Peridontal regeneration - a critical evaluation

Introduction

The over-all goal of periodontal therapy is to provide a dentition that functions in health and comfort for the life of the patient ^{(Zander et al., 1976)}. Periodontal therapy involves elimination or reduction of the periodontal infection allowing for the resolution of the related inflammatory process. Uncontrolled inflammatory processes will be responsible for periodontal tissue destruction. Traditionally, periodontal therapy has been aimed at arresting, rather than reversing the loss of the tooth-supporting structures. The tissue destruction remaining after active periodontitis may at large persist following traditional periodontal therapy as found clinically by increased probing pocket depth, loss of attachment and radiographically detectable bone loss. By regenerative procedures attempts are made to revert these defects to normal structure and function. Decreased probing pocket depth, improved clinical attachment levels and bone fill may be obtained after regenerative procedures in suprabony, intrabony, furcation and gingival recession defects. However, the predictability of significant repair or regeneration in periodontal defects seems questionable following any of the available therapies (for review, see ^{Egelberg, 1999}).

It should be emphasized that regenerative surgery is not a method to treat periodontitis. Prior to regenerative treatment the periodontal infection must be treated by eliminating or adequately suppressing the pathogenic microflora by conventional periodontal therapeutic strategies ^{(Karring and Cortellini, 1999)}.

Presently there is no information available on the advantage of periodontal regeneration for the longevity of the treated teeth. This review will summarize findings from selected research of periodontal regeneration. The role of biological mediators for periodontal regeneration are excluded from it since this area of research will be covered elsewhere in this issue.

Definitions

Regeneration is defined as the natural renewal of a structure, as of a lost tissue or part ^{(Dorland, 1988)}. Periodontal regeneration relates to the complete regeneration of lost tooth-supporting tissues, including alveolar bone, periodontal ligament, cementum and gingiva.

The term repair means the physical or mechanical restoration of damaged or diseased tissues by the growth of healthy new cells or by surgical apposition ^{(Dorland, 1988)}. Thus in wound healing by repair the part may not be fully restored to original structure and function. The term « new attachment » is defined as the union of connective tissue or epithelium with a root surface that has been deprived of its original attachment apparatus ^{(Garrett, 1996)}. New connective tissue attachment refers to a new connective tissue attachment, i.e. newly formed periodontal ligament, inserting directly to the root surface or inserted into newly formed cementum.

Reattachment relates to reunion of epithelium and/or connective tissue with the root surface that may occur after surgical incision or trauma. Bone fill means the clinical restoration of bone tissue in a periodontal defect. Bone fill may or may not be accompanied with additional periodontal regeneration. It is important to understand that when evaluating bone fill, as by clinical re-entry procedures or via radiography, epithelium may well be present between the bone tissue and root surface and thus no true periodontal regeneration has been achieved.

Assessment of periodontal regeneration

Clinical assessment criteria of periodontal regeneration includes periodontal probing, radiography and in selected cases re-entry procedures. Pocket depth, gingival recession and attachment level is measured using periodontal probing. Clinical attachment level (CAL) has become widely accepted as one of the primary clinical endpoints of regenerative attempts around natural teeth ^{(Garrett, 1996)}. CAL is defined as the distance from a fix reference point (normally cemento-enamel junction, CEJ) to the tip of the periodontal probe during periodontal probing ^{(AAP, 1992)}. Gain in clinical attachment after treatment may represent regeneration, but may also indicate resolution of inflammation and reformation of connective tissue, bone fill and attachment of a long junctional epithelium to the root surface.

The one and only component of periodontal regeneration that can be accurately assessed clinically is bone fill. Comparison of bone level prior to and following regenerative surgery can be assessed clinically by re-entry procedures. However, this assessment can not distinguish between bone attached to the root via periodontal ligament or attachment by a long junctional epithelium.

Radiographs are routinely used to evaluate changes in bone height and density, comparing pre- and postoperative images. Modern techniques involving digital radiography allows for more accurate and reproducible measurements of alterations involving bone tissue. Again, it is not possible to assess the true nature of the attachment to the root surface.

