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Flexion-Extension Gaps Balanced Using Navigation Assistance in TKA

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Abstract

This study evaluated the accuracy of navigation for balancing soft tissue and flexion-extension gaps in primary total knee arthroplasty (TKA). We evaluated 112 knees treated with TKA using the gap technique and navigation system. Flexion-extension gaps were measured before femoral cutting (precutting gaps) and before prosthesis insertion (final gaps). Balanced precutting flexion-extension gaps were shown in 45 cases (40.2%), and balanced final flexion-extension gap was achieved in 105 cases (93.8%). Precutting tight extension gaps resulted from preoperative extension lag. We found TKA using the gap technique with navigation to be an effective means of achieving balanced flexion-extension gaps.

It is well known that successful total knee arthroplasty (TKA) depends on several factors (eg, proper patient selection, appropriate implant design, correct surgical technique, and effective perioperative care). Accurate restoration of the mechanical leg axis and knee stability in extension and flexion have been cited as the 2 most important objectives to achieve for TKA to be successful. To achieve these objectives, medial-lateral and flexion-extension gaps must be balanced, because imbalanced gaps cause instability, pain, swelling, walking disturbances, abnormal wear, premature mechanical loosening of components, and patellofemoral problems.^1-5 To resolve imbalanced gaps, secondary additional procedures, such as posterior cruciate ligament (PCL) recession or sacrifice, or recutting of the distal femur, posterior femur, or proximal tibia, are performed, according to the surgeon’s experience. These may result in inadequate flexion-extension gaps or imbalanced PCL tension, which is difficult to correct.

When comparing TKA techniques, the gap technique is better than the measured resection technique for achieving balanced flexion-extension gaps. Furthermore, the navigation systems increasingly used for TKA provide excellent restoration of the mechanical axis and precise component positioning, which improve the accuracy of the balancing procedure by providing more objective and quantitative measures of flexion-extension gaps.⁶

This study evaluated the functionality of navigation-assisted TKA by measuring and comparing differences between flexion-extension gaps before and after femoral bone cutting in TKA using the gap technique with navigation.

Materials and Methods

Between March 2003 and April 2004, 88 patients (112 knees) who were followed up for at least 1 year after TKA for osteoarthritis were enrolled in this study. The version 4.0 or 4.2 OrthoPilot (B. Braun Aesculap, Tuttlingen, Germany) navigation system was used in all cases. Ethical approval was granted for the study, and all patients (9 men and 79 women; average age, 67.6 years [range, 52 - 79 years]) who participated provided informed consent.

All procedures were performed using the gap technique with navigation by a single surgeon, and the intraoperative details and measurements were recorded. Using a medial parapatellar approach, the knee joint was exposed, and the hip, knee, and ankle centers were navigated. Anatomic landmarks were registered by hand using a pointer to define the joint line and the mechanical axis of the leg. The mechanical axis was then restored to neutral (±2°) at full extension by stepwise meticulous medial soft tissue release and osteophyte removal (Figure 1). Proximal tibial bone cutting was performed under real-time navigation system control, and the PCL was preserved and confirmed to be functionally intact. Flexion-extension gaps were measured at full extension and at 90° of flexion using a V-STAT variable soft tissue alignment tensor (Zimmer, Warsaw, Indiana) and a special torque wrench set at 200 Nm before femoral bone cutting (precutting flexion-extension gaps) (Figure 2). Gap differences were classified as balanced, tight flexion, or tight extension gaps. A balanced gap was defined as having a flexion-extension gap difference of within 3 mm; a tight flexion gap was defined as an extension gap at least 3 mm more than the corresponding flexion gap; a tight extension gap was defined as a flexion gap at least 3 mm more than the corresponding extension gap. Levels of distal and posterior femoral cuts and amount of femoral component rotations were determined based on extension-flexion and medial-lateral gap differences (Figure 3). Following final bone cuts and soft tissue release completion, flexion-extension gaps were reassessed (final flexion-extension gaps) (Figure 4).

Figure 1: After anatomic and kinematic registration, the mechanical axis was restored to neutral (±2°) at full extension by incremental meticulous medial soft tissue release and osteophyte removal.

Figure 2: After proximal tibial bone cutting with preserving posterior cruciate ligament under navigation and before femoral bone cutting, flexion-extension gaps were measured at full extension and at 90° of flexion using a tensioning device and a special torque wrench set at 200 Nm (precutting flexion-extension gaps).

Figure 3: Levels of distal and posterior femoral cuts and amount of femoral component rotation were planned based on extension-flexion and medial-lateral gap differences.

