Computational simulations of stress shielding and bone resorption around existing and computer-designed orthopaedic screws

Stability Analysis of Plate-Screw Fixation for Femoral Midshaft Fractures

An understanding of the biomechanical characteristics and configuration of flexible and locked plating in order to provide balance stability and flexibility of implant fixation will help to construct and promote fast bone healing. The relationship between applied loading and implantation configuration for best bone healing is still under debate. This study aims to investigate the relationship between implant strength, working length, and interfragmentary strain (εIFM) on implant stability for femoral midshaft transverse fractures. The transverse fracture was fixed with a fragment locking compression plate (LCP) system. Finite element analysis was performed and subsequently characterised based on compression loading (600 N up to 900 N) and screw designs (conventional and locking) with different penetration depths (unicortical and bicortical). Strain theory was used to evaluate the stability of the model. The correlation of screw configuration with screw type shows a unicortical depth for both types (p < 0.01) for 700 N and 800 N loads and (p < 0.05) for configurations 134 and 124. Interfragmentary strain affected only the 600 N load (p < 0.01) for the bicortical conventional type (group BC), and the screw configurations that were influenced were 1234 and 123 (p < 0.05). The low steepness of the slope indicates the least εIFM for the corresponding biomechanical characteristic in good-quality stability. A strain value of ≤2% promotes callus formation and is classified as absolute stability, which is the minimum required value for the induction of callus and the maximum value that allows bony bridging. The outcomes have provided the correlation of screw configuration in femoral midshaft transverse fracture implantation which is important to promote essential primary stability.

Analytical Study of Stress Distributions around Screws in Flat Mandibular Bone under In-Plane Loading

A known complication for mechanically loaded bone implants is the instability due to screw loosening, resulting in infection and the non-union of fractures. To investigate and eventually prevent such bone degradation, it is useful to know the stress state in the bone around the screw. Considering only in-plane loadings and simplifying the mandibular bone into an orthotropic laminated plate, the analysis was reduced to a two-dimensional pin-loaded plate problem. An analytic model, based on the complex stress analysis, was introduced to the bone biomechanics field to obtain the stress distributions around the screw hole in the bone. The dimensionless normalized stresses were found to be relatively insensitive to the locations of the screw hole over the mandible. Parametric analyses were carried out regarding the friction coefficient and load direction. It was found that the load direction had a negligible influence. On the contrary, the friction coefficient had a significant effect on the stress distributions. Whether the screw was well bonded or not thus played an important role. The proposed analytic model could potentially be used to study bone failure together with stress-based failure criteria.

A novel graded-stiffness footwear device for heel ulcer prevention and treatment: a finite element-based study

Diabetic heel ulceration is a serious, destructive, and costly complication of diabetes. In this study, a novel "graded-stiffness" offloading method was proposed. This method consists of heel support with multi-increasing levels of stiffness materials, to prevent and treat heel ulcers. A three-dimensional finite element model of the heel was used to evaluate the novel "graded-stiffness" orthotic device compared to two existing solutions: (1) an insole with a hole under the active ulcer and (2) an insole with a hole filled with a soft material (elastic modulus of 15 kPa). Volumetric exposure evaluation of internal tissues to stress was performed at two volume-of-interests: (1) the area of the heel soft tissues typically at high risk for ulceration, and (2) the soft tissues surrounding the high-risk area. The models predict that the "graded-stiffness" offloading solution is more effective than existing solutions in distributing and reducing heel internal loads, considering both volume-of-interests. Comparing different material gradient combinations for the offloading support reveals considerable variation of the heel stress distribution. In clinical practice, the "graded-stiffness" technological solution enables to form an adaptable and flexible system that can be customized to a specific patient, through adequate selection of the offloading materials, to fit the shape and size of the ulcer. This solution can be made as an off-the-shelf product or alternatively, be manufactured by-demand using 3D printing tools. The proposed novel practical offloading solution has the potential for streamlining and optimizing the prevention and treatment of diabetic heel ulcers.

Development and initial validation of a novel undercut thread design for locking screws

BACKGROUND

Locking screws with a typical buttress thread have high levels of failure in patients with osteoporotic bones. This study aims to develop a novel thread design for the locking screw and compare its fixation stability with the typical buttress thread.

METHODS

Locking screws with a novel thread design that possess an undercut feature and locking screws with a typical buttress thread were manufactured from stainless steel. Their fixation stabilities were then evaluated individually under a lateral migration test and evaluated in pairs together with a locking plate (LP) in an osteoporotic bone substitute under cyclic craniocaudal and torsional loadings. A finite element analysis (FEA) model was constructed to analyze the stress distributions present in the bone tissue adjacent to the novel thread versus the buttress thread.

RESULTS

The biomechanical test revealed that the novel thread had a significantly higher lateral migration strength than the buttress thread. When applied to a LP, the locking screw with the novel thread requires more cycles and higher forces or torque to resist migration up to 5 mm or 10° than the buttress thread. The FEA simulation showed that the novel thread can make the stress distribute more evenly at the adjacent bone tissue when compared with the buttress thread.

CONCLUSIONS

The locking screw with the novel undercut thread had superior lateral migration resistance during both initial and continued migration and superior fixation stability when applied to a LP under both cyclic craniocaudal loading and torsional loading than the locking screw with a typical buttress thread.

The Regularity of Stress Shielding in Internal Fixation Characterized by Hydromechanics

Stress shielding is an important factor in the internal fixation of a fracture. To explore the regularity of stress shielding in internal fixation, a simplified model of a comminuted femoral shaft fracture bridged by a locking plate was established and finite element analysis was performed to analyze the load distribution between the plate and femur from the proximal end of the femur to the fracture line and investigate the stress shielding degree of the plate on the bone. The stress, deformation, and axial compressive force distribution of four internal fixation schemes under compression were obtained, and the stress shielding degrees on each section was calculated. To compare the regularity of stress shielding and flow distribution, the relationship between the compressive force increment and stress shielding degree was established. The normalized curves of compressive force increment with the plate section position were compared with the flow distribution in a Z-type manifold, a parallel pipe system similar to an internal fixation system in structure and working characteristics. For quantitative comparison, the similarity between normalized curves of the compressive force increment and simulated flow distribution was calculated. The regularity of load distribution along the section position of the plate was similar to the flow distribution in the Z-type manifold. Therefore, the flow distribution pattern of the Z-type manifold can be used to characterize the regularity of load distribution in internal fixation. This study provided a new method to characterize the stress shielding degree of a locking plate on bone.

