Effect of two types of shoulder prosthesis on the muscle forces using a generic multibody model for different arm motions.

BACKGROUND: This study aims to analyze the effects of a novel dual-bearing shoulder prosthesis and a conventional reverse shoulder prosthesis on the deltoid and rotator cuff muscle forces for four different arm motions. The dual-bearing prosthesis is a glenoid-sparing joint replacement with a moving center of rotation. It has been developed to treat rotator cuff arthropathy, providing an increased post-operative functionality. METHODS: A three-dimensional musculoskeletal OpenSim® model of an upper body, incorporating a natural gleno-humeral joint and a scapula-thoracic joint developed by Blana et al. (J Biomech 41: 1714-1721, 2008), was used as a reference for the natural shoulder. It was modified by integrating first a novel dual-bearing prosthesis, and second, a reverse shoulder prosthesis into the shoulder joint complex. Four different arm motions, namely abduction, scaption, internal and external rotation, were simulated using an inverse kinematics approach. For each of the three models, shoulder muscle forces and joint reaction forces were calculated with a 2 kg weight in the hand. RESULTS: In general, the maximal shoulder muscle force and joint reaction force values were in a similar range for both prosthesis models during all four motions. The maximal deltoid muscle forces in the model with the dual-bearing prosthesis were 18% lower for abduction and 3% higher for scaption compared to the natural shoulder. The maximal rotator cuff muscle forces in the model with the dual-bearing prosthesis were 36% lower for abduction and 1% higher for scaption compared to the natural shoulder. Although the maximal deltoid muscle forces in the model with the dual-bearing prosthesis in internal and external rotation were 52% and 64% higher, respectively, compared to the natural shoulder, the maximal rotator cuff muscle forces were 27% lower in both motions. CONCLUSION: The study shows that the dual-bearing shoulder prosthesis is a feasible option for patients with rotator cuff tear and has a strong potential to be used as secondary as well as primary joint replacement. The study also demonstrates that computer simulations can help to guide the continued optimization of this particular design concept for successful clinical outcomes.

Mechanical Effects of Heat Exposure From a Bipolar Radiofrequency Probe on Suture Under Simulated Arthroscopic Conditions.

PURPOSE: To determine conditions for the safe use of radiofrequency (RF) tissue ablation probes that avoid damaging suture material. METHODS: Four sutures made of 3 different materials commonly used in arthroscopic procedures were analyzed in a saline bath related to effects of RF-produced heat by proximity, duration, and intensity settings measuring burn-through time and ultimate load to failure. The parameters tested were electrode-to-suture distance, power setting, and the presence of tendon tissue or metallic anchor eyelets. Outcome variables were the burn-through time and the ultimate failure load of differently treated suture samples. RESULTS: Mean burn-through time for suture in direct contact with the RF probe ranged from 57.2 to 14.7 seconds for ultra-high-molecular-weight polyethylene (UHMWPE) sutures, 1.1 seconds for polydioxanone suture, and 0.8 seconds for polyethylene terephthalate suture. One of the UHMWPE sutures was capable of withstanding 3 seconds of direct contact with the RF probe without any compromise in tensile strength. No suture material tested had any mechanical change as long as the RF probe was kept 1 mm from the suture. CONCLUSIONS: Heat from RF tissue ablation probes can cause undetected damage. High-strength UHMWPE sutures were less sensitive to an RF treatment than polyester sutures. The use of different test substrates did not significantly influence the burn through time. CLINICAL RELEVANCE: Heat from RF probes can damage some suture material if direct contact is made even briefly. The use of RF devices may be safe for the suture when a distance between probe and suture of >1 mm is maintained. Suture made from UHMWPE may tolerate up to 3 seconds of RF probe contact and not sustain significant damage. Surgeons must use great care when using RF devices in the vicinity of suture placement.

Are asymmetric metal markings on the cone surface of ceramic femoral heads an indication of entrapped debris?

