Frequently Asked Questions

What does a CBMT scientific instrument do?  A CBMT scientific instrument performs a dynamic 3-point bending test to measure the mechanical properties (mass, stiffness, damping, quality factor, flexural rigidity and bending strength) of cortical bone at the mid-shaft of the ulna in the forearm of living persons.  A CBMT scientific instrument can also perform this test on cadaveric human arms, and on ulna bones excised from those arms.

Why measure cortical bone?  After age 60, most bone loss is cortical, and most fractures occur at cortical sites in the appendicular skeleton.  These fractures are not well explained by clinical measurements of bone mineral density (BMD).

Why measure the mechanical properties of cortical bone?  When a bone deforms too much, it breaks.  Mechanical properties quantify how bones deform in response to force.  Force = Mass ´ acceleration + Damping ´ velocity + Stiffness ´ Displacement.  (F = Ma + Dv + Kx)

Why do a functional test?  A functional test measures how a bone actually bends in response to applied force.

Why do a dynamic test?  A dynamic test applies rapidly changing force to cause a bone to deform rapidly.  This enables the test to measure the bone’s mass and damping as well as its stiffness.  Quasistatic mechanical tests (QMT) load a bone very, very slowly so that acceleration and velocity are near zero.  This prevents QMT from measuring bone mass and damping.  Also, QMT can only be performed on bones and bone specimens that have been removed from the body.

Why do a structural test?  A structural mechanical test measures the properties of a whole bone structure.  By contrast, material mechanical tests measure the properties of the material of which the bone structure is comprised.  When a bone fractures, it fractures as a structure (i.e., as a material of particular dimensions and geometry), not as a material.

Why do a bending test?  To get unambiguous information about cortical bone, you must measure bone that is unambiguously cortical.  Bending tests are specifically sensitive to mechanical properties at the mid-shaft of a long bone, where bone tissue is entirely cortical.  Any apparently trabecular bone there is actually “trabecularized” cortical bone, i.e., thin residues of cortical bone between cortical resorption spaces.

Why test the ulna?  Of all the bones in the body, the ulna can be most ideally supported for a bending test.

What are stiffness, mass and damping?  Bone bending stiffness (K) determines how much force is required to bend a bone a distance x (F = Kx).  Bone damping (D) determines how much force is required to bend a bone at velocity v (F = Dv).  Bone mass (M) determines how much force is required to bend a bone with acceleration a (F = Ma).

Why measure bending stiffness?  Bending stiffness (K = F/x) is a very accurate predictor of quasistatic bending strength (R2 = 0.99).  To compare bones of different lengths, one must calculate their flexural rigidity, which is independent of bone length:  flexural rigidity = EI = KL3/48.  Ulna EI is what predicts the bending strengths of ulna bones of different people so accurately.  Only CBMT measures K directly in vivo.

Why measure ulna damping and mass?  Few fractures occur under quasistatic loading conditions.  Most fractures occur under dynamic loading conditions, such as when impacting the ground in a fall.  Dynamic loads are distributed across inertial (Ma) and viscous (Dv) as well as elastic (Kx) components of motion.  Bone mass and damping reduce the amount of bone bending that occurs and are protective against fracture.

Why is the CBMT scientific instrument a robot?  Because of individual differences in ulna geometry and cortical porosity, and because the ulna is covered by soft tissue, it is impossible for a CBMT operator to recognize the correct place on the forearm to perform a CBMT test.  So, the CBMT robot performs many tests at many places and applies generally applicable, objective, quantitative criteria to recognize the data that came from the correct site.

How does a CBMT test feel?  A CBMT test applies two forces to the forearm, a static force and an oscillating force.  The static force is like the force on your fingertip when you press an elevator button.  The oscillating force is like the force in your hand as you hold an electric toothbrush or electric razor.