The Anatomy of the Saddle Joint

A saddle joint—also called a sellar joint—allows for many types of movements in many directions.

A saddle joint includes one concave bone—shaped like a saddle—and another convex bone—shaped like a rider sitting on a horse's saddle. The design of a saddle joint helps keep it stable and reduces the risk of damage from constant use.

There are only a few saddle joints in your body. These joints are found in the thumb, shoulder, and inner ear.

This article describes the anatomy and function of the saddle joint. It also describes problems that can occur with this type of joint and ways to treat them.

Anatomy

There are 360 joints in the adult body. A joint is a structure that connects two or more bones. Joints help in different types of movements and also bear weight. A saddle joint is moveable, highly flexible, and has a unique design.

Structure

A saddle joint is a type of synovial joint. A synovial joint is the most common type of joint in the human body.

The unique structure of a synovial joint includes two bony surfaces surrounded by a fibrous joint capsule. The capsule is lined with a thin layer of slippery, viscous fluid called synovial fluid. Synovial fluid lubricates and maintains the articular cartilage and bones inside the joint capsule.

The articular cartilage serves as a shock absorber that covers the ends of bones where they join to form joints. It allows the bones in a synovial joint to glide over one another when movement occurs.

A saddle joint consists of one concave bone shaped like a saddle that interlocks with another convex bone that resembles a rider on a horse resting on it. This configuration is called concavoconvex.

Location

There are three saddle joints in the human body. They can be found in the following locations:

• Base of the thumb: The joint at the base of your thumb is a saddle joint. This joint, called the first carpometacarpal joint, is located between the trapezium and the first metacarpal bone at the base of your thumb.
• Shoulder: The saddle joint in your shoulder is the sternoclavicular joint. It is one of four joints that form the shoulder and the only joint that connects your arm to your body. This joint is located where your clavicle, or collarbone, connects to your sternum, or breastbone, at the base of your neck.
• Ear: The incudomalleolar joint is a saddle-shaped joint, which is located in your inner ear. It sits between the incus (anvil) and the malleus (hammer) in your ear. These two small bones are key to the processes involved in hearing and interpreting sounds.
  

Function

Most joints, like saddle joints, allow movement. A saddle joint stabilizes the bones in the joint while providing flexibility to move in a wide range of motion that includes movement back and forth and side to side.

Motor Function

Saddle joints are classified as biaxial joints. In biaxial joints, movement occurs at two distinct axes at right angles to each other. These joints support movement on two planes known as flexion-extension and abduction-adduction, which allows for bending motions in several directions.

Flexion and extension are two opposite movements. Flexion describes a movement that decreases the angle between two bones, such as bending the joint. An extension is a movement that increases the angle between the two bones, such as straightening the joint.

Saddle joints also support abduction and adduction. Abduction is the movement of a structure from its midline reference point, while adduction is the movement of a structure when it is added back to the midline reference point. For example, in the carpometacarpal joint, abduction involves moving the thumb in an anterior motion away from your hand, while adduction involves returning or "adding" the thumb back to your hand.

The ability of the saddle joints to allow for flexibility in different directions supports the following functions:

The range of motion provided by the carpometacarpal joint at the base of your thumb gives you opposable thumbs that allow you to move your finger across your palm to touch the other fingers on your hand. This allows you to hold objects, establish strong grips, and manipulate various things, such as tools.

The range of motion of the sternoclavicular joint is involved in every movement of the upper limb. It is needed to perform the upper limb's full range of motion.

The range of motion of the incudomalleolar joint is dependent on sound. While it is functionally immobile at normal sound-pressure levels, its mobility depends on frequency, with the joint becoming more mobile at frequencies above 2 kilohertz (kHz).

Sensory Function

The incudomalleolar joint is crucial to accurate hearing. It is part of the chain of ossicle, which sends sound from the outer ear to the inner ear through mechanical vibration. These joints may also provide a protective filtering effect that reduces the possible damage of peak amplitudes on the inner structure of the ear.

Associated Conditions

Below are issues that can occur in the saddle joint.

