A Comparative Finite-Element Analysis of Bone Failure and Load Transfer of Osseointegrated Prostheses Fixations

An alternative solution to conventional stump–socket prosthetic limb attachment is offered by direct skeletal fixation. This study aimed to assess two percutaneous trans-femoral implants, the OPRA system (Integrum AB, Göteborg, Sweden), and the ISP Endo/Exo prosthesis (ESKA Implants AG, Lübeck, Germany) on bone failure and stem–bone interface mechanics both early post-operative (before bony ingrowth) and after full bone ingrowth. Moreover, mechanical consequences of implantation of those implants in terms of changed loading pattern within the bone and potential consequences on long-term bone remodeling were studied using finite-element models that represent the intact femur and implants fitted in amputated femora. Two experimentally measured loads from the normal walking cycle were applied. The analyses revealed that implantation of percutaneous prostheses had considerable effects on stress and strain energy density levels in bone. This was not only caused by the implant itself, but also by changed loading conditions in the amputated leg. The ISP design promoted slightly more physiological strain energy distribution (favoring long-term bone maintenance), but the OPRA design generated lower bone stresses (reducing bone fracture risk). The safety factor against mechanical failure of the two percutaneous designs was relatively low, which could be improved by design optimization of the implants.     

Initial stability of a new hybrid fixation hip stem: experimental measurement of implant-bone micromotion under torsional load in comparison with cemented and cementless stems

A new hybrid fixation stem, named cemented-locked uncemented (CLU), for total hip arthroplasty was developed to achieve good initial stability. Primary stability is guaranteed by the cement which is injected into two pockets in the lateral area. This leaves a large surface available for long-term biologic fixation (direct bone attachment on implant). This study evaluates in vitro the initial stability of the CLU prototype under torsional load, in comparison with cemented and cementless stems. The results show that the CLU stem is very stable in simulated stair climbing. Its micromotions are comparable to those of a cemented prosthesis, and significantly less (80-90% lower) than those for a cementless stem. These findings confirm the optimal initial stability expected from the CLU prototype. This new design, which employs hybrid fixation, should improve bone formation on the implant and reduce the risk of stem loosening.     

Bioactive glass microspheres as osteopromotive inlays in macrotextured surfaces of Ti and CoCr alloy bone implants: trapezoidal surface grooves without inlay most efficient in resisting torsional forces

We have tested the efficacy of porous bioactive glass (BG) inlays in enhancement of implant osseointegration. A total of 24 sheep underwent bilateral surgical implantation of three parallel implants on the anteromedial cortical surface of each tibia. The disc-shaped implants made of Ti6Al4V or cobalt chromium (CoCr) alloys had two parallel surface grooves (trapezoidal space with bottom widening) filled with sintered 100% bioactive glass microspheres or a selected mixture of bioactive and biocompatible glass microspheres. The surface of uncoated control implants was smooth, grit-blasted or had unfilled grooves. A subgroup of control smooth CoCr implants was coated with two or three BG layers. Implant incorporation with bone was evaluated using torque testing to failure, scanning electron microscopy and morphometry at 12 and 25 weeks. A total of 144 in vivo implants and 16 ex vivo cemented control implants were analyzed. Control Ti6Al4V implants with unfilled trapezoidal grooves showed highest torsional failure loads with excellent ingrowth of new bone and remodeling of ingrown bone into lamellar bone. Implants with BG inlays and microroughened control Ti6Al4V implants showed significantly lower torsional failure loads than control Ti6Al4V implants with unfilled grooves. In conclusion, BG inlays failed to enhance biological implant fixation. Macrotextured surface was more effective than grit-blasting in promotion of mechanical incorporation.     

