Kinematic vs Mechanical Alignment in TKA

For 40 years, the dogma of Total Knee Arthroplasty (TKA) was simple: "Make it straight." The survivorship of the implant was king, and mechanical alignment was the law. But as implant materials improved and survivorship ceased to be the primary failure mode, surgeons began to ask: "Why are 20% of my patients with 'perfect' X-rays still unhappy?"

This question ignited the most significant debate in modern arthroplasty: Mechanical Alignment (MA) vs Kinematic Alignment (KA). This guide dissects the philosophies, the evidence, and the new middle ground.

Visual Element: Resection plane visualizer. A split-screen diagram. Left: Mechanical Alignment (Cuts perpendicular to mechanical axis, large soft tissue release). Right: Kinematic Alignment (Cuts parallel to joint line, soft tissue preservation).

The Mechanical Axis Dogma (The History)

The Philosophy: Introduced by Insall, the goal of Mechanical Alignment is to create a neutral hip-knee-ankle (HKA) axis (0°).

• Femur: Cut perpendicular to the mechanical axis of the femur.
• Tibia: Cut perpendicular to the mechanical axis of the tibia.
• Gap Balancing: Soft tissues (ligaments) are released until the gaps are rectangular and equal in flexion and extension.

The Logic: A neutral limb distributes load evenly across the tibial polyethylene, minimizing edge-loading and wear. In the era of poor plastics, this was essential for survival.

The Problem: Humans are not robots. Constitutional Varus exists in up to 32% of men and 17% of women. Forcing a patient who has lived 60 years in 5° of varus into 0° neutral requires:

1. Changing their joint line obliquity.
2. Releasing the MCL (destabilizing the knee).
3. Alteration of patellofemoral tracking. This "over-correction" is hypothesized to cause the "forgotten knee" gap—where the knee feels unnatural, tight, or painful.

Kinematic Alignment (KA): The Restoration

The Philosophy: Championed by Howell, KA aims to resurface the knee, restoring the patient's pre-arthritic constitutional anatomy.

• Rule 1: The knee rotates around a specific cylindrical axis.
• Rule 2: The goal is to restore the native joint line obliquity and native ligament tension.
• Technique: Remove bone/cartilage equivalent to the thickness of the implant. No ligament releases are performed.

The Logic: If you put the joint back where it was born, the ligaments will be perfectly tensioned without releases. The knee will feel "normal" because the kinematics (roll-back, rotation) are physiological.

Restricted Kinematic Alignment (rKA): The Middle Ground

Pure KA has a risk: reproducing severe deformities (e.g., 15° varus) might be good for ligaments but bad for the implant (catastrophic edge loading). Restricted KA (rKA) is the compromise, widely adopted with robotics.

The Protocol:

1. Aim for KA principles (resurfacing).
2. The Guardrails:
   • Overall HKA must be within ± 3° (or 5°) of neutral.
   • Joint line obliquity < 5°.
3. If the patient's anatomy is outside the guardrails, correct them just enough to get inside the safe zone, but no further.

Visual Element: The CPAK (Coronal Plane Alignment of the Knee) Classification matrix. A grid showing the 9 phenotypes of knee alignment based on Joint Line Obliquity and Arithmetic HKA.

The Robotic Revolution

KA and rKA are difficult to perform with manual instruments (jigs differ by 1-2 degrees). Robotics (MAKO, ROSA, VELYS) has been the enabler of this philosophy.

• Precision: You can plan a 3.5° varus tibial cut and execute it within 0.5mm.
• Virtual Balancing: You can assess ligament tension before making cuts and adjust the plan to avoid releases.

The Evidence Corner

Key Study: Young et al. (The KA TKA Trial)

• Large RCT comparing MA vs KA.
• Result: No difference in Oxford Knee Score at 2 years. However, KA group had better range of motion.

Conclusion: The Paradigm Shift

We are moving from "Systematic Surgery" (make everyone straight) to "Personalized Surgery" (match the patient).

• For the straight knee: MA and KA are the same.
• For the constitutional varus knee: KA/rKA offers a more physiological reconstruction with less soft tissue trauma.

Exam Answer Structure: "While Mechanical Alignment has the longest track record of survivorship, I recognize the growing evidence for Kinematic principles in improving functional outcomes. My practice is to utilize Restricted Kinematic Alignment, respecting the native anatomy within safe limits (±3 degrees) to optimize soft tissue balance without compromising implant longevity."

Related Topics:

• Robotics in TKA
• Unicompartmental Knee Arthroplasty (The ultimate kinematic knee)
• Polyethylene Wear Mechanisms
• TKA Instability