Topic: You are to restore an upper full arch implant retained bridge that occludes against a partially dentate mandible. Explain the factors that you would consider in your occlusal management of this type of case. Support your discussion with a critical appraisal of the evidence.
Successful osseointegration of implant has become the optimal objective of partially and fully arch implant retrained prosthesis. An appropriate occlusal approach is a critical requirement for both long-term survival implants and prosthesis. The effects of mechanical loading on bone-implant contacts (BIC) must be noted (Marcián et al 2014). The excessive load of occlusion might affect the BIC and contribute to the peri-implantitis, bone loss, and even implant failure. Therefore, analysing of the implant occlusion and risk factors is important. To understand deeply occlusal overload, it is critical to differentiate early loading and occlusal overloading. Early loading is explained as implant/or prosthetics on implant bear occlusal force from 48 hours to 3 months after implant placement. This kind of force from early loading may affect to osseointegrate process. While occlusal overloading is the force transmission onto the BIC. To restore the fully arch implant retained bridge in maxilla jaw, the need to analysis implant occlusion, examine the risk factors of occlusal overloading, and finally suggest the appropriate occlusal management scheme and prosthesis design.
There are numerous differences between natural teeth and dental implant, which are summarized in table 1 (Kim et al 2005). In term of connection, natural teeth are supported by periodontal ligaments (PDL) while implants are suspended in osseointegration or function ankylose. PDL helps to adjust the occlusal stress toward the axis of the tooth and periodontal tissue. Overloading can cause injury with widening and repair the PDL, or even increasing mobility of the tooth and having periodontal disease. This is normal action of natural tooth and reversible process. Therefore, PDL of the natural teeth can make differences in loading and adaptation of force compared with implants. The axial mobility of natural teeth in the socket is 25-100 µm, while implant mobility is 3-5µm. However, the fibers around implants which run parallel to the axis and do not attach to the surface. Therefore, this is poor connection compared with the natural teeth.
Table 1 The difference between implant and natural teeth
In the study, Kim et al (2005) studied numerous academic articles and summarized that lateral and fulcrum force would focus on apical third of root in natural dentition, compared with focus on crestal bone in dental implant. So the stress on implant would concentrate on the cervical bone around implant. Due to excessive force loading, cervical bone loss will occur. Therefore, implant must be protected as placing in proper position and direction. In the natural teeth, malocclusion may occur, but it may be uneventful over years. However, malocclusion on implants can cause traumatic response and impact the crest of bone round implants. The signs of overloading will be screw loosening or fracture, prosthesis fracture, abutment disconnected and implant failure.
According to the result of this study above, it can be conducted that osseointegration of implants would be effected easily by overloading. As limited apical and lateral movement, adaptation to force, and low osseoperception, dental implant would be more susceptible to overloading factors.
Parafunctional habits such as bruxism, clenching and heavy occlusion may cause excess forces and overloading. It is possible generate fracture of both teeth and prosthesis, dislodge the screws, porcelain fracture, bone loss and implant failure. Bruxism is also parafunction which is considered a masticatory system’s disorders. This is one of the complex, destructive TMJ’s disorder. It has also suggested to have overload of implants and prosthesis. However, some studies shown that there is no significant impact on implant’s bone loss. Bruxism seems is not a risk factor of bone loss. Another studies shown that bone loss around implant and bruxism do not related significantly (Johansson et al,2008; Manfredini et al, 2010). In the contrast, Rangert et al. (1995) examined 90% of fracture implant cases had involved in posterior region while 30% of cases were fixed bridge retained supported by 1 or 2 implants. In this study, the fracture of implant prosthesis associated with overloading such as bruxism. Komiyama et al. (2012) conducted in the study that clinical management of bruxism for implant prosthesis will be crucial factor in immediate or early loading. A night guard and pharmacological approach were recommended to protect implant prosthesis form bruxism. However, recommendations and treatment planning for bruxism were preferred by clinical based rather than literature based.
