Radiographic Evaluation And Measurements Of The

To identify more recently described findings of FAI, our radiographic hip protocol deviates from common past screening hip radiographs. The AP weight-bearing view of the pelvis and AP view of the hip provides multiple measure ments to assess acetabular coverage and orientation [1,20,21]. Radiographic measurements should be performed after a thorough assessment for subtle fractures or tumors, soft tissue, and intrapelvic anomalies, as well as sacroiliac, pubic symphyseal, and lower lumbar pathologies.

A commonly used measurement to assess for readily apparent acetabular dysplasia is the lateral center edge angle (of Wiberg), which is obtained by drawing a line from the center of the femoral head to the lateral margin of the acetabulum (as its name implies) referenced to a vertical perpendicular line originating from the center of the femoral head (Fig. 6A). Normal values vary, but generally, values less than 20° to 25° are considered abnormal. The anterior lateral edge angle (or false profile view) has also been used, particularly when the center edge angle is abnormal. The horizontal toit externe (THE) angle, also known as acetabular index of the weight-bearing surface, is measured from a line parallel to the weight-bearing surface of acetabulum referenced to a horizontal line (Fig. 6B). Values greater than 10° are considered abnormal. Other measurements such as femoral head extrusion index (with normal values of less than 25%) and acetabular index of depth may also be useful.

However, to evaluate for FAI it is essential to detect more subtle abnormalities of the femoral head-neck junction and acetabulum that are associated with labral tears. Femoroacetabular impingement has been categorized into two basic types [22]. Type 1 is loss of femoral head neck offset, also known as cam-type or pistol-grip deformity, and is best identified on crosstable lateral view (or CT or MRI), but can be appreciated on some AP and frogleg lateral views depending on the severity. On the AP view the femoral head may appear nonspherical [23].

Lateral Center Edge Angle

Fig. 6. AP view of the pelvis in a patient with hip dysplasia shows that the lateral center edge angle is markedly decreased less than 20° (A) and the acetabular index of the weight-bearing surface is increased above 10° (B). Note also substantial lack of coverage of the femoral head, also indicative of a dysplastic hip. (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

Fig. 6. AP view of the pelvis in a patient with hip dysplasia shows that the lateral center edge angle is markedly decreased less than 20° (A) and the acetabular index of the weight-bearing surface is increased above 10° (B). Note also substantial lack of coverage of the femoral head, also indicative of a dysplastic hip. (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

Type 2, or pincer-type impingement, is associated with acetabular retroversion. A combination of cam and pincer types has been described, as well as impingement associated with a deep acetabular socket and morphologic changes of the proximal femur such as coxa vara [22,24,25].

Femoral head-neck offset can be assessed on plain films using the crosstable lateral view with the leg in neutral position (Fig. 7) [26,27]. This image is obtained by placing the film cassette adjacent to the hip of concern with the patient in the supine position while the opposite knee is bent to allow passage of the X-ray beam. Offset is measured by creating a line along the longitudinal axis of the femoral neck (which may or may not intersect the center of the femoral head). Two other parallel lines are placed at the level of the anterior femoral neck cortex and the most anterior margin of the femoral head. Distance or offset between the two most anterior lines has been shown to be less than 7.2 mm (SD 2.6) in abnormal cases and 11.5 mm (SD 2.2) in asymptomatic normal patients. A ratio can also be determined by dividing by the diameter of the femoral head. The cause of cam-type impingement is unclear but thought to result from physeal injury and extension of physeal scar [26]. However, it is possible that repeated abutment of the femoral head-neck junction may cause bony bump formation [27]. Also, the finding could be related to underlying initial soft tissue injuries such as labral tear or capsular instability, causing altered mechanics and bone remodeling, reminiscent of Fairbank's type changes in the knee. One immunohistologic analysis of perilesional capsular tissue suggested progenitor cells were recruited to this region of the bony bump, which would support the latter hypotheses [28]. The cause however, is likely multifactorial.

