The understanding of biological concepts in wound healing together with the evolution in biomaterials applied in periodontal regeneration allowed for improved, minimally invasive surgical techniques with a wider range of application and adapted to achieve multiple goals at the same time. Regenerating attachment was never the sole end point, but maintaining the patient’s own natural dentition in health and esthetics is becoming a feasible goal even in cases considered challenging just a few years ago. In this article we report on the evolution of techniques and biomaterials and their application in esthetic and challenging cases.
Key points
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Periodontal regenerative therapy has undergone several modifications over the year, including flap designs, barrier membranes, bone graft materials and biologic agents.
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Understanding the rationale for each surgical technique and biomaterial is crucial for planning the regenerative procedure.
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Minimally invasive flaps in combination with biologic agents can reduce the chance of papilla dehiscence/necrosis, post-operative morbidity and can enhance the regenerative outcomes.
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Recent techniques involve the regeneration of the intrabony defect together with the augmentation of the supracrestal soft tissue component.
Introduction
Just as osseointegration for endosseous dental implants, periodontal regeneration was a revolution for periodontal treatment. In an era in which bone resection was the only option to level unfavorable bone architecture for periodontal tissue maintenance, the possibility of leveling the defect by generating new attachments allowed for the proposal of several new techniques for maintaining the patients’ natural dentition in health. Moreover, following Nyman and coworkers’ study on 1982 it was demonstrated that not only repair of the lost periodontal tissues was possible but also regeneration of the complete periodontal architecture with new bone, new cementum, and new connective tissue with perpendicularly oriented fibers was achievable.
The full potential of this at-the-time novel therapy is yet to be uncovered.
From flap design and suturing techniques to biomaterials for wound stabilization, from the concept of compartmentalization to that of blood clot stabilization, from scaffolding technologies to biological agents and growth factors to cell therapy our understanding of periodontal regeneration is constantly evolving.
The early stages
Nyman and coworkers’ report of periodontal tissue regeneration adopted an extended flap sutured over a millipore filter placed to act as a barrier membrane between the soft tissues and the root surface. The investigators applied the occlusive to prevent the migration of dentogingival epithelium and gingival connective tissue cells into the defect along the curetted root surface. Progenitor cells originating from the adjacent periodontal ligament (PDL) and alveolar bone were therefore enabled to colonize the blood clot and induce periodontal regeneration. After 3 months of healing histologic analysis revealed the formation of new attachment in coronal direction to a level 5 mm coronal to the alveolar bone crest. This article reported for the first time on a human histologic evidence of new bone and new cementum formation together with the regeneration of a periodontal ligament with perpendicularly oriented fibers.
Earlier studies from Schallhorn and colleagues adopted autogenous grafts for periodontal tissue regeneration, and other reports attempted to apically position the flaps to allow bone cells to populate the defect before epithelial cells could. Nevertheless, the radiographic filling of the intrabony defect due to the properties of the bone graft could not be considered an evidence of periodontal regeneration. A preclinical study demonstrated that the healing of some procedures that were considered “regenerative” (including the modified Widman flap, with or without different bone grafts) occurred by repair, with the formation of long junctional epithelium.
Later on, Melcher introduced the concept of compartmentalization, speculating that different cell populations “compete” to repopulate the periodontal defect. To obtain periodontal regeneration, cells migrating from the epithelium and connective tissue layers have to be excluded from the defect. Based on this theory, it was demonstrated that barrier membranes can effectively favor the migration of cells from the periodontal ligament into the infrabony defect, resulting in a true regeneration of periodontal ligament, cementum, and alveolar bone. Different barrier membranes were developed with the idea of allowing for new tissue formation inside the vertical periodontal defect. Expanded polytetrafluoroethylene (e-PTFE) membranes were porous nonresorbable membranes characterized by the presence of an open microstructured collar, designed to stop epithelial cell migration via contact inhibition, and a partially occlusive device developed to allow compartmentalization of the defect while allowing for nutrients to sustain the flap integrity. Five weeks after the surgery these membranes were removed. Absorbable membranes eliminated the need for this second intervention and proved their proficiency in several studies.
These approaches were called guided tissue regeneration where the word “guided” maintained that the barrier membrane had the ability to direct the regeneration of tissues.
Despite the encouraging results these approaches had several limitations.
Flap design was not optimized for regenerative approaches and the tendency was to use extended flaps thus decreasing the stability during wound healing. In the early stages of periodontal regeneration interdental tissues were often discarded, bone substitutes were used as scaffolds and membranes as barriers, but the ability to maintain primary wound closure required the release of the flap and was hindered by the lack of an adequate amount of soft tissue to cover the interproximal areas. Despite these limitations guided tissue regeneration (GTR) proved to be superior to open flap debridement in terms of clinical attachment level gain and probing depth reduction and superior to osseous resective surgery in terms of clinical attachment levels. These results were confirmed in the short term, and long term-follow-up studies proved them to be stable even after 10 to 15 years.
