Anterior pelvic plane registration accuracy and cup position measurement using ultrasound- and pointer-based navigation in primary total hip arthroplasty

Abstract

BACKGROUND:

Incorrect cup positioning in primary total hip arthroplasty is known as a risk factor for early implant failure. The use of navigation systems leads to more accurate cup positioning.

OBJECTIVE:

The aim of this study was to compare the registration accuracy of the anterior pelvic plane and the measurement accuracy of the definite cup position for a pointer computer-assisted orthopaedic navigation system (P-CAOS) and an ultrasound-based navigation tool (US-CAOS) in an intra-individual study design.

METHODS:

Anterior pelvic plane registration was performed in 44 patients receiving a primary total hip arthroplasty with P-CAOS and US-CAOS. The cup implantation was performed using US-CAOS. Intraoperatively, the cup position was assessed using P-CAOS and US-CAOS. The postoperative cup position was determined via CT scan. Inclination and anteversion errors were calculated using intraoperative values and CT data. All operations were performed by a single, high-volume surgeon using a minimally invasive anterolateral approach.

RESULTS:

The mean inclination error was 0.9 ${}^{\circ}$ in the US-CAOS group and $-$ 1.1 ${}^{\circ}$ in the P-CAOS group. This was not statistically significant. The mean anteversion error was significantly reduced ( $p<$ 0.001) in the US-CAOS group (1.4 ${}^{\circ}$ ) compared to the P-CAOS group ( $-$ 8.0 ${}^{\circ}$ ). Significantly more cups (23 of 44; 52%) in the P-CAOS group were outliers regarding to the defined anteversion error range of 15 ${}^{\circ}$ $\pm$ 10 ${}^{\circ}$ . Outliers in the US-CAOS group amounted to two (of 44; 5%) ( $p<$ 0.001). The number of outliers regarding the inclination error range of 40 ${}^{\circ}$ $\pm$ 10 ${}^{\circ}$ , did not differ significantly between the P-CAOS (2; 5%) and US-CAOS (1; 2%) group.

CONCLUSION:

We were able to show a systematic anterior pelvic plane registration error in this intraindividual study design. US-CAOS based APP landmark registration showed to be significantly more precise compared to P-CAOS registration. The anteversion error of the cup using US-CAOS showed to be significantly reduced compared to the P-CAOS method.

Keywords

Primary total hip arthroplasty ultrasound-based navigation pointer-based navigation anterior pelvic plane registration error anteversion error inclination error orthopaedic navigation system cup positioning

1. Introduction

Accurate cup positioning is essential for achieving a good outcome in total hip arthroplasty (THA). An incorrect cup position can lead to an increased rate of early complications [1] such as a reduced range of motion (ROM), impingement, an increased rate of dislocation and furthermore to premature wear of the tribological pairing [2]. Depending on the type of bearing utilized, impingement can cause increased polyethylene wear, elevated serum ion levels with resulting adverse local tissue reaction, or inlay fracture of ceramic inlays as well as component dislocation [2, 3, 4].

With the aim of reducing above complications, various target zones were defined for cup positioning [5, 6, 7, 8]. The most frequently cited is the “safe zone” as described in 1978 by Lewinnek et al. [8]. The safe zone is defined by conventional radiological measurement with an inclination of 40 ${}^{\circ}$ $\pm$ 10 ${}^{\circ}$ and anteversion of 15 ${}^{\circ}$ $\pm$ 10 ${}^{\circ}$ [8]. Historically, a cup position within the Lewinnek safe zone [8] was targeted to reduce the rate of dislocation. However, current literature has shown that a cup position within the Lewinnek safe zone, even if targeted, is achieved in less than 50% of cases with free-handed implantation [9, 10]. Furthermore, cup positioning in the zone defined by Lewinnek does not preclude dislocation of THA [11]. Despite this, achieving a prior to surgery defined implant position remains essential.

