The effect of substrate roughness and hydroxyapatite coating thickness on implant shear strength

Implant Strength Contributes to the Osseointegration Strength of Porous Metallic Materials

Creating the optimal environment for effective and long term osseointegration is a heavily researched and sought-after design criteria for orthopedic implants. A validated multimaterial finite element (FE) model was developed to replicate and understand the results of an experimental in vivo push-out osseointegration model. The FE model results closely predicted global force (at 0.5 mm) and stiffness for the 50-90% porous implants with an r2 of 0.97 and 0.98, respectively. In addition, the FE global force at 0.5 mm showed a correlation to the maximum experimental forces with an r2 of 0.90. The highest porosity implants (80-90%) showed lower stiffnesses and more equitable load sharing but also failed at lower a global force level than the low porosity implants (50-70%). The lower strength of the high porosity implants caused premature plastic deformation of the implant itself during loading as well as significant deformations in the ingrown and surrounding bone, resulting in lower overall osseointegration strength, consistent with experimental measurements. The lower porosity implants showed a balance of sufficient bony ingrowth to support osseointegration strength coupled with implant mechanical properties to circumvent significant implant plasticity and collapse under the loading conditions. Together, the experimental and finite element modeling results support an optimal porosity in the range of 60-70% for maximizing osseointegration with current structure and loading.

Thermal spray processes influencing surface chemistry and in-vitro hemocompatibility of hydroxyapatite-based orthopedic implants

Orthopedic implants made from titanium are a popular choice in the medical field because of their remarkable strength-to-weight ratio. Nevertheless, they may not interact well with human blood, resulting in thrombosis and hemolysis. In fact, non-hemocompatibility is believed to be responsible for about 31 % of medical device failures in the US alone, requiring painful and expensive revision surgery. To address this issue, bioactive hydroxyapatite coatings are applied to Ti-6Al-4V implants using thermal spray techniques. However, the temperature used during thermal processing impacts the coating's surface properties, affecting the mechanical and biological properties. Furthermore, the effectiveness of HA coatings on titanium for orthopedic applications has not been validated by biocompatibility tests, particularly hemocompatibility. In this study, we aimed to investigate the relative efficacy of three thermal spray processes of different temperature ranges: Atmospheric plasma spray (APS) (high temperature), Flame spray (FS) (moderate temperature), and High-Velocity Oxy-Fuel spray (HVOF) (low temperature), and study their impact on coating's surface properties, affecting blood components and implant's strength. The crystallinity of the HA coating increased by 32 % with a decrease in the operating temperature (APS < FS < HVOF). HVOF coating exhibited a ~ 34 % and ~ 120 % improvement in adhesion strength and ~ 31 % and 59 % increment in hardness compared to APS and FS coating, respectively, attributed to its low porosity, low coating thickness (~55 μm), and high degree of crystallinity. The HVOF coating showcased a significant increase in non-hemolytic behavior, with hemolysis rates ~8 and ~ 11 times lower than APS and FS coatings, respectively, owing to its smooth texture and high degree of crystallinity (p < 0.05). Furthermore, the HVOF coating exhibited minimal blood clotting based on the whole blood clotting assay, again confirmed by PT and aPTT assays showing delayed clotting time, indicating its non-thrombogenic behavior. The number of platelets adhered to the three coatings showed no significant difference compared to Ti-6Al-4V. APS and FS coatings showed low platelet activation, unlike HVOF coating and titanium, which revealed round platelets, similar to the negative control. Neither titanium nor HA coatings exhibited antibacterial properties, which may be due to their high affinity for organic substances, which promotes bacterial adhesion and replication. Among the three thermal processes, HVOF coating displayed good apatite growth, non-hemolytic, and non-thrombogenicity with no platelet activation owing to its low processing temperature, high degree of crystallinity (89.7 %), hydrophilicity, smooth (~4 μm) and dense (~97 %) microstructural properties. The results demonstrated that the HVOF-HA coating presented in this work meets the hemocompatible requirements and shows promise for prospective application as an orthopedic implant. Furthermore, this study has the potential to significantly reduce the use of animals in in-vivo research and improve their welfare while also cutting costs.

There Are over 60 Ways to Produce Biocompatible Calcium Orthophosphate (CaPO4) Deposits on Various Substrates

A The present overview describes various production techniques for biocompatible calcium orthophosphate (abbreviated as CaPO4) deposits (coatings, films and layers) on the surfaces of various types of substrates to impart the biocompatible properties for artificial bone grafts. Since, after being implanted, the grafts always interact with the surrounding biological tissues at the interfaces, their surface properties are considered critical to clinical success. Due to the limited number of materials that can be tolerated in vivo, a new specialty of surface engineering has been developed to desirably modify any unacceptable material surface characteristics while maintaining the useful bulk performance. In 1975, the development of this approach led to the emergence of a special class of artificial bone grafts, in which various mechanically stable (and thus suitable for load-bearing applications) implantable biomaterials and artificial devices were coated with CaPO4. Since then, more than 7500 papers have been published on this subject and more than 500 new publications are added annually. In this review, a comprehensive analysis of the available literature has been performed with the main goal of finding as many deposition techniques as possible and more than 60 methods (double that if all known modifications are counted) for producing CaPO4 deposits on various substrates have been systematically described. Thus, besides the introduction, general knowledge and terminology, this review consists of two unequal parts. The first (bigger) part is a comprehensive summary of the known CaPO4 deposition techniques both currently used and discontinued/underdeveloped ones with brief descriptions of their major physical and chemical principles coupled with the key process parameters (when possible) to inform readers of their existence and remind them of the unused ones. The second (smaller) part includes fleeting essays on the most important properties and current biomedical applications of the CaPO4 deposits with an indication of possible future developments.

