The Anatomy of the Temporal Bone

The temporal bones are paired bones that help make up the sides and base of the skull (cranium). This places them lateral—to the side of—the temporal lobes of the brain’s cerebral cortex, ensuring that the cranium is properly supported and protecting the important structures there. As such, blunt trauma or fracture of these bones can lead to serious complications and brain injury, damage to the inner ear, as well as facial nerve problems. In addition, middle ear infections can spread to this bone. 

Anatomy

Structure & Location

In anatomy, each temporal bone is composed of five parts: the squama, the petrous, mastoid, and tympanic parts, as well as the styloid process. Here’s a quick breakdown:

• Squama: Forming the front, upper part of the temporal bone, the squama is thin, translucent, and described as “scale-like.” The outer surface is smooth and forms a convex shape, and, via the temporal line (a curved ridge running to the back and upward), it attaches to the temporalis muscle, which helps with chewing. The front end of the squama is serrated and connects with the zygomatic bone—one of two paired bones that form the cheeks and the sidewalls of the orbits (the openings where the eyeballs sit). The bottom end of this bone connects to the masseter, another major muscle for chewing. In addition, this part of the temporal bone is connected to ligaments regulating muscles in the upper portion of the mandible, or jawbone.
• Mastoid Part: Forming the rear part of the temporal bone, the mastoid part’s outer surface is rough and attaches to muscles that regulate the motion of eyebrows (occipitalis muscle) as well as those above the ear (superior auricular muscle). This portion is perforated and includes the mastoid foramen, an opening that allows a vein to access the transverse sinus (the area on each side of the head that drains blood from the back of the head) as well as an artery that provides oxygenated blood to the layer of tissue tasked with protecting the brain (dura mater). Moving downward, this portion becomes a conical projection—the mastoid process—which connects to muscles essential for head movement. The upper part of this portion has been observed to be hollow in places, but these openings grow smaller as you move down.
• Petrous Part: Also known as the “pyramid part,” because of its shape, the petrous part sits at the base of the skull at between the occipital (the part that represents the base) and sphenoid bones (the portion just beneath the temples). Moving upward, this portion has a base fused with the squama and mastoid portion, and an apex that sits between the occipital and sphenoid bones. The latter form the carotid canal, which allows important arteries to access the brain. Its anterior (front) surface forms the rear portion of the middle fossa (a cavity) in the base of the skull. A thinner portion goes over the tympanic cavity, the hole surrounding the middle ear bones. The posterior (rear) portion accesses the posterior fossa, which is the opening at the base of the skull that cradles the cerebellum and brain stem. At its center, there’s an opening—the internal acoustic meatus—that allows important nerves and arteries to pass through.
• Tympanic Part: The curved portion of bone beneath the squama and in front of the mastoid process, the tympanic part forms a major part of the external acoustic meatus, which is the pathway from outer to inner ear. Its middle portion contains the tympanic sulcus, a groove that attaches to the tympanic membrane, better known as the eardrum. At its rear surface, the tympanic part forms the rear boundary of the mandibular fossa, a groove which connects with the jaw bone. The outside surface is rough and attaches to the cartilage portion of the acoustic meatus, whereas its interior fuses with the petrous portion, just below the ear canal. Thin and sharp in its middle portion, it splits to enclose a portion of the styloid process (see below).
• Styloid Process: This is a narrow projection of bone coming out of the temporal bone. Variable in length, it is angled downward and forward, accessing on the inner side the tympanic part that encloses it, and on the outer side to ligaments that connect to the stylohyoid and other muscles involved with chewing motions. Its outer side is just next to the parotid gland (the source of saliva), which is also the site where the external carotid artery (which supplies features in the face and brain) crosses. This structure also attaches to the stylopharyngeus, a muscle on the underside of the head that connects to the pharynx.

Notably, the temporal bone attaches to the joint of the jaw bone—the temporomandibular joint—and is fused with other bones of the skull, including the occipital bone on the lower rear side, the parietal bone above that, the sphenoid bone on its front side, and the zygomatic (cheek) bone.

