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TKR: Depth of Proximal Tibial Cut

- Discussion:
    - see:
          - tibial component
          - extra-medullary cutting guide;
          - preparation for proximal tibial cut
          - posterior slope

    - this cut is made at right angles to anatomical axis of tibia;
    - proximal tibial cut is made 2 mm below the anterior surface of the more involved side;
          - in knee with fixed varus deformity more bone is resected from lateral tibial plateau than off the medial tibial plateau;
    - alternatively, the resection can be based off the less involved side;
    - cut should be < 5 mm below tibial plateau because tibial cancellous bone weakens rapidly as distance from articular surface increases;
    - due to complex geometries that may be present on degenerative tibial surface, it may be prudent to "eyeball" depth of proximal cut,
            rather than relying on stylus;
            - reliance on the stylus alone may result in a inadvertently deep cut on the lesser involved side;
    - quick trial:
          - if the femoral cuts have already been made (and the femoral trial is in place), insert a trial base plate  w/ an 8 mm spacer while 
                  knee is in full extension;
                  - if the 8 mm spacer cannot be easily inserted (in full extension) then it is unlikely that a 10 or 12 mm spacer will be able to
                           be inserted (and therefore further proximal tibial resection is required);
    - consequences of excessive proximal tibial resection:
          - lowering of the joint line;
          - loss of bone quality;
                 - quality of cancellous bone in the tibia is best in immediate subarticular area, and decreases rapidly w/ more distal resection;
          - loss of surface area:
                 - surface area of the proximal tibia decreases w/ more distal cut;
          - distal resection jeopardizes attachments of IT band & pes anserinus, MCL and PCL;
                 - PCL takes origin from posterior aspect of proximal tibia, so that its attachment is not lost when proximal tibia is resected
                           just below the articular surface;
                 - valgus instability may result from an excessively deep resection along w/ an excessively steep posterior slope (the later may
                           remove the posterior oblique ligament;
          - resection more than 1 cm below original articular surface:
                 - cuts at this depth require bone grafts or revision components;
                 - due to loss of PCL, a posterior stabilized prosthesis: will be required;
                 - in presence of severe deformity, proximal tip of fibula, normally located approx 1 cm below tibial articular surface, is good
                           guide for the level of maximum resection

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