Abstract:Strain energy-based adaptive material models are used to predict bone resorption resulting from stress shielding induced by prosthetic joint implants. Generally, such models are governed by two key parameters: a homeostatic strain-energy state (K) and a threshold deviation from this state required to initiate bone reformation (s). A refinement procedure has been performed to estimate these parameters in the femur and glenoid; this study investigates the specific influences of these parameters on resulting dens… Show more
“…The changes in strain energy density (ΔSED) for every element from the intact state to the reconstructed state was also calculated. The expected time‐zero bone response (resorb, no change, or remodel) for each element of bone in each slice and quadrant was then estimated using the methodology presented by Neuert et al 22 using a threshold value of 55% change in strain energy density (ΔSED), where a decrease in SED of more than 55% would result in an element being classified as having bone resorption potential, and an increase of more than 55% would result in bone remodeling potential. The resulting elemental response was volume‐weighted based on the volume of each individual element.…”
The use of uncemented humeral stems in total shoulder arthroplasty (TSA) is associated with stress shielding. Shorter length stems have shown to decrease stress shielding; however, the effect of stem varus-valgus alignment is currently not known. The purpose of this study was to quantify the effect of short stem distal humeral endosteal contact due to varus-valgus angulation on bone stresses after TSA. Three-dimensional models of eight male cadaveric humeri were constructed from computed tomography data. Bone models were reconstructed with a short stem humeral component implant in three positions (standard, varus, and valgus).Modeling was performed at 45°and 75°of abduction and the resulting differentials in bone stress compared to the intact state and the expected time-zero bone response were determined. In cortical and trabecular bone, the standard position (STD) altered bone stress less than the valgus (VAL) and varus (VAR) positions relative to the intact state. For both cortical (p = 0.033) and trabecular (p = 0.012) bone, the VAL position produced a larger volume of bone with resorbing potential compared to the STD position.
“…The changes in strain energy density (ΔSED) for every element from the intact state to the reconstructed state was also calculated. The expected time‐zero bone response (resorb, no change, or remodel) for each element of bone in each slice and quadrant was then estimated using the methodology presented by Neuert et al 22 using a threshold value of 55% change in strain energy density (ΔSED), where a decrease in SED of more than 55% would result in an element being classified as having bone resorption potential, and an increase of more than 55% would result in bone remodeling potential. The resulting elemental response was volume‐weighted based on the volume of each individual element.…”
The use of uncemented humeral stems in total shoulder arthroplasty (TSA) is associated with stress shielding. Shorter length stems have shown to decrease stress shielding; however, the effect of stem varus-valgus alignment is currently not known. The purpose of this study was to quantify the effect of short stem distal humeral endosteal contact due to varus-valgus angulation on bone stresses after TSA. Three-dimensional models of eight male cadaveric humeri were constructed from computed tomography data. Bone models were reconstructed with a short stem humeral component implant in three positions (standard, varus, and valgus).Modeling was performed at 45°and 75°of abduction and the resulting differentials in bone stress compared to the intact state and the expected time-zero bone response were determined. In cortical and trabecular bone, the standard position (STD) altered bone stress less than the valgus (VAL) and varus (VAR) positions relative to the intact state. For both cortical (p = 0.033) and trabecular (p = 0.012) bone, the VAL position produced a larger volume of bone with resorbing potential compared to the STD position.
