Classification of Joints Summary Questions

JOINTS

 

 

Without joints, body movements would be impossible.

(credit: Graham Richardson/flickr.com)

 

Chapter Objectives

After this chapter, you will be able to:

• Discuss both functional and structural classifications for body joints
• Describe the characteristic features for fibrous, cartilaginous, and synovial joints and give examples of each
• Define and identify the different body movements
• Discuss the structure of specific body joints and the movements allowed by each
• Explain the development of body joints

 

The adult human body has 206 bones, and with the exception of the hyoid bone in the neck, each bone is connected to at least one other bone.

 

Joints are the location where bones come together.

• Many joints allow for movement between the bones.
  • At these joints, the articulating surfaces of the adjacent bones can move smoothly against each other.
• However, the bones of other joints may be joined to each other by connective tissue or cartilage.
  • These joints are designed for stability and provide for little or no movement.

 

Importantly, joint stability and movement are related to each other.

• This means that stable joints allow for little or no mobility between the adjacent bones.
• Conversely, joints that provide the most movement between bones are the least stable.
• Understanding the relationship between joint structure and function will help to explain why particular types of joints are found in certain areas of the body.

 

The articulating surfaces of bones at stable types of joints, with little or no mobility, are strongly united to each other. 

• For example, most of the joints of the skull are held together by fibrous connective tissue and do not allow for movement between the adjacent bones.
  • This lack of mobility is important, because the skull bones serve to protect the brain.

 

Similarly, other joints united by fibrous connective tissue allow for very little movement, which provides stability and weight-bearing support for the body.

• For example, the tibia and fibula of the leg are tightly united to give stability to the body when standing.
• At other joints, the bones are held together by cartilage, which permits limited movements between the bones.
  • Thus, the joints of the vertebral column only allow for small movements between adjacent vertebrae, but when added together, these movements provide the flexibility that allows your body to twist, or bend to the front, back, or side.

 

In contrast, at joints that allow for wide ranges of motion, the articulating surfaces of the bones are not directly united to each other.

• Instead, these surfaces are enclosed within a space filled with lubricating fluid, which allows the bones to move smoothly against each other.
  • These joints provide greater mobility, but since the bones are free to move in relation to each other, the joint is less stable.
• Most of the joints between the bones of the appendicular skeleton are this freely moveable type of joint.
  • These joints allow the muscles of the body to pull on a bone and thereby produce movement of that body region.
  • Your ability to kick a soccer ball, pick up a fork, and dance the tango depend on mobility at these types of joints.

 

 

9.1 | Classification of Joints

By the end of this section, you will be able to:

• Distinguish between the functional and structural classifications for joints
• Describe the three functional types of joints and give an example of each
• List the three types of diarthrodial joints

 

A joint, also called an articulation, is any place where adjacent bones or bone and cartilage come together (articulate with each other) to form a connection.

• Joints are classified both structurally and functionally.
  • Structural classifications of joints take into account whether the adjacent bones are strongly anchored to each other by fibrous connective tissue or cartilage, or whether the adjacent bones articulate with each other within a fluid-filled space called a joint cavity.
  • Functional classifications describe the degree of movement available between the bones, ranging from immobile, to slightly mobile, to freely moveable joints.
• The amount of movement available at a particular joint of the body is related to the functional requirements for that joint.
  • Thus immobile or slightly moveable joints serve to protect internal organs, give stability to the body, and allow for limited body movement.
  • In contrast, freely moveable joints allow for much more extensive movements of the body and limbs.

 

Structural Classification of Joints

The structural classification of joints is based on whether the articulating surfaces of the adjacent bones are directly connected by fibrous connective tissue or cartilage, or whether the articulating surfaces contact each other within a fluid filled joint cavity.

• These differences serve to divide the joints of the body into three structural classifications.
  • A fibrous joint is where the adjacent bones are united by fibrous connective tissue.
  • At a cartilaginous joint, the bones are joined by hyaline cartilage or fibrocartilage.
  • At a synovial joint, the articulating surfaces of the bones are not directly connected, but instead come into contact with each other within a joint cavity that is filled with a lubricating fluid.
    • Synovial joints allow for free movement between the bones and are the most common joints of the body.

 

Functional Classification of Joints

The functional classification of joints is determined by the amount of mobility found between the adjacent bones.

• Joints are thus functionally classified as a synarthrosis or immobile joint, an amphiarthrosis or slightly moveable joint, or as a diarthrosis, which is a freely moveable joint (arthroun = “to fasten by a joint”).
• Depending on their location, fibrous joints may be functionally classified as a synarthrosis (immobile joint) or an amphiarthrosis (slightly mobile joint).
• Cartilaginous joints are also functionally classified as either a synarthrosis or an amphiarthrosis joint.
• All synovial joints are functionally classified as a diarthrosis joint.

