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

Is cement mantle thickness a primary cause of aseptic tibial loosening following primary total knee arthroplasty?

BACKGROUND

Aseptic tibial loosening following primary total knee replacement is one of the leading causes of long-term failure. Cement mantle thickness has been implicated as a source of aseptic tibial loosening. Therefore, the following study was designed to determine (1) what is the cement mantle thickness in patients that develop aseptic tibial loosening, and (2) is there a difference in cement mantle thickness based on the interface of failure?

METHOD

This retrospective cohort included 216 patients revised for aseptic tibial loosening. Patient demographics, operative data, and clinical outcomes were recorded. A preoperative radiographic assessment was performed to determine the interface of failure and the thickness of the cement mantle using the Knee Society Radiographic Evaluation System zones.

RESULTS

The average patient age was 65 years and body mass index was 33.7 kg/m². 203 patients demonstrated radiographic failure at the implant-cement interface and 13 patients demonstrated failure at the cement-bone interface. The average cement mantle thickness of each radiographic zone for the entire cohort on the AP and lateral views was 4.4 and 4.5 mm, respectively. The average cement mantle thickness of patients that developed failure at the implant-cement interface was significantly greater than patients that failed at the cement-bone interface in each radiographic zone (p < 0.001).

CONCLUSIONS

Patients that develop implant loosening at the cement-bone interface were noted to have a significantly decreased cement mantle compared to patients that failed at the implant-cement interface. Methods for decreasing tibial implant loosening should likely focus on improving the fixation at the implant-cement interface.

Chitranjan S. Ranawat Award: Motion During Total Knee Cementing Significantly Decreases Tibial Implant Fixation Strength

BACKGROUND

Aseptic tibial loosening following primary total knee arthroplasty persists despite technique and device-related advancements. The mechanisms for this mode of failure are not well understood. We hypothesized that knee movement while the cement was curing dispersed lipids at the implant-cement interface and would result in decreased tibial fixation strength.

METHODS

A cadaveric study was performed utilizing 32 torso-to-toe specimens (64 knees). Four contemporary total knee arthroplasty designs were evaluated. Each implant design was randomly assigned to a cadaveric specimen pair with side-to-side randomization. Specimen densitometry was recorded. Each tibial implant was cemented using a standard technique. On one side, the tibial component was held without motion following impaction until complete cement polymerization. The contralateral knee tibial implant was taken through gentle range of motion and stability assessment 7 minutes after cement mixing. Axial tibial pull-out strength and interface failure examination was performed on each specimen.

RESULTS

The average pull-out strength for the no motion cohort (5,462 N) exceeded the motion cohort (4,473 N) (P = .001). The mean pull-out strength between implant designs in the no motion cohort varied significantly (implant A: 7,230 N, B: 5,806 N, C: 5,325 N, D 3,486 N; P = .007). Similarly, the motion cohort inter-implant variance was significant (P ≤ .001). Intra-implant pull-out strength was significantly higher in implant A than D. The average pull-out strength was significantly lower in specimens that failed at the implant-cement interface vs bone failures (4,089 ± 2,158 N vs 5,960 ± 2,010 N, P < .0025).

CONCLUSION

Knee motion during cement polymerization is associated with significant decreases in tibial implant fixational strength. Reduction in implant pull-out strength was identified with each implant design with motion and varied between designs. Across all tested designs, we recommend limiting motion while cementing the tibial implant to improve fixation strength.

Varus Collapse in Total Knee Arthroplasty: Does Fixation or Bone Fail First?

INTRODUCTION

Aseptic tibial loosening is now considered the most common reason that total knee arthroplasties (TKA) fail long term. There are unique subsets of patients that fail into varus alignment of the tibial tray with collapse of the medial proximal tibia. It is currently unknown if the implant fixation fails first or if the proximal medial tibia collapses first.

MATERIALS

We performed a retrospective analysis of 88 patients that were revised at our institution secondary to aseptic varus collapse of the proximal tibia. Two fellowship-trained arthroplasty surgeons performed a retrospective analysis on sequential precollapse radiographs in each patient to determine which failed first: the implant fixation (implant-cement or cement-bone interface) or the medial proximal tibia.

DISCUSSION

36/88 (40.9%) patients had a series of precollapse radiographs that could be reviewed. Failure at the implant-cement interface before varus collapse in 23 vs 22 patients, failure at the implant-cement and cement-bone interface before varus collapse in two patients, and contemporaneous failure at the implant-cement interface and varus collapse in 11 vs 12 patients were identified by reviewers one and two, respectively.

CONCLUSION

The most frequent mechanism of failure identified was failure of the implant-cement interface followed by subsequent medial tibial varus collapse. Improving implant fixation may decrease the incidence of this unique failure mechanism. We advocate the use of supplemental stem fixation in high-risk patients and optimal cement techniques for all patients as methods of potentially avoiding tibial varus collapse, one of the most frequent modes of long-term failure.

Where Is the "Weak Link" of Fixation in Contemporary Cemented Total Knee Replacements?

BACKGROUND

Aseptic tibial loosening is a frequent cause of long-term failure following primary cemented total knee replacement. Failure of the tibial implant can occur at the implant-cement interface or at the cement-bone interface. Currently, it is unknown at which interface failure occurs in cases of aseptic tibial loosening. The following study was designed to determine which interface represents the "weak link" for tibial implant fixation.

METHODS

We performed a retrospective analysis of 149 patients who were revised secondary to aseptic tibial loosening at our institution from 2005 to 2017. Operative reports and radiographs were reviewed on each patient to determine the location and pattern of fixation failure.

RESULTS

Implant failure was more prevalent at the implant-cement than cement-bone interface, 140/149 (94.0%) vs 9/149 (6.0%). Additionally, we noted 2 distinct patterns of failure in patients that loosened at the implant-cement interface. Ninety of 140 (64.3%) patients developed varus collapse pattern of failure. Forty-nine of 140 (35.0%) patients developed failure between the implant-cement interface without angulation. All 149 patients had heterotopic bone formation anterior to the tibial baseplate, which was consistent regardless of which interface failed.

CONCLUSION

The most frequent interface failure identified in our study was at the implant-cement interface, 140/149 (94.0%). This finding has substantial clinical ramifications. Because failure was predominantly at the implant-cement interface there may be design opportunities for increasing implant fixation to cement. Implants with improved undersurface tibial tray features may be necessary to mitigate the risk of failure at this interface, especially in overly active patients or those with elevated body mass indices.

Laser-Induced Microgrooves Improve the Mechanical Responses of Cemented Implant Systems

The impact of a laser-induced microgroove (LIM) architecture on mechanical responses of two cemented implant systems was evaluated. One system consisted of two aluminum alloy rods bonded end-to-end by polymethylmethacrylate cement. The second system consisted of a custom-made, aluminum tibial tray (TT) cemented in an artificial canine tibia. Control specimens for each system were polished smooth at the cement interface. For LIM samples in the rod system, microgrooves were engraved (100 µm depth, 200 µm width, 500 µm spacing) on the apposing surface of one of the two rods. For TT system testing, LIM engraving (100 µm spacing) was confined to the underside and keel of the tray. Morphological analysis of processed implant surfaces revealed success in laser microgrooving procedures. For cemented rods tested under static tension, load to failure was greater for LIM samples (279.0 ± 14.9 N vs. 126.5 ± 4.5 N). Neither non-grooved nor grooved TT samples failed under cyclic compression testing (100,000 cycles at 1 Hz). Compared with control specimens, LIM TT constructs exhibited higher load to failure under static compression and higher strain at the bone interface under cyclic compression. Laser-induced microgrooving has the potential to improve the performance of cemented orthopedic implants.

The chitosan/tri-calcium phosphate bio-composite bone cement promotes better osteo-integration: an in vitro and in vivo study

Background

Polymethylmethacrylate bone cement has a variety of applications in orthopedic surgery, but it also has some shortcomings such as high heat generation during polymerization and poor integration with bone tissue. In this study, a bio-composite bone cement composed of tri-calcium phosphate and chitosan as additives to acrylic bone cement was developed. Our hypothesis is that this new bio-composite bone cement has a better osteo-integration than pure polymethyl methacrylate cement.

