The strength of the osteoporotic sacrum.
METHODS
Biomechanical cadaveric bench study.


OBJECTIVE
To determine the strength of the osteoporotic sacrum subjected to vertical force.


BACKGROUND
Sacral insufficiency fractures are thought to be caused by vertical shear forces acting in the sacral ala. The force required to fracture the osteoporotic sacrum is unknown.


METHODS
Eighteen osteoporotic cadaveric pelves were potted and mounted on a materials testing machine so that the anterior superior spine was aligned with the symphysis in the sagittal plane. The lumbar spine (L3-L4) was displaced vertically downward until failure occurred. Failure loads were recorded, and fracture patterns were identified via computed tomography.


RESULTS
Mean (+/-SD) failure load was 3200 +/- 1262 N. In 3 of 18 specimens, sacral fractures were not apparent on computed tomography. In 15 of 18 specimens, fractures were produced lateral to the neural foramina. Of those 15 specimens, 8 were unilateral (1 with horizontal extension) and 7 were bilateral. In 13 of 15 cases, fractures were located in Denis Zone 1.


CONCLUSIONS
The osteoporotic sacrum failed under vertical forces of approximately 4.5 times body weight. Imaged fracture patterns resembled sacral insufficiency fractures seen clinically. In 1 case, a horizontal fracture line was produced. Vertical compression forces appear to cause insufficiency fractures in the osteoporotic sacrum.
Waites
Matthew D
MD
International Center for Orthopaedic Advancement, Department of Orthopaedic Surgery, Johns Hopkins Bayview Medical Center, Baltimore, MD 21224-2780, USA.
Mears
Simon C
SC
Mathis
John M
JM
Belkoff
Stephen M
SM
D001696:Biomechanical Phenomena; D019245:Compressive Strength; D005260:Female; D005598:Fractures, Spontaneous / Q000209:etiology; D006801:Humans; D008297:Male; D010024:Osteoporosis / Q000503:physiopathology*; D012447:Sacrum / Q000503:physiopathology*; D016103:Spinal Fractures / Q000209:etiology; D016474:Weight-Bearing