The Effect of Hollow Sphered Structure on Stress Shielding Reduction
European Scientific Journal February 2018 edition Vol.14
The Effect of Hollow Sphered Structure on Stress Shielding Reduction
Mohammadreza Yazdifar Ibrahim Esat 0
0 Brunel University, UK Mahshid Yazdifar Coventry University , UK
Bone mechanics and traditional implant materials cause a frequent problem for patients of total hip arthroplasty (THA): the bone becomes shielded from the loading. This will result in loosening of the implant, pain, and therefore revision surgery will take place to correct the issue. The current study, a methodology is developed for creating an innovative structural design that extracts volume in the shape of spheres from the samples, in order to focus solely on expected behaviour within the samples and bone. The design decreases extreme stresses carried by samples and pass them onto the remaining bone. Finite element analysis was applied to various models with different complex internal structures that contain hollow spheres close to surface. Moreover, compression test was applied to solid sample and the experimental case containing hollow spheres. This approach was to investigate the effects of spherical hollow structure near the side surface and its bonesample interface. The models containing hollow spheres have smaller young modulus and strength in comparison to the solid sample. The hollow spherical structures reduce the stress shielding and they transfer more stress onto the bone compared to the solid model. This approach also re-structures a hard material such as stainless steel to enhance osseointegration. The reduction of the young modulus and stress directly depends on the volume of the spheres in the models. However, there is a range defined for the volume that could be extracted from solid structure to achieve the most effective outcome.
Hollowed Structure; Implant Design; Stress Shielding
-
main biomedical metals used for medical applications are Stainless steel,
Cobalt alloys and Titanium alloys
(Niinomi, 2008)
(Karanjai, Sundaresan,
Rao, Mohan , & Kashyap, 2007)
. Titanium alloys used in femoral stems have
certain problems while producing and articulating surfaces are no longer
recommended for biomedical applications
(Zhang, 2009)
.
One of the most important failure parameters that all implants face is
stress shielding
(Bitsakos, Kerner, Fisher, & Amis, 2005)
,
(Sumner & Galante,
1992)
. However, flexible stems decrease bone resorption if the interface bond
is strong. It could be concluded that flexible stems are the solution to bone
resorption but it may also result in increased loosening rates
(Huiskes,
Weinans, & van Rietbergen , 1992)
(Diegel, Daniels, & Dunn, 1989)
. Implant
stiffness depends on implant material and its cross sections.
There are studies
(Mattheck, Vorberg, & Kranz, 1990)
,
(Schmidt &
Hackenbroch, 1994)
,
(Chang , et al., 2001)
(Ridzwan, Shuib, Hassan, Shokri,
& Mohamad Ibrahim, 2007)
regarding factors which could lead into stress
shielding reduction.
A study in 2001 focused on optimising a hollow structure stem to
decrease stress shielding and also decreased the maximum stress in cement. In
this study, the inner diameter was the variable and cement stress was defined
as the design constraint. The obtained results were compared with a solid
structure stem, but, the implant was only cylindrical with simple boundary
conditions. The stem with hollow structure showed an increase in proximal
bone stress about 15% and it was 32% for the case with high strength cement
(Gross & Abel, 2001)
.
There have been two approaches about the relation between porosity
and young modulus that when porosity goes up, Young’s modulus will
decrease. In these studies, the cellular implant has a structure like a spongy
bone and it acts nearly as a solid femoral stem. The cellular implant
demonstrated a rise in the load-transfer mechanism in comparison to the solid
one. Therefore, metal foams may cause longer period for stress shielding to
happen
(Rahman & Mahamid, 2002)
(Smith, Szyniszewski, Hajjar, Schafer,
& Arwade, 2012)
.
Finally, honeycomb geometries were added to the stems design in new
total hip replacement implants. These geometries were analysed using finite
element method and auxetic stems showed reduction in stress shielding effect
(Sanami, 2015)
.
As the above studies show, stress shielding is a major problem that
reducing the young modulus could solve the issue. One of the ways to reduce
the young modulus is to have porous structure.
The aim of this paper is to develop the idea of having hollow voids
near the surface to reduce the localised stress on samples and increase the
stress on the surrounding area which is bone. This paper focuses on verifying
if hollow sphered structure near surface will decrease stress shielding. In
addition, this paper also identifies the best configuration in terms of the sphere
size and their distribution within the mass. Having reduced young modulus
improves displacement, as the displacement i (...truncated)