Friction coefficient and effective interference at the implant-bone interface

Advantages and limitations of mobile-bearing unicompartmental knee arthroplasty: an overview of the literature

INTRODUCTION

Interest in unicompartmental knee arthroplasty (UKA) has recently grown. Mobile bearing UKA, in which the bearing is not fixed but rather perfectly conforms with femoral and tibial components and moves completely passively between the femoral and tibial implant, has now been used for approximately half a century.

AREAS COVERED

Alongside the recognized advantages of UKA, the mobile-bearing variant benefits from an extremely low rate of polyethylene wear and tolerable minor malalignment. Revision rates for UKA have been reported to exceed those of total knee arthroplasty, but long-term survival rates and outcomes from mobile-bearing UKA have been found to be satisfactory. In addition to the lateral osteoarthritis and loosening, which are main complications of UKA, bearing dislocation is a specific complication of mobile bearing UKA. Fractures and valgus subsidence are more prevalent than in the cementless UKA. While these continue to be features to be addressed, they have been partially solved.

EXPERT OPINION

Given the manifold benefits of UKA, its application could be extended to a larger patient population. Successful outcomes rely on careful patient selection and the surgeon's extensive familiarity with the procedure. Looking ahead, the incorporation of robotic surgery, already a feature of some fixed-bearing UKAs, might shape the future trajectory of mobile-bearing UKA.

Press-Fit Placement of a Rectangular Block Implant in the Resorbed Alveolar Ridge: Surgical and Biomechanical Considerations

Rectangular Block Implant (RBIs) were manufactured, using computer-aided-design lathe turning, surface roughened with grit blasting and gamma irradiated. Implants were surgically placed into the resorbed edentulous mandibular ridges of both greyhound dogs (ex vivo and in vivo) and humans; the pooled total was 17 placements. The aim was to achieve mechanical stability and full implant submergence without damage to the mandibular canal and without bone fracture: fulfilment of all of these criteria was deemed to be a successful surgical outcome. Rectangular osteotomy sites were prepared with piezo surgical instrumentation. Sixteen implants were fully submerged and achieved good primary stability without bone fracture and without evidence of impingement of the mandibular canal. One implant placement was deemed a failure due to bone fracture: the event of a random successful outcome was rejected (p < 0.01 confidence, binomial analysis). Technique of placement yielded excellent mechanical retention: key biomechanical factors that emerged in this process included under preparation of the osteotomy site with the use of specifically designed trial-fit gauges, the viscoelastic property of the peri-implant bone, the flat faces and cornered edges of the block surfaces which enhance stress distribution and mechanical retention, respectively. It was concluded that the surgical protocol for the RBI placement in the resorbed alveolus is a predictable clinical procedure tailored to its specific, unique biomechanical profile.

3-D finite element model of the impaction of a press-fitted femoral stem under various biomechanical environments

BACKGROUND

Uncemented femoral stem insertion into the bone is achieved by applying successive impacts on an inserter tool called "ancillary". Impact analysis has shown to be a promising technique to monitor the implant insertion and to improve its primary stability.

METHOD

This study aims to provide a better understanding of the dynamic phenomena occurring between the hammer, the ancillary, the implant and the bone during femoral stem insertion, to validate the use of impact analyses for implant insertion monitoring. A dynamic 3-D finite element model of the femoral stem insertion via an impaction protocol is proposed. The influence of the trabecular bone Young's modulus (Et), the interference fit (IF), the friction coefficient at the bone-implant interface (μ) and the impact velocity (v0) on the implant insertion and on the impact force signal is evaluated.

RESULTS

For all configurations, a decrease of the time difference between the two first peaks of the impact force signal is observed throughout the femoral stem insertion, up to a threshold value of 0.23 ms. The number of impacts required to reach this value depends on Et, v0 and IF and varies between 3 and 8 for the set of parameters considered herein. The bone-implant contact ratio reached after ten impacts varies between 60% and 98%, increases as a function of v0 and decreases as a function of IF, μ and Et.

CONCLUSION

This study confirms the potential of an impact analyses-based method to monitor implant insertion and to retrieve bone-implant contact properties.

A finite element model for investigating the influence of keel design and position on unicompartmental knee replacement cementless tibial component fixation

OBJECTIVES

The cementless Oxford Unicompartmental Knee Replacement (OUKR) tibial component relies on an interference fit to achieve initial fixation. The behaviour at the implant-bone interface is not fully understood and hence modelling of implants using Finite Element (FE) software is challenging. With a goal of exploring alternative implant designs with lower fracture risk and adequate fixation, this study aims to investigate whether optimisation of FE model parameters could accurately reproduce experimental results of a pull-out test which assesses fixation.

MATERIALS AND METHODS

Finite element models of implants with three methods of fixation (standard keel, small keel, and peg) in a bone analogue foam block were created, in which implants were modelled using an analytical rigid definition and the foam block was modelled as a homogenous linear isotropic material. The total interference and elastic slip were varied in these models and optimised by comparing simulated and experimental results of pull-out tests for two (standard and peg) implant geometries. Then the optimised interference and elastic slip were validated by comparing simulated and experimental data of a third (small keel) implant geometry.

RESULTS

The optimisation of parameters established an interference of 0.16 mm and an elastic slip of 0.20 mm as most suitable for modelling the experimental force-displacement plots during pull-out. This combination of parameters accurately reproduced the experimental results of the small keel geometry. The maximum pull-out forces from the FE models were consistent with experimental data for each implant design.

CONCLUSIONS

This study shows that experimental pull-out tests can be accurately modelled using adjusted interference values and non-linear friction and outlines a method for determining these parameters. This study demonstrates that complex problems in modelling implant behaviour can be addressed with relatively simple models. This can potentially lead to the development of implants with reduced risk of failure.

Mechanical behaviors of titanium, nickel-titanium, and stainless elastic intramedullary nail in fixation of tibial diaphyseal fractures

INTRODUCTION

Elastic nails have been widely used in the diaphyseal fracture fixation of long bones in adolescents. However, high complication rates have been reported in cases involving weights exceeding 55 kg. The existing nails are fabricated with different metals in clinical settings; however, the effect of the materials on the mechanical responses of the fractured bone remains unclear. Hence, the present study is conducted to compare the mechanical responses of typically used metals, namely titanium, stainless, and nickel-titanium, for elastic nails in the fixation of tibial diaphyseal fractures.

MATERIAL AND METHODS

A sawbone tube is used to determine the contact force, which is developed after constraining the nail inside the narrow canal using different nail materials. Furthermore, a finite element (FE) model of the tibial diaphyseal fracture is developed to predict the fracture gap deformation based on different nail materials under axial compression and bending loads. The push-out force in the FE simulation is compared with that of a case without an end cap.