Alternatives and preferences for materials in use for pressure ulcer prevention: An experiment-reinforced literature review

Alleviation of localised, sustained tissue loads and microclimate management are the most critical performance criteria for materials in use for pressure ulcer prevention, such as in prophylactic dressings, padding or cushioning. These material performance criteria can be evaluated by calculating the extents of matching between the material stiffness (elastic modulus) and the thermal conductivity of the protective dressing, padding or cushioning with the corresponding properties of native skin, separately or in combination. Based on these bioengineering performance criteria, hydrocolloids, which are commonly used for prophylaxis of medical device-related pressure ulcers, exhibit poor stiffness matching with skin. In addition, there is remarkable variability in the modulus and thermal conductivity matching levels of different material types used for pressure ulcer prevention, however, it appears that among the materials tested, hydrogels provide the optimal matching with skin, followed by gels and silicone foams. The stiffness matching for hydrocolloids appears to be inferior even to that of gauze. This article provides quantitative performance criteria and metrics for these evaluations, and grades commonly used material types to biomechanically guide clinicians and industry with regards to the selection of dressings for pressure ulcer prevention, both due to bodyweight forces and as a result of applied medical devices.

Mechanical Analysis and Corrosion Analysis of Zinc Alloys for Bioabsorbable Implants for Osteosynthesis

Zinc alloys have recently been researched intensely for their great properties as bioabsorbable implants for osteosynthesis. Pure zinc (Zn) itself has relatively poor strength, which makes it insufficient for most clinical use. Research has already proven that the mechanical strength of zinc can be enhanced significantly by alloying it with silver. This study evaluated zinc silver alloys (ZnAg) as well as novel zinc silver titanium alloys (ZnAgTi) regarding their mechanical properties for the use as bioabsorbable implants. Compared to pure zinc the mechanical strength was enhanced significantly for all tested zinc alloys. The elastic properties were only enhanced significantly for the zinc silver alloys ZnAg6 and ZnAg9. Regarding target values for orthopedic implants proposed in literature, the best mechanical properties were measured for the ZnAg3Ti1 alloy with an ultimate tensile strength of 262 MPa and an elongation at fracture of 16%. Besides the mechanical properties, the corrosion rates are important for bioabsorbable implants. This study tested the corrosion rates of zinc alloys in PBS solution (phosphate buffered solution) with electrochemical corrosion measurement. Zinc and its alloys showed favorable corrosion rates, especially in comparison to magnesium, which has a much lower degradation rate and no buildup of hydrogen gas pockets during the process. Altogether, this makes zinc alloys highly favorable for use as material for bioabsorbable implants for osteosynthesis.

Comparison of zirconia degradation in dental implants and femoral balls: an X-ray diffraction and nanoindentation study

Background

New tetragonal zirconia polycrystal dental implants stabilized with yttria (Y-TZP) have appeared in the implantology market in the form of single piece or two-piece zircona implant system. These new type of implants improve the aesthetical properties compared to conventional commercially pure (c.p.) titanium used for implants, although the long term mechanical behavior of these new implants is not yet well known. In orthopaedics, the application of zirconia as femoral balls presented an important controversial use due to the premature fracture once implanted. Y-TZP dental implants can be affected by hydrothermal degradation and its behavior should be analysed to avoid a premature fracture. The scientific question behind the study is to analyse if the degradation mechanism observed in orthopaedics applications of Y-TZP is similar to that of Y-TZP for dental applications.

Materials and methods

For this purpose, 30 original Y-TZP dental implants and 42 Y-TZP femoral balls fractured in vivo have been studied. Dental implants were submitted to an accelerated hydrothermal degradation to compare with the femoral balls fractured in vivo. Phase transformation as well as the mechanical behaviour of the degraded samples was studied by X ray diffraction and nanoindentation tests, respectively.

Results

Results have shown that the fracture mechanism of dental implants does not resemble the mechanism observed in orthopaedic samples, presenting a good long-term behaviour.

Conclusion

The results ensure the good performance of zirconia dental implants, because the degradation of the ceramic is very limited and does not affect the mechanical properties.

The effect of humeral implant thickness and canal fill on interface contact and bone stresses in the proximal humerus

Background

Stem size is an important element for successful time zero primary fixation of a press-fit humeral stem in shoulder arthroplasty. Little basic science research, however, has been conducted on the effects of implant thickness and canal fill on load transfer, contact, and stress shielding. The purpose of this finite element study was to determine the effects of varying stem thickness on bone contact, bone stresses, and bone resorption owing to stress shielding.

Methods

Three generic short-stem implant models were developed and varied based on cross-sectional thickness (thinner - 8 mm, medium - 12 mm, thicker - 16 mm). Using a finite element model, three outcome measures were determined (1) the amount of bone-to-implant contact, (2) changes in cortical and trabecular bone stresses from the intact state, and (3) changes in cortical and trabecular strain energy densities which can predict bone remodeling or stress shielding.

Results

Increasing the size of the humeral stem had no significant effects on bone-to-implant contact during loading (P > .07). The thinner implant with the lowest canal fill ratio produced significantly lower changes in stress from the intact state in both cortical and trabecular bone (P < .002). In addition, the thinner implant resulted in a substantially lower volume of bone predicted to stress shield and resorb when compared with the medium and thicker stems.

Discussion

The results demonstrate that thinner implants and lower canal fill may be beneficial over thicker sizes, provided equal initial fixation can be achieved. The thinner implant has a greater degree of load sharing and increases the mechanical load placed on surrounding bone, reducing the risk of stress shielding and bone resorption.

