Transcript Joint Mechanics

Joint function is determined primarily
by the shape and contour of the contact
surfaces & the constraints of the
surrounding soft tissue(s).
•There are 3 classifications of joints:
•FIBROUS
•CARTILAGINOUS
•SYNOVIAL
FIBROUS joints are immoveable. e.g. cranial sutures.
CARTILAGINOUS joints are slightly moveable. The bones are
separated by cartilage and, at times, by ligaments e.g. intervetebral
discs, pubic symphisis, sacroiliac joint
SYNOVIAL joints are freely moveable. The joints consist of 2
bones in a cavity surrounded by a membrane that lubricates it with
synovial fluid.
-In synovial joints, the bones do NOT connect; there is articular
cartilage at the ends to reduce friction as the bones move
- Sometimes the joint cavity is divided by cartilaginous discs. e.g.
meniscus in the knee.
- Some synovial joints contain fluid-filled sacs called bursa between
points of contact; these act as shock absorbers.
Stability of the joint
refers to its ability to
resist unwanted action.
Ex: In a hinge joint it
resists abduction,
adduction, rotation, and
dislocation while
allowing range of motion
in flexion & extension.
The planes of motion are
determined in a joint by the
skeletal articulations and
the tensile forces of the
ligaments, the tendons &
muscles and the articulating
capsule alsprovid some
support.
PIVOT joints have
a ring of bone
around a peg
e.g. C1 of the
vertebral column
rotates around the
‘dens’ of C2
HINGE joints
allow movement
in only one plane:
flexion and
extension
e.g.: the ankle,
interphalangeal &
elbow joints
BALL & SOCKET
joints are best seen in
the hip and shoulder
joint where movements
go through all planes,
including flexion,
extension, abduction,
adduction & rotation.
e.g. Hip and shoulder
ELLIPSOIDAL is a
reduced ball and
socket in which
significant rotation is
largely excluded
e.g. radiocarpal
(wrist) joints &
temporomandibular
SADDLE has 2
concave articulating
surfaces permitting all
motions except
rotation
e.g. carpo-metacarpal
joint at the base of the
thumb
GLIDING joints have
flat articulating
surfaces
e.g. the facet joints of
the vertebrae, the
acromio-clavicular,
intercarpal or
intertarsal joints
The shoulder, or glenohumeral
joint, is an intricate and unstable
joint.This instability gives it the
huge range of motion needed for
arm movements.
The synovial ball & socket joint
is made up of 2 bones – the
scapula & humerus, and
indirectly, the clavicle.
The humeral head is held into the
glenoid fossa with ligaments &
the long head tendon of the
biceps brachii.
Skeletal muscles employ simple machines, such as levers, to
increase efficiency of their contractile work about a joint.
Mechanically, the degree of muscular effort needed to overcome
resistance to movement at a joint (the fulcrum) depends on the
force of that resistance (weight); the relative distances from the
anatomical fulcrum to the anatomical sites of muscular effort
(insertion); and the anatomical sites of resistance (joints)
The position of the joint relative to the site of muscle pull
(insertion) and the site of imposed load determines the class of
lever system in use.
In a 1st class lever, the joint lies
between the muscle and the load.
The 1st class lever is the most
efficient – it operates like a teeter
totter.
An example is flexing and extending the neck, the fulcrum is at
the C1 and C2 joints, load is the weight of the head, and the
effort is the muscular effort at the insertion on the occipital
bone.
In a 2nd class lever, the load lies
between the joint and the effort
(or pulling muscle).
The 2nd class lever operates like a
wheelbarrow.
An example is standing on your toes with the fulcrum on the
metatarsal heads at the metatarsophalangeal joints; load is the
weight of the body, and the effort is the muscular effort at the
insertion on the calcaneus.
In a 3rd class lever, the muscle
(effort) lies between the joint and
the load.
The 3rd class lever is less efficient
for strength – it is built more for
speed at the end of the lever. The
most common lever in the human
body is the 3rd class lever.
An example is the biceps curl. Because the
muscle has a poor mechanical advantage in this
type of lever, it would be impossible to lift a bag
of cement or your body weight with flexed arms.