Histology remains the ultimate method to measure the nature and extent of periodontal regeneration. In studies on human periodontal regeneration, for obvious reasons, only limited reports are available. The largest study available is by ^{Hiatt et al. (1978)}, describing sections of 100 treated root surfaces. Using histological sections from humans or animals is essential to the evaluation of periodontal regeneration. Once the potential of a new treatment to regenerate the periodontal tissues has been established, clinical and radiographic measurements are sufficient to assess the long-term outcome of the regenerative therapy in human patients ^{(Reddy and Jeffcoat, 1999)}.

Outcome of nonsurgical and nonregenerative surgical periodontal therapy

In sites with initial probing pocket depth of 7 mm and more, a pocket reduction of about 3 mm can be expected including a gain of probing attachment level of about 1.5 mm (^{Badersten et al., 1981} and ¹⁹⁸⁴). More gain of probing attachment was obtained in deeper than in shallower sites. Progression of disease can be prevented in the vast majority of non-molar sites. Sites with deep intraosseous defects may also show a favorable response to nonsurgical therapy (for review, see ^{Egelberg, 1999}). Surgical treatment resulted in more reduction in probing depth and gain of probing bone level than nonsurgical (root planing only) therapy ^{(Renvert et al., 1990)}. ^{Isidor and Karring (1986)} compared root planing and surgery in intraosseous lesions over 5 years. They reported similar changes for both methods. ^Westfelt reported gain in clinical attachment level of 2-3 mm following surgery in sites initially 7 mm or more. Similar results have been reported by ^Ramfjord and ^Nyman .

It can be concluded that periodontal therapy using different treatment modalities may prevent further overall breakdown at initially diseased sites and also result in improved levels of probing attachment. Nonsurgical root instrumentation may produce similar results as different types of surgical non-regenerative therapy. For deep sites the healing response seems to be more pronounced following surgical treatment (review by ^{Egelberg, 1999}).

The anatomic defect remaining following elimination of the periodontal infection may be corrected by regenerative approaches. The goal for periodontal regeneration is to eliminate periodontal defects by regenerating the lost periodontium including bone, cementum, periodontal ligament and other gingival soft tissues ^{(Garrett, 1996)}.

Critical elements in periodontal wound healing and regeneration

In periodontal wound healing new connective tissue attachment is precluded if the periodontal lesion heals with reepithelialization ^{(Karring et al., 1984)}. Thus, it seems important to exclude epithelium from proliferating along the root surface. Coronal positioning of the flap margins following surgery may be helpful in such efforts. Following use of coronally positioned flaps in experimental animal studies a majority of teeth showed new attachment and complete closure of furcation defects. This was contrasted with an absence of new attachment following replacement of the flaps near the cemento-enamel junction ^{(Klinge et al., 1984)}. Improved clinical attachment levels and bone fill has also been reported after regenerative surgery in humans ^{(Gantes et al., 1988} ; ^{Garret et al., 1990} ; ^{Fuentes et al., 1993)}. Following coronally positioned flaps epithelium is most likely excluded from the early healing events and new connective tissue attachment may be accomplished, whereas following a replaced flap procedure the margins will recede sooner and epithelium from the gingival margin will migrate along the root surface before connective tissue attachment has been accomplished ^{(Klinge, 1984)}.

Gingival recession of replaced flaps could be reduced with crown-attached sutures. This suturing technique resulted in new connective tissue attachment in experimental furcation defects. The distance from the gingival margin to the furcation fornix was less following crown-attached sutures than following coronally positioned flaps. The successful healing response following both techniques, however, indicate that the coverage achieved with crown-attached sutures is sufficient for new attachment to occur ^{(Klinge et al., 1984)}. This may indicate that a limited flap coverage is sufficient of establishment of new attachment, if the flaps stay stable during the early events of wound organization. These findings seems to concur with the report by ^{Polson and Proye (1983)} on early healing events following replantation of demineralized roots. Connective tissue attachment to the root surface appeared to be dependent upon a healing sequence related to fibrin and collagen interactions taking place within the first few days after replantation. By providing a stable wound healing support and preventing rupture of the delicate initial cell and matrix interactions, new connective tissue attachment may be achieved. This underscores the importance of securing and positioning the flap, minimizing tensile forces. In contrast to this, therapy may result in the formation of a long junctional epithelium at the tooth-mucogingival interface where maturation of a root-surface adhering fibrin clot has been disturbed by disruptive forces ^{(Wikesjö and Selvig, 1999)}.

