Auhtor: Dr. Harshitha Alva, Dr. Krishna Prasad D., Dr. Anupama Prasad D.
Nitte University, Deralakatte,Mangalore.
Abstract:
Mechanical consideration and the transfer of mechanical load across the interfaces between prosthesis and their supporting biological structures play significant roles in success of every type of Prosthodontic rehabilitative treatment modalities. In addition to the implants themselves with their various components and retained prosthetic superstructures, special attention must be focused on the transfer of mechanical load across the interface between implants and bone. Thus a close contact of bone and implants at the cellular level is a prerequisite for successful long term osseointegrated supported prosthesis function. Over the years various concepts of loading dental implants have been put forth by different authors.
This article discusses the various loading concepts, their advantages, disadvantages, indications and contra-indications.
Introduction:
Mechanical loading of dental implants is one of the key factors that influence osseointegration. Mechanical stimulation may be used to enhance bone strength and implant osseointegration. Various studies have concluded that amongst the numerous factors that influence the response to mechanical loading, rate and loading frequency are of prime concern.
Bone as a living mass has the ability to adapt its mass and structure to the demands of mechanical loading. Osteocytes, buried in the bone matrix and the lacuna-canalicular porosity are believed to be the professional mechanosensory cells of bone and structure that mediates mechanosensing respectively. Various concepts of loading have been put forth in the recent times and their selection varies from patient to patient depending on the clinical scenario.
Branemark’s original protocol:
The two-stage surgical protocol established by Branemark et al to accomplish osseointegration consisted of several prerequisites, which included countersink the implant below the crestal bone, Obtaining and maintaining a soft-tissue covering over the implant for 3 to 6 months and maintaining a minimally loaded implant environment for 3 to 6 months.
The primary reasons cited for the submerged, countersunk surgical approach to implant placement were to reduce and minimize the risk of bacterial infection, to prevent apical migration of the oral epithelium along the body of the implant, and to minimize the risk of early implant loading during bone remodelling. A second-stage surgery was necessary to uncover these implants and place a prosthetic abutment.
Various loading concepts in osseointegrated supported prosthesis:
Immediate loading:
Immediate loading refers to implant-based surgical technique in which the “implant supported restoration is placed into occlusal loading within atleast 48 hours after implant placement.” Immediate loading of a dental implant not only includes a non-submerged, one stage surgery but also actually loads the implant with a provisional restoration at the same appointment or shortly thereafter1. It is indicated when there is adequate bone quality (type I, II and III), sufficient bone height of approximately 12mm for a minimum length of 10 mm implant, sufficient bone width of approximately 6 mm and the ability to achieve an adequate antero-posterior(AP) spread between the implants. A poor antero-posterior spread decreases the mechanical advantage gained by splinting and the ability to cantilever the restoration. The contraindications include poor systemic health,severe parafunctional habits, bone of poor quality (e.g. type IV), less bone height, less Bone width and inability to achieve an adequate AP spread.
According to Gapski et al2, several factors influence the results of immediate implant loading which could be divided into the following four categories:
The immediate implant-loading concept challenges the conventional healing time of 3 to 6 months of no loading before the restoration of the implant. Often the risks of this procedure are perceived to be during the first week after the implant insertion surgery. In reality, the bone interface is stronger on the day of implant placement compared with 3 monthslater. The surgical process of the implant osteotomy preparation and implant insertion cause a regional accelerator phenomenon of bone repair around the implant interface. As a result of the surgical placement, organized, mineralized lamellar bone in the preparation site becomes unorganized, less-mineralized, woven bone of repair next to the implant.
The implant-bone interface is weakest and most at risk of overload at 3 to 6 weeks after surgical insertion because the surgical trauma causes bone remodeling at the interface that is least mineralized and unorganized during this time frame. However, this has proved to be sufficient in most bone types and clinical situations for two-stage healing and delayed implant loading. One method to decrease the risk of immediate occlusal overload is to decrease the surgical trauma and amount of initial bone remodeling at implant placement. A protocol for immediate load is to tighten the implant within the bone to 45 to 60 N-cm. Although this concept helps to ensure that the implant has rigid fixation and is in good quality bone, the additional torque used to secure or evaluate fixation of an implant in bone actually may increase the strain at the interface and therefore increase the amount of remodeling, which decreases the strength of the bone-implant interface. Hence, it is prudent to minimize factors related to thermal injury and surgical trauma when considering immediate load to the implant interface.
