Extremity Injury Criteria

Arm And Leg Dynamic Characteristics

The extremities or arms and legs are divided into three main areas due to their characteristics.

• The Long Bones – comprising of the major bones of the arms and legs.  These are jointed at the body, by the shoulder and hip joints, and to each other at the elbow, and knee, and then the wrist and ankle

• The shoulder, hip, elbow and knee joints are either ball, as in the shoulder and hip, or sliding synovial joints, as in the elbow and knee.  These joints have cartilage inner surfaces, are joined by ligaments across the joints, and the tendons which are attached to muscles to move the joints.  The knee has the patella, the largest sesamoid in the body, to give the upper leg muscles extra leverage to move the lower leg

• The hands and feet, consist of a complex assembly of small bones to give flexibility and dexterity and are all joined by ligaments and tendons to give stability and motion to the hands and feet

Long Bone Fracture Criterion

The mechanical properties of the major load bearing bones in both the arms Clavicle, Humerus, Radius and Ulna, and leg Femur, Tibia and Fibula are given in Appendix 1 Table 37.

As seen in fracture classifications, fractures are often caused by combinations of loads and, for the Diaphysis or shaft of the main load bearing bones, these will be a combination of bending moments and axial compression.  In the rail vehicle impact environment the

combination of axial load and bending moment could be used as the injury criterion for indirect inertial loads and direct impacts to the ends of the bones..

The tibia index, developed for the HIII instrumented lower leg, combines both axial load and bending moment to predict tibia fracture, using the following equation:

Tibia Index (TI) =         Actual Bending Moment  +      Actual Axial Compressive Force

                                    Ultimate Bending Moment       Ultimate Axial Compressive Force

Tibia fracture is being predicted if the Tibia Index is equal or greater than 1.

An Index value of 1 predicts a non-comminuted fracture.  Higher index values would predict the probability of comminuted fractures with the greater possibility of being open.  Although it would be difficult to assess the index value to predict such a comminuted fracture, an Index of 1.3 has been used as an increased level of injury in Automotive Legislation.

The equation is applicable to all the long bones, using the Ultimate Bending and Axial Compressive force for each specified bone.

Therefore, there are individual Long Bone Fracture or Index Injury Criteria for each of the main extremity long bones based on their respective ultimate compressive loads and bending moments which is shown in Appendix 1, Table 38.

• Humerus Index Criterion
• Ulna Index Criterion
• Radius Index Criterion
• Femur Index Criterion
• Tibia Index Criterion
• Fibula Index Criterion

Bending moments will vary considerably depending on the direction and location of the impact or indirect load, so the Index Criterion needs to be evaluated at both the top and bottom of each long bone.

The Index Criterion for long bone fracture can only be used for either indirect quasi-static loading or direct loading to the ends of the bones.  Direct impacts to the shaft of long bones produce very high concentrated loads.  At low velocities and energies then the Index Criterion may be applicable, but at moderate or high velocities and energies then an energy band criterion should be used.  Further biomechanical research is required to develop such a criterion for predicting the asset and severity of fractures.

Application

The long bone Fracture or Index Criterion can be applied to all direct and indirect impacts and loads.  For the forearm, comprising of the Ulna and Radius, and the lower leg, comprising of the Tibia and Fibula, the lowest index normally for the fibula and ulna is Injury Level 1, and then the Index for the Radius and Tibia is Injury Level 2 – 3.

Evaluation Techniques

Crash test dummies / physical simulators – To calculate the Fracture Index Criterion 6-axis load cells are required near to both ends of each simulated bone shaft.  The Ulna and Radius, tibia and fibula, being combined into a single shaft on crash test dummies.  6-Axis load cells are now available for all these locations for the HIII family of dummies.

Computer Simulations – MADMYO and Finite Element dummy simulations can measure all the forces and bending moments at all extremity joints, so the Bone fracture indexes can be calculated.

Joint Injury Criteria

Joint injuries occur either by two main mechanisms.

