We reported our experience using this device for intraoperative measurement with the PS TKA and further discussed the importance of the patellar orientation during the measurement [43, 49, 50]. First, we reported joint component gap kinematics in PS TKA with and without patellar eversion. The component gap showed an accelerated decrease during full knee extension. With the PF joint everted, the component gap increased throughout knee flexion. In contrast, the component gap with a reduced PF joint increased with knee flexion but decreased after 60° of flexion [49]. Second, we reported that intraoperative joint component gap kinematic assessment with a reduced PF joint has the possibility to predict the postoperative flexion angle and thus allows evaluation of the surgical technique throughout a range of knee motion. Both an increased value during the extension to flexion gap and a decreased value during the flexion to deep flexion gap with PF joint reduced, not everted, showed inverse correlations with the postoperative knee flexion angle, not with the preoperative flexion angle [43]. Third, we demonstrated that the correlations between the soft tissue balance, assessed by the tensor, and the navigation system were higher with a reduced PF joint than those with an everted PF joint. This suggests that surgeons should assess soft tissue balance during PS TKA with the PF joint reduced when using a navigation system [50]. In a series of intraoperative soft tissue balance assessments, we emphasized the importance of maintaining a reduced and anatomically oriented PF joint in order to obtain accurate and more physiologically relevant soft tissue balancing.

In addition to our reports, some recent studies have emphasized the importance of the physiological postoperative knee condition in assessing soft tissue balance with PF joint reduction [17, 26, 91]. Using our tensor with a 5-mm-long minute uniaxial foil strain gauge, Gejo et al. reported a similar kinematic pattern of the intraoperative joint component gap; when the patella was reduced, the joint gap was decreased at 90 and 135° of flexion (by 1.9 mm and 5.5 mm, respectively) compared with the gap with the patella everted. Patellar tendon strain at 90° of flexion, increasing with knee flexion, correlated with the joint gap difference with the patella in the everted and reduced positions. Based on their study, they concluded that the knee extensor mechanism might have an influence on the joint gap and be important in achieving the optimal joint gap balance during TKA [17]. With the use of an original tensor device that can measure the load of the spread joint gap, Yoshino et al. reported a significant difference between the loads in the patella-everted position and the reset position in flexion, but not in extension, in PS TKA. However, in CR TKA, they reported no significant difference between the loads in the patella-everted position and in patella-reset position at either extension or flexion. Therefore, they concluded that the load in the flexion gap will increase in PS TKA or, in other words, the flexion gap distance will decrease by resetting [91]. With the use of an offset-type tensor that has been developed based on our tensor, Kamei et al. reported a joint gap size and inclination measured intraoperatively on a knee in 90° flexion, with and without patellar eversion [26]. After the tibial and distal femoral cuts were made, they showed that the joint gap with the patella in situ (17.0 ± 3.4 mm) was significantly greater than that with patellar eversion (15.4 ± 3.0 mm), as was gap inclination at 90° flexion with the patella in situ (4.9° ± 3.1°) compared with that observed with patellar eversion (4.0° ± 2.9°). Based on these results, they speculated that the steeper flexion gap inclination obtained without patellar eversion induced more externally rotated femoral positioning in the absence of patellar eversion. They emphasized that these results ought to be taken into account by surgeons considering a switch from conventional to minimal incision surgery (MIS) TKA.

