Anatomy, Modeling and Biomaterial Fabrication for Dental and Maxillofacial Applications

Implants have not always enjoyed a favorable reputation despite the fact that they have been used for many years to support dental prostheses. The function of a dental implant system is to restore dentition by providing a means of transmitting masticatory forces to the mandibular or maxillary bone. The importance of understanding the way in which the stresses and distortion acting in a dental implant and its surrounding bone structure are distributed is of paramount importance in the field of prosthetic replacement where the principal aim is to replace a damaged tooth so that the patient can function effectively. In response to occlusal forces as well as establishing normal dimensions of the peri-implant soft tissues, bone remodeling will take place during the first year of function. Changes in the internal state of stress in bone due to occlusal forces determine whether destructive or constructive bone remodeling will occur. The careful planning of functional occlusal loading can lead to a possible increase in bone-to-implant contact and maintain osseointegration. On the other hand, bone loss and/or component failure can be the result of insufficient load transfer or excessive loading.

From a macroscopic point of view, human bone appears in two forms. The most obvious difference between these two types of bone is their volume fraction of solids or relative densities. The term cortical or compact is used to categorize bone with a volume fraction of solids greater than 70 percent. On the other hand, bone with a volume fraction of solids less than 70 percent is referred to as cancellous or trabecular. Typically, most bones within the human body possess both types: a core of spongy cancellous bone is surrounded by an outer shell composed of a dense compact bone. The constitutive properties of cancellous bone are of vital importance as it is this bone that is in direct contact with the implant or prosthesis.

The masticatory system is the functional unit of the body primarily responsible for chewing, speaking and swallowing. The system is made up of bones, joints, ligaments, teeth and muscles. The system of mastication is a highly refined and complex unit. Gaining an in-depth knowledge into its biomechanics and functional anatomy is necessary to the study of occlusion. The mandible is a horseshoe-shaped bone supporting the lower teeth and makes up the lower portion of the facial skeleton. It is suspended below the maxilla by ligaments, other soft tissues, and muscles, which provide the mobility required to function with the maxilla. The energy that moves the mandible and allows function of the masticatory system is provided by muscles. There are four pairs of muscles (left and right side of the mandible) that make up a group known as the muscles of mastication.

The masticatory system is the functional unit of the body primarily responsible for chewing, speaking and swallowing. The system is made up of bones, joints, ligaments, teeth and muscles. The system of mastication is a highly refined and complex unit. Gaining an in-depth knowledge into its biomechanics and functional anatomy is necessary to the study of occlusion. The mandible is a horseshoe-shaped bone supporting the lower teeth and makes up the lower portion of the facial skeleton. It is suspended below the maxilla by ligaments, other soft tissues, and muscles, which provide the mobility required to function with the maxilla. The energy that moves the mandible and allows function of the masticatory system is provided by muscles. There are four pairs of muscles (left and right side of the mandible) that make up a group known as the muscles of mastication.

The most frequently used biomechanical analogy for the mandible has been the Class III lever, in which the condyle acts as a fulcrum, the masticatory muscles as applied force, and the bite pressure as resistance. Various workers have suggested either directly or indirectly, that there is little or no reactive force at either mandibular condyle. Mandibular movement occurs as a complex series of interrelated three-dimensional rotational and translational activities. Mandibular movement is limited by the ligaments and the articular surfaces of the temporomandibular joints as well as by the morphology and alignment of the teeth. The collective activities of masticatory muscles in different configurations have enabled us to perform a variety of mandibular movements.

Numerous muscles acting simultaneously on the mandible are utilized to create a bite force. Defining the involvement of each muscle to the creation of this bite force is the biggest challenge as it cannot be resolved using the conditions of static equilibrium. Estimation of the temporomandibular joint (TMJ) forces from mathematical models has a long history but has led to conflicting results. The biggest disagreement has been over whether the TMJ is even load bearing. To model completely all of the forces involved in the production of the TMJ reaction force will be difficult as all force vectors need to be established in three-dimensions. Three different approaches have been proposed to calculate all the muscle forces and the forces in the temporomandibular joints during biting.

The finite element method is a mathematical approach used to examine continua and structures. Typically, the problem at hand is too difficult to be resolved in a satisfactory manner using classical analytical means. Introduced initially as an approach used to explain structural mechanics problems, finite element analysis was rapidly acknowledged as a universal technique of mathematical approximation to all physical problems that can be modelled by a differential equation description. The application of finite element analysis in dentistry related to the deformations during functional loadings and in the design and analysis of implants accelerated after the 1980’s. This method has been widely accepted and applied in engineering and biomedical systems with increasing frequency for stress analyses of soft and hard tissues, bone and bone-prosthesis structures, fracture fixation devices, and dental implants and devices ever since. The finite element approach has also been applied in nanomechanical testing and nanoindentation to assess the biomechanical properties of nanocoatings on implants and devices.

