TKA INSTABILITY
Flexion Gap | Extension Gap | Constraint | Revision
TYPES
Critical Must-Knows
- Gap imbalance most common cause
- Flexion instability: loose in flexion
- Extension instability: recurvatum tendency
- Component malposition key factor
- Revision with increased constraint if failed
Examiner's Pearls
- "Flexion gap: posterior femoral condyle resection
- "Extension gap: distal femur resection
- "PCL incompetent = flexion instability in CR
- "Increase constraint level for revision
Clinical Imaging
Imaging Gallery
Critical TKA Instability Concepts
Flexion Instability
Knee unstable in flexion. Caused by excessive posterior femoral condyle resection, flexion gap too large, PCL incompetence in CR TKA, component malrotation.
Extension Instability
Knee unstable in extension. Caused by excessive distal femur resection, extension gap too large, MCL/LCL laxity, recurvatum tendency.
Mid-Flexion Instability
Unstable 30-60 degrees. Femoral component too small or internally rotated. Polyethylene wear. Difficult to address.
Global Instability
Unstable in all positions. Severe ligamentous laxity. Requires higher constraint level (VVC or hinge).
At a Glance: Quick Decision Guide - TKA Instability
| Instability Type | Key Finding | First-line Treatment | Revision Approach |
|---|---|---|---|
| Flexion instability | Loose in flexion, stairs difficulty | Thicker polyethylene | PS conversion or downsize femur |
| Extension instability | Recurvatum, loose standing | Bracing trial | Distal femoral augment |
| Mid-flexion | 30-60 degree laxity | PT, bracing | Correct rotation, VVC |
| Global instability | All positions unstable | Hinged brace | VVC or rotating hinge |
Gap Imbalance Types
| Type | Cause | Solution |
|---|---|---|
| Flexion greater than extension | Excessive posterior condyle resection | Downsize femur or use PS |
| Extension greater than flexion | Excessive distal resection | Augment distal femur |
| Both gaps large | Global ligament laxity | Increase constraint level |
| Both gaps tight | Under-resection | Additional bone cuts |
PCPSFlexion Gap Components
Memory Hook:PCPS determines Posterior (flexion) Gap!
DMCExtension Gap Components
Memory Hook:DMC = Distal femur determines extension gap!
CPVHConstraint Level Selection
Memory Hook:CPVH = Constraint rises from C to H!
Overview and Epidemiology
Cause Analysis
Most TKA instability results from gap imbalance - mismatch between flexion and extension gaps, or global ligamentous laxity. Component malpositioning is the underlying cause.
Pathophysiology and Mechanisms
Gap Balancing Concepts:
The stability of TKA depends on equal and balanced flexion and extension gaps. Understanding the structures that contribute to each gap is essential for both prevention and treatment of instability.
Extension Gap:
- Structures removed: Distal femur resection
- Stabilizers: MCL, LCL, posterior capsule
- Assessment: Knee fully extended (0 degrees)
- Gap size: Determined by distal femur cut depth
Flexion Gap:
- Structures removed: Posterior femoral condyle resection
- Stabilizers: PCL (in CR), MCL, LCL
- Assessment: Knee at 90 degrees flexion
- Gap size: Determined by posterior condyle cut and femoral sizing
Key Biomechanical Principles:
- Distal femur resection primarily affects extension gap
- Posterior condyle resection primarily affects flexion gap
- Femoral sizing affects flexion gap (larger femur = smaller flexion gap)
- Posterior tibial slope affects flexion gap (more slope = larger flexion gap)
- Collateral ligaments contribute to both gaps
Constraint Mechanism:
TKA implants provide varying degrees of intrinsic constraint:
| Constraint Level | Mechanism | Indications |
|---|---|---|
| CR | Relies on intact PCL | Normal ligaments |
| PS | Cam-post replaces PCL | PCL deficiency |
| VVC | Taller post, deeper box | Collateral laxity |
| Hinge | Linked axis | Global instability |
Increasing constraint increases stress at fixation interface - use minimum constraint necessary.
Classification and Mechanism
Flexion Gap Too Large
Causes:
- Excessive posterior condyle resection
- Femoral component too small (anterior referencing)
- PCL rupture/incompetence in CR TKA
- Excessive posterior tibial slope
Presentation:
- Instability/giving way with stairs, sitting
- Knee feels loose in flexion
- Posterior subluxation
Address by reducing flexion gap or increasing constraint.
