TKA Cruciate Retention vs Substitution

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)

Timing: PCL can be removed before or after bone cuts (surgeon preference). Technique: Sharply excise PCL from tibial insertion using curved osteotome or scalpel. Remove entire substance including femoral attachment. Rationale: Creates space for tibial post and femoral box, removes potential impingement source.

Instrumentation: Use box cutting guide that references anterior and posterior femoral cuts. Dimensions: Box typically 18-20mm wide, 12-15mm deep. Creates 5-7mm posterior femoral bone loss. Accuracy critical: Asymmetric box causes cam-post maltracking and accelerated wear. Use multiple verification steps.

Engagement point: Cam-post contact should begin at 60-70 degrees flexion (design-specific). Jump height: Measure vertical distance from tibial polyethylene surface to top of post (typically 8-10mm). Clearance: Ensure 2-3mm clearance between post and femoral cam at full extension to avoid hyperextension impingement.

Flexion-extension balance: With trial components, verify equal tension through full ROM. No lift-off in extension, no excessive tightness in flexion. Cam-post function: At 90 degrees flexion, gentle posterior force should engage cam-post with solid feel. Ensures PS mechanism is functional.

Assessment: If PCL quality is questionable or balancing is unsuccessful after thorough attempts, make decision to convert early (before final implants). Communication: Inform team of plan change, ensure PS implants and instruments are available.

Technique: Remove PCL completely from both tibial and femoral attachments using curved osteotome or rongeur. Inspection: Ensure no PCL remnants that could cause impingement.

Challenge: Femoral cuts already made for CR (no box cut yet). Solution: Use PS box cutting guide that references existing femoral cuts. Carefully create box ensuring symmetry. Verification: Use trial PS femoral component to confirm adequate box clearance.

CR tibial cut: Already performed, but PS requires central post hole. Solution: Most modern tibial baseplates accommodate both CR and PS inserts. Verify tibial baseplate accepts PS poly. Alternative: If tibial cut inadequate, may need to recut tibia with slight increase in resection (1-2mm).

PS trial reduction: Insert PS trial components and assess balance through ROM. Cam-post function: Verify engagement at 60-70 degrees, ensure no hyperextension impingement. Final checks: Equal flexion-extension gaps, no lift-off, appropriate cam-post clearance.