Compromised teeth that were once sacrificed to maintain a positive architecture could then be maintained, and clinical attachment could be gained to improve tooth prognosis.
Tissue engineering developed new materials to attempt to overcome some of the limitations of this approach to achieve more predictable results and faster and better tissue regeneration.
Papilla preservation flaps
The importance of defect isolation by occlusion membranes on tissue regeneration was questioned in several articles. In GTR, the formation of a long junctional epithelium as a consequence of periodontal repair as opposed to regeneration has been suggested to be more closely related to wound failure rather than to failure of defect isolation per se . , Several studies reported on the critical role of an uncomplicated adsorption, adhesion, and maturation of the fibrin clot at the tooth-mucogingival flap interface to achieve a new connective tissue attachment and to prevent the downgrowth of the junctional epithelium.
In 1985, Takei and colleagues suggested a new flap design called papilla preservation flap with the intent of preserving the interdental papilla via a buccal semilunar incision. The vertical defects were accessed elevating the flap, and the interdental tissues were preserved attached to the palatal flap. After debridment and bone grafting the flaps were sutured with external crossed mattress sutures. Despite the innovative approach described by this article this flap design was not adopted by many practitioners.
Ten years later, in 1995, Cortellini and Tonetti described the modified papilla preservation flap. In their approach the incision line was moved lingually and a double-layer internal mattress suture technique was described. The importance of maintaining flap stability started to become more preeminent, and an increased attention to the suturing technique was adopted.
Soon after, in 1999, the simplified papilla preservation flap was described with the intent of allowing for interdental tissue maintenance even in areas with narrow interdental spaces (<2 mm). The simplified papilla preservation flap was not as performing as the modified papilla preservation flap, although it could be used in more clinical scenarios and was simpler to perform, therefore it became more and more accepted in the periodontal community.
These flaps were described to be used in combination with xenogeneic bone substitutes and resorbable membranes, and the flaps had to be elevated to receive.
Minimally invasive flaps
In the same year the simplified papilla preservation flap was described Rasperini and coworkers described the surgical technique for enamel matrix derivative (EMD). Three case reports documented flaps with reduced elevation adopted in combination with EMD alone in esthetic areas. A modified mattress suture was used. All cases showed complete radiographic bone fill and no gingival recession. Re-entry surgery demonstrated complete closure of the vertical defects.
For the first time it was demonstrated that the membrane could be avoided and more limited flap elevation could help wound healing in contained defects even in the absence of bone grafts.
This new understanding of wound healing biology switched the focus of the regenerative procedure from the compartmentalization concept proposed by Melcher and supported by Nyman and coworkers’ case report to the blood clot stability concept.
In 2003, Wachtel and colleagues reported on the use of minimally invasive flaps in combination with EMDs. Buccal and lingual sulcular incisions were performed with microsurgical blade, and the papilla above the periodontal defect was elevated using the modified papilla preservation technique.
In 2007, Cortellini and Tonetti proposed the minimally invasive surgical technique (MIST), a modification of the modifier papilla preservation flap designed to limit the mesiodistal flap extension and the coronoapical reflection to reduce the surgical trauma and increase flap stability and to be used in combination with the application of EMD in the treatment of isolated deep intrabony defects. The flap design was enhanced by the adoption of an operating microscope and microsurgical instruments. Thirteen cases were reported in this preliminary cohort study suggesting that excellent results could be achieved with low patient morbidity associated to the surgery.
Another advancement in the surgical techniques was suggested by Trombelli and coworkers with the single flap approach (SFA) first described in a case series report in an Italian paper and then in an international magazine the following year. Trombelli’s intuition was that if the defect could be accessed and degranulated elevating just one flap either buccal or lingual to the incision line, the elevation of the other flap was unnecessary. By not elevating both flaps wound closure was easier and blood clot stability enhanced by the maintenance of transgingival fibers in the notelevated flap and by the improved anchorage of the elevated one to the former thanks to an accurate suturing technique. In the same journal Cortellini and Tonetti further improved their MIST with the modified MIST, which again promoted the importance to elevate only the buccal flap while keeping the lingual one unelevated. In cases where the defect is not accessible from the buccal aspect, the investigators suggested not to use the modified version of the MIST.
These improved approaches to periodontal regeneration were diffused widely by many speakers in international meetings, although the technical difficulties and required attention to detail partially hindered their adoption by several periodontal schools that still preferred the guided tissue regeneration with extended flaps and membranes. Nevertheless, it should be noted that minimally invasive approaches cannot be adopted in all clinical scenarios.
