Alignment Philosophies in TKA: Mechanical, Kinematic and Functional

The shift away from "neutral for everyone" rests on the observation that constitutional (native) lower limb alignment is highly variable - many healthy people have constitutional varus or valgus. Mechanical alignment normalises all of these to neutral, which alters each individual's joint line and frequently requires ligament releases to balance. Individualised philosophies try to respect native anatomy instead. Robotics and navigation are the enabling tools that make precise, reproducible execution of these individualised targets possible.

CPAK characterises a knee using two parameters derived from radiographs:

• The arithmetic HKA (aHKA), which estimates the patient's constitutional limb alignment (varus, neutral, or valgus), and
• The joint line obliquity (JLO), the orientation of the joint line (apex distal, neutral, or apex proximal).

Combining three categories of each yields nine CPAK phenotypes. CPAK has become a common language for reporting alignment, for planning which knees might benefit from a kinematic approach, and for ensuring consistency between studies. In the validating work, kinematic alignment achieved optimal intra-operative soft-tissue balance more often than mechanical alignment across phenotypes, with the largest differences in specific (e.g. constitutional varus) phenotypes.

The honest synthesis - which is what a viva rewards - is that the evidence is nuanced and, at the highest level, equivocal:

• Short/medium-term signals favour individualised alignment on some measures. Meta-analyses of randomised trials report better WOMAC, Knee Society Scores and flexion for kinematic alignment versus mechanical, and restricted kinematic alignment similarly improves some functional scores - though many differences are small and not consistent across every outcome.
• The best long-term randomised evidence shows equivalence. A Level I RCT with 10-year follow-up found no difference in any patient-reported outcome or in survivorship between kinematic and mechanical alignment, mirroring its own 2-year and 5-year results, and concluded that mechanical alignment remains the reference standard.
• Functional alignment is comparable to mechanical at 2 years in randomised data, while requiring fewer soft-tissue releases, with benefits that appear subgroup-specific.

Global Practice Picture

Alignment philosophy is one of the most actively debated topics in international knee arthroplasty. Practice is genuinely divided: mechanical alignment remains the most widely taught and registry-validated approach, while kinematic, restricted-kinematic and (with the spread of robotics) functional alignment have substantial and growing adoption, particularly in Australasia and Europe where much of the constitutional-alignment and CPAK work originated.

Side-by-Side Evidence Synthesis

Registry & Practice Variation

National joint registries continue to validate the long-term survivorship of mechanically-aligned TKA, and long-term registry data for kinematic and functional alignment are still maturing. The pragmatic global position: mechanical alignment is the safe, validated default; individualised alignment is a reasonable, boundaried alternative that reduces soft-tissue releases and is increasingly executed with robotics, but its long-term superiority remains unproven and should be presented to patients as such.