The petrous bone is part of the human skull. It belongs to the temporal bone (os temporale) and is located at the base of the skull. It is also known as the Rock Pyramid because of its pyramid-like shape . Surrounded by the petrous bone is the inner ear, which surrounds the vestibular organ and 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 is found only in bones that are not yet fully developed: during embryonic development, bones form from woven bone to form the basic framework of the skeleton . However, parallel collagen fibers reinforce it in other bones, turning the woven bone into a lamellar bone. However, the petrous bone is different – even in adult humans it consists of the original network. As a result, it is less stable than other bones and can therefore break more easily.

Anatomy & Structure

The anatomical terms used to describe the petrous bone are based on its rough geometric shape, which resembles a three-sided pyramid. The tip of the petrous bone (apex) is located in the cranial 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 also belong to the temporal bone.According to 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 in the precise description of fractures. In its entirety, the petrous bone belongs to the temporal bone (Os temporale). The canal of the auditory tube (Canalis musculotubaris) is one of three main approaches in the petrous bone and connects it to the middle ear . Nerves can reach the pyramidal structure through the internal acoustic pore and the stylomastoid foramen.

Function & Tasks

As a bone, the petrous bone performs general protective and stabilizing functions. In its particular case, it protects the organ of balance and the cochlea that it surrounds. These two structures make up the inner ear. The vestibular system consists of semicircular canals that contain fluid and are lined with hair cells . In conjunction with loose, bone-like solid bodies that lie in the vestibular system, these sensory cells can determine whether the person is standing upright or adopting 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 system, but also in the cochlea or cochlea. It contains auditory cells that react sensitively to the pressure of sound waves and are therefore responsible for the perception of sounds. The pitch is encoded by the location of the stimulation: low frequencies consist of long sound waves that cannot penetrate far into the cochlea, while the highest audible tones with their very short sound waves penetrate to the core of the cochlea. This phenomenon is based on the physical properties of sound waves and the anatomy of the cochlea, which spirals and narrows inwards.

Diseases

If too much pressure is applied to the petrous bone, the bone may fracture. The petrous bone fracture often occurs together with other skull fractures and can be accompanied by other clinical pictures. The brain is also involved in craniocerebral trauma ; Physicians determine the degree of severity using three levels, with the lowest being the concussion .

It often proceeds without long-term sequelae, 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 produces permanent lesions. A petrous bone fracture can also occur in polytrauma involving many parts of the body. The petrous bone is more prone to fracture than other bones because it is a woven bone that is not stabilized by additional collagen lamellae. Burst fractures are therefore particularly common in petrous bone fractures.

Another clinical picture that specifically affects the petrous bone is Gradenigo syndrome or pyramidal apex syndrome. The name of the clinical picture goes back to the Italian physician Giuseppe Conte Gradenigo, who brought the syndrome into the specialist literature in 1904. Doctors understand it to be an inflammatory complication that can follow an acute middle ear infection . A purulent discharge due to the inflammation is typical. People with Gradenigo syndrome often experience pain behind the eyes, facial pain , and double vision due to paralysis of the eye muscles .

Symptoms are due to damage to the cranial nerves involved : the acute otitis media travels to the skull and either spreads to the cranial nerves or causes tissue swelling (ie, edema develops ) which in turn affects the cranial nerves. The nerves affected by Gradenigo syndrome are the trigemus nerve, the abducens nerve, and/or the occulomotor nerve.