Tissue response to porous-coated implants lacking initial bone apposition

Although initial bone apposition of a porous-surfaced implant is desirable, it is not always achieved surgically. A model to study the effect of a gap on the quantity and quality of bone growth in both the cancellous and cortical regions has been developed. Implants were surgically placed in the intramedullary canals of adult dogs producing uniform gap spaces 0.0-2.0 mm wide. Histologic and microradiographic evaluations were conducted after 3, 6, and 12 weeks in situ. The results demonstrate that the initial apposition of a porous implant to the surrounding bone surface is not necessary for fixation by bone ingrowth. New bone will grow up to and within the porous structure of an implant even when there is a gap as large as 2.0 mm. However, the rate and degree of maturity and mineralization is enhanced when the gap width is 0.5 mm or less. The amount of bone activity in the cortical region was greater than in the cancellous region at 3 and 6 weeks after operation. After 12 weeks in situ bone growth in gap spaces and into the porous coating was approximately equal.     

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.     

Effect of noninvasive low intensity ultrasound on bone growth into porous-coated implants

Noninvasive low intensity ultrasound has been shown to be an effective means of accelerating bone fracture healing in both animal and clinical studies. An in vivo canine study was designed to determine if noninvasive low intensity ultrasound could influence the rate and extent of bone growth into porous-coated implants. Twenty-two pairs of fully porous transcortical implants were inserted bilaterally into the femora of 12 dogs. In each dog, one femur served as a control and the other was subjected to daily ultrasound stimulation for 2, 3, or 4 weeks. Overall, the ultrasound-stimulated implants demonstrated an 18% increase in bone ingrowth compared with their contralateral controls (p = 0.02). Noninvasive low intensity ultrasound had its greatest effect in the first 2–3 weeks of stimulation. At 2 and 3 weeks, the ultrasound-stimulated implants showed 21 and 16% more ingrowth than their respective contralateral controls. Because noninvasive low intensity ultrasound had a positive effect on bone ingrowth in this experimental investigation, further research is suggested to assess the clinical potential for application to noncemented porous-coated total joint replacements.     

Progression of human bone ingrowth into porous-coated implants: Rate of bone ingrowth in humans

We report the measured progression of human cancellous bone ingrowth into load-bearing porous-coated titanium implants over 5 time periods (0,3,6, 9, and 12 months). There was a statistically significant progression of bone ingrowth into the implants over a 9-month period, but the 9- and 12-month data were not different. Investigators are advised to analyze time “0” implants in order to distinguish mechanical impaction of bone from the biological process of bone ingrowth.     

Mediators of bone resorption around implants.

An important cause of prosthetic loosening is bone resorption that results from the interaction of macrophages with implant surfaces and particulate debris. The mediators involved in this bone resorption were investigated in vitro. Medium conditioned by macrophages interacting with foreign materials was assayed for bone resorption stimulation and inhibition, and for prostaglandin E2 (PGE2). In some experiments, the medium was dialyzed, and in others indomethacin was added. Macrophages were found to release stimulators and inhibitors of bone resorption. The relative amount of these was variable. When macrophages interacted with foreign surfaces, their stimulatory effect was ten times their inhibitory effect. Further activation by phagocytosis caused a further 15-fold increase in stimulation, with no change in inhibition. It is probable that before macrophages encounter foreign materials their stimulatory and inhibitory effects are the same so that they do not cause net bone resorption. Important stimulatory and inhibitory mediators were not dialyzable and so were probably cytokines or possibly collagenase. Prostaglandins were neither important stimulators nor inhibitors. There was only enough PGE2 released to account for 2% of the bone resorption that was stimulated.     

Bone mineral density around femoral stems DXA measurements in 22 porous-coated implants after 5 years

In this cross-sectional study the bone mineral density (BMD) close to proximal porous-coated femur stems was measured by DXA 4.5-6 years after the stem implantation and compared to the contralateral non-operated femur in 22 cases. Measuring areas were defined by Gruen's method.

The average precision error varied according to the zone assessed and ranged from 2.2 to 4.9 percent. The median BMD values of the operated femur were in 5 of 7 areas lower than those of the contralateral side. The largest differences were noted in the calcar region and the greater trochanteric region (21 and 20 percent, respectively). No correlation was found between the femoral BMD differences and the stem diameter.     

