Author: Dr. Harshitha Alva, Dr. Krishna Prasad D., Dr. Anupama Prasad D..
Nitte University, Deralakatte, Mangalore
ABSTRACT:
In natural dentition, the force distribution is dependent on the micromovement induced by the periodontal ligament where as dental implants do not possess such an inherent movement to resist the load. The success or failure of osseointegrated supported prosthesis depends greatly on the forces that act on the prosthesis. Hence a thorough knowledge of the direction and distribution of forces acting on a prosthesis and appropriate management of deleterious forces determines the key to a successful prosthesis.
This article describes the biomechanical aspects that should be taken into consideration in designing osseointegrated supported prosthesis.
INTRODUCTION:
Forces that are involved in both mastication and para-function play a significant role in the design of prosthesis. These considerations act as determining factors for the success or failure of prosthesis. The fixation of oral implants to bone by means of osseointegration has allowed highly predictable, long-term functional performance of osseointegrated prosthesis. There are always differences between patients in terms of key factors affecting osseointegration such as the amount of available bone, bone quality, number of missing teeth, masticatory habits, etc. In designing a successful oral implant supported prosthesis, the main objective is to ensure that the implant can withstand the masticatory forces and transfer them safely to interfacial tissues over the long term. Biomechanics are central in this design problem.
Biomechanics are of two types: Reactive and Therapeutic. Reactive biomechanics refers to the interaction of isolated biomechanical factors which when combined, produce an accumulative effect and therapeutic refers to the clinical process of altering each biomechanical factor to reduce the cumulative response causing implant overload.
FORCES APPLIED ON OSSEOINTEGRATED IMPLANTS:
Stress and strain have been shown to be important parameters for crestal bone maintenance and implant survival. The higher the crestal stress, the higher the risk of crestal bone loss. Forces applied on dental implants may be characterized in terms of five factors:
Direction:
The anatomy of the mandible and maxilla places significant constraints on the ability to surgically place root form implants suitable for loading along their long axis. Resorptive patterns following prolonged edentulism exacerbates the normally occurring angulation challenges. Bone is strongest when loaded in its long axis in both compression and tensile forces. A 30-degree offset load reduces the compressive strength of bone by 11%, and reduces the tensile strength by 25%. As the angle of load increases, the stresses around the implant increase, particularly in the vulnerable crestal bone region. As a result, virtually all implants are designed for placement perpendicular to the occlusal plane. This placement allows a more axial load to the implant body and reduces the amount of crestal stress. Additionally, axial alignment places less stress on the abutment components and decreases the risk of short- and long-term fracture.
Duration:
The duration of bite forces on the dentition has a wide range. Under ideal conditions, the teeth come together during swallowing and mastication for only brief period of contacts. The total time of those brief episodes is less than 30 minutes per day. Patients who exhibit bruxism, clenching, or other parafunctional habits, however, may have their teeth in contact for several hours each day. Dental implants are designed for loading along their long axis and the implant body is particularly susceptible to fatigue fracture with bending load in the buccolingual plane. Such transverse bending loads may be caused by premature contacts, bruxism or angled implants. No root form implants are designed to withstand bending loads. Implant bodies are susceptible to fatigue fracture at the apical extension of abutment screw within the implant body or at the crest module around an abutment screw, which does not have direct contact.
Magnitude:
The magnitude of bite force varies as a function of anatomic region and the state of dentition. Following sustained periods of edentulism, the bone foundation often becomes less dense. Its ultimate strength is highly dependent on its density. As such, less dense bone may no longer be able to support normal physiologic bite forces on implants.
Magnification:
A surgical placement resulting in extreme angulation of the implant and/or a patient exhibiting parafunctional habits will likely exceed the capability of any dental implant design to withstand physiologic loads. Cantilevers and crown heights act as levers and therefore are force magnifiers.
Functional surface area actively serves to dissipate compressive and tensile non shear loads through the implant bone interface and provide initial stability of the implant following surgical placement. Total surface area is passive and do not participate in load transfer.
