Gap balancing versus measured resection in TKA
Evidence for/against measured resection
Measured resection is a TKA technique that relies on bony landmarks to set component placement and adapts the soft tissues to the chosen implant position. It is an effective method and commonly used by surgeons around the globe. In Part 3 of this article series we look at measured resection and explore the benefits and shortcomings of this technique.
Establishing acceptable component positioning and soft tissue tensioning are two important aspects of TKA that influence patient outcomes [1,2,3]. If these considerations are not adequately addressed then patients can experience pain and implant malfunction or wear . Studies have shown that between 8 and 19% of patients report being dissatisfied with their total knee arthroplasty (TKA) for various reasons, including pain and unmet expectations [5,6]. A few problems that could result from malrotation and/or incorrect soft tissue tensioning are: patellofemoral instability , anterior knee pain , arthrofibrosis , and flexion gap instability .
Measured resection is one technique commonly used by surgeons during TKA to attain correct alignment and soft tissue tension, and ideally deliver a pain-free knee that allows patients a return to daily activities . It has been suggested that North American surgeons predominantly use measured resection versus gap balancing, while the use of the techniques is likely more variable within Europe . Part 1 of this article series explains why surgeons would prefer one technique over the other.
Measured resection philosophy
Measured resection is characterized by the use of bony landmarks to determine femoral component rotation [7,13,14]. With this technique, and in contrast to gap balancing, bone cuts are made before soft tissue tensioning takes place. Generally, three bony landmarks are referenced [12,15]. These are the: transepicondylar axis (TEA) (surgical and anatomical); anterioposterior axis (AP) or ‘Whiteside’s line’; and posterior condylar axis (PCA). These axes should not be used singularly, but in combination  as unique variations in anatomy and/or deformity can skew femoral component placement . Additionally, surgeons may have difficulty accurately identifying these landmarks during TKA . Figure 1 shows these landmarks.
Transepicondylar axis (TEA)
The TEA connects the protuberance of the lateral epicondyle with the one on the medial side. A number of studies have shown that this landmark is useful intraoperatively. Placing the femoral component parallel to the TEA  has been shown to: more consistently help achieve a rectangular flexion gap ; a lower incidence of condylar lift-off ; and improved patellofemoral tracking and better femorotibial kinematics . However, there is also ample literature confirming that the TEA is regularly misidentified [19,20,21,22,23].
Anterioposterior axis (AP)
Sometimes referred to as “Whiteside’s line” [14,24], the AP axis is “derived from singular anterior and posterior points”  and connects the anterior trochlear sulcus center to the middle of the intercondylar notch’s posterior aspect . The effectiveness of this landmark is dependent on the absence of deformity in the distal femur’s trochlear groove and intercondylar notch [8,26]. The intraoperative goal is to establish femoral rotation perpendicular to the AP axis [14,24]. In a study by Katz et al., the AP axis was found to reliably define the flexion-extension axis and balance the flexion gap . Arima et al. called it a “reliable landmark” . Daines et al. point out that axis can also be effective if there is bone erosion of the posterior condylar or hypoplasia .
Posterior condylar axis (PCA)
The PCA is a line that runs along the bottom of the posterior femoral condyles. It has been called “the most reliable [rotational guide]”  and “easy to find” . The intraoperative goal is to place the femoral component around 3° to 4° externally rotated to the PCA . Simple, accurate instrumentation has been developed to help surgeons set external rotation relative to this axis; the alignment tool is best used in varus knees with little to no deformity and no femoral erosion. However, the unique anatomy of each patient’s femur influences the relationship between the TEA and the PCA which may result in malrotation if the standard, pre-set rotation delivered by instrumentation is uniformly applied . If the PCA is the only determinant for femoral component rotation, valgus deformed knees develop hypoplasia on the posterior aspect and varus knees can develop this on the posterior aspect of the medial femoral condyle (if the anterior cruciate ligament is insufficient prior to surgery) .