The main concepts of measurement using the new tensor are different from the conventional tensioning device with the femoral trial component in place as well as a reduced PF joint. As the next step, accordingly, we focused on the difference in soft tissue balancing between the placement of femoral trial component and the conventional osteotomized condition. In the intraoperative assessment of soft tissue balance, the joint gap showed a significant decrease in extension, but not flexion, after femoral trial prosthesis placement. Varus ligament balances were significantly reduced in extension and increased in flexion after femoral trial placement [58]. These changes in extension might be caused by the tensed posterior structures of the knee, associated with the posterior condyle of the externally rotated, aligned femoral trial. At knee flexion, a medial tension in the extensor mechanisms might be increased after femoral trial placement with PF joint repaired and increased ligament balance in varus. We measured the “joint component gap,” which is remarkably different from the more conventional gap measurement. The joint component gap is measured with the femoral component in place, whereas the conventional gap measurement is made between the cutting surfaces of the femur and tibia. By keeping the femoral component in place, the knee is afforded a greater degree of extension because of its curving arc. In this arrangement, the posterior condyles of the component tighten the posterior capsule, resulting in a smaller joint gap at full extension. In addition, because of the 7-degree posterior slope of the tibia and a slight femoral anterior bowing, we can consider the “conventional extension gap” to be at about 10° of the knee flexion angle. Mitsuyama et al. similarly reported on 80 varus-type osteoarthritic knees with the offset-type tensor that selecting a larger size femoral component as well as femoral component placement reduced the extension gap [55]. They reported that the placement of the femoral component reduced the medial and lateral extension gaps by 1.0 mm and 0.9 mm, respectively. The medial and lateral gaps further decreased by 2.1 mm and 2.8 mm, respectively, when a specially made femoral component with a posterior condyle enlarged by 4 mm was tested. Mihalko et al. found in a cadaver study that the release of more posterior structures had a greater effect on the extension gap than on the flexion gap, explaining the importance of the relationship between posterior structures and the extension gap [54]. Sugama et al. reported in their operative study that a bone cut from the posterior femoral condyles could change the tension of the posterior soft tissue structures and so alter the width and shape of the extension gap [75]. These previous reports support our hypothetical mechanism.

Considering the clinical significance of intraoperative assessment, we should confirm that intraoperative values assessed with the tensor reflect the postoperative soft tissue balance. Hence, we investigated the correlation between the intraoperative values assessed with the tensor and the 5-year postoperative values assessed with stress radiography in extension and flexion [47]. In CR TKA, postoperatively both the joint component gap and ligament balance in extension and flexion showed positive correlations with the intraoperative values of 10° and 90° of flexion. However, in PS TKA, whereas postoperatively both the joint component gap and ligament balance in extension showed positive correlation with the intraoperative values at 10° of flexion, postoperatively neither joint component gap nor ligament balance in flexion correlated with that in 90° of flexion. These results indicate that the intraoperative measurements of soft tissue balance by the tensor reflect the postoperative values assessed by the stress radiographs, even at the 5-year follow-up. However, despite existing correlations in extension, there were no correlations in flexion in either the joint component gap or the ligament balance between intra- and postoperative values in PS TKA. This discrepancy in PS TKA may be caused by flexion instability due to a flexion gap larger than the extension gap [42].

Acquisition of a high flexion angle after TKA is one of the factors leading to patient satisfaction. Therefore, we focused on the relationship between the postoperative flexion angle and the intraoperative soft tissue balance. In the series of studies in PS TKA, the joint gap change value (90–0°) with the PF joint reduced, not everted, showed an inverse correlation with the postoperative knee flexion angle and posterior condylar offset [43]. However, in another series of studies on CR TKA, the postoperative flexion angle was positively correlated with the joint gap change value (90–0°). In either case, multivariate regression analysis among various values, including various joint gap change values, ligament balance, and preoperative knee flexion angle, demonstrated that the preoperative knee flexion angle and the joint gap change value (90–0°) had a significant independent effect on the postoperative knee flexion angle [76]. One of the reasons for this discrepancy may be the different patterns of soft tissue balance between PS and CR TKA [42, 46]. In that report, CR TKA in comparison with PS TKA showed significantly smaller gaps when the arc of movement ranged from mid- to deep flexion [42]. PCL in the osteoarthritic knee is considered relatively rigid and shortened, despite being macroscopically intact. When we consider the flexion gap tightness, Ritter et al. reported that 30% of CR TKA required ligament balancing to obtain a smooth flexion arc [67]. If the PCL was too tight, excessive femoral rollback resulted in anterior lift-off of the tibial trial in flexion, leading to limitation of flexion [29]. Balancing the flexion gap can facilitate postoperative flexion to an increased angle and result in a satisfactory range of motion [3, 34]. In our studies on CR TKA, we found that a 16% increase in flexion gap tightness (smaller flexion gap than extension gap) resulted in a smaller flexion angle. Similarly, using a commercially available knee balancer with the measurement under 80 N distraction force, Higuchi et al. reported that flexion medial/lateral gap tightness led to restriction of the flexion angle [22]. Therefore, in these cases, surgeons are advised to release soft tissues such as the PCL to decrease flexion gap tightness by [29, 67, 90].

