Pelvis Injury Criteria

Pelvis Dynamic Characteristics

The pelvis consists of a rigid skeletal ring or girdle, which transfers the load from the legs via the hip joints to the spine.  In order to support the upper body the pelvis is surrounded by a complex arrangement of muscles providing a thick compliant covering over the pelvis.

The pelvis has evolved to be significantly stronger in the vertical and longitudinal directions, to take walking and running loads, than in the lateral direction where it is weaker, due to the smaller pubis bones at the front of the pelvic girdle.

Also the pelvis is rarely exposed to direct frontal impacts except potentially to standing occupants, where the hip joint and acetabulum are exposed to indirect loads, via the femur from knee impacts. These loads produce hip joint dislocations and localised fractures.

The pelvis is exposed to direct impact in lateral seating where the load is transferred via the femur head and hip joint into the pelvic girdle which fractures at the pubis symphysis.

Therefore two injury criteria, both based on loads to produce either pelvis fracture or hip joint dislocations, are required for frontal and lateral direct and indirect loads.

Pelvis Frontal Injury Criterion

Load (KN)

The majority of biomedical data available on pelvis fracture and dislocation has concentrated on indirect frontal loading from axial loads in the femur usually produced by knee impacts with knee bolsters.

As the pelvis is a rigid skeletal structure, the impact load correlates best to pelvic fracture rather than acceleration or compression based injury criteria.

There is very little biomechanical data for direct loading to the front of the pelvis. As for indirect knee impact data, using complete cadavers there is no clear indication of the level of impact loads which could either cause fracture to the acetabulum or traumatic hip dislocations.  Tests conducted show a very large impact range from 6.2 to 25.6 KN for the onset of hip fractures, the large variance being possibly due to the large variety of different impact speeds and impactor stiffness used.  Further biomechanical research, either by impact testing or computer simulation, is required before even rudimentary tolerance levels could be proposed.

Therefore a Pelvis Frontal Injury Criterion, based on load (KN,) is proposed.  However, no tolerance levels have been evaluated.

Application

The Pelvis Frontal Injury Criterion is applied to all indirect longitudinal loads from the knee, via the femur, to predict hip joint dislocation and pelvis acetabulum fracture.  Should always be used in conjunction with the Femur Index Injury Criterion.

Evaluation

Crash Test dummies / physical tests – the indirect longitudinal load into the hip joint is currently measured using the longitudinal femur load cell in the HIII dummy.  It would be proposed that 3-axis load cells, longitudinal (shear on joint), lateral (direct compression load) and vertical (shear on joint) at the hip joint itself would be required to accurately predict the probability of pelvis injury.

Computer Simulations – Theoretically both MADYMO and Finite Element computer simulation codes should be able to measure 3-axis loads, as described above, at the hip joint although some modifications to the programme codes may be required.