Synovial joints

Learning objectives

• Examine four of our major synovial joints of the body.

• Appreciate the structure of each of these synovial joints, both diagrammatically and physiologically.

• Learn about the directions of movement.• Understand examples of sporting activities

where these joints are used.

Introduction

• Most joints are freely moveable.• They allow different movement, depending on the shape

of the bones at the joint.• The shoulder, hip and ankle are susceptible to injury due

to the amount of movement possible at them

Ball-and-socket joint• Most moveable joint in the

body.• One bone has a bulge like a

a ball at the end.• This fits into a socket in the

other bone.• It can turn in many directions.

Examples• The hip joint• The shoulder joint

task

1. Why do you think it is difficult to dislocate a ball-and socket joint?

2. In what joint do you think a cricket bowler would be want to be very flexible?

The hinge joint• This works like a hinge on

a door.• The bone can swing

backwards and forward.• The end of one bone is

shaped like a cotton reel.• It fits into the hollow of

the other.• The joint will open until it

is straight and no further.

Examples• The elbow joint• The knee joint

The pivot joint

• One bone has a bit that juts out, like a peg or ridge.

• This fits into a ring or notch on the other bone.

• The joint only allows rotation Examples

• The joint between the atlas and axis.

• The joint between the radius and ulna.

The gliding joint

• Here the end of the bones are flat enough to glide over.

• There is little movement in all directions.

• Of all synovial joints this gives the least movement.

Examples• Joints between the carpals

(hand) and tarsals (foot).• Joints between most of the

vertebrae.

Cartilage

• Cartilage stops bones knocking together.• It is a gristly cushion between the bones at slightly

moveable joints.• It forms a smooth slippery coat on the ends of bones at

synovial joints

Ligaments

• Ligaments are strong cords and straps that lash bones together and hold joints in place.

• They are a bit elastic-enough to let the bones move.

Tendons

• Tendons are the cords and straps that join muscle to bone.

• The best known joins our calf muscle to our heel and is called the Achilles tendon.

• Achilles, the Heel

• Achilles was the son of Thetis and Peleus, the bravest hero in the Trojan war, according to Greek mythology.

• When Achilles was born, his mother, Thetis, tried to make him immortal by

dipping him in the river Styx. As she immersed him, she held him by one heel and forgot to dip him a second time so the heel she held could get wet too. Therefore, the place where she held him remained untouched by the magic water of the Styx and that part stayed mortal or vulnerable.

• To this day, any weak point is called an “Achilles’ heel”. We also refer to the strong tendon that connects the muscles of the calf of the leg with the heel bone as the “Achilles’ tendon”.

• Although the above rendition of the Achilles’ story is in current vogue, Michael Macrone, in his It’s Greek to Me, tells us that Achilles didn’t always have a vulnerable heel. Oh yes, he had a weak spot, but according to the original story about Achilles, Homer, in the Iliad, said it was his pride. Later versions indicate his weakness was his love for the Trojan princess Polyxena. In his Metamorphoses, Ovid suggested that Achilles

had a vulnerable spot on his body; but the Roman poet, Statius (c. A.D. 45-96), was the first to imply in a poem that it was his heel.

Joints and movement: a summary

TYPE OF JOINT MOVEMENT ALLOWED

Ball-and-socket Flexion and extensionAbduction and adduction

Hinge Flexion and extension

Pivot Only rotation

gliding A little movement in all directions (no bending or circular motions)

Key revision points

• You need to know the four different synovial joints (ball-and-socket, hinge, pivot and gliding joint).

• Understand the differences of each and the movements that occur at those joints.

• Again, understand the role of cartilage, ligaments and tendons.