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Reijo Lappalainen

Publications and source records attributed to Reijo Lappalainen.

14 recordsLinked to original sources
PubMed2006

Liquid boundary movements in cylindrical and convex hydrophobic matrix tablets: Effects on tablet cracking and drug release.

The aim of this study was to investigate liquid penetration into both cylindrical and convex hydrophobic matrix tablets and to relate the changes in tablet structure to drug release. Starch acetate with degree of substitution of 2.7 was used as a hydrophobic matrix former and anhydrous caffeine as a freely soluble model drug. Phenolred was used as a colouring agent to enhance the visual detection of the liquid boundary movements, which were examined in axial and radial directions for both types of tablets. The tablets started to expand during the dissolution, resulting in cracking as the liquid boundary penetrated into tablet. The cracking influences drug release by shortening the diffusion path and decreasing the tortuosity. The liquid boundaries proceed differently in cylindrical and convex tablets, this being attributable to differences in pore structure and density distribution. Cylindrical tablets are quite homogeneous in terms of density, but convex tablets have more porous areas at the domes of the tablet.

Algorithms↗
PubMed2006

Collagen network of articular cartilage modulates fluid flow and mechanical stresses in chondrocyte.

The extracellular matrix of articular cartilage modulates the mechanical signals sensed by the chondrocytes. In the present study, a finite element model (FEM) of the chondrocyte and its microenvironment was reconstructed using the information from fourier transform infrared imaging spectroscopy. This environment consisted of pericellular, territorial (mainly proteoglycans), and inter-territorial (mainly collagen) matrices. The chondrocyte, pericellular, and territorial matrix were assumed to be mechanically isotropic and poroelastic, whereas the inter-territorial matrix, due to its high collagen content, was assumed to be transversely isotropic and poroelastic. Under instantaneous strain-controlled compression, the FEM indicated that the fluid pressure within the chondrocyte increased nonlinearly as a function of the in-plane Young's modulus of the collagen network. Under instantaneous force-controlled compression, the chondrocyte experienced the highest fluid pressure when the in-plane Young's modulus of the collagen network was approximately 4 MPa. Based on the present results, the mechanical characteristics of the collagen network of articular cartilage can modify fluid flow and stresses in chondrocytes. Therefore, the integrity of the collagen network may be an important determinant in cell stimulation and in the control of the matrix maintenance.

Biophysical Phenomena↗
PubMed2006

Interaction of radio frequency electromagnetic fields and passive metallic implants--a brief review.

During the last decade, use of radio frequency (RF) applications like mobile phones and other wireless devices, has increased remarkably. This has triggered numerous studies related to possible health risks due to the exposure of RF electromagnetic (EM) fields. One safety aspect is the coupling of EM fields with active and passive implants in the human body. While interactions with active implants have been quite extensively researched, only a few studies have focused on passive implants. The present article reviews interaction mechanisms and studies of passive metallic, that is, conductive, implants in common external RF EM fields. It is found that implants have been mostly studied numerically, and experimental studies are rare. Furthermore, the studies cover mostly far-field conditions and only a few have studied implants in near fields. A summary of results indicates that a conductive object in tissues may cause notable local enhancement of the EM field and thus enhanced power absorption. The degree of enhancement depends, for example, on the orientation, the dimensions, the shape, and the location of the implant. However, in most of the cases, the field enhancement has not been strong enough to cause remarkable excess heating (more than 1 degrees C) of tissues.

Absorption↗
PubMed2005

The effect of geometry and abduction angle on the stresses in cemented UHMWPE acetabular cups--finite element simulations and experimental tests.

