The Hip Joint

The mechanical connection between the legs and themove them and the body in some cases quickly and
mass of the trunk occurs at the hip joints with theirwith force. The hip abductors, adductors and gluteal
large socket and ball. The hip socket or acetabulum ismuscles are all major stabilisers and movers of the
a deep rounded structure with a rim of cartilagejoint. The abductors play a large part in the sideways
which closely holds the head of the femur, the largestability of the pelvis and the gluteals, the body's
ball structure at the top of the thigh bone. As befitsmost powerful muscles, move the body weight
a joint where large forces are applied, the hip isaround.
stable, deep and has strong stabilising features. ToThe mechanical forces which are transmitted across
permit fluid movement under considerable bodily loadsthe hip joint when we are doing things such as going
the hip surfaces are covered with articular cartilageup stairs, jumping or running are much higher than
which confers almost friction-free motion under thesejust our body weight. The areas subject to the
loads. This is essential to the hip and the thickestgreatest stresses are consequently endowed with
cartilage is where the greatest forces are applied.much thicker cartilage to cope with these loads. The
The socket, head and neck of the hip joint arefeeding of the cartilage is provided by the synovial
completely enveloped by the hip capsule, afluid and through the underlying bone but tissue
ligamentous bag which is well strengthened by theturnover of cartilage is slow, new cells forming at the
large and powerful ligaments of the hip. Lined by thebase as the upper areas suffer stresses and slough
synovial membrane which secretes nutritional andoff.
lubricating synovial fluid, the capsule runs from theThe synovial membrane lines the capsule of the hip
margins of the socket, down over the head andand performs the secretion of small amounts of
neck to its base. The thigh bone travels upwardssynovial fluid. This fluid is not present in large amounts
towards the body but the hip socket is at the sidebut is thought to lubricate the movements of the
of pelvis and faces outwards and down, requiring thejoint, help particles of wear be absorbed to prevent
femur to make a 125 degree turn inwards at the topa grinding paste being developed and spread
to allow the hip joint to form. This facilitates themechanical loads throughout the joint. If the joint is
appropriate mechanical structure.under great load, the synovial membrane can respond
The bone's internal structure in the upper femurto the cyclical stresses by increasing its secretion
reflects the need of the hip to bear weight and torate to protect the joint and lubricate it during
move the weight of the body. Denser areas of boneperiods of high activity.
struts or trabeculae have developed which reinforceAny disturbance in the available movements of the
the bone at the points of greatest mechanical need.hip will have consequences on its most important
These strengthened areas function well in ensuringfunction, gait. An even stride length and balanced gait
we manage vigorous activities but this has left muchpattern is essential for normal daily function and to
weaker areas which show themselves, particularly inthe health of the hip joints themselves. We have a
elderly people, by the frequency of hip fractures. Thistypical and restricted pattern of joint movement
is a vital matter as many fractures in elderly peoplewhen we walk but the movement which often
are not survived well by this group.becomes limited first is extension, the ability to take
The hip is designed to perform locomotion of thethe leg behind as the other leg strides forward. At
body and to stand and manage the body weightaround 20 degrees there is much less hip extension
through the legs. To perform these actions the hipthat hip flexion, the ability to take the thigh towards
joints have very strong muscles to stabilise them andthe chest, which is about 130 degrees.