The Reliability of Navigation-guided Gap Technique in Total Knee Arthroplasty

Abstract

The OrthoPilot TKA navigation system (B. Braun Aesculap, Tuttlingen, Germany) offers software to optimize soft tissue balance using gap balance techniques. However, there are no studies on the reliability of the navigation-guided gap technique. The goal of this study is to establish the reliability of the navigation-guided gap technique. The investigators measured flexion and extension gap in the medial and lateral sides of the knee joint after bone resection to evaluate the reliability of navigation-guided soft tissue balancing. Gap data from 100 cases of navigation-guided total knee arthroplasty were analyzed. We defined trapezoidal gap (unsatisfactory soft tissue balance) as a gap difference > 3 mm between the medial and lateral sides in extension and a 5-mm difference in 90° of flexion. Furthermore, gap difference between flexion and extension greater than 3 mm on the medial side and 5 mm on the lateral side was also considered a trapezoidal gap. Among 100 cases, 84 showed rectangular (acceptable) gap, and 16 showed trapezoidal gap. We also evaluated the correlation between clinical results including range of motion and soft tissue balance as well as characteristics of trapezoidal gap. This study suggests that the navigation-guided gap technique is a reliable method for optimizing soft tissue balance.

Accurate limb alignment and well-controlled ligament balance after total knee arthroplasty (TKA) are critical to successful clinical outcomes and long-term prosthesis survival.^1,2 Malalignment and ligament imbalance could produce unequal load on the bearing surface and instability of the prosthetic joint, causing uneven wear of the polyethylene, early implant loosening, and poor clinical outcomes. ³ A close correlation between soft tissue balance and rotational alignment of the femoral component has been well understood.⁴ In addition, soft tissue balance is influenced by whether a balanced flexion and extension gap is achieved intraoperatively.⁵ However, soft tissue balancing remains a technically demanding and difficult part of TKA, because measuring the soft tissue tension is dependent on subjective surgeon assessment.⁶ Therefore, measurements of the flexion and extension gaps are unreliable. The OrthoPilot TKA navigation system (B. Braun Aesculap, Tuttlingen, Germany) offers software that optimizes soft tissue balance, principally through the gap technique.

Over the past 10 years, many surgeons have become interested in computer navigation systems to perform more precise TKA. The initial results of navigation- assisted TKA are promising with regard to the restoration of mechanical limb alignment.^7,8 However, only a few studies have addressed the effectiveness and reliability of the navigation system on soft tissue balance in TKA. The purpose of this study is to determine the reliability of the navigation-guided gap technique and to evaluate the clinical results from adequate soft tissue balance in TKA using the navigation-guided gap technique.

Materials and Methods

TKA was performed on 108 osteoarthritic knee joints from 78 patients using the navigation-guided gap technique in our institution between May 2004 and June 2006. Eight patients were excluded from the study; two died from causes unrelated to TKA, two converted to conventional TKA owing to registration failure, and four were lost to follow up. Consequently, 100 knees from 70 patients were included in the study. Raw data, including gender, age at the time of surgery, preoperative range of motion (ROM), and duration of follow-up, were obtained from the prospective TKA database at our institution. The study group included 66 women, mean age was 67.1 years (range, 52-81 years), and mean follow-up was 2.3 years. The mean coronal plane alignment was 11.2° ± 5.1° in varus, preoperatively. All procedures were performed by the same surgeon (SBH). Study subjects underwent cruciate-retaining TKA with e.motion TKA system (B. Braun Aesculap). The patients who underwent posterior- stabilized TKA were excluded because posterior cruciate ligament resection might result in an unexpected increase of the flexion gap.^9,10 In all cases, the image-free navigation system (OrthoPilot version 4.0 or 4.2, B. Braun Aesculap) was used. All knees were accessed in a similar manner, using a midline skin incision with a medial parapatellar approach and routine soft tissue exposure. The preliminary ligament (usually medial) release, which is essential for the gap technique, was performed guided by real-time feedback from the navigation system.

Figure 1: A simple device similar to a lamina spreader with a torque meter (A) and a tensor with slide ruler (B) was developed to measure joint gap.

