1 Introduction

The most magnificent characteristics of the hip joint are its remarkable range of motion, its high stability, and its nearly frictionless lubrication mechanism. These are unattainable without the morphology, orientation, cartilaginous surface, and arterial perfusion that are inherent in the normal, uninjured hip. However, once traumatic injury has disrupted this joint, the complex anatomy that underlies such an elegant mechanism contributes to the extraordinary difficulty involved in reconstruction. This task is impossible without a thorough appreciation of the normal anatomy [1].

2 Hip development

Early development of the hip begins with formation of the lower limb buds in the 4th week of embryological development, and development is practically complete by the 16th week [2, 3]. The femoral head and acetabulum form from an interlocked structure composed of primitive chondroblasts. The femoral head develops centrally from a dense sphere of chondroblasts and is surrounded by a cartilaginous “anlage” with three disc-shaped masses which form the ilium, ischium, and pubis [2−4]. The region of the future joint space forms via apoptosis of cells between the anlage and the femoral head. By the end of the 8th week, the blood supply of the developing hip is fully established [2]. By the 16th week, the centers of ossification of the ilium, ischium, and pubis emerge, and the triradiate cartilage is formally created [2, 4]. The hip enlarges during the remainder of fetal growth, but does not undergo other morphological changes until infancy.

The acetabulum and labrum develop much of their final morphological features during infancy and childhood [5, 6]. Eventually formed as a coalescence of the three innominate bones of the pelvis, the hip is entirely cartilaginous at birth and remains so during much of the first year. Between the innominate bones lies a cartilaginous T-shaped structure ( Fig 2.1-1 ) [6]. This structure, the triradiate cartilage, is responsible for the formation of the anterior wall, posterior wall, and the dome of the acetabulum. The triradiate cartilage is most responsible for the final depth of the acetabulum [6]. Laterally, the triradiate cartilage gives rise to a circumferential lip composed of hyaline cartilage centrally and fibrocartilage peripherally. This cartilaginous cup is the structure that will form most of the mature acetabulum when growth is complete and is the location where the acetabular epiphyses form, all under the constantly shaping stimulus of the femoral head. The os acetabuli form after 7 years and have completed growth and closed before 9 years [6]. The triradiate cartilage closes at 14−16 years, but the acetabular epiphyses can remain open as late as 18 years [7].

3 Acetabular landmarks

Surrounding the adult acetabulum are several bony landmarks that can be used internally and externally as a guide for the position of the native anatomy. The primary landmark of the hip and acetabulum is the anterior superior iliac spine (ASIS). This landmark lies superior and just lateral to the acetabulum in the coronal plane. It has been called the lighthouse of the hip because of its prominence, and it can be easily palpated, even in obese patients, making it an ideal anterior landmark for the hip. It is used to determine the location of the anterior column, the anterior wall, and the iliac crest. The lateral femoral cutaneous nerve lies about 2 cm medial and inferior on average from the ASIS [8]. The iliopectineal eminence, a residual of the pubic extension of the triradiate cartilage, is an important internal landmark marking both the medial border of the acetabulum in the coronal plane and the iliopectineal bursa ( Fig 2.1-2 , Fig 2.1-3 ) [1, 9].

4 Acetabular structure

The acetabulum is a complex geometric structure that can be conceptualized as being built from essentially six principal components. These components are as follows:

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  Anterior column

  
  
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  Posterior column

  
  
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  Anterior wall

  
  
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  Posterior wall

  
  
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  Acetabular dome or tectum (Latin for roof)

  
  
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  Medial wall

This structure provides coverage to approximately 170° of the femoral head, less than a hemisphere [10].

Supporting the acetabulum are two columns of bone, anterior and posterior columns, which connect the acetabulum to the rest of the pelvis and provide its structural support ( Fig 2.1-4 ) [11]. The acetabulum, which lies in the “concavity of the arch” [11] created by the two columns, transmits load superiorly via this arch. The two columns connect at an angle of about 60° anteroinferior to the sacroiliac joint and just superior to the greater sciatic notch. This junction is formed in an extremely dense area of bone almost never involved in fractures of the acetabulum [11]. Below, in the axilla of this junction, lies a “keystone” of dense cortical iliac bone that supports the dome of the acetabulum [11].

The anterior column is formed from a combination of the ilium and pubic bones. It extends from the posterosuperior ilium down along the pelvic brim to the pubic tubercle. Its width laterally extends to the ASIS. The underside of the anterior arch supports the anterior wall of the acetabulum.

