Abstract
Flexion instability is a well-defined, though often difficult to diagnose, type of TKA instability. It may also complicate posterior stabilised arthroplasties. It is one of three modes of tibial-femoral instability along with: 1. Varus-valgus or coronal plane instability and 2. Instability in the plane of motion that results from either fixed flexion contracture and buckling or recurvatum and collapse. The issues for correction of coronal instability are generally alignment and either ligamentous balance or constraint. For plane of motion instability it is full extension without hyperextension and restoration of extensor mechanism power. The issues for flexion instability are basically balanced flexion and extension gaps.
The diagnosis of flexion instability is made by history and physical examination. These patients, with a more spacious or lax flexion gap, initially do extremely well following surgery, achieving flexion rapidly and comfortably. They progress within months however, to a condition of chronic swelling and tenderness of peri-articular soft tissue, recurrent effusion and a feeling of unease up and down the stairs, as well as getting up out of a chair: anything that stresses the knee in the flexed position.
The diagnosis is confirmed by clinical examination. In gross cases, the patient sitting on the edge of the exam table with the legs dangling and flexed at 90 degrees will first of all close the flexion gap, bringing the tibial component into contact with the posterior femoral condyles when they contract the quadriceps muscle. This vertical motion that precedes extension can be observed. Similarly, if the patient is supine, with the knee flexed to 90 degrees, the examiner may grasp the ankle and with a hand under the thigh, distract the flexion gap and then allow it to close. The travel and the clunk can be appreciated. The standard ‘posterior drawer’ test that is appropriate for the non-arthroplasty knee will only be useful for relatively non-constrained, cruciate dependent prostheses. It will not be useful for flexion instability in the posterior stabilised prosthesis. It is useful to perform this distraction maneuver in flexion, during the arthroplasty with trial components in place to confirm that the arthroplasty is stable in flexion. The common maneuver to assess the flexion gap, of internally and externally rotating the femur to detect medial lateral instability in flexion seems to be less accurate.
The patients at greatest risk for this complication are those presenting for arthroplasty with a fixed flexion contracture. If a measured resection technique is employed without consideration of correcting the tighter extension gap, when a (relatively thinner) poly insert is selected to achieve full extension, it will not be thick enough to stabilise the larger/normal flexion gap. Flexion instability should not be confused with so-called “mid-flexion” instability, which is a poorly defined and much more subtle, clinical entity that has been described in case reports of revision surgery and the cadaver laboratory. Although more conforming articular polyethylene inserts may resolve this problem, even if revision is performed to a more constrained component, the essence of the solution is revision arthroplasty to balance the flexion and extension gaps.
