Histological and biomechanical analysis of bone and interface reactions around hydroxyapatite-coated intramedullary implants of different stiffness: a pilot study on the goat

We hypothesized that reduced stem stiffness of orthopaedic implants contributes to a high risk of loosening, since interface stresses and relative motions may exceed a tolerable range. To study this hypothesis, three types of load-bearing implant with different stiffnesses were inserted into the tibia of the goat. Histological analysis was performed of bone repair after insertion of the implant, bone ingrowth, interface disruption and loosening. A finite element model of the configuration provided the quantitative range of interface stresses and relative motions for the present experiment. The implants were made out of stainless steel, hollow titanium and a thin titanium core covered with a polyacetal coating. The stiffness ratios of these implants were approximately 10:4:1, respectively. All implants were coated with a layer of hydroxyapatite (HA) in order to minimize the possible biological effects of the different implant materials. Irrespective of the type of implant, there was a repair phase that lasted 6-12 weeks. The stiff implants functioned well. Large areas of bone bonding to the HA layer were found after the repair phase at 12 weeks postoperatively. After 24 weeks, some signs of loosening were observed. More loosening occurred with the hollow titanium and polyacetal implants, mainly during the repair phase. Three hollow titanium and three polyacetal coated implants survived this period, and were killed after 24 weeks. The integrity of the HA layer at the bone-implant interface of the titanium implants was good. In the polyacetal implants, the repair reaction of the cortical bone was incomplete. Bone ingrowth into HA was largely lacking. In conclusion, we found significant differences in the repair and interface reactions around implants of different stiffness. Stiff implants showed favourable initial interface conditions for bone ingrowth. Intermediate and flexible implants provoked unfavourable interface conditions for initial bone ingrowth. The finite element study showed that the flexible stems produce larger micromotions and higher interface stresses at the bone-prosthesis interface than the stiff stems, indicating an explanation for the histological findings.     

Mechanical and histological evaluation of amorphous calcium phosphate and poorly crystallized hydroxyapatite coatings on titanium implants

The effect of amorphous calcium phosphate (Ca/P) and poorly crystallized (60% crystalline) hydroxyapatite (HA) coatings on bone fixation to "smooth" and "rough" (Ti-6A1-4V powder sprayed) titanium-6Al-4V (Ti) implants was investigated. Implants were evaluated histologically, mechanically, and by scanning electron microscopy (SEM) after 4 and 12 weeks of implantation in a rabbit transcortical femoral model. Histological evaluation of amorphous vs. poorly crystallized HA coatings showed significant differences in bone apposition (for rough-coated implants only) and coating resorption (for smooth- and rough-coated implants) that were increased within cortical compared to cancellous bone. The poorly crystallized HA coatings showed most degradation and least bone apposition. Mechanical evaluation, however, showed no significant differences in push-out shear strengths between the two types of coatings evaluated. Differences between 4 and 12 weeks were significant for coating resorption and push-out shear strength but not for bone apposition. Significant enhancement in interfacial shear strengths for bioceramic coated as compared to uncoated implants were seen for smooth-surfaced implants (3.5-5 times greater) but not for rough-surfaced implants at 4 and 12 weeks. Rough implants showed greater mean interfacial strengths than uncoated smooth implants at 4 and 12 weeks (seven times greater) and to coated smooth implants at 12 weeks only (two times greater). Mechanical failure of the bone/coating/implant interface consistently occurred within the bone, even in the case of the poorly crystallized HA coatings, which had almost completely resorbed on rough implants. These results suggest that once early osteointegration is achieved biodegradation of a bioactive coating should not be detrimental to the bone/coating/implant fixation.     

Resorption of hydroxyapatite and fluorapatite coatings in man. An experimental study in trabecular bone

The clinical use of hydroxyapatite (HA) coating is controversial especially in regard to the long-term performance of the coating and the effects of resorption. In each of 15 consenting patients we inserted two implants, coated with either HA or fluorapatite (FA) into the iliac crest. They were harvested at a mean of 13.6 +/- 0.6 months after surgery. Histological examination showed that bone ongrowth on the HA-coated implants was significantly greater (29%) than that on the FA-coated implants. When bone was present on the coating surface the HA coating was significantly thicker than the FA coating. When bone marrow was present, the HA coating was significantly thinner than the FA coating. The reduction in coating thickness when covered by bone or bone marrow was 23.1 +/- 9.7 microm for HA and 5.1 +/- 1.7 microm for FA (p < 0.01) suggesting that FA is more stable than HA against resorption by bone marrow. The findings suggest that in man the osteoconductive properties of HA coating are superior to those of FA. Resorption rates for both coatings were approximately 20% of the coating thickness per year. Bone ongrowth appears to protect against resorption whereas bone marrow seems to accelerate resorption. No adverse reaction was seen in the surrounding bone.     