Bone mineral density around femoral stems DXA measurements in 22 porous-coated implants after 5 years

In this cross-sectional study the bone mineral density (BMD) close to proximal porous-coated femur stems was measured by DXA 4.5-6 years after the stem implantation and compared to the contralateral non-operated femur in 22 cases. Measuring areas were defined by Gruen's method.

The average precision error varied according to the zone assessed and ranged from 2.2 to 4.9 percent. The median BMD values of the operated femur were in 5 of 7 areas lower than those of the contralateral side. The largest differences were noted in the calcar region and the greater trochanteric region (21 and 20 percent, respectively). No correlation was found between the femoral BMD differences and the stem diameter.     

Inflammatory response in retrieved noncemented porous-coated implants

One hundred forty-six noncemented porous-coated hip and knee implants retrieved from 97 patients were evaluated histologically for the type, amount, and anatomic distribution of tissue ingrowth. The degree of inflammatory cell infiltrate present was also evaluated and the predominant cell type was identified. An inflammatory infiltrate was present in the components of 21 of 97 patients (22%). In 16 of the 21 cases the infiltrate was lymphocytes and histiocytes with a minor population of plasma cells. One of the remaining five cases had a predominately plasma cell reaction, and the other four had significant populations of plasma cells. Vascular proliferation was observed in nine of the 21 cases. Bone ingrowth was present in ten of the 21 cases. A 38% incidence of removal for persistent pain was present in cases with an inflammatory infiltrate. Seventeen of 87 patients (20%) with cobalt-chromium devices and four of ten patients (40%) with titanium devices were identified as having an inflammatory infiltrate. The origin of the inflammatory infiltrate is unclear. All patients with inflammatory infiltrates had noninfected implants, which were not loose roentgenographically or clinically at the time of removal. Hypersensitivity and allergic responses to metal ions may produce such infiltrates. It is impossible, however, in the present study to definitively determine the etiology of the infiltrates.     

Relative effects of wound healing and mechanical stimulus on early bone response to porous-coated implants

We hypothesized that early bone adaptation to well fixed porous-coated implants is influenced more by wound healing than by mechanical loading. To test this hypothesis, two groups of dogs with identical, hydraulically controlled porous-coated implants interference fit within distal femoral trabecular bone were used. One group had no load; the other had 35 N of load applied to the implants. At 5 weeks after surgery, the resulting adaptation of bone around the implants was quantified on a cellular basis by cytochemical analysis of type-I procollagen synthesis and on a structural basis using three-dimensional micro-computed tomography imaging. The percentage of trabecular surfaces covered by osteoblasts expressing type-I procollagen was significantly increased in bone surrounding the implant in both groups compared with contralateral control bone tissue. There was no difference between the groups with no load or 35 N of load. In addition, measures of trabecular bone structure did not differ significantly between the load and no-load groups. Taken together, these results suggest that wound healing plays a much greater role in the early response of bone to well fixed porous-coated implants than does mechanical stimulus.     

The evaluation of bone remodeling about orthopaedic implants with ultrasound

Total hip arthroplasty causes biomechanical changes in the normal femur, including a redistribution and concentration of stress. These mechanical alterations in the femur cause local remodeling and resorption that affect the geometry and mechanical properties of the bone. Two complementary ultrasonic techniques were used to study the local adaptive remodeling of bone due to prosthesis implantation. An ultrasonic wave propagation technique was used to determine elastic properties and a new scanning acoustic microscope (SAM) mapped the acoustic impedance profile of each section. The effects of the implantation of two types of hip prostheses, an uncemented bipolar prosthesis with an Austin-Moore type stem and a cemented Charnley prosthesis, were investigated. Both prostheses had a detrimental effect on local elastic properties as determined by acoustic velocity measurements. The SAM system provided information about local inhomogeneities in bone properties not obtainable by any other means. The acoustic impedance maps highlighted bone resorption and bone remodeling on a microstructural level.     

Superior sealing effect of hydroxyapatite in porous-coated implants: experimental studies on the migration of polyethylene particles around stable and unstable implants in dogs

Background Migration of wear debris to the periprosthetic bone is a major cause of osteolysis and implant failure. Both closed-pore porous coatings and hydroxyapatite (HA) coatings have been claimed to prevent the migration of wear debris. We investigated whether HA could augment the sealing effect of a porous coating under both stable and unstable conditions.

