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.     

Invasion of bone into porous fiber metal implants in cats

Bone ingrowth in porous Ti-6Al-4V implants in feline femurs was assessed for the amount and composition 4, 12 and 26 weeks after implantation. During the first 12 weeks there was a rapid ingrowth that levelled out during the ensuing weeks. Bone had penetrated the implants to an average depth of 0.5 mm over their entire surface at 26 weeks. The ingrown bone was deficiently mineralized at 4 and 12 weeks, but reached full mineralization at 26 weeks. A low calcium to phosphorus ratio persisted at all observation times. The ingrown tissue was compared to bone filling drill holes that had been made in the trochanters contralaterally to the implants. By means of radioactive strontium, the mineral deposits in the holes and implants could be compared. The overall results indicate that bone ingrowth in a porous implant and bone healing are similar processes both qualitatively and quantitatively.     

The effect of demineralized bone matrix gel on bone ingrowth and fixation of porous implants

The presence of demineralized bone matrix (DBM) gel did not enhance or accelerate attachment strength or bone ingrowth and resulted in a significant decrease in implant interface attachment strength at 3 weeks. Hydroxyapatite (HA) coating resulted in significant increases in interface shear strength and bone ingrowth compared with non-HA-coated porous implants at all time periods. The HA-coated implants achieved greater attachment strength and bone ingrowth at earlier time periods and maintained greater attachment strength at long-term periods. The results of this study indicate that in the presence of a good bone-implant interference fit, there is no beneficial effect in applying DBM gel to a porous-coated or HA-coated porous implant surface. The small amount that can be applied and the degree of osteoinductivity of DBM seem to preclude it from having a significant biologic effect.     

Effects of estrogen deficiency on the growth of tissue into porous titanium implants

The effect of systemic deficiency of estrogen on the growth of tissue into porous titanium-fiber implants was studied in ovariectomized Beagle dogs. Five dogs were ovariectomized and five dogs had a sham operation. After waiting four months to allow the levels of circulating estrogen to decline, a titanium-alloy implant that contained four surface pads of titanium-alloy mesh was implanted in the proximal part of the humerus of each dog bilaterally. Two months later, the implants were harvested and subjected to a mechanical push-out test and quantitative histological study. The push-out strength of the implants from the ovariectomized dogs was 31 per cent less than in the control animals. Ovariectomy caused no difference in the amount of ingrowth of bone but resulted in a significant increase in the amount of fibrous connective tissue within the porous pads. The presence of this fibrous tissue appeared to have an important effect on bone-implant fixation: in the control dogs, strength correlated positively with ingrowth of bone and negatively with ingrowth of fibrous tissue, whereas in the ovariectomized dogs, strength correlated positively with ingrowth of fibrous tissue and not at all with ingrowth of bone.     

Bone ingrowth observed in a cup removed during revision surgery for early dislocation after primary THA: A case report

Introduction and importanceRecently cementless total hip arthroplasty (THA) has shown good long-term results with excellent stability resulted from the porous coating of the implant. A hydroxyapatite-tricalcium phosphate (HA-TCP) coating on the porous surface is expected to promote bone ingrowth and to improve initial fixation of the implant. Here we report a case of bone ingrowth observed in a cup removed during revision surgery for early dislocation, 37 days after primary THA using a porous coating cup with HA-TCP.Case presentationA 61-year-old woman who has bilateral osteoarthritis underwent same-day bilateral THA. Both sides used porous coating cups with HA-TCP. Line-to-line technique and screw fixation were utilized. Anterior dislocation of the left hip occurred on days 27 and 31, and we performed cup revision on day 37 after surgery. We noted bone-like tissue on the posterior surface of the cup and in a vacant screw hole. Bone tissue was also confirmed in pathological findings.Clinical discussionStudies in animals have confirmed early bone ingrowth about 4 weeks after surgery with HA-TCP coated implants. In humans, the earliest report of bone ingrowth in cups is for a cup without HA-TCP coating, detected 5 weeks after surgery. In the present case, we used a porous coating cup with HA-TCP, and bone ingrowth was confirmed at approximately the same time as for the previous case.ConclusionEarly clinical bone ingrowth was confirmed in an HA-TCP coated cup, occurring at about the same time after surgery as in previous reports.     

