Principles & Technical Aspects

Clinical Goals

  1. Achieve a knee that is:
    • Pain-free
    • Provides functional range of motion (ROM)
    • Ensures longevity
  2. Achieved by:
    • Restoring correct mechanical alignment
    • Using good component design
    • Ensuring durable fixation
    • Restoring the joint line
    • Proper knee balancing

Mechanical Axis

  • Femoral Anatomic vs. Mechanical Axis:
    • The femoral anatomic axis is 6° off its mechanical axis.
  • Tibial Axis:
    • Mechanical and anatomic axes are identical.
  • Distal Femur:
    • In 6° valgus, allowing parallel articulation with the tibia.

Surgical Techniques

Distal Femoral Cut

  • Creates the extension gap.
  • Referenced from the femoral anatomic axis.
    • Cutting jig replicates normal 6° valgus.
    • Results in a rectangular extension gap.
  • Adjustments for:
    • Very tall (<6° valgus).
    • Very short patients (>6° valgus).
    • Previous femoral deformity (pre-operative measurement required).

AP Femoral Cut

  • Determines the flexion gap.
  • Normal tibial plateau is in 3° varus; tibial cut is perpendicular to the tibial axis.
  • To create a rectangular flexion gap:
    • Posterior condyles are cut in external rotation (removing more of the MFC).
    • Cutting jigs are typically offset by 3°.

Referencing Options

  1. Anterior Referencing:
    • Errors affect the posterior condyles, prioritizing PFJ space.
  2. Posterior Referencing:
    • Errors affect the anterior space, prioritizing flexion gap.

Joint Line Restoration

  • Critical for maintaining ligament function at normal tension.
  • Achieved by replacing the same amount of bone/cartilage resected.
  • Elevation or distalization of the joint line can result in:
    • Patella Baja:
      • Limits flexion and causes patella-poly impingement.
    • Patella Alta:
      • Increases contact pressures in flexion, leading to pain and wear.

Ligament Balancing

  • Essential for correcting deformities.
  • Varus Knee:
    • Releases are performed on the tibia (osteophytes, MCL, etc.).
    • Sequential releases of superficial MCL, avoiding complete release.
  • Valgus Knee:
    • Releases are mainly performed on the femur (lateral capsule, ITB, LCL).
    • Severe valgus deformities may require constrained prostheses.
  • Flexion Contracture:
    • Managed with posterior osteophyte removal, capsule release, or increased tibial slope.

Balancing the PCL

  • Too tight: Excessive rollback damages poly.
  • Too lax: Flexion instability and reduced ROM.
  • Managed by:
    • Sequential PCL recessing (1-2 mm increments).
    • Increasing posterior slope.
    • Switching to posterior-stabilized (PS) components if necessary.

Addressing Flexion/Extension Gap Mismatches

  1. Equalize flexion and extension gaps for stable tibial insert throughout ROM.
  2. Rules for sagittal plane balancing:
    • Symmetric issues: Address tibia first.
    • Asymmetric issues: Address femur first.

Common Scenarios

Scenario Problem Solution
Flexion & Extension tight Tibial cut too small Cut more tibia
Flexion & Extension loose Too much tibia cut Upsize poly or augment tibia
Flexion good, Extension loose Excessive distal femur resection Augment distal femur or downsize femur
Flexion good, Extension tight Inadequate distal femur resection Cut more distal femur
Flexion tight, Extension good Insufficient posterior femur resection Downsize femur or recess/substitute PCL
Flexion loose, Extension good Excessive posterior femur resection Upsize femur AP size

Patellofemoral Joint (PFJ)

Maltracking and Failure Causes

  • Common technical causes include:
    • Unequal gaps (instability/impingement).
    • Patella maltracking or overstuffing.

Strategies for Optimal Tracking

  1. Lateralize femoral component.
  2. Medialize patella button.
  3. Avoid internal rotation of tibia or femur.
  4. Restore mechanical alignment (valgus increases Q angle).

PFJ Pathologies

  • Patella Baja:
    • Pre-existing or due to distal femoral cut.
    • Managed by superior button placement or anterior poly trimming.
  • Patella Alta:
    • Caused by excessive tibial cut or lowered joint line.
    • Leads to early wear.

TKA After Patellectomy

  • Reduces quadriceps strength by ~50%.
  • Post-patellectomy TKR:
    • Use PS knee (prevents anterior femoral translation).
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