Abstract. Objectives. Young patients receiving metallic bone implants after surgical resection of bone cancer require implants that last into adulthood, and ideally life-long. Porous implants with similar stiffness to bone can promote bone ingrowth and thus beneficial clinical outcomes. A mechanical remodelling stimulus, strain energy density (SED), is thought to be the primary control variable of the process of bone growth into porous implants. The sequential process of bone growth needs to be taken into account to develop an accurate and validated bone remodelling algorithm, which can be employed to improve porous implant design and achieve better clinical outcomes. Methods. A bone remodelling algorithm was developed, incorporating the concept of bone connectivity (sequential growth of bone from existing bone) to make the algorithm more physiologically relevant. The algorithm includes adaptive elastic modulus based on apparent bone density, using a node-based model to simulate local remodelling variations while alleviating numerical checkerboard problems. Strain energy density (SED) incorporating stress and strain effects in all directions was used as the primary stimulus for bone remodelling. The simulations were developed to run in MATLAB interfacing with the commercial FEA software ABAQUS and Python. The algorithm was applied to predict bone ingrowth into a porous implant for comparison against data from a sheep model. Results. The accuracy of the predicted bone remodelling was verified for standard loading cases (bending, torsion) against analytical calculations. Good convergence was achieved. The algorithm predicted good bone remodelling and growth into the investigated porous implant. Using the standard algorithm without connectivity, bone started to remodel at locations unconnected to any bone, which is physiologically implausible. The implementation of bone connectivity ensures the gradual process of bone growth into the implant pores from the sides. The bone connectivity algorithm predicted that the full remodelling required more time (approximately 50% longer), which should be considered when developing post-surgical rehabilitation strategies for patients. Both algorithms with and without bone connectivity implementation converged to same final stiffness (less than 0.01% difference). Almost all nodes reached the same density value, with only a limited number of nodes (less than 1%) in transition areas with a strong density gradient having noticeable differences. Conclusions. An improved bone remodelling algorithm based on strain energy density that modelled the sequential process of bone growth has been developed and tested. For a porous metallic bone implant the same final bone density distribution as for the original adaptive elasticity theory was predicted, with a slower and more fidelic process of growth from existing surrounding bone into the porous implant. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

The effect of zoledronic acid on bone ingrowth was examined in an animal model in which porous tantalum implants were placed bilaterally within the ulnae of seven dogs. Zoledronic acid in saline was administered via a single post-operative intravenous injection at a dose of 0.1 mg/kg. The ulnae were harvested six weeks after surgery. Undecalcified transverse histological sections of the implant-bone interfaces were imaged with backscattered scanning electron microscopy and the percentage of available pore space that was filled with new bone was calculated. The mean extent of bone ingrowth was 6.6% for the control implants and 12.2% for the zoledronic acid-treated implants, an absolute difference of 5.6% (95% confidence interval, 1.2 to 10.1) and a relative difference of 85% which was statistically significant. Individual islands of new bone formation within the implant pores were similar in number in both groups but were 69% larger in the zoledronic acid-treated group. The bisphosphonate zoledronic acid should be further investigated for use in accelerating or enhancing the biological fixation of implants to bone

