INFLUENCE OF LOADING RATE AND MAGNITUDE ON TRABECULAR BONE ADAPTATION TO MECHANICAL STIMULUS

Participants: N.J. Caldwell, C.R. Maynard, M.R. Moalli, S.A. Goldstein, K.A. Sweet, B. Nolan, D.C. Kayner, M.W. Stock

Keywords: bone adaptation, trabecular bone, mechanical stimulus

Introduction

Despite numerous investigations, the quantitative nature of the relationship between mechanical factors and the formation, maintenance, and adaptation of bone has yet to be determined. It is believed that local mechanical factors have a direct effect at the cellular level on the bone remodeling response. The focus of this research is to identify the cellular events and the associated morphologic and architectural adaptation of trabecular bone that occur in response to experimentally controlled mechanical stimuli. Specifically, the aim of this project is to elucidate the relationship between the bone response and particular characteristics of the applied mechanical stimuli, namely the loading rate and load magnitude.

Materials and Methods

The large volume hydraulic bone chamber, 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 via an actuator pressurized through a sealed, servohydraulic, microcomputer-controlled system. 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. After an experimental time period, a specially designed coring tool is used to remove the tissue within the chamber for analysis, allowing new bone to form. This experimental procedure can be repeated numerous times in the same animal

The experimental design consists of two groups, one investigating the effect of loading rate (n=6), and the other load magnitude (n=6). The rate group compares four loading rates: 0 lbs/sec (no load), 20 lbs/sec, 40 lbs/sec, and 80 lbs/sec. The magnitude group compares four loading magnitudes: 0 lb. (no load), 1 lb., 2 lb., and 4 lb. peak load. A trapezoidal waveform (equal rise and decay times) at 1 Hertz for 1800 cycles per day is used for all load groups. In order to investigate both the cellular and morphologic changes in the bone and their temporal sequence, animals are further distinguished by loading duration within each experimental group. For each experimental group there are three load durations: three days (n=2, each group), two weeks (n=2), and eight weeks (n=2). The experimental flow is as follows. Bilateral chambers are inserted in the proximal tibia and four weeks later the initial bone is biopsied in order to create an equal baseline for all subsequent cycles. Tissue is then allowed to develop for eight weeks prior to loading. During the daily load period, the length of which is dependent on the duration group, the two chambers receive different load rates or magnitudes. The bone is then biopsied, frozen for processing, and the experimental cycle is repeated with another load comparison until five comparisons have been performed per dog. The specimens are scanned on a microcomputed tomography system and analyzed for morphologic and architectural parameters; then cryo-sectioned and stained for basic histology, type I procollagen, alkaline phosphatase, and acid phosphatase.

Progress

All animals have completed all five in vivo experimental cycles. Data analysis is in progress.