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Ultrasound-based Navigation and 3D CT Compared in Acetabular Cup Position

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Abstract

Intraoperative landmarks are used in image-free navigation systems. The ultrasound-based navigation systems try to overcome the problems of positional deviation associated with soft tissue. Our study analyzed the accuracy of ultrasound-based navigation of cup positioning compared with postoperative 3-dimensional (3D) computed tomography scans of cup positioning. Twenty-five ultrasound-navigated total hip arthroplasties (THAs) were analyzed. The difference between the intraoperative cup orientation (navigation) and the postoperative cup position (CT) was evaluated. The average difference between intraoperative navigation and postoperative CT measurements was 2.8° (SD±1.8°) for abduction and 2.2° (SD±1.6°) for anteversion. Therefore, we recommend ultrasound-based navigation as an exact tool for cup positioning in THA.

The longevity and clinical outcome after total hip arthroplasty (THA) depend on a large number of surgery- and implant-related factors.¹ Malposition of the acetabular component is a common surgical problem and can lead to hip dislocation after implantation, limit the range of motion, increase wear, and result in early loosening.^1-3 To avoid malpositioning, computer-assisted navigation systems have been developed.^4-7 Computer-assisted hip navigation uses the anterior pelvic plane as a pelvic coordinate system to determine the acetabular cup orientation. This coordinate system is defined using pelvic bone landmarks, the anterior-superior iliac spines, and the pubic symphysis. The landmarks define the anterior frontal plane of the pelvis.³ The landmarks are acquired by manual palpation through the overlying soft tissues. This step is a potential failure source and may influence the reliability of imageless navigation, especially in obese patients.⁸ Acquisition of landmarks by ultrasound promises to be beneficial in improving accuracy, so there is a need to evaluate the accuracy of intraoperative readings for the implant orientation obtained with the imageless computer navigation method.

A number of previous studies^4,6,9-12 have revealed that computer-assisted cup positioning allowed for more consistent placement compared with the conventional manual procedure. These studies were generally focused on the comparison of the acetabular cup orientation achieved using the navigation method with that using a conventional manual method. However, there is little information on the accuracy of the navigation system itself. Previous studies have used postoperative radiographic measurements to analyze the acetabular cup orientation. There has been wide variation in evaluation of the acetabular cup orientation when only radiographs were used.⁴ Because of the unknown influence of the patient’s position and pelvic tilt on the results of radiography, the reliability of such two-dimensional (2D) radiography measurements is still limited.^3,4 Radiographic measurement for evaluating cup orientation is not a reliable method, especially for cup anteversion.⁴ The use of computed tomography (CT) has the potential to provide more reliable assessment of acetabular cup position with a 3-dimensional (3D) analysis. Our study analyzed the accuracy of ultrasound-based navigation of cup positioning during surgery compared with postoperative CT scans. To avoid projection-related imaging errors in measurements of conventional 2D radiographic and CT images, we evaluated the position of the cup by 3D reconstructions of the 2D CT images. The goal of this study was to compare the use of ultrasound-assisted navigation of cup positioning with CT images.

Materials and Methods

Twenty-five patients were evaluated. No patient was excluded. All patients were implanted with the same kind of cementless cup (Plasmacup, Polyethylene-Layer; B. Braun Aesculap, Tuttlingen, Germany) using a minimally invasive operating technique (anterolateral approach in the supine position). Furthermore, the same surgeon performed all operations. All patients underwent CT scan of the pelvis between postoperative day 5 and postoperative day 10.

The navigation system OrthoPilot with the imageless software THAplus (B. Braun Aesculap) was used in all cases. The relevant bone points (anterosuperior iliac spines and pubic symphysis) to establish a coordinate system (anterior pelvic plane [APP]) were determined by ultrasound. With the navigation system, trackers were rigidly fixed only on the iliac crest. Using the bone landmarks that define the coordinate system and the position of the instruments and implants, the computer software calculated the position and orientation of the implants. The orientation of the acetabular cup (anteversion and abduction) was then determined in real time and provided to the surgeon.

Evaluation of Abduction and Anteversion

Three-dimensional implant position was determined by CT. In accordance with a standard protocol, we acquired scans of the pelvis in 1-mm-thick slices.