Type:
Three types of forces may be imposed on dental implants within the oral environment: compression, tension, and shear. Bone is strongest when loaded in compression, 30% weaker when subjected to tensile forces, and 65% weaker when loaded in shear. Endosteal root-form implants load the bone-to-implant interface in pure shear unless surface features are incorporated in the design to transform the shear loads to more resistant force types.
The character of force distribution depends on the stiffness of each member1. The implant abutment prosthesis interface causes minute degree of flexibility as the result of retaining screw deformation. These factors play a vital role on the concepts of force distribution and induce risk of clinical failure when teeth and implants are combined to provide support for a prosthesis without an understanding of the fundamentals. Bone undercuts constrain implant placement and thus force direction. Most undercuts occur on the facial aspects of the bone, with the exception of the submandibular fossa in the posterior mandible. Hence implant bodies are often angled to the lingual, to avoid penetrating the facial undercut during insertion. Bone is strongest when loaded in its long axis in both compression and tensile forces.
Load distribution in combined implant-tooth prosthesis:
The difference in differential mobility is of prime concern in this situation. In such conditions natural tooth intrusion separates internal attachments vertically. In the maxillary arch, buccally inclined forces are produced by the posterior working side forces and anterior incisal guidance which tends to cause separation of the prosthesis2,3. Thus splinted maxillary natural tooth tends to move away from adjacent free standing implant. This hazard can effectively be reduced by decreasing the incisal guidance, posterior cuspal inclines and optimising the bucco-lingual occlusal arrangement4. But in mandible, since the resultant forces are lingually inclined, they have fewer tendencies to horizontally separate the component.
Therapeutic biomechanics:
Therapeutic biomechanics involves alteration of each biomechanical factor in a corrective manner in the physiological chain of events so as to reduce the accumulative response that causes implant overload. Each of these biomechanical factors should be identified, and followed by suggested corrective changes.
Muscle Force:
Muscle force is a constant for each patient, except the fact that we should take into consideration that muscle force is greater posteriorly than anteriorly by a 4x1 ratio. To further complicate matters; most posterior bone contains less compact bone. Larger implants have been used posteriorly to provide an increased area of osseointegration to accommodate for the quality of bone and greater muscle force posteriorly.
Impact Area:
The impact area has a significant biomechanical effect on force distribution. It has been shown mathematically that the cusp inclination is the most potent factor in producing torque also known as moment. However, the location of the impact area, relative to the supporting bone, also plays a significant role.
Resultant Line of Force:
A resultant line of force is perpendicular to the incline when an occlusal force is applied. It should be evident that the more inclined the cusp, the resultant line of force falls further away from the supporting bone than a shallow cusp incline. In most instances, the cusp inclination is a clinical variable under the control of the clinician and should be reduced to improve force distribution to the supporting bone.
Implant Location:
Anatomical factors, such as the sinus and inferior alveolar nerve, require implants to be horizontally offset. Torque is increased approximately 15% for every 1mm of lateral offset.
Torque:
Moment or Torque is a way of expressing and measuring the result of force applied at an angle to an object and its supporting medium. For instance, a force applied to a cusp incline produces a resultant line of force perpendicular to that incline. Torque is expressed as force times the perpendicular distance from the resultant line of force. It is a powerful and potentially destructive force because it operates as a lever arm. Torque can be visually evaluated by comparing the length of the distance arm. In clinical terms, it is always best to have the resultant line of force pass as close as possible to the implant or supporting bone. The clinical objective of therapeutic biomechanics is to alter as many factors as possible to direct the resultant line of force as close to the implant and supporting bone as possible.
Physiologic Variation:
Research into the duplicability of centric relation has indicated that it varies in the range of approximately ± 0.4mm due to time, methods of recording, head position, neuromuscular coordination, and muscle tone. Most restorative occlusion uses standard occlusal anatomy, which can produce a cusp that articulates with a line angle rather than a shallow fossae. This "locking effect" causes lateral force to be produced during physiologic variation from the original centric occlusion.