Anatomical Variations

Variations in the anatomy of the temporal bone are not uncommon and usually have to do with the size and shape of its many openings. The most commonly observed variations are:

• High Riding Jugular Bulb: This is when the jugular bulb, a vein structure located near the inner ear, ascends higher into the temporal bone than usual. This asymptomatic variation is important for surgeons operating on the inner ear to note, and it’s been reported to occur in up to 32% of cases. 
• Körner’s Septum: In some people, this septum—a dense, bony plate in the mastoid process—separates the mastoid process from the squama. This variation is also very common and studies contend it occurs in 28% of people.
• Anteriorly Placed Sigmoid Sinus: Found 34% of the time, this is when the groove in the mastoid portion of the temporal bone runs deeper than usual and appears more forward than is typical.
• Low Hanging Dura In Ears: This case, which is seen in up to 26% of people, is characterized by the membrane surrounding the brain (the dura) hanging lower than usual, impacting surrounding bony structures of the auditory canal.
• Mastoid Aeration: Differences in the shape of the mastoid portion of the temporal bone can affect how well the inner ear can adjust to air pressure changes.

Function

The temporal bone provides structural support for the skull, while protecting the cerebrum of the brain and surrounding membranes. In addition, this bone surrounds the middle and inner portions of the ear. Its lower portion connects with the mandible or jawbone to allow the mouth to open and close. Notably, a majority of the cranial nerves—nerves associated with sensation and perception—pass over this bone.

Given their position on the sides and back of the skull, these bones connect to a number of important muscle groups. In particular, the temporalis and masseter—muscles involved with chewing motion—are connected to the squama and styloid process. Furthermore, the more rear-facing parts are linked to the sternocleidomastoid and splenius capitis muscles, associated with neck and head motion. Finally, via its mastoid process, the bone is connected to the suprahyoid muscle, essential for swallowing.

Associated Conditions

A number of medical issues can arise in this part of the skull. While the temporal bone is relatively thick, blunt trauma can cause a fracture of this bone. This can lead to a number of serious complications, including damage to hearing, vertigo, facial paralysis (due to damage to the facial nerve), and bleeding in the ear as well as bone bruising. Notably, fractures can also lead to leaking of cerebral spinal fluid.

More common are fractures of the pterion, which is where the temporal bone joins with other major bones of the skull: the parietal, frontal, and sphenoid. This juncture is the weakest point of the skull. The middle meningeal artery, which supplies the dura and skull, passes right behind it. If injured or lacerated, blood collects and dangerously increases intracranial pressure. This can lead to seizures, nausea, vomiting, and limb weakness, among other symptoms.

Because the mastoid portion of the temporal bone is porous, middle ear infections spread to it, leading to a condition called mastoiditis. If untreated, the infection can spread further into the middle cranial fossa, a major region of the interior of the skull, and even the brain, itself, causing meningitis.

Rehabilitation

Depending on the severity of trauma and fracture of the skull, surgery may be necessary to correct the issue and take on bleeding and other issues that may arise. If there’s damage to the facial nerve, as is often the case, a nerve decompression surgery may be necessary to repair it and ease pressure on it. This, along with more conservative approaches, is effective in taking on facial paralysis; however, the decision to move ahead needs to be carefully weighed.

Leaking of cerebrospinal fluid following a fracture of the temporal bone increase the risk of meningitis, a type of brain infection. In addition, this issue can also impact structures in the ear, and lead to fluid leaking from there and the sinuses. These cases are best handled without surgery through the use of antibiotics to take on any infectious cultures, rest and elevation of the head during healing, as well as the use of other means of returning cerebrospinal fluid levels to normal. If, however, the leak doesn’t close up, a surgery is needed to correct the problem.

In more extreme cases where the middle meningeal artery is lacerated due to fracture of the temporal bone, prompt surgery may be required to take on the resulting bleeding. If the case is more moderate, doctors may opt to prescribe diuretic drugs.