Acetabular Dysplasia

Acetabular dysplasia is characterized by a deficient acetabular weight-bearing zone that overloads the articular cartilage and labrum and can lead to OA. It predominantly affects women and may be a sequela of developmental dysplasia of the hip (DDH), or it may present in an adolescent or adult with no history of childhood hip disease. The severity of dysplasia ranges from a subluxated or dislocated hip to more subtle variants of a mildly shallow acetabulum that may go unrecognized until symptoms develop. Patients typically present with insidious onset of activity-related groin or lateral hip pain. Most have a positive impingement sign (pain with combined hip flexion, adduction, and internal rotation); many have a limp and positive Trendelenburg sign. Acetabular dysplasia is most commonly identified as deficient lateral femoral head coverage, but it may also involve significant anterior deficiency, which is identified on a false-profile view. Acetabular dysplasia is frequently associated with proximal femoral deformities, such as coxa valga, femoral anteversion, and femoral head abnormalities. With growing awareness of FAI, it is now recognized that many patients with acetabular dysplasia also have FAI morphology of decreased femoral head-neck offset. Among the hip deformities that lead to OA, the natural history of acetabular dysplasia is best understood. In 1939, Wiberg published an extensive review of hip dysplasia and was the first to quantify acetabular coverage by describing the center-edge (CE) angle. He defined a CE angle less than 20° as abnormal and noted a linear relationship with lower coverage angles and subluxation corresponding with earlier development of OA. In a series of 286 patients, Murphy and colleagues reported that no patient with a CE angle less than 16° and acetabular roof index greater than 15° reached age 65 years with a well-functioning hip.

FAI

In 1935, Smith-Petersen first described pathologic impingement between the acetabular rim and proximal femur in a case series of patients whose disorders included protrusio acetabuli, previous SCFE, and OA. He recommended acetabuloplasty, even for patients with predominantly femoral-sided lesions, because of concern about weakening the femoral neck with excessive resection. Although the association between proximal femoral deformity and OA was repeatedly observed in subsequent decades, it was not until Ganz and colleagues introduced the surgical dislocation in the early 1990s that operative access to manage FAI became feasible.

On the femoral side, the pathologic morphology involves a prominent or aspherical femoral head-neck junction. With flexion and internal rotation of the hip, the incongruent articulation between the femoral head-neck junction and the acetabulum results in a cam-type impingement that can cause acetabular chondral delamination and labral damage. Patients with normal femoral head-neck shape but increased femoral retroversion may impinge because of the limited capacity for combined flexion and internal rotation.

On the acetabular side, overcoverage can restrict motion because of pincer-type impingement. The excess coverage may be global and severe, such as in protrusio acetabuli, mild, as in coxa profunda, or focally anterior, as occurs in acetabular retroversion. Most cases of protrusio are idiopathic, although it is associated with other conditions, such as Marfan disease and rheumatoid arthritis. With pincer impingement, the femoral neck impacts the deep anterior acetabular rim in flexion, which causes outside-in labral damage. Often, there may be combined cam and pincer impingement. Patients with FAI present with insidious onset of activity-related groin pain and positive impingement test on physical examination.

There may be overlap in hips with dysplasia and impingement morphology. Acetabular retroversion has been reported to occur with acetabular dysplasia in a range from 17% to 38% ; the incidence of insufficient femoral head-neck offset with acetabular dysplasia is more than 70%.

Anteroposterior View

Plain film imaging to screen for hip abnormalities begins with an anteroposterior (AP) pelvis radiograph. For a supine film, both hips are internally rotated 15° to neutralize femoral anteversion, which gives a true AP image of the proximal femur in most patients. The x-ray beam is centered at the midpoint between the superior border of the pubic symphysis and a horizontal line connecting the anterior superior iliac spines with a tube-to-film distance of 120 cm. Quantitative measures vary with the position of the pelvis; therefore, the first step in evaluating an AP pelvis radiograph is to determine whether it is appropriately positioned with respect to tilt and rotation. To determine proper rotation around the longitudinal axis, the center of the sacrum and coccyx should be in line with the pubic symphysis and the ilia, obturator foramina and acetabular teardrops symmetric. Siebenrock and colleagues determined that the distance from the top of the pubic symphysis to the sacrococcygeal (SC) joint averages 32 mm in men and 47 mm in women. Distances of 2 to 5 cm from the top of the symphysis to the SC joint are considered acceptable for radiographic positioning of tilt. The SC joint can sometimes be obscure, so others have accepted a distance of 0 to 2 cm from the symphysis to the tip of the coccyx.