The use of lumbar fusion procedures in the USA and Europe has rapidly increased over the last decade and a large number of these procedures involve the use of bone grafts. Despite of technical progress of spinal surgery and operative materials the risk of vertebral fusion failure occurs in 5 – 35 % of cases. Autografting has been considered the gold standard for bone graft procedures. However, the harvesting from the iliac crest can be associated with short and long-term morbidity in up to 22 % of cases. Main goal of this experimental study was to compare newly developed hybrid biodegradable nanocomposit porous implant (HBNPI) against bone craft from iliac crest as a new and better alternative for lumbar interbody fusion. 24 male pigs 4 months old weighting around 40 Kg were included in our study. These pigs were divided into two study groups depending on fusion method. Group A – 12 pigs underwent lateral lumbal interbody fusion (L2/3) with implantation of iliac crest bonegraft. Group B - 12 pigs underwent lateral lumbal interbody fusion (L2/3) with newly developed HBNPI. Each group were divided into two subgroups from these 6 spines were harvested 8 weeks (subgroup A1, B1) and 6 spines 16 weeks (group A2, B2) after surgery. After sacrifice, the lumbar spines were taking out and micro-CT, biomechanical testing and histomorphological analysis in all groups were performed to evaluate a quality of intervertebral fusion. As controls (group N), 6 cadaveric intact lumbar spines underwent biomechanical, micro-CT and histological testing. All 24 animals recovered from general anesthesia without unusual events. The operations lasted between 50–90 minutes (mean 70) in Group A and between 35–72 minutes (mean 43) in Group B. All of the pigs from group A could stand up and were mobile within 20 hours (range 7–20). When bone graft harvesting was not necessary (group B) this time was shortened, ranging from 1 – 1,5 hour. All pigs from Group A were limping on the first postoperative day. No limping animal was observed in group B. Total body weight of the pigs increased from 37 kg (range 36–40) at the start to 85 (range 80–89) at sacrifice. Biomechanics evaluation shows that extension flexural stiffness values are statistically significantly different between A2 (16 weeks post-implant) and A1 (8 weeks post-implant). Group A2 achieves higher values than Group A1, which is attributed to the adhesion of the implant to the surrounding vertebrae. Similarly, this also applies to groups B2 and B1. The flexural stiffness at group B2 extension is statistically significantly higher than the A2 group and also than the native N group. Biomechanical evaluation supports findings on micro-CT and histological specimens, where both adjacent vertebrae are completely fused in groups B2, unlike in group A2, where there is no or incomplete fusion. Newly developed HBNPI represents new possibility how to do intervertebral fusion, and simultaneous become chance how to improve and accelerate bone healing process against standard procedures

Revision surgeries for orthopaedic infections are done in two stages – one surgery to implant an antibiotic spacer to clear the infection and another to install a permanent implant. A permanent porous implant, that can be loaded with antibiotics and allow for single-stage revision surgery, will benefit patients and save healthcare resources. Gyroid structures can be constructed with high porosity, without stress concentrations that can develop in other period porous structures [1] [2]. The purpose of this research is to compare the resulting bone and prosthesis stress distributions when porous versus solid stems are implanted into three proximal humeri with varying bone densities, using finite element models (FEM). Porous humeral stems were constructed in a gyroid structure at porosities of 60%, 70%, and 80% using computer-aided design (CAD) software. These CAD models were analyzed using FEM (Abaqus) to look at the stress distributions within the proximal humerus and the stem components with loads and boundary conditions representing the arm actively maintained at 120˚ of flexion. The stem was assumed to be made of titanium (Ti6Al4V). Three different bone densities were investigated, representing a healthy, an osteopenic, and an osteoporotic humerus, with an average bone shape created using a statistical shape and density model (SSDM) based on 75 cadaveric shoulders (57 males and 18 females, 73 12 years) [3]. The Young's moduli (E) of the cortical and trabecular bones were defined on an element-by-element basis, with a minimum allowable E of 15 MPa. The Von Mises stress distributions in the bone and the stems were compared between different stem scenarios for each bone density model. A preliminary analysis shows an increase in stress values at the proximal-lateral region of the humerus when using the porous stems compared to the solid stem, which becomes more prominent as bone density decreases. With the exception of a few mesh dependent singularities, all three porous stems show stress distributions below the fatigue strength of Ti-6Al-4V (410 MPa) for this loading scenario when employed in the osteopenic and osteoporotic humeri [4]. The 80% porosity stem had a single strut exceeding the fatigue strength when employed in the healthy bone. The results of this study indicate that the more compliant nature of the porous stem geometries may allow for better load transmission through the proximal humeral bone, better matching the stress distributions of the intact bone and possibly mitigating stress-shielding effects. Importantly, this study also indicates that these porous stems have adequate strength for long-term use, as none were predicted to have catastrophic failure under the physiologically-relevant loads. Although these results are limited to a single boney geometry, it is based on the average shape of 75 shoulders and different bone densities are considered. Future work could leverage the shape model for probabilistic models that could explore the effect of stem porosity across a broader population. The development of these models are instrumental in determining if these structures are a viable solution to combatting orthopaedic implant infections

Introduction

Many surgeons are reluctant to use a constrained liner at the time of acetabular component revision given concerns this might result in early acetabular component loosening. We hypothesized that with appropriate initial implant stabilization of highly porous acetabular components with supplemental screw fixation, constrained liners could be safely used at the time of acetabular revision.

