Biomechanical Assessment of the Validity of Sheep as a Preclinical Model for Testing Mandibular Fracture Fixation Devices

This study explores the suitability of sheep mandibles as models for testing fracture fixation devices designed for humans. Using 3D finite element modeling, the biomechanics of human and sheep mandibles were compared under various clenching tasks, in healthy and fractured scenarios. Results revealed significant differences in strain distribution and implant stresses between the two species. While the sheep mandibular body displayed lower strains and less similarity to human mandibles, the sheep mandibular diastema exhibited mechanical conditions closer to humans, particularly in strain patterns at the fracture site.

These findings suggest that the sheep mandibular diastema is a promising preclinical model for studying fixation devices and their impact on bone healing. In contrast, the sheep mandibular body may not adequately represent human biomechanics. This research highlights the importance of selecting appropriate anatomical regions for testing to ensure reliable and relevant results.

Reference: Vincenzo Orassi; Georg N. Duda; Max Heiland; Heilwig Fischer; Carsten Rendenbach; Sara Checa https://doi.org/10.3389/fbioe.2021.672176

In Silico Biomechanical Evaluation of WE43 Magnesium Plates for Mandibular Fracture Fixation

This study investigates the biomechanical performance of biodegradable magnesium WE43 alloy plates for mandibular fracture fixation, comparing them to titanium and polylactic acid (PLA) plates. Magnesium’s biodegradability and radiopacity make it a promising alternative to titanium, offering comparable stiffness without the drawbacks of imaging artifacts or implant removal. Using finite element analysis, four fracture scenarios were simulated under post-operative clenching loading conditions to evaluate primary fixation stability, thus mechanical strain within the healing regions, for all the devices. Moreover, two plate thicknesses were tested, 1 mm and 1.5 mm.

Key findings revealed that WE43 plates performed mechanically similar to titanium plates, both inducing comparable mechanics strains. PLA plates, in contrast, resulted in significantly higher strains, especially in highly loaded regions. Increasing the plate thickness did not remarkably change the strains within the healing regions. These results suggest magnesium WE43 alloy as a viable option for mandibular fracture fixation, supporting early healing while avoiding complications associated with traditional materials.

Reference: Vincenzo Orassi; Heilwig Fischer; Georg N. Duda; Max Heiland; Sara Checa https://doi.org/10.3389/fbioe.2021.803103

Mandible Reconstruction

Free osseus flaps are the gold standard for the reconstruction of combined bone and soft tissue defects following a continuity interrupting resection of the mandible. Depending on the type and size of a defect, the corresponding autologous graft is segmented and subsequently fixed using either segment-crossing titanium reconstruction or miniplates. 

Problems with current fixation systems are material failure with either screw loosening or screw/plate breaks due to masticatory forces up to 1000 Newton, non-consolidated osseous gaps after fixation and substantial metal artifacts with an impact on tumor after care by computed tomography or MRI. 

With this project, a cooperation of the TUHH and the Department of Oral and Maxillofacial Surgery, University Medical Center Hamburg Eppendorf, the mechanical strength, biomechanical behaviour and characteristics of material failure of different titanium systems has been exhibited. Further, alternative materials (fiber-reinforced glass, polylactide and magnesium) are under consideration for use in patients.  

Reference: C. Rendenbach, L. Gerbig, M. Boehme, K. Sellenschloh, M.M. Morlock, G. Huber

OMIBONE: Omics-driven computer model of bone regeneration for personalized treatment

Treatment of bone fractures are commonly standardized, neglecting individual differences due to patient's healing potential or accompanying diseases. This study introduces a novel framework that allows to predict bone regeneration outcome using combined proteomic and mechanical analyses in a computer model. The framework uses Ingenuity Pathway Analysis (IPA) software to link protein changes to alterations in biological processes and integrates these in an Agent-Based Model (ABM) of bone regeneration. The computer model predicted the progression of bone formation patterns in a mouse femur fracture stabilized with an intramedullary pin. The developed framework holds promise as a concept to enable personalized bone healing predictions.

