On the strength-safety factor (SSF) for load-bearing skeletal organs

The strength of healthy postnatal mammalian load-bearing bones, growth plates, joints, fascia, ligaments and tendons exceeds the minimum strength needed to keep voluntary mechanical usage from breaking or rupturing them or from causing arthroses. Thus, they have a strength-safety factor (SSF). Some general features of the physiology in the Utah paradigm of skeletal physiology can explain two things: (i) Why load-bearing bones should have an SSF, (ii) and why its numerical value should approximately 6 in healthy young adult mammals. The number and kinds of studies and facts that revealed those two things for load-bearing bones do not yet exist for the extraosseous load-bearing organs that are made with cartilage and collagenous tissue. However, clinical-pathologic observations suggest the latter organs' SSFs should depend on features analogous to those that create SSFs for load-bearing bones. If so, the physiology on which bone's SSF depends could suggest directions for future studies of the SSF determinants of load-bearing extraosseous organs. Biomechanicians currently favor strain above stress when discussing biomechanical roles in the functional adaptations of bones to mechanical loading. However, an SSF is best expressed in stress terms, so a Table in this article provides corresponding strain/stress/unit-load values for bone's three important thresholds, and for its ultimate strength.