The kinematic alignment (KA) technique for unicompartmental knee arthroplasty (UKA) has been successfully performed for decades and presents potential clinical benefits. The discussion and videos in this chapter assist the surgeon in understanding how to perform KA UKA with the calipered technique. The first section defines the KA UKA technique; the second section provides the rationale; the third lists the potential benefits of this technique and summarizes the evidence; the final section briefly describes the surgical technique for medial KA UKA. Our objective is to encourage surgeons to use the KA UKA technique because it is a simple, safe method with a more physiologic outcome. It presents multiple potential advantages over the traditional MA technique, and further investigations are needed to better define its clinical impact and the limits for alignment of components.
See Discussion on the definition of the kinematic alignment technique for unicompartmental knee arthroplasty.
Traditional techniques for partial or total knee arthroplasty (TKA) recommend positioning components in a systematic orientation, thereby neglecting the anatomic variation of the individual knee. The standard for decades has been to frontally align the components perpendicular to the femoral and tibial mechanical axes, and to produce an identical tibial slope for all patients; this is known as the mechanical alignment (MA) technique for TKA and UKA. Although MA prosthetic knees altered the knee anatomy and physiologic soft tissue balance, the technique was thought to improve the reproducibility of the implantation. Nevertheless, function and perception of these MA prosthetic knees have been disappointing, , despite great advancements in implant design and surgical precision. , This has led to the development of more personalized and physiologic techniques of implantation, which respect the individual knee anatomy and soft tissue balance, known as kinematic alignment (KA; Fig. 15.1 ). ,
Similar to the KA TKA, the goal of the KA UKA is to coalign UKA components with the kinematic axes that dictate native motion of the tibia around the femur. , Components are therefore aligned parallel and perpendicular to the cylindrical axis and longitudinal tibial axis, respectively ( Fig. 15.2 ). In simplistic terms, the KA technique for UKA aims to produce a “true resurfacing,” restoring the native joint line level and orientation in the implanted knee compartment ( Fig. 15.3 ). In the case of medial UKA, the individual medial and posterior slopes of the medial tibia plateau and the frontal and axial orientation of the medial femoral condyle are therefore restored ( Fig. 15.3 ). The KA technique for UKA was popularized decades ago under the name ‘Cartier Angle’ technique. , Philippe Cartier developed this technique in the 1970s and was influenced by works from Christophe Levigne and Michel Bonnin (Lyon School), who first described the proximal tibia metaphyseal-epiphyseal axis.
The KA and MA techniques differ at almost every step of the procedure (see the Surgical Technique section further in this chapter), with different goals for aligning UKA components. It is therefore important when assessing UKA component performance to consider the technique of alignment by distinguishing between KA and MA positioning of components ( Fig. 15.4 ). The terminology describing the alignment technique is derived from the reference landmark used to align UKA components: the KA and MA techniques respectively align the UKA components on the kinematic axes of the knee and the mechanical axis of long bones ( Figs. 15.4 and 15.5 ).
See Discussion on the rationale for the kinematic alignment technique for unicompartmental knee alignment.
KA is a personalized, physiologic technique that intends to restore the unique individual knee’s soft tissue balance and kinematics, and to optimize the bone loading and dynamic interaction between components ( Figs. 15.1 , 15.2 , 15.4 , and 15.5 ). This is in contrast to the MA technique for UKA, whereby posterior and frontal tibial slopes usually alter the medial knee compartment anatomy, balance, and biomechanics, , making the MA technique less physiologic.
By respecting the individual anatomy of the knee compartment, namely the joint line height and orientation of articular surfaces, the KA technique is biomechanically sound. The components are aligned on the knee’s native kinematic axes that dictate the physiologic motion of the tibia around the femur. Restoration of the natural medial collateral ligament tension and knee kinematics is enabled. In addition, the tibial component is positioned perpendicular to the subchondral trabeculae, the orientation of which are dictated by Wolff’s law, adapted to mechanical loads, and parallel to the ground during gait. The more physiologic the loading of the tibial bone, the more the shear stress at the bone-implant fixation interface is reduced, thereby potentially benefiting the implant’s life span.
Similarly to KA TKA, respecting knee anatomy when implanting UKA components adequately restores the native knee’s soft tissue balance and kinematics, thereby producing optimal clinical outcomes. , , This approach is echoed by many studies that have reported on the deleterious effect of altering the medial tibia plateau anatomy when implanting medial UKA.
Expected Benefits and Evidence
See Discussion on the expected benefits of and the evidence on kinematic alignment unicompartmental knee arthroplasty.
