Peri-implant stress correlates with bone and cement morphology: Micro-FE modeling of implanted cadaveric glenoids

Abstract

Aseptic loosening of cemented joint replacements is a complex biological and mechanical process, and remains a clinical concern especially in patients with poor bone quality. Utilizing high resolution finite element analysis of a series of implanted cadaver glenoids, the objective of this study was to quantify relationships between construct morphology and resulting mechanical stresses in cement and trabeculae. Eight glenoid cadavers were implanted with a cemented central peg implant. Specimens were imaged by micro-CT, and subject-specific finite element models were developed. Bone volume fraction, glenoid width, implant-cortex distance, cement volume, cement-cortex contact, and cement-bone interface area were measured. Axial loading was applied to the implant of each model and stress distributions were characterized. Correlation analysis was completed across all specimens for pairs of morphological and mechanical variables. The amount of trabecular bone with high stress was strongly negatively correlated with both cement volume and contact between the cement and cortex (r = -0.85 and -0.84, p < 0.05). Bone with high stress was also correlated with both glenoid width and implant-cortex distance. Contact between the cement and underlying cortex may dramatically reduce trabecular bone stresses surrounding the cement, and this contact depends on bone shape, cement amount, and implant positioning.

Keywords: cement morphology; finite element; glenoid implant; implant fixation.

Figure 1

(a) Morphological measurements were made from micro-CT images and 3D reconstructed models as shown. (b) The volume of interest for reporting stress results spanned from the bottom of the cement to beneath the subchondral bone, and included bone within 1.5 mm (red) of the cement mantle (blue), determined at each cross section. In the left image the blue dots represent locations of cement voxels (1/50 of total voxels shown), and in the right image, the cement outline is shown at a single axial cross section.

Figure 2

Micro-FE models generated from micro-CT scans of eight different implanted cadaver specimens. Cortical and trabecular bone appears in green, cement in gray, and implant in red. Specimens are ordered according to cement volume (Specimen 1 has lowest volume).

Figure 3

Macroscopic continuum FE model (sectioned in half) used to determine loads transferred to the top of central peg (yellow-shaded region), which were then applied to the eight micro-FE models.

Figure 4

Von Mises stress distributions at the central transverse plane in each micro-FE model. Locations of the implant (red), cement (gray), and bone (green) can be seen in the smaller inset pictures. Also shown in the inset pictures are locations of contact between the cement and cortex (circled white). Specimen 2, 5 and 8 graded as higher amounts of contact between cement and cortex, and had lower stresses in the trabeculae compared to the other specimens.

Figure 5

Scatter plots and linear regression of key pairs of variables having statistically significant (p < 0.05) correlations. (c)-(f) Less bone volume had high stress in specimens with smaller glenoid width, larger cement volume, larger cement-bone interface area, and larger amount of contact between cement and cortex.

Figure 6

Example of a specimen with minimal contact between cement and cortex (top), and specimen with the most contact (bottom). In the specimen with little contact, it appears the load is more supported by trabecular bone (with higher stresses).

Figure 7

Micro-CT image of cadaveric glenoids implanted with 3-peged implant after 100,000 cycles loading. Cracks (red arrows) were identified in trabeculae immediately surrounding cement mantle.