Computational simulation of bone fracture healing under inverse dynamisation.

Adaptive finite element models have allowed researchers to test hypothetical relationships between the local mechanical environment and the healing of bone fractures. However, their predictive power has not yet been demonstrated by testing hypotheses ahead of experimental testing. In this study, an established mechano-biological scheme was used in an iterative finite element simulation of sheep tibial osteotomy healing under a hypothetical fixation regime, "inverse dynamisation". Tissue distributions, interfragmentary movement and stiffness across the fracture site were compared between stiff and flexible fixation conditions and scenarios in which fixation stiffness was increased at a discrete time-point. The modelling work was conducted blind to the experimental study to be published subsequently. The simulations predicted the fastest and most direct healing under constant stiff fixation, and the slowest healing under flexible fixation. Although low fixation stiffness promoted more callus formation prior to bridging, this conferred little additional stiffness to the fracture in the first 5 weeks. Thus, while switching to stiffer fixation facilitated rapid subsequent bridging of the fracture, no advantage of inverse dynamisation could be demonstrated. In vivo data remains necessary to conclusively test this treatment protocol and this will, in turn, provide an evaluation of the model's performance. The publication of both hypotheses and their computational simulation, prior to experimental testing, offers an appealing means to test the predictive power of mechano-biological models.

Treatment of long bone defects and non-unions: from research to clinical practice.

The treatment of long bone defects and non-unions is still a major clinical and socio-economical problem. In addition to the non-operative therapeutic options, such as the application of various forms of electricity, extracorporeal shock wave therapy and ultrasound therapy, which are still in clinical use, several operative treatment methods are available. No consensus guidelines are available and the treatments of such defects differ greatly. Therefore, clinicians and researchers are presently investigating ways to treat large bone defects based on tissue engineering approaches. Tissue engineering strategies for bone regeneration seem to be a promising option in regenerative medicine. Several in vitro and in vivo studies in small and large animal models have been conducted to establish the efficiency of various tissue engineering approaches. Neverthelsss, the literature still lacks controlled studies that compare the different clinical treatment strategies currently in use. However, based on the results obtained so far in diverse animal studies, bone tissue engineering approaches need further validation in more clinically relevant animal models and in clinical pilot studies for the translation of bone tissue engineering approaches into clinical practice.

Fit optimisation of a distal medial tibia plate.

An iterative method for the fit optimisation of a pre-contoured fracture fixation plate for a given bone data set is presented. Both plate shape optimisation and plate fit quantification are conducted in a virtual environment utilising computer graphical methods and 3D bone and plate models. Two optimised shapes of the undersurface of an existing distal medial tibia plate were generated based on a dataset of 45 3D bone models reconstructed from computed tomography image data of Japanese tibiae. The existing plate shape achieved an anatomical fit on 13% of tibiae from the dataset. Modified plate 1 achieved an anatomical fit for 42% and modified plate 2 a fit for 67% of the bones. If either modified plate 1 or plate 2 is used, then the anatomical fit can be increased to 82% for the same dataset. Issues pertaining to any further improvement in plate fit/shape are discussed.

Custom-made composite scaffolds for segmental defect repair in long bones.

Current approaches for segmental bone defect reconstruction are restricted to autografts and allografts which possess osteoconductive, osteoinductive and osteogenic properties, but face significant disadvantages. The objective of this study was to compare the regenerative potential of scaffolds with different material composition but similar mechanical properties to autologous bone graft from the iliac crest in an ovine segmental defect model. After 12 weeks, in vivo specimens were analysed by X-ray imaging, torsion testing, micro-computed tomography and histology to assess amount, strength and structure of the newly formed bone. The highest amounts of bone neoformation with highest torsional moment values were observed in the autograft group and the lowest in the medical grade polycaprolactone and tricalcium phosphate composite group. The study results suggest that scaffolds based on aliphatic polyesters and ceramics, which are considered biologically inactive materials, induce only limited new bone formation but could be an equivalent alternative to autologous bone when combined with a biologically active stimulus such as bone morphogenetic proteins.

Ovine bone- and marrow-derived progenitor cells and their potential for scaffold-based bone tissue engineering applications in vitro and in vivo.

Recently, research has focused on bone marrow derived multipotent mesenchymal precursor cells (MPC) and osteoblasts (OB) for clinical use in bone engineering. Prior to clinical application, cell based treatment concepts need to be evaluated in preclinical, large animal models. Sheep in particular are considered a valid model for orthopaedic and trauma related research. However, only sheep aged > 6 years show secondary osteon formation characteristic of human bone. Osteogenic cells isolated from animals of this age group remain poorly characterized. In the present study, ex vivo expanded MPC isolated from ovine bone marrow proliferated at a higher rate than OB derived from tibial compact bone as assessed in standard 2D cultures. MPC expressed the respective phenotypic profile typical for different mesenchymal cell populations (CD14(-)/CD31(-)/CD45(-)/CD29(+)/CD44(+)/CD166(+)) and showed a multilineage differentiation potential. When compared to OB, MPC had a higher mineralization potential under standard osteogenic culture conditions and expressed typical bone related markers such as osteocalcin, osteonectin and type I collagen at the mRNA and protein level. After 4 weeks in 3D culture, MPC constructs demonstrated higher cell density and mineralization, whilst cell viability on the scaffolds was assessed > 90%. Cells displayed a spindle-like morphology and formed interconnected networks. In contrast, when implanted subcutaneously into NOD/SCID mice, MPC presented a lower osteogenic potential than OB. In summary, this study provides a detailed characterisation of ovine MPC and OB from a bone engineering perspective and suggests that MPC and OB provide promising means for future bone disease related treatment applications.

