Finite element method based stress analysis of zone I and zone II sacral fractures.

Background: Sacral bone fractures after direct traumas such as gunshot wounds and blunt sacral traumas are rarely, whereas those occurring after indirect traumas with vertical shear mechanisms ( car accidents or falls) are more frequently seen.
Methods: A cadaver sacrum was modelled 3-dimensionally using finite element software. Left sacroiliac joint was modeled to simulate a 75- kg man falling on one leg from a height of 5 meters. An impact load of 10 kN was transferred to the sacrum via intervertebral disc surface and two facets. Von Mises equivalent stress distribution was estimated.
Results: Von Mises stress distribution was calculated for sacral ala (430 MPa), S1 pedicle (225 MPa), and S1 lamina (35 MPa). Stress values were 200 Mpa, 130 Mpa, 105 Mpa, and 55 MPa on ventrolateral, dorsomedial, ventromedial and dorsolateral sides of S1 foramen respectively. Vertical shear injury caused stress distribution to concentrate at sacral ala and S1 pedicle (zone I). Local stress distribution around S1 foramen is concentrated ventrolaterally
Conclusion: High incidence of zone I sacral fractures and low incidence of neurological deficits could be explained by concentration of stress at sacral ala. Zone II fractures might be due to by transfer of triggered stresses from sacral ala and S1 pedicle to ventrolateral side of S1 foramen.