Loosening of total hip prosthesis

The problem of total hip prosthesis is an immediate and long-term stability. An insufficient bone anchorage of the prosthesis produces a tissue reaction around the implant that causes a loosening. The hight incidence of loosening of cemented total prosthesis has spurred the research into uncemented implants. The experience at the Busto Arsizio Hip Center (Director Prof. R. Bombelli) with more than 2,800 R.M. uncemented isoelastic total hip prostheses was favorable and the study with the cooperation of Pathology Department (Director Prof. P. Lampertico) of the 82 loosening hips has shown: 1) the importance of the elasticity for a stress transmission to the surrounding bone; 2) the pursuit of the most suitable stem calibre and length; 3) the necessity of a stable primary mechanical fixation, waiting for a biological reaction.     

The load carrying and fatigue properties of the stem-cement interface with smooth and porous coated femoral components

Porous coated surfaces for fixation of total hip replacement are a current trend in clinical orthopedics. Such devices are designed to be fixed by ingrowth of bony tissue, although in the absence of FDA approval for biologic fixation, fixation with PMMA cement is recommended by the implant manufacturers. In order to characterize the mechanical properties of the micro-interlocked stem-cement interface, we tested both porous coated and smooth femoral components in cement mantles of consistent overall geometry. Under conditions of increasing load the smooth stems demonstrated stepwise irreversible subsidence into the mantle. Axial and circumferential strains measured in the cement containment vessels with the smooth stems showed that stepwise increases in tensile hoop strain occurred concomitantly with the stepwise incidents of stem subsidence. When subjected to the same loading conditions, the porous coated stems did not undergo stepwise incidents of subsidence, and hoop strain generation was reduced. In addition, a twofold increase in the failure load of the stem-cement interface was measured with the porous coated stems. Fatigue loading for 10(7) loading cycles did not result in gross failure of either the micro-interlocked or smooth interfaces. However, the data showed that during fatigue loading, stepwise subsidence of the smooth stems again occurred. The final subsidence magnitude of the smooth stem-cement interface at 10(7) loading cycles was six times greater than the value associated with the porous coated stem. Thus the porous coating of femoral stems was shown to dramatically improve the load carrying capability and fatigue characteristics of the stem-cement interface.     

Implant fracture of the Regenerex® modular metal tibial component: A report of three cases

BackgroundImplant fractures are a very rare complication in primary total knee replacement (TKR) surgery and with modern implant designs and improved metals these events have nearly been eliminated. In this case series we report three cases of tibial metal baseplate fractures in uncemented Regenerex® TKR.MethodsCases originated from a prospective case series of 80 patients operated between 2013 and 2016. Five patients were pilot cases and 75 were participants in a prospective randomized double-blinded clinical trial that evaluated different adjuvant bone anti-resorptive medical therapies. All patients were treated with an uncemented press-fit Regenerex® Porous Titanium Construct tibial tray and matching cemented (Refobacin Bone Cement R) patella and femoral components (hybrid implant).ResultsWe report three cases of medial side metal baseplate fractures of a modular finned tibial stem. All three baseplate fractures were in male patients. Confirmed failure of the implant occurred after 10, 12 and 23 months, in situ, with a mean follow-up of 15 months (range 10-23).ConclusionsBased on the current case series we cannot make any causal inferences. Failures may represent a multifactorial process with a cascade of events with implant failure as the result. However, like in most other case reports of metal failures in the literature, the implant fractures in this report were located on the medial side of the tibial component in male patients.     

Does Increased Bone–Cement Interface Strength have Negative Consequences for Bulk Cement Integrity? A Finite Element Study

Implant loosening is one of the most important modes of failure of cemented total hip replacement. It may be related to the cement strength, cement–prosthesis interface, cement–bone interface, surgical technique, or stem design. The main purpose of this study is to investigate the effect of bone–cement interface mechanical properties on cement degradation. The computational methodology proposed herein combines a previously developed bone–cement interface damage model and an accumulative damage model for bulk cement. This has been applied to a finite element model of an Exeter cemented hip implant. A higher strength of the bone–cement interface due to a higher amount of interdigitated bone results in faster cement deterioration. Over time, damage both at the bone–cement interface and in the cement mantle worsens. Also, a larger debonded area was predicted proximally, as observed in clinical practice. We conclude that the computational model proposed herein allows a realistic simulation of the bone–cement interface debonding and cement degradation, being a useful tool in the design of this kind of medical devices.     

