**2.9 Particle debris**

172 Recent Advances in Arthroplasty

By minimizing the stresses associated with the types of damage that can occur or by increasing the strength of the polyethylene, the risk of surface damage can be decreased. We know that abrasive wear which occurs in hip joints can be minimized by a reduction in contact stress. Interestingly, both the range of the maximum principal stress and the maximum shear stress generally decrease when contact stress is decreased. Therefore, one can reduce the risk of abrasive wear and pitting and delamination by reducing the contact stress. The overall design goal is to choose the geometry of the articulating surfaces and

Contact stresses in acetabular components are affected by changes in loading, conformity of the articulating surfaces, thickness of the polyethylene, and stiffness of the material. Contact stresses increase with increasing load. If the same prosthesis (same conformity, thickness, and material) is used in patients with different weights, the stresses will be higher in the heavier patients. The stresses are not directly proportional to the load. As the load between the contacting surfaces increases, the contact area also gets larger. The contact stresses, of course, are not uniform over the contact area. The metal component of a total joint replacement is a rigid indenter. The contact stress will be greatest where the surface displacement of the polyethylene is greatest. Consequently, the displacement of the polyethylene surface in a direction normal to the surface will be determined by the shapes of the two contacting surfaces. For example, if the indenter and the polyethylene are both spherical, as they are in an ideal total hip replacement, then the maximum displacement of the polyethylene will occur at the center of contact. Therefore, the maximum contact stress will occur at the center of the contact area, and the minimum contact stress (zero) will occur at the edge of contact. Furthermore, the shape of the contact area in this ideal case will be circular. If the surfaces are not spherical because of either design or manufacturing variations, then the maximum contact stress may not be at the center of the contact area. The acetabular surface has ripples in it. As a result, when the femoral head is pressed into the polyethylene, the largest displacement normal to the surface of the polyethylene component will be at the apexes of the ripples. Therefore, the greatest contact stresses will also occur at these points. The stress in the valleys in the contact area will be small, because the deformation (difference between the dashed and solid lines) will be small at these points. Surface waviness can be caused by normal variations in manufacturing processes. Changes in conformity, thickness, and material properties cause changes in the contact area. In general, changes that decrease the contact area will increase the stresses, because the same load must be distributed over a smaller region. The contact area decreases when the conformity between the articulating surfaces decreases, when the thickness of the material decreases, and when the stiffness of the material increases. The effects of changes in thickness and elastic modulus on contact stresses may be understood as follows. Conceptually, the polyethylene may be considered to be supporting the metal indenter by a collection of parallel rods that are aligned along the direction of loading. Each rod supports

a portion, δP, of the total load P. The stiffness of a rod under axial load is given by

Krod = δP/Δ = EA/L (3)

material properties of the polyethylene that minimize contact stress.

**2.8 Reducing surface damage** 

**2.8.1 Minimizing contact stress** 

The generation of particulate debris is a central focus of attention in the arthroplasty literature. The biologic response to wear debris is currently heralded as the single most important factor limiting the long-term durability of contemporary total hip and total knee replacement arthroplasty.