Clinical experience with the medpor porous polyethylene implant

The Medpor porous polyethylene implant is a highly stable and somewhat flexible porous alloplast that has been shown to exhibit rapid tissue ingrowth into its pores. A total of 116 Medpor implants were placed in 70 patients over a four-year period. Implants were used for the chin, malar area, nasal reconstruction, ear reconstruction, orbital reconstruction, and the correction of craniofacial contour deformities. Many of these implants were placed in areas long considered problematic such as areas of thin soft tissue coverage, extensive scarring, and severe facial burns. Nine complications occurred including seven exposures, all of which occurred in areas of minimal soft tissue coverage. Because of the rapid vascularization of the implants, only two implants were removed, both from the columella. On the basis of our results, it is felt that the Medpor implant is an excellent alternative to existing implant materials. The implant is easy to shape; it is strong yet somewhat flexible; it is remarkably stable; and it exhibits tissue ingrowth into its pores.     

Bioactive porous titanium: an alternative to surgical implants

Abstract:  Porous titanium implants have been used to improve implant–bone attachment by the ingrowth of bone tissue within the porous structure. Despite the efficient bone adhesion of porous titanium implants, chemical bonds are required at bone–implant interface. These implants can become bioactive by a biomimetic precipitation process. The aim of this work was to enhance the bioactivity of pure porous titanium implants by biomimetic process. The samples immersed in a simulated body fluid promoted the nucleation and growth of calcium phosphate (Ca-P) crystals, such as hydroxyapatite (Hap), on the material surface. Scanning electron microscopy, energy dispersive spectroscopy, and Fourier transform infrared spectroscopy analyses revealed that a Ca-P deposition occurred without the need of pretreatments to improve the surface bioactivity. This present study indicates the potential for growing a bone-like Hap layer on porous titanium implants by biomimetic processes.     

A new model to assess tibial fixation in knee arthroplasty. I. Histologic and roentgenographic results

A model to assess tibial fixation in knee arthroplasty is described. Eighteen mongrel dogs were implanted with a right tibial hemiarthroplasty. Implantation was of a press-fit, smooth implant (eight dogs) or a titanium alloy beaded device (ten dogs). Animals were studied at six, 12, 18, and 24 weeks. Roentgenographic and histologic analysis was performed for all implants. All animals were ambulatory. Roentgenographically, smooth implants showed progressive radiolucencies with increasing trabecular sclerosis under the implants. These changes were less prominent under the porous implants. Histologically, fibrous interfaces were identified focally under all implants but were thicker under smooth devices. Eighteen- and 24-week specimens showed substantial ingrowth into the porous pegs but minimal ingrowth into the porous plateau. Concurrent histologic and biochemical evaluation in dogs demonstrates the possible fate of implants in knee arthroplasty in human beings. As has been observed, histologic analyses suggest that clinically satisfactory results do not constitute evidence of stabilization by bone ingrowth.     

The effect of proximally and fully porous-coated canine hip stem design on bone modeling

Porous coated canine femoral hip replacement implants were evaluated for biological fixation by bone ingrowth and the effect of the extent of porous coating on bone modeling. The Co-Cr alloy implants were either fully porous coated or coated only on the proximal 40% of the stem. Two implants of each type were studied 9, 16, and 36 months after surgery. Implant fixation and bone modeling were assessed radiographically throughout the implant periods and histologically after the test animals were killed. All 12 implants appeared stably fixed within the femur and were bone-ingrown in the porous region. Radiographic features such as proximal medial and anterior cortical thinning, proximal cancellous bone hypertrophy, and new endosteal bone formation near the stem tip were noted within the first postoperative year, with no appreciable change thereafter. The extent of proximal cortical thinning varied from virtually none to as much as 40%, being more prominent with the proximally coated implants at 16 months and with the fully coated implants at 36 months. Of consistent note was cancellous hypertrophy at the junction of porous and smooth implant surfaces with proximally coated implants and new endosteal bone formation and ingrowth at the stem tip of fully coated implants. These results indicate that the proximally porous-coated implant design causes increased proximal stress transfer, but this does not necessarily preclude proximal cortical resorption.     