BACKGROUND: The probability of in vivo failure of ceramic hip joint implants is very low (0.004-0.05%). In addition to material flaws and overloading, improper handling during implantation can induce fractures of the ceramic ball head in the long term. Identifying the causes of an in vivo fracture contributes to improved understanding and potentially to further reduction of the fracture probability for patients. Asymmetric metal markings on the cone surface of in vivo ball head fractures have been reported. The question, therefore, is whether asymmetric loading is the sole cause or whether additional factors, specifically contamination entrapped in the taper fit, also contribute or are even the main cause. METHODS: The influence of the asymmetric physiological load configuration on resulting metal markings in the cone surface of an alumina femoral ball head with and without biological contaminants was investigated. Static and cyclic tests on ball heads were carried out in a load configuration of 0° (axisymmetric) and 40° in a physiological environment. The analysis of the metal marking was carried out to gain a better understanding of the processes that contribute to the generation of metal marking. Fractography was carried out to determine the fracture initiation of failed ball heads. RESULTS: Different types and sizes of residuals entrapped in the conical surface are shown to yield strongly asymmetric metal marking patterns. All heads tested without contaminants exhibited an almost homogenous distribution of residual metal markings around the circumference of the ceramic cone surface at the proximal end of the bore hole. The failure of ball heads that contained entrapped contaminants revealed a common fracture pattern. The site of fracture initiation on two of the failed heads was in the entrance region of the bore hole on the superior half of the head. CONCLUSION: Asymmetric metal markings observed on the ball heads tested in this investigation are most probably caused by the presence of contaminants entrapped in the taper fit. Homogenous metal mark distributions around the circumference indicate proper assembly of the ball head without entrapped contaminants. It should, however, be noted that different taper designs may possibly result in different marking patterns.

Pelvic incidence-lumbar lordosis mismatch results in increased segmental joint loads in the unfused and fused lumbar spine.

PURPOSE: Symptomatic adjacent segment disease (ASD) has been reported to occur in up to 27 % of lumbar fusion patients. A previous study identified patients at risk according to the difference of pelvic incidence and lordosis. Patients with a difference between pelvic incidence and lumbar lordosis >15° have been found to have a 20 times higher risk for ASD. Therefore, it was the aim of the present study to investigate forces acting on the adjacent segment in relation to pelvic incidence-lumbar lordosis (PILL) mismatch as a measure of spino-pelvic alignment using rigid body modeling to decipher the underlying forces as potential contributors to degeneration of the adjacent segment. METHODS: Sagittal configurations of 81 subjects were reconstructed in a musculoskeletal simulation environment. Lumbar spine height was normalized, and body and segmental mass properties were kept constant throughout the population to isolate the effect of sagittal alignment. A uniform forward/backward flexion movement (0°-30°-0°) was simulated for all subjects. Intervertebral joint loads at lumbar level L3-L4 and L4-L5 were determined before and after simulated fusion. RESULTS: In the unfused state, an approximately linear relationship between sagittal alignment and intervertebral loads could be established (shear: 0° flexion r = 0.36, p < 0.001, 30° flexion r = 0.48, p < 0.001; compression: 0° flexion r = 0.29, p < 0.01, 30° flexion r = 0.40, p < 0.001). Additionally, shear changes during the transition from upright to 30° flexed posture were on average 32 % higher at level L3-L4 and 14 % higher at level L4-L5 in alignments that were clinically observed to be prone to ASD. Simulated fusion affected shear forces at the level L3-L4 by 15 % (L4-L5 fusion) and 23 % (L4-S1 fusion) more for alignments at risk for ASD. CONCLUSION: Higher adjacent segment shear forces in alignments at risk for ASD already prior to fusion provide a mechanistic explanation for the clinically observed correlation between PILL mismatch and rate of adjacent segment degeneration.

On interlayer stability and high-cycle simulator performance of diamond-like carbon layers for articulating joint replacements.