Methods We inserted porous-surfaced knee implants, with and without HA coating, in 16 dogs, according to a paired, randomized study design. 8 dogs had 2 implants inserted into each knee using a stable implant device and 8 dogs received 1 implant in each knee using a micro-motion (500 μm) implant device. Implants had a periimplant gap of 0.75 mm. We then injected polyethylene (PE) particles or a control solution into the knee joints on a weekly basis.

Results After 16 weeks, the rating of particles around stable implants was reduced by the HA coating from a median value of 2 (1-4) to 1 (0-1) (p = 0.01) and during micromotion from 3 (2-4) to 1 (0-3) (p = 0.002). HA-coated implants had superior bone ongrowth during stable and unstable conditions. We found no difference in bone ongrowth between PE-exposed and vehicle-exposed implants.

Interpretation Compared to a pure plasma-sprayed porous coating, a layer of HA coating provides better bone ongrowth and protects the bone-implant interface against the migration of wear debris under both stable and unstable conditions.     

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.     

膜引导骨再生技术在骨量不足牙种植术中的临床应用

目的:探讨膜引导骨再生技术解决牙种植术中种植区骨量不足的临床疗效,同时探究更为客观、精确的成骨生长效果评价方式。方法:种植手术中,植入人工种植体的同时于骨量不足区植入人工骨粉,可吸收膜覆于其表面。植骨手术前、手术后、手术后6个月通过临床检查、骨面直接测量、CT二维测量及CT三维容积测量评价临床疗效。结果:种植术后随访6个月,22枚种植体与周围骨结合情况良好,牙槽嵴的外形丰满,种植体无动度,未见明显骨吸收,膜引导骨再生技术在种植区骨量不足患者的应用近期疗效较好。成骨生长效果方面,CT三维容积测量法与其他两种测量方法的比较差异具有统计学意义(P<0.05)。结论:提示CT二维测量结合三维容积测量评价成骨生长效果更客观。对种植区骨量不足,牙种植术结合膜引导骨再生术成骨生长效果良好,可扩大牙种植手术适应证,值得在临床上进行推广。     

Fatigue failure of noncemented porous-coated implants. A retrieval study

The causes of mechanical failure of five noncemented porous-coated components were studied. There were two cobalt-chromium alloy and three titanium alloy implants which fractured after 12 to 48 months. The implants included one acetabular component, and one femoral condylar, one patellar and two tibial components. Examination of the fractured surfaces revealed fatigue to be the mechanism of failure in all cases. The porous coating and the processes required for its fabrication had resulted in weakening and reduction of substrate thickness. Additional factors were stress concentration due to limited, localised bone ingrowth, and some features of the design of the implants.     

Enhanced stabilization of porous-coated metal implants with tricalcium phosphate granules

Tricalcium phosphate (TCP) granules were tested as a possible alternative to bone graft as an interposition material to stabilize porous-coated metal implants in overreamed femoral intramedullary canals in dogs in vitro and in vivo. TCP granules provided as much stability as bone graft or a press fit in the in vitro testing. At six weeks in the in vivo system, the TCP stabilized specimens had an increased average resistance to tensile testing, but the increase was not significant. At six months, however, TCP-stabilized specimens exhibited a significant increase in resistance to pull-out force (p less than 0.05). Histologic examination revealed incorporation of the TCP granules in bone bridging from the endosteal cortex to the porous surfaces. TCP granules might be useful as a synthetic interposition material for stabilization of porous-coated metal components. TCP granules exhibited osteoconductive properties.     

Osteogenic protein 1 device increases bone formation and bone graft resorption around cementless implants

In each femoral condyle of 8 Labrador dogs, a non weight-bearing hydroxyapatite-coated implant was inserted surrounded by a 3 mm gap. Each gap was filled with bone allograft or ProOsteon with or without OP-1 delivered in a bovine collagen type I carrier (OP-1 device). 300 microg OP-1 was used in the 0.75 cc gap surrounding the implant. After 3 weeks, the OP-1 device enhanced implant fixation by 800% (p <0.05) in the ProOsteon group, but OP-1 bad no significant effect on implant fixation in the allograft group. By adding the OP-1 device, the volume fraction of woven bone close to the implant increased from 12% to 20% (p < 0.05) in the bone allograft group and from 6% to 25% (p < 0.05) in the ProOsteon group. The volume fraction of bone allograft decreased from 29% to 9% (p < 0.05) in the OP-1 treated group versus 33% to 30% in the allograft group not treated with OP-1. No resorption of ProOsteen was found. In conclusion, OP-1 accelerates resorption of bone allograft and enhances new bone formation around cementless implants grafted with bone allograft or semisynthetic hydroxyapatite bone substitute. Our findings do not support the use of ProOsteon alone around cementless implants.     

