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Appropriate tissue management is a vital part of the process of recording an acceptable fixed prosthodontic impression. In some cases, precise preparation of the tooth without iatrogenic damage to surrounding tissue is sufficient. Ideally, marginal gingiva should be healthy at the time of crown and bridge procedures.1,2 When factors such as esthetics, existing restorations, or fracture dictate that finish lines of the prepared tooth be placed equigingival or intracrevicular, some form of tissue retraction or displacement is necessary.
Several means of tissue retraction are commonly used by dentists to create sufficient depth and width of material for crown and bridge impressions. These include mechanical, chemical, and surgical.3-5 Often, a combination of these techniques is used. The purpose of this article is to review the local and systemic effects of mechanical and chemical tissue retraction for fixed prosthodontics. Specifically, the use of retraction cord with or without chemicals to control sulcular hemorrhage and moisture is discussed. The authors concentrate on the most common astringents, hemostatics, and vasoconstrictors used in dentistry as gingival retraction agents, and offer recommendations for modification of patient and treatment management.
Mechanical Retraction
The most common form of mechanical tissue displacement practiced by dentists to unmistakably record a conventional fixed prosthodontic impression involves the use of gingival retraction cord. Several studies have examined the use of gingival retraction techniques by dentists, and mechanical or mechanico-chemical were most commonly utilized.6-9 Mechanical retraction using gingival cord consists simply of the use of a string, usually made of cotton, silk, or yarn wool. Products can be fabricated into configurations of knitted, braided, or twisted cord of varying diameters, giving the practitioner numerous choices for easier placement, manipulation, absorbency, and tissue retraction.10 One unique product on the market offers another option that is composed of a braided cord with a thin metal filament designed to promote retention of the cord in the sulcus after placement.11,12 The characteristics of knitted, braided, and twisted cords are summarized in Table 1. Placement of retraction cord is accomplished using cord-packing instruments, with either a smooth or serrated end, and generally requires local anesthesia.
Clinical use of retraction cord typically involves either a single-cord or double-cord technique.1-3,5,10-14 In both cases the goal of mechanico-chemical retraction is to direct pressure into the sulcus to mitigate crevicular flow, achieve hemostasis, and create a physical space for impression material to flow and record the prepared tooth. The use of plain cord may have limited success in creating a dry, bloodless field for making impressions. Wöstman and colleagues showed that the use of nonimpregnated cotton cord caused increases in crevicular fluid flow,15 although Kumbuloglu et al showed no recurrence of bleeding after plain, untreated cord was used.4 Several studies have since concluded that consistently successful hemostasis and drying of the sulcus can only be achieved by a combination of mechanical and chemical means.6,16-18 Mechanico-chemical retraction involves cord that has been impregnated or soaked in astringents, hemostatics, or vasoconstrictors to achieve the clear field necessary for successful fixed prosthodontic impressions.
Retraction cord remains in direct contact with the thin monolayer of epithelial cells of the gingival sulcus and the connective epithelium at the bottom of the sulcus until effective shrinkage and displacement of free gingiva away from tooth structures and hemostasis is obtained. It is well known that placement of cord can lead to acute tissue injury and can be associated with marginal recession. In an early study by Ruel it was demonstrated that retraction cords impregnated with 0.1% (1:1000 concentration) epinephrine resulted in an average of 0.2 mm of gingival recession after crown preparation.19 Azzi et al showed no recession after cord placement but did emphasize that extreme care should be taken with tissue management, including the use of cord because irreparable tissue damage could result.20 In a more recent study by Kazemi, in agreement with Ruel, cord impregnated with aluminum chloride caused up to 0.2 mm of recession after 28 days.21
Mechanico-chemical gingival retraction can also have some negative local effects on the gingival connective tissues directly. In fact, many authors have observed an inflammatory response or even necrosis of the sulcular epithelium and subepithelial connective tissue induced by gingival cord with or without an applied chemical agent.4,20,22-27 Harrison examined the effect of mechanico-chemical retraction on the sulcular epithelium on dogs and found that retraction cord treated with epinephrine, alum, or zinc chloride caused tissue injuries varying from slight to severe, although most healed within 7 to 10 days.28 More recently, Feng and colleagues studied the effect of retraction cord on healthy human gingiva and concluded that pro-inflammatory mediators were released following placement, but that acute tissue injury healed within 2 weeks.29 This was in agreement with other studies that examined the effect of cord placement on gingival indices.19,20,30 It is generally recommended, then, that the smallest diameter cord should be used for gingival retraction and should be in place for 3 to 5 minutes, not to exceed 10 minutes. It is further recommended that cord be placed firmly but gently and should be wet with water or other chemical agents during placement and removal from the sulcus to prevent damage.31