BACKGROUND: Contact pressure of UHMWPE acetabular cup has been shown to correlate with wear in total hip replacement (THR). The aim of the present study was to test the hypotheses that the cup geometry, abduction angle, thickness and clearance can modify the stresses in cemented polyethylene cups. METHODS: Acetabular cups with different geometries (Link: IP and Lubinus eccentric) were tested cyclically in a simulator at 45 degrees and 60 degrees abduction angles. Finite element (FE) meshes were generated and two additional designs were reconstructed to test the effects of the cup clearance and thickness. Contact pressures at cup-head and cup-cement interfaces were calculated as a function of loading force at 45 degrees, 60 degrees and 80 degrees abduction angles. RESULTS: At the cup-head interface, IP experienced lower contact pressures than the Lubinus eccentric at low loading forces. However, at higher loading forces, much higher contact pressures were produced on the surface of IP cup. An increase in the abduction angle increased contact pressure in the IP model, but this did not occur to any major extent with the Lubinus eccentric model. At the cup-cement interface, IP experienced lower contact pressures. Increased clearance between cup and head increased contact pressure both at cup-head and cup-cement interfaces, whereas a decreased thickness of polyethylene layer increased contact pressure only at the cup-cement interface. FE results were consistent with experimental tests and acetabular cup deformations. CONCLUSION: FE analyses showed that geometrical design, thickness and abduction angle of the acetabular cup, as well as the clearance between the cup and head do change significantly the mechanical stresses experienced by a cemented UHMWPE acetabular cup. These factors should be taken into account in future development of THR prostheses. FE technique is a useful tool with which to address these issues.

Acetabulum↗
PubMed2005

Effect of bone mineral density and amorphous diamond coatings on insertion torque of bone screws.

In this study, the potential of high-quality amorphous diamond (AD) coatings in reducing the torque and failures of bone screws was studied. Torque values were recorded for 32 stainless steel screws, 2.7 or 3.5 mm in diameter and 60 mm in length. Half of the screw sets were coated with the AD coating before installing in predrilled holes of human cadaveric femoral bone samples. The bone samples were selected from two groups of four persons with mean ages of 34 years (range 25-41 years) and 75 years (range 73-77 years), respectively. The bone mineral density (BMD) values of the samples were determined exactly at the screw insertion site by peripheral quantitative computed tomography (pQCT). In the mechanical tests, insertion and removal torques were measured. BMD had a significant effect on insertion torque; the maximum torque (adjusted with respect to the screw diameter) was significantly higher for the young bone than for the old bone (p < 0.05). By using a polished AD coating, insertion torque was decreased even up to 50% in comparison with the screws without coating. The results suggest that AD coating provides a stable, smooth surface and reduces the risk of screw failures.

Adult↗
PubMed2005

Improvement of arthroscopic cartilage stiffness probe using amorphous diamond coating.

During arthroscopic evaluation of articular cartilage unstable contact and even slipping of the measurement instrument on the tissue surface may degrade the reproducibility of the measurement. The main aim of the present study was to achieve more stable contact by controlling the friction between articular cartilage surface and the arthroscopic cartilage stiffness probe (Artscan 200, Artscan Oy, Helsinki, Finland) using amorphous diamond (AD) coating. In order to obtain surfaces with different average roughnesses (R(a)), polished stainless steel disks were coated with AD by using the filtered pulsed arc-discharge (FPAD) method. Dynamic coefficient of friction (mu) between the articular cartilage (n = 8) and the coated plates along one non-coated plate was then determined. The friction between AD and cartilage could be controlled over a wide range (mu = 0.027-0.728, p < 0.05, Wilcoxon test) by altering the roughness. Possible deterioration of cartilage was investigated by measuring surface roughness after friction tests and comparing it with the roughness of the adjacent, untested samples (n = 8). Importantly, even testing with the roughest AD (R(a) = 1250 nm) did not damage articular surface. On the basis of the friction measurements, a proper AD coating was selected for the stiffness probe. The performance of coated and non-coated probe was compared by measuring bovine osteochondral samples (n = 22) with both instruments. The reproducibility of the stiffness measurements was significantly better with the AD-coated probe (CV% = 4.7) than with the uncoated probe (CV% = 8.2). To conclude, AD coating can be used to safely control dynamic friction with articular surface. Sufficient friction between articular surface and reference plate of the arthroscopic probe improves significantly reproducibility of the stiffness measurements.

Animals↗
PubMed2005

Potential of coatings in total hip replacement.

In total hip replacements, the bulk properties of materials, such as proper elasticity and hardness, are important. However, the material interacts with the body mainly at the surfaces. Wear and corrosion are initiated at the surfaces also. Therefore, the control of surface properties using different kinds of treatments or coatings may improve total hip replacements considerably. The most studied surface treatments include ion implantation and methods to control surface topography, such as grit or sand blasting or plasma treatments. Among the large variety of coatings, hydroxyapatite, titanium oxide and nitride, zirconium oxide, pyrolytic carbon, and diamondlike carbon coatings have shown the most promising results. These coatings mainly are used to enhance bone growth; to minimize friction, wear, and corrosion; and to improve biocompatibility of total joint prostheses. The potential of novel coatings to solve some present problems in joint prostheses is discussed based on the structure and properties of different kind of coatings. It can be concluded that currently, coating methods exist to improve the tribologic performance and longevity of the total hip replacements. However, coatings must fulfill two essential requirements: no delamination in biochemical and biomechanical environments and sufficient protection of substrate from corrosion.

