The question of how many joints there are in the human body is a difficult one to answer because it depends on a number of variables. This includes:
- The definition of joints. Some define a joint as a point where 2 bones connect. Others suggest it is a point where bones connect for the purpose of moving body parts.
- The inclusion of sesamoids. Sesamoids are bones imbedded in tendons, but not connected to other bones. The patella (kneecap) is the largest sesamoid. These bones vary in number from person to person.
- The age of the human. Babies start out with about 270 bones. Some of these bones fuse together during growth. Adults have about 206 named bones, with 80 in the axial skeleton and 126 in the appendicular skeleton.
In short, there’s no definite answer to this question. The estimated number is between 250 and 350.
The human body has three main types of joints. They’re categorized by the movement they allow:
- Synarthroses (immovable). These are fixed or fibrous joints. They’re defined as two or more bones in close contact that have no movement. The bones of the skull are an example. The immovable joints between the plates of the skull are known as sutures.
- Amphiarthroses (slightly movable). Also known as cartilaginous joints, these joints are defined as two or more bones held so tightly together that only limited movement can take place. The vertebrae of the spine are good examples.
- Diarthroses (freely movable). Also known as synovial joints, these joints have synovial fluid enabling all parts of the joint to smoothly move against each other. These are the most prevalent joints in your body. Examples include joints like the knee and shoulder.
There are six types of freely movable diarthrosis (synovial) joints:
- Ball and socket joint. Permitting movement in all directions, the ball and socket joint features the rounded head of one bone sitting in the cup of another bone. Examples include your shoulder joint and your hip joint.
- Hinge joint. The hinge joint is like a door, opening and closing in one direction, along one plane. Examples include your elbow joint and your knee joint.
- Condyloid joint. The condyloid joint allows movement, but no rotation. Examples include your finger joints and your jaw.
- Pivot joint. The pivot joint, also called the rotary joint or trochoid joint, is characterized by one bone that can swivel in a ring formed from a second bone. Examples are the joints between your ulna and radius bones that rotate your forearm, and the joint between the first and second vertebrae in your neck.
- Gliding joint. The gliding joint is also called the plane join. Although it only permits limited movement, it’s characterized by smooth surfaces that can slip over one another. An example is the joint in your wrist.
- Saddle joint. Although the saddle joint does not allow rotation, it does enable movement back and forth and side to side. An example is the joint at the base of your thumb.
The adult human skeletal system has a complex architecture that includes 206 named bones connected by cartilage, tendons, ligaments, and three types of joints:
- synarthroses (immovable)
- amphiarthroses (slightly movable)
- diarthroses (freely movable)
Although the actual number of joints in any one person depends on a number of variables, the estimated number is between 250 and 350.