ABSTRACT
Background: Replacement of tooth loss has been a topic of constant research in dentinstry, given the challenges that dentists face during the treatment through dental implants. Amongst such challenges is sinus lift surgery and controlling the stress encountered during the process. Alternatives to the sinus lift surgery include the use of short straight implants and tilted implants that allow dentists to control stress distribution and make a successful implant.
The Aim: The aim was to identify an alternative to sinus lift surgery in order to minimize its drawbacks through a comparison between two alternatives to sinus lift surgery (short vs tilted)’ around the maxillary sinus.
Material and Methods: A human jaw model of cortical bone layer, trabecular bone layer and commercially pure titanium implants was utilized in this regard and the analysis conducted on the maxilla region through a MSC NASTRAN FEA model. A 3D simulation of a human adult jaw was utilized to test the short and tilted implants using loads of 300N at a 25o inclination (oblique occlusal) to the bucco-palatinal axis of the implants in both models. The FE model of this study was derived from a C.T scan of an adult male patient to simulate the maxilla, maxillary sinus and alveolar bone.
Results: Through the simulation, Von Mises principal stress distribution was charted along with Cortical bone and Trabecular bone minimum and maximum principal stress distribution for both the models i.e. short straight implants and tilted dental implants. The use of the oblique occlusal loads increased the stress on the Trabecular bone, and had lower stress in the Cortical bone during the simulation of the tilted implants in the maxilla region.
Conclusion: It was found that the use of tilted implants is a better alternative over short implants, given its success rate in stress distribution as alternative to sinus lift surgery in the maxillary region given the simulation results. It is recommended that future research within this research area through the application of FE analysis be supported with a qualitative modelling for better and more reliable results.
Introduction to Tooth Loss & its Effect
The replacement of missing teeth has been one of the most important aspects of dentistry in the past decades (Irish, 2004). In order to reach a proper treatment plan, one must have a full understanding of dental anatomy; the science that studies appearance, development and morphology of teeth. Teeth are bone like structures composed of two parts, a root that is implanted in the jawbone and a crown that is exposed to the oral cavity (Saber et al., 2015). Jawbones are composed of two parts, a fixed part; the maxilla responsible for stability and a lower mobile jaw; the mandible. The two jaws perform the action of mastication. (American Dental Association, 2010). The alveolar bone is the part that surrounds the roots and it forms the socket, similar to all bones in the body they are continuously undergoing through cyclic resorption and reformation. (Linkow and Chercheve, 1970)
Tooth loss is multi-factorial and is mainly encountered in older patients. The case of complete loss of the dentition is called edentoulism, while the loss of a few numbers of teeth is called partial edentoulism (Ruth et al, 2001). There are various causes of tooth loss which includes Physical trauma or injury; Tooth decay; and Periodontal disease (Saber et al., 2015). The failure in replacing missing teeth has many drawbacks. Patients may suffer from aesthetical as well as functional problems (Shahrulet al, 2011). After the tooth loss, the alveolar bone undergoes a continuous regeneration. There is continuous osteoblastic (bone forming) and osteoclastic (bone resorbing) activity (Saber et al., 2015). However, this cycle is affected by the force transmitted by teeth from the mechanical action of chewing. After tooth loss this stimulus is absent, leading to a disturbed equilibrium. Bone loss becomes more prominent, which eventually decreases bone height and width leading to deformation (Verri et al., 2015).
The occlusion harmony is also disturbed by the movement of neighbouring teeth into the empty space; this action shifts the direction of force distribution from the long axis of the tooth. The opposing teeth lose their function due to the absence of mastication, their periodontal ligament becomes thinner and looser. Hence, these teeth will gradually creep into the empty space. (Shahrulet al, 2011)
Replacing Missing Teeth and Applied Approaches
To replace the tooth loss and the missing teeth, there are different approaches that are practiced by dentists. While the approaches have evolved over the years, there are two commonly used methods practiced widely: Fixed Denture and Removable Denture. As the name indicates, a fixed denture or a fixed bridge is cemented or bonded to the surface while a removable denture allows removability.
Dental Implants
Dental implants are applied to solutions for total or partial tooth loss, however, given the varying anatomy of positions as well as structures, they cannot be placed in the most appropriate position as analysed (Sarfaraz et al., 2015). Sometimes, they are used in a titled position and sometime in a straight position. The straight implants are commonly used in tiled position considering the effect of the anatomy (often referred to as the maxillary sinus). The application of dental implants has been and is subjected to numerous studies examining the most suitable methods to investigate the distribution of stress and the logical effects that arise of the treatment.
Research conducted earlier highlight on the point that there is no clinical difference in the use of tilted and straight implants (Rammelsberg et al., 2012; Guven et al., 2015). In those studies, the biomechanical characteristics were considered for evaluation and used in comparison against the design for straight implants (7, 8). Past research also indicates that the survival rates vary based on the type of implants applied in clinical studies (Guven et al., 2015; SalehSaber et al., 2015; Verri et al., 2015). One of the most widely applied framework for analysis pertaining to dental implant stress effects is the finite element analysis (Verri et al., 2015). Given the lower availability of data on the stress distribution on the maxillary posterior region through short or titled implants, this research area was identified.
