CHALLENGING CASES: Achieving Stability in Revision TKA
One of the primary goals in a revision total knee arthroplasty (TKA) is to restore stable knee kinematics and achieve a well-functioning knee. As Ormonde Mahoney, MD, emphasized at the ICJR South/RLO Course, this means reestablishing the joint line height and the center of rotation of the femur so that the knee will flex through its normal arc of motion and rotate appropriately.
Patellofemoral (PF) instability occurs in up to 35% of TKA patients, and tibiofemoral instability occurs in up to 6%.
The most common causes of PF instability are:
- Over-tightening of the lateral retinaculum
- Over-stuffing the PF joint
- Internal rotation of the femoral component
- Internal rotation of the tibial component
Dr. Mahoney, from Athens Orthopaedic Clinic in Georgia, recommends using bony landmarks such as Whiteside’s Line or the epicondylar axis to set the rotation of the femoral component. To set the rotation of the tibial component, Dr. Mahoney uses the tibial tubercle as his landmark and sets the center of the tibial tray close to the tibial tubercle.
Dr. Mahoney emphasized the importance of establishing the cause of tibiofemoral instability, such as soft tissue incompetence or complications with the implant. It is also important to determine whether the instability is global or is present only in extension, flexion, or mid-flexion. Appropriate radiographs are of great help in the evaluation. In addition, CT scans are valuable in evaluating rotational malalignment of the implant. Examination under fluoroscopy can help reveal subtle instabilities.
Instability in mid-flexion is most common and is caused by resecting too much bone from the distal femur to accommodate flexion contractures in TKA. Dr. Mahoney recommends testing for stability in extension with just a bit of flexion in the knee. Otherwise, the stability that is felt with the knee fully extended may not be from well-balanced collateral ligaments; it could be from tension in the posterior capsule.
Intraoperative examination is critically important as well, Dr. Mahoney said. Never stop checking.
In case of ligament insufficiency, Dr. Mahoney uses a component with some level of constraint. If both collateral ligaments are intact and provide physiological stability, a primary posterior stabilized (PS) implant is used. In cases in which 1 collateral ligament is incompetent, but the patient still has an intact extensor mechanism and posterior capsule, a constrained PS implant is used. If the extensor mechanism is compromised in addition to 1 of the collateral ligaments, Dr. Mahoney will use a rotating hinge prosthesis.
Dr. Mahoney concluded that revision outcomes approaching those of primary TKA are achievable in most cases with careful attention to technique and judicious use of constraint.
Click the image above to watch Dr. Mahoney’s presentation.
Dr. Mahoney disclosed that he is a consultant for and receives royalties from Stryker Orthopaedics and that he receives research funding from Arthrex and Stryker.