Reference: Mahdi Jaber, Johannes Schmidt, Stefan Kalkhof, Louis Gerstenfeld, Georg N Duda, Sara Checa OMIBONE: Omics-driven computer model of bone regeneration for personalized treatment. Bone. 2024 Oct 17:117288. doi: 10.1016/j.bone.2024.117288. PMID: 39426580.

PCL strut-like scaffolds appear superior to gyroid in terms of bone regeneration within a long bone large defect: An in silico study

The treatment of large bone defects represents a major clinical challenge, where 3D printed scaffolds appear as a promising strategy to support bone defect regeneration. This study investigated whether gyroid scaffolds, characterized by a zero mean surface curvature, present advantages over traditional strut-like scaffolds in terms of their bone regeneration potential in large bone defects. Using a validated in silico modeling approach, bone regeneration within both scaffolds designs was simulated. Simulation results showed that the large surface curvatures of the gyroid scaffold leads to a slower tissue formation dynamic and as a result reduced bone regeneration compared with strut-like designs. Contrary to initial assumptions, gyroid scaffolds did not improve healing. These results highlight the potential of computational approaches for optimizing scaffold designs in pre-clinical stages.

Reference: Mahdi Jaber, Patrina Poh, Georg Duda, Sara Checa. PCL strut-like scaffolds appear superior to gyroid in terms of bone regeneration within a long bone large defect: An in silico study. Front Bioeng Biotechnol. 2022 Sep 23;10:995266. doi: 10.3389/fbioe.2022.995266. PMID: 36213070; PMCID: PMC9540363.

External mechanical loading overrules cell-cell mechanical communication in sprouting angiogenesis during early bone regeneration

Sprouting angiogenesis – the process by which new blood vessel sprout off from existing vasculature – is essential for bone regeneration. Endothelial cells within vessels are known to interact mechanically with their surroundings, such as outer-vascular stromal cells, through cell-induced traction forces. Simultaneously, external physiological loads cause tissue deformations at the healing site impacting cellular arrangements. However, the relative influence of these two mechanical cues – cell-induced and external – on angiogenesis during early bone healing remains unclear. This study used an in silico modelling approach to explore how these mechanical signals shape sprout patterns during early bone healing. The model incorporated the mechano-regulation of sprouting angiogenesis and stromal cell organization, and its predictions were compared to in vivo experiments using a mouse osteotomy model stabilized with rigid or semirigid fixation. Results showed that the magnitude and orientation of principal strains within the healing region can explain experimentally observed sprout patterning, under both fixation conditions. Simulations revealed that external mechanical signals dominate over the local cell-mediated mechanical communication. Such findings illustrate the relevance of external mechanical signals in guiding angiogenesis and could inform fracture treatment strategies to enhance bone regeneration.

Reference: Chiara Dazzi,Julia Mehl,Mounir Benamar,Holger Gerhardt,Petra Knaus,Georg N. Duda,Sara Checa doi: 10.1371/journal.pcbi.1011647

Reduced Bone Regeneration in Rats With Type 2 Diabetes Mellitus as a Result of Impaired Stromal Cell and Osteoblast Function-A Computer Modeling Study

The ability of bones to self‐regenerate is impaired in Type 2 Diabetes Mellitus (T2DM). T2DM is a metabolic disease associated with reduced bone regeneration capability, high fracture risk, and a high incidence of non‐unions. Both mechanical and biological factors have been associated with the reduced bone regeneration response, however their relative contribution to the impair healing remains largely unknown. This study investigated whether reduced healing in T2DM results primarily from mechanical or cellular alterations. A previously validated in silico computer model of bone regeneration was further developed to incorporate T2DM-related mechanical and cellular alterations in bone regeneration.  The computer model showed that mechanical alterations had little effect on the reduced bone regeneration in T2DM and that alterations in MSC proliferation, MSC migration, and osteoblast differentiation had the highest effect. These findings could have clinical implications in the treatment of bone fractures in patients with T2DM.

Reference: Mahdi Jaber, Lorenz C Hofbauer, Christine Hofbauer, Georg N Duda, Sara Checa. Reduced Bone Regeneration in Rats With Type 2 Diabetes Mellitus as a Result of Impaired Stromal Cell and Osteoblast Function-A Computer Modeling Study. JBMR Plus. 2023 Oct 2;7(11):e10809. doi: 10.1002/jbm4.10809. PMID: 38025037; PMCID: PMC10652174.

