1 Introduction

Appropriate knee extension-flexion balance is a prerequisite for a successful outcome in total knee arthroplasty (TKA), because this balance affects joint stability,¹ range of motion,² and patient satisfaction.³ Previous studies have revealed that a flexion joint gap that is larger than the extension joint gap results in an improved postoperative knee flexion angle.² Chia et al.³ reported that a flexion joint gap balanced by being 0–2 mm larger than the extension joint gap is associated with improved clinical outcomes, and that objective and subjective clinical scores were significantly higher in the balanced group than in the unbalanced group 6 months after surgery. In contrast, extension-flexion imbalance is associated with an increased incidence of postoperative femoral condylar liftoff,¹ which leads to increased polyethylene wear.⁴

In this context, the target of extension-flexion balance in the current study was the flexion joint gap 0–2 mm larger than the extension gap. However, in clinical practice, targeted balance is frequently not achieved, and, in good agreement with our experience, several groups have reported that the frequency of extension-flexion imbalance is 34%–64%.^3,5–7 Modulation of the balance is exceptionally difficult once the extension-flexion balancing becomes inappropriate after bone resection; therefore, it is important to proceed with surgery by adjusting factors that affect extension-flexion balance during procedures. However, there have been limited studies on the factors that influence extension-flexion balance, and clarification of such factors will help surgeons to achieve targeted extension-flexion balance, and consequently improve clinical outcomes in TKA.

The purpose of the present study was to investigate preoperative physical findings, radiographic parameters, and intraoperative bone resection thickness to identify preoperative and intraoperative factors that influence extension-flexion balance. We hypothesized that the preoperative range of knee motion affects extension-flexion balance.

2 Materials and methods

This observational study was approved by the Institutional Review Board of our university. From June 2014 to September 2019, 93 consecutive primary TKAs performed for primary varus osteoarthritis using the Journey II Bi-cruciate Stabilized Total Knee System (Smith and Nephew, Memphis, Tennessee, USA) were included in this study. The exclusion criteria were TKAs for valgus osteoarthritis, rheumatoid arthritis, and TKA with other prostheses. Four knees with incomplete intraoperative measurement data were excluded. Finally, 93 knees (73 knees of female patients and 20 knees of male patients, with a mean age of 73.5 ± 7.1 years) were included in the study.

2.1

2.1 Surgical procedures and measurement of joint gaps and resected bone thickness

All patients underwent primary TKA performed by two specialist knee surgeons using the modified gap balancing technique.⁸ Resection of the cruciate ligaments, osteophyte removal, and detachment of soft tissue adhesion were performed through the medial parapatellar approach to balance the knee in extension. The proximal tibia was resected using an extramedullary alignment guide 11 mm below the lateral plateau, perpendicular to the mechanical axis of the lower leg, and with a posterior tibial slope of 4°. The distal femur was resected using an extramedullary alignment guide 9 mm above the most distal point, with a mechanical axis of the femur of 0° and a flexion angle of 3°.⁹ If the extension gap was not sufficient for the thinnest 9-mm-thick insert, the proximal tibia was cut by an additional 2 mm. The rotational alignment of the femoral component was determined by a central spreading tensor (Journey, Smith and Nephew) by applying a force of 120 N,¹⁰ and the medial posterior femoral condyle was cut according to the thickness of the femoral component. The thickness of the resected bone of the proximal tibia, distal femur, and posterior condyle of the femur was recorded (Fig. 1). After all bone resections, joint gaps were made at the medial and lateral compartments by a 60 N distraction force for each compartment, and were measured in extension and 90° flexion using force-controlled compartment-specific ligament tensioner.^11,12 The gap difference (ΔGap) was defined as the distance of the flexion gap subtracted from that of the extension gap.

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Fig. 1 Bone resection thickness. (a): Medial distal femur, (b): lateral distal femur, (c): medial proximal tibia, (d): lateral proximal tibia, (e): medial posterior condyle of the femur, and (f): lateral posterior condyle of the femur.

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2.3

2.3 Radiographic evaluation

The femoral condylar height (CH), femoral posterior condylar offset (PCO),¹⁴ and joint line convergence angle (JLCA)¹⁵ were radiographically assessed (Fig. 2). The CH was defined as the distance between the adductor tubercle of the femur and the articular surface of the medial femoral condyle in the anteroposterior view. The PCO was defined as the distance between the tangent line of the posterior cortex of the femur and the most posterior point of the posterior condyle in the lateral view of the knee. The JLCA was defined as the angle between the tangential lines of the femoral condyles and the tibial plateau.

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Fig. 2 Radiographic evaluations. Anteroposterior radiograph of the knee (a): femoral condylar height (CH). Lateral radiograph of the knee (b): femoral posterior condylar offset (PCO). Varus-stress radiographs (c): joint line convergence angle (JLCA) in varus-stress radiographs. Valgus-stress radiographs (d): JLCA in valgus-stress radiographs.

