The posterior portion of the sacrum is characterized as what type of joint?

• A. Synovial joint
• B. Ligamentous joint
• C. Cartilaginous joint
• D. Hinge joint

Answer: (B)

The posterior portion of the sacrum is characterized as a ligamentous joint because it is primarily held together by strong ligaments rather than a synovial or cartilaginous structure. In the case of the sacrum, the posterior sacroiliac ligaments connect the sacrum to the ilium of the pelvis, creating a stable but slightly movable joint. This ligamentous structure allows for some degree of flexibility and stability in the lower back and pelvis, accommodating for the stresses placed on the spine and pelvic region during movement.

Ligamentous joints, also known as syndesmoses, involve the connection of bones via dense connective tissue, which limits movement mainly to that of flexibility and stability rather than mobility. This is distinctly different from synovial joints, which have a fluid-filled joint cavity allowing for a broader range of movement, or cartilaginous joints, which permit more flexibility than ligaments but less than synovial joints.

Thus, recognizing that the primary role of the posterior sacral region is to provide support and stability through ligamentous connections is crucial in understanding the biomechanics of this area.