A simple device similar to a lamina spreader with a torque meter and tensor with slide ruler was developed to measure the joint gap during surgery (Figure 1). After the preliminary release and tibial cutting, soft tissue tension was measured and registered to the navigation system with use of these devices. The OrthoPilot navigation system version 4.2 offers the femoral planning step that allows simulation of the femoral component sizing and rotation for the balanced gap (Figure 2). After completion of the bone cutting guided by the femoral planning step, the gap measurement was performed at the full extension and 90° of flexion in the medial and lateral side of the knee joint; medial extension gap, medial flexion gap, lateral extension gap, lateral flexion gap (shown in Figure 3). In this study, joint distraction force, which was applied between the osteotomized tibia and femur and was set at 40 lbs (18.7 kg), the joint gap with 40 lbs of distraction force at full extension most closely corresponds to the thickness of the insert actually selected for the procedure.⁶

Figure 2: Femoral planning was performed to decide the size and rotation of the femoral component, which was a most useful step in OrthoPilot Navigation system.

We defined the trapezoidal gap (poorly balanced gap) as a gap difference greater than 3 mm between the medial and lateral sides in extension or 5 mm difference in 90° of flexion. Furthermore, difference between flexion and extension gap greater than 3 mm in the medial side or 5 mm in the lateral side was also considered a trapezoidal gap (Figure 3B). Knees that did not meet the criteria for a trapezoidal gap were defined as having a rectangular gap (well-balanced gap). Based on our criteria for soft tissue balancing, patients were divided into two groups: a rectangular (well-balanced gap) group and a trapezoidal (poorly balanced gap) group. Moreover, gap difference in 90° of flexion (medial flexion gap~lateral flexion gap), extension (medial extension gap~lateral extension gap), medial (medial flexion gap~medial extension gap), and lateral (lateral flexion gap~lateral extension gap) side was analyzed to detect outliers and evaluate the reliability of gap balancing based on Griffin’s method.¹¹

Figure 3: Gap measurements were composed of four folds. A, Medial and lateral extension gaps (MEG and LEG) and B, medial and lateral flexion gaps (MFG and LFG).

Hospital for Special Surgery (HSS) scores and ROM at latest follow-up were used for the clinical outcome assessment. Mechanical alignment of the limb was checked on a standing radiograph of the entire lower extremity obtained at the latest follow-up. Mechanical axis measurements were performed by one of the investigators, who was blinded to the adequacy of balancing and clinical outcome.

Statistical analysis was performed using SPSS version 12.0 (SPSS Inc., Chicago, Illinois) for Windows. Clinical outcomes and radiological data in the two groups (rectangular and trapezoidal group) were analyzed using Fisher’s exact test. Repeated measures ANOVA was used to compare the medial flexion gap, medial extension gap, lateral flexion gap, and lateral extension gaps. A P value < .05 was considered statistically significant.

Results

Gap Measurements

The mean intraoperative gaps were 21.8 ± 2.5 mm, 22.6 ± 2.3 mm, 21.3 ± .2 mm, and 22.0 ± 2.3 mm for medial flexion gap, lateral flexion gap, medial extension gap, and lateral extension gap, respectively. No statistically significant differences were found between the groups with regard to these four variables (P = .629, repeated measures ANOVA). According to our criteria, 84 knees were classified in the rectangular group, and the remaining 16 knees were classified in the trapezoidal group.

A summary of the gap differences in flexion, extension, and medial and lateral gaps is given in Table 1. Of the 100 knees, 72 knees had flexion gaps balanced (medial flexion gap~lateral flexion gap) within 1 mm. Nineteen knees had a side-to-side difference of 2 mm, 7 had a difference of 3 mm, and 2 had a difference greater than 3 mm. Seventy-four knees had extension gaps balanced (medial extension gap~lateral extension gap) within 1 mm. Of the remaining 26 knees, 14 had asymmetry of 2 mm, 10 had asymmetry of 3 mm, and 2 had asymmetry > 3 mm. With regard to medial gap differences (medial flexion gap~medial extension gap), 57 (57%) knees were balanced within 1 mm. Of the remaining knees, 17 had a mismatch of 2 mm, 12 had a mismatch of 3 mm, 8 had a mismatch of 4 mm, 3 had a mismatch of 5 mm, and 3 had a mismatch of > 5 mm. With respect to lateral gap differences (lateral flexion gap~lateral extension gap), 57 knees were balanced within 1 mm. Of the remaining knees, 20 had a mismatch of 2 mm, 12 had a mismatch of 3 mm, 4 had a mismatch of 4 mm, 6 had a mismatch of 5 mm, and 1 had a mismatch of greater than 5 mm.

Comparison of Clinicoradiologic Results Between the Rectangular and Trapezoidal Gap Group

Demographic data, mean body mass index, coronal alignment, and knee function (including ROM and HSS score) were not significantly different preoperatively between the two groups (Table 2).

