TKA EXTENSOR MECHANISM FAILURE
Catastrophic Complication | Quadriceps or Patellar Tendon
Types of Extensor Mechanism Failure
Critical Must-Knows
- Devastating complication with poor functional outcomes
- Prevention is critical - protect extensor mechanism during surgery
- Patellar tendon ruptures have worse outcomes than quadriceps
- Allograft reconstruction is treatment of choice for chronic ruptures
- Many patients cannot achieve active extension despite treatment
Examiner's Pearls
- "Risk factors: previous surgery, diabetes, steroid use, MUA
- "Acute: primary repair with augmentation
- "Chronic: allograft (Achilles, whole extensor mechanism)
- "Extension lag is functional measure of success
Critical Extensor Mechanism Exam Points
Patellar Tendon Rupture
Most devastating extensor mechanism injury. Risk factors: prior surgery, TTO, diabetes, steroid injection. Presents with inability to extend, patella alta on XR. Primary repair rarely successful - usually needs allograft.
Quadriceps Tendon Rupture
Better prognosis than patellar tendon due to better tissue quality. Can often achieve primary repair with augmentation. Still significant functional limitation expected.
Prevention
Protect the extensor mechanism: Avoid forceful manipulation, careful TTO technique with secure fixation, protect during exposure, avoid multiple surgeries when possible.
Reconstruction Options
Allograft options: Achilles tendon allograft (bone block to patella), whole extensor mechanism allograft, or synthetic mesh augmentation. Outcomes variable.
Quick Decision Guide
| Injury Type | Timing | Tissue Quality | Management |
|---|---|---|---|
| Acute quadriceps rupture | Less than 2 weeks | Good tissue | Primary repair with augmentation |
| Chronic quadriceps rupture | Greater than 6 weeks | Retracted, scarred | Allograft reconstruction |
| Acute patellar tendon | Less than 2 weeks | Usually poor | Primary repair (often fails) or early allograft |
| Chronic patellar tendon | Greater than 6 weeks | Retracted, scarred | Allograft extensor mechanism |
RUPTURERisk Factors
Memory Hook:These factors put the tendon at risk of RUPTURE!
GRAFTManagement Principles
Memory Hook:Use GRAFT principles for extensor mechanism reconstruction!
AWEAllograft Options
Memory Hook:AWE - Allografts are the mainstay of reconstruction!
Overview and Epidemiology
Extensor mechanism disruption after TKA is an uncommon but devastating complication. It includes rupture of the patellar tendon, quadriceps tendon, or patellar fracture extending into the tendon insertion. Outcomes are often poor, with many patients losing the ability to actively extend the knee.
Impact on Function
Loss of active extension prevents stair climbing, rising from chair, and normal gait. Even with successful reconstruction, many patients have permanent extension lag (10-30 degrees) and reduced functional capacity. This is a quality-of-life altering complication.
Risk Factors
- Previous knee surgery (strongest factor)
- Revision TKA: 3-4x higher risk
- Tibial tubercle osteotomy
- Diabetes mellitus
- Steroid use (systemic or local)
- Inflammatory arthritis
Mechanism of Injury
- Intraoperative: During exposure, manipulation
- Early postoperative: Fall, forceful flexion
- Late: Spontaneous (weakened tissue)
- MUA: Excessive force during manipulation
Pathophysiology and Mechanisms
Critical Anatomy
The extensor mechanism includes quadriceps muscle → quadriceps tendon → patella → patellar tendon → tibial tubercle. The patellar tendon has poorer blood supply than quadriceps tendon, especially after TKA with fat pad resection, explaining worse outcomes for patellar tendon injuries.
Anatomical Considerations
| Structure | Vascularity | Healing Potential | Repair Considerations |
|---|---|---|---|
| Quadriceps tendon | Good (muscle supply) | Better healing | Primary repair often possible |
| Patellar tendon | Poor (fat pad dependent) | Poor healing | Often needs allograft |
| Tibial tubercle | Moderate | Bone healing | Screw fixation of TTO |
Blood Supply Critical
Patellar tendon vascularity is compromised by: fat pad resection, previous surgery, diabetes, and lateral release. The tendon receives blood from inferior pole of patella and fat pad. Loss of these sources leads to poor healing potential.