Diamond like carbon (DLC) coatings have been proven to be an excellent choice for wear reduction in many technical applications. However, for successful adaption to the orthopaedic field, layer performance, stability and adhesion in physiologically relevant setups are crucial and not consistently investigated. In vitro wear testing as well as adequate corrosion tests of interfaces and interlayers are of great importance to verify the long term stability of DLC coated load bearing implants in the human body. DLC coatings were deposited on articulating lumbar spinal disks made of CoCr28Mo6 biomedical implant alloy using a plasma-activated chemical vapor deposition (PACVD) process. As an adhesion promoting interlayer, tantalum films were deposited by magnetron sputtering. Wear tests of coated and uncoated implants were performed in physiological solution up to a maximum of 101 million articulation cycles with an amplitude of ±2° and -3/+6° in successive intervals at a preload of 1200 N. The implants were characterized by gravimetry, inductively coupled plasma optical emission spectrometry (ICP-OES) and cross section scanning electron microscopy (SEM) analysis. It is shown that DLC coated surfaces with uncontaminated tantalum interlayers perform very well and no corrosive or mechanical failure could be observed. This also holds true in tests featuring overload and third-body wear by cortical bone chips present in the bearing pairs. Regarding the interlayer tolerance towards interlayer contamination (oxygen), limits for initiation of potential failure modes were established. It was found that mechanical failure is the most critical aspect and this mode is hypothetically linked to the α-ß tantalum phase switch induced by increasing oxygen levels as observed by X-ray diffraction (XRD). It is concluded that DLC coatings are a feasible candidate for near zero wear articulations on implants, potentially even surpassing the performance of ceramic vs. ceramic.

The influence of implant design and limb alignment on in vivo wear rates of fixed-bearing and rotating-platform knee implant retrievals.

Analysis of polyethylene (PE) wear in knee implants is crucial for understanding the factors leading to revision in total knee arthroplasty. Importantly, current experimental and computational methods for predicting insert wear can only be validated against true in vivo measurements from retrievals. This study quantitatively investigated in vivo PE wear rates in fixed-bearing (FB) (n = 21) and rotating-platform (n = 53) implant retrievals. 3D surface geometry of the retrievals was measured using a structured light scanner. Then, a reference surface that included the deformation, but not the wear that the retrievals had experienced in vivo, was constructed using a fully automatic surface reconstruction algorithm. Finally, wear volume was calculated from the deviation between the worn and reconstructed surfaces. The measurement and analysis techniques were validated and the algorithm was found to produce errors of only 0.2% relative to the component volumes. In addition to quantifying cohort-level wear rates, the effect of mechanical axis limb alignment on mediolateral wear distribution was examined for a subset of the retrievals (n = 14 + 26). Our results show that FB implants produce significantly (p = 0.04) higher topside wear rates (24.6 ± 10.1 mm3/year) than rotating-platform implants (15.3 ± 8.0 mm3/year). This effect was larger than that of limb alignment, which had a smaller and nonsignificant influence on overall wear rates (+4.5 ± 11.6 mm3/year, p = 0.43). However, increased varus alignment was associated significantly with greater medial compartment wear in both the FB and rotating-platform designs (+1.7 ± 1.3%/° and +1.8 ± 1.6%/°). Our findings emphasize the importance of implant design and limb alignment on wear outcomes, providing reference data for improving implant performance and longevity.

Immunomodulation Using BMP-7 and IL-10 to Enhance the Mineralization Capacity of Bone Progenitor Cells in a Fracture Hematoma-Like Environment.

Following biomaterial implantation, a failure to resolve inflammation during the formation of a fracture hematoma can significantly limit the biomaterial's ability to facilitate bone regeneration. This study aims to combine the immunomodulatory and osteogenic effects of BMP-7 and IL-10 with the regenerative capacity of collagen-hydroxyapatite (CHA) scaffolds to enhance in vitro mineralization in a hematoma-like environment. Incubation of CHA scaffolds with human whole blood leads to rapid adsorption of fibrinogen, significant stiffening of the scaffold, and the formation of a hematoma-like environment characterized by a limited capacity to support the infiltration of human bone progenitor cells, a significant upregulation of inflammatory cytokines and acute phase proteins, and significantly reduced osteoconductivity. CHA scaffolds functionalized with BMP-7 and IL-10 significantly downregulate the production of key inflammatory cytokines, including IL-6, IL-8, and leptin, creating a more permissive environment for mineralization, ultimately enhancing the biomaterial's osteoconductivity. In conclusion, targeting the onset of inflammation in the early phase of bone healing using BMP-7 and IL-10 functionalized CHA scaffolds is a promising approach to effectively downregulate inflammatory processes, while fostering a more permissive environment for bone regeneration.

