Density is not the sole measure that protects bones on the one hand or predisposes them to fracture on the other. As Dr. Susan Love, clinical professor of surgery at the University of California, Los Angeles, reminded us recently, “…bone density is only one aspect of bone health. It just happens to be the one we can measure.”
Quantifiable or otherwise, additional physical qualities that bones possess include their size, shape, and architecture. Bone architecture can be defined as the pattern of trabeculae (partitions) in the bone and associated structures. Along with density, these aspects of bones govern the ability to sustain the loads to which we subject the bones. Just as architects and engineers must design buildings to code to safely sustain environmental forces, Mother Nature has spent millennia perfecting her design specs for our bones.
What all designers understand is that form follows function. The theory of bone architecture that defines how bone in a healthy body will adapt to loads is known as Wolff’s law, and was developed over 100 years ago by the German bone anatomist and surgeon, Julius Wolff. This law states, “Every change in the form and the function of a bone, or in its function alone, is followed by certain definite changes in its internal architecture and secondary alterations in its external conformation.”
This simply means that the architecture of bone directly relates to its function. The bones in our extremities, for example, must support most of our weight and carry heavy loads, so they are longer, harder, and denser than those that must frequently expand and contract, like our rib cages do with our breathing, and our spines, which are articulated and pliant.
Bones are categorized as a type of connective tissue. Bone architecture can also be divided into two major types, trabecular and cortical
- Trabecular, or cancellous bone has a lower density and lower strength than cortical bone. This is the interior, spongy type of bone that has lots of partitions and lacunae(holes), and thus, a great deal more surface area than cortical bone. Trabecular bone is where most of the blood vessels, as well as the nerves, of bones are located. The outer layer of trabecular bone is where bone marrow can be found. It is less calcified (only 15-25%) than cortical bone, and makes up only one-fifth of our total bone tissue by volume.
- Cortical bone is the extremely hard, dense type of bone tissue forming the surface of bones. It is sometimes referred to as compact bone. Comprising four-fifths of our total bone in volume, cortical bone is 80-90% calcified. The primary functions it serves are support and protection. Orthopaedic scientists in The Netherlands have actually monitored the three-dimensional development of cortical bone using micro-CT technology. These images show how the pores of trabecular structures gradually fill in to form the compact cortical bone. They also confirm that trabecular and cortical bone share the same regulatory mechanisms for adapting to mechanical loads.
Yet anatomically and physiologically, these two bone types are distinctive. The fact that the spongiform trabecular bones have a much higher amount of surface area per volume means that they undergo a much higher rate of turnover than cortical bone. This marked difference in the anatomy and physiology of cortical and trabecular bone tissues is reflected in their susceptibility to fracture. It makes sense that trabecular bone’s larger surface area and higher rate of metabolic activity predispose it to fracture more easily. When conditions for bone turnover are sub-optimal, the incidence of spinal and rib fractures increases with age because the bones of the spine and ribs have a higher percentage of trabecular bone.
For more on the demographics of bone fracture, check out bone specialist Dr. Susan Ott’s graphs, which show the rates of fracture for different bone types, geographic regions, different races, and both genders across the lifespan.
Please click here for information on types of fractures — what different kinds of bone fractures can happen?.