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PubMed2001

Effects of an adhesion promoter on the debond resistance of a metal-polymethylmethacrylate interface.

Debonding and premature failure of prostheticpolymethylmethacrylate interfaces have been shown to be exacerbated by exposure to physiological environment. In efforts to counteract these hydrolytic degradation effects, two clinically relevant Co-Cr-Mo surface morphologies were treated with an organosilane adhesion promoter (gamma-methacyloxypropyltrimethoxy) before interface bonding. Samples were quantitatively characterized in terms of the adhesion (fracture) and subcritical debond growth-rate (fatigue) behavior of the interface. The steady-state interface debond resistance, Gss (J/m2), was shown to increase with application of the silane pretreatment both in air (20 degrees C, 45% relative humidity) and simulated physiological environment (37 degrees C, Ringer's). Similarly, positive shifts in the subcritical debond threshold, deltaG(TH), values are observed for silane pretreated interfaces. A shift in the debond path from primarily adhesive failure in untreated surfaces to cohesive failure between the silane layer and bulk polymethylmethacrylate for silane treated surfaces was observed. Silane pretreatment of Co-Cr-Mo surfaces was shown to effectively limit the degree of the environmental degradation. General insights to the effects of surface roughness, chemical enhancement, and the environmental effects on the thermodynamics at the interface and resulting debond behavior are discussed.

Adhesiveness↗
PubMed2006

Systemic immunosuppressive therapy inhibits in-stent restenosis in patients with renal allograft.

BACKGROUND: Cyclosporine is used routinely for prophylaxis for renal allograft rejection. In experimental animal studies, cyclosporine had been shown to inhibit smooth muscle cell proliferation during the arterial response to injury. We investigated whether systemic immunosuppression may inhibit in-stent restenosis in renal transplant patients undergoing coronary stenting. METHODS: From 1993 to 2003, 33 renal transplant patients with 45 coronary lesions and 37 dialysis patients with 52 lesions underwent coronary stenting using bare metal stents at our center. We followed all patients clinically for a mean period of 37 +/- 31 months and 40 patients angiographically at 14 +/- 15 months after coronary intervention. Cyclosporine was combined with corticosteroids in 32 patients and one patient received tacrolimus instead of cyclosporine. RESULTS: The baseline clinical and angiographical characteristics were similar and the success rate of the procedure was 100% in both groups. In renal transplant group, the mean dose of cyclosporine was 192.5 +/- 68 mg/day and the blood cyclosporine level at the time of procedure was 152.9 +/- 51.5 ng/mL. The rate of in-stent restenosis was 7.1% in renal transplant group and 57.1% in dialysis group (P < 0.0001). The mean late loss was 0.47 +/- 0.57 mm in renal transplant group when compared with 1.51 +/- 1.09 mm in dialysis group (P = 0.004). The overall rate of major adverse cardiac events (MACEs) was 6.1% in renal transplant group and 35.1% in dialysis group (P < 0.0001). CONCLUSIONS: Renal transplant patients receiving combined immunosuppressive agents showed markedly low rates of in-stent restenosis and MACE after coronary revascularization with stent. We consider that this result may be related to the ability of combined immunosuppressive therapy to inhibit inflammatory reaction and vascular smooth muscle cell proliferation induced by coronary stenting.

Coronary Angiography↗
PubMed2001

Quantitative analysis of mRNA expression of TIMPs in the periprosthetic interface tissue of loose hips by real-time PCR system.

Matrix metalloproteinases (MMPs) and their endogenous inhibitors, tissue inhibitors of metalloproteinases (TIMPs), have been reported to play a critical role in extracellular degradation around artificial hip joints. Although messenger ribonucleic acid (mRNA) expression patterns of several MMPs and TIMPs were reported, there is no report of quantitative mRNA analysis of TIMPs in periprosthetic tissues. In this study, mRNA expression of four different types of TIMPs in periprosthetic interface tissue of loose hips was analyzed by a quantitative polymerase chain reaction system. The mRNA expression level of TIMP-1, -2, and -3 in periprosthetic interface tissue was significantly higher than that in control. In contrast, the mRNA expression level of TIMP-4 in the periprosthetic interface tissue was lower. This study suggested that increased levels of TIMP-1, -2, and -3, and decreased levels of TIMP-4 may contribute to pathologic extracellular matrix degradation in combination with MMPs, thus leading to prosthetic loosening and osteolysis around artificial total hip joints.

