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12.7: Fibrous Joints - Biology

12.7: Fibrous Joints - Biology


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Learning Objectives

  • 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 (Figure 1). 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.

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. (See Figure 1a.) 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 (Figure 2).

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.

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 (see Figure 1b). 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 (see Figure 1c). Due to the immobility of a gomphosis, this type of joint is functionally classified as a synarthrosis.


Synovial Joint

A synovial joint is a connection between two bones consisting of a cartilage lined cavity filled with fluid, which is known as a diarthrosis joint. Diarthrosis joints are the most flexible type of joint between bones, because the bones are not physically connected and can move more freely in relation to each other. In synarthrosis and amphiarthrosis connections between bones, the bones are directly connected with fibrous tissue or cartilage, limiting their ultimate range of motion.


Classification of Joints on the Basis of Structure

There are two ways to classify joints: on the basis of their structure or on the basis of their function. The structural classification divides joints into bony, fibrous, cartilaginous, and synovial joints depending on the material composing the joint and the presence or absence of a cavity in the joint.

Fibrous Joints

The bones of fibrous joints are held together by fibrous connective tissue. There is no cavity, or space, present between the bones and so most fibrous joints do not move at all, or are only capable of minor movements. There are three types of fibrous joints: sutures, syndesmoses, and gomphoses. Sutures are found only in the skull and possess short fibers of connective tissue that hold the skull bones tightly in place (Figure 1a).

Syndesmoses are joints in which the bones are connected by a band of connective tissue, allowing for more movement than in a suture. An example of a syndesmosis is the joint of the tibia and fibula in the ankle. The amount of movement in these types of joints is determined by the length of the connective tissue fibers. Gomphoses occur between teeth and their sockets the term refers to the way the tooth fits into the socket like a peg (Figure 1b). The tooth is connected to the socket by a connective tissue referred to as the periodontal ligament.

Figure 1. (a) Sutures are fibrous joints found only in the skull. (b) Gomphoses are fibrous joints between the teeth and their sockets. (b credit: modification of work by Gray’s Anatomy)

Cartilaginous Joints

Figure 2. Synovial joints are the only joints that have a space or “synovial cavity” in the joint.

Cartilaginous joints are joints in which the bones are connected by cartilage. There are two types of cartilaginous joints: synchondroses and symphyses. In a synchondrosis, the bones are joined by hyaline cartilage. Synchondroses are found in the epiphyseal plates of growing bones in children. In symphyses, hyaline cartilage covers the end of the bone but the connection between bones occurs through fibrocartilage. Symphyses are found at the joints between vertebrae. Either type of cartilaginous joint allows for very little movement.

Synovial Joints

Synovial joints are the only joints that have a space between the adjoining bones (Figure 2). This space is referred to as the synovial (or joint) cavity and is filled with synovial fluid. Synovial fluid lubricates the joint, reducing friction between the bones and allowing for greater movement. The ends of the bones are covered with articular cartilage, a hyaline cartilage, and the entire joint is surrounded by an articular capsule composed of connective tissue that allows movement of the joint while resisting dislocation. Articular capsules may also possess ligaments that hold the bones together. Synovial joints are capable of the greatest movement of the three structural joint types however, the more mobile a joint, the weaker the joint. Knees, elbows, and shoulders are examples of synovial joints.


Synovial Joints

The most common joints are freely movable joints in the body called synovial joints. Synovial joints are surrounded by fibrous tissue or sac called the joint capsule. The lining of this capsule secretes synovial fluid, which lubricates the tissues and spaces within this capsule. There are several types of synovial joints that allow different forms of motion.  

Ball and Socket Joints

This type of joint allows for a wide range of rotation and movement, including rotation. Your shoulder and hip are examples of ball and socket joints.

Condyloid Joints

The jaw and fingers both have condyloid joints. These joints don't allow rotation, but are versatile think of the way a joystick moves on a video game console.

Gliding Joints

You have this kind of joint, which allows bones to glide around and past each other in your spine, ankles, and wrists.

Hinge Joints

Just like the name suggests, these joints work like hinges. Think of your knee and the part of your elbow that bends (the ulna). These are hinge joints.

Pivot Joints

Your neck and elbow both have pivot joints, which allow bones to pivot or twist around other bones.

Saddle Joint

The best example of a saddle joint and what it does is found in the base of the thumb. Saddle joints allow side to side and back and forth motion, but don't fully rotate.


