1 Anatomy of the Hip and Surgical Approaches
Basic Anatomy of the Hip and Pelvis
I. The femur bone is a near-cylindrical long bone with high cortical bone density, separated from each other by the breadth of the pelvis. The anterior femoral bow is angled medially for relative valgus alignment. There is a gender difference in pelvic breadth, with it being wider in females.
II. It is divided into the body and two extremities, upper and lower ( Fig. 1.1 ).
The upper extremity consists of the head, neck, and greater and lesser trochanter:
It is globular, convex, and hemispheric. It is superiorly, medially, and slightly anterior directed.
It has a smooth, articular surface covered in hyaline cartilage normally.
The fovea capitis is a small, concave, depression within the medial side of the head, directed superior to posteroinferiorly. It is deficient in articular cartilage and provides attachment for the ligamentum teres: the acetabular branch of the obturator artery runs within and disruption results in avascular necrosis of the head.
The neck is relatively flat and pyramidal. It connects the head to the shaft of the femur.
It is flattened anterior to posterior, constricted in the middle, and broadens from medial to lateral.
The anterior surface is perforated by numerous vascular foramina. Along the junction of the anterior surface with the head is a shallow groove. It is prominent in the elderly and provides attachment to orbicular fibers of the hip joint capsule.
The inferior border, long and narrow, curves slightly posteriorly and ends at the lesser trochanter.
The angle of inclination is formed by intersection of a line drawn along the shaft of the femur and a line drawn down the neck of the femur. It is widest in infancy and decreases with age. The normal angle is between 120 degrees and 125 degrees. It shows height and gender variability: less in shorter individuals and right angle in females than in males. Angle greater than 125 degrees results in coxa valga, and decreased angle results in coxa vara.
The trochanter is divided into lesser and greater trochanters; they are connected by the intertrochanteric line.
The lesser trochanter is a conical eminence. From its apex, three borders extend: medial, lateral, and inferior. The inferior border is continuous with the middle division of the linea aspera. The apex provides attachment to the psoas major muscle.
The greater trochanter is a large, irregular, quadrilateral eminence. It is located at the junction of the neck with the upper part of the body. It has two surfaces (medial and lateral surfaces) and four borders (superior, inferior, anterior, and posterior borders).
The distal end of the femur is cuboid and has a greater transverse diameter than the anteroposterior diameter. It is prominent on both sides as the medial and lateral condyles, separated by the intercondylar fossa.
Inferior to the lateral condyle is an oblique groove that provides attachment to the popliteus muscles.
The body of the femur is cylindrical, broader superiorly, flattens, and narrows downward. It is convex anteriorly and concave posteriorly. The prominent longitudinal ridge, linea aspera, lies posteriorly.
It has three borders (posterior, lateral, and medial) and three surfaces, separated by the borders.
The linea aspera is a crest on the posterior aspect of the femur. It is composed of the medial and lateral lips, and an intermediate line. The lateral ridge extends upward from the lateral lip to the base of the greater trochanter, forming the gluteal tuberosity and providing attachment to the gluteus maximus. The intermediate ridge extends upward as the pectineal line toward the base of the lesser trochanter, providing attachment to the pectineus muscle.
The lateral border runs from the greater trochanter to the anterior extremity of the lateral condyle. The medial border runs from the intertrochanteric line to the anterior extremity of the medial condyle.
The anterior surface is situated between the lateral and medial borders. it is smooth, convex, broader superiorly and inferiorly with a narrow center. It provides attachment for the vastus intermedius.
The lateral surface is the portion between the lateral border and the linea aspera. The superior three-fourths provide attachment for the vastus intermedius.
The medial surface includes the area between the medial border and the linea aspera; it provides attachment for the vastus medialis.
Pelvis and Acetabulum
Fig. 1.2 shows a three-dimensional reconstruction of the hip.
