- Definition:
- is space between the posterior coronal cut on the distal femur & transverse cut on proximal tibia, while knee is in flexion;
-
effect of resection of PCL (see
posterior stabilized prosthesis);
- in report by Y. Kadoya et al, effect of PCL resection on tibiofemoral joint gap was analyzed in 30 patients w/ varus DJD of knee who underwent TKR;
- medial soft tissue was released and the bone cut was made without preserving the bone segment of the tibia to which PCL was attached;
- medial and lateral joint gaps in full extension and 90° flexion were measured before and after PCL was resected using a tensioning device;
- after the resection, the flexion gap significantly increased in the medial and the lateral sides (4.8 ± 0.4 and 4.5 ± 0.4 mm,
respectively, mean ± standard error) compared with those seen in the extension gap (0.9 ± 0.2 and 0.8 ± 0.2 mm);
- there was no significant difference between the changes in the medial and lateral gaps;
- mean value of the flexion gap was 2 mm smaller than the extension gap before the resection and 1.7 mm larger after the sacrifice;
- posterior cruciate ligament resection resulted in an increase in the flexion gap and made space for approximately 3-mm thicker polyethylene;
- ref: Effects of Posterior Cruciate Ligament Resection on the Tibiofemoral Joint Gap. Clin Orthop 2001 October;2001(1):210-217
- Small Flexion Gap:
- causes of incomplete knee flexion are:
- tibial cut that is too proximal (w/ or w/o a
Posterior Slope);
- an anterior femoral coronal cut which is inadequate, ie too anterior
- patellar cut which is inadequate (too superficial) with a resultant increase in total patellar thickness (bone and implant);
- if the knee cannot flex normally and is unstable in extension there is situation in which the gap problems are diametrically opposed;
- this is due to surgeon error;
- if the knee cannot fully flex but extends normally;
- verify ROM with the patella reduced;
- either the
posterior slope is inadequate, the patella is too thick, or the anterior cut is too anteior;
- if knee cannot fully flex nor extend, then recutting the tibia more proximally will correct the problem;
- Large Flexion Gap (unstable in flexion)
- may be due to a tibial cut which is too distal
- posterior femoral coronal cut which was too anterior; (ie removing too much posteior femoral condyle);
- if
extension gap is also too small, then solution is to recut the femur more proximally;
- if
extension gap is normal (extends normally and is stable in extension)
- if instability is mild, then accept the deformity;
- if instability in flexion is gross, it is corrected by inserting a thicker tibial component, & resultant loss of extension is then corrected by recutting distal femor more proximally;
-
flexion space may be larger than the extension space;
- this most commonly occurs after release of large fixed varus or valgus contracture;
- in such a situation the thickness of implant that is required for flexion stability is too thick in extension and the knee does not fully extend;
- too correct this it is necessary to resect an additional amount of distal femur to equalize the spaces;
- one should not increase the size of extension space by a resection of distal femur since this would concomitantly increases
flexion space, perpetuating problem;
- flexion space may be smaller than the extension space;
- this may happen if too much distal femur is resected (which is almost always due to a surgical error in the use of distal femoral cutting jig;)
- correction of this problem consists of either resecting more posterior femur (and here you are limited by the actual size of posterior femur and the presence
of posterior femoral cortex) or of adding bone grafts to distal femur to decrease extension space;
- neither of these options works well;
- increased flexion gap (loose in flexion & tight in extension) is more common and can be corrected by resecting more of the distal femur;
- increased
extension gap (loose in extension & tight in flexion) is usually the result of a technical error and can be corrected by resecting posterior femoral condyles
or converting the flat tibial surface to posteriorly sloping surface;
- when the knee is tight in both flexion and extension, more proximal tibia should be resected;
- when knee is tight in both flexion extension, and more proximal tibia should be resection;
- patellar height can also result in tight flexion and may require additional patellar resection;
- w/
extension gap is adequate, but flexion gap is too narrow:
- resecting additional distal femur will increase
extension gap while preserving the flexion gap, resulting in excessive laxity in full extension;
- resecting more proximal tibia will increase the extension and flexion gaps simultaneously & will also result in excessive laxity in full extension.
- relying on fixation to correct problems as a result of malposition of components will eventually result in loosening of the implant and should be avoided;
- by increasing the posterior inclination of the proximal tibial resection, femoral rollback is facilitated, effectively increasing the flexion gap -
extension gap is unaffected;
The flexion gap in normal knees. An MRI study.
