- See:
-
TKR Menu:
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Flexion Contracture of TKR:
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Flexion Gap:
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Extension Gap:
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Joint Line Position
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Valgus Deformity
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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 a thicker tibial implant will stabilize the 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);
-
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;
- 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 the 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;
- References:
Soft-Tissue Balance in Revision Total Knee Arthroplasty. Michael D. Ries et al Surgical Techniques. J. Bone Joint Surg. Am., Mar 2004; 86: 81 - 86.
Flexion and extension gap balancing in revision total knee arthroplasty.
