The external occipital protuberance, also known as the inion, is a bony projection located at the posterior aspect of the skull. It is a key landmark in the study of human anatomy and has important functions in the body.
Structure
The external occipital protuberance is a small, triangular shaped projection located on the midline of the posterior skull. It is formed by the external occipital bone, which is a flat bone that forms the posterior part of the skull. The external occipital bone is composed of two parts: the occipital squama and the occipital condyles.
- The occipital squama is the larger, more posterior portion of the external occipital bone. It is a thin, flat bone that forms the base of the skull and contributes to the formation of the posterior cranial fossa. The occipital squama is marked by several prominent features, including the external occipital protuberance, the external occipital crest, and the superior nuchal lines.
- The occipital condyles are a pair of rounded protuberances located at the inferior aspect of the external occipital bone. They are the articulating surfaces for the first cervical vertebra (the atlas) and are important in the movement of the head.
Location
It is located at the midline of the posterior skull, just above the nuchal line. It is situated between the two external occipital condyles and is easily palpable through the skin.
Function
The external occipital protuberance has several important functions in the body. One of its main functions is to serve as an attachment point for several muscles and ligaments.
The trapezius muscle, a large, flat muscle that spans the neck, back, and shoulder, attaches to the external occipital protuberance. The trapezius muscle is responsible for moving and stabilizing the scapula, as well as assisting in neck movement.
The ligamentum nuchae, a strong band of connective tissue that runs along the midline of the neck, also attaches to the external occipital protuberance. The ligamentum nuchae helps to support the head and neck and is important in maintaining proper posture.
In addition to its role in muscle and ligament attachment, the external occipital protuberance also serves as a point of reference for measuring head size in infants and children. The occipitofrontal circumference, or head circumference, is used to assess the growth and development of infants and children. The external occipital protuberance is one of several points used to measure head size, along with the forehead, the temples, and the back of the head.
Clinical Significance
One of the main clinical uses of the external occipital protuberance is in the measurement of head size in infants and children. As mentioned above, the occipitofrontal circumference is used to assess the growth and development of infants and children. Abnormal head size, either too small or too large, can be a sign of abnormal development or underlying medical conditions.
The external occipital protuberance is also used as a reference point in the measurement of cranial suture closure. Cranial sutures are the fibrous joints that connect the bones of the skull. In infants and young children, these sutures are open, allowing for growth and expansion of the skull. As the child grows, the sutures begin to close and fuse together. The degree of suture closure can be used to determine the age of the individual and can be important in forensic cases. The external occipital protuberance is used as a reference point in suture closure measurement, along with other points such as the lambdoid suture and the coronal suture.
In addition to its use in measurement, the external occipital protuberance is also a key landmark in the identification of cranial fractures. Fractures of the skull can be difficult to diagnose and may require imaging studies such as CT or MRI to confirm the diagnosis. The external occipital protuberance is often used as a reference point in the identification and classification of cranial fractures.