Forces on Mechanism
- Quadriceps force: up to 7x body weight
- Patellofemoral contact: 3x body weight stairs
- Tendon stress concentrated at insertions
- Weakest point after TKA: patellar tendon
Factors Weakening Mechanism
- Fat pad resection (blood supply)
- Lateral release (destabilizes patella)
- Multiple surgeries (scar, vascularity)
- Steroid injection (collagen weakening)
- Component malposition (abnormal stress)
Classification Systems
Classification by Anatomical Site
| Location | Incidence | Prognosis | Primary Treatment |
|---|---|---|---|
| Quadriceps tendon rupture | 30-40% | Better | Primary repair with augmentation |
| Patellar tendon rupture | 40-50% | Worst | Allograft reconstruction |
| Patellar fracture with mechanism disruption | 20-30% | Variable | ORIF vs revision patella |
Location determines prognosis and treatment options.
Clinical Assessment
History
- Mechanism: Fall, sudden pop, intraoperative
- Timing: When did it occur relative to TKA
- Symptoms: Inability to extend, giving way
- Previous surgeries: Risk factor assessment
- Comorbidities: Diabetes, steroids, RA
Physical Examination
- Active extension: Complete loss or extension lag
- Palpable gap: At rupture site
- Patella position: Alta (patellar tendon) or baja (quadriceps)
- Swelling/ecchymosis: Around knee
- Wound assessment: Previous surgical scars
Key Clinical Finding
Inability to actively extend the knee is pathognomonic for complete extensor mechanism rupture. Test with patient supine, leg straight - ask to lift leg off bed. If unable to maintain straight leg raise = complete disruption.
Clinical Signs by Location
| Rupture Site | Patella Position | Palpable Defect | Extension Test |
|---|---|---|---|
| Patellar tendon | Patella ALTA (high-riding) | Below patella | Complete loss of extension |
| Quadriceps tendon | Patella BAJA (low) | Above patella | Complete loss of extension |
| Partial rupture | May be normal | Tenderness only | Weak extension, may have lag |
Extension Lag
Extension lag = can maintain extension if placed there, but cannot actively achieve full extension. This indicates partial disruption or weak/reconstructed mechanism. Measure lag in degrees (e.g., 15-degree extension lag).
Investigations
Diagnostic Workup
Lateral XR: Assess patella position (alta vs baja). Look for patellar fracture, avulsion fragments. Compare to previous films.
Can visualize tendon continuity, gap size, and retraction. Operator-dependent but useful for confirming clinical diagnosis.
Defines rupture site, gap size, tissue quality, and associated pathology. Useful for surgical planning. May have artifact from prosthesis.
If any suspicion of infection contributing to tissue failure. CRP, ESR, aspiration if needed.
Patella Position on XR
Insall-Salvati ratio (patellar tendon length / patella length): Normal 1.0-1.2. Patella alta (greater than 1.2) suggests patellar tendon rupture. Patella baja (less than 0.8) suggests quadriceps rupture. Compare to contralateral.
XR Findings
- Patella position (alta vs baja)
- Avulsion fragments
- Patellar fracture
- Component position (still important)
- Tibial tubercle fragment (TTO failure)
MRI Findings
- Tendon discontinuity
- Gap measurement
- Tissue quality assessment
- Associated muscle atrophy
- Planning for reconstruction
Management Algorithm
Acute Extensor Mechanism Rupture (Less than 2-4 Weeks)
| Location | Tissue Quality | Treatment | Prognosis |
|---|---|---|---|
| Quadriceps tendon | Good | Primary repair + augmentation | Fair |
| Quadriceps tendon | Poor/friable | Primary repair + allograft augmentation | Guarded |
| Patellar tendon | Any | Primary repair (often fails) or early allograft | Poor |
Acute Repair Principles
Primary repair with augmentation (autograft hamstrings, synthetic mesh, or allograft) is preferred when tissue allows. Repair is tensioned with knee in extension. Protected for 6-8 weeks in extension splint.
