Processing, surface integrity, and performance of biodegradable magnesium-calcium implants by laser shock peening

Abstract

Every year, several million people suffer bone fractures caused by accidents or age related diseases in the U.S. alone. The annual incidence of bone fractures is expected to escalate in the coming years due increased life expectancy. The aggregate cost of treating bone fractures is estimated at a staggering $220 billion per year with an annual growth rate of 12%. Many of those fractures have to be surgically fixed by orthopedic implants to replace and act as missing biological structures. However, some patients require a secondary surgery to revise or often remove an implant which has failed. This imposes a heavy burden on the national healthcare system. Implants fail because they are designed to be permanent fixtures attached to bone; however, current implant materials made from titanium, stainless steel, or cobalt-chromium alloys cause stress shielding when left permanently in place. Stress shielding arises when metal implants carry the majority of stress because of their much higher Young's modulus than that of bone. As a consequence, bones are in a reduced stress state which allows them to become brittle and weak over time. Weakened bones are more susceptible to re-fracture and could potentially dislodge an implant causing severe harm and discomfort to a patient. There is a need for a biomaterial that is not permanent because it has the capability to degrade. This reduces stress shielding over time as well as the need for a second removal surgery. The similar mechanical properties of magnesium (Mg) to bone indicate it is an ideal implant material to minimize the damaging effects of stress shielding. Mg alloy implants have the ability to gradually dissolve and absorb into the human body after implantation. Mg alloys as a biodegradable implant material have the potential to minimize stress shielding as well as eliminate the need for secondary surgery while providing both biocompatibility and adequate mechanical properties. The critical issue that hinders the application of a Mg alloy implant is its rapid corrosion in human body fluids. Since corrosion occurs too quickly, mechanical properties cannot be maintained long enough for bone to properly heal. Therefore, how to control the biodegradation rate of Mg-based implants to make them commercially viable for orthopaedic applications is a critical technical barrier to realizing its great socioeconomic benefits. Laser shock peening (LSP) is an innovative surface treatment to impart deep compressive residual stresses and a surface topography across a broad area on an implant. The high compressive residual stress has great potential to slow corrosion rates and improve wear and fatigue performance. Also, the peened surface topography has the potential to promote bone ingrowth and attachment. The goal of this work is to develop and evaluate LSP as an enabling manufacturing process to control the corrosion and fatigue performance of a degradable magnesium-calcium (MgCa) implant by imparting a unique surface integrity. Fabricating such a unique surface integrity for various types of orthopedic implants relies on the peening process parameters such as laser power and the peening overlap. Unique surface integrities were fabricated by changing the laser power from 3 W to 8 W as well as changing the dent overlap ratio from 25%, 50%, and 75%. The effects of LSP on surface integrity, corrosion, and fatigue were investigated. The surface integrity was characterized by topography, microstructure, microhardness, and residual stress. Corrosion rate was assessed by potentiodynamic polarization in Hank's solution. Fatigue life was measured by rotating bending fatigue test in air. LSP reduced the corrosion rate for every tested condition. Also, LSP increased the fatigue life for every tested condition. LSP at high peening overlap ratios reduced the tensile pile-up region which resulted in lower corrosion rates and the highest fatigue life. Low overlap ratios caused more surface area and more pile-up regions which translated to higher corrosion rates and reduced the fatigue life. Increasing the laser power increased the surface roughness and the size of the pile-up region which caused the corrosion rate to increase. Also, surface topography and corrosion rate models have been established based on finite element analysis (FEA) and linear regression analysis, respectively. Therefore, a manufacturing process was developed that controlled the performance of a degradable MgCa implant within the ranges needed for orthopedic applications. In addition, this research has begun modeling the relationship between surface modification and clinical performance in order to be able to develop the next generation of orthopedic implants that can be tailored to degrade to meet individual patient's needs.