Aims

Unicompartmental and total knee arthroplasty (UKA and TKA) are successful treatments for osteoarthritis, but the solid metal implants disrupt the natural distribution of stress and strain which can lead to bone loss over time. This generates problems if the implant needs to be revised. This study investigates whether titanium lattice UKA and TKA implants can maintain natural load transfer in the proximal tibia.

Abstract. Objectives. Currently, total hip replacement surgery is an effective treatment for osteoarthritis, where the damaged hip joint is replaced with an artificial joint. Stress shielding is a mechanical phenomenon that refers to the reduction of bone density as a result of altered stresses acting on the host bone. Due to solid metallic nature and high stiffness of the current orthopaedic prostheses, surrounding bones undergo too much bone resorption secondary to stress shielding. With the use of 3D printing technology such as selective laser melting (SLM), it is now possible to produce porous graded microstructure hip stems to mimics the surrounding bone tissue properties. Method. In this study we have compared the physical and mechanical properties of two triply periodic minimal surface (TPMS) lattice structure namely gyroid and diamond TPMS. Based on initial investigations, it was decided to design, and 3D print the gyroid and diamond scaffolds having pore size of 800 and 1100 um respectively. Scaffold of each type of structure were manufactured and were tested mechanically in compression (n=8), tension (n=5) and bending (n=1). Results. Upon FEA validation of the scaffold in Abaqus, the desired scaffold for hip implant application was evaluated to have a young's modules of 12.15 GPa, yield strength of 242 MPa and porosity of 55%. Topology and lattice optimization were performed using nTopology to design an optimised graded porous hip implant based on stress shielding reduction. It was understood that the designed optimised hip implant can reduce the stress shielding effect by more than 65% when compared to the conventional generic implant. Conclusions. The designed hip implant presented in this work shows clinical promise in reducing bone loss while having enhanced osseointegration with the surrounding cortical bones. Hence, this will help reduce the risk of periprosthetic fracture and the probability of revision surgery

The purpose of this study was to compare the clinical and radiological findings in patients with avascular necrosis of the femoral head after management with vascularized fibular graft (VFG) compared to porous tantalum implant. The study included 60 hips in 50 patients, who were followed prospectively for 2–4 years. 28 patients (30 hips) were treated with VFG and 22 patients (30 hips) were treated with a porous tantalum implant. The two patient groups were matched for age, gender, etiology, pre-operative stage and Harris Hip Score (HHS). Of the VFG group, 14 hips were stage II and 16 were stage III, while of the tantalum group, 1 hip was stage I, 11 hips were stage II and 18 hips were stage III, according to Steinberg classification system. Mean operative time was 3 hours for VFG and 30 min for porous tantalum. At final follow-up, there was no significant statistical difference in the radiological stage (p=0.246), and radiological progression of the disease (p=0.329) was observed between these two groups. Using HHS, the mean clinical results improved in the VFG group from 61 points preoperatively to 90 points at final follow-up, while in tantalum group HHS improved from 63 to 81 points (p=0.022). Three hips from each group underwent total hip arthroplasty. The results of the present study suggest that although the management of AVN with VFG appears to show better results compared to the use of porous tantalum using clinical evaluation Methods: (eg HHS), further controlled studies with larger patient groups and longer follow-up are required

The purpose of this study was to evaluate the clinical and radiological findings in patients with avascular necrosis of the femoral head after implantation of a porous tantalum implant combined with autologous growth factors (AGF). The study included 40 hips in 30 patients. Of the 40 hips, 1 hip was stage I, 9 hips were stage II, 22 hips were stage III, and 8 hips were stage IV, according to the Steinberg classification system. Follow-up was 2 to 4 years. The porous tantalum (diameter of 10 mm, length ranging from 70 –110 mm) was implanted in the center of the necrotic area under fluoroscopic control. Clinical evaluation using the Harris hip score showed an increase from 66 to 96 points preoperatively to postoperatively in hips with stage II disease, from 60 to 85 points in hips with stage III, and from 60 to 80 points in hips with stage IV (p< 0,001). Of the 40 hips, the radiological stage in 20 hips (50%) remained stable, 17 (42.5%) progressed to an advanced stage and 3 hips (7.5%) underwent total hip arthroplasty (p< 0,001). Of the 17 hips that progressed, 7 were stage II, 7 were stage III, and 3 were stage IV. No material damage or implant migration was observed. Although failures are still observed with the use of porous tantalum, the minimal invasive technique and short operative time make tantalum a reasonable alternative in the treatment of AVN of the femoral head

In rabbits and goats, test implants with a porous surface of two layers of Tl-6A;-4V beads were examined at intervals for bond strength with bone. Half of the implants were coated with hydroxyapatite by plasma spray. The bonding strength with bone in the coated specimens was about four times greater than that of the uncoated specimens at two weeks, and twice as strong at six weeks. Twelve weeks after implantation, the strengths were similar. The hydroxyapatite coating of the beads provided earlier and stronger fixation.