Implant designs and characteristics also are critical factors of implant occlusion. Manufactures of implant branches have to decrease the height of threads and increase the width of implant threads to attain more bone-implant contact. These changes can help to maximise osseointegration, and minimise bone loss. The micro and macro designs of implants can effect force distribution. V shaped design threads, spiral and square also increase transition of force and are more efficient in osseointegration. In addition, the micro-surface features, tapered implants, implant dimensions and position also lead to more bone contact and less stress. However, the design of implant platform also plays an importance role for force distribution. In the clinical study done by Shin et al (2006), Implant with smooth platform had significantly more marginal bone loss compared to the rough platform. Furthermore, the increasing of number threads may help to gain more bone-implant contact. In addition, Schwarz (2000) conducted that 45-degree bevel of implant platform help to minimize screw loosening in the implant screw retained prosthesis.
Prosthetic design for implant has significant impact on long-term successful of both prosthesis and implants. In term of prosthesis design, cantilevers, premature contacts, cusp inclination, occlusal table, available occlusal space and improper torque could be considered risk factors of overloading.
In fixed dental bridge retainer prosthesis, cantilevers can contribute to overloading and possibly can cause stress to the bone interface of implant. In term of cantilever’s length, in some studies shown that the length of the cantilever should be below 15mm. The larger cantilever can create overloading while the shorter cantilevers will minimize torque to abutment of implant (Laurell and Lundgren, 1987). Duyck et al (2001) shown that compared to prostheses with 5 or 6 implants, prostheses supported by only 3 implants can have the highest forces and bending on the distal implants. It is important that the shorter length of cantilever is suitable choice for implant prostheses. In the contact position or parafunction, the distribution of occlusal force could change and cause the greater force on cantilevers. The angled buccal cusp, or ridge contact from cantilever may be damaged during a period of time. Therefore, the evaluation of occlusion should be managed and eliminated the overloading forces.
Premature contacts are identified as occlusal contacts between prosthesis and dentition through movement. Premature contacts can occur from a normal movement of closure, or mandibular movement. The height of premature contacts is critical for marginal bone loss, or even peri-implantitis. As there is increasing in the height of crown, or occurring premature contact on cantilevers, the forces which are from lateral premature contacts load to cervical bone. Therefore, excessive lateral forces may be risk factor of marginal bone and implant loss. Lateral premature occlusal loads to the implant crestal region are further magnified when crown height is increased or when present on the cantilevered portion of the prosthesis.
In term of site-related factors, the quality of bone and ridge deficiency were the risk factors of implant complications. Some studies stated that poor quality of bone can be a crucial factor in the implant failure. Engquist et al. (1988) conducted that there were higher implants failure rates in poor quality of bone. Jaffin and Berman (1991) examined 35% of 1054 implants in type IV bone were failed while 3% of implants in other types of bone were failed. They stated that the soft and type IV bone were the major factor of implant failure.
To maximized bone contact of implant and reduced magnificent force of occlusion. It is important to place implant in the proper position. Non-axial occlusal force can increase the risk factors of overloading.
Occlusal complications of implant prosthesis are identified in some studies (Kim et al 2004; Chen et al 2008; Schwarz 2000). There are loosen screw, screw fractures, porcelain fractures, bone loss, implant fractures and implant loss. These complications resulted from excessive distribution force, or even overloading and lead to damage ossteointegration, peri-implant tissue, and even implant and prosthesis on implant. The impact of occlusal overloading has been contributed to cervical bone loss. Clinician conducted in the literature report that there is the effect of occlusal load on ossteointegration without the sign of inflammation. In healthy tissue condition, there was irreversible cervical bone loss around implant in the excessive overloading. In a literature review of publications for the past 10 years, it is poor little evidence to cause and effect relationship between traumatic bone loss and overloading force. However, excessive overload seems to trigger marginal bone loss and there is a sign of inflammation (Bertolini et al 2019). However, in the literature review done by Fu et al. (2012), peri-implant bone loss was associated with occlusal overloading. The mechanism of fixture bone loss can be identified in distribution of occlusal force and cause stress under overloading condition. Stress was concentrated on marginal bone, lead to bone resorption significantly and then loss of osseointegration.