Methods

Physiological composition, i.e., 65 wt% inorganic and 35 wt% organic components, of tri-calcium phosphate and chitosan contents was selected as degradable additives to replace acrylic bone cement. A series of properties such as exothermic temperature changes, setting time, bio-mechanical characteristics, degradation behaviors, and in vitro cytotoxicity were examined. Preliminary in vivo animal study was also performed.

Results

The results showed that the bio-composite bone cement exhibited lower curing temperature, longer setting time, higher weight loss and porosity after degradation, lower compressive Young’s modulus, and ultimate compressive strength as compared with those of pure polymethyl methacrylate cement. Cell proliferation tests demonstrated that the bio-composite bone cement was non-cytotoxic, and the in vivo tests revealed that was more osteo-conductive.

Conclusions

The results indicated that the modified chitosan/tri-calcium phosphate/polymethyl methacrylate bio-composites bone cement could be degraded gradually and create rougher surfaces that would be beneficial to cell adherence and growth. This new bio-composite bone cement has potential in clinical application. Our future studies will focus on long-term implantation to investigate the stability of the bio-composite bone cement in long-term implantation.

Microgroove and Collagen-poly(ε-caprolactone) Nanofiber Mesh Coating Improves the Mechanical Stability and Osseointegration of Titanium Implants

The effect of depositing a collagen (CG)-poly-ε-caprolactone (PCL) nanofiber mesh (NFM) at the microgrooves of titanium (Ti) on the mechanical stability and osseointegration of the implant with bone was investigated using a rabbit model. Three groups of Ti samples were produced: control Ti samples where there were no microgrooves or CG-PCL NFM, groove Ti samples where microgrooves were machined on the circumference of Ti, and groove-NFM Ti samples where CG-PCL NFM was deposited on the machined microgrooves. Each group of Ti samples was implanted in the rabbit femurs for eight weeks. The mechanical stability of the Ti/bone samples were quantified by shear strength from a pullout tension test. Implant osseointegration was evaluated by a histomorphometric analysis of the percentage of bone and connective tissue contact with the implant surface. The bone density around the Ti was measured by micro–computed tomography (μCT) analysis. This study found that the shear strength of groove-NFM Ti/bone samples was significantly higher compared to control and groove Ti/bone samples (p < 0.05) and NFM coating influenced the bone density around Ti samples. In vivo histomorphometric analyses show that bone growth into the Ti surface increased by filling the microgrooves with CG-PCL NFM. The study concludes that a microgroove assisted CG-PCL NFM coating may benefit orthopedic implants.

Plasma nitriding of titanium alloy: Effect of roughness, hardness, biocompatibility, and bonding with bone cement

Titanium (Ti) alloys have been widely used in orthopedics and orthodontic surgeries as implants because of their beneficial chemical, mechanical, and biological properties. Improvement of these properties of a Ti alloy, Ti-6Al-4V Eli, is possible by the use of plasma nitriding treatment on the Ti alloy. The novelty of this study is the evaluation of a DC glow discharge nitrogen plasma treatment method on the surface, mechanical and biological properties of Ti alloy. Specifically, this study measured the chemical states, roughness, hardness, and biocompatibility of plasma nitride treated Ti-6Al-4V Eli as well as determined the effect of plasma treatment on the fracture strength between the Ti alloy and bone clement. This study hypothesized that DC glow discharge nitrogen plasma treatment may alter the surface chemical and mechanical states of the Ti alloy that may influence the fracture strength of implant/cement interfaces under static load. This study found that plasma nitride treatment on Ti alloy does not have effect on the roughness and biocompatibility (P value > 0.5), but significantly effect on the hardness and fracture strength of Ti-bone cement interfaces compared to those values of untreated Ti samples (P value < 0.5). Therefore, the DC glow discharge nitrogen plasma treated Ti alloy can potentially be used for orthopedic applications.

Peen treatment on a titanium implant: effect of roughness, osteoblast cell functions, and bonding with bone cement

Implant failure due to poor integration of the implant with the surrounding biomaterial is a common problem in various orthopedic and orthodontic surgeries. Implant fixation mostly depends upon the implant surface topography. Micron to nanosize circular-shaped groove architecture with adequate surface roughness can enhance the mechanical interlock and osseointegration of an implant with the host tissue and solve its poor fixation problem. Such groove architecture can be created on a titanium (Ti) alloy implant by laser peening treatment. Laser peening produces deep, residual compressive stresses in the surfaces of metal parts, delivering increased fatigue life and damage tolerance. The scientific novelty of this study is the controlled deposition of circular-shaped rough spot groove using laser peening technique and understanding the effect of the treatment techniques for improving the implant surface properties. The hypothesis of this study was that implant surface grooves created by controlled laser peen treatment can improve the mechanical and biological responses of the implant with the adjoining biomaterial. The objective of this study was to measure how the controlled laser-peened groove architecture on Ti influences its osteoblast cell functions and bonding strength with bone cement. This study determined the surface roughness and morphology of the peen-treated Ti. In addition, this study compared the osteoblast cell functions (adhesion, proliferation, and differentiation) between control and peen-treated Ti samples. Finally, this study measured the fracture strength between each kind of Ti samples and bone cement under static loading. This study found that laser peen treatment on Ti significantly changed the surface architecture of the Ti, which led to enhanced osteoblast cell adhesion and differentiation on Ti implants and fracture strength of Ti–bone cement interfaces compared with values of untreated Ti samples. Therefore, the laser peen treatment method has the potential to improve the biomechanical functions of Ti implants.

Fracture toughness of titanium-cement interfaces: effects of fibers and loading angles

Ideal implant–cement or implant–bone interfaces are required for implant fixation and the filling of tissue defects created by disease. Micron- to nanosize osseointegrated features, such as surface roughness, fibers, porosity, and particles, have been fused with implants for improving the osseointegration of an implant with the host tissue in orthopedics and dentistry. The effects of fibers and loading angles on the interface fracture toughness of implant–cement specimens with and without fibers at the interface are not yet known. Such studies are important for the design of a long-lasting implant for orthopedic applications. The goal of this study was to improve the fracture toughness of an implant–cement interface by deposition of micron- to nanosize fibers on an implant surface. There were two objectives in the study: 1) to evaluate the influence of fibers on the fracture toughness of implant–cement interfaces with and without fibers at the interfaces, and 2) to evaluate the influence of loading angles on implant–cement interfaces with and without fibers at the interfaces. This study used titanium as the implant, poly(methyl methacrylate) (PMMA) as cement, and polycaprolactone (PCL) as fiber materials. An electrospinning unit was fabricated for the deposition of PCL unidirectional fibers on titanium (Ti) plates. The Evex tensile test stage was used to determine the interface fracture toughness (K_C) of Ti–PMMA with and without PCL fibers at 0°, 45°, and 90° loading angles, referred to in this article as tension, mixed, and shear tests. The study did not find any significant interaction between fiber and loading angles (P>0.05), although there was a significant difference in the K_C means of Ti–PMMA samples for the loading angles (P<0.05). The study also found a significant difference in the K_C means of Ti–PMMA samples with and without fibers (P<0.05). The results showed that the addition of the micron- to nanosize PCL fibers on Ti improved the quality of the Ti–PMMA union. The results of the study are essential for fatigue testing and finite-element analysis of implant–cement interfaces to evaluate the performance of orthopedic and orthodontic implants.

Cement-implant interface contamination: possible reason of inferior clinical outcomes for rough surface cemented stems

BACKGROUND

Shape-closed cemented implants rely on a stronger bond and have displayed inferior clinical outcomes when compared to force-closed designs. Implant contamination such as saline, bone marrow and blood prior to cement application has the potential to affect the cement-implant bond. The consequences of implant contamination were investigated in this study.

METHODS

Fifty Titanium alloy (Ti-6Al-4V) dowels were separated into ten groups based on surface roughness and contaminant, and then cemented in polyvinyl chloride tubes. Push-out testing was performed at 1mm per minute. The roughness of the dowel surface was measured before and after the testing. The dowel surface and cement mantel were analyzed using a Scanning Electron Microscopy (SEM) to determine the distribution and characteristics of any debris and contaminants on the surface.