In the past few years, an increase in the use of FEA tool has been found to predict dental implants stress level within the implant as well as the surrounding area of the bone. Using the FEA, more realistic and accurate structures can be assessed, with the models further being divided parts that are mathematically accurate (Geng et al., 2008). With the simulation process, the aspects covering strain, stress as well as displacement can be studied and calculated in detail (Beer, 1981). The use of oblique and vertical loads apply a mastication affect leading to actions such as bending leading to stress within the bone as well as the implant. In dental implants, success of the implants is tested in the form of gradient of stress that is transferred to the bone (Guven et al., 2015; Geng et al., 2008). The impact or density of the stress relates to the interface between the bone and the implant structure, the applied location (oblique or vertical), the type of prosthesis, implant length/diameter, shape and also, the implant surface characteristics. Another important factor is the quality /quantity of the bone area (Beer, 1981). To measure the stress, von Mises stress metric is utilized which combines shear and normal stress in all directions.
LITERATURE REVIEW:
Background on Dental Implants
History
Dental implants date back to ancient Egypt; wherein sea shells and stones were used to replace missing teeth, while noble metals were used by others to act like the natural roots (Abraham, 2014). In the 1930's, some archaeological excavations discovered that the Mayans used curved stones, shells, and gold to replace missing teeth over 2000 years ago (Jemat et al., 2015). In more recent times, teeth from the same individual (autograft), and teeth from a different individual of the same species (allograft) were used. However, its use was limited as a result of infectious diseases that sometimes led even to death. (Lee JH et al, 2005)
In 1957, a Swedish orthopaedic surgeon by the name PeringorBranemark discovered that bone could grow in proximity with Ti, without being rejected. This phenomenon was called “osseointegration’’ (Searson, 2005; Sullivan, 2001). The first dental implant was placed in 1965 by Branemark into a 34 year old male patient; it served for more than 40 years until the patient passed away. Branemark presented his 15 years of research in 1982 at the Toronto conference. The US Food and Drug Administration (FDA) approved the use of Titanium (Ti) dental implants in the same year. Ever since, there has been continuous development and different types of dental implant have been used.
The history of Ti dental implants began in 1940 (Adelleet al, 1981), Bothe et al. implanted a mix of Ti and other metals into laboratory animals, Ti was well tolerated due to its high corrosion resistance. In the early 1970’s, Ti and its alloys have been gradually accepted in the U.S, after the development of pre-fabricated Ti blade implants (Hiroshi and Toru, 1996). Grade four commercially pure Ti is typically used in dental implants because of its great strength and resistance to corrosion. However, recently Ti alloys, mainly Ti6A14V, has become popular because of its strength and fatigue resistance. (Le Guéhennecet al, 2007)
Abutment
They are made of Ti in different sizes and angulations specific to each case.. As seen in the figure 11, the classification of abutments is into three types: Rotational, manufacture version and retention of prosthesis. These are further classified into seven types.
Figure 11: Classification of Abutments
Abutments are available in two types: Angulated and Straight, as seen in the figure 12. The angulated abutments are further divided into 30˚Cad-cam milled structure, 6˚ is indicated for overdenture and bridges; Upto 20˚for immediate provisional prosthesis and 25˚ for narrow platforms and 28˚ for the rest of the platforms.
Figure 12: Types of Angulations
Abutment selection criteria
There is a particular selection criterion on which dental implants are planted. Due to the advancement in medical field, it has opened many doors for performing a dental implant in a variety of ways. The dental surgeon can specifically choose what material is to be used and what exact method of implants is going to be implemented during the dental surgery. The dental implants are also very stable in the long term if the procedure is done accurately (Cavallaro and Greenstein, 2011). Aesthetics is quite a difficult field and a lot of challenges are present in a successful restoration through aesthetics. It has been seen that anterior teeth are very hard to restore through aesthetics. The tissue and bone loss determine the success of the process and if these losses are kept at a minimum the procedure is considered successful. However it is even more difficult to replace multiple teeth instead of a single tooth through aesthetics. A lot of planning is needed and also a lot of emphasis is to be put on the overall betterment of the patient.Some of the commonly used abutment selection criteria include it being bio-compatible, accurate fit to prevent loosening, long-term stability and aesthetics.
Angulated Implant placement
It was first used by Dr: Paulo Malo in 1933, when he placed two vertical implants in the anterior and two in the posterior region at an angulation of 35˚-40˚ and named it “all in four”.
A two-dimensional safe distance between the vertical and tilted implant must be obtained, by multiplying the known length of each implant by a constant derived from the sine of the insertion angle. After complete assessment of the site of insertion, a three-dimensional guidance must be preformed. Extreme angulation must be avoided as it increases forces exerted at the bone-implant interface. For easier construction of the final prosthesis, inter-implant angulation must be confined to a single three-dimensional plane. Single tooth restoration should be avoided on angulated abutments (Geramipanahet al, 2010).