Observations of bony ongrowth and clinical fixation in two retrieved disc replacements

Total disc replacements utilize textured coatings to maximize bony ongrowth. However, the contribution of direct bony attachment to overall fixation for total disc replacements has not been reported. The goal of the present study was to document the extent of bony attachment to the surfaces of two clinically functional total disc replacements that were securely fixed at the time of revision. Two metal-and-polymeric disc replacements, one cervical and one lumbar, were evaluated following surgical retrieval. The cervical device was retrieved at 8 months and the lumbar device at 28 months post-operative. Both devices were reported well-fixed at the time of removal, with large bone masses attached to one endplate of each device. Visual inspections, non-destructive gravimetric measurements, and surface metrology were performed to assess fixation. These inspections suggested that both devices had been fixed at the time of removal with little in vivo mechanical damage, as surgical extraction damage was noted on both devices and provided imaging showed a lack of device migration. Devices were then embedded and sectioned to evaluate the bone-implant interface. High resolution photographs and contact microradiographs were taken to assess bony attachment. In contrast to initial analysis, these images revealed radiolucent gaps between the endplates and bone masses. Little direct contact between the bone and endplate surface was identified and the original surgical cuts were still visible. Both devices were clinically fixed at the time of removal and neither had complications associated with loosening. However, osseointegration was minimal in one of the devices and altogether absent from the other. The findings of the present study suggest that other factors may influence overall clinical fixation such as the surgical preparation of the vertebral bone or the surface roughness of the treated endplates. Despite the limitations of the present study, this information is unique to the current total disc replacement literature and the ongrowth and fixation of devices should be considered as a topic for future investigation.

Survival and performance of a dual tapered-wedge fully HA-coated press fit femoral stem

INTRODUCTION

Fully hydroxyapatite-coated titanium alloy double-tapered stems have been successful implant designs for THA. The Novation Element Stem (Exactech, Gainesville, FL) is one of several of these implants following a modified Corail design philosophy. Although a great deal is known about the results of the Corail stem, little is known about the more recent Corail-inspired implants. The authors evaluated the clinical outcomes, radiographic findings and survival statistics of the collarless version of the Element Stem when used routinely in a diverse patient population.

METHODS

A retrospective review was performed for all primary THR cases from 2010 to 2018. Patient demographics and HHS/OHS/patient satisfaction scores were obtained. Radiographs were evaluated for stem subsidence, radiolucencies, and bone responses to the implant.

RESULTS

One hundred and fifty seven Element Stems were implanted with greater than 2 years of radiographic follow-up, while 105 Element Stems implanted during the study period had a survival of 5 years or more. Average HHS was 91.7 and average OHS was 44.0. 54 of 157 hips had radiolucencies, all of which included a proximal zonal radiolucency. Average subsidence was 3.33 mm; 18 hips had subsidence > 4 mm. There were 8 revisions for: aseptic loosening (4), trauma-related peri-prosthetic fractures, and early proximal-medial fractures. At 5 years, the all-cause survival rate is 92.4%, 96.2% based only on aseptic loosening and 98.1% based only on proximal medial fractures.

CONCLUSION

Clinical outcomes using the collarless Novation Element Stem are good, but early proximal medial fracture is still a factor in patients with poor-quality bone. Proximal radiolucencies progressing to aseptic loosening post-operatively are also a concern. The 11.5% rate of subsidence is comparable to other fully HA-coated collarless stems. This study provides a thorough critical analysis of outcomes and midterm survival data of this dual tapered-wedge fully HA-coated collarless stem used routinely in a diverse patient population.

A Novel Nanostructured Surface on Titanium Implants Increases Osseointegration in a Sheep Model

BACKGROUND

A nanostructured titanium surface that promotes antimicrobial activity and osseointegration would provide the opportunity to create medical implants that can prevent orthopaedic infection and improve bone integration. Although nanostructured surfaces can exhibit antimicrobial activity, it is not known whether these surfaces are safe and conducive to osseointegration.

QUESTIONS/PURPOSES

Using a sheep animal model, we sought to determine whether the bony integration of medical-grade, titanium, porous-coated implants with a unique nanostructured surface modification (alkaline heat treatment [AHT]) previously shown to kill bacteria was better than that for a clinically accepted control surface of porous-coated titanium covered with hydroxyapatite (PCHA) after 12 weeks in vivo. The null hypothesis was that there would be no difference between implants with respect to the primary outcomes: interfacial shear strength and percent intersection surface (the percentage of implant surface with bone contact, as defined by a micro-CT protocol), and the secondary outcomes: stiffness, peak load, energy to failure, and micro-CT (bone volume/total volume [BV/TV], trabecular thickness [Tb.Th], and trabecular number [Tb.N]) and histomorphometric (bone-implant contact [BIC]) parameters.

METHODS

Implants of each material (alkaline heat-treated and hydroxyapatite-coated titanium) were surgically inserted into femoral and tibial metaphyseal cancellous bone (16 per implant type; interference fit) and in tibial cortices at three diaphyseal locations (24 per implant type; line-to-line fit) in eight skeletally mature sheep. At 12 weeks postoperatively, bones were excised to assess osseointegration of AHT and PCHA implants via biomechanical push-through tests, micro-CT, and histomorphometry. Bone composition and remodeling patterns in adult sheep are similar to that of humans, and this model enables comparison of implants with ex vivo outcomes that are not permissible with humans. Comparisons of primary and secondary outcomes were undertaken with linear mixed-effects models that were developed for the cortical and cancellous groups separately and that included a random effect of animals, covariates to adjust for preoperative bodyweight, and implant location (left/right limb, femoral/tibial cancellous, cortical diaphyseal region, and medial/lateral cortex) as appropriate. Significance was set at an alpha of 0.05.

RESULTS

The estimated marginal mean interfacial shear strength for cancellous bone, adjusted for covariates, was 1.6 MPa greater for AHT implants (9.3 MPa) than for PCHA implants (7.7 MPa) (95% CI 0.5 to 2.8; p = 0.006). Similarly, the estimated marginal mean interfacial shear strength for cortical bone, adjusted for covariates, was 6.6 MPa greater for AHT implants (25.5 MPa) than for PCHA implants (18.9 MPa) (95% CI 5.0 to 8.1; p < 0.001). No difference in the implant-bone percent intersection surface was detected for cancellous sites (cancellous AHT 55.1% and PCHA 58.7%; adjusted difference of estimated marginal mean -3.6% [95% CI -8.1% to 0.9%]; p = 0.11). In cortical bone, the estimated marginal mean percent intersection surface at the medial site, adjusted for covariates, was 11.8% higher for AHT implants (58.1%) than for PCHA (46.2% [95% CI 7.1% to 16.6%]; p < 0.001) and was not different at the lateral site (AHT 75.8% and PCHA 74.9%; adjusted difference of estimated marginal mean 0.9% [95% CI -3.8% to 5.7%]; p = 0.70).

CONCLUSION

These data suggest there is stronger integration of bone on the AHT surface than on the PCHA surface at 12 weeks postimplantation in this sheep model.

