Osseointegration in arthroplasty: can simvastatin promote bone response to implants?

Cementless fixation depends on bone ingrowth for long-term success. Simvastatin as a lipid lowering agent has been demonstrated to have osteoanabolic effects. This study was designed to measure the possible effect of simvastatin on implant osseointegration. Bilateral femoral implantation of titanium cylinders was performed in 20 rabbits. Blood lipid levels were measured pre- and postoperatively. Scanning electron microscopy (SEM) was used to measure the percentage of the surface of each implant in contact with bone and mechanical pull-out testing was performed. The blood lipid levels were significantly reduced in the simvastatin group. Histomorphometric examination revealed increased bone ingrowth and mechanical examination showed increased interface strength in the simvastatin group. Mechanical and histological data showed superior stability and osseous adaptation at the bone/implant interface for the simvastatin group. We conclude that simvastatin has potential as a means of enhancing bone ingrowth, which is a key factor in the longevity of cementless implants.     

Cementless fixation of prosthetic components in total arthroplasty of the knee and hip

An appropriately designed implant can be fixed without cement to accurately prepared bone surfaces so as to give results comparable with those of cemented fixation in the short term. The unchanging appearance of the roentgenograms suggests that these results may be maintained in the long term. If these conclusions are valid, polymethylmethacrylate (PMMA) may be thought of as an optional alternative in joint arthroplasty. Thus, theoretically, inert materials, e.g., carbon, can be directly interfaced with the skeleton. Since the authors have achieved interlock with viable bone without relying on bony ingrowth, these materials need not be porous. It is strongly suspected (but as yet unsupported by data) that adequate fixation can be obtained without relying even on the authors' relatively unsophisticated finned peg, provided that (1) the overall shape of the implant achieves interlock on the gross scale, with acceptably low shear stresses at the interface; and (2) the surgical technique achieves a correctly positioned implant applied to accurately cut bone surfaces. However, if for any reason accurate bone cuts can not be achieved, the authors advocate use of PMMA as a valuable fixation agent.     

Bone response to implant surface morphology

This study was designed to measure implant osseointegration using different surface treatments. Bilateral distal intramedullary implantation of titanium cylinders 25 mm × 5 mm was performed in 60 rabbits. The 3 surfaces tested were fiber mesh, mean pore size 400 μ; grit-blasted, mean surface roughness 6 μ; and acid-etched, mean surface roughness 18 μ. Scanning electron microscopy was used to measure the percentage of the surface of each implant in contact with bone at 2, 6, and 12 weeks postimplantation. Mechanical pull-out testing of the bone-implant interface was performed at 12 weeks. Overall, acid-etched surfaces demonstrated greater mean osseointegration than fiber mesh surfaces. All 3 surfaces demonstrated similar interface strengths. Acid etching has potential as a means of enhancing bony apposition in cementless fixation.     

Histology of the implant-bone interface in cemented and uncemented endoprostheses

The introduction of bone cement into endoprosthetic surgery was a significant step forward from the 'pre-cement' era. Nonetheless, its disadvantages soon stimulated the search for better means of anchoring implants without cement. Interest focuses on the so-called interface zone, which is the area between bone and foreign body. On the basis of light-microscopical findings our understanding of the biological and biomechanical interactions has influenced the principles of prosthetic design and the implant materials selected for use. The present article gives a synopsis of the histomorphology of the interface, with special reference to stability, tissue reactions, wear products, and implant fixation at bone surfaces (concerning bony ingrowth, osseointegration, and bonding). In all, 127 histological specimens of total hip and knee revisions were reviewed, in addition to a total of 24 non-cemented polyethylene cups retrieved at autopsy.     

