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Revision TKA: Femoral Component

- Discussion:
    - quick femoral bone cuts by eye:
            - usually the femoral bone under the prosthesis will be soft, and will impact when the femoral trial is applied;
            - therefore, there is no need for time consuming bone cuts, when "eyed cuts" are just as good;
            - the main issue is determining whether the femoral cut needs more external rotation;
            - prior to removal of components, patellar tracking should have already be assessed;
            - flexion and extension gaps:
                     - this is determined preoperatively by the height of the tibial surface and height of the posterior femoral condyle;
                     - flexion gap often ends up being larger than the extension gap, and this needs to be determined before the primary
                              components are removed;
                    - with the knee in flexion, ensure that the flexion gap is rectangular and not trapezoidal;
    - medullary reaming:
           - note that most revision hip systems base their femoral cuts off of the previously inserted straight medullary reamer;
           - if the reaming hole is inserted too far anteriorly, then their will be an excessively large flexion gap, and the anterior flange of the femoral component
                  will lie proud off of the femoral surface;
                  - likewise if the reaming hole is placed too far posteriorly, then the flexion gap will be narrowed;
           - consider measuring the distance between the stem of the femoral component and the inner edge of the anterior flange;
                  - this distance can then be used to help guide the proper position of the reaming hole (ie, by translating the reaming hole the proper  distance from
                             the anterior femoral surface);
           - after reaming to 12 mm, continue to procede slowly by 1 mm increments;
           - reaming should cease once firm resistance is encountered;
           - it is not necessary to have direct cortical contact, inorder to avoid excessive bone loss;
           - generally, stem diameter will be the same size as the reaming diameter;
    - depth of reaming:
           - if straight stems are used, do not ream past 190 mm inorder to avoid impinging the femoral anterior bow;
           - if a longer stem is required, then it should be curved;
    - bone will often be deficient distally and posteriorally, so be prepared to augment these areas;
    - insert IM rod with appropriate sleeve and place IM cutting guide (we prefer 5 degrees of valgus);
    - remove 1-2mm of bone distally
    - perform anterior and posterior cuts;
    - both the anterior and posterior cuts should be parallel to the epicondylar axis;
    - be aware that increased external rotation will improve patellar tracking, increase the medial flexion gap and decrease the lateral flexion gap;
    - insertion of the trial femoral component:
           - rotation of the femoral component:
                  - if their has been optimal patellar tracking, then the surgeon should accept the orientation of the pre-existing femoral bone cuts;
                  - if patellar tracking is not acceptable, then the surgeon will have to change the orientation of the anterior, posterior, and chamfer
                            cuts (ie, more external rotation is required);
                  - use the femoral epicondyles to judge neutral rotation of the femoral component;
           - flexion and extension gaps: final adjustment;
                  - as stated, the femoral medullary stem will have the most significant effect on flexion and extension gaps;
                  - if the flexion gap is greater than the extension gap, then there will be no easy solution;
                  - if the extension gap is greater than the flexion gap, then the femoral trial can be augmented w/ wedge spacers, but just
                            ensure that the patient can achieve full extension;
           - evaluate and manage bone defects:
                  - probably in most cases 4-6 mm distal femoral augments are required, especially when the extension gap is greater that 16-18 mm; 
                  - be prepared, to add wedges to the posterior condyles so that no gaps remain between posterior femoral components and the bone surface;
                  - use a caliper inorder to meansure bone gaps, and re-trial with an augmented femoral component; 
           - restoration of normal joint: (see malposition of joint line)
                  - consider inserting trial components and noting when the patellar component impinges on the tibial component;
                  - if patellar-tibial impingement at knee flexion angles less than 110 deg probably indicates patella baja or raising of the joint line;
                  - in this situation additional distal femoral augments are required;
                  - ref: Restoration of the Distal Femur Impacts Patellar Height in Revision TKA