Definition
Saddle joint is any joint where one bone forms a saddle or support for the other bone to rest on it, like a horse rider sitting on a saddle. It is a biaxial synovial joint which allows double motion due to the opposition of two surfaces, one of which is convex in one direction and concave in the other direction.
As compared to a hinge or gliding joint, a saddle joint provides more flexibility and stability to the bones.
It is one of the five types of synovial joints in the human skeletal system. It is also known as a sellar joint as bones meeting at the joint form an X shape.
Movements
Because saddle-shaped joint has articular surfaces which are concave in one direction and convex in another, so it allows movements in all directions except for axial rotation. It is the most versatile type of joint in human beings.
The bone resting on the saddle moves in an oval shape quite similar to a condyloid joint.
The types of movement a saddle joint allows are flexion, extension, adduction and abduction.
All types of gripping motion are provided by saddle joint such as using a pen, grasping your mobile phone and steering the wheel of your car.
Examples
The carpometacarpal joint at the base of thumb which is formed between the first metacarpal and trapezium bone is a saddle joint.
Both the thumbs have one saddle joint each which is the only example of this type of joint found in the human body.