Figure 4: Following final bone cuts and soft tissue release completion, flexion-extension gaps were reassessed (final flexion-extension gaps).

Preoperatively and postoperatively, patients were assessed using Hospital for Special Surgery (HSS) scores and Western Ontario MacMaster (WOMAC) scores. In addition, ranges of motion (ROMs), complications, and mechanical axes in standard weight-bearing plain radiographs were recorded for all patients. All assessments were performed by one author, who was not directly involved in the surgical procedures.

The 3 groups were compared using ANOVA, and statistical significance was accepted for P values <.05.

Results

According to precutting gap differences based on medial sides, 59 knees (52.7%) had a balanced gap, 32 knees (28.6%) had a tight flexion gap, and 21 knees (18.8%) had a tight extension gap (Table 1). In the tight flexion gap group, 2 (6.3%) knees showed a flexion-extension difference of >5 mm; in the tight extension gap group, 8 (25%) knees showed a difference of >5 mm. The final flexion-extension gap difference was 0.9 mm (range, -5-7 mm). According to final gap differences, 106 knees (94.6%) had a balanced gap, 2 knees (1.8%) had a tight flexion gap, and 4 knees (3.6%) had a tight extension gap (Table 1).

According to precutting gap differences based on lateral sides, 66 knees (58.9%) had a balanced gap, 20 knees (17.9%) had a tight flexion gap, and 26 knees (22.3%) had a tight extension gap (Table 2). In the tight flexion gap group, 6 (30%) knees showed a flexion-extension difference of >5 mm, and in the tight extension gap group, 10 (38.5%) knees showed a difference of >5 mm. The final flexion-extension gap difference was 0.4 mm (range, -5-6 mm). According to final gap differences, 108 knees (96.4%) had a balanced gap, 1 knee (0.9%) had a tight flexion gap, and 3 knees (2.7%) had a tight extension gap (Table 2).

Overall, 7 knees (6.3%) had imbalanced final flexion-extension gaps on the medial or lateral side; of these, 3 knees had imbalanced flexion-extension gaps on both sides. Therefore, 105 knees (93.7%) had balanced final medial-lateral flexion-extension gaps.

Preoperative mean extension lag was 9.9°. On the medial side, the precutting tight extension gap group had a significantly more preoperative extension lag than the precutting tight flexion (P=.000) and balanced gap groups (P=.000). On the lateral side, the precutting tight extension gap group also had a significantly more preoperative extension lag than the precutting tight flexion (P=.000) and balanced gap groups (P=.004). Preoperative mean ROM was 115.8°, mean HSS score was 61.6 points, and mean WOMAC score was 77.2 points. At last follow-up visit, mean extension lag and ROM were 0.8° and 131°, respectively (Table 3). The HSS and WOMAC scores were 93.3 and 31.4 points, respectively (Table 4). According to precutting gap differences of flexion-extension gaps on the medial or lateral sides among the 3 groups, no significant difference was observed in terms of preoperative or postoperative mean ROM or in HSS or WOMAC scores.

By radiographic evaluations, mean mechanical axis was 169.4° preoperatively, whereas it was 178.8° in 6 cases (5.4%) of outliers (>±3° of optimum) at the last follow-up. According to precutting gap differences of flexion-extension gaps on the medial or lateral sides among the 3 groups, no significant difference was observed in terms of mechanical axis.

Discussion

The 2 methods used to evaluate soft tissue tensioning are lamina spreading with maximum distraction power and the use of calibrated tensioning devices. Soft tissues surrounding the knee joint have viscoelastic characteristics; thus the knee joint space is dependent on the amount of distraction power applied to soft tissue. The lamina spreading method cannot be viewed as an objective method because the tensioning of soft tissue and the substantial forces applied are not quantitative.⁷ However, evaluation of soft tissue tension using calibrated tensioning devices is straightforward and yields quantitative information. Also, distraction power of 200 Nm is typically sufficient to measure extension and flexion gaps.^8,9 Therefore, in the present study, we used a torque of 200 Nm delivered by a tensioning device (V-STAT tensor) to measure both gaps.