CLINICAL RELEVANCE

Given that the AHT implants formed a more robust interface with cortical and cancellous bone than the PCHA implants, a clinical noninferiority study using hip stems with identical geometries can now be performed to compare the same surfaces used in this study. The results of this preclinical study provide an ethical baseline to proceed with such a clinical study given the potential of the alkaline heat-treated surface to reduce periprosthetic joint infection and enhance implant osseointegration.

A Review of Biomaterials and Associated Performance Metrics Analysis in Pre-Clinical Finite Element Model and in Implementation Stages for Total Hip Implant System

Total hip replacement (THR) is a common orthopedic surgery technique that helps thousands of individuals to live normal lives each year. A hip replacement replaces the shattered cartilage and bone with an implant. Most hip implants fail after 10–15 years. The material selection for the total hip implant systems is a major research field since it affects the mechanical and clinical performance of it. Stress shielding due to excessive contact stress, implant dislocation due to a large deformation, aseptic implant loosening due to the particle propagation of wear debris, decreased bone remodeling density due to the stress shielding, and adverse tissue responses due to material wear debris all contribute to the failure of hip implants. Recent research shows that pre-clinical computational finite element analysis (FEA) can be used to estimate four mechanical performance parameters of hip implants which are connected with distinct biomaterials: von Mises stress and deformation, micromotion, wear estimates, and implant fatigue. In vitro, in vivo, and clinical stages are utilized to determine the hip implant biocompatibility and the unfavorable local tissue reactions to different biomaterials during the implementation phase. This research summarizes and analyses the performance of the different biomaterials that are employed in total hip implant systems in the pre-clinical stage using FEA, as well as their performances in in vitro, in vivo, and in clinical studies, which will help researchers in gaining a better understanding of the prospects and challenges in this field.

In vivo evaluation of osseointegration ability of sintered bionic trabecular porous titanium alloy as artificial hip prosthesis

Hydroxyapatite (HA) coatings have been widely used for improving the bone-implant interface (BII) bonding of the artificial joint prostheses. However, the incidence of prosthetic revisions due to aseptic loosening remains high. Porous materials, including three-dimensional (3D) printing, can reduce the elastic modulus and improve osseointegration at the BII. In our previous study, we identified a porous material with a sintered bionic trabecular structure with in vitro and in vivo bio-safety as well as in vivo mechanical safety. This study aimed to compare the difference in osseointegration ability of the different porous materials and HA-coated titanium alloy in the BII. We fabricated sintered bionic trabecular porous titanium acetabular cups, 3D-printed porous titanium acetabular cups, and HA-coated titanium alloy acetabular cups for producing a hip prosthesis suitable for beagle dogs. Subsequently, the imaging and histomorphological analysis of the three materials under mechanical loading in animals was performed (at months 1, 3, and 6). The results suggested that both sintered bionic porous titanium alloy and 3D-printed titanium alloy exhibited superior performances in promoting osseointegration at the BII than the HA-coated titanium alloy. In particular, the sintered bionic porous titanium alloy exhibited a favorable bone ingrowth performance at an early stage (month 1). A comparison of the two porous titanium alloys suggested that the sintered bionic porous titanium alloys exhibit superior bone in growth properties and osseointegration ability. Overall, our findings provide an experimental basis for the clinical application of sintered bionic trabecular porous titanium alloys.

High-strength, porous additively manufactured implants with optimized mechanical osseointegration

Optimization of porous titanium alloy scaffolds designed for orthopedic implants requires balancing mechanical properties and osseointegrative performance. The tradeoff between scaffold porosity and the stiffness/strength must be optimized towards the goal to improve long term load sharing while simultaneously promoting osseointegration. Osseointegration into porous titanium implants covering a wide range of porosity (0%-90%) and manufactured by laser powder bed fusion (LPBF) was evaluated with an established ovine cortical and cancellous defect model. Direct apposition and remodeling of woven bone was observed at the implant surface, as well as bone formation within the interstices of the pores. A linear relationship was observed between the porosity and benchtop mechanical properties of the scaffolds, while a non-linear relationship was observed between porosity and the ex vivo cortical bone-implant interfacial shear strength. Our study supports the hypothesis of porosity dependent performance tradeoffs, and establishes generalized relationships between porosity and performance for design of topological optimized implants for osseointegration. These results are widely applicable for orthopedic implant design for arthroplasty components, arthrodesis devices such as spinal interbody fusion implants, and patient matched implants for treatment of large bone defects.

The effect of a novel pillar surface morphology and material composition demonstrates uniform osseointegration

Long-term survival of orthopedic implants requires a strong and compliant interface between the implant and surrounding bone. This paper further explores the in-vivo response to a novel, macro-scale osseointegration surface morphology. In this study, we examine the effects of material composition on osseointegration in relation to the controlled surface geometry. The pillared surface is constructed of discontinuous surface geometry which creates an open space for unencumbered bone migration. In creating an open, macro-scale morphology we have demonstrated a bone migration and integration that is less dependent on the underlying implant material and is substantially driven thru surface geometry. In this in-vivo study an established ovine model was used to examine the effects of implant material composition on bone ingrowth and mechanical performance. Cortical and cancellous sites in the tibia and distal femur were examined at 6 and 12 weeks with μCT, histology, histomorphometry, and mechanical performance. Implant materials tested included PEEK (Evonik, VISTAKEEP®), PEEK HA (Invibio, PEEK-OPTIMA HA Enhanced), Titanium coated PEEK, Titanium (Ti-6Al-4V, Grade 5), and Ultra-High Molecular Weight Polyethylene (UHMWPE). Extensive bone ingrowth was noted in all implant materials at 12 weeks with maturation of the bone within the pillar structure from 6 weeks to 12 weeks. Histology demonstrated little fibrous deposition at the implant interface with no adverse cellular reactions. Histomorphometric review of cortical sites revealed greater than 60% bone ingrowth at 6 weeks increasing to nearly 80% by the 12 week timepoint. Cancellous sites yielded a mean of 30% ingrowth at 6 weeks increasing to 35% by 12 weeks. Pushout testing of cortical site samples demonstrated increase in pushout force between the 6 and 12 week timepoints. Increases were significant in all but the UHMWPE samples. Stiffness likewise increased in all samples between the two times. These results demonstrated the effectiveness of the pillar morphology with full integrating from the surrounding bony tissue regardless of the material.

