Corpus: Hip joint
1. Definition
2. Anatomy
2.1. Overview
The hip joint is composed of the hip socket (acetabulum) and the head of the femur. The acetabulum is formed by parts of the ilium, pubis, and ischium, which are joined in a Y-shaped connection at the acetabular region. The acetabular rim is reinforced by a fibrocartilage lip called the acetabular labrum.
The femoral head, a roughly spherical structure at the top of the femur, fits into the acetabulum. Since the acetabulum surrounds the femoral head beyond its equator, this connection is classified as an enarthrodial joint, a variant of a ball-and-socket joint (spheroidal joint).
2.2. Joint capsule
The joint capsule originates at the bony rim of the acetabulum and the transverse acetabular ligament. It attaches anteriorly along the intertrochanteric line and the base of the greater trochanter. Posteriorly, the lower part of the femoral neck remains free of the capsule, which inserts about 1.5 cm closer to the femoral head than on the anterior side. The greater and lesser trochanters lie entirely outside the capsule.
When the hip is in slight flexion, external rotation, and abduction, the joint capsule is at its most relaxed. This is the typical joint position assumed in cases of hip joint inflammation (coxitis).
2.3. Ligaments
The ligament of the femoral head lies between the acetabulum and the femoral head. It contains a small artery that supplies blood to the femoral head. The ligament emerges from the pelvis at the acetabular notch and enters a foramen at the apex of the femoral head. The acetabular notch is bridged by the transverse acetabular ligament.
The hip joint is encased in a tight capsule that is further stabilized by three strong ligaments:
Deep fibers from all three ligaments merge into the zona orbicularis, a circular band surrounding the narrowest part of the femoral neck. The zona orbicularis helps prevent dislocation of the femoral head.
3. Function
The hip joint allows movement in three degrees of freedom, but the range of motion is limited by its bones, cartilage, and ligaments. Because of its structure, the hip joint is classified as an enarthrodial joint.
The hip's range of motion, measured using the neutral-zero method, is as follows:
- Extension and flexion: 15/0/140 degrees (flexion is limited by the thigh muscles and the iliofemoral ligament)
- External and internal rotation: 40/0/25 degrees
- Abduction and adduction: 45/0/35 degrees
4. Angle
Certain radiological angles are used to assess the hip joint's biomechanics:
- CCD angle
- CD angle
- Antetorsion angle
The CCD angle changes with age:
- Child: 140°
- Adult: 120–130°
- Seniors: 115°
The antetorsion angle measures the angle in the frontal plane between the femoral neck and the condylar axis. A normal antetorsion angle is approximately 12°. Deviations are classified as follows:
- Coxa antetorta: increased angle
- Coxa retrotorta: decreased angle
5. Pathophysiology
Repetitive stress on the cartilage covering the joint surfaces can lead to coxarthrosis over time. This condition is increasingly common due to factors such as sedentary lifestyles, obesity, and aging. Coxarthrosis represents a major burden in orthopedic medicine, both in terms of treatment and healthcare costs.
6. Diagnostics
Changes in the hip joint are diagnosed using imaging techniques. Depending on the clinical presentation, the following methods may be used:
- Conventional X-rays
- CT (computed tomography)
- MRI (magnetic resonance imaging)
7. Literature
- Anderhuber et al, Waldeyer - Anatomie des Menschen: Lehrbuch und Atlas in einem Band (19th updated edition), De Gruyter, 2012