A novel method to accurately recreate in vivo loads and kinematics in computational models of the knee.

Despite availability of in vivo knee loads and kinematics data, conventional load- and displacement-controlled configurations still can't accurately predict tibiofemoral loads from kinematics or vice versa. We propose a combined load- and displacement-control method for joint-level simulations of the knee to reliably reproduce in vivo contact mechanics. Prediction errors of the new approach were compared to those of conventional purely load- or displacement-controlled models using in vivo implant loads and kinematics for multiple subjects and activities (CAMS-Knee dataset). Our method reproduced both loads and kinematics more closely than conventional models and thus demonstrates clear advantages for investigating tibiofemoral contact or wear.

Inverse Finite Element Approach to Identify the Post-Necking Hardening Behavior of Polyamide 12 under Uniaxial Tension.

Finite-element (FE) simulations that go beyond the linear elastic limit of materials can aid the development of polymeric products such as stretch blow molded angioplasty balloons. The FE model requires the input of an appropriate elastoplastic material model. Up to the onset of necking, the identification of the hardening curve is well established. Subsequently, additional information such as the cross-section and the triaxial stress state inside the specimen is required. The present study aims to inversely identify the post-necking hardening behavior of the semi-crystalline polymer polyamide 12 (PA12) at different temperatures. Our approach uses structural FE simulations of a dog-bone tensile specimen in LS-DYNA with mesh sizes of 1 mm and 2 mm, respectively. The FE simulations are coupled with an optimization routine defined in LS-OPT to identify material properties matching the experimental behavior. A Von Mises yield criterion coupled with a user-defined hardening curve (HC) were considered. Up to the beginning of necking, the Hockett−Sherby hardening law achieved the best fit to the experimental HC. To fit the entire HC until fracture, an extension of the Hockett−Sherby law with power-law functions achieved an excellent fit. Comparing the simulation and the experiment, the following coefficient of determination R2 could be achieved: Group I: R2 > 0.9743; Group II: R2 > 0.9653; Group III: R2 > 0.9927. Using an inverse approach, we were able to determine the deformation behavior of PA12 under uniaxial tension for different temperatures and mathematically describe the HC.

Simulated Kick Injury to the Mandible in Horses: Study of Fracture Configurations and Physical Parameters of the Impact.

OBJECTIVE: The goal of this study was to generate mandibular fractures in three regions of the diastema using a metal impactor to simulate a kick from a horse and to determine the mean deceleration in the initial phase of the impact event, the maximum contact force, the impact energy necessary to create a fracture and the duration of the impact. STUDY DESIGN: Thirty heads of horses aged between 5 and 20 years and euthanatized for various reasons were used. The heads were attached to a steel bar at the occiput at an axial angle of 45 degrees so that the body of the mandible was positioned horizontally and directly under the trajectory of the impactor. A 2 kg solid impactor was dropped with velocities of 6 to 14 m/s to simulate a kick from a horse. The impact was recorded using a high-speed video camera with a frame rate of 30,000 frames per second. Radiographs of the heads were obtained before and after the simulated kick. RESULTS: Mandibular fractures with configurations similar to those seen in clinical practice were generated at all three locations. The mean deceleration increased with impact velocity and with more cranially located impact positions. Absorbed energy increased with increasing impact velocity when no fracture was generated. CONCLUSION: The susceptibility to experimental fracture of the diastema increased from rostral to caudal locations, which is most probably caused by decreasing mandibular bone strength and an increase in the curvature at the lateroventral aspect of the mandible in that region. Physical parameters depended on fracture occurrence and type.

Shape fidelity, mechanical and biological performance of 3D printed polycaprolactone-bioactive glass composite scaffolds.

