INTRODUCTION

Root canal treatment enjoys high favourable outcome rates (Ng et al., 2011a, 2011b). However, as with any healthcare procedure, failure (non-healing) remains an unfortunate possibility. The persistence, development or recurrence of apical periodontitis with or without clinical symptoms is indicative of failure, and further intervention is necessary if the tooth is to be retained. Thus, to prevent tooth loss, root canal retreatment is often carried out to salvage a failed, previously root treated tooth.

A principal difference between retreatment and initial treatment is the need to remove the previous root filling material, to permit access to the root canal space and address technical deficiencies and possible causes of failure (Siqueira & Rôças, 2011). Safe, efficacious and complete removal of root filling materials is beneficial to successful retreatment (Chong, 2004).

Removal of root filling material is dependent on various factors, including the previous root filling technique and materials used, and root canal anatomy. Removal of single-cone gutta-percha root fillings is relatively straightforward compared with canals filled with well-compacted cold lateral, or warm vertical, gutta-percha techniques (Khedmat et al., 2016). However, the larger amount of sealer used for single-cone root fillings may mean they are more difficult to remove (Kim et al., 2019a, 2019b; Uzunoglu et al., 2015). Therefore, clinically, the complete removal of root filling material is a challenging process; a wide variety of manual and automated instruments, irrigants and solvents in combination with various removal techniques have been used (Table 1) (Duncan & Chong, 2008). Due to the tubular nature of dentine and root canal anatomical features such as fins, isthmuses and lateral canals, complete removal of well-compacted gutta-percha and sealer is impossible in most cases (Barreto et al., 2016; Campello et al., 2019); the only recourse is to employ thorough disinfection and filling of the prepared space as it may help to entomb and prevent any regrowth of residual intracanal microbes (Ørstavik, 2005). Therefore, where feasible, and without the introduction of additional procedural errors, complete removal of the root filling material will help improve access to the canal spaces to permit thorough disinfection and filling, thereby contributing to a favourable outcome (Ng et al., 2011a).

TABLE 1. Instruments, irrigants, solvents and supplementary protocols used during removal of root filling materials Techniques Instruments Irrigants Solvents Supplementary protocols Manual hand files Mechanical engine-driven files Heat Ultrasonic Lasers Gates-Glidden burs Hedström files Canal Finder System (Socièté Endo Technic) ProFile (Dentsply Sirona) ProTaper Retreatment (Dentsply Sirona) Mtwo-R (VDW) D-RaCe (FKG Dentaire) R-Endo (Micro-Mega) Reciproc (VDW) Nd:YAG & Er:YAG lasers System B (SybronEndo) Ultrasonic files Sodium hypochlorite Chlorhexidine 17% EDTA 10% Formic acid Chloroform Eucalyptus oil Xylene/Xylol Methyl chloroform Tetrahydrofuran Methylene chloride Halothane Rectified turpentine Orange oil Sonic activated irrigation Passive ultrasonic irrigation Laser activated irrigation XP Endo files (FKG Dentaire) Self-Adjusting File (ReDent Nova) GentleWave (Sonendo Inc)

Since a variety of materials and techniques have been used for filling root canals (Chong & Chandler, 2021), there is an abundance of studies on different techniques to remove various root filling materials. Excluding post-operative pain and bacterial level studies, the vast majority of these studies are laboratory-based, and thus, it may be argued, of limited clinical significance. The main difficulty with designing an in vivo clinical study assessing root canal retreatment procedures is the inability to control and standardize all aspects of the initial canal preparation and filling. These confounding factors in laboratory studies may be removed or reduced by standardization, allowing for a more accurate assessment of the effect of the investigated variable/s; however, it is not possible to directly extrapolate the results or draw conclusions on the relevance to clinical practice from non-clinical studies. To guide future research, which can better inform clinical practice, there is a necessity to classify and condense the varied information on this topic. Thus, the main focus of this narrative review is on the research methods and experimental models to study the removal of root filling materials. A brief perspective is also included on more appropriate and valid experimental methodologies that have direct application, relevance and translation to everyday clinical endodontics. Specific clinical procedures and techniques to remove different root filling materials, and related outcome rates are not within the scope of this review.

METHODOLOGY

A literature search was performed using MEDLINE and PubMed databases ending in June 2021. The following search items were used: “Root Canal Filling,” “Removal,” “Root Filling,” “Remov*,” “Retreat*,” “Root Canal Filling Materials.” A grey literature search was performed using the British Dental Association Library. Systematic reviews, case reports and studies in languages other than English were excluded. From the search, 1524 abstracts were obtained of which 340 were related to removal of root filling materials, 296 described research and experimental models, and 44 were not relevant. From the articles that were included for further analysis, 288 were laboratory studies and eight were in vivo studies.