The posterior column is composed of thick, dense bone and forms a wedge shape in the transverse plane. Extending from superior to the greater sciatic notch to the ischial tuberosity, it is through this column that the greater and lesser sciatic notches are stabilized via tension trusses, the sacrospinous, and sacrotuberous ligaments. Anterolaterally, the column supports the posterior half of the acetabular articular surface. Medial to the posterior column is the quadrilateral plate. The posterior wall extends laterally from this structure.

The acetabular socket itself is formed by the anterior wall, the posterior wall, the dome, and the medial wall. The anterior wall is directly connected to the pubis, and the superior pubic ramus extends anteriorly from its medial border. The anterior wall and anterior rim of the acetabulum have a variable morphology. Proceeding medially along the anterior wall, there is an indentation in the anterior wall adjacent to a prominent groove on the pelvic rim. This groove, just lateral to the iliopectineal eminence, provides a track for the iliopsoas. The iliopectineal eminence lies just anterior to the inferior half of the anterior wall on the pubic brim ( Fig 2.1-5 ). Thus, it is an important landmark representing the anterior and medial boundary of the joint.

The posterior wall is larger and projects more laterally than the anterior wall. Its lateral edge has a nearly vertical but slightly curved route. The posterior wall is the most vulnerable portion of the acetabular structure, lying farthest from the support of the arch of the two columns. It is the most commonly and easily fractured, and the most important for stability [12].

The medial wall includes the fossa of the acetabulum laterally and the quadrilateral plate medially. The fossa is a central cavity where no articulation occurs, and it is filled with a fat pad (called the pulvinar) and the ligamentum capitis femoris (or ligamentum teres). These structures receive relatively little attention because their function is unclear. As for the bony fossa and the fat pad, both appear to be involved in evenly distributing forces across the articular contact surfaces [13]. Multiple foramina serve as access for the small arterioles of the acetabular branch of the obturator artery, which runs through the fat pad to both walls and to the dome area [14].

5 Radiographic anatomy

The ability to interpret and recognize acetabular anatomy on plain x-rays of the pelvis is paramount. The following six radiographic landmarks have been defined on the conventional trauma AP pelvis x-ray ( Fig 2.1-6 ) [11]:

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  Iliopubic line (or arcuate line)

  
  
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  Ilioischial line

  
  
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  Teardrop (or roentgenographic U)

  
  
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  Sourcil

  
  
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  Anterior lip

  
  
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  Posterior lip

Disruption of any of these radiographic landmarks suggests a fracture of the underlying structure, so it is important to carefully identify each item when evaluating an AP pelvis x-ray. While the iliopubic line corresponds to the medial cortical border of the anterior column, the ilioischial line corresponds to the medial cortical border of the posterior column. The teardrop outlines an anterior portion of the quadrilateral plate medially, and an anteroinferior portion of the acetabular fossa laterally. This radiographic landmark identifies the position of the medial wall of the acetabulum, and displacement suggests protrusio acetabuli. The sorseal represents the superior most portion of the acetabular dome. Finally, the anterior and posterior lips represent the lateral most cortical border of the anterior and posterior acetabular walls.

When further evaluating the acetabulum using plain x-rays, oblique views (Judet views) can be indispensable in showing the anterior and posterior columns and walls. Oblique views, coupled with the AP view, must be obtained and understood, even with the advent of computed tomography, so that the surgeon may assess them intraoperatively. The internal oblique (or obturator oblique) view brings the anterior column and posterior wall into profile ( Fig 2.1-7 ). This view is obtained by rotating the pelvis 45° away from the side of interest. The external oblique (or iliac oblique) view brings the posterior column and the anterior wall into profile ( Fig 2.1-8 ). This view is obtained by rotating the pelvis 45° toward the side of interest.

6 Hip stability

The posterior wall is the major bony contributor to stability of the hip. Trauma research has shown that hip stability depends principally on an intact posterior wall, and to a lesser extent, an intact capsule. In a cadaver study, 100% of hips were stable with as much as 25% of the posterior wall disrupted, but when 33% of the posterior wall was disrupted only 75% of hips were stable ( Table 2.1-1 ) [12]. When 50% of the posterior wall was disrupted, all specimens were found to be unstable [12]. When the posterior capsule was removed, stability was reduced to 89−14% of hips in the 25−33% posterior wall disruption groups, respectively ( Table 2.1-2 ).