TKA CRUCIATE RETENTION VS SUBSTITUTION
CR vs PS Design | Biomechanical Differences | Evidence-Based Selection
TKA Design Philosophy
Critical Must-Knows
- CR preserves PCL for normal kinematics; PS sacrifices PCL and uses cam-post mechanism
- No clinically significant difference in survivorship or functional outcomes at 10+ years
- PS preferred for PCL deficiency, severe deformity (greater than 15 degrees varus/valgus), or inflammatory arthropathy
- CR requires intact, functional PCL and normal bone stock
- Cam-post impingement and polyethylene wear are PS-specific complications
Examiner's Pearls
- "PCL balancing in CR requires careful soft tissue releases to avoid flexion instability
- "PS cam-post engages at 60-70 degrees flexion, providing posterior femoral rollback
- "Australian registry data shows equivalent revision rates for CR vs PS at 10 years
- "Higher flexion range possible with PS due to cam-post mechanism (130-140 degrees vs 120-130 degrees)
Critical TKA Design Exam Points
Biomechanical Fundamentals
PCL function in normal knee: Prevents posterior tibial translation and guides femoral rollback during flexion. In CR designs, preserved PCL must be balanced to neutral tension. In PS designs, cam-post mechanism substitutes this function from 60-70 degrees flexion onward.
Selection Criteria
Choose PS for: PCL deficiency, severe deformity (greater than 15 degrees), inflammatory arthritis, revision TKA. Choose CR for: Normal PCL, mild-moderate OA, desire to preserve bone stock. Both achieve excellent long-term outcomes.
Design-Specific Complications
CR complications: PCL imbalance causing flexion instability, PCL avulsion. PS complications: Patellar clunk syndrome, cam-post wear and dissociation, posterior femoral bone loss from box cut, anterior tibial post fracture.
Kinematic Differences
Femoral rollback: CR relies on PCL tension (variable, 10-15 degrees). PS uses cam-post contact (consistent, 15-20 degrees). Clinical impact: Greater flexion potential with PS (130-140 degrees vs 120-130 degrees), but no functional difference in daily activities.
Quick Decision Guide: CR vs PS Selection
| Clinical Scenario | PCL Status | First Choice | Key Rationale |
|---|---|---|---|
| Primary OA, mild-moderate deformity (under 15 degrees) | Intact, functional PCL | CR or PS (surgeon preference) | Equivalent outcomes, CR preserves bone stock |
| Severe deformity (over 15 degrees varus/valgus) | PCL may be contracted or attenuated | PS (posterior stabilized) | Easier balancing, predictable kinematics |
| Inflammatory arthropathy (RA, psoriatic) | PCL often attenuated or dysfunctional | PS (posterior stabilized) | Avoid reliance on diseased soft tissues |
| Revision TKA with PCL loss | PCL absent or insufficient | PS or CCK (constrained condylar) | Cannot rely on absent PCL, need mechanical stability |
Mnemonics and Design Frameworks
ROLLPCL Functions (Why CR Needs Intact PCL)
Memory Hook:PCL makes the femur ROLL back - CR designs need this intact!
FACESPS Design Advantages (Cam-Post Mechanism)
Memory Hook:PS design FACES the challenge of PCL loss with cam-post mechanics!
SLIMCR Design Requirements (What You Need)
Memory Hook:Keep it SLIM - CR needs intact structures and mild deformity!
Overview and Design Philosophy
Historical Context
The cruciate retention vs substitution debate has evolved over 40+ years. Early TKAs sacrificed both ACL and PCL, leading to instability. CR designs emerged in the 1980s to preserve "normal" kinematics. PS designs followed to address PCL balancing difficulties and deformity correction challenges. Modern evidence shows equivalent long-term survivorship and patient-reported outcomes, making the choice primarily surgeon preference and patient-specific factors.
Fundamental Design Principles
CR Philosophy
Goal: Preserve native PCL to maintain normal knee kinematics.
Design features:
- Flat or slightly conforming tibial bearing
- No femoral box cut (preserves bone stock)
- Smaller intercondylar notch clearance
- Relies on PCL for posterior stability
Biomechanical reliance: PCL must guide femoral rollback and prevent posterior tibial translation. If PCL is tight, flexion instability occurs. If too loose, posterior translation and wear increase.