Orthopaedic navigation systems were developed with the aim of achieving a more accurate and reproducible implant positioning [12]. Navigated THA is able to achieve a significant reduction in the number of cup outliers in comparison to free-handed implantation [12]. Pointer (P-CAOS) and ultrasound-based computer assisted orthopaedic navigation systems (US-CAOS), both “Non-Robotic” devices and not requiring preoperative imaging, are common tools in use. In the P-CAOS and US-CAOS navigation procedures, the anterior pelvic plane (APP), defined by the percutaneously palpable landmarks of both anterior superior iliac spines (ASIS) and the symphysis pubis (SP), serves as reference point for the navigation system throughout surgery. However, navigation systems can produce errors in the definite cup position, mainly due to registration errors of the APP.

Several studies showed that the incidence of cup malpositioning increases significantly in patients with a body-mass index (BMI) $>$ 27 kg/m ${}^{2}$ despite the use of navigation tools. This is indebted to an imprecise registration of the APP landmarks through the overlying soft tissues over the symphysis pubis (SP) [13, 14]. For this reason, US-CAOS based navigation was developed to visualize the bony APP landmarks, thus significantly improving the precision of the APP registration [14]. An earlier prospective randomized study was able to demonstrate the superiority of US-CAOS, in terms of a significant reduction in outliers, compared to the P-CAOS method [15] in an interindividual study design.

To the best of our knowledge, there has not been a prospective study published to date to investigate P-CAOS and US-CAOS workflows in an intra-individual study design.

The aim of this study was therefore to compare the registration accuracy of the anterior pelvic plane and the measurement accuracy of the definite cup position in one individual using both a P-CAOS and US-CAOS based navigation system.

Figure 1.

Intraoperative pointer-based landmark registration (OrthoPilot, Aesculap AG, Tuttlingen, Germany).

2. Materials and methods

2.1 Patients

Prior approval for the study was obtained from the local Ethics Committee. We conducted a prospective clinical study involving 44 consecutive patients in total. Nineteen males and 25 females with an average age of 70 years [range 51–85; SD 7.5] and BMI of 28 kg/m ${}^{2}$ [SD 3.8] were included.

The following inclusion criteria applied: primary THA for primary or secondary hip osteoarthritis; patient age $\geqslant$ 50 years; ASA score $\leqslant$ 4; given consent for study inclusion.

The exclusion criteria in addition to severe bone deformities of the pelvis or femur included: local and systemic infections; lack of follow-up over a period of three months ( $n=$ 0); the incapacity to grant consent; revoke of consent ( $n=$ 1); an accumulated radiation dose in the patient over the course of the past 10 years of over 10 mSv ( $n=$ 1). The postoperative follow-up was three months.

2.2 Operative technique

All operations were performed in a supine position by a single high-volume surgeon with a press-fit acetabular cup (Plasmacup SC, Aesculap AG, Tuttlingen, Germany) via a minimally invasive anterolateral approach, utilizing the interval between the M. gluteus medius and M. tensor fasciae latae [16]. Each procedure started with the registration of the APP firstly by P-CAOS (OrthoPilot, Aesculap AG, Tuttlingen, Germany) followed by US-CAOS (OrthoPilot, Aesculap AG, Tuttlingen, Germany). After sterile dressing, an infrared reflective tracker was attached to the middle of the ipsilateral iliac crest to serve as a reference point for both navigation systems. The osseous landmarks for the APP were acquired in the following order: ipsilateral ASIS, contralateral ASIS and SP.

P-CAOS registration (Fig. 1) preceded registration with US-CAOS (Fig. 2) to prevent the soft tissue over the SP from being falsified by previous manipulation via the US-probe and the surgeon being biased by the previous visualization of the SP via US on the display. Both data sets were collected and stored. The definite cup implantation was performed using US-CAOS in all cases. We choose a target value of 40 ${}^{\circ}$ for cup inclination and 15 ${}^{\circ}$ for cup anteversion and an error margin of $\pm$ 10 ${}^{\circ}$ for both values. During reaming and cup positioning, the US-CAOS supplied the surgeon with real-time information regarding cup inclination and anteversion.