RESULTS

In the sawbone tube, the results indicate that the contact force developed by the titanium nail is significantly higher than those developed by stainless and nickel-titanium nails. The contact forces developed by the titanium, stainless steel, and nickel- titanium nails are 385 (SD 34), 358 (SD 49), and 258 (SD 42) N, respectively. In the FE simulation, the titanium nail yields the highest push-out force when an end cap is not used, and the push-out forces in axial compression are 201, 183, and 87 N in the titanium, stainless, and nickel-titanium nails under axial compression, respectively. By contrast, the stainless nail yields the smallest gap deformation when an end cap is used.

CONCLUSION

Results of the present study show that the end cap is an important factor affecting the mechanical responses of nails fabricated using different materials. Titanium nails are preferred when an end cap is not used, whereas stainless nails are preferred when an end cap is used.

Pre-clinical characterization of a novel flexible surface stem design for total knee replacements

Primary stability is crucial for implant osseointegration and the long-term stability of cementless total joint replacements. Biomechanical studies have shown the potential of femoral stems for total knee replacements to reduce micromotions at the bone-implant interface. However, approaches such as focusing on the structural elasticity of the femoral stems are rarely described. Three groups with different femoral stem designs were investigated: group 1: flexible surface stem, group 2: flexible surface stem with open-porous structured lamellas, and group 3: solid stem (reference). The stems were implanted into bone substitute material and dynamically loaded for 1000 cycles. Relative movement and subsidence were measured optically, and axial pull-out forces were determined after dynamic testing. Relative movements increased to 0.10 mm (groups 1 and 2) compared to 0.03 mm (group 3). Subsidence increased to 0.08 mm (group 1) and 0.11 mm (group 2) compared to 0.06 mm (group 3). For each group, subsidence mainly occurred during the first 500 cycles. A similar convergence was observed in the further course. Pull-out forces increased to 1815.0 N (group 1) and 1347.1 N (group 2) compared to 1306.4 N (group 3). The flexible surface stem design resulted in higher relative movements and subsidence, but also exhibited increased pull-out forces. The relative movements were below the critical limit of 0.15 mm and represent a superposition of the elastic deformations of the interacting implant components as well as the micromotion at the bone-implant interface. Therefore, the novel flexible surface stem design appears to offer promising primary implant fixation.

Influence of stem design parameters on periprosthetic femoral fractures examined by subject specific finite element analyses

Due to the increasing number of periprosthetic femoral fractures (PFF), the optimisation of implant design gains importance. For the presented research a validated, subject specific finite element model of a human femur with an inlying total hip stem was used to compare the influence of different geometrical implant parameters on the development of PFF. The heterogeneous bone tissue was modelled on the basis of computed tomography scans. A ductile damage model with element deletion was applied to simulate bone fracture in a load case re-enacting a stumbling scenario. The results were compared in terms of fracture load, subsidence and fracture pattern to analyse the influence of friction at the implant-bone interface, implant size and stem length. The results showed that higher friction coefficients lead to an increase of fracture load. Also, the usage of an oversized implant has a negligible effect while an undersized implant reduces the fracture load by 48.9% for the investigated femur. Lastly, a higher fracture load was reached with an elongated stem, but the bending and change in fracture path indicate a more distal force transmission and subsequent stress shielding in the proximal femur.

Numerical analysis of the mechanical response of novel swelling bone implants in polyurethane foams

In this study, a numerical framework was developed in order to analyze the swelling properties, mechanical response and fixation strength of swelling bone anchors. Using this framework, fully porous and solid implants, along with a novel hybrid design (consisting of a solid core and a porous sleeve), were modeled and studied. Free swelling experiments were conducted to investigate their swelling characteristics. The finite element model of swelling was validated using the conducted free swelling. Compared with the experimental data, results obtained from the finite element analysis proved the reliability of this frame-work. Afterwards, the swelling bone anchors were studied embedded in artificial bones with different densities with two different interface properties: considering frictional interface between the bone anchors and artificial bones (simulating the stages prior to osteointegration, when the bone and implant are not fully bonded and the surface of the implant can slide along the interface), and perfectly bonded (simulating the stages subsequent to osteointegration, when the bone and implant are fully bonded). It was observed that the swelling considerably decreases while the average radial stress on the lateral surface of the swelling bone anchor surges in the denser artificial bones. Ultimately, the pull-out experiments and simulations of the swelling bone anchors from the artificial bones were conducted to look into the fixation strength of the swelling bone anchors. It was found that the hybrid swelling bone anchor exhibits mechanical and swelling properties close to those of solid bone anchors, while also bone in-growth is expected to happen, which is an integral factor to these bone anchors.

Debonding of coin-shaped osseointegrated implants: Coupling of experimental and numerical approaches

While cementless implants are now widely used clinically, implant debonding still occur and is difficult to anticipate. Assessing the biomechanical strength of the bone-implant interface can help improving the understanding of osseointegration phenomena and thus preventing surgical failures. A dedicated and standardized implant model was considered. The samples were tested using a mode III cleavage device to assess the mechanical strength of the bone-implant interface by combining experimental and numerical approaches. Four rough (Sa = 24.5 μm) osseointegrated coin-shaped implants were left in sheep cortical bone during 15 weeks of healing time. Each sample was experimentally rotated at 0.03°/sec until complete rupture of the interface. The maximum values of the torque were comprised between 0.48 and 0.72 N m, while a significant increase of the normal force from 7-12 N to 31-43 N was observed during the bone-implant interface debonding, suggesting the generation of bone debris at the bone-implant interface. The experimental results were compared to an isogeometric finite element model describing the adhesion and debonding phenomena through a modified Coulomb's law, based on a varying friction coefficient to represent the transition from an unbroken to a broken bone-implant interface. A good agreement was found between numerical and experimental torques, with numerical friction coefficients decreasing from 8.93 to 1.23 during the bone-implant interface rupture, which constitutes a validation of this model to simulate the debonding of an osseointegrated bone-implant interface subjected to torsion.