Clinical Assessment
History
- Giving way episodes
- When does instability occur
- Stairs vs walking vs rising
- Pain location and character
- Time from primary surgery
- Previous surgeries
Timing of symptoms helps classify.
Examination
- Varus/valgus stress at 0 and 30 degrees
- Anterior/posterior drawer
- Recurvatum assessment
- Compare to contralateral
- ROM assessment
- Gait evaluation
Document degree and position of laxity.
Investigations
Radiographic Assessment
Standard views:
- Weight-bearing AP
- Lateral
- Skyline
Assess:
- Component position
- Joint line position
- Polyethylene wear
- Alignment
Stress views may demonstrate laxity.
Management Algorithm
Conservative Trial
Indications:
- Mild instability
- Poor surgical candidate
- Recent surgery (give time to stabilize)
Options:
- Bracing (hinged knee brace)
- Physical therapy (quad strengthening)
- Activity modification
Limited success for true mechanical instability.
Surgical Technique
Implant Constraint Options
| Constraint | Indication | Mechanics |
|---|---|---|
| CR (cruciate-retaining) | Normal knee | Relies on PCL |
| PS (posterior-stabilized) | PCL deficient | Cam-post mechanism |
| VVC (varus-valgus constrained) | Collateral laxity | Taller post, more constraint |
| Hinge | Global instability | Linked mechanism |
Increase constraint as needed for stability.
Constraint Level
Use minimum constraint necessary. Higher constraint transfers more stress to fixation interface, potentially increasing loosening risk. Balance with need for stability.
Complications
| Complication | Incidence | Prevention/Management |
|---|---|---|
| Recurrent instability | 5-10% | Appropriate constraint, good balance |
| Stiffness | Variable | Aggressive early ROM |
| Aseptic loosening | Increased with constraint | Adequate fixation, stems |
| Infection | 2-3% revision | Prophylaxis, staged if indicated |
Complication Prevention Strategies
Intraoperative Principles:
- Confirm gap balance before final cementation
- Assess stability through full ROM under anesthesia
- Document constraint level selection rationale
- Use adequate stem fixation for constrained implants
- Consider staged approach if any infection concern
Postoperative Monitoring:
- Regular clinical and radiographic follow-up
- Early identification of recurrent symptoms
- Low threshold for aspiration if effusion recurs
- Long-term outcomes depend on initial balance
Postoperative Care
Revision TKA Rehabilitation
Weight-bearing as tolerated. Brace if needed. ROM exercises.
Progressive strengthening. ROM focus. Stairs training.
Full activities. Quad and hamstring focus. Balance training.
Return to normal activities. Long-term follow-up.
Outcomes and Prognosis
Prognostic Factors
Better outcomes: Correct diagnosis, appropriate constraint, good bone stock.
Worse outcomes: Global instability, multiple prior surgeries, poor soft tissues.
Evidence Base and Key Studies
Causes of TKA Instability
- Gap imbalance most common
- Component malposition underlying
- Flexion instability most frequent
- Revision with constraint effective
Constraint Level Selection
- Match constraint to laxity
- VVC effective for moderate instability
- Hinge for severe cases
- Minimum necessary constraint
VVC vs Rotating Hinge Outcomes
- VVC preferred for mild-moderate instability
- Rotating hinge for global instability, 80% satisfied at 10 years
- Both options superior to non-operative management
- Stem fixation critical for both constraint levels
AOANJRR Revision Data
- Instability accounts for 15-20% of TKA revisions
- Revision for instability has 85% 10-year survival
- Higher constraint associated with lower re-revision for instability
- Younger patients have higher re-revision risk
Gap Balancing vs Measured Resection
- Gap balancing reduces instability rates
- Measured resection has higher reported flexion instability
- Both techniques acceptable with experience
- Surgeon experience key factor
Additional Evidence Considerations
Key Studies in TKA Instability:
-
Pagnano et al (1998): Established association between component malrotation and instability. Internal rotation of femoral component leads to mid-flexion instability.
-
Berger et al (1998): Defined rotational landmarks for TKA. Transepicondylar axis now standard reference for femoral rotation.
-
Dennis et al (2003): Fluoroscopic kinematic studies showing different motion patterns in stable vs unstable TKAs.
