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Ultrasound vs Pointer Palpation Based Method in THA Navigation: A Comparative Study

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Abstract

Intraoperative registration of the anterior pelvic plane is necessary for navigation during total hip arthroplasty (THA). Ultrasound referencing of bony landmarks was implemented in a THA navigation system (OrthoPilot, B. Braun Aesculap; Tuttlingen, Germany) as an alternative to pointer palpation. This study compared the accuracy of manual pointer palpation with ultrasound registration for determining anterior pelvic plane registration in a consecutive series of 37 cases. Findings showed postoperative radiographic anteversion was lower than ultrasound and pointer navigated anteversion. Because ultrasound registers the bony landmarks of the anterior pelvic plane with a high degree of accuracy, we conclude radiographs 2 weeks post-operatively are not suited to evaluate anteversion, because they still display a pelvic tilt due to a pre-operative flexion contraction.

Navigation systems are designed to measure and display several parameters of implant position and to support the surgeon during bone preparation and implant alignment. Studies have shown the use of navigation systems reduces the variability of cup position.^1-4

Navigation systems for total hip arthroplasty (THA) require intraoperative registration of the anterior pelvic plane as a reference for cup implantation.^5,6 Commonly, the manual palpation technique is used to reference the anterior pelvic plane with a pointer.⁷ However, the precision of this technique is questionable, especially in obese patients.⁸

In our practice, ultrasound referencing of bony landmarks has been implemented in our THA navigation system (OrthoPilot, B. Braun Aesculap; Tuttlingen, Germany) as an alternative to pointer palpation. The purpose of this study was to compare the accuracy of anterior pelvic plane registration obtained with manual pointer palpation to that obtained with ultrasound registration.

Materials and Methods

The OrthoPilot navigation system references the instruments with a 3-dimensional infrared-camera with passive tracking technology. Using the OrthoPilot THA workflow version 2.1 with an acetabular bone sensor reference at the anterior iliac spine and a femoral c-clamp sensor at the greater trochanter,⁹ we performed 40 consecutive cementless THAs with a straight hip stem and a press-fit acetabular cup. Patient position was supine and a standard lateral approach was used. Special software assisted in the acquisition of bony landmarks of the anterior pelvic plane using either pointer palpation or ultrasound palpation. For ultrasound registration, the shape of the spina iliaca anterior and the os pubis are visible on the navigation screen. These three points that define the anterior pelvic plane are registered in three consecutive steps by marking the bone references with a virtual pointer (Figure 1).

All navigation data obtained with either pointer palpation or ultrasound palpation are displayed on the navigation screen. All intraoperative data are stored in a file that is available postoperatively for comparison with the preoperative plan and the postoperative radiographs. The cup position was measured as described elsewhere.¹⁰

Radiographs were taken prior to patient discharge from the hospital. Intraoperative cOrthoPilot data (pointer or ultrasound registration) were compared to the radiographic cup inclination and anteversion postoperatively.

Results

Of the 40 consecutive cases, 37 records had complete data sets and were analyzed. Mean patient age was 71.2 years (range, 45-85 years); 65% of the patients were women. Mean body mass index (BMI) was 28.2 (range, 19-38). Average operative time (complete navigated THA procedure for cup and stem implant) was 108 minutes (range, 60-59 minutes).

There were no early implant-related postoperative complications, defined as dislocation or implant instability at hospital discharge. There were no complications associated with the navigation procedure.

All acetabular cups were positioned within a desired safe zone of 42.5±10° radiographic inclination and 12.5±10° radiographic anteversion (Figure 2). During surgery, navigation data (Figure 3) for cup position were chosen outside the safe zone in one case with pointer palpation (inclination<35°) and in three cases with ultrasound registration (anteversion>22.5°).

The standard deviations of navigated data for pointer and ultrasound registration were similar with radiographic measurements, ranging between 2.6° and 4.2° for inclination, and between 3.4° and 5.3° for anteversion (Table). There was no influence of the BMI on the results or accuracy of pointer or ultrasound registration.

Comparison of the values for inclination revealed no visible differences (Figure 4A). However, the Wilcoxon signed rank test demonstrated ultrasound values were significantly higher compared with pointer values (P=.004).