In addition, popular implant complications such as screw loosen or fractures, porcelain fractures can be repaired. In literature study, porcelain fracture was the popular technical failure, more than 30% incidence were loosening retentive of overdenture, and 22% of resin veneer fractures (Goodacre et al 2003).
As the specific condition of implant, occlusion on implant should be minimized, less stress on both implant and prosthesis. Higher occlusion on cantilevers or other segments of fixed prosthesis on implants could be eliminated and equalized. The occlusal goals for implant prosthesis were bilateral continuous contact, no contact on retruded contact position (RCP), anterior guidance as possible, equal distribution of force, and non-working interferences (Chapman, 1989). There are modified basic principles of implant prostheses including balance stability in centric occlusion, anterior guidance, no interfere from centric and retruded position, evenly distributed occlusal contacts, stability of bilateral occlusal and proper force distribution (Beyron, 1969).
According to those risk factors of implant occlusion mentioned above, there are numerous consideration for implant occlusion including three major factors. To protect occlusion in implant prosthesis, the increasing bone quality of implant is needed including extend healing time and loading, other factors of implant size, length, and surfaces to achieve osseointegration. Another goals of implant occlusion are to improve force direction, occlusal force should be along the implant axis and focus on centre contacts. In term of occlusal morphology, there are necessary of flatting central fossa, decreasing cusp inclination and occlusal table. In addition, to reduce magnificent force, distribution and proper position of occlusal contacts should be eliminated.
Kim et al (2005) summarized the occlusal guidelines for numerous clinical applications. There is full-arch fixed prosthesis, overdenture, posterior implant supported fixed prosthesis, single implant prosthesis, and prosthesis on poor quality and grafted bone as the table 2 below.
Table 2 Clinical guidelines for implant prosthesis
Decision making of fully edentulous planning must be based on economic, psychosocial and physical factors of the patients. There are numerous considerations of fully arch fixed implant supported prosthesis including relation of maxilla and mandibular jaws, the quality and residual ridges, vertical and horizontal dimensions of bone, supporting anatomy, aesthetic concerns, lip support and so on. These are significant factors to determine for the position of implant and prosthesis outcome and occlusal scheme design and also called individual clinical determinants (IDCs). Kim et al (2005) recommended that bilateral balanced occlusal contacts with opposite full denture. In case the opposite partial denture, it is recommended to have a group function occlusion. There is no working and balancing contact on the cantilever of prosthesis. Clinical guideline of full arch fixed implant supported prosthesis need to follow as shown on the figure 3 below (Gross, 2008).
Table 3: Considerations and clinical guidelines for fully arch fixed implant support restoration
These guidelines focused on implant-related and prosthesis factors that the clinicians should consider and manage. The number of implant in full arch fixed implant supported prosthesis is controversial. There are 6-8 implants in maxilla jaw and 5-8 implants in mandibular which are acceptable by clinicians. However, the angulation of implant should be right angle to plan of occlusion. As following the prosthesis-related factors, distal cantilevers of the fixed bridge should be one premolar unit and do not contact in loading. It is necessary to reduce cantilever length and forces on prosthesis. The ratio of crown and implant should be 1:1, if the ratio over 1:1, it will increase biomechanical risk. As the design of prosthetic, guiding cusp inclination should be flat as possible and minimize vertical overlap. Depend on specific application of the individual clinical determinants (ICDs), implant occlusal scheme should be modified. Protrusive guidance should be flat to prevent damaging and unevenly contact in posterior region. These will depend on the ICDs of the patients. Group function guidance should be achieved in lateral excursion in order to optimal force distribution. In case of opposing natural dentition, anterior guidance in lateral excursion should have to reduce the unexpected lateral force. In this excursion, it is necessary to avoid the non-working and working interferences in posterior regions (Lundgren & Laurell 1994). Kim et al (2005) summarized from number of literature articles and stated that the need of mutually protected occlusion in which molar protect the anterior teeth in maximum intercuspation (MI) and anterior teeth also protect the back teeth in other excursion. On the occlusion adjustment of the full arch bridge, there are no balance and working contact on these cantilever. Infraocclusion on a cantilever unit should be around 100 µm to decrease overload and fracture for the prosthesis. In addition, the length of cantilever should be kept under 15mm as minimize the torque to posterior abutment. The freedom in the centric should be from 1 to 1.5 mm (Chapman 1989).
