In the past, the area missing a tooth could be filled by bridge implants by eroding the enamel on neighboring teeth. In cases where there was a large gap or no teeth to use as connectors, removable implants were used. In other words, present healthy teeth would be treated to compensate for the missing teeth; if the number and health of the remaining teeth was not sufficient, removable implants would be produced that gave the “ready to fall out” feeling. The imperfections of this type of implant led scientists to pursue new ideas.

Scientists have tried to fill in missing teeth with artificial roots and with implants mounted over these by mimicking the existing tooth root. This way, there is no need to treat and erode the healthy enamel of neighboring teeth in the gap. These artificial roots, which are positioned into the jaw bone by a surgical operation, are called a “tooth implant.” Modern-day implants are generally composed of two main parts.

One of these is the fixture part which is secured into the jaw bone by screws; the other is the support part (abutment) which mimics the visible parts of the tooth in the mouth. Approximately 3-4 months of curing time is required for the fusion of bone and implant (osseointegration), which is screwed together by a surgical operation. Towards the end of this time, the implant is built over the abutment. The length of time necessary for the implant to integrate with the bone to replace a tooth which can be extracted so quickly is rather noteworthy despite ongoing studies to reduce the wait time and successful attempts to do so.

Employed materials

Even though the main logic for implants is the mimicry of the tooth and the root, the materials utilized are quite variable. The history of such implants goes back thousands of years. The employment of bamboo sticks in tooth restorations could be seen in some civilizations, such as ancient China. Mayans used sea shells, which were recently shown to be biologically suitable as implant material. Maggiolo produced a tooth root by using gold, in 1809. In the beginning of the 20th century, Lambotte prepared implants using materials such as aluminum, silver, brass, copper, and gold.

These newer implant methods have failed because of the insufficient strength of the majority of the materials and the failure to fit into biological tissues. The initial employment of titanium and its alloys in the 1950’s almost opened a new age in implantology and enabled the successful use of these materials up until today. However, these titanium implants, which are considered successful, have a very limited ability to mimic tooth roots:

• Once the implant is in place, for reasons such as aging and improper tooth brushing, tooth gum recession and resurfacing of the bright faces of the implants which mimic the root surface can take place. This leads to undesirable displays. Even though gum recession can also occur in natural teeth for similar reasons, this situation is not as disturbing as with implants. This is because the tooth root color is similar to that of the tooth surface.
• Under normal conditions, implants should be fused to integrate with the bone (osseointegration). Integrated implants are considered successful clinically. A similar case to this bondage, which is unwanted clinically, is the fusing of tooth and bone which can be seen generally in dead teeth and is called ankylosis. Many hardships can be encountered when the tooth that is fused with the jaw bone is required to be removed. The same thing also applies to the implant. If an implant is set to be extracted for a reason, even if just a small amount must be removed, the surrounding jaw bone must be removed as well. However, a natural tooth is not bonded so tightly to the jaw bone, as if the tooth is healthy, it is suspended over the jaw bone through small suspension-like structures. When a tooth is needed to be pulled as a result of trauma or tooth rotting, only these suspensions are detached; thus any damage to the jaw bone is averted. Present efforts to design similar mechanisms for implants have not been successful yet.
• When a disease develops in our natural teeth, we feel pain and take action before it is too late. However, diseases within implants are rarely encountered as pain; generally, when the pain is felt, the implant is already disconnected from the bone and it is lost. Toothache often felt as a strong pain is a blessing granted to us. If the implants were built with pain mechanisms, just like in our natural teeth, we could take action at the beginning of the disease and avoid the time and financial losses.
• Even a healthy tooth, which we think of as immobile, can move via microscopic ligaments for 0.1 mm around its root on a type of pad. The tooth root is created in a perfect fashion to resist the strong chewing power that can be generated from many directions in the mouth through these ligaments, which are wrapped around different sides and which come from different angles. However, these ligaments, which act as a suspension, do not exist in between the implant and jaw bone. Therefore, implants are almost immobile. Even though it may seem like a good thing for them to be immobile and tightly fixed, there may be intense power spots generated on the implant or jaw bone since these forces acting on the implants are transmitted to the jaw bone without being dispersed. Forces densely acting on small areas can lead to fractures of the material, including tiny screws or porcelain plating, and can damage the bone tissue. Investigators once understood that these tiny, suspension-like structures won’t form around the implant, so they engaged in mounting tiny Teflon or spring pieces inside the implant to generate suspension. However, these imitated suspensions failed to resist the chewing forces that can go up to 400 kg at times, and their use was aborted after a short time.
• Teeth can be examined by the method of tapping with small hand tools (percussion). These small impacts cause pain in case of an abscess at the tooth root. By taking necessary actions, the tooth can survive longer. It is impossible to do such determination and diagnosis with implants.