Biomechanical failures:
These failures range from screw loosening to fracture of implant itself or any component parts. The few biomechanical failures commonly encountered are screw loosening, abutment screw fracture or facture of prosthesis. Peri-implantitis is a common biological complication observed. These failures can be avoided with proper treatment planning, a good understanding of screw joint mechanics and knowledge of the implant system used.
Discussion:
Implant dentistry is now a fast developing field which has a wide area of application. To assure a definite treatment plan it is essential to examine the patient thoroughly. Both dental and medical history plays a vital role in the diagnosis and treatment planning.
The discipline of implant dentistry has enhanced dental health care to a large extent. Various treatment options are available for restoration of an edentulous region. Osseointegrated supported implant restorations have been shown to have the highest survival rate as compared to any other type of conventional prosthesis for the replacement of missing teeth.
Implants can be placed in both partially dentulous and edentulous individuals regardless of age or gender. It can be used to replace a single tooth or more. It may also be used to support a fixed or a removable prosthesis. Management of surgical procedures as well as the bone overload encountered with the use of osseointegrated supported implants is always a challenging task.
In missing teeth replacement modalities, implants have become the treatment of choice in most situations, if not all. Studies on the interaction between implant-supported restorations and the surrounding oral environment appear to support the conclusion that the human host responses to oral implants are favourable.
The treatment planning for an implant restoration is unique regarding the number of variables that may influence the therapy. Of prime importance is recognition of the fact that a definitive treatment plan should be developed sequentially in order to ensure the best possible service.
Biomechanical failures do occur due to deficient knowledge on the forces the implant would be subjected to. Hence it is always necessary for the team professionals to have a thorough knowledge on the basic principles of biomechanics and plan the treatment accordingly. The ability to reliably identify patients and conditions with greater potential for failure would be valuable. One of the basic trends of restoring the lost dentition with dental implants is to protect them from overload during function or parafunction5. Occlusal load also plays a significant role in biomechanics of successful implants.
The primary goal of an occlusal scheme is to maintain the occlusal load that has been transferred to the implant body within the physiologic limits of each patient. These limits are not identical for all patients or restorations. The forces generated by a patient are influenced by parafunction, masticatory dynamics, tongue size, implant arch position and location, and implant arch form and crown height. A dental surgeon can best address these force factors by selecting the proper implant size, number, and position, using stress-relieving elements, increasing bone density by progressive loading and selecting the appropriate occlusal scheme.
Screw retained components of implants have been proven to withstand less non-axial forces compared to forces along the long-axis of the implant6.
The use of cantilevers in implant supported fixed prosthesis is largely anecdotal. However Branemark and colleagues have demonstrated success on the use of fixed, cantilevered prosthesis to treat edentulous mandible particularly when opposed by conventional complete dentures7,8.
CONCLUSION:
The key to understanding biomechanical and functional behaviour at osseointegrated supported prosthesis interface is to control the extent of anticipated modelling and remodelling behaviour through an optimal implant design combined with a thorough understanding of how biological tissues respond to the mechanically active environment. A proper diagnosis and treatment planning is essential for good prognosis and success of endosseous oral osseointegrated supported prosthesis.
References:
BIOMECHANICAL CONSIDERATIONS IN OSSEOINTEGRATED SUPPORTED PROSTHESIS
Nitte University, Deralakatte, Mangalore
ABSTRACT:
In natural dentition, the force distribution is dependent on the micromovement induced by the periodontal ligament where as dental implants do not possess such an inherent movement to resist the load. The success or failure of osseointegrated supported prosthesis depends greatly on the forces that act on the prosthesis. Hence a thorough knowledge of the direction and distribution of forces acting on a prosthesis and appropriate management of deleterious forces determines the key to a successful prosthesis.
This article describes the biomechanical aspects that should be taken into consideration in designing osseointegrated supported prosthesis.
