INCREASED DISTRACTION RATES INFLUENCE PRECURSOR TISSUE COMPOSITION WITHOUT AFFECTING BONE REGENERATION

Participants: M. Richards, K.M. Kozloff, J.A. Goulet, S.A. Goldstein

Keywords: distraction osteogenesis, chondrogenesis, bone regeneration, strain, damage

Introduction

Distraction osteogenesis (DO) is an increasingly popular technique used to stimulate new bone formation to treat orthopaedic disorders resulting from bony defects and deficits. Slow, controlled separation of the two bony ends at a rate of about one mm/day has been accepted as critical to the technique’s success, however faster distraction rates might reduce the amount of time required for lengthening and subsequent consolidation. This leads to the question of whether modest increases in the distraction rate prove detrimental to bone regeneration, or affect precursor tissue composition within the gap. The objectives of this study were to determine whether increases in distraction rate: 1) affect new bone volume; and 2) alter the amount or distribution of cartilaginous tissue within the distraction gap.

Materials and Methods

Four threaded K-wires (0.067") were placed in the femora of adult male Sprague-Dawley rats, and sub-periosteal osteotomies were performed at mid-diaphysis. After a six-day latency period, one randomly chosen femur was distracted 0.50 mm/day for 12 days, and the contralateral femur was distracted at 1.50 mm/day for 4 days. Distraction for both legs began on the same day following the six-day latency period. Total femoral lengthening was 6.0 mm on both sides, and was completed on day 10 for the increased rate leg and day 18 for the contralateral limb. Group I rats (n=9) were sacrificed on day 18 for the investigation of soft tissue development within the distraction callus. Group II rats (n=7) were sacrificed on day 36 to determine mineralized bone volumes within the gap. Distraction zones were harvested with fixators and neighboring bone intact and fixed in neutral-buffered formalin before being placed in 70% ethanol. While in ethanol, Group II distraction zones were scanned by three-dimensional micro-computed tomography at a resolution of 50 µ/voxel to obtain histograms of voxel count vs. voxel intensity. Group I rats were decalcified and embedded in paraffin, sectioned at 7 µm, and stained with toluidine blue and safranin-O/fast green. A second set of slides were stained with type II antibody for detection of cartilage. Group II rats were dehydrated and embedded undecalcified in PMMA, sectioned at 1000 µm, mounted on radiolucent plastic, polished, and surface-stained with toluidine blue for qualitative histologic analysis.

Results

Histograms from MCT images showed measures of new bone volume to be increased slightly in rat femora distracted at the higher rate (Figure 1) for both BVGap (tissue within the gap bounded by the radial dimensions of the original cortex) and BVFull (extended area including the entire distraction callus). However, these differences were small and not statistically significant (BVGap- 42.69 mm3 vs. 46.13 mm3, p=0.63; BVFull- 75.05 mm3 vs. 82.21 mm3, p=0.53 ).

Digital color analysis of Group I rats demonstrated mild differences in the amount of cartilage between femora distracted at the two rates. Analyses of slides stained with type II antibody indicated a nearly two-fold increase in cartilaginous tissue area (1.51 mm2 vs. 0.83 mm2, p=0.10; Figure 2). The trend did not achieve statistical significance.

Discussion

Higher distraction rates induced increases in cartilage formation at 18 days post-surgery. The presence of cartilage was always associated with bone formation activity and was usually in a periosteal location. The three-fold increase in distraction rate had no effect on new bone volumes at 36 days post surgery; thus femora distracted at both rates appeared to make bone at similar rates.

Increased distraction rates might lead to higher peak stresses and strains immediately after distraction, and more rapid tension accumulation during the full course of lengthening may occur at higher distraction rates. These effects may decrease vascularity or oxygen tension to the tissues, by either exceeding the growth of blood vessels within the area, or damaging existing blood vessels, rendering the tissue more amenable to cartilage differentiation than other tissue types [1,2].

While increased distraction rates did influence cartilaginous tissue production, they did not have any effect on new bone volume at 36 days post-surgery in this animal model. New bone volume was unaffected by any potential prior differences in cartilaginous tissue formation. The high distraction rate may have initially caused damage reflected in the increased cartilage levels, however it is possible that the extra consolidation time, seen by the increased rate leg, may have allowed more healing to occur than the contralateral femur. Increased endochondral bone formation may have contributed to the levels of bone seen at the final time point.

Our observations support the currently held view than an intermediate distraction rate is optimal. By increasing distraction rate to a value that may increase cartilage formation, but not cause permanent damage to the tissue, nor hinder new bone formation, the final desired length might be reached at an earlier time point. However, additional consolidation time may then be required in order to achieve required bony filling of the gap.

References

1. Ashhurst DE 1992 Macromolecular Synthesis and Mechanical Stability during Fracture Repair. In: Hall BK (ed) Bone, 1st ed, vol 5. CRC Press, Inc., Ann Arbor, pp. 80-87.

2. Li G, Simpson A, Kenwright J, Triffit J 1997 Assessment of cell proliferation in regenerating bone during distraction osteogenesis at different distraction rates. J Orthop Res 15: 765-772.