The Hip

Contents of this page:

• Introduction
• Hip capsule
• Biomechanics
• Clinical points

Introduction

The hip joint is the most mobile joint in the lower limb and one of the most mobile in the body. It is capable of: flexion and extension, abduction and adduction, medial and lateral rotation and all of these in a circular motion - circumduction

The hip joint is a ball and socket synovial joint

The ball is the head of the femur. It consists of 2/3 of a sphere and is covered in life by hyaline articular cartilage - except for a small area - the fovea, where the ligament of the head of the femur is attached

The socket is the acetabulum of the hip bone. In life it contains an upper area shaped like an inverted horseshoe (the lunate surface) which is covered in hyaline cartilage and an inferior area which is filled with fat (the acetabular fossa).

The socket is made deeper still by a fibrocartilaginous rim - the labrum (meaning 'lip'). This prevents the head of the femur leaving the acetabulum and enhances the already excellent bony fit. The labrum bridges across the inferior deficiency of the acetabulum (acetabular notch), from this bridge arises the ligament of the head of the femur (ligamentum teres) which carries blood vessels to the head.

The articular cartilage is resricted to the top and sides of the acetabulum because gravity determines that this is where weight will be transferred from the hip bone to the head of the femur when standing upright.

Hip capsule

• proximally to the bone surrounding the margin of the acetabulum.
• distally to the trochanteric line.

• to the margins of the acetabulum and surrounding bone
• to the neck of the femur (it does not reach down as far as the trochanteric crest).

The capsule has specialised thickenings which give added stability. In particular, the iliofemoral ligament which is attached proximally to the anterior inferior iliac spine and distally to the 2 ends of the trochanteric line.

Minor thickenings include pubofemoral and ischiofemoral ligament. The deepest fibres of the latter form a circular layer - the zona articularis.

Biomechanics

There is a general point here: all the major joints (hip, knee, ankle) become close-packed at full extension and this coincides with the limb becoming a rigid, vertical, weight-bearing pillar. This is clearly the essential prerequisite for standing upright on two legs i.e. the adoption of bipedal stance.

When standing erect the centre of gravity passes behind the hip joint. This should result in hyperextension i.e. the trunk falling backwards at the hip. This is prevented by a more slouching stance, in which the centre of gravity is bought forwards, and by the iliofemoral ligament (one of the strongest ligaments in the body) which resists hyperextension.

Clinical points

The hip joint is one of the most stable synovial joints in the body. It has all three features which confer stability on a joint.

• good bony fit
• strong ligaments
• a strong surrounding muscle mass

Congenital dislocations are occasionally encountered but in traumatic injury, dislocation is most commonly a posterior fracture dislocation. For example in a car crash, where the knee impacts and the femur is forced backwards, knocking off the posterior rim of the acetabulum. An x-ray of a posterior dislocation is shown.

In a posterior dislocation the sciatic nerve is at risk as it lies just behind the capsule of the hip joint.

Very occasionally injury to the side of the hip (e.g. in a fall) results in an internal dislocation in which the head of the femur is driven through the floor of the acetabulum. The obturator nerve (amongst other structures) may be at risk.

Comments to: