Introduction
Dental implants are increasingly becoming popular because a large section of clinicians believe that natural tooth restoration techniques are inferior to dental implants as implants have a longer life than natural tooth and are less susceptible to biological changes. There are various types of dental implants that support crown, bridges, and dentures, and act as orthodontic anchors and facial prosthesis. The basis of the modern dental implant is a biological process in which titanium forms a bond with bone. Dental fixture is first implanted and then a prosthetic is added.
Over the years, various implant impression techniques, such as transfer, pick-up and splint, have been introduced. Some of these techniques are more accurate surgically, but are biologically and technically inferior. These procedures depend on several factors, such as the depth of implants, opposite denture, age, sex and the number of implants. Success rate of implants depends on many factors such as the condition of the patient, drugs used, the health of the tissue around the implant location, selection of the type of implant (splint or non-splint), the position of the implant and biomechanical forces created during chewing (occlusion related). Risks involved in the fixed implant process can be divided into two major categories, 1) surgical (i.e. excess bleeding or nerve damage during surgery) and 2) long-term biological and technical (implant fracture, soft tissue damage etc.). This paper will mainly focus on the long term functioning and delivery of the fixed implants and how that can lead to biological and technical issues. The paper will also discuss how some of those issues can be resolved.
Long-term Biological and Mechanical Issues in Fixed Implants
One of the first issue faced by patients who undergoes fixed implant is the stability of a dental implant. As will be discussed in the subsequent sections, the instability of fixed implant screws is the largest contributor among all type of technical issues. For single crown implants, peri-implantitis (vertical bone loss and soft tissue damage) and screw loosening are the two most frequently observed risks. For fixed complete dentures (FPDs), peri-implantitis, superstructure damage and prosthetic screw loosening are the three most frequently observed risks. In the long run, (over 10 years), fracture and wear of tooth structure become the most common form of risk for fixed implant prosthesis. However, the risk level decreases significantly for metal ceramic prostheses than the traditional gold-acrylic prostheses.
Splint and non-Splint Techniques: Comparison
Lee et al. (2009) did a meta-data analysis using the already published articles to find out the accuracy of implant impressions. Lee et al. analyzed seventeen studies to compare between splint and non-splint techniques. Seven out of seventeen studies advocated splint technique while only three advocated non-splint techniques. Among the seventeen studies used for the comparison, fourteen studies compared between pick-up and transfer techniques. Five studies indicated that pick-up technique is more accurate in terms of implant impression. Only two studies indicated transfer to be a better technique while seven studies indicated no significant difference between the two. From a clinical perspective, pick-up and transfer are recommended when the number of implants is three or less. In fact, transfer techniques are not recommended for not easy to reach teeth.
Main Issues: Fixed Implant Crowns and FPDs
Fixed implant prosthesis has both surgical and prosthodontics issues. The survival rate of fixed implants and implant supported crowns is very high. However, in the long run, implant-supported crowns can develop many biological and technical issues. Especially, issues such as developing soft tissues near the crowns, bone loss and loosening of the screw are common, requiring additional chair hours for the clinicians after the procedure of fixed implants is completed.
Solely implant-supported implants and combined tooth-implant-supported fixed partial dentures are two methods used by the clinicians. Orthodox method still suggests combined tooth-implant-supported method over solely implant-supported FPDs. However, after several studies indicated that the survival rate of implants is lower for combined tooth-implant-supported fixed partial dentures than solely implant-supported FPDs, many clinicians are now using solely implant-supported FPDs. Nevertheless, combined tooth-implant-supported still remains more popular among patients.
Many studies have been conducted to understand the performance of combined tooth-implant-supported FPDs and the main biological and technical issue involved with this process. Nicklaus Lang et al. did a study on combined tooth-implant-supported fixed partial dentures (FPDs) and solely implant-supported FPDs. It was observed that the survival rate of implants for combined tooth-implant-supported FPDs was high (90.1%) after 5 years, but significantly lower than solely implant-supported FPDs (95.4%). The survival rate of FPDs for solely implant-supported FPD process is 95%, which is slightly higher than combined tooth-implant-supported process (94.1%) after 5 years. The study found no significant difference between the survival rate of the tooth (97.8%) and implant abutments (97.6%) after 5 years. Implant supported single crowns require surgical precision as well as a superior understanding of the implant and prosthesis process. Fixed implant prosthesis can pose several challenges in the long term. Ronald E Jung et al. (2007) did a meta-data analysis on the survival rate of implant-supported single crowns. The study found that 96.8% of the implants supporting single crowns and 94.5% of the single crowns survived after 5 years. There is a trend for using hybrid single crowns instead of using only metal or ceramic crowns. Jung et al. (2007) found that the survival rate of metal-ceramic hybrid crowns was much higher (95.4%) than all the ceramic crowns (91.2%). This meta-data analysis also found several biological and technical issues after 5 years of implant.
