Advantages of Kinematically Aligned Total Knee Arthroplasty: A Biomechanical Perspective

Overview

The fundamental principle of calipered kinematically aligned total knee arthroplasty (KA TKA) is to reproduce the flexion-extension axis of the prearthritic knee and to maintain the original collateral ligament balance and joint line with controlled, symmetric bony resection. A growing number of articles suggest that modification of the conventional neutral mechanical alignment (MA) strategy toward a more native kinematic alignment (KA) strategy with preservation of the original joint line and rotational axis of the knee is required to achieve better functional performance after TKA. ^, Recent systematic reviews and meta-analyses comparing KA TKA to MA TKA have indicated that the short-term outcomes of KA TKA are comparable or superior to those of MA TKA. ^, Moreover, long-term clinical outcome and survivorship have been reported to be promising at 10 years after KA TKA.

Although evidence of better patient satisfaction after KA TKA has recently been published, biomechanical evidence justifying superiority of KA TKA over MA TKA remains limited. The fundamental reasons why a strategy to restore the patient-specific joint line, rotational axis, and ligament balance, otherwise designated as ‘kinematic alignment,’ is superior to traditional MA should be clarified from the biomechanical perspective. To date, numerous biomechanical studies have tried to assess the beneficial effects of KA TKA. Some of these articles compared KA TKA to MA TKA and others to nonimplanted native knees. Biomechanical approaches for measuring knee kinematics are roughly classified into five types: (1) gait analysis based on skin markers, (2) roentgen stereophotogrammetry, (3) computer simulation, (4) in vivo fluoroscopy, and (5) the measurement of contact stress or ligament strain in situ using cadaveric knees. Regardless of the methodology, all studies were conducted with the assumption that kinematic differences between implanted knees and native knees might play a role not only in objective clinical outcomes such as loosening or wear but also in variance in patient satisfaction following TKA. Currently, an increasing number of surgeons are adopting a KA strategy, many with various modifications to alignment, joint line, and soft tissue balance. It is important therefore for surgeons to understand the biomechanical differences in both kinematic and kinetic profiles between KA TKA, MA TKA, and native knees. This chapter provides readers with the pertinent information necessary to understand KA TKA kinematics and kinetics.

Gait analysis studies

Skin marker-based stereophotogrammetry is the most widely used technique for gait analysis after TKA. To date, there have been several studies dealing with gait analysis of KA TKA. Among these studies, various surgical techniques were used for KA TKA, as well as different referent controls. Blakeney et al. reported the results of gait analysis of 45 restricted KA (rKA) TKAs versus 45 MA TKAs performed using an optical computer navigation system and calliper measurements of resected bone. By study design, the rKA-TKA arm was limited to bone cuts in the coronal plane within 5 degrees of the neutral mechanical axis and a final hip-knee-ankle (HKA) angle that was within 3 degrees of neutral alignment. Most gait parameters in the rKA TKA group more closely resembled those of normal knees than did those in the MA TKA group. Knee adduction angles and knee external rotation angles were significantly smaller in KA TKA than in MA TKA. Increased external rotation of the tibia in MA TKA is problematic, as it may reflect a pivot-shift avoidant gait because of the absence of the anterior cruciate ligament (ACL) following TKA, as noted by another investigator. McNair et al. compared gait profiles of 14 KA TKAs performed using patient-specific instrumentation with those of 15 MA TKAs performed with optical computer navigation. With respect to knee kinetics in the sagittal plane, KA TKAs tended to exhibit larger knee flexion moments than MA TKAs, whereas in the coronal plane, the knee adduction moment (KAM) was comparable between the two types of TKAs. In the transverse plane, the internal rotation moment was smaller in KA TKAs.

It is noteworthy that a comparative analysis of the KAM between KA TKA with an average –3-degrees HKA and MA TKA with an average 0-degrees HKA revealed significantly larger KAM in KA-TKA than in MA TKA. Generally, varus component alignment has been assumed to increase external KAM. Moreover, when the effects of joint line obliquity were neglected, varus limb alignment substantially increased medial contact stress, in addition to KAM, suggesting that reproducing constitutional varus alignment might lead to a risk of premature loosening of the tibial component. Actually, limb alignment and joint line obliquity should be considered independently when the effects of these elements on KAM are discussed. According to our previous study, varus joint line orientation substantially decreased KAM and thus any increase in KAM induced by reproducing constitutional varus alignment may be canceled after KA TKA. In another study, Yeo et al. performed a series of robotic-assisted TKAs using a version of rKA concepts. Component alignment for the medial proximal tibial angle (MPTA) was set to 2 degrees, and the lateral distal femoral angle was also set at 2 degrees, with 2 degrees of internal rotation relative to the transepicondylar axis. At a minimum of 8 years’ follow-up, gait analysis revealed that knee varus angle and medial-lateral ground reaction force (GRF; i.e., horizontal GRF) were significantly reduced in KA TKA compared with MA TKA. These results corroborated our theory that a medially inclined joint line after KA TKA orients the joint line parallel to the ground so that the center of pressure (COP) is more medial ( Fig. 11.1 ). This results in the center of mass (COM)-COP line becoming oriented more vertically relative to the ground, and the length of the lever arm to the knee center and horizontal GRF becoming smaller than with MA TKA. This mechanism is more evident during single-leg stance (e.g., walking or running) ( Fig. 11.2 ) resulting in a KAM that is ultimately reduced. In addition to joint line obliquity, step width during gait affects the location of the COP and subsequently the KAM. Step width was reported to be related to balance control in elderly patients and dull anticipatory postural control by dorsal muscles, when postural instability was experimentally induced by lateral perturbations of the pendulum. As step width increases with increasing age, elderly patients having unstable trunk balance may favor MA TKA in which step width is relatively wide, to maintain the joint line parallel to the floor.