Impaction procedure influences primary stability of acetabular press-fit components

The aim of the study was to investigate whether the primary stability of press-fit acetabular components can be improved by altering the impaction procedure. Three impaction procedures were used to implant acetabular components into human cadaveric acetabula using a powered impaction device. An impaction frequency of 1 Hz until complete component seating served as reference. Overimpaction was simulated by adding ten strokes after complete component seating. High-frequency implantation was performed at 6 Hz. The lever-out moment of the acetabular components was used as measure for primary stability. Permanent bone deformation was assessed by comparison of double micro-CT (µCT) measurements before and after impaction. Acetabular component deformation and impaction forces were recorded, and the extent of bone-implant contact was determined from 3D laser scans. Overimpaction reduced primary acetabular component stability (p = 0.038) but did not significantly increase strain release after implantation (p = 0.117) or plastic deformations (p = 0.193). Higher press-fits were associated with larger polar gaps for the 1 Hz reference impaction (p = 0.002, R² = 0.77), with a similar trend for overimpaction (p = 0.082, R² = 0.31). High-frequency impaction did not significantly increase primary stability (p = 0.170) at lower impaction forces (p = 0.001); it was associated with smaller plastic deformations (p = 0.035, R² = 0.34) and a trend for increased acetabular component relaxation between strokes (p = 0.112). Higher press-fit was not related to larger polar gaps for the 6 Hz impaction (p = 0.346). Overimpaction of press-fit acetabular components should be prevented since additional strokes can be associated with increased bone damage and reduced primary stability as shown in this study. High-frequency impaction at 6 Hz was shown to be beneficial compared with 1 Hz impaction. This benefit has to be confirmed in clinical studies.

Finite element analysis of screw fixation durability under multiple boundary and loading conditions for a custom pelvic implant

Despite showing promising functional outcomes for pelvic reconstruction after sarcoma resection, custom-made pelvic implants continue to exhibit high complication rates due to fixation failures. Patient-specific finite element models have been utilized by researchers to evaluate implant durability. However, the effect of assumed boundary and loading conditions on failure analysis results of fixation screws remains unknown. In this study, the postoperative stress distributions in the fixation screws of a state-of-the-art custom-made pelvic implant were simulated, and the risk of failure was estimated under various combinations of two bone-implant interaction models (tied vs. frictional contact) and four load cases from level-ground walking and stair activities. The study found that the average weighted peak von Mises stress could increase by 22-fold when the bone-implant interactions were modeled with a frictional contact model instead of a tied model, and the likelihood of fatigue and pullout failure for each screw could change dramatically when different combinations of boundary and loading conditions were used. The inclusion of additional boundary and loading conditions led to a more reliable analysis of fixation durability. These findings demonstrated the importance of simulating multiple boundary conditions and load cases for comprehensive implant design evaluation using finite element analysis.

Influence of the biomechanical environment on the femoral stem insertion and vibrational behavior: a 3-D finite element study

The long-term success of cementless surgery strongly depends on the implant primary stability. The femoral stem initial fixation relies on multiple geometrical and material factors, but their influence on the biomechanical phenomena occurring during the implant insertion is still poorly understood, as they are difficult to quantify in vivo. The aim of the present study is to evaluate the relationship between the resonance frequencies of the bone-implant-ancillary system and the stability of the femoral stem under various biomechanical environments. The interference fit IF, the trabecular bone Young's modulus [Formula: see text] and the bone-implant contact friction coefficient [Formula: see text] are varied to investigate their influence on the implant insertion phenomena and on the system vibration behavior. The results exhibit for all the configurations, a nonlinear increase in the bone-implant contact throughout femoral stem insertion, until the proximal contact is reached. While the pull-out force increases with [Formula: see text], IF and [Formula: see text], the bone-implant contact ratio decreases, which shows that a compromise on the set of parameters could be found in order to achieve the largest bone-implant contact while maintaining sufficient pull-out force. The modal analysis on the range [2-7] kHz shows that the resonance frequencies of the bone-implant-ancillary system increase with the bone-implant contact ratio and the trabecular bone Young's modulus, with a sensitivity that varies over the modes. Both the pull-out forces and the vibration behavior are consistent with previous experimental studies. This study demonstrates the potential of using vibration methods to guide the surgeons for optimizing implant stability in various patients and surgical configurations.

Nanomechanical tribological characterisation of nanostructured titanium alloy surfaces using AFM: A friction vs velocity study

Medical-grade titanium alloys used for orthopaedic implants are at risk from infections and complications such as wear and tear. We have recently shown that hydrothermally etched (HTE) nanostructures (NS) formed on the Ti6AlV4 alloy surfaces impart enhanced anti-bacterial activity which results in inhibited formation of bacterial biofilm. Although these titanium alloy nanostructures may resist bacterial colonisation, their frictional properties are yet to be understood. Orthopaedic devices are encapsulated by bone and muscle tissue. Contact friction between orthopaedic implant surfaces and these host tissues may trigger inflammation, osteolysis and wear. To address these challenges, we performed simulation of the contact behaviour between a smooth control Ti6Al4V alloy and HTE surfaces against a hardwearing SiO₂ sphere using Atomic Force Microscopy (AFM) in Lateral Force Microscopy mode. The friction study was evaluated in both air and liquid environments at high (5 Hz) and low (0.5 Hz) scan velocities. Lower scan velocities demonstrated opposing friction force changes between the two mediums, with friction stabilising at higher velocities. The friction measured on the NS alloy surfaces was reduced by ~20% in air and ~80% in phosphate buffered saline, in comparison to the smooth control surface, displaying a non-linear behaviour of the force applied by the AFM tip. Changes in friction values and cantilever scan velocities on different substrates are discussed with respect to the Stribeck curve. Reduced friction on nanostructured surfaces may improve wear resistance and aid osseointegration.

A parametric numerical analysis of femoral stem impaction

Press-fitted implants are implanted by impaction to ensure adequate seating, but without overloading the components, the surgeon, or the patient. To understand this interrelationship a uniaxial discretised model of the hammer/introducer/implant/bone/soft-tissues was developed. A parametric analysis of applied energy, component materials and geometry, and interactions between implant and bone and between bone and soft-tissues was performed, with implant seating and component stresses as outcome variables. To reduce the impaction effort (energy) required by the surgeon for implant seating and also reduce stresses in the hardware the following outcomes were observed: Reduce energy per hit with more hits / Increase hammer mass / Decrease introducer mass / Increase implant-bone resistance (eg stem roughness). Hardware stiffness and patient mechanics were found to be less important and soft tissue forces, due to inertial protection by the bone mass, were so low that their damage would be unlikely. This simple model provides a basic understanding of how stress waves travel through the impacted system, and an understanding of their relevance to implantation technique and component design.

