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.

Figure 11.1, Left : Schematic representation of bilateral leg standing after kinematically aligned ( KA ) and mechanically aligned ( MA ) total knee arthroplasties ( TKAs ). Lever arm and direction of ground reaction force ( GRF ) are depicted. The line connecting the center of pressure (COP) and the center of mass (COM) indicates putative GRF. The lever arm of GRF is shorter in KA TKA than in MA TKA. Right : Representative radiograph of KA TKA. As in the left panel, the joint line is parallel to the ground, and the COP is medially positioned. Consequently, the lever arm of GRF becomes short and knee adduction moment is reduced, despite slight varus limb alignment.

Figure 11.2, Schematic representation of single-leg standing after kinematically aligned total knee arthroplasty. Joint line is parallel to the ground, and the center of pressure ( COP ) of single-leg standing is more medially positioned than that of bilateral leg standing. Knee adduction moment and horizontal ground reaction force ( hGRF ) are decreased. COM, Center of mass.

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