The effect of orthopedic screw profiles on the healing time of femoral neck fracture

One possible treatment for femoral neck fractures, especially in young people, is the use of bone screws or Lug screws. The design of these implants requires taking into account the biocompatibility of materials, mechanical properties plus surface properties, and thread's geometric, as well as chemical properties, etc. Various profiles are designed for fracture fixation. The most famous of these profiles, which are introduced by the ISO standard, are HB, HC, and HD type profiles. This article investigates the performance of these profiles in reducing or increasing the healing time. This study is based on the rule of bone remodeling and using a set of three-dimensional computational (finite element) models. The study revealed that the HB profile outperformed the other two profiles. Meanwhile, HD profile was also better than HC profile.

Revealing the influence of electron beam melted Ti-6Al-4V scaffolds on osteogenesis of human bone marrow-derived mesenchymal stromal cells

Porous Titanium-6Aluminum-4Vanadium scaffolds made by electron beam-based additive manufacturing (AM) have emerged as state-of-the-art implant devices. However, there is still limited knowledge on how they influence the osteogenic differentiation of bone marrow-derived mesenchymal stromal cells (BMSCs). In this study, BMSCs are cultured on such porous scaffolds to determine how the scaffolds influence the osteogenic differentiation of the cells. The scaffolds are biocompatible, as revealed by the increasing cell viability. Cells are evenly distributed on the scaffolds after 3 days of culturing followed by an increase in bone matrix development after 21 days of culturing. qPCR analysis provides insight into the cells' osteogenic differentiation, where RUNX2 expression indicate the onset of differentiation towards osteoblasts. The COL1A1 expression suggests that the differentiated osteoblasts can produce the osteoid. Alkaline phosphatase staining indicates an onset of mineralization at day 7 in OM. The even deposits of calcium at day 21 further supports a successful bone mineralization. This work shines light on the interplay between AM Ti64 scaffolds and bone growth, which may ultimately lead to a new way of creating long lasting bone implants with fast recovery times.

Biomechanical analyses of one-piece dental implants composed of titanium, zirconia, PEEK, CFR-PEEK, or GFR-PEEK: Stresses, strains, and bone remodeling prediction by the finite element method

This work aimed to assess the biomechanics, using the finite element method (FEM), of traditional titanium Morse taper (MT) dental implants compared to one-piece implants composed of zirconia, polyetheretherketone (PEEK), carbon fiber-reinforced PEEK (CFR-PEEK), or glass fiber-reinforced PEEK (GFR-PEEK). MT and one-piece dental implants were modeled within a mandibular bone section and loaded on an oblique force using FEM. A MT implant system involving a Ti6Al4V abutment and a cp-Ti grade IV implant was compared to one-piece implants composed of cp-Ti grade IV, zirconia (3Y-TZP), PEEK, CFR-PEEK, or GFR-PEEK. Stress on bone and implants was computed and analyzed while bone remodeling prediction was evaluated considering equivalent strain. In comparison to one-piece implants, the traditional MT implant revealed higher stress peak (112 MPa). The maximum stresses on the one-piece implants reached ~80 MPa, regardless their chemical composition. MT implant induced lower bone stimulus, although excessive bone strain was recorded for PEEK implants. Balanced strain levels were noticed for reinforced PEEK implants of which CFR-PEEK one-piece implants showed proper biomechanical behavior. Balanced strain levels might induce bone remodeling at the peri-implant region while maintaining low risks of mechanical failures. However, the strength of the PEEK-based composite materials is still low for long-term clinical performance.

Predicting the Failure Risk of Internal Fixation Devices in Chinese Patients Undergoing Spinal Internal Fixation Surgery: Development and Assessment of a New Predictive Nomogram

The current study is aimed at developing and validating a nomogram of the risk of failure of internal fixation devices in Chinese patients undergoing spinal internal fixation. We collected data from a total of 1139 patients admitted for spinal internal fixation surgery at the First Affiliated Hospital of Guangxi Medical University from May 2012 to February 2019. Of these, 1050 patients were included in the spinal internal fixation group and 89 patients in the spinal internal fixation device failure group. Patients were divided into training and validation tests. The risk assessment of the failure of the spinal internal fixation device used 14 characteristics. In the training test, the feature selection of the failure model of the spinal internal fixation device was optimized using the least absolute shrinkage and selection operator (LASSO) regression model. Based on the characteristics selected in the LASSO regression model, multivariate logistic regression analysis was used for constructing the model. Identification, calibration, and clinical usefulness of predictive models were assessed using C-index, calibration curve, and decision curve analysis. A validation test was used to validate the constructed model. In the training test, the risk prediction nomogram included gender, age, presence or absence of scoliosis, and unilateral or bilateral fixation. The model demonstrated moderate predictive power with a C-index of 0.722 (95% confidence interval: 0.644-0.800) and the area under the curve (AUC) of 0.722. Decision curve analysis depicted that the failure risk nomogram was clinically useful when the probability threshold for internal fixation device failure was 3%. The C-index of the validation test was 0.761. This novel nomogram of failure risk for spinal instrumentation includes gender, age, presence or absence of scoliosis, and unilateral or bilateral fixation. It can be used for evaluating the risk of instrumentation failure in patients undergoing spinal instrumentation surgery.

Tibial implant fixation in TKA worth a revision?-how to avoid stress-shielding even for stiff metallic implants

In total knee arthroplasty (TKA), force is transmitted into the tibia by a combined plate-stem device along with cemented or cementless stem fixation. The present work analyzes this force transmission in finite element simulations with the main aim to avoid reported postsurgical bone density reduction as a consequence of a reduced tibial bone loading. In the numerical analysis different implant materials, stem/extension lengths and implant-to-stem interface conditions are considered, from a stiff fully cemented fixation to sliding contact conditions with a low friction coefficient. The impact of these variations on bone loading changes are measured by (i) decomposing the total force into parts mediated by the plate and by the stem and by (ii) post-surgery strain energy density (SED) deviations. Based on a bionics-inspired perspective on how nature in pre-operative conditions carries out force transfer from the knee joint into the tibia, a modified implant-bone interface is suggested that alters force transmission towards physiological conditions while preserving the geometries of the standard plate-stem endoprosthesis design. The key aspect is that the axial force is predominantly transmitted through the plate into proximal bone which requires a compliant bone-stem interface as realized by sliding friction conditions at a low friction coefficient. These interface conditions avoid stress shielding almost completely, preserve pre-surgery bone loading such that bone resorption is not likely to occur.

