Cup Positioning in Primary Total Hip Arthroplasty Using an Imageless Navigation Device: Is There a Learning Curve?

Abstract

In this study, the success of cup positioning in total hip arthroplasty (THA) using an imageless navigation system was analyzed (1) during the learning period and (2) after the learning period for using the navigation system. Sixty THAs were performed in which threaded cups were placed with use of a computer-assisted navigation device (B. Braun Aesculap, Tuttlingen, Germany). Half of the procedures (30), group A, were done by the same surgeons under the learning curve for using the navigation system; the other half (30), group B, were done by surgeons who were no longer considered under the learning curve. Intraoperative acetabular component parameters (inclination, anteversion) for both groups were compared with postoperative radiographic alignment values. In group A, significant differences were seen between intraoperative and postoperative cup orientation. In group B, no significant differences were seen between intraoperative and postoperative cup orientation. Additionally, the percentage of outliers decreased in group B. Operating and anesthesia times significantly decreased with the surgeon’s experience in imageless cup navigation. There was an individual increase of precision during the learning curve for all surgeons. Imageless navigation is a dependable and accurate method of cup positioning during THA. However, accuracy of cup placement and length of operating room time were affected by surgeons’ experience using the system. Imageless navigation may lead to a reduction in the length of the learning curve for surgeons beginning to perform THAs, improvement in the surgeon’s ability to perform this procedure safely, and minimization of outliers.

Orientation of cup positioning directly affects the success and survival of total hip arthroplasty (THA). Besides surgical approach, component materials (eg, ceramic-ceramic, metal-ceramic, ceramic-polyethylene), and fixation techniques, the orientation of the components plays a significant role in long-term survival of the endoprosthesis. Malpositioning of the acetabular component is often associated with higher rates of dislocation, femoroacetabular impingement, and wear in patients with THA.^1-5

Especially for the modern hard-hard bearing surfaces, such as ceramic- ceramic and metal-metal, an exact positioning of the acetabular components is important to avoid squeaking, ceramic fracture due to edge loading, or increased release of metal ions in metal-on-metal bearings. Lewinnek et al⁶ defined a “safe zone” for cup orientation (inclination: 45°±10°, anteversion: 15·±10°) to minimize complications such as dislocation. Some authors report a high rate of revision (one-third of all revisions) within the first 5 years after THA following recurrent dislocations.^5,7,8

In the past, many companies developed mechanical guides for cup positioning to help surgeons and to minimize outliers with high or low inclination and anteversion. However, most mechanical guides had poor precision and accuracy.⁹ These guides require an exact knowledge of patient orientation on the operating table. This is more complicated in patients in the lateral decubitus position than in patients in the supine position. Furthermore, the surgeon relies on experience to modify the guides intraoperatively to avoid malalignment of the acetabular component. Especially in obese patients, orientation for adequate cup positioning can be difficult and may lead to suboptimal implant orientation. This may result in a wide discrepancy between planned implant positioning and final orientation.¹⁰

In minimally invasive or less-invasive THA surgery with a limited view of the operative field, perfect implant positioning is difficult to achieve.^11,12 Especially in these cases, special tools, such as computer-aided navigation systems, facilitate component positioning despite the inability to directly visualize anatomic landmarks.^10,13

In recent years, many companies have developed computer navigation technology to assist surgeons in THA. The OrthoPilot (B. Braun Aesculap, Tuttlingen, Germany) uses passive trackers to register intraoperatively the orientation of the pelvis in relation to bony landmarks. Furthermore, the reaming process and implantation of the acetabular component is guided throughout the implantation process.

Figure 1: Implanted threaded cup and Metha short stem.

In the past, there were few reports about computer navigation in THA. Reasons for not using computer navigation in THA were (1) the relatively slow adoption in orthopedic departments, (2) the expense, (3) the prolonged operative time, (4) the need for a learning curve, and (5) the belief that the acetabular component can be adequately positioned in most cases without computer navigation.

In this study, we analyzed the learning curve of imageless computer navigation in THA regarding the intraoperative cup positioning achieved by computer navigation compared with postoperative results as seen on radiograph study.

Materials and Methods

Thirty-five women and 25 men underwent THA with the Metha short stem (B. Braun Aesculap) and a threaded cup (threaded SC Cup, B. Braun Aesculap) (Figure 1). The patients were divided into 2 groups: group A (n=30) comprised patients who underwent THA during the learning period; group B (n=30) comprised patients who underwent THA after the learning period. In all cases, surgery was done using the imageless computer navigation system OrthoPilot. Patient demographic data and surgical parameters (eg, operating time, anesthesia time) were recorded.

The goal of acetabular component positioning was 45° inclination and 20° anteversion. Intraoperative acetabular component parameters (cup inclination and anteversion) were recorded automatically with the OrthoPilot software. The final acetabular component position was determined by postoperative anteroposterior radiographic examination.¹⁴ The intraoperative navigation data and the postoperative radiograph data for acetabular orientation (inclination and anteversion) were compared, and analysis of variance testing was used to assess statistical significance between intraoperative and postoperative acetabular component orientation. The means, variances, and standard deviations were calculated using OriginPro software (OriginLab Corporation, Northampton, Massachusetts).

Surgical Procedure

All surgeries were performed using a minimized lateral transgluteal approach with the patient in supine position. After the acetabulum was exposed, a rigid body was fixed to the ipsilateral iliac crest. The bony landmarks (both anterosuperior iliac spines, pubic symphysis, and fovea of acetabulum) were recorded with a passive tracker to determine the pelvic plane and depth of the acetabulum. A passive tracker was attached to the reamer to monitor inclination, anteversion, and depth on a computer screen during the reaming process. Afterward, the acetabular component was implanted using an instrument that was also equipped with a passive navigation tracker. The position was recorded automatically with the navigation software.