Although 3D-printed porous dental implants may possess improved osseointegration potential, they must exhibit appropriate fatigue strength. Finite element analysis (FEA) has the potential to predict the fatigue life of implants and accelerate their development. This work aimed at developing and validating an FEA-based tool to predict the fatigue behavior of porous dental implants. Test samples mimicking dental implants were designed as 4.5 mm-diameter cylinders with a fully porous section around bone level. Three porosity levels (50%, 60% and 70%) and two unit cell types (Schwarz Primitive (SP) and Schwarz W (SW)) were combined to generate six designs that were split between calibration (60SP, 70SP, 60SW, 70SW) and validation (50SP, 50SW) sets. Twenty-eight samples per design were additively manufactured from titanium powder (Ti6Al4V). The samples were tested under bending compression loading (ISO 14801) monotonically (N=4/design) to determine ultimate load (F. ult. ) (Instron 5866) and cyclically at six load levels between 50% and 10% of F. ult. (N=4/design/load level) (DYNA5dent). Failure force results were fitted to F/F. ult. = a(N. f. ). b. (Eq1) with N. f. being the number of cycles to failure, to identify parameters a and b. The endurance limit (F. e. ) was evaluated at N. f. = 5M cycles. Finite element models were built to predict the yield load (F. yield. ) of each design. Combining a linear correlation between FEA-based F. yield. and experimental F. ult. with equation Eq1 enabled FEA-based prediction of F. e. . For all designs, F. e. was comprised between 10% (all four samples surviving) and 15% (at least one failure) of F. ult. The FEA-based tool predicted F. e. values of 11.7% and 12.0% of F. ult. for the validation sets of 50SP and 50SW, respectively. Thus, the developed FEA-based workflow could accurately predict endurance limit for different implant designs and therefore could be used in future to aid the development of novel porous implants. Acknowledgements: This study was funded by EU's Horizon 2020 grant No. 953128 (I-SMarD). We gratefully acknowledge the expert advice of Prof. Philippe Zysset

Aims. The purpose of this study was to evaluate the biological fixation of a 3D printed porous implant, with and without different hydroxyapatite (HA) coatings, in a canine model. Materials and Methods. A canine transcortical model was used to evaluate the characteristics of bone ingrowth of Ti6Al4V cylindrical implants fabricated using laser rapid manufacturing (LRM). At four and 12 weeks post-implantation, we performed histological analysis and mechanical push-out testing on three groups of implants: a HA-free control (LRM), LRM with precipitated HA (LRM-PA), and LRM with plasma-sprayed HA (LRM-PSHA). Results. Substantial bone ingrowth was observed in all LRM implants, with and without HA, at both time periods. Bone ingrowth increased from 42% to 52% at four weeks, to 60% to 65% at 12 weeks. Mechanical tests indicated a minimum shear fixation strength of 20 MPa to 24 MPa at four weeks, and 34 MPa to 40 MPa at 12 weeks. There was no significant difference in the amount of bone ingrowth or in the shear strength between the three implant types at either time period. Conclusion. At four and 12 weeks, the 3D printed porous implants exhibited consistent bone ingrowth and high mechanical shear strength. Based on the results of this study, we confirmed the suitability of this novel new additive manufacturing porous material for biological fixation by bone ingrowth. Cite this article: Bone Joint J 2019;101-B(6 Supple B):62–67