Rationale for immediate loading is not only to reduce the risk of fibrous tissue formation which may result in clinical failure, but also to minimize woven bone formation and promote lamellar bone maturation to sustain occlusal load.
The Branemark novum concept3:
The Novum Conceptwas conceived in 1980. Branemark Novum, an essentially new treatment modality is based on with the Branemark Classic osseointegration procedure, a two-step surgical approach with varying time intervals between the steps. The distinctive feature of Novum is that it requires only 6 to 8 hours for the entire reconstruction and thus gives the patient a third dentition in just 1 day. Novum system is a one-day treatment of the edentulous or periodontally hopeless mandibular dentition. There are four drill templates and eight drill guides that precisely position three implants which are totally parallel and level. A prefabricated lower bar is placed on the three implants, and an upper bar fits on the lower bar. The restorative dentist has previously selected denture teeth and recorded the vertical dimension of occlusion. The case is waxed up, adjusted, processed and fit and insertion done on the same day.
The advantage of this procedure is completion of the surgery and reconstruction in one day, with rigid stabilization at the time of implant placement and also the reduced cost. Disadvantages include appearance of the lower bar when the patient pulls down his or her lower lip, surgical procedure is very labour-intensive and much more demanding than routine implant surgery, limited patient selection due to anatomic limitations and also the surgical template might not fit all mandibles.
Factors that reduce the risk of immediate loading:
Early loading:
Early loading refers to an implant supported restoration that is in occlusion between 2 weeks and 3 months after implant placement. A fundamental goal of early loading is improving bone formation in order to support occlusal loading at two months.
Delayed loading:
It refers to implant prosthesis with an occlusal load after more than 3 months after implant placement. The delayed occlusal loading approach may use either a two-stage surgical procedure that covers the implants with soft tissue or a one-stage approach that exposes a portion of the implant at the initial surgery.
The rationale behind this approach is that premature loading of implants would lead to implant micro movement, caused by functional force, around the bone-implant interface during wound healing and may induce fibrous tissue formation rather than bone contact, leading to clinical failure. In addition, coverage of an implant has also been thought to prevent infection and epithelial down growth. Initial exposure or biomechanical stimuli often induced a fibrous connective tissue interface between implants and bone. Hence the submerged implants were preferable for initial rigid fixation4.
Progressive loading:
Branemark first proposed the concept of progressive or gradual bone loading during prosthetic reconstruction to decrease crestal bone loss and early implant failure of endosteal implants in 19805.
Factors affecting Progressive loading of implants:
A review of the literature of in vivo and in vitro studies6,7 has shown that dynamic or cyclic loading is necessary to cause a significant metabolic change in the bone cell population. The greater the rate of change of applied strain in bone, the more bone formation is increased. The effect of applied strains on bone is dictated not only by the rate of the applied load but also by the magnitude and duration. Cyclic loading is necessary to cause a significant metabolic change in bone cell population. Lower-magnitude loads applied for many cycles can cause the same anabolic effects of larger loads applied for a limited number of cycles. Therefore a range of clinical conditions may equate to an increase in bone density.
The bone strength is related directly to density, with Division Dl bone being 10 times stronger than D4 bone to stresses that cause micro-fracture8. Therefore increasing bone density around an implant increases the strength of bone, which in turn can help avoid crestal bone loss and implant failure.
Elements of progressive loading protocol :
Discussion:
Marginal bone resorption around dental implants can jeopardize the stability of peri-implant tissue which may lead to peri-implantitis or unesthetic implant restorations.