Arthritis

Saddle joints are susceptible to arthritis, a common disorder affecting all synovial joints. There are more than 100 types of arthritis. These conditions can result from overuse, wear-and-tear from aging, autoimmune disease, metabolic disease, genetics, or infection.

Osteoarthritis is the most common type of arthritis. It can cause the cartilage to wear down, leaving the ends of the two bones in the saddle joint unprotected. The friction of the bone on bone during movement can cause joint pain, stiffness, and inflammation.

Other types of arthritis that can affect the saddle joints include the following:

• Rheumatoid arthritis
• Psoriatic arthritis
• Spondyloarthritis
• Gout
• Infectious arthritis

The effect of arthritis can worsen and lead to debilitating pain and eventual deterioration of the bones in the joint. As a result, the condition can interfere with your ability to use the affected joint and function normally. Arthritis can impact saddle joints in the following ways:

• Thumb arthritis in the carpometacarpal joint
• Arthritis of the sternoclavicular joint
• Arthritis of the incudomalleolar joint
• Hearing loss when rheumatoid arthritis is present

Sprained thumb

A sprained thumb is a condition that occurs when the ligaments that support the thumb are stretched past their normal limits. It can happen in a fall when you bend your thumb backward, away from the palm of your hand, or fall onto an outstretched hand.

A sprained thumb is often called "skier's thumb" because it often results when you fall with your hand strapped to a ski pole. When this injury occurs as a result of overuse over time, it is called "gamekeeper's thumb."

Sprained thumbs usually involve injury to the ulnar collateral ligament (UCL), located inside the carpometacarpal joint. The injury can make the carpometacarpal joint unstable.

Sternoclavicular Injury

Injuries to the sternoclavicular joint can range from a mild sprain to a fracture of the clavicle. A strong blow can dislocate the joint from its normal position. Injuries to this joint are often caused by a high-impact event such as a hard fall or collision during contact sports or an auto accident.

When the sternoclavicular joint is dislocated backward, deep into the upper chest, it can present life-threatening problems with breathing or blood flow.

Incudomalleolar Injury

The most common causes of injury to the incudomalleolar joint include the following:

• Insertion of a foreign body into the external ear canal
• Incudomalleolar joint dislocation
• Skull trauma
• Head injuries with or without fractures
• Ear infections
  

Rehabilitation

The type of rehabilitation advised for a saddle joint varies based on the extent of your disease or injury and the joint affected. Conservative methods are often advised when the problem does not pose a life-threatening or other danger to your long-term well-being.

Rehabilitation for the carpometacarpal and sternoclavicular joint may include:

• Icing the joint for 5 to 15 minutes daily
• Nonsteroidal anti-inflammatory drugs (NSAIDs), such as aspirin, ibuprofen, or naproxen, to help reduce inflammation and pain
• A splint or cast to keep the thumb or shoulder to promote healing
• Physical therapy and/or prescribed exercises to help stabilize the affected joint, increase flexibility, and build endurance
• Therapeutic ultrasound
• Corticosteroid injection to relieve pain and inflammation
• Closed reduction (manipulation of the clavicle back into place without surgery)

When surgery is necessary to repair a joint injury or restore a joint damaged by disease, the following options may be used:

• Joint arthroscopy to fix tears in the soft tissue and remove free-floating cartilage
• Joint resurfacing to replace part of the affected joint
• Ostomy to cut and remove bone or add a wedge of bone near a damaged joint
• Synovectomy to remove most or all of the damaged synovium via open surgery or arthroscopy
• Arthrodesis, or fusion, to join two bones together using pins, plates, or rods to make one continuous joint
• Total joint replacement to replace the natural joint with an implant made from a combination of metal, plastic, and/or ceramic materials

Dislocation of the incudomalleolar joint may require the following types of treatment:

• Bone cement to repair traumatic incudomalleolar joint separation
• Total or partial prostheses
• Surgical reconstruction of the ossicular chain
  

Summary

A saddle joint is a type of synovial joint that supports a wide range of movement. Saddle joints can move up and down and back and forth.

While there are over 300 joints in your body, only three are saddle joints. These are found in your thumb, shoulder, and ear.