Impact of screw thread shape on stress transfer in bone: a finite element study

Gradual screw loosening is a well-known failure mechanism in internal fixation. Loosening is primarily due to progressive bone loss caused by stress shielding, a phenomenon in which a medical device absorbs a disproportionate amount of load within the screw-bone construct. The proximity of elastic moduli of magnesium and bone presents the potential for alleviating screw loosening by allowing optimum stress to be transferred between screw and bone, and in turn, supporting bone remodeling around the screw. In this study, the effect of thread profile on stress transfer in a magnesium fixation was simulated using a 2-D finite element model. Modified stress parameters from a previous study were used to estimate stress transfer across three thread profiles. Results showed highest stress transfer in trapezoidal-shaped magnesium screw thread. In accordance, this study corroborates the potential for magnesium as an ultimate screw material to eliminate progressive screw loosening.

The Effects of Various Metallic Surfaces on Cellular and Bacterial Adhesion

The effects of Ti, Nb, Ta, Zr, and Ag on cellular and bacterial adhesion were investigated in this study. Moreover, the relationships between surface compositions, metal ion release behaviors, and biological responses were examined. As a result, MC3T3-E1 cells and S. aureus were able to better attach to Ti and Zr rather than the Nb and Ta specimens. For the Ag specimen, the amount of Ag ions released into Hanks’ solution was the largest among all the specimens. Cellular and bacterial adhesion onto the Ag specimen was inhibited compared with the other specimens, because of Ag ion release. Alternatively, Nb and Ta specimens exhibited specific biological responses. Cellular adhesion on Nb and Ta specimens was similar to that on Ti, while bacterial adhesion on Nb and Ta specimens was inhibited compared with that on Ti. This study proved that Nb and Ta inhibited bacterial adhesion and exhibited no harmful effects on cellular adhesion. In addition, these results indicate that the passive layer on Nb and Ta plays a key role in the inhibition of bacterial adhesion.

Titanium-Tissue Interface Reaction and Its Control With Surface Treatment

Titanium (Ti) and its alloys are widely used for medical and dental implant devices-artificial joints, bone fixators, spinal fixators, dental implant, etc. -because they show excellent corrosion resistance and good hard-tissue compatibility (bone formation and bone bonding ability). Osseointegration is the first requirement of the interface structure between titanium and bone tissue. This concept of osseointegration was immediately spread to dental-materials researchers worldwide to show the advantages of titanium as an implant material compared with other metals. Since the concept of osseointegration was developed, the cause of osseointegration has been actively investigated. The surface chemical state, adsorption characteristics of protein, and bone tissue formation process have also been evaluated. To accelerate osseointegration, roughened and porous surfaces are effective. HA and TiO2 coatings prepared by plasma spray and an electrochemical technique, as well as alkalinization of the surface, are also effective to improve hard-tissue compatibility. Various immobilization techniques for biofunctional molecules have been developed for bone formation and prevention of platelet and bacteria adhesion. These techniques make it possible to apply Ti to a scaffold of tissue engineering. The elucidation of the mechanism of the excellent biocompatibility of Ti can provide a shorter way to develop optimal surfaces. This review should enhance the understanding of the properties and biocompatibility of Ti and highlight the significance of surface treatment.

Bone resorption triggered by high radial stress: The mechanism of screw loosening in plate fixation of long bone fractures

Screw loosening is a common complication in plate fixation. However, the underlying mechanism is unclear. This study investigated screw loosening mechanisms by finite element analysis (FEA) simulation and clinical X-ray feature analysis. Two FEA models incorporated bone heterogeneity and orthotropy, representing fracture fixation using dynamic compression plate (DCP) and locking compression plate (LCP), were developed. These models were used to examine the volume of bone exceeding a certain stress value around each screw under physiologically-relevant loading conditions. These damaged bone was then separated and compared by the axial stress and radial stress of each screw. In addition, features of patients' X-ray images showing screw loosening were analyzed to validate the loosening features simulated by the models. The FEA study showed that more damaged bone was found at the central two screws which gradually decreased toward the two end screws in all groups. More bone was damaged by the radial stress of each screw than by the axial stress. The radiological analysis of screw loosening showed that bone loss occurred at the screw closest to the fracture line first then subsequent bone loss at the screws further away from the fracture line occurred. This study found that the two screws nearest to the fracture line are more vulnerable to loosening. The radial stress of the screw plays a larger role in screw loosening than the axial stress. Bone resorption triggered by the high radial stress of screws is indicated as the mechanism of screw loosening in the diaphyseal plate fixation. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1498-1507, 2019.

The biomechanical protective effects of a treatment dressing on the soft tissues surrounding a non-offloaded sacral pressure ulcer

Patients who are immobile endure prolonged bodyweight-related compressive, tensional and shear loads at their body-support contact areas that over time may lead to the onset of pressure ulcers (PUs). Approximately, one-third of the common sacral PUs are severe and classified as category 3 or 4. If a PU has occurred, off-loading is the basic, commonly accepted clinical intervention; however, in many situations, complete off-loading of sacral PUs is not possible. Minimising the exposure of wounds and their surroundings to elevated mechanical loads is crucial for healing. Accordingly, in the present study, we aimed to investigate the biomechanical effects of the structural and mechanical properties of different treatment dressings on stresses in soft tissues surrounding a non-offloaded sacral PU in a supine patient. Using a novel three-dimensional anatomically realistic finite element modelling framework, we have compared performances of three dressing designs: (a) The Mepilex Border Sacrum (MBS) multilayer anisotropic silicone foam dressing (Mölnlycke Health Care), (b) an isotropic stiff dressing, and (c) an isotropic flexible dressing. Using our newly developed protective efficacy index (PEI) and aggravation index (AI) for assessing prophylactic and treatment dressings, we identified the anisotropic stiffness feature of the MBS dressing as a key design element.

