The main goals of total knee arthroplasty (TKA) are to alleviate pain and improve function, while restoring alignment, balance, and knee motion. Indeed, one of the most impactful technical factors to optimize the outcome of TKA is proper balancing of the soft tissues, in both the coronal plane (varus/valgus) as well as the sagittal plane (flexion and extension) gaps.¹,² Creating equal rectangular gaps in flexion and in extension after bone resection and surface preparation ensures a stable knee throughout the range of motion, while providing full extension and maximal flexion.

Most arthritic knees will present with some modicum of underlying soft-tissue imbalance, often with a flexion contracture, which will need to be corrected at the time of surgery. A balanced knee is important in order to attain proper knee function, compatible with an active lifestyle, and to optimize the longevity of the prosthesis by avoiding uneven strain that could lead to early failure by implant loosening or wear of the polyethylene insert, even in the setting of suboptimal bone resections.³ This chapter will focus on standard methods of flexion/extension gap balancing.

Proper preoperative planning vis-à-vis flexion-extension gap balancing begins during preoperative evaluation. The physical examination should note (and document) the preoperative knee range of motion, paying particular attention to the presence and magnitude of flexion contracture, extension lag or recurvatum deformity, and assessment of coronal and sagittal instability and coronal alignment. Standard imaging includes weight-bearing anteroposterior, flexed posteroanterior, lateral, as well as sunrise radiographs. These are evaluated for coronal deformity, presence and size of osteophytes, loose bodies, tibial translation relative to the femur, and coronal alignment. Taken together, these assessments will help the surgeon plan resections and anticipate required soft-tissue releases and types of implants to be available.

Evaluation of flexion and extension gaps during surgery can be done with the use of spacer blocks, quantified ligament-tensioning devices or pressure sensors, laminar spreaders, or manual traction with visual evaluation. The goal is to have equally sized and balanced rectangular gaps (coronal balancing) in both full extension and 90° of flexion. Gap assessment by manual varus/valgus stress with trial implants in place after bony resection, and anterior/posterior drawer testing, is also performed. Scott and Chmell described the “pull-out lift-off test” (POLO test) for proper balancing of posterior cruciate ligament (PCL) tightness in cruciate retaining (CR) TKA with use of a stemless tibial component trial.⁴ A positive pullout test reflects a loose flexion gap: the tibial trial can easily be pulled out and/or slid into place in 90° of flexion. Conversely, a tibial trial that lifts anteriorly off of the tibial bone surface with the knee in flexion reflects a PCL that is too tight (positive lift-off test). Alternatively, a femoral component that rolls back excessively also suggests a flexion gap that is too tight (Fig. 30-1). An everted patella should be reduced into place when assessing for a lift-off test because it may lead to a false-positive result due to an eccentric proximal pull of the patellar tendon on the tibia.

Proper intraoperative assessment of adequate coronal and sagittal balancing can also be aided with the use of a sensor-embedded tibial insert trial.⁵ These devices can quantify contact pressures in the medial and lateral compartments both in extension and flexion positioning, identifying the maximal contact points in the medial and lateral compartments and assisting in guiding soft-tissue releases or bone resection adjustments (Fig. 30-2). Kinetic tracking can identify patterns of femoral rollback with passive flexion. In a knee with adequate sagittal balance, contact points will cluster in the central third of the bearing surface in 90° of flexion. In cases with a tight flexion gap, there will be posterior positioning of the contact points with increased loads and minimal excursion during posterior drawer testing. On the other hand, if a loose flexion gap is present, there will be increased excursion of the femoral contact points during posterior drawer testing and anterior translation of the femoral contact points.

This technology is used by some surgeons to objectively assess soft-tissue balancing instead of relying on subjective feel. Indeed, Elmallah et al demonstrated improved balancing with the use of an electronic sensor over manual balancing by an experienced arthroplasty surgeon.⁶ Whether this translates to improved kinematics and functional outcomes has yet to be determined.

As previously stated, the target for a “balanced knee” is equal tightness medially and laterally throughout the range of motion. This is achieved by obtaining equal and rectangular gaps between the resected bone surfaces in extension (between the distal femoral cut and the tibial cut) and flexion (between the posterior femoral and the proximal tibial resection) (Fig. 30-3). Two techniques have been traditionally described to achieve a balanced knee: the measured resection technique and the gap balancing technique. In measured resection, the surgeon aims to remove equivalent bone and cartilage as will be replaced by the prosthesis, and the bone cuts are made in reference to anatomical landmarks. In the true gap balancing method, bone cuts are made based on soft-tissue tension as opposed to anatomical landmarks. The most important difference is the way femoral component rotation is determined. In measured resection, the femoral cutting block determining femoral component rotation is placed in reference to Whiteside line, the posterior condylar axis, and/or the transepicondylar axis. In gap balancing, the cutting guide is placed parallel to the tibial cut with use of a tensioning device or spacer block. In reality, whether acknowledging it or not, most surgeons use a combination of the two methods, although the sequences may vary. In any case, soft-tissue releases may still be necessary after bone cuts are made to ensure proper balancing. Indeed, a well-balanced knee is achieved through combined modification of bone cuts, soft-tissue releases, and/or component modification (sizing, augments, alignment, and rotation).