“…Thus, the intact and reconstructed states resulted in identical humeral geometries allowing for direct element-to-element comparison of changes in bone stress. 20 The bone and implant were then meshed with quadratic tetrahedral elements with a maximum edge length of 2 mm and maximum deviation factor of 0.06 mm. 22 Figure 2 For each humeral implant stem investigated, partitions were created by cutting and reaming the trabecular bone with the desired implant size.…”
Section: Methodsmentioning
confidence: 99%
“…The capacity of bone resorption was determined using a threshold value of change in SED of 55%, where bone would be expected to remodel and become stronger, remain the same, or resorb if the change in SED was greater than, equal to, or less than this threshold value, respectively. 20 Each element in the slice of interest was placed into one of these three categories depending on its change in SED. To determine the overall percent volume of bone with resorbing potential, the volume of the elements that exhibited change in SED less than the 55% threshold were divided by the sum of the total volume of the elements in all categories.…”
Section: Methodsmentioning
confidence: 99%
“…To better predict the capacity of bone resorption and stress shielding in orthopedic models, various studies have investigated changes in strain energy density (SED). 1 3 , 11 , 12 , 14 , 20 , 26 , 27 Changes in SED can be measured across several bone sites using finite element (FE) computational analysis, and can be used to predict regions of bone that are likely to resorb when the change in SED is less than the threshold value of 55%. 20 As such, the objective of this FE study was to determine changes in bone-to-implant contact (BIC), cortical and trabecular bone stresses, as well as the changes in cortical and trabecular SED from the intact state when implant thickness was increased within the proximal humerus using a composite short-stem humeral implant.…”
Background
Stem size is an important element for successful time zero primary fixation of a press-fit humeral stem in shoulder arthroplasty. Little basic science research, however, has been conducted on the effects of implant thickness and canal fill on load transfer, contact, and stress shielding. The purpose of this finite element study was to determine the effects of varying stem thickness on bone contact, bone stresses, and bone resorption owing to stress shielding.
Methods
Three generic short-stem implant models were developed and varied based on cross-sectional thickness (thinner – 8 mm, medium – 12 mm, thicker – 16 mm). Using a finite element model, three outcome measures were determined (1) the amount of bone-to-implant contact, (2) changes in cortical and trabecular bone stresses from the intact state, and (3) changes in cortical and trabecular strain energy densities which can predict bone remodeling or stress shielding.
Results
Increasing the size of the humeral stem had no significant effects on bone-to-implant contact during loading (
P
> .07). The thinner implant with the lowest canal fill ratio produced significantly lower changes in stress from the intact state in both cortical and trabecular bone (
P
< .002). In addition, the thinner implant resulted in a substantially lower volume of bone predicted to stress shield and resorb when compared with the medium and thicker stems.
Discussion
The results demonstrate that thinner implants and lower canal fill may be beneficial over thicker sizes, provided equal initial fixation can be achieved. The thinner implant has a greater degree of load sharing and increases the mechanical load placed on surrounding bone, reducing the risk of stress shielding and bone resorption.
“…From these data, we calculated: (1) the volume-weighted average change in the magnitude of von Mises stress of the bone following reconstruction, calculated with respect to the intact state ( σ Δ VWA ), 7 (2) the volume-weighted average deviatoric component of the change in stress tensor with respect to the intact state (ΔS), to account for any changes in the direction of stress, 16 below the intact state occurred, based on strain-adaptive bone remodeling theory. 28,29 Corresponding equations for each cal-…”
Stress shielding of the proximal humerus following total shoulder arthroplasty (TSA) can promote unfavorable bone remodeling, especially for osteoporotic patients. The objective of this finite element (FE) study was to determine if a hollow, rather than solid, titanium stem can mitigate this effect for healthy, osteopenic, and osteoporotic bone. Using a population-based model of the humerus, representative average healthy, osteopenic, and osteoporotic humerus FE models were created. For each model, changes in bone and implant stresses following TSA were evaluated for different loading scenarios and compared between solid versus hollow-stemmed implants. For cortical bone, using an implant decreased von Mises stress with respect to intact values up to 34.4%, with a more pronounced effect at more proximal slices. In the most proximal slice, based on changes in strain energy density, hollow-stemmed implants outperformed solid-stemmed ones through reducing cortical bone volume with resorption potential by 11.7% ± 2.1% (p = .01). For cortical bone in this slice, the percentage of bone with resorption potential for the osteoporotic bone was greater than the healthy bone by 8.0% ± 1.4% using the hollow-stemmed implant (p = .04). These results suggest a small improvement in boneimplant mechanics using hollow-stemmed humeral implants and indicate osteoporosis could exacerbate stress shielding to some extent. The hollow stems maintained adequate strength and using even thinner walls may further reduce stress shielding. After further developing these models, future studies could yield optimized implant designs tuned for varying bone qualities.
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