Biological Issues
Biological issues, such as bone loss and soft tissue near the fixed implant, are not uncommon. Jung et al. (2007) found that 9.7% of the fixed implant single crowns develop soft tissue near the single crowns and 6.3% shows bone loss more than 2 mm over a period of 5 years. Like fixed implant single crowns, FPDs also develop soft tissues around the implant. In about 8.6% of the cases, soft tissue related complications develop in FPDs after 5 years. As discussed above, the survival rate of implant for solely implant-supported FPDs is more than combined tooth-implant-supported FPDs. It is preferred from the perspective of rehabilitation. However, patient-centered issues, risk assessments of the residual dentition and anatomical aspects still justify combined tooth-implant-supported FPDs over solely implant-supported FPDs.
Mechanical and Technical Issues
Implant fracture is not uncommon and only a small percentage (0.14%) of fixed implants develops fracture after 5 years. The most common form of technical problem in fixed implants is screw and abutment loosening. In more than 12.7% of the implant-supported single crowns, screw and abutment loosening happens within first 5 years of implant. In the case of FPDs, the main technical issues are implant fracture, the connection between tooth and implant and superstructure related complications. The number of cases of implant fracture is low (0.4%) after 5 years. Connection related problems (Loose screw) and superstructure related problems are the two most (7.3% and 14% respectively) frequent technical problem for FPDs.
Material
Apart from biological and technical factors, material is another important factor that can significantly increase or decrease the survival and success rates of fixed implants. For implant abutments, two most used materials are zirconia and titanium. Both of these materials are known to last for a long time without decay. Zembic et al. (2009) in their study on abutment materials found that after three years of fixed implant, zirconia and titanium both have 100% survival rate. However, the result of this study is ineffective as it only captures results after three years. For abutments, it is important to understand the performance of zirconia and titanium after 10 years or more. Zirconia and titanium also affect the soft tissues discoloration around the abutment in a similar way.
Fixed implant crowns have a high failure rate. However, it is not easy to understand the causes of crown failure. Ceramic-metal crowns that are used by most of the clinicians develop complications in over 10% of the cases over a 5 year period after the implant. It is believed that factors such as the number of supporting implants, type of restoration, location in the dental arch, type of occlusion, patient habits, gender, age and opposing dentition influence the wear and tear of the fixed implant crowns.
Occlusion: Issues stemming from Fixed Implants
Occlusion is one of the most common problems among fixed implant patients. Occlusion can happen due to many reasons. In fact, in many cases, clinicians are not sure why occlusion happened after a fixed implant. If a tooth is removed from a complete arch, some defects of the teeth may trigger malocclusion. Occlusion is a controversial area with clinicians mostly disagreeing with each other on the causes and the treatments for malocclusion. However, most of the clinicians now agree that occlusion is a dynamic process that changes over the lifetime of the patient. There is no consensus among clinicians on the possible reasons behind these dynamic changes in occlusion. Local teeth health, general health as well as biomechanical factors may have a significant influence on occlusion. Almost no research exists in this area, and so little is known about it. Carlsson (2009) in his paper concluded that occlusion may have less impact or importance on implant restorations outcomes. Additionally, he concluded that no single occlusion design is proven to be better than others.
Kinsel and Lin (2009) found out that opposing dentition, bruxism and not wearing implant protective device are the three most significant causes of ceramic crown failure. The study found that when metal-ceramic crowns have an opposing dentition that is another implant-supported restoration, then the odds of ceramic fracture goes up thirteen times in comparison to an opposing dentition that is natural teeth. Patients with bruxism habit have a 100% higher chance of ceramic crown fracture compared to patients without a bruxism habit. One of the reasons for such failure is the load coming from occlusion on the implants. Kim et al. (2004) in their study found that occlusion impacts fixed implants differently than it does natural teeth. For different types of clinical situations, the impact is different. For example, in the case of the full arched fixed prosthesis, it is important to ensure that there is no working and balancing contact on the cantilever. However, occlusion is patient specific and the main idea is to maintain implant load within the physiological limit for long-term survival of the implant prostheses.
Increasing the Survival rate of Fixed Implants
Decrease Chances of Screw Loosening
As discussed above, for both fixed implant single crowns and FPDs, the most common technical problem is screw loosening, fracture and fracture of the superstructure (framework). There are several methods used to improve the process. 3i dental implants are one of the techniques that can reduce the screw loosening and superstructure related defects significantly. 3i Implants come with a bone-bonding component. This means that implant screw can be placed even in low bone density areas. This helps in several ways. Firstly, patients no longer need to wear poorly fitted dentures. Secondly, the 3i bone bonding component helps in quicker recovery. Finally, 3i implants reduce screw loosening and superstructure related problem significantly. 3i dental implants are sprayed with calcium phosphate to enhance the bone integration process. 3i screws actually contain calcium phosphate, which reduces chances of delamination. Kreissl et al. (2007) conducted a study on edentulous patients who were fitted with 3i-implants. These patients received different types of 3i implants, namely splint, FPD bridges and cantilevers, and single crowns. Without 3i-implants, the survival rate of FPDs was 90.1% after 5 years. However, patients with 3i-implants showed a survival rate of 94.5% after an observation period of 5 years. The incidence of screw loosening went down from 7.4% to 6.7% and implant porcelain fracture went down from 8.4% to 5.7%. In fact, superstructure related complications went down from 14% to less than 1%. FPD bridges and splinted crowns showed maximum survival and success rates for implants. However, even with 3i-implants, the success rate for FPD cantilevers was poor.