The Use of Customized Three-Dimensionally Printed Mandible Prostheses with a Pressure-Reducing Device: A Finite Element Analysis in Different Chewing Positions, Biomechanical Testing, and In Vivo Animal Study Using Lanyu Pigs

Segmental bony defects of the mandible constitute a complete loss of the regional part of the mandible. Although several types of customized three-dimension-printed mandible prostheses (CMPs) have been developed, this technique has yet to be widely used. We used CMP with a pressure-reducing device (PRD) to investigate its clinical applicability. First, we used the finite element analysis (FEA). We designed four models of CMP (P1 to P4), and the result showed that CMP with posterior PRD deployment (P4 group) had the maximum total deformation in the protrusion and right excursion positions, and in clenching and left excursion positions, posterior screws had the minimum von Mises stress. Second, the P4 CMP-PRD was produced using LaserCUSING from titanium alloy (Ti-6Al-4V). The fracture test result revealed that the maximum static pressure that could be withstood was 189 N, and a fatigue test was conducted for 5,000,000 cycles. Third, animal study was conducted on five male 4-month-old Lanyu pigs. Four animals completed the experiment. Two animals had CMP exposure in the oral cavity, but there was no significant inflammation, and one animal had a rear wing fracture. According to a CT scan, the lingual cortex of the mandible crawled along the CMP surface, and a bony front-to-back connection was noted in one animal. A histological examination indicated that CMP was significantly less reactive than control materials (p = 0.0170). Adequate PRD deployment in CMP may solve a challenge associated with CMP, thus promoting its use in clinical practice.

Subject specific finite element modelling of periprosthetic femoral fractures in different load cases

Periprosthetic femoral fractures (PFF) around total hip replacements are one of the biggest challenges for orthopaedic surgeons. To understand the risk factors and formation of these fractures, the development of a reliable finite element (FE) model incorporating bone failure is essential. Due to the anisotropic and complex hierarchical structure of bone, the mechanical behaviour under large strains is difficult to predict. In this study, a state-of-the-art subject specific FE modelling technique for bone is utilised to generate and investigate PFF. A bilinear constitutive law is applied to bone tissue in subject specific FE models of five human femurs which are virtually implanted with a straight hip stem to numerically analyse PFF. The material parameters of the models are expressed as a function of bone ash density and mapped node wise to the FE mesh. In this way the subject specific, heterogeneous structure of bone is mimicked. For material mapping of the parameters, computed tomography (CT) images of the original fresh-frozen femurs are used. Periprosthetic fractures are generated by deleting elements on the basis of a critical plastic strain failure criterion. The models are analysed under physiological and clinically relevant conditions in two different load cases re-enacting stumbling and a sideways fall on the hip. The results of the analyses are quantified with experimental data from previous work. With regard to fracture pattern, stiffness and failure load the simulations of the load case stumbling delivered the most stable and accurate results. In general, mapping of material properties was found to be an appropriate way to reproduce PFF with finite element models.

A model of uniaxial implant seating by impaction

Implants anchored by press-fit are predominantly implanted by impaction. This method allows sufficiently high forces to be generated easily by the surgeon. Suitable impaction should provide adequate implant seating without damaging the patient (tissues), the implant and implantation system, or the surgeon. However, issues have been documented for all of these factors. In this study a model to predict implant seating is developed, given an applied impaction impulse, the mass of the accelerated components and the push-in resistance force. The model was validated against experimental data for a contemporary femoral stem implanted in a polyurethane foam surrogate for bone, with the input parameters varied. The model tended to overestimate seating but represented seating patterns well. The model can be used to estimate implant seating using easily measured parameters and could be useful in the design of implantation systems, and in optimising impaction strategies.

Fracture Fixation Technique and Chewing Side Impact Jaw Mechanics in Mandible Fracture Repair

Lower jaw (mandible) fractures significantly impact patient health and well‐being due to pain and difficulty eating, but the best technique for repairing the most common subtype—angle fractures—and rehabilitating mastication is unknown. Our study is the first to use realistic in silico simulation of chewing to quantify the effects of Champy and biplanar techniques of angle fracture fixation. We show that more rigid, biplanar fixation results in lower strain magnitudes in the miniplates, the bone around the screws, and in the fracture zone, and that the mandibular strain regime approximates the unfractured condition. Importantly, the strain regime in the fracture zone is affected by chewing laterality, suggesting that both fixation type and the patient's post‐fixation masticatory pattern—ipsi‐ or contralateral to the fracture— impact the bone healing environment. Our study calls for further investigation of the impact of fixation technique on chewing behavior. Research that combines in vivo and in silico approaches can link jaw mechanics to bone healing and yield more definitive recommendations for fixation, hardware, and postoperative rehabilitation to improve outcomes. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Renatured hydrogel painting

Hydrogel coatings pave an avenue for improving the lubricity, biocompatibility, and flexibility of solid surfaces. From the viewpoint of practical applications, this work establishes a scalable method to firmly adhere hydrogel layers to diverse solid surfaces. The strategy, termed as renatured hydrogel painting (RHP), refers to adhering dehydrated xerogel to a surface with appropriate glues, followed by the formation of a hydrogel layer after rehydration of the xerogel. With the benefits of simplicity and generality, this strategy can be readily applied to different hydrogel systems, no matter what the substrate is. Hydrogel adhesion is demonstrated by its tolerance against mechanical impact with hydrodynamic shearing at 14 m/s. This method affords powerful supplements to renew the surface chemistry and physical properties of solid substrates. In addition, we show that the RHP technique can be applied to living tissue, with potential for clinical applications such as the protection of bone tissue.

An Early Periprosthetic Fracture of a Cementless Oxford Unicompartmental Knee Arthroplasty: Risk Factors and Mitigation Strategies

Introduction:

The cementless Oxford partial knee arthroplasty is associated with low perioperative complications and good long-term survival rates. However, perioperative fractures remain a serious morbidity for patients.

Case Report:

This case report describes an early post-operative tibial periprosthetic fracture through the keel slot, which we believe may be contributed by the deep implant keel design and the presence of a narrow metaphysis in the Asian knee. The patient subsequently underwent a revision total knee replacement and fixation of the periprosthetic fracture.

Conclusion:

This paper discusses the ways to identify patients at high risk of developing periprosthetic fractures and to minimize such occurrences, including adopting a modified tibial preparation, doing precise saw cuts, and considering a cemented tibial implant.

Development and Characterization of a Probe Device toward Intracranial Spectroscopy of Traumatic Brain Injury

Traumatic brain injury is a leading cause of mortality worldwide, often affecting individuals at their most economically active yet no primary disease-modifying interventions exist for their treatment. Real-time direct spectroscopic examination of the brain tissue within the context of traumatic brain injury has the potential to improve the understanding of injury heterogeneity and subtypes, better target management strategies and organ penetrance of pharmacological agents, identify novel targets for intervention, and allow a clearer understanding of fundamental biochemistry evolution. Here, a novel device is designed and engineered, delivering Raman spectroscopy-based measurements from the brain through clinically established cranial access techniques. Device prototyping is undertaken within the constraints imposed by the acquisition and site dimensions (standard intracranial access holes, probe’s dimensions), and an artificial skull anatomical model with cortical impact is developed. The device shows a good agreement with the data acquired via a standard commercial Raman, and the spectra measured are comparable in terms of quality and detectable bands to the established traumatic brain injury model. The developed proof-of-concept device demonstrates the feasibility for real-time optical brain spectroscopic interface while removing the noise of extracranial tissue and with further optimization and in vivo validation, such technology will be directly translatable for integration into currently available standards of neurological care.

