Understanding the Effect of Thermal Treatment on the Physico-Mechanical Properties of Light-Cured Composites for use in Indirect Restorations

OBJECTIVES

To study the potential benefits of a post-cure thermal treatment on key physico-mechanical properties of light-cured resin-based composites for use in indirect restorations, a CAD/CAM composite block being used as control.

MATERIAL AND METHODS

Six commercial composites were light-cured before being thermally treated in a furnace at 90°C during 15 minutes (CAD/CAM composite used as a control). The properties measured with or without thermal treatment were: degree of conversion, flexural strength, elastic modulus, Vickers microHardness, organic mass content and eluted and absorbed mass before and after storage in ethanol. The data were analysed using one-way ANOVA, and Weibull distributions.

RESULTS

A general increase in the properties measured was observed for all materials after thermal treatment, except a general decrease in mass elution and absorption (most statistically significant: p⟨0.05). Weibull analysis showed a tendency (p⟩0.05) of increased reliability of the flexural strength after thermal treatment for all materials.

CONCLUSION

The present data revealed clear physico-mechanical improvements after thermal treatment of light-cured composites. Such method could hence be beneficially used to produce indirect restorations as compared to stratifying and light-curing the same composites in situ. However, most properties of the control CAD/CAM composite were higher, but CAD/CAM technologies aren't available everywhere.

Investigating the limits of resin-based luting composite photopolymerization through various thicknesses of indirect restorative materials

OBJECTIVE

To determine the limitations of using light-curable resin-based luting composites (RBLCs) to bond indirect ceramic/resin-composite restorations by measuring light transmittance through indirect restorative materials and the resulting degree of conversion (DC) of the luting-composites placed underneath.

METHODS

Various thicknesses (0-4mm) and shades of LAVA Zirconia and LAVA Ultimate were prepared and used as light curing filters. A commercial, light curable RBLC, RelyX Veneer (control) was compared with four experimental RBLCs of the following composition: TEGDMA/BisGMA (50/50 or 30/70wt%, respectively); camphorquinone/amine (0.2/0.8wt%) or Lucirin-TPO (0.42wt%); microfillers (55wt%) and nanofillers (10wt%). RBLCs covered with the LAVA filter were light-cured for 40s, either with the dual-peak BluephaseG2 or an experimental device emitting either in the blue or violet visible band. The samples were analyzed by Raman spectroscopy to determine DC. Light transmittance through the filters was measured using a common spectroscopy technique.

RESULTS

All the factors studied significantly influenced DC (p<0.05). RBLCs with increased TEGDMA content exhibited higher DC. Only small differences were observed comparing DC without filters and filters ≤1mm (p>0.05). For thicknesses ≥2mm, significant reductions in DC were observed (p<0.05). Transmittance values revealed higher filter absorption at 400nm than 470nm. A minimal threshold of irradiance measured through the filters that maintained optimal DC following 40s irradiation was identified for each RBLC formulation, and ranged between 250-500mW/cm².

SIGNIFICANCE

This work confirmed that optimal photopolymerization of RBLCs through indirect restorative materials (≤4mm) and irradiation time of 40s is possible, but only in some specific conditions. The determination of such conditions is likely to be key to clinical success, and all the factors need to be optimized accordingly.

Reduced polymerization stress of MAPO-containing resin composites with increased curing speed, degree of conversion and mechanical properties

OBJECTIVES

The degree and rate of photopolymerization in resin-based dental composites will significantly affect polymer network formation and resultant material properties that may determine their clinical success. This study investigates the mechanical properties, the generation of stress from polymerization, tooth cusp deflection and marginal integrity of experimental resin composites that contain different photoinitiators.

