Chapter 8
Joints
255
8
Factors Influencing the Stability
of Synovial Joints
Because joints are constantly stretched and compressed, they
must be stabilized so that they do not dislocate (come out of
alignment). Te stability of a synovial joint depends chiefly on
three factors: the shapes of the articular surfaces; the number
and positioning of ligaments; and muscle tone.
Articular Surfaces
Te shapes of articular surfaces determine what movements are
possible at a joint, but surprisingly, articular surfaces play only
a minor role in joint stability. Many joints have shallow sock-
ets or noncomplementary articulating surfaces (“misfits”) that
actually hinder joint stability. But when articular surfaces are
large and fit snugly together, or when the socket is deep, stabil-
ity is vastly improved. Te ball and deep socket of the hip joint
provide the best example of a joint made extremely stable by the
shape of its articular surfaces.
Ligaments
Te capsules and ligaments of synovial joints unite the bones
and prevent excessive or undesirable motion. As a rule, the
more ligaments a joint has, the stronger it is. However, when
other stabilizing factors are inadequate, undue tension is placed
on the ligaments and they stretch. Stretched ligaments stay
stretched, like taffy, and a ligament can stretch only about 6%
of its length before it snaps. Tus, when ligaments are the major
means of bracing a joint, the joint is not very stable.
Muscle Tone
For most joints, the muscle tendons that cross the joint are the
most important stabilizing factor. Tese tendons are kept taut at
all times by the tone of their muscles. (
Muscle tone
is defined as
low levels of contractile activity in relaxed muscles that keep the
muscles healthy and ready to react to stimulation.) Muscle tone
is extremely important in reinforcing the shoulder and knee
joints and the arches of the foot.
Sacroiliac
Pubic symphysis
Hip (coxal)
Knee
(tibiofemoral)
Ankle
Intertarsal
Inferior
tibiofibular
Tarsometatarsal
Metatarso-
phalangeal
Interpha-
langeal (toe)
Knee
(femoropatellar)
Superior
tibiofibular
ILLUSTRATION
JOINT
ARTICULATING BONES
STRUCTURAL TYPE*
FUNCTIONAL TYPE; MOVEMENTS ALLOWED
 
Sacrum and coxal
bone
Synovial; plane in
childhood, increasingly
fibrous in adult
Diarthrotic in child; amphiarthrotic in adult;
(more movement during pregnancy)
 
Pubic bones
Cartilaginous;
symphysis
Amphiarthrotic; slight movement (enhanced
during pregnancy)
 
Hip bone and femur
Synovial; ball-and-
socket
Diarthrotic; multiaxial; flexion, extension,
abduction, adduction, rotation,
circumduction of thigh
 
Femur and tibia
Synovial; modified
hinge
(contains
articular discs)
Diarthrotic; biaxial; flexion, extension of leg,
some rotation allowed in flexed position
 
Femur and patella
Synovial; plane
Diarthrotic; gliding of patella
 
Tibia and fibula
(proximally)
Synovial; plane
Diarthrotic; gliding of fibula
 
Tibia and fibula
(distally)
Fibrous; syndesmosis
Synarthrotic; slight “give” during dorsiflexion
 
Tibia and fibula with
talus
Synovial; hinge
Diarthrotic; uniaxial; dorsiflexion, and plantar
flexion of foot
 
Adjacent tarsals
Synovial; plane
Diarthrotic; gliding; inversion and eversion
of foot
 
Tarsal(s) and
metatarsal(s)
Synovial; plane
Diarthrotic; gliding of metatarsals
 
Metatarsal and
proximal phalanx
Synovial; condylar
Diarthrotic; biaxial; flexion, extension,
abduction, adduction, circumduction
of great toe
 
Adjacent phalanges
Synovial; hinge
Diarthrotic; uniaxial; flexion; extension of
toes
*
Fibrous joints
indicated by orange circles;
cartilaginous joints
by blue circles;
synovial joints
by purple circles.
These modified hinge joints are structurally bicondylar.
Table 8.2
(continued)
previous page 289 Human Anatomy and Physiology (9th ed ) 2012 read online next page 291 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off