Component malalignment or malposition are complications related to surgical technique. Correct frontal and lateral alignment is readily evaluated with radiography. However rotational placement of the components represent the third dimension of alignment which may be difficult to assess operatively or with conventional radiography.
Patients with painful total knee replacements or early failure without obvious implant failure or signs of infection should be evaluated for malrotation of components.
Malrotation is usually related to excessive internal rotation of the components, most commonly the tibial component. This results in patellar tilt, subluxation, dislocation, and component failure.24 Early after surgery, patients begin to complain of anterior knee pain.25 Poor positioning can result in catastrophic wear and implant failure.
Catastrophic component wear secondary to component placement in rotation.
Photograph of total knee replacement demonstrating rotated components
Close-up photograph of tibial component demonstrates marked asymmetric wear of polyethylene tray. The medial portion was worn down to the underlying metal surface and there is wear of the underlying metal tray (arrow).
Photograph of femoral component demonstrates associated wear of the femoral condyle.
Berger et al24 have described methodology to quantitatively measure component rotational alignment using CT scanning.
With the knee in full extension, axial images are obtained through the knee.
Femoral component rotation is determined as follows:
- Use axial image through femoral condyles
- Draw line (A) through the surgical epicondylar axis which connects the lateral epicondylar prominence (arrow) and the medial sulcus of the medial epicondyle (arrowhead)
- Draw second line (B) through the posterior portions of the medial and lateral prosthetic condylar surfaces
- Angle formed by these two lines is called the posterior condylar angle
- In this example, the posterior condylar angle measures 10 degrees, indicating that the femoral component is 10 degrees internally rotated relative to the surgical epicondylar axis
- Women 0.3 +/- 1.2 degrees internal rotation
- Men 3.5 +/- 1.2 degrees internal rotation
Tibial component rotation is determined as follows:
Geometric center of proximal tibial plateau is obtained.
- Oval is sized and rotated to best fit using CT scanner software
- Center of oval is determined by software
- A line is drawn through the posterior aspect of the tibial tray
- A second perpendicular line (C) is made centrally respecting the tibial component axis
- Geometric center (arrow) transposed distally to axial image of tibial tubercle
- Line (D) is drawn from geometric center to tubercle
- Tibial component axis line (C) is transposed from above image
- Tibial component rotation is measured by the angle formed by the tibial tubercle axis (D) and the tibial component axis (C)
- Tibial component axis normal value = 18 degrees
- In this case, the angle measures 28 degrees, indicating the tibial component is placed in 10 degrees of excessive internal rotation