The significance of space provision for periodontal regeneration to be successful has been evaluated. One basic requirement in tissue regeneration is to provide and maintain a space to allow for regeneration from designated cells or tissues and preventing non-regenerative wound healing ^{(Karring et al., 1993)}.

Gain in clinical attachment and decreased probing pocket depth have been related to initial defect depth, elimination or suppression of bacterial infection, number of defect walls, defect width, oral hygiene level and smoking ^{(Becker and Becker, 1999} ; ^{Tonetti et al., 1995} ; ^{Trombelli et al., 1997)}. As in all periodontal therapy, the importance of proper plaque control on the outcome of therapy is well established. Higher plaque scores was observed in non-responding sites following regenerative treatment ^{(Hugoson et al., 1995} ; ^{Machtei et al., 1994)}.

Less attachment gain has been reported in smokers than in nonsmokers ^{(Rosenberg et al., 1994} ; ^{Tonetti et al., 1995)}. The deeper the initial defect, the greater the gain of clinical attachment can be expected ^{(Garrett et al., 1988} ; ^{Tonetti et al., 1993} ; ^{Falk et al., 1997)}. ^{Becker and Becker (1999)} recently suggested that defects 5 mm or deeper and surrounded by three bony walls can be considered good candidates for regenerative procedures. In evaluating various techniques and procedures for periodontal regeneration it may be considered that the role of coronal flap positioning, membranes for guided tissue regeneration as well as that of various bone fillers may primarily be related to the essential condition of providing and maintaining space and to secure a stable bed for appropriate wound healing to advance.

Root surface conditioning

Periodontitis-affected root surfaces may be contaminated by endotoxin and have shown to harbor bacteria ^{(Selvig, 1969} ; ^{Adriaens et al., 1988}). In the treatment of the periodontally involved teeth, current concepts hold that the root surface therefore should be mechanically instrumented in order to remove bacterial deposits, calculus and cementum contaminated by bacteria and endotoxins (for review, see ^{Adriaens et al., 1988}). Recent studies indicates that the binding of endotoxin to the root surface appears weak. The need for cementum removal for the sake of endotoxin elimination has been questioned ^{(Nyman et al., 1988)}. Cell and tissue irritants, like endotoxins, may be removed from the root surface by means of chemically active substances ^{(Blomlöf et al., 1987)}. Mechanical instrumentation of the root surface results in the formation of a smear layer of organic and mineralized debris. This smear layer has been suggested to act as a physical barrier, inhibiting new attachment formation and acting as a substrate for bacterial growth ^{(Polson et al., 1984} ; ^{Hanes et al., 1987)}. To overcome the limitations of using only mechanical root instrumentation, chemical root surface conditioning has been introduced. This conditioning is intended to decontaminate, detoxify and demineralize the root surface, removing the smear layer and exposing collagen matrix. For chemical root surface treatment a variety of compounds have been used : bile salts, citric acid, detergents, EDTA, phosphoric acid and tetracycline HCl. Root surface conditioning is often used in periodontal regenerative attempts ^{(for review, see Klinge, 1997)}. The rationale for this conditioning is based on numerous encouraging in vitro and experimental animal studies. Hitherto, most clinical trials using surface conditioning have failed to result in new probing attachment. Explanations for these inconsistent findings may include variations in experimental design, including varying application of the root surface conditioner, inconsistent flap adaptation, inadequate demineralization of periodontitis-affected root surfaces, and on the organic matrix of the cementum and/or dentine. At present chemical root surface conditioning does not seem to provide any adjunctive effects that can be observed clinically (review by ^{Egelberg, 1999}).

