Image Registration Demonstrates the Growth Plate has a Variable Affect on Vertebral Strain

Abstract

Characterizing the biomechanical behavior of the vertebrae is important in understanding the impact of structural and material changes on spinal growth and fracture risk. The growth plate is critical for the normal development of the skeleton, with abnormalities leading to uneven maturation. Little is known about how growth plates affect the stress and strain experienced by the surrounding bone. Concentrated strain within the growth plate may influence mechanical cell signaling during development, lead to increased fracture risk at this site and may influence average bone strain measures. It is hypothesized that the growth plates and adjacent bony areas will take up a large amount of the strain within rat-tail vertebrae under axial compressive loading, leading to increased average bone strain measures. The sixth caudal vertebrae of 8 rnu/rnu rats were muCT scanned in both loaded (20-32 N axial compression) and unloaded configurations. Image registration was used to calculate strain in the bone due to the applied load by finding a spatial mapping between the two scans. In seven of the eight rats, the majority of the strain measured within their vertebrae was concentrated in the growth plates. Five of the specimens had growth plates that demonstrated rigid behavior in contrast to compliant growth plate behavior seen in the other three rats. The presence of a compliant growth plate led to higher average (-0.03 vs. -0.01) and maximum (-0.13 vs. -0.02) strains. The strain within the growth plate is important to consider when interpreting apparent tissue level biomechanical data commonly reported in the literature as this study suggests strains are not uniformly distributed with high concentrations in and around the growth plate. This strain distribution may provide insight into the mechanical signals that cells experience during the formation of new bone, with the higher strains near the growth plate signaling cells to lay down more bone, while also leading to increased risk of fracture in this region.

Can micro-imaging based analysis methods quantify structural integrity of rat vertebrae with and without metastatic involvement?

“…Spinal metastases can present as osteolytic, osteoblastic or with a mixed osteolytic/osteoblastic pattern of involvement. Non-invasive quantitative evaluation of the effect of osteolytic spinal metastases on biomechanical stability has been conducted through image-based strain measurement (Hardisty and Whyne, 2009;Hardisty et al, 2010;Hojjat et al, 2011), structural rigidity analyses (Snyder et al, 2009;Entezari et al, 2011;Cory et al, 2010) and continuum-level finite element (FE) modeling (Hardisty and Whyne, 2009;Whyne et al, 2003;Eswaran et al, 2007;Ito et al, 2002;Kim et al, 2007;O'Reilly and Whyne, 2008). However, to date, limited work has been completed to directly compare these analytical tools or been applied to vertebrae with more complex mixed metastatic disease.…”

Non-Destructive Evaluation of the Effects of Combined Bisphosphonate and Photodynamic Therapy on Bone Strain in Metastatic Vertebrae Using Image Registration

“…The apparent axial strains measured within this study are on the order of previously found apparent level strains in trabecular bone and strains measured under failure of individual trabeculae. 6,8,10 The qualitative and quantitative strain patterns in our study together with the morphological measurements conducted in previous work, 15,16 suggest that combined BP + PDT treatment is able to reduce the structural deficit that occurs during rat vertebral metastatic involvement with human MT1 breast cancer cells. Thus in addition to destroying tumor tissue, combined BP + PDT is able to maintain/restore the structural integrity of the rat spine.…”

“…As such, the bone adjacent to the soft tissue of the growth plate appears overly strained; this phenomenon is referred to as a ''bleeding effect''. 6 As well, the spatial resolution limits the ability to precisely localize areas of maximum strain, thereby limiting the accurate identification of specific locations at elevated risk for failure.…”

“…The spatial resolution of strain measurement possible with the presented image registration technique is limited, however, based on the texture of the material being analyzed. 5,6 The deformable registration algorithm requires high contrast to accurately register the trabecular structure. At the current resolution, there is not sufficient contrast to properly resolve strain in the bony areas adjacent to the growth plate.…”

A study of the progression of damage in an axially loaded Branta leucopsis femur using X-ray computed tomography and digital image correlation

“…There are many examples where X-ray computed micro-tomography and DIC have been used to examine 3D local trabecular strains for small cubes of trabecular bone in mammals (McDonald et al, 2011), rodents (Christena et al, 2012; Hardisty et al, 2010) and humans (Libertiaux, Pascon & Cescotto, 2011; Pan, Wu & Wang, 2012). Such cubic specimens have also been used to provide the geometry for accurate computer simulations of bone (Levrero-Florencio et al, 2016a, 2016b, 2017).…”