Results

In group A (learning curve) the intraoperative cup orientation was found to be 43.7°±4.2° for inclination and 15.1°±5.7° for anteversion. The postoperative inclination was 47.3°±6.5° and anteversion was 20.9°±3.1° (Figure 2). Intraoperative and postoperative inclination (P<.05) and anteversion (P<.05) showed significant difference in group A (learning curve) (Table). This means that the goal of intraoperative navigation was not reached and navigation and radiographic data differed significantly in inclination and anteversion.

Figure 2: Group A (learning curve). Statistically significant differences existed between navigation and radiograph for inclination and anteversion (P<.05).

Table: Cup Orientation

In group B (after the learning curve), the intraoperative cup orientation was found to be 46.5°±3.7° for inclination and 18.6°±9.7° for anteversion. The postoperative inclination was 48.7°±6.6° and anteversion was 20.8°±5.8° (Figure 3). Intraoperative and postoperative inclination (P=.12) and anteversion (P=.31) showed no significant difference in group B (after the learning curve) (Table). This means that the goal of intraoperative navigation was reached and navigation and radiographic data did not differ significantly regarding inclination and anteversion.

Figure 3: Group B (after the learning curve). No statistically significant differences existed between navigation and x-ray for inclination and anteversion (P >.05).

Prolonged navigation time, additional to the total operative time, was significantly lessened with the surgeon’s experience in imageless cup navigation: group A=13.2±5.2 minutes, group B=4.8±3.8 minutes (P<.05). The mean body mass index of all patients was 28.8 kg/m² with no significant difference between the 2 groups (P>.05).

The authors found no complications during the study period. There were no revisions, dislocations, or aseptic loosening in any patients.

Discussion

Cup orientation is a major factor influencing the success of long-term survival of the endoprosthesis in THA. Especially in young patients with a greater physiologic demand on their THA, failures in orientation of the acetabular component may lead to greater wear, dislocation, or loosening of the endoprosthesis. This may necessitate early revision surgery and incur higher health care costs. Therefore, improving the longevity of the THA is increasingly important.

One way to increase the longevity of the THA is to prevent malpositioning of the acetabular component. Nogler et al¹⁵ have shown that imageless navigation techniques reduce the variability of cup orientation in vitro compared with manual techniques. The use of an image-based navigation system is time consuming, infers greater irradiation, and requires more technical effort. One criticism of the imageless navigation system is that the surgeon works with less anatomic information than is available with an image-based system; however, the technically easier, faster procedure compensates for that drawback.¹⁶

Two limitations of this study are (1) the variability of anteroposterior pelvic radiographic measurements of the final cup anteversion and (2) the recording of only acetabular position rather than both stem and cup orientation.

Patient positioning on the radiograph table, especially in pelvic tilt, is highly variable. Patients relax their muscles more on the operating table than on the radiograph table. Furthermore, a higher or lower lumbar lordosis in awake patients can cause differences between intraoperative and postoperative anteversion. Several authors have established mathematical methods to determine cup anteversion on plain radiographs.^14,17 However, these determinations are inaccurate compared with data from postoperative computed tomographic measurements.^18-20 Nevertheless, plain radiographs are the standard investigation method for follow-up examination in outpatient clinics. Additionally, postoperative computed tomography is expensive and patients are exposed to further radiation.

Another limitation is that only the acetabular position—not both stem and cup orientation—is recorded, although the absolute orientation of both cup and stem is less important than the relative orientation of both components to each other.²¹ Further studies will analyze the combined inclination and anteversion on the orientation of both components.

The purpose of this study was to analyze the learning curve of surgeons using an imageless navigation system for THA. Two groups were compared using the intraoperative recorded navigation data of cup orientation and the postoperative radiographs. In group A (learning curve), there was a significant difference in inclination and anteversion of intraoperative and postoperative cup orientation. The surgeon was not able to achieve the planned orientation of the acetabular component, which was seen on the postoperative radiographic study. In group B (after the learning curve), no significant differences were found between navigation and radiograph data. The surgeon was able to place the cup in the desired orientation more precisely. (These results coincide with those of other studies of other navigation systems.²²) Furthermore, the number of outliers can be reduced with an imageless navigation technique, increasing the reliability and accuracy with which components can be implanted.

Prolonged navigation time, additional to the total operative time—a major complaint regarding use of a navigation system—was 4.8±3.8 minutes after the learning curve. The authors believe that the nominal increase in operative time incurred with use of an imageless navigation system is justified by the ability to achieve perfect cup orientation and its associated long- and short-term benefits.

Conclusion

Imageless navigation is a dependable and accurate method of cup positioning during THA, as measured by intraoperative and postoperative cup inclination and anteversion. However, accuracy of cup placement and length of operating room time were affected by the surgeons’ experience using the system. Imageless navigation may lead to a reduction in the length of the learning curve for surgeons beginning to perform THAs, improvement in the surgeon’s ability to perform this procedure safely, and minimization of outliers.

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Authors

Drs Thorey, Klages, Lerch, Flörkemeier, Windhagen, and von Lewinski are from the Department of Orthopaedic Surgery, Hannover Medical School, Hannover, Germany.

Drs Thorey, Klages, Lerch, Flörkemeier, Windhagen, and von Lewinski have no relevant financial relationships to disclose.

Correspondence should be addressed to: Fritz Thorey, MD, Department of Orthopaedic Surgery, Hannover Medical School, Anna-von-Borries-Str 1-7, 30625 Hannover, Germany.

doi: 10.3928/01477447-200915-52