Novel bone adhesives: a comparison of bond strengths in vitro

Abstract

Fracture fixation using adhesive is a promising alternative in craniofacial surgeries, replacing the plates and screws system. The advantages include the ease of application and avoidance of drilling holes that may weaken the bone and cause fractures. In this study the bond strengths of selected adhesives were evaluated and compared with resorbable plates and screws. Four adhesives, octyl-cyanoacrylate, N -butyl-cyanoacrylate, a novel methyl-methacrylate, and a novel cyanoacrylate derivative, were tested for their microtensile and shear bond strengths. The bone samples were cut into rectangular bars and bonded with selected adhesives for microtensile testing. For the shear bond test, paired bars were bonded at the overlap, while two other sets of bars were attached by a Lactosorb plate using either adhesive or screws. Data were analysed by analysis of variance (ANOVA). The microtensile bond strengths of N -butyl-cyanoacrylate, novel cyanoacrylate derivative, and novel methyl-methacrylate derivative were significantly greater than octyl-cyanoacrylate. When bone sections were fixed with resorbable plates and adhesives, shear bond strength was significantly greater for N -butyl-cyanoacrylate than plate and screws, while the bond strengths of other adhesives were comparable with the plate and screws. N -Butyl cyanoacrylate was shown to have the greatest potential for fixation of fractured bone in craniofacial surgical applications.

Recent advancements in craniofacial surgical procedures have provided a number of surgical innovations and new biomaterials for internal fixation methods. While conventional rigid fixation with metal plates and screws is ideal for stable internal fixation, the disadvantages include extrusion, migration, palpability, and growth restriction. The plate fixation system with screws requires the drilling of holes that may cause additional trauma and may weaken the bone, causing further fractures. Furthermore, the load is mainly transferred onto the site of the screws, thereby leading to punctual stress overload and fixation failures.

Resorbable fixation systems have been found to be a good choice for fracture fixation in recent years. The limitation of the resorbable plates with screws system is that it still requires drilling before screw fixation. Moreover, during traumatic injuries, fixing the bone fragments is difficult using plates and screws. Adhesives remain a promising alternative in cases where the fixing of bone fragments is difficult using plates and screws. The advantages of using adhesive lie in the ease of application, better biomechanical properties, biodegradability, and the ability to support bone healing.

In order for an adhesive to be considered as part of an alternative fixation method, it must fulfill certain criteria: (1) have sufficient strength to be useful, (2) be inert, non-toxic, non-carcinogenic, and non-teratogenic, (3) adhere to moist surfaces, (4) not interfere with the natural healing process, (5) have chemical stability, (6) be economically feasible, (7) be bioresorbable, and (8) be as easy to use as other conventional methods. Out of the existing adhesives used in biomedical applications, only a few meet these criteria; these include cyanoacrylates and methacrylates.

Cyanoacrylates are the most commonly used group of adhesives for bone fixation. They have been found to have good biomechanical strength and also work in a wet environment. Additionally, they have bacteriostatic and homeostatic properties. Studies on butyl- and ethyl-cyanoacrylate have demonstrated an in vivo half-life of 24–48 weeks. It has been reported that the use of cyanoacrylate adhesives does not hinder the vascularization of newly formed bone.

Methyl-methacrylate is commonly used as a bone cement and has been used to fill in traumatic skull defects for many years. Methyl-methacrylate is also a safe implant material that is used in the craniofacial region. Methyl-methacrylate is resorbable in the body, with an aqueous aerobic degradation half-life of 1–4 weeks.

In a recent in vitro study, the biomechanical strength and biocompatibility of methyl-methacrylate was assessed and it was recognized as a promising biomaterial for bone adhesion and bone regeneration. Furthermore, methacrylates do not represent a barrier for the vasculature during the fracture fixation process, and some of them show enhanced angiogenesis.

The present investigation focused on the use of acrylate-based adhesives for the bonding of resorbable fixations to craniofacial bone. The microtensile and shear bond strength of cyanoacrylate- and methyl-methacrylate-derived adhesives alone were evaluated. The shear bond strengths of adhesives and adhesives in combination with resorbable plates were evaluated and compared to the bond strengths of resorbable plates and screws.

Materials and methods

In this study, bond strengths of two types of novel adhesives, viz. a novel cyanoacrylate (NCA) and methyl-methacrylate (NMMA) derivative, were evaluated and compared with N -butyl-cyanoacrylate (BCA; provided by Biomet Microfixation, Jacksonville, FL, USA) and the commercially available adhesive octyl-cyanoacrylate (OCA; Dermabond ® , Ethicon Inc., Somerville, NJ, USA) which is currently used in clinical practice. All of the specimens in this study were randomly allocated for testing.

Human cadaver parietal bone samples were obtained from the National Disease Research Interchange, Philadelphia, PA, USA. Bone samples were stored at −70 °C (−94 °F) until they were used.

