(occurs without apparent family history of the
condition), there are no measures of prevention.
See also GENETIC DISORDERS.
meniscectomy A surgical OPERATION to remove
part or all of a damaged meniscus in the knee.
Each knee has two menisci, C-shaped pads of CAR-
TILAGEthat cushion the ends of the femur (thigh
BONE) and tibia (shin bone) as they come together
within the knee JOINT. Meniscus tears are common
ATHLETIC INJURIESand occur when there is torsion
under pressure—the body above the knee twists
suddenly during movement but the foot remains
planted. Most of the time the orthopedic surgeon
can perform a partial meniscectomy with
ARTHROSCOPY (MINIMALLY INVASIVE SURGERY using a
specialized endoscope), which allows rapid recov-
ery and return to regular activities.
Cartilage, which is very dense, does not have its
own BLOOD supply but rather draws necessary
NUTRIENTSfrom surrounding tissues and fluids. Tears
near the outer edge of the meniscus are more likely
to heal than tears in the center of the meniscus. The
surgeon may attempt to repair an outer tear though
will likely need to remove the damaged segments
of meniscus when the tear is interior. The goal is to
remove as little of the meniscus as possible because
without it the bone ends loose protection. It is
equally important to remove any pieces of the
meniscus that are torn or fragmented to prevent
them from “jamming” the joint.
The risks of meniscectomy include excessive
bleeding during surgery and postoperative INFEC-
TION, both of which are rare. Full recovery after
arthroscopic surgery takes about six weeks and
after OPEN SURGERYmay take up to six months.
Even after HEALING, complete meniscectomy may
limit some athletic activities that place significant
stress on the knee, such as downhill skiing.
See also KNEE INJURIES; PHYSICAL THERAPY; SURGERY
BENEFIT AND RISK ASSESSMENT.
muscle Contractile fibers or the structures these
fibers form. Muscles move the body, some under
voluntary control and others reflexively. The gas-
trointestinal tract, genitourinary tract, and BLOOD
vessels contain smooth (nonstriated) muscle,
which is under involuntary control of the auto-
nomic NERVOUS SYSTEM. The HEARTcontains a spe-
cialized form of muscle called myocardial, also
under control of the autonomic nervous system.
The bulk of the muscle tissue in the body is skele-
tal (striated) muscle, which responds to voluntary
control through the CENTRAL NERVOUS SYSTEM.
The skeletal muscles are responsible for move-
ment and account for about 40 percent of the
body’s mass. They generally appear in opposing
pairs attached to BONEvia tendons. When one
muscle contracts, its opposing muscle relaxes. This
allows smooth, balanced movement. The SKELETON
provides resistance and leverage for the muscles as
they contract and relax. The body contains about
650 muscles, the largest of which is the gluteus
maximus (main muscle of the buttocks) and the
smallest of which is the stapedius in the middle
EAR(moves the stapes bone).
Movement requires interaction between neu-
rons (NERVEcells) and muscle fibers. This interac-
tion occurs at the neuromuscular junction, a
synapse where the NEURON’s axons end (terminate)
and the muscle fiber begins. When conveying a
nerve impulse to activate a skeletal muscle fiber,
the motor neuron releases a molecule of acetyl-
choline, a NEUROTRANSMITTER. The acetylcholine
molecule binds with an acetylcholine receptor on
the muscle fiber, forming a biochemical bridge
that allows the nerve impulse to travel from the
neuron to the muscle fiber. The impulse creates an
action potential in the muscle fiber—a cycle of
activation, discharge, and recovery—that becomes
a muscle contraction.
Skeletal muscles contain two types of fiber that
dictate how rapidly and with what intensity they
complete an action potential. Type 1 fibers, also
called slow-twitch or red fibers (red because they
have a high myoglobin content), have a slow and
steady response. Type 1 fibers are in a constant
state of partial contraction; they provide muscle
tone and are essential for maintaining the body’s
posture. Type 2 fibers, also called fast-twitch or
white fibers (white because they contain very lit-
tle myoglobin), have a rapid response. Type 2
fibers are responsible for muscle STRENGTH. Most
skeletal muscles contain a combination of type 1
and type 2 fibers. Exercise to extend ENDURANCE
increases the percentage of type 1 fibers; exercise
to improve strength increases the percentage of
type 2 fibers.336 The Musculoskeletal System