The supine AP image is helpful for qualitative assessment of OA and femoral head shape, as well as qualitative and quantitative measures of acetabular coverage and orientation. Pelvic tilt, the forward (flexion or inclination) or backward (extension or reclination) rotation of the pelvis around the transverse axis, varies by individual and position. The pelvis extends with movement from supine to standing, which alters acetabular and joint space measures. Because of this variation in tilt, some investigators recommend standing AP pelvis radiographs to evaluate functional joint space width and to quantify acetabular parameters in dysplasia.

When reviewing the AP pelvis radiograph, it is helpful to develop a systematic routine to screen for acetabular and femoral abnormalities. The following are commonly used measures of acetabular morphology:

• 1.
  

  Lateral CE angle. This angle is formed by the intersection of a line from the center of the femoral head to the lateral rim of the acetabulum and a second line that is perpendicular to a line connecting the center of the femoral heads ( Fig. 2 ). Wiberg defined a CE angle greater than 25° as normal and less than 20° as dysplastic. The upper limit of normal has not been clearly defined; however, CE angle greater than 40° indicates a deep acetabulum that may be at risk for pincer impingement ( Figs. 3 and 4 ).

  
  

  
  

  
  

  

  
  

  
  

  Lateral CE angle. This angle is determined by the intersection of a line from the center of the femoral head to the lateral rim of the acetabulum and a second line that is perpendicular to a line connecting the center of the femoral heads. Normal lateral CE angle is 25° to 35°.

  
  

  
  

  
  

  
  

  
  

  Coxa profunda. In this patient, the lateral CE angle is high, the Tönnis angle is negative, and the acetabular teardrop is medial to the ilioischial line ( arrow ).

  
  

  
  

  
  

  
  

  
  

  Protrusio acetabuli. In this patient, the lateral CE angle is greater than 40°, the Tönnis angle is negative, and the femoral head is medial to the ilioischial line. In addition, this example shows the teardrop medial to the iliopectineal line ( arrow ), another indicator of protrusio.

  
  
  
  
• 2.
  

  Acetabular roof angle, or Tönnis angle. This angle is determined by a line between the femoral head centers (or parallel to it) and a second line that connects the most medial and lateral margins of the sclerotic acetabular weight-bearing zone, or the sourcil (the original description used a line perpendicular to the vertical axis of the sacrum to represent the transverse pelvic axis but, in our experience, this axis is more accurately denoted by a line connecting the centers of the femoral heads). Normal acetabular roof angles range between 0° and 10°. Dysplastic hips have a steeper roof, with Tönnis angles greater than 10° ( Fig. 5 ), whereas overcoverage is characterized by a downsloping or negative roof angle (see Figs. 3 and 4 ).

  
  

  
  

  
  

  
  

  
  

  Tönnis angle, or acetabular roof angle. This angle is the intersection of a line connecting the center of the femoral heads with a line parallel to the sourcil of the acetabulum. Normal Tönnis angle is 0° to 10°.

  
  
  
  
• 3.
  

  Shenton line. This line describes the curve from the top of the obturator foramen to the medial, inferior femoral neck. In a normal hip, this arc maintains a smooth contour. Disruption of the Shenton line greater than 5 mm indicates dysplasia with femoral head subluxation ( Fig. 6 ).

  
  

  
  

  
  

  
  

  
  

  Shenton line. The Shenton line describes the arc from the inferior femoral neck to the superior obturator foramen, which should form a smooth contour. In this example of a patient with bilateral acetabular dysplasia, there is disruption of the Shenton line on the left hip.

  
  
  
  
• 4.
  

  Cross-over sign. The anterior acetabular wall is a continuation of the superior public ramus and is more horizontally oriented. The posterior wall extends from the lateral ischium and is more vertical. In a normal hip, the anterior and posterior acetabular walls meet at the lateral rim on an AP radiograph. The cross-over sign is an indicator of acetabular retroversion or focal anterior overcoverage; it is positive if the anterior acetabular wall projects lateral to the posterior wall ( Fig. 7 ). This measure may be falsely positive with increased pelvic inclination.

  
  

  
  

  
  

  
  

  
  

  Cross-over, posterior wall, and ischial spine signs. The anterior ( dotted line ) and posterior walls ( solid lines ) should meet at the lateral rim of acetabulum on the AP view. The cross-over sign is an indication of acetabular retroversion and is considered positive when the anterior wall is lateral to the posterior wall. The ischial spine sign is also associated with acetabular retroversion and is positive if the ischial spine projects medially to the iliopectineal line ( arrows ). The posterior wall sign is positive if the rim of the wall is medial to the center of the head, which indicates insufficient posterior coverage.

