Biomechanics

The field of biomechanics falls into two separate but complementary areas: Orthopaedic biomechanics and the biomechanics of human injury. Orthopaedics deals with the study of basic biological processes related to bone and muscle, and the application of mechanical engineering to prosthetic and fixator design. Injury biomechanics is a specialization that endeavors to understand how humans are injured in accidents, what the thresholds of injury are, and what can be done to minimize injury in risky activities. These two fields are complementary because knowledge of the strength of human tissues is key to understanding how those tissues are injured; and, the mechanism of injury helps othopaedists design prosthetic devices that will survive daily activities.

Principia's engineering staff has over 20 years of experience in both orthopaedic biomechanics and the biomechanics of human injury. We have worked on numerous projects for many of the largest medical device makers to help them design better products and also to assist in the regulatory approval processes.

Some representative products include:

• Fracture fixation devices
• Prosthetic joints such as hips and knees
• PCA pumps and other medical pumps
• Pneumatic tourniquets
• Catheters
• Cardiac pacemakers
• Dental prophylaxis devices
• Stents

The study of injury is central to the practice of failure analysis and accident reconstruction. Often, accidents and product failures are accompanied by human injury or death. Investigation of these failures and accidents involves not only the examination of broken mechanical parts but also the examination of the physical evidence present in the injures caused by the accident. Principia's staff applies extensive knowledge of engineering, biology and medicine to understand the cause and severity of injuries. This is accomplished through examination of the forces and motions experienced on the human body during an incident and comparing that to the broad knowledge of human injury tolerances.

Some representative areas of expertise include:

• Injury mechanisms, potential and prevention
• Occupant motion in automobile accidents
• General human dynamics
• Injury potential in automobile accidents
• Restraint systems
• Bicycle and motorcycle accident injuries
• Occupational injuries
• Recreation and sports injuries
• Slip, trips and falls

Our consulting staff is experienced in the use of computer modeling in biomechanics for injury analysis and general human dynamics. In conjunction with the Accident Reconstruction service area, we analyze the human's overall role in a case, from injury severity and likelihood to accident causation.

Example project: Tibial Plateau Fracture Mechanism

Some injuries require a certain magnitude and direction of loading to produce that injury. This magnitude and loading directions are termed mechanism of injury. In this example there were two versions of how a tibia fracture occurred. Scenario one was that the man was being pulled down and the weight and resulting moment on the tibial plateau resulted in the fracture. Scenario two was while there was a compressive weight down on the tibial plateau there was an additional force applied to the lateral side of the tibia from a kick or blow. Principia was asked to determine which of these scenarios was the most likely. In scenario one a simple lateral tibial plateau fracture with no involvement of the tibial shaft would occur, while scenario two would result in not only the plateau fracture but a comminuted fracture of the tibial shaft.

We started by creating a 3-D model of the tibia from the CT sections as shown in the figure to the left. It was shown that not only was there the lateral plateau fracture but a comminuted fracture of the proximal tibial shaft. The tibial shaft fracture was indicative of a high energy focal impact such as a kick or impact with a swinging baton such as in scenario two. We also conducted experiments to determine the swinging speed of a baton and the speed of a kick to show these would deliver enough energy to produce a comminuted fracture of the tibial shaft.

Example project: Occupational Injuries from Vibration

Occupational hazards exist in practically every work environment. The hazards that exist, however, vary significantly depending on the work environment. The occupational hazards that exist in an office environment are very different than those that exist in a construction site or railroad environment. Operators of heavy industrial machinery (cranes, bull dozers, forklifts, jack hammers, etc.) are exposed to relatively high levels of vibration and harshness. When chronic injuries occur to people in these work environments, a big question that arises is the contribution of the work environment to the injury.

The following example illustrates the tools and techniques that Principia utilizes to assist our clients in understanding and quantifying the harshness of a particular work environment and how it relates to a particular injury. The case involves an operator of a railway tamper who was suffering from tarsal tunnel syndrome, a repetitive stress injury that is the ankle equivalent of the common wrist injury carpal tunnel. The figure above shows the tamper, a large machine that rides on the railway and compacts the gravel ballast below the railway ties. This process of compacting the gravel ballast creates a good deal of vibration. Is this vibration high enough to cause a repetitive stress injury such as tarsal tunnel?

To answer this question, Principia measured the vibration transmitted to the operator’s feet during the most extreme operations of the railway tamper. The photograph at the left shows a tri-axial accelerometer, a device used to measure acceleration, mounted on the footrest used by the operator. The accelerations in three perpendicular directions were recorded during all phases of the tamper’s operation.

The charts below show an example of the measured accelerations. In order to produce meaningful results, a frequency analysis was done on the acceleration signals to compare the magnitudes of the signal with published standards on vibration exposure from the International Organization for Standardization (ISO). The figure on the right below is an example of the frequency-domain acceleration levels during the harshest operation of the tamper compared to the standards for impaired working efficiency. It turns out that the harshest conditions experienced by the tamper operator can be endured for between 4 and 8 hours with no impairment of working efficiency. In fact, the standard gives an exposure limit for safety that is twice as high as the limit for working efficiency making the most extreme tamper vibrations safe for an entire 8-hour working day.