The Impact of Calcium Hydroxide on the Mechanical Properties of Radicular Dentin: Mini-Review

Calcium hydroxide is a biocompatible material that has been used as intracanal medicament. It is applied in endodontics to disinfect the root canal system as antibacterial that inhibits the growth of microorganisms between appointments. Calcium hydroxide may also reduce the inflammation of periapical area. However, this material may adversely affect the integrity of tooth structure. Hence, the purpose of this paper is to review the effects of calcium hydroxide on some of the mechanical properties of radicular dentin.

In 1920 Hermann first defined the time-honored material; Calcium Hydroxide [CH] to be used in endodontic treatment [1]. CH has been commonly applied in contemporary root canal therapy as an intracanal medicament. The pH of CH is about 12.5, rendering it as a strong alkaline material that causes different biological effects; antimicrobial activity, tissue dissolving ability, inhibition of tooth resorption, and induction of repair by hard tissue formation. For these reasons CH has been suggested to apply in various clinical cases [2]. 

While the various benefits of CH, a high failure has been observed as an incidence of immature roots after being treated with intracanal CH. This may be due to the changes in the mechanical properties of dentin. Exposure of radicular dentin to the biological properties of this material causes alteration of mechanical characteristics of dentin. This may also have significant clinical consequences for the therapy of traumatized teeth and immature roots with necrotic pulp [2]. In the same context, Sano in1994 stated that CH as intracanal medication will lead to weakening the endodontically treated teeth. Furthermore, the results of other studies revealed that the CH application into the canals may weaken the dentin in comparison to non-treated samples [3].

Hence, the aim of this mini review was to identify and analyze the studies related to the effect of CH on various mechanical properties of radicular dentine. However, three of the most significant properties will be discussed in detail. 

Fracture Strength 

Fracture strength is the ability of the material to resist failure and is designated specifically rendering to the mode of applied loading, such as tensile, compressive, or bending [4]. 

Cvek in 1992 treated 885 luxated, immature incisor teeth were dressed intracanal with CH for long-term and reported 40 Percentage cervical root fracture rate [5]. Similarly, White et al. [2002] presented an informative study to compare the impact of CH, MTA and NaOCl on the fracture strength of radicular dentin. The results revealed 59% reduction of fracture strength for NaOCl, 33% reduction for MTA, and 32% reduction for CH. After five weeks all the tested materials caused significant reduction in fracture strength [6].

Andreasen et al. (2002] showed an inverse proportion between the destructive effect of CH on the fracture strength of dentin and the time of dressing. The study recommended that, if the results at the hand were confirmed by further research, alternative endodontic materials such as inter appointment dressing other than CH should then be suggested [7].

Glen Doyon et al (2005] stated that, the CH reduce the fracture resistance of dentin by about 10-20% [8]. Rosenberg et al (2007) as well, measured the effect of CH on the microtensile fracture strength (MTFS] of roots after 7, 28, and 84 days. The results revealed a reduction of 13.9 Mpa pre 77 days with an average of 0.157 Mpa/day. There was a significant difference between the MTFS of CH filled dentin after 84 days (31.8 Mpa] and the gutta perch filled teeth (41.3 Mpa) when used as a control root filling material. A statistical difference was observed between the mean MTFS of 7 days (45.7 Mpa) and 28 days (35.6 Mpa] and between 7 and 84 days (31.8 Mpa). The weakening of the dentin of 23-43.9 Percentage after application of CH as intracanal dressing provides convincing suggestions to reconsider the regular usage of CH in root canal treatment [9].

Sahebi et al. (2010] proved that 30 days contact with CH minimizes the strength of tooth structure of mature teeth by about 15% [10]. Yassen et al. (2013] revealed that CH required 90 days to significantly reduce the fracture resistance of dentin in comparison to 7 days dressing. The study represented that the fracture resistance of the tooth depends on the protein structure of dentin, explaining the side effect of CH that causes deterioration of tooth strength. This happened due to its high alkalinity that dissolves these proteins and consequently the frailer of dentin [11]. The studies clarified that CH as a long-term intracanal medicament exhibited an important reduction of fracture strength of the teeth [12].