PS Philosophy
Goal: Mechanically substitute PCL function with cam-post articulation.
Design features:
- Central tibial polyethylene post
- Femoral box cut with cam surface
- Higher conformity tibial bearing
- Cam-post engages at 60-70 degrees flexion
Biomechanical independence: No reliance on PCL quality. Cam-post provides predictable femoral rollback (15-20 degrees) and prevents posterior tibial subluxation. Greater constraint allows management of severe deformity.
Pathophysiology and Biomechanics
Normal Knee Kinematics (Intact PCL Function)
Flexion Arc Kinematics
Femoral motion: Minimal translation, primarily rotation. PCL role: Minimal tension, ACL is primary stabilizer. Clinical relevance: Both CR and PS behave similarly in this range.
Femoral motion: Progressive posterior rollback begins (5-10mm). PCL role: Increasing tension guides femoral condyle rollback. Clinical relevance: CR relies on PCL tension; PS cam-post begins engagement at 60-70 degrees.
Femoral motion: Maximal posterior rollback (10-15mm total). PCL role: Maximum PCL tension, prevents posterior translation. Clinical relevance: PS cam-post fully engaged, provides consistent 15-20mm rollback. CR variable depending on PCL tension.
Kinematic Differences: CR vs PS
| Parameter | Cruciate Retaining (CR) | Posterior Stabilized (PS) | Clinical Significance |
|---|---|---|---|
| Femoral rollback | 10-15mm (PCL-dependent, variable) | 15-20mm (cam-post, consistent) | PS provides more predictable posterior femoral translation |
| Flexion range | 120-130 degrees (average) | 130-140 degrees (average) | 10-20 degrees greater flexion potential with PS, but minimal functional impact |
| Contact stress | Lower conformity, higher stress | Higher conformity, distributed stress | PS has lower peak contact stress but higher constraint |
| Paradoxical anterior slide | More common (up to 20%) | Rare (under 5%) | CR may show anterior femoral translation in mid-flexion if PCL too loose |
Clinical Presentation and Indications
Patient Assessment for Implant Selection
The choice between CR and PS TKA begins with thorough patient assessment. Key factors include:
Preoperative Evaluation
Clinical examination:
- PCL integrity: Posterior drawer test at 90 degrees flexion
- Deformity assessment: Varus/valgus alignment and degree
- Range of motion: Flexion contracture and maximum flexion
- Ligamentous stability: Assess collateral ligament competence
Radiographic evaluation:
- Standing AP and lateral radiographs: Measure deformity, bone stock
- Varus/valgus stress views: Assess ligament integrity
- Skyline view: Evaluate patellofemoral joint
Patient Factors
Demographics:
- Age: Younger patients may benefit from CR (bone preservation)
- Activity level: High-demand patients may prefer PS (predictable kinematics)
- Cultural practices: Deep flexion requirements (kneeling, squatting)
Comorbidities:
- Inflammatory arthropathy: Strong indication for PS
- Neurological conditions: May benefit from PS constraint
- Previous trauma or surgery: May compromise PCL quality
Investigations and Preoperative Assessment
Imaging Studies for PCL Assessment
| Investigation | Purpose | Key Findings | Influence on Design Choice |
|---|---|---|---|
| Standing AP radiograph | Assess deformity and bone stock | Varus/valgus angle, joint space narrowing, bone defects | Severe deformity (over 15 degrees) favors PS |
| Lateral radiograph | Evaluate PCL calcification and posterior bone stock | PCL calcification, posterior osteophytes, femoral bone loss | PCL calcification suggests poor function (favors PS) |
| MRI (if available) | Detailed PCL and soft tissue assessment | PCL integrity, fiber continuity, signal changes indicating degeneration | Attenuated or torn PCL is absolute indication for PS |
| Varus/valgus stress radiographs | Assess ligamentous laxity and deformity correctability | Degree of deformity correction with stress, ligament competence | Fixed deformity over 15 degrees favors PS for easier balancing |
Intraoperative Assessment
Final decision on CR vs PS often made intraoperatively. After exposure and initial bone preparation, directly palpate PCL substance. If PCL feels soft, attenuated, or calcified, plan for PS. After trial component insertion, perform posterior drawer test at 90 degrees flexion - if translation exceeds 5mm with soft endpoint, convert to PS. Always have PS instruments available as backup for planned CR cases.
