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TKR: Trial Components and Tracking


- Trial Components:
    - component fit:
         - evaluate femoral and tibial trial components and determine areas that will need additional cement;
    - flexion & extension gaps
          - range of motion: use the largest polyethylene spacer that allows full extension;
          - persistent flexion contracture
                  - consider stripping posterior capsule;
                  - need for further resection of distal femur;
                  - if all of soft tissue contractures have been relieved & knee fails to fully extend, then consider whether there is too little resection of distal
                         femur (ie there has been lowering of the joint line) or whether there is an excessively large polyethylene component;
          - inadequate flexion;
                  - consider need for more posterior slope rather than more proximal tibia resection;
    - soft tissue balance:
          - needs to be addressed prior to final determination of stability;
          - "apparent valgus instability" means that the MCL is cut (unlikely) or that the tibia has be cut in varus (which is unacceptable);
          - apparent lateral flexion instability may indicate that the femoral component is internally rotated; (see femoral component rotational alignment)
          - test knee's stability in flexion;
                - if there is excessive laxity, increase the height of the tibial trial component until stability is achieved;
          - test the stability in extension;
    - alignment:
          - standard tibial tray is checked w/ the long alignment rod and should pass thru the 2nd metatarsal of the foot;

- Tracking of Patella:
    - patellar tracking is assessed prior to reaming of the patellar component;
    - if patellar tracking is optimal, then it is acceptable to allow the tibial component to seek its own rotation (with the knee in extension)
            as long as there is no component overhang;
            - use cautery to mark out the center of component on the tibia, so that its rotation can
                   be reproduced when the tibial trial stem is inserted;
    - management of patellar subluxation
            - several options are availabe to the surgeon when patellar subluxation is present;
            - the surgeon can consider individual adjustment of tibial component rotation, lateralization of the femoral and tibial components, 
                   medialization of the patellar component, and finally lateral retinacular release;
            - tibial component
                   - if there is tendency for lateral patellar subluxation, position of tibial component can be adjusted to more external rotation,
                           producing relative internal rotation of tibial tubercle, lessening Q angle;
                          - taken to excess, this may internally rotate the leg and may cause incongruenty in the femoral-tibial articulation;
                   - in addition to adjusting tibial rotation, the tibial component can be translated slightly laterally;
            - femoral component:
                   - when a PCL retaining component is used, there is the option of translating the component slightly laterally;
                          - w/ a posterior stabilized prosthesis, the box cuts will have already been cut, and therefore additional lateralization is not possible;
                   - although lateral translation of femoral componenet will decrease the effective Q angle, it is important to avoid lateral overhang 
                          of anterior flange because of negative effect this will have on the extensor mechanism;
            - patellar component:
                   - if lateral patellar subluxation is present, then medialize the patellar component as much as possible;
                   - in this situation, it is also important to be sure that the patella is not under-reamed, since this will increase the patellar thickness (which
                           worsens the tendency for patellar subluxation);
                   - if there is no tendency for patellar subluxation, then be careful not to over-medialize component, since there have been examples of medial patellar subluxation;
            - lateral retinacular release
                   - the final option is lateral retinacular release;
                   - the decision to perform a lateral retinacular release should be delayed until the final patellar component has been placed

Overhang of the femoral component in total knee arthroplasty: risk factors and clinical consequences. 

Unique relationship between osteophyte and femoral-tibia component size mismatch in determining polyethylene wear in primary TKR: a case report