Direct ink writing (DIW) is a promising extrusion-based 3D printing technology, which employs an ink-deposition nozzle to fabricate 3D scaffold structures with customizable ink formulations for tissue engineering applications. However, determining the optimal DIW process parameters such as temperature, pressure, and speed for the specific ink is essential to achieve high reproducibility of the designed geometry and subsequent mechano-biological performance for different applications, particularly for porous scaffolds of finite sizes (total volume > 1000 mm3) and controlled pore size and porosity. The goal of this study was to evaluate the feasibility of fabricating Polycaprolactone (PCL) and bio-active glass (BG) composite-based 3D scaffolds of finite size using DIW. 3D-scaffolds were fabricated either as cylinders (10 mm diameter; 15 mm height) or cubes (5 × 5 × 5 mm3) with height/width aspect ratios of 1.5 and 1, respectively. A rheological characterization of the PCL-BG inks was performed before printing to determine the optimal printing parameters such as pressure and speed for printing at 110 °C. Microstructural properties of the scaffolds were analyzed in terms of overall scaffold porosity, and in situ pore size assessments in each layer (36 pores/layer; 1764 pores per specimen) during their fabrication. Measured porosity of the fabricated specimens-PCL: x¯ =46.94%, SD = 1.61; PCL-10 wt%BG: x¯ = 48.29%, SD = 5.95; and PCL-20 wt% BG: x¯=50.87%, SD = 2.45-matched well with the designed porosity of 50%. Mean pore sizes-PCL [x¯ = 0.37 mm (SD = 0.03)], PCL-10%BG [x¯ = 0.38 mm (SD = 0.07)] and PCL-20% BG [x¯ = 0.37 mm (SD = 0.04)]-were slightly fairly close to the designed pore size of 0.4 mm. Nevertheless there was a small but consistent, statistically significant (p < 0.0001) decrease in pore size from the first printed layer (PCL: 0.39 mm; PCL-10%BG: 0.4 mm; PCL-20%BG: 0.41 mm) to the last. SEM and micro-CT imaging revealed consistent BG particle distribution across the layers and throughout the specimens. Cell adhesion experiments revealed similar cell adhesion of PCL-20 wt% BG to pure PCL, but significantly better cell proliferation - as inferred from metabolic activity - after 7 days, although a decrease after 14 days was noted. Quasi-static compression tests showed a decrease in compressive yield strength and apparent elastic modulus with increasing BG fraction, which could be attributed to a lack of adequate mechanical bonding between the BG particles and the PCL matrix. The results show that the inks were successfully generated, and the scaffolds were fabricated with high resolution and fidelity despite their relatively large size (>1000 mm3). However, further work is required to understand the mechano-biological interaction between the BG particle additives and the PCL matrix to improve the mechanical and biological properties of the printed structures.

European Society of Biomechanics S.M. Perren Award 2022: Standardized tibio-femoral implant loads and kinematics.

Knowledge of both tibio-femoral kinematics and kinetics is necessary for fully understanding knee joint biomechanics, guiding implant design and testing, and driving and validating computational models. In 2017, the CAMS-Knee datasets were presented, containing synchronized in vivo implant kinematics measured using a moving fluoroscope and tibio-femoral contact loads measured using instrumented implants from six subjects. However, to date, no representative summary of kinematics and kinetics obtained from measurements at the joint level of the same cohort of subjects exists. In this study, we present the CAMS-Knee standardized subject "Stan", whose reference data include tibio-femoral kinematics and loading scenarios from all six subjects for level and downhill walking, stair descent, squat and sit-to-stand-to-sit. Using the peak-preserving averaging method by Bergmann and co-workers, we derived scenarios for generally high (CAMS-HIGH100), peak, and extreme loading. The CAMS-HIGH100 axial forces reached peaks between 3022 and 3856 N (3.08-3.93 body weight) for the five investigated activities. Anterior-posterior forces were about a factor of ten lower. The axial moment around the tibia was highest for level walking and squatting with peaks of 9.4 Nm and 10.5 Nm acting externally. Internal tibial rotations of up to 8.4° were observed during squat and sitting, while the walking activities showed approximately half the internal rotation. The CAMS-HIGH100 loads were comparable to Bergmann and co-workers', but have the additional benefit of synchronized kinematics. Stan's loads are +11 to +56% higher than the ISO 14243 wear testing standard loads, while the kinematics exhibit markedly different curve shapes. Along with the original CAMS-Knee datasets, Stan's data can be requested at cams-knee.orthoload.com.