 

Synarthrosis

An immobile or nearly immobile joint is called a synarthrosis.

• The immobile nature of these joints provide for a strong union between the articulating bones. This is important at locations where the bones provide protection for internal organs.
  • Examples include sutures, the fibrous joints between the bones of the skull that surround and protect the brain, and the manubriosternal joint, the cartilaginous joint that unites the manubrium and body of the sternum for protection of the heart.

 

Suture Joints of Skull The suture joints of the skull are an example of a synarthrosis, an immobile or essentially immobile joint.

 

 

Amphiarthrosis

An amphiarthrosis is a joint that has limited mobility.

• An example of this type of joint is the cartilaginous joint that unites the bodies of adjacent vertebrae.
  • Filling the gap between the vertebrae is a thick pad of fibrocartilage called an intervertebral disc.
    • Each intervertebral disc strongly unites the vertebrae but still allows for a limited amount of movement between them. However, the small movements available between adjacent vertebrae can sum together along the length of the vertebral column to provide for large ranges of body movements.
  • Another example of an amphiarthrosis is the pubic symphysis of the pelvis.
    • This is a cartilaginous joint in which the pubic regions of the right and left hip bones are strongly anchored to each other by fibrocartilage.
      • This joint normally has very little mobility. The strength of the pubic symphysis is important in conferring weight-bearing stability to the pelvis.

 

 

Intervertebral Disc An intervertebral disc unites the bodies of adjacent vertebrae within the vertebral column. Each disc allows for limited movement between the vertebrae and thus functionally forms an amphiarthrosis type of joint. Intervertebral discs are made of fibrocartilage and thereby  structurally form a symphysis type of cartilaginous joint.

 

 

Diarthrosis

A freely mobile joint is classified as a diarthrosis.

• These types of joints include all synovial joints of the body, which provide the majority of body movements.
• Most diarthrotic joints are found in the appendicular skeleton and thus give the limbs a wide range of motion.
• These joints are divided into three categories, based on the number of axes of motion provided by each.
  • An axis in anatomy is described as the movements in reference to the three anatomical planes: transverse, frontal, and sagittal.
  • Thus, diarthroses are classified as uniaxial (for movement in one plane), biaxial (for movement in two planes), or multiaxial joints (for movement in all three anatomical planes).

 

Diarthrosis joints can then be classified in degrees of motion: 

1. A uniaxial joint only allows for a motion in a single plane (around a single axis).
   • The elbow joint, which only allows for bending or straightening, is an example of a uniaxial joint.
2. A biaxial joint allows for motions within two planes.
   • An example of a biaxial joint is a metacarpophalangeal joint (knuckle joint) of the hand.
     • The joint allows for movement along one axis to produce bending or straightening of the finger, and movement along a second axis, which allows for spreading of the fingers away from each other and bringing them together.
3. A joint that allows for the several directions of movement is called a multiaxial joint (polyaxial or triaxial joint).

• • This type of diarthrotic joint allows for movement along three axes.
    • The shoulder and hip joints are multiaxial joints.
    • They allow the upper or lower limb to move in an anterior-posterior direction and a medial-lateral direction. In addition, the limb can also be rotated around its long axis. This third movement results in rotation of the limb so that its anterior surface is moved either toward or away from the midline of the body.

 

 

Multiaxial Joint A multiaxial joint, such as the hip joint, allows for three types of movement: anterior-posterior, medial-lateral, and rotational.

 

 

9.2 | Fibrous Joints

By the end of this section, you will be able to:

• Describe the structural features of fibrous joints
• Distinguish between a suture, syndesmosis, and gomphosis
• Give an example of each type of fibrous joint

 

At a fibrous joint, the adjacent bones are directly connected to each other by fibrous connective tissue, and thus the bones do not have a joint cavity between them.

• The gap between the bones may be narrow or wide.
• There are three types of fibrous joints.
  • A suture is the narrow fibrous joint found between most bones of the skull.
  • At a syndesmosis joint, the bones are more widely separated but are held together by a narrow band of fibrous connective tissue called a ligament or a wide sheet of connective tissue called an interosseous membrane.
    • This type of fibrous joint is found between the shaft regions of the long bones in the forearm and in the leg.
  • Lastly, a gomphosis is the narrow fibrous joint between the roots of a tooth and the bony socket in the jaw into which the tooth fits.