Peri-implant bone formation and implant integration strength of peptide-modified p(AAM-co-EG/AAC) interpenetrating polymer network-coated titanium implants

Interpenetrating polymer networks (IPNs) of poly (acrylamide-co-ethylene glycol/acrylic acid) functionalized with an -Arg-Gly-Asp- (RGD) containing 15 amino acid peptides, derived from rat bone sialoprotein (bsp-RGD(15), were grafted to titanium implants in an effort to modulate bone formation in the peri-implant region in the rat femoral ablation model. Bone-implant contact (BIC) and bone formation within the medullary canal were determined using microcomputed tomography at 2 and 4 weeks postimplantation. BIC for bsp-RGD(15)-IPN implants was enhanced relative to hydroxyapatite tricalcium phosphate (HA-TCP) coated implants, but was similar to all other groups. Aggregate bone formation neither indicated a dose-dependent effect of bsp-RGD(15) nor a meaningful trend. Mechanical testing of implant fixation revealed that only the HA-TCP coated implants supported significant (>1 MPa) interfacial shear strength, despite exhibiting lower overall BIC, an indication that bone ingrowth into the rougher coating was the primary mode of implant fixation. While no evidence was found to support the hypothesis that bsp-RGD(15)-modified IPN coated implants significantly impacted bone-implant bonding, these results point to the lack of correlation between in vitro studies employing primary osteoblasts and in vivo wound healing in the peri-implant region.     

Time-lapsed imaging of implant fixation failure in human femoral heads

The failure mechanisms of bone–implant constructs are still incompletely understood, because the role of the peri-implant bone in implant stability is unclear. We hypothesized that implant fixation failure is preceded by substantial peri-implant bone failure. A new device was developed that combines mechanical testing of large bone–implant constructs with high-resolution peripheral quantitative computed tomography, following the principles of image-guided failure assessment (IGFA). In this study, we investigated the push-in failure behavior of dynamic hip screws (DHS) implanted in human cadaveric femoral heads. For the first time the fixation failure of a clinically used implant in human trabecular bone could be experimentally visualized at the microstructural level. The ultimate force was highly correlated with the peri-implant bone volume fraction (R² = 0.85). We demonstrated that primary fixation failure of DHS implants was accompanied by trabecular bone failure in the immediate peri-implant bone region only. Such experimental data are crucial to enhance the understanding on the quality of the bone–implant interface and of the trabecular bone in the process of implant fixation failure. We believe that this newly developed device will be beneficial for the development of new implant designs, especially for use in osteoporotic bone.     

A comparison between cemented, press-fit, and HA-coated interfaces in Kinemax total knee replacement

There are possible advantages of using uncemented fixation in total knee replacement. In this prospective randomised multi-centre study, a comparison was made between cemented and two types of uncemented fixation for the Kinemax design. There were 12-14 cases in each group. Beads were inserted in the bones from which component migration was measured at time intervals up to 2 years. The axial migrations were significantly less for cemented and HA-coating, compared with press-fit, at all time intervals. The clinical data showed no differences at 2 years except for more cases of pain in the uncemented groups. Radiographically, the cemented interfaces showed the least change, press-fit showed a radiolucent line and a radiodense line, and HA showed a diffuse radiodensity adjacent to the components. It was concluded that for the Kinemax design of tibial component, press-fit was inferior to cemented, but that there was the potential for designing a special component for uncemented fixation for which HA-coating would be an advantage.     