Vascular endothelial growth factor enhances vascularization in microporous small caliber polyurethane grafts

Neoarterial regeneration in an implanted small caliber vascular prosthesis is complexly controlled by many structural and biologic factors, such as cytokines. The authors designed an artificial graft, which was prepared as follows. Segmented polyurethane tubular film (inner diameter, 1.5 mm; wall thickness, 100 microns; length, 20 mm), in which micropores (pore size, 100 microns) were fabricated by an excimer laser ablation technique, were coated with a mixed solution of photoreactive gelatin and heparin with or without cytokines (vascular endothelial growth factor [VEGF]: 5 or 50 micrograms/ml, basic fibroblast growth factor [bFGF]: 1 microgram/ml). These coated grafts were irradiated by ultraviolet light, and were implanted in aortas of rats for 4 weeks; the VEGF (5 micrograms/ml) group, n = 6; the bFGF group, n = 6; the VEGF (5 micrograms/ml)/bFGF group, n = 11; the VEGF (50 micrograms/ml)/bFGF group, n = 5; and the control group, n = 9. Control grafts were treated without cytokines. Endothelial coverage was greater for the cytokine immobilized groups (approximately equal to 50-60%) than for the control group (approximately equal to 30%). At the midportion of the triple VEGF immobilized group, many capillaries were seen in the neoarterial intima, and in the micropores, although such capillaries were rarely observed in the bFGF and control groups. Thus, impregnation of VEGF in the gelatinous layer of grafts enhanced transanastomotic tissue ingrowth and transmural capillary ingrowth.     

Dynamics of Gradient Bioceramic Composite Coating on Surface of Titanium Alloy by Wide-Band Laser Cladding

Hydroxyapatite (HA) ,akindofbioactivematerial,haswideapplicationprospectinhardtissuereplacementandrepairbecauseofsimilarchemicalcompositionandcrystallographicstructuretothoseofbonemineral[1,2 ] .TheHAcoatingwithbioactivityandbiocompatibilityonthesur faceo…     

Mechanical and histological fixation of hydroxylapatite-coated pyrolytic carbon and titanium alloy implants: a report of short-term results

Historically, pyrolytic carbon has been a material for cardiovascular applications, but it has several properties suited for orthopedic uses as well. Pyrolytic carbon has an elastic modulus similar to bone and is highly fatigue resistant, but has not been used in orthopedics because of poor fixation to bone. Plasma sprayed hydroxylapatite (HA) has significantly improved the bonding of bone to titanium alloy implants. The effect of plasma-sprayed HA on pyrolytic carbon implants was investigated in this study. Cylindrical samples were implanted through a single cortex in Beagle femurs. The animals were sacrificed after 8 weeks, and a mechanical push-out test was performed on the implants immediately after explantation. Samples were microradiographed, stained for histology, and examined histomorphometrically. Interface strength for each type of implant was calculated. Pyrolytic carbon showed almost no attachment strength with an average strength value of 1.59 MPa. HA-coated pyrolytic carbon (8.71 MPa) yielded the same interfacial strength as HA-coated titanium (8.71 MPa). Histology revealed that bone was in direct apposition to all implants, both HA coated and noncoated. Failures occurred between the core material and the coating, or within the coating, but not at the bone/HA interface. Histomorphometry results confirmed that the two types of HA-coated implants had more bone apposition than the uncoated pyrolytic carbon implants. It was concluded that a plasma sprayed HA coating significantly improves the bone fixation of pyrolytic carbon.     

