The occipital bone is a trapezoidal-shaped bone membrane located at the inferior and posterior section of the skull. It appears like a saucer and curves in itself so as to be able to hold the back portion of the brain as well as protect it.
The occipital bone is one of the 7 bones that fuse with one another to make the skull and is adjacent to 5 of the cranial bones. It also features the foramen magnum, an oval-shaped large opening via which the cavity in the cranium articulates with the vertebral canal. The occipital bone is placed on the spinal column and creates a joint which helps the head move.
Occipital bone – Location, Anatomy, and Function
Located at the lower back part of the skull, the occipital bone is categorized into 4 areas, i.e., basioccipital, squama occipitalis, and ex-occipitalis with 2 condylar parts. These 4 regions occur around the foramen magnum and are integral to the anatomy of the occipital bone.
Each of the above 4 parts grow separately after birth and are attached via cartilage, consequently creating a ring around the foramen magnum. The 4 bones unite into single bone only after a child has become 6 years old. Most of the occipital bone is made up of the squamous region which is created by a postero-superior concave plate located at the back of the foramen magnum. The 2 lateral regions are situated on either side of the large opening, while the basilar region is situated anteriorly.
The foramen magnum acts as an entry and exit passageway for the brainstem’s medulla oblongata and the spinal cord, thereby facilitating the communication between the spinal cord and the brain. Other structures that pass via the foramen magnum include the accessory nerve’s spinal roots, the vertebral artery, meninges, ligaments, spinal vein, subarachnoid space, meningeal branches, the cerebellum tonsils, the posterior and anterior spinal arteries, and the sympathetic plexuses.
The occipital bone also offers connection to many neck muscles as well as muscles of the upper back, such as rectus capitis lateralis, longus capitis, rectus capitis posterior minor, splenius capitis, and sternocleidomastoid, etc.
The occipital bone features inferior and superior angles and borders. The inferolateral borders connect on either side with the temporal bone’s mastoid section, while the superolateral borders connect to the parietal bone’s back border. The superior angles lies at the junction of the superolateral borders. In adults, the inferior angle connects to the sphenoid bone’s body, while the superior angle communicates with the parietal bone’s occipital angles. The superior angle occurs at the confluence of the skull’s lamboid and sagittal sutures around the lambda. The lateral angle is present on either side of the junction of the inferolateral and superolateral borders.
The basic structure of the occipital bone is discussed below.
- The squamous region/Squama: This expanded plate section of the occipital bone has the following anatomical features:
- The external surface: The external occipital protuberance is situated on the external surface’s midline; it acts as a connection point for the trapezius muscle. The external surface also features 4 nuchal lines which act as an attachment point for the back and neck ligaments and muscles. Nuchal lines are ridges or curved lines that form due to elevation of the bone caused by excess stress exerted on the specific areas by the attached muscles. The 4 nuchal lines are:
- The apex nuchal line acts as a connection site for galea aponeurotica and continues on both sides from the external protuberance to the temporal bone’s mastoid process.
- The medial nuchal line acts as a connection point for the nuchal ligament occurring at the back of neck. It extends from the occipital protuberance downwards to the foramen magnum.
- The superior nuchal line, lying below the apex nuchal line, acts as a connection point for the splenius capitis, trapezius, and sternocleidomastoid muscles.
- The inferior nuchal line acts as a connection point for the semispinalis capitis muscle. It starts laterally from the middle section of the medial nuchal line, on both sides, and continues alongside the superior nuchal line.
- The internal surface: Concave-shaped internal surface of the squamous region features the cruciate eminence which is divided into 4 fossae. The lower and upper divisions are respectively quadrilateral and triangular in shape. The cerebrum’s occipital lobes occur in the upper 2 divisions, while the cerebellar hemispheres occur in the lower 2 divisions. The intersection point of the 4 divisions is home to the internal occipital protuberance.
- The occurrence of dural venous sinuses causes grooves to form on the internal surface.
- Torcular herophili, the angle formed via union of the transverse and superior sagittal sinuses, occurs as a dent on either side of the internal occipital protuberance. Transverse sinus ridges extend horizontally and rightwards from the protuberance and run into the sigmoid sinus grooves.
- The transverse occipital sulcus borders act as a connection point for tentorium cerebelli, a compartment made by dura mater, which separates the cerebellum and the cerebrum. The falx cerebri, another dural fold, also connect to the sulcus margins and separate the brain’s left and right hemispheres.
