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Objectives: Fracture risk in the elderly osteoporotic patient is a major clinical concern. Fractures experiencing delayed union may result in pseudarthroses (false joints) which are often associated with chronic pain and disability. Understanding the role of mechanical factors in the development of pseudarthroses will give further insight to clinicians treating these debilitating fractures. A tissue differentiation hypothesis developed in our laboratory proposes that: 1) hydrostatic pressure directs the pluripotential mesenchymal tissue of a fracture callus down a chondrogenic pathway; 2) significant shear or tensile strain leads to fibrogenesis; and 3) given adequate vascularity, minimal levels of hydrostatic stress and shear/tensile strain allow direct intramembranous bone formation.1, 3 The objective of the present study was to test this tissue differentiation hypothesis with a finite element (FE) model of an oblique fracture to determine if delayed union and pseudarthrosis formation could be predicted based on stress and strain distributions within the fracture callus.
Methods: Model I: An idealized 2-D FE model of an oblique fracture was created based upon the geometry of a typical oblique pseudarthrosis.2 A compressive axial force was applied to the cortical bone ends and plane strain analysis was performed to determine patterns of hydrostatic stress and maximum tensile strain. Model II: A contact model was then developed incorporating sliding contact surfaces within the interfragmentary gap corresponding to locations of high tensile strain and regions of callus failure predicted in Model I.
Results: Model I: Stress distributions showed low levels of hydrostatic tension at two periosteal corners of the fracture ends, high levels of hydrostatic pressure at the opposing periosteal corners, and intermediate levels of hydrostatic pressure throughout the interiragmentary gap. Maximum tensile strains were highest within the interiragmentary gap and lowest within the external callus. Model II: Hydrostatic stress distributions in the contact model were similar to those of Model I. Maximum tensile strain distributions, however, were quite different. Tensile strains decreased within the interiragmentary gap and increased within the external callus. These results would predict fibrocartilage maintenance within the interfragmentary gap and bone formation at the two periosteal corners experiencing low hydrostatic tension and low tensile strain. These results are consistent with in vivo patterns of bone and fibrocartilage formation in oblique pseudarthroses.3
Conclusions: We have predicted interfragmentary tissue failure, fibrocartilage formation, and locations of bone formation and resorption4 consistent with initial stages of pseudarthrosis development seen in vivo. These results provide us with a better understanding of how the stresses and strains at a fracture site may cause delayed union, nonunion, and pseudarthrosis formation. This information may lead to improved fixation techniques and clinical outcomes for osteoporotic patients undergoing fracture treatment.
References: 1) Carter et al. (1998) CORR 355S:S41; 2) McLean and Urist (1968) Bone. Chicago, Univ of Chicago Press 234; 3) Pauwels (1980) Biomechanics of the Locomotor Apparatus. Berlin, Springer- Verlag 106-137; 4) Robertsson et al. (1997) Acta Orthop Scand 68(3):231.
Acknowledgments: Supported by VA Rehabilitation R&D grant A501-4RA.