RESEARCH METHODS

From the literature assessed, the research methods to assess the removal of root filling materials can be broadly divided into two distinct categories; exposure of the root canal space to permit direct visualization, and non-destructive examination relying on imaging techniques (Table 2). Other methods include the binary (Yes/No) measurement for the successful removal of root filling material, and weight difference following root filling material removal and canal preparation.

TABLE 2. Assessment methods to study removal of root filling materials Assessment methods Studies Direct visualization Tooth splitting

Wilcox et al. (1987)

Friedman et al. (1993)

Moshonov et al. (1994)

Imura et al. (1996)

Bertrand et al. (1997)

Hülsmann and Stotz (1997)

Frajlich et al. (1998)

Baldassari-Cruz and Wilcox (1999)

Wolcott et al. (1999)

Imura et al. (2000)

Sae-Lim et al. (2000)

Baratto Filho et al. (2002)

Barrieshi-Nusair (2002)

Vidučić et al. (2003)

Hülsmann and Bluhm (2004)

de Oliveira et al. (2006)

Ezzie et al. (2006)

Kosti et al. (2006)

Zmener et al. (2006)

Hassanloo et al. (2007)

Saad et al. (2007)

Bodrumlu et al. (2008)

Boutsioukis et al. (2008)

Giuliani et al. (2008)

Scelza et al. (2008)

Só et al. (2008)

Somma et al. (2008)

Taşdemir et al. (2008b)

Aydın et al. (2009)

de Mello Junior et al. (2009)

Horvath et al. (2009)

Pirani et al. (2009a, 2009b)

Ring et al. (2009)

Takahashi et al. (2009)

Zarei et al. (2009)

Bramante et al. (2010)

Fenoul et al. (2010)

Ramzi et al. (2010)

Fariniuk et al. (2011)

Hess et al. (2011)

Nica et al. (2011)

Yılmaz et al. (2011)

Akhavan et al. (2012)

Dadresanfar et al. (2012)

Jayasenthil et al. (2012)

Kumar et al. (2012)

Xu et al. (2012)

Marques da Silva et al. (2012)

Só et al. (2012)

Voet et al. (2012)

Khalilak et al. (2013)

Müller et al. (2013)

Reddy et al. (2013)

Shivanand et al. (2013)

Zuolo et al. (2013)

Alves et al. (2014)

Grischke et al. (2014)

Iriboz and Sazak Öveçoğlu (2014)

Keleş et al. (2014b)

Kok et al. (2014)

Rached-Júnior et al. (2014a, 2014b)

Rios et al. (2014)

Simsek et al. (2014b)

Agrafioti et al. (2015)

Capar et al. (2015a, 2015b)

Colaco and Pai (2015)

de Souza et al. (2015)

Kim et al. (2015)

Nevares et al. (2015)

Uzunoglu et al. (2015)

Colombo et al. (2016)

Joseph et al. (2016)

Kanaparthy and Kanaparthy (2016)

Kasam and Mariswamy (2016)

Keleş et al. (2016)

Koçak et al. (2016)

Niemi et al. (2016)

Özyürek and Demiryürek (2016a, 2016b)

Vidal et al. (2016)

Bhagavaldas et al. (2017)

Bueno et al. (2017)

Das et al. (2017)

Ferreira et al. (2017)

Gomes et al. (2017)

Gorduysus et al. (2017)

Rodrigues et al. (2017)

de Castro et al. (2018)

Donnermeyer et al. (2018)

Ekici et al. (2018)

Fatima et al. (2018)

Kakoura and Pantelidou (2018)

Keskin et al. (2018)

Madarati et al. (2018)

Raj et al. (2018)

Agrawal et al. (2019)

Eymirli et al. (2019)

Kim et al. (2019b)

Sadat Shojaee et al. (2019)

Salgado et al. (2019)

Fracchia et al. (2020)

Nasiri and Wrbas (2020)

Park et al. (2020)

Purba et al. (2020)

da Silva Machado et al. (2021)

Clearing Schirrmeister et al. (2006b, 2006c, 2006d)

Gu et al. (2008)

Taşdemir et al. (2008a)

Akpınar et al. (2012)

Chauhan et al. (2012)

Patil et al. (2018) Imaging techniques 2D radiographs

Ladley et al. (1991)

Ferreira et al. (2001)

Valois et al. (2001)

Masiero and Barletta (2005)

Bueno et al. (2006)

Gergi and Sabbagh (2007)

Zanettini et al. (2008)

Çelik Ünal et al. (2009)

Abramovitz et al. (2012)