INTRODUCTION:
Forces that are involved in both mastication and para-function play a significant role in the design of prosthesis. These considerations act as determining factors for the success or failure of prosthesis. The fixation of oral implants to bone by means of osseointegration has allowed highly predictable, long-term functional performance of osseointegrated prosthesis. There are always differences between patients in terms of key factors affecting osseointegration such as the amount of available bone, bone quality, number of missing teeth, masticatory habits, etc. In designing a successful oral implant supported prosthesis, the main objective is to ensure that the implant can withstand the masticatory forces and transfer them safely to interfacial tissues over the long term. Biomechanics are central in this design problem.
 |
Biomechanics are of two types: Reactive and Therapeutic. Reactive biomechanics refers to the interaction of isolated biomechanical factors which when combined, produce an accumulative effect and therapeutic refers to the clinical process of altering each biomechanical factor to reduce the cumulative response causing implant overload.
FORCES APPLIED ON OSSEOINTEGRATED IMPLANTS:
Stress and strain have been shown to be important parameters for crestal bone maintenance and implant survival. The higher the crestal stress, the higher the risk of crestal bone loss. Forces applied on dental implants may be characterized in terms of five factors:
Direction:
The anatomy of the mandible and maxilla places significant constraints on the ability to surgically place root form implants suitable for loading along their long axis. Resorptive patterns following prolonged edentulism exacerbates the normally occurring angulation challenges. Bone is strongest when loaded in its long axis in both compression and tensile forces. A 30-degree offset load reduces the compressive strength of bone by 11%, and reduces the tensile strength by 25%. As the angle of load increases, the stresses around the implant increase, particularly in the vulnerable crestal bone region. As a result, virtually all implants are designed for placement perpendicular to the occlusal plane. This placement allows a more axial load to the implant body and reduces the amount of crestal stress. Additionally, axial alignment places less stress on the abutment components and decreases the risk of short- and long-term fracture.
Duration:
The duration of bite forces on the dentition has a wide range. Under ideal conditions, the teeth come together during swallowing and mastication for only brief period of contacts. The total time of those brief episodes is less than 30 minutes per day. Patients who exhibit bruxism, clenching, or other parafunctional habits, however, may have their teeth in contact for several hours each day. Dental implants are designed for loading along their long axis and the implant body is particularly susceptible to fatigue fracture with bending load in the buccolingual plane. Such transverse bending loads may be caused by premature contacts, bruxism or angled implants. No root form implants are designed to withstand bending loads. Implant bodies are susceptible to fatigue fracture at the apical extension of abutment screw within the implant body or at the crest module around an abutment screw, which does not have direct contact.
Magnitude:
The magnitude of bite force varies as a function of anatomic region and the state of dentition. Following sustained periods of edentulism, the bone foundation often becomes less dense. Its ultimate strength is highly dependent on its density. As such, less dense bone may no longer be able to support normal physiologic bite forces on implants.
Magnification:
A surgical placement resulting in extreme angulation of the implant and/or a patient exhibiting parafunctional habits will likely exceed the capability of any dental implant design to withstand physiologic loads. Cantilevers and crown heights act as levers and therefore are force magnifiers.
Functional surface area actively serves to dissipate compressive and tensile non shear loads through the implant bone interface and provide initial stability of the implant following surgical placement. Total surface area is passive and do not participate in load transfer.
Type:
Three types of forces may be imposed on dental implants within the oral environment: compression, tension, and shear. Bone is strongest when loaded in compression, 30% weaker when subjected to tensile forces, and 65% weaker when loaded in shear. Endosteal root-form implants load the bone-to-implant interface in pure shear unless surface features are incorporated in the design to transform the shear loads to more resistant force types.
 |
The character of force distribution depends on the stiffness of each member1. The implant abutment prosthesis interface causes minute degree of flexibility as the result of retaining screw deformation. These factors play a vital role on the concepts of force distribution and induce risk of clinical failure when teeth and implants are combined to provide support for a prosthesis without an understanding of the fundamentals. Bone undercuts constrain implant placement and thus force direction. Most undercuts occur on the facial aspects of the bone, with the exception of the submandibular fossa in the posterior mandible. Hence implant bodies are often angled to the lingual, to avoid penetrating the facial undercut during insertion. Bone is strongest when loaded in its long axis in both compression and tensile forces.