Comparative in vitro assessment of the primary stability of cementless press-fit acetabular cups

Long-term stable osseointegration of porous-coated acetabular cups depends on bony ingrowth within their porous surface. For ingrowth to take place, one must ensure rigid initial fixation of the implant, by means of screws or by impaction or using a threaded ring. Primary stability is a prerequisite for long term stability through bony ingrowth. We tested several cups commonly used in our department to assess their primary stability. The study was done using synthetic EP-Dur polyurethane resin blocks (Bayer, Leverkusen, Germany). The blocks were fixed at a 45 degrees angle to the horizontal. They were subsequently reamed using the appropriate reamers and the cups tested were impacted into the resin blocks. Eleven 52-mm cups were tested. The pull out force necessary to extract each cup was measured. The pull-out strength ranged from 7.63 to 55.46 Nm. We noted that the closer the cup was to a hemisphere, the better was the initial stability. The contact zone was at the periphery, and the greater the contact was with the resin, the better was the stability. Micromovements exceeding 150 microns prevent any bony ingrowth in vivo. Solid osseointegration can thus only be achieved if movements between implant and bone can be prevented. Our study indicated that initial fixation is essentially peripheral and that those cups that demonstrated the highest pull-out values also had the best peripheral contact. Our observations suggest that the geometry of the cup is more important than its surface macrostructure in terms of primary stability. To achieve stable fixation, we recommend using an oversized cup with a flattened dome to allow maximum peripheral contact.     

Influence of a platelet concentrate on prosthetic bone ingrowth in a rabbit model

Recent studies have shown that an increase in bone ingrowth by addition of osteogenic growth factors can reduce micro motion and gross implant motion and contribute to joint implant stability through osseointegration. Platelet-rich plasma (PRP) has the potential to provide growth factors that may be conducive to osteointegration at the bone-implant interface. This study analyzed the influence of PRP on bone ingrowth upon a beaded metal implant in distal femurs of 22 rabbits. Rabbit limbs were randomly assigned to receive an implant plus PRP or plain implant. Half of the specimens were randomly assigned to a 2-week group (n = 20) or a 5-week group (n = 20). Histologic and histomorphometric comparison between implant alone and implant plus PRP, at 2 and 5 weeks, was performed. In both the 2- and 5-week comparisons, there was no statistical difference (p > .05) in bone ingrowth between the control and PRP group, despite a slight increase in trabecular bone growth in PRP groups. This study suggests that PRP is not a major contributing factor to bone ingrowth at the bone-implant interface. This supports growing evidence in the literature that PRP can lead to variable bone growth stimulation in vivo.     

Bioactive titanium calcium phosphate coating for disc arthroplasty: analysis of 58 vertebral end plates after 6- to 12-month implantation

Background contextFrom a biomechanical perspective, the successful outcome of total disc replacement is largely based on the mechanisms of acute fixation obtained at the index procedure and the extent of porous biological osseointegration at the prosthesis-bone interface, ensuring long-term device fixation.PurposeThe present retrospective investigation quantifies the extent of porous osseointegration in cervical and lumbar disc arthroplasty implants containing a bioactive titanium/calcium phosphate coating.Study designBased on radiographic analysis and quantitative histomorphometry, the study was designed to determine the extent of porous osseointegration and whether osseointegration was affected by arthroplasty implant position.Outcome measuresQuantitative histomorphometric analysis of trabecular apposition in metallic backed cervical and lumbar arthroplasty devices.MethodsTwenty-nine disc arthroplasty devices underwent radiographic and histomorphometric analysis after 6- to 12-month implantation. The specimens included 12 cervical porous-coated motion devices implanted in a caprine model, and 17 lumbar Charité devices implanted in a non-human primate baboon. The two prosthetic-bone surfaces (superior and inferior) of each implant were examined for a total of 58 vertebral end plates. The operative motion segments were processed using undecalcified histologic technique with production of high-resolution light photomicrographs and microradiographs used for histomorphometric quantification of trabecular bone area at the implant interface. Based on plain film radiographs and histologic microradiographs, the technical accuracy of implant placement was classified as Ideal, Suboptimal, or Poor, with alignment referenced to the sagittal and coronal planes.ResultsThe technical accuracy of implant placement in the cervical spine based on histologic microradiographs ranged from poor=8% (2 out of 24), suboptimal=17% (4 out of 24), to ideal=75% (18 out of 24), whereas accuracy of lumbar disc arthroplasty ranged from poor=20% (7 out of 34), suboptimal=52% (18 out of 34), and ideal=26% (9 out of 34). Based on histomorphometric analysis of the inferior and superior end plate surfaces, the trabecular apposition ranged from poor placement 21%±30% ingrowth, suboptimal 26%±33%, to ideal=44%±23% (p>.05). Similar findings were observed for the lumbar region; however, the suboptimal and ideal positions were closer in values with regard to trabecular apposition. Poor placement was 34%±29%, suboptimal 49%±19%, and ideal 51%±13%, but this was not statistically significant (p>.05).ConclusionsThe present study represents the largest analysis to date of any retrieved porous ingrowth disc replacement prostheses. A trend was observed of increase porous osseointegration with improved implant positioning; however, the small sample size and high standard deviations account for lack of statistical significance. Although osseointegration occurs despite nonideal intraoperative positioning, it remains imperative that surgeons strive for Ideal implant position.     

