Cortical Bone Mechanics Technology™ (CBMT)
CBMT performs a non-invasive, dynamic 3-point bending test on the ulna bone in the forearms of living people. The ulna bone is chosen for its superficial location and for its unique, almost ideal biomechanics in bending. The technology comprises two steps: data collection and data analysis.
For data collection, the subject lies supine with the arm to be tested resting in the “tomahawk” position. The proximal end of the ulna is supported by the articulating humerus while the distal end is supported via the radius on a rigid platform. A lateral support on the elbow allows the arm to completely relax in this position.
Static and oscillatory forces are then applied via a blunt probe to the skin overlying the ulna bone. The static force feels like the force on one’s fingertip when pressing an elevator button, while the oscillatory force (spanning a frequency range of 40-1200 Hz) is similar to the force one feels when holding an electric toothbrush or electric razor.
It is impossible for a CBMT operator to recognize by sight or touch where to put the force probe on the forearm, because:
1. The ulna is covered by soft tissue,
2. Ulnas differ greatly in shape from one person to another, and
3. The pattern of intra-cortical destruction of the ulna also differs greatly from person to person.
So, the operator positions the probe mid-way along the length of the ulna and starts the data collection process. An algorithm in the system’s computer then automatically positions and repositions the probe at many sites, while it collects and analyzes data at each site, until it recognizes distinctive features of data from the correct place.
Figure 1. Typical complex accelerance FRF data.
This FRF contains two resonances. The higher frequency resonance (~600 Hz in Figure 1) is determined primarily by the mechanical properties of the skin, while the lower frequency resonance (~200 Hz in Figure 1) is determined primarily by the mechanical properties of the underlying ulna bone.
Figure 2. Mechanical Model.
Modeling of Forearm Dynamics.
1. The compliance and stiffness FRFs are computed by
1.1. Integrating the accelerance FRF twice to obtain the complex compliance FRF (Figure 3).
1.2. Inverting the complex compliance FRF to obtain the complex compliance FRF (Figure 4).
2.1. The compliance TF to the complex compliance FRF; and
2.2. The complex stiffness TF to the complex stiffness FRF
3. The values of the 7 model parameters are then calculated from the simultaneous solution of the algebraic equations using coefficients from:
3.1. The compliance FRF, and
3.2. The stiffness FRF.
4. Invariably, the parameter values derived from the fits to the complex compliance and complex stiffness FRFs do not agree. The system software then:
4.1. Applies proprietary, non-arbitrary, objective, quantitative criteria to determine whether the FRF came from the correct site, and if so
4.2. Reports the values of the 7 parameters, including MB, KB and DB.