Navigated non-image-based registration of the position of the pelvis during THR. An accuracy and reproducibility study

Abstract

The precise recording of the position of the pelvis is a prerequisite for total hip replacement (THR). The anterior pelvic plane is an accepted reference for determining the 3D pelvic orientation. We hypothesized that cutaneous palpation of this plane was accurate and reproducible.

Ten consecutive navigated implantations of THR prostheses were studied. In each case, four palpations of both anterior iliac spines and the pubic symphysis were performed with a navigated stylus. The first palpation was made on actual bone contours through a skin puncture and was considered as the reference. The other three palpations were made over the intact skin as a normal intra-operative procedure.

There was no significant difference between the pelvic orientations measured by the three cutaneous palpations, or between the orientations measured by transcutaneous palpation and the mean result with cutaneous palpation.

Cutaneous palpation can be considered as a reliable technique for the definition of pelvic orientation during THR with the non-image-based system employed.

• Total hip replacement
• pelvis position
• navigation

Introduction

The positioning of the cup of a total hip replacement (THR) prosthesis is considered critical to the short- and long-term outcomes of the procedure [1]. Precise recording of the position of the pelvis is a prerequisite for this procedure in order to obtain a reliable reference for cup orientation. It has been demonstrated that conventional, non-navigated measurements are less than optimal [2],[3], whereas CT-based navigation systems improve the accuracy of recording of the pelvic position [2]. However, a pre-operative CT scan is not regarded as a routine examination and involves additional costs. Non-image-based navigation systems might provide the same accuracy at lower costs.

The anterior pelvic plane was defined by Lewinnek as the plane containing both antero-superior iliac spines (ASIS) and the pubic symphysis [4], and it has been widely accepted as a reference for determining 3D pelvic orientation [5–8]. Other reference frames might be used [5],[9], but these have not yet been validated on a large scale. We hypothesized that intra-operative cutaneous palpation of these three relevant anatomical landmarks was accurate and reproducible.

Materials and methods

The study was approved by our local institutional board, and all patients gave written informed consent.

At our institution, we are using the OrthoPilot® system (Aesculap, Tuttlingen, Germany) (Figure 1) for THR on a routine basis. This system is non-image-based as it relies only on anatomic and kinematic data collected intra-operatively, with no pre-operative additional imaging. The operative technique has already been described elsewhere [10]. Briefly, the patient was placed in the supine position, and an infrared localizer was fixed with a percutaneous screw to the anterior iliac crest on the operated side. The first author performed four palpations of three anatomical landmarks with a navigated stylus: both antero-superior iliac spines and the pubic symphysis. The first palpation was made on actual bone contours through a skin puncture, and was considered as the reference for pelvic orientation. The other three palpations were made over the intact skin. The 3D positions of the pelvic localizer and stylus were recorded by the infrared Polaris® camera (Northern Digital, Inc., Waterloo, Ontario, Canada). The 3D pelvic orientation was calculated in the three reference directions relative to the anterior pelvic plane: inclination as a rotation along the antero-posterior axis, rotation as a rotation along the vertical axis, and flexion as a rotation along the horizontal axis.

Figure 1. The OrthoPilot navigation system.

We studied 10 consecutive navigated implantations of THR prostheses. The inclusion criterion was the navigated implantation of a THR prosthesis; there were no exclusion criteria. Preliminary power analysis showed that this number of cases was sufficient to detect a 4° difference between two measurements (with an expected standard deviation of 3°) with an α risk of 0.05 and a β risk of 0.20.

The range of the three cutaneous registrations for the same patient in all three planes was calculated. The difference between the three cutaneous registrations for the same patient in all three planes was studied with a Friedmann test at a 0.05 level of significance, with post-hoc individual comparison for all possible couples using a paired Wilcoxon t-test at a 0.025 level of significance. Paired correlation between all possible couples was studied with a Spearman correlation coefficient calculation at a 0.025 level of significance.

The mean value of the three cutaneous registrations for the same patient in all three planes was calculated and compared to the reference frame of the transcutaneous position (defined by a 0° orientation in all planes) using a paired Wilcoxon t-test at a 0.05 level of significance. Correlation between the mean value of the three cutaneous registrations and the transcutaneous position for the same patient was studied using a Spearman correlation coefficient calculation at a 0.05 level of significance.

Results

Ten patients were eligible for this study. They comprised 4 men and 6 women, with a mean age of 68 years (range: 58 to 79 years). Mean BMI was 29.3 (range: 21.5 to 42.1).

Results are presented in Figures 2 to 4, with the transcutaneous registration serving as the reference position.

Figure 2. Results for pelvic inclination. [Color version available online.]

Figure 3. Results for pelvic rotation. [Color version available online.]

Figure 4. Results for pelvic flexion. [Color version available online.]

The mean range for the three cutaneous palpations was 2° for inclination (range: 0 to 6°), 4° for rotation (range: 2 to 9°), and 1° for flexion (range: 0 to 4°). Inclination measurements were within a range of 4° for 9 of 10 cases. Rotation measurements were within a range of 4° for 8 of 10 cases. Flexion measurements were within a range of 4° for all cases.

There was no significant difference between the three measurements for one patient for each direction (inclination: p = 0.12; rotation: p = 0.245; flexion: p = 0.08). There was no significant difference in all possible couples (Tables I to III), and there was a significant correlation between all possible couples (Tables IV to VI).

Table I.  Cutaneous palpation: inclination measurements. Paired Wilcoxon t-test.

	Second	Third
First	p = 0.40	p = 0.07
Second		p = 0.06

Table II.  Cutaneous palpation: rotation measurements. Paired Wilcoxon t-test.