High Short-Term Loosening Rates with the Wagner Standard Cup

The stability of prosthetic fixation is to a large extent dependent on component design. The purpose of this study is to analyze the short-term radiological results obtained with the Wagner Standard Cup in primary hip arthroplasty. An assessment was made of one hundred primary hip arthroplasties. The radiological evaluation revealed bone ingrowth in 37 of cases, fibrous integration in 49 and loosening in 14. In summary, osseointegration of the Wagner Standard Cup was unsuccessful in a high percentage of cases. This finding, which was unrelated to the type of stem or bearing surface used, bore a statistically significant relation (P < 0.05) with an observed poor bone coverage of the acetabular component. Although insufficient bone coverage could arguably be attributable to a poor surgical technique, we would tend to agree with other authors that it is rather likely to result from a flawed implant design that impedes osseointegration.     

Enhancement of the fixation of pyrolytic carbon implants by using atomic oxygen texturing

The purpose of this study was to examine the effect on the biocompatibility and biologic fixation of atomic oxygen-textured pyrolytic carbon. The implants consisted of unalloyed pyrolytic carbon rods, with half the length of the rod textured by atomic oxygen, and the other half retaining the as-deposited surface that normally occurs during the manufacturing process. The rods were implanted transcortically across the distal portion of the femurs of 6 adult male rabbits for 8 weeks. The implants were assessed mechanically by measuring the interface strength between the implant and the bone. The implant-bone interface was also examined by light microscopy. No adverse reaction to either the as-deposited or the textured pyrolytic carbon was seen. Percent bone apposition was greater for textured implants than for the as-deposited implants; however, it was not significant. The bone apposition efficiency factor, calculated by dividing the interface strength by the fraction of bone apposition, was greater for the textured implants than for the as-deposited implants. This indicates that the fixation obtained was more effective for the textured implants. The findings of this study suggest that biologic fixation of pyrolytic carbon implants can be enhanced by surface texturing by using direct exposure to atomic oxygen, without compromising its biocompatibility.     

Powder metal-made orthopedic implants with porous surface for fixation by tissue ingrowth

Powder metal-made orthopedic implants with a porous coating provide an effective means for implant fixation by tissue ingrowth. Additionally, the use of metal alloy powders for forming porous surfaces offers the advantage of uniform coatings on complex part shapes. With proper processing, implants with strong, porous surface layers and good substrate mechanical properties can be formed. The in vivo tests have demonstrated the need for initial implant stability to achieve bony ingrowth. Animal studies indicate an optimum pore size range of 50-400 mu, and human hip prostheses with pores in this range appear to function well. A porous surface integrated with a compatible implant design avoids undesirable bone remodeling with these types of implants.     