A specific clinical scenario that is at times encountered especially in the lower anterior mandible is represented by loss of attachment in teeth with particularly close root proximity. In such cases even an incision line as the simplified papilla preservation flap would cut through a very narrow papilla that would be difficult to suture and would anyway have a hard time maintaining its stability throughout the healing process. To reduce suffering of this fragile area the entire papilla preservation technique was developed. This flap design makes use of a “J”-shaped vertical incision one tooth apart from the defect to be treated. The defect can be accessed avoiding cutting the papilla at all, and biological agents can be applied through this lateral access. Sutures are then only put on the vertical incision in an area that is well vascularized and has a tendency to heal with minimal complication thus maintaining excellent stability of the clot during wound healing.
By designing flaps specifically to boost the potential of periodontal regeneration new attachment was shown to be possibly formed even in severely compromised teeth with attachment loss to the apex, teeth considered hopeless just a few years before. Despite being technique and operator sensitive and therefore not applicable in anybody’s hands, the improvement in flap design had a tremendous impact on the possibility to preserve patients’ natural dentition in health and function, which is eventually the primary goal of periodontal therapy.
Combined intrabony- and soft tissue- regenerative approaches
Some investigators suggested that periodontal regeneration could benefit from the use of flaps described for root coverage procedures. Rasperini and colleagues introduced the “soft tissue wall technique” for the regenerative treatment of noncontained intrabony defects. This technique involves a trapezoidal coronally advanced flap that is first stabilized with sling sutures and then with an internal mattress suture for achieving primary intention healing and closure of the papilla.
Based on the claimed advantages of the tunnel technique for root coverage (high esthetic outcomes, blood supply, graft nutrition, and quick healing , ), some techniques avoiding the incision of the papilla have been described. , In particular, Najafi and coworkers suggested the use of modified vestibular incision subperiosteal tunnel access. On the other hand, Moreno Rodriguez and colleagues claimed that a better access and clinical outcomes can be achieved if an apical horizontal incision is made in the buccal aspect of the alveolar mucosa (on cortical healthy bone) instead of at the interproximal area. The investigators showed that this nonincised papillae surgical approach was not only able to provide a significant improvement in clinical attachment level but also a significant recession reduction (0.25 ± 0.44 mm) and a coronal advancement of the tip of the papillae after 1 year (0.4 ± 0.5 mm). This technique can also be combined with a connective tissue graft for the treatment of intrabony defects.
Trombelli and colleagues further demonstrated that adding a connective tissue graft can improve the outcomes of SFA, in terms of coronal position of the gingival margin and increase in soft tissue volume. Based on the connective tissue graft wall technique introduced for improving root coverage and clinical attachment level in RT3 gingival recessions, Zucchelli and coworkers proposed to use a connective tissue graft obtained from the de-epithelialization of a free gingival graft and stabilized on the buccal aspect of noncontained infrabony defects as a biological barrier for promoting, in combination with EMDs, periodontal regeneration.
Biomaterials
Barrier Membranes
Periodontal regeneration started with the introduction of barrier membranes. Nonresorbable membranes include titanium foils and ePTFE with or without a titanium reinforcement. These membranes are able to maintain the space necessary for both periodontal and bone regeneration. Nevertheless, the high incidence of membrane exposure and the necessity of an additional surgery for removing the membrane are the main drawback of these materials. These disadvantages led clinicians to explore the use of biodegradable membranes for periodontal regeneration. Resorbable membranes include polyesters (ie, polyglycolic acid, polylactic acid) and tissue-derived collagens. Polymeric resorbable membranes maintain their maximum stability for about 14 days and then gradually lose their structural and mechanical properties within 1 month, but they have limited biocompatibility. , On the other hand, collagen membranes have a great biocompatibility and also poor mechanical properties. Although some studies showed greater outcomes for nonresorbable membranes (when not exposed) compared with resorbable membranes, only the latter are nowadays used for periodontal regeneration purposes. Indeed, the use of resorbable membranes is now supported by a larger evidence and increased experience levels.
Last, it has to be mentioned that e-PTFE membranes were replaced by high-density PTFE (d-PTFE) membranes, characterized by smaller pore size that may reduce the drawbacks of early membrane exposure. Nevertheless, d-PTFE membranes are mostly used in guided bone regeneration.
Filling Materials
The rationale on the use of filling materials for periodontal regenerative procedures is mainly related to their scaffolding, space maintenance, and blood clot-stabilizing properties. Several filling materials have been used in periodontal regeneration, including autogenous bone grafts, xenografts, allografts, and synthetic bone grafts. Autogenous bone graft is the only bone filler that has osteogenic, osteoinductive, and osteoconductive properties at the same time. Nevertheless, its harvesting often requires an additional surgical site with increased patient morbidity, and a significant remodeling has also been considered one of the main drawbacks of autogenous grafts. On the other hand, the resorption rate of bone substitutes instead is quite slow. , In the regeneration of tooth-supporting structures, autologous grafts demonstrated high potential for periodontal growth, but the current tendency of performing minimally invasive approaches limits the use of this scaffold material.