METHODS

Experimental light-activated resin composites (60vol% particulate filled in 50/50mass% bis-GMA/TEGDMA) were formulated using a monoacylphosphine oxide (MAPO) photoinitiator and compared with a conventional camphoroquinone (CQ)-based system. Similar radiant exposure was used (18Jcm(-2)) for polymerization of each material although the curing protocol was varied (400mWcm(-2) for 45s, 1500mWcm(-2) for 12s and 3000mWcm(-2) for 6s). Degree and rate of polymerization was calculated in real-time by near infrared spectroscopy and the generation of stress throughout polymerization measured using a cantilever beam method. Flexural strength and modulus were acquired by three-point bend tests. Standardized cavities in extract pre-molar teeth were restored with each material, the total cuspal deflection measured and post-placement marginal integrity between the tooth and restoration recorded.

RESULTS

Generally, MAPO- exhibited a significantly higher degree of conversion (72±0.8 to 82±0.5%) compared with CQ-based materials (39±0.7 to 65±1.6%) regardless of curing protocol (p<0.05) and MAPO-based materials exhibited less difference in conversion between curing protocols. CQ-based materials exhibited between ∼85 and 95% of the maximum rate of polymerization at <15% conversion, whereas MAPO-based RBCs did not approach the maximum rate until >50% conversion. Higher irradiance polymerization had a significant deleterious effect on the mechanical properties of CQ-based materials (p<0.05) whereas MAPO-based materials exhibited increased strength and modulus and were less affected by the curing method. Total cuspal deflection in restored extracted teeth was higher for CQ- compared with MAPO-based materials cured at the lowest irradiance curing protocol (12.9±4.0 and 8.3±1.5μm) and similar at 3000mWcm(-1) for 6s (10.1±3.5 and 9.0±1.5μm). A significant decrease in marginal integrity was observed for CQ-based RBCs cured at high irradiance for short exposure time compared with that of the MAPO-based RBC cured using a similar protocol (p=0.037).

SIGNIFICANCE

Polymer network formation dictates the final properties of the set composite and the use MAPO photoinitiators may provide an effective restorative material that exhibits higher curing speeds, increased degree of conversion, strength and modulus without compromise in terms of polymerization stress and marginal integrity between tooth and restoration.

Benefits and limitations of adding hyperbranched polymers to dental resins

Volumetric shrinkage reduction is a constant challenge in the improvement of dental resins. The inclusion of hyperbranched polymers (HBPs) with modified functionalities (hydroxyl, propionate, and methacrylate) instead of conventional dimethacrylate monomers has the potential to reduce shrinkage, but can also affect other properties. The null hypothesis was that the addition of HBPs (from 5 to 40 mass%) to a 50/50 mass% Bis-GMA/TEGDMA mixture reduces shrinkage without affecting degree of conversion, elastic modulus, glass transition temperature, Wallace hardness (before/after ethanol storage), and viscosity. This hypothesis was rejected, since HBP incorporation significantly affected most properties either negatively or positively. When HBP amounts in the resin were increased, the following general trends were observed: Volumetric shrinkage decreased significantly (p < 0.0001), down to about one-third of the control value at 40% HBP; Wallace hardness (both before and after ethanol) and viscosity increased progressively, while elastic modulus showed a parabolic profile, with a maximum at 10% HBP; and finally, degree of conversion and glass transition temperature were relatively stable, regardless of the HBP content. These results indicate that HBPs with modified end groups might be interesting substitutes for Bis-GMA/TEGDMA.

Irradiation Modes’ Impact on Radical Entrapment in Photoactive Resins

Different irradiation protocols are proposed to polymerize dental resins, and discordances remain concerning their impact on the material. To improve this knowledge, we studied entrapment of free radicals in unfilled Bis-GMA/TEGDMA (50:50 wt%) resin after light cure. The tested hypothesis was that various irradiation parameters (curing time, irradiance, and radiant exposure) and different irradiation modes (continuous and pulse-delay) led to different amounts of trapped free radicals. The analysis of cured samples (n = 3) by electron paramagnetic resonance (EPR) revealed that the concentrations of trapped free radicals significantly differed according to the curing protocol. When continuous modes with similar radiant exposure were compared, higher concentrations of trapped free radicals were measured for longer times with lower irradiance. Concerning pulse modes, the delay had no influence on trapped radical concentration. These results give new insights into the understanding of the photopolymerization process and highlight the relevance of using EPR when studying polymerization of dimethacrylate-based materials.