Acid-etched microtexture for enhancement of bone growth into porous-coated implants

We designed an in vivo study to determine if the superimposition of a microtexture on the surface of sintered titanium beads affected the extent of bone ingrowth. Cylindrical titanium intramedullary implants were coated with titanium beads to form a porous finish using commercial sintering techniques. A control group of implants was left in the as-sintered condition. The test group was etched in a boiling acidic solution to create an irregular surface over the entire porous coating. Six experimental dogs underwent simultaneous bilateral femoral intramedullary implantation of a control implant and an acid etched implant. At 12 weeks, the implants were harvested in situ and the femora processed for undecalcified, histological examination. Eight transverse serial sections for each implant were analysed by backscattered electron microscopy and the extent of bone ingrowth was quantified by computer-aided image analysis. The extent of bone ingrowth into the control implants was 15.8% while the extent of bone ingrowth into the etched implants was 25.3%, a difference of 60% that was statistically significant. These results are consistent with other research that documents the positive effect of microtextured surfaces on bone formation at an implant surface. The acid etching process developed for this study represents a simple method for enhancing the potential of commonly available porous coatings for biological fixation.     

Osteointegration in Compliant Self-Adjusting Compression Fixation Shown by Backscatter Electron Microscopy: A Case Report

Compliant self-adjusting compression implants are a novel approach to increase the durability of megaprosthesis fixation. However, there is no report of current implant designs that documents the bone-prosthetic interface of this implant. A well-fixed compliant, self-adjusting distal femoral replacement was retrieved from a patient undergoing revision unrelated to fixation. The prosthesis-bone interface was preserved, embedded in poly(methyl methacrylate), and sectioned into 2–4-mm slices. Slices were then imaged using backscatter electron microscopy, and ongrowth and ingrowth were quantified using imaging software. The average percentage of bony ongrowth from five successive sections was 52.5%, and the average percentage of ingrowth into the porous titanium surface was 13.5%. We found that bone ongrowth on the cortex between anchor plug and spindle averages more than 50% and up to 70% depending upon the slice examined with backscatter electron microscopy. Bone ingrowth was consistently around 13% on every slice examined. This is a new finding compared with prior spindle designs, likely due to the addition of hydroxyapatite-coated porous metal titanium surface on the spindle. This report is an important step in understanding the mechanism of bony fixation generated by this implant and supports its increased use in oncological and complex reconstructive situations.

     

A comparison of bone remodelling around hydroxyapatite-coated, porous-coated and grit-blasted hip replacements retrieved at post-mortem

We investigated the implant-bone interface around one design of femoral stem, proximally coated with either a plasma-sprayed porous coating (plain porous) or a hydroxyapatite porous coating (porous HA), or which had been grit-blasted (Interlok). Of 165 patients implanted with a Bimetric hip hemiarthroplasty (Biomet, Bridgend, UK) specimens were retrieved from 58 at post-mortem. We estimated ingrowth and attachment of bone to the surface of the implant in 21 of these, eight plain porous, seven porous HA and six Interlok, using image analysis and light morphometric techniques. The amount of HA coating was also quantified. There was significantly more ingrowth (p = 0.012) and attachment of bone (p < 0.05) to the porous HA surface (mean bone ingrowth 29.093 +/- 2.019%; mean bone attachment 37.287 +/- 2.489%) than to the plain porous surface (mean bone ingrowth 21.762 +/- 2.068%; mean bone attachment 18.9411 +/- 1.971%). There was no significant difference in attachment between the plain porous and Interlok surfaces. Bone grew more evenly over the surface of the HA coating whereas on the porous surface, bone ingrowth and attachment occurred more on the distal and medial parts of the coated surface. No significant differences in the volume of HA were found with the passage of time. This study shows that HA coating increases the amount of ingrowth and attachment of bone and leads to a more even distribution of bone over the surface of the implant. This may have implications in reducing stress shielding and limiting osteolysis induced by wear particles.     