The ability to extract data from computed tomography (CT) or any other appropriate imaging technology to generate the patient’s own model is already a common practice. For example, by integrating computerized modeling with medical imaging, it would be possible to determine the correct location, configuration, size and number of implants needed to address the patient’s functional and restorative needs. Furthermore, this approach can be used to define the form and mechanical requirements of implants and prostheses employed in the treatment of mandibular and maxillary fractures with fixation and reduction of the fracture obtained with minimal osteosynthesis plate bulk, number and size. This integrated system can be coupled with modern rapid prototyping such as 3D printing and laser sintering to produce superstructures and patient matched dental devices and guides.

The bone remodeling sequence is composed of three consecutive phases: resorption, reversal, and formation. The utilization of numerical models to simulate the fracture healing process may prove to be advantageous in determining the ideal mechanical-based reconstruction or treatment after an illness or accident.

During functional movements such as chewing, forces on the prosthesis will be transferred to the implants and this will result in stresses being generated within the bone surrounding the implants. The bone-implant interface is of great significance to osseointegration as the utilization of dental implants may alter the mechanical environment of the mandible. Bone remodeling occurs in the first year of function in response to occlusal forces and establishment of the normal dimensions of the periimplant soft tissues. The type of bone remodeling taking place in the bone tissue surrounding the implant will be governed by the variations in the internal stress state. Stress shielding and bone resorption will occur when no load is being transferred to the supporting tissues, while abnormally high stress concentration can lead to implant failure. For these reasons, it is essential to consider the effect of bone remodeling on the performance of dental implants and prostheses in order to improve its efficiency. The bone remodeling process around dental implants has been simulated in a number of studies using a variety of models. Mathematical algorithms such as nano-interactions and mechanosensory mechanisms have been incorporated into numerical models to describe bone formation and osseointegration of dental implants. Furthermore, strain energy density algorithm has been adapted to illustrate bone remodeling induced by implants and fixed partial dentures.

Bioceramics prior to the 1970s were utilized as implants to perform singular and biologically inert roles. The limitations with these manufactured materials as tissue substitutes were emphasized with the growing realization that tissues and cells of the human body function other different metabolic and regulatory roles. The demands of bioceramics have changed from sustaining a fundamentally physical function without provoking a host response to providing a more positive interaction with the host ever since. This has been complemented by increasing demands on medical devices to extend the duration of life in addition to improving its quality. More importantly, the exciting and potential opportunities associated with the use of nanobioceramics as body interactive materials, facilitating the body to heal, or promoting the regeneration of tissues, therefore restoring physiological functions. Major factors in determining the potential applications of a biomaterial are its biocompatibility and functionality. Furthermore, the bioceramic should not suffer any deformation when loaded under physiological situations. In terms of mechanical properties, the safety of ceramic components is related to their mechanical strength. As a result, improving the mechanical strength of ceramics is the primary objective as well as all the properties which are interrelated to strength.

Tissue engineering in recent years has taken a new direction by seizing the advantage of combining the use of living cells with three-dimensional ceramic scaffolds to deliver vital cells to the damaged site of the patient. Productive and feasible strategies have been initiated during the last few decades to combine bioceramic implants with biogenic materials to the field of cell growth and differentiation of osteogenic cells. The reconstruction of bone tissue utilizing nanocomposite bone grafts with composition, biomechanical, biological, physiochemical and structural features that mimic those of natural bone is an objective to be pursued.

Over the years, the research and applications of calcium phosphate materials as nanocoatings for dental and biomedical applications have undergone a revolution to become a state-of-the-art approach for improving osseointegration of implants and devices. Determination of stresses within a nanocoating is vital as its mechanical stability is governed by factors such as deposition method and heat treatments applied. The presence of external mechanical loading as well as the possibility of the coating to crack and spall because of inbuilt stresses (whether they are tensile or compressive) will affect the successful deployment of biomaterial implants. The most commonly used methods for characterizing the performance of micro- and nanocoatings on substrates can generally be divided into the measurement of coating properties and adhesion strength. Several excellent methods that can be used in thin film mechanical properties evaluation, and some of the commonly used methods are nanoindentation, tensile testing, scratch testing, adhesion and wear testing, pin-on-disk testing, pull-out test, and bending and bulge testing. Furthermore, the biomechanical characteristics of nanocoatings such as hydroxyapatite deposited on metallic substrates have also been examined using nanomechanical testing and nanoindentation simulated via the finite element approach.