Total Hip Arthroplasty

When confronting a CT-scan of a Total Hip Replacements, it is expected that the radiologist evaluates the presence of signs suggestive of loosening, component wear, osteolysis, fractures, component dislocation and protusio, as the most common complications. More information on the evaluation of some of these entities may be found under ..\osteolysis\osteolysis__hystyocytic_respons.htm .

However, the following measurements may also be included in the report, particularly in cases where they are requested by the referring physician, or there is a clinical concern about instability, prior dislocation or impingement.

Acetabular Anteversion:

The acetabular cup should be anteverted approximately 15 degrees to avoid instability. The measurement is not absolute, because the femoral neck anteversion can compensate for variations in the acetabular version. Therefore, the report should not include subjective assessments, but rather be limited to recording the numeric anteversion angle.

[Normal acetabular anteversion in the native pelvis = 19 +/- 6 degrees].

Acetabular Anteversion:

Step 1: Draw a line tangent to both ischial tuberosities. Copy this line across all the scans in the series.

Acetabular Anteversion:

Step 2: Trace a perpendicular line to the the bi-ischial line at the level of the acetabular component. Measure the angle formed between this perpendicular line and the edge of the acetabular cup.

Femoral Anteversion:

The version is the angle formed between the femoral neck and the transcondylar line at the knee. The femoral component tends to dislocate posteriorly; therefore, anteversion of the femoral neck will place the prosthesis in a biomechanical advantageous position. The caveat is that excessive anteversion will produce limitations in external rotation.

[Femoral anteversion in the native femur is 5-15 degrees for adults].

Impingement: The referring physician was concerned about impingement, which results from marked anteversion of the acetabular cup. Notice the lack of separation between the femoral neck and the acetabular cup.

Femoral Version:

Step 1: Obtain a 5 - 10 mm. scan through the knee at the level of the femoral condyles, and trace the inter-condylar line. Select the line, and copy it across the series. For more reproducible results, the line between the posterior aspect of the femoral condyles may be used.

Femoral Version:

Step 2: Measure the angle formed between the intercondylar line and the angle of the neck of the prosthesis.

Femoral Offset:

Distance between the center of the femoral head and line drawn along femoral shaft. Increased offset will increase the leverage effect of the abductor musculature; decreased offset may result in impingement.

Abduction Angle:

Angle measured on the coronal reconstructions, and formed between the bi-ischial line and the lateral edges of the acetabular component. Angles of less than 30 degrees will result in impingement; angles greater than 50 degrees tend to have the highest degree of component wear.

Abduction angle: Draw a bi-ischial line in the coronal plane, and measure the angle formed with the edge of the acetabular component.