Laboratory Studies
For most primary OA cases, extensive laboratory workup is not required. However, specific scenarios warrant investigation:
- Inflammatory arthropathy: ESR, CRP, RF, anti-CCP antibodies (strong PS indication)
- Infection screening: ESR, CRP, synovial fluid analysis if effusion present
- Metabolic bone disease: Vitamin D, calcium, parathyroid hormone if suspected
- Coagulopathy screening: INR, PT/PTT if on anticoagulation
Management Principles and Implant Selection
Ideal Indications for CR TKA
Absolute requirements:
- Intact PCL: No significant attenuation, calcification, or dysfunction
- Adequate bone stock: Posterior femoral condyles intact, no large defects
- Mild-moderate deformity: Under 15 degrees varus or valgus angulation
- Primary OA: Non-inflammatory etiology with preserved soft tissue quality
Relative advantages:
- Preservation of bone stock (no femoral box cut or tibial post)
- Theoretical proprioception benefit from PCL mechanoreceptors
- Lower polyethylene conformity allows greater rotational freedom
- Avoids cam-post complications (clunk, wear, dissociation)
PCL Integrity Assessment
Intraoperative assessment: After trial reduction, flex knee to 90 degrees and apply posterior drawer. If excessive translation (greater than 5mm) or loose PCL feel, consider converting to PS. Tight PCL requires careful posterior capsule release; if still tight after releases, consider PS to avoid flexion instability.
Patient factors favoring CR:
- Younger, higher-demand patients (preserves bone stock for future revision)
- Patients with good PCL on preoperative imaging (MRI showing intact fibers)
- Cultural practices requiring deep flexion (kneeling, squatting) - controversial, PS may actually be better
These patient factors help guide CR selection when PCL is intact.
Surgical Technique Considerations
PCL Balancing in CR TKA
PCL Assessment and Balancing Steps
Imaging evaluation: Assess PCL integrity on lateral radiograph (calcification?) and MRI if available (intact fibers?). Physical exam: Posterior drawer test at 90 degrees flexion - should be firm endpoint. Excessive laxity suggests PCL incompetence.
Exposure: After arthrotomy and synovectomy, palpate PCL substance. Should feel firm, taut, and continuous. Red flags: Soft, attenuated, or calcified PCL suggests poor function. Consider converting to PS if concerns about PCL quality.
Flexion gap test: With trial components in place, flex knee to 90 degrees. Apply posterior drawer force. Normal PCL: Minimal translation (under 5mm), firm endpoint. Tight PCL: Flexion gap opens posteriorly, difficult to flex to 90 degrees. Loose PCL: Excessive posterior translation (over 5mm), soft endpoint.
If PCL too tight:
- First, release posterior capsule (most common cause)
- If still tight, consider "pie-crusting" PCL with 11-blade (controversial)
- If persistently tight, convert to PS (avoid flexion instability)
If PCL too loose:
- Increase tibial insert thickness (1-2mm increments)
- Ensure adequate femoral component size (not too small)
- If persistently loose, convert to PS (avoid posterior tibial translation and wear)
PCL Balancing Pitfalls
Overtightening the PCL causes flexion instability, pain, and limited ROM. Patients cannot flex beyond 90 degrees comfortably. Undertightening the PCL leads to posterior tibial subluxation, paradoxical anterior slide, and accelerated polyethylene wear. Golden rule: If in doubt about PCL quality or balance after careful assessment, convert to PS intraoperatively.
CR-Specific Technical Pearls
Posterior Capsule Release
Technique: Use blunt Hohmann retractor to elevate posterior capsule off posterior femoral condyles. Start medially, work laterally. Release should be subperiosteal to avoid popliteal vessel injury. Goal: Create 1-2mm of additional flexion gap without overstretching PCL.
PCL-Tibial Attachment
Danger: PCL inserts just posterior to posterior edge of tibial cut. Overly posterior tibial resection can detach PCL, forcing conversion to PS. Protection: Keep tibial cut at approximately 0 degrees posterior slope (or slightly anterior). Mark PCL insertion before cutting.
These techniques ensure optimal PCL balance and implant function.