Bone response to a novel highly porous surface in a canine implantable chamber

Long-term survival of uncemented hip components is dependent upon successful biological fixation. This study examined a new prosthetic surface treatment consisting of a highly porous open structure of commercially pure titanium, Tritanium Dimensionalized Metal; its overall porosity is approximately 65-70%. With the use of an implantable chamber in dogs, the effects of this treatment on bone ingrowth and strength of attachment were compared to both titanium (overall porosity of 30-35%) and cobalt chrome beads (overall porosity of 35-40%), with and without hydroxyapatite coating. At 6 and 12 weeks, chambers were explanted and specimens underwent high-resolution radiographic imaging and mechanical testing. At 12 weeks, Tritanium surfaces had greater bone penetration and tensile strength than remaining surface types. Over 40% of the Tritanium specimens had a tensile strength greater than 500 N, exceeding the testing capability of the servohydraulic equipment. The highly porous Tritanium surfaces allow for a far greater amount of bone ingrowth than beaded surfaces, and may create a geometry that enhances mechanical strength. Tritanium Dimensionalized Metal surface treatment may result in a clinically valuable implant fixation surface to induce rapid ingrowth and a strong bone-implant interface, contributing to increased implant survivorship.     

Anisotropic bone remodeling of a biomimetic metal-on-metal hip resurfacing implant

Hip resurfacing (HR) is a highly attractive option for young and active patients. Some surgeons have advocated cementing the metaphyseal stem of the femoral component to improve fixation and survivorship of HR. However, extending component fixation to the metaphysis may promote femoral head strain shielding, which in turn may reduce survival of the femoral component. Replacing the metallic metaphyseal stem by a composite material with bone-matching properties could help to alleviate this phenomenon. This study uses finite element analysis to examine the strain state in the femoral head for three types of implant fixation: an unfixed metallic stem, an osseointegrated biomimetic stem and a cemented metallic stem. Bone remodeling is also simulated to evaluate long-term bone resorption due to strain shielding. Results show that the unfixed stem causes strain shielding in the femoral head, and that cementing the stem increases strain shielding. The biomimetic stem does not eliminate the strain shielding effect, but reduces it significantly versus the metallic cemented version. The current finite element study suggests that an osseointegrated metaphyseal stem made of biomimetic material in hip resurfacing implants could become an interesting alternative when fixation extension is desired.     

Assessment of the primary stability of revision tibial trays augmented with a cementless sleeve in AORI Type III defects

BackgroundPrevious studies have evaluated the initial stability of uncemented tibial components in revision total knee replacement (rTKR) in the presence of an Anderson Orthopaedic Research Institute (AORI) Type II tibial defect. This study sought to evaluate similar metrics in the severe Type III (AORI TIII) defects with combined uncemented stem and sleeve fixation, specifically, the effect of varying the stem’s length and tibial canal engagement upon stability and bone strain.MethodFinite element models generated from the CT scans of 4 tibias with Type III defects were used to investigate the primary stability, in terms of the bone–implant composite peak micromotion (CPM) and microstrains (CPS), achieved after virtual implantations with and without stems.ResultsA stemless rTKR had increased metaphyseal CPM and CPS compared to all stemmed implants. Significant area of the bone supporting the stemless rTKR had CPS greater than bone yield (7000 με). Short engaging stems (≤150 mm construct length), could not achieve reliable engagement in the diaphysis (canal fill ≤ 50%), leading to insufficient reduction of CPS (≥5000 με). Longer engaging stems (170–220 mm construct length), were able to reliably engage the diaphysis (fill ratio ≥ 75%) resulting in CPS ≤ 5000 με. Although, non-engaging stems resulted in increased CPM and CPS compared to engaging stems, long non-engaging stems (170–220 mm construct) appeared to provide additional stability to the rTKR compared to stemless rTKR.ConclusionThe results indicate a likely correlation between uncemented stem engagement and metaphyseal CPS in Type III defects. Excessive strain within the supporting metaphyseal bone is likely to lead to rTKR migration and loosening.     

Preliminary studies on the effects of direct current on the bone/porous implant interfaces

The effect of small direct current (∼8μA) upon the tensile strength of the interfacial union between porous metal and polymer implants (100–200 μm diam pores) and bone was studied in the femur of dogs. The present results show that the electrical stimulation accelerates the rate of bony tissue growth into pores of the implants regardless of the nature of the materials tested. It is particularly exciting to find that the porous metal can be used as an electrode as well as an implant. This implies that an ordinary prosthesis made of metal can be made porous and the fixation time period can be shortened by the electrical stimulation. This has an enormous potential for all the direct fixation prostheses for rapid achievement of fixation.     