As it is seen, making less successful artificial implants as opposed to real teeth is quite a cumbersome, costly, painful, and time consuming job. Despite all of these imperfections, because of their numerous advantages over traditional implants, the popularity of dental implants continues to increase, and is preferred by dentists and patients. In addition, implants are seen as an inevitable treatment option when tooth loss occurs, and may ultimately provide patient relief in the long run.

Despite progress made in implants, it’s clear that our teeth were created in such a perfect way, down to the very smallest detail, that they cannot be replaced. Science is well aware of the fact that we will never fully be able to replicate tooth structure. Therefore, studies are shifting towards stem cell research. This way, instead of foreign substances, the missing tooth will be compensated as if by planting tooth seeds by using human’s own cells. Nonetheless, the opportunities that are discovered via all these efforts will never replace the natural teeth that were so perfectly created for us.

The discipline in which medicine and engineering truly meet and face new challenges is the field of “biomedical engineering,” an emerging discipline that is only a few decades old. In each sub-specialty of biomedical engineering, researchers study the human body, develop new materials and structures using engineering sciences, either as a treatment method for disease (e.g. artificial bone implants, artificial blood, vascular stents, cardiac valves, etc.) or to diagnose them (e.g. imaging methods and other diagnostic instruments in hospitals). Biomedical engineers face the incredible challenge of developing materials and devices that are compatible with biological systems and capable of working inside the human body to substitute bodily functions. Needless to say, the extreme complexity of the human body makes it impossible to mimic the original system or function of the organs in any way. However, even a poor replacement part or a functional improvement provides great benefit to the patients.

One of the most complex systems of the human body is the nervous system, which consists of the central area (the brain and the spinal cord) and the peripheral parts. The branch of biomedical engineering that deals with the nervous system is “neural engineering.” In this article, we will touch upon a specific subject in the broader area of neural engineering, that is, “neural prosthetics.”1 As the name implies, neural prosthetics is an area where engineering knowledge is utilized to treat neural disorders.

The building blocks of the nervous system are called “neurons.” Neurons generate electric pulses to communicate with each other. The fact that these electric pulses can be elicited by artificial means, i.e. by applying small electric currents to the neurons externally, forms the very foundation of the field of neural prosthetics. Neural engineers can input information into the nervous system by taking advantage of this phenomenon, called “neural stimulation.” Likewise, the information content of neuronal activity can be deciphered by recording the electrical pulses from the neurons and interpreting them according to neuronal function. This two way traffic, monitoring and controlling the neural activity, allows researchers in this field to develop methods of treatment for some sensory, motor, and psychological disorders.

Some of the most successful neural prosthetic applications have been in deep brain stimulation in Parkinson’s disease,

cochlear prosthesis in hearing impairment, bladder emptying and respiration in spinal cord injury, and vagus nerve (10th cranial nerve) stimulation in epilepsy and psychological depression. These are neural prostheses that are readily available as a treatment method for the given ailments. There is a whole host of others that are in the research and development phase. We will review a couple of examples.

In certain diseases of the inner ear hearing is lost as a result of damage to the hair cells inside the cochlea. In normal cochlea the sound information reaches these hair cells after traveling through the ear drum (tympanic membrane) and the structures of the middle ear, causing them to vibrate. This vibration of the hair cells is mechanically transported to the spiral ganglion cells that form the hearing (auditory) nerve. The hearing nerve carries the sound information to the brain in the form of electric pulses. The ganglion cells are healthy and functional even if the entire population of hair cells has been lost as a result of disease. Neural engineers take advantage of the fact that the spiral ganglion cells (which normally accept input from the hair cells) can be electrically stimulated, thus mimicking the function of the hair cells and producing the sensation of sound.2 During a simple surgical operation, the surgeon inserts an electrode into the ear canal which spirals into the lumen of the cochlea so that the sites where the electric current emits from the electrode are adjacent to the spiral ganglion cells (Figure 1). To summarize the principle of the operation; the audio signals are captured by a microphone, processed, converted into electric pulses (A in Figure 1), and transmitted to the implant over a transmitting antenna (B in Figure 1), or headpiece, held in place by magnets. The implant (C in Figure 1) applies the signals to the ganglion cells in the cochlea through tiny electrodes (D in Figure 1). The hearing nerve (auditory nerve) carries the sound information to the brain, where it is “heard.”

Even though the human spiral ganglion has tens of thousands of nerve cells that provide a rich sense of hearing, the cochlear implant, using only six stimulation contacts, can produce auditory perception with sufficient fidelity to enable a deaf individual to use an ordinary telephone.3 Individuals with cochlear implants can also improve their hearing with practice. Thousands of patients have been implanted with cochlear prostheses to date, including children.