Fatigue Crack Growth and Fracture of Internal Fixation Materials in In Vivo Environments-A Review

The development of crack patterns is a serious problem affecting the durability of orthopedic implants and the prognosis of patients. This issue has gained considerable attention in the medical community in recent years. This literature focuses on the five primary aspects relevant to the evaluation of the surface cracking patterns, i.e., inappropriate use, design flaws, inconsistent elastic modulus, allergic reaction, poor compatibility, and anti-corrosiveness. The hope is that increased understanding will open doors to optimize fabrication for biomedical applications. The latest technological issues and potential capabilities of implants that combine absorbable materials and shape memory alloys are also discussed. This article will act as a roadmap to be employed in the realm of orthopedic. Fatigue crack growth and the challenges associated with materials must be recognized to help make new implant technologies viable for wider clinical adoption. This review presents a summary of recent findings on the fatigue mechanisms and fracture of implant in the initial period after surgery. We propose solutions to common problems. The recognition of essential complications and technical problems related to various approaches and material choices while satisfying clinical requirements is crucial. Additional investigation will be needed to surmount these challenges and reduce the likelihood of fatigue crack growth after implantation.

The effect of different interference fits on the primary fixation of a cementless femoral component during experimental testing

Cementless femoral total knee arthroplasty (TKA) components use a press-fit (referred to as interference fit) to achieve initial fixation. A higher interference fit could lead to a superior fixation, but it could also introduce more damage to the bone during implantation. The purpose of the current study was to investigate the effect of interference fit on the micromotions and gap opening/closing at the bone-implant interface. Experimental tests were performed in six pairs of cadaveric femurs implanted with femoral components using a low interference fit of 350 μm and a high interference fit of 700 μm. The specimens were subjected to the peak loads of gait and squat, based on the Orthoload dataset. Digital Image Correlation (DIC) was used to measure the micromotions and opening/closing in different regions of interest (ROIs). Two linear mixed-effect statistical models were created with micromotions and gap opening/closing as dependent variables. ROIs, loading conditions, and implant designs as independent variables, and cadaver specimens as random intercepts. The results revealed no significant difference between the two interference fit implants for micromotions (p = 0.837 for gait and p = 0.065 for squat), nor for the gap opening/closing (p = 0.748 for gait and p = 0.561 for squat). In contrast, significant differences were found between loading and most of the ROIs in both dependent variables (p < 0.0001). Additionally, no difference in bone deformation was found between low and high interference fit. Changing interference between either 350 μm or 700 μm did not affect the primary stability of a femoral TKA component. There could be an interference fit threshold beyond which fixation does not further improve.

BIOMECHANICAL EVALUATION OF RECONSTRUCTED EXTENSIVE MANDIBULAR DEFECTS BY DIFFERENT MODELS USING FINITE ELEMENT METHOD

Rehabilitation of major mandibular defects after tumor resection has become a serious challenge for surgeons. In this research, four various models were designed to repair a critical mandibular lateral defect. Biomechanical behavior of the models was assessed by Finite Element Method. These models are including Fibular-Free Flap (FFF), Customized Prosthesis (CP), Tray Implant without Bone Graft (TI-wo-BG), and Tray Implant with Bone Graft (TI-w-BG). FFF is a subset of microvascular free flap technique in which some segments of patient’s fibula bone are used to restore mandibular defects. CP is a hollow and light prosthesis which is fabricated using Additive Manufacturing technology from Ti alloy powder. TI-wo-BG is similar to a crib which is designed according to the geometry of the patient’s mandible. TI-w-BG, in fact, is a TI-wo-BG which is filled with small cortico-cancellous chips in order to benefit potential profit of bone grafting. The chewing operation and loading on the mandible was simulated considering the three mandibular muscular forces including masseter, medial pterygoid, and temporalis.

The result of FEM analysis of TI-wo-BG and TI-w-BG showed that in both models, screw number 6 endured a strain of 5684 and 2852[Formula: see text][Formula: see text]m/m which exceeded pathological and mild overload risk, respectively. This may increase the probability of screw loosening and system failure. The results proved the stability of the FFF and CP models. In addition, it can be concluded that stress and strain on the screw’s interfaces can decrease by improving the plate and increasing the friction at the interface of plate, bone and screw.

Measurement of Internal Implantation Strains in Analogue Bone Using DVC

The survivorship of cementless orthopaedic implants may be related to their initial stability; insufficient press-fit can lead to excessive micromotion between the implant and bone, joint pain, and surgical revision. However, too much interference between implant and bone can produce excessive strains and damage the bone, which also compromises stability. An understanding of the nature and mechanisms of strain generation during implantation would therefore be valuable. Previous measurements of implantation strain have been limited to local discrete or surface measurements. In this work, we devise a Digital Volume Correlation (DVC) methodology to measure the implantation strain throughout the volume. A simplified implant model was implanted into analogue bone media using a customised loading rig, and a micro-CT protocol optimised to minimise artefacts due to the presence of the implant. The measured strains were interpreted by FE modelling of the displacement-controlled implantation, using a bilinear elastoplastic constitutive model for the analogue bone. The coefficient of friction between the implant and bone was determined using the experimental measurements of the reaction force. Large strains at the interface between the analogue bone and implant produced localised deterioration of the correlation coefficient, compromising the ability to measure strains in this region. Following correlation coefficient thresholding (removing strains with a coefficient less than 0.9), the observed strain patterns were similar between the DVC and FE. However, the magnitude of FE strains was approximately double those measured experimentally. This difference suggests the need for improvements in the interface failure model, for example, to account for localised buckling of the cellular analogue bone structure. A further recommendation from this work is that future DVC experiments involving similar geometries and structures should employ a subvolume size of 0.97 mm as a starting point.

Tibial shape and size predicts the risk of tibial plateau fracture after cementless unicompartmental knee arthroplasty in Japanese patients

AIMS

Cementless unicompartmental knee arthroplasty (UKA) has advantages over cemented UKA, including improved fixation, but has a higher risk of tibial plateau fracture, particularly in Japanese patients. The aim of this multicentre study was to determine when cementless tibial components could safely be used in Japanese patients based on the size and shape of the tibia.