Arthroplasty, Replacement, Hip↗
PubMed2004

Ultrasonic quantitation of superficial degradation of articular cartilage.

Ultrasound (US) has been suggested as a means for the quantitative detection of early osteoarthrotic changes in articular cartilage. In this study, the ability of quantitative US 2-D imaging (20 MHz) to reveal superficial changes in bovine articular cartilage after mechanical or enzymatic degradation was investigated in vitro. Mechanical degradation was induced by grinding samples against an emery paper with the grain size of 250 microm, 106 microm, 45 microm or 23 microm. For enzymatic degradation, samples were digested with collagenase, trypsin or chondroitinase ABC. Variations of the US reflection coefficient induced by the degradation were investigated. Furthermore, two novel parameters, the US roughness index (URI) and the spatial variation of the US reflection coefficient (SVR), were established to quantitate the integrity of the cartilage surface. Statistically significant decreases (p < 0.05) in US reflection coefficient were observed after mechanical degradations or enzymatic digestion with collagenase. Increases (p < 0.05) in URI were also revealed after these treatments. We conclude that quantitative US imaging may be used to detect collagen disruption and increased roughness in the articular surface. These structural damages are typical of early osteoarthrosis.

Animals↗
PubMed2003

Reduction of wear in total hip replacement prostheses by amorphous diamond coatings.

Wear-debris-induced periprosthetic bone loss and aseptic loosening is regarded as the main long-term problem of total hip replacements (THRs). In this study the amount of wear debris from articulating surfaces of THRs was reduced so as to be close to negligible by the use of high-quality amorphous diamond (AD) coatings deposited by pulsed plasma arc techniques. The wear rates determined in a commercial hip simulator for 15 million walking cycles (corresponding to about 15 years of clinical use) in serum lubrication were even 1,000,000 times lower than the clinical values for conventional THRs (polyethylene-metal or metal-metal pairs). Elimination of wear debris and good biomechanical performance should provide a substantial improvement of the clinical longevity of the THRs.

Biocompatible Materials↗
PubMed2003

Tantalum as a buffer layer in diamond-like carbon coated artificial hip joints.

The acid resistance of tantalum coated and uncoated human hip joint prostheses was studied with commercial CrCoMo acetabular cups. The samples were exposed to 10% HCl solution and the quantities of dissolved Cr, Co, and Mo were measured with proton-induced X-ray emission (PIXE). The absolute quantities were obtained with the use of Cr and Se solution standards. Tantalum coatings (thicknesses 4-6 microm) were prepared in vacuum with magnetron sputtering. Tantalum coating decreased the corrosion rate by a factor of 10(6). As a spinoff from recent wear tests on artificial hip joints it was shown that tantalum has excellent mechanical properties as an intermediate layer of diamond-like carbon (DLC) coatings. When tantalum was tested together with DLC on three metal-on-metal hip joint pairs in a hip simulator, no observable defects occurred during 15 million walking cycles with a periodic 50-300-kg load (Paul curve).

Acids↗
PubMed2003

Fibril reinforced poroelastic model predicts specifically mechanical behavior of normal, proteoglycan depleted and collagen degraded articular cartilage.

