Porous versus solid shoulder implants in humeri of different bone densities: A finite element analysis

Porous metallic prosthesis components can now be manufactured using additive manufacturing techniques, and may prove beneficial for promoting bony ingrowth, for accommodating drug delivery systems, and for reducing stress shielding. Using finite element modeling techniques, 36 scenarios (three porous stems, three bone densities, and four held arm positions) were analysed to assess the viability of porous humeral stems for use in total shoulder arthroplasty, and their resulting mechanobiological impact on the surrounding humerus bone. All three porous stems were predicted to experience stresses below the yield strength of Ti6Al4V (880 MPa) and to be capable of withstanding more than 10 million cycles of each loading scenario before failure. There was an indication that within an 80 mm region of the proximal humerus, there would be a reduction in bone resorption as stem porosity increased. Overall, this study shows promise that these porous structures are mechanically viable for incorporation into permanent shoulder prostheses to combat orthopedic infections.

Humeral short stem varus-valgus alignment affects bone stress

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.

Structural analysis of hollow versus solid-stemmed shoulder implants of proximal humeri with different bone qualities

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 bone-implant 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.

The effect of humeral implant thickness and canal fill on interface contact and bone stresses in the proximal humerus

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.

The effect of short-stem humeral component sizing on humeral bone stress

BACKGROUND

Several humeral stem design modifications for shoulder arthroplasty, including reduced stem length, changes to metaphyseal geometry, and alterations to implant surface texture, have been introduced to reduce stress shielding. However, the effect of changes in the diametral size of short-stem humeral components remains poorly understood. The purpose of this finite element study was to quantify the effect of varying the size of short-stem humeral components on the changes in bone stress from the intact state to the reconstructed state.

METHODS

Three-dimensional models of 8 male cadaveric humeri (mean age, 68 ± 6 years; all left-sided humeri) were constructed from computed tomography data using Mimics software. Each humerus was then reconstructed with 2 short-stem components (Exactech Preserve), one having a larger diametral size (SH+) and one having a smaller diametral size (SH-). Modeling was conducted for loading states consistent with 45° and 75° of abduction, and the resulting changes in bone stress compared with the intact state and the expected bone response were determined.

RESULTS

The smaller (SH-) short-stem implant produced humeral cortical and trabecular bone stresses that were closer to the intact state than the larger (SH+) short-stem implant at several locations beneath the humeral head resection (P ≤ .032). A similar trend was observed for expected bone response, where the smaller (SH-) short-stem implant had a smaller proportion of bone that was expected to resorb following reconstruction compared with the larger (SH+) short-stem implant for several slice depths in the medial quadrant (P ≤ .02).

DISCUSSION

These findings may indicate that smaller short-stem components are favorable in terms of stress shielding.

The Effect of Inhomogeneous Trabecular Stiffness Relationship Selection on Finite Element Outcomes for Shoulder Arthroplasty

An important feature of humeral orthopedic finite element (FE) models is the trabecular stiffness relationship. These relationships depend on the anatomic site from which they are derived; but have not been developed for the humerus. As a consequence, humeral FE modeling relies on relationships for other anatomic sites. The variation in humeral FE outcomes due to the trabecular stiffness relationship is assessed. Stemless arthroplasty FE models were constructed from CT scans of eight humeri. Models were loaded corresponding to 45 deg and 75 deg abduction. Each bone was modeled five times with the only variable being the trabecular stiffness relationship: four derived from different anatomic-sites and one pooled across sites. The FE outcome measures assessed were implant-bone contact percentage, von Mises of the change in stress, and bone response potential. The variance attributed to the selection of the trabecular stiffness relationship was quantified as the standard deviation existing between models of different trabecular stiffness. Overall, variability due to changing the trabecular stiffness relationship was low for all humeral FE outcome measures assessed. The variability was highest within the stress and bone formation potential outcome measures of the trabecular region. Variability only exceeded 10% in the trabecular stress change within two of the eight slices evaluated. In conclusion, the low variations attributable to the selection of a trabecular stiffness relationship based on anatomic-site suggest that FE models constructed for shoulder arthroplasty can utilize an inhomogeneous site-pooled trabecular relationship without inducing marked variability in the assessed outcome measures.

The effect of stemless humeral component fixation feature design on bone stress and strain response: a finite element analysis

BACKGROUND

Since the advent of stemless implants, several different fixation feature designs have been used to improve primary implant stability. These stemless designs are diverse, and the rationale for their selection and design has not been thoroughly studied. Accordingly, this investigation assessed the effect of stemless implant geometry on the simulated stress and strain response of the proximal humerus.

METHODS

Five humeral finite element models were used to examine 10 generic stemless implants with variable fixation features (2 central, 4 peripheral, and 4 boundary crossing). Loads representing 45° and 75° of shoulder abduction were simulated. Implants were compared based on the percentage of implant-bone surface area that remained in contact, the change in bone stress relative to the intact state, and the simulated potential for bone to resorb, remodel, or remain unchanged after reconstruction.

RESULTS

The implant-bone contact area was greatest for peripheral, followed by central and boundary-crossing designs. All implants elicited similar bone stress variations, which were greatest 0 to 5 mm beneath the resection and laterally. The simulated potential cortical response was also similar for all implants, with the greatest simulated resorbing potential 0 to 15 mm beneath the resection, and very little expected remodeling. Differences between implants were most prominent within the simulated potential trabecular response, with the central implants having the least bone volume percentage expected to resorb.

CONCLUSIONS

Simulated humeral bone response after stemless anatomic shoulder replacement depends on fixation feature geometry. Trade-offs exist between implant types. Centrally pegged implants produced the lowest simulated resorbing potential, whereas peripheral implants had the greatest percentages of implant-bone contact area.

An assessment of proximal humerus density with reference to stemless implants

BACKGROUND

Shoulder arthroplasty humeral components have undergone several generational changes, with systems now offering shorter stems and stemless options. The stemless humeral implants obtain fixation in the trabecular bone of the proximal humerus through elaborate fixation features. To optimize implant design, the regional variations in bone density within the proximal humerus should be determined. As such, the purpose of this computed tomography-based study was to map the regional variations in bone density of the proximal humerus.

METHODS

The trabecular-canal of the proximal humerus was extracted from computed tomography scans of 98 subjects and divided into 13 slices and 5 subsections (central, anterior, posterior, medial, and lateral). The average apparent density (ρ_AVG) was then quantified in each subsection of the trabecular-canal.

RESULTS

Slice depth, subsection, and gender were all significant main effects, with additional significant interactions between slice depth, subsection, and osteoarthritic condition. The slices above the resection plane had the greatest ρ_AVG, with densities decreasing down the canal. The central subsection had significantly lower ρ_AVG than the peripheral sections, and the medial subsection tended to have the highest ρ_AVG (P < .001). Furthermore, the ρ_AVG of male subjects was significantly greater than that of female subjects (P < .001).

CONCLUSIONS

The apparent density of the proximal humerus' trabecular-canal is nonuniform. This has implications for the design of stemless implants, indicating that implants seeking purchase in higher density bone should take advantage of the peripheral regions of the trabecular-canal within the first 15-20 mm beneath the humeral head resection plane.