Finally, postoperative kinematics such as tibial internal rotation and tibial anterior translation are important to achieve better clinical outcomes, including a high knee flexion angle. With regard to achieving high flexion after TKA, some studies have emphasized that an increase in postoperative tibial internal rotation is observed during knee flexion [31, 88]. Therefore, we investigated the correlation between intraoperative soft tissue balance (assessed by the tensor) and postoperative knee kinematics (assessed by a navigation system) following all prostheses implanted [53]. The results confirmed a positive correlation between varus ligament balance and tibial internal rotation, which may indicate that looseness of the lateral compartment in relation to the medial side at 60° and 90° of flexion permits rotational mobility and results in increased tibial internal rotation. In fact, the positive correlation between the lateral compartment gap and tibial internal rotation from mid- to deep knee flexion was a more sensitive factor than the joint component gap, and the fact that there was no relationship between the medial compartment gap and tibial internal rotation supported this result. Moreover, in another study assessing the correlation between intra- and postoperative knee flexion angle and knee kinematics, postoperative as well as preoperative knee flexion angle was significantly correlated with the postoperative tibial internal rotation [52]. In addition, we reported a positive correlation between intraoperative lateral laxity in flexion and postoperative flexion angle in CR TKA, indicating that medial stability with appropriate lateral laxity was important for achievement of a high flexion angle [60]. Similarly, Kobayashi reported, using postoperative stress radiography, that lateral laxity in flexion (flexion-valgus, 3.4°; flexion-varus, 6.2°) showed a positive correlation with the postoperative knee flexion angle [30]. Other studies also support these findings, indicating that the flexion gap in healthy knees is not rectangular and that the lateral joint gap is significantly lax [39, 56, 72, 80]. In summary, to reproduce medial pivot motion after TKA, medial stability with moderate lateral laxity during flexion might lead to appropriate tibial internal rotation and result in a high flexion angle.

The most important aspect of soft tissue balancing is not just an assessment but also the close interaction between the surgical technique and the assessment, in which surgeons should reflect the surgical technique to attain final soft tissue balance. With the measured resection technique for CR TKA, we recently reported the importance of minimal medial release (osteophyte removal and release of the deep layer of the medial collateral ligament) for varus-type osteoarthritis to maintain an appropriate tibial internal rotation and to gain a high flexion angle [51]. Recently, an offset-type tensor was developed to use with the gap technique as well as the measured resection technique during TKA. With this new system, FuZion™ (Zimmer, Inc.) (Fig. 13.3), surgeons can assess and correct soft tissue balance after making the distal femoral and proximal tibial cut, then adjust femoral rotation based on the tensor measurement, and confirm the final balance throughout the range of motion with femoral component placement. The information, made available by the use of the tensor during surgery, is useful in a real-time manner and is essential for providing insight regarding the true postoperative kinematics. It allows the surgeon to adjust the soft tissue balance more accurately and thereby to expect a better postoperative outcome.

To achieve successful clinical outcomes, accurate osteotomy/implantation and soft tissue balancing are essential in TKA. Appropriate bone cut and prosthetic implantation have improved due to advances in surgical instrumentations, such as the computer-assisted navigation system, preoperative image-matching technique, or patient-specific instrumentation. Similarly, appropriate soft tissue balancing has become more important than it was previously. With the recent advances in this field described here, improved patient satisfaction after TKA is expected in the near future.