Bones, Muscles, and Joints

Bones provide support for our bodies and help form our shape. Although they're very light, bones are strong enough to support our entire weight.

Bones also protect the body's organs. The skull protects the brain and forms the shape of the face. The spinal cord, a pathway for messages between the brain and the body, is protected by the backbone, or spinal column. The ribs form a cage that shelters the heart and lungs, and the pelvis helps protect the bladder, part of the intestines, and in women, the reproductive organs.

Bones are made up of a framework of a protein called collagen, with a mineral called calcium phosphate that makes the framework hard and strong. Bones store calcium and release some into the bloodstream when it's needed by other parts of the body. The amounts of some vitamins and minerals that you eat, especially vitamin D and calcium, directly affect how much calcium is stored in the bones.

Bones are made up of two types of bone tissues:

  1. Compact bone is the solid, hard outside part of the bone. It looks like ivory and is extremely strong. Holes and channels run through it, carrying blood vessels and nerves.
  2. Cancellous (KAN-suh-lus) bone, which looks like a sponge, is inside compact bone. It is made up of a mesh-like network of tiny pieces of bone called trabeculae (truh-BEH-kyoo-lee). This is where bone marrow is found.

In this soft bone is where most of the body's blood cells are made. The bone marrow contains stem cells, which produce the body's red blood cells and platelets, and some types of white blood cells. Red blood cells carry oxygen to the body's tissues, and platelets help with blood clotting when someone has a cut or wound. White blood cells help the body fight infection.

Bones are fastened to other bones by long, fibrous straps called ligaments (LIG-uh-mentz). Cartilage (KAR-tul-ij), a flexible, rubbery substance in our joints, supports bones and protects them where they rub against each other.

How Do Bones Grow?

The bones of kids and young teens are smaller than those of adults and contain "growing zones" called growth plates. These plates consist of multiplying cartilage cells that grow in length, and then change into hard, mineralized bone. These growth plates are easy to spot on an X-ray. Because girls mature at an earlier age than boys, their growth plates change into hard bone at an earlier age.

Bone-building continues throughout life, as a body constantly renews and reshapes the bones' living tissue. Bone contains three types of cells:

  1. osteoblasts (AHS-tee-uh-blastz), which make new bone and help repair damage
  2. osteocytes (AHS-tee-o-sites), mature bone cells which help continue new born formation
  3. osteoclasts (AHS-tee-o-klasts), which break down bone and help to sculpt and shape it

What Are Muscles and What Do They Do?

Muscles pull on the joints, allowing us to move. They also help the body do such things as chewing food and then moving it through the digestive system.

Even when we sit perfectly still, muscles throughout the body are constantly moving. Muscles help the heart beat, the chest rise and fall during breathing, and blood vessels regulate the pressure and flow of blood. When we smile and talk, muscles help us communicate, and when we exercise, they help us stay physically fit and healthy.

Humans have three different kinds of muscle:

  1. Skeletal muscle is attached by cord-like tendons to bone, such as in the legs, arms, and face. Skeletal muscles are called striated (STRY-ay-ted) because they are made up of fibers that have horizontal stripes when viewed under a microscope. These muscles help hold the skeleton together, give the body shape, and help it with everyday movements (known as voluntary muscles because you can control them). They can contract (shorten or tighten) quickly and powerfully, but they tire easily.
  2. Smooth, or involuntary, muscle is also made of fibers, but this type of muscle looks smooth, not striated. We can't consciously control our smooth muscles rather, they're controlled by the nervous system automatically (which is why they're also called involuntary). Examples of smooth muscles are the walls of the stomach and intestines, which help break up food and move it through the digestive system. Smooth muscle is also found in the walls of blood vessels, where it squeezes the stream of blood flowing through the vessels to help maintain blood pressure. Smooth muscles take longer to contract than skeletal muscles do, but they can stay contracted for a long time because they don't tire easily.
  3. Cardiac muscle is found in the heart. The walls of the heart's chambers are composed almost entirely of muscle fibers. Cardiac muscle is also an involuntary type of muscle. Its rhythmic, powerful contractions force blood out of the heart as it beats.

How Do Muscles Work?

The movements that muscles make are coordinated and controlled by the brain and nervous system. The involuntary muscles are controlled by structures deep within the brain and the upper part of the spinal cord called the brain stem. The voluntary muscles are regulated by the parts of the brain known as the cerebral motor cortex and the cerebellum (ser-uh-BEL-um).

