Balancing the Rates of New Bone Formation and Polymer Degradation Enhances Healing of Weight-Bearing Allograft/Polyurethane Composites in Rabbit Femoral Defects

Abstract

There is a compelling clinical need for bone grafts with initial bone-like mechanical properties that actively remodel for repair of weight-bearing bone defects, such as fractures of the tibial plateau and vertebrae. However, there is a paucity of studies investigating remodeling of weight-bearing bone grafts in preclinical models, and consequently there is limited understanding of the mechanisms by which these grafts remodel in vivo. In this study, we investigated the effects of the rates of new bone formation, matrix resorption, and polymer degradation on healing of settable weight-bearing polyurethane/allograft composites in a rabbit femoral condyle defect model. The grafts induced progressive healing in vivo, as evidenced by an increase in new bone formation, as well as a decrease in residual allograft and polymer from 6 to 12 weeks. However, the mismatch between the rates of autocatalytic polymer degradation and zero-order (independent of time) new bone formation resulted in incomplete healing in the interior of the composite. Augmentation of the grafts with recombinant human bone morphogenetic protein-2 not only increased the rate of new bone formation, but also altered the degradation mechanism of the polymer to approximate a zero-order process. The consequent matching of the rates of new bone formation and polymer degradation resulted in more extensive healing at later time points in all regions of the graft. These observations underscore the importance of balancing the rates of new bone formation and degradation to promote healing of settable weight-bearing bone grafts that maintain bone-like strength, while actively remodeling.

d-amino Acid Inhibits Biofilm but not New Bone Formation in an Ovine Model

“…The primary outcome was bone volume/total volume measured by lCT in the middle region of the defect. An a priori power analysis indicated that a sample size of four would detect a 7.5% difference in bone volume/total volume between groups assuming a mean and SD of 30% and 5%, respectively in the middle region of interest of the graft (2.5 mm from the center-line) [13,37]. This is with a power of 80% and an alpha level of 0.05 using a two-tailed t-test of two independent samples.…”

“…Osteoclast precursor cells were prepared from spleens of four adult C57 WT mice by the Ficoll (Lymphocyte Separation Medium; MP Biomedicals, Solon, OH, USA) gradient method [6] and plated separately at a density of 2. [13,14,37]. An advantage of low-viscosity grafts compared with calcium phosphate cements or allograft is its ability to support diffusion-controlled sustained release of biologics [20,41].…”

“…The three inner regions incorporated the composites, whereas the outer region included the host bone interface. Ossified tissue was segmented using a threshold of 340 mg hydroxyapatite per cm À3 to include new bone and ceramic as reported previously [13,37]. Bone volume fraction (bone volume/total volume), trabecular separation, trabecular number, and trabecular thickness were measured for each annular region and plotted versus the mean radial distance from the core of the defect (R m ) [13,37].…”

“…Ossified tissue was segmented using a threshold of 340 mg hydroxyapatite per cm À3 to include new bone and ceramic as reported previously [13,37]. Bone volume fraction (bone volume/total volume), trabecular separation, trabecular number, and trabecular thickness were measured for each annular region and plotted versus the mean radial distance from the core of the defect (R m ) [13,37]. These morphometric parameters were chosen to quantitatively assess the remodeling of trabecular bone architecture in three dimensions assuming a fixed-structure model [5].…”

“…Sections were stained with Stevenel's Blue and Van Gieson's and imaged at 92 and 920 magnification with an Olympus DP71 camera and SZX16 microscope (Olympus Corp, Tokyo, Japan). New bone formation and residual ceramic were quantified based on color analysis from one slide per defect (two data points per slide) using MetaMorph 1 software (Version 7.0.1) in an area of interest (6 9 14.6 mm) located in the center of the defect [13,14,37]. The rectangular area of interest was subdivided in regions 1.83 mm wide corresponding to the size and location of the lCT regions of interest [13,37].…”