A Quantitative Approach for the Bone-implant Osseointegration Assessment Based on Ultrasonic Elastic Guided Waves

Quantitative and reliable monitoring of osseointegration will help further evaluate the integrity of the orthopaedic construct to promote novel prosthesis design and allow early mobilisation. Quantitative assessment of the degree or the lack of osseointegration is important for the clinical management with the introduction of prosthetic implants to amputees. Acousto-ultrasonic wave propagation has been used in structural health monitoring as well as human health monitoring but so far has not extended to osseointegrated implants or prostheses. This paper presents an ultrasonic guided wave approach to assess the osseointegration of a novel implant. This study explores the potential of integrating structural health monitoring concepts into a new osseointegrated implant. The aim is to demonstrate the extension of acousto-ultrasonic techniques, which have been widely reported for the structural health monitoring of engineering structures, to assess the state of osseointegration of a bone and implant. To illustrate this potential, this paper will report on the experimental findings which investigated the unification of an aluminium implant and bone-like geometry surrogate. The core of the test specimen is filled with silicone and wrapped with plasticine to simulate the highly damped cancellous bone and soft tissue, respectively. To simulate the osseointegration process, a 2-h adhesive epoxy is used to bond the surrogate implant and a bone-like structure. A series of piezoelectric elements are bonded onto the surrogate implant to serve as actuators and sensors. The actuating piezoelectric element on an extramedullary strut is excited with a 1 MHz pulse signal. The reception of the ultrasonic wave by the sensing elements located on the adjacent and furthest struts is used to assess the integration of this implant to the parent bone structure. The study shows an Osseointegration Index can be formulated by using engineering and acousto-ultrasonic methods to measure the unification of a bone and implant. This also highlights a potential quantitative evaluation technique regardless of bone-implant geometry and soft tissue damping.

How can a short stem hip implant preserve the natural, pre-surgery force flow? A finite element analysis on a collar cortex compression concept (CO4)

The present work proposes a simple, novel fixation concept for short stem hip endoprostheses, which preserves the pre-surgery force flow through femoral bone to an unprecedented extent. It is demonstrated by finite element analyses that a standard implant model endowed with minor geometrical changes can overcome bone loading reduction and can achieve almost physiological conditions. The numerical results underpin that the key aspect of the novel, so-called "collar cortex compression concept CO4" is the direct, almost full load transmission from the implant collar to the resected femur cortex, which implies that the implant stem must be smooth and therefore interacts mainly by normal contact with the surrounding bone. For a stem endowed with surface porosity at already small areas, it is mainly the stem which transmits axial forces by shear, whereas the collar shows considerable unloading, which is the standard metaphyseal fixation. Only in the latter case the implant-bone stiffness contrast induces stress shielding, whereas for CO4 stress shielding is avoided almost completely, although the implant is made of a stiff Ti-alloy. CO4 is bionics-inspired in that it mimics force transmission at implant-bone interfaces following the natural conditions and it thereby preserves pre-surgery bone architecture as an optimized solution of nature.

Microstructures, mechanical, and biological properties of a novel Ti-6V-4V/zinc surface nanocomposite prepared by friction stir processing

Background

The interaction between the material and the organism affects the survival rate of the orthopedic or dental implant in vivo. Friction stir processing (FSP) is considered a new solid-state processing technology for surface modification.

Purpose

This study aims to strengthen the surface mechanical properties and promote the osteogenic capacity of the biomaterial by constructing a Ti-6Al-4V (TC4)/zinc (Zn) surface nanocomposites through FSP.

Methods

FSP was used to modify the surface of TC4. The microstructures and mechanical properties were analyzed by scanning electron microscopy, transmission electron microscopy, nanoindentation and Vickers hardness. The biological properties of the modified surface were evaluated by the in vitro and in vivo study.

Results

The results showed that nanocrystalline and numerous β regions, grain boundary α phase, coarser acicular α phase and finer acicular martensite α′ appeared because of the severe plastic deformation caused by FSP, resulting in a decreased elastic modulus and an increased surface hardness. With the addition of Zn particles and the enhancement of hydrophilicity, the biocompatibility was greatly improved in terms of cell adhesion and proliferation. The in vitro osteogenic differentiation of rat bone marrow stromal cells and rapid in vivo osseointegration were enhanced on the novel TC4/Zn metal matrix nanocomposite surface.

Conclusion

These findings suggest that this novel TC4/Zn surface nanocomposite achieved by FSP has significantly improved mechanical properties and biocompatibility, in addition to promoting osseointegration and thus has potential for dental and orthopedic applications.

On stress/strain shielding and the material stiffness paradigm for dental implants

BACKGROUND

Stress shielding considerations suggest that the dental implant material's compliance should be matched to that of the host bone. However, this belief has not been confirmed from a general perspective, either clinically or numerically.

PURPOSE

To characterize the influence of the implant stiffness on its functionality using the failure envelope concept that examines all possible combinations of mechanical load and application angle for selected stress, strain and displacement-based bone failure criteria. Those criteria represent bone yielding, remodeling, and implant primary stability, respectively MATERIALS AND METHODS: We performed numerical simulations to generate failure envelopes for all possible loading configurations of dental implants, with stiffness ranging from very low (polymer) to extremely high, through that of bone, titanium, and ceramics.

RESULTS

Irrespective of the failure criterion, stiffer implants allow for improved implant functionality. The latter reduces with increasing compliance, while the trabecular bone experiences higher strains, albeit of an overall small level. Micromotions remain quite small irrespective of the implant's stiffness.

CONCLUSION

The current paradigm favoring reduced implant material's stiffness out of concern for stress or strain shielding, or even excessive micromotions, is not supported by the present calculations, that point exactly to the opposite.

The improvement of corrosion resistance, biocompatibility and osteogenesis of the novel porous Mg–Nd–Zn alloy

A magnesium scaffold is a promising biodegradable bone repair material.

Simulation of mechanical behavior and optimization of simulated injection molding process for PLA based antibacterial composite and nanocomposite bone screws using central composite design

In this study, injection molding of three poly lactic acid (PLA) based bone screws was simulated and optimized through minimizing the shrinkage and warpage of the bone screws. The optimization was carried out by investigating the process factors such as coolant temperature, mold temperature, melt temperature, packing time, injection time, and packing pressure. A response surface methodology (RSM), based on the central composite design (CCD), was used to determine the effects of the process factors on the PLA based bone screws. Upon applying the method of maximizing the desirability function, optimization of the factors gave the lowest warpage and shrinkage for nanocomposite PLA bone screw (PLA9). Moreover, PLA9 has the greatest desirability among the selected materials for bone screw injection molding. Meanwhile, a finite element analysis (FE analysis) was also performed to determine the force values and concentration points which cause yielding of the screws under certain conditions. The Von-Mises stress distribution showed that PLA9 screw is more resistant against the highest loads as compared to the other ones. Finally, according to the results of injection molding simulations, the design of experiments (DOE) and structural analysis, PLA9 screw is recommended as the best candidate for the production of biomedical materials among all the three types of screws.