Ersev et al. (2012)

Mollo et al. (2012)

Rehman et al. (2013)

Vale et al. (2013)

Marfisi et al. (2015)

Silva et al. (2015)

Preetam et al. (2016)

Silva et al. (2016)

Kaşıkçı Bilgi et al. (2017)

Dhaimy et al. (2021)

Serefoglu et al. (2021)

CBCT

Barletta et al. (2007)

Barletta et al. (2008)

Dall'Agnol et al. (2008)

Neelakantan et al. (2013)

Yadav et al. (2013)

Chandrasekar et al. (2014)

Madani et al. (2015)

Akbulut et al. (2016)

Dass et al. (2016)

Khedmat et al. (2016)

Pawar et al. (2016b)

Yürüker et al. (2016)

Azim et al. (2018)

Kapasi et al. (2020)

MCT

Hammad et al. (2008)

Roggendorf et al. (2010)

Ma et al. (2012)

Rödig et al. (2012)

Solomonov et al. (2012)

Asheibi et al. (2014)

Cavenago et al. (2014)

Fruchi et al. (2014)

Helvacioglu-Yigit et al. (2014)

Rödig et al. (2014a, 2014b)

Sağlam et al. (2014)

Simsek et al. (2014a)

Keleş et al. (2014a)

da Rosa et al. (2015)

Keleş et al. (2015)

Alves et al. (2016)

Barreto et al. (2016)

Crozeta et al. (2016a, 2016b)

de Siqueira Zuolo et al. (2016)

Nevares et al. (2016)

Rodrigues et al. (2016)

Martins et al. (2017)

Suk et al. (2017)

AlShwaimi (2018)

Endal et al. (2018)

Rivera-Peña et al. (2018)

Rödig et al. (2018)

Silva et al. (2018a, 2018b)

Silveira et al. (2018)

Yılmaz et al. (2018)

Aksel et al. (2019)

Athkuri et al. (2019)

Bago et al. (2019)

Campello et al. (2019)

De-Deus et al. (2019a, 2019b, 2019c)

Delai et al. (2019)

Kaloustian et al. (2019a, 2019b)

Machado et al. (2019)

Pedullà et al. (2019)

Wright et al. (2019)

Kim et al. (2019a)

Alakabani et al. (2020)

Azevedo et al. (2020)

Bago et al. (2020)

Garrib and Camilleri (2020)

Romeiro et al. (2020)

Silva et al. (2020)

Volponi et al. (2020)

Pinto et al. (2021)

Silva et al. (2021)

Tavares et al. (2021)

Combination Direct visualization and imaging techniques

Bramante and Betti (2000)

Betti and Bramante (2001)

de Carvalho Maciel and Zaccaro Scelza (2006)

Schirrmeister et al. (2006a)

Cunha et al. (2007)

Betti et al. (2009)

Betti et al. (2010)

Duarte et al. (2010)

Marfisi et al. (2010)

Tachinami and Katsuumi (2010)

Kfir et al. (2012)

Bernardes et al. (2016)

Palhais et al. (2017)

Borges et al. (2019)

Nguyen et al. (2019)

Crozeta et al. (2020)

Prati et al. (2020)

Crozeta et al. (2021)

Clearing and splitting Fariniuk et al. (2017) MCT and CLM Canali et al. (2019) Others Binary (Yes/No) Hülsmann (1990) Ibarrola et al. (1993) Kesim et al. (2017) Weight Yiangou (2021) Abbreviations: 2D, two-dimensional; CBCT, cone-beam computed tomography; CLM, confocal laser microscopy; MCT, microcomputed tomography. Direct visualization

Direct visualization involves the optical inspection and subsequent quantification of the contents within the root canal space following a root filling material removal procedure. It is typically achieved by either splitting the tooth root or chemically “clearing” the hard tissues rendering them transparent.

Following root filling removal, tooth splitting involves the careful longitudinal splitting of the root and an assessment of residual material in the exposed canal space. In most studies, to enable direct assessment of the root canal space, the roots were divided longitudinally in a bucco-lingual direction by first cutting a groove using a steel disc (Imura et al., 2000) or a diamond bur (Iriboz & Sazak Öveçoğlu, 2014), after which a chisel and mallet were used for splitting. When making the initial groove, great care must be exercised to avoid encroaching into the root canal space to avoid accidental damage to the canal walls as these samples will then need to be excluded and/or replaced (Joseph et al., 2016; Zmener et al., 2006).

Images of the two halves of split roots can be captured using a camera (Bramante & Betti, 2000) or a stereomicroscope (Bodrumlu et al., 2008) to help with further assessment of the root filling material remaining in the root canal space (Figures 1 and