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Sizing and Seating of the Tibial Tray



- Sizing of the Tibial Component:
    - generally, the tibial component is either larger or at least the same size as the femoral component;
           - femoral overhang is a significant source of postoperative TKR pain;
           - ref:  Overhang of the Femoral Component in Total Knee Arthroplasty: Risk Factors and Clinical Consequences
    - generally, the surgeon chooses the largest tibial component which allows for appropriate tibial rotation.
           - ref: Maximizing Tibial Coverage Is Detrimental to Proper Rotational Alignment

    - rotation of tibial component:
           - avoid internal rotation of the tibial component:
           - medially rotated tibial & femoral components have a high incidence of assoc postop patellar dislocation;
           - if there is tendency for lateral subluxation, position of tibial component can be adjusted to more external rotation, producing
                   relative IR of tibial tubercle, lessening angle between quadriceps & patellar tendon, & thus lessening tendency of patella
                   to dislocate laterally; (see Q angle);
           - note that too much external rotation of the tibial component can overhang laterally and cause impingement on the popliteus tendon;
    - medial lateral translation:
           - goal is to provide maximum coverage &, when possible, have component extend to the cortical margins;
           - laterally dislocated soft tissues tend to push trial component medially, which is resolved by adequate retraction;
           - overhang:
                   - medial overhang is usually avoided but the literature shows that overhang by itself does not correlate with pain;
                   - slight lateral translation of the tibial component helps to centralize patellar tracking;
                   - note that too much external rotation of the tibial component can overhang laterally and cause impingement on
                          the popliteus tendon;
                   - of note, femoral overhang has been shown to be a much more important cause of pain than tibial overhang.  
                   - references:
                          - Tibial component overhang in total knee replacement: incidence and functional outcomes.
                          - The effect of tibial component sizing on patient reported outcome measures following uncemented total knee replacement.
                          - The impact of tibial component overhang on outcome scores and pain in total knee replacment.
                          - Overhang of the femoral component in total knee arthroplasty: risk factors and clinical consequences.

    - anterior posterior translation:
           - goal is to provide maximum coverage;
           - if symmetric component is sized to fit AP length of medial plateau, comonent will either overhang posteriorly on the lateral
                   plateau or have a tendency to internally rotate;
                   - the solution is to compromise in AP coverage, rather than allow overhang or internal rotation;
           - appropriately sized component is then placed as posteriorly as possible;
                   - this will in effect shift the tibial tubercle anteriorly, which will decrease patello-femoral contact pressures
           - ref: Anteroposterior positioning of the tibial component and its effect on the mechanics of patellofemoral contact.