Biomechanik ist die Wissenschaft von der Bewegung eines lebenden Körpers, einschließlich der Art und Weise, wie Muskeln, Knochen, Sehnen und Bänder bei der Bewegung zusammenarbeiten . Die Biomechanik ist Teil des umfassenderen Fachgebiets der Kinesiologie und konzentriert sich speziell auf die Bewegungsmechanik.

Biomechanik ist die Wissenschaft von der Bewegung eines lebenden Körpers, einschließlich der Art und Weise, wie Muskeln, Knochen, Sehnen und Bänder bei der Bewegung zusammenarbeiten . Die Biomechanik ist Teil des umfassenderen Fachgebiets der Kinesiologie und konzentriert sich speziell auf die Bewegungsmechanik.

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.

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.

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).

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

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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.

https://doi.org/10.1371/journal.pone.0303682

Mandible Reconstruction

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

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.  

Spinal fatigue strength

G. Huber, K. Nagel, D.M. Skrzypiec, A. Klein, K. Püschel, M.M. Morlock

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. 

Funding of FIOSH, Germany (F2059, F2069) is kindly acknowledged. 

Micromations at the tape junction of modular hip prostheses

S. Jauch, G. Huber, M.M. Morlock

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.

This study is financially supported by Aesculap AG, Tuttlingen.

Audible Vibrations of total Hip Replacements

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

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.

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

Ceramic bearing safety

J. Gührs, M.M. Morlock, G. Huber

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.

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Biomechanik ist die Wissenschaft von der Bewegung eines lebenden Körpers, einschließlich der Art und Weise, wie Muskeln, Knochen, Sehnen und Bänder bei der Bewegung zusammenarbeiten . Die Biomechanik ist Teil des umfassenderen Fachgebiets der Kinesiologie und konzentriert sich speziell auf die Bewegungsmechanik.

Biomechanik ist die Wissenschaft von der Bewegung eines lebenden Körpers, einschließlich der Art und Weise, wie Muskeln, Knochen, Sehnen und Bänder bei der Bewegung zusammenarbeiten . Die Biomechanik ist Teil des umfassenderen Fachgebiets der Kinesiologie und konzentriert sich speziell auf die Bewegungsmechanik.