In the same context, Batur et al. (2013) revealed a statistically significant drop on the MTFS of endodontically treated teeth because of CH on the root dentin along different periods (180, 270, 360, and 540 days) related to the control group [13]. Within the same line, another study stated that, the dressing with CH for long time as an intracanal medication exhibited a significant decline in fracture resistance when compared with the control group that were left empty at the end of the 1st, 3rd, and 6th months of treatment and would recommend that application of CH for periods longer than one month should be used with carefulness [12].

Lee in 2013 stated that daily application for long period CH must be avoided if possible. It is true that CH is a time-honored material, but it should be used carefully for longer than one month particularly in traumatized immature teeth with thin root walls, because they are more susceptible to root fracture [14].

Pranav and Chaudhry (2015) concluded that, in comparison to control group, a reduction in the fracture strength of dentin happened when exposed to CH after two weeks. They stated that CH is not a material of choice for more than two weeks and if dressed for long period then tooth strengthening must be carried out [15].

Valera et al. (2015) assessed the fracture resistance of root dentin dressed in CH as interterm medication with different irrigations (group 1: physiological solution, group 2: 2% CHX gel, and group 3: 1% NaOCl). The assessment has been done at different intervals (15, 60, 90, 180 and 360 days]. A reduction in fracture resistance was detected throughout the times of interterm medication. The maximum mean of fracture resistance was noticed in group 1 with 15 days of dressing. Flushing by NaOCl accompanied with CH for 15 days offered the least fractured resistance; nevertheless, without significant change compared with CHX in the same period. With extended times of CH dressing [180 and 360 days], samples irrigated by NaOCl and CHX presented significantly lower fracture resistance [16]. 

Said and Moskovitz (2018) noticed significant changes of the fracture strength of CH filled roots after three months (19.1 Mpa) in comparison to the control group (35,8 Mpa). Dentin microtensile fractured strength of the CH filled teeth declined at an average of 0.142 MPa/day [17]. Olcay et al. (2018) concluded that, the detrimental effect of CH started after two weeks or within 1,2,3,9 or 12 months after dressing [18]. 

In 2019, Richhawal et al. concluded that, the application of CH into the root canal decreased the compressive strength of dentin rendering the root more liable to fracture. The study recommended reevaluating the usage of CH in root canal treatment and endodontic materials that would not deteriorate the tooth structure have to be sought for as a feasible replacement [19].

In the study of Hedgire et al. in 2021, the teeth had been exposed to CH for two weeks and resulted in a significant reduction of fracture resistance [20]. Hiyasat et al. (2021) as well presented an interesting study, examining the impact of non-setting CH on fracture resistance of teeth at different periods of time (7, 14, 30, and 90 days). The data represented a significant and advanced reduction in fracture resistance with longer time of contact with CH, then the fracture resistance in samples contact for 90 days was significantly less than all other periods. Scanning electron microscopy (SEM) showed cracks on the surface of dentin due to contact with CH. Fourier transform infrared spectroscopy (FTIR) revealed a significant reduction of inorganic to organic ratio in radicular dentin exposed to CH for 30 and 90 days compared to samples exposed for shorter periods.  X-ray diffraction (XRD) investigation represented lowered crystallinity of dentin. The study concluded that the overexposure to CH significantly reduced the fracture resistance of root dentin, minimized the inorganic to organic ratio and decreased dentin crystallinity [21].

Yet, the exact mechanism that maximizes the susceptibility of dentin to failure is uncertain, but it may be regarded as multifactorial. Hence, several theories have been proposed [22].

Early studies suggested that the CH disturbed the bonding between organic and inorganic components of dentin [23, 24]. Andreasen et al. [2006] proposed that the high alkalinity of CH causes conformational alteration of proteoglycan components and applies a dissolving change by elevation of matrix metalloproteinase action [25].

Dentin strength is determined by the connection between the organic and inorganic components of the dentin. The alkaline pH of CH may rapture this connection, cause denaturation of phosphate and carboxylate groups and trigger the failure of dentin. High alkalinity may dissolve, denature, or neutralize the proteoglycans and acid proteins that act as bonding agents between the hydroxyapatite crystals and collagen fibrils network [7]. This interaction of CH with dentin may clarify the decrease in the fracture resistance of compression next to the endodontic treatment where the teeth filled with CH [26,27]. 