Short-term alendronate treatment does not maintain a residual effect on spinal fusion with interbody devices and bone graft after treatment withdrawal: an experimental study on spinal fusion in pigs

Purpose

Whether alendronate treatment has a residual effect on bone ingrowth into porous biomaterial in humans or experimental animals after treatment withdrawal is still unknown. The purpose of this study was to investigate bone ingrowth into porous tantalum and carbon fiber interbody implants after discontinuing alendronate treatment in experimental spinal fusion in pigs.

Methods

Twenty-four pigs were randomly divided into two groups of each 12 pigs. The pigs underwent anterior intervertebral lumbar arthrodeses at L2–3, L4–5 and L6–7. Each level was randomly allocated to one of the three implants: a porous tantalum ring with pedicle screw fixation, a porous tantalum ring or a carbon fiber cage with anterior staple fixation. The central hole of implants was packed with an autograft. Alendronate was given orally for the first 3 months to one of the two groups. The pigs were observed for 6 months postoperatively. Histology and micro-CT scans were done at the endpoint.

Results

The spinal fusion rates of each implant showed no differences between two treatment groups. Furthermore, no differences were found between two groups as for bone ingrowth into the central holes of implants and bone–implant interface in each implant, or as for the pores of tantalum implants. Trabecular bone microarchitecture in the central hole of the carbon fiber cage did not differ between two treatment groups.

Conclusion

The application of ALN, with a dose equivalent to that given to humans during the first 3 months after surgery, does not maintain a residual effect on spinal fusion with porous tantalum ring and autograft after treatment withdrawal in a porcine ALIF model.     

Influence of disodium (1-hydroxythylidene) diphosphonate on bone ingrowth into porous,titanium fiber-mesh implants

The influence of disodium (1-hydroxythylidene) diphosphonate on the bonding between bone and porous, titanium fiber-mesh implants was studied. Rectangular, porous, titanium fiber-mesh implants (15 × 10 × 2.4 mm) were implanted into the tibial bone of mature male rabbits. The rabbits were divided into six groups. Disodium diphosphonate was administered daily by subcutaneous injection to groups 1–5. Groups 1–4 received doses of 5.0, 2.5, 1.0, and 0.1 mg per kilogram of body weight per day for 8 weeks, respectively. Group 5 received a dose of 5 mg per kilogram of body weight per day for 4 weeks. Group 6 (control group) was given saline injections. At 8 weeks after implantation, the rabbits were killed. The tibiae containing the implants were dissected out and subjected to detachment tests. The failure load, when an implant became detached from the bone or when the bone itself broke, was measured. The interface of the bone and implant was investigated by Giemsa surface staining and contact microradiography. Giemsa surface staining and contact microradiography showed that porous implant bonding to bone tissue was inhibited by a high dose of disodium diphosphonate in groups 1, 2, and 5. Soft tissue was observed at the interface. In groups 3, 4, and 6, bone tissue ingrowth was observed at the interface between the porous implant and bone tissue. Growth of bone into the porous fiber-mesh implant of a cementless prosthesis is possible if a low dose of diphosphonate below 1.0 mg per kilogram of body weight is given subcutaneously     

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.

     

Hip arthroplasty with a Moore prosthesis with porous coating. A five-year study

The five-year clinical experience with Moore hip prostheses with a porous coating has been encouraging. The results have been equivalent to those of a simultaneous experience using cemented prostheses. The patient population chosen for hip arthroplasty using prostheses with a porous coating was a group at high risk for failure with cemented prostheses. The mean Harris hip ratings for the 26 patients have improved rather than deteriorated with time. Based on current knowledge, it is the author's opinion that the implants in these 26 patients were firmly fixed by a combination of fibrous and bony tissue ingrowth. The presence of radiopaque lines in one-third of the patients at a two-year follow-up examination indicates that micromotion often was present. Close inspection of the three- and four-year follow-up roentgenograms reveals that the tissue ingrowth seems to have provided implant fixation sufficiently uniform to prevent the roentgenographic signs of localized high-stress concentration so frequently seen with smooth-stem Moore prostheses of the same design. The tissue ingrowth also has produced stress transfers sufficiently uniform to provide roentgenographic signs of healthy bone modeling. With four-year follow-up roentgenograms now available in ten patients, there has been no roentgenographic evidence of bone resorption due to femoral stress shielding by the relatively high-modulus implant with a porous surface.     