Although both KA and MA techniques for UKA aim to restore the constitutional standing lower limb and knee alignment, the KA technique is likely to better replicate the natural dynamic limb/knee frontal alignment (particularly during knee flexion), as the individual tibial slope is not altered. As a consequence, KA enables more physiologic knee biomechanics through decreased alteration of anatomy and soft tissue balance.
The KA UKA technique presents multiple theoretical benefits over the MA technique; however, some of these have yet to be demonstrated scientifically. This relative absence of evidence is rooted in the fact that alignment of UKA components has been poorly debated thus far, and no study on UKA has ever focused on comparing the value of alignment philosophies. However, there are several aspects of KA principles that are theoretically beneficial to UKA.
First, the KA technique preserves tibial bone stock and loads the supporting tibial bone more physiologically (toward the metaphysis) than MA techniques. This reduces the risk of stress and excessive tibial cortex loading, thus potentially preventing complications such as tibia plateau fracture and residual pain from bone remodeling that some authors have identified in UKA. Second, by restoring the native tibial plateau’s posterior slope, the KA technique potentially generates a more physiologic soft tissue balance by preventing mismatch between flexion and extension (more precisely 90-degrees and 10-degrees knee flexion) gap laxities. This may reduce complications such as residual pain and stiffness, and clinically detrimental poor prosthetic knee kinematics. Third, the KA technique enables components to remain perpendicular relative to one another throughout the knee’s range of motion, therefore creating optimal dynamic component interaction that may reduce complications related to liner edge loading (dislocation for mobile liner, accelerated polyethylene wear for fixed liner). Fourth, it has been shown that the Oxford® UKA components better fit the supportive bone (less compromise regarding implant overhang and bone undercoverage) when kinematically aligned. Again, this could be clinically beneficial by lowering the risk of residual pain and optimizing implant life span. Finally, it is biomechanically sound to perform KA UKA that places the tibial component perpendicularly aligned to the subchondral trabeculae and orientated parallel to the ground in weight-bearing positions (reduction of shear stress on implant fixation). By reducing shear stress on implant fixation and reproducing the physiologic metaphyseal bone loading, the KA technique could benefit long-term UKA outcomes.
Many studies have reported good outcomes for KA UKA over the long term, demonstrating acceptable implant life span, excellent functional performance, restored joint perception, and high patient satisfaction after KA-UKA. , , By simulating KA medial Oxford® UKA on 40 unselected osteoarthritic knee models, Rivière et al. found that the orientation of components was always within the acceptable range of alignment, as recommended by the Oxford Group, and that the shape-fit between KA components and supportive bone was improved compared with the MA technique. Three radiostereometric studies found KA component fixation to be reliable, with low component migration up to 2 years postoperatively and that up to 6 degrees of varus orientation for the tibial component is acceptable. Conversely, many studies have reported on the deleterious effect of altering the medial tibia plateau anatomy when implanting UKA. A recent systematic review found nine studies having indirectly compared KA and MA UKA. Those studies included 593 KA UKAs with follow-up between 3.2 and 12 years. The findings reported high Knee Society Score (KSS) (from 87 to 95), in addition to satisfaction scores of 88% for KA UKA patients. There were low revision rates reported for KA UKA for tibial plateau fracture (0%, no case), unexplained pain (0.8%), tibial component loosening (2%), and any causes of aseptic failures (5.6%). The prosthetic lower limb and tibial implant alignments were found to be in varus (mean of 3 degrees to 5 degrees), while the postoperative joint line and tibial component were found to be parallel to the floor when standing. The authors concluded KA UKA had good mid- to long-term safety and efficacy but could not determine if one technique was superior to the other. The authors highlighted the need for research specifically focused on comparing the outcomes of KA and MA techniques for UKA.
See Discussion on the surgical steps for performing medial calipered kinematic alignment unicompartmental arthroplasty with the Oxford®.
See Illustrating the execution of a medial calipered kinematic alignment unicompartmental arthroplasty with the Oxford® components.
Similar to KA TKA, the KA technique for UKA is a measured resection technique that can simply and reliably be performed with manual instrumentation and a caliper-based check of the bone cut thickness. Technological assistance allowing three-dimensional planning (e.g., personalized surgical instrumentation and robotics) may also be of interest, but is unlikely to be a gamechanger, as the KA technique relies on easily accessible intraarticular anatomic landmarks. KA UKA does not need to establish extraarticular landmarks to identify the long bones’ mechanical axes, and the surgeon has the intraoperative ability to quality assure alignment by checking and potentially correcting bone cuts. The KA and MA surgical techniques differ at every step of the preparation, with the exception of the axial and sagittal rotations of the tibial and femoral components, respectively ( Table 15.1 ).