We developed a tool in AMIRA software (Mercury Computer Systems, Chelmsford, Massachusetts) for acquiring the abduction and anteversion of the acetabulum from CT image slices. For a reference system, the left and right anterosuperior iliac spines (LASIS and RASIS) and the pubic symphysis were identified to ultimately calculate the APP (Figure 1). To calculate the APP, the normal vector (X in Figure 2) on the plane is the cross-product of the vector, pointing from LASIS to RASIS and from LASIS to the pubic symphysis. Moreover, the 2 in-plane vectors of the APP were assigned to be 1 normalized transversal vector pointing from LASIS to RASIS (Y in Figure 2) and 1 normalized vector pointing cranially (Z in Figure 2), which was calculated as the cross-product of the transversal vector and the plane normal.

Figure 1: Determination of the left and right anterosuperior iliac spines (LASIS and RASIS) and the pubic symphysis to calculate the anterior pelvic plane.

Figure 2: Determination of the coordinate system from the anterior pelvic plane.

The orientation of the acetabular cup was determined by acquiring a sufficient but undefined number of points on the acetabular cup rim to recreate the plane of the socket face (Figure 3). A least-square-approximated plane was fitted through these points and the plane normal to the acetabular cup rim was then calculated. This vector n (Figures 4, 5) was orthogonally projected onto 2 planes: (1) the APP, which is defined by the vectors X and Y (Figure 2), and (2) the sagittal plane spanned by the vectors Y and Z (Figure 2).

Figure 3: Orientation of the acetabular cup determined by acquiring points on the acetabular cup rim.

Figure 4: Determination of the anteversion.

Figure 5: Determination of the abduction.

The angle between the acetabulum’s normal n and its projection on the APP is the anteversion (the angle ? in Figure 4) and the angle between the acetabulum’s normal n and the sagittal plane is the abduction (the angle ? in Figure 5). The APP (Figure 1) and the socket face plane (Figure 3) recreate an image of corresponding planes (Figure 6).

Figure 6: Orientation of the acetabular cup relative to the anterior pelvic plane.

Statistical Analysis

The arithmetic mean, standard deviation, and distribution of values were determined for the abduction, the anteversion, and the single errors between the different measures. Results were considered to be significant at P<.05 and were highly significant at P<.005. All statistical analyses were performed using SPSS 11.5 (Statistical Package for Social Sciences, Inc, Chicago, Illinois).

Results

Mean age of the 25 patients (19 women; 6 men) was 66.1 years. In all cases, primary osteoarthritis of the hip joint was the reason for surgery. There was no severe deformity of the hip joint or rheumatoid arthritis in this group. We did not evaluate body mass index (BMI) or thickness of soft tissue in this study.

The Table shows the arithmetic mean, standard deviation, and distribution of values of abduction and anteversion error of intraoperatively displayed and postoperatively measured cup position.

Analysis of all navigation protocols shows a mean abduction of 43.7° (range, 34°-49°; SD±4.5°). Postoperative 3D analysis shows an abduction of 44.8° (range, 37.4°-51.5°; SD±3.5°). The mean error between these methods was 2.8° (range, 0.3°-6.5°; SD±1.8°).

Analysis of all navigation protocols shows a mean anteversion of 24.8° (range, 19°-33°; SD±3.3°). Postoperative 3D analysis of anteversion of cups shows an angle of 25.6° (range, 16° - 35°; SD±4.1°). There was a mean difference of 2.2° (range, 0.19-7; SD±1.8°).

In terms of the general accuracy (±10° outside the range of the desired goal), navigation methods had a total of 2 (8%) outliers in anteversion and 2 (8%) in abduction.

Furthermore, there were no postoperative complications such as dislocations of the hip, loosening of implants, or local complications.

Discussion

Computer-assisted navigation systems have been demonstrated to be valuable in reducing the standard deviation of positioning of the acetabular component in THA.^4,5 However, the accuracy of these systems is directly dependent on the data input into the computer. The percutaneous acquisition of the APP seems to be a source of error and may lead to inaccuracy in the final alignment of the acetabular component.⁸ Therefore, we used the ultrasound-based acquisition of the bone APP to limit the effect of the surrounding soft tissue; however, there is little information about the error of this method.

The aim of this study was to compare cup positioning between intraoperative navigation (ultrasound-based image-free navigation) and postoperative 3D CT.

Tannast et al¹³ reported postoperative measurement of the cup position using conventional radiographs; measurements of anatomic angles differed significantly when the APP was not used as the reference. To overcome these discrepancies, the APP could be measured intraoperatively by ultrasound-based acquisition of the bone landmarks, and postoperatively the APP could be used as the reference for CT-based measurement of the cup position.

