Biomechanics-related research addresses both fundamental and applied problems. On the fundamental level, we are seeking to understand the adhesion of chondrocytes and osteoblasts (bone-forming cells) to various implant surfaces, with an ultimate goal to design better knee and hip prostheses. Also of interest is to understand how adhesion controls the cell’s own biochemical processes. Figure 1 shows a successful experiment in which cells have been removed from a polystyrene substrate surface by the laser-generated stress waves.To appreciate the loading environment, the anatomy and physiology of the joints is also considered in calculating the local joint forces using applied mechanics.

Figure 1. Fibroblast /Protein Interface Adhesion

In another research, we are looking at how to design novel material systems for impact management so as to design more efficient hip protectors with an ultimate goal to help the elderly in preventing fall-induced fractures. A fully instrumented hip model for measuring the dynamic forces on the joint in a typical fall also exists in the laboratory. Details of the experiments are given below - "Performance based testing and characterization of industrial grade hip protectors".

Lasers-based research includes, use of a novel Nd:YAG laser-generated GHz frequency ultrasound technology for mechanical cavitation of subcutaneous fat for aid in liposuction, and development of longer-lasting tribological PMMA and ceramic coating interfaces for knee prostheses.

Finally, biomechanics-based finite element modeling research includes study of soft tissue and nerve injury mechanisms under dynamic loading as in automobile collisions, calculation of seat belt forces and its role in causing shoulder joint trauma, calculation of stresses in lumbar and cervical spine under dynamic flexion and extension motion of the head for determining the risk for disk pathology (herniation, bulging, etc), design and development of surgical instruments, and design of newer-types of transposition flaps that do not pin cushion or trapdoor post-operatively for facial reconstruction.

The purpose of this research was to test in vitro the energy (or force) absorbing characteristics of the foams that are used inside various industry produced hip protectors. Hip protectors are medical devices sold as undergarments (usually) that protect the hip during a fall. When worn correctly, the energy absorbing material, typically a polyurethane/polyethylene foam or a hard shell or a combination of both, covers the hip so that any impact will be reduced to safer levels such that a hip fracture will be averted

The experiments were conducted using a drop weight apparatus that simulated the dynamic forces transferred to the greater trochanter area of the femur during a typical fall. Impacts were conducted using an Instron 8250 DynaTup apparatus. A surrogate hip model (Fig. 1) was fabricated and impact conditions were simulated for an elderly female subject. The average energy introduced into the apical surface of the substrate was 80 J, with an impact velocity of 2.14 m/s. The crosshead was outfitted with the approximate effective mass (35 kg) during a fall onto the lateral side of a patient, specifically the greater trochanter of the femur.

The dimensions of the impactor head were chosen so as to transfer the forces to the hip over an area that represented the actual contact between the hip and the impacting surface in a typical fall. Thus, this work overcomes the serious deficiencies in previous works where the area of contact was chosen unrealistically high (e.g., see, Robinovitch SN, Hayes WC, McMahon TA. Energy-shunting hip padding system attenuates femoral impact force in a simulated fall. J. Biomech Eng. 1995; 13:956-62.) Researchers following such deficient test protocols to test the effectiveness of various hip protectors have reached completely wrong and misleading conclusions that the lateral area of the protective pad would be a factor in shielding the impact force. It should be obvious to any student who has taken high school physics or have some common sense that any lateral portion of the protective pad that is beyond the greater trochanter area (side protrusion of the hip bone near the hip joint) will not help in shielding the impact force during the fall on a flat surface. This is because the projection of the bone will contact the flat impacting surface first and the impact force will be transferred over the contact area of the greater trochanter area irrespective of the lateral dimensions of the falling surface. In the same fashion, if the person falls on a protective pad that is placed on the falling surface, only the area of the pad that covers the projection of the bone will be effective in shielding the dynamic force. Another way of explaining this to a layperson will be to consider falling over a nail head that has a protective foam pad placed over it and then asking if the area of the foam (which is like a sponge) that is beyond the nail head will protect him from the nail head piercing his hip during a fall? The answer is obvious. Unfortunately prior researchers in this area with no background in dynamics were unable to appreciate that the dynamic force transfer during impact will not allow the lateral area of the pad that is beyond the greater trochanter area to participate in impact management.

In conclusion, the area of the pad beyond the projected area of the greater trochanter does very litter to manage impact. On the contrary, the thickness of the pad is an important parameter that controls impact force. Our research results now remove this fatal thinking that has infected this narrow field. Details of the experimental setup and the results on tests done on the foams inside various commercial hip protectors can be found in the MS thesis of Bimal Gandhi.

Figure 1. A surrogate hip model