Figure 2.

Intraoperative ultrasound-based landmark registration of the ipsilateral anterior superior iliac spine with displayed ultrasound image (OrthoPilot, Aesculap AG, Tuttlingen, Germany).

Figure 3.

3D model of radiological definition of the anterior pelvic plane as described by Murray [17].

Figure 4.

Intraoperative ultrasound-based (US-CAOS) cup position measurement (in red) compared to postoperative cup position as determined by 3D CT-based reconstruction (in blue).

2.3 Intra- and postoperative evaluation of the cup position

For comparison with the target parameters suggested by Lewinnek’s safe zone [8], we used the radiological definition according to Murray [17] (Fig. 3), characterizing the inclination as the angle between the longitudinal axis of the body and the acetabular axis. The anteversion is defined by the angle between the acetabular axis and the coronal plane.

To evaluate the final cup position, a pelvic CT scan was conducted three months postoperatively on all patients displaying RASIS, LASIS and SP. A slice thickness of 1 mm was chosen, and the final cup position was analysed using a standardized and previously validated 3D software (Amira Software; Mercury Computer Systems, Chelmsford, MA, USA). After calculation of the final cup inclination and anteversion from the CT data, the inclination and anteversion errors were calculated from the discrepancy between the postoperative CT data and the intraoperatively stored inclination and anteversion values from both the P-CAOS and US-CAOS navigation systems.

2.4 Statistical analysis

Means and standard deviations were calculated for patient age, BMI, cup inclination and anteversion values as well as inclination and anteversion errors, and tested for statistical significance using the Mann-Whitney U Test. All the statistical analyses were conducted with SPSS 24 (IBM Inc.; Armonk, NY, USA). Statistical significance was defined as $p\leqslant$ 0.05.

Figure 5.

Intraoperative pointer-based (P-CAOS) cup position measurement (in green) compared to postoperative cup position as determined by 3D CT-based reconstruction (in blue).

3. Results

The mean error of inclination was $-$ 1.1 ${}^{\circ}$ for the P-CAOS and 0.9 ${}^{\circ}$ for the US-CAOS group. This difference was not significant. Mean anteversion error was significantly lower in the US-CAOS with 1.4 ${}^{\circ}$ in comparison to $-$ 8.0 ${}^{\circ}$ in the P-CAOS group ( $p<$ 0.001) (Figs 4 and 5). Twenty-three cups (52%) in the P-CAOS and two cups (5%) in the US-CAOS group were outside of the previously defined error margin of 10 ${}^{\circ}$ anteversion $\pm$ 10 ${}^{\circ}$ . This result was statistically significant ( $p<$ 0.001). The measured inclination error, with a deviation of $>$ 10 ${}^{\circ}$ in the case of two cups (5%) in the P-CAOS and one cup (2%) in the US-CAOS group, was not shown to be significant. No intra- or postoperative adverse events occurred. The mean surgical time was 77 minutes [range 50–137; SD 22].

4. Discussion

Precise cup positioning is known as a factor for the success of THA [2, 3, 4], although THA stability and functionality is multifactorial. Navigation systems help to improve the accuracy of cup positioning and reducing the number of overall outliers compared to the freehand method. This is repeatedly emphasized in the current literature [10, 15, 18, 19]. Comparing available navigation tools is therefore needful.

To our knowledge this is the first study quantifying the magnitude of the APP acquisition error and the consecutively resulting deviation in cup position measurement between P-CAOS and US-CAOS within an intraindividual study design.