The significance of morphological and biochemical changes in the root surface, produced by chemical root surface conditioning, is presently poorly understood. Chemical root surface conditioning may well prove to be of future importance in regenerative periodontal procedures ; however, further studies are needed to support this assumption.

Grafts and fillers

Osseous grafts and bone graft substitutes have been used as methods of enhancing regeneration following treatment of periodontal osseous defects. If bone graft is defined as a transplantation of bone tissue with vital cells, fresh or cryopreserved autografts and allografts can be considered as true bone grafts ^{(Ouhayoun, 1997)}. When the bone-derived material is processed so that the cells no longer are vital it is considered as bone graft substitute. In this category is included freeze-dried bone, decalcified or non-decalcified and other non-osseous (alloplastic) biomaterials or bone fillers. Bone substitutes includes dense and porous hydroxyapatites, bioglass, coral and polymers.

In relation to grafting the terms osteoinduction and osteoconduction are widely used, however the definition of the terms seems to vary with different publications. Osteoinduction (induction - causing to occur) relates to the stimulation of phenotypic shift of progenitor cells within the wound bed to those cells that can form bone. Presently demineralized freeze-dried bone (DFBA) and bone morphogenetic proteins (BMPs) are supposed to fulfill the criteria of osteoinduction ^{(Nasr et al., 1999)}. Osteoconductive materials (conduction - transfer of) provide a scaffold, a ladder or mesh, to allow bone ingrowth and deposition. Most bone substitutes are supposed to be osteoconductive.

The reports on the use of various grafts are numerous. Several reports demonstrate successful healing following the use of graft materials. In other studies where grafted lesions have been compared to non-grafted control sites the results are often inconclusive ^{(Ouhayoun, 1997)}. In a review of the literature on 94 human specimens following bone grafting it was concluded that new attachment is more likely to occur when various grafts are used than in non-grafted sites ^{(Bowers et al., 1982)}. This conclusion was supported by ^{Egelberg (1999)} in his recent review of regenerative treatment of intraosseous defects. Osseous grafting and the use of ceramic grafts seem to enhance the results after regenerative surgery in intraosseous defects. However, the predictability of attainment of significant repair in intraosseous defects seems questionable also following filling of the defect.

Guided tissue regenereration

New connective tissue attachment is precluded if the cells from the oral epithelium proliferate along the root surface forming a long junctional epithelium. ^Björn used a surgical technique whereby the epithelium was excluded from the root surface during healing. The crowns of the experimental teeth were cut off and the roots were submerged under a mucosal tissue flap. The investigation demonstrated the formation of new cementum, periodontal ligament and alveolar bone. These results were confirmed in a similar study by ^Cook . ^{Melcher (1976)} proposed a theory on the repair potential of periodontal tissues, postulating that the cells repopulating the exposed root surface will determine the nature of the attachment that forms.

A membrane technique to prevent epithelium and dentogingival connective tissue from reaching the curetted root surface during healing was described by ^Nyman . This technique, later to be termed guided tissue regenereration (GTR), allowed for the periodontal cells to repopulate the wound area, so enhancing regeneration.

In a series of investigations ^{Gottlow (1986)} and ^{Gottlow et al. (1986)} developed the principle of GTR. They demonstrated that new connective tissue attachment forms if cells from the periodontal ligament are given preference to repopulate the root surface during healing. This was accomplished by the placement of a membrane which excluded the gingival connective tissue and the dentogingival epithelium from the healing process adjacent to the root surface. Studies on guided tissue regeneration in treatment of mandibular degree II furcation defects, intrabony defects and in gingival recession defects generally demonstrates significant gain of clinical attachment level compared with that of debridement and replaced flap surgery (^{Egelberg, 1999}). Regarding degree II maxillary furcation defects the results are inconsistent and the treatment of degree III furcation defects is unpredictable ^{(Karring and Cortellini, 1999)}.

