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Effect of cast Co-Cr and acetal resin removable clasp on the surface of enamel

Objetive of the study was to evaluate the effect of cast Co-Cr and acetal resin clasp on the surface of tooth. Methodology: Ten extracted human mandibular molars were selected for the study. Clasp holding surfaces of the teeth were demarcated and examined by stereomicroscope and optical profilometer to evaluate the surface roughness qualitatively and quantitatively. The molars were then mounted on a cast and on which cast Co-Cr and acetal resin clasps were fabricated with an attached vertical strut. The assembly was placed on a masticatory simulator. Three thousand cycles of insertion and removal were carried out to simulate 2 years of usage. After the experiment was completed, the molars were evaluated for the surface changes by stereomicroscope and optical profilometer. Results: Qualitative analysis by stereomicroscope showed linear scratches and grooves on the surface of enamel caused by the use of cast Co-Cr and acetal resin clasp. Minimal scratches were seen on the enamel when acetal resin clasp was used. Optical profilometry showed statistically significant results (P < 0.0001) with the Co-Cr clasp. Acetal resin clasps showed insignificant results. The retentive force of cast Co-Cr clasp showed a decrease from 12.4 N to 8.1 N. Whereas, the retentive force of acetal resin clasp reduced from 5.2 N to 4.03 N at the completion of the experiment. Conclusions: Acetal resin clasps do not abrade the surface of tooth and maintain retention. Acetal resin clasps are esthetic and are available in 16 different shades.

Clinical Relevance to Interdisciplinary Dentistry Top

Removable partial dentures are frequently used as a treatment option for long span edentulous spaces. Clasps are an integral part of removable prosthesis. With advancement in the materials, numerous options are available for fabrication of partial denture and clasps. Since these clasps can have long term effect on the tooth surface, it is essential to evaluate the effect of these materials on the tooth surface.

Introduction Top

Removable partial dentures are retained by carefully designed clasps. A cast clasp flexes nearly 10 times a day and applies a force of 10 N., A closely bracing clasp can abrade the enamel surface eventually resulting into retention loss. Both the tooth surface loss and the retention loss are matters of concern while using a cast removable partial denture. Acceptability of a metallic clasp is very limited because of its inherent color. The use of an esthetic and tooth friendly substitute in the fabrication of a retainer has been unanswered till the introduction of acetal resin by Herman Staudinger, a German chemist, in the 1920s. However, acetal resin clasps were fabricated much later in 1971 with the advent of rapid injection systems.[4] They are superior in esthetics and are available in sixteen shades of vita shade guide. Acetal resin clasps are reasonably strong, resist wear and are flexible. Hence, they are friendly to enamel. A functional comparison of conventional Co-Cr clasps and the acetal resin clasps has not been tried in the past. Hence, this study was designed to find out and compare the abrasive effect ofCo-Cr and acetal resin clasps on the tooth surface and the resultant change in retention.

Methodology Top

Ten extracted human mandibular molars were selected, and two were incorporated in a standard cast made of dental stone. Altogether five casts were made . This was duplicated in dental stone, and circumferential clasp assemblies were designed. The clasp assembly consisted of retentive arm, reciprocal arm, occlusal rest, 1.5 mm guide plate and a vertical strut to enable holding of the clasp assembly in the universal testing machine. Five clasps were made in acetal resin and five in cobalt chromium alloy. These clasps were to be tried on the extracted teeth incorporated in the model.

The area of the tooth where the clasp arms were positioned was demarcated with a marker.a and b. The clasp assembly and the tooth containing cast were placed in a universal testing machine which served as a simulator (Nano plug and play-ISN 67). Three thousand cycles of insertion and removal were carried out, and this was equivalent to 2 years of usage a, b and a, b. The demarcated tooth surface was examined under stereomicroscope (Labovision, Haryana, India) to find out the surface irregularities a. The same area was subjected to optical profilometry (Veeco NT1100, Tucson, USA) to quantify the roughness b. This provided the preexperiment data. After the experiment was completed, the tooth surfaces were evaluated for the changes using stereomicroscope and optical profilometer. The same masticatory simulator was used to measure the retention of clasps (Test Builder version 2.3) before the experiment and after every 1000 cycles. Pre- and post-experiment values were compared and statistically evaluated using Student’s t-test SISA (Simple Interactive Statistical Analysis).

Results Top

Qualitative analysis by stereomicroscope showed visible surface changes on the enamel with both cast Co-Cr and acetal resin clasps. After the experiment, eosin stain was applied on the tooth surface to demarcate the surface change. Linear scratches and grooves were more evident on the enamel surface with cast Co-Cr clasp, whereas when acetal resin clasps were used, the scratches were minimal a, b, c, d, e, f, g, h and a, b, c, d, e, f, g, h. The pre- and post-experiment surface roughness values were significantly different with Co-Cr clasps, whereas it was not so with acetal resin clasp [Table 1] and [Table 2]. To find out the statistical power, the values obtained for Co-Cr clasp and acetal resin clasps were considered as two groups. The calculations are given in [Table 3]. There is a distinctive difference in the effect of both the clasps on enamel. The power is 1.0. The retentive force of cast Co-Cr clasp was 12.4 N at the start of the experiment and it decreased to 8.1 N at the completion of the experiment. Whereas, the retentive force with acetal resin clasp was 5.2 N at the start of the experiment and 4.03 N at the completion of the experiment.

Discussion Top

The use of cast Co-Cr clasp resulted in abrasion of the enamel and which can result in accumulation of plaque and discoloration of the tooth. Loss of retention of the partial denture can also be attributed to the tooth surface loss caused by the abrasivity of Co-Cr clasp. On the other hand, the effect of acetal resin clasp on enamel was negligible in terms of abrasion and loss of retention. Abrasion of tooth surface is due to the difference in hardness of the materials.[5] Hardness of cast Co-Cr is 391 KHN which is higher than that of enamel (343 KHN). Hardness of acetal resin is 82–۸۵ Shore D,[6] which is comparatively very friendly to the enamel. Acetal resin clasp caused less roughness to enamel when compared to cast Co-Cr clasp. Loss of retention can also be attributed to fatigue and deformation of the clasp arm. As the acetal resin clasps are more flexible, fatigue and deformation was not observed.[3] Retention provided by the acetal resin clasp was inferior to that of Co-Cr clasp. However, the loss of retention observed during the experimental period with acetal resin clasp is very low when compared to that of Co-Cr clasp. Loss of retention can be either due to tooth surface loss or due to wear of the clasp material. Tooth wear caused due to acetal resin clasp can be equated to that of tooth brush. For this connection, it may be noted that the most common cause of cervical abrasion is tooth brushing.[6] Hardness of bristle of toothbrush is 74–۷۶ Shore D. An acceptable range of retention for a removable partial denture is 2.9 N–۷٫۳ N.[7] Despite the loss of retention, acetal resin clasp provided a retention of 4.03 N. Acetal resin clasp can, therefore, be successfully used for partial denture frameworks.

Conclusions Top

It was concluded that acetal resin clasps do not abrade tooth surface when compared to that of Co-Cr clasp
Although inferior to Co-Cr clasp, the acetal resin clasp provided adequate retention of 4.03
Acetal resin clasps have superior esthetics.

Source: Journal of Interdisciplinary Dentistry

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