A study of force extension and force degradation of orthodontic latex elastics: An in vitro study

INTRODUCTION

Latex elastics have remained one of the most commonly used force delivery systems. The advantages of latex elastics include an ability to exert a continuous force, convenience of use, compatibility in the oral environment and cost- effectiveness.^[1]

One of the inherent disadvantages of elastics is that the force levels decrease over time. This property is termed “force decay.” For optimum orthodontic tooth movement this force decay has to be within acceptable limits.

The standard index employed by a majority of orthodontic elastic manufacturers says that when the elastic is stretched to 3 times the inner lumen diameter (3×), it will exert the force levels stated on the package.^[1]

The purpose of this study was to evaluate the initial force and force decay pattern of elastics at 25 mm, 2× and 3× of inner lumen diameter. This will help in determining the appropriate time for change of elastics for maintaining optimum orthodontic force. The present study also compares the force decay values of elastics manufactured by 4 companies.

MATERIALS AND METHODS

Orthodontic latex elastics marketed by 4 companies (HP company-Calicut, Kerala India; JJ company-Chalakudy, Kerala India; AO company-Sheboygan USA; TP company- Laporte, USA) were tested. An acrylic board was prepared having sets of stainless steel pins set at a fixed distance of 25 mm between them, which was considered to cover the range that the elastic would be stretched under clinical conditions and at 2 times and 3 times (i.e., 2× and 3×) their inner lumen diameter for 5/16” diameter medium (green) and ¼” diameter medium elastics (blue). These particular sizes were selected as they are the most commonly used elastics in orthodontic practice.

Correx force gauge (HP company-Calicut, Kerala, India) was used to measure the force in grams exerted by the elastics. The force range of the gauge is 25-250 g.

The Correx force gauge was mounted together with a ruler so that force exerted by the elastic at given length could be noted.

Two tests were carried out.

RESULTS

The results of the present study were as follows:

• Force values obtained in this study at 3× extension were found to be closer to the force values mentioned by the manufacturer rather than force values at 2× [Table 1].

• A considerable margin of error was found between the actual force value of the elastic and value indicated by the manufacturer with JJ and AO Company. Their elastics showed lesser force values than those mentioned by manufacturer [Table 1].

• HP and TP Company elastics show nearly the same force values as those mentioned by the manufacturer [Table 1].

• The property of “Force Degradation” was found in all types of elastics. Force degradation increases with time [Tables 2, 3, 4 and 6].

• Greatest force degradation of nearly 20% occurs during the 1^st day [Tables 2 and 3].

• 5-10% force degradation was found during the 2^nd day [Tables 2, 3, 4 and 6].

On comparing, the force degradation values of HP and JJ Company, i.e., two Indian companies it was seen that 5/16 inch lumen medium HP elastics show lower force degrading qualities than 5/16 inch lumen medium JJ elastics [Table 5].

Conversely, ¼ inch lumen medium JJ elastics show lower force degrading qualities than ¼ inch lumen medium HP elastics [Table 7].

On comparing the force degradation values of HP and TP company, i.e., one Indian and one American company it was seen that 5/16 inch lumen medium HP elastics have lower force degrading qualities than 5/16 inch lumen medium TP elastics [Table 5]. Conversely, ¼ inch lumen medium TP elastics show lower force degrading qualities than ¼ inch lumen medium HP elastics [Table 7].

Similarly when the elastics of HP and AO company, i.e., one Indian and one American company were compared it was observed that 5/16 inch lumen medium elastics of HP company show nearly similar force degradation qualities than 5/16 inch lumen medium AO elastics [Table 5]. Although ¼ inch lumen medium AO elastics show lower force degrading qualities than ¼ inch lumen medium HP elastics [Table 7].

On comparison of elastics of JJ Company and TP Company, it was observed that 5/16 inch lumen medium JJ elastics show lower force degradation than 5/16 inch lumen medium TP elastics [Table 5]. Conversely, ¼ inch lumen medium TP elastics show lower force degrading qualities than ¼ inch lumen medium JJ elastics [Table 7].

