Gap balancing versus measured resection in TKA
Evidence for/against gap balancing
When it comes achieving correct femoral component positioning during TKA, one commonly used technique is gap balancing. This method is successfully employed by many surgeons and involves balancing tension and flexion gaps before femoral bone cuts are made. Here, in part 2 of our article series, we explore gap balancing and some of the positive and negative commentary surrounding this technique.
Some schools of thought maintain that the success of total knee arthroplasty (TKA) relies - among other things - on achieving symmetric, balanced flexion and extension gaps . Indeed, soft tissue management has been identified by some as more important to outcomes than bone management .
If femoral component malrotation is present, it can result in decreased patient satisfaction (pain) and implant disfunction  via altered tibiofemoral kinematics [4,5]. Malrotation can also cause patellofemoral complications and instability. This was directly correlated to the combined (femoral and tibial) internal component rotation by Berger et al.  It has also been suggested that if relative rotational mismatch is kept within ±5° (surgical norms), then knee axial rotation during flexion is more controlled and more likely to deliver a better outcome .
Gap balancing has been shown to produce desirable outcomes, particularly because balancing is done independent of bony anatomy, eliminating confusion around intraoperative landmarking that could be compromised by bone deformities and individual anatomical variation [7,8].
Matthew Abdel, orthopedic surgeon at the Mayo Clinic in Rochester, US, finds the technique, “particularly useful for patients with distal femoral fractures where anatomic landmarks are displaced and may make a measured resection technique difficult.” Philipp von Roth, orthopedic surgeon at the Charité-University Hospital in Berlin, Germany, feels that the “gap balancing technique is optimal for redressable varus- or valgus deformities and in case of difficulties identifying the anatomical landmarks with intact ligaments.”
Two gap balancing techniques
While measured resection is a technique used to determine femoral component rotation, this article will examine gap balancing and its two recognized sequences, looking at the pros and cons. See our interview on why surgeons use one or the other technique.
In gap balancing, ligaments are released before femoral bone cuts are made . One technique we will discuss involves balancing the flexion gap first; the other technique first balances the joint in extension, then returns to address the flexion gap in relation to the previously balanced extension gap . Both require sequential soft tissue balancing .
Remove all osteophytes
Both techniques demand the removal of all osteophytes (including posterior femoral and tibial) before soft tissue releases. Ramappa et al. reminds surgeons about “the importance of meticulous osteophyte excision” . These bony projections tension adjacent ligaments, which may produce flexion gap asymmetry and femoral component malrotation , and increase the likelihood of interaction between implant components and surrounding tissue, to the detriment of the patient .
Success tied to accurate tibial cuts
The success of gap balancing, for both sequences, hinges on a precise proximal tibial cut; correct depth and alignment on the coronal and sagittal planes is critical to avoid femoral component malrotation . If the resection is varus or valgus then either internal or external rotation, respectively, of the femoral component will occur [7,8]. This can result in “a mismatch of flexion and extension gap dimensions” . The tibial cut is used as a reference point for femoral resections, further reaffirming the importance of accuracy in the tibial resection.
If you start with the flexion gap
If a surgeon chooses to balance the extension gap first they are looking for parallel alignment of the tibial cut with the transepicondylar axis, which will be perpendicular to the anteroposterior axis. Required soft tissue releases are performed after the joint is correctly tensioned in flexion, using the above alignments to guide releases (Fig. 1.). After this, the extension gap addressed .
If you start with extension balancing
This gap balancing approach is inverse to the one described above; here, the extension gap is balanced before the flexion gap. The dimensions established in extension are duplicated in flexion. Daines et al. find this method preferable (versus balancing in flexion first) for its ability to deliver “more precise and reproducible gap balance”. Laminar spreaders and cutting/spacer blocks are maneuvered to equally tension ligaments (Fig.2) .