Enamel matrix protein

Enamel matrix proteins (Emdogain^®) have been suggested to stimulate periodontal regeneration. The knowledge in this area is still limited, but interesting findings have been described. In a experimental non-human primate model, porcine enamel matrix derivates were applied to the instrumented root surface prior to flap placement ^{(Hammarström et al., 1997)}. The results from these and other similar experiments suggests that periodontal regeneration can be achieved. It is suggested that factors in the applied proteins will promote osteoblast-cementoblast differentiation and that subsequently new periodontal ligament will form. Undifferentiated mesenchymal cells in the immediate environment will thus allow for cell transformation and the formation of new periodontal tooth supporting structures. In clinical studies, periodontal intraosseous defects treated with enamel matrix protein showed more radiographic bone fill and limited additional probing attachment gain than control defects ^{(Heijl et al., 1997)}. It is noteworthy that the lack of bone fill in control defects differs from what has been reported from other studies. Bone fill following enamel matrix protein application has been described to increase over time. Topical application of enamel matrix proteins in addition to conventional periodontal surgery may thus result in gain in periodontal attachment and radiographic bone fill. The future therapeutic potential for these matrix proteins hopefully will be clarified in additional controlled experiments.

The biologic potential to achieve true periodontal regeneration seems to be established.

Infection and periodontal regeneration

In periodontal regenerative techniques graft materials or barrier membranes are often advocated. These regenerative devices are placed in an environment likely to harbor remnants of infection. Periodontal regeneration is contingent upon the elimination or marked suppression of pathogens at the treated periodontal site ^{(Slots et al., 1999)}. Barrier membranes and grafts may be colonized by microbial pathogens. In the surfaces of retrieved membranes bacteria were identified, despite postsurgical antibiotics and chlorhexidine rinsing ^{(Selvig et al., 1990)}. In a subsequent study an inverse relationship was shown between gain in clinical attachment level and the extent of membrane microbial contamination ^{(Selvig et al., 1992)}. The effect of local application of metronidazole gel after guided tissue regenerative surgery was studied by ^Sander . Significant gain of bone occurred in the sites treated with a combination of membrane and metronidazole. This was contrasted to the results where membranes only were used. The authors suggested that the microbiological benefit of the metronidazole gel was confined to the very early regeneration phase. One approach to antimicrobial therapy in periodontal therapy has been suggested by ^{Slots et al. (1999)}. The approach includes application of antimicrobial agents to the instrumented root surface, antibiotic therapy, chlorhexidine rinse and frequent professional plaque control during the healing period. There seems however to be few studies critically evaluating this approach.

Biology of periodontal regeneration : possible future implications

Limited information is available on molecular and cellular interactions occurring during the early healing events at the interface between the root surface and the adjacent blood clot, surrounding bioliquid and mesenchymal periodontal precursor cells. The chemical composition of the root surface is most likely critically important for tissue interaction to occur. Following mechanical instrumentation or chemical conditioning of the root surface, areas are instantly exposed to a medium containing blood, proteins and other biomolecules. Some of these molecules will rapidly be attached to the root surface. It is likely that the initially attached molecules are later replaced due to biochemical alterations in the surrounding tissues or at the root surface. To this date limited attention has been focused on the cementum - or dentine - bioliquid interface. The primary molecular interactions will take place at this interface. Tentatively, a cell-to-matrix dialogue will be established in successful regenerative procedures.

Critical to success of periodontal regeneration is an understanding of the cells necessary for regeneration, their origin and response characteristics. The periodontal undifferentiated stem cells can be triggered to achieve their terminal differentiation. The periodontal regenerating tissues seems to retain the ability to express phenotypes compatible with hard tissue turnover, thereby recapitulating embryonic periodontal development ^{(Amar and Chung, 1994)}.

Assuming that cells capable of periodontal regeneration reside in bone and periodontal ligament, it is of future interest to find out the factors that attract these lineage's towards the root surface, allow their selective attachment and promote their proliferation and differentiation. Extracellular matrix constituents and cytokines are possible candidates to exert this effect on precursor cells.

