Retention of 3-D-Printed, Milled, and Conventionally Fabricated Occlusal Splints

Introduction

The occlusal splint (OS) is an intraoral device used to treat patients with craniomandibular dysfunctions (CMD) and to reduce signs and symptoms associated with TMD. It also protects teeth from wear and tear, improves movements of the mandible and opening of the mouth, and achieves neuromuscular equilibrium by stabilizing occlusion [1]. Its most crucial roles include keeping the relationship of the condyle and the glenoid cavity in a firm musculoskeletal position, reducing cellular hypoxia, moderating proprioception of the ligaments of periodontium, shielding the teeth/implant-supported restorations and associated structures from bruxism, and providing symptomatic evidence of clenching or grinding – for example, by enabling the clinician to watch while the patient wears it [2,3].

Occlusal devices are commonly used in the treatment of temporomandibular disorders (TMDs). According to the glossary of prosthodontic terms (GPT), an occlusal device is characterized as “any removable artificial occlusal surface that influences the spatial relationship between the mandible and the maxilla, serving either diagnostic or therapeutic purposes.” The term “occlusal splint” is often employed to describe various types of occlusal devices, including hard stabilization splints like the Michigan, flat, and relaxing splints, along with anterior repositioning splints, anterior/posterior bite planes, pivoting appliances, and soft resilient appliances. Nevertheless, the term “occlusal splint” may not be as precise as “occlusal orthosis or orthotic device” owing to the differing etymological roots of those words. Recently, Bhargav et al recommended replacing the term “occlusal splint” with “oral orthotic occlusal appliance” [4].

In all situations, the first dental intervention that is recommended consists of a reversible procedure using occlusal splints, which reduces and relieves tension and discomfort in 50–80% of patients [5–7]. There are various types of splints available, with Michigan splints being one of the most widely utilized. In terms of the risk-benefit ratio, these centric splints with customized occlusal surfaces are regarded as the best [6–8]. Splint therapy provides an occlusion that harmonizes and relaxes the masticatory system, which in turn reorganizes muscular and articular functional patterns with related counterparts and relieves pain in the stressed muscles [8].

Occlusal splints are a type of removable prosthesis, providing functional and therapeutic benefits, particularly in managing temporomandibular joint disorders and preventing dental wear [9–11]. Removable prostheses play a crucial role in dental treatment by replacing missing teeth and associated structures and maintaining dental stability [12]. Similarly, OSs provide support and stability to the dentition while also facilitating management of various occlusal disorders [13].

Splints are prepared by using an acrylic resin. A uniform bite plane is used in all supporting regions during its fabrication. The OSs have canine-guided occlusion and work on the “freedom in centric” principle [14,15]. One of the key aspects of any occlusal splint is that it should be retentive, properly adjusted, and stabilized without causing any sensitivity in the dental tissues [16]. It should be made of a material that is biocompatible and does not harm the surrounding structures.

Improperly fabricated OSs or even improper use of occlusal splints can lead to a variety of problems, including discomfort, misalignment of teeth, deterioration of oral health, and decreased effectiveness. These complications can result in further dental problems, increased treatment costs, and decrease overall quality of life [17]. Furthermore, improperly fabricated OSs can increase the risk of developing temporomandibular joint (TMJ) disorders, exacerbating symptoms such as pain and restricted movement [18]. This can result in chronic discomfort and difficulty in jaw function and can necessitate further dental interventions to correct the misalignment. Inadequate retention of occlusal splints can exacerbate these issues, leading to a cycle of discomfort and worsening dental health [18,19].

The materials used to create any intraoral device/occlusal splint should be comparable to the dentin’s micro-hardness, ranging from 250 to 800 Mpa, and its modulus of elasticity (10 to 20 GPa). The materials with such values offset the stiffness of enamel, provide adequate surface finish, have good wear resistance, and have can cushion masticatory impacts. This prevents occlusal instability, micro-porosity, growth of biofilm, and discoloration of the appliance [20,21]. The occlusal splints can be classified into 3 categories based on the method of their fabrication: conventional lost-wax process, milled, and 3D-printed.

The conventionally-fabricated occlusal splints are made of cold-cured methyl methacrylate (polymer and monomer). It requires impression-making, casts, interocclusal check records, mounting, wax-up, and related laboratory processes [20,22,23]. This kind of splint has several benefits, including being inexpensive in comparison to other types and being simple to fabricate. Nevertheless, it has certain drawbacks, including shrinkage during polymerization, fragility, dimensional instability, and lengthy laboratory delivery times [22].

With the advent of advanced digital dentistry methods, the fabrication of intraoral devices like occlusal splints has become easy and accurate. The use of CAD/CAM systems (computer-aided design and manufacturing) has revolutionized the way dentistry is practiced, from the manual methods to digital methods [12,24,25]. The digital method allows registration of data directly from the patient by an intraoral scanner, or the impression of the cast can be used to digitalize patient data using a desktop scanner [26,27].