Back to the Research Overview

The influence of hip revision stem spline design on the torsional stability in the presence of major proximal bone defects

The study evaluates how hip revision stem design influences torsional stability in patients with significant bone loss. Specifically, it compares an established stem (Reclaim®) with a prototype featuring two sets of splines, one being less prominent. Five pairs of human femurs were used in the study, simulating large bone defects, common for revision surgeries. The prototype stem showed improved contact with the femoral cortex, resulting in 54% more cortical contact area and significantly higher torsional stability (35.2 Nm vs. 28.2 Nm). Although both stems demonstrated similar implantation characteristics, the prototype required slightly more force for proper seating. No significant differences in implant depth or angular misalignment during implantation were observed between the two designs. The study concluded that adding less prominent splines can enhance contact between the implant and bone, providing greater resistance to torsional loads and potentially improving the long-term success of hip revision surgeries.

Reference: Julius M. Boettcher ,Kay Sellenschloh,Gerd Huber,Benjamin Ondruschka,Michael M. Morlock https://doi.org/10.1371/journal.pone.0291599

Strength of the taper junction of modular revision hip stems

The study investigates the impact of contamination and improper assembly on the stability of modular hip stem connections in revision surgeries. Modular components offer flexibility in fitting implants to the patient's bone structure but are vulnerable to relative motion, which can cause fretting corrosion and implant failure. The research tested 48 neck-stem connections under various conditions of contamination (native, contaminated, cleaned) and assembly (secured, pre-tensioned). Results showed that contamination, particularly combined with improper assembly, significantly increased neck rotation (35.3° vs. 2.4°), micromotion (67.8 μm vs. 5.1 μm), and axial displacement (34.1 μm vs. 4.3 μm). A significant reduction in mechanical stability was observed in improperly secured connections. Proper cleaning with a new instrument and pre-tensioning of components reduced these adverse effects. The study highlights the importance of thorough cleaning and correct assembly to prevent early implant failure and potential complications.

Reference: Julius M. Boettcher, Kay Sellenschloh, Anna Strube, Gerd Huber & Michael M. Morlock https://doi.org/10.1007/s00132-023-04459-2

Post Mortem Retrievals

Rare and valuable feedback can be gained by studying retrieved post mortem specimens. This is possible on a large scale in Hamburg. Implanted specimens are analysed with the implant in situ, whereby sections are made either using histological techniques, or non-invasively using CT scanning (Titanium only) and documented for quality of implant anchorage. Mechanical tests are applied to measure implantation stability or strength (all histological sections to be displayed on this website in 2013).

Small design modifications can improve the primary stability of a fully coated tapered wedge hip stem

In this study small design modifications of a hip stem were experimentally compared to the established stem design (Emphasys™ vs. Corail®, DePuy Synthes, n = 6 per design) to identify whether they can increase the primary stability without increasing the periprosthetic fracture risk (PFF). The design modifications include a wider proximal section, smaller tip, and shorter length (Figure 1b). Additionally, new surgical equipment was introduced combining compaction and sharp extraction broaching (Figure 1a). Broaching and implantation were performed by an experienced surgeon followed by cyclic loading (1 Hz, 600 cycles @ 80 to 800 N, 600 cycles @ 80 to 1600 N) recorded with digital image correlation (Zeiss GOM). Broaching and implantation forces for the modified stem were up to 40% higher (p = 0.024), resulting in a 23% larger contact area between stem and bone (R2 = 0.694, p = 0.039). This led to a fourfold reduction in subsidence during loading (p = 0.028). The slight design modifications in this in-vitro study resulted in a higher primary stability, which suggests a reduced risk of loosening. While the higher forces during preparation and implantation might increase the PFF risk, there were no PFFs observed.

