Covering, Support, and Movement of the Body
as those that occur during blocking and tackling in football and
in ice hockey.
When thinking of common knee injuries, remember the
3 Cs: collateral ligaments, cruciate ligaments, and cartilages
(menisci). Most dangerous are
lateral blows
to the extended
knee. Tese forces tear the tibial collateral ligament and the
medial meniscus attached to it, as well as the anterior cruci-
ate ligament
(Figure 8.9)
. It is estimated that 50% of all pro-
fessional football players have serious knee injuries during
their careers.
Although less devastating than the injury just described, in-
juries that affect only the anterior cruciate ligament (ACL) are
becoming more common, particularly as women’s sports be-
come more vigorous and competitive. Most ACL injuries occur
when a runner changes direction quickly, twisting a hyperex-
tended knee. A torn ACL heals poorly, so repair usually requires
a graF taken from either the patellar ligament, the hamstring
tendon, or the calcaneal tendon.
Shoulder (Glenohumeral) Joint
In the shoulder joint, stability has been sacrificed to provide
the most freely moving joint of the body. Te shoulder joint
is a ball-and-socket joint. Te large hemispherical head of the
humerus fits in the small, shallow glenoid cavity of the scapula
(Figure 8.10)
, like a golf ball sitting on a tee. Although the gle-
noid cavity is slightly deepened by a rim of fibrocartilage, the
glenoid labrum
lip), it is only about one-third the
size of the humeral head and contributes little to joint stability
(±igure 8.10d).
Te articular capsule enclosing the joint cavity (from the mar-
gin of the glenoid cavity to the anatomical neck of the humerus) is
remarkably thin and loose, qualities that contribute to this joint’s
freedom of movement. Te few ligaments reinforcing the shoulder
joint are located primarily on its anterior aspect. Te superiorly
coracohumeral ligament
mer-ul) pro-
vides the only strong thickening of the capsule and helps support
the weight of the upper limb (±igure 8.10c). Tree
mer-ul) strengthen the front of the capsule
somewhat but are weak and may even be absent (±igure 8.10c, d).
Muscle tendons that cross the shoulder joint contribute most
to this joint’s stability. Te “superstabilizer” is the tendon of the
long head of the biceps brachii muscle of the arm (±igure 8.10c).
Tis tendon attaches to the superior margin of the glenoid la-
brum, travels through the joint cavity, and then runs within the
intertubercular sulcus of the humerus. It secures the head of the
humerus against the glenoid cavity.
±our other tendons (and the associated muscles) make up
rotator cuff
. Tis cuff encircles the shoulder joint and
blends with the articular capsule. Te muscles include the sub-
scapularis, supraspinatus, infraspinatus, and teres minor. (Te
rotator cuff muscles are illustrated in ±igure 10.15, p. 351.) Te
rotator cuff can be severely stretched when the arm is vigorously
circumducted; this is a common injury of baseball pitchers. As
noted in Chapter 7, shoulder dislocations are fairly common.
Because the shoulder’s reinforcements are weakest anteriorly
and inferiorly, the humerus tends to dislocate in the forward
and downward direction.
is somewhat lax when the knee is flexed, and taut when the knee
is extended.
Te stronger
posterior cruciate ligament
is attached to the
intercondylar area of the tibia and passes anteriorly, me-
dially, and superiorly to attach to the femur on the lateral side of
the medial condyle (±igure 8.8a, b). Tis ligament prevents back-
ward displacement of the tibia or forward sliding of the femur.
Te knee capsule is heavily reinforced by muscle tendons. Most
important are the strong tendons of the quadriceps muscles of the
anterior thigh and the tendon of the semimembranosus muscle
posteriorly (±igure 8.8c and d). Te greater the strength and tone
of these muscles, the less the chance of knee injury.
Te knees have a built-in locking device that provides steady
support for the body in the standing position. As we begin to
stand up, the wheel-shaped femoral condyles roll like ball bear-
ings across the tibial condyles and the flexed leg begins to extend
at the knee. Because the lateral femoral condyle stops rolling be-
fore the medial condyle stops, the femur
(rotates) medially
on the tibia, until the cruciate and collateral ligaments of the knee
are twisted and taut and the menisci are compressed. Te tension
in the ligaments effectively locks the joint into a rigid structure
that cannot be flexed again until it is unlocked. Tis unlocking
is accomplished by the popliteus muscle (see ±igure 8.8d and
²able 10.15, p. 370). It rotates the femur laterally on the tibia,
causing the ligaments to become untwisted and slack.
Homeostatic Imbalance
Of all body joints, the knees are most susceptible to sports in-
juries because of their high reliance on nonarticular factors for
stability and the fact that they carry the body’s weight. Te knee
can absorb a vertical force equal to nearly seven times body
weight. However, it is very vulnerable to
blows, such
Hockey puck
Figure 8.9
A common knee injury.
Anterior view of a knee
being hit by a hockey puck. Such blows to the lateral side tear both
the tibial collateral ligament and the medial meniscus because the
two are attached. The anterior cruciate ligament also tears.
previous page 298 Human Anatomy and Physiology (9th ed ) 2012 read online next page 300 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off