The resections of the tibia and femur during knee arthroplasty must result in rectangular flexion and extension gaps (equal medial and lateral soft tissue tensions) without changing the anatomical joint line [8].

Traditionally, resection of the femur and tibia can be done through three approaches, namely, the measured resection technique, the gap balancing technique, or a combination of the two techniques. The major difference between measured resection and gap balancing is the way in which femoral rotation is determined. During the measured resection technique, bony landmarks (Whiteside line, surgical epicondylar axis, posterior condylar axis, and the anterior-posterior axis) are used to set femoral component rotation, whereas the gap balancing technique relies on symmetrical tensioning of the medial and lateral soft tissues in flexion to set femoral rotation. The former technique may result in a wide range of soft tissue balance due to the difficulty of reproducibly identifying the bony landmarks intraoperatively [9]. This can lead to flexion gap asymmetry and condylar lift-off. To remedy the situation, the correct course of action depends on whether joint stiffness increase or decrease during flexion and the degree of asymmetry in the medial-lateral soft tissues and its variability with flexion [3]. Although the gap balancing technique provides better chances of achieving proper ligament balance in full extension and 90° flexion, midflexion stability is not guaranteed. This can be attributed to the risk of getting an incorrect tibial cut which serves as the platform from which the flexion gap is established [9]. Secondary to that is the uncertainty in the application and magnitude of the correct distraction force [9]. Since soft tissue balance can be manipulated by varying the medial-lateral extension and flexion gaps, incorrect resection may result in instability due to ligament imbalance.

Varus or valgus instability refers to a trapezoidal extension gap due to asymmetric contracture or laxity in the collateral ligaments (Fig. 4.2). This type of laxity can be either symmetric or asymmetric [8]. Symmetric instability may result due to excessive cartilage loss on the affected condyle. Alternatively, the patient might have had a varus or valgus alignment before the pathology set in. For these cases, a rectangular extension gap may then result in a pronounced varus or valgus alignment even though the ligaments might be balanced. On the other hand, asymmetric instability refers to contracture or excessive laxity of one of the collateral ligaments. Traditionally, surgeons employing gap balancing have relied on spacer blocks and distractors to achieve proper soft tissue balance [3]. Since these techniques rely solely on tactile feedback and subjective assessment [4, 10], success is strongly related to the skill level and experience of the surgeon. An attempt to circumvent this has seen the introduction of instrumented tibial trials and distractors with which the medial-lateral load components can be objectively measured [3]. Unfortunately, these new developments still shed little light on what the surgical steps should be to achieve a balanced condition [3].

Although there are no clear guidelines, e.g., it is still unclear what level of extension gap tightness is appropriate to avoid postoperative flexion contracture [2], some values have been found to produce good outcomes. The amount of laxity should be governed by the patient’s perception of stability. A medial extension gap of 1–3 mm has been found to result in a stable feeling as well as not causing flexion contracture, whereas the lateral side should be 2.5° laxer than the medial side [2]. The medial flexion gap should be similar or close to the extension gap. This will achieve near-normal articulation, function, and patient satisfaction [2]. Unfortunately, there are no clear evidence on what constitutes a safe range for the lateral flexion gap other than some degree of laxity being acceptable [2]. Instrumented distractors and tibial trials have necessitated the need to quantify the flexion and extension gap balance in terms of force values.

Ideal force target values have not as of yet been validated; a medial-lateral ratio ranging between 0.5 and 0.55 is suggested [3, 4]. A case series (n = 189) has shown that a medial-lateral force differential less than 60 lb will result in good outcomes [4], whereas a more conservative ratio of less than 15 lb has also been ascribed [11]. It is however difficult to maintain this ratio throughout flexion [3] and furthermore unclear whether it is important to maintain the same ratio throughout flexion [10]. A recent case series (n = 12) measured the differential at 10°, 45°, and 90° [5]. The differentials (medial load min lateral load) were, respectively, 5.6, 9.8, and 4.3 lb. Laxities in the native knee are not uniform, and there is a need for more in depth analysis to determine appropriate target values [12]. Fortunately, there are qualitative measures and guidelines to address varus deformities, valgus deformities, flexion contracture, and genu recurvatum through soft tissue balancing.

A varus-deformed knee requires release of the deep medial collateral ligament and removal of osteophytes [8]. Persistent contracture may require release of the distal superficial medial collateral ligament in combination with the posterior-medial capsule and semimembranosus insertion [8]. Sacrifice of the posterior cruciate ligament will significantly increase the flexion gap on the medial side with little influence on the extension gap [13]. In cases with persistent deformity, it may be necessary to advance the lateral collateral ligament. It has been shown that lateral soft tissue laxity increased with increasing severity of knee deformities, while the medial side did not contract with increasing varus deformity [2]. This result suggests that release on the medial side may be unnecessary to make a space for implant replacement, even in severely deformed knees. Contrary to this, release of different parts of the medial collateral ligament will increase laxity at discrete ranges of flexion [14].

Valgus deformity is associated with tight lateral stabilizers and abnormal femoral lateral condylar anatomy. There is no consensus on what approach should be followed to address this type of deformity [8]. In general, the sequence of release starts off with the lateral collateral ligament followed by the posterior-lateral capsule, iliotibial band, posterior cruciate ligament, popliteus tendon, and biceps femoris. It should be noted that in one study (n = 37), valgus deformity was addressed solely through over resection of the distal femur and a constrained total knee arthroplasty system with no reported cases of loosening or instability at a 7.8-year follow-up [15]. This approach has merit, since it has been shown that lateral tissue release to address valgus deformities frequently produces asymmetric flexion-extension gaps and ligament instability [16]. The lateral flexion gap is affected most by the lateral collateral ligament, whereas the iliotibial band influences the extension gap size the most [16]. In the same study, a release sequence starting with the posterior cruciate ligament, posterior-lateral capsule, iliotibial band, popliteus tendon, and lateral collateral ligament resulted in a symmetric flexion-extension gap. The best approach however is to examine the flexion and extension gap after each step in a release sequence regardless of what sequence is used [8].