Osteogenic Phenomena Across Endosteal Bone-Implant Spaces with Porous Surfaced Intramedullary Implants

Porous surfaced femoral components of hip prostheses stabilized by tissue ingrowth are often situated a certain distance away from the endosteal cortex in the diaphysis. The purpose of this study was to examine the significance of this space between an implant and the cortex on bone growth into the porous surface of the implant.

Intramedullary rods of different diameters with porous surface regions made of powder metal were inserted into the femurs of adult beagles. The rods had outside diameters of 2.5, 3.2, 4.5, and 5.5 millimeters; this variation produced endosteal bone-implant surface spaces ranging from 0 to 4 millimeters. The animals were sacrificed at 4, 8, 12, and 16 weeks.

Histological sections revealed that by 12 weeks the implants became generally surrounded by a thin shell of spongy bone which was joined to the endosteal cortex by bony trabeculae. This feature was most prominent for implants which were approximately 2 millimeters or less from the endosteum. Denser, more haversian-like bone developed up to and within those areas of implants which were in contact with the cortex.

The development of this intramedullary type of bone could significantly contribute to the fixation strength of clinical porous surfaced prostheses whose stems do not completely fill the medulla.     

Mechanical evaluation of cadaver retrieval specimens of cementless bone-ingrown total hip arthroplasty femoral components

Early retrieval prior to gross failure of implants can provide valuable information for critical issues in total joint arthroplasty. To evaluate fixation of the femoral component in total hip arthroplasty, two femoral specimens—one Mallory-Head (Biomet, Warsaw, IN) specimen and one AML (DePuy, Warsaw, IN) specimen—were retrieved after 2–8 years of successful use in active patients. Radiographs were made and evaluated for trabecular apposition to the porous-coated areas of the stem, then torsional and axial load tests were performed for each specimen to determine micromotion and displacement at the bone-implant interface. Both implants had radiographic signs of bone ingrowth. No permanent rotational displacement was found in either specimen during torsional load testing, but rotational and axial micromotion were found in both. These findings indicate excellent fixation of implant to bone, and no slippage at the bone-implant interface. The Mallory-Head implant had much greater elastic displacement than the AML, and histological examination showed cancellous bone ingrowth into the porous-coated portion of the Mallory-Head stem. The AML implant, which withstood much higher torsional loads, was found upon histological evaluation to have dense cortical-cancellous bone ingrowth. Strength of attachment of the metal implant to bone was good in both specimens, and neither had slippage at this interface. Differences in mechanical behavior can be attributed to the type of bone supporting the implant. The Mallory-Head implant had a severely worn titanium femoral head, so the joint was full of particulate metal debris. Particle migration appeared to be especially well controlled by the closed-pore type of porous coating of the Mallory-Head stem. Since the AML implant had a cobalt-chromium femoral head, and consequently had no detectable metallic wear debris, a comparison of the barrier effect of the two types of porous coating could not be made. The beaded porous surface of the AML femoral component also seemed to be a barrier to polyethylene particle migration. Osteolysis was not found around either implant, and neither implant appeared to be clinically affected by particulate debris despite long-term service.     

Implant fixation by bone ingrowth

The term osseointegration referred originally to an intimate contact of bone tissue with the surface of a titanium implant; the term bone ingrowth refers to bone formation within an irregular (beads, wire mesh, casting voids, cut grooves) surface of an implant. The section dealing with the historical background describes the development of macroporous, microporous, and textured surfaces with an emphasis on the evolution of porous and textured metal surfaces. The principal requirements for osseointegration and bone ingrowth are systematically reviewed as follows: i) the physiology of osseointegration and bone ingrowth, including biomaterial biocompatibility with respect to cellular and matrix response at the interface; ii) the implant surface geometry characteristics; iii) implant micromotion and fixation modes; and iv) the implant-bone interface distances. Based on current methods of bone ingrowth assessment, this article comparatively reviews and discusses the results of experimental studies with the objective of determining local and systemic factors that enhance bone ingrowth fixation.     

Hydroxyapatite coating modifies implant membrane formation. Controlled micromotion studied in dogs.

