By B. Arakos. University of Hawai`i, West O`ahu.
Radiation Biology of the blood vessels ﬁnd themselves closer to the blood supply and so reoxygenate buy avana 100 mg online. When cells are treated with these drugs for several days before irradiation with x- or g-rays order 200mg avana overnight delivery, cells become highly sensitive to radia- tion buy avana 200mg on-line. For optimal therapeutic gain in radiotherapy generic 100 mg avana otc, patients should be treated for a period of time extending over several cell cycles to maximize drug incorporation into the cells. Others Radiosensitizers such as actinomycin D, puromycin, methotrexate, and 5- ﬂuorouracil have been successfully used in combination with radiation to treat cancer. Whether these agents truly increase radiosensitivity or are simply toxic to the cells is still not clear. Investigators have been trying to explore radiosensitizing chemicals to substitute for oxygen that requires the use of a high-pressure technique. Metronidazole (Flagyl), having a structure with high electron affnity, is a good radiosensitizer for hypoxic cells. Another useful radiosensitizer for hypoxic cells is misonidazole, which also has high electron affnity. Mis- onidazole is almost ten times more effective than metronidazole in sensi- tizing hypoxic cells. Another radiosensitizer of this kind is etanidazole, which is less toxic than misonidazole, and has great potential in radio- therapy. These agents protect normal cells from radia- tion damage by combining with free radicals that are produced by radia- tion and would be toxic to normal cells. However, these compounds cause severe adverse reactions such as nausea and vomiting. Experimental evidence showed that Classiﬁcation of Radiation Damage 245 these products concentrate more in normal cells and less in tumor cells. As a result, normal cells are protected better than tumor cells if these agents are administered immediately before the radiation dose is given. Expo- sure of cells to 100 to 1000rad (100 to 1000cGy) causes delay in the G2 phase to M phase transition. An exposure of 1000rad (1000cGy) inhibits the progression of the S phase cells by 30%, whereas the S phase to G2 phase transition is not affected by such an exposure (Prasad, 1995). Classiﬁcation of Radiation Damage Cell death is a measure of extreme radiation damage. All these damages are relevant in clinical radiation therapy as to the effective- ness of treatment. Sublethal damage occurs in mammalian cells, when a radiation dose is given in fractions at different time intervals rather than a single dose. Repair involves the healing of the radiation-induced damage in the time interval between the two fractions of the dose. If the second dose is applied too soon after the ﬁrst application, the damage does not have enough time to repair and the cell will die. In the redistribution process, the cells are desynchronized and sensitized to show increased damage. Following irradiation, the radiosensitive cells will die, and one would expect the proportion of radioresistant cells and hence the surviving fraction to increase. Regeneration is a mechanism of response to depopulation of a cell cohort due to radiation damage, and depends on the types of tissue and their proliferating capacity. Protracting a fractionated dose should be beneﬁcial to normal tissues and somewhat harmful to regenerating tumor cells. Reoxygenation discussed earlier is an effect that makes the hypoxic cells more radiosensitive in the presence of oxygen in fractionated radiotherapy. Sublethal damage repair depends very much on the dose rate and in which stage of the cell cycles the cells are. For example, the testis of male rats is most radiosensitive, whereas the small intestine seems to be less affected by radiation. The repair is signiﬁcant with x-rays and g-rays and almost nonex- istent for neutrons and a-particles. For example, the survival of the HeLa cells increased after irradiation, when the cells were treated with excess thymidine or hydro- xyurea for a period of 4hr. Stochastic and Deterministic Effects Two categories of radiation effects on biological systems are encountered: stochastic and deterministic. Stochastic effects are the biological effects that occur randomly, the probability of which increases with increasing dose without a threshold. Radiation-induced hereditary effects and cancer inci- dences are examples of stochastic effects. The assumption of no threshold is made on the belief that radiation damage to a few cells or a single cell could theoretically induce the genetic disorder or cancer, and the severity of the disease will be the same, if it ever occurs. Acute Effects of Total Body Irradiation 247 The deterministic or nonstochastic effects are induced by high radiation doses and the severity of the damages, rather than their probability of occurrence, increases with the dose. Cataracts, skin erythema, sterility, and ﬁbrosis are examples of deterministic effects induced by high radiation doses. Acute Effects of Total Body Irradiation Different tissues of the body respond differently to radiation, due to varying degrees of radiosensitivity. When an adult subject is irradiated over the entire body, various syndromes are manifested depending on the dose applied. The effects of radiation are characterized by the survival time of the species and various stages of acute syndromes following the total- body irradiation. Cell survival time varies with mammal species depending on the indi- vidual radiosensitivity. Acute radiation syndromes appear in four stages: prodromal, latent, manifest illness, and recovery or death. A minimum of 200 to 300rad (200 to 300cGy) is required for all four stages to be seen and can cause death. In the prodromal stage, major symptoms are nausea, vomiting, and diarrhea and they occur in the early phase, lasting for only a short period of time depending on the dose. In the latent stage, biological damage slowly builds up without manifestation of any syndromes, again lasting for hours to weeks, depend- ing on the dose. During the manifest illness stage, radiation syndromes appear as a result of the damage to the organs involved after the latent period, and the subject becomes ill. Hemopoietic Syndrome Hemopoietic or bone marrow syndromes appear at a total body dose of 250 to 500rad (250 to 500cGy) following irradiation. Also, the number of lymphocytes are greatly depressed, whereby the immune system of the body is sup- pressed. Loss of blood cell counts can be noticed at a dose as low as 10 to 15rad (10 to 15cGy).
