Anatomy

The hip is a diarthrodial joint and is an articulation between the head of the femur and the acetabulum. The acetabulum is formed by the union of the ilium, ischium, and the pubis. The tri-radiate cartilage usually fuses by 15 to 16 years of age, and is oriented approximately 45° caudally and has 15° of anteversion. A variety of normal radiographic indices have been described to differentiate normal from abnormal bony anatomy and play an important role in understanding why some patients develop instability. The Tonnis angle is determined by marking a horizontal line along the inferior aspect of the ischial tuberosities (line 1), and another line parallel to the first line but through the center of the femoral head (line 2). Finally, a third line (line 3) is drawn from the medial and lateral aspects of the weight-bearing portion of the superior ace-tabulum (the "Sourcil"). Where this intersects with line 2 is the Tonnis angle, which normally measures <10° [2,3] (Fig. 1). Increased Tonnis angles are associated with lateral subluxation of the femoral head in the acetabulum and increased forces directed across the weight-bearing zone of the socket.

The center-edge angle of Wiberg is determined by again drawing a horizontal line along the inferior aspect of the ischial tuberosities (line 1) and then a line parallel to line 1 that passes through the center of the femoral head (line 2). Another line is drawn perpendicular to the second line and passes through the center of the femoral head (line 3). A fourth line (line 4) is then drawn from the center of the femoral head to the lateral aspect of the acetabulum and is normally >25° with 20° to 25° considered borderline [3,4] (Fig. 2).

The acetabular version can also be estimated based on an anteroposterior (AP) radiograph of the pelvis. The posterior rim is identified by extending a line from the ischial tuberosity superiorly and laterally along the posterior wall to the

Fig. 1. An AP radiograph demonstrating the method for measuring the Tonnis angle of the hip. A normal Tonnis angle is <10°. Increased Tonnis angles are associated with lateral subluxation of the hip and increased contact pressures of the femoral head on the anterosuperior weight-bearing zone of the acetabulum.

Fig. 1. An AP radiograph demonstrating the method for measuring the Tonnis angle of the hip. A normal Tonnis angle is <10°. Increased Tonnis angles are associated with lateral subluxation of the hip and increased contact pressures of the femoral head on the anterosuperior weight-bearing zone of the acetabulum.

Fig. 2. An AP radiograph demonstrating the method for measuring the center-edge angle of Wiberg. The center-edge angle is normally >25°, with 20° to 25° considered borderline (From Delaunay S, Dussault RG, Kaplan PA, et al. Radiographic measurements of dysplastic adult hips. Skeletal Radiol 1997;26(2):75-81.)

Fig. 2. An AP radiograph demonstrating the method for measuring the center-edge angle of Wiberg. The center-edge angle is normally >25°, with 20° to 25° considered borderline (From Delaunay S, Dussault RG, Kaplan PA, et al. Radiographic measurements of dysplastic adult hips. Skeletal Radiol 1997;26(2):75-81.)

roof of the acetabulum. A second line is then drawn along the anterior acetabular rim by extending a line from the acetabular teardrop superolaterally along the margin of the rim to the roof. If the lines do not cross, the acetabulum is anteverted with normal values ranging between 15° to 20°. A "crossover" sign is present if the lines cross, which represents a retroverted acetabulum [2,4,5] (Fig. 3). The degree of retroversion can be estimated by the height of the crossover, with lower crosses suggestive of increased retroversion. Although important in understanding the complete bony anatomy of the hip joint, femoral anteversion

Fig. 3. An AP radiograph of the pelvis demonstrates the crossover sign indicative of a retroverted acetabulum. In a retroverted acetabulum, the anterior acetabular rims (solid lines) crosses over the posterior acetabular rim (dashed lines) on the AP radiograph of the pelvis.

is difficult to determine on standard plain radiographs. For complete evaluation of femoral anteversion, either a CT scan or MRI of the hip joint with a spot view of the distal femoral epicondyles is necessary for an accurate calculation.

