The human skull, or cranium, serves as the protective bony casing for the brain. The thickness of this bony vault is not a single, static measurement but varies widely based on location, age, and individual factors. Understanding this variability is important for grasping how the skull protects the delicate neural tissue inside from external forces.
Average Measurements and Layered Structure
The total thickness of an adult human skull generally falls within a range of approximately 5 to 10 millimeters, though extremes can reach up to 14 millimeters in certain areas. An overall average measurement for the cranial vault often centers around 6.5 millimeters for males and 7.1 millimeters for females, demonstrating a slight difference between sexes.
The skull bone is made up of three distinct layers that provide strength and shock absorption. The outermost layer is the external table, a thick plate of compact cortical bone. Beneath this lies the inner table, which is thinner, denser, and more brittle, sometimes called the vitreous table due to its fragility.
Separating these two hard layers is the diploë, a layer of spongy, cancellous bone that resembles a honeycomb structure. The diploë contributes significantly to the overall thickness, accounting for approximately 54% of the bone’s volume in some regions. This porous middle layer is a defining feature of the cranial vault.
Regional Differences in Skull Thickness
The skull’s thickness is not uniform across its surface; it varies considerably depending on the specific bone and location. The thickest regions are typically the occipital bone (back of the head) and parts of the frontal bone (forehead). These areas, which are generally more exposed to potential impact, can average over 10 millimeters in thickness.
In contrast, the thinnest area of the cranial vault is the squamous part of the temporal bone, located above the ear. Measurements here can be as low as 3 to 4 millimeters. This thinning occurs because the temporal bone houses delicate ear structures and provides attachment points for the powerful temporalis muscle.
The overall pattern of thickness follows a functional design, with the posterior and anterior regions offering the greatest mechanical protection. The internal surface of the skull also features ridges and grooves that contribute to its structural integrity. Protection is managed through shape and composition, not just simple thickness.
How Thickness Changes Throughout Life
A human skull begins development as a thin, incomplete structure in infancy, characterized by soft spots known as fontanelles. These fibrous membranes allow for rapid brain growth and flexibility during birth. They gradually close as the bony plates expand and fuse, and total skull thickness increases rapidly during childhood and adolescence, establishing adult dimensions.
As a person moves into old age, the skull undergoes further remodeling, although overall thickness remains relatively stable. Studies indicate a relationship between age and the thinning of the inner and outer cortical tables, particularly in females, with decreases ranging from 36% to 60% between the ages of 20 and 100 years. The diploë layer can also become more porous and irregular due to decreased bone mineral density.
While total skull thickness does not drastically diminish, the internal architecture of the diploë changes, with its density increasing over time due to progressive ossification. Males tend to maintain a thicker diploë layer, especially in the frontal region.
The Relationship Between Thickness and Injury Resistance
The layered structure of the skull is a sophisticated adaptation for dissipating energy and is directly related to its injury resistance. When an external force impacts the head, the tough outer table initially absorbs the blow, bending inward slightly. The inner table, being more brittle, tends to fracture first in tension as the outer layer deforms.
The diploë layer functions like a biomechanical crumple zone, similar to the design of a car. Its spongy, porous nature allows it to absorb and distribute impact energy by fracturing and deforming, preventing the force from being transmitted directly to the brain. This energy absorption minimizes the deformation of the entire skull, which is a major factor in preventing traumatic brain injury.
Skull thickness is one component of the fracture threshold, which is the amount of force required to cause a break. Studies using finite element modeling show that increased thickness decreases the deformation of the skull upon impact. However, injury severity is also dependent on the velocity and area of contact; a high-velocity impact concentrated on a small area requires less force to cause a fracture than a low-velocity impact spread over a large area. Fracture risk is closely correlated with the tensile strain experienced by the outer table, with forces between 980 and 1334 Newtons capable of causing penetration in certain areas of the parietal bone.