The second neural prosthesis application we will review is the retinal prosthesis, which, unlike the cochlear implants, is still in the research phase. Retinitis pigmentosa and age-related macular degeneration both lead to photoreceptor degeneration in the eye and result in a significant visual deficit or blindness.4 A growing body of research supports the feasibility of replacing the function of the photoreceptors with an electronic device. 5–7 A retinal prosthesis is analogous to the cochlear implant in many ways. In a healthy retina, the photoreceptors initiate a neural signal in response to light. In a retinal prosthesis, electrical pulses are utilized to initiate a neural response in the remaining cells of the retina, the bipolar and ganglion cells. It is hypothesized that the perception of shapes and images will be possible through pattern stimulation of the retina. Initial results are encouraging, but the quality of vision that can be attained with this approach is still a question to be answered. A conceptual retinal prosthesis system is shown in Figure 2. The system consists of an external unit coupled to an implanted stimulator with a wireless link. A video camera in the external unit captures an image and converts it to digital data. The implanted unit receives the signal, recovers power and data from the signal, and generates the stimulating current. The stimulus pattern is applied to the retina via the electrode array, which contains distinct electrodes that interface at many locations on the retinal surface. Recent implants in human subjects suggest the feasibility of this approach where individuals attain perception of bright dots in the visual field called ‘phosphenes.’ Furthermore, blind subjects are able to perceive edges when a few of these bright dots are lined up in their visual field.

Figure 3: A conceptual diagram of a Brain-Computer Interface for high level spinal cord injury or patients with ‘locked-in syndrome’ (adapted from Wolpaw et al., see note 9). The recorded neural activity from the motor cortex is processed and converted into command signals to control, for instance, a wheelchair, or to generate electrical signals to activate hand muscles for grasping an object.

Both cochlear and retinal implants are sensory prostheses, i.e. aids for sensory impairments. Another family of neural prostheses deals with motor impairments. Severe motor disability results from high level spinal cord injuries (quadriplegia) where most of the body is paralyzed, sparing only some face, neck, and shoulder muscles. Quadriplegic individuals are in extreme need of a means to control their environment; they need to be in control of their wheelchairs, bed, the room temperature, lights, TV, etc. Because of the level of paralysis it is impossible for them to generate any control signal, except perhaps by sipping or puffing on the end of a tube, which produces a very poor control signal. In the case of a ‘locked-in syndrome’ the condition of the patient is even more serious, with only some functions remaining in the facial muscles. The term ‘brain-computer interface’ has been coined to refer to attempts whereby the motor output of the brain is recorded and interpreted to generate the control signals needed by these patients (Figure 3,9). The ultimate objective of this research will be accomplished when the patients are able to control anything they need to control in their environment, including a computer. The brain-computer interfaces vary in the invasiveness of the approach. The least invasive methods utilize the electroencephalogram (EEG) signals recorded from the scalp. Unfortunately, the signal quality is poor and only ‘on/off’ type of command signals can be generated using this method. In the most invasive, yet most successful applications, an array of electrodes is implanted directly into the motor cortex of the brain at a depth of a couple of millimeters. The recorded signals contain volitional information as the patient makes intentions to move their arms or legs. These signals can be controlled by the patient, and they can in turn be used to control their environment. The current level of success in this type of BCI allows the user to have three dimensional control of a robot arm. This is of invaluable benefit to a quadriplegic individual.

Concluding Remarks: Reflections on Divine Wisdom

Even the subtlest parts of the nervous system are extremely complex. Just to name a few examples, from the highest centers in the brain down to the skeletal muscles in a descending order; the neural circuits of the short-term memory in the hippocampus, fine motor control circuits of the cerebellum, central pattern generators in the spinal cord, and even the control of skeletal muscles in graceful movements of the limbs are impossible to reproduce by artificial means. The Seal of Divine Design is clearly visible in these neural systems, as they are far more complex, far more compact, and far more functionally efficient than any system engineered by mankind. If anything, the growing experience in neurosciences teaches us that the vertebrate nervous system is full of wonders of engineering design. Therefore, it is a great blessing to be a student of both neurosciences and engineering disciplines. This bestows neural engineers with a unique perspective to understand the beauty embroidered into the human nervous system and contemplate on the Divine Wisdom. In spiritual terms, we may think of the human nervous system as a window opening to the works of Divine Wisdom, with manifestations of His Beautiful Names at the brightest level. It is an overwhelming joy to be able to open this window a crack, once in a while, and take a little peek.

References

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