Nevertheless, according to a Bayesian network meta-analysis of Tu and coworkers, combination therapies performed better than single therapies, with GTR and bone grafts showing the greatest defect fill. However, it has to be mentioned that the additional benefits of combination therapies were considered small.
Last, scaffold technology has rapidly evolved in the last years. Creating personalized three-dimensional printed polymeric scaffolds may represent the future direction of periodontal regeneration. Our group described the first-in-human application use of a three-dimensional bioprinted scaffolding matrix to treat a periodontal defect. The scaffold was made of polycaprolactone biomaterial to custom fit the osseous architecture of a patient presenting with a large, localized periodontal osseous defect of critical size. This defect could not have been treated with traditional scaffolding approaches. The treated area remained covered and showed evidence of early clinical reattachment.
Biological Agents
Enamel matrix proteins are deposited on the developing tooth roots before the formation of the cementum. , It has been shown that EMDs obtained from porcine fetal tooth biomimetically stimulate cementogenesis by enhancing proliferation and migration of periodontal ligament cells and osteoblasts, mimicking the natural process of tooth development. A recent review summarizing the properties and outcomes of EMDs highlighted the large evidence supporting its efficacy in periodontal regeneration. Nevertheless, it is still unclear whether this biological agent would benefit from the utilization of carrier systems.
Platelet-derived growth factor-BB (PDGF-BB) is one of most investigated growth factors in periodontal tissue engineering; its properties of promoting bone, cementum, and PDL regeneration have been confirmed in animal and clinical studies. This growth factor is able to enhance the proliferation and chemotaxis of cells from the periodontal ligament and alveolar bone cells. , A large multicenter randomized controlled trial demonstrated the safety and efficacy of PDGF-BB in periodontal regeneration, with significantly higher clinical attachment gain and bone fill compared with the carrier alone. Interestingly, no barrier membranes were used in combination with PDGF-BB.
Other biological agents include platelet concentrates and fibroblast growth factor. There is limited (and controversial) evidence regarding the role of platelet concentrates for periodontal regeneration. , Although some initial promising results have been reported for the fibroblast growth factor, , more studies are necessary to evaluate the regenerative properties of this biological agent.
Concluding remarks
We reviewed here the evolution of periodontal regenerative approaches over a period of almost 40 years. This article highlights the revolution that periodontal regeneration allowed in periodontal therapy.
Periodontal regeneration allows for maintenance of teeth that could not be preserved when open flaps and osseous resective surgery were the only options to re-create a positive architecture that is easy to clean. This is clearly a revolution not only because of the many teeth we can now maintain but also because it determines a switch in an important paradigm of periodontal therapy. The definition of periodontal disease was once that of gingival inflammation causing an irreversible loss of clinical attachment level. The classification of periodontal disease itself differentiates the stage of the disease according to loss of interproximal attachment. We recently published an article highlighting how by regenerating interproximal attachment we can now reverse the periodontal condition of our patients and bring them back to a less-severe stage of periodontal disease.
Several advanced tissue engineering approaches are being studied, which may keep evolving our understanding and ability to regenerate lost tissues.
Unfortunately, the evolution of periodontal regeneration led to enhanced difficulty of some of the proposed techniques requiring not only a basic periodontal setting but also the availability of operating microscopes, scaffolding biomaterials, and biological agents increasing the overhead and making the logistic of these treatments more complicated. These drawbacks are reducing the diffusion of such techniques making them accessible just to a limited patient population while many practitioners choose to rely on the least technique-sensitive and more predictable traditional approach. It is imperative that the periodontal community sticks together and actively puts its maximum effort toward increasing the awareness on the potential of periodontal regeneration in the maintenance of natural dentition, especially in the education of younger generations of periodontal specialists. New advancements are continuously evolving our knowledge of the biological mechanisms lying behind the regeneration of a complex structure such as the periodontal ligament and related tissues. By improving our knowledge, we are able to provide enhanced possibilities for the treatment of periodontal patients. We cannot walk back to earlier stages. We must only look forward.
Clinics care points
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Several techniques have been progressively introduced for periodontal regeneration.
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While at the beginning the surgical techniques for periodontal regeneration were aimed at preserving the integrity of the papilla and providing good visibility and access for utilizing barrier membranes, the introduction of biologic agents has allowed for minimally invasive surgical approaches.
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Nowadays, a large variety of bone graft materials, collagen membrane and biologic agents is available on the market and clinicians should choose the most appropriate biomaterial(s) based on the characteristics of the defect.
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Utilizing soft tissue grafts at the same time of the periodontal regenerative therapy can further enhance the interproximal attachment gain and esthetic outcomes, which is particularly crucial for infrabony defects in the esthetic area.
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