Use of Image-free Navigation in. Determination of Acetabular Cup. Orientation: Analysis of Factors. Affecting Precision. TOMOKAZU FUKUI, MD; SHIGEO ...
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Use of Image-free Navigation in Determination of Acetabular Cup Orientation: Analysis of Factors Affecting Precision TOMOKAZU F UKUI, MD; SHIGEO FUKUNISHI, MD; S HOJI N ISHIO, MD; N AO S HIBANUMA, MD; S HINICHI YOSHIYA, MD
abstract We have been using an image-free total hip arthroplasty (THA) navigation system (OrthoPilot; Aesculap, Tuttlingen, Germany) to ensure accurate and reproducible acetabular cup orientation. In this study, the accuracy of this system in the assessment of acetabular cup orientation was evaluated by comparing the intraoperative inclination and anteversion angles presented by the navigation system and the corresponding postoperative values obtained by computed tomography measurement. In the intraoperative accuracy analysis, we additionally examined the influence of factors such as body mass index and soft tissue thickness on assessment error. Intraoperative and postoperative results obtained from 115 consecutive navigated THAs were compared and analyzed. In both inclination and anteversion angles, good agreement was observed; a discrepancy of more than 5° was observed in 1 and 3 cases, respectively. In the analysis of factors potentially affecting the accuracy of the intraoperative assessment, no correlation between each parameter and the intraoperative and postoperative discrepancy was demonstrated.
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se of a computer-assisted navigation system in total hip arthroplasty (THA) has been introduced with expectations of achieving optimum implant alignment.1-8 Current THA navigation systems can be divided into 2 groups based on the operation principle: image-based or image-free. In the image-free system, the position and orientation of the patient’s pelvis are recognized intraoperatively by palpation and registration of the bony landmarks. Thus, a discrepancy between the locations of the bony landmark and the registered point may cause assessment errors. In several
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clinical studies, the accuracy of the imagefree THA navigation system has been assessed and reported.3-5,9-12 In most studies, satisfactory agreement between the intraoperative navigation results and the postoperative values obtained from radiologic examination has been reported.3-5,13,14 We have been using the OrthoPilot image-free navigation system (B. Braun Aesculap, Tuttlingen, Germany) since 2006. In order to examine the accuracy and potential source of error in the use of this navigation system, we conducted a study comparing the intraoperative cup positioning angles shown by the
navigation and the corresponding values calculated from postoperative computed tomography (CT) data. In the CT evaluation, we analyzed Digital Imaging and Communications in Medicine (DICOM) data using the Medical Image Processing, Analysis and Visualization (MIPAV) computer program developed by the National Institutes of Health (Bethesda, Maryland), which allows the consistent creation of an analysis coordinate system.15,16 We reported preliminary results of the analysis for the initial 25 THAs in 2008.17 Although a reasonably high accuracy with the use of this system was observed, the small sample size in the study precluded us from analyzing the factors affecting the accuracy and presenting a conclusive statement.
Drs Fukui, Fukunishi, Nishio, and Yoshiya are from the Department of Orthopaedic Surgery, Hyogo College of Medicine, Nishinomiya, and Dr Shibanuma is from the Department of Orthopaedic Surgery, Kobe Kaisei Hospital, Kobe, Japan. Drs Fukui, Fukunishi, Nishio, Shibanuma, and Yoshiya have no relevant financial relationships to disclose. Correspondence should be addressed to: Tomokazu Fukui, MD, Department of Orthopaedic Surgery, Hyogo College of Medicine, 1-1 Mukogawa-cho Nishinomiya, Hyogo 6638501, Japan. doi: 10.3928/01477447-20100510-61
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IMAGE-FREE NAVIGATION FOR ACETABULAR CUP ORIENTATION | FUKUI ET AL
One of the potential factors leading to assessment error in the use of the image-free system is inaccurate localization of the bony landmarks.12,18,19 In particular, the presence of thick subcutaneous tissue in an obese patient can cause a discrepancy. In this study, we increased the number of patients and examined the effects of soft tissue on assessment error in the intraoperative evaluation. Moreover, it was expected that the accuracy of this system could be more substantially assessed than in our previous report17 due to the analysis of a larger patient population.
