A range of simple radiographic measurements can be obtained to accurately diagnose many structural disease of the hip,
congenital or acquired.
1. Acetabular coverage
There are three methods to measure the degree of acetabular coverage:
- The Center-edge angle of Wilberg or Lateral center-edge angle allows assessment of the superolateral coverage of the femoral head by the acetabular roof.
It is an angle formed between a vertical line through the center of the femoral head and a line from the center of the femoral head connecting the most superolateral margin of the acetabulum,
in the AP view.
Normal angle must be above 25º.
(Fig.5) If below 20º it is diagnostic of hip dysplasia - this is the most important measurement for diagnosing dysplasia.
- The Femoral head extrusion index: indicates the percentage of the femoral head not covered by the acetabulum.
It is calculated by dividing the horizontal distance of the part of the femoral head that is lateral to the edge of the acetabulum by the total horizontal width of the femoral head and multiplying by 100.
It is considered normal if less than 25%.
(Fig.6) It is increased in dysplasia and reduced in PINCER impingment.
- With the Center-edge angle of Lequesne or Anterior center-edge angle,
in the false profile view,
the anterior coverage of the femoral head can be assessed.
It is the angle between the vertical line from the center of the femoral head and the most anterior point of the acetabular sourcil. The normal range is from 20-45 degrees.
(Fig.7) If this angle is smaller than 20º the anterior coverage of the acetabulum is considered insufficient and associated with acetabular dysplasia.
It is increased in the pincer-type impingment.
2. Acetabular inclination
There are two methods to evaluate acetabular inclination:
- Sharp’s angle or Acetabular index measures overall acetabular inclination. It is the angle formed between an horizontal line at the inferior aspect of both pelvic teardrops and a line from the inferior margin of the teardrop to the lateral margin of the acetabular roof.
The normal range in adults is 33-38º.
(Fig.8) If the angle is above 45º it suggests potential acetabular developmental dysplasia and neuromuscular disorders.
3. Acetabular depth
The positions of the acetabular fossa and femoral head are judged on the basis of the ilioischial line or the acetabular index of depth to width.
- The Ilioischial line/Kohler’s line is a line along the outer border of the obturator foramen to the medial border of the iliac wing,
that should pass through the acetabular teardrop.
In normal conditions the floor of the acetabular fossa is lateral to the ilioischial line – 2mm in men and 1 mm in women.
(Fig.10) If the acetabular fossa meets the ilioischial line it is diagnosed as coxa profunda whereas protusio acetabuli means that the femoral head overlaps or projects medial to ilioischial line.
- The Acetabular quotient/index of depth to width is the ratio of the acetabular width (distance from the lateral edge of the acetabular roof to the most inferior point of the acetabulum) an the acetabular depth (perpendicular distance from the midpoint of the width line to the acetabular dome) on an AP pelvic view.
(Fig.11) An acetabular quotient with the ratio depth/width x 1000 of less than 250 is seen is dysplasia.
4. Acetabular version
- Anteversion is defined as the absence of an intersection between the line connecting the anterior margin of the acetabulum and a line connecting its posterior margin.
Under normal conditions the acetabulum is oriented in anteversion.
(Fig.12)
- Retroversion is defined as the presence of such intersection,
when the posterior wall of the acetabulum crosses the anterior wall before reaching the acetabular roof – the cross-over or figure-of-eight sign.
The deficient posterior wall is medial to the center of the femoral head and there is a prominent projection of the ischial spine into the pelvic cavity. It is associated with the PINCER type of impingement.
(Fig.13)
5. Femoral head sphericity
The head shape can be classified into either spherical or aspherical: if the epiphysis of the femoral head deviates more than 2 mm from the reference circle it is classified as aspherical.
(Fig.14)
6. Femoral-acetabular joint space
The distance between the cortex of the femoral head and the acetabulum should not excede 6 mm superiorly,
7 mm axially or 13 mm medially.
