Summary Kinematic alignment (KA) is a technique for total knee arthroplasty that aims to reconstruct patient-specific limb alignment and knee biomechanics based on the pre-arthritic kinematic axes of the patient’s femur. KA aims to restore the patient’s pre-arthritic limb alignment and joint lines by resurfacing the distal femur and proximal tibia. KA is considered a “ligament sparing” technique as it relies on bone cuts to recreate pre-arthritic knee laxities and compartment forces. Anatomic Rationale Humans have alignments of the Hip-Knee-Ankle (HKA) axis that range from significant varus to neutral to significant valgus. The obliquity of the femoral-tibial joint line also ranges from varus to neutral to valgus. The HKA axis and the joint line obliquity are independent anatomic considerations that can be present in 9 different combinations Only approximately 15% of native knees, arthritic or otherwise, have a 180-degree HKA axis and joint line perpendicular to that axis. Therefore, traditional mechanical alignment surgical techniques alter approximately 85% of patients from their normal anatomy and biomechanics. Foundational Principles Outside of extreme circumstances there is minimal to no femoral bone wear in the arthritic process there is minimal to no attenuation or contracture of the ligaments and soft tissues the PCL is usually intact and functional Varus and valgus arthritic deformities result from cartilage loss and tibial bone wear The normal biomechanics of the human knee are defined by the three kinematic axes found in the femur Therefore, a resurfacing of the femur will restore the three femoral axes Restoring tibial wear and cartilage wear will result in a “balanced” joint with appropriate patellar tracking without the need for releases of healthy ligaments Normal “balance” of the knee comprises different medial and lateral laxities in extension and flexion medial and lateral laxities are the least when the knee is in extension when the knee is in flexion, the lateral compartment usually exhibits more laxity than the medial compartment Elevation or depression of the joint line is avoided to reduce the risk of patellar baja/alta The Three Kinematic Axes of the Femur The flexion-extension axis The tibia flexes and extends around one axis located across the medial condyle This axis is oriented differently than the transepicondylar axis (TEA) The internal-external rotation axis The tibia internally and externally rotates around one axis located in the medial compartment (i.e., a medial pivot) While the knee flexes, internal rotation of the tibia contributes to the “roll back” of the lateral femoral condyle’s contact point on the lateral tibial plateau The patellar tracking axis The patella travels through the femoral trochlea around an isometric rotation axis located across the medial condyle This is analogous to Schottle’s start point on the medial cortex in MPFL reconstruction Surgical Techniques in Kinematic Alignment Femoral surgical technique First, the surgeon computes the femoral resection KA target to restore the joint surfaces The KA target is the thickness of the condyle of the femoral component minus 1 mm for the saw blade's kerf and 2 mm for complete cartilage loss "Balancing" the tibia in kinematic alignment After resurfacing the femur, the tibial bone cut must be adjusted to achieve the desired balance and account for any tibial bony wear (See Algorithm Chart) Gap Assessment: In extension, a rectangular gap should be created to achieve equal medial and lateral compartment balance with negligible varus-valgus laxity Flexion: In flexion, there should be comparable laxities in the medial compartment compared to full extension However, the lateral side should progressively loosen as the knee flexes, thereby forming a trapezoidal gap in the flexed position Optimizing the varus/valgus and slope of the tibial cut is used to balance the knee Soft tissue releases are not used to balance the knee The PCL is not sacrificed or recessed to balance the knee Methods of Performing Kinematic Alignment Caliper-Verified Technique Benefits Minimal change from traditional surgical workflows Can be performed with any instrument set as long as a bone caliper is used to verify the cut thicknesses Intraoperative caliper measurements determine whether the thicknesses of the bone resections are within ± 0.5 mm of the KA target Can be utilized with any vendor or implant design, although medial pivoting implants are helpful to restore the internal-external rotation axis Limitations Difficult to accurately restore the native sagittal alignment (i.e., flexion-extension of the femoral component) using an intramedullary rod Tibial recuts are commonly needed to optimize varus/valgus and slope Difficult to pre-operatively determine component sizes and positions Patient-Specific Cutting Guides Benefits Detailed pre-operative planning to optimize component position and size Minimal trays and instruments are required Limitations Tibial recuts are still necessary Advanced imaging is necessary for planning Typically closed-vendor systems where only the implant of the PSI guide vendor can be used Navigation (Infrared, Bluetooth, Augmented Reality, etc.) Benefits Detailed pre-operative planning to optimize component position and size Intra-operative adjustments can be made to avoid the need for recuts Limitations May require pre-operative advanced imaging Typically closed-vendor systems where only the implant of the navigation vendor can be used High purchase and maintenance costs for the navigation hardware Robotics Benefits Detailed pre-operative planning to optimize component position and size Intra-operative adjustments can be made to avoid the need for recuts Limitations May require pre-operative advanced imaging Typically closed-vendor systems where only the implant of the robotic vendor can be used Heavy equipment burden intraoperatively High purchase and maintenance costs for the robotic hardware Kinematic Alignment versus Mechanical Alignment Common differences between mechanical and kinematic alignment Varus knees in kinematic alignment Varus tibia cuts or varus limb alignment are not associated with bony or hardware failures of the reconstruction A well-reconstructed varus knee may still be in residual varus which represents the correct pre-arthritic condition of the patient’s knee Valgus knees in kinematic alignment Valgus knees do not have hypoplastic lateral femoral condyles; rather, valgus knees have distal femur joint lines in natural positions of valgus with comparably sized condyles Valgus arthritic alignment is a combination of lateral compartment cartilage wear and lateral tibial wear A well-reconstructed valgus knee may still be in residual valgus which represents the correct pre-arthritic condition of the patient’s knee Component rotation in kinematic alignment External rotation of the femoral component and tibial component are not necessary to optimize patellar tracking Many native trochleae, and therefore many kinematically placed femoral components, are internally rotated when referenced to traditional landmarks like the TEA Patellar tracking is optimized by: The correct HKA, joint line obliquity, and joint line height The internal rotation of the tibial tubercle as the knee flexes Implant design in kinematic alignment It is difficult to achieve normal knee biomechanics without utilizing a medial-pivot implant design that facilitates isolated lateral compartment roll-back There are mechanical concerns about sheer loading a cam-and-post mechanism of a PS knee Related Techniques Restricted KA Limits are placed on the magnitude of the varus or valgus position of the femoral component and tibial component Limits are placed on the magnitude of the varus or valgus alignment of the overall HKA axis When necessary, minor soft tissue releases are utilized to achieve the desired balance as opposed to cutting more bone beyond the pre-set limits Functional Alignment The bony resections of both the femur and tibia are adjusted intraoperatively to achieve either symmetric or asymmetric joint spaces Soft tissue releases are avoided Patient-Specific Alignment (i.e., “Inverse” KA) A pre-determined tibial cut is made based on patient-specific factors, and then the femoral component is gap balanced to achieve either symmetric or asymmetric joint spaces Soft tissue releases are avoided