In-Vivo response to a novel pillared surface morphology for osseointegration in an ovine model

Primary stability and secondary fixation of orthopedic implants to bony tissues are important for healing and long-term functionality. Load sharing and stress transfer are key requirements of an effective implant/tissue interface. This paper presents a novel, macro-scale osseointegration surface morphology which addresses the implant/tissue interface from both the biologic as well as biomechanical perspective. The surface morphology is a controlled, engineered, open topography manifested as discrete pillars projecting from the implant enabling continuous bone ingrowth. The pillared surface is distinct from other porous surfaces and can be differentiated by the localization of the implant material into discrete pillars enabling a continuous mass of bone to freely and easily interdigitate into the pillared structure. Traditional porous structures distribute the implant material throughout the surface forcing the bone to grow in a discontinuous manner. Creating an open and continuous space or "open porosity" in and around the pillar structure allows the bone to easily interdigitate with the implant surface without encumberment from a continuous porous structure. An in-vivo study, using an established ovine model, was undertaken examining the effects of pillar morphology on bone ingrowth and mechanical performance. Cortical and cancellous sites were evaluated utilizing histology, histomophometry, and mechanical pushout, at 4 and 12 weeks. Robust bone ingrowth occurred for all morphologies as was noted in review of the study results. An increase in volume and maturity of bone was noted between the intermediated and final time points. Histomophometry demonstrated over 40% and 80% new bone occupied the available "ingrowth" area at 12 weeks for cancellous and cortical sites (respectively). Histologic review showed little fibrous tissue ingrowth at the interface with no adverse cellular reactions. Testing of cortical samples demonstrated a significant increase in pushout load between the 4 and 12 week timepoints and a 4-8 fold increase in pushout load as compared to the grit blast control. These results demonstrated the effectiveness of the novel interface for orthopedic applications in an in-vivo ovine model.

Bone ongrowth and mechanical fixation of implants in cortical and cancellous bone

BACKGROUND

What is the right surface for an implant to achieve biological fixation? Surface technologies can play important roles in encouraging interactions between the implant surface and the host bone to achieve osseointegration. Preclinical animal models provide important insight into in vivo performance related to bone ongrowth and implant fixation.

METHODS

A large animal model was used to compare the in vivo response of HA and plasma-sprayed titanium coatings in a well-reported adult ovine model to evaluate bone ongrowth in terms of mechanical properties in cortical sites, and histology and histomorphometry in cortical and cancellous sites at 4 and 12 weeks.

RESULTS

Titanium plasma-sprayed surfaces outperformed the HA-coated samples in push-out testing in cortical sites while both surfaces supported new bone ongrowth and remodeling in cortical and cancellous sites.

CONCLUSIONS

While both HA and Ti plasma provided an osteoconductive surface for bone ongrowth, the Ti plasma provided a more robust bone-implant interface that ideally would be required for load transfer and implant stability in the longer term.

Relationship between the Surface Roughness of Biodegradable Mg-Based Bulk Metallic Glass and the Osteogenetic Ability of MG63 Osteoblast-Like Cells

Mg-based bulk metallic glass materials have been investigated for their large potential for application in orthopedic implants due to their biocompatibility, low degradation rate, and osteogenetic ability. As an orthopedic implant, initial cell adhesion has been a critical issue for subsequent osteogenesis and bone formation because the first contact between cells and the implant occurs upon the implants surface. Here, we aimed to create Mg-based bulk metallic glass samples with three different surface roughness attributes in order to understand the degradation behavior of Mg-based bulk metallic glass and the adhesion ability and osteogenetic ability of the contact cells. It was found that the degradation behavior of Mg₆₆Zn₂₉Ca₅ bulk metallic glass was not affected by surface roughness. The surface of the Mg₆₆Zn₂₉Ca₅ bulk metallic glass samples polished via #800 grade sandpaper was found to offer a well-attached surface and to provide a good cell viability environment for Human MG63 osteoblast-like cell line. In parallel, more calcium and mineral deposition was investigated on extracellular matrix with higher surface roughness that verify the relationship between surface roughness and cell performance.

In vivo osseointegration of a randomized trabecular titanium structure obtained by an additive manufacturing technique

The additive manufacturing techniques (AM) are able to realize three-dimensional trabecular structures that mimic the trabecular structure of the bone. An in vivo study in sheep was carried out with the aim of assessing the bone response and the trend of osteointegration of a randomized trabecular titanium structure produced by the AM technique. In 6 sheep were implanted 84 specimens with a trabecular titanium structure (4 implants in the femur distal epiphysis; 4 implants in the tibial plate; 6 implants in the tibial shaft). Sheep were sacrificed at 3 postoperative time-points: 6 weeks, 10 weeks, 14 weeks. Histomorphometric analysis was performed for the evaluation of Bone Implant Contact, and Bone Ingrowth. A standard push-out test was used to analyze the mechanical characteristics of the bone-implant interface. The histomorphometric data and biomechanical tests showed a fast osseointegration of the specimens both in the cancellous and in the cortical bone. The quantitative analysis of osseointegration data in cancellous bone showed the percentage of the surface of the implant in direct contact with the regenerated bone matrix significantly improved from 28% at 6 weeks to 54% at 14 weeks. An early osseointegration occurred in cortical bone showing that 75% of surface of implant was in direct contact with regenerated bone after 6 weeks; this value increased to 85% after 14 weeks. Mechanical tests revealed an early improvement of mean peak load of implants at 10 weeks (4486 N ± 528 N) compared to values at 6 weeks (2516 N ± 910 N) confirming the high rate of progression of osseointegration in the cortical bone. The non-mineralized matrix followed an increasing process of mineralization almost completely after 14 weeks. The results of this study have showed a rapid osseointegration and excellent biocompatibility for a randomized trabecular titanium structure that should be confirmed by clinical investigations.

Does implantation site influence bone ingrowth into 3D-printed porous implants?

BACKGROUND CONTEXT

The potential for osseointegration to provide biological fixation for implants may be related to anatomical site and loading conditions.

PURPOSE

To evaluate the influence of anatomical site on osseointegration of 3D-printed implants.

STUDY DESIGN

A comparative preclinical study was performed evaluating bone ingrowth in cortical and cancellous sites in long bones as well as lumbar interbody fusion with posterior pedicle screw stabilization using the same 3D-printed titanium alloy design.