Conclusion

Improved clinical attachment levels and bone fill may be obtained after regenerative periodontal surgery in intraosseous defects, in mandibular degree II furcation defects and in gingival recession defects. Several surgical procedures including coronally positioned flaps, placement of membranes, grafts and fillers seems to improve the defect characteristics. Debridement and flap repositioning seems to provide little if any improvement. The predictability appears limited or questionable following any of the available regenerative therapies.

Comments

At selected sites, where regenerative efforts can be considered successful, the magnitude of periodontal regeneration is surprisingly similar, irrespective of the regenerative technique used. This condition directs ideas towards a presumption that the techniques and materials currently used for periodontal regeneration primarily will contribute in creating an immediate environment compatible with or promotive of periodontal regeneration, rather than being the primary cause of this process.

The predominant concept on periodontal regeneration implies that existing periodontal ligament cells are essential for true periodontal regeneration. New insights and conjectures relating to the role of signaling systems to stimulate a phenotypic shift of suitable progenitor cells related to the periodontium, may lead to a shift in this paradigm.

Future research on periodontal regeneration should focus on an understanding of the cells necessary for regeneration, their origin and response characteristics.

BIBLIOGRAPHIE

• AAP. Glossary of periodontal terms. Chicago : The American Academy of Periodontology, 1992.
• ADRIAENS et al. Bacterial invasion in root cementum and radicular dentin of periodontally diseased teeth in humans. A reservoir of periodontopathic bacteria 1988;59(4):222-230.
• AMAR S, CHUNG KM. Clinical implications of cellular biologic advances in periodontal regeneration. Curr Opin Periodontol 1994;128-140.
• BADERSTEN A, NILVÉUS R, EGELBERG J. Effect of nonsurgical periodontal therapy. I. Moderately advanced periodontitis. J Clin Periodontol 1981;8:57-72.
• BADERSTEN A, NILVÉUS R, EGELBERG J. Effect of nonsurgical periodontal therapy. II. Severely advanced periodontitis. J Clin Periodontol 1984;11:63-76.
• BECKER W, BECKER BE. Periodontal regeneration : a contemporary re-evaluation. Periodontol 2000 1999;19:104-114.
• BJÖRN H, HOLLENDER L, LINDHE J. Tissue regeneration in patients with periodontal disease. Odontol Rev 1965;16:317-326.
• BLOMLÖF L, LINDSKOG S, APPELGREN R, JONSSON B, WEINTRAUB A, HAMMARSTRÖM L. New attachment in monkeys with experimental periodontitis with and without removal of the cementum. J Clin Periodontol 1987;14:136-143.
• BOWERS GM, SCHALLHORN RG, MELLONIG JT. Histologic evaluation of new attachment in human intrabony defects. A literature review. J Periodontol 1982;53:509-514.
• COOK RT, HUTCHENS LH, BURKES EJJ. Periodontal osseous defects associated with vitally submerged roots. J Periodontol 1977;48:249-260.
• DORLAND. Dorland's illustrated medical dictionary. Philadelphia : Saunders, 1988.
• EGELBERG J. Periodontics. Malmö : OdontoScience, 1999.
• FALK H, LAURELL L, RAVALD N, TEIWIK A, PERSSON R. Guided tissue regeneration therapy of 203 consecutively treated intrabony defects using a bioabsorbable matrix barrier. Clinical and radiographic findings. J Periodontol 1997;68:571-581.