Complications
| Complication | CR-Specific | PS-Specific | Incidence | Management |
|---|---|---|---|---|
| Flexion instability | PCL too loose or avulsed | Inadequate cam-post engagement | CR: 2-5%, PS: under 1% | CR: Thicker insert or convert to PS; PS: Check post height, consider revision |
| Patellar clunk syndrome | Rare (no femoral box) | Common (5-10% incidence) | PS-specific complication | Arthroscopic debridement of fibrous nodule (90% resolution) |
| Post-cam wear/dissociation | Not applicable | Tibial post fracture (under 1%) | PS-specific late failure | Revision with thicker insert or constrained condylar knee |
| Posterior tibial translation | PCL incompetence (1-3%) | Rare with functional cam-post | CR: Higher polyethylene wear | CR: Consider revision to PS if symptomatic wear |
| Bone loss at revision | Minimal (no box cut) | Posterior femoral defect from box | PS: 5-7mm bone loss | Both: Augments or metaphyseal sleeves at revision |
Outcomes and Prognosis
Key outcomes (AOANJRR 2023, Cochrane meta-analysis):
- Survivorship: CR 5.8%, PS 5.9% revision rate at 10 years (no significant difference)
- Function: WOMAC and KSS scores equivalent at 5-10 years, patient satisfaction 85-90% for both
- ROM: CR 120-130°, PS 130-140° flexion (not functionally significant)
- Complications: PS higher patellar issues (0.8% vs 0.5%), CR higher instability (0.6% vs 0.4%)
Evidence Base and Key Studies
Key Evidence
Cochrane Review: CR vs PS TKA
- Meta-analysis of 17 RCTs (1810 patients, 2206 knees)
- No difference in functional outcomes (WOMAC, KSS) between CR and PS at 5-10 years
- No difference in ROM: CR 112 degrees vs PS 113 degrees (not clinically significant)
- No difference in revision rates at mean 6.8 year follow-up
- Quality of evidence: Moderate due to heterogeneity and small sample sizes
Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR)
- Over 400,000 primary TKAs analyzed (CR vs PS comparison)
- 10-year cumulative revision rate: CR 5.8%, PS 5.9% (no significant difference)
- PS slightly higher revision rate for patellar complications (0.8% vs 0.5%)
- CR slightly higher revision rate for instability (0.6% vs 0.4%)
- Both designs show improving survivorship with modern implants (over 96% at 10 years)
Biomechanics of CR vs PS During High-Flexion Activities
- In vivo fluoroscopic analysis of CR vs PS kinematics during kneeling and squatting
- PS showed more consistent posterior femoral rollback (15-20mm vs 10-15mm in CR)
- CR demonstrated greater variability in femoral rotation (PCL-dependent)
- PS achieved 5-10 degrees greater maximal flexion in high-demand activities
- Neither design replicated normal knee kinematics perfectly
Patellar Clunk Syndrome in PS TKA
- Incidence: 3-12% in early PS designs, reduced to 2-5% in modern designs
- Mechanism: Fibrous nodule forms on proximal pole of patella, catches on superior aspect of femoral box during extension
- Clinical presentation: Painful, palpable clunk during terminal extension (100-40 degrees)
- Treatment: Arthroscopic debridement successful in 85-95% of cases
- Prevention: Adequate patellar resection (8-10mm), avoid patellofemoral overstuffing
Long-Term Survivorship: CR vs PS in Primary OA
- Multicenter study: 2,233 primary TKAs followed for 10-15 years
- 15-year survivorship: CR 93.4%, PS 94.1% (not statistically different)
- Functional outcomes (KSS, WOMAC) equivalent at all time points
- Complication profile differs: PS higher patellar issues, CR higher instability revisions
- Patient satisfaction equivalent between groups (85-90%)
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Implant Selection for Primary OA
"A 68-year-old woman with severe medial compartment osteoarthritis presents for TKA. She has 12 degrees varus deformity and an intact PCL on clinical examination. She asks about the difference between cruciate retaining and posterior stabilized implants. How would you counsel her and what are the key factors in your implant selection?"
Scenario 2: Intraoperative PCL Assessment and Balancing
"You are performing a CR TKA on a 62-year-old man with primary OA. After inserting the trial components, you notice excessive posterior tibial translation (approximately 8mm) on posterior drawer testing at 90 degrees flexion. The PCL feels somewhat loose. Walk me through your assessment and management options."