RESULTS

Contaminants largely decreased stem-cement interfacial shear strength, especially for rough surfaces. Saline produced the greatest decrease, followed by blood. The effect of bone marrow was less pronounced and similar to that of oil. Increasing surface roughness increased the interfacial bonding strength, even with contaminants. There was a non-significant increase in mean bonding strength for smooth surfaces with bone marrow and oil contamination. SEM showed that contaminants influence the interfacial bond by different mechanisms. More debris was found on rough samples following testing.

CONCLUSIONS

The results of this study underscore the importance of keeping an implant free from contamination, and suggest if contamination does occur, a saline rinse may further decrease the stability of an implant. The deleterious effects of contamination on rough surface cement bonding were considerable, and indicate that contamination at the time of surgery may, in part, contribute to inferior clinical outcomes for rough surfaced cemented stems.

Development of strong and bioactive calcium phosphate cement as a light-cure organic-inorganic hybrid

In this research, light cured calcium phosphate cements (LCCPCs) were developed by mixing a powder phase (P) consisting of tetracalcium phosphate and dicalcium phosphate and a photo-curable resin phase (L), mixture of hydroxyethylmethacrylate (HEMA)/poly acrylic-maleic acid at various P/L ratios of 2.0, 2.4 and 2.8 g/mL. Mechanical strength, phase composition, chemical groups and microstructure of the cured cements were evaluated at pre-set times, i.e. before and after soaking in simulated body fluid (SBF). The proliferation of Rat-derived osteoblastic cells onto the LCCPCs as well as cytotoxicity of cement extracts were determined by cell counting and 3-{4,5-dimethylthiazol-2yl}-2,5-diphenyl-2H-tetrazolium bromide assay after different culture times. It was estimated from Fourier transforming infrared spectra of cured cements that the setting process is ruled by polymerization of HEMA monomers as well as formation of calcium poly-carboxylate salts. Microstructure of the cured cements consisted of calcium phosphate particles surrounded by polymerized resin phase. Formation of nano-sized needlelike calcium phosphate phase on surfaces of cements with P/L ratios of 2.4 and 2.8 g/mL was confirmed by scanning electron microscope images and X-ray diffractometry (XRD) of the cured specimen soaked in SBF for 21 days. Also, XRD patterns revealed that the formed calcium phosphate layer was apatite phase in a poor crystalline form. Biodegradation of the cements was confirmed by weight loss, change in molecular weight of polymer and morphology of the samples after different soaking periods. The maximum compressive strength of LCCPCs governed by resin polymerization and calcium polycarboxylate salts formation was about 80 MPa for cement with P/L ratio of 2.8 g/mL, after incubation for 24 h. The strength of all cements decreased by decreasing P/L ratio as well as increasing soaking time. The preliminary cell studies revealed that LCCPCs could support proliferation of osteoblasts cultured on their surfaces and no cytotoxic effect was observed for the extracts of them.

Similar TKA designs with differences in clinical outcome: a randomized, controlled trial of 77 knees with a mean follow-up of 6 years

BACKGROUND AND PURPOSE

To try to improve the outcome of our TKAs, we started to use the CKS prosthesis. However, in a retrospective analysis this design tended to give worse results. We therefore conducted a randomized, controlled trial comparing this CKS prosthesis and our standard PFC prosthesis. Because many randomized studies between different TKA concepts generally fail to show superiority of a particular design, we hypothesized that these seemingly similar designs would not lead to any difference in clinical outcome.

PATIENTS AND METHODS

82 patients (90 knees) were randomly allocated to one or other prosthesis, and 39 CKS prostheses and 38 PFC prostheses could be followed for mean 5.6 years. No patients were lost to follow-up. At each follow-up, patients were evaluated clinically and radiographically, and the KSS, WOMAC, VAS patient satisfaction scores and VAS for pain were recorded.

