Courtesy: Dr Eugene E K, Ashok Shyam, Ortho TV

Functional Alignment in Knee Replacement Surgery

Introduction

• Alignment strategies in total knee replacement have evolved significantly over the years.

• Many surgeons initially adopted mechanical alignment, which has been the traditional and widely practiced method for several decades.

• Over time, alternative approaches such as kinematic alignment and functional alignment have been introduced to improve patient outcomes and joint balance.

• Functional alignment is a relatively newer concept that aims to combine the advantages of traditional methods while minimizing their limitations.

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Traditional Mechanical Alignment

Basic Principle

• Mechanical alignment has been the standard approach in knee replacement surgery for many years.

• In this method:

  • The distal femur is cut perpendicular to the mechanical axis of the femur.

  • The proximal tibia is cut perpendicular to the mechanical axis of the tibia.

• The goal is to create a neutral mechanical axis from the hip to the ankle.

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Advantages

• Simple and reproducible surgical technique.

• Well-documented long-term outcomes.

• Historically considered the safest alignment strategy for implant longevity.

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Limitations

• Achieving ideal gap balance can sometimes be difficult.

• Flexion and extension gaps may not match naturally.

• Surgeons frequently need to perform ligament releases to balance the knee.

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Kinematic Alignment Concept

Surgical Objective

• Kinematic alignment attempts to restore the patient’s native knee anatomy before arthritis developed.

• The procedure aims to:

  • Reproduce the natural joint surfaces.

  • Preserve native ligament tension.

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Potential Benefits

• Faster functional recovery in some patients.

• Improved knee kinematics.

• Reduced need for extensive ligament releases.

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Concerns

• In patients with extreme anatomical deformities, restoring native alignment may place implants in unusual positions.

• This could potentially increase the risk of implant failure or early wear.

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Restricted Kinematic Alignment

Concept

• Restricted kinematic alignment was introduced to address the risks associated with unrestricted kinematic techniques.

Key Principles

• Maintain implant alignment within safe limits.

• Slightly correct severe deformities when necessary.

• Maintain a balance between restoring anatomy and ensuring implant safety.

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Typical Alignment Boundaries

• Hip-knee-ankle alignment within approximately three degrees of neutral.

• Femoral and tibial cuts generally kept within five degrees of varus or valgus.

These limits aim to prevent excessive implant malalignment while preserving natural joint mechanics.

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Functional Alignment Philosophy

Core Idea

• Functional alignment builds upon the principles of restricted kinematic alignment.

• The primary goal is to achieve optimal ligament balance while preserving as much natural alignment as possible.

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Key Surgical Strategy

Instead of performing additional ligament releases:

• The implant position is slightly adjusted within safe limits.

• These adjustments allow the surgeon to achieve balanced flexion and extension gaps.

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Advantages

• Minimizes soft tissue damage.

• Reduces the need for extensive ligament releases.

• Improves gap balance during surgery.

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Clinical Example

Patient Profile

• Female patient aged sixty one years.

• Significant varus knee deformity.

Preoperative Measurements

• Mechanical varus alignment: twelve degrees.

• Lateral distal femoral angle: ninety two degrees.

• Medial proximal tibial angle: eighty six degrees.

These measurements indicate a pronounced varus alignment of the knee.

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Intraoperative Gap Assessment

Initial Measurements

During surgery:

• Extension gap measured approximately twenty four millimeters.

• Flexion gap measured approximately twenty seven millimeters.

This indicates that the flexion gap is slightly larger than the extension gap, which is a common situation.

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Initial Surgical Adjustments

Steps performed:

• Slight reduction in planned bone resection.

• Removal of medial osteophytes.

• Mild medial ligament release using the pie-crusting technique.

Pie-crusting involves multiple small punctures in the ligament using a needle to gradually reduce tightness.

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Reassessment After Initial Balancing

• Trial components were inserted.

• Valgus stress testing was performed to evaluate alignment and ligament balance.

Findings:

• Residual varus alignment remained.

• The lateral side appeared slightly more lax.

Additional minor pie-crusting adjustments were performed to improve balance.

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Flexion Gap Evaluation

At ninety degrees of knee flexion:

• Controlled distraction forces were applied medially and laterally.

• The medial and lateral gaps remained unequal.

Traditionally, further ligament release might be performed at this stage.

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Functional Alignment Adjustment

Instead of further ligament release:

• The position of the femoral component was slightly adjusted.

• A small change in component alignment improved both:

  • Extension gap balance

  • Flexion gap balance

Additionally:

• A slight increase in femoral external rotation helped optimize the flexion gap.

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Postoperative Evaluation

Radiographic assessment confirmed:

• Proper implant positioning.

• Balanced alignment with slight residual varus that remained within acceptable limits.

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Comparative Clinical Study

A comparison study was performed involving:

• One hundred total knee replacement procedures.

Two alignment strategies were evaluated:

• Modified kinematic alignment

• Functional alignment

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Results

Flexion Gap Balance

• Modified kinematic alignment achieved satisfactory flexion gap balance in approximately forty three percent of cases.

• Functional alignment achieved satisfactory flexion gap balance in all cases studied.

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Component Rotation

• Functional alignment typically resulted in slightly greater external rotation of the femoral component, which improved flexion gap symmetry.

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Key Takeaways

• Several alignment philosophies exist in total knee replacement surgery.

• There is still no universal agreement regarding the best alignment strategy.

• Mechanical alignment remains reliable with long-term results.

• Kinematic alignment focuses on restoring native anatomy.

• Functional alignment aims to optimize ligament balance while preserving anatomical alignment.

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Conclusion

• Functional alignment offers a promising approach for achieving balanced knee mechanics.

• By adjusting implant position rather than extensively releasing ligaments, surgeons can achieve better gap balance while preserving soft tissues.

• Continued research and long-term studies are necessary to determine the optimal alignment strategy for knee replacement surgery.