Comparative study of human cancellous bone remodeling to titanium and hydroxyapatite-coated implants

The human cancellous bone response was compared in weight-bearing porous hydroxyapatite (HA) and titanium-coated implants placed in the distal medial femoral condyles of consenting staged bilateral knee patients. The Institutional Review Board approved study quantified the amount of bone ingrowth, the mineral apposition rate, and the bone mineral content. Results showed that the osteoconductive HA coating increased the amount of bone ingrowth by 8% (P=.018). The HA coating did not effect the mineral apposition rate of the bone but had an 8% lower bone mineral content at the implant interface (P=.042). The influence of HA coatings on human cancellous bone appears highly focal along the coating surface. Gaps of 50–500 μm filled with fibrous connective tissue were observed along the porous-coated surfaces of both implant types suggesting that HA coatings still require precision placement adjacent to human cancellous bone.     

Massive acetabular bone loss: Limits of trabecular metal cages

Massive acetabular bone loss (more than 50% of the acetabular area) can result in insufficient native bone for stable fixation and long-term bone ingrowth of conventional porous cups. The development of trabecular metal cages with osteoconductive properties may allow a more biological and versatile approach that will help restore bone loss, thus reducing the frequency of implant failure in the short-to-medium term. We report a case of massive bone loss affecting the dome of the acetabulum and the ilium, which was treated with a trabecular metal cage and particulate allograft. Although the trabecular metal components had no intrinsic stability, they did enhance osseointegration and incorporation of a non-impacted particulate graft, thus preventing failure of the reconstruction. The minimum 50% contact area between the native bone and the cup required for osseointegration with the use of porous cups may not hold for new trabecular metal cups, thus reducing the need for antiprotrusio cages. The osteoconductive properties of trabecular metal enhanced allograft incorportation and iliac bone rebuilding without the need to fill the defect with multiple wedges nor protect the reconstruction with an antiprotrusio cage.     

Local alendronate increases fixation of implants inserted with bone compaction: 12-week canine study.

Bone compaction has been shown to increase initial implant fixation. Furthermore, bone compaction creates a peri-implant zone of autograft that exerts osteoconductive properties. We have previously shown that locally applied bisphosphonate (alendronate) at 4-week observation can preserve the autograft generated by bone compaction. We now investigate whether the increased amount of autograft, seen at 4 weeks, can increase implant osseointegration and biomechanical fixation. Porous-coated titanium implants were bilaterally inserted with bone compaction into the proximal part of tibia of 10 dogs. On the right side, local bisphosphonate was injected into the bone cavity prior to bone compaction immediately prior to implant insertion. On the left side, saline was used as control. Observation period was 12 weeks. Locally applied bisphosphonate significantly increased biomechanical implant fixation (approximately twofold), bone-to-implant contact (1.2-fold), and peri-implant bone volume fraction (2.3-fold). This study indicates that local alendronate treatment can increase early implant osseointegration and biomechanical fixation of implants inserted by use of bone compaction. Long term effects remain unknown.     

Bone ingrowth to implant surfaces in an inflammatory arthritis model

Many studies have shown enhanced bone apposition to implants coated with hydroxyapatite, but the optimum implant texture, especially in abnormal trabecular bone, is unclean The purpose of this project was to evaluate the histological and mechanical properties of cylindrical implants with three different surface textures that were placed in the cancellous bone of the distal femur of the rabbit after the production of an inflammatory knee arthritis. The three implant surfaces included a beaded surface (Group A), a beaded surface coated with hydroxyapatite (Group B), and a smooth surface coated with hydroxyapatite (Group C). The right knees of 36 rabbits were injected with carrageenan twice a week for 2 weeks. Then bilateral implantations were performed, with 12 rabbits in each group receiving identical implants in the right and left knees. The rabbits were killed 6 weeks after surgery. Mechanical (push-out test) and histomorphometric analyses were performed to determine the quality and quantity of bone ingrowth. In Group A, there was virtually no direct contact (a 20–60-μm clearance) between the bone and the beaded surfaces. Direct contact between the bone and the implant surfaces was seen in Groups B and C. The thickness and number of trabeculae were smaller on the arthritic side than on the control side for all groups but were not different between groups for either the control or the arthritic side. Mechanical testing showed that the shear strength of the interface was weaker on the arthritic side in all groups. The results suggest that inflammatory arthritis induced by carrageenan may influence the quality of local bone (osteopenic changes) and hence compromise the bone apposition and mechanical stability of the interface between the implant and bone.     