Pulpal-temperature Rise and Polymerization Efficiency of LED Curing Lights

This paper provides practitioners with useful information on the importance of aligning the spectra of the LCU and the material in terms of polymerization efficiency and temperature rise in the pulp chamber.

Antibacterial activity of glass-ionomer cements, compomers and resin composites: relationship between acidity and material setting phase

Agar diffusion testing was used with four different media to evaluate the antibacterial activity of six products (one conventional glass-ionomer cement (GIC), two light-activated glass-ionomers, two polyacid-modified resin composites and one resin composite) on Streptoccoccus mutans. Their respective antibacterial activities were also compared during and after setting. The relationship between product acidity and antibacterial activity was evaluated. All the GICs demonstrated antibacterial properties in contrast to the polyacid-modified resin composites and resin composite which did not shown any antibacterial effects. Vitrebond GIC exhibited higher antibacterial action, probably because of a cytotoxic photo-initiator diphenyliodoniumchloride. A direct relationship between material acidity and growth inhibition of S. mutans was observed.

Characterization of the inorganic fraction of resin composites

The purposes of this study were to determine the weight fraction of filler in thirty-nine resin-based materials including flowable and packable composites, and to examine the morphology of the filler particles. The percentages of inorganic fillers by weight were determined by Thermogravimetric Analysis and by ashing in air technique at 900 degrees C. The size and shape of the filler particles were examined using scanning electron microscopy (SEM) after dissolution of the organic matrix. The weight fraction of inorganic fillers ranged between 41.6 and 84.6%. Wide variations were found among materials of the same category. Values found in the present study were sometimes different from those given by the manufacturers. The SEM photomicrographs showed various shapes, and sizes of inorganic fillers. Compared with universal hybrid restorative materials, flowable composites have lower filler loading and packable resin composites did not show higher values as claimed by some manufacturers. Various factors may explain the observed discrepancies between the manufacturer's data and our results. The silane treatment as well as the incorporation of organic material as part of the fillers of the composite could be responsible for those differences.

Treatment with removable partial dentures: a longitudinal study. Part I

The aim of this study was to evaluate the effectiveness of an oral rehabilitation by removable partial denture (RPD). Between 1983 and 1994, 629 patients were provided with an RPD at the Dental School of the Université catholique de Louvain. All the RPDs were constructed with a cobalt-chromium framework. All the treatments were provided by dental students under the supervision of clinical instructors. At recall time (1998-2000), 269 patients could not be reached neither by telephone nor by mail and 27 had died. Consequently, 333 patients were called for clinical examination and 254 of these (76.3%) actually attended. For a total of 292 RPDs fitted for these 254 patients, 218 (74.7%) were still being worn at the time of the check-up. Seventy-four dentures were considered to be 'failures', either because they were replaced by another RPD or by a complete denture, or because they had actually never been worn. The statistical analysis (Mantel-Haenszel and Kaplan-Meier) shows that the number of failures is significantly higher at the lower jaw compared with the upper jaw. Most of the failures are attributable to RPDs with free-end saddles and, in particular, to class I mandibular dentures. The patients are wearing their denture(s) mostly continuously (63.6%) and award a high degree of satisfaction to their denture. In general, the results recorded may be considered as very satisfactory, all the more so as we have no regular recall procedures established at our school and as check-up asked for spontaneously by the patients in the course of the period of observation are most of the time occasional or non-existent.