We studied the influence of controlled micromovements between bone and porous titanium alloy implants with and without hydroxyapatite coating. A dynamically loaded unstable device producing approximately 150-microns axial translation of knee implants during each gait cycle was developed. Stable implants served as controls. Matched stable and unstable implants with either porous titanium (Ti) or hydroxyapatite (HA) coating surrounded by a gap of 0.75 mm were inserted into the weight-bearing regions of the medial femoral condyles in 14 mature dogs. Histologic analysis after 4 weeks showed a fibrous membrane surrounding both types of implants subjected to micromovements, whereas various amounts of bone ingrowth was obtained in the stable implants. The membrane around unstable HA implants was thinner than that around unstable Ti implants. Islands of fibrocartilaginous tissue characterized the membrane around unstable HA implants, whereas fibrous connective tissue surrounded unstable Ti implants. The collagen concentration of the fibrous membranes was higher around unstable HA implants compared with Ti implants. Instability reduced the shear strength of the implants. However, the shear strength of unstable HA implants exceeded that of the Ti implants, both unstable and stable. The greatest shear strength was obtained by stable HA implants, i.e., tenfold greater than that of stable Ti implants. The gap-healing capacity around stable HA implants increased toward the HA surface, and was greater than that around Ti implants. Our study demonstrates that micromovements between bone and implant inhibit bone ingrowth and lead to the development of a fibrous membrane. The superior fixation of unstable HA implants compared with unstable Ti implants may be ascribed to the presence of fibrocartilage, a higher collagen concentration, and radiating orientation of collagen fibers in the membrane. The strongest mechanical anchorage and the greatest amount of bone ingrowth was obtained by stable implants coated with hydroxyapatite.     

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.     

Bone ingrowth into two porous ceramics with different pore sizes: an experimental study

Many properties of porous calcium phosphate ceramics have been described, but how pore size influences bony integration of various porous ceramics remains unclear. This study was performed to quantify the bony ingrowth and biodegradability of two porous calcium phosphate ceramics with four different pore size ranges (45-80 microm, 80-140 microm, 140-200 microm, and 200-250 microm). Hydroxyapatite (HA) and beta-tricalcium phosphate (TCP) cylinders were implanted into the femoral condyles of rabbits and were left in situ for up to 12 months. The percentage of bone ingrowth and the depth of ingrowth within the pores were determined. Biodegradability of the implants was also evaluated. Bone ingrowth occurred at a higher rate into the TCP than into the HA ceramics with the same pore size ranges. The amount of newly formed bone was statistically smaller (p < 0.05) into ceramics with 45-80 microm pore size than with larger pore size, whatever the implantation time for HA and until four months for TCP. No statistical difference was noted between the three highest pore size ranges. No implant degradation was noted up to four months. Our results suggest that a pore size above 80 microm improves bony ingrowth in both HA and TCP ceramics. Bone formation was higher in the TCP than in the HA implants.     

Ultrastructural and Electron Diffraction of the Bone-Ceramic Interfacial Zone in Coral and Biphasic CaP Implants

We investigated the influence of natural coral implants used as a bone substitute on the quality of bone ingrowth in rabbits 2, 3, and 6 weeks after implantation. Explants were characterized by transmission electron microscopy and electron diffraction. Bone ingrowth has been previously demonstrated by light microscopy, however, few have been performed in electron microscopy to compare mineralized tissue ingrowth in coral implants which occurs at the expense of calcium carbonate to that of calcium phosphate (CaP) implants. The interface between coral aragonite and mineralized tissue or bone was abrupt, with no invasion of the aragonite structure by newly formed crystals, as occurs in micropores when biphasic CaP (BCP) ceramics were used. The restoring process appears to be different from that induced by BCP implants. Precipitation of needle-like apatite crystals on the CaCO₃ implant surface was not observed. Instead, apatitic smooth-shaped crystals formed in aggregates. The coral dissolution process does not release phosphate and so precipitation of apatite does not occur in the micropores of the coral implant, thereby limiting the formation of an apatite layer and hence bone bonding to the outer surface of the implant. In addition, on the outer surface of the implant, close to bone and a phosphorus source, the CaP crystals that do form are in aggregates presumably due to the carbonate and mismatch between the aragonite and the apatite. This seems to result in a delayed bone attachment or weaker bone bonding than CaP implants which encourage an epitaxial biological crystal deposition. Received: 21 July 1995 / Accepted: 7 August 1997