- The falx cerebelli is attached to the lower division and the occipital sinus is present in its attached border. The lower section of the internal occipital crest features a depression that houses the cerebellum vermis.
- The lateral part/condylar sections
- The condylar parts are situated laterally on either side of the foramen magnum and consist of 2 kidney-shaped projections known as occipital condyles with communicate with the C1 or the atlanto-occipital joint. The capsules of the C1 articulations are connected to condyles’ borders, the medial side both these condyles feature a bony prominence for connecting to the alar ligament, and the bony and short hypoglossal canal occurs at the bottom part of the condyles. The canal hypoglossal acts as the entry pathway for the ascending pharyngeal artery’s meningeal branch and the exit passageway for the twelfth cerebral nerve.
- The condylar fossa occurs at the back of both condyles, while the condylar canal’s opening occurs at the fossa’s base. The side of the condyle features the jugular notch and near it lies the sigmoid sinus groove on the bone’s cerebral surface. Emissary veins from the transverse sinus pass across these canals.
- A quadrilateral-shaped bony plate called the jugular process runs laterally form the condyle’s back half and its coarse underside acts as a connection point for the lateral atlanto-occipital ligament and the rectus capitis lateralis muscle. The paramastoid process projecting from the surface may communicate with the atlas transverse process. The superior surface of the jugular process feature a deep ridge which bends towards the front and middle and runs alongside the jugular notch. It acts as the home of the last part of the transverse sinus. Additionally, the notch forms the back part of the jugular foramen.
- The superior surface of the lateral region features an oval-shaped protuberance known as the jugular tubercle, over the hypoglossal canal. Glossopharyngeal, accessory, and vagus nerves run along the oblique ridges present at the back of the tubercle.
- Basilar part
- The basilar part of the occipital bone runs upwards and forwards from the opening of the foramen magnum. The pharyngeal tubercle is located around one cm in front of the opening. It acts as a connection site for the pharynx’s fibrous raphé. The anterior atlanto-occipital membrane joins just in front of the opening, while the rectus capitis anterior and longus capitis insert on each side of the middle line.
- The basilar part’s upper surface features a shallow groove or ridge that runs forwards and upward from the foramen magnum. It offers support to the medulla oblongata and also acts as a connection point for the membrana tectoria, next to foramen magnum’s border. The lateral borders of this surface also contain tiny indentations for the inferior petrosal sinus. In children, the sphenoid bone’s body attaches to the basilar part via a cartilaginous plate. This plate undergoes ossification only after a person becomes 25 years old, thereby making the sphenoid and occipital bones continuous.
Occipital bone pain
Occipital bone pain or occipital headaches can occur due to varied causes such as nerve disorders, blood vessel anomalies, and severe traumatic injuries, etc. A few common causes of occipital bone pain are listed below:
- Occipital neuralgia: It is a condition marked by occipital nerve irritation which in turn triggers intense pain in the occipital bone region at the back of the head. The occipital nerves come out from the spinal cord at the back-top section of the neck and spread across the scalp. Headaches associated with occipital neuralgia are usually restricted to one side of the head and follow the pathways of the nerves.
- The condition does not have any brain-linked symptoms and may possibly occur due to vasculitis, neck tumors, forward titling of the head for long periods, injury, and arthritis.
- Primary treatments involve neck rest and intake of medications like anti-inflammatory drugs and muscle relaxants.
- Subarachnoid hemorrhage: The brain is protected by 3 tissue layers of the meninges, i.e., the outermost meningeal layer, the middle arachnoid mater layer, and the innermost pia mater layer which covers the surface of the brain. The condition of bleeding between the arachnoid and pia mater is referred to as subarachnoid hemorrhage. It is a medical emergency.
- The most distinctive symptom of subarachnoid hemorrhage is an intense headache at the back of the head in the occipital bone region. Other associated symptoms include vomiting, nausea, visual problems, disorientation, and reduced consciousness levels.
- Basilar skull fracture: A fracture that affects the lowest section of the cranium near the entry point of the spinal cord is known as basilar skull fracture. It usually affects the occipital bone and is a serious injury that requires hospitalization.
- Common symptoms include intense headache near the occipital bone area, hearing loss, sense of smell loss, visual problems, and bruise development around the eyes or behind the ears.
- The occipital bone region is tough and thick. Hence, considerable force is needed to cause basilar skull fractures. This is one of the reasons why the condition comes with a high risk of brain injury. Leakage of spinal fluid due to meningeal rupture is a common complication of the condition. It is possible for spinal fluid to drain out from the nose or the ear.