METHODS

3D-printed dowels were implanted in cortical bone and cancellous bone in adult sheep and evaluated at 4 and 12 weeks for bone ingrowth using radiography, mechanical testing, and histology/histomorphometry. In addition, a single-level lumbar interbody fusion using cages based on the same 3D-printed design was performed. The aperture was filled with autograft or ovine allograft processed with supercritical carbon dioxide. Interbody fusions were assessed at 12 weeks via radiography, mechanical testing, and histology/histomorphometry.

RESULTS

Bone ingrowth in long bone cortical and cancellous sites did not translate directly to interbody fusion cages. While bone ingrowth was robust and improved with time in cortical sites with a line-to-line implantation condition, the same response was not found in cancellous sites even when the implants were placed in a press fit manner. Osseointegration into the porous walls with 3D porous interbody cages was similar to the cancellous implantation sites rather than the cortical sites. The porous domains of the 3D-printed device, in general, were filled with fibrovascular tissue while some bone integration into the porous cages was found at 12 weeks when fusion within the aperture was present.

CONCLUSION

Anatomical site, surgical preparation, biomechanical loading, and graft material play an important role in in vivo response. Bone ingrowth in long bone cortical and cancellous sites does not translate directly to interbody fusions.

The in vivo response to a novel Ti coating compared with polyether ether ketone: evaluation of the periphery and inner surfaces of an implant

BACKGROUND CONTEXT

Increasing bone ongrowth and ingrowth of polyether ether ketone (PEEK) interbody fusion devices has the potential to improve clinical outcomes.

PURPOSE

This study evaluated the in vivo response of promoting new bone growth and bone apposition with NanoMetalene (NM) compared with PEEK alone in a cancellous implantation site with an empty aperture.

STUDY DESIGN

This is a randomized control animal study.

METHODS

Implants and funding for this study were provided by SeaSpine (60,000 USD). Cylindrical dowels with two apertures were prepared as PEEK with a sub-micron layer of the titanium (NM). The titanium coating was applied over the entire implant (Group 1) or just the apertures (Group 2). Polyether ether ketone implants with no coating served as controls (Group 3). Implants were placed in the cancellous bone of the distal femur or proximal tibia with no graft material placed in the apertures in eight adult sheep. Bone ongrowth to the surface of the implant and ingrowth into the apertures was assessed at 4 and 8 weeks after surgery with micro-computed tomography (CT) and undecalcified histology.

RESULTS

The apertures in the implants were notably empty in the PEEK group at 4 and 8 weeks. In contrast, new bone formation into the apertures was found in samples coated with NM even though no graft material was placed into the defect. The bone growing into the aperture tracked along the titanium layer. Apertures with the titanium coating demonstrated significantly more bone by micro-CT qualitative grading compared with PEEK with average bone coverage scores of Group 1 (NM) 1.62±0.89, Group 2 (NM apertures only) 1.62±0.77, and Group 3 (PEEK) 0.43±0.51, respectively, at 4 weeks (p<.01) and Group 1 (NM) 1.79±1.19, Group 2 (NM apertures only) 1.98±1.18, and Group 3 (PEEK) 0.69±0.87, respectively, at 8 weeks (p<.05). The amount of bone in the apertures (ingrowth) quantified using the volumetric data from the micro-CT supported an overall increase in bone volume inside the apertures with the titanium coating compared with PEEK. Histology showed newly formed woven bone tracked along the surface of the titanium in the apertures. The PEEK interface presented the typical nonreactive fibrous tissue inside the apertures at 4 weeks and some focal contact with bone on the outside at 4 weeks and 8 weeks.

CONCLUSIONS

Micro-CT and histology demonstrated bone ongrowth to the surfaces coated with NM where the newly formed bone tracked along the thin titanium-coated surfaces. Polyether ether ketone surfaces presented the nonreactive fibrous tissue at the interface as previously reported in preclinical scenarios.

Does PEEK/HA Enhance Bone Formation Compared With PEEK in a Sheep Cervical Fusion Model?

BACKGROUND

Polyetheretherketone (PEEK) has a wide range of clinical applications but does not directly bond to bone. Bulk incorporation of osteoconductive materials including hydroxyapatite (HA) into the PEEK matrix is a potential solution to address the formation of a fibrous tissue layer between PEEK and bone and has not been tested.

QUESTIONS/PURPOSES

Using in vivo ovine animal models, we asked: (1) Does PEEK-HA improve cortical and cancellous bone ongrowth compared with PEEK? (2) Does PEEK-HA improve bone ongrowth and fusion outcome in a more challenging functional ovine cervical fusion model?

METHODS

The in vivo responses of PEEK-HA Enhanced and PEEK-OPTIMA^® Natural were evaluated for bone ongrowth in the form of dowels implanted in the cancellous and cortical bone of adult sheep and examined at 4 and 12 weeks as well as interbody cervical fusion at 6, 12, and 26 weeks. The bone-implant interface was evaluated with radiographic and histologic endpoints for a qualitative assessment of direct bone contact of an intervening fibrous tissue later. Gamma-irradiated cortical allograft cages were evaluated as well.

RESULTS

Incorporating HA into the PEEK matrix resulted in more direct bone apposition as opposed to the fibrous tissue interface with PEEK alone in the bone ongrowth as well as interbody cervical fusions. No adverse reactions were found at the implant-bone interface for either material. Radiography and histology revealed resorption and fracture of the allograft devices in vivo.

CONCLUSIONS

Incorporating HA into PEEK provides a more favorable environment than PEEK alone for bone ongrowth. Cervical fusion was improved with PEEK-HA compared with PEEK alone as well as allograft bone interbody devices.

CLINICAL RELEVANCE

Improving the bone-implant interface with a PEEK device by incorporating HA may improve interbody fusion results and requires further clinical studies.

Improve the performance of coated cemented hip stem through the advanced composite materials

Design of hip joint implant using functionally graded material (FGM) (advanced composite material) has been used before through few researches. It gives great results regarding the stress distribution along the implant and bone interfaces. However, coating of orthopaedic implants has been widely investigated through many researches. The effect of using advanced composite stem material, which mean by functionally graded stem material, in the total hip replacement coated with the most common coated materials has not been studied yet. Therefore, this study investigates the effect of utilizing these two concepts together; FGM and coating, in designing new stem material. It is concluded that the optimal FGM cemented stem is consisting from titanium at the upper stem layers graded to collagen at a lower stem layers. This optimal graded stem coated with hydroxyapatite found to reduce stress shielding by 57% compared to homogenous titanium stem coated with hydroxyapatite. However, the optimal functionally graded stem coated with collagen reduced the stress shielding by 51% compared to homogenous titanium stem coated with collagen.