When you decide to move, the motor cortex sends an electrical signal through the spinal cord and peripheral nerves to the muscles, making them contract. The motor cortex on the right side of the brain controls the muscles on the left side of the body and vice versa.

The cerebellum coordinates the muscle movements ordered by the motor cortex. Sensors in the muscles and joints send messages back through peripheral nerves to tell the cerebellum and other parts of the brain where and how the arm or leg is moving and what position it's in. This feedback results in smooth, coordinated motion. If you want to lift your arm, your brain sends a message to the muscles in your arm and you move it. When you run, the messages to the brain are more involved, because many muscles have to work in rhythm.

Muscles move body parts by contracting and then relaxing. Muscles can pull bones, but they can't push them back to the original position. So they work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint. Then, when the movement is completed, the flexor relaxes and the extensor contracts to extend or straighten the limb at the same joint. For example, the biceps muscle, in the front of the upper arm, is a flexor, and the triceps, at the back of the upper arm, is an extensor. When you bend at your elbow, the biceps contracts. Then the biceps relaxes and the triceps contracts to straighten the elbow.

What Are Joints and What Do They Do?

Joints are where two bones meet. They make the skeleton flexible &mdash without them, movement would be impossible.

Joints allow our bodies to move in many ways. Some joints open and close like a hinge (such as knees and elbows), whereas others allow for more complicated movement &mdash a shoulder or hip joint, for example, allows for backward, forward, sideways, and rotating movement.

Joints are classified by their range of movement:

  • Immovable, or fibrous, joints don't move. The dome of the skull, for example, is made of bony plates, which move slightly during birth and then fuse together as the skull finishes growing. Between the edges of these plates are links, or joints, of fibrous tissue. Fibrous joints also hold the teeth in the jawbone.
  • Partially movable, or cartilaginous (kar-tuh-LAH-juh-nus), joints move a little. They are linked by cartilage, as in the spine. Each of the vertebrae in the spine moves in relation to the one above and below it, and together these movements give the spine its flexibility.
  • Freely movable, or synovial (sih-NO-vee-ul), joints move in many directions. The main joints of the body &mdash such as those found at the hip, shoulders, elbows, knees, wrists, and ankles &mdash are freely movable. They are filled with synovial fluid, which acts as a lubricant to help the joints move easily.

Three kinds of freely movable joints play a big part in voluntary movement:


Condyloid Joints

Condyloid joints consist of an oval-shaped end of one bone fitting into a similarly oval-shaped hollow of another bone (Figure 5). This is also sometimes called an ellipsoidal joint. This type of joint allows angular movement along two axes, as seen in the joints of the wrist and fingers, which can move both side to side and up and down.

Figure 5. The metacarpophalangeal joints in the finger are examples of condyloid joints. (credit: modification of work by Gray’s Anatomy)


Bone Development and Growth:

1. _______________________________bones = broad, flat bones of the skull

2. Endochondral bones = all other bones.

Bones first form as hyaline cartilage. The cartilage then gradually changes into bone tissue - a process called ____________________________,

primary ossification center - diaphysis, increase diameter
secondary ossification center - epiphysis, increase length

epiphyseal disk ( growth plate) between the diaphysis and the epiphysis.

_____________________- produce bone cells called OSTEOCYTES
_____________________ - dissolve bone tissue - a process called RESORPTION.


Articular Cartilage Function

Articular cartilage function is based upon its composition of hyaline cartilage, which is practically frictionless due to the glass-like surface and ability to self-lubricate via lubricating glycoproteins within the extracellular matrix. When articulation is smooth, less stress is exercised on the cartilage surface and the tissue is more resistant to wear, in the same way oil added to a squeaky door hinge prevents the erosion of the touching surfaces.

The structure of articular cartilage into three zones with different characteristics allows for an efficient, load-bearing surface which distributes compressive forces generated during diarthroidal joint loading and diarthroidal joint motion. Wrong movement in load bearing cartilage, for example at a joint between the long bones, is the reason why the knee (between the femur and tibia) is the location of frequent articular cartilage injury. The knee joint can undergo damage through excessive rotation a common football injury is the dreaded meniscus tear.