MATERIALS AND METHODS Patient Selection In 2005, we began using the OrthoPilot CT-free navigation system. After an initial series of 38 hips, we designed and started this study in April 2006 after approval by the Institutional Review Board and with informed consent from the patients. From April 2006 to March 2008, both acetabular alignment data based on navigation system and acetabular alignment data based on postoperative CT evaluation were obtained and compared in 115 consecutive navigated THAs in 110 patients. Mean patient age was 63.4 years (range, 21-88 years). Fifteen patients were men and 95 were women. Preoperative diagnosis was osteoarthritis in 85, osteonecrosis in 25, rheumatoid arthritis in 3, and hemophilic arthropathy in 2 patients. Sequence of the Navigation Procedure Preoperatively, 2 metal markers were attached to the bilateral anterosuperior iliac spines, and the location of the markers was confirmed by a plain anteroposterior pelvic radiograph (Figure 1). These markers showed the location of the bony landmarks both in the intraoperative and postoperative evaluations. Before surgery, the tracker was inserted and fixed to the iliac crest through a stab incision with the patient in the supine position. As the anatomic landmarks, the bilateral
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2
Figure 1: AP radiograph of the pelvis with 2 markers attached onto the skin at the bilateral anterior superior iliac spines before surgery.
Figure 2: Postoperative CT scan with 2 markers at the bilateral anterosuperior iliac spines.
anterosuperior iliac spines and the upper margin of the symphysis were located by percutaneous palpation of the markers and bony prominence. The reference plane in the analysis was based on the triangle (anterior pelvic plane, APP) made by these 3 landmarks, and the orientation of the pelvis was determined.
the patient in the same position as for the registration at surgery (Figure 2). A helical CT scanner (Somatom; Siemens, Germany) produced images with 2-mm slice intervals. The CT data were transferred as DICOM files to a desktop computer. The DICOM data were recognized by the MIPAV application for specification of the 3-dimensional coordinate. The 3 anatomic landmarks (bilateral anterosuperior iliac spines and the upper margin of the pubic symphysis) were specified as the reference points according to the location of the markers observed in the CT images. Additionally, 3 points at the periphery of the cup were located on the images for determination of the 3-dimensional cup orientation. Three anatomic landmarks and 3 points at the periphery of the cup were digitized 3 times by 1 examiner (T.F.). Thereafter, the radiologic anteversion and inclination angles were calculated. The average value was adopted for the subsequent analysis.
Surgical Technique Surgeries were performed with routine techniques with patients in the lateral position. All procedures were either performed by or supervised by 1 of the senior authors (S.F.). We used MIS Modified Hardinge approach or MIS Modified Watson-Jones approach with a skin incision of ⭐8 cm. All THAs were performed with cementless cup (Plasmacup; B. Braun Aesculap) and stem (Bicontact; B. Braun Aesculap). For cup positioning, the inclination and anteversion angles were targeted at 40⬚ and 15⬚, respectively. Thus, optimal cup alignment within the “safe zone” proposed by Lewinnek was assured with adequate bony coverage and press fit stability of the cup in each case.20 However, for 7 elderly patients with considerable posterior pelvic tilt, the anteversion angle of the cup was intentionally reduced to ⬍5⬚. Computed Tomography Evaluation of Cup Alignment Postoperative CT scan was done with the 2 skin markers attached to the bilateral anterosuperior iliac spines with
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Definition of Acetabular Orientation In the intraoperative navigation, the inclination and anteversion angles of the cup were presented according to the radiographic definition described by Murray.7 In the comparison of intraoperative and postoperative values, anatomic angles obtained by CT measurement were converted to the angle in the radiologic definition and compared with the intraoperative value indicated by the navigation system.
