Navigated and Nonnavigated Total Hip Arthroplasty: Results of Two Consecutive Series Using a Cementless Straight Hip Stem

Abstract

The purpose of this study was to compare conventional and navigated technique and a recently developed straight hip stem for uncemented primary total hip arthroplasty. The results of two consecutive implantation series of 42 patients (nonnavigated) and 42 patients (navigated) were analysed for implant positioning and complications. All surgeries were performed by the investigator. Radiographic analysis of cup position showed a significant improvement with reduced radiologic inclination (53° nonnavigated /44° navigated; P < .001) and higher anteversion (7° nonnavigated /12° navigated; P <.001). The mean postoperative limb length difference was 6.2 mm (SD, 9.0 for nonnavigated) and 4.4 mm (SD, 6.4 for navigated). Intraoperative and early postoperative complications were not different. No dislocation occurred in either group. There was one intraoperative trochanter fracture that was not revised (nonnavigated) and one revision because of a periprosthetic fracture caused by fall down during rehabilitation (navigated). We conclude that acetabular implant positioning can be significantly improved by the use of navigated surgery technique. The data for postoperative limb length difference were still similar but within the expected range in both groups. The effect of improved cup positioning on mid- and long-term results for both groups has to be investigated further.

Figure 1: Straight cementless hip stem.

Cementless total hip arthroplasty (THA) should lead to survival rates of > 95% at 10 years.¹ Implant positioning of cup and stem components may influence long-term results, and early results can be improved by reducing postoperative complications due to impingement,² joint dislocation³, or wear and acetabular component failure.⁴ The definition of appropriate implant positioning of THA components may be controversial and is always influenced by individual patient circumstances. Preoperative planning considers indication, bone morphology, and the degree of joint destruction to position the implant components. Cementless implants must be fixed with appropriate primary stability and should adjust limb length discrepancies. Cup positioning must have a positive anteversion value of 10º to 20º and an average inclination of 40º to 45º without extreme flat inclinations (<35º) or extreme high inclination (>55º). The radiographic evaluation of implant positions is difficult due to projection and magnification errors. Pelvic ante- or retroversion ⁵ or different femoral rotation⁶ and flexion may lead to deviations of projected implant positions.

In conventional implantation technique, surgeons experience has a major influence on preoperative plan and intraoperative implementation. Navigated THA implantation technique is an intraoperative tool to assist the surgeon with data on implant positioning for the implementation of a surgical plan to achieve an optimal implant position for the individual patient. Because the investigator is currently using routine THA navigation the purpose of the study was to compare implant positioning and short-term complications in two consecutive cementless THA implantation series without and with the use of this navigation technology.

Figure 2: A, Semilateral decubitus patient position of 30° to 45° for a direct lateral THA approach registration of the anterior pelvic plane. B, Pointer registration of the anterior pelvic plane.

Material and Methods

In 2000, we introduced a recently developed straight cementless hip stem (Excia; B. Braun Aesculap, Tuttlingen, Germany) that had been designed for primary THA (Figure 1). We introduced THA navigation technology (OrthoPilot; B. Braun Aesculap) for cup (2005) and stem (2006) positioning. The registration of the anterior pelvic plane is performed with a pointer, and a distal femoral fixation of the navigation sensor is used.

Two consecutive implantation series of 42 THAs with appropriate postoperative radiograph projections in non-navigated technique (April 2000 to December 2001) and 42 THAs in navigated technique (September 2006 to December 2007) were compared. The group of 42 navigated patients was retrospectively included backward from December 2007. All 84 surgeries were performed by the investigator. Implant position was evaluated on postoperative anteroposterior radiography between 2 and 3 months after the index surgery. All pelvic radiographs were made in strict standing position of the patient. The operated hip joints were classified on the preoperative radiographs into three subgroups: group 1 with a preoperative leg shortening (>5 mm), group 2 with a preoperative leg length equality (±5 mm), and group 3 with preoperative lengthening of the operated leg (>5 mm). Additionally, the projected values of the caput collum diaphysis (CCD) angle were classified into three subgroups (<125º, 125º to 135º, and >135°). Changes in leg length were measured using the most distal line between the teardrop figure and the proximal corner of the lesser trochanter as anatomical landmarks.⁷ The distance between the two teardrops and the head diameter of the hip replacement was used to scale preoperative and postoperative radiographs. Radiographic cup positions were also measured for inclination in relation to the teardrop line. Anteversion was calculated with the method of Pradhan.⁸ All THA surgeries were performed with a direct lateral approach with the patient in semilateral decubitus position of 30º to 45º (Figure 2a). A distal pin with an infrared sensor on the femur was used for the femoral reference (Figure 2b). The navigated THAs were performed using a minimally invasive surgical technique. Intraoperative and postoperative complications were documented, and all patients achieved immediate postoperative full-load bearing, which was possible without pain.

Results

The general data for the two groups are comparable according to the Mann-Whitney U test for the nonparametrical values (CCD angle, age, BMI) and the chi-square test for the distribution of the operated leg, gender, and indication. All values are summarized in Table 1. Patients age at time THA showed a statistical difference because the P value was slightly below .05.

Cup Positioning

Radiographic analysis of cup position showed a significant difference. Radiologic inclination (53°: SD, 8.1 [non-navigated]; 44°: SD, 5.6 [navigated]; P < .001) was reduced by an average of 8° by the use of navigation. The radiologic anteversion (7°: SD, 4.6 [non-navigated]; 12°: SD, 5.3 [navigated]; P < .001) was increased by 6°. The number of cup positions within a safe zone definition of radiographic inclination/anteversion of 45°/15°±10° (Figure 3) was also significantly improved by navigation (21 of 42, 50% non-navigated /38 of 42, 90% navigated; P < .001).