Effect of sucralfate and basic fibroblast growth factor on fibrovascular ingrowth into hydroxyapatite and porous polyethylene alloplastic implants using a novel rabbit model

This study investigated the effects of sucralfate and basic fibroblast growth factor (bFGF) on fibrovascular ingrowth into porous implant materials. Seven white female New Zealand rabbits underwent bilateral abdominal incisions through which porous orbital spherical or and disc-shaped implants were inserted between their abdominal muscles. Eighty hydroxyapatite (HA) and porous polyethylene (PP) implants, each material of different pore sizes, were implanted. These implants were either uncoated or coated with suspensions of polyhydroxymethylmethacrylate (hydron); hydron and sucralfate; or hydron, sucralfate, and bFGF. Implants were harvested after 1, 3, or 6 weeks. Observers classified the extent of fibrovascular ingrowth in a blind manner using light microscopy. All discs and spheres showed fibrovascular ingrowth; at 6 weeks, almost all implants were fully vascularized. Although demonstrating different degrees of fibrovascular maturity, all 3- and 6-week discs showed complete cellular ingrowth. Overall, the most extensive and mature fibrovascularization was found in HA implants, regardless of shape, duration of implantation, or angiogenic enhancing agent used. Thus, this study indicates that fibrovascular ingrowth into porous implants is more greatly affected by implant porosity and composition than by addition of angiogenic enhancing agents. Further in vivo study, using other potential angiogenesis-promoting agents as well as implants with different pore characteristics, is warranted using this reliable and predictable animal model.     

Porous-coated titanium implant impregnated with a biodegradable protein delivery system

Tissue ingrowth into porous-coated orthopedic and dental implants is commonly used as a means to achieve long-term fixation of these prostheses. However, the degree of tissue ingrowth is often inadequate and inconsistent. If the pores of these implants are impregnated with a controlled drug release system delivering relevant growth factors, then it might be possible to stimulate more tissue ingrowth. The present study introduces such a system based on biodegradable polymers and investigates its protein release profile and polymer degradation characteristics. Porous coated titanium implants were impregnated with a mixture of a 50%-50% polylactic acid-polyglycolic acid copolymer and a model protein, soybean trypsin inhibitor. Control implants contained only the polymer and no protein. The implants were subjected to hydrolytic degradation in phosphate buffered saline at 37 degrees C for periods of 3, 6, and 11 weeks. The protein release and the mass and molecular weight of the polymer were monitored. The results indicate that the protein is released in three distinct phases and the polymer loses almost all its mass and molecular weight by 11 weeks. There was a significant difference in the polymer degradation characteristics between the control and test implants, which might be the result of some complex polymer-protein interactions.     

Titanium and bioactive glass-ceramic coated titanium as materials for keratoprosthesis

The current problem with keratoprosthesis is the ingrowth of corneal or conjunctival epithelium into the anterior chamber. This may lead to infections and extrusion of the prosthesis as well as to the development of retroprosthetic membrane and secondary glaucoma. Glass-ceramic coated and uncoated titanium has been tested as material for the keratoprosthesis to prevent epithelial ingrowth. Twenty-two Supra-Descemet's membrane keratoprostheses were inserted in the eyes of 22 rabbits for 1, 2, 4, 8, or 12 months. The prosthesis had an optic part made of polymethylmetacrylate (PMMA). The support for the optic part and the flange of the prosthesis were made of titanium. Eleven of the prostheses were coated with glass-ceramic. The histological sections of the enucleated eyes were prepared through the central part of the cornea and the prosthesis using a cutting-grinding method. The histological analysis was made on both halves of the implants separately giving two analysis areas in each eye. All 11 titanium prostheses were retained for the time period planned. Two glass-ceramic coated prostheses were lost at 2 and 4 weeks, respectively. This was caused by difficulties at surgery due to a thick coating. These eyes were excluded from the histological analysis. No significant ingrowth of epithelium was seen in 15/18 (83%) and in 16/22 (73%) of the analysed areas of the glass-ceramic coated and titanium prostheses, respectively. Titanium appears to be a suitable material for the keratoprosthesis. The ingrowth of the epithelium may be hindered further by coating the titanium with bioactive glass-ceramic.     

Preclinical testing of total hip stems. The effects of coating placement

The long-term fixation endurance of noncemented hip stems in total hip arthroplasty is subject to incompatible design goals. To reduce stress shielding and periprosthetic bone loss, proximal fixation and load transfer are indicated. However, to prevent interface motion and promote interface-bonding security, fixation preferably should be maximized over the entire stem surface. In this study, the authors questioned whether hydroxyapatite coatings could be applied in patterns that reduce bone resorption, while maintaining safe interface stress levels. For that purpose, strain-adaptive bone-remodeling theory was applied in 3-dimensional finite element models, to simulate the long-term postoperative bone resorption process. During the process, the adaptation of interface stresses was monitored, and its effects on interface failure probability evaluated. This analysis was done for a fully coated stem, a 1/3 proximally coated stem, a smooth uncoated, press-fitted stem, and a stem with 5 proximal patches of circumferential stripes. The uncoated stem reduced bone loss dramatically, but promoted interface motions and distal pedestal formation. In all cases, the gradual bone-remodeling process increased the interface security of the coated stems. Bone loss and interface failure probability were not very different for the fully and 1/3-coated stems. Stripe coating reduced bone resorption considerably, while increasing long-term interface failure probability only slightly. The investigators concluded that the initial stability and the ingrowth potential of such a stem design are likely to be inadequate.     