Reference: Katja Glismann ,Tobias Konow,Frank Lampe,Benjamin Ondruschka,Gerd Huber,Michael M. Morlock https://doi.org/10.1371/journal.pone.0300956

BoneStress

TUHH BoneStress is an interactive model demonstrating the effects of impaction and joint loading on bone stresses for different hip stem designs. It describes radial load transfer between implant and bone in uncemented press-fit femoral stem implantations.

This app provides a visual representation of the stress states between press-fit implant and bone directly post-operatively, and after bone ingrowth.

Stem design (stem and neck lengths, neck angle), impaction force, and joint loading can be varied to limit implant-bone separation while maintaining bone stresses within “normal” or “osteoporotic” limits. This will allow bone ingrowth and reduced stresses.

Its intuitive use makes TUHH BoneStress the perfect learning tool for surgeons and their patients by demonstrating the influence of stem design, implantation conditions, and patient activity, on the local stress situation, and its effect on successfull bone ingrowth.

It is noted that the simple model used cannot be directly transferred to an individual patient Situation

This App has been designed for the iPhone and for the iPad

Price: 0,89 €; Category: Medicine; Published: 21.10.2013; Version: 1.0; Size: 2.6 MB; Language: English;

Developer: Solutionline.de Agency for Online-Service GmbH; © TuTech innovation GmbH

Back to the Research Overview

The force at the implant cannot be assessed by the mallet force – Unless supported by a model

The study investigated how the forces applied during the assembly of the head taper junction of hip prostheses attenuate along the force transmission path. Forces were measured in vitro at the tip of the mallet, directly above the polymer tip of the impactor and below the stem taper. Additionally, a semi-empirical model was developed and validated to simulate force transmission and predict the effects of different surgical instruments. Both experimental and numerical analyses were conducted, with the result that peak forces at the tip of the impactor and the stem taper were found to be significantly attenuated, reaching only 35% and 21% of the force applied by the mallet, respectively. The study highlights that accurate assessment of the forces at the implant requires knowledge of the entire transmission path, and comparisons across studies are only valid when setups are consistent.

Reference: Peter J. Schlieker ,Michael M. Morlock,Gerd Huber https://doi.org/10.1371/journal.pone.0303682

Spinal fatigue strength

Spinal in-vitro testing is commonly performed quasi-statically, but the slow loading velocity does not mimic in-vivo conditions well. During occupational activities subjects are exposed to vibrations with high numbers of loading cycles – e.g. during handling and driving of heavy machinery. The purpose of this study is to provide an estimation of fatigue failure strength of human functional spinal units using Wöhler equation.  

41 lumbar specimens were loaded in axial compression for 300,000 cycles with different load levels and the results combined with data of 70 thoracic and lumbar specimens of Brinckmann et al. (1988) loaded with up to 5,000 cycles. Fatigue force of each specimen was normalized (Fnorm) by individual geometrical (endplate area) or material specific parameters (age, BMD) and used to derive a Wöhler equation with cycles to failure as independent variable.  

The fatigue strength of spinal specimens after cyclic loading is significantly lower than their ultimate strength. Including the upper compressive peak, endplate area and age for normalization explained 28% of variation in fatigue force (p<0.001). Introducing BMD instead of age improved the prediction to 61% explained variance of Fnorm (p<0.001). 

Superimposing movements (e.g. bending forward or twisting) might probably lead to smaller numbers of cycles to bone failure or also soft tissue failure. Since it is possible to determine those individual parameters for living subject, the risk of occupational activities can be estimated in combination with numerical models for the appraisal of occupational diseases or further the determination of duty cycles for spinal implants. 

Reference: G. Huber, K. Nagel, D.M. Skrzypiec, A. Klein, K. Püschel, M.M. Morlock Funding of FIOSH, Germany (F2059, F2069) is kindly acknowledged. 

Micromations at the tape junction of modular hip prostheses

Since 1972 modular hip prostheses were used in orthopaedic surgery. Modularity of the femoral component of total hip implants became popular because neck length and femur offset could be adjusted intraoperatively. Furthermore it is possible to combine different materials like metals and ceramics. The downside of this flexibility is an additional joining area which bears the risk of incorrect assembly as well as fretting and crevice corrosion.