In a preceding study we have been able to show US-CAOS APP landmark registration to result in a significant lower anteversion error compared to P-CAOS (US-CAOS anteversion error 1.2 ${}^{\circ}$ ; P-CAOS anteversion error 8.7 ${}^{\circ}$ ; $p=$ 0.001) in an interindividual, postoperative CT-validated study design. No significant difference was found concerning the inclination error between both methods (US-CAOS inclination error 4.2 ${}^{\circ}$ ; P-CAOS inclination error 6.6 ${}^{\circ}$ ; $p=$ 0.08) [15]. These findings were traced back on an APP registration inaccuracy. In the present study the prior results [13, 15, 20], in means of a significant reduction of anteversion error with use of US-CAOS compared to P-CAOS, were confirmed. Likewise, the overall outliers to the prior defined target zone were significantly reduced using US-CAOS. The US-CAOS method lead to 5% outliers while outliers in the P-CAOS group amounted to 52%.

Soft tissue coverage of the bony landmarks is known to cause inaccuracy in APP landmark registration. Parratte and Argenson [13] and Lee and Yoon [14] were able to show that BMI-values $>$ 27 kg/m ${}^{2}$ and therefore an increasing soft tissue thickness [20] can be a source of systematic landmark acquisition and subsequently cup orientation error.

Differences pertain not only to the thickness, but also to the consistency of the soft tissue layer. Richolt and Rittmeister [21] showed that the soft tissue mantle over the SP is by comparison on average three times as thick as that of the ASIS and can be compressed but cannot be repositioned. A correspondingly inaccurate registration of the APP landmarks leads to the above-mentioned deviation in implant position, however completely and correctly conducted the navigation workflow. Wolf et al. [22] showed that a registration error of 4 mm in the APP leads to a 7 ${}^{\circ}$ error in cup anteversion and 2 ${}^{\circ}$ in cup inclination [22].

One point of criticism when conducting navigated THA regards to the higher costs in setting up the system. One limitation of this study can be seen in the fact that all definite cup implantations were conducted using US-CAOS. With the knowledge of the above mentioned investigation [15] we knowingly chose this method to benefit from a more precise cup position using US-CAOS. Another concern one may see can be the lengthening of operation time by 12 to 18 minutes caused by the navigation workflow [23, 24, 25]. The mean surgical time in this study indicated with 77 minutes appears long, considering that all operations were performed by a long-standing experienced surgeon. This can be traced back on the peculiarity of carrying out two navigation workflows in one patient. In our opinion it may not be considered as a deficiency because this is not standard. This study did not include stem anteversion so that no statement can be made concerning the combined anteversion of the THA [6]. Given the importance of the combined anteversion for the success of the THA, this may be seen as a limitation.

Up to today, medium-term results following navigated cup implantation have been a subject of controversy in the relevant literature [26, 27].

A global use of navigation tools for cup positioning seems not to be necessary. Nonetheless there are, in our opinion, good indications for US-CAOS assisted cup implantation when considering patients with a high risk of cup malpositioning in case of elevated BMI, minimal invasive approach and low-volume surgeons [10], not least by increasing confidence [28]. In comparison, US-CAOS navigation shows itself to be superior and should be preferred to P-CAOS, in that we are faced with ever greater numbers of overweight patients, for whom P-CAOS based navigation has been identified as significantly error-prone [15, 20]. Inexperience in navigation on the part of the surgeon, and a high BMI of the cadaver did not lead to worse results within the framework of a previous cadaveric study at our department [29].

5. Conclusion

In conclusion, we have been able to prove a significant occurrence of registration error in the APP landmark acquisition when the P-CAOS method was used. We showed that significantly greater accuracy in measurement was achieved for cup anteversion in the US-CAOS navigated group in this intraindividual study design.

Long-term follow-up will furthermore be delivering detailed results on the outcome regarding navigated prostheses. The further development of navigation systems and their application to potentially modified and patient-specific implant positioning [11, 30], will, in our view, be an area to pursue.

Footnotes

Conflict of interest

None to report.

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