Superantigenic exotoxin production by isolates of Staphylococcus aureus from the Kawasaki syndrome patients and age-matched control children

Ten screw-shaped commercially pure titanium implants were inserted in 10 rabbit tibia. The implants with surrounding bone were harvested after 12 weeks. The samples were processed to be cut and ground. Histomorphometrical analyses of the bone-to-metal contact and the bone area around the entire implants were performed on transvertically cut sections at intervals, starting with 100 microns thick sections; then the same sections were further ground down to 50, 30 and 10 microns. The thicker the section the more bone-to-metal contacts were demonstrated. A statistically significant difference was obtained when comparing the 100-30, 100-10, 50-30 and the 50-10 microns thick sections. Comparing the area measurements of 100-10, 50-30, 50-10 and 30-10 microns revealed a statistically significant difference. Bone-to-metal contact measurements on ground sections that are too thick (over 30 microns) may result in overestimations of the 'true' bony contacts.     

A new microporous polyurethane vascular graft prepared by an excimer laser ablation technique

Three types of polyurethane (PU) based tubes (internal diameter, 2 mm; wall thickness, 100 microns) with micropores of well controlled size and arrangement were fabricated using an excimer laser (KrF) ablation technique. The pore size (100 microns) and the longitudinal pore-to-pore distance (200 microns) were constant, and the circumferential pore-to-pore intervals were 60 degrees (type 1), 30 degrees (type 2), and 15 degrees (type 3). The surface of the fabricated tube was photochemically modified with photoreactive gelatin. Scanning electron microscopy showed that pore size and arrangement were precisely controlled as designed, and that a gelatinous layer thoroughly covered the luminal surface. The stiffness parameter (beta), inversely related to compliance, was determined from the change in external diameter against intraluminal pressure. An increase in the number of pores around the circumference decreased the beta value. The type 1 tubes implanted preliminarily in rats for 4 weeks showed good patency (80%). The combination of excimer laser-directed microporing and photochemical surface processing techniques enabled the development of a novel compliant small caliber vascular graft, which is expected to show enhanced transmural tissue ingrowth in vivo.     

真空等离子喷涂HA/Ta复合涂层

羟基磷灰石(HA)具有良好的生物相容性和骨传导能力,采用等离子体喷涂技术在钛合金等金属基材上制备的HA涂层材料已在临床医学上被广泛使用。但是,HA涂层与钛合金基材之间较低的结合强度影响了植入物在体内的长期使用。本文采用真空等离子体技术制备钽(Ta)掺杂的HA涂层,以期在保持其良好生物学性能的同时提高其结合强度。HA/Ta复合涂层的微观形貌、元素组成和相组成由SEM及配套的能谱仪(EDS)和XRD分析技术表征。按ASTMC-633标准对涂层的结合强度进行了测试。将涂层试样浸泡于模拟体液中以评估其生物活性。结果表明:Ta增强HA涂层具有粗糙的表面和层状结构,其结合强度随着Ta含量的增加而增加。掺60%Ta(H4T6)涂层的结合强度达到37.2MPa,约为HA涂层的1.9倍。模拟体液浸泡试验显示,掺钽HA涂层表面形成了类骨磷灰石,表明具有良好的生物活性。     

In vitro crystallisation systems for the study of urinary stone formation

Hydroxyapatite orbital implants undergo early ingrowth of fibrovascular tissue after enucleation. This animal study determined whether control and osteogenin-impregnated hydroxyapatite orbital implants vary in their osteogenic response at 6 and 52 weeks. Rabbits underwent enucleation with implantation of control or osteogenin-impregnated hydroxyapatite spheres. Light microscopy determined fibrovascular ingrowth, and histomorphometry quantitated the amount of bone produced. Osteogenin implants vascularized at a faster rate and contained bony foci by 6 weeks that became confluent at 1 year. Spontaneous osteogenesis was not seen in control animals at 6 weeks. After 1 year they contained bone, although less than in the osteogenin implants. Mixed cell inflammation was observed at the hydroxyapatite-tissue interface in both groups. No inflammation was noted at the interface of hydroxyapatite and bone. These are the first controlled observations that bone-specific differentiation occurs in the pores of spherical hydroxyapatite implants within the soft tissues of the socket. This vascularized process can be enhanced with osteogenin to occur earlier and more uniformly in the implants at one year.     