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50
30
25 45
*
P< .001
20
*
40
P < .001
15
Avg 40.1˚63.8˚ 35
(range, 30.6˚250.0˚)
Avg 41.3˚64.0˚ 10
(range, 31.6˚255.0˚)
5
30
Avg 14.0˚64.0˚ (range, 4˚2524.6˚)
Avg 16.3˚64.9˚ (range, 5.0˚235.4˚)
0
Intraoperative navigation data
Postoperative CT data
3
Figure 3: Inclination angle in the intraoperative and postoperative evaluations.
Intraoperative navigation data
Postoperative CT data
4
Figure 4: Anteversion angle in the intraoperative and postoperative evaluations.
5 Figure 5: Postoperative cup anteversion and inclination in each patient. Range of Lewinnek’s “safe zone” is shown by the solid square.
Assessment of Factors Affecting Intraoperative Assessment Regarding the factors that may affect the accuracy of the intraoperative assessment in the navigation system, body mass index (BMI) and thickness of the soft tissue over the bony landmark were adopted as variables for analysis. In the qualitative evaluation of the thickness of the intervening soft tissue, the distance between the pubic symphysis and the corresponding skin surface was measured in the CT axial image. Then, correlation between the BMI and the soft tissue thickness values and discrepancy between the intraoperative and postoperative cup orientation values were analyzed. Statistical Analysis For correlation analysis, Spearman’s rank correlation coefficient was calculated.
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In the comparison between the intraoperative and postoperative values in each case, Wilcoxon signed-rank test was used to assess any differences. Comparison between the patient groups was made with MannWhitney U test. In all analyses, P⬍.05 was considered statistically significant.
RESULTS Agreement of Intraoperative and Postoperative Cup Alignment Angles In comparison of the acetabular inclination, the mean inclination angle as indicated by the navigation system was 40.1°⫾3.8° (range, 30.6⬚-50⬚), whereas the mean value determined by postoperative CT measurement was 41.3°⫾4.0⬚ (range, 31.6⬚-55⬚). In the comparison of intraoperative navigated and postoperative CT evaluated values, significant difference was observed between 2 groups,
but a discrepancy of ⬎5⬚ was observed only in 1 hip. The mean difference between the intraoperative and postoperative values was 1.2⬚ (Figure 3). For the anteversion, the mean intraoperative and postoperative values were 13.0⬚⫾5.2⬚ (range, 0⬚-24.6⬚) and 15.4⬚⫾5.7⬚ (range, 1⬚-35.4⬚), respectively. The 2 angles obtained by intraoperative and postoperative assessments corresponded well (P⬍.001). In the comparison of intraoperative navigated and postoperative CT evaluated values in each case, although the magnitude of the difference was small, the postoperative value was higher than the intraoperative value in 89 of the 115 cases (77.3%). As a result, the postoperative anteversion angle was statistically significantly higher than the intraoperative value (P⬍.05). However, a discrepancy of ⬎5⬚ was observed only in 3 hips. The mean difference between the intraoperative and postoperative values was 2.3⬚ (Figure 4). In the assessment of the final cup positioning based on the Lewinnek “safe zone” criteria, data from 7 patients in whom the intraoperative anteversion was intentionally set at less than 5⬚ were excluded. In the remaining 108 hips, 2 hips (1.9%) were deemed to be outside the “safe zone” (Figure 5). Analysis of Factors Affecting Precision Measured BMI and soft tissue thickness (STT) values ranged from 17.2 to 34.2 (mean, 23.7) and 17.8 to 65.5 mm (mean, 33.1 mm) respectively. In the correlation analysis, no correlation was observed between either BMI or STT and the intraoperative and postoperative discrepancy in cup inclination and anteversion angles (BMI, r ⫽ ⫺0.036; STT, r ⫽ 0.129). Additionally, mean BMI and STT values were compared between the patients with an intraoperative and postoperative anteversion discrepancy of ⭓4⬚ and those with the discrepancy of ⬍4⬚. Average BMI and STT were 21.9⫾4.3
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(17.2-34.2) and 36.2⫾11.4 mm (14.864.5 mm) in 22 cases with an indicated discrepancy of ⭓4⬚ (4⬚-19.6⬚), whereas the corresponding values were 20.4⫾4.1 (17.2-32.7) and 35.5⫾13.2 mm (17.064.7 mm) in 93 cases with discrepancy of ⬍4⬚. Consequently, no significant difference in the BMI and STT values was noted between the 2 groups.