Flexion-extension gap balancing should be performed in accordance with medial and lateral collateral ligament balancing, but degree of laxity remains controversial. Scott and Volatile¹⁰ and Zalzal et al¹¹ reported that approximately 2 mm of laxity is acceptable, whereas Sugama et al¹² favored <2 or 3 mm of laxity. However, Winemaker¹³ defined an imbalanced gap as one with more than 3 mm of laxity, and observed flexion-extension gaps of >3 mm on the medial and lateral sides in 8.4% of knees. Griffin et al¹⁴ defined an imbalanced gap as one with >3 mm of laxity, and imbalance of >3 mm was observed in 13.5% of knees on the lateral sides and in 10.6% of knees on the medial sides. In our series, we regarded a balanced gap to be one with a flexion-extension difference of <3 mm, a tight flexion gap as an extension gap of at least 3 mm more than the corresponding flexion gap, and a tight extension gap as a flexion gap at least 3 mm more than the corresponding extension gap.

In our study, flexion-extension gaps were measured during TKA using a navigation system. Navigation systems are being increasingly used in TKA and provide excellent restoration of the mechanical axis and precise component positioning, which improve the accuracy of the balancing procedure by providing more objective and quantitative measures of flexion-extension gaps.⁶ Saragagila et al¹⁵ reported that navigation is a useful tool for checking the reducibility of deformity before performing any release and for predicting the amount of medial release. Our study shows that approximately 60% of knees had balanced gap before femoral cutting. Patients with a greater preoperative extension lag were found to be more likely to have precutting tight extension. Moreover, the precutting balanced, tight flexion, and tight extension gap groups showed no differences in postoperative clinical outcomes, because almost all cases in the 3 groups had good results due to achieved final balanced flexion-extension gaps. However, because the number of knees with a final imbalanced gap was small, the clinical results of the 3 groups could not be meaningfully compared. Further study of this issue is needed.

Two surgical techniques can be used to establish ligament balance: the gap technique and the measured resection technique. The latter technique may have accompanying problems in imbalanced cases. To resolve an imbalanced gap, multiple additional bone cuts or soft tissue releases are needed after complete tibial and femoral bone cuts have been made. However, multiple bone cuts or soft tissue releases could create more imbalance. Using the gap technique, when a tight flexion gap is encountered, the following modalities should be considered: (1) cut more at the posterior condyle of the distal femur, (2) release or sacrifice the PCL, or (3) increase the posterior slope of the proximal tibia. On the other hand, when a tight extension gap is encountered, more bone is excised from the distal femur. Furthermore, a calibrated torque wrench and tensor system that provide reproducible measures of flexion-extension gap differences could be used to assist surgeons in determining optimal balance in individual knees earlier during operations.

Unitt et al¹⁶ measured flexion-extension gaps in 218 TKAs using the measured resection technique before and after soft tissue release and defined an imbalanced gap as one with >3 mm of laxity. Before soft tissue release, 110 knees (50.5%) had a balanced gap and 108 (49.5%) an imbalanced gap. After soft release, 92 of the 110 balanced knees (83.6%) still had a balanced gap, and 83 of the 108 imbalanced knees (76.9%) became balanced. Based on these results, balanced flexion-extension gaps in TKA using the measured resection technique was achieved in 175 knees (80.3%). In the present study, in terms of precutting gap differences, a balanced gap was achieved in approximately 60%. However, when final flexion-extension gaps were measured, 105 knees (93.7%) had balanced final medial-lateral flexion-extension gaps and seven cases (6.3%) did not. When the present study was compared with Unitt’s study, measured flexion-extension gaps before soft tissue release in the measured resection technique and before femur cutting in the gap technique were similar. TKA using the gap technique with navigation produced better results for balanced flexion-extension gaps than TKA using the measured resection technique; this was especially the case in knees with imbalanced flexion-extension gaps before soft tissue release using the measured resection technique or before femur cutting using the gap technique.

Conclusion

Balanced final flexion-extension gap could be achieved in 105 cases (93.8%) by using the navigation system, although precutting flexion-extension gaps were balanced only in 45 cases (40.2%). Therefore navigation could be considered as a useful tool for TKA using the gap technique to achieve balanced flexion-extension gaps. Navigation-assisted TKA using the gap technique also provided accurate alignment of the lower leg radiologically and excellent midterm clinical outcome. It remains to be seen whether balancing these knees will lead to a better outcome in the long term.

References

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Authors

Drs Song, Seon, and Park are from the Department of Orthopedic Surgery, Chonnam National University Hwasun Hospital, Chonnam, Korea.

Dr Song is a consultant for B. Braun Aesculap. Drs Seon and Park have no relevant financial relationships to disclose.

Correspondence should be addressed to: Sang-Jin Park, MD, PhD, Center for Joint Disease, Chonnam National University Hwasun Hospital, 160, Ilsim-Ri, Hwasun-Eup, Hwasun-Gun, Jeonnam, 519-809, Korea.

doi: 10.3928/01477447-20090915-55