Matsumoto et al looked at 135 gap-balanced TKAs using navigation and compared them to 120 measured resection TKAs.⁷ They found significantly higher sagittal gap imbalance in PCL-sacrificing/-substituting (PS) knees compared to CR knees, mostly consisting of an increased flexion gap, with both measured resection and gap balancing methods. CR knees were also found to have a significantly higher rate of achieving equalized rectangular gaps in both flexion and extension as compared to PS knees both with measured resection and gap balancing methods. Despite these differences, functional outcomes and range of motion at minimum 2-year follow-up were similar among all groups. Moon et al conducted a meta-analysis comparing soft-tissue balancing, femoral component rotation, and joint line change between gap balancing and measured resection methods.⁸ They found no significant overall differences in flexion and extension gaps between the two techniques, although medial/lateral gaps in extension were found to be more balanced using gap balancing. The gap balancing method resulted in more external rotation of the femoral component and more joint line elevation compared to the measured resection technique. These changes had minimal clinical relevance, as they were typically in the 1 mm or 1° range. Another meta-analysis in 2017 found similar results, with no difference in flexion gap, extension gap, or flexion/extension differences between the two approaches and more joint line elevation when gap balancing.⁹ However, the study found that patients who received TKA with the gap balancing technique had significantly higher functional scores 2 years postoperatively. On the contrary, a 5-year follow-up report of their randomized controlled trial by Babazadeh et al showed no difference in functional outcomes between measured resection and gap balancing techniques despite greater joint line elevation in the gap balancing group.¹⁰ Of note, the joint line elevation was still well below 8 mm, which is the classically described threshold for either optimal or poor functional outcome after TKA with joint line elevation.¹¹

Depending on one’s preference, the surgeon can choose to balance the extension gap first and then the flexion gap at 90° of flexion, or the flexion gap can be
determined and balanced first. The extension gap is balanced first by many, because the bone cuts are often (but not always) made independent of the soft tissues. The distal femoral cut is typically made at a predetermined angle of valgus relative to the axis of the femur, although it may be individualized according to preoperative alignment measures or based on whether the planned tibial cut is made perpendicular to the mechanical axis or using a kinematic alignment method. Others may prefer to prepare the flexion gap first, arguing that it reduces the risk of joint line elevation; however, contemporary implant size intervals, appropriate femoral sizing, and maintenance of posterior condylar offset reduce this risk.²,¹²

Clinically, a tight extension gap will present as an inability to achieve full extension, leading to a flexion contracture. Persistent flexion contracture postoperatively can have detrimental effects on the outcome after TKA with increased pain, altered gait, decreased functional scores, increased energy expenditure during gait, and diminished quality of life.¹³,¹⁴,¹⁵ Intraoperatively, a small flexion contracture is first addressed by resection of osteophytes from the posterior femoral condyles, which may tent the posterior capsule and may prevent terminal extension. They may also potentially lead to coronal asymmetry if the posterior condylar osteophytes are unilateral or larger on one side of the knee than the other (Fig. 30-4A). Resection of the posterior femoral osteophytes has to be handled with care, in order to avoid excessive posterior condylar resection (which may compromise implant support) or inadvertent injury to the neurovascular bundle. Usually this is done with the knee in flexion and a laminar spreader between the tibia and the posterior femur. Pushing back on the posterior tibia (as if performing a posterior drawer test) can also optimize visualization of, and access to, posterior osteophytes and loose bodies. The osteophytes are removed with the use of a ¾″ curved osteotome (Fig. 30-4B).

In addition to affecting the extension gaps, posterior osteophytes can also affect flexion range by causing impingement on the posterior aspect of the tibial component in deep flexion. Indeed, Yau et al showed that residual posterior osteophytes after TKA were an independent risk factor for decreased flexion at 1 year after PS TKA, in addition to limited preoperative range of motion and overstuffing of the patellofemoral joint.¹⁶ Sriphirom et al in a recent prospective study on patients undergoing computer-assisted TKA showed a significant increase in both flexion and extension gaps after removal of posterior osteophytes.¹⁷ This further underlines the necessity to excise these osteophytes early on during the surgery in order to avoid unnecessary soft-tissue releases and excessive bony resection, especially if a gap balancing method is used and tension is assessed with the presence of these osteophytes.

If flexion contracture persists after resection of posterior osteophytes and loose bodies in the posterior recess, the posterior capsule is released subperiosteally using a Cobb elevator from the posterior femur in a controlled fashion to avoid injury to the superior geniculate arteries or to the popliteal neurovascular bundle (Fig. 30-5). In severe concomitant varus deformity with flexion contracture, adequate medial release should be ensured before too aggressive of a posterior release. This should include the posteromedial capsule and the semimembranosus insertion and that should help decrease extension gap tightness. In revision cases, actual excision of scar tissue posteriorly may be necessary.