Soft Tissue Balance, Kinematics, and Patient Satisfaction

Fig. 17.1

Factors affecting clinical results after total knee arthroplasty

In this chapter, clinical and biomechanical studies are introduced to discuss the relationship among soft tissue balance, kinematics, and clinical symptoms, including patient satisfaction.

17.1 Intraoperative Soft Tissue Balance and Patient Symptoms

Intraoperative soft tissue balance is related to postoperative knee kinematics, which affect patient symptoms. Also, knee kinematics affect ROM, which can be significantly related to patient satisfaction and knee function [1]. In this chapter we first review studies on extension and flexion gaps and then discuss medial-lateral gap balancing (Table 17.1).

Table 17.1

The effect of gap and medial-lateral balance on clinical results

	Gap	Medial-lateral balance
Extension	Tight gap: flexion contracture More than 1 mm was necessary to avoid flexion contracture [2]. Loose gap: uncertain A 2–3mm medial gap can be acceptable based on normal knee laxity [3] and clinical study [4].	Medial-opening imbalance: possibly symptomatic More than 1.5 mm of imbalance was related to pain [5]. Lateral-opening imbalance: uncertain No studies have shown that a large lateral gap is symptomatic. 2–3° of imbalance can be acceptable based on normal knee laxity [3].
Flexion	Tight gap: decreasing flexion angle [6–9] Loose gap: variable results A 2.5mm larger gap than extension was related to higher JFS in LCS [10]. A 3.0mm larger gap than extension was related to lower function score in CR [11]. Very large flexion gap causes symptomatic knees [12].	Medial-opening imbalance: possibly symptomatic ≥3° of imbalance was related to dissatisfaction in CS [13]. Lateral -opening imbalance: positive effect on flexion angle [7, 14, 15]

17.1.1 Extension Gap

The degree of extension gap necessary to avoid postoperative flexion contracture remains unclear [16]. Few studies have evaluated the relationship between intraoperative soft tissue tension and postoperative extension angle [17]. Usually, varus osteoarthritic knees show a smaller extension gap at the medial side than the lateral side. Therefore, we previously evaluated the effect of a medial extension gap on postoperative flexion contracture by evaluating the intraoperative extension gap in 75 knees with varus deformity undergoing TKA with the NexGen LPS (Zimmer, Warsaw, IN, USA) [2]. The extension gap was measured in the presence of a femoral component using a tension device applying a distraction force of 178 N. A “component gap” was defined as the distance calculated by subtracting the selected thickness of the tibial component, including the polyethylene liner, from the measured gap. The postoperative extension angle was measured by radiography. Flexion contracture was defined when the angle between the anatomical axis of the distal femur and the proximal tibia exceeded 5°. The knees with >1 mm medial component gap showed no flexion contracture at 1 year after surgery. Nagai et al. also reported that the postoperative active knee extension angle was positively correlated with the medial compartment gap at 0° [18]. These studies suggest that achieving an adequate medial extension gap is very important to avoid flexion contracture.

How loosely can the knee be left in extension? First and foremost, the patient should not feel any instability. Thus one possible benchmark is the stability of normal knees. We previously measured knee laxity in normal knees by stress radiography and found that the medial opening angle was 2.4° when a valgus stress of 147 N was applied [3]. Ishii et al. reported that excellent clinical results were achieved after TKA in patients with 3–4° of valgus laxity [4]. On the basis of these studies, 2–4° laxity does not lead to feelings of instability in patients who have undergone TKA. Therefore, we suggest that medial extension laxity should be 1–3 mm to avoid flexion contracture and subjective instability (note: 1° medial laxity equals approximately 1.05 mm when the transverse diameter of the tibia is 80 mm).

17.1.2 Flexion Gap

Many studies have evaluated the relationship between the intraoperative flexion gap and postoperative flexion angle. A clinical study by Takayama et al. indicated that flexion gap tightness decreases range of motion after cruciate-retaining (CR) TKA [6]. Nakano et al. evaluated the lateral and medial flexion gaps separately and found that the lateral compartment gap at 90° of flexion was positively correlated with postoperative knee flexion angle in knees after CR-TKA [7]. Hasegawa et al. reported that increased medial-lateral laxity from 90° to 120° showed a positive correlation with the postoperative flexion angle in posterior-stabilized (PS) TKA [8]. Other studies focused on deep knee flexion. For instance, Niki et al. divided patients who had undergone posterior-stabilized (PS) TKA into those who could achieve Seiza sitting (Japanese-style very deep knee flexion) and those who failed to and found that the gap length at 135° of flexion was significantly larger in the former group than the latter [9]. Watanabe also reported that in PS-TKA knees between 135° and 0° of extension, larger gap differences were associated with larger postoperative flexion angles [19]. In these studies, gaps were measured without patellar eversion so the assessments might have been affected by quadriceps tightness in deep knee flexion.