Pincer-type FAI is associated with acetabular retroversion. Although acetabu-lar dysplasia is often associated with anteversion, several recent studies estimate that acetabular retroversion can be seen in one sixth to one third cases of acetabular dysplasia, a finding that may influence surgical techniques and approaches (ie, adjustment of acetabular realignment procedures) [20,29]. When the anterior rim abnormally crosses over the posterior rim (usually superiorly) on plain film radiographs, this finding has been termed the crossover sign, and represents a marker of acetabular retroversion (Fig. 8) [30]. The posterior acetabular rim should also lie medial to the center of the femoral head as well (posterior wall sign). These findings must be measured on a well-centered AP view of the pelvis with the distance between the sacrococcygeal joint and pubic

Fig. 7. (A) Thirty-two-year-old professional football player with femoroacetabular impingement seen on crosstable lateral view. (B) There is loss of normal femoral head-neck offset, which is confirmed with an MR arthrogram. MRI also shows a complex intrasubstance predominant anterior labral tear (arrow) treated arthroscopically with intrasubstance suture banding. A normal crosstable lateral view shows good offset between lines B and C. Line A is drawn along the femoral shaft, lines B and C are drawn parallel to line A along the anterior femoral neck cortex and anterior femoral head respectively (C). (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

Cortex Femoral

Fig. 8. (A) Acetabular retroversion with positive crossover sign. (B) The same patient with annotations marking crossover sign of the anterior rim (black line) of the acetabulum superiorly over the posterior rim (white line). Note relationship of coccyx with pubic symphysis. This radiograph shows the pelvis is slightly reclined, which can minimize appearance of retroversion. One must account for reclination, inclination, and rotation to properly assess the degree of acetabular retroversion. (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

Fig. 8. (A) Acetabular retroversion with positive crossover sign. (B) The same patient with annotations marking crossover sign of the anterior rim (black line) of the acetabulum superiorly over the posterior rim (white line). Note relationship of coccyx with pubic symphysis. This radiograph shows the pelvis is slightly reclined, which can minimize appearance of retroversion. One must account for reclination, inclination, and rotation to properly assess the degree of acetabular retroversion. (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

symphysis measuring about 3 cm to 5 cm (3.2 cm male, 4.7 cm female) [31]. Others consider the radiograph well centered when the coccyx is about 1 cm from the pubic symphysis [29]. Reclination of the pelvis can underestimate the appearance of retroversion (crossover sign), and inclination can overestimate the finding. One must also account for rotation of the film as well (ie, rotation of the pelvis to the right increases appearance of retroversion on right and decreases on left).

Normal Variants or Pathologic Process?

Periacetabular ossicles are often anecdotally considered normal variants of secondary ossification centers of the acetabulum. However, when we see small superolateral periacetabular ossicles we raise suspicion for underlying labral pathology (Fig. 9). "Os acetabuli" have also been described in dysplastic hips with anterior rim syndrome where the labrum was detached along with an avulsed a piece of the acetabular rim [32].

Radiographic findings of the synovial herniation pit of the hip were first described in 1982, and have a characteristic appearance [33]. Despite past considerations as a normal anatomic variant in about 5% of the population, the original description hypothesized that the finding could be a pathologic abnormality in the setting of a painful hip. Subsequent reports suggesting a pathologic nature as well, described soft tissue impingement, enlargement over time, and increased uptake on bone scan [34,35]. Currently when seen, we report hernia-tion pits as compatible with femoroacetabular impingement (Fig. 9).

Ossicle Hip Ray

Fig. 9. Synovial herniation pit with periacetabular ossicle in the setting of cam type 1 FAI on AP view (A) and frogleg lateral view rather than crosstable lateral view (B). Note loss of sphero-city of the femoral head on both views. (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

Fig. 9. Synovial herniation pit with periacetabular ossicle in the setting of cam type 1 FAI on AP view (A) and frogleg lateral view rather than crosstable lateral view (B). Note loss of sphero-city of the femoral head on both views. (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

A recent publication retrospectively reviewed 117 hips with femoroace-tabular impingement and found fibrocystic changes (ie, synovial herniation pit) on AP radiographs in one third of the cases. Dynamic MR and intraoperative observations of the same patients demonstrated close proximity of the fi-brocystic lesion with area of impingement suggesting a causal relationship [36]. One recent presentation of radiographic analysis of 54 patients with findings suggestive of FAI on frogleg lateral view reported 15% had synovial herniation pits and 30% had periacetabular ossicles [37].

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  • lana
    What is a periacetabular ossicle?
    5 years ago

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