Some investigators [7,11] proposed that the alkalinity pH dissolves the collagen fibrils, and since the collagen fibrils are enveloped by the hydroxyapatite crystals, it needs a while of time [e.g., four weeks in some research] for CH to infiltrate, causing the tooth structure to be more fragile and vulnerable to failure.

Doyon et al. (2005) [8], Rosenberg et al. (2005) [9] explained the reduction of fracture strength due to the interaction of CH with tooth structure. Some articles have clarified that organic matrix alterations by long time contact to CH, then the mechanical characteristics of tooth are changed. Additionally, the rise of pH observed after contact to CH could decrease the collagen support of dentin matrix. The bonding of organic and inorganic components can control the dentin strength. The high alkalinity of CH may dissolve the phosphate and carboxylate groups causing failure of dentin structure. This interaction could disrupt the bonding between the organic and inorganic parts that may harmfully affect the mechanical properties of tooth structure. This may be clarified by the concept of Andresen where the dissolving effect of CH could decline the fracture of dentin equal to 50% through 1 year.

Additionally, Richhawal et al. (2019) stated that, the direct exposure of dentin with CH may initiate and propagate the surface cracks, rendering dentin more susceptible to collapse [19]. 

On the other hand, many researchers presented a non-significant effect of CH on the fracture resistance of dentin. Andreasen et al. (2002) confirmed that, the strength of the root was not significantly reduced with a 30-day application of CH. Therefore, it appears that the standard protocol of up to 30-day application of CH for infected mature teeth with apical periodontitis is safe and need not to be adjusted [7].

Moreover, the findings of the study by Doyon et al. (2005) demonstrated that a 30-day exposure to CH had no significant effect on the fracture resistance of dentin disks [8]. Additionally, Moazami et al. (2014) reported that the differences between the fracture loads were not significant for CH in a 7-day period [3]. 

One more in vitro study on ovine teeth discloses non-significant changes in fracture resistance of samples filled with CH compared to control group after one year [27].

Hawkins et al. found insufficient evidence to support a decrease in fracture resistance of dentin within a 6-month period, using Vitapex, Ultracal XS, or Pulpdent [three commercial CH formulations] in deciduous lamb incisors [28]. 

Additionally, Hashim in 2021 concluded that CH had no effect on fracture strength of mature or immature teeth. CH application periods of 1 and 12 months had no effect on fracture strength of mature or immature teeth [29]. 

Hardness

Hardness is the resistance to permanent surface indentation or penetration. According to this meaning, it is clear why this property is so important to dentistry [30]. Microhardness was used as a parameter to evaluate the strength of the teeth, because it is not a measure of a single property. It is affected considerably by other important properties of the material such as modulus of elasticity, tensile strength, yield strength and crystal structure stability. It can be used as an indicator of the general strength or resistance to deformation when compared with baseline information [31]. Hence, hardness measurements are commonly used to obtain an indication of the mineral content of the hard tissue of teeth [32].

Evaluating the microhardness of dentin is considered as secondary sign for gaining or losing of inorganic component in the dental hard tissues. Thus, any material can cause alteration in dentin component could change the microhardness, in addition to the solubility and permeability of radicular dentin [33]. 

Osol and Hoover (1975) as well as Alacam et al. (1998), proposed that the reduction of the dentin microhardness related with CH may be elevated if glycerin is mixed with CH as a vehicle. This decline may be associated with the hygroscopic property of glycerin and with minor dissolving of CH in glycerin [2]. 

Andreasen in 2002 proposed the decline of hardness by CH because of the collapse of the bonds between organic and inorganic components [7]. Yoldas et al. [2004] treated dentin with either CH-glycerin mixture or CH-distilled water for one, three or seven days to assess the effect of these materials on the microhardness of dentin. They determined a significant reduction of dentin microhardness due to both mixtures after three and seven days. CH-glycerin mixture significantly decreased the microhardness more than CH-distilled water mixture after the same periods. Hence the dressing of CH mixtures as intracanal medicament soften the human dentin [34].

Yoldas et al. [34] and Seyed et al. [35] have revealed that the reduction in the hardness of dentin due to the treatment of CH may happen because of the changes of dentin structure. It was found that CH denatures pulp tissue could result from dissolving along with hydrolysis. Furthermore, the high pH of CH could decrease the collagen support of the dentin matrix. This high alkalinity may cause collapse of organic matrixes that negatively affect the mechanical properties of tooth structure.