Flexion and extension gaps should be symmetrical, rectangular, and evenly tensioned  (Fig. 3). It is preferable that bone cuts are not used to achieve this balance . However, one study notes that even with meticulous efforts, perfectly balanced gaps are not always attainable .
A 2017 study published in the Journal of Arthroplasty points out that gap balancing of the posteriorly stabilized TKA can be influenced by posterior tibial translation and result in “artifactual widening of the flexion gap” to the extent that it changes the size selection of femoral components in some patients . Heesterbeek et al. report that even if the gaps are balanced, femoral component malrotation may still occur since “patient variability and variation in ligament releases” play a role as well .
The case for gap balancing
Gap balancing has been shown to be a successful technique for soft tissue balancing in TKA and has been used by surgeons for many years . In and of itself, gap balancing will deliver positive outcomes and is regularly included in comparative studies [16,17,18] with measured resection, another technique that is discussed in this article series.
A major consideration that supports the use of this method is that gap balancing is less dependent on intraoperative bony landmarks and therefore not influenced by irregular anatomical characteristics or deformities . Matthew Abdel comments that, “Gap balancing techniques allow the surgeon to balance the flexion and extension gaps through bony cuts, minimizing the amount of soft-tissue releases that need to be completed.”
Further, the technique is, “Dependent on the integrity of the collateral ligaments…as long as the collateral ligaments are in a good shape, the gap balancing technique is preferable in case[s] of bony deformation,” says Philipp von Roth.
The case against gap balancing
Discussions and research regarding gap balancing do not find the technique inappropriate or inadequate. However, there is a subtler question about when to employ this technique (See Table 1). Surgeon skill and experience and patient characteristics play a role in the effectiveness, however, as a sole tactic, it may not always deliver the intended results. “True gap balancing technique may result in internal rotation of the femoral component, contributing to patellofemoral issues,” says Matthew Abdel.
The fact that the technique requires accurate tibial resection could be seen as a detractor from its selection as it becomes more difficult to adjust for resection inaccuracies. Over or under resection of either the femoral or tibial bone prevent symmetrical gap dimensions. Adjusting component sizes and over-resection of the posterior femoral condyles is then needed . “The technique is less forgiving,” says Philipp von Roth. “If the tibial cut is not performed properly, you may experience major problems during the implantation process—femoral rotation errors and relevant changes in the joint line.”
When not to use gap balancing
A couple of considerations should be reflected upon when evaluating if gap balancing is right for a patient. Firstly, collateral integrity should be present or rotational errors could occur , as mentioned above by Philipp von Roth. Secondly, if non-reduceable, fixed deformities are found in a patient then gap balancing may not be the appropriate choice as it hampers correct bone cuts and component positioning . Table 1 summarizes a number of critiques both for and against the use the gap balancing. It is by no means intended to be an exhaustive list for either category.
Advantages of gap balancing
Disadvantages of gap balancing
Delivers good femoral component rotation [19,20]
Tensioners are used when the patella is subluxed which means the gap any be wrongly sized or misbalanced and produce excessive internal rotation
Can result in coronal stability [21,22]
Difficult to control distraction forces 
Provides flexion gap stability 
Does not always allow for midflexion balancing; equal tension of ligament at 0° and 90° not enough to prevent midflexion instability [8,24]
Can lead to good patellar tracking 
Collateral ligament integrity needed, or rotational errors may result 
Compensates for lateral laxity by adjusting rotation under tension 
Needs precise proximal tibial resection 
Allows accommodations for unpredictable effect releases can have on flexion space [8,25]
In cases of fixed, non-reducible deformities the technique might be limited 
Lower incidence of condylar lift off 
Numerous studies and successful surgical outcomes support the use of gap balancing. However, there is also support for the measured resection technique as it similarly delivers positive outcomes [26,27]. Part 3 takes a deeper look of measured resection and its pluses and minuses. See Part 1 of this article series for further comparison of gap balancing and measured resection. Is one superior to the other? Which one should you use in TKA?
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