Load distribution in combined implant-tooth prosthesis:
The difference in differential mobility is of prime concern in this situation. In such conditions natural tooth intrusion separates internal attachments vertically. In the maxillary arch, buccally inclined forces are produced by the posterior working side forces and anterior incisal guidance which tends to cause separation of the prosthesis2,3. Thus splinted maxillary natural tooth tends to move away from adjacent free standing implant. This hazard can effectively be reduced by decreasing the incisal guidance, posterior cuspal inclines and optimising the bucco-lingual occlusal arrangement4. But in mandible, since the resultant forces are lingually inclined, they have fewer tendencies to horizontally separate the component.
Therapeutic biomechanics:
Therapeutic biomechanics involves alteration of each biomechanical factor in a corrective manner in the physiological chain of events so as to reduce the accumulative response that causes implant overload. Each of these biomechanical factors should be identified, and followed by suggested corrective changes.
Muscle Force:
Muscle force is a constant for each patient, except the fact that we should take into consideration that muscle force is greater posteriorly than anteriorly by a 4x1 ratio. To further complicate matters; most posterior bone contains less compact bone. Larger implants have been used posteriorly to provide an increased area of osseointegration to accommodate for the quality of bone and greater muscle force posteriorly.
Impact Area:
The impact area has a significant biomechanical effect on force distribution. It has been shown mathematically that the cusp inclination is the most potent factor in producing torque also known as moment. However, the location of the impact area, relative to the supporting bone, also plays a significant role.
Resultant Line of Force:
A resultant line of force is perpendicular to the incline when an occlusal force is applied. It should be evident that the more inclined the cusp, the resultant line of force falls further away from the supporting bone than a shallow cusp incline. In most instances, the cusp inclination is a clinical variable under the control of the clinician and should be reduced to improve force distribution to the supporting bone.
Implant Location:
Anatomical factors, such as the sinus and inferior alveolar nerve, require implants to be horizontally offset. Torque is increased approximately 15% for every 1mm of lateral offset.
Torque:
Moment or Torque is a way of expressing and measuring the result of force applied at an angle to an object and its supporting medium. For instance, a force applied to a cusp incline produces a resultant line of force perpendicular to that incline. Torque is expressed as force times the perpendicular distance from the resultant line of force. It is a powerful and potentially destructive force because it operates as a lever arm. Torque can be visually evaluated by comparing the length of the distance arm. In clinical terms, it is always best to have the resultant line of force pass as close as possible to the implant or supporting bone. The clinical objective of therapeutic biomechanics is to alter as many factors as possible to direct the resultant line of force as close to the implant and supporting bone as possible.
Physiologic Variation:
Research into the duplicability of centric relation has indicated that it varies in the range of approximately ± 0.4mm due to time, methods of recording, head position, neuromuscular coordination, and muscle tone. Most restorative occlusion uses standard occlusal anatomy, which can produce a cusp that articulates with a line angle rather than a shallow fossae. This "locking effect" causes lateral force to be produced during physiologic variation from the original centric occlusion.
Biomechanical failures:
These failures range from screw loosening to fracture of implant itself or any component parts. The few biomechanical failures commonly encountered are screw loosening, abutment screw fracture or facture of prosthesis. Peri-implantitis is a common biological complication observed. These failures can be avoided with proper treatment planning, a good understanding of screw joint mechanics and knowledge of the implant system used.
Discussion:
Implant dentistry is now a fast developing field which has a wide area of application. To assure a definite treatment plan it is essential to examine the patient thoroughly. Both dental and medical history plays a vital role in the diagnosis and treatment planning.
