Synarthrosis: Characteristics, Suture, Fontanelles, Interosseous Membrane, Syndesmosis and Gonfosis

A joint is a location where two or more bones make contact.

They are constructed to allow movement (except for the bones of the skull), provide mechanical support, and are classified structurally and functionally.

The structural classification is determined by how the bones connect, while the functional classification is determined by the degree of movement between the articulated bones. There is a significant overlap between the two types of categories in practice.

The bones that articulate to form the cartilaginous joints are held together by hyaline cartilage or fibrocartilage.

The joints characterized by hyaline cartilage between the bones that form them are called synchondrosis, and those joined by fibrocartilage, symphysis. The cartilaginous joints only allow minimal movements between the bones and specific circumstances.

What is synarthrosis?

Joints of the synarthrosis type (fibrous) are articulations not equipped with movement, such as the joints of the bones of the skull. They are joined by dense irregular connective tissue rich in collagen fibers.

Characteristics of the synarthrosis

The joints of the synarthrosis type are connected by dense connective tissue consisting mainly of collagen. These joints are also called fixed or immovable because they do not move.


The fibrous joints do not have a joint cavity and are connected through fibrous connective tissue. The bones of the skull are connected by fibrous joints called sutures.

The bones of a fetus’ skull are not fused, so they can move slightly over each other to compress the size of the head during birth.

After birth, the bones begin to fuse slowly to attach, making the bones of the skull immobile to protect the brain from impact.

Syndesmosis of long bones and teeth gonfosis are also synarthrosis type joints. The movement of the root within a gonfosis has a triple effect. Decreases part of the impact between the upper and lower teeth when biting.

It also pumps blood and lymph from the periodontal membrane into the dental veins and lymph channels. It stimulates the sensory nerve terminals in the membrane to send signals to the brain centers that control chewing muscles.


A suture is a fibrous joint (synarthrosis) joined by Sharpey fibers that only occurs in the skull.

A suture is a fibrous joint that only occurs in the skull, where it joins the bone plates. The sutures are bound by a matrix of connective tissues called Sharpey fibers, which grow from each bone to the adjacent one.

A small amount of movement is allowed in the sutures, which contributes to the compliance and elasticity of the skull. These joints are synarthrosis (immovable joints).

Cranial sutures

Most sutures are named for their articulate bones, but some have unusual proper names. Sutures mainly visible from the side of the skull (norm lateralis) include:

  • Coronal suture.
  • Sutura lambdoidal.
  • Suture ocipitomastoidea.
  • Parietomastoid suture.
  • Sphenophrontal suture.
  • Sphenoparietal suture.
  • Sphenoesquimal suture.
  • Sphenozygomatic suture.
  • Scaly suture.
  • Zygomaticotemporal suture.
  • Zygomatofrontal suture.

Sutures mainly visible from the front of the skull (frontal norm) or above the head (norm verticalis) include:

Frontal suture / metopic suture: between the two frontal bones, before fusion of the two in a single bone.

Sagittal suture: along the midline, between the parietal bones.

Sutures mainly visible from below the skull (baseline) or inside the head include:

  • Fronto-aural suture.
  • Petrosquamous suture.
  • Sphenoethmoidal suture.
  • Sphenopotrophic suture.

Fibrous connective tissue in a suture (to join or sew) strongly bonds adjacent skull bones and, therefore, helps protect the brain and form the face.

In adults, the skull bones are closely opposed, and the fibrous connective tissue fills the narrow space between the bones. The suture is often complicated, forming a tight joint that prevents most movement between the bones.


A syndesmosis is a 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 the syndesmosis joints. The membrane is essential in creating compartments to separate different structures, distributing the impact of the forces and dividing the joints.

For example, the long bones of the lower arm and leg have joined interosseous membranes. In the portion, the interosseous membrane extends between the tibia and the fibula, running along the ridges of the bones.

The leg muscles are separated into sections in the front and back with this membrane. The strength of the membrane allows the absorption and distribution of impacts to any of the bones.

The interosseous membrane in the lower arm extends between the radius and the ulna. It is involved in the elbow joint and helps stabilize the lower arm bones for strength, durability, and flexibility.

Like other joint tissues, it is designed to be deformed and flexed instead of crushed or fractured on impact, allowing the joint to absorb considerable stress before damage occurs.


Along with the symphysis joints, the syndesmosis is classified as amphetarthrosis joints because they allow a slight movement.

Joints of this type are found at various points in the human body, including the intermediate radioulnar joint where the radius and ulna meet above the wrist and on the spine between the spinous processes of several adjacent vertebrae above the joint. Of the ankle where the tibia and the fibula converge.

Located directly above the ankle joint, which is a synovial joint of the hinge, the syndesmosis of the ankle is held together by four ligaments. The inferior anterior tibiofibular ligament is crossed in front of the bones of the tibia and the fibula.

The posterior inferior tibiofibular ligament and the transverse ligament connect the two bones, and the interosseous ligament runs between the contiguous bone surfaces of the two bones.

Due to the limited flexibility in these joint structures, ligamentous lesions in syndesmosis joints are common, particularly in the wrist and ankle.

A sprain or even a break in these ligaments may occur when the wrist or ankle joint is bent beyond its normal range of motion. The mild injury due to syndesmosis may involve the sprain of a single ligament.

More severe injuries can involve damage to multiple ligaments at a time or even separation of the bones in the joint (known as diastasis).

Hard-sports players such as football or rugby have a greater risk of fracturing their fibulas and breaking the interosseous ligament between it and the tibia. The surgeon temporarily replaces the ligament with a syndesmotic screw when that happens.


A gomphosis is a fibrous joint that joins the teeth to the bony sockets in the bones of the maxillary jaw.

A gomphosis is a joint that anchors a tooth to its socket. The genesis is aligned in the upper and lower jaw in each socket and is also known as peg and socket joints.

These joints have a minimal range of mobility, so the teeth stay firmly in place. However, it is possible to move them gradually over time.

Each tooth has protuberances or bony pins hooked in the alveolus with the help of gonfosis. Disorders of the mouth sometimes involve these joints.

This particular joint is an example of synarthrosis, a joint with limited movement or without movement. Several other joints of this type can be found in the body, including the connections between the skull plates.

Gonfosis is composed of fibrous tissue, a collection of complex ligaments that join the alveolus and the tooth’s base. As people age and lose their initial set of baby teeth, the new teeth develop gonfosis to anchor them in the jaw.

A disorder that can affect gonorrhea is scurvy, a connective tissue disease. Like the ligaments around the teeth, the connective tissues begin to dissolve.

Patients with untreated scurvy develop loose teeth that eventually can 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 can loosen the ligaments and lead to loss or instability of the teeth.

Patients with braces and retention devices take advantage of gomphosis’s limited range of motion to bring teeth to new positions.

This may be necessary for a variety of reasons. The goal is to align the teeth evenly to create a solid and healthy bite.

The clamps gradually adjust overtime to pull and push the teeth into place. Between each adjustment, the teeth and jaw have time to recover.