RESULTS

With total Knee Society score (KSS) as primary endpoint, there was a difference in favor of the PFC group at final follow-up (p = 0.04). Whereas there was one revision in the PFC group, there were 6 revisions in the CKS group (p = 0.1). The survival analysis with any reoperation as endpoint showed better survival in the PFC group (97% (95% CI: 92-100) for the PFC group vs. 79% (95% CI: 66-92) for the CKS group) (p = 0.02).

INTERPRETATION

Our hypothesis that there would be no difference in clinical outcome was rejected in this study. The PFC system showed excellent results that were comparable to those in previous reports. The CKS design had differences that had considerable negative consequences clinically. The relatively poor results have discouraged us from using this design.

Free form fabricated features on CoCr implants with and without hydroxyapatite coating in vivo: a comparative study of bone contact and bone growth induction

The current study evaluates the in vivo response to free form fabricated cobalt chromium (CoCr) implants with and without hydroxyapatite (HA) plasma sprayed coatings. The free form fabrication method allowed for integration of complicated pyramidal surface structures on the cylindrical implant. Implants were press fit into the tibial metaphysis of nine New Zealand white rabbits. Animals were sacrificed and implants were removed and embedded. Histological analysis, histomorphometry and electron microscopy studies were performed. Focused ion beam was used to prepare thin sections for high-resolution transmission electron microscopy examination. The fabricated features allowed for effective bone in-growth and firm fixation after 6 weeks. Transmission electron microscopy investigations revealed intimate bone-implant integration at the nanometre scale for the HA coated samples. In addition, histomorphometry revealed a significantly higher bone contact on HA coated implants compared to native CoCr implants. It is concluded that free form fabrication in combination with HA coating improves the early fixation in bone under experimental conditions.

Fabrication of bioactive hydroxyapatite/bis-GMA based composite via three dimensional printing

Hydroxyapatite/bis-GMA composites were processed by new technique which comprised three dimensional printing (3DP) of hydroxyapatite and then impregnation by bis-GMA based resin. Two types of composites which used either as-fabricated green 3DP samples or 1,300 degrees C sintered 3DP samples were studied. It was found that both 3DP composites have higher flexural modulus, strength and strain at break than the initial 3DP hydroxyapatite sample. Composites produced from sintered sample has higher hydroxyapatite content, higher density and greater modulus, but lower strength and strain at break than composite produced from green 3DP sample. In vitro toxicity shows that 3DP hydroxyapatite/bis-GMA based composites are non-toxic. Osteoblast cells were observed to attach and attain normal morphology on the surface of composites.

Poly(ethylene terephthalate)/hydroxyapatite biomaterials: preparation, characterization, and in vitro bioactivity

Poly(ethylene terephthalate)/hydroxyapatite (PET/HAp) composites were prepared by mixing HAp powder with a mixture solution of cyclic oligo(ethylene terephthalate) (C-OET) and dibutyl tinoxide catalyst in dichrolomethane, and then shaping the precomposites in cylindrical pellets. The C-OET in the precomposites was ring-opening polymerized (ROP) to PET under vacuum at 250 degrees C for 24 h. The PET/HAp composites were formulated with HAp to PET ratios of 60:40 (H6P4) and 50:50 (H5P5). The ROP-PET in the composites was present as a thin-layer coating on the HAp grains and evenly distributed throughout the samples. Compressive strength of the PET/HAp composites was significantly increased from 8 MPa of the H10P0 to 17 and 29 MPa for H6P4 and H5P5, respectively. In vitro bioactivity of the PET/HAp composites was studied by soaking in simulated body fluid (SBF) at 36.5 degrees C for 7-28 days. After prolonged soaking, the HAp nanocrystals precipitated from the SBF solution and formed as a layer of globular aggregates, coated on the composite surfaces. The H6P4 composite showed faster formation rate of nano-HAp than the H5P5 composite, indicating that the bioactivity of PET/HAp composites depended on the amount of HAp.