EXPANSION JOINT SPECIFICATION DATA AND SIZE INFORMATION

CONFORMS TO OR MEETS SPECIFICATIONS

THICKNESS
WIDTHS*

SLAB
WIDTHS

STANDARD
LENGTHS

•ASTM D 994
•FEDERAL SPECIFICATION HH-F-341 F
•AASHTO M 33
•FAA SPECIFICATION Item P-610-2.7 1/4″ (6.35 mm)
3/8″ (9.53mm)
1/2″ (12.7 mm)
3/4″(19.05mm)
1″ (25.4 mm) •ASTM D 1751
•AASHTO M 213
•FAA SPECIFICATION Item P-610-2.7
•Corps of Engineers CRD-C
•Federal Specification HH-F-341 F, Type I 3/8″ (9.53mm)
1/2″ (12.7 mm)
3/4″(19.05mm)
1″ (25.4 mm) •ASTM D 1752, Sections 5.1 through 5.4 with the compression requirement modified to 10 psi
(7.03 g/mm²) minimum and 25 psi
(17.58 g/mm²) maximum
ASTM D 5249, Type 2
ASTM D 7174-05 1/4″ (6.35 mm)
3/8″ (9.53mm)
1/2″ (12.7 mm)
3/4″(19.05mm)
1″ (25.4 mm) •ASTM D 1752, Type I
•FEDERAL SPECIFICATION HH-F-341 F, TYPE II, Class A
•AASHTO M 153, Type I
•FAA SPECIFICATION Item P-610-2.7
•Corps of Engineers CRD-C 509, Type I 1/4″ (6.35 mm)
3/8″ (9.53mm)
1/2″ (12.7 mm)
3/4″(19.05mm)
1″ (25.4 mm) •ASTM D 1752, Type II
•FEDERAL SPECIFICATION HH-F-341 F, TYPE II, Class B
•AASHTO M 153, Type II
•FAA SPECIFICATION Item P-610-2.7
•Corps of Engineers CRD-C 509, Type II 1/4″ (6.35 mm)
3/8″ (9.53mm)
1/2″ (12.7 mm)
3/4″(19.05mm)
1″ (25.4 mm) •ASTM D 1752, Type III
•FEDERAL SPECIFICATION HH-F-341 F, TYPE II, Class C
•AASHTO M 153, Type III
•FAA SPECIFICATION Item P-610-2.7
•Corps of Engineers CRD-C 509, Type III 1/2″ (12.7 mm)
3/4″(19.05mm)
1″ (25.4 mm)

*Pre-cut joint furnished in any desired width


2. Joints Can Be Grouped By Their Structure into Fibrous, Cartilaginous, and Synovial Joints

Fibrous Joints. Between the articulations of fibrous joints is thick connective tissue, which is why most (but not all) fibrous joints are immovable (synarthroses). There are three types of fibrous joints:

(1) Sutures are nonmoving joints that connect bones of the skull. These joints have serrated edges that lock together with fibers of connective tissue.

(2) The fibrous articulations between the teeth and the mandible or maxilla are called gomphoses and are also immovable.

(3) A syndesmosis is a joint in which a ligament connects two bones, allowing for a little movement (amphiarthroses). The distal joint between the tibia and fibula is an example of a syndesmosis.

Cartiliginous Joints. Joints that unite bones with cartilage are called cartilaginous joints. There are two types of cartilaginous joints:

(1) A synchrondosis is an immovable cartilaginous joint. One example is the joint between the first pair of ribs and the sternum.

(2) A symphysis consists of a compressable fibrocartilaginous pad that connects two bones. This type of joint allows for some movement. The hip bones, connected by the pubic symphysis, and the vertebrae, connected by intervertebral discs, are two examples of symphyses.

Synovial Joints. Synovial joints are characterized by the presence of an articular capsule between the two joined bones. Bone surfaces at synovial joints are protected by a coating of articular cartilage. Synovial joints are often supported and reinforced by surrounding ligaments, which limit movement to prevent injury. There are six types of synovial joints:

(1) Gliding joints move against each other on a single plane. Major gliding joints include the intervertebral joints and the bones of the wrists and ankles.

(2) Hinge joints move on just one axis. These joints allow for flexion and extension. Major hinge joints include the elbow and finger joints.

(3) A pivot joint provides rotation. At the top of the spine, the atlas and axis form a pivot joint that allows for rotation of the head.

(4) A condyloid joint allows for circular motion, flexion, and extension. The wrist joint between the radius and the carpal bones is an example of a condyloid joint.

(5) A saddle joint allows for flexion, extension, and other movements, but no rotation. In the hand, the thumb’s saddle joint (between the first metacarpal and the trapezium) lets the thumb cross over the palm, making it opposable.

(6) The ball-and-socket joint is a freely moving joint that can rotate on any axis. The hip and shoulder joints are examples of ball and socket joints.


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