Treatment with removable partial dentures: a longitudinal study. Part II

The aim of this study was to evaluate the effectiveness of an oral rehabilitation by removable partial denture (RPD). Between 1983 and 1994, 629 patients were provided with an RPD at the Dental School of the Université catholique de Louvain. All the RPDs were constructed with a cobalt-chromium framework. All the treatments were provided by dental students under the supervision of clinical instructors. At recall time (1998-2000), 269 patients could not be reached neither by telephone nor by mail and 27 had died. Consequently, 333 patients were called for clinical examination and 254 of these (76.3%) actually attended. For a total of 292 RPDs checked 1893 persistent teeth were listed, with 804 serving as abutments; more than half (57.6%) of these were crowned, for which 51 debondings were recorded. For the period of observation we listed 79 abutments lost (37 at the upper jaw and 42 at the lower jaw), i.e. 9.8% of the total number of abutments. No relation could be established between the type of the abutments (natural or crowned teeth) and the losses observed (Test of Mantel-Haenszel: P=0.9496). In the upper jaw, the percentage of abutments lost was significantly higher in the presence of free-end edentulous areas as compared with bounded edentulous areas (Test of Mantel-Haenszel: P=0.0002); this difference does not appear for the lower jaws (P=0.9558). If we deduct the 25 abutments related with the 11 non-worn RPDs and the 79 abutments lost, no change becomes apparent for 92.2% of the maxillary abutments and for 85.8% of the mandibular abutments. For the 1089 other teeth, we observed the loss of 40 teeth and the appearance of caries or new fillings for 95 teeth. The fractures of cast clasps represent 3.4%.

Interfacial layer characterization in dental composite

Nowadays, dental resins find increasing use by practitioners. However, photopolymerization of such resins is limited to a so-called 'depth of cure'. To face this problem, dentists superimpose resin layers of limited depth. This technique raises the problem of the quality of interlayer. This paper aims at screening different cases of adhesion at the interface. Shear strength of such interfacial layers is tested in shear mode in various conditions (with or without an oxygen-inhibited layer, or contaminated by saliva or water). Unexpectedly, the presence of an oxygen-inhibition on the first layer, as assessed using micro-Raman spectroscopy, induces higher shear strength. In this case, a cohesive break occurs while an adhesive one is observed in all the other cases.

Raman scattering determination of the depth of cure of light-activated composites: influence of different clinically relevant parameters

The purpose of this research was to determine the depth of cure of light-activated composites in relation with different clinically relevant parameters. A Raman spectroscopic method has been used. The measurement of cure is made on a relative basis by comparing the vibration band of the residual unpolymerized methacrylate C=C bond at 1640 cm-1 against the aromatic C=C stretching band at 1610 cm-1 used as an internal standard. The information gained draw attention to the importance of light transmission during the exposure. The influence of sample's thickness on the depth of cure is illustrated by a second order polynomial regression. The shade and translucency of the resin composite also modify the light transmission and thus have a significant influence on the degree of conversion. Moreover the light-source intensity and the distance from the curing tip are important parameters of influence. A significant reduction of the depth of cure is observed for all sample thickness of resin composite tested when using a light device with an intensity of 300 mW cm-2 as well as using a distance from the curing tip higher than 20 mm.

Dynamic and static moduli of elasticity of resin-based materials

OBJECTIVES

The purpose of this study was to assess and compare the elastic moduli of 34 resin-based materials using a dynamic and a static method. The effect of water storage was also studied up to 6 months.

METHODS

Five samples of each material were prepared according to ISO-4049. The dynamic moduli were first determined non-destructively from the fundamental period of the vibrating specimen, then the static moduli were determined by a three-point bending test. The percentages of fillers by weight were determined by ashing in air at 900 degrees C.

RESULTS

Low values were obtained with flowable composites as well as with two packable resin composites. Correlations were found between the static and the dynamic modulus of elasticity (r = 0.94; p = 0.0001) as well as between the weight percentage of fillers and the moduli of elasticity (r = 0.82; p < 0.05 for static modulus and r = 0.90; p < 0.05 for the dynamic modulus) both at 24h. Water storage significantly affected both static and dynamic moduli of elasticity (ANOVA two factors; p < 0.05).

