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Current Status of Navigated Cup Replacement in Dysplastic Cases

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Abstract

From the authors’ experiences, several challenges exist in creating accurate cup positioning in patients with dysplastic hip due to anatomic abnormalities. B. Braun Aesculap (Tuttlingen, Germany) developed an advanced OrthoPilot software module for dysplastic cup positioning that has several advantages over the normal cup software as a solution to cup inaccuracies. The following study is a preliminary report on the new software version, which provides more specific functionality than the previous software system, a precondition for successful dysplastic cup navigation. This new functionality increases the usefulness of navigation and the safety of the procedure.

In total hip arthroplasty, cup and stem positioning is of considerable importance with regard to postoperative dislocation and durability. During the past 50 years, surgeons have observed marked advances in artificial joint materials and fixation methods. The operative technique, which also affects the durability of a prosthesis, must still be chosen by the individual surgeon, however.¹ Malpositioning of the cup reduces range of motion and induces stem impingement, resulting in dislocation. Navigation systems are designed to prevent such problems.²

Initially, the author (K.K.) used the original OrthoPilot cup navigation software (B. Braun Aesculap, Tuttlingen, Germany) to prevent dislocations caused by impingement between the neck of the stem and the posterior edge of the cup, with the lower extremity in outward rotation, and achieved good results with long durability.

In Japan, surgeons encounter many patients with dysplasia and only a few patients with primary osteoarthrosis of the hip. The incidence of hip dysplasia in Japan accounts for approximately 90% of the total caseload of hip osteoarthrosis, whereas in Western countries, most cases are of primary osteoarthrosis of the hip. In the latter case, replacing the cup using the original OrthoPilot cup software is relatively easy because cup positioning can be set to create a larger and deeper cup base at the original position. However, inclination and anteversion require careful consideration.³ In contrast, creating the new cup at the original position is difficult in Japanese patients with a dysplastic hip. The difficulty is attributable to anatomic abnormalities such as a shallow or narrow cup superior from the original area.^4,5

As a solution, B. Braun Aesculap developed an OrthoPilot software module for dysplastic cup positioning that has several advantages over the normal cup software. Additional measurements, such as the reamer distance to the inner wall and the reamer distance to the posterior rim of the acetabulum, which are registered prior to the reaming stage, are available with the dysplastic software module.

More recently, they developed a second generation of the dysplastic software module with additional improvements. The new version offers an index and a clear, user-friendly display. The author’s (K.K.) institution was the first worldwide to use the new OrthoPilot dysplastic cup navigation software, with the first surgery performed in early May 2006. This preliminary report on the new system is based on several cases.

Materials and Methods

Preoperative planning based on preoperative anteroposterior (AP) radiography is essential in finding two reference points. The first point is the teardrop, and the second point is the lateral top of the superior osteophytes. Two lines are drawn, one connecting the bilateral teardrops and one perpendicular to the first line, from the lateral top of the superior osteophytes. Drawing the two lines creates a new cup center and measures the distance and width from the reference points. Finally, the diameter of the planned cup is determined.

The following data were computed for radiographic planning: measured caudal cranial distance from the teardrop point to the planned cup center and measured medial lateral distance from the appropriate reference point to the planned cup center (Figure 1). The result was a positive value if the reference point was lateral from the planned rotational center and a negative value if the reference point was medial from the planned center. Also, the magnification factor of the radiographic view and a planned cup diameter was computed in the radiograph planning stages.

Plane registration was performed in a manner similar to that with the previous software. Three points were registered, including bilateral points on the anterosuperior iliac crest and a point on the syndesmosis pubis. The pointer, however, was modified in the new software version. Registering the opposite side of the anterosuperior iliac crest in the lateral position was difficult using the original pointer, because it pointed away from the infrared camera. With the new version, the rigid body of the pointer can be turned toward the camera, and the point can be easily registered.

Teardrop point palpation was performed in the same manner as with the previous software. The point is recorded with reference to the cranial-caudal direction.

Palpations of additional points were conducted as references for the medial-lateral position of the cup, according to preoperative planning, along the lateral top of the superior osteophytes. New registration locates the mediolateral direction more clearly than was possible with the previous software version.