• Crushing from high indirect loading leading, to either damage to the cartilage or bone load bearing surface

• Dislocation from shear or bending forces damaging and rupturing ligaments, tendons and soft tissue

Many of these injuries are produced by indirect loads from impacts on other body parts by stumbling or falling, and therefore are produced by a combination of different shear, axial and bending moment forces.  This, coupled with the lack of biomechanical data, makes proposing injury criteria extremely difficult.  The only injury criterion developed for this type of injury is the sliding knee criterion. The injury criterion predicts the probability of rupture to the cruciate ligaments, which link the femur and tibia, causing knee dislocation.  This is produced from an indirect shear force over the knee joint from a direct impact to the upper tibia.  As such it can be applied to any knee or tibia impact to a seated occupant. 

Knee Dislocation Injury Criterion

Displacement (mm)

Rupture to ligaments and tendons occur when they are stretched beyond their breaking strain, so a maximum displacement injury criterion is applied.  The criteria was developed for knee and upper tibia impacts with knee bolsters in cars. This results in a direct impact to the upper tibia, forcing the tibia rearwards in relation to the femur.  This in turn induces a high longitudinal shear force across the knee stretching the cruciate ligaments.  To simulate this loading mechanism a special sliding knee was developed for the HIII dummy, which allows the knee pin joint to displace longitudinally to the tibia, not the femur.  Rubber blocks control the displacement characteristic and the displacement is monitored using a linear potentiometer.

Application

The knee dislocation injury criterion can be applied to all seated occupants having a direct impact to the tibia with a seat back or front edge.

Evaluation

Crash Test dummies / physical simulators – The sliding knee attachment can be applied to a HIII crash test dummy for frontal impacts.

Computer Simulations – Both MADYMO and Finite Element dynamic modelling codes allow the knee displacement to be analysed.

Shoulder Impact Criterion

Impact Velocity (ms-1)

Dislocations or serious damage to the shoulder, elbow, knee and ankle can obviously impair an occupant’s ability of egress.  Although there is little published biomechanical data available, it is proposed that an injury criterion to predict the probability of shoulder injury from direct impact can be defined.  With no biomechanical data it is proposed to link the level of injury directly to the velocity of impact.

Application

The Shoulder Impact Criterion can be applied to all direct impacts to the shoulder, which could lead to dislocation or direct injury.  Therefore it applies to all occupants whether sitting or standing.

Evaluation Techniques

As it does not measure a direct internal parameter to the occupant it can be evaluated by both physical and computer simulations in which the occupants kinematics or motion is being analysed.

Hand Injuries

Fractures and severe lacerations to the hands and arms although not life threatening or serious disabling, if a number of them occur to an occupant it will effect their ability of egress.  The complexity of impacts and loading mechanisms preclude the use of a direct occupant measured parameter, therefore the best method for predicting the level of injury is based on the velocity of impact and impactor shape.  Two injury criteria have been proposed.

Hand Fracture Criterion

Impact Velocity (ms-1)

Fractures to the metatarsal and digits occur due to impact loads produced from direct impacts with rigid blunt objects.  The severity of the injury being dependent on the impact loads which will be directly related to the velocity of impact.  Therefore, the Hand Fracture Injury Criterion is based on the relative velocity of the hand with the impacted object.

Application

All hand direct impacts with rigid blunt objects.

Evaluation Techniques

As it does not measure a direct internal parameter to the occupant, it can be evaluated by both physical and computer simulations in which the occupants kinematics or motion is being analysed, and so the hand impact velocity can be predicted or evaluated.

Hand Laceration Criterion

Impact Velocity (ms-1)

Lacerations can occur due to either direct penetration of the skin with a sharp object, or tearing of the skin from impact with a blunt object.  A sharp object being defined as any edge with a radius of < 5 mm, while a blunt object is 5 – 40 mm radius.  The severity of the injury is dependent of the velocity of the impact, therefore, the Hand Laceration Criterion is based on the relative velocity of the hand with the impacted object and the shape of the object.

Application

All hand direct impacts with rigid sharp or blunt objects.

Evaluation Techniques

As it does not measure a direct internal parameter to the occupant, it can be evaluated by both physical and computer simulations in which the occupants kinematics or motion is being analysed, and so the hand impact velocity can be predicted or evaluated.