How does pathology affect the crystalline structure of mammalian bone?
On left, µCT render and slice of a giant anteater vertebra showing signs of osteoarthritis (FMNH 60688, Myrmecophaga tridactyla). On right, EDD slices showing location of normal bone, which is difficult to see because of its low mineral content relative to the pathological tissue. Bright spots in the upper right corner of each slice are cross-sections of pathological bone; each color indicates an area with a particular directionality of hydroxyapatite (hAp) crystal.
My work with student Hartrich Zack on the effects of captivity on mammalian bone gave rise to a new collaboration concerning the pathological bone growth often found in the vertebral column of captive animals. This collaboration among Stuart Stock (Northwestern University), Jun-Sang Park (Argonne National Labs Advanced Photon Source [APS]), Haiyan Chen (Argonne National Labs COMPRES), Ken Angielczyk (Field Museum), Hartrich, and myself aims to understand how the crystallographic texture of bone differs between normal, healthy tissue, and tissue that grows in response to a pathology. Bone pathologies like osteoarthritis and diffuse ideopathic skeletal hyperostosis (DISH) produce secondary bone growth, and we found several instances of these types of pathologies in captive anteater specimens at the Field Museum. To investigate the material and potential mechanical consequences of exostotic growth, we subjected healthy and pathological specimens of two types of anteater (Tamandua and Myrmecophaga) to Energy Dispersive Diffraction (EDD) crystallography. EDD uses a polychromatic synchrotron X-ray source and multiple detectors arranged in a semicircle, with a conical slit to maintain constant 2θ across all readings. Using the 6-BM-B beamline at APS, we executed EDD experiments and obtained maps showing directionality of bioapatite in both healthy and pathological bone tissue.
Our results show striking differences between normal and pathological bone. Pathological bone diffracts more intensely than normal bone, likely due to higher mineral content and volumetric bone density. Additionally, pathological bone shows a high degree of texture: that is, there are large areas of uniform bioapatite directionality, which are adjacent other areas with uniform directionality in a different direction (shown as colored areas on the EDD maps). The mechanical consequences of this arrangement are unclear, but our continuing work will investigate internal strain within both types of tissue.