Scenario 3: PS Complication Management - Patellar Clunk
"A 55-year-old woman returns 18 months after PS TKA complaining of a painful, palpable clunk in her knee during extension from deep flexion. She describes it as occurring around 60-70 degrees of flexion during activities like standing from a chair. Examination demonstrates a reproducible clunk with active extension from 100 to 40 degrees. What is your diagnosis and management approach?"
MCQ Practice Points
Biomechanics Question
Q: At what degree of flexion does the cam-post mechanism in PS TKA typically engage to provide posterior femoral rollback? A: 60-70 degrees of flexion. The cam (on the femoral component) contacts the post (on the tibial polyethylene insert) at this flexion range, initiating the mechanical substitute for PCL function. This engagement provides consistent posterior femoral rollback of 15-20mm compared to the variable 10-15mm seen in CR designs (dependent on PCL tension).
Indications Question
Q: What is the deformity threshold (varus or valgus angulation) above which PS TKA is generally preferred over CR TKA? A: Greater than 15 degrees of varus or valgus deformity. Severe deformities require extensive soft tissue releases which may compromise PCL function. PS designs eliminate reliance on PCL quality and provide predictable stability through the cam-post mechanism, making soft tissue balancing more straightforward in these challenging cases.
Complications Question
Q: What is the mechanism and incidence of patellar clunk syndrome in PS TKA? A: Mechanism: Fibrous nodule forms on the superior pole of the patella and catches on the superior aspect of the femoral box during extension from deep flexion, producing a painful, palpable clunk between 100-40 degrees. Incidence: 5-10% in early PS designs, reduced to 2-5% in modern designs with improved femoral box geometry. Treatment: Arthroscopic debridement (85-95% success rate).
Outcomes Question
Q: According to the Australian registry (AOANJRR) and international evidence, what is the 10-year survivorship comparison between CR and PS TKA in primary osteoarthritis? A: Equivalent survivorship: Both CR and PS designs achieve approximately 96% survivorship at 10 years. Cochrane meta-analysis of 17 RCTs and AOANJRR data both confirm no clinically significant difference in revision rates (CR 5.8%, PS 5.9% at 10 years). Functional outcomes (ROM, KSS, WOMAC scores) are also equivalent. Complication profiles differ slightly (PS higher patellar issues, CR higher instability) but overall outcomes are comparable.
Surgical Technique Question
Q: What are the critical steps in assessing PCL integrity and balance during CR TKA trial reduction? A: Assessment technique: (1) With trial components in place, flex knee to 90 degrees. (2) Apply posterior drawer force to tibia. (3) Normal PCL should limit translation to under 5mm with firm endpoint. (4) Excessive translation (over 5mm) indicates PCL incompetence. (5) Troubleshoot by increasing insert thickness or checking femoral size. (6) If translation remains over 5mm despite adjustments, convert to PS intraoperatively. Key principle: A loose or insufficient PCL will cause posterior tibial subluxation and accelerated wear in CR designs.
Evidence Question
Q: What does the Cochrane systematic review (Verra 2013) conclude about functional outcomes and ROM differences between CR and PS TKA? A: No clinically significant difference. Meta-analysis of 17 RCTs showed: (1) Functional scores (WOMAC, KSS) equivalent between CR and PS at 5-10 years. (2) ROM: CR 112 degrees vs PS 113 degrees (1-degree difference, not clinically meaningful). (3) Revision rates equivalent at mean 6.8 years follow-up. (4) Quality of evidence: Moderate due to heterogeneity. Clinical implication: Choice between CR and PS should be based on patient-specific factors (PCL quality, deformity) and surgeon experience, not expected functional superiority.
Australian Context and Registry Data
AOANJRR Data: CR vs PS
Australian registry findings (2023 Annual Report):
- Over 400,000 primary TKAs analyzed with CR vs PS comparison
- 10-year cumulative revision rate: CR 5.8%, PS 5.9% (no significant difference)
- Patellar revision: PS 0.8% vs CR 0.5% (patellar clunk and complications)
- Instability revision: CR 0.6% vs PS 0.4% (PCL imbalance issues)
- Overall survivorship: Both exceed 96% at 10 years with modern implants
Key message: Australian data confirms international findings of equivalent long-term outcomes. Small differences in complication profiles align with biomechanical expectations.
