Michael J. Yaszemski

Evolution of bone transplantation: molecular, cellular and tissue strategies to engineer human bone *

Bone defects occur in a wide variety of clinical situations, and their reconstruction to provide mechanical integrity to the skeleton is a necessary step in the patients rehabilitation. The current gold standard for bone reconstruction, the autogenous bone graft, works well in many circumstances. However, autograft reconstruction, along with the available alternatives of allogenous bone graft or poly(methylmethacrylate) bone cement, do not solve all instances of bone deficiency. Novel materials, cellular transplantation and bioactive molecule delivery are being explored alone and in various combinations to address the problem of bone deficiency. The goal of these strategies is to exploit the bodys natural ability to repair injured bone with new bone tissue, and to then remodel that new bone in response to the local stresses it experiences. In general, the strategies discussed in this paper attempt to provide the reconstructed region with appropriate initial mechanical properties, encourage new bone to form in the region, and then gradually degrade to allow the new bone to remodel and assume the mechanical support function. Several of the concepts presented below are already finding clinical applications in early patient trials.

Biodegradable polymer scaffolds to regenerate organs

The problem of donor scarcity precludes the widespread utilization of whole organ transplantation as a therapy to treat many diseases for which there is often no alternative treatment. Cell transplantation using biodegradable polymer scaffolds offers the possibility to create completely natural new tissue and replace organ function. Tissue inducing biodegradable polymers can also be utilized to regenerate certain tissues and without the need for in vitro cell culture. Biocompatible, biodegradable polymers play an important role in organ regeneration as temporary substrates to transplanted cells which allow cell attachment, growth, and retention of differentiated function. Novel processing techniques have been developed to manufacture reproducibly scaffolds with high porosities for cell seeding and large surface areas for cell attachment. These scaffolds have been used to demonstrate the feasibility of regenerating several organs.

In vitro degradation of a poly(propylene fumarate)-based composite material

We investigated the in vitro degradation of a novel degradable polymeric composite material being developed to function as a temporary replacement for trabecular bone. This material is based on a mixture of poly(propylene fumarate) cross-linked by N-vinyl-pyrrolidone and includes sodium chloride and beta-tricalcium phosphate. Using an in vitro test in simulated body fluids, the compressive strengths and compressive moduli of two composite materials increased with degradation time and remained above the minimum values acceptable for trabecular bone substitutes. A compressive strength of 21.3 (+/- 0.4) MPa and a compressive modulus of 696 (+/- 53) MPa were measured after twelve weeks for a composite material with initial strength of 18.0 (+/- 4.6) MPa and initial modulus of 113 (+/- 40) MPa. This unexpected phenomenon may prove to be useful for orthopaedic applications.

The Ingrowth of New Bone Tissue and Initial Mechanical Properties of a Degrading Polymeric Composite Scaffold

Trabecular bone deficiency causes a dilemma at surgery in a variety of clinical situations, including trauma, tumor resection, and reconstruction. A synthetic material to replace trabecular bone would be biocompatible, provide temporary mechanical strength to the reconstructed region, and serve as a scaffold upon which new bone could grow (i.e., osteoconduction). In addition, it should serve as a carrier for osteoinductive biomolecules, degrade into nontoxic materials that the body can excrete via normal metabolic pathways, and allow the new bone to remodel along lines of local stress. A particulate filled composite based on an unsaturated linear polyester was designed as a candidate material for this application. The components are mixed with a monomer that cross links the double bonds of the unsaturated polyester. Degradation occurs via hydrolytic degradation of the backbone polymers ester linkages. This strategy of prepolymer synthesis via condensation polymerization in the laboratory followed by cross linking the unsaturated prepolymer via radical polymerization at surgery offers design flexibility. The radical polymerization allows curing during surgery to facilitate reconstruction of various shaped defects. The laboratory synthesis of the prepolymer allows alterations of its composition and physical properties to effect desired properties in the resulting composite. This study investigates the effect of several composite material formulations on the in vitro mechanical properties and the associated in vivo histologic characteristics of the resulting material. The prepolymer molecular weight, presence of a leachable salt, and amount of cross linking monomer had strong effects on the resulting strength and modulus of the composite. These strengths were on the order of 5 MPa, a magnitude appropriate for consideration of the material as a temporary trabecular bone substitute. The in vivo studies in a rat proximal tibia model demonstrated progressive growth of new bone against the receding surface of the degrading material, and ingrowth of new bone trabeculae into the interior of the degrading specimen. The specimen was also well integrated with the surrounding bone, with no internal fibrosis. There was an absence of a foreign body inflammatory response to the presence of this material over a 5-week time span. This material may thus be an attractive candidate for temporary replacement of trabecular bone, facilitating both osteoconduction and osteoinduction.

Poly(α-Hydroxy Ester)/Short Fiber Hydroxyapatite Composite Foams for Orthopedic Application

A process has been developed to manufacture biodegradable composite foams of poly(DL-lactic- co-glycolic acid) (PLGA) and hydroxyapatite short fibers for use in bone regeneration. The processing technique allows the manufacture of three-dimensional foam scaffolds and involves the formation of a composite material consisting of a porogen material (either gelatin microspheres or salt particles) and hydroxyapatite short fibers embedded in a PLGA matrix. After the porogen is leached out, an open-cell composite foam remains which has a pore size and morphology defined by the porogen. The foam porosity can be controlled by altering the volume fraction of porogen used to make the composite material. Foams made using NaCl particles as a porogen were manufactured with porosities as high as 0.84±0.01 (n=3). The short hydroxyapatite fibers served to reinforce the PLGA. The compressive yield strength of foams manufactured using gelatin microspheres as a porogen was found to increase with fiber content. Foams with compressive yield strengths up to 2.82±0.63 MPa (n=3) with porosities of 0.47±0.01 (n=3) were manufactured using 30% by weight hydroxyapatite fibers in the initial composite prior to leaching. These composite foams with improved mechanical properties may also be expected to have enhanced osteoconductivity and hence provide a novel material which may prove useful in the field of bone regeneration.

The growth and phenotypic expression of human osteoblasts.

The care of patients with a skeletal deficiency currently involves the use of bone graft or a non-biologic material such as a metal or polymer. There are alternate possibilities in development which involve the growth of bone cells (osteoblasts) on degradable polymer scaffolds. These tissue engineering strategies require production of the polymeric scaffold, cellular harvest followed by either ex vivo or in vivo growth of the cells on the scaffold, and exploration of the interaction between the cell and scaffold. Research into these strategies utilizes cells from a variety of species, but clinical applications will likely require human osteoblasts. This study explores the process whereby human osteoblasts are harvested under sterile conditions during joint replacement surgery from normally discarded cancellous bone, transported from the operating room to the lab, and grown in culture. This process is feasible, and the cells express their phenotype via the production of alkaline phosphatase and collagen in culture.