Extension Gap


- Discussion:
    - see flexion gap
    - is space between transverse cut on distal femur & transverse proximal tibial cut while the knee is in complete extension;
    - while minor loss of knee flexion or mild instability in flexion can be tolerated, priority is always be given to extension gap adjustments;
    - although in terms of sequence of adjustments one must always correct flexion gap first, extension gap status is functionally more important;
    - certain soft tissues such as the iliotibial band predominately affect the extension gap;

- Extension Gap Too Large:
    - resultant instability in extension while the flexion gap may be too small, with resultant loss of flexion;
    - may be from excessive ligament lengthening or excessive removal of bone from the femur, the tibia, or both;

- Extension Gap Too Small: loss of extension

- Residual Flexion Contracture (Extension Gap Too Tight)
    - w/ trials in place, good flexion is obtained, however full extension is not;
    - additional 2 or 4 mm of distal femur are resected;
    - distal cut is accordingly revised;
    - notch cut and chamfers are subsequently revised to maintain correct configuration, the anterior and posterior cuts are not;
    - this maneuver affects ligamentous tension in extension but not in flexion;
    - avoid elevation of joint line:
    - few degrees of hyperextension at knee are advantage because this over the center mechanism allows the patient to stand w/ quads relaxed and the knee joint stable;
    - extensive hyper-extension makes the leg feel uncomfortable because posterior capsule is under too much tension

- References:

The significance of an asymmetric extension gap on routine radiographs after total knee replacement: A new sign and its clinical significance.
Influence of a secondary downsizing of the femoral component on the extension gap: a cadaveric study.
Recutting the distal femur to increase maximal knee extension during TKA causes coronal plane laxity in mid-flexion.

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Original Text by Clifford R. Wheeless, III, MD.

Last updated by Data Trace Staff on Monday, August 12, 2013 9:51 am