Calcium orthophosphate deposits: Preparation, properties and biomedical applications

Since various interactions among cells, surrounding tissues and implanted biomaterials always occur at their interfaces, the surface properties of potential implants appear to be of paramount importance for the clinical success. In view of the fact that a limited amount of materials appear to be tolerated by living organisms, a special discipline called surface engineering was developed to initiate the desirable changes to the exterior properties of various materials but still maintaining their useful bulk performances. In 1975, this approach resulted in the introduction of a special class of artificial bone grafts, composed of various mechanically stable (consequently, suitable for load bearing applications) implantable biomaterials and/or bio-devices covered by calcium orthophosphates (CaPO4) to both improve biocompatibility and provide an adequate bonding to the adjacent bones. Over 5000 publications on this topic were published since then. Therefore, a thorough analysis of the available literature has been performed and about 50 (this number is doubled, if all possible modifications are counted) deposition techniques of CaPO4 have been revealed, systematized and described. These CaPO4 deposits (coatings, films and layers) used to improve the surface properties of various types of artificial implants are the topic of this review.

Plasma-sprayed titanium coating to polyetheretherketone improves the bone-implant interface

BACKGROUND CONTEXT

Rapid and stable fixation at the bone-implant interface would be regarded as one of the primary goals to achieve clinical efficacy, regardless of the surgical site. Although mechanical and physical properties of polyetheretherketone (PEEK) provide advantages for implant devices, the hydrophobic nature and the lack of direct bone contact remains a limitation.

PURPOSE

To examine the effects of a plasma-sprayed titanium coated PEEK on the mechanical and histologic properties at the bone-implant interface.

STUDY SETTING

A preclinical laboratory study.

METHODS

Polyetheretherketone and plasma-sprayed titanium coated PEEK implants (Ti-bond; Spinal Elements, Carlsbad, CA, USA) were placed in a line-to-line manner in cortical bone and in a press-fit manner in cancellous bone of adult sheep using an established ovine model. Shear strength was assessed in the cortical sites at 4 and 12 weeks, whereas histology was performed in cortical and cancellous sites at both time points.

RESULTS

The titanium coating dramatically improved the shear strength at the bone-implant interface at 4 weeks and continued to improve with time compared with PEEK. Direct bone ongrowth in cancellous and cortical sites can be achieved using a plasma-sprayed titanium coating on PEEK.

CONCLUSIONS

Direct bone to implant bonding can be achieved on PEEK in spite of its hydrophobic nature using a plasma-sprayed titanium coating. The plasma-sprayed titanium coating improved mechanical properties in the cortical sites and the histology in cortical and cancellous sites.

Osseointegration of multiphase anodic spark deposition treated porous titanium implants in an ovine model

Modification of titanium oxide by multiphase anodic spark deposition (ASD) has the potential to increase bioactivity and hasten osseointegration and biological fixation in uncemented arthroplasty. This study assessed the in vivo performance of control (Ti), plasma-sprayed HA-coated (TiHA) and ASD (Biospark) treated (TiAn) porous titanium implants with a solid core using a standard uncemented implant fixation sheep model. Cortical interfacial shear-strength and bone ingrowth in cortical and cancellous sites were quantified following 12 weeks in situ. Ultimate shear-strength for the Ti, TiHA and TiAn coatings was 33±9.5, 35.4±8.4 and 33.8±7.8 MPa, respectively, which was limited by coating delamination. ASD treatment was associated with significantly higher mean bone ingrowth at both sites. These results support the osteoconductive potential of the BioSpark treatment of porous titanium.

The effect of APH treatment on surface bonding and osseointegration of Ti-6Al-7Nb implants: an in vitro and in vivo study

This study investigated the effects of anodization-cyclic precalcification-heat (APH) treatment on the bonding ability of Ca-P coating to the parent metal and osseointegration of Ti-6Al-7Nb implants. Eighteen Ti-6Al-7Nb discs, 9 untreated and 9 APH-treated, were cultured with osteoblast cells in vitro, and the cellular differentiation ability was assayed at 1, 2, and 3 weeks. For in vivo testing, 28 Ti-6Al-7Nb implants (14 implants of each group) were inserted to rat tibias, and after each 4 and 6 weeks of implantation, bone bonding, and osseointegration were evaluated through removal torque and histological analysis. Osteoblast-culturing showed twice as much of the alkaline phosphatase activity on the treated surface at 3 weeks than on the untreated surface (p < 0.05). The treated implants exhibited higher removal torque values than the untreated ones (15.5 vs. 1.8 Ncm at 4 weeks and 19.7 vs. 2.6 Ncm at 6 weeks, p < 0.05). Moreover, the excellent bonding quality of coats was confirmed by the existence of cohesive fractures on the surface of removed APH implants (field emission scanning electron microscopy and histological observation). Within the limits of this study, it can be concluded that the APH treatment significantly enhanced osseointegration of the Ti-6Al-7Nb implant, with the stable bonding between the coating and the implant surface.

Modification of Ti6Al4V substrates with well-defined zwitterionic polysulfobetaine brushes for improved surface mineralization

Osteoconductive mineral coatings are beneficial for improving the osteointegration of metallic orthopedic/dental implants, but achieving adequate structural integration between the surface minerals and underlying metallic substrates has been a significant challenge. Here, we report covalent grafting of zwitterionic poly(sulfobetaine methacrylate) (pSBMA) brushes on the Ti6Al4V substrates to promote the surface-mineralization of hydroxyapatite with enhanced surface mineral coverage and mineral-substrate interfacial adhesion. We first optimized the atom transfer radical polymerization (ATRP) conditions for synthesizing pSBMA polymers in solution. Well-controlled pSBMA polymers (relative molecular weight up to 26 kD, PDI = 1.17) with high conversions were obtained when the ATRP was carried out in trifluoroethanol/ionic liquid system at 60 °C. Applying identical polymerization conditions, surface-initiated atom transfer radical polymerization (SI-ATRP) was carried out to graft zwitterionic pSBMA brushes (PDI < 1.20) from the Ti6Al4V substrates, generating a stable superhydrophilic and low-fouling surface coating without compromising the bulk mechanic property of the Ti6Al4V substrates. The zwitterionic pSBMA surface brushes, capable of attracting both cationic and anionic precursor ions during calcium phosphate apatite mineralization, increased the surface mineral coverage from 32% to 71%, and significantly reinforced the attachment of the apatite crystals on the Ti6Al4V substrate. This facile approach to surface modification of metallic substrates can be exploited to generate multifunctional polymer coatings and improve the performance of metallic implants in skeletal tissue engineering and orthopedic and dental care.

