Total Knee Arthroplasty and Alignment Philosophies

Introduction

• Focus: Restoring natural or constitutional alignment in knee arthroplasty using CPAK classification.
• Key Insight: Natural alignment isn't always zero degrees mechanical; it is individual.
• Study Reference: Findings published in 2012 showed an average alignment of 1.3 degrees varus.

Natural Knee Alignment

• Distribution: Gaussian distribution around 1.3 degrees varus, standard deviation 2.3 degrees.
• Population: About one-third have natural varus alignment, termed "constitutional varus."

Challenges in Knee Replacement

• Standard Mechanical Alignment Issues:
  • Conflict with medial soft tissues when corrected to zero degrees.
  • Asymmetric extension space requires releasing or stretching medial collateral ligaments.

CPAK Classification

• Purpose: Classifies knee phenotypes based on alignment and joint line orientation.
• Matrix Design:
  • Columns: Overall alignment (varus, neutral, valgus).
  • Rows: Joint line orientation (apex distal, neutral, apex proximal).
• Formulations:
  • MPTA - LDFA: Defines alignment (varus, neutral, valgus).
  • MPTA + LDFA: Defines joint line orientation (apex distal, neutral, apex proximal).

Population Data

• Western European Data: Most knees are CPAK types 1-5.
• Arthritic Patients: Distribution is similar to healthy populations.

Surgical Implications

• Kinematic Alignment: Suggested approach to restore constitutional alignment.
  • Risks: Severe axis deviation or bone erosion makes kinematic alignment less reliable.

CPAK Type Strategies

• Evaluation: Pre-operative full-leg radiographs to determine CPAK type using MPTA and LDFA.
• Adjustments:
  • For each type, restore knee to its pre-disease or constitutional configuration.
  • Restricted Boundaries: Specific limits for MPTA and LDFA adjustments.

CPAK Types Detailed Strategies

• Type 1 (Varus, apex distal):
  • Standard: MPTA 87, LDFA 89; adjust to max 92 LDFA if needed.
• Type 2 (Neutral, apex distal):
  • Restore to MPTA 87, LDFA 87.
• Type 3 (Valgus, apex distal):
  • Keep MPTA 89, LDFA reduce to 87.
• Type 4 (Varus, neutral):
  • MPTA 87, LDFA 90; adjust to max 92 LDFA if necessary.
• Type 5 (Neutral, neutral):
  • MPTA 90, LDFA 90; avoid valgus over-correction.
• Type 6 (Valgus, neutral):
  • Restore MPTA to 90, LDFA to 88.
• Type 7 (Varus, apex proximal):
  • Keep MPTA 90, LDFA 93; adjust tibia to max 87 MPTA.
• Type 8 (Neutral, apex proximal):
  • Avoid increasing valgus; maintain current alignment if encountered.
• Type 9 (Valgus, apex proximal):
  • Restore MPTA to 92, LDFA to 90.

Conclusion

• CPAK Classification: Provides a framework to restore constitutional alignment effectively in knee arthroplasty.
• Future Work: Further investigations needed for rare CPAK types.