Diagnosis of heel pad injuries

The biomechanics of in vivo heel pads has been investigated for more than 30 years, but unfortunately numerical results from the many individual investigators cannot be compared due to the different methodologies used, and the sometimes modest number of subjects investigated. The overall aim of the present thesis is to obtain a thorough understanding of the mechanical properties of in vivo human heel pad by studying the anatomical and physiological structure of healthy and diseased tissue, and to develop quantitative methods for diagnosing injuries.

A compression device was built in order to record load-displacement curves from in vivo heel pads. To ensure applicability also for pathological feet, the device uses force levels lower than those needed to reproduce the physiological conditions of walking. One hundred twenty seven healthy volunteers were enrolled for compression tests and ultrasound investigation of heel pad thickness, so that three biomechanical parameters could be investigated: Heel Pad Compressibility Index (HPCI), Elastic modulus (E), and Energy Dissipation Ratio (EDR).

Statistical analysis, based on linear regression models, showed that intrinsic subject factors such as age, weight, height and gender did influence HPCI, E and EDR, and that there was a significant statistically difference between males and females in E, but not in EDR and HPCI. In order to attempt a verification of the method used above, compression tests and ultrasound investigations on artificial heel pad models were conducted in two additional studies.

In vivo experimental tests were used to validate a numerical 3D subject-specific heel pad model subjected to an external compression, and to further investigate the visco-elastic nature of the heel pad. Ultrasound distance measurements were compared with corresponding distance measurements with MRI and true values in order to assess its reliability in heel pad thickness measurements.

The latter study confirmed the need to investigate the real speed of sound for the heel pad tissues, in order to obtain realistic measurements when dealing with human heel pad.