Comparing Biocompatibility of Nanocrystalline Titanium and Titanium-Oxide with Microcrystalline Titanium

Titanium (Ti) is the material of choice for orthopaedic applications because it is biocompatible and encourages osteoblast ingrowth. It was shown that the biocompatibility of Ti metal is due to the presence of a thin native sub-stoichiometric titanium oxide layer which enhances the adsorption of mediating proteins on the surface [1]. The present studies were devised to evaluate the adhesion, survival, and growth of cells on the surface of new engineered nano-crystal films of titanium and titanium oxides and compare them with orthopaedic-grade titanium with microcrystals. The engineered nano-crystal films with hydrophilic properties are produced by employing an ion beam assisted deposition (IBAD) technique. IBAD combines physical vapor deposition with concurrent ion beam bombardment in a high vacuum environment to produce films (with 3 to 70 nm grain size) with superior properties. These films are “stitched” to the artificial orthopaedic implant materials with characteristics that affect the wettability and mechanical properties of the coatings.

To characterize the biocompatibility of these nano-engineered surfaces, we have studied osteoblast function including cell adhesion, growth, and differentiation on different nanostructured samples. Cell responses to surfaces were examined using SAOS-2 osteoblast-like cells. We also studied a correlation between the surface nanostructures and the cell growth by characterizing the SAOS-2 cells with immunofluorescence and measuring the amount alizarin red concentration produced after 7 and 14 days. The number of adherent cells was determined by means of nuclei quantification on the nanocrystalline Ti, TiO2, and microcrystalline Ti and analysis was performed with Image J. Our experimental results indicated that nanocrystalline TiO2 is superior to both nano and microcrystalline Ti in supporting growth, adhesion, and proliferation. Improving the quality of surface oxide, i.e. fabricating stoichiometric oxides as well as nanoengineering the surface topology, is crucial for increasing the biocompatibility of Ti implant materials.

Structural influence from calcium phosphate coatings and its possible effect on enhanced bone integration

OBJECTIVE

The aim of this review was to summarize our present knowledge about calcium phosphate (CaP) coatings on implants with respect to their topographical appearance at micrometer as well as nanometer level and also the reported influence on bone healing.

MATERIAL AND METHODS

The PubMed database was used with the key words - surface roughness, CaP coating, implant, bone integration, clinical studies, experimental studies - used in different combinations. Only in vivo studies were taken into consideration.

CONCLUSIONS

A significantly improved healing capacity associated with CaP-coated implants is often reported, but individual importance of the several modes of surface changes introduced, deliberately or not, is usually very difficult to interpret. Several studies claim this difference to be due to altered chemistry, but in many the result may equally well be dependent on the surface topography. The few studies that have been published indicate that nanometer structures have an impact on early bone healing. However, the optimal size and distribution of nanometer-sized particles or pores applied on implant surfaces is still unknown, as are the evaluation effects of micrometer roughness. Improved surface characterization is needed if we are to reveal effects dependent on isolated nanometer alterations.

Influence of electron beam melting manufactured implants on ingrowth and shear strength in an ovine model

Arthroplasty has evolved with the application of electron beam melting (EBM) in the manufacture of porous mediums for uncemented fixation. Osseointegration of EBM and plasma-sprayed titanium (Ti PS) implant dowels in adult sheep was assessed in graduated cancellous defects and under line-to-line fit in cortical bone. Shear strength and bony ingrowth (EBM) and ongrowth (Ti PS) were assessed after 4 and 12 weeks. Shear strength of EBM exceeded that for Ti PS at 12 weeks (P = .030). Ongrowth achieved by Ti PS in graduated cancellous defects followed a distinctive pattern that correlated to progressively decreasing radial distances between defect and implant, whereas cancellous ingrowth values at 12 weeks for the EBM were not different. Osteoconductive porous structures manufactured using EBM present a viable alternative to traditional surface treatments.

Osseointegration of porous titanium implants with and without electrochemically deposited DCPD coating in an ovine model

Background

Uncemented fixation of components in joint arthroplasty is achieved primarily through de novo bone formation at the bone-implant interface and establishment of a biological and mechanical interlock. In order to enhance bone-implant integration osteoconductive coatings and the methods of application thereof are continuously being developed and applied to highly porous and roughened implant substrates. In this study the effects of an electrochemically-deposited dicalcium phosphate dihydrate (DCPD) coating of a porous substrate on implant osseointegration was assessed using a standard uncemented implant fixation model in sheep.

Methods

Plasma sprayed titanium implants with and without a DCPD coating were inserted into defects drilled into the cancellous and cortical sites of the femur and tibia. Cancellous implants were inserted in a press-fit scenario whilst cortical implants were inserted in a line-to-line fit. Specimens were retrieved at 1, 2, 4, 8 and 12 weeks postoperatively. Interfacial shear-strength of the cortical sites was assessed using a push-out test, whilst bone ingrowth, ongrowth and remodelling were investigated using histologic and histomorphometric endpoints.

Results

DCPD coating significantly improved cancellous bone ingrowth at 4 weeks but had no significant effect on mechanical stability in cortical bone up to 12 weeks postoperatively. Whilst a significant reduction in cancellous bone ongrowth was observed from 4 to 12 weeks for the DCPD coating, no other statistically significant differences in ongrowth or ingrowth in either the cancellous or cortical sites were observed between TiPS and DCPD groups.

Conclusion

The application of a DCPD coating to porous titanium substrates may improve the extent of cancellous bone ingrowth in the early postoperative phase following uncemented arthroplasty.