So far only few studies have experimentally investigated the relative motion in the taper lock interface, which might play a role for occasionally observed prostheses failures. Micromotions in the taper lock interface can lead to a constant abrasion of the neck piece`s passivation layer resulting in fretting and fatigue fracture of the prosthesis. Contamination of the taper joining area might considerably increase the phenomenon.

In this study the fretting and crevice corrosion caused by micro motions at the mating surface is investigated. Inside into the failure mechanism and the influencing factors might help to prevent implant failures in the future.

After quantification of relative motions at the connecting element with neck pieces made of titanium and CoCrMo for different joining conditions suitable test methods for preclinical testing should be developed. A finite-element-model highlights areas with diminished contact forces which are in accordance with zones with increased probability of huge relative motions at the modular interface. Thereby it will be possible to test modular hip prostheses before clinical application in order to avoid prostheses fractures.

Presently, modular hip prostheses are embedded in methyl methacrylate according to ISO 7206-4 and loaded by a servohydraulic test machine (MTS MiniBionixII) with an axial sinusoidal load from 230N to 2300N. To determine the relative motion between the stem and the neck of the prosthesis three eddy current sensors (Micro-Epsilon) are screwed into a holder which is mounted on the stem. As a counterpart a clamp is fixed at the neck piece of the prosthesis.

Reference: S. Jauch, G. Huber, M.M. Morlock
This study is financially supported by Aesculap AG, Tuttlingen.

Audible Vibrations of total Hip Replacements

Recently, squeaking of total hip replacements with ceramic on ceramic bearing is a frequently discussed phenomenon. Although numerous publications have discussed clinical factors which are potentially essential for its occurrence, the responsible mechanisms are not yet well understood.

Squeaking occurs when structural vibrations are excited in a manner that the vibration amplitudes allow the emission of audible sounds. This is typically possible when the frequency of the excitation matches the natural frequency of a part of the system or the system’s entity. The prosthesis stems are the main vibrating components detuned by the surrounding bone stock and the induced load.

In an artificial hip joint system the friction between the ceramic bearing surfaces due to relative movement during gait cycles induce energy into the vibrating system – the lower the friction the lower the application of energy. An increased friction coefficient allows the excitation of vibration amplitudes which are high enough to emit sounds which are clearly audible from outside the patient’s body.

To study this phenomenon a hip simulator was built to reproduce squeaking in-vitro and to analyze the dynamic events under realistic conditions. Lubrication, bearing clearance, stem and cup design, loading, bearing surface roughness, component orientation, bone quality are just examples for various factors influencing the susceptibility and characteristics of squeaking. Modal analyses, frequency analysis of acoustic noise, analysis of transfer paths utilizing laser vibrometry, high precise microphones and accelerometers as well as explicit and implicit FE analyses are performed to understand the dynamic behavior of prosthesis-tissue systems.

The results from this study will help to understand the responsible mechanisms and influencing factors for squeaking and to develop a remedy.

Reference: A. Hothan, G. Huber, C. Weiß, K. Sellenschloh, N. Hoffmann, M.M. Morlock

This study is finically supported by Ceramtec. Components were donated by Aesculap, Biomet, DePuy, Eska, Mathys, Plus and Smith&Nephew.

Ceramic bearing safety

Ceramic-on-ceramic bearings are frequently used in total hip arthroplasty to avoid the negative long-term effects of metal or polyethylene wear particles. Concerns remain regarding component safety since ceramic is a brittle material and the rare event of component fracture is a devastating scenario for the patient making revision surgery inevitable. Aim of this study is to investigate different worst case scenarios regarding implantation, implant handling and load bearing for ceramic femoral and acetabular bearing components in order to further increase implant safety and reliability.

Laboratory and cadaveric testing is used to simulate and understand failure mechanisms. Microscopic surface analyses as well as numerical and analytical investigations are carried out to derive conclusions how to further improve implant design and implantation handling techniques.

So far, implant re-use and component mismatch experiments with ceramic femoral ball heads have shown that ceramics require careful component matching and handling in accordance with manufacturers’ guidelines to avoid premature failure.

Reference: J. Gührs, M.M. Morlock, G. Huber

Back to the Research Overview