Courtesy: Dr Brett A Fritsch, Ashok Shyam, Ortho TV

Personalized Alignment in Total Knee Arthroplasty

Introduction

A fundamental concept in knee surgery is that no two knees are identical.
Each patient demonstrates unique variations in:
- Bone morphology
- Limb alignment
- Soft tissue balance
Historically, surgeons often treated osteoarthritic knees with a uniform surgical approach, assuming similar anatomy.
Modern research shows that significant anatomical variability exists, requiring more individualized surgical strategies.

Variability in Knee Anatomy

Evidence from Population Studies

Research evaluating knee alignment across populations demonstrates:

Wide variation in coronal alignment
Differences in axial rotation
Differences in sagittal alignment

Large datasets of knees undergoing arthroplasty reveal:

Femoral coronal alignment variation of over 20 degrees
Tibial alignment variation of more than 25 degrees
Rotational differences of over 10 degrees

These findings confirm that knee anatomy varies widely across individuals.

Frequency of Anatomical Variations

Analysis of large datasets shows:

Approximately 16% of knees fall outside traditionally defined “normal” anatomical ranges.
More than 50% of patients demonstrate at least one anatomical deformity.
Around 12–13% of individuals have two or more anatomical variations.

This highlights that anatomical diversity is the rule rather than the exception.

Knee Alignment Phenotypes

Concept of Knee Phenotyping

To better understand anatomical diversity, knees are classified based on two primary measurements:

Lateral Distal Femoral Angle (LDFA)
Medial Proximal Tibial Angle (MPTA)

Using these parameters, knee alignment patterns can be categorized into distinct phenotypes.

Key Alignment Concepts

Mechanical Alignment

Represents the overall limb alignment from hip to ankle.
Influenced by:
- Bone anatomy
- Cartilage thickness
- Degenerative changes

As osteoarthritis progresses:

Cartilage loss alters mechanical alignment.
Alignment may shift toward varus or valgus depending on the compartment affected.

Constitutional Alignment

Reflects the native alignment of the femur and tibia independent of cartilage loss.
Remains stable even as osteoarthritis progresses.
Allows surgeons to estimate the patient’s original knee alignment before degeneration occurred.

Joint Line Obliquity

Describes the orientation of the joint line between the femur and tibia.
Determined by combining femoral and tibial alignment angles.

Joint line orientation can be:

Neutral
Apex distal
Apex proximal

Importantly:

Joint line obliquity is independent of the mechanical axis.

Knee Phenotype Classification

By combining:

Limb alignment
Joint line orientation

A total of nine distinct knee phenotypes can be identified.

Population studies show:

Around 65% of knees fall into three common phenotypes
Approximately one-third fall outside these typical patterns

Natural Mechanical Alignment

Only about 15% of individuals naturally have a neutral mechanical axis.

Implication:

When surgeons perform a mechanically aligned knee replacement, only 1 in 8 patients receives their original alignment.
The remaining patients undergo intentional alteration of their native anatomy.

Differences Across Populations

Alignment phenotypes vary geographically.

Examples from research:

Some populations show higher prevalence of neutral alignment patterns.
Others demonstrate greater rates of varus alignment.

This reinforces the need for population-specific considerations in knee arthroplasty.

Importance of Three-Dimensional Analysis

The knee functions in three planes:

Coronal plane
Sagittal plane
Axial plane

Modern technologies allow better analysis of these planes, including:

CT scans
MRI
Computer navigation
Robotic systems

Two knees may appear similar in coronal alignment but differ significantly in rotational or sagittal anatomy, affecting surgical outcomes.

Alignment Philosophies in Knee Replacement

Two major approaches exist.

Fixed Mechanical Alignment

Characteristics:

Implants positioned in standard orientation.
Soft tissues are released to accommodate implant positioning.

Typical parameters:

Femoral component perpendicular to mechanical axis
Tibial component perpendicular to mechanical axis
Standard rotational alignment

Advantages:

Predictable implant positioning
Historically well-established

Limitations:

Native anatomy often altered
Higher need for soft tissue releases

Patient-Specific (Kinematic) Alignment

Goal:

Restore the patient’s original knee anatomy and ligament balance.

Principles include:

Reproducing natural joint line orientation
Preserving native soft tissue tension
Accepting slight variations in implant alignment

This approach treats knee arthroplasty more like an anatomical resurfacing procedure.

Alignment Spectrum

Alignment strategies exist on a continuum.

Strict Mechanical Alignment

Fixed implant positioning
Maximum reliance on soft tissue releases

Anatomical Alignment

Slight deviation from mechanical alignment to mimic average anatomy

Restricted Kinematic Alignment

Attempts to restore patient anatomy
Accepts limited implant deviations within safe boundaries

Unrestricted Kinematic Alignment

Fully replicates native anatomy
Allows significant implant deviations if necessary

Most surgeons practice somewhere between mechanical alignment and unrestricted kinematic alignment.

Trade-offs Between Approaches

When selecting alignment strategy, surgeons balance several factors:

Restoration of Native Anatomy

Highest with unrestricted kinematic alignment
Lowest with strict mechanical alignment

Need for Soft Tissue Releases

High in mechanical alignment
Low in kinematic alignment

Implant Position Safety

More predictable in mechanical alignment
Greater variability in kinematic alignment

Surgical Philosophy

Total knee arthroplasty involves balancing multiple competing goals:

Recreate native limb alignment.
Avoid extreme implant positioning.
Preserve ligament balance.
Achieve stable joint mechanics.

This process resembles tailoring a suit for the patient rather than forcing the patient to fit a standard template.

Clinical Results Comparing Alignment Strategies

Studies comparing mechanical alignment with restricted kinematic alignment show:

Bone Resection

Kinematic alignment requires less bone removal.

Soft Tissue Releases

Mechanical alignment required releases in over 70% of cases.
Kinematic alignment required releases in only a small minority of cases.

Implant Alignment

Slight increase in varus positioning with kinematic alignment.
No significant increase in extreme outliers.

Clinical Outcomes

Recent clinical studies using robotic systems demonstrate:

Excellent patient-reported outcomes.
High functional scores.
Minimal need for major ligament releases.

Reported results include:

High Oxford Knee Scores
Good functional recovery
Very low revision rates in early follow-up.

Key Messages

1. Knee anatomy varies greatly among individuals.

2. Only a small percentage of patients naturally have neutral mechanical alignment.

3. Standardized implant positioning alters native anatomy in most patients.

4. Personalized alignment strategies may improve soft tissue balance and function.

5. Modern technologies enable more precise patient-specific surgery.

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