METHODS

The study involved 212 cementless Oxford UKAs which were undertaken in 174 patients in six hospitals. The medial eminence line (MEL), which is a line parallel to the tibial axis passing through the tip of medial intercondylar eminence, was drawn on preoperative radiographs. Knees were classified as having a very overhanging medial tibial condyle if this line passed medial to the medial tibial cortex. They were also classified as very small if a size A/AA tibial component was used.

RESULTS

The overall rate of fracture was 8% (17 out of 212 knees). The rate was higher in knees with very overhanging condyles (Odds ratio (OR) 13; p < 0.001) and with very small components (OR 7; p < 0.001). The OR was 21 (p < 0.001) in those with both very overhanging condyles and very small components. In all, 69% of knees (147) had neither very overhanging nor very small components, and the fracture rate in these patients was 1.4% (2 out of 147 knees). Males had a significantly reduced risk of fracture (OR 0.13; p = 0.002), probably because no males required very small components and females were more likely to have very overhanging condyles (OR 3; p = 0.013). 31% of knees (66) were in males and in these the rate of fracture was 1.5% (1 out of 66 knees).

CONCLUSION

The rate of tibial plateau fracture in Japanese patients undergoing cementless UKA is high. We recommend that cemented tibial fixation should be used in Japanese patients who require very small components or have very overhanging condyles, as identified from preoperative radiographs. In the remaining 69% of knees cementless fixation can be used. This approach should result in a low rate of fracture. Cite this article: Bone Joint J 2020;102-B(7):861-867.

Fabrication of Microporous Coatings on Titanium Implants with Improved Mechanical, Antibacterial, and Cell-Interactive Properties

The success of an orthopedic implant therapy depends on successful bone integration and the prevention of microbial infections. In this work, plasma electrolytic oxidation (PEO) was performed to deposit TiO₂ coatings enriched with Ca, P, and Ag on titanium to improve its surface properties and antibacterial efficacy while maintaining normal biological functions and thus to enhance the performance of orthopedic implants. After PEO treatment, the surface of Ti was converted to anatase and rutile TiO₂, hydroxyapatite, and calcium titanate phases. The presence of these crystalline phases was further increased with an increased Ag content in the coatings. The developed coatings also exhibited a more porous morphology with an improved surface wettability, roughness, microhardness, and frictional coefficient. In vitro antibacterial assays indicated that the Ag-doped coatings can significantly prevent the growth of both Staphylococcus aureus and Escherichia coli by releasing Ag⁺ ions, and the ability to prevent these bacteria was enhanced by increasing the Ag content in the coatings, resulting in a maximal 6-log reduction of E. coli and a maximal 5-log reduction of S. aureus after 24 h of incubation. Moreover, the in vitro cytocompatibility evaluation of the coatings showed that the osteoblast (MC3T3) cell integration on the PEO-based coatings was greatly improved compared to untreated Ti and no notable impact on their cytocompatibility was observed on increasing the amount of Ag in the coating. In conclusion, the coating with favorable physicochemical and mechanical properties along with controlled silver ion release can offer an excellent antibacterial performance and osteocompatibility and can thus become a prospective coating strategy to face current challenges in orthopedics.

Patient and surgical variability in the primary stability of cementless acetabular cups: A finite element study

Aseptic loosening is the most common indication for revision of cementless acetabular cups and often depends on the primary stability achieved following surgery. Cup designs must be capable of achieving primary stability for a wide variety of individuals and surgical conditions to be successful. Typically, preclinical finite element (FE) testing of cups involves assessing the performance in a single patient and under a limited set of idealized conditions. The aim of this study was to assess the effect of patient and surgical parameters on the primary stability of an acetabular cup design in a set of subject-specific FE models. Interference fit was varied in a representative set of 12 patient-specific models of the implanted hemipelvis. Linear mixed models showed a significant association with micromotion for interference fit (P < .0001), acetabular bone elastic modulus (P < .001), native acetabular diameter (P  = .03), and the interference fit-elastic modulus interaction (P = .01). There were no significant associations between the polar gap and any of the parameters considered. The significant interference fit-elastic modulus interaction suggests that increasing the interference fit in patients with low bone quality leads to a greater reduction in micromotion than in patients with higher bone quality. However, the significant association between percentage bone yielding and interference fit (P < .0001) suggests a higher periacetabular fracture risk at higher interference fits. This work supports the development of preclinical testing of cup designs for the broad range patients and surgical conditions a cup may face following surgery.

Bactericidal coating to prevent early and delayed implant-related infections

The occurrence of an implant-associated infection (IAI) with the formation of a persisting bacterial biofilm remains a major risk following orthopedic biomaterial implantation. Yet, progress in the fabrication of tunable and durable implant coatings with sufficient bactericidal activity to prevent IAI has been limited. Here, an electrospun composite coating was optimized for the combinatorial and sustained delivery of antibiotics. Antibiotics-laden poly(ε-caprolactone) (PCL) and poly`1q`(lactic-co glycolic acid) (PLGA) nanofibers were electrospun onto lattice structured titanium (Ti) implants. In order to achieve tunable and independent delivery of vancomycin (Van) and rifampicin (Rif), we investigated the influence of the specific drug-polymer interaction and the nanofiber coating composition on the drug release profile and durability of the polymer-Ti interface. We found that a bi-layered nanofiber structure, produced by electrospinning of an inner layer of [PCL/Van] and an outer layer of [PLGA/Rif], yielded the optimal combinatorial drug release profile. This resulted in markedly enhanced bactericidal activity against planktonic and adherent Staphylococcus aureus for 6 weeks as compared to single drug delivery. Moreover, after 6 weeks, synergistic bacterial killing was observed as a result of sustained Van and Rif release. The application of a nanofiber-filled lattice structure successfully prevented the delamination of the multi-layer coating after press-fit cadaveric bone implantation. This new lattice design, in conjunction with the multi-layer nanofiber structure, can be applied to develop tunable and durable coatings for various metallic implantable devices. This is particularly appealing to tune the release of multiple antimicrobial agents over a period of weeks to prevent early and delayed onset IAI.