Many studies about navigation in THA have used pointer-based navigation for acquiring these landmarks.^12,14 It is well known, however, that the pointer-based acquisition of the APP is influenced by the thickness of subcutaneous soft tissue, which is described as the “cutaneous Lewinnek plane.”¹⁵

Parratte and Argenson¹⁵ reported about deviation of pointer-based navigation and “free-hand” cup positioning, comparing their measured cup position to the “safe zone” of Lewinnek.³ They found 20% outliers in the navigation group and 57% outliers in the “free-hand” group.

Poor correlation between intraoperative and postoperative cup position in the pointer-based navigation group was found when patient BMI was >27.¹⁵

In our study, we found 8% outliers, thus demonstrating the greater precision of the ultrasound-based navigation system.

The limitation of this study was that we did not evaluate the effect of the BMI on the postoperative cup position and the resulting error.

Using a kinematic model, Wolf et al¹⁶ demonstrated that a minor error in correctly identifying the anatomic landmarks can lead to improper alignment of the acetabular component. An error in the frontal plane directly modifies the angle of abduction, and an error in the sagittal plane affects the angle of anteversion.¹⁶

A total error of 4 mm in measuring the anterosuperior iliac spine and the pubic tubercles would result in a final cup position error of 2° abduction and 7° anteversion.¹⁶

These errors in the kinematic model can result in large differences in the final cup alignment, which should be considered when using pointer-based acquisition of the APP.¹⁶

An in vitro study evaluated the error of percutaneous and ultrasound-based computer-assisted THA in 2 cadavers. ¹⁷ The researchers found a mean version error of the APP of 6.2° with the ultrasound acquisition and a significantly higher version error with percutaneous methods (16.2°).¹⁷ However, errors in the final position of the cup were not evaluated by analyzing anteversion and abduction.¹⁷ To avoid this error with percutaneous methods, ultrasound systems were developed to take advantage of ultrasound visualization of bone landmarks and to make it possible to acquire landmarks regardless of obesity.

Kalteis et al¹⁸ reported about significant reduction of outliers in a prospective randomized study in cases using pointer-based or CT-based navigation systems. In a previous study,¹⁰ we showed similar results in a comparison of pointer-based and ultrasound-based navigation. A disadvantage was that only radiographs for evaluating anteversion and abduction were used. All studies are focused on comparing techniques or different systems, but there is little information about the accuracy of the systems. Lin et al¹⁹ validated pointer-based navigation in a cadaver study. They found a deviation between intraoperative and postoperative anteversion up to 2.1° and between intraoperative and postoperative abduction up to 3.2°. Blendea et al²⁰ reported about deviation between 2.1° and 16.7° in abduction.

Our analysis of accuracy of the ultrasound-based navigation system revealed only a small, clinically tolerable deviation in cup anteversion (2.2°) and cup abduction (2.8°) between intraoperatively displayed and postoperatively measured cup position. The evaluated image-free navigation system appears to be a practical and reliable alternative to the pointer-based navigation system for placing acetabular cups in THA.

Conclusion

Most surgeons cannot position the components of a total hip prosthesis perfectly every time. Malposition has been linked to early failure and postoperative complications. Conroy et al²¹ reported about relevant early dislocation rate after THA. Malposition of implants is one of the most important risk factors. For this study, more than 65,000 THAs were evaluated. Cup position in regard to wear is a relevant factor. Perka et al²² reported on the significant increase of wear when cup inclination was >45°. Because of increased wear, we must be aware of higher rates of osteolysis and earlier loosening of implants. In 8% of all revision cases of acetabular components in the United States, periprosthetic osteolysis is the reason for revision surgery.²³ Bearing surface wear accounts for another 8%. In 24% of cup revision cases, mechanical loosening is described. Therefore, we can conclude that malposition and wear are important factors in failure of THA. Correct position of implants is much more important for mechanical stability and survival of implants, especially because of the increasing number of younger and active patients.

Computer-assisted navigation technology is a new tool that enables the surgeon to accurately measure, in real time, intraoperative implant alignment. Our study confirms the usefulness of ultrasound acquisition for THA navigation and shows only small errors between intraoperatively displayed and postoperatively measured cup position.

References

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Authors

Drs Hasart, Perka, and Wassilew are from Charité Orthopaedic Department, Center for Musculoskeletal Surgery. Mr Poepplau is from Julius Wolff Institute, and Dr Asbach is from Charité Department of Radiology, Charité Universitätsmedizin, Berlin, Germany.

Drs Hasart and Perka are consultants for Aesculap. Mr Poepplau and Drs Asbach and Wassilew have no relevant financial relationships to disclose.

Correspondence should be addressed to: Olaf Hasart, MD, Orthopaedic Department, Charité, Charitéplatz 1, 10117 Berlin, Germany.

doi: 10.3928/01477447-20090915-50