A review of biomaterials in bone defect healing, remaining shortcomings and future opportunities for bone tissue engineering: The unsolved challenge

Abstract

Despite its intrinsic ability to regenerate form and function after injury, bone tissue can be challenged by a multitude of pathological conditions. While innovative approaches have helped to unravel the cascades of bone healing, this knowledge has so far not improved the clinical outcomes of bone defect treatment. Recent findings have allowed us to gain in-depth knowledge about the physiological conditions and biological principles of bone regeneration. Now it is time to transfer the lessons learned from bone healing to the challenging scenarios in defects and employ innovative technologies to enable biomaterial-based strategies for bone defect healing. This review aims to provide an overview on endogenous cascades of bone material formation and how these are transferred to new perspectives in biomaterial-driven approaches in bone regeneration. Cite this article: T. Winkler, F. A. Sass, G. N. Duda, K. Schmidt-Bleek. A review of biomaterials in bone defect healing, remaining shortcomings and future opportunities for bone tissue engineering: The unsolved challenge. Bone Joint Res 2018;7:232-243. DOI: 10.1302/2046-3758.73.BJR-2017-0270.R1.

Keywords: Biomaterials; Bone; Fracture; Healing phases; Mineralization; Scaffold.

Radiological time course of 59-year-old female patient after bilateral open distal femoral fractures (a, c, e: right leg; b, d, f: left leg). a) and b) radiographs at admission with bilateral femoral comminuted fractures. Of note is the bone deficiency on the right femur immediately after injury. c) and d) The status after initial stabilization. Note the shortening and comminution of the right distal femur and the gentamicin beads in the femoral canal of the left femur indicating post-traumatic infection therapy. e) Images after removal of the plate and external stabilization of the right femur. Note the atrophic nonunion, indicating biological inhibition of bone healing. f) Radiographs after removal of avital femoral bone and temporary replacement with antibiotic-loaded bone cement of the left femur. g) Latest radiograph of the follow-up showing both knees replaced with distal femoral replacements; on the left limb, a total femoral replacement was necessary due to the too small residuum of the femur after resecting all infected and dead bone.

Atrophic pseudarthrosis of the radial diaphysis in a 17-year-old girl. a) Anteroposterior and b) lateral radiograph of the right forearm showing plate breakage at the site of nonunion (arrow and arrowhead). Also seen is an impingement of the carpus due to flexion contracture and a radiocarpal collapse.

Radiological follow-up of a 44-year-old male patient with infected pseudarthrosis of the right femoral diaphysis and large bone defect after shotgun injury: a) anteroposterior radiograph of the right femur after plate stabilization and implantation of a calcium phosphate bone substitute (indicated by arrowheads) into the defect area without signs of integration or remodelling; b) CT scan after debridement of the defect and removal of most of the calcium phosphate, and implantation of an antibiotic-loaded bone cement spacer (indicated by arrowheads) augmented by a metal nail for local antimicrobial therapy; c) anteroposterior radiograph after removal of the spacer and implantation of a vascularized fibular graft (the fibula was harvested from the ipsilateral leg and fixed with screws into the defect in two parts nurtured by the same artery); d) anteroposterior radiograph from the ipsilateral lower leg demonstrates the donor site morbidity after removal of the fibula; e) anteroposterior radiograph after one-year follow-up. The fibular graft shows good integration and the implant is without loosening or failure.

Fig. 4

Consecutive phases of bone healing are described in view of conditions, processes occurring in normal healing and the biomaterial properties which could support these phases. The lower row depicts the healing bone tissue in the different phases. Images from left to right: haematoma, haematoxylin eosin (H&E) staining, one day after injury, sheep; granulation tissue, immune histology for alpha smooth muscle with a methylene green counterstain, seven days after injury, sheep; cartilage, Movat pentachrome staining, 14 days after injury, mouse; woven bone, Movat pentachrome staining, 21 days after injury, mouse; remodelling, Movat pentachrome staining, six weeks after injury, rat.

In a long bone defect model in a rat femur, gap size 5 mm, stabilized with an external fixator, bone healing was compared between a group with an empty defect and a group receiving an artificial blood clot from allogenic peripheral rat blood (n = 6). Animals were sacrificed after two, four and six weeks, and analyzed with micro-CT to determine the percentage of mineralized tissue. A) 3D images of µCT evaluation, b) tissue volume in mm3 of the volume of interest analyzed by µCT c) bone volume in mm3 of the volume of interest analyzed by µCT d) the quotient of bone and tissue volume of the volume of interest analyzed by µCT. No significant difference was found between the groups, confirming the usability of such a biomaterial to add cells or factors during the early bone healing phase (animal experiments were approved by the local legal representative (G 0428/07) and carried out according to the policies and principles established by the Animal Welfare Act).

a) Growth and differentiation factor 5 (GDF5); b) bone morphogenetic protein (BMP)-2 application. Histological staining was done with Movat pentachrome to depict different tissues: yellow, bone; green, cartilage; orange, muscle; light blue, connective tissue; dark red, bone marrow (adapted from Wulsten et al). c) Graph showing that, when applied within a critical-sized defect in a rat femur osteotomy model, GDF5 with a collagen scaffold favoured cartilage formation within the callus after six weeks, as compared with BMP2. d) Graph showing that tissue stiffness within the defect showed solidification after two weeks with BMP, while GDF5 still had not reached bone stability after six weeks of healing. Control confirms the non-healing of this model without growth factor application.

Fig. 7

Mesenchymal stromal cells (MSCs) are able to differentiate into adipose, fibrous or bone tissue depending on the substrate stiffness; a stiffer substrate favours bone formation. Successful bone formation also depends on the oxygen tension. A lack of oxygen leads to cartilage formation. These differentiation properties have prompted numerous research projects in which the substrate qualities were used to steer cell differentiation.