Bone regeneration strategies: Engineered scaffolds, bioactive molecules and stem cells current stage and future perspectives

Abstract

Bone fractures are the most common traumatic injuries in humans. The repair of bone fractures is a regenerative process that recapitulates many of the biological events of embryonic skeletal development. Most of the time it leads to successful healing and the recovery of the damaged bone. Unfortunately, about 5-10% of fractures will lead to delayed healing or non-union, more so in the case of co-morbidities such as diabetes. In this article, we review the different strategies to heal bone defects using synthetic bone graft substitutes, biologically active substances and stem cells. The majority of currently available reviews focus on strategies that are still at the early stages of development and use mostly in vitro experiments with cell lines or stem cells. Here, we focus on what is already implemented in the clinics, what is currently in clinical trials, and what has been tested in animal models. Treatment approaches can be classified in three major categories: i) synthetic bone graft substitutes (BGS) whose architecture and surface can be optimized; ii) BGS combined with bioactive molecules such as growth factors, peptides or small molecules targeting bone precursor cells, bone formation and metabolism; iii) cell-based strategies with progenitor cells combined or not with active molecules that can be injected or seeded on BGS for improved delivery. We review the major types of adult stromal cells (bone marrow, adipose and periosteum derived) that have been used and compare their properties. Finally, we discuss the remaining challenges that need to be addressed to significantly improve the healing of bone defects.

Keywords: Bioactive; Biomaterial; Fracture; Nonunion; Scaffold; Stem cells.

Figure 1

The major fracture sites in the body where strategies using synthetic bone graft substitutes, bioactive molecules and/or stem cells are needed to repair bones in difficult clinical situations.

Figure 2

Healing of a non-stabilized long bone fracture through the formation of a cartilaginous callus. The major biological phases during healthy fracture healing go through the chronological stages of inflammation, the formation of a cartilaginous callus and remodeling of the callus into bone. The primary cell types that are found at each stage include inflammatory cells, chondrocytes, osteoblasts, osteoclasts, hematopoietic cells and osteocytes. (A) Upon fracture, the hematoma forms, associated with reduced O₂ and pH levels as well as increased lactate. At this stage, the inflammatory cells remove injured tissue and secrete stimulatory factors to recruit cells from the environment including the periosteum. (B) A callus forms due to the massive progenitor cell expansion leading to cellular condensation and initiation of chondrogenic differentiation. (C) Hypertrophic chondrocytes in the callus mineralize and osteoblasts enter and subsequently form woven bone. The woven bone remodels through osteoclast-osteoblast coupling and the lamellar bone eventually bridges the fracture (D).

Figure 3

The three major strategies currently used and developed to repair bone. The first (left column) relies on using only synthetic bone graft substitutes (BGS). The second (middle column) relies on combining bioactive molecules with a carrier that is mostly an extracellular matrix protein or a ceramic-based carrier. The third (right column) consists of combining stem cells with a carrier, possibly with the use of additional bioactive molecules. Each of these approaches is more appropriate for the healing of bone defects depending on their severity. When healing of a defect is compromised, there is a need to have a biological functionality in addition to the BGS, which is provided either by bioactive molecules, stem cells, or a combination of both.

Figure 4

Biology as a guide in the development of a cell-based construct for the treatment of compromised bone fractures. The natural healing process of a bone fracture is largely dependent on the mechanical environment. (A) A mechanically unstable long bone fracture heals through the formation of an intermediate cartilaginous callus that subsequently remodels into bone and the native bone structure and shape. The initial cartilaginous callus is mainly formed by cells recruited from the periosteum, and provides initial stabilization to the fracture. This allows blood vessel ingrowth closely followed by remodeling by cartilage-resorbing chondroclasts. Thereafter, progenitor cells recruited from the periosteum and bone marrow differentiate into osteoblasts that deposit new bone. (B) A compromised calvarial fracture in a mechanically stable environment mainly heals through direct ossification. In this process, cells from the periosteum, bone marrow (long bones) and dura mater (calvarial) contribute to the defect healing. (C) Consequently, the type of fracture to be repaired/healed determines the cell source, media, stimulatory factors and 3D environment that should be used in the design of a cell-based construct.