Computer simulation studies

Blakeney et al. performed bone resection simulation on 1000 knee computed tomography (CT) scans from a database using an rKA TKA protocol and compared the results to an MA TKA resection protocol. On the three-dimensional planning software, the study calculated bone resection thicknesses and resulting soft tissue imbalances throughout the knee range of motion from full extension to maximum flexion for both techniques. The results indicated that knees with less than 3 mm of imbalance in any plane could be obtained in 92% of rKA TKA knees versus only 49% of MA TKA knees. Similar results for MA TKA simulation were reported in the past studies. ^, Kang et al. conducted a computer simulation of the stance-phase gait and deep knee bend and evaluated mechanical forces applied on KA and MA TKAs. The results indicated that the forces on the medial collateral ligament in KA TKA were lower than those in MA TKA and the maximum contact stress on the medial polyethylene insert in KA TKA was lower than that in MA TKA. However, two separate computer simulation studies proposed contradictory results. Ishikawa et al. reported that greater femoral rollback and more internal rotation of the tibia were observed with KA TKA than with MA TKA, but patellofemoral and medial tibiofemoral contact stresses were increased with KA TKA. Chen et al. simulated biomechanical loads and long-term wear volume after 10-million cycles of knee extension/flexion. They reported that peak medial tibiofemoral contact stresses were increased by 8.2%, whereas overall wear volume was decreased by 5.5% in KA TKA, compared with MA TKA. In addition, patellofemoral contact stress was increased by 40% in KA TKA. They concluded that there were no noticeable advantages in KA TKA from the biomechanical perspectives.

Cadaveric studies

Roth et al. performed KA TKAs in 13 cadaveric knees using manual instruments, without soft tissue release. The tibial forces and femorotibial contact positions were measured using a custom tibial force sensor. After KA TKA, net posterior translation of the contact point of the lateral femoral condyle on the lateral tibia was sufficient for deep knee flexion, and average differences in tibial contact stresses between compartments were small and similar to the native knee, despite the variance of the joint line from neutral in the coronal plane. This paper substantially contradicts the markedly increased medial contact stresses reported by a few computer simulation studies, ^, suggesting that perhaps the computerized software may not have accurately modeled all the complex forces acting on the knee. The same investigators further analyzed knee kinematics on the premise that shapes of articular surface geometry should match soft tissue tension, otherwise kinematic conflict may occur after TKA. So they compared knee laxities and neutral positions between three conditions: native, ACL-deficient, and KA TKA. The largest difference in laxity found in the KA TKA knees versus the native knees averaged 1.6-mm anterior translation at 30-degrees flexion. At 0-degrees flexion, the tibia was 3.8 mm and 1.2 mm more anterior to the femur in the KA TKA knee than in the native knee and ACL-deficient knee, respectively, which highlights the limitation of current ACL-resecting TKA designs. Koh et al. tested seven pairs of cadaveric knees. One knee in each pair was randomly assigned to KA-TKA and the other to MA-TKA. The results indicated that KA TKA better restored femoral rollback and varus soft tissue tension when compared with the contralateral MA TKA, and better maintained medial pivot motion with less paradoxical motion of the medial femoral condyle.

Fluoroscopic two- and three-dimensional shape-matching analyses

Nicolet‑Petersen et al. assessed in vivo anterior-posterior (A-P) tibial contact locations of KA TKA knees using a two- and three-dimensional (2D-3D) fluoroscopic model and compared the data with the patient’s healthy contralateral native knees. Some 25 patients underwent calipered KA TKA with controlled bony resection. With the knee in 0 degrees of flexion, the mean A-P tibial contact locations of the KA TKA knees were 4 mm more posterior in the medial compartment and 7 mm more posterior for the lateral compartment than those of the native knees, which is a nearly identical result to that reported by Roth et al. Murakami et al. analyzed the dynamic coronal orientation of the tibial component and the joint line over the stance phase of gait by using the 2D-3D shape-matching technique. The study compared joint line orientation relative to the floor between 3 degrees of KA TKA knees and MA TKA knees. The lateral tilt of the joint line was smaller in KA TKA, accommodating 3 degrees more varus inclination than in MA TKA. The results indicated that 3 or more degrees of medial inclination of the joint surfaces are needed to keep parallel to the ground during the stance phase of gait, which supports the gait analysis data reported by Blakeney et al.

Summary

A growing amount of evidence favoring KA TKA over MA TKA is being accumulated. Articles not in favor of KA TKA are few. In current biomechanical studies, both MA TKA knees and native-aligned knees serve as controls, with few, if any, papers comparing KA to balanced gap techniques. One of the key findings has been that coronal joint line inclination of the tibia (MPTA <87 degrees) alone does not seem to increase KAM and medial contact stress, and overall varus limb alignment is at risk of increasing KAM.

Some surgeons perform rKA TKA in their clinical practice, limiting the LDFA (lateral distal femoral angle) and MTPA to parameters that maintain a neutral or nearly neutral HKA and limited varus in the coronal plane. However, the appropriate boundary condition for a varus tibial cut relative to the mechanical axis should be based on a comprehensive biomechanical perspective and not arbitrary alignment parameters. Further, although the femorotibial contact position and loads of TKA components in the sagittal plane indicated that KA TKA is much closer to the native knee than MA TKA, the anterior translation of the tibial component in the sagittal plane still evident in KA TKA knees when fully extended because of ACL deficiency suggests that there are still opportunities for improvement. Further studies are warranted to determine whether a bicruciate-retaining TKA design will provide a closer match to natural kinematics and whether an intact ACL will function properly in such TKA designs.