LIMITATIONS OF QMT

 

The current Gold Standard method for measuring stiffness and strength of bone is quasistatic mechanical testing (QMT).  Since the first step in QMT is to remove the bone of interest from the body, this method cannot be used in research involving living human subjects.  So, its use is limited to animal studies and studies of cadaveric human bone.

In QMT a very slowly increasing mechanical force is applied to an excised bone or bone specimen while the applied force and the associated deformation of the bone are recorded.  Bone stiffness K is taken as the slope of the linear elastic relationship between the applied force F and the deformation x.

F = Kx

The strength of the bone is taken as the peak load before fracture.  Although there is no physical theory relating strength to stiffness, the measurement of a bone’s stiffness enables the bone’s strength to be calculated very accurately ― under the quasistatic conditions of the test.

A SECOND LIMITATION OF QMT

is that it does not measure the viscous and inertial properties of bone.  The problem with such quasistatic data is that few people fracture under quasistatic conditions.  Under the dynamic conditions of real fractures, the relationship between force and deformation of the bone is more complicated:

F = Kx + Dv + Ma

where v is the velocity of bone deformation, D is the bone’s damping coefficient, a is the acceleration of deformation, and M is bone mass.  The spread of the applied dynamic force across these three terms reduces the amount of dynamic force available to cause elastic deformation, thereby protecting the bone against fracture.  QMT is unable to provide information on these mechanical properties of bone.