-
Fehring et al (2001): Registry analysis identifying gap imbalance as most common technical error leading to revision.
Surgical Technique Evidence:
The evidence strongly supports:
- Balanced flexion and extension gaps
- Appropriate constraint selection based on ligament status
- Use of stems in revision for improved fixation
- Staged approach if infection suspected
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Flexion Instability
"A patient 6 months after primary CR TKA reports instability going downstairs and difficulty rising from a chair. Examination shows increased anterior-posterior translation in flexion but stable in extension. X-rays show well-fixed components. What is your diagnosis and management?"
Scenario 2: Global Instability
"A 70-year-old woman with severe RA has had 2 previous TKA revisions. She now has gross instability in all positions and cannot walk without aids. What are your options?"
MCQ Practice Points
Flexion Gap Question
Q: What does resecting more posterior femoral condyle do? A: Increases the flexion gap. Posterior condyle resection primarily affects flexion gap.
Extension Gap Question
Q: What does resecting more distal femur do? A: Increases the extension gap. Distal femur resection primarily affects extension gap.
PCL Question
Q: What happens if PCL is incompetent in CR TKA? A: Flexion instability. PCL is primary flexion restraint. Need to revise to PS.
Constraint Question
Q: What constraint level for moderate collateral laxity? A: VVC (varus-valgus constrained). Taller post provides more coronal stability.
Component Rotation
Q: What does internal rotation of femoral component cause? A: Mid-flexion instability. The tibia externally rotates relative to internally rotated femur.
Constraint and Fixation
Q: Why use stems with constrained TKA revision? A: To bypass stress transfer to metaphysis. Higher constraint increases fixation stress.
Australian Context
AOANJRR Data on TKA Instability:
The Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) provides valuable data on revision rates and outcomes. Understanding this data is essential for the Orthopaedic examination and clinical practice.
Key Statistics from AOANJRR:
- Instability accounts for 15-20% of TKA revisions in Australia
- Revision for instability has stable outcomes with 85% 10-year survival of revision TKA
- VVC constraint level most commonly used for instability revision
- Younger age at revision associated with higher re-revision rate
- Component malposition is frequently identified at revision surgery
Australian Practice Points:
- Two-stage approach: Many Australian surgeons favor excluding infection before revising for instability with aspiration and inflammatory markers
- Constraint selection: VVC implants available from all major manufacturers in Australia (Stryker, DePuy, Smith and Nephew, Zimmer)
- Registry reporting: All revisions should be reported to AOANJRR
- PBS considerations: Revision implants covered under PBS when clinically indicated
- GAP balancing: Many Australian arthroplasty surgeons favor gap balancing technique rather than measured resection
Fellowship Exam Relevance: The Orthopaedic Orthopaedic Viva frequently tests:
- AOANJRR data on revision TKA for instability
- Constraint level selection based on Australian guidelines
- Indications for rotating hinge vs VVC constraint
- Gap balancing principles for prevention
- Interpretation of CT rotation studies
Implant Considerations in Australia: All major VVC and hinge designs are available in Australia including options from DePuy, Stryker, Zimmer, and Smith Nephew. Surgeons should be familiar with available constraint options and their indications. Revision implants are covered under PBS when clinically indicated.
TKA INSTABILITY
High-Yield Exam Summary
Types
- •Flexion: loose in flexion, stairs/sitting
- •Extension: recurvatum, loose standing
- •Mid-flexion: 30-60 degree laxity
- •Global: all positions unstable
Gap Balancing
- •Posterior condyle = flexion gap
- •Distal femur = extension gap
- •Equal gaps for stability
- •Flexion gap tight = limited bend
- •Extension gap tight = flex contracture
Causes
- •Gap imbalance most common
- •Component malposition
- •PCL incompetence (CR)
- •Ligament laxity
Constraint Levels
- •CR: relies on PCL
- •PS: cam-post for PCL deficiency
- •VVC: collateral laxity
- •Hinge: global instability
Treatment Principles
- •Minimum constraint needed
- •Balance gaps at revision
- •Address malposition
- •Use stems in revision
Outcomes
- •80-85% good after revision
- •Re-revision 5-10%
- •Match constraint to laxity
- •Early instability: technical error likely
- •Late instability: polyethylene wear/laxity