SIGNIFICANCE

The low moduli of the flowable composites do not allow their use in posterior cavities under high stress. However, this does not exclude their use for minimally-invasive Class I cavities when the opposing tooth is stabilized to a large amount on the natural enamel. The Grindosonic method is very useful and simple for determining the dynamic moduli although it gives higher values than the static one. The elastic modulus evolution of resin-based materials after water storage is unpredictable since different patterns were observed as a function of time.

Meta-analytical review of factors involved in dentin adherence

Literature data on adherence tests of dentin-bonding systems (DBS) may differ widely, even for the same DBS. The problem of bond testing is that materials are seldom compared with a standard, and experimental conditions often vary. We sought to identify the parameters that influence this variability. Using inclusion and exclusion criteria, we conducted a meta-analytical review of 75 articles, published between 1992 and 1996 in SCI reviews, that give bond strength data for 15 dentin-bonding agents of the so-called third and fourth generations. Seventeen selected parameters were classified into four groups: Group A includes factors related to the dentin substrate (i.e., nature of teeth); group B, composite and bonding area (i.e., composite stiffness); group C, storage conditions of the bonded samples (i.e., thermocycling); and group D, test design (i.e., crosshead speed). For each report, the experimental features, the bond strength means and standard deviations, and the failure mode were extracted and tabulated. Statistical Analysis System software was used to perform Pearson correlation analysis and analysis of variance, with bond strength as the dependent variable and experimental conditions as the independent variables. The meta-analytical review highlighted the significant influence of various parameters in the different groups: origin of dentin, types of teeth, pulpal pressure, tooth storage temperature, maximum storage time of teeth, and dentin depth in group A; type and stiffness of composite and bonding area in group B; storage of bonded samples (medium, temperature, and time) in group C, and testing mode and crosshead speed in group D. A significant positive correlation was observed between the mean bond strength and the rate of cohesive failure. It can be concluded from this study that some of these parameters should be controlled by the use of a standardized protocol. Unfortunately, the substrate-related variables are more difficult to control, even though their influence is consistent.

Fluoride release from glass-ionomer cements, compomers and resin composites

The short and long-term fluoride release of 16 products (seven conventional glass-ionomers, five light-activated glass-ionomers, two polyacid-modified resin composites and two resin composites) commercialized as fluoride-releasing materials were measured. A potential link between the material type and its level of fluoride release was researched. The fluoride release was evaluated after different time intervals. Initial fluoride release from all materials was highest during the first 24 h and decreased sharply over the first week. Some groups of materials appeared to be significantly different after, respectively, 7 and 91 days. However, it was impossible to correlate the fluoride release of the materials by their type (conventional or resin-modified glass-ionomers, polyacid-modified resin composite and resin composite) except if we compared the products from the same manufacturer. The link between fluoride release and an acid-base reaction seems to be confirmed. The glass-ionomer composition (glass particles and polyacid's type, powder/liquid ratio) should have more influence on fluoride release than material type.

The micro-Raman spectroscopy, a useful tool to determine the degree of conversion of light-activated composite resins

Light-activated composites are now among the most popular dental restorative materials. Nevertheless, concerns exist about the so-called depth of cure. Infrared spectroscopy (FTIR) has traditionally been used to quantify this problem by evaluating the degree of conversion of dental resins. However, Raman scattering provides an alternate method. This article describes the advantages and the limitations of micro-Raman spectroscopy, as compared to FTIR and other techniques, for calculating the local degree of conversion and the depth of cure of light-cured composites.

[Evaluation technics of the anticaries potential of fluoride-releasing materials]

