Strain shielding in trabecular bone at the tibial cement-bone interface

Abstract

Aseptic loosening of the tibial component remains the leading cause for revision surgery in total knee arthroplasty (TKA). Understanding the mechanisms leading to loss of fixation can offer insight into preventative measures to ensure a longer survival rate. In cemented TKA, loosening occurs at the cement-trabecular interface probably due to a stress-shielding effect of the stiffer implant material in comparison with bone. Using finite element models of lab-prepared tibial cement-trabeculae interface specimens (n=4) based on micro-CT images, this study aims to investigate the micromechanics of the interlock between cement and trabecular bone. Finite element micromotion between cement and trabeculae and bone strain were compared in the interdigitated trabeculae as well as strain in the bone distal to the interface. Lab-prepared specimens and their FE models were assumed to represent the immediate post-operative situation. The cement layer was removed in the FE models while retaining the loading conditions, which resulted in FE models that represented the pre-operative situation. Results showed that micromotion and bone strain decrease when interdigitation depth increases. Bone-cement micromotion and bone strain at the distal interdigitated region showed a dependence on bone volume fraction. Comparing the immediate post-operative and pre-operative situations, trabeculae embedded deep within the cement generally showed the highest level of strain-shielding. Strain shielding of interdigitated bone, in terms of reduction in compressive strains, was found to be between 35 and 61 % for the four specimens. Strain adaptive remodeling could thus be a plausible mechanism responsible for loss of interdigitated bone.

Trabecular resorption patterns of cement–bone interlock regions in total knee replacements

“…In the image processing approach used here, there are errors associated with the estimate of resorbed bone volume due to the fact that the cement surface at the completion of surgery does not completely conform to the interlocking trabeculae. Previous work has shown that errors in estimates of resorbed bone volume are 0.02 mm 3 /mm 2 ; this represents about 10% of the mean predicted resorbed bone volume measured in the current study.…”

“…The mechanism (or mechanisms) responsible for the trabecular resorption process in interdigitated cement–bone constructs is not known. Recent work using finite element models of interlocked cement–bone regions that were loaded axially, across the cement–bone interface, indicates that the trabeculae are stress shielded and that the amount of stress shielding increases for bone deeper in the cement layer. The authors also noted that there is more micromotion near the cement border and this micromotion was diminished deeper in the cement layer.…”

“…Recent work using finite element models of interlocked cement–bone regions that were loaded axially, across the cement–bone interface, indicates that the trabeculae are stress shielded and that the amount of stress shielding increases for bone deeper in the cement layer. The authors also noted that there is more micromotion near the cement border and this micromotion was diminished deeper in the cement layer. If resorption were driven only by stress shielding, one would anticipate that resorption would start deep in the cement layer and move towards the cement border, but this is not consistent with the findings here.…”

A review of biomaterials in bone defect healing, remaining shortcomings and future opportunities for bone tissue engineering

“…Balancing implant stability against stress-shielding is an important factor in the biomaterial design. If a biomaterial is stiffer than bone, the consequent stress-shielding can lead to loosening of the implant 70 due to bone loss at the interface. A possible strategy to counteract this could be a 3D-printed fully porous implant where the material architecture is tuned to reduce bone resorption due to stress-shielding.…”

Mechanical anisotropy of titanium scaffolds

“…Effects like stress-shielding [1] are the consequences of a mechanical mismatch between the stiff implant material and the softer surrounding bone tissue with negative impact on the naturally occurring bone remodelling yielding in a further weakening of the bone. In many spinal and cranio-maxillofacial applications, the mechanical stresses are essentially directed along a specific axis and an anisotropic response from the implant or scaffold would be advantageous.…”