  
  
  
  
• 5.
  

  Posterior wall sign. The rim of the posterior wall of the acetabulum should be in line with or lateral to the center of the femoral head on an AP radiograph. The posterior wall sign is positive if the rim is medial to the center of the head, which denotes insufficient posterior coverage and occurs with acetabular retroversion or global acetabular dysplasia (see Fig. 7 ).

  
  
• 6.
  

  Ischial spine sign. This sign strongly correlates with the cross-over sign as another indicator of acetabular retroversion, but it is less affected by pelvic tilt. It is considered positive if the ischial spine projects medial to the iliopectineal line into the pelvis (see Fig. 7 ).

  
  
• 7.
  

  Ilioischial line. The floor of the acetabular fossa, which corresponds radiographically with the acetabular teardrop, and the position of the femoral head should be examined relative to the ilioischial line as additional measures of acetabular depth in patients with a high lateral CE angle or negative roof angle. In coxa profunda, the teardrop touches or is medial to the ilioischial line (see Fig. 3 ). The teardrop can be medial to the ilioischial line in dysplastic hips; therefore, this radiographic sign must be interpreted in context. In protrusio acetabuli, the femoral head is medial to this line (see Fig. 4 ).

False-Profile View

The false-profile view is a lateral image of the acetabulum tangential to the anterior acetabular rim that allows measurement of anterior femoral head coverage. It is sensitive to detect mild dysplasia or early joint space narrowing that may not be evident on the AP view. The image is obtained standing with the affected hip positioned against the cassette and the ipsilateral foot parallel to it. The pelvis is rotated 65° away from the cassette and the x-ray beam is centered on the femoral head with a tube-to-film distance of 90 cm. The anterior center-edge angle is determined by a vertical line through the center of the femoral head and a second line from the center of the head to the edge of the sclerotic weight-bearing zone of the anterior acetabulum. Similar to the lateral CE angle, normal values are 25° to 35°, with less than 20° considered dysplastic, and greater than 40° indicating overcoverage ( Fig. 8 ).

( A ) Anterior CE angle. This measure is obtained on the false-profile view and is the angle between a vertical line through the center of the femoral head and a second line from the center of the head to the sclerotic anterior edge of the acetabular sourcil. In this example, the anterior CE angle is 0°. The dashed line indicates where the sourcil should extend to achieve normal coverage, which corresponds with an angle of 25°. ( B ) The same patient after periacetabular osteotomy with improved anterior coverage as noted by a normal anterior CE angle ( red lines ).

Abduction-Internal Rotation (von Rosen) View

Although initially described for the evaluation of DDH in children, the von Rosen view is a functional radiograph that is useful for preoperative planning, as well as a screening tool to identify patients with incongruence or advanced OA who may fare poorly with joint-preserving procedures. The image is obtained with both legs in maximal abduction and internal rotation, which reflects the joint congruence that can be achieved with rotation of the acetabular fragment from a periacetabular osteotomy ( Fig. 9 ).

( A ) von Rosen view. Taken with the hips in maximal abduction and internal rotation, this image shows the congruence that can be expected between the weight-bearing acetabulum and the femoral head when planning periacetabular osteotomy. ( B ) Postoperative AP view of the same patient.

Lateral Views

Lateral views of the hip are useful for visualizing proximal femoral morphology. The most commonly used lateral radiographs are the cross-table lateral and the frog-leg lateral. The cross-table view is obtained with the patient supine and the uninvolved hip and knee flexed greater than 80°. The x-ray beam is directed parallel to the table at a 45° angle to the symptomatic hip and centered on the femoral head. The affected hip is internally rotated 15° to target the beam tangential to the anterosuperior head-neck junction. The frog-leg lateral view is obtained with the patient supine; the affected hip is abducted 45° with the knee flexed 30° to 40° and the heel resting against the medial side of the contralateral knee. The beam is directed at a point midway between the anterior superior iliac spine and the pubic symphysis with a tube-to-cassette distance of 90 cm.

