[Steel or titanium for osteosynthesis : A mechanobiological perspective]. / Stahl oder Titan bei der Osteosynthese : Eine mechanobiologische Perspektive.

BACKGROUND: An implant used for stabilizing a fracture creates a mechanical construct, which directly determines the biology of bone healing. The stabilization of fractures places high mechanical demands on implants and therefore steel and titanium are currently almost exclusively used as the materials of choice. OBJECTIVES: The possible range of attainable mechanobiological stimulation for mechanotherapy as a function of plate stiffness depending on the selection of the plate material and the physical and mechanical properties of the material options are discussed. MATERIAL AND METHODS: An overview of the material properties of steel and titanium is given. For dynamically fixed long bone fractures as examples, various finite element models of plate osteosynthesis (steel/titanium) are created and the plate working length (PWL, screw configuration close to fracture) is varied. The interfragmentary movement (IFM) as a measure of mechanobiological stimulation is evaluated. RESULTS: Stimulation in the form of IFM varies across the fracture and also as a function of the osteosynthesis material and the configuration. The influence of the material appears to be notably smaller than the influence of PWL but both lose their influence largely over a bridged fracture situation (contact). With a flexible titanium plate and large PSS, a greater mechanobiological stimulation is produced. CONCLUSION: An essential prerequisite for the secondary fracture healing is an appropriate mechanobiological environment, which can be controlled by the osteosynthesis material and the configuration and is also affected by the type of fracture and load.

Microstructure and homogeneity of distribution of mineralised struts determine callus strength.

Non-invasive assessment of fracture healing, both in clinical and animal studies, has gained favour as surrogate measure to estimate regain of mechanical function. Micro-computed tomography (µCT) parameters such as fracture callus volume and mineralisation have been used to estimate callus mechanical competence. However, no in-depth information has been reported on microstructural parameters in estimating callus mechanical competence. The goal of this study is to use differently conditioned mice exhibiting good and impaired fracture healing outcomes and investigate the relationship between µCT imaging parameters (volume, mineralisation, and microstructure) that best estimate the callus strength and stiffness as it develops over time. A total of 99 mice with femoral fracture and intramedullary stabilisation were divided into four groups according to conditioning: wild type, NF1 knock-out, RAG1 knock-out and macrophage depleted. Animals were sacrificed at 14, 21, 28 or 35 days and µCT parameters and torsional stiffness and strength were assessed post-sacrifice. Using linear regression for all groups and time points together, torsional stiffness could be estimated with strut thickness, strut number and strut homogeneity (R² = 0.546, p < 0.0001); torsional strength could be estimated using bone mineral density, strut thickness and strut homogeneity (R² = 0.568, p < 0.0001). Differently conditioned mice that result in different fracture healing outcomes have been shown to result in varying structural, material and volumetric µCT parameters which can be used to estimate regain of bone strength. This study is the first to demonstrate that microstructure and strut homogeneity influence callus stiffness and strength.

Biomechanical comparison of polylactide-based versus titanium miniplates in mandible reconstruction in vitro.

OBJECTIVES: Evaluation of the mechanical integrity and reliability of polylactide-based miniplates for osseous free flap fixation at the mandible in an experimental study setup of a mandible reconstruction model. MATERIAL AND METHODS: 1.0mm titanium miniplates (group TI) (MatrixMandible, DePuy Synthes, Umkirch, Germany) and 1.5mm polylactide miniplates (group PL) (Inion CPS, Inion Oy, Tampere, Finland) were used to fix a polyurethane (PU) fibula segment to a PU mandible reconstruction model using monocortical non-locking screws. Mastication was simulated via unilateral cyclic dynamic loading at 1Hz with increasing loads (+ 0.15N/cycle, Bionix, MTS, USA). A 3D optical tracking system (Aramis, GOM, Braunschweig, Germany) was used to determine interosteotomy movements (IOM). RESULTS: IOM were higher in the polylactide group (distal: P=0.001, mesial: P=0.001). Differences in mean stiffness (titanium: 478±68N/mm; polylactide: 425±38N/mm, P=0.240) and mean force at a vertical displacement of 1.0mm (titanium: 201.6±87.1N; polylactide: 141.3±29.9N, P=0.159) were not significant. CONCLUSIONS: The results of this study suggest that polylactide-based miniplates provide reduced mechanical integrity and higher interosteotomy movements in comparison to titanium miniplates in vitro. Indications for clinical use of polylactide-based miniplates in mandible reconstruction have to be placed critically. Future studies will focus on clinical complications of polylactide-based plates in risk patients.