 

Fibrous Joints Fibrous joints form strong connections between bones. (a) Sutures join most bones of the skull. (b) An interosseous membrane forms a syndesmosis between the radius and ulna bones of the forearm. (c) A gomphosis is a specialized fibrous joint that anchors a tooth to its socket in the jaw.

 

Suture

All the bones of the skull, except for the mandible, are joined to each other by a fibrous joint called a suture.

• The fibrous connective tissue found at a suture (“to bind or sew”) strongly unites the adjacent skull bones and thus helps to protect the brain and form the face.
  • In adults, the skull bones are closely opposed and fibrous connective tissue fills the narrow gap between the bones.
• The suture is frequently convoluted, forming a tight union that prevents most movement between the bones.
• Thus, skull sutures are functionally classified as a synarthrosis, although some sutures may allow for slight movements between the cranial bones.

 

In newborns and infants, the areas of connective tissue between the bones are much wider, especially in those areas on the top and sides of the skull that will become the sagittal, coronal, squamous, and lambdoid sutures.

• These broad areas of connective tissue are called fontanelles.
• During birth, the fontanelles provide flexibility to the skull, allowing the bones to push closer together or to overlap slightly, thus aiding movement of the infant’s head through the birth canal.
• After birth, these expanded regions of connective tissue allow for rapid growth of the skull and enlargement of the brain.
• The fontanelles greatly decrease in width during the first year after birth as the skull bones enlarge.
• When the connective tissue between the adjacent bones is reduced to a narrow layer, these fibrous joints are now called sutures.
• At some sutures, the connective tissue will ossify and be converted into bone, causing the adjacent bones to fuse to each other.
• This fusion between bones is called a synostosis (“joined by bone”).
  • Examples of synostosis fusions between cranial bones are found both early and late in life.
  • At the time of birth, the frontal and maxillary bones consist of right and left halves joined together by sutures, which disappear by the eighth year as the halves fuse together to form a single bone.
  • Late in life, the sagittal, coronal, and lambdoid sutures of the skull will begin to ossify and fuse, causing the suture line to gradually disappear. 

 

 

The Newborn Skull The fontanelles of a newborn’s skull are broad areas of fibrous connective tissue that form fibrous joints between the bones of the skull.

 

 

Syndesmosis

A syndesmosis (“fastened with a band”) is a type of fibrous joint in which two parallel bones are united to each other by fibrous connective tissue.

• The gap between the bones may be narrow, with the bones joined by ligaments, or the gap may be wide and filled in by a broad sheet of connective tissue called an interosseous membrane.
  • In the forearm, the wide gap between the shaft portions of the radius and ulna bones are strongly united by an interosseous membrane.
  • Similarly, in the leg, the shafts of the tibia and fibula are also united by an interosseous membrane.
    • In addition, at the distal tibiofibular joint, the articulating surfaces of the bones lack cartilage and the narrow gap between the bones is anchored by fibrous connective tissue and ligaments on both the anterior and posterior aspects of the joint.
    • Together, the interosseous membrane and these ligaments form the tibiofibular syndesmosis.
  • The syndesmoses found in the forearm and leg serve to unite parallel bones and prevent their separation.
    • However, a syndesmosis does not prevent all movement between the bones, and thus this type of fibrous joint is functionally classified as an amphiarthrosis.
  • In the leg, the syndesmosis between the tibia and fibula strongly unites the bones, allows for little movement, and firmly locks the talus bone in place between the tibia and fibula at the ankle joint.
    • This provides strength and stability to the leg and ankle, which are important during weight bearing.
  • In the forearm, the interosseous membrane is flexible enough to allow for rotation of the radius bone during forearm movements.
    • Thus in contrast to the stability provided by the tibiofibular syndesmosis, the flexibility of the antebrachial interosseous membrane allows for the much greater mobility of the forearm.

 

The interosseous membranes of the leg and forearm also provide areas for muscle attachment.

• Damage to a syndesmotic joint, which usually results from a fracture of the bone with an accompanying tear of the interosseous membrane, will produce pain, loss of stability of the bones, and may damage the muscles attached to the interosseous membrane.
• If the fracture site is not properly immobilized with a cast or splint, contractile activity by these muscles can cause improper alignment of the broken bones during healing.

 

Gomphosis

A gomphosis (“fastened with bolts”) is the specialized fibrous joint that anchors the root of a tooth into its bony socket within the maxillary bone (upper jaw) or mandible bone (lower jaw) of the skull.