Reduce Chances of Malocclusion
As discussed above, malocclusion is not uncommon among patients who undergo fixed implant. Especially, those who use prefabricated implant bridges due to the low cost of the implant are exposed to a higher risk of malocclusion. Tooth extraction without subsequent restorative surgery is the most common cause of tooth movement. Therefore, to reduce the chances of occlusion related complications, it is important to use restorative materials that have better wear characteristics. Traditional resin based composites used for fixed implant restorations are prone to wear, which may lead to malocclusion. In fact, ceramics used in fixed implant crowns opposing teeth can cause quick wear of the ceramic material. Therefore, it is important to use only those materials that have wear characteristics similar to that of enamel. Additionally, simple and traditional techniques such as jaw registration and different occlusal concepts can help in better occlusion of implant supported prosthesis. However, the best way to reduce occlusion related complications is to monitor patients on an ongoing basis after fixed implant, which is a factor affecting occlusion, changes over time.
Increase Life of Prostheses
As discussed in the previous section that dental implants are most commonly restored using fixed implant metal-ceramic crowns. However, metal-ceramic crowns have a high risk of fracture in implant supported restorations than tooth-supported restorations. Therefore, to reduce the chances of failure, clinicians should select the most appropriate prosthesis based on the ancillary factors to increase the likelihood of successful prosthodontic treatment without the need for future remedial corrections. For example, a clinician can select fixed implant-supported restoration over tooth-supported restoration when the opposing dentition is natural teeth. Additionally, to maintain a load on the prosthesis within a physiologically tolerable limit coming from occlusion, increased support area, reduced force magnification and improved force direction are indispensable factors. However, as cited multiple times within the paper that full mystery around occlusion is still not fully understood. Therefore, the best way to increase the survival rate of fixed implant prostheses is to create customized treatment plan for individuals and use implant occlusion highly individualized to suit the need. For patients who cannot afford customized fixed-implants and may go for standard arches, regular reevaluation and adjustment are required for long term success.
Future Scope
Dental implant is a very active area of research. Dental erosion is a common phenomenon and so is dental implants. Dental implants will see a huge improvement in the coming days. Due to huge advancement in the CAD/CAM technology and machining capability, custom made implants are becoming easier and less costly (Gaviria et al., 2014). Usage of individualized fixed implants will certainly help increase the success rate of all types of fixed implant procedures.
Other technologies such as micro casting and nanotechnology-based implants will help in better protein absorption and cell adhesion. Using nanotechnology to deposit zirconia, alumina or calcium phosphate to coat titanium implant will be more precise in future. Additionally, nanotechnology can help in coating the titanium surfaces with bone stimulating agents such as bone morphogenetic proteins (Gaviria et al., 2014). This will help reduce peri-implantitis, a major risk associated with fixed implant.
As discussed in the previous section that bone density is crucial to dental implants and a good density can significantly reduce the risk of screw loosening in the long term. This can be improved by finding out a dense bone location in the jaw using techniques such as densitometric measurements, computer tomography, dental cone beam CT and dual energy X-ray (Gaviria et al., 2014). These will help determine the jaw bone density and best possible locations for fixed implants.
Conclusion
Tooth loss is a very common problem and dental implant as a result is also becoming a common practice. This has led to a significant increase in research on dental implant materials, techniques and designs. However, still there is 10% failure rate for fixed implants within 5 years of the implant. Loosening of the implant screw, and peri-implantitis is the two most common risks associated with fixed implants in the long run. There are many factors that can cause these problems. Some of the major factors include the bone density around the implant, health of soft tissues, occlusion habits, the health of the patient, fixed implant process used, bruxism and not wearing implant protective device.
Splint is a better technique when the number of implants required is high but pick-up and transfer work equally well when number of implants required are 3 or less. From a technical perspective, erosion and fracture of crown material are also an issue in the long run. Even the widely used metal-ceramic prostheses have very high fracture rate beyond 5 years. As gum arch, dental health and occlusion habits are very person specific, standard made fixed implants may not be as effective as custom made ones. Due to recent advancement in nanotechnology and imaging techniques, customized implants based on the patient needs are available at a lower cost. Design requirements such as shape, type of implant, the biomechanical behavior of the material, bone quality and medical condition of the patient are important to determine the best possible fixed implant technique. More research is required in the area of the implant and its long-term impacts, to better understand design parameters, surface treatment technologies and analysis techniques.
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