The α angle is widely used to quantify the femoral head-neck junction. It was originally described from oblique axial magnetic resonance (MR) images but is now commonly measured on lateral radiographs. The angle is formed by the intersection of a line along the axis of the center of the femoral neck passing through the center of the femoral head and a second line from the center of the femoral head to the point on the femoral head-neck junction where the head ceases to be spherical ( Fig. 10 ). In the initial study, the mean α angle was 42° (range 33°–48°) in the control group and 74° (range 55°–95°) in the group with symptomatic impingement. In general, an α angle of 55° or more is considered abnormal when supported by other clinical findings of FAI. In a retrospective comparison, the frog-leg lateral view was found to be more accurate than AP and cross-table lateral images for assessing femoral head-neck offset and α angle. Meyer and colleagues examined 6 radiographic views (not including the frog-leg lateral) and determined that the 45° Dunn-Rippstein lateral (45 degree Dunn) view was the most sensitive for assessing the α angle (see Fig. 10 B). A recent report noted 96% sensitivity to detect a cam lesion with a Dunn view compared with 71% on a cross-table lateral image using radial magnetic resonance imaging (MRI) as the standard. The 45 degree Dunn view is obtained with the patient supine and the affected hip in 45° flexion, 20° abduction, and neutral rotation. The beam direction and distance are identical to the frog-leg lateral radiograph.

( A ) α Angle, frog-leg lateral view. The α angle is formed by a line along the center of the femoral neck passing through the center of the femoral head and a second line from the center of the femoral head to the point where the head-neck junction becomes aspherical. This example is a cam lesion with α angle greater than 55°. The dotted line depicts the threshold for normal, which is less than 50°. ( B ) α Angle, 45° Dunn-Rippstein lateral view. This radiograph view shows the superolateral femoral head-neck region, which is the most common location of diminished offset. In this example of a patient with bilateral cam lesions, the femoral head is posteriorly translated relative to the center of the femoral neck. Thus, the α angle is drawn with a line parallel to the center of the femoral neck that intersects the center of the femoral head. If it were drawn through the center of the femoral neck, it would be anterior to the center of the femoral head, which is frequently the case in patients with prior SCFE.

Tables 1 and 2 summarize normal and pathologic measures on plain radiographs.

Computed Tomography

Computed tomography (CT) scans can be reformatted in multiple planes to provide a more accurate and comprehensive rendering of bony structure compared with two-dimensional radiography. CT has added a great deal to the knowledge of normal and dysplastic acetabular anatomy. Several investigations have delineated acetabular version in asymptomatic populations and consistently show that acetabular anteversion is greater in women than men, a finding that corresponds with measures obtained from osteological collections of human skeletons. Anda and colleagues described the acetabular sector angles, which divide the acetabulum into anterior and posterior sections, to define more accurately global acetabular coverage ( Fig. 11 ). Although anteversion and sector angles are generally measured at the center of the femoral head, anteversion increases from cephalad to caudad in the normal hip and has wide variability in dysplastic hips. In a CT investigation of acetabular retroversion, Perreira and colleagues concluded that retroversion is not caused by isolated posterior insufficiency or anterior overcoverage but reflects torsion of the acetabulum and ischial spine segment. More recent investigations suggest that it is more accurate to define the acetabular opening based on measures of an acetabular rim plane, rather than from a single axial slice.

Axial CT measurement of acetabular anteversion and sector angles. The anterior (AASA) and posterior acetabular sector angles (PASA) are determined by a line connecting the center of the femoral heads and a line to the anterior and posterior acetabular rims, respectively. Acetabular anteversion (AA) is the angle formed between a line perpendicular to the line from the center of the femoral heads and a line contacting the anterior and posterior rim.

Klaue and colleagues described a CT method to determine femoral head coverage to simulate correction and aid surgical planning of both pelvic and femoral osteotomies. Using a three-dimensional (3D) graphics program, their technique creates a topographic map of the acetabulum and femoral head by outlining overlapping contours on sequential axial images. CT-derived surface contour maps to define femoral head coverage have shown that the deficiency in dysplasia is global, rather than isolated to the anterolateral acetabulum as had previously been thought. Janzen and colleagues described a technique to measure femoral head coverage by reformatting vertical planar images from 3D CT to create CE angles in 10° increments circumferentially around the acetabular rim. This method was subsequently used to compare graphically the coverage before and after periacetabular osteotomy. Ito and colleagues reported good correlation between 3D CT and conventional radiographic measures for CE angle and Tönnis roof angles; however, they recommended the detailed evaluation of 3D CT to individualize surgical planning because of the wide variability in the location and degrees of deficiency in patients with acetabular dysplasia.

CT is a powerful tool to assess bony anatomy. Understanding of the variability in normal and dysplastic pelvic morphology is important when planning reorientation procedures of the acetabulum, as well as implant positioning in arthroplasty. However, each of the studies discussed earlier used graphics programs to manipulate CT image data after reformatting, which is time consuming and rarely available in the clinical setting. Moreover, CT scans can be subject to error. As previously discussed, pelvic inclination changes from supine to standing, but current CT imaging can only be performed with a patient supine. Pelvic obliquity and tilt are determined by the patient’s position in the gantry, which can affect acetabular measures. 3D reformatting is required to correct positioning and improve accuracy but is not routinely performed with standard CT imaging. Apart from these concerns, the primary limit to widespread use of CT imaging for acetabular dysplasia is the increasing worry about radiation exposure, particularly in this population of young, female patients in whom radiation-associated cancer risks are highest.

