Synovial Joints and Dislocations

by Robert Tallitsch, PhD | June 9, 2022

Synovial Joints and Joint Dislocation Brain Builder

BodyViz Brain Builder on Synovial Joints and Dislocation with a trauma patient case example, article, and activity!

Written by: Robert Tallitsch, PhD

The definition of a joint, as it applies to the skeletal system, is “the junction of two or more bones”. Any joint can dislocate, but the chances of dislocation are not the same for all joints. Simply put, the chances for dislocation of a joint are directly proportional to the possible range of motion at that joint. Synovial joints allow for the greatest range of motion. Therefore, synovial joints have the greatest possibility for dislocation.

In this Brain Builder we will briefly review the anatomy of a synovial joint, and then look at some of the anatomical factors that contribute to joint stability. (For a more complete review of synovial joints see the Brain Builder titled “Joints and Arthritis”.)

Synovial Joint Characteristics

A synovial joint allows for considerable range of motion. All synovial joints demonstrate the following seven anatomical characteristics.

A fibrous capsule.
A synovial membrane lining the inner surface of the joint capsule.
Secretion of synovial fluid by the synovial membrane. This fluid fills the joint capsule, nourishes, and lubricates the hyaline cartilage covering the articulating surfaces of the bones of the joint.
Hyaline cartilage covering the articulating surfaces of the joint.
Presence of localized connective tissue thickenings of the joint capsule. These thickenings are termed ligaments, and they serve to limit extreme range-of-motion at the joint.
Small blood vessels and capillaries penetrating the joint capsule.
Sensory nerves supplying the ligaments and the external layer of the joint capsule with receptors for proprioception and pain.

The more mobile a joint is the less stabile it is. Joint mobility and joint stability are antagonists of each other — the more of one means the less of the other.

The three most important factors contributing to the stability of a synovial joint are:

the shape, size, and arrangement of the articulating surfaces,
the structure of the joint capsule and ligaments associated with the joint, and
the musculature surrounding the joint.

Shape, Size, and Arrangement of the Articulating Surfaces

How the shape, size, and arrangement of the articulating surfaces affects joint stability is most easily understood by comparing the articulating surfaces of the shoulder (glenohumeral) and the hip joints.

The head of the humerus is much larger than the glenoid fossa of the scapula. In addition, the glenoid fossa is a very shallow surface. As a result, the glenohumeral joint exhibits a large range of motion but also is a joint that is easily dislocated.

In contrast, the acetabulum of the pelvis is a deep socket that almost completely surrounds the head of the femur. This arrangement causes the hip joint to have a smaller range of motion and more stability than the glenohumeral joint. Clinical data shows that approximately 50% of the dislocations seen in an emergency room involve the glenohumeral joint as compared to only 7% for hip dislocations.

Structure of the Joint Capsule and Ligaments Associated with the Joint

The joint capsule and associated ligaments serve to prevent extreme range-of-motion movements at the joint. The tighter the joint capsule and associated ligaments the smaller the range of motion at the joint and, therefore, the greater the stability of the joint. The glenohumeral joint has a very lax joint capsule and comparatively loose ligaments. In contrast, the hip joint has a very tight joint capsule and associated ligaments, both of which significantly reduce the range of motion at the hip as compared to the glenohumeral joint.

Muscle Structure

The tone and bulk of the muscles surrounding a joint contribute significantly to the stability of the joint. If muscles associated with a joint lose their tone the probability of a dislocation at that joint increases proportionately.

In addition, the location of a muscle’s insertion relative to a joint’s axis of rotation gives some indication as to the function played by the muscle at that joint. A muscle that inserts further from the axis of rotation at the joint will stabilize the joint in addition to moving the joint, as compared to a muscle that inserts close to the axis of rotation. Two examples of this are the brachioradialis and biceps brachii muscles and their actions on the elbow joint. The brachioradialis muscle inserts more distally from the axis of rotation than does the biceps brachii. When the brachioradialis contracts it flexes the elbow while, at the same time, pulls the head of the radius closer to the capitulum of the humerus, thus stabilizing the elbow. Contraction of the biceps brachii flexes the elbow but does not significantly alter the distance between the head of the radius and the capitulum of the humerus.

Joint Dislocation

A joint dislocation (also termed a joint luxation) occurs when bones that participate in a joint become separated. Symptoms of a dislocation include pain, muscle spasms, reduced range of motion, swelling, and a deformation of the joint.