A Surface-to-Surface Finite Element Algorithm for Large Deformation Frictional Contact in febio

This study formulates a finite element algorithm for frictional contact of solid materials, accommodating finite deformation and sliding. The algorithm uses a penalty method regularized with an augmented Lagrangian scheme to enforce contact constraints in a nonmortar surface-to-surface approach. Use of a novel kinematical approach to contact detection and enforcement of frictional constraints allows solution of complex problems previously requiring mortar methods or contact smoothing algorithms. Patch tests are satisfied to a high degree of accuracy with a single-pass penalty method, ensuring formulation errors do not affect the solution. The accuracy of the implementation is verified with Hertzian contact, and illustrations demonstrating the ability to handle large deformations and sliding are presented and validated against prior literature. A biomechanically relevant example addressing finger friction during grasping demonstrates the utility of the proposed algorithm. The algorithm is implemented in the open source software febio, and the source code is made available to the general public.

Dependence of the primary stability of cementless acetabular cup implants on the biomechanical environment

Biomechanical phenomena occurring at the bone–implant interface during the press-fit insertion of acetabular cup implants are still poorly understood. This article presents a nonlinear geometrical two-dimensional axisymmetric finite element model aiming at describing the biomechanical behavior of the acetabular cup implant as a function of the bone Young’s modulus Eb, the diametric interference fit ( IF), and the friction coefficient µ. The numerical model was compared with experimental results obtained from an in vitro test, which allows to determine a reference configuration with the parameter set: μ*  = 0.3, [Formula: see text], and IF*  = 1 mm for which the maximal contact pressure tN = 10.7 MPa was found to be localized at the peri-equatorial rim of the acetabular cavity. Parametric studies were carried out, showing that an optimal value of the pull-out force can be defined as a function of μ, Eb, and IF. For the reference configuration, the optimal pull-out force is obtained for μ  = 0.6 (respectively, Eb = 0.35 GPa and IF = 1.4 mm). For relatively low value of µ ( µ < 0.2), the optimal value of IF linearly increases as a function of µ independently of Eb, while for µ > 0.2, the optimal value of IF has a nonlinear dependence on µ and decreases as a function of Eb. The results can be used to help surgeons determine the optimal value of IF in a patient specific manner.

Biomechanical behaviours of the bone-implant interface: a review

In recent decades, cementless implants have been widely used in clinical practice to replace missing organs, to replace damaged or missing bone tissue or to restore joint functionality. However, there remain risks of failure which may have dramatic consequences. The success of an implant depends on its stability, which is determined by the biomechanical properties of the bone-implant interface (BII). The aim of this review article is to provide more insight on the current state of the art concerning the evolution of the biomechanical properties of the BII as a function of the implant's environment. The main characteristics of the BII and the determinants of implant stability are first introduced. Then, the different mechanical methods that have been employed to derive the macroscopic properties of the BII will be described. The experimental multi-modality approaches used to determine the microscopic biomechanical properties of periprosthetic newly formed bone tissue are also reviewed. Eventually, the influence of the implant's properties, in terms of both surface properties and biomaterials, is investigated. A better understanding of the phenomena occurring at the BII will lead to (i) medical devices that help surgeons to determine an implant's stability and (ii) an improvement in the quality of implants.

The effect of plate design, bridging span, and fracture healing on the performance of high tibial osteotomy plates: An experimental and finite element study

Objectives

Opening wedge high tibial osteotomy (HTO) is an established surgical procedure for the treatment of early-stage knee arthritis. Other than infection, the majority of complications are related to mechanical factors – in particular, stimulation of healing at the osteotomy site. This study used finite element (FE) analysis to investigate the effect of plate design and bridging span on interfragmentary movement (IFM) and the influence of fracture healing on plate stress and potential failure.

Materials and Methods

A 10° opening wedge HTO was created in a composite tibia. Imaging and strain gauge data were used to create and validate FE models. Models of an intact tibia and a tibia implanted with a custom HTO plate using two different bridging spans were validated against experimental data. Physiological muscle forces and different stages of osteotomy gap healing simulating up to six weeks postoperatively were then incorporated. Predictions of plate stress and IFM for the custom plate were compared against predictions for an industry standard plate (TomoFix).

Results

For both plate types, long spans increased IFM but did not substantially alter peak plate stress. The custom plate increased axial and shear IFM values by up to 24% and 47%, respectively, compared with the TomoFix. In all cases, a callus stiffness of 528 MPa was required to reduce plate stress below the fatigue strength of titanium alloy.

Conclusion

We demonstrate that larger bridging spans in opening wedge HTO increase IFM without substantially increasing plate stress. The results indicate, however, that callus healing is required to prevent fatigue failure.

Cite this article: A. R. MacLeod, G. Serrancoli, B. J. Fregly, A. D. Toms, H. S. Gill. The effect of plate design, bridging span, and fracture healing on the performance of high tibial osteotomy plates: An experimental and finite element study. Bone Joint Res 2018;7:639–649. DOI: 10.1302/2046-3758.712.BJR-2018-0035.R1.

Instruments to reduce the risk of tibial fracture following cementless unicompartmental knee replacement

BACKGROUND

Tibial plateau fracture is an important complication of cementless Oxford unicompartmental knee replacement. The press fit between the keel and the bone's keel slot is responsible for primary fixation and also contributes to fracture risk. This study investigates whether the fracture risk could be reduced without compromising primary fixation, by using different instruments to widen the keel slot.

METHODS

Keel slots were made in polyurethane blocks (n = 60) using the standard keel cut saw blade or a new blade that was 0.2 mm wider, with adjuvant use of the cemented pick or a prototype rasp. A tibial component was pushed into and pulled out of the slots using a Dartec materials testing machine. It was assumed that the 'push-in' force was related to the risk of fracture and 'pull-out' force was related to fixation. Reproducibility studies with 10 different tibial components were undertaken.

RESULTS

The new blade required significantly lower push-in forces than the standard blade (789 N SD 130, 1411 N SD 180; P < 0.001), but the pull-out forces were not different (240 N SD 47, 230 N SD 56; P > 0.999). With the standard blade the pick decreased the push-in (818 N SD 318; P < 0.001) and pull-out (128 N SD 58; P < 0.001) forces, but the rasp had no effect. With the new blade the pick had no effect, but the rasp increased the push-in force (1390 N SD 202; P < 0.001).

CONCLUSIONS

This study suggests the fracture risk will be reduced with the new blades, with no compromise in fixation. If the new blades are used routine use of the cemented pick appears to be of no benefit.

Optimal interference of the tibial component of the cementless Oxford Unicompartmental Knee Replacement

Objectives

The primary stability of the cementless Oxford Unicompartmental Knee Replacement (OUKR) relies on interference fit (or press fit). Insufficient interference may cause implant loosening, whilst excessive interference could cause bone damage and fracture.

The aim of this study was to identify the optimal interference fit by measuring the force required to seat the tibial component of the cementless OUKR (push-in force) and the force required to remove the component (pull-out force).