Chemical Retraction Agents
The use of chemical agents, used alone or in combination with cords, is typical in clinical practice for fixed prosthodontic impressions. The agents are supplied in the form of gingival retraction fluids, gels, or pastes (Table 2).32,33 Cords may be preimpregnated (ie, the chemical agent is incorporated by the manufacturer) or the chemical agent may be applied to the cord by the clinician prior to placement. If chemical retraction is applied to the cord at chairside it should be allowed to soak for approximately 20 minutes to achieve proper saturation.34 Both Shillingburg and Kumbuloglu et al described the most desirable chemical agents to be used in gingival retraction procedures as meeting three criteria: the drug must be effective; it should not cause significant and irreversible tissue damage; and it should not produce potentially harmful systemic effects.4,29 With respect to the pharmacologic effects of the active substance, they belong either to class 1 (vasoconstrictors) or class 2 (hemostatics, astringents).35
Classification of Agents
A variety of chemicals have been used with gingival retraction cords to act as vasoconstrictors, hemostatics, or astringent agents capable of enhancing the effectiveness of mechanical tissue displacement. Some techniques utilizing only gingival retraction fluids, gels, or pastes are intended to create less traumatic tissue management and hemostasis.
Vasoconstrictors do not produce coagulation of blood but act by constricting blood vessels. The most commonly used vasoconstrictor in dentistry, epinephrine, exerts its effect through stimulation of alpha-, beta 1-, and beta 2-adrenergic receptors. Epinephrine provides vasoconstriction of the small blood vessels in submucosal tissue by stimulating alpha adrenergic receptors; this allows for delayed local anesthetic absorption and improved hemostasis in the operative field. These local effects are considered desirable by most oral healthcare providers and are the main reasons that epinephrine is employed topically as a gingival retraction fluid. Csillag et al advise the use of low-concentration epinephrine (0.01%) for gingival retraction due to its superior effect in keeping the gingival sulcus dry during the impression procedure.17 Originally, 8% racemic epinephrine was the vasoconstrictor of choice and was preferred over some of the early astringents such as zinc chloride and alum because they often caused adverse tissue reactions. Racemic epinephrine has since been the subject of many studies primarily because of the controversy regarding its possible hemodynamic effects.6,25,36 Racemic epinephrine differs from epinephrine USP because it contains a 50:50 mixture of the d- and l-isomers; epinephrine USP contains only the l-isomer. The racemic form of epinephrine is used in retraction cord because the chemical is more stable and allows for efficacy under varying conditions of storage.37 Racemic epinephrine is sometimes listed as dl-epinephrine on retraction cord product labels.
Astringents are chemicals that precipitate proteins to make the superficial layer of the mucosa mechanically stronger. They do not typically penetrate cells but rather toughen the mucosal surface to increase gingival resistance against infection.38 Because these drugs have poor cell permeability, they are particularly useful in prosthodontics for the management of bleeding during gingival retraction without concern for systemic effects; they also decrease exudation and crevicular fluid flow.38 Some examples include alum, aluminum chloride, zinc chloride (8% to 20%), and tannic acid. The term “styptic” is sometimes used to describe the concentrated form of astringents. Styptics cause superficial and local coagulation and are, therefore, often referred to as hemostatic agents. Some examples are ferric chloride and ferric sulfate.
Epinephrine
While the primary class 1 gingival retraction agent, epinephrine, exerts its effect through stimulation of alpha-, beta 1-, and beta 2-adrenergic receptors as mentioned above, it is the alpha-stimulation that is most desirable in dentistry (ie, vasoconstriction of the small blood vessels in submucosal tissue and improved hemostasis in the operative field). However, too much epinephrine, prolonged exposure, or patients with unique anatomy may present risk for some negative local effects of the vasoconstrictor (ie, tissue blanching and reduced blood flow). Given the very short half-life of epinephrine (about 2 minutes), these local effects tend to be transitory and non–life-threatening, such that many practitioners seldom consider them to be significant.