I. The pelvis is formed by the bones of the ilium, ischium, and pubis. It is a large and flat bone.
II. Ossification is from three primary centers for the ilium, ischium, and pubis.
III. The primary centers fuse by age 13 to 14 at a Y-shaped triradiate cartilage at the center of the acetabulum. 1
IV. The right and left hemipelvis articulate with each other anteriorly at the pubic symphysis and posteriorly at the sacral ala to form the sacroiliac joint.
V. The pelvis offers the primary connection between the axial skeleton and the bones of the lower limb, forming a bridge for structures passing from the axial skeleton to the lower limb.
VI. Bones of the hip offer stability and attachment for soft tissues.
I. The ilium is the widest and the largest of the three parts of the hemipelvis.
II. It is divided into the ala and the body, which are separated by the arcuate line anteriorly and the acetabular margin externally.
The ilium expands superiorly to form the ala:
The concave inner surface of the ala forms the iliac fossa, giving attachment to the iliacus.
The convex external surface forms the gluteal fossa from which originates the gluteal muscles.
The superior margin thickens to form the iliac crest. The crest projects forward and backward, forming the anterior and posterior iliac spines.
The superior margin provides an inferior attachment for the abdominal wall muscles.
The anterior and posterior iliac spines further subdivide into the superior and inferior spines.
The anterosuperior iliac spine (ASIS) is an important landmark; it provides attachment for the inguinal ligament and the sartorius muscle.
The anteroinferior iliac spine (AIIS) projects outward from the wing of the ilium. The superior portion of the AIIS provides attachment to the direct head of the rectus femoris. Avulsion fracture may occur at this site of attachment. The inferior portion provides attachment to the iliofemoral ligament of the hip joint, slightly superior to the acetabular rim. In some individuals, the AIIS may project distally to impinge on the femoral neck during motion. Subspine impingement may limit hip motion and cause labral injuries. 2
The body of the ilium forms a part of the acetabulum and provides attachment to the obturator internus.
I. It is inferior to the ileum and posterior to the pubis. The superior portion forms one-third of the acetabulum.
II. Parts: superior, inferior rami, and the body:
It extends inferiorly and posteriorly from the body.
Its three surfaces are the posterior, inner, and external surfaces.
It extends anteriorly to form the posteroinferior margin of the obturator foramen.
It expands posteriorly to form the ischial tuberosity.
It is flat and thin and ascends from the superior ramus to join the pubis anteriorly.
It has two surfaces, inner and external, and two borders, medial and lateral.
Body of the ischium:
It contributes to the formation of the acetabulum.
It has two surfaces, external and inner surfaces, and two borders, posterior and anterior.
I. The anterior aspect of the hip and pelvis comprises three parts: the body and the inferior and superior pubic rami.
The body forms a part of the acetabulum; it projects anteromedially toward midline to connect with the opposite body of the pubis at the pubic symphysis.
The superior and inferior rami form a part of the obturator foramen.
I. It is formed by the fusion of the three hip bones marked by the triradiate cartilage.
II. The acetabular rim surrounds the fossa and is limited inferiorly by the acetabular notch.
III. The fossa provides attachment for the ligamentum teres.
IV. The acetabular notch is converted into a foramen by the transverse acetabular ligament.
V. The acetabular labrum is attached to the rim. The labrum deepens the acetabular surface for articulation with the femoral head.
VI. The standard clock-face reference provides reliable surgical landmark of the intra-articular hip structures. 3 Irrespective of laterality, the 3 o’clock position always marks the anterior aspect, the 9 o’clock position the posterior aspect, the 12 o’clock position the superior aspect, and the 6 o’clock position the inferior aspect ( Fig. 1.3 ).
I. Hip joint:
Osseous structures: The ball-and-socket synovial joint is formed by the head of the femur and the acetabulum.
The articular surfaces are lined by the hyaline cartilage.
The acetabulum is deepened by the acetabular labrum and articulates with the head of the femur.
The hip joint connects the trunk and pelvis to the bones of the lower extremity.
Capsule: It is attached to the acetabulum superiorly and the neck of the femur inferiorly, and blends anteriorly with the iliofemoral ligament. 4 It is composed of two sets of fibers: circular fibers (that invest the femoral neck) and longitudinal fibers.