Preparation of adhesives

Among the four adhesives tested, three were liquid adhesives (derivatives of cyanoacrylates) and one was a composite (derivative of methyl-methacrylate). The composite adhesive was prepared by hand mixing the liquid monomer with the powder filler. The mixing proportions were as per the instructions of the manufacturer.

Bone sample preparation for microtensile test

Bone samples were allowed to thaw for about 1 h at room temperature and were then cut into rectangular sections (2 mm × 2 mm × 20 mm) using a low-speed saw (IsoMet; Buehler, Lake Bluff, IL, USA) with a diamond-rim blade (15 HC IsoMet Wafering Blade; Buehler). Specimen size was determined based on the available literature. The recommended size for preparing specimens for microtensile testing is 1.6–1.8 mm 2 . This was determined as the minimal area that would produce a uniform distribution of stresses. However, due to the minimum width and thickness of the parietal bone sample provided for this study, microtensile bars were cut with a minimal area of 4 mm 2 . The rough edges of the bone surface were rounded using 320 grit abrasive paper (CarbiMet; Buehler) in order to obtain samples suitable for bond strength testing. The rectangular bars were sectioned perpendicular to the long axis and made into equal halves and glued together with the selected adhesives ( Fig. 1 ). A sample size of 10 specimens per group was selected for the microtensile test. Octyl-cyanoacrylate was chosen as the control as it is currently used in clinical applications.

Fig. 1
Example of microtensile bond strength specimen.

Bone sample preparation for shear bond strength

Samples were assigned to three different groups, as outlined below.

Group 1—adhesives alone

Bone samples were cut into 2 mm × 2 mm × 20 mm rectangular sections using a low-speed saw. A lap shear test (ASTM D5868) was conducted with minor modifications ( Fig. 2 A) , with the specimen size similar to that for microtensile testing for comparison between the two groups. A custom-made fixture ( Fig. 2 B) was used for shear bond testing. The rectangular bone pieces were overlapped (∼10 mm) onto each other and the adhesive was applied in the region of contact ( Fig. 2 B).

Fig. 2
(A) Schematic figure of lap shear test. (B) Set-up for shear bond test.

Group 2—resorbable plates with adhesive

Lactosorb resorbable plates (Biomet Microfixation) measuring 6 mm × 22.8 mm were used. The bone sample size was determined based on the size of the plates. Bone samples measuring 6 mm × 6.5 mm × 40 mm were prepared using the low-speed saw. Resorbable plates were cross-hatched with a scalpel to obtain a rough surface in order to increase the retention of the adhesive onto the plate. The bone sections were secured together with the plate and adhesive ( Fig. 3 A).

Fig. 3
Example of resorbable plate shear specimens. (A) Resorbable plate and adhesive. (B) Resorbable plate and screw.

Group 3—resorbable plates and screws

Bone samples measuring 6 mm × 6 mm × 40 mm were prepared according to the size of plates and screws (Ø1.5 mm × 3 mm; Biomet Microfixation). The fixation was performed using Lactosorb plates with holes, fixed to the outer cortex of each bone segment with the Lactosorb screws (bone samples were secured with the plates fixed with a total of four screws, i.e., two screws on each segment) ( Fig. 3 B). Screws were placed after drilling a pilot hole slightly smaller than the screw diameter.

For all of the experiments, the setting time of the samples with adhesives was kept to between 10 and 20 min and at 37 °C (98.6 °F) under humid conditions. Cured bone sections were secured in a linear configuration onto a universal testing machine (Model 8831; Instron, Canton, MA, USA) using a cyanoacrylate ester (Rocket Heavy and Accelerator; Dental Ventures of America Inc., Corona, CA, USA). Load was applied to the fixture with a crosshead speed of 1 mm/min until failure. The maximum tensile and shear bond strengths were recorded on a computer attached to the machine and represented in a graph. Bond strength was calculated in megapascals (MPa) from the division of the maximum force (in Newtons) by the known area of adhesive applications. A sample size of five specimens per group was selected for all three shear testing groups.

Sample preparation for failure analysis

After the specimens were tested for microtensile bond strength, they were removed from the testing apparatus and the sites of failure were observed under a stereo zoom microscope (SMZ 140; VWR International, West Chester, PA, USA) to identify the mode of failure. Bone samples were air-dried overnight and mounted with the fracture surface facing up on aluminium stubs using a carbon tape. They were then gold sputter coated and observed using a scanning electron microscope (Quanta 200; FEI, Hillsboro, OR, USA). The mode of failure was determined as being adhesive (failure at the bonding surface, i.e., between bone and adhesive), cohesive failure (failure in the material itself, i.e., failure within the adhesive or failure within the bone), or mixed mode failure.

Data analysis

Data were analysed with a one-way analysis of variance (ANOVA) using GraphPad PRISM (GraphPad Software, Inc., La Jolla, CA, USA). Dunnett’s post hoc test and Tukey’s multiple comparison test were performed for inter-group comparisons. The criterion for statistical significance was P < 0.05.

Jan 24, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Novel bone adhesives: a comparison of bond strengths in vitro
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