Porous-coated metal-backed patellar components in total knee replacement. A postmortem retrieval analysis

The use of porous-coated metal-backed patellar components to achieve consistent fixation by bone ingrowth and to provide relief of pain warrants serious scrutiny. We conducted a quantitative postmortem investigation of eleven consecutively retrieved components with use of high-resolution contact radiographs, electron microscopy, and histological analysis. The implants had been in situ for a mean (and standard deviation) of 45+/-36 months (range, one to eighty-four months). Analysis of the high-resolution contact radiographs revealed that a mean of 86+/-12 per cent (range, 61 to 100 per cent) of the porous coating was in contact with the host bone. Backscattered electron imaging showed that the mean volume fraction of bone ingrowth was 13+/-9 per cent (range, 0 to 30 per cent). No significant difference was detected, with the numbers available, between the volume fraction of the bone ingrowth measured in the porous coating and that of the host cancellous bone in the patellae.     

Basic fibroblast growth factor promotes bone ingrowth in porous hydroxyapatite

The effect of basic fibroblast growth factor on tissue ingrowth and differentiation in porous hydroxyapatite of coralline origin was studied in a bone chamber model. The hydroxyapatite with or without basic fibroblast growth factor was placed in 22 mm3 titanium bone conduction chambers implanted bilaterally in rat tibiae. Ingrowing bone could enter the cylindrical interior of the chamber only at 1 end. It then penetrated the porous hydroxyapatite inside the chamber. The distance that the ingrown tissue had reached into the material then was measured on histologic slides. Because fibrous tissue always reached further into the material than did bone, both total tissue ingrowth and bone ingrowth distances were measured. In implants supplemented with 0.04 microg basic fibroblast growth factor in a hyaluronate gel carrier, the bone ingrowth distance was increased by 70% at 6 weeks, as compared with paired controls in the contralateral leg. The total tissue ingrowth distance also was increased by 58%. When the dose of basic fibroblast growth factor was increased to 1.0 microg, still using the hyaluronate carrier, there was no difference in bone ingrowth compared with controls, but this dose still increased the total tissue ingrowth. In hydroxyapatite with 1.5 microg basic fibroblast growth factor without hyaluronate gel at 4 weeks, no increase in bone ingrowth was shown, but total tissue ingrowth was increased. At 6 weeks, bone ingrowth and total tissue ingrowth were increased by 41% and 33%, respectively. With a lower dose of 0.15 microg without carrier, only the total ingrowth distance was increased. The results suggest that basic fibroblast growth factor may promote tissue ingrowth into porous hydroxyapatite and that bone ingrowth may be increased by appropriate doses. The hyaluronate gel carrier reduced the optimal dose.     

Effect of chronic bile duct obstruction and LPS upon targeting of naproxen to the liver using naproxen-albumin conjugate

In order to avoid the potential risks of disease transmission in allograft surgery, numerous substitute materials have been described. As the biological response to implant materials is different, we undertook the following study to assess type and amount of bone ingrowth in CaP-ceramics. 105 cylindrical bone defects with a diameter of 5.4 mm were created surgically in the femoral condyles of 53 skeletal mature NZW rabbits. The defects were filled with crushed coralline hydroxyapatite (HA) implants (n = 21), synthetically produced hydroxyapatite (n = 21) and surface-modified alpha-Tricalciumphosphate (TCP) grains (n = 21). 21 defects were left empty and other drill holes were filled with rabbit cancellous bone cylinders (n = 21) after 3 months of cryopreservation at -78 degrees C without sterilization. Following observation periods of 2, 4, 6, 8, 12, 26 and 52 weeks the femoral condyles were harvested for histological evaluation and quantitative analysis of bone ingrowth. Woven bone formation at implant periphery can be observed in all substances as early as 2 weeks postoperatively. At 4-week-intervals cryopreserved allografts show new bone apposition on surfaces of necrotic trabeculae and graft-host junctions by a predominantly osteoblastic reaction at the periphery of all cylinders, while in HA- and TCP-grains early bone formation in the center of drill holes is detectable as well. There is a direct contact between HA-/TCP-particles and newly formed bone without fibrous tissue formation at the implant surfaces. Central new bone formation in rabbit allografts can be observed after 6 to 8 weeks together with a secondary osteoclastic resorption of necrotic transplant trabeculae. The result of this remodeling process is a complete degradation of transplant cylinders with reorganization of vital trabeculae oriented in a mature pattern after 12 to 26 weeks. In contrast the HA- and TCP-implants did not show any signs of resorption.     

