Study of the morphology and biomechanics of sacral fracture.

Objective: To observe the morphological characteristics of sacral fracture under different impact loads.

Method: Ten fresh pelvic specimens were loaded in dynamic or static state. A series of mechanical parameters including the pressure strain and velocity were recorded. Morphological characteristics were observed under scanning electron microscope.

Results: The form of sacral fracture was related to the impact energy. Under low energy impact loads, ilium fracture, acetabulum fracture and crista iliaca fracture were found. Under high energy impact loads, three types of sacral fracture occurred according to the classification of Denis: sacral ala fracture, Type I fracture; sacral foramen cataclasm fracture, Type II fracture; central vertebral canal fracture, Type III fracture. Nerve injury of one or two sides was involved in all three types of sacral fracture. The fracture mechanism of sacrum between the dynamic impact and static compression was significantly different. When the impact energy was above 25 J, sacral foramen cataclasm fracture occurred, involving nerve root injury. When it was below 20 J, ilium and sacral fracture was most likely to occur. When it was 20 approximately 25 J, Type I fracture would occur. While in the static test, most of the fracture belonged to ilium or acetabulum fracture. The cross section of sacrum was crackly and the bone board of Haversian system was brittle, which could lead to separation of bone boards and malposition of a few of cross bone boards.

Conclusions: In dynamic state, sacrum fracture mostly belongs to Type I and Type II, and usually involves the nerve roots. Sacrum fracture is relevant to the microstructures, the distribution of the bone trabecula, the osseous lacuna and the Haversian system of sacrum. The fracture of ilium and acetabulum more frequently appears in static state, with slight wound of peripheral tissues.