The effect of the flexion gap on postoperative symptoms has also been evaluated. Lampe et al. reported that larger flexion gaps (more than 2.9 mm) led to statistically lower Knee Society function scores and knee scores at 1 year after CR-TKA [11]. In CR-TKA, a large flexion gap results in function loss of the posterior cruciate ligament (PCL), which possibly worsens clinical results. In PCL-sacrificing TKA, the surgeon can control the flexion gap using the gap-balanced technique. In a study of knees with low-contact stress prostheses, Ismailidis et al. compared one group in which the flexion gap was intentionally 2.5 mm larger than the extension gap with another group in which these gaps were equal. They found that the former group achieved good ROM and showed a significantly higher Forgotten Joint Score-12[10]. In normal knees, knee laxity in flexion is slightly larger than in extension, by about 1–2 mm [20]. One biomechanical study demonstrated that when the flexion gap was 2 mm greater than the extension gap, tibial forces were decreased in deep knee flexion [21].

Based on these clinical and biomechanical studies and normal knee evaluations, we should definitely avoid tighter gaps in knee flexion than extension to achieve good range of motion, and about 2 mm greater laxity in flexion can result in more normal feeling. But care should be taken to avoid excessively large flexion gaps because some patients with flexion instability after PS-TKA may present with pain, especially while negotiating stairs, as well as recurrent joint effusions, both of which can be causes of revision surgery [12].

17.1.3 Medial-Lateral Balancing

17.1.3.1 In Knee Extension

First, we discuss ligament balance in osteoarthritic knees. We previously investigated knee laxity in osteoarthritic knees during TKA [22]. In that study, the extension gap was measured after the distal part of the femur and the proximal part of the tibia were resected. The patients were divided into mild, moderate, and severe varus groups, based on preoperative hip-knee-ankle angles of <10°, 10–20°, and >20°, respectively. Measurements were made after removing osteophytes with a distraction force of 178 N. The results showed greater lateral soft tissue laxity with increasing severity of knee deformities. However, the medial side did not contract with increasing varus deformity. These results suggest that release on the medial side is unnecessary to create a space for implant replacement, even in severely deformed knees. However, gap imbalance increased up to 5 mm with increasing knee deformity. Therefore, we should determine the answer to this question: “How much of an imbalance can be tolerated in knee extension?”

As for intraoperative gap measurement, Lampe et al. reported that higher medial-lateral gap inequality (more than 2 mm) in both extension and flexion did not worsen Knee Society function or knee scores at 1 year after CR knee [11]. In postoperative stress radiograph evaluation, Nakahara et al. reported that varus laxities (5.9 ± 2.7°) or valgus laxities (5.0 ± 1.6°) under static stress in extension were not related to patient-reported outcomes after well-aligned PS-TKA [23]. Liebs et al. evaluated postoperative radiographs of gap imbalance (without any stress) and found that patients with an asymmetric medial opening extension gap of ≥ 1.5 mm had significantly higher pain scores at 3 and 6 months’ follow-up, whereas a gap on the lateral side was associated with less pain [5]. Our study of normal knees showed 2.5° greater laxity on the lateral side than the medial side [3]. The results of these studies suggest that in knee extension, a couple of degrees of ligament imbalance, especially in lateral opening imbalance, can be tolerated from the standpoint of knee symptoms.

17.1.3.2 Knee Flexion

Many clinical and cadaveric studies have shown that in normal knees, soft tissue is laxer on the lateral side than the medial side in knee flexion [3, 20, 24]. Corroborating this, an MRI study by Tokuhara et al. showed that the lateral side was 4.6 mm laxer than the medial side [25].

We evaluated the effect of looseness in knee flexion on clinical outcome in 50 patients after TKA with a cruciate-sacrificed design (Kyocera Bisurface Knee) [13]. Stress radiographs were taken with a lateral traction force of 50 N applied perpendicular to the lower leg at 80° knee flexion. We measured the angle between a line tangential to the femoral condyles and a line through the tibial joint surface. Patient satisfaction, symptoms, and knee function according to the new Knee Society scoring system were compared between the knees with ≥3° medial flexion laxity (medial loose group) and knees with <3° medial flexion laxity (medial tight group). The scores of the medial loose and tight groups were 22 and 30 (out of 40) for satisfaction, 16 and 20 (out of 25) for symptoms, and 19 and 24 (out of 30) for standard activities, respectively. These results show that the knees with a medial opening imbalance had worse clinical outcomes after CS-TKA. Another clinical study by Seon et al. reported no difference between knees with rectangular and non-rectangular flexion gaps with respect to knee score [26].