Modular stimuli-responsive hydrogel sealants for early gastrointestinal leak detection and containment.

Millions of patients every year undergo gastrointestinal surgery. While often lifesaving, sutured and stapled reconnections leak in around 10% of cases. Currently, surgeons rely on the monitoring of surrogate markers and clinical symptoms, which often lack sensitivity and specificity, hence only offering late-stage detection of fully developed leaks. Here, we present a holistic solution in the form of a modular, intelligent suture support sealant patch capable of containing and detecting leaks early. The pH and/or enzyme-responsive triggerable sensing elements can be read out by point-of-need ultrasound imaging. We demonstrate reliable detection of the breaching of sutures, in as little as 3 hours in intestinal leak scenarios and 15 minutes in gastric leak conditions. This technology paves the way for next-generation suture support materials that seal and offer disambiguation in cases of anastomotic leaks based on point-of-need monitoring, without reliance on complex electronics or bulky (bio)electronic implantables.

The Effect of Reducing the Bone to Cast Distance in an Equine Transfixation Pin Cast: An Ex Vivo Biomechanical Study.

OBJECTIVE: The aim of this study was to evaluate the effect of reducing the bone to cast distance on the resistance of the pin to cyclic loading in equine transfixation pin casts. STUDY DESIGN: Eleven pairs of cadaveric equine third metacarpal bones were prepared and one 6.3/8.0 mm transfixation pin was placed in standard fashion 10 mm proximal to the distal physeal scar into each bone. One metacarpus of each pair was tested with a distance of 10 mm (10 mm group) and the contralateral metacarpus with a distance of 20 mm (20 mm group) between the outer cortex of the bone and the fixation of the pin. Eight pairs were tested using a simplified test set-up in which the pins were fastened at both ends to polyoxymethylene-copolymer sleeves. The pins of the remaining three pairs of bones were incorporated into a fibreglass cast. All specimens were tested under cyclic loading until failure of the pin in axial compression. RESULTS: All pins failed uni- or bilaterally at clinically relevant load levels. Pins of the 10 mm group endured significantly (p < 0.05) higher load levels and total number of cycles until failure compared with the pins of the 20 mm group. CONCLUSION: The distance between the bone surface and the cast at the location of pin insertion has a significant effect on resistance of the pins to cyclic loading. Therefore, the amount of padding applied underneath an equine transfixation pin cast can have an influence on the overall stability and durability of the construct.

Estimating lumbar passive stiffness behaviour from subject-specific finite element models and in vivo 6DOF kinematics.

Passive rotational stiffness of the osseo-ligamentous spine is an important input parameter for estimating in-vivo spinal loading using musculoskeletal models. These data are typically acquired from cadaveric testing. Increasingly, they are also estimated from subject-specific imaging-based finite element (FE) models, which are typically built from CT/MR data obtained in supine position and employ pure rotation kinematics. We explored the sensitivity of FE-based lumbar passive rotational stiffness to two aspects of functional in-vivo kinematics: (a) passive strain changes from supine to upright standing position, and (b) in-vivo coupled translation-rotation kinematics. We developed subject-specific FE models of four subjects' L4L5 segments from supine CT images. Sagittally symmetric flexion was simulated in two ways: (i) pure flexion up to 12° under a 500 N follower load directly from the supine pose. (ii) First, a displacement-based approach was implemented to attain the upright pose, as measured using Dynamic Stereo X-ray (DSX) imaging. We then simulated in-vivo flexion using DSX imaging-derived kinematics. Datasets from weight-bearing motion with three different external weights [(4.5 kg), (9.1 kg), (13.6 kg)] were used. Accounting for supine-upright motion generated compressive pre-loads ≈ 468 N (±188 N) and a "pre-torque" ≈2.5 Nm (±2.2 Nm), corresponding to 25% of the reaction moment at 10° flexion (case (i)). Rotational stiffness estimates from DSX-based coupled translation-rotation kinematics were substantially higher compared to pure flexion. Reaction Moments were almost 90% and 60% higher at 5° and 10° of L4L5 flexion, respectively. Within-subject differences in rotational stiffness based on external weight were small, although between-subject variations were large.