Woven bone formation around implants and the effect of bacterial infection.

Several implant materials used in dental and orthopedic surgery were placed in rat tibial bones to study their effects on mineralization. The implants consisted of bone bonding and non-bonding materials. Changes in mineralization were defined by morphometric analysis of matrix vesicle distribution at the implant interface and in normal bone healing following marrow injury. Bone-bonding materials induced an increase in matrix vesicle activity. This finding was supported by study of the biochemical changes in the same model that manifested high correlations to the morphometrical observations with regard to enhancement or delay of primary mineralization. In addition, the study of healing using nuclear methods indicated that implants alter bone healing as shown by the different uptakes of 99mTc and 32P in the different bone compartments. Decreased 32P uptake by the organic phase in the presence of bone-bonding implants suggested that cleavage of 99mTc-MD32P into its technetium and methylene diphosphonate moieties was inhibited by administration of implants. Further studies on the effect of bacterial infection on the peri-implant tissues revealed a decrease in woven bone formation due to infection.     

The case for porous-coated hip implants. The femoral side

A series of 1163 total hip arthroplasties (THAs) using porous-coated femoral components were roentgenographically assessed for implant fixation. For 959 primary THAs followed from two to 12 years, the femoral revision rate was 1% and the ten-year survivorship rate was 96.4%; 150 young patients had a fixation failure incidence of only 1.3% at a mean follow-up period of 6.4 years; in 204 revision THAs, the femoral re-revision rate was 4% at a mean follow-up period of 53.4 months. Failures were largely related to inadequate femoral canal filling. Because of refinements in implant design and surgical techniques, a press fit of the implant is currently achieved in 94% of cases compared to 36% during the first five years. Porous-coated femoral components have yielded results equivalent to those with cement in primary THAs. Excellent results were observed in relatively young patients and patients with revisions.     

High-turnover periprosthetic bone remodeling and immature bone formation around loose cemented total hip joints.

Aseptic loosening and periprosthetic osteolysis are the major problems awaiting solution in total hip surgery. The clinical investigation focused on the analysis of periprosthetic bone remodeling to clarify one important key event in the cascade of periprosthetic connective tissue weakening and osteolysis around loose artificial hip joints. Twelve acetabular bone samples adjacent to granulomatous synovial-like membrane of loose hip prosthesis were retrieved at revision surgery and processed for Villanueva bone staining for morphological observation and bone histomorphometric analysis. Eight well-fixed bony samples were used as control. Although osteoclastic surface and eroded surface by osteoclasts were evident in the periprosthetic bone from loose hip joints (p = 0.003 and p = 0.027), increased osteoid/low-mineralized bone matrix (p < 0.001) and osteoid width (p < 0.001) also were significant findings in structural analysis. In addition, not only elevated mineral apposition rate (MAR; p = 0.044) but also increased mineralizing surface (p = 0.044) and bone formation rate (BFR; p = 0.002) in loose periprosthetic bones were shown in dynamic data analysis. These results were confirmed by precise morphological observation by confocal laser scanning microscopy. Active coupling of bone formation and resorption and increased osteocytes with abundant bone canalicular projections were found in combined with the presence of immature bone matrices (osteoid and low-mineralized bone areas) in periprosthetic bones from loose hip joints. These results indicated that active osteoclastic bone resorption and/or defective bone formation are coupled with monocyte/macrophage-mediated foreign body-type granuloma in the synovial-like interface membrane of loose hip joints. Thus, this unique high-turnover periprosthetic bone remodeling with bad bone quality probably is caused by the result of cellular host response combined with inappropriate cyclic mechanical loading. The fragile periprosthetic bone may contribute to hip prosthesis loosening.