Intramembranous Bone Regeneration Research Articles

Saline irrigation does not affect bone formation or fixation strength of hydroxyapatite/tricalcium phosphate‐coated implants in a rat model

Intramembranous bone regeneration is critical to implant fixation. In cementless joint replacement (as opposed to cemented joint replacement), saline irrigation is not typically performed during surgery so that the osteogenic stimulus provided by the marrow is preserved. Several groups are now using the rat marrow ablation model to study intramembranous bone regeneration and implant fixation. In this model, the marrow contents are mechanically disrupted, and debris is often cleared by saline irrigation, a step that appears inconsistent with the clinical situation. Furthermore, in contrast to conventional wisdom, it has been reported that saline irrigation enhanced bone-implant contact and peri-implant bone formation in the rat model (Ishizaka et al. Bone 1996;19:589-594), although mechanical fixation of the implant was not investigated. Accordingly, the present study was performed to determine if saline irrigation leads to enhanced mechanical fixation of implants in the rat model. Forty-eight 400 to 450 g male rats were divided equally into two groups. The treatment group, in contrast to the control group, received saline irrigation in the ablated medullary canal prior to placement of hydroxyapatite/tricalcium phosphate-coated implants. Eight animals in each group were killed at 2, 4, or 8 weeks after implantation, at which time the specimens were analyzed by micro computed tomography to measure bone formation around the implant, followed by a mechanical pull-out test to measure the strength of fixation of the implant. As expected, there was increased fixation strength over time, but there were no significant differences in peri-implant bone volume, bone-implant contact, or implant fixation strength between the two groups. Thus, we found no effect of saline irrigation on bone formation or implant fixation strength in this study in which the implant had an osteoconductive coating.

Local application of rhTGF-β2 modulates dynamic gene expression in a rat implant model

Various anabolic agents, including transforming growth factor-beta (TGF-β), have been shown to enhance intramembranous bone regeneration and strengthen the mechanical connection between implant and host skeleton, a prerequisite for clinical success with orthopedic and dental implants. Mechanisms underlying these observations at the level of the gene have received little attention. A rat model was used to examine levels of gene transcription for 21 “osteogenic” genes by real-time polymerase chain reaction at days 1, 3, 5, 7, 10, 14, and 28 in a control group and a group in which the implant was treated with 1 μg recombinant human TGF-β2 ( n = 42, equally divided among the 2 groups and 7 time points). Genes were chosen to represent three functional categories: (1) growth factors, their receptors and antagonists; (2) bone differentiation markers; and (3) inflammation markers. Examination of the transcription profiles showed that nine genes had up-regulated or down-regulated expression levels without a change in timing and 12 genes had accelerated or delayed expression profiles with or without a concomitant change in maximal or minimal expression. The earliest changes (days 1–3) involved accelerated expression profiles for IGF-1R and VEGF and up-regulation of TGF-β2, TβRI, BMP-2, BMP-7, and Cbfa1. Furthermore, principal components analyses showed that some subsets of genes were co-expressed in both groups, although the temporal relationship of these subsets was altered following growth factor treatment. Thus, in addition to changes in individual transcription profiles, the regulatory connections between sets of co-expressed genes may also be affected by exogenously delivered anabolic agents during bone regeneration.

Aging does not lessen the effectiveness of TGFbeta2-enhanced bone regeneration.

Controversy exists over the potency of bone healing in the aged skeleton, and there is concern that enhancement of bone regeneration after use of bone-stimulating growth factors may not be effective in the aged. In this study, 30 skeletally mature beagles (1-2 or 10-12 years old) had titanium implants placed bilaterally in the proximal humerus for a period of 4 weeks in a model of intramembranous bone regeneration. A bony defect made at the time of surgery created a 3-mm gap between the implant surface and the host bone. Some of the implants were treated with recombinant human TGFbeta2 (rhTGFbeta2) at various does (0.32-35 microg per implant), and some served as paired controls. The dose response was similar in young and old animals. The most effective dose, 35 microg, led to a 3-fold increase in the volume fraction of new bone within the gap in both the young (p = 0.001) and old (p = 0.002) animals. At this dose, there was a 5-fold increase in osteoblast surface. While age did not significantly affect the quantity of new bone formed as assessed by backscatter scanning electron microscopy, the older animals had thinner regenerated trabeculae that tended to be spaced more closely than the younger animals. Coupled with the finding that the increase in osteoid was greater in the old animals compared with the young animals, these qualitative differences suggest that there may have been a slight delay in the rate or a defect of mineralization in the old animals.

The effects of magnitude and frequency of distraction forces on tissue regeneration in distraction osteogenesis of the mandible.

Callus distraction has become an accepted treatment procedure to lengthen hypoplastic mandibles in humans. For this purpose, extraoral and intraoral devices have been applied successfully. The effects of the distraction vector, distractor stability, and rate and frequency of callus distraction on the regenerating tissues have been examined in various studies. In an experimental animal trial on pigs (n = 12), a newly developed microhydraulic osteodistractor was tested. The animals were evenly assigned to two groups to perform a continuous and noncontinuous osteodistraction of the mandible. Initially, the forces necessary to distract the pig mandibles were recorded from a noncontinuous distraction procedure; the results were then used to perform continuous bone distraction. Besides testing the new distractor, the study proved that in continuous osteodistraction, intramembranous bone regeneration occurred, whereas in noncontinuous osteodistraction the bone regeneration process followed a chondroid ossification. In continuous osteodistraction, the bone regeneration proceeded at a higher speed with the lower distraction forces compared with noncontinuous distraction, thereby reducing the consolidation period. Clinical and microscopical results are presented.

Proteoglycan synthesis during intramembranous bone regeneration following avulsive wounding in guinea pig long bones.

Information on proteoglycan synthesis by bone cells and tissue is largely limited to studies of developing fetal bone. The present investigation focuses on proteoglycan synthesis during the intramembranous type of bone regeneration seen within avulsive (puncture-type) defects placed in guinea pig tibiae. [35S] Sulfate-labeled proteoglycans were extracted from tissue within regenerating tibial avulsive defects seven days following surgical wounding and also from xiphisternal cartilage utilized as an internal control. Labeled proteoglycans in 4M guanidine HCl extracts of regenerating bone and cartilage were purified by DEAE-Sephacel chromatography and further analyzed by chromatography and appropriate enzyme digestions. Regenerating bone tissue contained a proteoglycan relatively small in size (Kav = 0.56 following chromatography on Sepharose CL-2B) compared to proteoglycan from xiphisternal cartilage (Kav = 0.17). Alkaline borohydride treatment degraded this bone proteoglycan (Kav = 0.4 on Sepharose CL-6B), indicating an average molecular weight of glycosaminoglycan chains approximating 50,000. Enzymatic digestions followed by Sepharose CL-6B chromatography showed that glycosaminoglycan side chains of regenerating bone proteoglycan contained dermatan sulfate, with 60% chondroitinase AC II-resistant but chondroitinase ABC-sensitive material. This bone proteoglycan did not interact with hyaluronic acid to form aggregates under conditions where such aggregates were formed by xiphisternal cartilage proteoglycan. The regenerating bone proteoglycans are therefore similar to other bone proteoglycans in hydrodynamic size and glycosaminoglycan chain size, but differ in the per cent of iduronic acid within glycosaminoglycan side chains. This guinea pig bone proteoglycan may be associated with the large mesenchymal cell population noted histologically within the bone defects at seven days of regeneration.