Regarding knee flexion angle, Niki et al. compared a group of patients who achieved Seiza sitting with those who failed and found no significant differences in gap inclination between PS knees [9]. On the other hand, some studies reported the importance of a certain amount of lateral laxity (i.e., lateral opening imbalance) for achieving a good flexion angle. Nakano et al. showed that the lateral opening imbalance at 90° of flexion was positively correlated with postoperative knee flexion angle in CR-TKA [7]. Kobayashi et al. reported that lateral laxity during knee flexion was related to good range of motion [14].

Thus, to date, few clinical studies have indicated that it is important to achieve a medial-lateral flexion gap in order to improve clinical results. A certain amount of lateral laxity can improve knee flexion, but medial laxity will lead to inferior clinical results. It is important to recognize that medial opening and lateral opening imbalances are quite different in terms of the effect on clinical results. We believe that a certain degree of lateral laxity in flexion is close to the normal condition and is also related to better range of motion.

17.2 Soft Tissue Balance and Kinematics

Soft tissue balance and the articular geometry of the implant are two major factors determining knee kinematics after total knee arthroplasty. However, not many studies have evaluated the effects of soft tissue conditions on knee kinematics (Table 17.2).

Table 17.2

The effect of gap and medial-lateral balance on knee kinematics

	Gap	Medial-lateral balance
Extension	No studies have evaluated the effect of extension gap alone on knee kinematics.	No correlation between ligament balance and lift-off [12, 27].
Flexion	Large gap difference was related to paradoxical motion in CR [14]. Larger gap difference between extension and flexion was related to near-normal kinematics in PS [8].	A greater medial flexion gap caused larger anterior translation [28]. Lateral opening imbalance was related to near-normal kinematics [26].

Some studies have evaluated the effects of gap and balance on knee motion in the anterior-posterior and rotational directions. Watanabe et al. reported that following PS-TKA, the gap difference in knee flexion at 135° minus 0° was correlated with the total posterior translation of the lateral femoral condyle and femoral external rotation during squatting, and these knees had larger flexion angles [19]. This study suggested that to achieve near-normal kinematics in PS-TKA, a tight flexion gap should be avoided. In CR-TKA, the situation is slightly different because a loose flexion gap results in PCL dysfunction. Fujimoto et al. divided patients with CR-TKA into two groups according to their 90° minus 0° component gap changes: the wide flexion gap group was defined by a change of >3 mm, while the narrow flexion gap group was defined by a change of <3 mm. The authors found that under non-weight-bearing conditions, the wide flexion gap group showed significant anterior displacement of the medial femoral condyle compared with the narrow flexion gap group [15]. Another clinical study also found that worse clinical scores were associated with larger flexion gaps in CR knees [11].

What are the effects of medial-lateral balance? Matsuzaki et al. evaluated intraoperative knee kinematics using navigation and found that varus ligament balance at 90° of flexion was positively correlated with tibial internal rotation at 60° and 90° of flexion, and the lateral compartment gap was positively correlated with tibial internal rotation at 60°, 90°, and 120° of knee flexion in CR knees [28]. This is one of the reasons why a large lateral flexion gap is related to good range of motion, as shown in clinical studies [7, 8, 14]. Our fluoroscopic analysis showed that a greater medial flexion gap caused larger anterior translation in knee flexion in CS-TKA [29], but lateral static instability at knee flexion did not cause any abnormal motion. CS-TKA controls AP stability via a curved articular surface and joint gap tightness, without a post-cam mechanism. Therefore, achieving an adequate flexion gap in CS-TKA is more important than in PS-TKA in order to achieve proper AP stability in flexion.

Knee motion in the coronal direction is also clinically important. Because joint laxity theoretically increases the risk of lift-off motion, we focus on that motion here. Since lift-off motion of the femoral component possibly increases wear of the articular surface [27], it should be avoided after TKA. Hamai et al. [30] used fluoroscopic stress radiography to evaluate the effect of post-CR-TKA static knee instability on dynamic lift-off motion and found that neither static varus-valgus laxity nor differences in laxity (i.e., imbalance) influenced lift-off motion. Nakahara et al. reported that no correlations were found between femoral condylar lift-off during walking and varus-valgus laxities under static stress in extension after well-aligned PS-TKA [23]. We also evaluated the effects of alignment and ligament balance on lift-off motion using computer simulations, which have recently been validated in the field of TKA [31–37], with KneeSIM software (LifeMOD/KneeSIM 2010; LifeModeler Inc., San Clemente, CA, USA). Our results showed that lift-off motion occurred with 5° varus alignment or with a combination of 2° varus deformity and 2 mm lateral laxity [38]. However, no lift-off motion was detected in knees with neutral to 1° varus malalignment, even when the knees had 5 mm lateral laxity. These findings show that alignment is also very important in terms of its impact on knee kinematics. Therefore, tolerance of ligament imbalance will be different depending on knee alignment.

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