Review Question - QID 219961

The patient presents with signs of flexion instability, which can be caused by increased posterior femoral resection, under-resection of the distal femur, or excessive posterior tibial slope, as shown by line 5.

Flexion instability after total knee arthroplasty (TKA) occurs due to an increased flexion gap relative to the extension gap, leading to mechanical dysfunction. Patients often report recurrent effusions, subjective instability (notably during activities like descending stairs), quadriceps weakness, and diffuse peri-retinacular pain. Diagnosis is commonly confirmed through manual laxity testing in flexion, though there is variability in testing methods and grading. Non-operative treatments include quadriceps strengthening exercises and bracing to improve knee stability. Surgical management typically focuses on correcting the underlying mechanical imbalance by increasing posterior condylar offset, decreasing the tibial slope, raising the joint line, using a thicker polyethylene insert, and optimizing component rotation. However, patient outcomes after revision TKA for flexion instability tend to show the least improvement compared to revisions for other TKA failure causes.

Stambough et al. review flexion instability after total knee arthroplasty (TKA). The authors describe the definition and causes of flexion instability, emphasizing a flexion space that is larger or more lax than the extension gap. Patients typically present with a constellation of symptoms that often include a sense of instability and may even “distrust” their knee. Revision surgery aims to correct technical mistakes in the index procedure.

Abdel et al. provide a stepwise approach to surgical correction of flexion instability after total knee replacement. The authors recommend the following steps: reduction of tibial slope, correction of malalignment, and improvement of condylar offset. Additional joint line elevation is needed if the above steps do not equalize the flexion and extension gaps.

Schwab et al. review their series of flexion instability without dislocation after posterior stabilized total knee arthroplasties. The authors report 10 patients who had a revision of a well-fixed posterior stabilized total knee arthroplasty for isolated symptomatic flexion instability. Each patient exhibited typical flexion instability with a sense of instability without giving way, recurrent knee effusions, multiple areas of soft tissue tenderness about the knee, and substantial anterior tibial translation at 90 degrees of flexion. The authors demonstrated good outcomes with improved knee society scores, stability, and patient satisfaction.

Figure A depicts a lateral of a knee with line 1 showing a resurfacing cut of the patella, line 2 anterior chamfer cut, line 3 the distal femur cut, line 4 native tibial slope, and line 5 an increased or excessive tibial slope cut.

Incorrect answers:
Answers 1-4: Each line represents a reasonable resection location for total knee arthroplasty, and would not be expected to contribute to flexion instability.