Comparison of the values for anteversion demonstrated statistically significant differences (Figure 4B). Ultrasound anteversion was significantly higher than pointer anteversion (P=.000001), and pointer anteversion was significantly higher than radiographic anteversion (P=.0001). Radiographic anteversion generally was lower than ultrasound anteversion (P=.000001).

Ultrasound registration showed significantly lower external and internal free range of motion (ROM) after navigated stem implantation than pointer registration (P=.000001). Ultrasound values were closer to the expected ratio of 2:1 for external:internal free ROM compared with pointer palpation. Ultrasound data also demonstrated a lower standard deviation.

Discussion

Registration of the bony landmarks of the anterior pelvic plane in cup or stem THA navigation represents the current standard of technology.¹¹ The OrthoPilot navigation system has been in clinical use since 2001 for cup navigation and since 2005 for stem navigation.⁹

According to our experience with more than 1500 navigated hip replacement procedures, palpation of the anterior pelvic plane with a pointer requires practice. Pointer palpation can be used routinely in surgeries with patients in a supine position. However, in obese patients we have observed less precision in cup anteversion measurements, due to difficulties with the pointer palpation of the pubic bone. Postoperative radiographic anteversion sometimes is reduced compared with the intraoperative navigated data.

Cup position measurements on standard radiographs provide only limited accuracy.^12,13 In particular, measurement of cup anteversion is unreliable if the pelvis is in a tilted position. Therefore, the anterior pelvic plane is not parallel to the radiograph. Correction of pelvic tilt is theoretically possible, but there is no standard procedure to validate all navigated data. The use of computer-assisted tomography is possible but unnecessarily exposes patients to high amounts of radiation.

In this study, a direct comparison of pointer and ultrasound registration was performed to determine whether certain aspects of the palpation procedure could be improved using ultrasound technology. The accuracy of ultrasound is approximately five times higher than pointer palpation and is not influenced by soft-tissue coverage. In fact, more soft tissue supports the visualization of bone shape with ultrasound technology.

The finding that BMI had no influence on the accuracy of anterior pelvic plane reference may be explained by our experience and familiarity with pointer registration. The navigated data for cup inclination were similar for the pointer and ultrasound methods. Ultrasound registration of inclination had less standard deviation and higher inclination values than pointer registration (P=.004).

We found statistically significant differences in cup anteversion data. The ultrasound registration showed increased values compared with pointer registration (P=.000001) and radiographic anteversion (P=.000001) postoperatively.

The accuracy of radiographic anteversion measurements on postoperative radiographs was limited, as postoperative pelvic tilt was still present because of preoperative flexion contraction. Examination of patients after longer follow-up would lead to improved radiographic analysis. In addition, better positioning of patients during radiography might influence these results.

The free ROM data from the navigation system also were analyzed. Typically, the ratio of external and internal ROM, which should be approximately two, can be used as an indirect test of system and application accuracy. Using this standard, ultrasound referencing led to more reliable ROM data of the navigation system.

The high referencing precision of ultrasound technology and the remaining inaccuracies of preoperative and postoperative radiographic analysis may contribute to a better understanding of pre- and postoperative pelvic tilt.

Conclusion

Ultrasound and pointer registration yielded comparable results for cup inclination values. Anteversion on postoperative radiograph was lower than ultrasound navigated anteversion.

Since ultrasound registers the bony landmarks of the anterior pelvic plane with a high degree of accuracy, we conclude that radiographs 2 weeks postoperatively offer a contortion, because they still display a pelvic tilt due to a pre-operative flexion contraction.

References

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Authors

Drs Kiefer and Othman are from the Department of Orthopaedic and Trauma Surgery, Lukas Krankenhaus, Bünde, Germany.

Correspondence should be addressed to: Prof Dr Hartmuth Kiefer, Dept of Orthopaedic and Trauma Surgery, Lukas Krankenhaus, Hindenburgstraße 56, D-32257 Bünde, Germany.

Dr Kiefer is a consultant for and is a member of the speaker’s bureau of B. Braun Aesculap. Dr Othman has no financial relationships to disclose.