Palpation of the posterior rim of the acetabulum was executed with the hammer pointer. Surgeons who want to avoid over-reaming can palpate up to five points. The software automatically detects the closest point to the reamer surface, and the distance is shown in real time during the reamer navigation. Rather than palpating the rim intraoperatively, palpating the thinnest point, which will represent the new cup position, is preferred. The thinnest point may be located at the bottom of the cup base of the posterior rim.

The next step was the monitored palpation of the pilot-hole entry point. The cranial-caudal distance between the teardrop transversal plane and the tip of the hammer pointer was displayed. In addition, the distance between the posterior plane (including the most posterior point registered on the posterior rim) and the tip of the pointer was shown. Usually, the AP distance to the most posterior rim point will be shown as the planned cup radius, and the cranial-caudal distance to the teardrop transversal plane will be shown as the planned cup center.

A pilot hole was drilled manually, perpendicular to the bottom of the cup. To identify the sagittal plane, pilot-hole palpation was performed with a hook pointer, registering the medial side of the inner wall of the pelvis. Making the pilot hole at the point of the thinnest area at the base of the acetabulum was essential. Theoretically, the planned position of the cup center and the position of the pilot-hole entry point are identical, because, in a dysplastic hip, reaming will first take place parallel to the teardrop transversal plane, down to the thickness of the medial wall plus several millimeters, by using a smaller reamer than planned (Figure 2). After reaming, other steps such as anteversion and inclination were finally decided. The reaming method is the safest and easiest for determining the planned cup position.

The navigated reaming screen is displayed with seven indicative measurements. Two distances with reference to the pilot hole and the reamer surface include the minimum distance from pilot-hole base to the reamer surface, minimum distance from the reamer surface to the sagittal plane through the pilot-hole base, and the distance from the reamer surface to the point closest to the posterior rim. The distance from the reamer surface to the point closest to the posterior rim depends on the reamer position (the closest point may change), the caudal-cranial distance from the reamer center to the teardrop level (entered planning value displayed in gray), the mediolateral distance from the reamer center to the relevant reference point (entered planning value displayed in gray), and also inclination and anteversion.

Cases and Results

The author (K.K.) performed only a few procedures using the new dysplastic software and cannot discuss data regarding the effects of the new software in detail. The author performed 35 procedures using the previous software version. In these, the investigators calculated the thickness of the medial wall and the cup position, including inclination and anteversion. To calculate the thickness of the medial wall, the real distance was measured directly during surgery and compared with the distance displayed on the navigation screen. To calculate cup position, the values displayed on the navigation screen to postoperative radiographs and computed tomography were compared. The data for this comparison are shown in the Table. No significant discrepancies were observed.

The first case involved a 68-year-old woman with a previous right hip osteotomy at the base of the minor trochanter. The goal of the osteotomy was to establish contact between the minor trochanter and the inferior part of the acetabulum; however, the head was not taken into consideration at that time. Until recently, the patient was comfortable with activities of daily living, but currently suffers from severe pain in the left hip. The leg length discrepancy was approximately 2 cm preoperatively.

Three radiographs show the difference in the pelvic tilt in various body positions. In standing position, the pelvis is rotated and tilted posteriorly, and the right minor trochanter is in contact with the original cup edge. The patient experiences no pain in the right hip disturbing activities of daily living. In the supine position, there is no contact between the minor trochanter and the inferior part of the acetabulum.

Preoperative planning and final implantation radiography evolved by connecting the baseline with the bilateral teardrop line. The bottom of the cup will then be at least 10 mm away from this line. The center line for reaming guidance is determined using a 50-mm diameter or 52-mm diameter cup, as required. The distance from the bottom line of the cup to this point is approximately 35 mm.

The displayed distance to the medial wall was 7 mm. Final implantation data showed an inclination of 43° and an anteversion of 13°. Measurements on postoperative radiography yielded an inclination of 45° and anteversion of 14.5°. The leg length was increased by approximately 2 cm.

Discussion

The new software version includes significant modifications to the previous software such as registration of the cup center and cup diameter-planning data, palpation of the lateral top of the superior osteophytes, and an additional palpation of the posterior cup rim.

Additional data registration just prior to surgery is considered problematic. The data provide the tentative size and position of the new cup. Regardless, preoperative measurements and sketching on the radiograph image must be performed and must be reflected in the navigation process. Furthermore, it should be noted that the data given are examples; the surgeon can still modify the data and cup size during reamer navigation.

