Flexion and Extension Spaces : Wheeless' Textbook of Orthopaedics

- See:
- TKR Menu
              - Flexion Contracture of TKR
              - Flexion Gap / Extension Gap
              - Joint Line Position
       - Valgus Deformity / Varus Deformity

- Discussion:
- first: Malalignment should always be corrected first;
- second: soft tissue adjustments which must be made before bone is recut;
- for example, if extension gap is too small (ie knee does not extend normally), it is vital to release the soft tissue structures
             posteriorly (and to remove any osteophytes when present), before removing more bone;
- also it is important to note that if the extension gap is too small whether more bone will be removed from the femur or the tibia will depend
             on the status of flexion gap (always "normalized" flexion gap before extension gap);
- flexion gap:
- posterior femoral cut and the proximal tibial cut mark the upper and lower limits of the flexion space;
- needs to be normalized prior to normalization of the extension gap:
- insertion of thicker tibial implant will stabilize knee in flexion but will decrease knee in extension, which is then corrected by recutting
                      distal femur more proximally;
- following the insertion of the trial prosthesis, it may be noted that gap in flexion and/or extension may be either too small (w/ resultant
                      loss of motion), or too large (w/ resultant instability);
             - ref: The Use of Navigation to Obtain Rectangular Flexion and Extension Gaps During Primary Total Knee Arthroplasty and Midterm Clinical Results
   - extension gap:
- distal femoral cut and proximal tibial cut mark the upper and lower limits of the extension space;
- extension gap must be rectangular in configuration (where it is trapezoidal, medial & lateral soft tissues must be balanced);
- bone cuts are not altered in order to create rectangular extension gap;
- whenever distal femoral cut is moved proximally, size of only the extension space is increased;
- whenever the tibia cut is moved distally, however, both the flexion and the extension spaces are increased;
             - ref: The significance of an asymmetric extension gap on routine radiographs after total knee replacement: A new sign and its clinical significance.

- Residual Tightness:
- where tension is correct in extension but tight in flexion, and appropriate soft tissue releases have been performed, 5 deg posterior slope is created on tibial plateau;
- steinmann pins are returned to their original holes in anterior cortex and the 5 deg cutting block positioned on the pins, using the hole designated 0 deg;
- tibia is most likely to sublux or dislocate in flexion as a result of flexion gap that is larger & more lax than extension gap;
- 3 common causes of AP instability in posterioir stabilized arthroplasty;
- collateral ligament laxity in flexion (mismatched gaps)
- collateral ligaments are lax in flexion due to removing excess bone from femoral condyles, rendering ligaments functionally lax
by increasing the size of the flexion gap relative to extension gap;
- prior patellectomy