However 100mg avana overnight delivery, if there is an element of attrition or signs of parafunction composite onlays will not be adequately durable in the posterior segments cheap avana 100 mg otc, so cast restorations are recommended buy avana 200 mg. Long-term review All patients with tooth surface loss should be reviewed regularly for three reasons: (1) to monitor future tooth surface loss; (2) to maintain the existing restorations; and (3) to provide support for the patient cheap 100 mg avana. Patients with eating disorders in particular are prone to periods of relapse and the dentist is in an ideal position to diagnose these periods. People change their diet as they get older⎯one example is the young adolescent who manages to stop drinking Coca Cola but starts drinking lager to excess instead and the erosion continues! In view of the wide variety of presentations and degree to which each individual case is affected, it is difficult to make generalizations. Early diagnosis of these conditions is important to their long-term prognosis; parents need to be educated as to the implications of the condition; monitoring of the amount of tooth wear can start, and, where necessary, teeth can be protected. There are four main clinical problems associated with inherited enamel and dentine defects: (1) poor aesthetics; (2) chipping and attrition of the enamel; (3) exposure and attrition of the dentine causing sensitivity; and (4) poor oral hygiene, gingivitis, and caries. While it is impossible to draw up a definitive treatment plan for all cases, it is possible to define the principles of treatment planning for this group of patients. It is important to realize that not all children with amelogenesis imperfecta or dentino- genesis imperfecta are affected equally. Many will not have marked tooth wear or symptoms, and will not require advanced intervention. Key Points Main treatment objectives for dental anomalies: • to alleviate symptoms; • to maintain/restore occlusal height; and • to improve aesthetics. Prevention Prevention is an essential part of the management of children with enamel and dentine anomalies. Oral hygiene in these children is often poor, due in part to the rough enamel surface which promotes plaque retention and to the sensitivity of the tooth to brushing. The combination of gingival swelling and enamel hypoplasia can result in areas of food stagnation and a generally low level of oral health. Oral hygiene instruction must be given sympathetically, with plenty of encouragement, and should be continually reinforced. In some cases it may be necessary to carry out some restorative/cosmetic treatment before good oral hygiene measures can be practised. For example, the placement of anterior composite veneers may reduce dentine sensitivity and improve the enamel surface so that the patient can brush their teeth more effectively. Conventional caries prevention with diet advice, fluoride supplements, and topical fluoride applications is mandatory. In this group of children it is particularly important to preserve tooth tissue and not allow caries to compromise further the dental hard tissues. Restoration Restorative treatment varies considerably depending on the age of the child and extent of the problem. If there is sensitivity or signs of enamel chipping, techniques to cover and protect the teeth should be considered. In the very young child it is often impossible to carry out extensive operative treatment, but the placement of glass ionomer cement over areas of enamel hypoplasia is simple and effective. In older/more co-operative children stainless-steel (or nickel/chrome) preformed crowns should be placed on the second primary molars to minimize further wear due to tooth on tooth contact (Chapter 8587H ). It is advisable (and usually possible) to place such restorations with minimum tooth preparation because of the pre-existing tooth tissue loss. The teeth undergo such excessive wear that they become worn down to gingival level and are unrestorable. Teeth affected by dentinogenesis imperfecta are also prone to spontaneous abscesses due to the progressive obliteration of the pulp chambers. In these cases pulp therapy is unsuccessful and extraction of the affected teeth is necessary. As the permanent dentition develops close monitoring of the rate of tooth wear will guide the decision about what intervention is needed. Cast occlusal onlays on the first permanent molars not only protect the underlying tooth structure but also maintain function and control symptoms. The resulting increase in the vertical dimension is associated with a decrease in the vertical overlap of the incisors. Within a few weeks full occlusion is usually re-established, the whole procedure being well tolerated by young patients. Alternatively, localized composite or glass ionomer cement restorations may be placed over areas of hypoplasia. The emphasis should remain on minimal tooth preparation until the child gains adulthood. At this point, if clinically indicated, full mouth rehabilitation may be considered and should have a good prognosis in view of the conservative approach that has been adopted throughout the early years (Fig. The characteristic form of the teeth in this condition is unfavourable for crowning; the teeth being supported by short, thin roots. The permanent dentition, like the primary dentition, is prone to spontaneous abscesses and the prognosis for endodontic treatment is very poor. The long-term plan for these patients is often some form of removable prosthesis, either an overdenture placed over the worn permanent teeth or a more conventional complete denture. Where the child is sufficiently co-operative the use of glass ionomer cements to restore and improve the appearance of primary incisors can be useful in gaining the respect and support from the patient and parent. In a few exceptional cases the loss of primary teeth may cause upset, but can be compensated for by constructing dentures. In cases of dentinogenesis imperfecta where the teeth are very worn but remain asymptomatic, overdentures can be constructed to which young children adapt remarkably well. As the permanent incisors erupt they must be protected from chipping of the enamel. The placement of composite veneers not only improves the appearance but also promotes better gingival health and protects the teeth from further wear. In a few cases the quality of the enamel is so poor that the bond between composite and tooth will be unsuccessful. It should be noted that in these cases porcelain veneers are also likely to be unsuccessful and full coronal restorations are the only option. Early consultation with an orthodontist is advisable in order to keep the orthodontic requirements simple. Treatment for these patients is possible and in many cases proceeds without problems. The use of removable appliances, where appropriate, and orthodontic bands rather than brackets will minimize the risk of damage to the abnormal enamel. The problem is twofold: there may be frequent bond failure during active treatment or the enamel may be further damaged during debonding. Some orthodontists prefer to use bands even for anterior teeth, while others will use glass ionomer cement as the bonding agent in preference to more conventional resin-based agents.