Twati et al. (2009) investigated the influence of non-setting CH on the microhardness as well as modulus of elasticity of human dentin. They concluded that there is a significant reduction of microhardness of dentin after three and six months. The reduction of hardness increased with time of application [36].

Koshy et al. [2009] assessed the effect of CH-glycerin as intracanal medicament on the microhardness of human dentin at different periods of time [one and three months]. The study revealed a significant reduction in the hardness after one month (57.86 HV) and after three months (44.53 HV) in comparison to control group (68.91 HV) that untreated with CH. Similar results were obtained in other research proposed that elongated periods of contact of CH with dentin may lead to decline of the microhardness [2].

Jun et al. (2013) evaluated the effect of CH-iodoform mixture, CH-distilled water in comparison to control group [no medicament]. The interval times for assessment were one, seven, thirty, and ninety days. Radicular dentin microhardness declined with further time of application with CH. Microhardness of the control group reduced rendering to the assessment time and displayed statistically significant [37]. 

Flexural Strength

Flexural strength is a parameter that determines the material resistance to the fracture in bend test [38]. This test is measured by applying a load in the middle of a beam that is simply supported [not fixed] at each end. This test is called a three-points bending or flexure test and the maximum stress measured in the test is called flexure strength [30].

American Society for Testing and Materials stated that, the flexural properties of rigid or semi-rigid materials can be determined using a three-points loading system, utilizing central loading on a simply supported beam.  A lot of studies in dental literature used three-points tests to investigate the effects of different chemicals on the flexural strength of tooth structure [39].

At the Pedodontic Meeting in Norway (1988), a very alarming comment was presented by Stormer et al. [40]. They revealed that 60% of root canal treated teeth with immature apices have cervical fractures due to slight impacts. Occasionally spontaneous fractures happened, which were confirmed with similar results by Cvek [5]. Such research led to the doubt that the endodontic treatment had weakened the tooth structure. This doubt was supported by histological demonstration of circumpulpal dentin changes of replanted teeth after dressing with CH [41].

Partially, the flexure strength of dentin may depend on a close linkage between its two main components, the organic and inorganic. Portion of the organic matrix is composed of acid proteins and proteoglycans containing carboxylate and phosphate groups. These materials could act as linking factors between hydroxyapatite crystals and collagen fibers. The high alkalinity of CH, neutralize, dissolve, or denature some of the acidic components, acting as linking agents and thus weaken the dentin [7].

CH dressing causes a 35% reduction of the mean flexure strength and keeps on constant after ten days [42]. Grigoratos et al. [43] instituted that the dentin samples saturated with CH have less flexure strength than the control group. This study was evaluated under different ways of force application. 

The study of Ratih in 2011 displayed that, CH (UitraCal®) caused less effect on the flexure strength than CH mixed with saline. Hence, the depth of solution and paste penetration must be taken into consideration. The combination of CH with saline penetrated more than commercial CH into radicular dentin because of the less surface tension of the combination [44].

The penetration ability of the material into dentinal tubules increases with the reduction of the surface tension, then causes harmful effect on the mechanical properties of tooth structure [45]. Direct contact with CH significantly minimizes the flexural strength in comparison to normal dentin. The clinical implications of Ratih study indicated that the application of CH as an intracanal medicament is suggested no longer than seven days because of its deleterious effect on flexure strength of dentin [44]. 

Application of CH as an intracanal medicament for seven days represents the clinical situation of this material. It was speculated that the extended period of dressing could increase the dissolving capability of CH. The more time of application to CH, two weeks and one month, caused the reduction of flexure strength [44]. Contact with CH for seven days did not result in different ability to eradicate microorganisms than the extended times of dressing [46]. Therefore, it is suggested to apply CH as intracanal medicament for seven days instead of the longer times of contact because of the disadvantageous effect on the mechanical properties of dentin. Tatsua et al., reported that the intracanal application of CH left for seven days may reduce the flexure strength of radicular dentin [47].

Calcium Hydroxide (CH)

Power Of Hydrogen (pH)

Mineral Trioxide Aggregate (MTA)

Sodium Hypochlorite (NaOCl)

Microtensile Fracture Strength (MTFS)

Magapascal (Mpa)

Chlorhexidine (CHX)

Vickers Hardness (HV)