3D printing acetabular cups offers the theoretical advantage of enhanced bony fixation due to greater design control of the porous implant surfaces. Analysing retrieved 3D printed implants can help determine whether this design intent has been achieved. We sectioned 14 off-the-shelf retrieved acetabular cups for histological analysis; 7 cups had been 3D printed and 7 had been conventionally manufactured. Some of the most commonly used contemporary designs were represented in both groups, which were removed due to either aseptic loosening, unexplained pain, infection or dislocation. Clinical data was collected for all implants, including their age, gender, and time to revision. Bone ingrowth was evaluated using microscopic assessment and two primary outcome measures: 1) bone area fraction and 2) extent of bone ingrowth. The additively manufactured cups were revised after a median (IQR) time of 24.9 months (20.5 to 45.6) from patients with a median (IQR) age of 61.1 years (48.4 to 71.9), while the conventional cups had a median (IQR) time to revision of 46.3 months (34.7 to 49.1, p = 0.366) and had been retrieved from patients with a median age of 66.0 years (56.9 to 68.9, p = 0.999). The additively and conventionally manufactured implants had a median (IQR) bone area fraction of 65.7% (36.4 to 90.6) and 33.9% (21.9 to 50.0), respectively (p < 0.001). A significantly greater amount of bone ingrowth was measured into the backside of the additively manufactured acetabular cups, compared to their conventional counterparts (p < 0.001). Bone occupied a median of 60.0% and 5.7% of the porous depth in the additively manufactured and conventional cups, respectively. 3D printed components were found to achieve a greater amount of bone ingrowth than their conventionally manufactured counterparts, suggesting that the complex porous structures generated through this manufacturing technique may encourage greater osteointegration

Bioabsorbable metals hold a lot of potential as orthopaedic implant materials. Three metal families are currently being investigated: iron (Fe), magnesium (Mg) and zinc (Zn). Currently, however, biodegradation of such implants is poorly predictable. We thus used Direct Metal Printing to additively manufacture porous implants of a standardized bone-mimetic design and evaluated their mechanical properties and degradation behaviour, respectively, under in vivo-like conditions. Atomized powder was manufactured to porous implants of repetitive diamond unit cells, using a ProX DMP 320 (Layerwise, Belgium) or a custom-modified ReaLizer SLM50 metal printer. Degradation behaviour was characterized under static and dynamic conditions in a custom-built bioreactor system (37ºC, 5% CO. 2. and 20% O. 2. ) for up of 28 days. Implants were characterized by micro-CT before and after in vivo-like degradation. Mechanical characterization (according to ISO 13314: 2011) was performed on an Instron machine (10kN load cell) at different immersion times in simulated body fluid (r-SBF). Morphology and composition of degradation products were analysed (SEM, JSM-IT100, JEOL). Topographically identical titanium (Ti-6Al-4V, Ti64) specimen served as reference. Micro-CT analyses confirmed average strut sizes (420 ± 4 μm), and porosity (64%), to be close to design values. After 28 days of in vivo-like degradation, scaffolds were macroscopically covered by degradation products in an alloy-specific manner. Weight loss after cleaning also varied alloy-specifically, as did the change in pH value of the r-SBF. Corrosion time-dependent changes in Young's moduli from 1200 to 800 MPa for Mg, 1000 to 700 MPa for Zn and 48-8 MPa for iron were statistically significant. In summary, DMP allows to accurately control interconnectivity and topology of implants from all three families and micro-structured design holds potential to optimize their degradation speed. This first systematic report sheds light into how design influences degradation behaviour under in vivo-like conditions to help developing new standards for future medical device evaluation