Orderly oriented wire mesh: a novel porous coating

As an alternative to commercially available porous beads and fiber metal mesh, a new porous coating, orderly oriented wire mesh (OOWM), was developed. Rectangular plugs, 10 x 5 x 5 mm with porous-coated beads, and four different OOWM configurations were inserted into bilateral femoral condyles of adult beagles. Dogs were sacrificed immediately after implantation, and at 4 weeks, 8 weeks, and 12 weeks postimplantation. Mechanical pullout strength of plugs revealed that porous beads are equivalent to the simplest OOWM at 12 weeks postimplantation. Of the four OOWMs tested, the 25 x 25 single layer was significantly more stable than others at 4 weeks postimplantation. These results indicate that in an in vivo unloaded model, OOWM is just as effective as the porous beads in achieving early bone ingrowth and stability.     

Ultrastructural study of the A-W GC-bone interface after long-term implantation in rat and human bone

The interface between apatite- and wollastonite-containing glass-ceramic (A-W GC) and bone after long-term implantation was studied by scanning and transmission electron microscopy (SEM and TEM) using rat and human specimens. First, particles of A-W GC (100-220 microns in diameter) were implanted into rat tibiae, and specimens were prepared for observation at 24, 48, 72, and 96 weeks after the operation. These long-term specimens showed an A-W GC-bone interface different from that at an earlier stage, which was investigated in our previous studies. SEM showed that the Ca-P-rich layer was wider, suggesting that leaching of ions from the A-W GC had continued even after bonding with bone. In some regions, the material particles were evidently replaced by the bone. TEM showed that the intervening apatite layer had become indistinct, and that A-W GC had intermingled with bone at the interface. In some regions, the surface of the A-W GC was degraded. These findings suggest that the surface region of A-W GC is slowly replaced by bone. Second, a human bone specimen, which included A-W GC particles (300-700 microns in diameter) implanted as a bone filler for about 75 weeks was harvested and investigated. Excellent A-W GC-bone bonding was observed, and the ultrastructure of the interface was similar to that in rats after long-term implantation. This finding demonstrated that A-W GC possibly worked in human bone in the same way as in rat bone, showing excellent bioactivity.     

Performance of small diameter synthetic vascular prostheses with confluent autologous endothelial cell linings

Autologous grafts are superior to their synthetic counter-parts for grafting arteries smaller than 6-mm diameter both in terms of acute thrombogenicity and chronic intimal hyperplasia. Endothelial cell (EC) coating of the blood contacting surface may reduce thrombogenicity of synthetic small diameter vascular prostheses. In this study, the survival of EC monolayers on synthetic 4-mm diameter arterial prostheses over short-term implantations (< or = 6 weeks) was examined. Graft types examined were expanded polytetra-fluoroethylene (ePTFE) and microporous polyurethane (PU). Lumenal coverage with ECs was achieved by culturing ovine ECs on prostheses treated by either physical adsorption or covalent binding of ovine fibronectin (Fn). An ovine carotid interposition model was used to examine the performance of EC coated ePTFE and microporous PU over implantation periods of 1, 3, and 6 weeks. Outcomes assessed at the end of each experiment were graft patency, area covered by ECs, and thrombus free surface area (TFSA). Fn concentration, cell density at the time of coating and prostacyclin production in vitro were similar for both graft types. Occlusion occurred more frequently in unseeded grafts compared with EC coated grafts over 3 and 6 week implantation periods; however, the difference was not significant (p = 0.099). In prostheses precoated with ECs, approximately 40-60% of the surface area remained covered with endothelial-like cells following the first postoperative week. Recovery of EC layers occurred rapidly thereafter with 80-90% coverage at 3 weeks. TFSA remained low in comparison to EC cover in these prostheses until between 3 and 6 weeks postoperatively, suggesting a lag phase in recovery of EC function of seeded cells. In contrast, EC cover of unseeded prostheses only achieved 10-30% at 3 weeks, primarily by pannus EC ingrowth from the adjacent artery. TFSA of unseeded grafts increased in direct proportion to EC cover over time suggesting that there was no lag phase in function of these ingrowing cells.