Australian Practice Patterns
Implant usage trends in Australia:
- PS dominance: Approximately 65-70% of primary TKAs use PS designs
- CR usage: 25-30% of primary TKAs, declining over past decade
- Regional variation: Metropolitan centers favor PS, rural areas more variable
- Training influence: Younger surgeons trained in PS era prefer PS designs
Rationale for PS preference: Ease of balancing, predictable outcomes, institutional standardization, and avoiding PCL-related complications drive the shift toward PS in Australian practice.
Medicolegal Considerations
Informed consent must include:
- Discussion of implant type (CR vs PS) and rationale for selection in patient's specific case
- Potential for intraoperative conversion from CR to PS if PCL quality inadequate
- Design-specific complications (patellar clunk for PS, potential instability for CR)
- Expected outcomes: 96% survivorship at 10 years, equivalent functional results for both designs
- Alternative options: unicompartmental knee replacement if isolated compartment disease
Documentation requirements:
- Preoperative PCL assessment (clinical exam, imaging if available)
- Intraoperative findings (PCL quality, deformity correction, balancing)
- Rationale for final implant choice or conversion decision
- Any intraoperative complications or unexpected findings
Common litigation issues:
- Flexion instability in CR TKA from inadequate PCL balance
- Patellar clunk syndrome in PS TKA without proper counseling
- Failure to convert from CR to PS despite intraoperative evidence of PCL insufficiency
- Component malposition (femoral box cut errors in PS designs)
TKA CRUCIATE RETENTION VS SUBSTITUTION
High-Yield Exam Summary
Design Fundamentals
- •CR = Preserves PCL for physiologic rollback (10-15mm), requires intact PCL and mild deformity (under 15 degrees)
- •PS = Sacrifices PCL, cam-post mechanism provides consistent rollback (15-20mm), independent of PCL quality
- •CS = Intermediate design with deep dish polyethylene, no post (less common)
- •PCL function: Prevents posterior tibial translation, guides femoral rollback, maintains quadriceps lever arm
Selection Criteria
- •CR indications: Intact PCL, mild-moderate OA, deformity under 15 degrees, desire to preserve bone stock
- •PS indications: PCL deficiency, severe deformity (over 15 degrees), inflammatory arthritis, revision TKA, flexion contracture over 15 degrees
- •Neutral zone: Primary OA with mild deformity (under 10 degrees) - either CR or PS acceptable, surgeon preference
- •Intraoperative conversion: CR to PS if PCL loose (over 5mm translation), attenuated, or avulsed
Biomechanics
- •Cam-post engagement: 60-70 degrees flexion in PS designs, provides mechanical PCL substitute
- •Femoral rollback: CR variable 10-15mm (PCL-dependent), PS consistent 15-20mm (cam-post)
- •Flexion range: CR 120-130 degrees average, PS 130-140 degrees (10-20 degrees greater with high-flex designs)
- •Paradoxical anterior slide: More common in CR (up to 20%) if PCL too loose, rare in PS (under 5%)
Surgical Technique
- •CR PCL balancing: Posterior drawer at 90 degrees should be under 5mm; if tight, release posterior capsule; if loose, thicker insert or convert to PS
- •PS femoral box cut: 18-20mm wide, 12-15mm deep, creates 5-7mm posterior femoral bone loss
- •Cam-post clearance: 2-3mm at full extension to avoid hyperextension impingement, engagement begins 60-70 degrees
- •Conversion CR to PS: Excise PCL, create femoral box cut, verify tibial baseplate accepts PS insert, trial balance
Complications
- •PS-specific: Patellar clunk syndrome (5-10%, fibrous nodule on superior patella catches on box), cam-post wear/dissociation (under 1%)
- •CR-specific: Flexion instability from PCL imbalance (2-5%), PCL avulsion, posterior tibial translation and wear if PCL loose
- •Patellar clunk treatment: Conservative 3-6 months (physio), then arthroscopic debridement (85-95% success)
- •Both: Infection, aseptic loosening, stiffness, periprosthetic fracture - no difference in rates between CR and PS