As the alpha effect causing vasoconstriction wears off, beta adrenergic stimulation dominates, resulting in relaxation of smooth muscle within the bronchial tree, cardiac stimulation (increasing myocardial oxygen consumption), and dilation of skeletal muscle vasculature; small doses can cause vasodilation via beta 2-vascular receptors; large doses may produce constriction of skeletal and vascular smooth muscle.39 Potential systemic side effects specifically related to the hemodynamic and cardiovascular influence of epinephrine are of greatest concern primarily in the at-risk cardiovascular population (ie, hypertensive, angina, myocardial infarction, and heart failure patients). In fact, in a study by Woycheshin, it was shown that the use of 1:1000 (0.1%) epinephrine cord caused high systemic concentrations, and the authors recommended that it should not be used in large areas of tissue laceration or abrasion.40
The systemic effects of epinephrine-impregnated gingival retraction cord in hypertensive patients has not been reported, although several authors have studied the effect of retraction cord in normotensive patients.36,41-44 In general, mean effects on blood pressure and heart rate were minimal.45 Regardless, given the short elimination half-life for epinephrine, the possible systemic effects occur within minutes of absorption and will have completely subsided in 10 to 15 minutes. Perhaps the most rational suggestion in regard to modifications of patient management should be based on patient assessment, and not on absolute amounts of epinephrine administered. For example, for patients with a diagnosed cardiac condition, a sensible protocol is to record baseline heart rate and blood pressure preoperatively and then every 5 minutes for 15 minutes following administration of a class 1 gingival retraction agent. While epinephrine is an effective vasoconstrictor both on retraction cords and in local anesthetics, practitioners should limit doses to minimize negative sequelae or consider alternate therapy. For this reason local anesthesia containing epinephrine in 1:50,000 concentrations should be administered judiciously if utilized for local hemostasis.
Aluminum Chloride, Aluminum Sulfate, Alum, Ferric Sulfate, and Others
Chemically, all the retraction agents containing astringents are characterized by a relatively high level of acidity, with their original concentrations ranging from pH 1 to pH 3 for solutions.46 In-vivo and in-vitro observations have shown these agents to induce undesirable local side effects on gingival margin tissues in addition to their desired activity.4,20,23-27,47-50 Studies in both human and animal models using various research methods have confirmed an inflammatory response of the surrounding soft tissues. The inflammatory response was normally transitory and its severity depended on the type and concentration of the retraction agent used. Results obtained by scanning electron microscope and energy dispersive x-ray spectroscopy (SEM-EDX techniques) reported an altered morphology of prepared human dentin surface after exposure to conventional astringents containing gingival retraction fluids.51-53
Of the class 2 gingival retraction agents (hemostatics, astringents), aluminum chloride, aluminum sulfate, alum, and ferric sulfate tend to be the most commonly used, with zinc chloride and potassium sulfate being used much less often.54 Unlike epinephrine, there are no known contraindications to their use and they have minimal systemic effects due to their poor cell permeability.24
The use of ferric sulfate or other ferrous compounds (eg, ferric chloride) at the impression or delivery stage has been reported to cause the development of grayish black discolorations under translucent porcelain restorations.2,55 The mechanism for this reaction is believed to be removal of the dental smear layer by the acidic ferric sulfate, causing decreased bond strengths, microleakage, and marginal discoloration.51,52 If discoloration occurs, the restorations must be removed and remade.2
Chlorhexidine
Despite adequate tissue management and displacement around the prepared tooth, voids in the impression may still occur and can be the result of blood or other liquid around the teeth or blood leakage from unhealthy gingiva adjacent to the tooth being impressed.2,3 Christensen suggested one additional method to increase gingival health and reduce bleeding, utilizing 0.12% chlorhexidine mouth rinse. It is recommend that patients rinse twice a day for at least 2 weeks before the preparation appointment (once in the morning after eating and once in the evening immediately before retiring).2 The rinse is used for a total of 6 weeks: 2 weeks before the procedure, 2 weeks during the provisional restoration stage, and 2 weeks after cementation of the restoration.