The discipline of implant dentistry has enhanced dental health care to a large extent. Various treatment options are available for restoration of an edentulous region. Osseointegrated supported implant restorations have been shown to have the highest survival rate as compared to any other type of conventional prosthesis for the replacement of missing teeth.
Implants can be placed in both partially dentulous and edentulous individuals regardless of age or gender. It can be used to replace a single tooth or more. It may also be used to support a fixed or a removable prosthesis. Management of surgical procedures as well as the bone overload encountered with the use of osseointegrated supported implants is always a challenging task.
In missing teeth replacement modalities, implants have become the treatment of choice in most situations, if not all. Studies on the interaction between implant-supported restorations and the surrounding oral environment appear to support the conclusion that the human host responses to oral implants are favourable.
The treatment planning for an implant restoration is unique regarding the number of variables that may influence the therapy. Of prime importance is recognition of the fact that a definitive treatment plan should be developed sequentially in order to ensure the best possible service.
Biomechanical failures do occur due to deficient knowledge on the forces the implant would be subjected to. Hence it is always necessary for the team professionals to have a thorough knowledge on the basic principles of biomechanics and plan the treatment accordingly. The ability to reliably identify patients and conditions with greater potential for failure would be valuable. One of the basic trends of restoring the lost dentition with dental implants is to protect them from overload during function or parafunction5. Occlusal load also plays a significant role in biomechanics of successful implants.
The primary goal of an occlusal scheme is to maintain the occlusal load that has been transferred to the implant body within the physiologic limits of each patient. These limits are not identical for all patients or restorations. The forces generated by a patient are influenced by parafunction, masticatory dynamics, tongue size, implant arch position and location, and implant arch form and crown height. A dental surgeon can best address these force factors by selecting the proper implant size, number, and position, using stress-relieving elements, increasing bone density by progressive loading and selecting the appropriate occlusal scheme.
Screw retained components of implants have been proven to withstand less non-axial forces compared to forces along the long-axis of the implant6.
The use of cantilevers in implant supported fixed prosthesis is largely anecdotal. However Branemark and colleagues have demonstrated success on the use of fixed, cantilevered prosthesis to treat edentulous mandible particularly when opposed by conventional complete dentures7,8.
CONCLUSION:
The key to understanding biomechanical and functional behaviour at osseointegrated supported prosthesis interface is to control the extent of anticipated modelling and remodelling behaviour through an optimal implant design combined with a thorough understanding of how biological tissues respond to the mechanically active environment. A proper diagnosis and treatment planning is essential for good prognosis and success of endosseous oral osseointegrated supported prosthesis.
References:
- Brunski J. Biomaterials and biomechanics in dental implant design. Int J Oral Maxillofac Implants 1988;3:85-97.
- Weinberg LA. Force distribution in splinted anterior teeth. Oral Surg Oral Med Oral Pathol 1957;10:484-494.
- Weinberg LA. Force distribution in splinted posterior teeth. Oral Surg Oral Med Oral Pathol 1957;10:1268-1276.
- Weinberg L A. The biomechanics of force distribution in implant-supported prosthesis. Int J Oral Maxillofac Implants 1993;8:19-31
- Taylor T. D. Implant prosthodontics: current perspective and future directions. Int J Oral Maxillofac Implants 2000;15:66-75
- McGlumphy EA, Robinson DM, Mendel DA. Implant superstructures: A comparison of ultimate failure force. Int J Oral Maxillofac Implants 1992;7:35–39.
- Falk H, Laurell L, Lundgren D. Occlusal force pattern in dentitions with mandibular implant-supported fixed cantilever prostheses occluded with complete dentures. Int J Oral Maxillofac Implants 1989;4:55–62.
- Lundgren D, Falk H, Laurell L. The influence of number and distribution of occlusal cantilever contacts on closing and chewing forces in dentitions with implant-supported fixed prostheses occluding with complete dentures. Int J Oral Maxillofac Implants 1989;4:277–283
In Association With
