The petrous bone is a bone and is part of the human skull. It is located at the base of the skull and is part of the temporal bone (Os temporale). In its pyramid-like basic shape lies the inner ear with the organ of equilibrium and the cochlea. Of clinical importance for the petrous bone are mainly the petrous bone fracture as well as the Gradenigo syndrome.
What is the petrous bone?
The petrous bone is a part of the human skull. It is part of the temporal bone (Os temporale) and is located at the base of the skull. It is also known as the petrous pyramid because of its pyramid-like shape. Surrounded by the petrous bone is the inner ear, which surrounds the organ of balance as well as the cochlea. A special feature of the petrous bone is its bone structure: it forms a so-called woven bone. Normally, this type of bone tissue occurs only in bones that are not yet fully developed: During embryonic development, bones form from woven bone to form the basic skeletal framework. However, collagen fibers running in parallel reinforce it in other bones, turning the braided bone into a lamellar bone. In the case of the petrous bone, however, it is different – even in adult humans it consists of the original meshwork. As a result, it is less stable than other bones and, accordingly, can break more easily.
Anatomy and structure
The anatomical names used to describe the petrous bone are based on its rough geometric shape, which resembles a three-sided pyramid. The petrous bone apex is located in the skull bone between the occipital bone (Os occipitale) and the sphenoid bone (Os sphenoidale). The base of the petrous bone is not clearly demarcated from other parts of the bone, but in adult humans it merges smoothly into the pars squamose and the pars mastoidea; both parts are also part of the temporal bone. In accordance with the pyramid analogy, medicine also speaks of surfaces or facies and angles or anguli in order to make more precise statements about the petrous bone. This plays an important role especially in the precise description of fractures. In its entirety, the petrous bone belongs to the temporal bone (Os temporale). The canal of the Eustachian tube (Canalis musculotubaris) is one of three main accesses in the petrous bone and connects it with the middle ear. Nerves can reach the pyramidal structure through the porus acusticus internus and the foramen stylomastoideum.
Function and Tasks
As a bone, the petrous bone generally performs protective and stabilizing functions. In its specific case, it protects the organ of balance and the cochlea, which are surrounded by it. These two structures form the inner ear. The organ of balance is composed of arcuate ducts containing fluid and clothed with hair cells. In conjunction with loose, bone-like solids located in the organ of equilibrium, these sensory cells can determine whether a person is holding himself upright or assuming a different position in space, depending on the direction in which the fine extensions of the hair cells bend. This type of sensory cell is found not only in the vestibular organ, but also in the cochlea. It contains auditory cells that are sensitive to the pressure of sound waves and are thus responsible for the perception of tones. The pitch of the sound is coded by the location of the stimulus: low frequencies consist of long sound waves that cannot penetrate far into the cochlea, while the highest audible sounds penetrate to the innermost part of the cochlea with their very short sound waves. This phenomenon is due to the physical properties of sound waves as well as the anatomy of the cochlea, which spirals and narrows inward.
Diseases
If too much pressure is applied to the petrous bone, the bone may fracture. Fracture of the petrous bone often occurs with other skull fractures and may be accompanied by other medical conditions. Craniocerebral trauma also involves the brain; physicians determine the severity based on three levels, the lowest being concussion. It often has no long-term consequences, whereas severe traumatic brain injury or brain contusion is associated with prolonged unconsciousness immediately after the trauma (at least 60 minutes) and in many cases causes permanent lesions.A fracture of the petrous bone may also be present in polytrauma with many body parts involved. The petrous bone is more susceptible to fracture than other bones because it is a woven bone that has no additional collagen lamellae to stabilize it. Burst fractures are therefore particularly common in the petrous bone fracture. Another clinical presentation that specifically affects the petrous bone is gradenigo syndrome or pyramid tip syndrome. The clinical picture is named after the Italian physician Giuseppe Conte Gradenigo, who introduced the syndrome into the medical literature in 1904. Doctors understand it to be an inflammatory complication that can follow an acute middle ear infection. Purulent discharge due to the inflammation is typical. People with Gradenigo syndrome often suffer from pain behind the eyes as well as facial pain and can see double because of paralysis of the eye muscles. Symptoms are due to damage to the cranial nerves involved: acute otitis media travels into the skull either spreads to the cranial nerves or causes the tissue to swell (i.e., edema develops), which in turn affects the cranial nerves. The nerves affected by gradenigo syndrome are the trigeminal nerve, the abducens nerve, and/or the occulomotor nerve.