• FUENTES P, GARRETT S, NILVÉUS R, EGELBERG J. Treatment of periodontal furcation defects. Coronally positioned flaps with or without citric acid root conditioning in class II defects. J Clin Periodontol 1993;20:425-430.
• GANTES BG, MARTIN M, GARRETT S, EGELBERG J. Treatment of periodontal furcation defects. II. Bone regeneration in mandibular class II defects. J Clin Periodontol 1988;15:232-239.
• GARRETT S. Periodontal regeneration around natural teeth. In : Genco RJ, ed. Annals of periodontology. Chicago : AAP, 1996:621-666.
• GARRETT S, LOOS B, CHAMBERLAIN D, EGELBERG J. Treatment of intraosseous periodontal defects with a combined therapy of citric acid conditioning, bone grafting, and placement of collagenous membranes. J Clin Periodontol 1988;15:383-389.
• GARRETT S, MARTIN M, EGELBERG J. Treatment of periodontal furcation defects. Coronally positioned flaps versus dura mater membranes in Class II defects. J Clin Periodontol 1990;17:179-185.
• GOTTLOW J. New attachment formation by guided tissue regeneration. Göteborg : thesis, 1986:1-30.
• GOTTLOW J, NYMAN S, KARRING T, LINDHE J, WENNSTRÖM J. New attachment formation in the human periodontium by guided tissue regeneration. Case reports. J Clin Periodontol 1986;13:604-616.
• HAMMARSTRÖM L, HEIJL L, GESTRELIUS S. Periodontal regeneration in a buccal dehiscence model in monkeys after application of enamel matrix proteins. J Clin Periodontol 1997;24:669-677.
• HANES PJ, POLSON AM, FREDERICK GT. Root and pulpal dentin after surface demineralization. Endod Dent Traumatol 1987;2:190-195.
• HEIJL L, HEDEN G, SVÄRDSTRÖM G, ÖSTGREN A. Enamel matrix derivative (Emdogain^®) in the treatment of intrabony periodontal defects. J Clin Periodontol 1997;18:111-116.
• HIATT WH, SCHALLHORN RG, AARONIAN AJ. The induction of new bone and cementum formation. IV. Microscopic examination of the periodontium following human bone and marrow allograft, autograft and nongraft periodontal regenerative procedures. J Periodontol 1978;49:495-512.
• HUGOSON A, RAVALD N, FORNELL J. Treatment of class II furcation involvements in humans with bioresorbable and non-resorbable guided tissue regeneration barriers. A randomized multi-center study. J Periodontol 1995;66:624-634.
• ISIDOR F, KARRING T. Long-term effect of surgical and non-surgical periodontal treatment. A 5-year clinical study. J Periodontal Res 1986;21:462-472.
• KARRING T, CORTELLINI P. Regenerative therapy : furcation defects. Periodontology 2000 1999;19:115-137.
• KARRING T, NYMAN S, GOTTLOW J, LAURELL L. Development of the biological concept of guided tissue regeneration - Animal and human studies. Periodontol 2000 1993;1:26-35.
• KARRING T, NYMAN S, LINDHE J, SIRIRAT M. Potentials for root resorption during periodontal wound healing. J Clin Periodontol 1984;11:41-52.
• KLINGE B. Regeneration of experimental periodontal furcation defects. Lund : thesis, 1984:1-32.
• KLINGE B. Root surface conditioning. In : Lang N, Karring T, Lindhe J, eds. Proceedings of the 2nd European Workshop on Periodontology. Berlin : Quintessence, 1997:276-283.
• KLINGE B, NILVÉUS R, EGELBERG J. Effect of crown-attached sutures on healing of experimental furcation defects in dogs. J Clin Periodontol 1985;12:369-373.
• KLINGE B, NILVÉUS R, KIGER RD, EGELBERG J. Effect of flap placement and defect size on healing of experimental furcation defects. J Periodontal Res 1981;16:236-248.
• MACHTEI EE, CHO MI, DUNFORD R, NORDERYD J, ZAMBON JJ, GENCO RJ. Clinical, microbiological, and histological factors which influence the success of regenerative periodontal therapy. J Periodontol 1994;65:154-161.