The digitalized data can be used to make the occlusal splints, either by milling/subtractive or additive/3D-printing techniques. These digital approaches have superior speed, stability in dimensions, improved retention, and increased repeatability [23,28,29]. Problems with the subtractive process include wastage of material, intrinsic limitations imposed by the machining equipment, and characteristics of the materials used. On the other hand, the 3-D printing technology divides the 3-D data of the samples (CAD design) into multi-slice images and then builds a designed shape using a simple layering process [22]. Furthermore, 3-D printing uses much less material than milling does, it essentially eliminates material loss, and it allows future used of excess material [17,23,28].

For milling, various types of machines are available, including 2-axis and 5-axis; it is recommended to use the 4- or 5-axis machines for milling complex structures like occlusal splints [30]. Similarly, various types of 3-D printing are available, such as fused deposition modelling (FDM), stereolithography (SLA/SLG), selective laser sintering (SLS), selective laser melting (SLM), powder binder printers (PBP), and digital light processing (DLP), but the ones most commonly used in dentistry are the SLA and DLP types [31,32].

Digital workflow is preferred by patients due to its ease of use and repeatability is case of loss or fracture of the splint, as the digital data stored can be used without needing to make new impressions [9,15,33,34]. However, it has been documented that the effectiveness of splint therapy is independent of the manufacturing techniques. For the treatment of CMD, the various splint manufacturing techniques are equally effective [5–7,9]. Previous studies [21,22] have also assessed the wear resistance of the different occlusal splints made by different fabrication techniques, but there is limited information on the retention of occlusal splints. The retention in any removable prosthesis, such as complete or partial dentures, can be defined as the quality inherent in the prosthesis acting to resist the forces of dislodgement along the path of insertion/vertical movement away from the tissues. Likewise, OSs is a removable prosthesis, and retention is very important for its effective use. A common discussion topic is the importance of adequate retention in occlusal splints. Occlusal splints work by repositioning the mandible to the maxilla. However, to carry out this process effectively, an occlusal splint needs to be retained properly. Improper retention of the OSs leads to improperly directed force on the teeth and associated structures, resulting in more harm than benefit, and can negatively affect patient acceptance of the appliance. Various clinical and laboratory methods have been suggested for evaluation of retention. Basically, it is the amount of pull/push force required to remove the OSs from its seated position. Previously, authors used various devices to assess retention; for example. Avant [35] and Chandu et al [36] used a T apparatus, and Thombare et al [37] developed a retention-measuring apparatus.

Thus, the present study aimed to evaluate retention of occlusal splints fabricated by conventional, 3D-printed, and milled methods. The null hypothesis was that there would be no difference in the amount of force required to pull occlusal splints, fabricated using different methods, out of the cast.

Material and Methods

STUDY DESIGN:

The study was designed to evaluate retention of occlusal splints fabricated by conventional, 3D-printed, and milled methods. It was conducted at the College of Dentistry, King Khalid University, Abha KSA, and ethics clearance was obtained from the institute committee (IRB/KKUCOD/ETH/2022-23/058).

The study was conducted following a pre-determined protocol, as described in the flow chart (Figure 1). A maxillary typodont teeth jaw set was taken with the full complement of teeth (up to the second molar).

SAMPLE SIZE AND GROUP DIVISION:

To determine the number of samples needed, an appropriate sample size was calculated as 40 in each group using power analysis (G*Power v3.1.9.4, Heinrich-Heine-Universität, Dusseldorf, Germany) based on the results of 10 pilot experiments with effect size [f]=0.86; actual power=99.94%; power=99.9%; α=0.05. A total of 120 study models were made and divided into 3 groups based on the method of fabrication used for occlusal splints. Group A – Conventional Occlusal Splints (COSs); Group B – Milled Occlusal Splints (MOSs); and Group C – 3-D-Printed Occlusal Splints (3-DP OSs) (n=40/group).

FABRICATION OF CONVENTIONAL OCCLUSAL SPLINT:

The primary impression with irreversible hydrocolloid was made for the maxillary and mandibular jaw set with complete dentition (up to second molar). It was poured and the primary casts were made. The base of the cast was poured with the base-former. The occlusal splint was planned to be made on the maxillary cast, so the surveying of the maxillary cast was done using Ney’s Surveyor and an undercut of 0.02 inch was marked; up to this marking, the occlusal splint extension was determined. The marking was scribed on the cast, the cast was duplicated, and 40 casts were made and coded. Similarly, the mandibular cast was duplicated and coded. The casts were mounted on an articulator with average values (Whipmix Corporation, Louisville, KY) and the duplicated mandibular cast was mounted.