Establishment of a preclinical ovine model for tibial segmental bone defect repair by applying bone tissue engineering strategies.

Currently, well-established clinical therapeutic approaches for bone reconstruction are restricted to the transplantation of autografts and allografts, and the implantation of metal devices or ceramic-based implants to assist bone regeneration. Bone grafts possess osteoconductive and osteoinductive properties; however, they are limited in access and availability and associated with donor-site morbidity, hemorrhage, risk of infection, insufficient transplant integration, graft devitalization, and subsequent resorption resulting in decreased mechanical stability. As a result, recent research focuses on the development of alternative therapeutic concepts. The field of tissue engineering has emerged as an important approach to bone regeneration. However, bench-to-bedside translations are still infrequent as the process toward approval by regulatory bodies is protracted and costly, requiring both comprehensive in vitro and in vivo studies. The subsequent gap between research and clinical translation, hence, commercialization, is referred to as the "Valley of Death" and describes a large number of projects and/or ventures that are ceased due to a lack of funding during the transition from product/technology development to regulatory approval and subsequently commercialization. One of the greatest difficulties in bridging the Valley of Death is to develop good manufacturing processes and scalable designs and to apply these in preclinical studies. In this article, we describe part of the rationale and road map of how our multidisciplinary research team has approached the first steps to translate orthopedic bone engineering from bench to bedside by establishing a preclinical ovine critical-sized tibial segmental bone defect model, and we discuss our preliminary data relating to this decisive step.

Cyclic strain disrupts endothelial network formation on Matrigel.

Most forms of tissue healing depend critically on revascularisation. In soft tissues and in vitro, mechanical stimuli have been shown to promote vessel-forming activity. However, in bone defects, increased interfragmentary motion impairs vascular regeneration. Because these effects seem contradictory, we aimed to determine whether a range of mechanical stimuli exists in which angiogenesis is favoured. A series of cyclic strain magnitudes were applied to a Matrigel-based "tube formation" assay and the total lengths of networks formed by human microvascular endothelial cells measured at 24 h. Network lengths were reduced at all strain levels, compared to unstretched controls. However, the levels of pro-angiogenic matrix metalloproteases-2 and -9 in the corresponding conditioned media were unchanged by strain, and vascular endothelial growth factor was uniformly elevated in stretched conditions. By repeating the assay with the addition of conditioned media from mesenchymal stem cells cultivated in similar conditions, paracrine stimuli were shown to increase network lengths, but not to alter the negative effect of cyclic stretching. Together, these results demonstrate that directly applied periodic strains can inhibit endothelial organisation in vitro, and suggest that this may be due to physical disruption rather than biochemical modulation. Most importantly, the results indicate that the straining of endothelial cells and their assembly into vascular-like structures must be studied simultaneously to adequately characterise the mechanical influence on vessel formation.

The challenge of establishing preclinical models for segmental bone defect research.

A considerable number of international research groups as well as commercial entities work on the development of new bone grafting materials, carriers, growth factors and specifically tissue-engineered constructs for bone regeneration. They are strongly interested in evaluating their concepts in highly reproducible large segmental defects in preclinical and large animal models. To allow comparison between different studies and their outcomes, it is essential that animal models, fixation devices, surgical procedures and methods of taking measurements are well standardized to produce reliable data pools and act as a base for further directions to orthopaedic and tissue engineering developments, specifically translation into the clinic. In this leading opinion paper, we aim to review and critically discuss the different large animal bone defect models reported in the literature. We conclude that most publications provide only rudimentary information on how to establish relevant preclinical segmental bone defects in large animals. Hence, we express our opinion on methodologies to establish preclinical critically sized, segmental bone defect models used in past research with reference to surgical techniques, fixation methods and postoperative management focusing on tibial fracture and segmental defect models.

Fit assessment of anatomic plates for the distal medial tibia.

OBJECTIVES: With the development and popularization of minimally invasive surgical methods and implants for fracture fixation, it is increasingly important that the available implants are precontoured to the specific anatomic location for which they are designed. The objective of this study was to develop a noninvasive method and criteria for quantifying the fit of a distal periarticular medial tibia plate and to test the method on a small set of tibia models. METHODS: The undersurface of the plate was extracted from a digital model of the plate. The surface of the plate was fitted to 21 computer tomography (CT)-based 3-dimensional (3-D) models of human tibiae. Four criteria were defined that constitute an anatomic plate fit and subsequently were applied for the quantitative fit assessment. The fitting of the plate undersurface to the bone was entirely conducted in a virtual environment. RESULTS: An anatomic fit of the plate was achieved for 4 of the models (19%). The individual categories generated fits of 62% (n = 13) for the proximal end; 43% (n = 9) for the proximal angle; 57% (n = 12) for the middle distance; and 57% (n = 12) for a distal fit. CONCLUSIONS: Although for the 4 individual criteria plate fits of 43%-62% were achieved, a global/anatomic fit only occurred for 19% of the bone models. This outcome is likely a result of bone morphology variations, which exist in a random population sample combined with the effects of a nonoptimized plate shape. Recommendations for optimizing the fit of the plate are discussed.