Chlorhexidine has activity against Gram-positive and Gram-negative organisms, facultative anaerobes, aerobes, and yeast; it is both bacteriostatic and bactericidal, depending on its concentration.56,57 The bactericidal effect of chlorhexidine is a result of the binding of this cationic molecule to negatively charged bacterial cell walls and extramicrobial complexes. At low concentrations, this causes an alteration of bacterial cell osmotic equilibrium and leakage of potassium and phosphorous, resulting in a bacteriostatic effect. At high concentrations of chlorhexidine, the cytoplasmic contents of the bacterial cell precipitate and result in cell death. Key adverse events related to its use on dental patients include increased calculus accumulation on teeth, altered taste perception, staining of oral surfaces (mucosa, teeth, dorsum of tongue), and oral/tongue irritation. Staining may be visible as soon as 1 week after treatment begins and is more pronounced when there is a heavy accumulation of unremoved plaque and on nonpolished restorative surfaces. The stain potentially caused by chlorhexidine does not have a clinically adverse effect other than it being esthetically unpleasing; patients should be informed of the possible negative results.58,59
Cordless Mechanico-Chemical Retraction Agents
To overcome the challenges of traditional mechanical retention—the need for anesthesia, risk of damage to gingival and epithelial attachment, possible gingival recession, gingival inflammation, and postoperative discomfort—a new class of cordless gingival retraction materials has been introduced.10,13,29,60-65 In addition, cordless retraction systems usually contain an astringent to aid with hemostasis and fluid control.
In 2001 a clay-based retraction paste containing aluminum chloride was introduced that relied on hygroscopic expansion of the primary ingredient, kaolin, to achieve mild tissue displacement in approximately 2 minutes.66 Al Hamad et al studied the effects of this retraction paste product compared with conventional gingival retraction cords and found that both techniques caused gingival inflammation. Surprisingly, the clay-based retraction paste caused the highest gingival index scores after 1 and 7 days, was slowest to heal, and was associated with dentin sensitivity. The authors attributed patient sensitivity to the high concentration of aluminum chloride, the acidity of the material, and the dryness produced.27 In a later study by Kazemi, this same product caused significantly less inflammation than cord after 7 and 14 days.21 Gingival retraction resulting from cord and the clay-based retraction paste was also examined. It was determined that although less than cord, gingival retraction caused by the paste created enough sulcular width to allow minimum impression material thickness, as reported by previous studies.67,68
Recently, Bennani et al compared the pressure generated after placement of cord versus this same retraction paste.69 It was concluded that the pressure of the paste in the sulcus was one-tenth of that of cord, and manipulation of the material after placement would further reduce the pressure. It is interesting to note that directions for other clay-based retraction systems recommend direct pressure on the material-filled sulcus by cotton compression caps or by placement of cord. According to the Bennani study, this may compromise chemical retraction in favor of direct pressure on the sulcus and pharmacologic effects of the astringent (ie, drying of crevicular fluid and hemostasis).
Conclusions
Methods of tissue management for recording fixed prosthodontic impressions include mechanical, chemical, and surgical. In many cases, a combination of techniques may be utilized. Regardless of technique used, risk of injury to the surrounding gingiva exists. The careful practitioner must understand the potential local and systemic effects of mechanico-chemical retraction. Although mechanical or mechanico-chemical retraction is often the most cost-effective means of tissue management, it can be the most traumatizing. Cordless mechanico-chemical agents may result in less tissue injury, but are usually more expensive than cord. Given today’s practice overhead, it is incumbent on practitioners to be efficient and use materials to produce consistent positive outcomes. Understanding the risks and benefits of each retraction system can help dentists select the right materials for each clinical situation.
DISCLOSURE
Dr. Goodchild is an employee of DENTSPLY Caulk.
ABOUT THE AUTHORS
Mark Donaldson, Pharm D
Director of Pharmacy Services, Kalispell Regional Medical Center, Kalispell, Montana
Clinical Professor, University of Montana, Skaggs School of Pharmacy, Missoula, Montana
Clinical Associate Professor, Oregon Health & Science University, School of Dentistry, Portland, Oregon
Jason H. Goodchild, DMD
Clinical Associate Professor, Department of Oral Medicine, University of Pennsylvania School of Dental Medicine, Philadelphia, Pennsylvania
Adjunct Assistant Professor, Division of Oral Diagnosis, Department of Diagnostic Sciences, Rutgers School of Dental Medicine, Newark, New Jersey
Research Dentist, Clinical Research and Education, DENTSPLY Caulk, Milford, Delaware
Private Practice, Havertown, Pennsylvania
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