Aged↗
PubMed2004

Massive wear of an incompatible metal-on-metal articulation in total hip arthroplasty.

We report on a case of massive wear because of an incompatible metal-on-metal combination. In a 62-year-old man, a cobalt-chromium (CoCr) inlay and a stainless steel head were paired by accident. Because of persistent pain, revision surgery was performed 7 months later. Histologic analysis of the surrounding tissue revealed massive metallosis. The wear volume was increased by a factor of 18 for the head and 2 for the cup compared with normal metal-on-metal articulation. The serum concentrations of chromium and cobalt were increased by a factor of 20 and 4 over levels of a healthy population, respectively. Incompatible metal-on-metal combinations should be revised immediately. In case of delayed diagnosis, no metal-on-metal articulation should be implanted because of the high volume of metal in the human body.

Aged↗
PubMed2004

Early detachment of titanium particles from various different surfaces of endosseous dental implants.

Titanium (Ti) endosseous dental screws with different surfaces (smooth titanium--STi, titanium plasma-sprayed-TPS, alumina oxide sandblasted and acid-etched--Al-SLA, zirconium oxide sandblasted and acid etched--Zr-SLA) were implanted in femura and tibiae of sheep to investigate the biological evolution of the peri-implant tissues and detachment of Ti debris from the implant surfaces in early healing. Implants were not loaded. Sections of the screws and the peri-implant tissues obtained by sawing and grinding were analysed by light microscopy immediately after implantation (time 0) and after 14 days. All samples showed new bone trabeculae and vascularised medullary spaces in those areas where gaps between the implants and host bone were visible. In contrast, no osteogenesis was induced in the areas where the implants were initially positioned in close contact with the host bone. Chips of the pre-existing bone inducing new peri-implant neo-osteogenesis were surrounded by new bone trabeculae. The threads of some screws appeared to be deformed where the host bone showed fractures. Ti granules of 3-60 microm were detectable only in the peri-implant tissues of TPS implants both immediately after surgery and after 14 days, thus suggesting that this phenomenon may be related to the friction of the TPS coating during surgical insertion.

Animals↗
PubMed2005

Role of hydroxyapatite coating in resisting wear particle migration and osteolysis around acetabular components.

The main problem facing the longevity of total joint replacements is wear particle-induced osteolysis, particularly around the acetabular component. Ovine Total Hip Replacement surgery was performed with roughened femoral heads in order to enhance wear debris generation in vivo. The resistance to aseptic loosening of acetabular components with different surface coatings was investigated. Implants remained in vivo for 1 year. Sheep were randomly assigned to one of six experimental groups where the acetabular cup was fixed utilising an: (a) cemented polyethylene acetabular cup, (b) metal backed grit blasted surface, (c) metal backed plasma sprayed titanium porous coating, (d) metal backed sintered beaded coating, (e) Hydroxyapatite (HA)-coated grit blasted surface (f) and HA-coated porous components. Ground Reaction Force (GRF) was used to asses the functional performance of the implants and data was collected pre-operatively and at 12, 24, 36 and 52 weeks post op. Wear debris generated was analysed and radiographs taken prior to preparation of thin sections. Fibrous tissue (FT) thickness and bone contact at 1 mm intervals along the acetabular bone-implant interface was calculated. GRF data demonstrated significant differences between experimental groups. In all groups there was an increase in the function of the hip after surgery and up to 24 weeks but thereafter the function of the group with the grit blasted surface reduced whereas the function of the other groups did not significantly change. Average wear particles generated were <1 microm in size. The cemented group demonstrated a significantly thicker average FT layer (2.69 mm) when compared with all other groups (p<0.05 in all cases) except the grit blasted group (1.56 mm). HA porous coated cups demonstrated significantly least fibrous tissue adjacent to its interface when compared with all other groups (cemented p<0.05, grit blasted p=0.029, porous p<0.05, sintered beads p<0.05 and HA grit blasted p<0.05). Significantly increased bone contact to HA-coated porous cups (73.33%) when compared with all groups was identified except HA-coated grit blasted cups where no significant difference was demonstrated. Radiographic signs of loosening were visible in all groups except the HA-coated porous group. Results demonstrated that HA porous coated acetabular components significantly enhanced bone ingrowth in the presence of wear particles, preventing their migration and reducing osteolysis. Non-HA-coated porous and sintered beaded components provided a more effective seal against the ingress of wear debris when compared with cemented cups.

