Basic considerations for determining the amount of press fit in acetabular cup endoprostheses as a function of the elastic bone behavior

Abstract

Acetabular cup endoprostheses are frequently placed in pelvic bone, employing the mechanical principle of press fit. While a sufficiently stable bone-implant connection is desirable, deformation of the cup and fracture of the pelvis should be avoided. The goal of this work is to demonstrate the importance of the elastic properties of bone on the amount of press fit achievable in a specific situation. On the basis of previous work describing the relation between relative bone mineral density and relative elastic modulus for cortical and trabecular bone, mechanical equations were used for analyzing the press-fit loading situation of an acetabular cup. Additionally, a two-dimensional finite element model was used for visualizing the stress and strain situation in the host bone occurring as a consequence of implant insertion, as well as the effect of moment loads acting on the acetabular cup. Given the fact that oversizing the implant for a specific recipient site is the only clinical means of optimizing press fit, knowledge of the elastic properties of the host bone before implant selection would be beneficial. Such information could, for instance, be derived from intraoperative compressive testing of the host bone.

Effect of impaction energy on dynamic bone strains, fixation strength, and seating of cementless acetabular cups

“…However, this study shows that these factors are inextricably linked—to generate fixation you must generate strain. The force of the “pinch” of bone on cup relates to the amount of fixation and stability the cup receives . Cup deformation might be reduced by increasing implant wall stiffness.…”

“…The force of the "pinch" of bone on cup relates to the amount of fixation and stability the cup receives. 15,16,41,42 Cup deformation might be reduced by increasing implant wall stiffness. However, it is well documented that excessive implant stiffness leads to strain shielding, an undesirable function of bone.…”

Biomimetics of Bone Implants: The Regenerative Road

“…This is a drilling process designed to facilitate the implant, which is sometimes known to fracture the host bone further. 11 It is especially problematic in the context of osteoporotic bone, which can exhibit low “pull out strength” of an implant once in situ due to insufficient cortical bone. 12 Quite common also is the issue of poor nutrient diffusion, borne of compromised blood supply.…”

Microindentation – a tool for measuring cortical bone stiffness?

“… 3 However, when seated in stiffer bone, the implant will be less likely to loosen because the radial stresses effectively gripping the implant and known as the “elastic grip”, 4 will be greater for a given amount of elastic deformation. 5 Similarly, in less stiff bone, less force is required to seat the implant, but loosening is more likely as the radial stresses are weaker for the same amount of elastic deformation. 5 In the elderly population, the structural stiffness and fracture resistance of bone is reduced due to increasing porosity.…”

“… 5 Similarly, in less stiff bone, less force is required to seat the implant, but loosening is more likely as the radial stresses are weaker for the same amount of elastic deformation. 5 In the elderly population, the structural stiffness and fracture resistance of bone is reduced due to increasing porosity. 6 , 7 This causes a looser fit for a given amount of elastic deformation, and leads to difficulty in achieving a safe press fit without causing a fracture.…”