Early surgery (within 2 weeks) gives best chance of primary repair.
Surgical Technique
Primary Repair with Augmentation
Surgical Steps
Midline incision through previous scar. Identify rupture site and tendon ends. Debride non-viable tissue. Assess tissue quality.
Freshen tendon ends. Create bone tunnels in patella or tibial tubercle as needed. May need to mobilize retracted tendon.
Heavy non-absorbable suture (Krackow or Bunnell technique). Pass through bone tunnels. Tension with knee in full extension.
Augment the repair: Options include semitendinosus autograft, synthetic mesh (e.g., polypropylene), or allograft tissue. Provides additional strength.
Augmentation Rationale
Primary repair alone often fails in TKA setting due to poor tissue quality and vascularity. Augmentation provides: mechanical reinforcement, scaffold for healing, and protection during rehabilitation.
Protect repair with extension splint for 6-8 weeks.
Complications
Complications of Treatment
| Complication | Incidence | Risk Factors | Management |
|---|---|---|---|
| Re-rupture | 10-30% | Poor tissue, non-compliance | Repeat reconstruction or accept |
| Persistent extension lag | 50-70% | Expected outcome | PT, accept residual lag |
| Infection | 5-15% | Multiple surgeries, diabetes | Debridement, antibiotics |
| Allograft failure/resorption | 10-20% | Immune response, poor incorporation | Revision reconstruction |
| Stiffness | Variable | Immobilization, fibrosis | PT, possible MUA (cautious) |
Re-rupture Risk
Re-rupture occurs in 10-30% of reconstructions. Risk factors: inadequate protection, early aggressive ROM, poor tissue quality, allograft failure. Always protect repair for full 6-8 weeks with extension splint.
Expected Outcomes
Even with successful reconstruction, expect: extension lag 10-30 degrees in most patients, reduced stair climbing ability, need for assistive devices for some. Full extension rarely achieved. Set realistic expectations preoperatively.
Postoperative Care
Rehabilitation Protocol
Extension splint or cylinder cast full-time. No active flexion. Weight bearing as tolerated with brace locked. Prevent flexion forces on repair.
Begin gentle passive flexion (0-30, progress to 0-60). Continue brace for ambulation. Start isometric quadriceps. Gradual progression.
Progressive active ROM. Quadriceps strengthening (closed chain). Gait training. Wean from brace gradually based on strength.
Continue strengthening. Assess final ROM and extension lag. Adapt to residual deficits. May need long-term brace for some activities.
Key Principles
- Protect repair - extension splint mandatory
- Delay flexion - avoid stress on repair
- Gradual progression - no rushing
- Accept limitations - extension lag likely
Red Flags
- Sudden loss of extension ability
- Palpable new gap
- Significant swelling
- Unable to maintain extension
Outcomes and Prognosis
Outcomes by Rupture Type
| Rupture Type | Primary Repair Success | Allograft Survival (5yr) | Functional Outcome |
|---|---|---|---|
| Quadriceps (acute) | 60-70% | N/A if repair holds | Fair-Good |
| Quadriceps (chronic) | Low | 60-70% | Fair |
| Patellar (any) | Low | 50-60% | Poor-Fair |
Functional Outcome Measures
Extension lag is the key functional measure. Patients with less than 15-degree lag can usually manage stairs and transfers. Greater than 30-degree lag often requires assistive devices. Complete active extension is rarely achieved.
Evidence Base
- Review of extensor mechanism complications after TKA. Patellar tendon ruptures have worse outcomes than quadriceps. Allograft reconstruction is treatment of choice for chronic ruptures.
- Whole extensor mechanism allograft for patellar tendon rupture. 75% retained some extensor function at 5 years. 50% had significant extension lag.
- Achilles tendon allograft with bone block technique for extensor mechanism reconstruction. 70% success rate at intermediate follow-up.
- Patellar tendon rupture has poor prognosis. Risk factors include multiple surgeries and steroid use. Primary repair often fails.
- Surgical technique critical for prevention. Avoid excessive retraction, careful TTO fixation, minimize lateral release, preserve fat pad vascularity.