Balancing incompatible endoprosthetic design goals: a combined ingrowth and bone remodeling simulation

In order to design a good cementless femoral implant many requirements need to be fulfilled. For instance, the range of micromotions at the bone-implant interface should not exceed a certain threshold and a good ratio between implant-bone stiffness that does not cause bone resorption, needs to be ensured. Stiff implants are known to evoke lower interface micromotions but at the same time they may cause extensive resorption of the surrounding bone. Composite stems with reduced stiffness give good remodeling results but implant flexibility is likely to evoke high micromotions proximally. Finding a good balance between these incompatible design goals is very challenging. The current study proposes a finite element methodology that employs subsequent ingrowth and remodeling simulations and can be of assistance when designing new implants. The results of our simulations for the Epoch stem were in a good agreement with the clinical data. The proposed implant design made of porous tantalum with an inner CoCrMo core performed slightly better with respect to the Epoch stem and considerably better with respect to a Ti alloy stem. Our combined ingrowth and remodeling simulation can be a useful tool when designing a new implant that well balances mentioned incompatible design goals.     

Enhanced fixation of implants by bone ingrowth to titanium fiber mesh: effect of incorporation of hydroxyapatite powder

Tight fixation between bone and implant materials is of great importance for a successful outcome of procedures such as total knee arthroplasty (TKA) and total hip arthroplasty (THA). Titanium fiber mesh is an attractive structure for the establishment of tight fixation between bone and implant by bone ingrowth into the spaces among the fibers. Enhancement of bone ingrowth is desired not only for tight fixation but also for a fast recovery. Our hypothesis is that just the presence of hydroxyapatite (HA) particles ensures improved bone ingrowth, and that long-term stability can be obtained by mechanical anchoring of bone in the spaces among titanium fibers. In this study, we examine our hypothesis by in vivo experiment using dog femur. HA particles were incorporated in titanium fiber mesh coated on titanium alloy rod by dipping in a slurry of HA with hydroxy-propyl-cellulose in an ethanol solution. Specimens were implanted for 3, 5, and 8 weeks, and were then compared with the results from specimens without the use of HA. Bonding strength was evaluated by push-out test, and histomorphometric measurements were made with analysis software to calculate the average value of bone ingrowth. A significantly higher bonding strength was observed for the specimens with HA-incorporated implant at 3 and 5 weeks, and larger bone ingrowth deep inside the titanium fiber mesh was measured at 3 weeks. Our proposed method has the additional advantage of not requiring a high temperature that may result in changes in characters of HA powder such as phase transition, grain growth, and decomposition. Moreover, this technique of HA powder incorporation without high-temperature treatment allows the use of several types of metallic fiber mesh, as well as the application to fiber mesh made of organic polymers. We conclude that this simple modification of titanium fiber mesh with HA powder can improve the fixation of implant to bone in the initial stage after operation.     

Cemented versus uncemented fixation of the femoral component of the NexGen CR total knee replacement in patients younger than 60 years: A Prospective Randomised Controlled RSA Study

The optimal mode of femoral fixation in total knee arthroplasty (TKA) remains controversial, especially for the young patient. In a prospective randomised study we compared the magnitude and pattern of the fixation of cemented versus uncemented femoral components during 2 years in patients younger than 60 years. Forty-one knees in 41 patients were randomised to receive a NexGen (Zimmer, Warsaw, USA) cruciate-retaining TKA with either a cemented or an uncemented non HA-coated femoral component. The patients were examined by radiostereometric analysis (RSA), as well as clinical and radiological evaluation. The magnitude and pattern of migration as measured by RSA did not differ significantly between the cemented and uncemented fixation during the 2-year follow-up, nor were there any differences between the groups in clinical parameters. These findings suggest that an uncemented and non HA-coated femoral component may behave equally as well as a cemented one in the long-term.     