Animals↗
PubMed2006

Partially cemented AncaDualFit hip stems do not fail in simulated active patients.

BACKGROUND: Partially cemented hip stems have been introduced to offer the advantages of cemented stems on the short-term, and of cementless ones on the long-term. One such device, the AncaDualFit, has been thoroughly validated pre-clinically under average loading conditions. Concerns recently arose concerning the long-term endurance of such implants in active patients. In fact, it was suspected that cyclic loads applied by demanding patients could lead to fixation failure. METHODS: The long-term performance of the AncaDualFit partially cemented stem was studied in vitro, using a validated protocol that simulated the loads of 24 years of an active patient. Inducible and permanent micromotions were measured in five specimens and compared against two well-established cemented stems, and a cementless stem (the original design from which was derived). Cement damage (fatigue cracks) due to cyclic loading was quantified and compared against the cemented stems. FINDINGS: Inducible and permanent micromotions of the AncaDualFit partially cemented stem were slightly larger that the cemented stems, but much smaller than the cementless one. The migration, however, indicated a clear trend towards stabilization. Cement damage was minimal, even if compared to the most successful cemented stems. Short cracks were observed only near the cement inlet, but did not propagate in the two cement pockets. INTERPRETATION: The partially cemented AncaDualFit stem can safely withstand very severe loading without significant damage, thanks to the design of the two anterior and posterior cement pockets. Results are reassuring also in comparison with other successful designs. Thus, it can be safely used, even in active patients.

Arthroplasty, Replacement, Hip↗
PubMed2004

Radiographic evaluation of osseointegration and loosening of titanium implants in the MCP and PIP joints.

PURPOSE: Good to excellent clinical results have been shown in the initial studies on osseointegrated metacarpophalangeal (MCP) and proximal interphalangeal (PIP) joint implants that consist of a flexible silicone spacer connected to osseointegrated titanium fixtures. A high fracture rate of the silicone spacer, however, has been reported, diminishing the value of this implant system. The purpose of this study was to evaluate the osseointegration of the titanium fixture itself in a large study group and compare patients with rheumatoid arthritis with those with osteoarthrosis as a base for further development of the implant concept. METHODS: Two hundred thirty-nine implants (27 PIP, 212 MCP) in 86 patients were retrospectively evaluated radiographically. Two hundred implants were in patients with rheumatoid arthritis. Radiographic evaluation included measuring the length and width of resorption zones around the titanium fixtures. A scoring system was used that grouped observations in 4 groups, from 0 (little or no change) to 3 (loosening). Status of the silicone spacer was also noted. RESULTS: At follow-up evaluation an average of 41 months (range, 12-103 months) after surgery, complete osseointegration was found in 450 of 478 fixtures (94%). Loosening was found in 10 proximal and 18 distal fixtures. The proximal fixtures showed loosening mainly during the first 3 years after surgery and the distal fixtures also had loosening. In the 31 MCP joint implants that were evaluated for more than 5 years, the osseointegration rate was 97%. CONCLUSIONS: Implant stem fixation with osseointegrated titanium fixtures is a valuable method that works well in rheumatoid arthritis patients. Further development will focus on a more durable constrained joint mechanism.

Adolescent↗
PubMed2001

Why incongruous knee replacements do not fail early.

Rapid failure of knee prostheses does not usually occur, despite the non-conforming nature of the articulation between femoral and tibial components and the associated large contact pressures. This theoretical study examines the likelihood of fatigue fracture of a layered elastic model loaded by a sliding cylindrical indenter. Cracks (line-defects) were assumed to have nucleated within the layer. The stress intensity factors (SIFs) associated with these cracks were calculated. The values obtained for the SIFs are quite low, with a corresponding low likelihood of crack-growth. When taken in conjunction with the experimentally derived fatigue laws of previous investigators, they suggest that short line-cracks should not grow. It seems that early failures have not simply been due to large shear stresses which occur beneath the prosthesis surface. Other factors, such as the degradation of material through heat-pressing, sterilisation or oxidation, or the deleterious effect of fusion defects, may be required to drive the cracks to delamination.