Clinical evaluation of anticariogenic potential of fluoride releasing materials seems to be the best way to evaluate products effectiveness. This is an expensive, lengthy and difficult process to realise. Yet there is a need to determine the relative effectiveness of different materials so that clinicians can make rational decision on what materials to use. Fluoride release measurement and fluoride uptake in dental tissues, antibacterial effect evaluation and caries-like lesion inhibition constitute a part of different model investigations use to study fluoride releasing materials. Even if the results of these different in vitro model systems may not be directly transposed to clinical reality, they give some useful information to determine anticariogenic potential of some dental materials. Three materials are analyzed: glass ionomer cements (conventional and resin-modified), polyacid modified resin composites and fluorated resin composites. Results obtained are different for the three kind of materials. Glass ionomer cements show much more anticariogenic properties than polyacid modified resin composites and fluorated resin composites. These two last products are more difficult to distinguish. It is probably due to their similar chemical properties. Even if these products don't have a large anticariogenic effect, it could be enough to prevent secondary caries. The anticariogenic action is principally due to the fluoride release of the materials. That's why it is necessary to define the acceptable level of fluoride release, which is required to have anticariogenic action.

Structural integrity of resin-modified glass ionomers as affected by the delay or omission of light activation

Since light activation of resin-modified glass ionomers as a means of polymerizing the HEMA is usually done shortly after mixing occurs, the acid-base reaction will proceed mainly within a formed HEMA-polymer matrix. Delaying or omitting light activation may alter the structure and consequently its integrity. The aim of this study was to investigate the effect on the structural integrity of Fuji II LC, Photac-Fil, and Vitremer by delaying or omitting light initiation as compared with the integrity when light activation is performed 2 min after mixing occurs. We evaluated integrity by three-body wear experiments, conducted 8 hrs after sample preparation, to establish the integrity in the early phase of hardening, as well as after 1 wk and after 4 mos, to follow the materials throughout the process of maturation. When light activation was delayed for 1 hr, the structural integrity of Fuji II LC and Photac-Fil improved significantly in the early stages of hardening. In the case of Vitremer, an hour's delay of light activation significantly decreased integrity, which declined further when light activation was omitted. Fuji II LC was not affected by the omission of light activation, while Photac-Fil was markedly weakened. After 4 mos of aging, most of the samples of each product which had been cured by the different methods attained equal integrity, with the exception of the non-light-activated Vitremer samples, which remained weaker. We concluded that the structural integrity of resin-modified glass-ionomer cements benefits from a chemical integration of the polyalkenoate and poly-HEMA networks, as in Vitremer. Improvement in the structural integrity in the early phase for cements with a mechanical entanglement of the matrices, as in Fuji II LC and Photac-Fil, requires an acid-base reaction, a considerable portion of which may take place before activation of the HEMA polymerization.

Bone resorption and response to calcium-regulating hormones in the absence of tissue or urokinase plasminogen activator or of their type 1 inhibitor

Plasminogen activators (PA) are implicated in cell migration and tissue remodeling, two components of the bone resorption processes. Using mice with inactivated tissue PA (tPA), urokinase PA (uPA), or type 1 PA inhibitor (PAI-1) genes, we evaluated whether these processes, or their stimulation by parathyroid hormone (PTH) or 1,25-dihydroxyvitamin (1,25[OH]2D3) are dependent on these genes. Two culture models were used, one involving 19-day fetal calvariae, to evaluate the direct resorptive activity of osteoclasis, and the other involving 45Ca-labeled 17-day fetal metatarsals, in which this activity depends on preliminary (pre)osteoclast migration. PTH similarly increased (about 10-fold) PA activity in calvariae from wild-type tPA+/+ and uPA+/+ or deficient uPA-/- and PAI-/- mice; it affected only tPA, not uPA. In tPA-/- bones, the low PA levels, due to uPA, were not influenced by PTH. Calcitonin did not affect PA responses to PTH. No differences were observed between tPA+/+, tPA-/-, uPA+/+, and uPA-/- calvariae for any parameter related to bone resorption (development of lacunae, release of calcium and lysosomal enzymes, accumulation of collagenase, loss of hydroxyproline), indicating similar responses to PTH or calcitonin. The progressive 45Ca release was largely similar in cultures of tPA+/+, tPA-/-, uPA+/+, uPA-/-, PAI+/+, or PAI-/- metatarsals and it was similarly enhanced by PTH or 1,25(OH)2D3. However, uPA-/- metatarsals released 45Ca at a slower rate at the beginning of the cultures, suggesting an impaired recruitment of the (pre)osteoclasts, which migrate at that time from the periosteum into the calcified cartilage. Thus, it appears that the direct resorptive activity of the osteoclasts does not necessitate the presence of either tPA or uPA, but uPA is likely to facilitate the migration of the (pre)osteoclasts toward the mineralized surfaces. Although considerably enhanced by PTH, tPA does not mediate the actions of PTH (nor of 1,25[OH]2D3) evaluated in these models.