At the latest follow up, mean ROM was 123.1° (range, 80°-150°) in the rectangular group and 120.3° (range, 85°- 150°) in the trapezoidal group. There was no statistically significant difference between the two groups (P = .528). Neither improvement of ROM and HSS score nor correction of coronal alignment was found to be significantly different between the two groups (Table 3).

Discussion

Recently, navigation-guided TKA has been widely used for enhanced precision. Studies^12,13 have addressed the reliability of navigation-guided TKA in terms of restoration of mechanical axis of the limb, but, to our knowledge, there have been only a few reports that address the effectiveness and reliability of the navigation-guided technique regarding the soft tissue balancing.

A well-balanced gap is important to successful TKA. In this study, rectangular flexion and extension gaps were brought within 1 mm in about 70% of cases (72% in flexion, 74% in extension). Only two cases showed gaps greater than 4 mm. A rectangular gap, according to our definition, was achieved in 84% of all knees, with trapezoidal gaps achieved in the remaining 16%. Even the knees in the trapezoidal group had a relatively small amount of asymmetry in soft tissue balance, and only five cases showed an asymmetry greater than 5 mm.

Although the gaps measured in this study lacked sufficient statistical significance, there was a tendency for the flexion gap to be larger than the extension gap and a tendency for the lateral gap to be greater than the medial gap when there was inequality. Regarding the tendency for a larger flexion gap than an extension gap, a possible explanation is that we intended the equal or larger gap in flexion than in extension on both the lateral and medial sides. Although the knee ROM after TKA was influenced by various factors such as the flexion contracture, body mass index, degree of deformity, and preoperative ROM,^14,15 we believed that postoperative knee flexion could be improved to a certain extent by increasing the flexion gap slightly.^16,17 The degree to which we increased the flexion gap did not result in flexion instability. There is a report that several millimeters of laxity in the flexion gap resulted in increased patient satisfaction after TKA.¹⁸

It is not surprising that, in this study, the lateral gap tended to be larger than the medial gap, because the lateral ligaments of the normal knee are almost always slacker than the medial ligaments.¹⁹ Therefore, noted above, this trend toward a larger gap on the lateral side compared with the medial side was also shown from our intention in favor of the theories that the tibiofemoral flexion gap at 90° of flexion in the normal knee was not rectangular and that lateral joint laxity was significantly more than medial joint laxity in in vivo study using magnetic resonance imaging.²⁰

In this study, there was no statistically significant difference in ROM, HSS score, improvement of ROM, or restoration of limb alignment after TKA between the rectangular and trapezoidal groups. This finding could be attributed to the fact that even the knees in trapezoidal group had a relatively small amount of asymmetry in soft tissue balance.

The primary limitations of our study were relatively small sample size and short follow-up period. It is possible that some of the differences between the two groups might have reached statistical significance had the sample size been larger and the follow-up period been longer. Regarding the navigation-guided gap balancing in TKA, to our knowledge, there had been only two published reports in the English-language literature. Moreover, those two studies^21,22 evaluated the gap balancing indirectly by measuring opening angles on varus and valgus stress radiographs. In contrast to previous reports, this study directly quantified the soft tissue balance by the gap measurements.

In conclusion, soft tissue balancing using navigation-guided gap technique was a reliable method for achieving symmetrical flexion and extension gaps. With the relatively short follow-up period, no significant difference in clinicoradiologic outcomes was noted between the rectangular and the trapezoidal groups. However, because our results could not extrapolate long-term factors such as wear and loosing of components, the interpretation of these clinical findings should be considered with caution.

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Authors

Drs Han, Lee, and Chae are from the Department of Orthopedics, Korea University Medical Center, Seoul, Korea; Dr Nha is from the Department of Orthopaedic Surgery, Inje University, Ilsanpaik Hospital, Ilsan, Korea; and Dr Yoon is from the Department of Orthopedic Surgery, Seoul Veterans Hospital, Seoul, Korea.

Dr. Han is a speaker on behalf of B. Braun Aesculap. Drs Nha, Chae, Yoon, and Lee have no relevant financial relationships to disclose.

Correspondence should be addressed to: Dae-Hee Lee, MD, Department of Orthopedics, Korea University Medical Center, Korea University College of Medicine, 126-1, 5-ga, Anam-dong, Seongbuk-gu, 136-705, Seoul, Korea.