Digital planning approach and surgical guide should be recommended for implant placement to achieve the right position of implant and eliminate shear forces. The article shown that the size of occlusal table, the inclination of cusp and the number of contact with the opposite teeth or prosthesis were risk factors for overloading. Therefore, it was recommended to have narrow and steady cusp inclination and occlusal table in order to reduce magnitude of forces (Sheridan et al 2016).
Occlusal force may be a critical selection and planning tool for both implant placement and implant prosthesis. In an excessive occlusal load situation, the need of appropriate implant size, the number of implant, occlusal and prosthesis design scheme to prevent overloading (Flanagan 2017). In the report, although Flanagan et al stated that it is high risk for late fracture of implant prosthesis with a high occlusal force and low occlusal force may have a successful result even if with poor bone quality. But there is no correlation of any overload would be indicated by any kind of occlusal force.
In fully edentulous implant support prosthesis, managing the consequenes of bruxism is challenging for clinicians. Bruxism is clenching and grinding the teeth lead to traumatic natural dentition as well as prosthesis. In the article done by Galindo (2016), first step of treatment planing is identify parafunctional activity. There are several degress of this activity. It is difficult to identify the early stage of bruxism due to the unwareness of the patients. However, clinicians could found significant wear surfaces, fractures of dentition structures and prosthesis, diminishing vertical dimension of occlusion.
BruxZir® Implant prosthesis is the durable alternative solution for screw-retained fully arch bridge, and hydrid dentures (Kosinski and Schaefer 2015). As construction by 100% of monolithic zirconia and more than 1,100 MPa of flexural strength, BruxZir® Implant prosthetic can minimum chips, stains and porcelain fractures. In addition, it has pypoallergenic feature, this kind of prosthetic wear compatible with natural dentition. It is recommended to restore the fully arch implant supported prosthesis, and the ideal solution for bruxers who have a strong bite, clenching and grinding (Galindo 2016).
Moreover, pharmacological approach and night guard are also recommended for bruxism treatment. The hard splint should be worn nightly to help optimized distribution forces from clenching and grinding teeth. Another kind of splint, for example, soft splint could be ultilized to ease the stress and force from the prosthesis.
In addition, a pharmacological treatment was proposed for bruxers with prosthetic on implant, in the severe situations, low dose of the dopamin or botox treatment could be ultilized to control bruxism in under furture study ( Komiyama et al, 2012).
As the success of implant and prosthetic on implant depends on the osseointegration which is ankylosed to the bone. The overloading for occlusion is considered as a critical factor of prosthetic/ implant loss and failure. There are numerous implant and prosthesis complications, for instance, screw break, porcelain chips and fractures, peri-implantitis, bone loss, implant failure. The complication could be predictable and prevent by proper treatment plan and occlusion management. The objectives of occlusal management, especially in fully arch fixed implant supported prosthesis, are to minimize overloading on both implants and prosthetics, to achieve the long term stability and longevity of implant and prosthesis. According to four main risk factors of occlusion mentioned above, there are limiting patient-related factors such as parafunctional habits, improving the site-related factors including bone quality, and ridge deficiency, managing the occlusion force and distribution through implant and prosthesis related factors to protect prosthesis from overloading. In the fully arch implant retained bridge, the need of occlusal splint should be fabricated after loading to protect occlusion and prevent bruxism. The management of occlusion are including of the implant or prosthesis components such as replacing with new devices, retightening the screw after occlusal equilibration as well as managing the marginal bone loss. For instance, mechanical debridement and antimicrobial approach should be considered to maintain the healthy condition for implant and prosthesis to achieve long-term survival.
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