Joint dislocations are typically caused by sports injury or trauma, muscle imbalances, a congenital laxity in the joint, or a direct impact, such as in a fall.

Treatment for a dislocation depends upon the joint affected and the severity of the injury. Immediate application of ice and immobilization of the joint is highly recommended until medical examination is possible. Further treatment often includes:

medication to reduce pain
manipulation of the joint in order to return the bones to their proper places
rest and temporary immobilization of the joint
rehabilitation in the form of physical therapy to strengthen muscles, tendons, and ligaments around the joint in an effort to reduce the possibility of future dislocations
surgery for severe dislocations

Anyone can suffer a dislocation. Those at higher risk include anyone prone to falls, anyone with an inherited joint disease, and anyone involved in contact sports. Most dislocations will heal completely, and pain is significantly reduced once the bones are put back into their proper place.

Did you find this article and video helpful and want to learn more about the 3D Anatomy visualizations from the video?

Learn more about BodyViz 3D Dissection Software, used to create this Brain Builder, where instructors and students can dissect the real anatomy of a patient! BodyViz converts MRI and CT scans into virtual dissections that can be sliced, filtered, annotated, and more! Schedule a demo with one of our team members to learn more!

Schedule Demo

Key Terms:

Joint - Junction of two or more bones.

Luxation - Alternative term for dislocation.

Acetabulum - One of the structures involved in the hip joint.

Glenoid fossa - The portion of the scapula where the head of the humerus forms the glenohumeral joint.

Synovial joint - The type of skeletal joint with the greatest range of motion and, therefore, the greatest possibility for dislocation.

Glenohumeral joint - Joint between the humerus and scapula.

Relationship between joint mobility and joint stability - The relationship between joint mobility and stability is an inverse one. This means that the more stabile a joint is the less mobile it is.

Stability of a joint - The ability of a joint to resist dislocation.

Questions:

True or False? A joint with a smaller range of motion will be more susceptible to dislocation than a joint with a larger range of motion.
A: False. The greater the range of motion the greater the chance for dislocation.
List the three anatomical factors that contribute to the stability of a joint.
A: (1) Structure, size, and arrangement of the articulating surfaces; (2) musculature surrounding the joint; (3) structure of the joint capsule and ligaments associated with the joint.
True or False? Shoulder dislocations occur twice as frequently as dislocations of the hip.
A: False. Shoulder dislocations are more than seven times as common as dislocations of the hip.
True or False? One of the factors that contributes to the stability of the hip joint is the depth of the acetabulum of the pelvis.
A: True
List three possible symptoms of a dislocated glenohumeral joint.
A: Any three of the symptoms of a joint dislocation listed below would be a correct answer regarding symptoms of a dislocated glenohumeral joint: pain, muscle spasms, reduced range of motion, swelling, and deformation of the joint.
True or False? Immediately after the dislocated bones are repositioned properly the amount of pain felt by the patient should decrease significantly.
A: True
True or False? Almost immediately after the dislocated bones are repositioned properly the patient may resume normal activity and range of motion at that joint.
A: False. One of the common forms of treatment for a dislocated joint is rest and temporary immobilization of the joint.
List the seven characteristics of a synovial joint.
A: The seven characteristics of a synovial joint are as follows:
- A fibrous capsule.
- A synovial membrane lining the inner surface of the joint capsule.
- Secretion of synovial fluid by the synovial membrane.
- Hyaline cartilage covering the articulating surfaces of the joint.
- Localized connective tissue thickenings (ligaments) of the joint capsule.
- Small blood vessels capillaries penetrating the joint capsule.
- Sensory nerves supplying the ligaments & the external layer of the joint capsule with receptors for proprioception & pain.
True or False? Only synovial joints dislocate.
A: False. Any joint can dislocate.
True or False? The location of a muscle’s insertion relative to a joint’s axis of rotation gives some indication as to the function played by the muscle at that joint.
A: True

Download our BodyViz Brain Builder Activity!

This activity supports this article and video with instructions, links to the Brain Builder Video, key vocabulary terms with definitions, 10 questions, and the answer key. Brain Builder Activities are ready-to-use and can be multi-purposed as homework assignments, study guides, lecture aids, team projects, and more!

Download Brain Builder Activity

Schedule a demo to see our virtual anatomy software and talk with our team about how we can help you to improve your anatomy class and become your students' favorite class!