Introduction. The ability to create patient-specific implants (PSI) at the point-of-care has become a desire for clinicians wanting to provide affordable and customized treatment. While some hospitals have already adopted extrusion-based 3D printing (fused filament fabrication; FFF) for creating non-implantable instruments, recent innovations have allowed for the printing of high-temperature implantable polymers including polyetheretherketone (PEEK). With interest in FFF PEEK implants growing, it is important to identify methods for printing favorable implant characteristics such as porosity for osseointegration. In this study, we assess the effect of porous geometry on the cell response and mechanical properties for FFF-printed porous PEEK. We also demonstrate the ability to design and print customized porous implants, specifically for a sheep tibial segmental defect model, based on CT images and using the geometry of triply periodic minimal surfaces (TPMS). Methods. Three porous constructs – a rectilinear pattern and gyroid/diamond TPMSs – were designed to mimic trabecular bone morphology and manufactured via PEEK FFF. TPMSs were designed by altering their respective equation approximations to achieve desired porous characteristics, and the meshes were solidified and shaped using a CAD workflow. Printed samples were mCT scanned to determine the resulting pore size and porosity, then seeded with pre-osteoblast cells for 7 and 14 days. Cell proliferation and alkaline phosphatase activity (ALP) were evaluated, and the samples were imaged via SEM. The structures were tested in compression, and stiffness and yield strength values were determined from resulting stress-strain plots. Roughness was determined using optical profilometry. Finally, our process of porous structure design/creation was modified to establish a proof-of-concept workflow for creating PSIs using geometry established from segmented sheep tibia CT images. Results. ALP activity measurements of the porous PEEK samples at 7 and 14 days were significantly greater than for solid controls (p < 0.001 for all three designs, 14 days). No difference between the porous geometries was found. SEM imaging revealed cells with flat, elongated morphology attached to the surface of the PEEK and into the pore openings, with filopodia and lamellipodia extensions apparent. mCT imaging showed average pore size to be 545 ± 43 µm (porosity 70%), 708 ± 64 µm (porosity 68%), and 596 ± 94 µm (porosity 69%) for the rectilinear, gyroid, and diamond structures, respectively. The average error between the theoretical and actual values was −16.3 µm (pore size) and −3.3 % (porosity). Compression testing revealed elastic moduli ranging from 210 to 268 MPa for the porous samples. Yield strengths were 6.6 ± 1.2 MPa for lattice, 14.8 ± 0.7 MPa for gyroid, and 17.1 ± 0.6 for diamond. Average roughness ranged from 0.8 to 3 µm. Finally, we demonstrated the ability to design and print a fully porous implant with the geometry of a sheep tibia segment. Assessments of implant geometrical accuracy and mechanical performance are ongoing. Discussion. We created porous PEEK with TPMS geometries via FFF and demonstrated a positive cellular response and mechanical characteristics similar to trabecular bone. Our work offers an innovative approach for advancing point-of-care 3D printing and PSI creation

Direct metal printed (DMP) porous iron implants possess promising mechanical and corrosion properties for various clinical application. Nevertheless, there is a requirement for better co-relation between in vitro and in vivo corrosion and biocompatibility behaviour of such biomaterials. Our present study evaluates absorption of porous iron implants under both static and dynamic conditions. Furthermore, this study characterizes their cytocompatibility using fibroblastic, osteogenic, endothelial and macrophagic cell types. In vitro degradation was performed statically and dynamically in a custom-built set-up placed under cell culture conditions (37 °C, 5% CO2 and 20% O2) for 28 days. The morphology and composition of the degradation products were analysed by scanning electron microscopy (SEM, JSM-IT100, JEOL). Iron implants before and after immersion were imaged by μCT (Quantum FX, Perkin Elmer, USA). Biocompatibility was also evaluated under static and dynamic in vitro culture conditions using L929, MG-63, HUVEC and RAW 264.7 cell lines. According to ISO 10993, cytocompatibility was evaluated directly using live/dead staining (Live and Dead Cell Assay kit, Abcam) in dual channel fluorescent optical imaging (FOI) and additionally quantified by flow cytometry. Furthermore, cytotoxicity was indirectly quantified using ISO conform extracts in proliferation assays. Strut size of DMP porous iron implants was 420 microns, with a porosity of 64% ± 0.2% as measured by micro-CT. After 28 days of physiological degradation in vitro, dynamically tested samples were covered with brownish degradation products. They revealed a 5.7- fold higher weight loss than statically tested samples, without significant changes in medium pH. Mechanical properties (E = 1600–1800 MPa) of these additively manufactured implants were still within the range of the values reported for trabecular bone, even after 28 days of biodegradation. Less than 25% cytotoxicity at 85% of the investigated time points was measured with L929 cells, while MG-63 and HUVEC cells showed 75% and 60% viability, respectively, after 24 h, with a decreasing trend with longer incubations. Cytotoxicity was analysed by two-way ANOVA and post-hoc Tukey's multiple comparisons test. Under dynamic culture conditions, live-dead staining and flow cytometric quantification showed a 2.8-fold and 5.7-fold increase in L929 and MG-63 cell survival rates, respectively, as compared to static conditions. Therefore, rationally designed and properly coated iron-based implants hold potential as a new generation of absorbable Orthopaedic implants