Innovative approach in the development of computer assisted algorithm for spine pedicle screw placement

Pedicle screws are typically used for fusion, percutaneous fixation, and means of gripping a spinal segment. The screws act as a rigid and stable anchor points to bridge and connect with a rod as part of a construct. The foundation of the fusion is directly related to the placement of these screws. Malposition of pedicle screws causes intraoperative complications such as pedicle fractures and dural lesions and is a contributing factor to fusion failure. Computer assisted spine surgery (CASS) and patient-specific drill templates were developed to reduce this failure rate, but the trajectory of the screws remains a decision driven by anatomical landmarks often not easily defined. Current data shows the need of a robust and reliable technique that prevents screw misplacement. Furthermore, there is a need to enhance screw insertion guides to overcome the distortion of anatomical landmarks, which is viewed as a limiting factor by current techniques. The objective of this study is to develop a method and mathematical lemmas that are fundamental to the development of computer algorithms for pedicle screw placement. Using the proposed methodology, we show how we can generate automated optimal safe screw insertion trajectories based on the identification of a set of intrinsic parameters. The results, obtained from the validation of the proposed method on two full thoracic segments, are similar to previous morphological studies. The simplicity of the method, being pedicle arch based, is applicable to vertebrae where landmarks are either not well defined, altered or distorted.

The in vitro and in vivo performance of a strontium-containing coating on the low-modulus Ti35Nb2Ta3Zr alloy formed by micro-arc oxidation

The β-titanium alloy is thought to be a promising alloy using as orthopedic or dental implants owing to its characteristics, which contains low elastic modulus, high corrosion resistance and well biocompatibility. Our previous study has reported that a new β-titanium alloy Ti35Nb2Ta3Zr showed low modulus close to human bone, equal tissue compatibility to a traditional implant alloy Ti6Al4V. In this study, micro-arc oxidation (MAO) was applied on the Ti35Nb2Ta3Zr alloy to enhance its surface characteristics and biocompatibility and osseointegration ability. Two different coatings were formed, TiO2 doped with calcium-phosphate coating (Ca-P) and calcium-phosphate-strontium coating (Ca-P-Sr). Then we evaluated the effects of the MAO coatings on the Ti35Nb2Ta3Zr alloy through in vitro and in vivo tests. As to the characteristics of the coatings, the morphology, chemical composition, surface roughness and contact angle of MAO coatings were tested by scanning electron microscopy, energy dispersive spectroscopy, atomic force microscopy, and video contact-angle measurement system respectively. Besides, we performed MTT assay, ALP test and cell morphology-adhesion test on materials to evaluate the MAOed coating materials' biocompatibility in vitro. The in vivo experiment was performed through rabbit model. Alloys were implanted into rabbits' femur shafts, then we performed micro-CT, histological and sequential fluorescent labeling analysis to evaluate implants' osseointegration ability in vivo. Finally, the Ca-P specimens and Ca-P-Sr specimens exhibited a significant enhancement in surface roughness, hydrophilicity, cell proliferation, cell adhesion. More new bone was found around the Ca-P-Sr coated alloy than Ca-P coated alloy and Ti35Nb2Ta3Zr alloy. In conclusion, the MAO treatment improved in vitro and in vivo performance of Ti35Nb2Ta3Zr alloy. The Ca-P-Sr coating may be a promising modified surface formed by MAO for the novel β-titanium alloy Ti35Nb2Ta3Zr.

Implant design and its effects on osseointegration over time within cortical and trabecular bone

Healing chambers present at the interface between implant and bone have become a target for improving osseointegration. The objective of the present study was to compare osseointegration of several implant healing chamber configurations at early time points and regions of interest within bone using an in vivo animal femur model. Six implants, each with a different healing chamber configuration, were surgically implanted into each femur of six skeletally mature beagle dogs (n = 12 implants per dog, total n = 72). The implants were harvested at 3 and 5 weeks post-implantation, non-decalcified processed to slides, and underwent histomorphometry with measurement of bone-to-implant contact (BIC) and bone area fraction occupied (BAFO) within healing chambers at both cortical and trabecular bone sites. Microscopy demonstrated predominantly woven bone at 3 weeks and initial replacement of woven bone by lamellar bone by 5 weeks. BIC and BAFO were both significantly increased by 5 weeks (p < 0.001), and significantly higher in cortical than trabecular bone (p < 0.001). The trapezoidal healing chamber design demonstrated a higher BIC than other configurations. Overall, a strong temporal and region-specific dependence of implant osseointegration in femurs was noted. Moreover, the findings suggest that a trapezoidal healing chamber configuration may facilitate the best osseointegration. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1091-1097, 2016.

Three Dimensional Printing of Titanium for Bone Tissue Engineering Applications: A Preliminary Study

One of the main goals of bone tissue engineering is the development of scaffolds that mimic both functional and structural properties of native bone itself. This study describes the preliminary work carried out to assess the viability of using three dimensional printing (3DP) technology for the fabrication of porous titanium scaffolds with lowered modulus and improved biocompatibility. 3DP enables the manufacturing of three dimensional (3D) objects with a defined structure directly from a Computer Aided Design (CAD). The overall porosity of the 3D structures is contributed by the presence of both pores-by-process (PBP) and pores-by-design (PBD). This study mainly focuses on the PBP, which are formed during the sintering step as the result of the removal of the binding agent polyvinyl alcohol (PVA). Sintering temperatures of 1250oC, 1350oC and 1370oC were used during the fabrication process. Our results showed that by varying the binder percentage and the sintering temperature, pores with diameters in the range of approximately 17-24 μm could be reproducibly achieved. Other physical properties such as surface roughness, porosity and average pore size were also measured for all sample groups. Results from subsequent cell culture studies using adipose tissue-derived mesenchymal stem cells (ASCs) showed improved attachment, viability and proliferation for the 3DP titanium samples as compared to the two-dimensional (2D) dense titanium samples. Hence, based on our current preliminary studies, 3DP technology can potentially be used to fabricate customized, patient-specific metallic bone implants with lowered modulus. This can effectively help in prevention of stress-shielding, and enhancement of implant fixationin vivo. It is envisioned that an optimized combination of binder percentage and sintering temperature can result in the fabrication of scaffolds with the desired porosity and mechanical properties to fit the intended clinical application.