Leg Length

Independent of the preoperative circumstance, the postoperative values for the change of the limb length of the operated hip were not significantly different (Mann-Whitney U test P = .7) and showed a mean leg lengthening of 9.2 mm (SD, 6.2 mm for non-navigated) and 8.5 mm (SD, 5.4 for navigated). Considering also the preoperative leg length difference, the resulting postoperative leg length difference was 6.2 mm (SD, 9.0) in the nonnavigated group and 4.4 mm (SD, 6.4) in the navigated group (Table 2).

Dependant of the preoperative classification of preexisting leg length difference the values for navigated and nonnavigated procedures showed a slight decrease of intraoperative leg lengthening in the navigated technique (Table 3).

The matched graph of the preoperative CCD angle and the results of leg lengthening showed a trend of increased leg lengthening for smaller CCD angles (Figure 4).

Figure 3: Safe zone result for radiographic cup position (navigated/non-navigated).

Figure 4: Matched graph of preoperative CCD angle and postoperative leg lengthening (navigated /non-navigated).

The classified values of leg lengthening within the three groups of preoperative CCD angles, however, did not reflect this tendency (Table 4).

Complications

There were no implant related or navigation technology related complications. There were no joint dislocations in both groups. One intraoperative trochanter fracture was treated with a cerclage (non- navigated). One revision was caused by a periprosthetic fracture after fall down during rehabilitation (navigated).

Discussion

The comparison of patient groups in navigated and non-navigated THA technique is a possible method to obtain additional information about the benefits and the possible improvement of implant positioning. This study design has general limitations because the measurement of implant position on radiograph is less precise than CT-based measurements. Our measurements of radiologic inclination and anteversion were taken in a straight standing position with anteroposterior radiographs, which should not exceed a deviation of 5 mm compared with CT.⁹ Our mean limb length data of the not operated hip joint were also small (+1.3 mm nonnavigated and -1.3 mm navigated). The method of cup position measurements can be improved if the radiograph is taken while the patient is in a standing position. Here the preoperative flexion contraction is reduced as it is during postoperative rehabilitation in the first weeks after THA.¹⁰

The comparison of two nonrandomized implantation series operated by a single surgeon might include a certain bias but can exclude the influence of different surgical techniques. Our non-navigated patient group is under continuous clinical and radiographic follow-up to document the mid- and long-term results of our cementless straight hip stem, which was developed when THA navigation for the hip stem was not available. The retrospective definition of the navigated patient group represents our current surgical procedure with this implant. Our results show a very clear and significant improvement of acetabular cup positioning by the use of THA navigation. Therefore, we also support the use of the anterior plane reference¹¹ for THA navigation.

Our results do not clearly show a change of leg lengthening data using navigated or non-navigated technique. In our two series, the average total limb lengthening of the operated hip joint was below 10 mm (9.2 mm non-navigated and 8.5 mm navigated) and therefore below a value of clinical relevance ¹² and well comparable with other studies with mean lengthening of 7 mm. ¹³ Postoperative limb discrepancy after THA has been analyzed with the same radiographic technique as in our study¹⁴ in 408 cases with 97% <10 mm and 86% <6 mm. Using a templating technique, ¹⁵ the mean postoperative limb discrepancy was 3.9 mm in a series of 420 cases. Our result of a mean limb length discrepancy is comparable with this data and is close to a level of 5 mm (6.2 mm non-navigated and 4.4 mm navigated).

“Unexpected” postoperative leg length discrepancies can be reduced, for example, by using mechanical devices¹⁶ or intraoperative radiograph. ¹⁷ This should also be the case for THA navigation technology. A navigation system generally records the absolute intraoperative lengthening between pelvis and femur. The aspect of limb length discrepancy/equalization to the contralateral hip joint has to be considered preoperatively. We aim for a minimum intraoperative leg lengthening of 4 to 5 mm in cases of primary osteoarthritis to compensate for cartilage destruction. The “tendency” to lengthen the leg to increase stability is less common because the head diameters have increased. In our navigated series, we used 32-mm heads compared with 28-mm components in the earlier nonnavigated series, which did not influence the dislocation rate. The larger head size might explain the tendency of slightly smaller values for intraoperative leg lengthening in the navigated series. On the other hand, the navigated series had a higher preoperative limb length discrepancy (-5.5 mm) compared with the nonnavigated group (-2.3 mm). Our explanation for the similar values in leg length discrepancy in both series is certain intraoperative inconsistencies due to the distal femoral reference of the used navigation software. This aspect leads to an intraoperative decision for leg lengthening independent of the available navigation data. We know today that a femoral referencing closer to the hip joint or even a pinless femoral referencing leads to more accurate data for limb length and femoral offset changes. The significant improvement of cup positioning encourages us to continue our navigated THA procedure, and we will investigate additional improved navigation workflows for intraoperative limb lengthening and offset data.

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Author

Dr. Mainard is from Centre Hospitalier Universitaire Nancy, France.

Dr. Mainard is a lecturer for and holds a patent under B. Braun Aesculap.

Correspondence should be addressed to Didier Mainard, MD, Department for Orthopedics and Traumatology, CHU Nancy, France; didier.mainard@ciril.fr.