A bony prominence is an anatomical term describing an area of the body where a bone lies very close to the skin’s surface. These locations lack substantial protective layers of fat or muscle tissue between the skeletal structure and the external environment. This proximity makes these specific locations vulnerable to external forces that can lead to tissue breakdown and injury. This article explores the physical characteristics of these landmarks and the physiological process that makes them susceptible to damage.
Anatomical Features
A bony prominence is defined by the minimal soft tissue coverage over the underlying bone structure. Unlike other areas of the body, these sites lack the substantial cushioning provided by thick layers of subcutaneous fat and muscle. This thin separation means that external force applied to the skin is rapidly transmitted directly to the skeletal surface.
The bone itself is covered by a thin but dense membrane called the periosteum, which is rich in blood vessels and nerves. This membrane provides nourishment to the bone and serves as an attachment point for tendons and ligaments. The skin overlying the prominence consists of the epidermis and dermis, which offer a limited protective barrier against mechanical stress.
The relative absence of deep, compliant tissue means the bone acts as a fixed, unyielding anchor point. When external pressure is applied, the soft tissues are compressed between this hard, internal surface and a hard, external surface. This compression is the primary structural factor that predisposes the area to damage.
Most Common Locations
Bony prominences are found throughout the body wherever the skeleton is near the surface without significant muscle mass. They are typically grouped into regions based on their location and the common positions that expose them to pressure.
Common locations include:
- The sacrum (the triangular bone at the base of the spine) and the ischial tuberosities (sit bones) in the trunk area.
- The greater trochanters on the sides of the hips and the malleoli (bony protrusions on either side of the ankle).
- The heels, where the calcaneus is close to the skin and bears significant weight when lying down.
- Upper body prominences like the elbows (olecranon process) and the scapulae (shoulder blades).
- The occiput, located at the back of the skull, which is exposed to pressure when lying flat on the back.
Mechanism of Injury Vulnerability
The vulnerability of a bony prominence stems from the destructive mechanism of tissue compression. When a sustained external load is applied, the soft tissue layers are forcefully squeezed between the bone and the supporting surface. This compression exerts pressure that is often concentrated and intense, initiating a cascade of biological damage.
The external force must overcome the natural pressure within the body’s microcirculation to cause injury. The pressure must exceed the arterial capillary pressure (around 32 millimeters of mercury, or mm Hg) to fully impede blood flow. Pressures above the venous capillary closing pressure (8 to 12 mm Hg) can still impair the return of deoxygenated blood, leading to congestion.
When blood flow is compromised (ischemia), the tissue is deprived of necessary oxygen and nutrients. This lack of oxygen (hypoxia) forces the cells to shift to anaerobic metabolism, which leads to the accumulation of waste products and a drop in cellular pH. If the pressure is not relieved, this metabolic crisis rapidly results in cell death and tissue necrosis, potentially occurring within just one to two hours of sustained loading.
Pressure injuries that form over bony prominences often originate as a “bottom-up” phenomenon. Deep tissue injury starts at the bone-muscle interface where the compression and shear forces are greatest. The muscle tissue closest to the bone is highly metabolically active and sensitive to ischemia, making it the first layer to sustain damage.
Damage to this deeper tissue can be extensive while the overlying skin remains intact or only minimally discolored. This makes the injury difficult to detect early, as the outward appearance of the skin does not accurately reflect the severity of the underlying destruction. The combination of direct compression, shear (a parallel force that distorts tissue layers), and resulting ischemia creates the unique vulnerability of bony prominences.