A split cast technique was Whip mix used to remount the splint for adjustment of occlusion after laboratory processing. Wherever required, slight changes were made to the pin before waxing the splint to ensure there was enough material (2–4 mm) over the premolars and molars. All samples were made following conventional flat surface occlusal splint design with canine-guided disocclusion for the upper arch, and were waxed in the same way to facilitate anterior guidance and posterior disocclusion. Each splint was invested on its master cast and underwent the standard process of flasking, dewaxing, and packing. The PMMA [Trevalon (Dentsply, Weybridge, England)] resin splint was heat cured following the manufacturer’s instructions.

After deflasking, the splints were mounted again in the same position in the articulator. Any change in the vertical occlusal dimension due to processing was documented. The distance between a fixed point on the upper and lower members of the articulator was measured using vernier callipers before and after processing. Any difference in measurement indicated errors in processing. After readjusting the articulator to ensure proper occlusal contact, the splints were carefully removed from their segmented casts, preventing any distortion of the acrylic resin. No adjustments were made for fit, except for removing any excess stone, acrylic resin bubbles, and flash from the surface. After minimal finishing and polishing, the splints were fitted with a C-shaped hook on the pre-molar molar area and in the anterior zone in relation to the incisal edges of central and lateral incisors for retention assessment. Any splints with defects (cracks, voids, porosity) were rejected and new splints were created (Figure 2A).

DESIGNING OF DIGITAL OCCLUSAL SPLINTS: Fabrication of milled and 3-D-printed occlusal splints was performed using computer numerical control (CNC), which is a manufacturing method. This was based on designing of the occlusal splint (CAD) and then fabrication (CAM/3-D Printing). The primary casts (both maxillary and mandibular jaw set) of the typodont jaw set were scanned using a laboratory desktop scanner (3Shape A/S, Holmens Kanal 7, 1060 Copenhagen, Denmark) individually (Figure 3A) and with intercuspation using the wax bite record.

The process for bite registration closely mirrored the traditional workflow. Following the bite registration, one side of the wax plate was trimmed to expose the buccal tooth surfaces and buccal cusps of the upper and lower jaw for scanning. The modified bite plates were then used to perform a buccal bite scan.

The scanned data were saved in standard tessellation language (STL) format. Then, occlusal splint (conventional design type with canine-guided disocclusion) over the virtual cast model was designed using 3Shape Splint Studio (Splint Studio 2020.3 s), which helped in designing occlusal splints in a fully-guided workflow (Figure 3B, 3C).

The designing of the occlusal splints started with marking the 3 points on the occlusal surface of the maxillary cast as guided by the software. These points helped in placing the cast over the virtual articulator and adjustment of the occlusal plane on it. Following this, the bite configuration was done by using the tooth outlines on the default plane in the software and the control spheres. The software provided the opportunity to adjust the virtual articulator setting, including the incisal table, incisal pin, and condylar segments (in the present study, the settings were done to achieve the occlusal splint thickness of 2–4 mm). After completion of design, the Run Articulation command was used to assess the minimum thickness of OSs. Then, the insertion direction as suggested by the software was used and undercut values were adjusted to 0.02 inch, similar to the OSs of the conventional method. This setting allowed the occlusal splint to cover the undercut only 0.02 inch. Once the design was considered satisfactory and was approved, the scan was stored in STL file format. Along with this, a C-shaped hook on the pre-molar molar area and mesial tips of the incisal edges of central incisors were designed as in the conventional method for the assessment of retention. The STL file of the designed occlusal splint was transferred to the software for milling and 3D printing (Figure 4A–4C).

MILLED OCCLUSAL SPLINTS: For the fabrication of milled OSs, the designed OS STL file was selected and transferred to CAD software (Ceramill Mind, Amann Girrbach, Koblach, Austria) and then to the milling software (Ceramill Motion 2; Amann Girrbach). The milled OSs were milled using a 5-axis machine (Ceramill Motion 2; Amann Girrbach) and poly- (methyl methacrylate) and methacrylic acid ester-based cross-linked resin blanks (Ceramill A-Splint, Amann Girrbach – 98.5×20.0 mm). After the milling was completed, the OSs were removed from the disk by cutting them from the blank, and the pins for support were cut manually (Figure 2B).