Relationship of the plasminogen activator/plasmin cascade to osteoclast invasion and mineral resorption in explanted fetal metatarsal bones

An attempt was made to establish whether the activation of plasminogen into plasmin is necessary either for the preparatory phases to bone resorption, involving the recruitment of osteoclast precursors, their migration toward mineralized surfaces, and their final differentiation, or for the subsequent osteoclastic resorption phase. 45Ca-labeled fetal (17 day) mouse metatarsals were cultured under conditions in which they pursue their modeling for a few days. In this model, the resorption phase, monitored by the release of 45Ca into the medium, is entirely dependent on the preparatory phases affecting osteoclast precursors. It was, as expected, stimulated by parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D3 and inhibited by calcitonin. PTH also enhanced the activity of tissue-type plasminogen activator (PA) in extracts of metatarsals but not that of urokinase (which is, however, the main PA present in the mouse fetal metatarsal culture model). The resorption processes were not dependent on the presence of plasminogen in the media, even when the rudiments were precultured with tranexamic acid to remove their endogenous plasminogen. Moreover, they were not influenced by inhibitors of plasmin, either the plasma inhibitors alpha 2-antiplasmin, alpha 2-macroglobulin, and alpha 1-antitrypsin, or aprotinin, which was tested under a variety of conditions. Aprotinin also did not influence the resorption (loss of calcium and hydroxyproline) of 19 day fetal mouse calvariae cultured with PTH in a medium devoid of plasminogen. It is concluded that the various steps implicated in the bone resorption processes that occur in the metatarsals and in the calvariae culture models are not dependent on the activity of plasmin. The function of PAs in bone, however, could be exerted through direct proteolysis of extracellular proteins other than plasminogen or be mediated by a molecular structural domain distinct from their catalytic domain.

Tissue and urokinase plasminogen activators in bone tissue and their regulation by parathyroid hormone

The identification of the plasminogen activator (PA) types present in bone and the regulation of their activity by parathyroid hormone (PTH) were investigated in cultures of fetal mouse calvariae with the use of either a chromogenic substrate or a zymographic assay. PA was detected essentially in the tissue extracts of the explanted bones, with only 1-2% of the total activity released in the surrounding culture media. From their electrophoretic behavior compared to PAs of other mouse tissues and from their response to a specific antibody raised against the tissue type PA (tPA), two major molecular species, of 70 and 48 kD were identified as tPA and urokinase (uPA), respectively, a third minor species of 105 kD being likely to correspond to complexes between tPA and an inhibitor; the culture fluids, moreover, contained enzymatically active degradation products of uPA of 42 and 29 kD. The PA activity of the bone extracts was only minimally affected by the addition of fibrinogen fragments to the chromogenic assays. PTH induced bone resorption and stimulated in parallel the accumulation of PA in the tissue; other bone-resorbing agents, 1,25-dihydroxyvitamin D3 and prostaglandin E2, had similar effects. Densitometric scanning of the zymograms of the bone extracts indicated that PTH stimulated only the production of tPA and had no effect on that of uPA. However, PTH also enhanced the release of uPA (both the 48 kD and the 29 kD forms) from the bones into the media. Although inhibiting bone resorption, calcitonin had no effect on the PTH-induced accumulation of PA in bone or on the release of tPA, but it prevented the PTH-induced accumulation of 29 kD uPA in the culture fluids. Thus these studies support the view that tPA and possibly also uPA may have a role in the physiology of bone; the nature of this role remains to be elucidated, however.