MRI

Similar to CT, MR images can be reformatted in multiple planes to provide a thorough analysis of the 3D anatomy of the hip and pelvis. In addition to comprehensive assessment of bony disorders, MRI provides superior detail of cartilage, labrum, synovium, and soft tissues. Clinically relevant labral tears are infrequent in the absence of a bony anomaly, such as a proximal femoral cam lesion. MRI radial views can simultaneously detect labral disorders and subtle abnormalities of the femoral head-neck junction that may be missed on radiographs ( Fig. 12 ). Acetabular and femoral version can be measured with axial views through the roof of the acetabulum and the femoral neck and condyles, respectively.

MRI radial view. This image shows an area of diminished femoral head-neck offset ( arrow ) in a patient in whom AP and cross-table lateral radiographs were normal.

Because of the spherical shape of the articular surfaces and thin cartilage, MRI in the hip remains complex and requires high resolution and contrast/noise ratio to differentiate bone, cartilage, capsule, and labrum. MR arthrography is the gold standard for identifying labral disorders but has a higher false-positive rate than noncontrast MRI. Techniques using small pixel size and higher field resolution are improving the accuracy to detect labral and cartilage disorders without the need for contrast, but sensitivity to distinguish acetabular chondral delamination is still lacking.

Biochemical MRI

Biochemical MRI reflects the structure and molecular composition of cartilage. These evolving techniques have improved the ability to detect degenerative changes not appreciable on plain MRI. Currently, the primary modalities are T2 mapping, T1rho, and delayed gadolinium-enhanced MRI of cartilage (dGEMRIC).

Normal articular cartilage is composed of 3 layers: a superficial layer with collagen fibers arranged parallel to the cartilage surface, high collagen and water content, and low proteoglycan concentration; a middle zone with obliquely oriented collagen fibers, high proteoglycan content, and lower collagen and water volumes than the superficial layer; and a deep zone with densely packed collagen fibers oriented perpendicular to the articular surface, the highest proteoglycan content, and lowest water concentration.

T2 relaxation times are sensitive to the interaction between collagen and water molecules and can be mapped to show the layered architecture of cartilage. In normal cartilage, T2 mapping reveals a zonal gradation with low signal intensity in the deep layer that increases in the intermediate and superficial layers. T2 signal increases with disruption of this highly organized structure and the increased water content that occurs with cartilage degeneration. Interpretation of T2 results can be complicated by competing tissue factors and structural responses to loading. T2* mapping has shorter acquisition time and the capacity for 3D imaging; however, it is more susceptible to artifacts at the bone-cartilage interface and from foreign body particles.

One of the first events in cartilage degeneration is loss of proteoglycan content, which is not detected on T2 images. Thus, T2 mapping is not sensitive to early stages of OA. T1rho is a technique that is responsive to changes in the collagen structure and water content, as well as proteoglycan concentration. However, similar to T2, because T1rho is influenced by alterations in various components, the competing effects make accurate interpretation of T1rho values difficult and prone to error.

Of the available biochemical MRI modalities, dGEMRIC has been most widely studied in clinical settings, particularly the hip. This method takes advantage of the highly negatively charged glycosaminoglycan (GAG) molecules that comprise the proteoglycan component of cartilage. With loss of proteoglycan in early OA, there is a decrease in charge density that can be measured by a mobile ion probe, such as gadolinium pentetate (Gd-DTPA ^2- ). The technique involves injection of intravenous contrast material and then MRI imaging after a delay of 30 minutes to allow time for contrast penetration into the articular cartilage. This method yields an indirect arthrogram in addition to enhancement of articular cartilage that can be quantitatively measured for GAG content after T1 mapping ( Fig. 13 ). Kim and colleagues found that dGEMRIC measures correlate with pain and the severity of acetabular dysplasia and may be an early indicator of OA in this population. Subsequent work showed that the dGEMRIC index was the most important predictor of early outcome after periacetabular osteotomy compared with standard MRI, radiographs, and clinical measures. This modality is sensitive to early cartilage deterioration and has been used to delineate patterns of articular damage in FAI, Perthes disease, and SCFE. Both hips can be imaged with one intravenous contrast injection, which is beneficial in disease entities that are often bilateral, such as dysplasia and FAI. The primary drawback of dGEMRIC is the risk associated with gadolinium injection, which requires caution in patients with renal impairment.

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