Problem of Pin Breakage in Equine Transfixation Pin Casting: Biomechanical Ex Vivo Testing of Four Different Pins.

OBJECTIVE: The aim of this study was to evaluate cyclic fatigue behaviour of a new pin with a thread run-out design in comparison with three other types of pins commonly used for equine transfixation pin casting. MATERIALS AND METHODS: Twenty-four pairs of equine cadaveric third metacarpal bones (MC3) equipped with one transfixation pin placed horizontally in the distal metaphysis were tested using a simplified model, mimicking the biomechanical situation of equine transfixation pin casting. A 6.3/8.0-mm Imex Duraface pin with thread run-out design (ITROP) was compared with a 6.1-mm smooth Steinmann pin (SSP), a Securos 6.2-mm, positive-profile pin (SPPP) and an Imex 6.3-mm, positive-profile pin (IPPP) under cyclic loading until failure in axial compression of MC3. RESULTS: All pins broke at clinically relevant load levels and cycle numbers. The SSP endured significantly (p = 0.0025) more cycles before failure (mean: 48685) than the ITROP (mean 25889). No significant differences in cycles to failure were observed comparing the SPPP versus ITROP, and the IPPP versus ITROP, respectively. CLINICAL SIGNIFICANCE: A thread run-out design does not necessarily lead to higher resistance against pin breakage under cyclic loading conditions. The SSP was most resistant against cyclic failure in these testing conditions, even though it was associated with more lateromedial displacement and cortical wear-out. This could outweigh reported disadvantages of the SSP such as reduced resistance to axial extraction and pin loosening.

Sensitivity of intervertebral joint forces to center of rotation location and trends along its migration path.

Translational vertebral motion during functional tasks manifests itself in dynamic loci for center of rotation (COR). A shift of COR affects moment arms of muscles and ligaments; consequently, muscle and joint forces are altered. Based on posture- and level-specific trends of COR migration revealed by in vivo dynamic radiography during functional activities, it was postulated that the instantaneous COR location for a particular joint is optimized in order to minimize the joint reaction forces. A musculoskeletal multi-body model was employed to investigate the hypotheses that (1) a posterior COR in upright standing and (2) an anterior COR in forward flexed posture leads to optimized lumbar joint loads. Moreover, it was hypothesized that (3) lower lumbar levels benefit from a more superiorly located COR. The COR in the model was varied from its initial position in posterior-anterior and inferior-superior direction up to ±6 mm in steps of 2 mm. Movement from upright standing to 45° forward bending and backwards was simulated for all configurations. Joint reaction forces were computed at levels L2L3 to L5S1. Results clearly confirmed hypotheses (1) and (2) and provided evidence for the validity of hypothesis (3), hence offering a biomechanical rationale behind the migration paths of CORs observed during functional flexion/extension movement. Average sensitivity of joint force magnitudes to an anterior shift of COR was +6 N/mm in upright and -21 N/mm in 30° forward flexed posture, while sensitivity to a superior shift in upright standing was +7 N/mm and -8N/mm in 30° flexion. The relation between COR loci and joint loading in upright and flexed postures could be mainly attributed to altered muscle moment arms and consequences on muscle exertion. These findings are considered relevant for the interpretation of COR migration data, the development of numerical models, and could have an implication on clinical diagnosis and treatment or the development of spinal implants.

Fusion angle affects intervertebral adjacent spinal segment joint forces-Model-based analysis of patient specific alignment.

