INFLUENCE OF TRABECULAR TISSUE DEVELOPMENT TIME ON ADAPTATION TO MECHANICAL STIMULUS

Participants: N.J. Caldwell, C.R. Maynard, M.M. Moalli, S.A. Goldstein,

K.A. Sweet, B. Nolan, D.C. Kayner, M.W. Stock

Keywords: bone adaptation, trabecular bone, mechanical stimulus

Introduction

At the tissue level, trabecular bone has two types of structural organization. Lamellar bone, the primary type found in adult human trabecular bone, is characterized by a very organized arrangement of collagen fibers and apatite crystals- the main constituents of bone. Woven bone, on the other hand, contains randomly oriented constituents and is found during periods of rapid bone formation, such as growth and fracture healing. The effect of initial tissue maturity and structure on the adaptation potential of trabecular bone has not previously been investigated, and could be critical to understanding the process of bone adaptation in both normal and pathologic states. For example, during fracture healing and surgical reconstruction, where woven bone formation is dominant, the adaptive potential of bone may be markedly different than during states of primarily lamellar bone, such as normal adult bone maintenance or during disease conditions such as osteoporosis. Thus, further understanding of the relationship between the structure of bone and its functional adaptation capability is extremely relevant to a number of clinical situations, as well as to expanding our knowledge of basic bone physiology. This work is investigating the hypothesis that the existing tissue structure (lamellar vs. woven) of the trabecular bone will influence the subsequent adaptation response to mechanical stimuli.

Materials and Methods

This hypothesis is being investigated utilizing an experimental model that allows controlled mechanical forces to be applied to a volume of trabecular bone tissue in vivo. The large volume bone chamber, which is a hollow cylindrical titanium implant with large transverse infiltration portals, is implanted in the metaphyseal region of the canine proximal tibia, and trabecular bone forms within the hollow chamber. The design allows for controlled compressive force to be delivered to the tissue within the chamber, and previous studies have shown that the chamber adequately shields the tissue from loads transmitted from the joint, so the tissue within the chamber is essentially mechanically isolated. Thus, the model contains the necessary in vivo biologic factors, which can enter via the infiltration portals, as well as allowing the application of known mechanical loading histories to the tissue. The type of tissue that is present within the chamber is dependent on the time allowed for development, with mainly immature woven bone present at earlier time points, and remodeled lamellar bone persisting later. After an experimental time period, which can be varied to produce differing tissue types, a specially designed coring tool is used to remove the tissue within the chamber for analysis.

The experimental design consists of the implantation of bilateral tibial chambers in adult mongrel dogs. Following a four week biopsy (to diminish the effect of the implantation), bone develops within the chambers for sixteen weeks. At that point, a single (one-side) biopsy is performed, followed by eight more weeks of maturation, thus producing one chamber with tissue that has developed for 24 weeks (predominately lamellar), while the bilateral chamber has predominantly woven eight-week-old tissue. The loading piston and cap will then be inserted, and daily loading initiated. This design allows for direct comparison of woven and lamellar responses within a dog.

Progress

To date, six animals have gone through five cycles of long-term tissue growth and in vivo loading. The MCT data has been acquired and analysis is in progress. Histologic processing is also in progress.