The hip joint is an intrinsically stable joint. Its deep acetabulum allows the hip to withstand joint reactive forces that may be in excess of five times body weight during athletic activities [6]. The femoral head normally forms two-thirds of a sphere and is flattened in the area where the acetabulum applies the greatest load. In the neutral, anatomic position, the anterior part of the femoral head is not engaged in the acetabulum, and the labrum augments the femoral head coverage by its extension from the bony acetabulum [7]. In other situations, there is natural variation in the acetabular depth and femoral head geometry. The soft tissue anatomy of the hip consists primarily of the capsuloligamentous structures, ligamentum teres, labrum, transverse acetabular ligament, pulvinar, and the articular surfaces of the femoral head and acetabulum. Inclination and version of the weight-bearing surface may affect the joint capsule and ligaments of the hip, the labrum, the ligamentum teres, as well as the suction effect of the hip [7]. In cases where there is deficiency of the bony acetabulum (dysplasia) there is more reliance on these surrounding soft tissue structures. More specifically, McKibbin has observed an association between femoral and acetabular version that leads to increased stress to the anterior capsulolabral structures. He defined the McKibbin index as the sum of the angles of femoral and acetabular anteversion with a total of >60 denoting severe instability [2,8].

The fibrous hip capsule has three discrete thickenings which form the main capsular ligaments: the iliofemoral (Y-Ligament of Bigelow), the pubofemoral, and the ischiofemoral (Fig. 4). The Y-ligament of Bigelow is the strongest of the three ligaments and prevents anterior translation ofthe hip during extension and external rotation. The terminal fibers of this ligament form a deep circular orientation surrounding the femoral neck in a leash-like fashion and are termed the zona orbicularis. These fibers tighten during extension but unwind or loosen during hip flexion which leads to a "screw home" effect in full extension.

Labral tissue, unlike capsular tissue, is made predominantly of fibrocartilage. The labrum runs circumferentially around the acetabular perimeter and becomes attached to the transverse acetabular ligament posteriorly and anteriorly. The labrum plays a role to help contain the femoral head in extremes of range of motion, especially flexion. The labrum is also involved with limiting fluid expression from the joint space, which has an important sealing function. The absence of the labrum causes increases contact pressure of the femoral head against the acetabulum [9-11]. The labrum appears to enhance joint stability and preserve joint congruity; thus, there is a significant concern about the potential for rotational instability or hypermobility in patients with a labral deficient hip [1]. This instability may lead to redundant capsular tissue, which can create a potential abnormal load distribution due to a transient incongruous joint resulting from subtle subluxation.

The ligamentum teres runs from the fovea capitus, a small depressed bare spot located at the medial aspect of the femoral head, and inserts adjacent to the

Anterior Posterior

Fig. 4. Anatomical constraints of the hip. The anterior ligamentous constraints of the hip our seen in the anterior view and include the iliofemoral and pubofemoral ligaments. The ischio-femoral ligament is the primary posterior restraint. (From Kelly BT, Williams 3rd RJ, Philippon MJ. Hip arthroscopy: current indications, treatment options, and management issues. Am J Sports Med 2003 Nov-Dec;316:1020-37, with permission. © 2003 American Orthopaedic Society for Sports Medicine.)

Anterior Posterior

Fig. 4. Anatomical constraints of the hip. The anterior ligamentous constraints of the hip our seen in the anterior view and include the iliofemoral and pubofemoral ligaments. The ischio-femoral ligament is the primary posterior restraint. (From Kelly BT, Williams 3rd RJ, Philippon MJ. Hip arthroscopy: current indications, treatment options, and management issues. Am J Sports Med 2003 Nov-Dec;316:1020-37, with permission. © 2003 American Orthopaedic Society for Sports Medicine.)

transverse acetabular ligament in the acetabular fossa. In the presence of a deficient labrum or a dysplastic hip, it may have a secondary stabilizing effect on the hip joint [12]. It is routinely observed clinically that tension on the ligamentum teres occurs as the hip is brought into external rotation (Fig. 5A and B). The transverse acetabular ligament runs from the base of the anterior and posterior labrum and acts as a conduit to the obturator foramen.

Fig. 5. (A, B) Dynamic hip arthroscopy demonstrates significant tightening of the ligamentum teres during external rotation (B) compared with internal rotation of the hip (A). These findings support the biomechanical role of the ligamentum teres in the stabilization of the hip.

The psoas major muscle originates from the vertebral bodies of T12 through L5 and the transverse processes of L1 through L5 and crosses anterior to the hip capsule as it inserts onto the lesser trochanter. As it crosses the anterior-medial aspect of the hip joint, it helps protect the anterior intermediate portion of the capsule. The tendon may be subjected to increased load during athletic activities, which can be further exacerbated in athletes with associated intraarticular pathology [13]. The psoas tendon may also become shortened and inflamed in patients with underlying instability as it attempts to provide dynamic stabilizing effects to the anterior aspect of the hip joint in the presence of static ligament deficiency. The coexistence of hip instability and secondary internal coxa saltans is not unusual to encounter [14].

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