Osteology and Arthrology
The hip joint or coxafemoral joint (CFJ) consists of the articulation between the convex head of the femur and the concave surface of the acetabulum pelvis. It is a diarthrodial, ball-in-socket joint with three degrees of freedom.
Two thirds of the femoral head is covered in hyaline cartilage. The center of the head lacks cartilage (termed the fovea centralis) where the insertion of the teres ligament and it's neurovascular structures exists. The cartilage tends to be thickest in a broad region above and anterior to the fovea (Kurrat 1978).
The acetabulum faces anteriorly, inferiorly, and laterally. The hyaline cartilage surface within the acetabulum is half mooned, or lunate shaped. The cartilage is thickest along the superior-anterior region of its dome, which corresponds to the regions of highest joint pressures when walking (Dalstra 1995). The acetabular notch lies inferior to this cartilage and is bridged by the transverse acetabular ligament. Fat is found within the floor, or inferior fossa, of the acetabulum, where the ligamentum teres emerges. The acetabular labrum is a ring of fibrocartilage that surrounds the circumfrence of the acetabulum. The labrum increases congruency, serves as an attachment for the capsule, and provides proprioceptive input via sensory organs. These sensory organs include Ruffini endings, Pacidnian corpuscles, Golgi-like organs, Krausse endings, and free nerve endings. It possesses a vascularity similar to that of the knee meniscus, whereby the outer margin is relatively vascularized and the inner margin is lacking blood supply.
The angle of inclination is the angle formed between the axis of the femoral head and neck and the axis of the femoral shaft in the frontal plane. It measures roughly 150 degrees in infants and decreases to 125 degrees in adults. The angle is slightly smaller in women due to their increased pelvic width. Coxa valga is a pathological increase in the angle of inclination (>150 degrees). Coxa vara is a pathological decrease (<120 degrees).
The angle of torsion is the angle between the axis of the femoral condyles and the axis of the femoral head and neck in the transverse plane. The plane of the head and neck is anterior to the plane of the condyles. It is approximately 40 degrees in infants and 12-15 degrees in adults. An increase in the angle of torsion is termed anteversion, while a decrease is called retroversion. The angle can be assessed clinically by using Craig's test.
The Center-edge angle (or angle of Wiberg) describes the extent to which the acetabulum covers the femoral head within the frontal plane. It varies, but on average measures 35-40 degrees. A smaller angle, or more vertical alignment offers less containment of the femoral head.
The Acetabular Anteversion Angle describes the extent to which the acetabulum surrounds the femoral head within the horizontal plane. Normal anteversion is about 20 degrees, which allows some exposure of the anterior femoral head, but it is covered by the anterior ligaments and iliopsoas tendon.
There are gender differences in the anatomy of the hip. First, in women, the acetabula are shallower. The female also possesses a wider pelvis with a greater pubic arch angle.
Capsule and Ligaments
The capsule of the hip joint has four different fiber orientations: longitudinal, oblique, arcuate, and circular. The capsule attaches medially to the acetabular rim and laterally to the femoral neck along the trochanteric line. A synovial membrane lines the internal surface of the hip capsule.
There are two intra-articular ligaments in the hip. The teres ligament courses from the fovea centralis of the femoral head to an insertion on the acetabular rim in the floor of the fenestra under the acetabular labrum (to the transverse ligament). It is a tubular sheath of synovial-lined connective tissue. This ligament provides minimal aid in maintaining the head of the femur within the acetabulum. A small branch of the obturator artery is located within this ligament, but this provides very little blood flow to head of the femur in adults. The transverse ligament bridges the acetabular fossa making a complete ring around the periphery along with the labrum.
The iliofemoral ligament (Y-lagament of Bigelow) is very thick and resembles an inverted Y. It posesses two branches. The pars inferioris, which constrains hip extension, courses from the iliac outer wall of the acetabulum to the insertion on the intertrachanteric line on the anterior proximal femur. The pars superioris, which contrains hip extension, adduction, and external rotation, begins in the same region as the other arm and courses inferolaterally to the intertrachanteric line just anterior to the greater trochanter.
The pubofemeral ligament courses from the pubic outer wall of the acetabulum to the same insertion as the pars interarticularis of the iliofemoral ligament, which also constrains hip extension, abduction, and external rotation.
The ischiofemoral ligament attaches from the psoterior and inferior aspects of the acetabulum, primarily form the adjacent ischium. These fibers spiral superiorly and laterally to attach near the apex of the greater trochanter.
- Iliocapsularis: originates on the anteromedial hip capsule and the inferior border of the anterior inferior iliac spine and inserts distal to the lesser trochanter.
- This muscle may tighten the anterior hip capsule to increase stability of the femoral head. The muscle is a landmark during hip surgery in order to expose the anteromedial hip capsule and the psoas tendon interval.
The femoral nerve sends fibers to the anterior capsule. The posterior joint capsule receives innervation from all roots of the sacral plexus (Inman et al 1944)
Primary supply to the femoral head are the medial and lateral femoral circumflex arteries, which are branches of the profunda femoris artery.
The extracapsular arterial ring is formed posteriorly by a large branch of the medial femoral circumflex artery and anteriorly by the lateral circumflex circumflex femoral artery. This extracapsular ring supplies most of the head and neck of the femur. These arteries surround and ascend along it, forming rings around the upper neck.
Ascending cervical branches, formed by the lateral circumflex artery, travel into the joint capsule and run along the neck of the femur, deep to the synovial lining of the neck.
Artery of the ligamentum teres contributes very little, if any significant blood supply to the femoral head.
There are more than a dozen bursae present in the hip area. The three most clinically relevant are the iliopectineal, the trochanteric, and the ischiogluteal. The iliopectineal bursa reduces the friction between the iliopsoas muscle and the iliopectineal eminence. The trochanteric bursa reduces the friction between the gluteus medius, gluteus minimus, piriformis and the greater trochanter. The ischiogluteal bursa reduces the friction between the hamstring tendons and the ischial tuberosity.