Standard conventional radiographic imaging in patients with FAI includes an anteroposterior pelvic view and an axial cross-table view of the proximal femur.
If these radiographic projections are not correctly performed,
the measurements cannot be correctly evaluated.
For the anteroposterior pelvic radiograph,
the patient is in the supine position with the legs 15º internally rotated.
The central beam is directed to the midpoint between a line connecting both anterosuperior iliac spines and the superior border of the symphysis.
The cross-table view of the proximal femur is taken with the leg internally rotated,
with the central beam directed to the inguinal fold.
An alternative to the axial view,
a Dunn view,
preferably in 45º of flexion,
can be obtained.
On an anteroposterior projection of the pelvis (Fig.1),
the distance from the superior margin of the pubic symphysis to the tip of the coccyx should be 1–3 cm or to sacrococcygeal joint 3–5 cm (no tilting).
Measurements above normal ranges indicate pelvic inclination.
Measurements below normal ranges indicate pelvic reclination.
The tip of the coccyx should point toward the midpoint of the superior aspect of the symphysis pubis,
and the obturator foramina should be symmetric (no rotation).
Radiographic Findings
The current radiological measurements to evaluate FAI include:
- General and Focal acetabular overcoverage (Lateral center edge angle,
Acetabular index,
Femoral head extrusion index,
Cross-over sign and Posterior wall sign)
- Sphericity of femoral head (Alpha angle,
Anterior femoral head-neck offset).
Assessment for PINCER morphology begins with qualitative assessment of the global acetabular depth.
Three normal landmarks are the medial border of femoral head,
the medial border of acetabular fossa and the ilioischial line (Fig.2).
General Acetabular Overcoverage
There are 2 distinct subtypes of general acetabular overcoverage,
best avaluated on AP pelvic radiographs:
· Coxa profunda occurs when the medial border of acetabular fossa contacts or is medial to the ilioischial line.
· Protrusio acetabuli occurs when the medial border of the femoral head overlaps or is medial to the ilioischial line (Fig.3).
Both forms relate to an increased depth of the acetabular fossa.
General acetabular overcoverage can also be assessed quantitatively using the lateral center edge angle (of Wiberg) and acetabular índex (Tonnis angle).
o The lateral center edge angle (of Wiberg): is the angle formed by a vertical line and a line connecting the femoral head center with the lateral edge of the acetabulum.
A normal value varies between 25° and 39°.
An angle <25° is likely to represent dysplasia and and values above 39° indicates acetabular overcoverage (Fig.4).
o The acetabular index (Tonnis angle): is the angle formed by a horizontal line and a line connecting the most medial point of the sclerotic zone of the acetabulum with the lateral edge of the acetabulum.
A normal value varies between 0° and 10°.
In hips with coxa profunda or protrusio acetabuli,
the acetabular index is 0° or negative (Fig.5).
o The femoral head extrusion index defines the percentage of femoral head that is uncovered (normally less than 25%).
Focal Acetabular Overcoverage
In a patient with a normal hip,
the anterior rim of the acetabulum lies medially to the posterior rim and the outline of the posterior acetabular rim runs approximately through the femoral head center (Fig.6).
There are 2 distinct subtypes of focal acetabular overcoverage:
· Anterior (acetabular retroversion): the anterior acetabular rim line lies lateral to the posterior rim in the cranial part of the acetabulum (Cross-over sign) (Fig.7).
· Posterior (prominent posterior wall): the outline of the posterior acetabular rim projects laterally to the femoral head center (Posterior wall sign) (Fig.8).
Assessment for CAM morphology
Sphericity of femoral head (Fig.9 / Fig.10): quantification of the amount of asphericity can be done by measuring the alfa angle and the anterior femoral head-neck offset.
o Alfa angle: is the angle between the femoral neck major axis and a line from the femoral head center to the point where asphericity of the head-neck contour begins.
The normal value is less than 55°.
An angle >55° is considered abnormal (Fig.11).
o Anterior femoral head-neck offset is defined as the difference in radius between the anterior femoral head and anterior femoral neck on a cross-table axial view of the proximal femur.
A value less than 10 mm suggests CAM impingement (Fig.12).
Radiographic evaluation can also detect indirect signs of FAI.
Secondary signs of FAI such as: synovial herniation pits,
labral ossification,
and os acetabuli can also be helpful indicators.
Extra-Articular Factors
Not only intra-articular factors lead to a conflict between the femur and acetabulum.
It’s important to consider extra-articular anatomical factors as well,
such as femoral torsion,
pelvic tilt and femoral neck angle.
Femoral torsion is defined by the angle formed by a horizontal plane tangent to the posterior femoral condyles and a plane defined by the center of the femoral head and the axis through the femoral neck.
Usually the femoral neck is pointing in an anterior direction compared with the femoral condyles,
termed femoral antetorsion / anteversion.
The normal femoral antetorsion is 13° ± 10°.
A decreased femoral torsion (<5°) is associated with CAM-type FAI and it reduces the internal hip rotation in these patients.
Although femoral torsion initially was measured with standard radiographs,
this method has been replaced by more precise measurements with CT and MRI.
Pelvic tilt influences the extent of focal acetabular overcoverage.
In a patient with increased anterior pelvic tilt,
there is an increase of acetabular coverage when compared with a normal pelvic tilt.
To determine accurately the individual pelvic tilt of a patient,
a standing lateral radiograph of the pelvis should be obtained,
in which the angle between the longitudinal axis of the body and a line connecting the femoral head center with the midpoint of the sacral plate is measured.
A normal pelvic tilt is 5° ± 6°.
An increased anterior pelvic tilt results in a reduced internal rotation of the hip and can be a contributing factor to FAI (Fig.13).
The pelvic tilt is also closely connected to the sagittal balance of the spine.
However,
in current clinical practice it is not used regularly.
The Center Collum Diaphyseal Angle (CCDA) is defined in the AP view radiographs by the angle between the neck of the femur and its shaft.
An angle of 130° ± 5° is reported as the normal value.
Patients with a valgus hip (>135°) have a decreased internal rotation of the hip joint,
especially if combined with other osseous factors such as decreased femoral torsion (Fig.14).
However,
the mechanical influence of the valgus hip in the development of FAI is minimal in most patients.