Live Webinar: Revisiting occlusion in implant dentistry

In the second live webinar, Dr. Sang J. Lee reviews with us the occlusion for dental implants. FOR Education council member Dr. David Kim moderates the webinar.

Dr. Lee opens the webinar by pointing out that there are numerous theories related to the occlusion scheme and the use in complete dentures, tooth supported removal partial denture, and implant restoration. However, there is a lacking in literature to determine the level of scientific evidence on implant occlusion. With his lecture Dr. Lee is trying to solve and clarify some myth of occlusion in implant dentistry and provide clinical guidance which can be applied to daily practice. He starts with some complications that we see with implant restorations. Pretty often we see screw loosening in cement-retained implant crowns or screw-retained implant crowns. There are different reasons for this complication, so torque could be an issue, a deformed screw, screw material, or a damage the hex connection. And lastly, we can also think that this could be also related to occlusion, says Dr. Lee. He continues with an x-ray, on which we cannot really see if the implant has failed or not. In the present case, the implant was mobile. And the etiology is that some improper premature contact and overload could be an issue in this case. Even though that there is no visible inflammation around the implant, after removal granulation tissue is present. This could be related to implant occlusion or overload. Another very typical situation that we see in daily practices too and are large, very heavy prosthesis, and failures and fractures. The etiology could be also misfit, lack of vertical dimension, material framework, and lack of maintenance. Dr. Lee points out that occlusion also plays a big part when we get to see these fractures. 

Dr. Lee continues to look at the differences in biomechanics between an implant and a natural tooth. The big difference is that there is no PDL (periodontal ligament) present with a dental implant. The PDL allows the tooth to move around 25 to 100 microns. The implant has a direct bone contact which does not really allow movement. But literature shows a small movement of 5-10 microns which results from the bone resiliency. The PDL provides tactile sensitivity, which is higher with a natural tooth compared to an implant. It also works as a sharp resorption, which is absent with implant so occlusion or heavy load will be transmitted directly on to the implant. So, the size of the overload or occlusal issues on tooth and implant, because of the difference in implant biomechanics, are quite different. With overload on the teeth, we can see a thickening of the PDL, and then we can also see mobility, the wear facet, fremitus, and pain. However, in implants since there is no PDL, we see more mechanical complications first, such as screw loosening, fracture, abutment fracture, prosthesis fracture, implant fracture, and bone loss, too, highlights Dr. Lee. What about the occlusal force? he asks, and his lecture will focus more on the occlusal force and occlusal scheme. During normal biting, the anterior tooth has a force of around 30-40 Ncm, the premolar has 40-70 and the molar has 80-120 Ncm. According to literature, a person who has a bruxism or clenching can develop a force that is 3-10 times higher than a normal person. The correlation between parafunctional habits and implant complications is clear. Bruxers can generate forces that can cause fractures of bone or prosthesis. What is the definition? Depending on the implant position, the overload can be caused by parafunctional habit of occlusal force. According to Dr. Lee , overload is a strong force and parafunctional habit. At closer look, it consists of four components:

1. Magnitude – the force we consider
2. Duration
3. Distribution
4. Direction

These four factors affect and influence overload in the patient’s mouth. Depending on the position of the implant different forces will be applied. Parafunctional habits affect the magnitude as well. If an implant has a large implant crown or a cantilever, there will be more force applied. Duration is critical too, as the application can happen for hours and hours, i.e., during nocturnal bruxism. Lastly, the direction as in an implant placed off-axis which can lead to more cantilever leverage action and higher forces from the lateral. He emphasizes that overload is not only a magnitude, but those four different components. Overload can happen frequently and could be detrimental and could cause a lot of stress in the daily practice, such as mechanical and structural complications. Dr. Lee moves on to biological complications, asking what are typical biological complications related to overload and implant complication? There is prosthesis failure, abutment failure, veneering material, and screw fracture. He demonstrates them with clinical images of a maxillary fixed complete denture, also known as hybrid denture. After a couple of weeks of wear the veneering porcelain chipped off, which is an obvious sign of uncontrolled occlusion or guidance in the canine area. Other points are screw loosening or implant fracture. It is very unusual to see an implant fracture on the body of a tissue level implant with a regular size. Overload can cause more mechanical failures first before an implant failure occurs. Another biological complication related to overload is bone loss. Even if there is a good interproximal contact between the implant and the tooth, after years this contact can open. This is a common problem and a common complication, according to literature 52-75% of implant contacts show this. Why is this contact important? The contact protects the hard and soft tissue around the tooth and is maintaining the integrity and alignment of the arch. Food impaction can follow and the risk for peri-implant mucositis and peri-implantitis is present. This is a complication difficult to manage. The next point is bone loss and peri-implantitis, they could also be affected by the occlusion. This has been a controversial topic, though, shares Dr. Lee. He has written an article about this,, which has been reviewed and accepted, that concludes that occlusion is a predisposing factor for peri-implant disease. But how does this work? He shows a diagram that illustrates that plaque accumulation, poor oral hygiene, lack of maintenance or care, or prosthetic design could cause inflammation in the peri-implant mucosa. These factors, together with risk factors like history of periodontal disease, systematic disease, parafunctional habits, or a risk of occlusal overload, could cause more or accelerate peri-implantitis. Dr. Lee returns to the topic of overload, mentioning that one part of it is distribution, another is direction and duration as well. Together with magnitude they provide a better definition for overload. Associated factors involved are poor bone quantity, quality of the bone, existing systematic conditions that affect the patient’s healing ability or other systematic diseases or existing pathologies and inflammation. With these factors, more biological failures could occur, leading to bone loss around the implant, peri-implantitis and implant failure. This concept can help to analyze and evaluate a patient in a different way based on their medical and dental history, so we can control the occlusion or assess the risk factors of each patient. Dr. Lee mentions that there are two schools of thought, and they are quite controversial. One school of thought shows that literature does not support the belief that occlusal factors lead to loss of osseointegration and implant failure. It sees the mechanical failure as the first issue. There is currently insufficient and limited scientific evidence on implant occlusion. The other school of thought is that in the presence of plaque-induced inflammation or other systemic diseases, it can lead to bone loss around the implant or loss of osseointegration. This school of thoughts tells us that bone loss and de-osseointegration are associated with a biological complication. Reviewing classic literature on overload and implant failure, Dr. Lee summarizes some findings like if the occlusal force is between a certain threshold it helps to remodel bone. However, if this threshold is crossed the occlusal force can cause bone micro-fractures which will interfere with bone remodeling and cause absorption. There is a clear link between occlusal overload and peri-implant disease. How does this affect our daily practice? What can we apply in our fixed implant restorations? Overall, the biomechanically controlled occlusion is essential for implant success and longevity of implants. 