Humans↗
PubMed2003

Early cement damage around a femoral stem is concentrated at the cement/bone interface.

This study aimed to improve understanding of the mechanical aspects of cemented implant loosening. After aggressive fatigue loading of stem/cement/femur constructs, micro-cracks and stem/bone micro-motions were quantified to answer three research questions: Are cracks preferentially associated with the stem/cement interface, the cement/bone interface or voids? Is cement damage dependent on axial position? Does cement damage correlate with micro-motion between the stem and the bone? Eight Charnley Cobra stems were implanted in cadaveric femora. Six stem/cement/femur constructs were subjected to "stair-climbing" loads for 300 kcycles at 2Hz. Loads were normalized by construct stiffness to avoid fracture. Two additional constructs were not loaded. Transverse sections were cut at 10mm intervals, stained with a fluorescent dye penetrant and examined using epi-fluorescence stereomicroscopy. Crack lengths and cement areas were recorded for 9 sections per specimen. Crack length-density was calculated by dividing summed crack length by cement mantle area. To isolate the effect of loading, length-density data were offset by the baseline length-density measured in the non-loaded specimens. Significantly more cracks were associated with the interdigitated area (35.1%+/-11.6%) and the cement/bone interface (31.0%+/-6.2%) than with the stem/cement interface (11.0%+/-5.2%) or voids (6.1%+/-4.8%) (p<0.05). Load-induced micro-crack length-density was significantly dependent on axial position, increasing proximally (p<0.001). Micro-motions were small, all stems rotated internally. Cement damage did not correlate with micro-motion.

Aged↗
PubMed2003

Prevention of mesh-dependent damage growth in finite element simulations of crack formation in acrylic bone cement.

Peak stress levels predicted in finite element analysis (FEA) usually depend on mesh density, due to singular points in the model. In an earlier study, an FEA algorithm was developed to simulate the damage accumulation process in the cement mantle around total hip replacement (THR) implants. It allows cement crack formation to be predicted, as a function of the local cement stress levels. As the simulation is driven by mesh-dependent peak stresses, predicted crack formation rates are also likely to be mesh dependent. The aim of this study was to evaluate the mesh dependence of the predicted crack formation process, and to present a method to reduce the mesh dependence. Crack-propagation experiments were simulated. Experimental specimens, representing transverse slices of cemented THR reconstructions, were subjected to cyclic torsional loading. Crack development around the corners of the stem was monitored. The experiments were simulated using three meshes with increasing levels of mesh refinement. Crack locations and orientations were accurately predicted, and were virtually independent of the level of mesh refinement. However, the experimental crack propagation rates were overestimated considerably, increasing with mesh refinement. To eliminate the effect of stress singularities around the corners of the stem, a stress averaging algorithm was applied in the simulation. This algorithm redistributed the stresses by weighted spatial averaging. When damage accumulation was computed based on averaged stresses, the crack propagation rates predicted were independent of the level of mesh refinement. The critical distance, a parameter governing the effect of the averaging algorithm, was optimized such that the predicted crack propagation rates accurately corresponded to the experimental ones. These results are important for the validity and standardization of pre-clinical testing methods for orthopaedic implants.

Artifacts↗
PubMed2003

An improved method of computing the wear factor for total hip prostheses involving the variation of relative motion and contact pressure with location on the bearing surface.

A new method of computing the wear factor for total hip prostheses is presented. In the conventional method, only the resultant contact force and the track drawn by the point of its application are considered so that the product of the instantaneous force and sliding increment is integrated over one motion cycle. In the present, improved, method the contact pressure distribution is discretized by a large number of smaller normal forces, and the contribution of each is summed. This is important because the relative motion and contact pressure vary strongly with location, and because the transverse pressure component is substantial. Hence, the present surface integral represents the large contact surface better than the conventional line integral. A prerequisite for the surface integral was the method of computing the relative motion correctly anywhere on the contact surface, developed and published earlier by the present authors. For the pressure discretization, the contact surface was divided into nearly equal-sized surface elements. The contact pressure was modelled with ellipsoidal, paraboloidal and sinusoidal distributions. Two load cases were studied, double-peak and static. When an ellipsoidal contact pressure distribution extending over a hemisphere was discretized by 1000 element forces, the computed wear factor for double-peak load in a biaxial hip wear simulator was 30% lower than in the conventional resultant force case. The present method can be later developed further to involve the temporal variation of size and location of the contact surface.