Degradation of collagen network and proteoglycan (PG) macromolecules are signs of articular cartilage degeneration. These changes impair cartilage mechanical function. Effects of collagen degradation and PG depletion on the time-dependent mechanical behavior of cartilage are different. In this study, numerical analyses, which take the compression-tension nonlinearity of the tissue into account, were carried out using a fibril reinforced poroelastic finite element model. The study aimed at improving our understanding of the stress-relaxation behavior of normal and degenerated cartilage in unconfined compression. PG and collagen degradations were simulated by decreasing the Young's modulus of the drained porous (nonfibrillar) matrix and the fibril network, respectively. Numerical analyses were compared to results from experimental tests with chondroitinase ABC (PG depletion) or collagenase (collagen degradation) digested samples. Fibril reinforced poroelastic model predicted the experimental behavior of cartilage after chondroitinase ABC digestion by a major decrease of the drained porous matrix modulus (-64+/-28%) and a minor decrease of the fibril network modulus (-11+/-9%). After collagenase digestion, in contrast, the numerical analyses predicted the experimental behavior of cartilage by a major decrease of the fibril network modulus (-69+/-5%) and a decrease of the drained porous matrix modulus (-44+/-18%). The reduction of the drained porous matrix modulus after collagenase digestion was consistent with the microscopically observed secondary PG loss from the tissue. The present results indicate that the fibril reinforced poroelastic model is able to predict specifically characteristic alterations in the stress-relaxation behavior of cartilage after enzymatic modifications of the tissue. We conclude that the compression-tension nonlinearity of the tissue is needed to capture realistically the mechanical behavior of normal and degenerated articular cartilage.

Animals↗
PubMed2003

Speed of sound in normal and degenerated bovine articular cartilage.

The unknown and variable speed of sound may impair accuracy of the acoustic measurement of cartilage properties. In this study, relationships between the speed of sound and cartilage composition, mechanical properties and degenerative state were studied in bovine knee and ankle cartilage (n = 62). Further, the effect of speed variation on the determination of cartilage thickness and stiffness with ultrasound (US) indentation was numerically simulated. The speed of sound was significantly (n = 32, p < 0.05) dependent on the cartilage water content (r = -0.800), uronic acid content (per wet weight, r = 0.886) and hydroxyproline content (per wet weight, r = 0.887, n = 28), Young's modulus at equilibrium (r = 0.740), dynamic modulus (r = 0.905), and degenerative state (i.e., Mankin score) (r = -0.727). In addition to cartilage composition, mechanical and acoustic properties varied significantly between different anatomical locations. In US indentation, cartilage is indented with a US transducer. Deformation and thickness of tissue are calculated using a predefined speed of sound and used in determination of dynamic modulus. Based on the simulations, use of the mean speed of sound of 1627 m/s (whole material) induced a maximum error of 7.8% on cartilage thickness and of 6.2% on cartilage dynamic modulus, as determined with the US indentation technique (indenter diameter 3 mm). We believe that these errors are acceptable in clinical US indentation measurements.

Animals↗
PubMed2003

Alternative materials to improve total hip replacement tribology.

An improvement in tribology of bearing surfaces is an effective means of increasing the longevity of total hip replacement (THR). Currently, 3 approaches are available to achieve this aim: first, use of highly cross-linked UHMWPE; second, aluminum oxide ceramic bearings, and third, metal-on-metal bearings. Cross-linking reduces the wear resistance of UHMWPE markedly without impairment of other significant properties of the material. Simulator studies and some clinical long-term (10-22 years) follow-up surveys suggest an almost immeasurable wear of the highly cross-linked UHMWPE-based acetabular components during an expected clinical life span. Bioinert alumina ceramic (aluminum oxide) was introduced 3 decades ago for THR-bearing surfaces to improve performance and longevity. Alumina ceramic is entirely biostable and bioinert and has good mechanical properties. For correctly positioned alumina-on-alumina bearings, the annual linear wear rate has been reported to be 3.9 microm. Alumina heads have been successfully used in combination with polyethylene sockets, but as regards wear, the best results have been obtained with alumina-on-alumina bearings. In ceramic THR bearings, precise manufacture and contact surface geometry, including optimal clearance, are most important. For the currently available products, the component fracture risk is almost nonexistent (less than 1 per 1000). Metal-on-metal bearings were used in the early stage of THR surgery, although not all old designs were successful. More recent analyses of the early series have shown the advantages of metal-on-metal to be better and have led to a renaissance of this articulation. Initially, stainless steel was used because it was easy to manufacture and polish. Current metal-on-metal bearings are based on cobalt-chromium-molybdenum alloys with varying carbon contents. Such bearings are self-polishing. Linear wear rates remain at the level of a few microm a year. An improvement in technology has increased the life span of the above three THR-bearing systems. Although the technical solutions differ considerably, they all seem to improve clearly the tribology and longevity of the THR. Each of these bearing concepts will probably permit the use of larger head sizes, to reduce the risk of impingement and luxations.

Aluminum Oxide↗