Influence of bone insertion level of the implant on the fracture strength of different connection designs: an in vitro study

OBJECTIVE

The aim of the present in vitro study was to assess resistance to static fatigue of implants with different connections at various insertion levels.

MATERIALS AND METHODS

Sixty implants and abutments were used with the smallest diameter of each model. Four groups (n = 15) were created on the basis of the implant design and connection: cylindrical external hexagon Ø3.30 mm (group 1), cylindrical internal hexagon Ø3.30 mm (group 2), conical internal hexagon Ø3.50 mm (group 3), and conical Morse taper Ø3.50 mm (group 4). Three insertion levels in resin were tested, 0 mm at the platform level (l1), 3 mm (l2), and 5 mm (l3) above the platform of the resin. All groups were subjected to quasi-static loading at 30° to the implant axis in a universal machine.

RESULTS

The mean fracture strengths for group 1 were 1,991 N (l1), 1,020 N (l2), and 767 N (l3); for group 2: 2,119 N (l1), 1,034 N (l2), and 903 N (l3); for group 3: 2,373 N (l1), 1,407 N (l2), and 929 N (l3); and for group 4: 1,710 N (l1), 1,680 N (l2), and 1,182 N (l3).

CONCLUSIONS

Resistance to loading decreases significantly with the loss of insertion, and the connection design between the implants and abutments can change the performance and resistance of the system.

CLINICAL RELEVANCE

When implants are used in areas where there is a possibility of bone loss, the selection of a connection type is an important consideration for the longevity of the system.

Biodegradable Orthopedic Magnesium-Calcium (MgCa) Alloys, Processing, and Corrosion Performance

Magnesium-Calcium (Mg-Ca) alloy has received considerable attention as an emerging biodegradable implant material in orthopedic fixation applications. The biodegradable Mg-Ca alloys avoid stress shielding and secondary surgery inherent with permanent metallic implant materials. They also provide sufficient mechanical strength in load carrying applications as opposed to biopolymers. However, the key issue facing a biodegradable Mg-Ca implant is the fast corrosion in the human body environment. The ability to adjust degradation rate of Mg-Ca alloys is critical for the successful development of biodegradable orthopedic implants. This paper focuses on the functions and requirements of bone implants and critical issues of current implant biomaterials. Microstructures and mechanical properties of Mg-Ca alloys, and the unique properties of novel magnesium-calcium implant materials have been reviewed. Various manufacturing techniques to process Mg-Ca based alloys have been analyzed regarding their impacts on implant performance. Corrosion performance of Mg-Ca alloys processed by different manufacturing techniques was compared. In addition, the societal and economical impacts of developing biodegradable orthopedic implants have been emphasized.

Surface integrity of biodegradable Magnesium-Calcium orthopedic implant by burnishing

Magnesium-Calcium (MgCa) alloy as an emerging biodegradable implant material has received considerable attention in orthopedic fixation applications. The biodegradable MgCa alloys avoid stress shielding and secondary surgery inherent with permanent metallic implant materials. They also provide sufficient mechanical strength in load carrying applications as opposed to biopolymers. However, the key issue facing a biodegradable MgCa implant is the fast corrosion in the human body environment. The ability to adjust the degradation rate of MgCa alloys is critical in the successful development of biodegradable orthopedic materials. Burnishing as a low plastic deformation process is a promising technique to tune surface integrity of MgCa implant surface for biodegradation control. However, the poor ductility of MgCa alloys imposes a great challenge for burnishing. This study focuses on the basic understanding of surface mechanical behavior of burnished biodegradable MgCa0.8 (wt%) alloy. The effects of burnishing parameters, i.e., pressure, feed, speed, number of path, and burnishing pattern on surface integrity factors such as surface topography, roughness, microhardness, microstructure, and residual stresses are investigated. The burnished surfaces are shinier and smoother than the as-machined ones. The MgCa alloy can be safely burnished at suitable burnishing conditions since no cracks are produced at the surface and in the subsurface. The microstructure including grain size does not show a noticeable change after burnishing. The machined surfaces are harder than the burnished ones down to the deep subsurface (∼200 μm) as opposed to the shallow hardened depth (∼50 μm) in cutting. Residual stresses are highly compressive especially at low burnishing pressure.

Fabrication and Characterization of Surface Texture for Bone Ingrowth by Sequential Laser Peening Biodegradable Orthopedic Magnesium-Calcium Implants

Biodegradable magnesium-calcium (Mg–Ca) implants have the ability to gradually dissolve and absorb into the human body after implantation. The similar mechanical properties to bone indicate that Mg–Ca is an ideal implant material to minimize the negative effects of stress shielding. Furthermore, using a biodegradable Mg–Ca implant prevents the need for a secondary removal surgery that commonly occurs with permanent metallic implants. The critical issue that hinders the application of Mg–Ca implants is the poor corrosion resistance to human body fluids. The corrosion process adversely affects bone ingrowth that is critical for recovery. Therefore, sequential laser shock peening (LSP) of a biodegradable Mg–Ca alloy was initiated to create a superior surface topography for improving implant performance. LSP is an innovative treatment to fabricate functional patterns on the surface of an implant. A patterned surface promotes bone ingrowth by providing a rough surface texture. Also, LSP imparts deep compressive residual stresses below the surface, which could potentially slow corrosion rates. Unique surface topographies were fabricated by changing the laser power and peening overlap ratio. The resultant effects on surface topography were investigated. Sequential peening at higher overlap ratios (75%) was found to reduce the tensile pileup region by over 40% as well as compress the overall surface by as much as 35 μm.

