Mark Richards

Bone regeneration and fracture healing. Experience with distraction osteogenesis model.

The relation between physical forces and the processes of bone regeneration and healing remains incompletely understood. Gaps in understanding of these processes stem in part from models that produce inadequate amounts of new bone for study. Bone created through the use of distraction osteogenesis provides an attractive substrate for the study of mechanical forces and their effects on bone formation because this technique produces large volumes of new bone in a controlled fashion. The optimal mechanical environment in which bone formation occurs clinically has not been fully determined. In laboratory studies, however, the mechanical environment can be manipulated, and resultant changes in bone formation can be measured. To investigate how changes in strain environment influence patterns of bone formation, a bilateral distraction osteogenesis was implemented. When a stiffener was applied to the external distractor, computation analyses predicted a sevenfold to eightfold decrease in all strain measures. These reductions in gap strains appeared to induce significant decreases in bone volume fraction and mean trabecular thickness. When osteotomies were created at a 30° angle to the bony axis to generate more shear within the gap tissue, changes in the distribution of gap strains and resultant new bone architecture were observed. Specific correlations between changes in tissue level strains and the pattern of bone regeneration were seen in both experiments. These results provide direct in vivo evidence that pluripotential gap tissues are sensitive to their physical surroundings. Mechanisms responsible for this sensitivity might include vascularity, stem cell supply, and scaffolding architecture. The process of bone formation in distraction osteogenesis appears to be related to bone formation processes associated with more common conditions. The distraction osteogenesis model described suggests a mechanism for bone formation that seems applicable to other more common processes associated with bone formation, including fracture healing and impaired fracture healing.

Evaluation of the mechanical environment during distraction osteogenesis

Physical forces have been hypothesized to direct the process of bone regeneration during distraction osteogenesis. However, despite significant clinical experience, relatively little is known about how the mechanics of distraction influence bone formation. This study investigated net fixator forces and strains in the distraction callus during bilateral lengthening of tibiae in New Zealand White rabbits. Distractions yielded a classic viscoelastic response with a sharp increase in fixator force, followed immediately by significant relaxation. Tension acting on mesenchymal gap tissue caused by distraction was estimated to reach more than 30 N by the time full lengthening was achieved. Average maximum cyclic strains within the distraction zone during ambulation were estimated to be 14% to 15% and supported by the results of fluoroscopic imaging. Paradigms for fracture healing have hypothesized that such strains are incompatible with new bone formation. The documented clinical success of distraction osteogenesis at stimulating large volumes of new bone suggests that other mechanisms that warrant additional investigation may be at work during distraction.

Trabecular bone adaptation to variations in porous-coated implant topology

Trabecular bone adaptation adjacent to porous-coated platen implants embedded within canine distal femoral metaphyses was evaluated following 24 weeks of daily compressive loading. The in vivo experimental model delivered controlled loads to five different platen implant topologies with variations in platen shape and porous coating distribution. Adaptive changes were evaluated based on three-dimensional stereological analyses of trabecular bone architecture underneath each platen and non-destructive mechanical tests of platen construct stiffness. Fully coated cylindrical platen designs possessed the highest construct stiffness in both tension and compression. Changes in local trabecular bone morphology were also found to be significantly influenced by platen implant topology. Cylindrical platens with fully coated bottom surfaces were associated with greater decreases in trabecular bone volume and connectivity than cylindrical platens with smooth bottom surfaces or fully coated conical platens. Comparisons to site-matched contralateral control bone volumes across all platen designs indicated significant decreases in the average bone volume fraction, trabecular plate number, and connectivity within experimental samples, but no change in trabecular plate thickness. In addition, analyses of microstructural anisotropy revealed a 20 degrees or 20.2 degrees trabecular reorientation towards the axis of loading in experimental tissue. This study demonstrates that trabecular bone adaptation near porous-coated surfaces is influenced by variations in local implant topology and provides insight into specific mechanisms of implant-mediated microstructural adaptation.

Temporal and Spatial Characterization of Regenerate Bone in the Lengthened Rabbit Tibia