Evaluation of sleeved implant fixation using a rat model

Cementless hip implants have some osteolysis, stress shielding, and/or revision problems while the cemented ones have cement fracture problems. To overcome these problems, a new concept of implant fixation using a sleeve between the implant and the bone is proposed. This is a cementless fixation using a sleeve in place of the cement with a press-fit stem. The sleeved implant fixation is compared with the conventional cemented fixation by measuring the bone volume fraction and interface strength in a rat model. The tapered implant was manufactured using a stainless-steel Kirschner-wire with a diameter of 1.6mm, and PEEK (polyetheretherketone) was used for the sleeve. The Sprague-Dawley rats had implantations in their bilateral diaphyseal cut femurs using cemented and sleeved fixation, respectively. The rats were sacrificed at 2, 4, and 12 weeks after the implantation, and micro-CT scanning as well as push-out testing were performed. There was no significant difference between the cemented fixation and the sleeved fixation in areas of both BV/TV (bone volume/total volume) (p=0.838) and the apparent interface strength (p=0.530). However, the decrease in the BV/TV over time was significant in the cemented fixation (p=0.021) while not in the sleeved fixation (p=0.603). The sleeved fixation showed a gain in strength with time while the cemented fixation showed a reverse tendency but the difference was not significant (p=0.125). The results showed the feasibility of the sleeved fixation even though the advantage of sleeved fixation was not clearly confirmed because of the small sample size and high variance as well as the suboptimal surface finish of the sleeve.     

Micromotion-induced strain fields influence early stages of repair at bone–implant interfaces

Implant loading can create micromotion at the bone–implant interface. The interfacial strain associated with implant micromotion could contribute to regulating the tissue healing response. Excessive micromotion can lead to fibrous encapsulation and implant loosening. Our objective was to characterize the influence of interfacial strain on bone regeneration around implants in mouse tibiae. A micromotion system was used to create strain under conditions of (1) no initial contact between implant and bone and (2) direct bone–implant contact. Pin- and screw-shaped implants were subjected to displacements of 150 or 300 μm for 60 cycles per day for 7 days. Pin-shaped implants placed in five animals were subjected to three sessions of 150 μm displacement per day, with 60 cycles per session. Control implants in both types of interfaces were stabilized throughout the healing period. Experimental strain analyses, microtomography, image-based displacement mapping, and finite element simulations were used to characterize interfacial strain fields. Calcified tissue sections were prepared and Goldner trichrome stained to evaluate the tissue reactions in higher and lower strain regions. In stable implants bone formation occurred consistently around the implants. In implants subjected to micromotion bone regeneration was disrupted in areas of high strain concentrations (e.g. >30%), whereas lower strain values were permissive of bone formation. Increasing implant displacement or number of cycles per day also changed the strain distribution and disturbed bone healing. These results indicate that not only implant micromotion but also the associated interfacial strain field contributes to regulating the interfacial mechanobiology at healing bone–implant interfaces.     

Stress distribution in porous surfaced medullary implants

Finite element stress analysis has been applied to examine the stress patterns in a prosthesis requiring fixation in the medullary shaft of a long bone. No specific prosthesis is considered but rather a generally applicable geometry has been chosen. This consists of a cylindrical section of cortical bone within which is implanted a prosthesis composed of a solid central rod surrounded by a porous coating. The finite element analysis utilized an axisymmetric model to determine the distribution of stresses throughout the system. The effect of changes in length of prosthesis, thickness of porous coating, depth and type of tissue ingrowth, and type of porous coating material were studied under conditions of axisymmetric loading. The results indicate that with complete bone ingrowth, the maximum shear stress and the distance necessary for load transfer are both independent of implant length. However, with incomplete ingrowth, increasing implant length reduces shear. Incomplete growth also produces lower shear stresses but higher shear strains in areas without ingrowth. In addition, a porous polyethylene coating gives a more even load transfer and lower shear than a porous coating of a high modulus material.