Effect of surgical fit on integration of cancellous bone and implant cortical bone shear strength for a porous titanium

Porous scaffold dowels of Ti(6)Al(4)V were prepared and implanted into cancellous and cortical bone sites in adult sheep. Cancellous implants were examined under gap, line-to-line, and press-fit conditions, whereas line-to-line implantation was used in cortical sites. Cortical shear strength increased significantly with time and reached 26.1 ± 8.6 MPa at 12 weeks, accompanied by a concomitant increase in bone integration and remodeling. In cancellous sites, bone integration was well established at 4 and 12 weeks under conditions of press-fit and line-to-line match between implant and surgical defect. New bone growth was also found in the gap conditions, although to a lesser extent. These findings suggest that the porous Ti(6)Al(4)V could prove an effective scaffold material for uncemented fixation in cortical and cancellous sites.

Tibial fixation without screws in cementless knee arthroplasty

This study reexamines the clinical and radiologic results of the first 216 cases of a cementless fixed-bearing tibial tray without screw holes in 191 patients at 7.6-year follow-up (range, 5-10 years). Mean Hospital for Special Surgery scores improved from 54/100 preoperatively to 84/100 postoperatively. Four tibial baseplates were revised. Five polyethylene inserts were revised. The overall reoperation rate was 12%. Four percent of tibial baseplates had incomplete nonprogressive radiolucent lines involving 1 or 2 of 10 defined zones. Overall tibial osteolysis rate was 4.1%. All tibial components were stable by radiographic technique. We conclude that this tibial tray provides reliable stability without the use of screws at medium-term follow-up. The polyethylene failure rate is a concern.

An electrodeposition method of calcium phosphate coatings on titanium alloy

Calcium phosphates coatings were deposited onto titanium alloy discs via en electrodeposition method. Titanium alloy discs were blasted with calcium phosphate particles, then etched in a mixture of nitric and fluoric acids and rinsed in demineralized water. The titanium alloy disc (cathode) and platinum mesh (anode) were immersed in a supersaturated calcium phosphate electrolyte buffered at pH 7.4 and connected to a current generator. The microstructure, chemical composition and crystallinity of the electrodeposited coatings were studied as function of time 10-120 min, temperature 25-80 degrees C, current density 8-120 mA/cm(2), magnesium and hydrogen carbonate amounts (0.1-1 mM). Uniform calcium phosphate coatings were obtained in 30 min but coating thickness increased with deposition time. Raising the temperature of electrolyte resulted in more uniform coatings as ionic mobility increased. Low current density was preferable due to hydrogen gas evolving at the cathode, which disturbed the deposition of calcium phosphate crystals on titanium. The amounts of magnesium and hydrogen carbonate ions affected both the homogeneity and morphology of the coatings. This study showed that the electrodeposition method is efficient for coating titanium with osteoconductive calcium phosphate layers.

Bone response to 3D periodic hydroxyapatite scaffolds with and without tailored microporosity to deliver bone morphogenetic protein 2

Three types of model hydroxyapatite (HA) scaffolds were implanted in the metacarpal and metatarsal bones of goats. Scaffolds, consisting of a latticed pattern of rods, were fabricated with a solid freeform fabrication (SFF) technique. All scaffolds contained macropores; some were also fabricated with micropores (5.2 +/- 2.0 microm). Recombinant human bone morphogenetic protein-2 (rhBMP-2) was added to some microporous scaffolds. rhBMP-2 caused increased percent filled with bone tissue compared to microporous scaffolds without rhBMP-2. Lamellar bone in the scaffolds was aligned perpendicular to the long axis of the bone near the junctions of the rods that make up the scaffold but was more random away from the junctions of rods. Microporous scaffolds stained beneath areas of contact with new bone. This staining might indicate either extracellular matrix (ECM) in the rods, byproducts of ECM production, or reaction of cellular products with the scaffold.

Effect of hydroxyapatite/tricalcium-phosphate coating on osseointegration of plasma-sprayed titanium alloy implants

This study determined the effects of a plasma-sprayed hydroxyapatite/tricalcium phosphate (HA/TCP) coating on osseointegration of plasma-sprayed titanium alloy implants in a lapine, distal femoral intramedullary model. The effects of the HA/TCP coating were assessed at 1, 3, and 6 months after implant placement. The HA/TCP coating significantly increased new bone apposition onto the implant surfaces at all time points. The ceramic coating also stimulated intramedullary bone formation at the middle and distal levels of the implants. Fluorescent bone labeling indicated that new bone formation occurred primarily during the first 3 months after implantation, with comparatively little activity detected in the latter stages of the study. There was no associated increase in pullout strength at either 3 or 6 months; however, post-pullout evaluation of the implants indicated that the HA/TCP coating itself was not the primary site of construct failure. Rather, failure was most commonly observed through the periprosthetic osseous struts that bridged the medullary cavity. The demonstrated osteoconductive activity of HA/TCP coating on plasma-sprayed titanium alloy implant surfaces may have considerable clinical relevance to early host-implant interactions, by accelerating the establishment of a stable prosthesis-bone interface.

Cemented fixation with PMMA or Bis-GMA resin hydroxyapatite cement: effect of implant surface roughness

Implant surface roughness is an important parameter governing the overall mechanical properties at the implant-cement interface. This study investigated the influence of surface roughness using polymethylmethcrylate (PMMA) and a Bisphenol-a-glycidylmethacyrlate resin-hydroxyapatite cement (CAP). Mechanical fixation at the implant-cement interface was evaluated in vitro using static shear and fatigue loading with cobalt chrome alloy (CoCr) dowels with different surface roughness preparations. Increasing surface roughness improved the mechanical properties at the implant-cement interface for both types of cement. CAP cement fixation was superior to PMMA under static and dynamic loading.

Influence of abrasion on the surface characteristics of thermally sprayed hydroxyapatite coatings

The surface condition of hydroxyapatite thermal sprayed coatings can be measured in terms of smooth melted and unmelted regions. Both these areas collectively contribute to abrasion resistance of a coated implant. The purpose of this work was to determine the influence of abrasion on the surface characteristics of coatings. Coatings with three degrees of roughness (R(a) of 7, 10 and 24 microm) were produced by thermal spraying. These coatings were subjected to abrasion in a pin-on-disk test. It was found that the majority of weight loss occurs within the first minute of the abrasion. Raised areas from the coating are removed and the bone analogue pin spreads amongst the elevated areas to produce a smoother coating. Further, abrasion is dependent upon the applied load. Small loads produce a smaller or negligible increase in coating loss, however, a higher load is able to displace more material from the coating surface. It is expected that the coating loss from plasma sprayed coatings amounts to a value of less than 3 wt% during the insertion into bone. Modification of the coating surface from abrasion is dependent upon the surface roughness of the coating.