.9 mm 8.8 mm (Transepicondylar axis: IR vs ER) Center of the proximal tibia (Specimen 3 excluded) Medial lateral 5.8 mm 7.8 mm mm 9.6 mm 7.7 mm (Mechanical axis of tibia: varus-valgus) Anterior-posterior 9.5 mm 10 mm mm 12.9 mm 10.8 mm (Mechanical axis of tibia: flexion-extension) Center of the talus (Specimen 3 excluded) Medial-lateral 5.6 mm 5.3 mm mm 11.8 mm 7.4 mm (Mechanical axis of tibia: valgus-varus) Anterior-posterior 7.1 mm 13.1 mm mm 24.4 mm 13.5 mm (Mechanical axis of tibia: extension-flexion) ER indicates external rotation; IR, internal rotation. Registration Errors in Computer Assisted TKA! Yau et al 1155 (anterior-posterior direction) for the center of the talus, 0.3 F 5.4 mm for the medial femoral epicondyle, and 7.5 F 6.3 mm for the lateral femoral epicondyle (Figs. 4 and 5). The details of the registration errors of anatomic landmarks for different specimens are listed in Table 2. The repeatability was low for the registration of all of the landmarks (Tables 2 and 3). The average range of registration error among the different specimens was 5.5 mm in the medial-lateral direction and 10.6 mm in the anterior-posterior direction for registration of the center of the distal femur; 7.7 mm in the medial-lateral direction and 10.8 mm in the anterior-posterior direction for the registration of the center of the proximal tibia; 7.4 mm in the medial-lateral direction and 13.5 mm in the anterior-posterior direction for registration of the center of the talus; 12 mm in the registration of medial femoral epicondyle and 8.8 mm in the registration of lateral femoral epicondyle. The projected maximum errors in the mechanical axis of the femur (due to registration error of the center of the distal femur) were 0.78 in the coronal and 1.48 in the sagittal plane. The projected maximum errors in the mechanical axis of the tibia arising from error in the registration of the center of the proximal tibia were 1.38 in the coronal and 28 in the sagittal plane. The projected maximum errors in the mechanical axis of the tibia arising from error in the registration of the medial and lateral malleolus were 1.38 in the coronal and 2.38 in the sagittal plane. The projected maximum errors in the transepicondylar axis were 9.18 (due to errors in the registration of the medial femoral epicondyle) and 7.28 (due to errors in the registration of the lateral femoral epicondyle) (Table 3). Except in the registration of the center of proximal tibia and lateral femoral epicondyle, there were significant interobserver differences in all of the landmarks studied (Figs. 6-9). The average projected errors in the femoral mechanical axis for the varus were 0.18F 0.28 in the coronal plane and 0.38flexion F 0.68 in the sagittal plane. In terms of the tibial mechanical axis, the average projected errors arising from registration error of the center of the proximal tibia for the

7 1156 The Journal of Arthroplasty Vol. 22 No. 8 December 2007 Fig. 6. Interobserver error in registration of the center of the distal femur. varus were 0.28 F 0.38 in the coronal plane and 0.68 flexion F 0.88 in the sagittal plane. The average projected errors in the mechanical axis of the tibia arising from registration error of the center of the talus for the valgus were F 0.68 in the coronal plane and flexion F 0.88 in the sagittal plane. In regard to the transepicondylar axis, the average projected errors arising from error in the registration of the medial femoral epicondyle and lateral femoral epicondyle were 1.38 externally rotated F 58 and 5.88 externally rotated F 58, respectively (Table 4). Discussion The current experiment investigated the errors in selecting anatomical landmarks, both visually and by palpation, in a fresh frozen cadaveric knee model during the registration process of non image-based navigation-assisted total knee arthroplasty. A similar experiment was previously conducted by studying the registration errors in a piece of cadaveric bone [24]. However, it was pointed out in that report that the absence of a baseline comparison was a major weakness of the study. Computed tomography was performed in the current experiment. This provided the requisite baseline comparison and allowed investigation of potential bias among the different anatomic landmarks registered. Fresh frozen cadaveric lower limbs with an intact soft tissue envelope were used in this study. This allowed a more realistic reproduction of the intra-operative setting during total knee arthroplasty. The distributions of the error in the registration of different anatomical landmarks are shown in Figs. 4 and 5. The average error ranged from 0.3 mm in the registration of the center of the distal femur to 24.3 mm in the lateral femoral epicondyle (Table 1). The error was much larger than that reported in the previous experiment in which a single piece of cadaveric bone was studied [24]. Significant bias was noted during registration of the lateral femoral epicondyle in the current experiment (Fig. 5). It was observed that the bias was toward a proximal and posterior direction for both