A note about intraoperative identification of axes
While the use of measured resection, via one or more of the described axes, has many proponents, it has also been repeatedly noted that surgeons may not correctly identify key bony landmarks intraoperatively [8,33]. This leads to malrotation and other problems as components are not positioned correctly. It is recommended that multiple axes be referenced during TKA with measured resection [8,11,24].
Additionally, even normal (versus deformed) anatomy is highly variable between individuals. Arthritic changes and deformity may mask the true landmark by altering the surface characteristics of the bones. It is interesting to note that a 2017 study by Shao et al. demonstrated that race impacts the anatomy of the tibial shaft and therefore should impact the design (and placement) of stemmed tibial TKA components; East Asians have statistically higher tibial mediolateral offset than Caucasians (East Asians 9.9 ± 2.7 mm, Caucasians 7.7 ± 3.1 mm, p < 0.001) .
The pros of measured resection
Measured resection is a technique that is widely used . It can certainly be said to equally have proponents and critics, somewhat fueling a debate around its effectiveness [12,33]. One reoccurring comment about this technique is that it is the skill and experience of the surgeon that dictates its success [12,33]. Using a patient’s anatomy to guide the procedure is highly recommended , as well as incorporating all three of the bony landmarks to avoid inaccuracies. Matthew Abdel, orthopedic surgeon at the Mayo Clinic in Rochester, US, tells us that, “The advantages of a measured resection technique primarily focus around the fact that in most routine primary TKAs, anatomic landmarks are reliable and simple.”
Orthopedic surgeon Philipp von Roth, from the Charité-University Hospital in Berlin, Germany, finds that, “For me, the measured resection technique is optimal in the case of clearly identifiable anatomical landmarks, large posterior osteophytes and contract varus- or valgus deformities.” He states that, “Judging the integrity of the collateral ligaments can be very challenging. In particular, after a history of a ligamentous injury. In these cases, the major advantage of the measured resection technique is that you rely on the bony anatomy only.”
The cons of measured resection
The dependence on bony landmarks opens the door to errors stemming from unique individual anatomy, arthritic changes, and/or deformity. Each of the TEA, AP axis, and PCA are subject to specific criticism about under what conditions they may not be the best landmark. For example, the TEA is commonly misidentified [19,20], the sole use of the AP axis can introduce error to femoral component rotation [4,21,31], and the PCA can be influenced by anatomical variation (Fig. 3) .
The success of measured resection also hinges on a surgeons’ ability to accurately identify these landmarks intraoperatively. For example, Jerosch et al. showed that when surgeons had to mark the epicondyles in experimental conditions the medial side position varied 22.3 mm and the lateral side varied 13.8 mm . When it comes to identifying anatomical landmarks, Philipp von Roth points out that, “identification can be difficult and even if clearly identifiable, the landmarks show a large anatomical variance.”
Even if anatomical landmarks are easy to identify in many routine TKA’s, Abdel reminds us that, “This can lead to flexion and extension gaps that are not initially balanced, as well as variations between the medial and lateral side, in regard to stability, that require additional soft-tissue releases.”
Table 1 shows a selection of advantages and disadvantages for measured resection. It is by no means intended to be an exhaustive list for either category.
Advantages of measured resection
Disadvantages of measured resection
Ligament balancing in the case of fixed deformities is better accommodated 
Can be difficult to identify landmark axes intraoperatively [12,19,32]
Results in less reduction of the post-operative joint line position 
Soft tissue releases are made after trial component placement; this might introduce difficult to correct laxity and asymmetry 
Indicated as preferable in cases of patellar infera 
Combining bony landmarks optimizes femoral component position 
Measured resection has been validated as durable and successful in establishing soft tissue tension and femoral component rotation in TKA . See Part 2 for further information about gap balancing, another common technique employed for the same ends. In Part 1 we compare these two techniques and present the cases for and against each one; which technique do you think will come out on top?
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