In Vitro Study Regarding the Biomechanical Behaviour of Bone, Fibre Reinforced Polymer and Wire Composite Periodontal Splints. II.Model Analysis

This paper is the second part of a study regarding the biomechanical behaviour of mandibular bone in the context of different periodontal splinting systems, occlusal forces and load distributions. Electric resistive tensometry method was used to measure the strains developed in mandibular bone replica. The tests were carried out on six mandibular acrylic models, each with 8 natural teeth. The experimental groups were defined corresponding to the bone condition and splint type: normal height bone; bone resorption without splint; bone resorption and wire-composite splint; bone resorption and polyethylene fiber-reinforced composite splint. Each sample was subjected to three similar loading cycles, the force being applied successively on four incisors, two central incisors and canines, and the specific deformation values were read for four loading forces: 30 N, 50 N, 100 N and 150 N. In case of bone loss, the bone deformations are up to 110%. Periodontal splinting redistribute forces, reducing incisors bone strains associated with a slight increase in canine bone strains.


Introduction
Periodontitis is a chronic infectious disease of the tissues surrounding the teeth caused by specific microorganisms or groups of specific microorganisms, characterized by gingival inflammation, loss of connective tissue attachment and destruction of alveolar bone [1][2][3]. With the reduction of periodontal attachment, mobility and dental migration appear, resulting in incorrectly distributed occlusal forces, which overload the already affected periodontal system. The relationship between occlusal trauma and tooth mobility depends on the intensity and frequency of occlusal forces [4].
The treatment of dental mobility in periodontal disease is determined by the degree of bone resorption. For teeth with increased mobility due to widening of periodontal space induced by the adaptation to the functional conditions of mastication, the treatment is a combination of occlusal adjustments and periodontal therapy. For teeth affected by gingival inflammation and increased mobility due to bone resorption, the treatment is a combination of periodontal therapy, occlusal adjustments and teeth immobilization [5][6][7][8][9]. Stabilization is achieved by periodontal splinting, which The first splinting system was wire-composite resin (WRC). The steps were as follows: (1) measuring the inter-canine distance and cutting the appropriate length of the twisted wire; (2) adapting and conforming the wire on the model; (3) degreasing of lingual surfaces; (4) etching of lingual and proximal accessible surfaces with orthophosphoric acid 37% for 30 seconds; (5) acid removal, surface washing and drying; (6) applying of the adhesive and light cure for 20 seconds / tooth with the tip of the light beam placed at a distance of 5 mm from the sample; (7) wire positioning and applying of the packable composite layer at the level of each tooth, followed by light curing for 40 seconds / tooth ( Figure 1). After performing the mechanical tests on the wire-composite splint model, the splint was carefully removed in order to avoid the damage of the model and of the strain gauges. The second immobilization system was the polyethylene fiber composite (FC). The steps were as follows: (1) measuring the inter-canine distance and cutting the corresponding strip of fiber band; (2) impregnation of the polyethylene fiber with the Construct resin; (3) degreasing of lingual surfaces; (4) etching of lingual surfaces with orthophosphoric acid 37% for 30 seconds; (5) acid removal, surface washing and drying; (6) adhesive applying and light curing for 20 seconds / tooth; (7) applying of a 0.2 mm layer of Premise composite; (8) adapting the impregnated polyethylene fiber and removing the excess of composite; (9) light-curing for 40 seconds / tooth; (10) applying a final layer of Premise flow composite to fully covering the fiber and light curing for 40 seconds / tooth ( Figure 2).

Mechanical testing of the samples
The mechanical testing protocol was also described in detail in the first part of this study [34]. It is presented briefly below. https://doi.org/10.37358/MP.20.1.5334 6 mm length and 2 mm width strain gauges (SG) (EA-06-240LZ-120/E, Micro-Measurements Group, Vishay, Batch No.: R-A59AF524) were selected for quantifying the bone deformation. Their electrical resistance was 120 ± 0.03Ω at 24ºC. They were placed on the mandibular replica, corresponding to coronal-radicular axis as it follows: three on the buccal surface (SG 1 -right lateral incisor/RLI, SG 2 -left central incisor/LCI, SG 3 -left canine/LC) and three on the lingual surface (SG 4 -LC, SG 5 -LCI, SG 6 -RLI). A WDW-5CE High Performance Electronic Universal Testing Machine (Bairoe, Shanghai, China) was used to perform the compression tests and record the timeforce variation. With a specially adapted device, the model was mounted on the test machine allowing an occlusal load orientation of 135° on the mandibular incisors that replicated the normal interincisal angle, and an individual loading on incisors and canines.
The load distribution was at the level of: (i) four incisors -4I; (ii) two central incisors -2I; (iii) canine -C. The samples were three times tested for each load distribution, with a load speed of 0.5 mm / min, 150 N maximum load, at room temperature (23° C) and with a 5 minute break before each test to allow for recalibration of the marks. The specific deformations provided by the strain gauges (in relation to time) were recorded with two strain gauge bridges P3 model (Vishay). The displacement values were read for the four loading forces: 30 N, 50 N, 100 N and 150 N, respectively.
A dial comparator was positioned in contact with the labial middle third of RLI, which allowed the horizontal tooth displacement measurements with an accuracy of ± 0.01 mm ( Figure 3).
The experimental groups were defined corresponding to the bone condition and splint type as it follows: models with normal height bone (NHB); models with bone resorption without splint (BR); models with bone resorption and wire-composite splint (WRC); models with bone resorption and polyethylene fiber-reinforced composite splint (FRC). Each sample was subjected to three similar loading cycles, the force being applied successively on 4I, 2I and C, and the specific deformation values expressed in µε (equivalent to µm/m) were read for four loading forces: 30 N, 50 N, 100 N and 150 N.

