Sagittal Plane Imbalance
Flexion instability is a common indication for revision knee surgery, accounting for up to 22% of TKA revisions, the majority of which are undertaken within a few years of the index primary procedure1-5, due to imbalance. Relative instability is present when the knee is tighter in extension than in flexion. Late instability typically occurs due to PCL rupture due to traumatic or attritional insufficiency. While most cases of instability are reported in cruciate retaining knees, instability in posterior-stabilized knees may also occur. The following techniques were established to utilize intraoperative sensors to quantify flexion gap kinematics in order to mitigate the incidence of complications and to reduce the risk of revision due to instability:
Correcting an Unstable Flexion Gap
1) Tight in Extension Only (Symmetrical)
The medial and lateral compartments may show excessive concurrent loading in extension. The posterior capsule or posterior MCL may be excessively tight or contracted. The center of peak load* is located in the mid-third of the tibial insert trial.
Loading 20-40lbf.: It is advised that the surgeon release the posterior capsule from the posterior tibia or the posterior femur. The knee should be cycled through a range of motion and re-evaluated for loading conditions. Further release should be performed until desired loading is achieved.
Loading > 40lbf.: In this scenario, distal femoral recut is recommended as soft-tissue release may need to be extensive in order to achieve balance, and bony correction is more reliable. After recutting the distal femur and chamfers, the knee should be cycled through a range of motion several times and then the loads should be re-evaluated at 10, 45, and 90 degrees. Mid-flexion and 90-degree loads should be unaffected.
2) Tight In Flexion Only (Symmetrical)
The medial and lateral compartments may show excessive concurrent loading during flexion, with femoral center of load* location located posterior on the sensor interface for both compartments. Excessive posterior roll-back will also be observed when the knee is taken through a range of motion – this may or may not be combined with anterior lift-off of the sensor. An anterior-posterior drawer test may show very limited translation of the femur.
If the flexion loads are in excess of 40 lbf., the surgeon is advised to increase the amount of tibial slope, incrementally (2 degrees), until desired loading and contact point kinematics are achieved. For smaller increments of tightness, selective puncture of tight PCL fibers with an 18-gauge needle or release of the PCL from the posterior tibia may be effective.
3) Looseness in flexion and extension
The medial and lateral compartments exhibit limited loads (<10 lbf) during extension and flexion. Femoral contact points** are seen in the mid- to anterior-third of the sensor during flexion. An anterior-posterior drawer test shows excessive translation of the femoral contact points** across the tibial insert.
In order to create more physiologic soft-tissue tension and to decrease the femoral AP excursion across the tibia, a thicker shim be added to the sensorized tibial trial. The loads should be reassessed, with target range 15-40 lbf and translation should be tested by application of an anterior-posterior drawer test to confirm stability. The shim size should be increased until desired loads and AP stability are achieved.
*For reference only
**Femoral contact points for reference only
Dr. Gregory J. Golladay is a paid consultant to OrthoSensor, Inc.
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5 Callaghan JJ, O’Rourke MR, Saleh KJ. Why knees fail: lessons learned. J Arthroplasty. 2004;19(4 Suppl 1):31-4.