Computer Simulation↗
PubMed1998

Hip arthroplasty in chronic dialysis patients.

We retrospectively reviewed 15 patients (24 hips) on chronic renal dialysis who underwent hip arthroplasty between 1970 and 1990. The average age at surgery was 39 years; the average follow-up was 8 years (range, 1-19 years). All follow-up of less than 5 years relates to those patients who died. Of these 24 hips, 14 (58%) failed or were failing due to loosening; the average time to revision was 7 years (range, 1.5-14 years). A complicated course was experienced in 16 hips (66%), primarily related to medical difficulties. There was one perioperative death. The following orthopedic complications afflicted 5 hips (21%): one femur fracture during revision; one femur fracture at 2 months after revision; one dislocation during seizure; one displacement of acetabular cup requiring recementing; and late generalized septic death of one patient (with both hips involved). Within an average of 3 years (range, 15 months to 7 years) after their index surgery, 6 of the 15 patients (40%) died. The patients who lived were chronically ill, and all but three remained on long-term dialysis. The functional level of all those remaining on dialysis steadily declined, and none reached a quality of life comparable to an osteoarthritic patient. This study confirms a previously reported high mortality and morbidity rate in this population. Despite their difficulties, 22/24 primary hips were relieved of pain and increased in function; six patients returned to work. We see no better alternative for pain relief in total hip arthroplasty, particularly in view of contemporary surgical techniques and improved medical management.

Adult↗
PubMed1989

Advantages and limitations of current soft intraocular lenses.

We report our clinical experience with six soft intraocular lenses: (1) STAAR silicone, (2) CooperVision/Schlegel silicone, (3) IOLAB silicone, (4) Allergan Medical Optics (AMO) silicone, (5) Fyodorov silicone, and (6) Alcon hydrogel (IOGEL). We found that these soft intraocular lenses have several common design problems. The single-size design of five of the six soft lenses can lead to a windshield-wiper decentration effect in lenses too small for larger eyes. We observed several instances in which the one-piece lenses became decentered, subluxed, and vaulted anteriorly against the iris or posteriorly away from the iris as postoperative capsulozonular changes occurred. We found that the insertion methods for these soft lenses need refining to reduce the risk of intraocular damage and wound stretching if the lenses are to be folded and inserted through a small 3.0 mm to 3.5 mm incision. The surface and physical characteristics of these lenses may also require further study. We observed folding grooves, sebacium deposits, and rust from the insertion instrument on some lenses inside the postoperative eye and pigment dispersion associated with hydrogel lenses. We conclude that small diameter polymethylmethacrylate implants are the best choice for small incision cataract surgery at this time.

Cataract Extraction↗
PubMed2003

The relationship between cement fatigue damage and implant surface finish in proximal femoral prostheses.

The majority of cemented femoral hip replacements fail as a consequence of loosening. One design feature that may affect loosening rates is implant surface finish. To determine whether or not surface finish effects fatigue damage accumulation in a bone cement mantle, we developed an experimental model of the implanted proximal femur that allows visualisation of damage growth in the cement layer. Five matte surface and five polished surface stems were tested. Pre-load damage and damage after two million cycles was measured. Levels of pre-load (shrinkage) damage were the same for both matte and polished stems; furthermore damage for matte vs. polished stems was not significantly different after two million cycles. This was due to the large variability in damage accumulation rates. Finite element analysis showed that the stress is higher for the polished (assumed debonded) stem, and therefore we must conclude that either the magnitude of the stress increase is not enough to appreciably increase the damage accumulation rate or, alternatively, the polished stem does not debond immediately from the cement. Significantly (P=0.05) more damage was initiated in the lateral cement compared to the medial cement for both kinds of surface finish. It was concluded that, despite the higher cement stresses with debonded stems, polished prostheses do not provoke the damage accumulation failure scenario.

Arthroplasty, Replacement, Hip↗