Unicompartmental Knee Arthroplasty: What is the optimal alignment correction to achieve success? The role of kinematic alignment

Unicompartmental knee arthroplasty (UKA) is in many ways the ultimate kinematic operation, as the express aim is to resurface the diseased side of the joint and restore pre-arthritic alignment and balance while maintaining integrity of both cruciate ligaments. An increasing body of knowledge relates the outcomes of UKA to pre-arthritic anatomy rather than an arbitrarily defined neutral. The Coronal Plane Alignment of the Knee (CPAK) classification provides a validated technique for calculating pre-arthritic limb alignment (the arithmetic hip-knee-ankle angle (aHKA)) and joint line obliquity (JLO) and will enable a greater understanding of the interactions between pre-arthritic anatomy, choice of prosthetic position and outcomes. When pre-arthritic alignment is not taken into consideration a post-operative limb alignment of mild to moderate varus for medial UKA and moderate valgus for lateral UKA appears to produce the best outcomes. When pre-arthritic anatomy is taken into account, superior results have been reported with restoration of pre-arthritic limb alignment and joint line obliquity. Restriction boundaries have yet to be clearly defined for tibial component coronal and hip-knee-ankle (HKA) angles when applying this new paradigm, but existing evidence would suggest a 6 degrees varus limit for the tibial coronal angle may be a reasonable starting point. Lateral UKA has inherent differences in terms of tibial component positioning and ligament balance targets. Mobile bearing UKA demands a three-dimensional understanding of the effect of implant position on bearing stability. Modification of technique is necessary to produce anatomic tibia component angles with equipment designed for mechanical alignment. Robotic technology allows accurate understanding of pre-arthritic anatomy, precise reproduction of patient specific virtual planning, equally precise manipulation of soft tissue balance, and future research using these platforms is likely to further clarify in terms of ideal patient- specific component and limb alignment targets.