• A gomphosis is also known as a peg-and socket joint.
• Spanning between the bony walls of the socket and the root of the tooth are numerous short bands of dense connective tissue, each of which is called a periodontal ligament.
• Due to the immobility of a gomphosis, this type of joint is functionally classified as a synarthrosis.

 

 

9.3 | Cartilaginous Joints

By the end of this section, you will be able to:

• Describe the structural features of cartilaginous joints
• Distinguish between a synchondrosis and symphysis
• Give an example of each type of cartilaginous joint

 

As the name indicates, at a cartilaginous joint, the adjacent bones are united by cartilage, a tough but flexible type of connective tissue.

• These types of joints lack a joint cavity and involve bones that are joined together by either hyaline cartilage or fibrocartilage.
• There are two types of cartilaginous joints.
  • A synchondrosis is a cartilaginous joint where the bones are joined by hyaline cartilage.
    • Also classified as a synchondrosis are places where bone is united to a cartilage structure, such as between the anterior end of a rib and the costal cartilage of the thoracic cage.
  • The second type of cartilaginous joint is a symphysis, where the bones are joined by fibrocartilage.

 

 

Cartiliginous Joints At cartilaginous joints, bones are united by hyaline cartilage to form a synchondrosis or by fibrocartilage to form a symphysis. (a) The hyaline cartilage of the epiphyseal plate (growth plate) forms a synchondrosis that unites the shaft (diaphysis) and end (epiphysis) of a long bone and allows the bone to grow in length. (b) The pubic portions of the right and left hip bones of the pelvis are joined together by fibrocartilage, forming the pubic symphysis.

 

 

Synchondrosis

A synchondrosis (“joined by cartilage”) is a cartilaginous joint where bones are joined together by hyaline cartilage, or where bone is united to hyaline cartilage.

• A synchondrosis may be temporary or permanent.
• A temporary synchondrosis is the epiphyseal plate (growth plate) of a growing long bone.
  • The epiphyseal plate is the region of growing hyaline cartilage that unites the diaphysis (shaft) of the bone to the epiphysis (end of the bone).
  • Bone lengthening involves growth of the epiphyseal plate cartilage and its replacement by bone, which adds to the diaphysis.
  • For many years during childhood growth, the rates of cartilage growth and bone formation are equal and thus the epiphyseal plate does not change in overall thickness as the bone lengthens.
  • During the late teens and early 20s, growth of the cartilage slows and eventually stops.
  • The epiphyseal plate is then completely replaced by bone, and the diaphysis and epiphysis portions of the bone fuse together to form a single adult bone.
  • This fusion of the diaphysis and epiphysis is a synostosis.
  • Once this occurs, bone lengthening ceases.
  • For this reason, the epiphyseal plate is considered to be a temporary synchondrosis.
  • Because cartilage is softer than bone tissue, injury to a growing long bone can damage the epiphyseal plate cartilage, thus stopping bone growth and preventing additional bone lengthening.
• Growing layers of cartilage also form synchondroses that join together the ilium, ischium, and pubic portions of the hip bone during childhood and adolescence.
  • When body growth stops, the cartilage disappears and is replaced by bone, forming synostoses and fusing the bony components together into the single hip bone of the adult.
  • Similarly, synostoses unite the sacral vertebrae that fuse together to form the adult sacrum.
• Examples of permanent synchondroses are found in the thoracic cage.
  • One example is the first sternocostal joint, where the first rib is anchored to the manubrium by its costal cartilage. (The articulations of the remaining costal cartilages to the sternum are all synovial joints.)
  • Additional synchondroses are formed where the anterior end of the other 11 ribs is joined to its costal cartilage.
  • Unlike the temporary synchondroses of the epiphyseal plate, these permanent synchondroses retain their hyaline cartilage and thus do not ossify with age.
  • Due to the lack of movement between the bone and cartilage, both temporary and permanent synchondroses are functionally classified as a synarthrosis.

 

Symphysis

A cartilaginous joint where the bones are joined by fibrocartilage is called a symphysis (“growing together”).

• Fibrocartilage is very strong because it contains numerous bundles of thick collagen fibers, thus giving it a much greater ability to resist pulling and bending forces when compared with hyaline cartilage.
• This gives symphyses the ability to strongly unite the adjacent bones, but can still allow for limited movement to occur.
• Thus, a symphysis is functionally classified as an amphiarthrosis.

 

The gap separating the bones at a symphysis may be narrow or wide.

• Examples in which the gap between the bones is narrow include the pubic symphysis and the manubriosternal joint.
  • At the pubic symphysis, the pubic portions of the right and left hip bones of the pelvis are joined together by fibrocartilage across a narrow gap.
  • Similarly, at the manubriosternal joint, fibrocartilage unites the manubrium and body portions of the sternum.

