STIFFENING EFFECTS OF CORTICAL BONE ON VERTEBRAL CANCELLOUS BONE IN-SITU

Study Design. This study tested a theory about vertebral cancellous bo ne stiffness by performing experimental tests and comparing the result s with the theoretical predictions. Objectives. To test experimentally a theoretical prediction that vertebral cancellous bone appears stiff er than would be expected from isolated tests because of the constrain ing effects of the cortical bone, to measure the magnitude of this str engthening effect and its dependency on tissue composition and density . Summary of Background Data. Vertebral bodies are composed mainly of cancellous bone surrounded by a thin shell of much stronger cortical b one. Little is known of the ways in which these two materials function synergistically to produce strong but light structures and why someti mes extensive damage to the cancellous bone has apparently little outw ard effect on vertebral body strength. Methods. Cancellous bone from 4 5 lumbar vertebrae from a homogeneous group of pigs was tested in comp ression both in situ in the vertebral body and as an excised cylinder. The density and composition of the bone were then measured and correl ation tested with both of the stiffness measurements. Results. The can cellous bone in situ appears much stiffer than when isolated by a fact or of about 4 (range, 1.6-12). No correlation was found between stiffn ess, either in situ or in isolation, and density, although density is predicted entirely by the volume fractions of water, organic, and mine ral phases. Conclusions. Combining low density cancellous bone with st iffer, more dense cortical bone leads to a lightweight structure that is much stronger than might be expected from the isolated properties o f its components.