Controlled laser texturing of titanium results in reliable osteointegration

We have developed a laser‐textured superhydrophilic Ti‐6Al‐4V surface with unique surface chemistry and topography that substantially promotes osteoblast adhesion in culture. Here we investigate the osteointegration of laser‐textured implants in an ovine model. Our hypothesis was that laser‐textured implants, without any surface coating (LT), would encourage comparable amounts of bone‐implant contact and interfacial strength when compared with widely accepted hydroxyapatite (HA) coated implants. Additionally, we hypothesized that LT would significantly increase bony integration compared with machine‐finished (MF) and grit‐blasted (GB) implants. Forty‐eight tapered transcortical pins were implanted into six sheep. Four experimental groups (LT, HA, MF, and GB) were investigated (n = 12) and implants remained in vivo for 6 weeks. Bone apposition rates, interfacial shear strength, and bone‐implant contact (BIC) were quantified. The interfacial strength of LT and HA implants were found to be significantly greater than GB (p = 0.032 and p = 0.004) and MF (p = 0.004 and p = 0.004, respectively), but no significant difference between LT and HA implants was observed. Significantly increased BIC was measured adjacent to HA implants when compared with both LT and GB implant surfaces (p = 0.022 and p = 0.006, respectively). No significant difference was found when LT and GB implants were compared. However, all surface finishes encouraged significantly increased BIC when compared with the MF surface. Maximizing implant fixation to host bone is vital for its long‐term success. The production of an LT surface is a simple and cheap manufacturing process and this study demonstrated that laser‐textured implants are a very promising technical development that warrants further research. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:820–828, 2017.

     

Biologic fixation

One of the major problems of implant surgery is the failure of the bone-cement interface. Because of such failures, observed with increasing frequency with the passage of time, biologic fixation of total joint implants by means of bone ingrowth has become the focus of considerable interest among orthopedic surgeons. Actual bone ingrowth has been demonstrated into porous metals, resulting in a strong interface between metal and bone. Many clinical trials of biologic fixation, including endoprostheses and total hip, shoulder, and knee prostheses, are being conducted. The brief experience of such fixation in humans supports the hypothesis that bone ingrowth will provide stable fixation for load-bearing prostheses. Many questions remain to be answered about biologic fixation and the burden that falls upon the clinical scientist to identify the proper niche for this process in the care of the musculoskeletal patient.     

The optimal fixation of the cementless acetabular component in primary total hip arthroplasty.

The optimal fixation of the acetabular component in primary total hip arthroplasty remains controversial. Long-term follow-up studies show that significant loosening rates occur with cemented acetabular components and that these problems persist despite attempts to improve cementing technique. Cementless acetabular components that rely on biologic fixation can have lower rates of radiographic loosening at 10 years compared with cemented acetabular components. Although revision rates for both modes of fixation are largely equivalent at 10 years, the superior radiographic performance of cementless acetabular components at 10 years suggests that biologic fixation through bone ingrowth may provide more durable long-term implant survival compared with cemented fixation. Osteolysis is the major obstacle to long-term cementless acetabular component survival. Potential future options that may inhibit osteolysis include decreasing bone resorption that results from debris-stimulated foreign body response through the use of medications; decreasing the number of particles generated by using alternative bearing surfaces; and improving bone ingrowth, particularly through the use of growth factors and improved implant materials and designs.