Maxillary sinus lift surgery
Sinus lift is a well-accepted technique to treat the loss of the VBH (vertical bone height) inside posterior maxilla either performed through a lateral window (Summers 1994) or by an osteotome sinus floor elevation technique with bone-graft material located within maxillary sinus region in order to enhance the width and height of the bone available.
Table 3: A guideline towards sinus floor elevation
Alternatives to sinus lift surgery
There is an alternative present to the sinus augmentation technique, short implants have some potential as they are brought forward by some researchers (Renouardet al, 2006). The implants with a relatively small length of 5-8 mm are known as short implants. Although there is some ongoing discussion on this topic as some authors also claim that 7-10 mm are the short implants (Das et al, 2006). It has been seen through Finite element (FEA) that the occlusal forces are distributed mainly to the crestal bone, they are not evenly spread over the surface area of an implant. Therefore the short implants are viable and can be used (Lumet al, 1999).
The short implants faced a lot of questions as the implants previously used had a lot of history and research behind them although over the years short implants have proven to be more successful. The success rates of short implants have steadily climbed and in 2004 it was 95.1% to be exact (Renouard et al, 2006). The patients with missing teeth have benefitted from this as the survival rate of normal implants is somewhat similar to the survival rate of short implants.
The earlier writings and researches stated the short implants had low survival rate when compared to standard ones and they failed more often than them. However in recent times where the textured-surfaced implants are more common, the survival rate has climbed and is almost the same to the standard implants. Research from different sources has determined that implants with a length of 6 mm have had the same survival rate as the standard ones (Bruggenkate et al, 1998). Although it is still recommended to not use shorter implants on their own but to synergize them with the longer ones to make them more stable.
A system review was conducted where a total of 16344 implants were reviewed and the failure rate was 4.8%. Implants that were 3.75 mm wide and 7 mm long failed more often as they had the failure rate of 9.7% while the failure rate of 3.75-10mm implants was at 6.3% (Das et al, 2006). The difference in the failure rates can be noted as the 4 mm implant in diameter proved to be more successful and had little failures. The long term effects of short implants as opposed to bone augmentation are not known at this time. The residual bone was about 6 to 7 mm with a standard implant while it was placed there at the same time with the sinus augmentation procedure (Pieri et al, 2012). Outcome of both of these techniques were similar. The clinical and radiographic review therefore proved the short implants are viable too.
Implants can also be placed in a disto-angulated direction as an alternative to the sinus surgery which can in turn avoid disturbing maxillary sinus. Pterygomaxilla can also be used as a location to place implants. Zygoma implants are another alternative to sinus augmentation procedure. (Graves et al, 1994)
Finite Element analysis
Many methods were implemented in the study of stress distribution around dental implants eg: strain gauge, two and three dimensional photo elasticity and finite element analysis (FEA) (Shenet al, 2010). FEA is a computer analysis that involves numerical techniques in order to calculate the strength as well as the behaviour of the structures engineered. It was developed in 1956 in the aircraft industry and was then introduced in the field of dentistry in the1970’s, when W. Farah, Thresher R.W, and Yattran A.L studied the stress in a tooth structure they modelled.
In the year 2000, Zhang L, et al conducted a study on the correlation between implant length and diameter, with proper stress distribution using three- dimensional FEA. The uses of three dimensional FEA can assess biomechanical problems before they occur. Each element is assigned with the properties of the material being modelled. The model should mimic the exact physical properties of the actual structure or as close as possible to ensure the quality of FEA. The most difficult part of the simulation is to mimic the properties of a living tissue. Mostly the properties are either drawn from a detailed anatomy book or a cone beam C.T scan of the jaws. CBCT offers not only the anatomical structure, but also can give a better idea about the material properties in relation to different bone density. Depending on the study intended, different bone modelling is initiated, in some studies bone is modelled as simple rectangle ellipsoids, or u-shaped (Sagatet al, 2010; Gröninget al, 2012).
As natural bone density occurs in different densities, specific density is modelled according to the study conducted, a more detailed geometric might be required sometimes like in cancellous bone, especially in the posterior maxilla as it has a wide variety of densities in the same jaw (Georgiopouloset al, 2007). As for implants, many ways were used to simulate both implants and abutment materials, but the two main ways were well studied in the literature. One way is to obtain all required information from the manufacturer eg: length, diameter, macro- micro thread configuration, the second way is by obtaining a digital scan of the used implant and abutment and to use the scanned model. While crowns are modeled relying on the morphology of the natural tooth to be replaced after calculation of both mesiodistal and buccolingual dimensions (Wheeler, 1963).
After completion of bone and implant simulation, masticatory forces are simulated and applied. Masticatory forces are defined 1. Compressive forces that attempt to push materials towards each other. 2. Tensile forces that pull objects apart. 3. Shear forces which cause sliding. Tensile and shear forces are the forces that can increase stress around implant-bone interface and prosthetics. In actual mastication two different types of forces are present, cyclic and static. Most of FEA studies use static forces as they are more accurate to simulate (Mischet al, 2005).The stress is then measured in a cross sectional area in neuton (force) per square meter (unit area), that’s N/m2. Usually different colour figures are used to illustrate the amount of stress around the implant and prosthetic structures (Genget al, 2001).