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Acute Patellar Tendon Rupture
"A 68-year-old man falls 3 weeks after revision TKA and cannot extend his knee. Examination shows a palpable gap below the patella and patella alta on XR. He has diabetes."
Scenario 2: Intraoperative Tibial Tubercle Avulsion
"During revision TKA with difficult exposure, you perform a tibial tubercle osteotomy. When you flex the knee to deliver the tibia, the tubercle avulses completely from the remaining tibia. What do you do?"
Scenario 3: Chronic Quadriceps Rupture
"A 70-year-old woman presents 4 months after primary TKA. She has had progressive difficulty extending her knee. She now has a 40-degree extension lag and cannot climb stairs. MRI confirms chronic quadriceps tendon rupture with 4cm retraction."
Scenario 4: Prevention During MUA
"You are about to perform MUA for stiffness at 10 weeks post-TKA. The patient has diabetes and had a previous TKA revision. How do you minimize risk of extensor mechanism rupture?"
MCQ Practice Points
Most Common Rupture Site
Q: Which extensor mechanism component has the worst prognosis when ruptured after TKA? A: Patellar tendon - Has poorest blood supply (depends on fat pad) and worst healing potential. Primary repair often fails, requiring allograft reconstruction with guarded outcomes.
Patella Position
Q: What patella position is expected with patellar tendon rupture? A: Patella alta (high-riding patella) - The patella migrates proximally when patellar tendon is disrupted. Insall-Salvati ratio greater than 1.2 indicates patella alta.
Allograft of Choice
Q: What is the most commonly used allograft for extensor mechanism reconstruction? A: Achilles tendon allograft with calcaneal bone block - The bone block is fixed to the patella and the tendon portion is sutured to the quadriceps, bridging the defect.
Protection Period
Q: How long should the knee be protected in extension after extensor mechanism repair/reconstruction? A: 6-8 weeks - This period allows healing of repair or incorporation of allograft. Active flexion is avoided during this time. Premature mobilization risks re-rupture.
Extension Lag
Q: What residual extension lag is commonly seen after extensor mechanism reconstruction? A: 10-30 degrees - Most patients have permanent extension lag even with successful reconstruction. Full active extension is rarely achieved after allograft reconstruction.
Australian Context
Clinical Practice
- Revision TKA at tertiary centers
- Registry tracks extensor mechanism complications
- Allograft availability through tissue banks
- Multidisciplinary approach to complex cases
Healthcare Setting
- Complex reconstruction at high-volume centers
- Extended rehabilitation required
- Long-term orthotics may be needed
- Registry data informs practice
Orthopaedic Exam Focus
Australian examiners will expect: Recognition of risk factors (revision, diabetes, TTO), understanding of blood supply issues with patellar tendon, knowledge of allograft options (Achilles vs whole mechanism), and realistic counseling about outcomes including expected extension lag.
TKA EXTENSOR MECHANISM FAILURE
High-Yield Exam Summary
Key Facts
- •Patellar tendon rupture = WORST prognosis
- •Quadriceps tendon = better tissue quality, better outcomes
- •Primary repair often fails in TKA setting
- •Allograft reconstruction is treatment of choice for chronic
Risk Factors (RUPTURE)
- •R = Revision surgery (3-4x higher risk)
- •U = Unusual approaches (TTO)
- •P = Poor vascularity (diabetes, smoking)
- •T = Tension (MUA, forceful manipulation)
Clinical Signs
- •Patellar tendon rupture = patella ALTA
- •Quadriceps rupture = patella BAJA
- •Inability to actively extend = complete rupture
- •Extension LAG = partial or reconstructed
Management
- •Acute: Primary repair + AUGMENTATION
- •Chronic: ALLOGRAFT reconstruction (Achilles or whole mechanism)
- •Protection: 6-8 weeks extension splint
- •Non-op if unfit for surgery
Outcomes
- •50-70% 5-year allograft survival
- •10-30 degree extension lag EXPECTED
- •Full extension rarely achieved
- •PREVENTION is the best treatment