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3 Results

The mean total follow-up period, age, sex, and KSS are summarized in Table 1. The KSS was significantly improved after surgery. Patients were stratified into three groups based on the magnitude of ΔGap (flexion joint gap minus extension joint gap): group L (loose flexion joint gap), >2 mm; group N (normal joint gap), 0–2 mm; and group T (tight flexion joint gap), <0 mm. There were 18 knees (19.3%) in group L, 45 knees (48.3%) in group N, and 30 knees (32.2%) in group T. The mean preoperative maximum extension angle was significantly lower in group L than in group N and group T (P < 0.005). There were no significant differences in preoperative maximum flexion angle between the three groups (Table 2).

4 Discussion

In this study, we investigated the preoperative and intraoperative factors that influence extension-flexion gaps. We found that ΔGap increased when the preoperative extension angle or the bone resection thickness of the medial distal femur decreased, or when the thickness of the resected medial proximal tibia increased. Therefore, bone should be resected bearing in mind that the resection thickness affects ΔGaps. Furthermore, decreasing the bone resection thickness in the medial proximal tibia and increasing the bone resection thickness in the medial distal femur may help to achieve a balanced ΔGap in knees with preoperative flexion contracture.

Notably, group L was associated with flexion contracture, as shown by the smaller maximum extension angle and thicker bone resection of the medial proximal tibia than those of the other groups. These results might be attributed to our surgical strategy to avoid joint line elevation, wherein the proximal tibia was additionally cut by 2 mm if the extension gap was not achieved by the first cut of the distal femur and the proximal tibia. In fact, the thickness of bone resection of the medial proximal tibia in group L was significantly higher than that in group T. In contrast, the thickness of the bone resection of the distal medial femur in group T was significantly higher than that in group L, suggesting that an increase in bone resection thickness of the distal medial femur leads to a decrease in ΔGaps. Consistent with our result, Cross et al. reported that recut of the distal femur can increase the extension gap required to achieve full knee extension.¹⁶ Taken together, at the stage of obtaining the extension gap in the knee with flexion contracture, it is important to determine whether the distal femur or proximal tibia is additionally cut in order to decide the extension-flexion balance.

Joint line elevation alters the biomechanics of the knee, and thereby leads to a change in the isometry of the medial collateral ligament,¹⁷ mid-flexion instability due to the change in flexion angle, and extensor mechanism strength by the reduced posterior condylar offset,¹⁸ resulting in poor clinical outcomes.¹⁹ Therefore, although the priority in our surgical strategy was not to induce joint line elevation, it turned out that such a strategy (i.e., the first additional cut of the proximal tibia) results in a loose flexion gap, as shown in our results. Whether the relationship between the degrees of joint line elevation and extension-flexion balance leads to better clinical outcomes should be studied in the future.

Varus-stress JLCA was significantly higher in group N than in group T, suggesting a relatively tight posterolateral ligamentous complex and popliteus tendon in group T. Since the medial flexion gap becomes larger after popliteus tendon resection,²⁰ this surgical procedure might be useful to adjust the extension-flexion balance in cases with a tight flexion gap.

Consistent with previous reports, about half of the knees in this study showed an imbalanced gap.^3,5–7 In cases of flexion contracture, to avoid tight or loose gap knees, we currently measure extension-flexion gaps before posterior femoral condyle resection and after the first resection of the distal femur and proximal tibia to create an extension gap. If a flexion gap is thought to be relatively tight, the proximal tibia is additionally cut, and conversely, if the flexion gap is considered to be relatively loose, the distal femoral resection was performed. Together, these procedures help to achieve a targeted gap balance.

This study has several limitations. First, we did not evaluate the relationship between the ΔGap and clinical outcomes, although we took note of the preoperative and intraoperative factors that influence the ΔGap. Therefore, it is not certain that the results in this study affect clinical outcomes. Second, we did not discuss the presence of osteophytes in the posterior compartment of the femoral condyle. Because removing these osteophytes is known to affect the extension gap in TKA,^21,22 bone should only be resected after considering these effects. Third, we only evaluated Bi-cruciate Stabilized type implants; thus, the results of this study could not be applied to other types of prostheses.

5 Conclusions

We investigated preoperative physical findings, radiographic parameters, and intraoperative bone resection thickness to identify the preoperative and intraoperative factors that influence ΔGaps. Preoperative flexion contractures were shown to cause larger ΔGaps. Furthermore, decreased bone resection thickness in the medial proximal tibia, and increased bone resection thickness in the medial distal femur led to decreased ΔGaps.

Ethical approval

Ethical approval for this study was obtained from the Institutional Review Board at the authors’ institution (approval no.: 2019-238).

Informed consent

Informed consent was obtained from all individual participants included in the study.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.