Tibiofemoral Instability

Key Concepts

Instability accounts for approximately 10 to 25% of revision total knee arthroplasties (TKAs).
Types of tibiofemoral instability include:
- Sagittal (anteroposterior) instability (Figure 67.1) due to flexion-extension mismatch such as flexion instability (Figure 67.2).
- Coronal (varus-valgus) instability
  - Iatrogenic from
    - Ligamentous release
    - Condylar fracture (Figure 67.3)
  - Attritional damage to ligaments, such as from
    - Polyethylene wear synovitis (Figure 67.4)
    - Cement synovitis from debonded tibial component (Figure 67.5A and B)
    - Periprosthetic joint infection
- Midflexion instability
- Global instability (Figure 67.6)
- Hyperextension (recurvatum) instability (Figure 67.7)
Accurate diagnosis of the etiology causing instability is essential to appropriately manage this complication.

Sterile Instruments and Implants

Instruments

Routine revision TKA instruments
Large and small sagittal saws
Large and small flexible osteotomes
High-speed 6.5-mm and pencil-tip burrs
Square tip impactor

Implants

Implants with varying levels of constraint should be available during all cases, from posterior-stabilized to varus-valgus constraint (VVC) to rotating-hinge constructs.

Surgical Approaches

In most cases, a medial parapatellar approach can be used. Occasionally, an extensile exposure may be required, such as a quadriceps snip.

Figure 67.1 ▪ Lateral radiograph of a 55-year-old man with a dislocated knee after a revision TKA completed at an outside institution.

Figure 67.2 ▪ Lateral radiograph of a 69-year-old woman with radiographic findings concerning for flexion instability including excessive posterior tibial slope and decreased condylar offset.

Preoperative Planning

A full complement of knee radiographs are recommended, including a long-leg hip-knee-ankle radiograph, standing anteroposterior (AP) radiograph, standing posteroanterior flexion radiograph, lateral radiograph, and patellar view.
Prior radiographs for comparison are often very helpful.
As with any revision TKA, periprosthetic joint infection (PJI) should be excluded with inflammatory markers (erythrocyte sedimentation rate and C-reactive protein) and a joint aspiration.

Figure 67.3 ▪ Anteroposterior radiograph of an 89-year-old woman with an intraoperative medial femoral condyle fracture at the time of reimplantation treated with open reduction and internal fixation. Lack of adequate treatment would result in coronal instability.

Figure 67.4 ▪ Intraoperative picture of a 25-year-old man with marked polyethylene wear 8 years after a distal femoral replacement for oncologic purposes. Even with a hinged construct, this resulted in instability.

Figure 67.5 ▪ Anteroposterior (A) and lateral (B) radiographs of a 60-year-old man revealing asymmetric gaps, indicative of coronal instability.

Bone, Implant, and Soft Tissue Techniques

The patient should be positioned supine.
The knee should be examined under anesthesia to determine the true form and extent of instability (Figure 67.8).