Parametric analysis of orthopedic screws in relation to bone density

A global study of geometry and material properties of orthopedic screws was performed, considering not only the effect of each single factor (screw pitch, number of threads, fillet angle, etc.) but also their interactions with respect to bone density.

The stress patterns resulting from different screw geometries and bone densities were analyzed using finite element techniques, taking into account different levels of osseointegration between the screw and the bone. These numerical models where validated through experimental pull-out tests, where a pull out force of 120 N produced localized failure of the last thread (stresses above 0.42 MPa). The results of the numerical simulations were then summarised using a multi-factorial parametric analysis. This demonstrated the great relevance of the interaction between bone density and screw pitch, showing that the optimal screw pitch can vary by more than 25% for different densities (0.35 g/cm³ and 0.47 g/cm³, respectively).

The parameters calculated by means of the multi-factorial analysis allow the pull out force to be estimated for different osseointegration levels, different screw geometries and material properties, and for different bone densities. The final objective is to determine the best choice of implant for each individual patient.

Contribution of loading conditions and material properties to stress shielding near the tibial component of total knee replacements

This communication reports important preliminary results of a parametric analysis into the stress shielding effects of loading conditions and material properties of a total knee replacement (TKR) prosthesis. A previously developed finite element (FE) model of the proximal tibia that incorporated orthotropic and heterogeneous bone properties was used. Tibiofemoral joint compression and soft tissue (ligament and muscle) forces were also included to better represent the loading condition in the tibia. Stress shielding effects were studied for a prosthesis similar to a commercially available model. Results from the model show that the hypothesis of relatively higher Young's modulus of implant compared to bone as the primary cause of stress shielding is not sufficiently descriptive. Loading conditions as a result of altered bone or implant condylar surface geometry, load placement on the condylar surface, and load pattern created by the TKR are at least as important or, in some cases, more important factors in observed stress shielding immediately post-operation. This finding can be used to focus new implant design on altered loading conditions as well as material selection.

A method of quantification of stress shielding in the proximal femur using hierarchical computational modeling

Stress shielding is a biomechanical phenomenon causing adaptive changes in bone strength and stiffness around metallic implants, which potentially lead to implant loosening. Accordingly, there is a need for standard, objective engineering measures of the "stress shielding" performances of an implant that can be employed in the process of computer-aided implant design. To provide and test such measures, we developed hierarchical computational models of adaptation of the trabecular microarchitecture at different sites in the proximal femur, in response to insertion of orthopaedic screws and in response to hypothetical reductions in hip joint and gluteal muscle forces. By identifying similar bone adaptation outcomes from the two scenarios, we were able to quantify the stress shielding caused by screws in terms of analogous hypothetical reductions in hip joint and gluteal muscle forces. Specifically, we developed planar lattice models of trabecular microstructures at five regions of interest (ROI) in the proximal femur. The homeostatic and abnormal loading conditions for the lattices were determined from a finite element model of the femur at the continuum scale and fed to an iterative algorithm simulating the adaptation of each lattice to these loads. When screws were inserted to the femur model, maximal simulated bone loss (17% decrease in apparent density, 10% decrease in thickness of trabeculae) was at the greater trochanter and this effect was equivalent to the effect of 50% reduction in gluteal force and normal hip joint force. We conclude that stress shielding performances can be quantified for different screw designs using model-predicted hypothetical musculoskeletal load fractions that would cause a similar pattern and extent of bone loss to that caused by the implants.

Osteoprogenitor response to semi-ordered and random nanotopographies

In bone tissue engineering, it is desirable to use materials to control the differentiation of mesenchymal stem cell populations in order to gain direct bone apposition to implant materials. It has been known for a number of years that microtopography can alter cell adhesion, proliferation and gene expression. More recently, the literature reveals that nanotopography is also of importance. Here, the reaction of primary human osteoprogenitor cell populations to nanotopographies down to 10 nm in size is considered. The topographies were originally produced by colloidal lithography and polymer demixing on silicon and then embossed (through an intermediate nickel shim) into polymethylmethacrylate. The biological testing considered cell morphology (image analysis of cell spreading and scanning electron microscopy), cell cytoskleton and adhesion formation (fluorescent staining of actin, tubulin, vimentin and vinculin) and then subsequent cell growth and differentiation (fluorescent staining of osteocalcin and osteopontin). The results demonstrated that the nanotopographies stimulated the osteoprogenitor cell differentiation towards an osteoblastic phenotype.

Biomechanics of bone-fracture fixation by stiffness-graded plates in comparison with stainless-steel plates

BACKGROUND

In the internal fixation of fractured bone by means of bone-plates fastened to the bone on its tensile surface, an on-going concern has been the excessive stress-shielding of the bone by the excessively-stiff stainless-steel plate. The compressive stress-shielding at the fracture-interface immediately after fracture-fixation delays callus formation and bone healing. Likewise, the tensile stress-shielding of the layer of the bone underneath the plate can cause osteoporosis and decrease in tensile strength of this layer.

METHOD

In order to address this problem, we propose to use stiffness-graded plates. Accordingly, we have computed (by finite-element analysis) the stress distribution in the fractured bone fixed by composite plates, whose stiffness is graded both longitudinally and transversely.

RESULTS

It can be seen that the stiffness-graded composite-plates cause less stress-shielding (as an example: at 50% of the healing stage, stress at the fracture interface is compressive in nature i.e. 0.002 GPa for stainless steel plate whereas stiffness graded plates provides tensile stress of 0.002 GPa. This means that stiffness graded plate is allowing the 50% healed bone to participate in loadings). Stiffness-graded plates are more flexible, and hence permit more bending of the fractured bone. This results in higher compressive stresses induced at the fractured faces accelerate bone-healing. On the other hand, away from the fracture interface the reduced stiffness and elastic modulus of the plate causes the neutral axis of the composite structure to be lowered into the bone resulting in the higher tensile stress in the bone-layer underneath the plate, wherein is conducive to the bone preserving its tensile strength.

CONCLUSION

Stiffness graded plates (with in-built variable stiffness) are deemed to offer less stress-shielding to the bone, providing higher compressive stress at the fractured interface (to induce accelerated healing) as well as higher tensile stress in the intact portion of the bone (to prevent bone remodeling and osteoporosis).