ArticleIn this article, biomechanics expert, Carolyne Albert introduces the fundamental concepts pertaining to the mechanics of bone injuries. The biomechanical experts at Robson Forensic are engaged when the cause or mechanism of an injury is not known, is not understood, or is in dispute.
Investigating Causation of Bone Fractures
Investigating injury causation often entails the comparison of loads involved in an incident to the body’s threshold for injury. Biomechanics is an interdisciplinary science that deals with the human body and its response to applied mechanical loads. This article provides a basic introduction to key concepts pertaining to the mechanics of bone injuries.
All structures subjected to physical loading experience mechanical stress. Stress is a physical quantity that describes the internal forces within loaded structures. There are three main types of mechanical stress – tension (pulling), compression (crushing), and shear (sliding).
Failure of a structure occurs when the applied mechanical stresses exceed the material’s critical value. This critical value is a material property called strength.
Bones of the skeleton experience fluctuating mechanical stresses in everyday life, and are generally well adapted to carry those stresses. However, bone injuries occur when the loads experienced generate stresses that exceed bone tissue strength.
For many materials, such as most metals, material properties are relatively uniform throughout the material. The properties of bones, however, are more complex and depend on factors such as the anatomic site and loading direction.
Bones that make up the human skeleton are non-uniform structures composed of dense compact-bone tissue and porous spongy-bone tissue regions. Each type has differing complex directional microstructures.
Compact-bone (cortical bone) is made up of dense, hard tissue and is found in the outer layer of bones as well as inside the shafts of long bones. This type of bone tissue accounts for roughly 80% of the skeletal mass. Spongy-bone (trabecular bone) has a porous, lattice-like structure and makes up the insides of most bones.
Because of their complex internal structure, the strength of bones is highly dependent on anatomic location and loading direction. The following are examples illustrating variations in bone strength within the human skeleton:
- Compact-bone tissues are generally over 10 times times stronger than spongy-bone tissues;
- The strength of a femur (thigh bone) is usually higher in the shaft region (i.e., mid-thigh) than at the hip and knee joints;
- Compact-bone in a long-bone shaft is stronger in the direction along shaft length than it is perpendicular to the shaft; and
- Bone tissues typically are stronger in compression than they are in tension.
Factors such as age and musculoskeletal health can also affect bone tissue strength and their response to applied loads. For example, in young children, bones tend to be more deformable and they can absorb more energy as they fracture compared to adult bones. Additionally, bone fragility disorders, such as osteoporosis, can lead to decreased bone strength particularly in regions composed largely of spongy-bone tissues such as around the hip, knee, and spine.
Thus, when investigating bone fractures, forensic biomechanical experts must analyze the loads generated in the incident in question, and rely on available scientific research related to the diagnosed injury and the specific anatomic site of the injury.
Types of Bone Fractures
Depending on the types of loading sustained during an incident, certain injury patterns can be generated. Here are some examples of types of fractures that can occur within the shaft of a femur:
These types of bone shaft fractures are typically associated with different loading patterns:
- Transverse fractures are perpendicular to the long axis of the bone shaft, and are often the result of bending loads.
- Oblique fractures are diagonal to the long bone axis, and tend to result from combined loading.
- Spiral fractures have a twisted shape and are typically caused by torsional loading.
- Comminuted fractures are when the bone is broken into more than two pieces, and often involve high-energy impacts.
Biomechanical Analyses of Injury Causation
Biomechanical experts at Robson Forensic offer a scientific approach to injury causation analysis. Forensic biomechanical investigations can be applied to various scenarios where the cause of an injury is in dispute, such as motor vehicle crashes, sports and workplace events, and falls.
By combining principles of mechanical physics with an understanding of injury biomechanics, our experts can determine whether an injury is consistent with the physical evidence and descriptions of an event. Our experts can also help elucidate whether an injury resulted from a defective product or device, and whether it could have been prevented.
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Mechanical Engineer & Biomechanics Expert
Dr. Carolyne Albert is a mechanical engineer with expertise in pediatric and adult orthopedic and musculoskeletal biomechanics, as well as materials engineering. Dr. Albert applies principles of biomechanics, dynamics, and mechanics of materials to investigate injury causation related to slips and trips, motor vehicle crashes, falls, sports, and workplace-related incidents.