(Fig.15)
The superior joint space is usually reduced in degenerative diseases as the axial space is commonly narrowed by inflammatory arthritis. The medial space is affected in both inflammatory and degenerative disease.
7. Neck-shaft angle or Caput-collum-diaphyseal angle
It allows the evaluation of the hip alignment.
It is an angle formed between the line along the axis of the femoral neck which passes through the center of the femoral head and the longitudinal axis of the shaft with the apex at the intetrochanteric line in the AP view.
Normal angle is 125º.
(Fig.16)
If the angle is less than 120º it's called coxa vara and if it is more than 130º it is coxa valga.
Secondary causes of osteoarthritis can be either congenital,
developmental or post-traumatic deformities of the hip bones.
The more common developmental pathologies of the hip will be adressed and illustrated.
Developmental dysplasia of the hip
It is a spectrum disorder characterized by an underdeveloped or shallow and upwardly sloping acetabulum with undercoverage of the femoral head.
Although commonly associated with hip dislocation in childhood,
mild dysplasia may be subtle and go undiagnosed until adulthood in a significant number of cases.
Surgical procedures that reorient the acetabulum thereby restoring previously abnormal biomechanics can delay or even prevent the development of osteoarthritis.
Radiological findings (Fig.17):
- Greater acetabular inclination: Sharp's angle > 45º; Tonnis angle > 10º
- Acetabular undercoverage: Lateral CE angle <20º,
Femoral head extrusion index > 25%,
Anterior CE angle <20º
- Wider teardrop-femoral head distance (>11mm)
- Shallow acetabulum with an acetabular quotient <250
Acetabular dysplasia can be associated with varying degrees of morphologic abnormalities of the proximal fémur such as excessive anteversion,
coxa valga and CAM deformity.
"Borderline" hip dysplasia
This entity refers to young patients,
especially women,
who have not been classified as developmental dysplasia in childhood and show subtle changes in radiographs in early adulthood with symptons similiar to femoral-acetabular impingment.
They don’t have CAM or PINCER radiological signs and don’t fulfill the criteria for dysplasia.
These patients are not candidates for osteotomies or the standard surgical protocols for true hip dysplasia.
Most of them are undergoing arthroscopic treatments.
Radiological findings (Fig.18):
- Slightly greater obliquity of the acetabular roof
- Discrete anterior or lateral undercoverage: Lateral CE angle 20-25º; Anterior CE angle 20-25º
- Subtle loss of sphericity of the femoral head,
with or without neck shortening
- Neck-shaft angle 135-140º
- Compensatory hypertrophy of the acetabular labrum,
that ossifies and is detected as a prominent acetabular rim that compensates the femoral coverage.
Femoroacetabular impingement
Recent studies have suggested that more subtle developmental abnormalities at the femoral head-neck juction or the acetabulum play a substantial role in the development of arthrosis in cases that formerly would have been classified as primary.
The 3 types of impingement are PINCER,
CAM ou mixed type.
1.
CAM
There is loss of bone concavity at the femoral head-neck juction or femoral offset by a bony protuberance,
mostly located at the anterosuperior aspect,
that leads to an aspherical shape of the femoral head and a pistol grip deformity.
The abnormal contour of the femoral head-neck junction causes impingement anteriorly against a normal acetabulum.
The alpha angle measured in a cross-table lateral view of the hip is nowadays more used in MRI as it is more accurately measured.
2. PINCER
It is essentially an over-coverage of the femoral head anteriorly by the acetabulum from either coxa profunda or a retroverted acetabulum.
Radiological signs (Fig.20 and 21):
- Acetabular retroversion: cross-over sign; posterior wall sign; sciatic spine sign (Fig.13)
- Overcoverage of the femoral head by the acetabulum: increased anterior and lateral CE angle; reduced acetabular extrusion index
- Reduced offset of the femoral head-neck junction
- Reduced inclination of the sclerotic acetabular sourcil with a very low Tonnis angle
- Greater acetabular depth: Coxa profunda or Protusio acetabuli (Fig.22)