Introduction
TKA poses significant challenges to achieving fixation, stability, alignment, and joint line restoration. Meticulous surgical technique with the appropriate implants is needed for satisfactory surgical outcome. With improvements in instrumentation and prosthetic design, the goals of revision TKA can be achieved with the use of modular augments, metaphyseal cones, and stems. However, obtaining flexion and extension gap balance remains the cornerstone of a successful arthroplasty.
Numerous causes of early TKA failure have been identified. Instability is a common mode of failure that can be largely attributed to poor management of the flexion/extension gaps. The reported incidence of postoperative instability leading to revision TKA ranges from 13% to 33%. Implant malpositioning and malrotation of the components further complicate revision TKA stability. Regardless of the cause, flexion/extension gap balancing determines the success and longevity of revision surgery. Failure to obtain balanced flexion and extension gaps may lead to catastrophic complications, including recurrent instability and, in the worst case, knee dislocation.
Surgical Concepts
The objectives of revision TKA are the same as those of primary arthroplasty—re-creating femoral anatomy, achieving appropriate component alignment in all planes, securing component fixation, and providing ligamentous stability. The challenges associated with revision TKA correlate with the remaining femoral and tibial bone stock and the integrity of the collateral ligaments. Although cases of component loosening with subsidence, osteolysis, and fracture are associated with bone loss, iatrogenic bone loss should be minimized. Component removal of well-fixed implants should be accomplished with as little bone loss as possible, because the residual bone acts as the building block and lays the foundation for subsequent reconstruction. After component removal, the degree of bone loss is assessed, and the need for augmentation to reestablish the joint line and the mechanical axis of the tibia and the femur is determined.
The key to successful reconstruction is to create equal and symmetric flexion and extension gaps. This usually is achieved with soft tissue releases to correct fixed angular deformities and augmentation of the tibia and femur in an effort to reestablish the appropriate tension in the soft tissue envelope that supports the knee. In addressing bone defects, adjustments on the femoral side can affect the knee in flexion or extension, whereas adjustments on the tibial side affect the flexion and extension gaps equally. Joint line restoration affects patella height and influences the clinical outcome.
The principles of revision TKA are accomplished by following a three-step algorithm Step 1 addresses the tibia and creates a flat, stable tibial surface perpendicular to the mechanical axis in the coronal and sagittal planes. Reconstruction of the tibia influences the flexion and extension gaps. Step 2 determines the appropriate size of the femoral component, which is set in the correct rotational position to reestablish the femoral posterior condylar offset. This step influences the flexion gap. Step 3 establishes the distal joint line by the proximal or distal positioning of the femoral component and also affects the extension gap.
Tibial Reconstruction
Because the tibia affects the flexion and extension gaps equally, it is addressed in the first step of the reconstruction and establishes the foundation for the subsequent steps. A tibial surface perpendicular to the mechanical axis must be created, and it may require modular augments such as blocks or wedges, metaphyseal cones, or structural allografts. Management of bone defects is discussed elsewhere in this textbook. For secure fixation in revision TKA and for assisting with component alignment, stem extensions (i.e., cemented stems or uncemented press-fit diaphyseal stems) should be employed. The method of stem fixation depends on the quality of the residual bone. Care should be taken to ensure the correct rotational position of the tibial component based on the tibial tubercle and anteromedial aspect of the tibial plateau. By following these basic steps, a solid tibial platform is established on which further reconstruction can be based.
Femoral Reconstruction
Femoral reconstruction is the second part of the procedure. First, the appropriate size for the femoral component must be determined. Bone loss should be taken into consideration to avoid undersizing the femoral component, which would result in loss of the posterior condylar offset and a larger flexion gap that could compromise flexion stability. It is prudent to select a component that has an anteroposterior (AP) dimension larger than the residual bone and to augment the posterior condyles to restore the original AP dimension. The rotational position of the femoral component also affects the flexion gap. The medial and lateral epicondyles provide fixed anatomic landmarks for setting femoral rotation and are usually present in revision surgery. If not, the femoral component rotation should be set parallel to the tibial platform at 90 degrees of flexion. Failure to set proper rotation leads to flexion gap asymmetry and can adversely affect stability and patellofemoral kinematics.
If the original femoral component was internally rotated, adjustments should be made to ensure that the femoral component is externally rotated as described earlier. This usually requires additional bone resection from the anterolateral and posteromedial femur. Augmentation of the posterolateral femoral condyle also helps to ensure correct external rotation of the femoral component. The loss of posterior femoral bone influences this step because posterior condylar bone loss may require equal or different thicknesses of the posterior condylar augments. These adjustments focus on reestablishing the posterior condylar offset and setting the rotational position of the femoral component, which help to establish the flexion gap.
After the appropriately sized femoral component is set in correct rotation, the distal joint line must be established. The femoral epicondyles provide a valuable landmark. On average, the joint line is 25 mm from the lateral epicondyle and 30 mm distal to the medial epicondyle. Other landmarks for the tibiofemoral joint line are the meniscal scar and fibular styloid process (1 cm above the joint line). With the correct distal femoral augmentation, the component is set provisionally to reestablish the distal joint line and the extension gap. Adjustments to the distal femoral augmentation influence the extension gap with the intention of creating an extension gap that is equal to the flexion gap.
Gap Balancing
With the provisional femoral and tibial components in place, the thickest polyethylene articular surface that fills the flexion space is inserted. The knee is brought through the range of motion and checked for alignment, full extension, patellar tracking, and stability in the coronal and sagittal planes. Final adjustments to the position and size of the femoral component may be needed as flexion and extension gaps are balanced. Small adjustments to the tibial polyethylene thickness may also be needed. When there is a discrepancy between the flexion and extension spaces, additional adjustments are necessary. The decision-making matrix shown in Table 17.1 should be followed.
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