Doctoral Dissertation: Collagen Fiber Orientation and Locomotor Loading in Primates

Kent State University, Kent OH

Investigators

Abstract

This study examines some important microscopic structural details of bone. Results can potentially improve our understanding of the tissue's microscopic architecture and how forces imposed on it during upright walking and running are distributed. Collagen fibers are a large component of bone microstructure. Their orientation reflects the types of forces sustained by bone over the time period during which it is being deposited. By microscopically analyzing histological sections, this study will provide new data about forces imposed on specific areas of the human skeleton during walking and running. Bone microstructure in animals that practice highly specialized forms of locomotion and which have been studied by other means such as in vivo strain gauge examination will first be reviewed. This will permit detailed standardization of methodology and also serve as a means of testing the capacity of collagen orientation to accurately reflect bone forces. It will then qualitatively and quantitatively assess the orientation of collagen fibers in human and non-human primate bone taken from highly localized parts of the skeleton. The first site to be studied will be the gibbon forelimb. Gibbons practice brachiation, which is a locomotor pattern in which the body is alternatively suspended by each forelimb. Strain in the gibbon ulna is primarily tensile, whereas, that in its companion bone of the forearm, the radius, is primarily bending. These force characteristics should specifically affect the collagen orientation in the two bones. This study will quantify and qualify the collagen fiber orientation in the gibbon forelimb and compare the results with the strains which have been measured in the limb. The second site that will be investigated in this study is the femoral necks of humans and chimpanzees. Human femoral necks are unique in that they have a very thin cortical bone shell at the superior portion. This region is very sensitive to bone loss from osteoporosis and is a frequent site of fracture in elderly females. An improved knowledge of the forces and structure of this region are therefore of potentially great importance. In contrast to those of humans, chimpanzee femoral necks are distinct because they have a thick cortical layer of bone on the superior neck. The different pattern of cortical bone development in humans and chimps is thought to be due to the forces sustained during their respective modes of locomotion and the result of different muscular anatomies in the hip. Humans practice bipedal locomotion during which the hip muscles function to stabilize the body on the pelvis, but chimpanzees only infrequently walk bipedally. The hip musculature in chimps therefore differs anatomically from that of humans. In humans the hip muscles (abductors) are thought to relieve forces on the upper part of the femoral neck. This results in only minimum bone formation in this region, and may be the primary reason for the thin cortical shell typical of human femora. However, chimp hip muscles do not have as strong an abductor function and therefore permit their femoral neck to suffer higher forces resulting in a thicker cortex. By examining collagen fiber orientation in chimp and human femoral necks, this study will determine what forces occur in the two species' femoral necks. These results will test whether the distribution of cortical bone is the result of different forces acting on the chimp and human femoral necks. This investigation will also examine the microanatomical construction of bone. The arrangement of collagen fibers in bone remains a highly debated topic in bone biology. Most researchers believe that collagen fibers possess particular orientations in bone. However, there are those who believe that collagen fibers are randomly oriented without any particular orientations. This study will examine collagen fibers in three orthogonal planes of bone in order to determine if specific orientations do indeed exist. The findings from this investigation will further our understanding of bone microstructure and its relationship to loads suffered by bone. This study will add critical new empirical evidence to the question of bone's microstructural adaptation to mechanical stimuli. In addition, new evidence will be produced that will help elucidate the microstructural architecture of bone by examining collagen fiber orientations. These results will have implications for physical anthropology, bone biology, orthopaedics, tissue engineering, and structural biology.