In the previous version of the software, six indicators for reamer navigation were displayed, specifically the inclination and anteversion angles and the distance between the pilot hole and the teardrop in the transversal and axial planes. The two displays were not easy to understand clearly and quickly enough, because one showed a lateral view whereas the other showed an image taken in the sagittal plane of the pelvis. Two other indicators were displayed: the distance from the reamer surface to the medial wall and the distance from the reamer surface to the baseline, which was registered as the teardrop line. The former was not the real distance from the registered medial wall point to the reamer surface because the direction of reaming was not the same as the direction of measurement from the medial wall point. In the new software, this point was redefined to be in the same direction so that the real distance from the reamer surface to the registered medial wall point could be acquired. The latter indicated the protrusion from the inferior rim of the cup. In fact, the indicator was not significantly useful for cup positioning. A normal hip has only a minor protrusion, and setting the cup slightly lower is easy. The leg length was adjusted by changing the neck length.

The new software offers seven indicators in the reamer navigation step. The setting angle of the cup is unchanged. The cup position is expressed more clearly and understandably than in the previous version, because the baseline is changed from the teardrop line and the pilot hole to the teardrop line and the lateral edge of the cup. In average practice, surgeons determine cup-position parameters such as the degree of medialization or inferiorization on the basis of AP-oriented radiography, which is now reflected in the new software version. In addition, the posterior rim indicator provides a safety area in the posterior reaming zone. In primary osteoarthrosis, reaming is started with an angled position (inclination of 45° and anteversion of 12°). In a dysplastic hip, however, reaming should be started perpendicular to the cup base and continued to several millimeters short of the medial wall on the outer side, with a smaller reamer. Subsequently, reaming with the planned reamer angle can be started, because reaming with the planned reamer angle is a safe way to avoid breaking through the posterior wall. The indication is useful. In the previous software version, only the minimal distance from the registered medial wall, which is the same as the distance from the bottom of the pilot hole to the reamer surface, was shown in the sagittal plane display. This distance was not the real distance between the reamer surface and the inner side of the medial wall. In the new system, however, an additional display, which is oriented in the same direction as the reamer shaft, is available. This is the actual distance the surgeon wants to know during surgery to avoid penetration of the medial wall.

Conclusion

In summary, the new software module provides more specific functionality, which is a precondition for successful dysplastic cup navigation, compared with the previous software version. This new functionality increases the usefulness of navigation and the safety of the procedure. For example, over-reaming of the posterior rim, which would seriously compromise the cup implant stability, is now easily avoidable.

Furthermore, screen ergonomics were improved, and the values and parameters displayed are now self-explanatory.

The additional information displayed request the palpation of additional landmarks. Additional monitoring of the learning curve is necessary to observe whether additional palpation will generally lead to longer operating times.

The next step should be an extension of the dysplastic cup navigation system to complete hip navigation. The inclusion of stem navigation will provide information about the leg length, offset, and range of motion of the new hip joint. Stem navigation will enable the surgeon to adjust the leg length exactly, even when the cup position is changed, and to optimize the range of motion by choosing the appropriate stem rotation. In this manner, dislocation can be prevented and maximum durability could be achieved.

The author (K.K.) predicts that the use of a navigation system will soon become a standard procedure in complicated interventions such as dysplastic hip surgery.

References

1. Hassan DM, Johnston GH, Dust WN, et al. Accuracy of intraoperative assessment of acetabular prosthesis placement. Arthroplasty. 1998; 13:80-84.
2. DiGioia AM, Jaramaz B, Blackwell M, et al. The Otto Aufranc Award. Image guided navigation system to measure intraoperatively acetabular implant alignment. Clin Orthop Relat Res. 1998; 355:8-22.
3. Kiefer H. OrthoPilot cup navigation-how to optimize cup positioning? Orthopedics. 2003; 27:S37-S42.
4. Anwar MA, Sugano N, Masuhara K, et al. Total hip arthroplasty in neglected congenital dislocation of the hip. A five- to 14-year follow-up study. Clin Orthop Relat Res. 1993; 295:127-134.
5. Herold HZ. Congenital dislocation of the hip treated by total hip arthroplasty. Clin Orthop Relat Res. 1989; 242:195-200.

Authors

Dr Kanesaki and Hieda are from the Department of Orthopedics, Moji Rosai Hospital, Fukoka; and Dr Nagata is from the Orthopedic Department at Kurume University, Japan.