On comparison of elastics of JJ Company and AO Company, it was observed that 5/16 inch lumen medium AO elastics show lower force degradation than 5/16 inch lumen medium JJ elastics [Table 5]. Similarly, ¼ medium AO elastics show lower force degrading qualities than ¼ medium JJ elastics [Table 7].

Finally on comparison of elastics of AO Company and TP company, i.e., two American companies it was observed that 5/16 inch lumen medium AO elastics show lower force degradation than 5/16 inch lumen medium TP elastics [Table 5]. Similarly, ¼ inch lumen medium AO elastics show lower force degrading qualities than ¼ inch lumen medium TP elastics [Table 7].

Thus, from the above comparison we can conclude that 5/16 inch lumen medium elastics are more resistant to force. Conversely, ¼ inch lumen medium elastics show poor force degradation. In ¼ inch lumen medium elastics least force degradation is present in AO elastics.

DISCUSSION

The present study has measured force values of elastics of 2 sizes at 25 mm, which covers the range that the elastic will be stretched under clinical conditions. The elastics were also stretched at 2 times (2×) and at 3 times (3×) their inner lumen diameter and tested for force value changes in a simulated oral environment.

Bales et al.^[2] had investigated the accuracy of force values at 3× inner lumen diameter index and had reported that a closer relationship of observed force values occurred at 2× stretch rather than at 3× stretch. Gupta et al.^[3] stated that the expected force values of elastic as stated in the manufacturers catalogue will be delivered at 3× stretch rather than at 2× stretch. It was observed in our study that force values at 3× were closer to the force values mentioned by the manufacturers on the elastic packet than force values at 2×. Thus, our study shows that the elastics exert forces as specified by the manufacturers standard extension index.

The present study showed that all the elastics regardless of brand or size underwent degradation in force with time. The average force degradation after 1 day ranged from a minimum of 16.04% for ¼ inch lumen medium elastics of TP Company to a maximum of 27.14% for ¼ inch lumen medium elastics of HP Company. This was in accordance with the results of other studies.^[4-7]

Similarly, average force degradation after 2^nd day ranged from a minimum of 19.01% for ¼ inch lumen medium elastics of AO Company to a maximum of 32% for ¼ inch lumen medium elastics of AO Company. On considering force degradation percentile values of all elastics there was about 20.84% during the 1^st day, which increased to 25.54% on the 2^nd day. This was in accordance with the various studies^[4-7] carried out on force degradation of latex elastics with a range of 7-30% from 1 h to 3 days. The difference in force degradation values can be attributed to different experimental conditions, stretching criteria and sample differences.

The factors that influence the force degradation characteristics of elastics are — Plant to plant variation, regional and seasonal variation before the manufacturing process and also the presence of impurities in raw material.^[8] The difference in force degradation characteristics of the elastics compared in this study can be attributed to differences during manufacture, i.e., the type, extent and the material that has been used for cross linking, inclusion of protective materials such as antioxidants and procedures such as bleaching. Before use, of the elastics after manufacture, exposure to sunlight, moisture, heat, ozone and radiations can affect the characteristics of the elastics; whereas during use, the masticatory forces, temperature variations, dietary habits and stretching cycles also play a role.^[9]

CONCLUSION

• Force values obtained in this study at 3× extension were found to be closer to the force values mentioned by the manufacturer rather than force values at 2×.

• The property of “Force Degradation” was found in all types of elastics. Force degradation increases with time with the highest i.e. nearly 20% occurring during the 1^st day and 5-10% occuring during the ^2nd day.

• A considerable margin of error was found between the actual force value of the elastic and value indicated by the manufacturer with JJ and AO Company. HP and TP Company elastics show nearly the same force values as those mentioned by the manufacturer.