3-D-PRINTED OCCLUSAL SPLINTS:

The 3-D-printed OS were fabricated using 3-D printers from Formlab (Formlabs Inc located at 35 Medford St. Suite 201, Somerville, MA 02143, USA). The designed OS stl file was transferred to PreForm software (Formlabs, Inc) for print preparation. The prepared file was then sent to the 3-D printer (Form 3 3-D printer from Formlabs, Inc), which uses stereolithography (SLA) technology and has a build platform measuring 14.5×14.5 cm. The occlusal splints were made by printing a biocompatible photopolymer hybrid nanoceramic resin (Formlabs Dental SG Resin), with a layering thickness of 0.05 mm and a maximum laser speed of 5000 mm/s. The OSs were position at a 90° angle and the software determined the necessary support structures and their positioning. Supports were strategically placed on the buccal surface to ensure accurate reproduction of the occlusal and intaglio surfaces. After printing, the OSs were removed from the build platform and supports were taken off. The printed OSs were washed using Form Wash, which is a 96% ethanol solution in an ultrasonic bath, and then dried. The OSs were post-cured in Form Cure and then sterilized using gamma-ray sterilization.

Similar to the conventional technique, no adjustment was made for fit other than minimum finishing procedures (eg, removal of supports) for 3D-printed and milled OSs. The splints with any faults (cracks) were rejected and new splits were made (Figure 2C)

ASSESSMENT OF RETENTION OF OCCLUSAL SPLINTS:

For assessing the retention of the conventional, milled, and 3D-printed OSs, an electronic vertical pull device was used. This device helped in quantitatively calculating the amount of force required to pull the OSs from the cast when completely seated over it. The device consists of a platform over which the cast with the corresponding OS was kept. It has a vertical shaft attached to the digital recording monitor that shows the force values required to pull the OS from the cast. The rounded end of the C-shaped hook was attached to the vertical shaft of the device. After all the attachments were connected, the device was switched on. It slowly increases the pull force on the occlusal splint, and the moment the OS is pulled out from the cast the highest force shown on the monitor was recorded and considered as the retentive force for the sample. All the fabricated samples were assessed in a similar manner and the data were recorded and tabulated in an Excel sheet as retentive forces of the samples and subjected to statistical analysis (Figure 5A–5D).

STATISTICAL ANALYSIS:

For data analysis and performing statistical tests, we used jamovi, a free and open-source computer program [jamovi (Version 2.5)-computer software, retrieved from https://www.jamovi.org]. Quantitative data are presented as medians, means, and standard deviations to compare independent variables between different groups. One-way ANOVA was used to compare variables between 2 groups, followed by Tukey’s post hoc test for pairwise comparison [38]. A P value less than 0.05 was considered statistically significant.

Results

The results of the study showed that highest retentive force was recorded for conventional OSs 24.2 N and minimum in 3-D Printed Occlusal Splint as 1.87 N. Figure 6 the Box and Whisker plot and Table 1 shows the median, minimum and maximum values in 3 groups. The distribution of the outcome variable (retentive forces) by Shapiro Wilk normality test showed P value >0.05 (0.293).

Hence, one-way ANOVA test was applied to compare the mean retentive forces among the group. Table 2 shows mean±standard deviation for conventional, milled, and 3D-printed occlusal splints as 16.4±4.94; 6.42±2.13; 4.42±1.53, respectively.

Tukey’s post hoc test was used for pairwise comparison among groups (Table 3). A statistically significant difference was found between the retentive forces for conventional and digitally fabricated OS. The difference was less between milled and 3-D-printed occlusal splints, but it was statistically significant (P<0.001).

Discussion

LIMITATIONS AND FUTURE RECOMMENDATIONS:

Even though the study was performed with utmost accuracy, certain factors act as limitations to it. The in vitro nature of the study, accuracy of conventional (manual error, material properties, remaining monomer, processing errors), milled (material and machine limitations) and 3D-printed (materials which would be influenced by the number of layers, layer intensity, total thickness, UV intensity, post-processing method, number, and placement of support structures) splints were not taken into consideration. It is recommended that future studies take these points into consideration, along with the ease of fabrication in terms of cost, time, and manpower. Also, aging of samples and the effect of thermal fluctuation in the oral cavity were not considered. We recommend performing in vivo assessment for these splints. The occlusal splint is used as an occlusal appliance in bite treatment. Because the patient is requested to wear the occlusal splint almost all the time, it is important that the splint be properly seated in the mouth, without any interspaces between the splint and the tooth; otherwise, the inhibition effect is reduced, and the therapeutic effect becomes uncertain. Therefore, it is necessary to understand the effects of repeated insertion and removal of the occlusal splint on tooth arrangement and retention. Thus, further studies are needed that focus on which occlusal splint design is best for maintaining the effect, as well as research into temporomandibular joint changes after removal of the splint.

Conclusions

Based on the current in vitro study, it can be concluded that the retentive force for the conventionally-produced splints was stronger than those produced digitally. The result showed significant variations in retention between conventionally-made, milled, and 3D-printed splints, with digitally-made OSs achieving clinically adequate retention (3D-printed occlusal splint retention was 4.42±1.53, which can be considered close to the lower limit of the clinically acceptable range) to maintain its position without causing trauma to the oral structures.