8 Registration Errors in Computer Assisted TKA! Yau et al 1157 Fig. 7. Interobserver error in registration of the center of the proximal tibia. observers in all of the specimens studied. We postulated that it was due to 2 reasons: the design of the registration pointer of the navigation system used and the obstruction of the pivoting action of the pointer by the patella. The navigation system used in this experiment was a passive optical system. In order for the computer to recognize the input anatomical landmark, pivoting of the pointer on the targeted landmark was required during the registration process and subsequent experiment. The pointer had a ball tip diameter of 2 mm. Slipping of the ball-tipped pointer during registration of the prominent point of lateral femoral epicondyle might have taken place. The chance of slipping of the pointer was increased when the tip of the pointer was not stabilized by the hand of the surgeon. This was particularly a problem in the registration of the lateral femoral epicondyle because the patella was reflected laterally to the lateral para-patella gutter. This created added difficulty in pivoting the pointer. We believed that these were the reasons leading to the consistent bias in the registration of the lateral femoral epicondyle in all the specimens. This seems to be likely to be unique to the navigation system used in this experiment. However, the best way to find out the cause of the bias in the registration of the lateral femoral epicondyle was to conduct a separate experiment comparing the registration errors in a passive optical navigation system and an active navigation system (either optical or electromagnetic). But this was beyond the scope of the current study. Concerning the registration of centers of the distal femur and proximal tibia, the results were comparable to those reported in the previous experiment (Table 3) [24]. However, the range of registration error of the medial and lateral femoral epicondyle nearly doubled from the results reported in the previous experiment (Table 3) [24]. The increase in range of error was likely related to the presence of thick soft tissue around the femoral epicondyles in the fresh frozen cadaveric specimens. This led to an increased difficulty in accurate identification of the 2 femoral epicondyles by palpation. The result of the current experiment is thus a better reflection of the potential errors in the intra-operative situation.

9 1158 The Journal of Arthroplasty Vol. 22 No. 8 December 2007 There was statistically significant difference between the 2 observers in identification of most of the targeted anatomical landmarks (Figs. 6-9). However, the average differences of the projected errors in the mechanical axes of the femur and tibia between the 2 observers were less than 0.58 for all the landmarks registered (Table 4). These differences were unlikely to carry clinical significance. A particular strength of the current experiment was the use of CT to provide the baseline calculation of the errors in the registration process in a non image-based computer navigation total knee arthroplasty system. Unfortunately, this was itself also another source of potential errors when the results were interpreted. The plastic markers can become loose and introduce errors into matching the CT data and computer navigation data. The results of the tibial portion of specimen 3 were excluded for this reason. In addition, errors can arise during the identification of the anatomical landmarks in the 3-D CT reconstruction image. It was observed that the 2 observers (one orthopedic surgeon and one radiologist) had difficulty in reaching consensus during the identification of the 2 malleoli in the 3-D CT image in most of the specimens. As a result, the center of the talus was identified as an alternative. The coordinate of the navigation data of the center of the talus was computed from the coordinates of the navigation data of the 2 malleoli according to the assumptions of the navigation system used (ie, the coordinate of the center of the talus = the coordinate of the medial malleolus the coordinate of the lateral malleolus 0.46). Hence, the error in the registration of the center of the talus reported in this study represents a combination of the errors in identification of the 2 malleoli and the potential error in the assumptions of the system itself. Readers of research reports of this type need to be aware of these potential pitfalls when the results of such experiments are carried over into the clinical situation. Besides, as neither interobserver nor intra-observer testing was performed in the identification of Fig. 8. Interobserver error in registration of the center of the talus.

10 Registration Errors in Computer Assisted TKA! Yau et al 1159 Fig. 9. Interobserver errors in registration of the medial and lateral femoral epicondyles. the landmarks in the computed tomogram, the bgold standardq provided by the CT may have hidden errors. The original cadaveric femur was osteotomized at a point 20 cm proximal to the knee joint and it was rigidly fixed to a piece of commercially available Table 4. Interobserver Error of the Projected Mechanical Axes of Femur and Tibia and Transepicondylar Axis Surgeon 1 Surgeon 2 Average Error in the mechanical axis of femur due to registration error of the center of the distal femur Frontal plane F F F 0.28 Sagittal plane 0.58 F F F 0.68 Error in the mechanical axis of tibia due to registration error of the center of the proximal tibia Frontal plane 0.18 F F F 0.38 Sagittal plane 0.68 F F F 0.88 Error in the mechanical axis of tibia due to registration error of the center of the talus Frontal plane 0.28 F F F 0.68 Sagittal plane 0.38 F F F 0.88 Error in the transepicondylar axis due to registration 1.18 F F F 5.08 error of the medial femoral epicondyle Error in the transepicondylar axis due to registration error of the lateral femoral epicondyle 5.88 F F F 5.08 Frontal plane: varus, positive; valgus, negative. Sagittal plane: extended, positive; flexed, negative. Rotation: externally rotated, positive; internally rotated, negative.

11 1160 The Journal of Arthroplasty Vol. 22 No. 8 December 2007 plastic femur to restore the original length of the femur. This facilitated the handling of the specimen in the whole experiment. However, as full lower extremity cadavers were not used, it was not an exact reproduction of the intra-operative conditions. To summarize the findings reported here, the average errors in the registration of visually selected anatomic landmarks in a non image-based computer navigation total knee arthroplasty system were from 0.2 to 7.5 mm (Table 2). The maximum potential error could be as high as 13.5 mm (Table 3). There was statistically significant interobserver difference in the registration of the centers of the distal femur, proximal tibia, and talus (Figs. 6-9). However, the projected errors in the mechanical axes of the femur and tibia due to errors in the registration of the visually selected anatomic landmarks were unlikely to bear clinical significance (Table 4). 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