Classifying joints

Fibrocartilage is packed with thick collagen bundles, so it resists both squeezing and pulling. It forms the intervertebral discs and the knee menisci. Hyaline cartilage covers bone ends, and elastic cartilage forms the ear flap.

• Hyaline cartilage:
• Correct: Fibrocartilage:
• Elastic cartilage:

A ligament is a cord of dense regular connective tissue that joins bone to bone. A tendon joins muscle to bone.

• Muscle to bone:
• Correct: Bone to bone:
• Skin to muscle:

A synovial membrane is a connective tissue membrane that lines the spaces inside movable joints and releases a thick, slippery fluid. The periosteum covers bone, and serous membranes line body cavities.

• Correct: Lines the cavities of movable joints and releases a slippery fluid:
• Covers the outer surface of a bone:
• Lines the body cavities and covers the organs in them:

A joint is defined by two structures meeting and being held together, not by movement. Many joints barely move.

• Correct: Any place where a bone meets another bone, a cartilage or a tooth: Correct. Any meeting point of bone with bone, cartilage or tooth is a joint.
• A place where two bones move freely against each other: This describes only freely movable joints. Skull sutures are joints too, and they do not move in an adult.
• A cord of dense connective tissue that joins two bones: This describes a ligament, which can help hold a joint together but is not the joint itself.
• The smooth cartilage that covers the end of a bone: This describes the cartilage on a bone end, one part found in some joints, not the joint itself.

The periodontal ligament is the thin layer of collagen fibers running from the tooth root to the socket wall. It forms the gomphosis.

• Interosseous membrane: An interosseous membrane is the broad sheet of fibers between two long bones, such as the radius and ulna.
• Correct: Periodontal ligament: Correct. The fibers around the tooth root are the periodontal ligament.
• Articular cartilage: Articular cartilage is hyaline cartilage covering bone ends, not fibers around a tooth root.
• Periosteum: The periosteum is the membrane covering the outer surface of a bone, not the fibers tying a tooth to its socket.

Sutures, gomphoses and syndesmoses are the three fibrous joints. The pubic symphysis and the epiphyseal plate are cartilaginous.

• Correct: Coronal suture: Yes. A suture is a fibrous joint between skull bones.
• Correct: Tooth in its socket: Yes. A tooth in its socket is a gomphosis, a fibrous joint.
• Correct: Radius and ulna joined by an interosseous membrane: Yes. Bones joined by an interosseous membrane form a syndesmosis, a fibrous joint.
• Pubic symphysis: No. The pubic symphysis joins bones with fibrocartilage, so it is cartilaginous.
• Epiphyseal plate: No. The epiphyseal plate is hyaline cartilage, a synchondrosis.

A fibrocartilage pad makes a symphysis, and slight movement makes it an amphiarthrosis. The intervertebral discs and the pubic symphysis fit this pair.

• Correct: Symphysis; amphiarthrosis: Correct. Fibrocartilage pad plus slight movement is a symphysis and an amphiarthrosis.
• Synchondrosis; synarthrosis: A synchondrosis uses hyaline cartilage and barely moves. This joint uses fibrocartilage and bends.
• Suture; synarthrosis: A suture is a fibrous joint between skull bones with no cartilage pad.
• Syndesmosis; diarthrosis: A syndesmosis is fibrous, not cartilaginous, and it is not freely movable.

A suture is an immobile joint, a synarthrosis. An intervertebral disc allows slight movement, so it is an amphiarthrosis.

• Correct: Amphiarthrosis: Correct. The disc is slightly movable.
• Diarthrosis: A diarthrosis is freely movable, like a synovial joint. One disc allows only slight movement.
• Synostosis: A synostosis is a joint that has turned to bone. A healthy disc is fibrocartilage.
• Syndesmosis: A syndesmosis is a structural subtype (a fibrous joint), not a functional class, and a disc is cartilaginous.

Sutures and gomphoses are immobile fibrous joints, while syndesmoses are slightly movable fibrous joints. One structural class spans two functional classes.

• The joint between the sacrum and hip bone moves freely: The joint between the sacrum and hip bone is synovial but held so tightly by ligaments that it barely moves, so it is close to an amphiarthrosis.
• Synchondroses are slightly movable amphiarthroses: Synchondroses such as the epiphyseal plate are nearly immobile, so they are synarthroses, not amphiarthroses.
• Correct: Fibrous joints include immobile and slightly movable ones: Correct. Fibrous joints span synarthroses and amphiarthroses.
• The two schemes are two names for one grouping: The two schemes are separate. Structure predicts function most of the time, but that joint and the synchondroses show the groupings differ.