The effect of water uptake on the behaviour of hydrophilic cements in confined environments

Physiological fluids will be in contact with the implant components from the first moments after a surgery. Therefore, the study of the effect of water on the properties of the bone cements that are part of the arthroplasty procedure is of critical importance to predict the long-term performance of the whole system. In our research group, we have developed a novel concept, the hydrophilic, partially degradable and bioactive cements which uptake considerably more water than standard bone cements. In this paper, we aimed to study the effect of water uptake (WU) by these cements on their behaviour. The tests were carried out in confined cavities, which represent more accurately the in vivo situation the cement will face (constrained by the bone and prosthesis surfaces). We observed that the equilibrium WU decreased up to 60% (as compared to non-confined situations), depending of the formulation. This decrease resulted in a latent tendency of the cements to swell, and the hindering of such swelling generated a swelling pressure against the constraining walls. The pressure, and consequent press-fitting effect, could be controlled by a number of mechanisms, and resulted in higher stability of the hydrophilic cements, expressed as an increase in the push-out force, required to extract the specimens from such constrained cavities. This effect was only observed in hydrophilic cements, not in commercial, hydrophobic ones used as controls. We conclude that such cements will provide an additional and very useful source of immediate adhesion in the short-term after surgery: water induced press fitting.

Nano-mechanics of bone and bioactive bone cement interfaces in a load-bearing model

Many bioactive bone cements were developed for total hip replacement and found to bond with bone directly. However, the mechanical properties at the bone/bone cement interface under load bearing are not fully understood. In this study, a bioactive bone cement, which consists of strontium-containing hydroxyapatite (Sr-HA) powder and bisphenol-alpha-glycidyl dimethacrylate (Bis-GMA)-based resin, was evaluated in rabbit hip replacement for 6 months, and the mechanical properties of interfaces of cancellous bone/Sr-HA cement and cortical bone/Sr-HA cement were investigated by nanoindentation. The results showed that Young's modulus (17.6+/-4.2 GPa) and hardness (987.6+/-329.2 MPa) at interface between cancellous bone and Sr-HA cement were significantly higher than those at the cancellous bone (12.7+/-1.7 GPa; 632.7+/-108.4 MPa) and Sr-HA cement (5.2+/-0.5 GPa; 265.5+/-39.2 MPa); whereas Young's modulus (6.3+/-2.8 GPa) and hardness (417.4+/-164.5 MPa) at interface between cortical bone and Sr-HA cement were significantly lower than those at cortical bone (12.9+/-2.2 GPa; 887.9+/-162.0 MPa), but significantly higher than Sr-HA cement (3.6+/-0.3 GPa; 239.1+/-30.4 MPa). The results of the mechanical properties of the interfaces were supported by the histological observation and chemical composition. Osseointegration of Sr-HA cement with cancellous bone was observed. An apatite layer with high content of calcium and phosphorus was found between cancellous bone and Sr-HA cement. However, no such apatite layer was observed at the interface between cortical bone and Sr-HA cement. And the contents of calcium and phosphorus of the interface were lower than those of cortical bone. The mechanical properties indicated that these two interfaces were diffused interfaces, and cancellous bone or cortical bone was grown into Sr-HA cement 6 months after the implantation.

Modified PMMA cements for a hydrolysis resistant metal-polymer interface in orthopaedic applications

Amongst the many factors influencing the long-term stability of cemented hip prostheses, the interface between the implant and bone cement is considered to be one of the most susceptible to failure. Osteolysis and loosening of the implant can occur by the interaction of mechanically and/or hydrolytically induced bond failure of the metal-cement interface. In this work, an improvement of the hydrolysis resistance of the titanium-bone cement interface was obtained by cement modification with a bifunctional coupling agent combined with a tribochemical TiO2-modification of the metal surface. Methacryloxypropyl-trimethoxysilane was added as coupling agent to the PMMA monomer in concentrations between 5 and 20 wt.% followed by the testing the shear bond strength of PMMA/titanium joints before and after ageing in physiological saline solution. It was found that the hydrolysis resistance of the metal-PMMA interface could be significantly improved by the modification of the cement. At the same time, the mechanical properties (compressive and bending strength) of the modified cement were not altered by the addition of the coupling agent. The advantage of the modification of the cement matrix is an easy clinical applicability of the procedure maintaining the processing and implantation techniques of the cement material.