A rabbit model of bilateral tibial lengthening was used to investigate temporal and spatial changes in new bone volume and architecture during regenerate bone formation. Tibiae were lengthened 9.0 mm at 0.75 mm/day after a 6‐day latency period. Animals were euthanized at four time points, and new bone volume and architecture within the distraction gap were assessed by microcomputed tomography and histomorphometry. New bone formation began before day 18 postsurgery and increased markedly between day 18 (completion of distraction) and day 24. This period of high bone formation activity might therefore be optimal for biologic and mechanical interventions aimed at enhancing bone regeneration. Regions of both endochondral and intramembranous bone formation were observed throughout the consolidation period. Significant increases in bone volume fraction were observed early in the consolidation period and were attributed to significant increases in trabecular thickness. This suggested that increased mineral density in the gap tissue with time was a consequence of increased osteoblast activity and associated trabecular thickening. New bone formation was shown to be highly oriented toward the distraction axis throughout lengthening. More bone formed consistently in lateral and proximal regions of the distraction gap, perhaps due to improved blood supply or progenitor cell availability in these areas. No differences in trabecular architecture were detected between regions having more or less bone volume, suggesting that bony tissue differentiation in all regions of the distraction gap was similar. Homotypical variations in measures of bone architecture were small; thus, these outcome variables seem appropriate for determining the effects of biological and mechanical interventions on bone regeneration in this animal model.

Increased distraction rates influence precursor tissue composition without affecting bone regeneration

The effect of increased distraction rate on bony tissue differentiation was studied using a paired bilateral model of rat femur lengthening. After a 6‐day latency period, one randomly selected femur for each rat was distracted at 0.5 mm/day (normal rate) for 12 days, and the contralateral femur was distracted at 1.5 mm/day (increased rate) for 4 days. Femoral lengthening for each side was 6.0 mm, leaving the increased rate leg with an extra 8 days of consolidation compared with the normal rate limb. Group I rats (n = 9) were killed at day 18 postsurgery and analyzed for cartilage tissue composition and distribution. Group II rats (n = 7) were killed on day 36 postsurgery and analyzed by three‐dimensional microcomputed tomography (MCT) for changes in new bone volume. Digital color analysis of slides stained with type II collagen antibody showed increases in cartilaginous tissue formation on the increased rate side (1.51 mm2 vs. 0.83 mm2; p = 0.10). No differences in new bone volume were detected between increased rate limbs and their contralateral controls (46.13 mm3 vs. 42.69 mm3; p = 0.63). These findings suggest that intermediate distraction rates may influence precursor tissue composition without affecting the final amount of new bone formed. Because damage to the tissue was not detected at either time point, these changes in chondrogenesis may reflect sensitivity of the pluripotential gap tissue to tension accumulation during lengthening. Future work with this in vivo model is focused on improving our understanding of the mechanisms behind this strain sensitivity. (J Bone Miner Res 2000;15:982–989)

Viscoelastic Characterization of Mesenchymal Gap Tissue and Consequences for Tension Accumulation During Distraction

Nonlinear viscoelastic analysis was used to characterize the time-dependent behavior of mesenchymal gap tissue generated during distraction osteogenesis. Six (n = 6) lengthened tibiae were harvested from New Zealand white rabbits at 18 days. This gap tissue was subjected to a series of step displacement tests of increasing magnitude, and force relaxation behavior was monitored. Isochrones in stress-strain space were fit to odd cubic functions of strain. An analytic expression, linear in both e and e3, was developed to predict stress accumulation within the gap tissue as a function of time during distraction. Stress relaxation functions were described well by two-term Prony series. The two time constants determined from mechanical testing results were consistent, suggesting the presence of two fundamental physiologic relaxation processes. Gap tissue stresses were predicted to rise considerably during early stages of lengthening when distraction magnitudes exceeded the clinical norm of 0.25 mm. These differences in tension accumulation were less pronounced by the time lengthening was completed. Specifically, these results may in part explain clinical observations of decreased bone regeneration and altered tissue proliferation and differentiation at higher distraction rates. More generally, this work provides a framework for the rigorous characterization of the viscoelastic properties of biologic tissues ordinarily exposed to step strains.

Reduced gap strains induce changes in bone regeneration during distraction.

A bilateral New Zealand white rabbit model of distraction osteogenesis (DO) was used to investigate the relationship between strain environment and bone regeneration during limb lengthening. In seven (n = 7) rabbits, a stiffener was applied to the fixator on one side to reduce strains within the gap tissue after lengthening was completed. Animals were euthanized six days later and their distraction zones were harvested and analyzed for changes in new bone volume and architecture. Nonlinear finite element analyses (FEA) were performed to predict changes in the gap strain environment. FEA results predicted a nearly uniform sevenfold decrease in average strain measures within the distraction zone. No change in total average new bone volume and significant decreases in both bone volume fraction (BV/TV) and trabecular thickness (Tb.Th) were observed in tibiae in which gap strains were reduced experimentally, compared to contralateral controls. These results suggest that fixator stiffening influenced the architecture but not the amount of newly formed bone. This animal model of distraction might be used to study the mechanisms by which strain fields affect events in bone repair and regeneration, such as cell proliferation, precursor tissue differentiation, and altered growth factor and nutrient delivery to tissues.