Statistical analysis
Statistical data analysis was performed with STATISTICA 11.0 (Stat-Soft, Tulsa, OK), at a significance level of 0.05. Factorial analysis of variance (ANOVA) was employed to evaluate the effect of four variables: bone condition and splint type (BC), mandibular bone surface (BS), load distribution (LD), and tooth position (T) on the bone strain in the context of different occlusal loading.

Results and discussions
The dental mobility was re-evaluated after applying each immobilization system and the results are presented in Table 2 The horizontal displacement values of RLI are presented in Table 3.
Bone strains values expressed in µε (equivalent with µm/m) for every situation referred above were read for the four forces 30N, 50N, 100N and 150N (Table 4).    The factorial ANOVA indicated significant differences between the four factors (load distribution, bone condition, bone surface, tooth; P <0.001), irrespective of load level. For the 2-factor interactions, the following interactions were significant for all load values (P <0.001): bone condition and load distribution, bone condition and tooth, load distribution and tooth, and bone surface and tooth. Of the 3-factor interactions, only the bone condition, load distribution, and tooth interaction was statistical significant irrespective of load level. describe some particular situations when splints are subjected to much larger forces due to lateral edentulism and consecutive occlusal forces concentration in frontal dental area [8].
Deformations values in the anterior region of the mandible were directly proportional to the load values of the four forces. Even small forces applied cyclically over a period of time, can cause a phenomenon of fatigue or interfere with tissue healing processes, taking into account the small size of the bone structure in this region. In all simulated situations, higher values of deformations were observed on the buccal surface, aspect also reported by Soares et al. This can be explained by a smaller thickness of the labial bone compared to lingual bone [36].
It was noticed that, regardless of the value and of the loaded area, strains in group BR were significantly greater than strains in the group NHB, and those in groups WRC and FRC had intermediate values, but closer to NHB group. In addition, strains at the central incisor were higher by 60-85% than at the lateral incisor, except when the contact was made on the canine.
Comparing the BR to NHB groups, a severe increase of strains was observed, both on lingual and labial surfaces, the highest difference (73%) was recorded for 100N force, on buccal for central incisor in BR2I group.
It can be noticed that in the groups with periodontal splinting (WRC and FRC), occlusal forces were distributed to all tooth in the splint, as demonstrated by the modified strain values.
Another quantified parameter with clinical relevance is the tooth contact. When the force is applied to the central incisors, the strains values are 60% higher than when force is balanced applied at the four incisors.
Another interesting aspect is that when force is applied to the incisors, bone strains in FRC group were higher than in WRC group with no significant differences. This suggests a more elastic behaviour of polyethylene fibers than of the wire-composite system.
When contact was at the canine, comparing the distribution of bone strains from the canine, it was observed that in WRC group the strains were higher by 18 % compared with FRC group, regardless of the force. Due to the intern stiffness of the wire, a reduction in incisors strains can be achieved, which has as disadvantage the consecutive increase of canine strain. In the context of constant overloading, the adaptive level of canine support tissues may be exceeded, thus favouring the progression of bone resorption.
Differences between wire-composite splint and fibre-reinforced composite splints may occur also from clinical behaviour point of view. Thus, the wire -composite resin splint has an interface between two materials with different modules of elasticity (stainless steel and composite resin) without chemical adhesion. Thus, in this area fracture initiation may occur, because the splint exhibits low fatigue strength [16,25].
Regardless the contact between the loading device and the dental surfaces, higher values are observed to lateral incisor in the group BR, versus group NHB, for all four forces. For groups WRC and FRC, horizontal displacement value decreased, but remained higher than in NHB group regardless of force level. No significant differences of displacement values were observed between the two splinting groups. The displacement values in the group BR2I were higher up to 300% compared to BR4I, which reinforce the need of balanced occlusion in order to achieve multiple contacts on the four incisors for an even distribution of occlusal forces.
In a study by Soares et al.
[36], based on electric tensometry method, the bone strains were compared when adhesives splinting systems (composite resin, wire resin and fiber-reinforced composites) and wire splinting were applied. The authors noted that bone strains values in case of wire splinting were significant higher compared with the FRC splinting. To a force of 150 N, the wire did not achieved significant stabilization of mobile teeth. According to these results, the use of the wire without application of a composite resin and an adhesive system is not suitable in the splinting periodontal treatment. In the same study, the authors pointed out that dental splints with adhesive system and composite resin produced lower bone strains irrespective of occlusal load. https://doi.org /10.37358/Mat.Plast.1964