Introduction. Cementless total knee arthroplasty (TKA) has several advantages compared to the cemented approach, including elimination of bone cement, a quicker and easier surgical technique, and potentially a stronger long-term fixation. However, to ensure the successful long-term biological fixation between the porous implant and the bone, initial press-fit stability is of great importance. Undesired motion at the bone-implant interface may inhibit osseointegration and cause failure of biological fixation. Initial stability of a cementless femoral implant is affected by implant geometry, bone press-fit dimension, and characteristics of the porous coating. The purpose of this study was to compare the initial fixation stability of two types of porous femoral implants by quantifying the pull-out force using a paired cadaveric study design. Methods. The two types of cementless TKA femoral implants evaluated in this study had identical implant geometry but different porous coatings (Figure 1). The first type had a conventional spherical-bead coating (Type A), while the second type had an innovative irregularly-shaped-powder coating (Type B). The porous coating thickness was equivalent for both types of implants, thus the dimensional press-fit with bone was also equivalent. Three pairs of cadaveric femurs were prepared using standard TKA surgical technique, with each pair of the femurs receiving one of each porous implant type. An Instron 3366 load frame (Norwood, MA, USA) was used to pull the femoral implant out from the distal femur bone (Figure 2). The testing fixture was designed to allow free rotation between the implant and the actuator. The pullout was performed under a displacement control scheme (5 mm/min). Peak pull-out force was recorded and compared between the two implant groups. Results. Mean pull-out force for the Type B porous femoral implants (512 ± 246 N) was greater than that of the Type A porous femoral implants (310 ± 185 N), although the difference was not statistically significant (p>0.05) (Figure 3). Discussion. This paired cadaveric study showed that the innovative Type B porous coating provides equivalent and potentially greater pull-out force than the conventional Type A porous coating. Lack of statistical significance could be attributed to the limited sample size. Although pull-out testing is not a physiological loading scenario for TKA implant, it provides a relevant assessment of the implant-bone press-fit stability. With all other factors the same, the greater pull-out force observed in the Type B implants is likely related to the higher roughness and friction of the new porous coating. Previous experiments have shown that the Type B porous coating has significantly greater friction against Sawbones surface (coefficient of friction 0.89) compared to Type A porous coating (coefficient of friction 0.50), which was consistent with the findings in this study. Greater initial fixation stability is more favorable in cementless TKA as it reduces the risk of interface motion and better facilitates long-term biological fixation

Surface coatings have been introduced to total joint orthopaedics over the past decades to enhance osseointegration between metal implants and bone. However, complications such as aseptic loosening and infection persist. Inadequate osseointegration remains a complication associated with implants that rely on osseointegration for proper function. This is particularly challenging with implants having relatively flat and small surface areas that have high shear loading, such as noncemented uni and total condylar knee tibial trays. Faster osseointegration can enhance recovery as a result of improved load distribution and a more stable bone-implant interface. Traditionally noncemented porous bone ingrowth coatings on knee, hip and shoulder implants are typically texturised by thermal plasma spray coating, sintered metal bead coatings, or 3-D additive manufactured structures that provide porous surface features having the rough texture with pore sizes on the order of 150 to 300 micrometers. These surfaces are often further chemically enhanced with hydroxyapatite (HA) deposition. This provides macro-mechanical (millimeter scale) and micro-mechanical (micrometer scale) bone remodeling into the implant surface. However, at the nanoscale and cellular level, these surfaces appear relatively smooth. More recent studies are showing the importance of controlling the macro, micro, and the nano (nanometer scale) surface topographies to enhance cell interaction. In vitro and in vivo research shows surfaces with nanoscale features in the metal substrate result in enhanced osseointegration, greater bone-implant contact area and pullout force, and potentially bactericidal. One surface modification treatment technique of particular promise is nano-texturing via electrochemical anodization to bio-mimicking TiO2 nanotube arrays that are superimposed onto existing porous surface microstructures to further enhance the already known bone ingrowth properties of these porous structures by superimposing onto the existing microstructure arrays of nanotubes approximately 100 nanometers in outside diameter and 300–500 nanometers in height. In an ovine model, 3-D printed Direct Metal Laser Deposition (DMLS) additive manufactured porous Ti-6Al-4V implant with and without TiO2 nanotube array nano-texturing were compared to similar sized implants with commercially available sintered beads with HA coating and additive manufactured cobalt chrome implants. The average bond strength was significantly higher (42%) when the implants were nano-texturised and similarly stronger (53%) compared to HA coated sintered bead implants. Histology confirms over 420% more direct bonded growth of new bone from 0.5mm to 1.0mm deep into the porosity on the implants when the same implants are nano-texturised. Nano-texturing also changes the surface of the implant to repel methicillin-resistant staphylococcus aureus (MRSA) in an in vivo rabbit model limiting biofilm formation on the porous surface compared with non-treated porous surfaces. Since nano-texturizing only modifies the nano-morphology of the surface and does not add antibiotics or other materials to the implant, these animal studies shows great promise that nano-texturizing the TiO2 coating may not only enhance osseointegration, but also repels bacteria from porous implant surfaces. As such, we believe nano-texturing of porous implants will be the next advancement in surface coating technology