 

The intervertebral symphysis is a wide symphysis located between the bodies of adjacent vertebrae of the vertebral column.

• Here a thick pad of fibrocartilage called an intervertebral disc strongly unites the adjacent vertebrae by filling the gap between them.
  • The width of the intervertebral symphysis is important because it allows for small movements between the adjacent vertebrae.
  • In addition, the thick intervertebral disc provides cushioning between the vertebrae, which is important when carrying heavy objects or during high-impact activities such as running or jumping.

 

 

9.4 | Synovial Joints

By the end of this section, you will be able to:

• Describe the structural features of a synovial joint
• Discuss the function of additional structures associated with synovial joints
• List the six types of synovial joints and give an example of each

 

Synovial joints are the most common type of joint in the body.

• A key structural characteristic for a synovial joint that is not seen at fibrous or cartilaginous joints is the presence of a joint cavity.
• This fluid-filled space is the site at which the articulating surfaces of the bones contact each other.
• Unlike fibrous or cartilaginous joints, the articulating bone surfaces at a synovial joint are not directly connected to each other with fibrous connective tissue or cartilage.
• This gives the bones of a synovial joint the ability to move smoothly against each other, allowing for increased joint mobility.

 

Watch this video: 

Joint Crash Course (Links to an external site.)

 

Shoulder Joint

The shoulder joint is called the glenohumeral joint.

• This is a ball-and-socket joint formed by the articulation between the head of the humerus and the glenoid cavity of the scapula.
• This joint has the largest range of motion of any joint in the body.
  • However, this freedom of movement is due to the lack of structural support and thus the enhanced mobility is offset by a loss of stability.

 

The glenohumeral (shoulder) joint is a ball-and-socket joint that provides the widest range of motions. It has a loose articular capsule and is supported by ligaments and the rotator cuff muscles.

 

The large range of motions at the shoulder joint is provided by the articulation of the large, rounded humeral head with the small and shallow glenoid cavity, which is only about one third of the size of the humeral head.

• The socket formed by the glenoid cavity is deepened slightly by a small lip of fibrocartilage called the glenoid labrum, which extends around the outer margin of the cavity.
• The articular capsule that surrounds the glenohumeral joint is relatively thin and loose to allow for large motions of the upper limb.
• Some structural support for the joint is provided by thickenings of the articular capsule wall that form weak intrinsic ligaments.
  • These include the coracohumeral ligament, running from the coracoid process of the scapula to the anterior humerus, and three ligaments, each called a glenohumeral ligament, located on the anterior side of the articular capsule. These ligaments help to strengthen the superior and anterior capsule walls.

 

However, the primary support for the shoulder joint is provided by muscles crossing the joint, particularly the four rotator cuff muscles.

• These muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) arise from the scapula and attach to the greater or lesser tubercles of the humerus.
• As these muscles cross the shoulder joint, their tendons encircle the head of the humerus and become fused to the anterior, superior, and posterior walls of the articular capsule.
  • The thickening of the capsule formed by the fusion of these four muscle tendons is called the rotator cuff.
• Two bursae, the subacromial bursa and the subscapular bursa, help to prevent friction between the rotator cuff muscle tendons and the scapula as these tendons cross the glenohumeral joint.
• In addition to their individual actions of moving the upper limb, the rotator cuff muscles also serve to hold the head of the humerus in position within the glenoid cavity.

• By constantly adjusting their strength of contraction to resist forces acting on the shoulder, these muscles serve as “dynamic ligaments” and thus provide the primary structural support for the glenohumeral joint.

 

Injuries to the shoulder joint are common.

• Repetitive use of the upper limb, particularly in abduction such as during throwing, swimming, or racquet sports, may lead to acute or chronic inflammation of the bursa or muscle tendons, a tear of the glenoid labrum, or degeneration or tears of the rotator cuff.

• Because the humeral head is strongly supported by muscles and ligaments around its anterior, superior, and posterior aspects, most dislocations of the humerus occur in an inferior direction.
• This can occur when force is applied to the humerus when the upper limb is fully abducted, as when diving to catch a baseball and landing on your hand or elbow.

• Inflammatory responses to any shoulder injury can lead to the formation of scar tissue between the articular capsule and surrounding structures, thus reducing shoulder mobility, a condition called adhesive capsulitis (“frozen shoulder”).

 

Watch these Videos:

Shoulder Anatomy (Links to an external site.)

 

Rotator Cuff Surgery (Links to an external site.)