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Total Knee Replacement (TKR/TKA) - Surgical Steps and Bone Cuts

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PATIENT POSITIONING & SETUP

• Supine with thigh support/tourniquet applied
• Knee flexed to ~90° for incision; leg hangs freely
• Tourniquet inflated after limb exsanguination (optional)

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STEP 1 - SURGICAL APPROACH

Skin Incision

• Anterior midline longitudinal incision - the standard
• Made with the knee in flexion so subcutaneous tissue falls medially and laterally, improving exposure
• Incision must be long enough to avoid excessive skin tension (risk of skin necrosis)
• If prior scars exist, use the most lateral usable scar (blood supply comes predominantly from medial side)

Deep Approach (Retinacular Incision)

1. Incise through a medial cuff of the quadriceps tendon (leave 3-4 mm cuff on vastus medialis for closure)
2. Continue around the medial border of the patella
3. Extend 3-4 cm onto the anteromedial tibial surface along the medial patellar tendon border
4. Subperiosteally elevate anteromedial capsule and deep MCL off the tibia to the posteromedial corner
5. Extend and evert the patella (or sublux laterally if obese/stiff)

Alternatives: midvastus approach (splits vastus medialis obliquus muscle), subvastus approach (preserves quadriceps entirely - better patellar tracking, harder exposure).

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STEP 2 - JOINT EXPOSURE AND SOFT TISSUE PREPARATION

1. Flex knee to ~90°
2. Remove the ACL and anterior horns of medial and lateral menisci
3. Remove any osteophytes (especially posterior condylar osteophytes - they block flexion and tent posterior capsule causing flexion contracture)
4. Posterior meniscal horns are removed after bone cuts

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STEP 3 - BONE CUTS (MEASURED RESECTION TECHNIQUE)

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CUT 1: DISTAL FEMORAL CUT (Extension Gap Cut)

• Made using an intramedullary (IM) guide (entry point: ~1 cm anterior and medial to intercondylar notch)
• Cut angle: 5-7° of valgus relative to the femoral anatomic axis (= perpendicular to the mechanical axis)
  • This angle is measured off a long-standing radiograph (angle between mechanical and anatomic femoral axes)
• Amount of resection = thickness of distal femoral component (typically ~9-10 mm)
• Establishes the extension gap
• Key rules:
  • Avoid elevating the joint line >4 mm
  • For PS (posterior-stabilized) implants: add 2 mm extra distally to compensate for PCL sacrifice
  • If a flexion contracture exists: additional resection can help, but avoid over-resection

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CUT 2: PROXIMAL TIBIAL CUT (Foundation Cut - Affects Both Gaps)

• The most important cut in TKA - it affects both the flexion and extension gap
• Made using intramedullary or extramedullary guide, cut perpendicular to the tibial mechanical axis (0° in the coronal plane)
• Posterior slope: most systems incorporate 3° into the polyethylene insert; cutting guide set accordingly
• Rotation: referenced to the medial 1/3 of the tibial tubercle proximally and slightly medial to the center of the ankle distally (cross-referenced with 2nd ray of the foot and tibial crest)
• Resection amount:
  • Referenced off the less arthritic side: ~8-10 mm (equals implant thickness)
  • Referenced off the more arthritic side: typically 2 mm or less
• Protect patellar tendon and collateral ligaments with retractors during this cut

Note: Tibial cut can be performed before or after distal femoral cuts (surgeon preference).

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CUT 3: FEMORAL SIZING AND ROTATION ASSESSMENT

• Component size (AP measurement of femur)
• Rotation of femoral component - this defines the shape of the flexion gap

Rotation Reference Options:

• Internal rotation of femoral component causes lateral patellar tilt or patellofemoral instability
• Excessive external rotation widens the medial flexion gap causing flexion instability

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CUT 4: 4-IN-1 FEMORAL CUTS (Flexion Gap + Finishing Cuts)

• Posterior referencing: jig feet sit flush on posterior condyles; removes fixed 9 mm posteriorly regardless of component size; smaller jigs remove more anteriorly
• Anterior referencing: boom sits on anterior cortex; consistent anterior cut depth; prevents notching/overstuffing

After these cuts, assess the flexion gap - it should equal the extension gap (symmetric rectangle). A rectangular flexion gap = correct rotation.

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CUT 5: INTERCONDYLAR NOTCH CUT (PS implants only)

• Performed perpendicular to the transepicondylar axis
• Removes bone from the intercondylar box to accommodate the post-and-cam mechanism of a posterior-stabilized implant
• CR (cruciate-retaining) implants skip this cut

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CUT 6: PATELLAR CUT

• Standard patellar button thickness = 9 mm
• Measure native patella thickness; resect to leave enough bone
  • Example: 20 mm patella → set guide to 11-12 mm resection (some surgeons use 14 mm to protect against thin patella)
• Maintain symmetric thickness to avoid patellar tilt
• Not universally performed - patellar resurfacing is standard in the USA but not in many other countries

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STEP 4 - GAP BALANCING AND SOFT TISSUE RELEASES

1. Insert spacer blocks or a tensioning device in both flexion (90°) and extension (0°) gaps
2. Gaps should be:
   • Symmetric (equal medially and laterally)
   • Equal in flexion and extension
3. If extension gap tight medially: release posterior oblique ligament, semimembranosus, posteromedial capsule
4. If flexion gap tight medially: release anterior superficial MCL, pes anserinus insertion
5. If PCL is too tight (flexion gap tight): recess or release PCL; if PCL no longer functions - convert to PS implant

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STEP 5 - TRIAL REDUCTION

1. Impact femoral and tibial trial implants
2. Insert provisional polyethylene spacer trial
3. Reduce the knee and assess:
   • Stability in full extension
   • Mid-flexion stability
   • Patellar tracking ("no thumb" test)
   • Range of motion
4. Final sizing and rotational alignment confirmed

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STEP 6 - COMPONENT IMPLANTATION

1. Thorough irrigation and debridement
2. Cement tibial baseplate (and/or femoral component) - depending on system (cemented vs cementless)
3. Impact femoral component
4. Insert final polyethylene insert
5. Cement or press-fit patella button
6. Confirm final ROM, stability, patellar tracking

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STEP 7 - CLOSURE

1. Thorough irrigation
2. Close retinacular layer (medial parapatellar cuff) with interrupted sutures
3. Subcutaneous layer closed
4. Skin closure (staples or suture)
5. Drain placement (optional)

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SUMMARY: ORDER OF BONE CUTS

1. Distal Femoral Cut      → sets extension gap (5-7° valgus IM guide)
2. Proximal Tibial Cut     → foundation cut (perpendicular to mech. axis)
3. Size & Rotate Femur     → determine AP size + rotation reference
4. Anterior Femoral Cut    → 4-in-1 jig
5. Posterior Femoral Cuts  → 4-in-1 jig (9 mm posteriorly)
6. Anterior Chamfer Cut    → 4-in-1 jig
7. Posterior Chamfer Cut   → 4-in-1 jig
8. Box Cut (PS only)       → intercondylar notch
9. Patellar Cut            → 9 mm button resurfacing

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ALIGNMENT GOALS

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