	Second	Third
First	p = 0.64	p = 0.41
Second		p = 0.11

Table III.  Cutaneous palpation: flexion measurements. Paired Wilcoxon t-test.

	Second	Third
First	p = 0.18	p = 0.12
Second		p = 0.75

Table IV.  Cutaneous palpation: inclination measurements. Spearman correlation test.

	Second	Third
First	rho = 0.753	rho = 0.809
	p = 0.03	p = 0.02
Second		rho = 0.803
		p = 0.02

Table V.  Cutaneous palpation: rotation measurements. Spearman correlation test.

	Second	Third
First	rho = 0.870	rho = 0.809
	p = 0.01	p = 0.02
Second		rho = 0.833
		p = 0.01

Table VI.  Cutaneous palpation: flexion measurements. Spearman correlation test.

	Second	Third
First	rho = 0.936	rho = 0.927
	p = 0.005	p = 0.005
Second		rho = 0.876
		p = 0.01

The mean difference between the measurements obtained with transcutaneous palpation and the mean cutaneous palpation result was 1° for inclination (range: -4 to 8°), 3° for rotation (range: -5 to 9°), and 1° for flexion (range: -4 to 2°). These differences were not significant (inclination: p = 0.84; rotation: p = 0.08; flexion: p = 0.76). Inclination measurements were within a range of 4° for 8 of 10 cases; rotation measurements were within a range of 4° for 6 of 10 cases; and flexion measurements were within a range of 4° for all cases. There was no significant correlation between the measurements obtained with transcutaneous palpation and the mean cutaneous palpation result for the three directions (inclination: rho = 0.515, p = 0.12; rotation: rho = 0.506, p = 0.13; flexion: rho = 0.512, p = 0.12); however, as the reference frame was defined with a 0° angle in all planes, this correlation has little relevance from a mathematical point of view.

Discussion

The positioning of the cup of a THR prosthesis is considered critical to the short- and long-term outcomes of the procedure [1]. There is no general agreement concerning the ideal reference for measuring cup orientation. The most accepted reference was defined by Lewinnek et al. [4] as the anterior pelvic plane. This is the plane containing both anterior-superior iliac spines and the pubic symphysis.

Conventional techniques involve manual orientation of the cup according to anatomical landmarks and/or pelvis positioning. However, it has been demonstrated that there are significant pelvic movements during THR [2], and that the pelvic position might be difficult to localize for use as a reliable intra-operative reference for cup orientation. DiGioia et al. [2] observed that 78% of cups were oriented outside the safe zone when they controlled the positioning of the cup intra-operatively with a CT-based navigation system. Kalteis et al. [11] found that 50% of cups were oriented outside the safe zone when they controlled the positioning of the cup post-operatively with a CT scan. There is an inherent inaccuracy associated with the conventional techniques, and navigation systems might address this issue.

The OrthoPilot® system relies on an anatomic registration of the anterior pelvic plane for definition of the pelvic orientation. Most navigation systems for THR rely on matching pre-operative CT images with intra-operative registration of points or surfaces [2]. However, a CT scan is not recognized as a routine pre-operative investigation, and these CT-based techniques result in additional radiation exposure for the patient and additional cost for the healthcare system compared to the standard manual technique of THR. In contrast, non-image-based systems rely on fully intra-operative registered data, and their use involves no additional pre-operative costs.

A navigation system will only be effective if the reference frame can be defined with confidence. Cutaneous palpation of the pelvic anatomic landmarks might be inaccurate because of the distance between the skin surface and the real bone contour. Richolt et al. [12] calculated that the mean error in the measurement of flexion of the pelvis was 3° because of the thickness of the soft tissue. Furthermore, there might be inaccuracy in defining the actual top of the ASIS. However, the technique used is still robust, as a 10-mm error in the location of the top of the ASIS induces only a 3° error in inclination and a 4° error in flexion [13].

We observed that the intra-operative palpation of this plane by a cutaneous technique was reproducible, with a mean variation of less than 5° between different registrations in all three directions. These small variations are probably clinically irrelevant. In particular, the larger variation in the rotation measurement might be irrelevant, as the intra-operative measurement of cup orientation relies mainly on inclination and flexion. The cutaneous technique can be considered to be reproducible.

We also observed that there was no significant difference between results obtained with this cutaneous palpation technique and those obtained by palpation of the actual bone contours using a transcutaneous technique. For the two directions considered to be critical during an operation (inclination and flexion), 8 of 10 and 10 of 10 cases, respectively, had a range of variation of less than 5°. These variations are probably clinically irrelevant too. There is little doubt that direct palpation of the bone coutours is an accurate way to define the anterior pelvic plane. There is currently no generally accepted reference technique for intra-operative definition of pelvic orientation. We believe that this technique of direct bone palpation can be considered as a true reference technique. It thus makes sense to compare the orientation of the pelvis as measured with this transcutaneous technique to the orientation measured with the conventional cutaneous technique in order to study the accuracy of the latter technique. As there was no significant or clinically relevant difference between the transcutaneous and cutaneous definitions of pelvic orientation, we consider that the cutaneous technique is also accurate.

There are still some weak points in this study. We did not address the issue of inter-observer variation, and a second study has been designed to control for this specific aspect. We also made no attempt to measure the thickness of the fat tissue, which would enable our results to be compared to those from a previously published study [12]; however, the small range of variation observed in our study leads us to believe that this factor may have less importance than expected.

Conclusion

Cutaneous palpation can be considered as a reliable technique for the definition of pelvic orientation during implantation of THR prostheses with the non-image-based system used in this study.