This study addresses the hypothesis that adjacent segment intervertebral joint loads are sensitive to the degree of lordosis that is surgically imposed during vertebral fusion. Adjacent segment degeneration is often observed after lumbar fusion, but a causative mechanism is not yet clearly evident. Altered kinematics of the adjacent segments and potentially nonphysiological mechanical joint loads have been implicated in this process. However, little is known of how altered alignment and kinematics influence loading of the adjacent intervertebral joints under consideration of active muscle forces. This study investigated these effects by simulating L4/5 fusions using kinematics-driven musculoskeletal models of one generic and eight sagittal alignment-specific models. Models featured different spinopelvic configurations but were normalized by body height, masses, and muscle properties. Fusion of the L4/5 segment was implemented in an in situ (22°), hyperlordotic (32°), and hypolordotic (8°) fashion and kinematic input parameters were changed accordingly based on findings of an in vitro investigation. Bending motion from upright standing to 45° forward flexion and back was simulated for all models in intact and fused conditions. Joint loads at adjacent levels and moment arms of spinal muscles experienced changes after all types of fusion. Hypolordotic configuration led to an increase of adjacent segment (L3/4) shear forces of 29% on average, whereas hyperlordotic fusion reduced shear by 39%. Overall, L4/5 in situ fusion resulted in intervertebral joint forces closest to intact loading conditions. An artificial decrease in lumbar lordosis (minus 14° on average) caused by an L4/5 fusion lead to adverse loading conditions, particularly at the cranial adjacent levels, and altered muscle moment arms, in particular for muscles in the vicinity of the fusion. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:131-139, 2017.

Intervertebral reaction force prediction using an enhanced assembly of OpenSim models.

OpenSim offers a valuable approach to investigating otherwise difficult to assess yet important biomechanical parameters such as joint reaction forces. Although the range of available models in the public repository is continually increasing, there currently exists no OpenSim model for the computation of intervertebral joint reactions during flexion and lifting tasks. The current work combines and improves elements of existing models to develop an enhanced model of the upper body and lumbar spine. Models of the upper body with extremities, neck and head were combined with an improved version of a lumbar spine from the model repository. Translational motion was enabled for each lumbar vertebrae with six controllable degrees of freedom. Motion segment stiffness was implemented at lumbar levels and mass properties were assigned throughout the model. Moreover, body coordinate frames of the spine were modified to allow straightforward variation of sagittal alignment and to simplify interpretation of results. Evaluation of model predictions for level L1-L2, L3-L4 and L4-L5 in various postures of forward flexion and moderate lifting (8 kg) revealed an agreement within 10% to experimental studies and model-based computational analyses. However, in an extended posture or during lifting of heavier loads (20 kg), computed joint reactions differed substantially from reported in vivo measures using instrumented implants. We conclude that agreement between the model and available experimental data was good in view of limitations of both the model and the validation datasets. The presented model is useful in that it permits computation of realistic lumbar spine joint reaction forces during flexion and moderate lifting tasks. The model and corresponding documentation are now available in the online OpenSim repository.

Improvement of the reliability of ceramic hip joint implants.

The aim of this article is to present the optimization of a proof test procedure of ceramic hip joint ball heads. The proof test rejects defective samples in the production line before being implanted into human body. Thereby on every ceramic ball head a static load is applied, which is somewhat higher than the maximum physiological load. The magnitude of the applied load should not damage the samples which are free of flaws in the high stress area. The configuration of the proof test influences the stress distribution in the ball head, which should be similar to the physiological case. To determine the stress distribution, a non-linear finite element (FE) analysis was performed and the results were validated by measurements. With an iterative approach based on FE calculations the proof test configuration was optimized in such a way that the stress distribution in the ball head is similar to the stress distribution in vivo. In this study all ball heads showed very high fatigue resistance after being proof tested and fulfilled the requirements of the FDA (Food and Drug Administration, USA) described in the Guidance Document for the Preparation of Premarket Notifications for Ceramic Ball Hip System. The probability of a fracture of an implanted ceramic ball head can be decreased by the presented optimized proof test procedure. Latter can thus improve the reliability of ceramic hip joint ball heads. The study was supported by the KTI (Commission for Technology and Innovation, Switzerland).