The hip joint ligament is divided into the intracapsular and extracapsular ligaments ( Fig. 1.4 ).
The extracapsular ligaments are divided into the “Y”-shaped iliofemoral (anterior), ischiofemoral (posterior), and pubofemoral (inferior) ligaments.
The ligamentum teres forms the intracapsular ligament.
Lateral center-edge angle:
The angle between a vertical line and a line from the center of the femoral head to the most lateral bony part of the acetabulum. 5
Normal: 25 to 40 degrees; less than 20 degrees indicate developmental dysplasia of the hip (DDH). 6
It is used in femoroacetabular impingement syndrome (FAIS) diagnosis.
It evaluates the acetabular lateral coverage.
Anterior center-edge angle:
On false profile, the angle formed by intersection of a vertical line through the center of the femoral head and a line extending through the center of the femoral head to the anterior sourcil. 7
It is obtained from the false profile view (allows assessment to the degree of femoral head anterior coverage), measures anterior dysplasia.
Normal: 25 to 40 degrees; less than 20 degrees indicates DDH. 8
It evaluates the acetabular anterior coverage.
Transverse (acetabular) angle: It is the angle between a line drawn from the superior to the inferior acetabular rim and the horizontal plane.
Each limb is measured individually.
On axial computed tomography (CT), find the slice that best reveals the femoral neck and the condylar alignments.
Measure the condyle-horizontal angle (CH) and the neck horizontal angle (NH) ( Fig. 1.5 ).
Calculate the angle of the neck relative to the condyles (NC = NH – CH).
In internal rotation, the CH is added to the NH. In external rotation, the CH is subtracted from the NH angle.
Tonnis angle: on anteroposterior (AP) plain radiograph. The angle is formed between a horizontal line and a line extending from the medial to lateral edges of the sourcil. Normal: less than 10 degrees. 6 , 9
On axial CT, each limb is measured individually. Find the slice that best reveals the deepest floor of the acetabulum.
The angle is measured between a line drawn tangent to the anterior and posterior walls of the acetabulum and a true sagittal line.
Acetabular angle (of Sharp):
It measures the acetabular inclination on the AP plain radiograph.
The angle is formed between a horizontal line and a line from the teardrop to lateral acetabulum. Normal: 33 to 38 degrees. 6
Femoral neck angle: It is the widest in infancy and reduces to average of 125 degrees in the adult. It varies among individuals and gender.
Coxa vara: reduced angle (<120 degrees).
Coxa valga: increased angle (>135 degrees).
Movements: Varying ranges of motion—flexion, extension, abduction, adduction, medial, and lateral rotations and circumduction.
Blood supply: Medial and lateral circumflex arteries; branches off the deep artery of the thigh. Sometimes it may directly branch off the femoral artery. The foveal artery lies within the ligamentum teres, branches off the posterior division of the obturator artery, and supplies the femoral head. Disruption of blood supply from the foveal artery may lead to avascular necrosis.
II. Sacroiliac joint:
It is formed by the sacrum (triangular bone formed from fused lower vertebrae) and the ilium on either side of the posterior midline.
Movement about the joint is planar. Reinforced by joint capsule and numerous ligaments: anterior, posterior, and interosseous sacroiliac ligaments, and sacrotuberous and sacrospinous ligaments. 10
It connects the axial skeleton to the pelvis and transmits upper body weight to the lower extremities.
III. Sacrococcygeal symphysis:
It is formed between the coccyx and the sacrum. The interosseous ligament connecting both structures is analogous to the intervertebral disk. 11
It is reinforced by the anterior, posterior and lateral sacrococcygeal ligaments.
Movements are limited to flexion and extension. 11
IV. Pubic symphysis:
It is an amphiarthrodial joint formed in the midline by the left and right pubic bones. Between the articulation is the fibrocartilaginous disk that is strengthened by the superior and inferior pubic ligaments. 12
It allows limited movement.