Dr. Lee moves on to the next topic of clinical guidelines. He explains that there a different concepts of occlusion, so we need to look at the definitions. The first is mutually protected occlusion, which is applied to fixed prosthodontics and some removable partial denture (RPD) cases. Mutually protected occlusion includes four factors: anterior guidance (during protrusive movements the posterior teeth separate). The next is posterior stops and posterior disocclusion during excursion. Next is that occlusion is a group function. Group function is a multiple contact relation between the maxillary and mandibular teeth in lateral movements on the working side, whereby the simultaneously contact of several teeth act as a group of distribution occlusal force. So, we see simultaneous multiple contacts in the working side, but not in the non-working side contacts. The third concept that he usually applies with complete dentures and some cases of removable partial denture, is balanced occlusion. Balanced occlusion is a cross-arch bilateral contact during excursive and protrusive movements. Dr. Lee illustrates these explanations with clinical images. Returning to the topic of fixed implant restorations, he shares that when a diagnostic work-up is created, these three factors need to be considered: vertical dimension, occlusal plane, and the arch form. He points out when there are excursive movements or protrusive movements, there should also be a good separation, but the separation needs to be very shallow. This requires short contacts and short cusps on the posterior teeth. This concept is accepted in dentistry since many years. What is the scientific evidence on why do we need to apply anterior guidance and mutually protected occlusion in fixed prosthodontics or implant prosthodontics? The literature shows that anterior teeth have more perception, that means that discretionary perception of axial or lateral load with anterior teeth is 5 to 10 times greater sensitivity than posterior teeth. Therefore, we want to guide with the anterior teeth that will allow the patients and us to see how much force is applied. Classic literature shows that with the anterior guidance and mutually protected occlusion, it causes a least traumatic occlusal scheme as shown in the EMG study. If we apply canine guidance, it will lessen and reduce the muscle of mastication activity and lessen the tension and decrease the force magnitude. According to Dr. Lee. this is the evidence why we want to apply mutually protected occlusion and anterior guidance in most of the fixed implants. How do we achieve this mutually protected occlusion? What are the things involved? Anterior guidance is dictated by facial aesthetic, phonetic, and impending function. And posteriorly, the components involved are intercondylar guidance, such as condylar guidance that is related to an angle and then also a protrusive interocclusal record. These are all factors in the posterior to allow us to achieve posterior disocclusion. 

