43 Fibrous Joints

Fibrous Joints

Fibrous joints are also called fixed or immovable joints because they do not move.

A joint is the location at which two or more bones make contact. They are constructed to allow movement (except for skull bones), provide mechanical support, and are classified structurally and functionally. Structural classification is determined by how the bones connect to each other, while functional classification is determined by the degree of movement between the articulating bones. In practice, there is significant overlap between the two types of classifications.

Fibrous joints are joined by dense irregular connective tissue that is rich in collagen fibers.

Characteristics of Fibrous Joints

Fibrous joints are connected by dense connective tissue consisting mainly of collagen. These joints are also called fixed or immovable joints because they do not move. Fibrous joints have no joint cavity and are connected via fibrous connective tissue. The skull bones are connected by fibrous joints called sutures. The skull bones of a fetus are unfused so that they can move over each other slightly to compress skull size during birth. After birth, the bones slowly begin to fuse to become fixed, making the skull bones immovable in order to protect the brain from impact.

Syndesmoses of long bones and gomphoses of teeth are also types of fibrous joints. The movement of the root within a gomphosis has a threefold effect. It lessens some of the impact between the upper and lower teeth in biting. It also pumps blood and lymph from the periodontal membrane into the dental veins and lymph channels and stimulates sensory nerve terminals in the membrane to send signals to the brain centers that control the muscles of mastication.

Sutures

A suture is a type of fibrous joint (synarthrosis) bound by Sharpey’s fibers that only occurs in the skull (cranium).

A suture is a type of fibrous joint which only occurs in the cranium, where it holds bony plates together. Sutures are bound together by a matrix of connective tissues called Sharpey’s fibers, which grow from each bone into the adjoining one. A tiny amount of movement is permitted at sutures, which contributes to the compliance and elasticity of the skull. These joints are synarthroses (immovable joints).

Cranial Sutures

Most sutures are named for the bones they articulate, but some have special names of their own. Sutures primarily visible from the side of the skull (norma lateralis) include:

• Coronal suture: between the frontal and parietal bones
• Lambdoid suture: between the parietal, temporal, and occipital bones
• Occipitomastoid suture
• Parietomastoid suture
• Sphenofrontal suture
• Sphenoparietal suture
• Sphenosquamosal suture
• Sphenozygomatic suture
• Squamosal suture: between the parietal and the temporal bone
• Zygomaticotemporal suture
• Zygomaticofrontal suture

Sutures primarily visible from front of the skull (norma frontalis) or above the skull (norma verticalis) include:

• Frontal suture / Metopic suture: between the two frontal bones, prior to the fusion of the two into a single bone
• Sagittal suture: along the midline, between parietal bones.

Sutures primarily visible from below the skull (norma basalis) or inside the skull include:

• Frontoethmoidal suture
• Petrosquamous suture
• Sphenoethmoidal suture
• Sphenopetrosal 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.

Fontanelles

It is normal for many of the bones of the skull to remain unfused at birth. The fusion of the skull’s bones at birth is known as craniosynostosis. The joint between the mandible and the cranium, the temporomandibular joint, forms the only non-sutured joint in the skull. 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.

Synostosis

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.

Syndesmoses

Syndesmoses are slightly movable joints formed where an interosseous ligament joins two bones.

A syndesmosis is a type of articulation or joint in which two adjacent bones are joined by an interosseous membrane.

Interosseous Membrane

The interosseous membrane is a type of connective tissue found between certain bones, such as those in syndesmosis joints. The membrane is important in creating compartments to separate different structures, distributing the impact of forces and separating the joints. For example, the long bones of the lower arm and the leg both have attached interosseous membranes. In the leg, the interosseous membrane extends between the tibia and the fibula, running along the crests of the bones. The muscles in the leg are separated into sections in the front and back with this membrane. The strength of the membrane allows absorption and distribution of impacts to either bone.

The interosseous membrane in the lower arm extends between the radius and the ulna. It is involved in the elbow joint and helps to stabilize the lower arm bones for strength, durability, and flexibility. Like other joint tissue, it is designed to be able to deform and flex rather than shred or fracture on impact, allowing the joint to absorb considerable stress before damage occurs.

Syndesmoses and Amphiarthrosis Joints

Along with symphysis joints, syndesmoses are classified as amphiarthrosis joints in that they allow slight movement. Joints of this kind are found at several points in the human body, including the intermediate radioulnar joint where the radius and ulna meet above the wrist, in the spine between the spinous processes of various adjacent vertebra, and above the ankle joint where the tibia and fibula converge.

Located directly above the ankle joint, which is a synovial hinge joint, the ankle syndesmosis is held together by four ligaments. The anterior inferior tibiofibular ligament crosses in front of the tibia and fibula bones. The posterior inferior tibiofibular ligament and the transverse ligament connect the two bones from behind, and the interosseous ligament runs between the contiguous bony surfaces of the two bones.

Due to the limited flexibility in these joint structures, ligament injuries in syndesmosis joints are common, particularly at the wrist and ankle. When the wrist or ankle joint is bent beyond its normal range of motion, a sprain or even a tear in these ligaments can occur. Mild syndesmosis injury may involve the sprain of a single ligament.

More severe injuries can involve damage to multiple ligaments at once or even the separating of the bones at the joint (known as diastasis). Players of rough sports such as football or rugby have an increased risk of fracturing their fibulas and tearing the interosseous ligament between it and the tibia. When that happens, the surgeon temporarily replaces the ligament with a syndesmotic screw.

Gomphoses

A gomphosis is a fibrous joint that binds the teeth to bony sockets in the bones of the maxilla mandible.

A gomphosis is a joint that anchors a tooth to its socket. Gomphoses line the upper and lower jaw in each tooth socket and are also known as peg and socket joints. These joints have a very limited range of mobility so the teeth are held firmly in place. However, as illustrated with braces, it is possible to move them incrementally over time. Each tooth has bony protrusions or pegs that latch into the socket with the assistance of the gomphosis. Disorders of the mouth sometimes involve these joints.

This particular joint is an example of a synarthrosis, a joint with limited to no movement. Several other joints of this type can be found in the body, including the connections between the plates of the skull. The gomphosis is made up of fibrous tissue, a collection of tough ligaments that attach to the socket and base of the tooth. As people age and lose their initial set of baby teeth, the new teeth develop gomphoses to anchor them in the jaw.

One disorder that can affect the gomphosis is scurvy, a disease of connective tissue. Connective tissues such as the ligaments around the teeth start to dissolve. Patients with untreated scurvy develop loose teeth that may eventually fall out because the joints are too unstable. Periodontal infection and inflammation can also damage the joint, causing pain and erosion in the soft tissue. Chronic dental problems may loosen the ligaments and lead to tooth loss or instability.

Patients with braces and retainers take advantage of the limited range of movement offered by the gomphosis to pull teeth into new positions. This may be necessary for a variety of reasons. The goal is to align the teeth evenly to create a strong, healthy bite. Braces are adjusted incrementally over time to pull and push the teeth into place. Between each adjustment, the teeth and jaw have time to recover.