Materials and Methods

Six cementless OUKR tibial components were implanted in 12 new slots prepared on blocks of solid polyurethane foam (20 pounds per cubic foot (PCF), Sawbones, Malmo, Sweden) with a range of interference of 0.1 mm to 1.9 mm using a Dartec materials testing machine HC10 (Zwick Ltd, Herefordshire, United Kingdom) . The experiment was repeated with cellular polyurethane foam (15 PCF), which is a more porous analogue for trabecular bone.

Results

The push-in force progressively increased with increasing interference. The pull-out force was related in a non-linear fashion to interference, decreasing with higher interference. Compared with the current nominal interference, a lower interference would reduce the push-in forces by up to 45% (p < 0.001 One way ANOVA) ensuring comparable (or improved) pull-out forces (p > 0.05 Bonferroni post hoc test). With the more porous bone analogue, although the forces were lower, the relationship between interference and push-in and pull-out force were similar.

Conclusions

This study suggests that decreasing the interference fit of the tibial component of the cementless OUKR reduces the push-in force and can increase the pull-out force. An optimal interference fit may both improve primary fixation and decrease the risk of fracture.

Cite this article: S. Campi, S. J. Mellon, D. Ridley, B. Foulke, C. A. F. Dodd, H. G. Pandit, D. W. Murray. Optimal interference of the tibial component of the cementless Oxford Unicompartmental Knee Replacement. Bone Joint Res 2018;7:226–231. DOI: 10.1302/2046-3758.73.BJR-2017-0193.R1.

Additive manufactured push-fit implant fixation with screw-strength pull out

Additive manufacturing offers exciting new possibilities for improving long-term metallic implant fixation in bone through enabling open porous structures for bony ingrowth. The aim of this research was to investigate how the technology could also improve initial fixation, a precursor to successful long-term fixation. A new barbed fixation mechanism, relying on flexible struts was proposed and manufactured as a push-fit peg. The technology was optimized using a synthetic bone model and compared with conventional press-fit peg controls tested over a range of interference fits. Optimum designs, achieving maximum pull-out force, were subsequently tested in a cadaveric femoral condyle model. The barbed fixation surface provided more than double the pull-out force for less than a third of the insertion force compared to the best performing conventional press-fit peg (p < 0.001). Indeed, it provided screw-strength pull out from a push-fit device (1,124 ± 146 N). This step change in implant fixation potential offers new capabilities for low profile, minimally invasive implant design, while providing new options to simplify surgery, allowing for one-piece push-fit components with high levels of initial stability. © 2017 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. on behalf of the Orthopaedic Research Society. J Orthop Res 36:1508-1518, 2018.

Evaluation of interference fit and bone damage of an uncemented femoral knee implant

BACKGROUND

During implantation of an uncemented femoral knee implant, press-fit interference fit provides the primary stability. It is assumed that during implantation a combination of elastic and plastic deformation and abrasion of the bone will occur, but little is known about what happens at the bone-implant interface and how much press-fit interference fit is eventually achieved.

METHODS

Five cadaveric femora were prepared and implantation was performed by an experienced surgeon. Micro-CT- and conventional CT-scans were obtained pre- and post-implantation for geometrical measurements and to measure bone mineral density. Additionally, the position of the implant with respect to the bone was determined by optical scanning of the reconstructions. By measuring the differences in surface geometry, assessments were made of the cutting error, the actual interference fit, the amount of bone damage, and the effective interference fit.

FINDINGS

Our analysis showed an average cutting error of 0.67mm (SD 0.17mm), which pointed mostly towards bone under-resections. We found an average actual AP interference fit of 1.48mm (SD 0.27mm), which was close to the nominal value of 1.5mm.

INTERPRETATION

We observed combinations of bone damage and elastic deformation in all bone specimens, which showed a trend to be related with bone density. Higher bone density tended to lead to lower bone damage and higher elastic deformation. The results of the current study indicate different factors that interact while implanting an uncemented femoral knee component. This knowledge can be used to fine-tune design criteria of femoral components to achieve adequate primary stability for all patients.

Biomechanical Effects of Various Bone-Implant Interfaces on the Stability of Orthodontic Miniscrews: A Finite Element Study

INTRODUCTION

Osseointegration is required for prosthetic implant, but the various bone-implant interfaces of orthodontic miniscrews would be a great interest for the orthodontist. There is no clear consensus regarding the minimum amount of bone-implant osseointegration required for a stable miniscrew. The objective of this study was to investigate the influence of different bone-implant interfaces on the miniscrew and its surrounding tissue.

METHODS

Using finite element analysis, an advanced approach representing the bone-implant interface is adopted herein, and different degrees of bone-implant osseointegration were implemented in the FE models. A total of 26 different FE analyses were performed. The stress/strain patterns were calculated and compared, and the displacement of miniscrews was also evaluated.

RESULTS

The stress/strain distributions are changing with the various bone-implant interfaces. In the scenario of 0% osseointegration, a rather homogeneous distribution was predicted. After 15% osseointegration, the stress/strains were gradually concentrated on the cortical bone region. The miniscrew experienced the largest displacement under the no osseointegra condition. The maximum displacement decreases sharply from 0% to 3% and tends to become stable.

CONCLUSION

From a biomechanical perspective, it can be suggested that orthodontic loading could be applied on miniscrews after about 15% osseointegration without any loss of stability.

Influence of trabecular bone quality and implantation direction on press-fit mechanics

Achieving primary stability of uncemented press-fit prostheses in patients with poor quality bone can involve axial implantation forces large enough to cause bone fracture. Radial implantation eliminates intraoperative impaction forces and could prevent this damage. Platens of two commercial implant surfaces ("Beaded" and "Flaked") were implanted onto trabecular bone specimens of varying quality in a press-fit simulator. Samples were implanted with varying interference, either axially (shear) or radially (normal). Push-in and pull-out forces were measured to assess stability. Microstructural changes in the bone were determined from μCT analysis. For force-defined implantation analysis, push-in and pull-out forces both increased proportionally with increasing radial force, independent of implantation direction, bone quality or implant surface. For position-defined implantation analysis, pull-out forces were generally found to increase with interference and to be greater for radial than axial implantation direction, and to be lower for poor quality bone. Bone density increased locally at the tested interface due to implantation, in particular for the Beaded surface under axial implantation. If a safe radial stress can be determined for cortical bone in a particular patient, the associated implantation force, and pull-out force which represents primary stability, can be directly derived, regardless of implantation direction, bone quality or implant surface. Radial implantation delivers primary stability that is no worse than that for axial implantation and may eliminate potentially damaging impaction forces. Development of implant designs based on this principal might improve implant fixation. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:224-233, 2017.