How do we achieve ideal occlusion with implants? Dr. Lee summarizes in one slide the clinical guidance for optimal implant occlusion. It could be applied depending on the treatment type, and guidance on teeth or implants and parafunctional habits. With a prosthesis or complete denture, we want to achieve a bilateral balanced occlusion. Sharing the load on occlusion contacts is important. Therefore, if canines or anterior natural teeth are present that we can guide on we prefer to guide on those natural teeth and provide a canine guidance. However, implants are involved in guidance such as in the canine, we prefer and apply the group function, explains Dr. Lee. Even contacts, freedom in centric, reduced inclination, and size of cantilever are also the key factor. An occlusal night or day guard is very important. Next is the design of the prosthetics. Splinting implants is a question here. Literature doesn’t show clear differences in clinical and histological views between individual or splinted implant restorations. So therefore, the recommendation is to leave multiple implants next to each other as separate implants and not to splint them. There is an increase in oral hygiene, but repair or replacement will be much easier. If the implants are short, the crown-to-implant ratio is big, if bone quality is an issue or the patient shows very heavy parafunctional habits, splinting the implants is recommended. Nowadays most of the implants have an internal hex connection. Implants placed in the 1990s and early 2000s may have an external hex and if several of these implants are next to each other they should be splinted to avoid mechanical complications due to load on the external connection. A proper interproximal contact between multiple implants is difficult to establish as the implants do not move and there is no PDL. In these cases, splinting may be a good option. What about cantilever and occlusal design? Cantilever have risk factors involved such as non-axial load causing biomechanical complications. The risk is more on the cuspal inclination or the size of the occlusal table. A higher cusp inclination and large occlusal table may cause more force on the implants and cause biomechanical complications. The implant position should be along the axis of the load, along the axis of the bit force and the angulation is important, too. Also, the distance the teeth or the implant position would need to be evenly distributed and positioned where the teeth would be to minimize the cantilever. The size of the occlusal table is critical. When a patient with an implant is asked to bite down, do they feel that they are biting on the restoration? The patient feels a proprioception around the tooth because of the PDL and the cells involved. The sensory cells involved are around 24 microns. With implants no PDL is involved and osseoperception is present, through peripheral receptors, nerve endings in the periosteum and the muscles of mastication, the oral mucosa and the temporomandibular joint may compensate to some degree to absence of the PDL. Therefore, even though there is a direct implant contact and osseointegration, a patient still feels a contact, which call it osseoperception. The osseoperception is less sensitive and is about 15 microns. This is important to consider especially in full arch rehabilitations. The literature shows that in the early stage of osseointegration, osseoperception is very minimal. During the rehabilitation process with the implant supported prosthesis, we should not rely on the patient's perception of occlusion, especially with the immediate loading protocol. There is a gradual increase in tactile perception over time after a certain period, approximately after three months. Dr. Lee reminds us that when we do immediate loading and check our contacts, it's important to understand the patient may not have osseoperception during that time. During the time of provisionalization, the occlusal analysis and occlusal adjustments should be made to ensure a bilateral balanced occlusion is present in the final prosthesis. Dr. Lee shares another clinical case of a patient restored with implants in the maxillary arch. An x-ray shows severe periodontal disease, and the plan was to remove the implants and go for a full arch rehabilitation. The mandible was restored with a fixed complete denture and individual crowns. The canine guidance in the maxilla is on implants, so in this case group function should be considered. In this case group function means that during laterotrusion, there are multiple contacts on the posterior teeth on the working side, however, not in non-working side. He summarizes and closes with a diagram:  so, if there is anterior sextant and including canine or excluding canine, then you want to have anterior guidance, and if implants are included as a part of canine, that you want to have a group function. If the natural teeth are involved and the canine, we should aim for a canine guidance and shallow guidance, too. Posterior, if there are single units, and we still want to have anterior guidance on the patient existing function or the teeth or restoration. If there are multiple units involved in the posterior, still, we want to follow the existing patient's function to achieve mutually protected occlusion. If it's one arch, such as one arch of treatment is in complete denture or RPD, complete denture, we want to achieve the biologic balance in form of lingualized occlusion. He demonstrates this with a clinical case: the case is a no. 3 restoration, fixed single implant on the posterior tooth. So, you want to have the mutually protected occlusion, that means that there shouldn't be any working side and non-working side contacts. If the patient has a group function, maintain the group function or if the patient has a canine guidance, maintain the canine guidance because this is just only one posterior tooth implant restoration. He recommends using these three different articulating papers. The first one is a Shimstock which is 12 microns. The second one is the regular size articulating paper, which is 21 microns, the third one is 150 microns it has its own different purposes using three different articulating papers for occlusal adjustment in implant restoration. First, he removes the red markings, so more contacts are coming up on other areas. Blue markings represent the excursive movements, which should not be visible in order to keep the patient’s existing canine guidance and in this case, group function. We should aim for good contacts with the 21 microns articulating paper.  First, we will use Shimstock and then to make sure that there is no hyperocclusion on the implant or not.  And if we identify any hyperocclusion with the Shimstock, then we want to use 150 microns for any excursive movements, and then use the red, which is 21 microns just to identify the centric stops. As this patient has a hyperocclusion on the implant, we need to remove red and blue, which is both centric stops and excursive movement at the same time. And the Shimstock? If a patient is missing the Shimstock marks on the posterior teeth, in several years these teeth will need to be replaced as they will cause more occlusal force in the anterior. Dr. Lee shares that he applies this differently from previous and classic adjustments. Next, he shows the use of a machine to track jaw movements and color map those. The machine was developed at HSDM. According to Dr. Lee, this shows that now that we can track mandibular movements with digital dentistry because of advances in digital dentistry. So, the mandibular movements, the parameters that we want to understand in order to achieve this occlusal scheme, and then gear and customize for each patient is a sagittal and horizontal condylar inclination, lateral condylar inclination, immediate side shift, and occlusal stability.

Dr. Lee closes the webinar by summarizing that it's important to understand the overload and contributing factors. Biological differences between the implants of teeth are determining factor for design occlusion, and it's important that we understand and apply biomechanically controlled occlusion, which is actually essential for success and longevity of dental implants. Occlusion is not a myth but the foundation of prosthodontics.