Family Doctor Books
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Published in association with the British Medical
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Family
Doctor Books |
Preview of Understanding Hip & Knee Surgery
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At each end of the long thigh bone (femur)
there is a large weight-bearing joint. At the bottom is the knee and
at the top is the hip. Although both joints are essential for walking,
each looks totally different.
Hips
Each hip is a ball and socket joint. At the top end
of the femur is the rounded femoral head that lies in the spherical hip
socket (acetab-ulum). To enable the joint to move with limited friction,
the surfaces that lie against each other (artic-ulating surfaces) are
covered with gristle (articular cartilage). The gristle is lubricated
by a tiny quantity of a yellowish fluid (syn-ovial fluid), allowing the
joint to move with less friction than even a skate on ice.
The femoral head is connected to the main shaft of
the femur by a firm bridge of bone called the femoral neck. When elderly
people fracture their hip joints, it is actually the femoral neck that
is damaged. At the junction of the femoral neck and the femoral shaft
is a large bony protrusion called the greater trochanter. This is the
hard lump of bone that can be felt on the outside of the hip and to which
most people point when asked to indicate their own hip joint. In fact,
the greater trochanter is not the hip joint at all, but it is connected
to the hip joint by the femoral head and neck.
The femoral head is kept within the acetabulum by strong
lig-aments. If these ligaments are div-ided (for example, at surgery)
or ruptured (for example, in a car accident) then the hip can disloc-ate.
Dislocation can damage the blood supply to the femoral head
by tearing the blood vessels, and the poor blood flow that results can,
in turn, lead to the development of arthritis in later years.
Surrounding the hip joint are three major nerves: the
femoral nerve, the sciatic nerve and the obturator nerve. These nerves
transport nerve impulses back and forth from the hip, groin and lower
limbs to the brain through the spinal cord, so that movement can be controlled
and sensations felt. These nerves are, in turn, surrounded by blood vessels
and major muscles.
The most powerful muscles supporting the hip joint
are the three muscles known as glutei (gluteus maximus, gluteus medius
and gluteus minimus) behind and the rectus femoris and iliopsoas muscles
in front. If the glutei weaken, as happens when arthritis progresses,
then a patient begins to limp.
The hip joint develops in the embryo after only eight
weeks in the womb. Bones do not initially appear in bony form at all,
but start as cartilage (firm, jelly-like sub-stance) and then gradually
turn into bone (hard substance filled with calcium). This process is
known as ossification, and involves the removal of cartilage, which is
re-placed by calcium-rich bone, laid down by special bone-producing cells.
The centres of most bones become ossified in childhood, but the ends
remain undeveloped till puberty to leave room for growth. At some stage
between the ages of 15 and 25 years the hip joint becomes fully developed
and growth in that area ceases.
As people grow older, so their bones can become thinner,
and in certain circumstances smaller. This is known as osteoporosis,
a form of bone weakening that can lead to fractures, particularly of
the hip, wrist and spine. Osteoporosis is different to osteoarthritis,
though the two terms are often confused.
Each hip
is a ball and socket joint; the femoral head (the ball) is held
within the acetabulum (the socket) by strong ligaments. |
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Knees
The knee is a very complex joint, formed by three bones:
the shin bone (tibia), thigh bone (femur) and kneecap (patella). The
fibula is near to the knee joint but does not specifically form part
of it.
The knee is a hinge joint, that is it mostly allows
movement in a single plane, like the hinge of a door, although some rotation
is possible. The lower end of the femur (thigh bone) is rounded to form
the femoral condyles. The upper end of the tibia (shin bone) is flattened
to form the tibial plateau. Logically, it seems a wonder that a joint
of this shape can provide balance at all, but it does do so with the
support of strong ligaments, which connect the lower femur to the upper
tibia.
The ligaments of the knee include the collateral ligaments
at each side and the cruciate ligaments in the centre of the joint. The
cruciates, particularly the anterior cruciate ligament, are the structures
so frequently relied upon, and damaged, by athletes.
On the front of the knee lies the patella (kneecap).
This lies on the front of the lower femur, forming a joint called the
patellofemoral joint. Injuries and diseases of this joint are a frequent
cause of pain. At the top end of the kneecap is attached the quadriceps
muscle (the large bulky muscle in front of the thigh) and at the lower
end is a tendon that inserts into the upper tibia. The patella helps
reinforce the muscles responsible for knee straightening, which happens
when the quad-riceps contracts and pulls on the patella, which pulls
on the tibia or shin. Any activity that involves forcible straightening
of the knee under pressure, for example, run-ning up and down stairs
two at a time, puts a great deal of strain on the patella.
As with the hip joint, much of the joint surface of
the knee is lined with articular cartilage. Synovial fluid is also found
within the joint.
There are many arteries around the knee, which bring
blood from the heart to the structures of the leg. The largest is the
popliteal artery, found directly behind the joint, and is a continuation
of the femoral artery that has passed by the hip higher up.
Each
hip is a ball and socket joint; the femoral head (the ball) is
held within the acetabulum (the socket) by strong ligaments. |
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Two major nerves are also found near the knee joint.
These are the tibial nerve behind and the common peroneal nerve to the
outside. These blood vessels and nerves are important because any trauma
to the knee joint may also disrupt the blood or nerve supply to the leg.
The common peroneal nerve is particularly important because damage to
this results in a drop foot deformity, that is, the patient is unable
to lift the toes or ankle upwards. It can sometimes be seen after fracture,
or occasionally as a complication of surgery.
As with the hip, the knee first appears as a cartilaginous
structure, later ossifying. The cartilaginous femur can first be seen
in the womb eight weeks after conception, the tibia appearing a little
later. The kneecap does not appear in bony form until at least the age
of three years in girls, even later in boys. The knee joint as a whole
ceases growth between the ages of 17 and 20 years.
In early years bones tend to be softer and more pliable
than later
in life. They are harder to fracture and heal faster if they do break.
Sometimes childhood injuries can lead to arthritis in future years. There
are also certain types of arthritis that are specific to children which,
in turn, can lead to major surgery being required at a young age.
There
are many arteries, veins and nerves around the knee, and extreme
care is exercised during surgery to prevent damage. |
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KEY POINTS
- The hip is a ball and socket joint kept within
the acetabulum by strong ligaments
- The hip joint stops growing between the ages of
15 and 25
- The knee is a hinge joint, although some rotation
is possible
- The knee joint stops growing between the ages
of 17 and 20
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