Joint Models

Introduction

Joints are present wherever two bones meet. They allow our skeleton to rotate and twist—without joints, our bodies would not be able to move. Joints differ in structure and the way they allow movement. In this demonstration, students will observe simple wooden models of joints in order to understand the structures of joints and how they perform. 

Concepts

• Skeletal system
• Joints
• Joint structure

Background

Joints differ greatly in both structure and function. One method of classifying joints is by the type of tissue that binds the bones. Based on this methodology, joints are classified as fibrous, cartilaginous and synovial.

Two of the most familiar joints are knees and elbows, which are considered synovial joints. Synovial joints may be further classified into six major types based on their component shapes and the movements they allow. These six categories of joints are ball-and-socket, condyloid, gliding, hinge, pivot and saddle. Ball-and-socket joints are comprised of an egg-shaped bone cavity and a spherically shaped second bone that inserts into this cavity. The ball-and-socket joint allows for radial motion in almost all directions. The hips and shoulders are examples of ball-and-socket joints. Condyloid joints, also referred to as ellipsoid, are similar to ball-and-socket joints. One bone is oval-shaped and fits into an elliptically-shaped cavity. Condyloid joints allow the same type of motion as a ball-and-socket joint, but with a smaller range of motion. The wrist is an example of a condyloid joint. Gliding joints generally have a slight curve. The components of gliding joints slide back and forth against each other. The vertebrae of the spine are examples of gliding joints. Hinge joints are somewhat similar to ball-and-socket joints except they do not allow for radial movement. They allow 180˚ extension and retraction as opposed to the radial motion of the ball-and-socket. The elbow and knee are hinge joints. Pivot joints allow rotation around an axis. A cylindrical shaped bone rotates within a ring shaped bone. The neck has pivot joints. Saddle joints are formed between bones whose surfaces are both convex and concave. They allow movement back and forth and up and down but not a full rotation. The joint between the carpal and metacarpal of the thumb is an example of a saddle joint. Students will gain a more thorough understanding of how joints work by observing the movement of the four synovial joint models in this activity.

The purpose of this demonstration is for students to investigate the features of four of the most recognizable synovial joints using models made of common wooden pieces. The models can be manipulated in order to gain a better understanding of joint anatomy.

Materials

Safety Precautions

Although this activity is considered nonhazardous, please follow all laboratory safety guidelines.

Disposal

Save the joint models for future use.

Prelab Preparation

The joint models will need to be assembled prior to the class display.

Gliding Joint

1. Obtain two 5⁄16" dowel rods and two 3" x 4" wood blocks.
2. Place wood glue on the tip of one dowel rod and insert the rod into the hole cut in the wooden block.
3. Hold the dowel rods in place until the glue has adhered enough to support the rods. Allow the glue to set at least one hour before using the model.
4. Repeat steps 2 and 3 using the second rod and block.

Hinge Joint

1. Obtain the half napkin ring and a ⅛" dowel rod.
2. Place wood glue on the tip of the dowel rod and insert the rod into the hole in the outer side of the napkin ring.
3. Hold the dowel rod in place until the glue has adhered enough to support the rod. Allow the glue to set at least one hour before using the model.
4. Obtain the 1¼" dowel rod with the hole and a 5⁄16" dowel rod.
5. Place wood glue on the tip of the 5⁄16" dowel rod and insert the rod into the hole on the 1¼" dowel rod.
6. Repeat step 3.

Pivot Joint

1. Obtain the napkin ring and 1¼" dowel rod. No further assembly required.

Ball and Socket Joint

1. Obtain the plastic half sphere, ball knob, and ⅛" dowel rod.
2. Place wood glue on the end of the dowel rod and insert into the hole in the ball knob.
3. Allow glue to dry.

Procedure

1. Obtain the assembled components of the gliding joint model.
2. Demonstrate to students how the joint moves.
3. Explain to students where gliding joints are found and their range of motion.
4. Allow students enough time to sketch the gliding joint on the Joint Models Worksheet.
5. Repeat steps 1 through 4 with the remaining three joint models.

Student Worksheet PDF

10849_Student1.pdf

Teacher Tips

• This kit contains four synovial joint models. There are two more synovial joints, the condyloid and the saddle joint. Optional Activity: Ask students if they can think of items in the everyday world that could represent these joints.
• After the joint models have been displayed and explained to the class, pass the models around so the students can manipulate the joints themselves.
• This laboratory activity can reasonably be completed in one 50-minute class period.
• Joints move by muscle contraction and relaxation. The demonstration kit, Levers of the Body, (Flinn Catalog No. FB1631), can be used to explore how muscles work.

Sample Data

(Student data may slightly vary in the location column.)

Answers to Questions

1. What are two examples of ball-and-socket joints found in the human body?

Both the shoulders and hips are classified as ball-and-socket joints.

2. Why do most joints have muscles attached to both sides of the bone to make up a joint?

Muscles expand and contract to pull the joints and allow movement.

3. Having examined the four synovial joint models, list three other common items that could be used to demonstrate the motion of a synovial joints as well as the joint that each item resembles.

Students should give examples such as a metal door hinge, which resembles a hinge joint.

4. How would our range of motion be different if the ball-and-socket joints in our hips were replaced with hinge joints?

Ball-and-socket joints allow for the widest range of motion of all types of synovial joints. They allow for movement in all planes as well as rotation around a central axis. If the hips were connected by a hinge joint instead of a ball-and-socket joint, they would only move in one direction.

5. Gliding joints are found between the vertebrae in the spine. Name at least two ways that gliding joints allow the back to move.

Gliding joints enable the spine to bend forward, backward and sideways. They also allow rotation.

6. Arthritis is a condition that results in inflammation of the joints. (a) Having examined the four types of synovial joint models, describe the effects arthritis might have on a person? (b) Arthrodesis, also known as bone fusion, is a surgery performed on patients with arthritis—the surgery joins two bones where a joint was present. In what ways would this surgery help patients and in what ways would it hinder patients?

(a) Since arthritis typically causes joints to be inflamed, it is very painful. The range of motion is also limited as the swollen joint takes up more space in the cavity; the bones are not able to move as well. (b) Arthrodesis helps patients by eliminating the pain of swollen joints and allows better weight support. A drawback of this surgery is that patients lose joint flexibility.

References

Fong, E. Body Structures and Function; Delmar Publishers Inc; Albany, NY; 1993; Eighth Edition, p. 67.

Hole, J. W. Human Anatomy and Physiology; Wm. C. Brown Publishers; Dubuque, IA; 1990; Fifth Edition, p. 263–267.