• MELCHER AH. On the repair potential of periodontal tissues. J Periodontol 1976;47:256-260.
• NASR HF, AICHELMANN-REDDY ME, YUKNA RA. Bone and bone substitutes. Periodontology 2000 1999;19:74-86.
• NYMAN S, LINDHE J, KARRING T, RYLANDER H. New attachment following surgical treatment of human periodontal disease. J Clin Periodontol 1982;9:290-296.
• NYMAN S, WESTFELT E, SARHED G, KARRING T. Role of « diseased » cementum in healing following treatment of periodontal disease. J Clin Periodontol 1988;15:464-468.
• OUHAYON JP. Bone grafts and biomaterials used as bone graft substitutes. In : Lang N, Karring T, Lindhe J, eds. Proceedings of the 2nd European Workshop on Periodontology. Berlin : Quintessence, 1997:313-358.
• POLSON AM, FREDERICK GT, LADENHEIM S, HANES PJ. The production of a root surface smear layer by instrumentation and its removal by citric acid. J Periodontol 1984;55:443-446.
• POLSON AM, PROYE MP. Fibrin linkage : a precursor for new attachment. J Periodontol 1983;54:141-147.
• RAMFJORD SP, CAFFESSE RG, MORRISSON EC, HILL RW, KERRY GJ, Appleberry EA et al. 4 modalities of periodontal treatment compared over 5 years. J Clin Periodontol 1987;14:445-452.
• REDDY MS, JEFFCOAT MK. Methods of assessing periodontal regeneration. Periodontology 2000 1999;19:87-103.
• Renvert S, Nilvéus R, Dahlén G, Slots J, Egelberg J. 5-year follow up of periodontal intraosseous defects treated by root planing or flap surgery. J Clin Periodontol 1990;17:356-363.
• ROSENBERG ES, DENT HD, CUTLES SH. The effect of cigarette smoking on the long-term success of guided tissue regeneration : a preliminary study. Ann Aust Coll Dent Surg 1994;112:89-93.
• SANDER L, FRANDSEN EVG, ARNBJERG D, WARRER K, KARRING T. Effect of local metronidazole application on periodontal healing following guided tissue regeneration. Clinical findings. J Periodontol 1994;65:914-920.
• SELVIG KA. Biological changes at the tooth saliva interface in periodontal disease. J Dent Res 1969;48:846-855.
• SELVIG KA, KERSTEN BG, CHAMBERLAIN ADH, WIKESJÖ UME, NILVÉUS RE. Regenerative surgery of intrabony periodontal defects using ePTFE barrier membranes : scanning electron microscopic evaluation of retrieved membranes versus clinical healing. J Periodontol 1992;63:974-978.
• SELVIG KA, NILVÉUS R, FITZMORRIS L, KERSTEN B, KHORSANDI SS. Scanning electron microscopic observations of cell population and bacterial contamination of membranes used for guided periodontal tissue regeneration in humans. J Periodontol 1990;61:515-520.
• SLOTS J, Smith MacDonald E, Nowzari H. Infectious aspects of periodontal regeneration. Periodontology 2000 1999;19:164-172.
• TONETTI MS, PINI-PRATO G, CORTELLINI P. Periodontal regeneration of human intrabony defects. IV. Determination of healing response. J Periodontol 1993;64:934-940.
• TONETTI MS, PINI-PRATO G, CORTELLINI P. Effect of cigarette smoking on periodontal healing following GTR in infrabony defects. A preliminary retrospective study. J Clin Periodontol 1995;22:229-234.
• TROMBELLI L, KIM CK, ZIMMERMAN GJ, WIKESJÖ UME. Retrospective analysis of factors related to clinical outcome of guided tissue regeneration procedures in intrabony defects. J Clin Periodontol 1997;24:366-371.
• WESTFELT E, BRAGD L, SOCRANSKY SS, HAFFAJEE AD, NYMAN S, LINDHE J. Improved periodontal conditions following therapy. J Clin Periodontol 1985;12:283-293.
• WIKESJÖ UME, SELVIG KA. Periodontal wound healing and regeneration. Periodontology 2000 1999;19:21-39.
• ZANDER H, POLSON A, HEIJL LC. Goals of periodontal therapy. J Periodontol 1976;47:261-266.