Osteocyte lacunae tissue strain in cortical bone

Abstract

Current theories suggest that bone modeling and remodeling are controlled at the cellular level through signals mediated by osteocytes. However, the specific signals to which bone cells respond are still unknown. Two primary theories are: (1) osteocytes are stimulated via the mechanical deformation of the perilacunar bone matrix and (2) osteocytes are stimulated via fluid flow generated shear stresses acting on osteocyte cell processes within canaliculi. Recently, much focus has been placed on fluid flow theories since in vitro experiments have shown that bone cells are more responsive to analytically estimated levels of fluid shear stress than to direct mechanical stretching using macroscopic strain levels measured on bone in vivo. However, due to the complex microstructural organization of bone, local perilacunar bone tissue strains potentially acting on osteocytes cannot be reliably estimated from macroscopic bone strain measurements. Thus, the objective of this study was to quantify local perilacunar bone matrix strains due to macroscopically applied bone strains similar in magnitude to those that occur in vivo. Using a digital image correlation strain measurement technique, experimentally measured bone matrix strains around osteocyte lacunae resulting from macroscopic strains of approximately 2000 microstrain are significantly greater than macroscopic strain on average and can reach peak levels of over 30,000 microstrain locally. Average strain concentration factors ranged from 1.1 to 3.8, which is consistent with analytical and numerical estimates. This information should lead to a better understanding of how bone cells are affected by whole bone functional loading.

Fig. 1

An in situ microscopy specimen loading stage was used to apply a controlled macroscopic displacement to each cortical bone specimen while imaging the specimen surface under an optical microscope. The load frame is designed to apply displacements or loads to the specimen in equal and opposite directions allowing the center of the specimen to remain centered under the microscope objective. A strain gage attached to the underside of the specimen was used to monitor the global specimen strain and a load cell incorporated into the load frame was used to monitor the load applied to the specimen.

Fig. 2

(a) Optical digital micrographs of the surface of a typical cortical bone specimen imaged at a total magnification of 500 ×. Superimposed on the image is the grid at which displacement and strain measurements were made to determine the average microstructural strain. Shown at the top left corner of the displacement grid are graphical representations of the correlation train and search parameters. The inner square represents the sub-image train size and the outer square represents the search size. The 24 × 18 grid spacing is 50 pixels both vertically and horizontally. (b) Typical 24 × 18 measurement grid spaced at 10 pixels used to quantify the local perilacunar bone matrix strains using images acquired at 500 ×. Indicated are three osteocyte lacunae where perilacunar strain measurements were made. (c) Digital micrograph of a cortical bone specimen taken at 200 × magnification. As with the 500 × magnification micrograph, a 24 × 18 grid spaced at 50 pixels is used to quantify the average microstructural strain over the imaged area. (d) A local perilacunar 14 × 10 measurement grid spaced at 10 pixels was used to quantify strains adjacent to osteocyte lacuna. Indicated are three osteocyte lacunae where perilacunar strain measurements were made.

Fig. 3

An image correlation parameter convergence analysis was performed using a measurement grid of 24 × 18 on two successive images of an unloaded specimen. The mean and standard deviation of the measured displacements in the x direction (DX) and y direction (DY) and the two-dimensional strain tensor (EXX, EYY GXY) for all measurements points on the image (432 grid points) were calculated for each successively increasing sub-image train size. The sub-image train template size converged to 61 × 61 pixels for both the in-plane displacements (3a) and the two-dimensional strain tensor (3b). Data plotted are mean values ± standard deviation.

Fig. 4

(a) The mean microstructural strain (x-component) measured over the entire imaged area correlated well with the macroscopic strain measured using a strain gage. The average perilacunar strain was significantly greater than both the average microstructural strain and the macroscopic strain as determined by an analysis of covariance (p < 0.05). The data were pooled for each global strain level by calculating the mean over all measurement grid points for each microstructural (entire image area) and perilacunar region. Data plotted is mean ± SEM. (b) The average strain concentration factor ranges from 3.6 to 1.1 at varying levels of global strain. The experimentally derived concentration factors are in agreement with analytical estimates based on ellipsoidal holes in a continuum.

Fig. 5

(a) The distribution of microstructural tissue strains for a typical specimen indicates large variability. This distribution is typical of all seven specimens tested. As the global strain increases, the variability of the microstructural strain increases and shifts toward larger local strain values. (b) Similar variability in tissue strains is found for the perilacunar regions. As the global strain is increased, perilacunar tissue strain variability increases with at least one peak measurement grid value of over 20,000 microstrain at each global strain value. Count refers to the number of grid points in the measurement grid whose measured strain falls within each bin.

Fig. 6

Perilacunar strain for individual osteocyte lacunae shows non-monotonic behavior compared to the applied macroscopic strain that may indicate local redistribution of bone matrix strain due to microdamage.

Fig. 7

Microstructural strain field overlaid on a digital micrograph of a cortical bone specimen that was loaded to a macroscopic strain of 1500 με in the horizontal direction. Each color represents a specific level of maximum principal strain as indicated by the legend. The local microstructural strain field is highly heterogeneous and strain is found to localize around osteocyte lacunae as well as between lacunae. Local perilacunar strain reaches peaks of over 15,000 με in this specimen. Subsurface lacunae affect the local strain fields in a similar manner as lacunae on the surface. The arrows indicate osteocyte lacunae below the machine surface of the specimen. Original magnification: 500 ×.