DISCUSSION Use of a computer-assisted navigation system in THA procedure has been shown to help achieve consistent and accurate positioning of the acetabular component in THA.1,2,6,21 Among the systems used in the current procedure, the image-free system has the clear advantage of not requiring preoperative CT examination. However, its accuracy and reproducibility are concerns because the 3-dimensional alignment is determined based on anatomic landmarks located by palpation. Accuracy analysis of the image-free navigation system has been performed in in-vitro studies using cadaver and anatomic models. Blendea et al22 analyzed the accuracy of this navigation system using pelvic phantoms and showed the error to be ⬍5⬚ both in inclination and anteversion. By contrast, Spencer et al23 assessed the intraobserver and interobserver reliability in a cadaveric study and showed substantial interobserver variability, especially in the assessment of anteversion, raising a question about the measurement accuracy of this system. Clinical studies examining the performance of the image-free navigation system have also been reported. In a majority of these studies, the navigation system has been shown to be effective in achieving more consistent acetabular cup alignment compared with the mechanically guided procedure.3,17,24 Moreover, Kalteis et al10 compared the CT-based and image-free systems in a clinical series and showed comparable results for both systems.
In our study, we evaluated postoperative cup positioning using postoperative CT data to assess the accuracy of the navigation system in our clinical series. In the assessment of 3-dimensional CT images, we used the MIPAV program developed by NIH. Shibanuma et al15,16 showed that this image analyzing program could effectively support the spatial transformation of an image set from the fixed imager coordinate system to an MIPAV-derived analysis coordinate system, thus enabling consistent and accurate assessment of 3dimensional images. The results of the image analysis in this study showed good agreement between the intraoperative navigation results and the postoperative values derived from CT evaluation. Discrepancy of ⬎5⬚ was observed in only 1 patient for inclination and in 3 patients for anteversion. Regarding the factors affecting the accuracy of the image-free navigation system, the presence of soft tissue between the skin and the bony landmark has been suggested to be a major source of registration error.19 Thus, effect of the subcutaneous tissue thickness on the intraoperative assessment error was analyzed using BMI and soft tissue thickness as parameters. Our results showed no correlation between the calculated values of these parameters and the intraoperative and postoperative assessment discrepancy. Only 1 patient in this study had an intraoperative and postoperative discrepancy of ⬎10⬚; this patient’s BMI and STT values were 34.2 and 64 mm. Thus, for patients with marked obesity, surgeons should be aware of the possibility of assessment error. In the comparison of intraoperative and postoperative values, the postopertave anteversion was shown to be higher than the corresponding intraoperative value in 89 of 115 cases (77.3%) with a mean difference of 2°. Richolt et al18 measured the thickness of subcutaneous tissue in their clinical cases
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and showed that the navigation system would have underestimated the anteversion by 2.8⬚ on average. Our result appears to correspond with their results. In the use of this system in evaluating the cup anteversion angle, this tendency (underestimation by a few degrees regardless of the thickness of the subcutaneous tissue) should be taken into consideration. Attempts to reduce this potential source of error in determining the reference points should be made to further improve the accuracy and reproducibility of the image-free navigation system. Recently, Kiefer and Othman11 developed an ultrasound palpation method for registration to manage these problems. However, further studies are still required to improve the accuracy of this system. Several issues remain. First, desirable cup alignment can be different for each patient, especially for those with pelvic tilt and inclination in a standing position, although basically uniform alignment angles are targeted in all cases. Second, only the acetabular position was investigated in this study, and the relative relationship between the cup and stem is also a matter of clinical significance.
CONCLUSION This study showed good agreement between the intraoperative values presented by the navigation system and those in the postoperative CT evaluation. Even though a slight underestimation in the intraoperative assessment of anteversion angle by navigation was demonstrated, this intraoperative and postoperative discrepancy (average, 2°) was consistent regardless of the thickness of the soft tissue.
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