The success of cementless orthopaedic implants relies on bony ingrowth and active bone remodelling. Much research effort is invested to develop implants with controllable surface roughness and internal porous architectures that encourage these biological processes. Evaluation of these implants requires long-term and costly animal studies, which do not always yield the desired outcome requiring iteration. The aim of our study is to develop a cost-effective method to prescreen design parameters prior to animal trials to streamline implant development and reduce live animal testing burden. Ex vivo porcine cancellous bone cylinders (n=6, Ø20×12mm) were extracted from porcine knee joints with a computer-numerically-controlled milling machine under sterile conditions within 4 hours of animal sacrifice. The bone discs were implanted with Ø6×12mm additive manufactured porous titanium implants and were then cultured for 21days. Half underwent static culture in medium (DMEM, 10% FBS, 1% antibiotics) at 37°C and 5% CO. 2. The rest were cultured in novel high-throughput stacked configuration in a bioreactor that simulated physiological conditions after surgery: the fluid flow and cyclic compression force were set at 10ml/min and 10–150 N (1Hz,5000 cycles/day) respectively. Stains were administered at days 7 and 14. Samples were evaluated with widefield microscopy, scanning electron microscopy (SEM) and with histology. More bone remodelling was observed on the samples cultured within the bioreactor: widefield imaging showed more remodelling at the boundaries between the implant-bone interface, while SEM revealed immature bone tissue integration within the pores of the implant. Histological analysis confirmed these results, with many more trabecular struts with new osteoid formation on the samples cultured dynamically compared to static ones. Ex vivo bone can be used to analyse new implant technologies with lower cost and ethical impact than animal trial. Physiological conditions (load and fluid flow) promoted bone ingrowth and remodelling

As compared to magnesium (Mg) and iron (Fe), solid zinc (Zn)-based absorbable implants show better degradation rates. An ideal bone substitute should provide sufficient mechanical support, but pure Zn itself is not strong enough for load-bearing medical applications. Modern processing techniques, like additive manufacturing (AM), can improve mechanical strength of Zn. To better mimic the in vivo situation in the human body, we evaluated the degradation behavior of porous Zn implants in vitro under dynamic conditions. Our study applied selective laser melting (SLM) to build topographically ordered absorbable Zn implants with superior mechanical properties. Specimens were fabricated from pure Zn powder using SLM and diamond unit cell topological design. In vitro degradation was performed under both static and dynamic conditions in a custom-built set-up under cell culture conditions (37 °C, 20% O2 and 5% CO2) for up to 28 days. Mechanical properties of the porous structures were determined according to ISO 13314: 2011 at different immersion time points. Modified ISO 10993 standards were used to evaluate biocompatibility through direct cell seeding and indirect extract-based cytotoxicity tests (MTS assay, Promega) against identically designed porous titanium (Ti-6Al-4V) specimens as reference material. Twenty-four hours after cell seeding, its efficacy was evaluated by Live-Dead staining (Abcam) and further analyzed using dual channel fluorescent optical imaging (FOI) and subsequent flow cytometric quantification. Porous Zn implants were successfully produced by means of SLM with a yield strength and Young's modulus in the range of 3.9–9.6 MPa and 265–570 MPa, respectively. Dynamic flow significantly increased the degradation rate of AM porous Zn after 28 days. Results from Zn extracts were similar to Ti-6Al-4V with >95% of cellular activity at all tested time points, confirming level 0 cytotoxicity (i.e., This study clearly shows the great potential of AM porous Zn as a bone substituting material. Moreover, we demonstrate that complex topological design permits control of mechanical properties and degradation behavior