Certain studies have also evaluated the effect of loading on the success of dental implants. Henry and Rosenberg9 used Branemark implants with bicortical anchorage. After a time period of 6-7 weeks before loading the implants, a success rate of 100% was obtained whereas Salama et al.10 found no difference in success rate between the randomly applied immediate and delayed loading. Scortecci 11 also studied the immediate loading of implants with bicortical anchorage. They demonstrated that bicortical anchorage and the placement of a rigid prosthesis allows the immediate loading of implants, with a predictable outcome.
Horiuchi et al.12 also studied the immediate loading of Branemark implants and suggested it was as predictable as delayed loading in the placement of overdentures, both in the maxilla and mandible. Jo et al.13 concluded that the main factor influencing the success of immediate loading is the primary stability of the implants at the time of the loading. Vercruyssen and Quirynen showed in their long-term study, that some factors such as smoking, guided bone regeneration, the presence of dehiscence and bone quantity clearly showed a significant impact on the marginal bone loss around the dental implants14.
With the trend of shortening treatment time and reducing patient discomfort, immediate loading implants has emerged. However, meticulous selection is needed to integrate this treatment into daily practice. Regular maintenance may be another factor to ensure the long-term success of immediately loaded implants. In addition, factors that may influence the outcome of this approach such as surgery related, host factors, implant and occlusion-related factors should be considered and analyzed prior to initiation of treatment.
The loading concepts in osseointegrated supported prosthesis along with their relative indications, contraindications, advantages and disadvantages can be summarised as follows:
Conclusion :
Various loading protocols have been applied over the past few decades. Immediate loading has achieved similar success rated when compared to other loading protocols. Primary implant stability is a key factor to be considered before attempting immediate implant loading along with other factors like patient’s medical and psychological condition, and the team experience to decide the type of loading protocol that will be used for a particular situation. A proper diagnosis, treatment planning and analysis of bone quantity and quality as well as careful selection and application of loading concept are necessary for long term success of Osseointegrated supported prosthesis.
References:
LOADING CONCEPTS IN OSSEOINTEGRATED SUPPORTED PROSTHESIS - AN INSIGHT
Nitte University, Deralakatte,Mangalore.
Abstract:
Mechanical consideration and the transfer of mechanical load across the interfaces between prosthesis and their supporting biological structures play significant roles in success of every type of Prosthodontic rehabilitative treatment modalities. In addition to the implants themselves with their various components and retained prosthetic superstructures, special attention must be focused on the transfer of mechanical load across the interface between implants and bone. Thus a close contact of bone and implants at the cellular level is a prerequisite for successful long term osseointegrated supported prosthesis function. Over the years various concepts of loading dental implants have been put forth by different authors.
This article discusses the various loading concepts, their advantages, disadvantages, indications and contra-indications.
Introduction:
Mechanical loading of dental implants is one of the key factors that influence osseointegration. Mechanical stimulation may be used to enhance bone strength and implant osseointegration. Various studies have concluded that amongst the numerous factors that influence the response to mechanical loading, rate and loading frequency are of prime concern.
Bone as a living mass has the ability to adapt its mass and structure to the demands of mechanical loading. Osteocytes, buried in the bone matrix and the lacuna-canalicular porosity are believed to be the professional mechanosensory cells of bone and structure that mediates mechanosensing respectively. Various concepts of loading have been put forth in the recent times and their selection varies from patient to patient depending on the clinical scenario.
Branemark’s original protocol:
 |
The two-stage surgical protocol established by Branemark et al to accomplish osseointegration consisted of several prerequisites, which included countersink the implant below the crestal bone, Obtaining and maintaining a soft-tissue covering over the implant for 3 to 6 months and maintaining a minimally loaded implant environment for 3 to 6 months.
The primary reasons cited for the submerged, countersunk surgical approach to implant placement were to reduce and minimize the risk of bacterial infection, to prevent apical migration of the oral epithelium along the body of the implant, and to minimize the risk of early implant loading during bone remodelling. A second-stage surgery was necessary to uncover these implants and place a prosthetic abutment.
Various loading concepts in osseointegrated supported prosthesis:
Immediate loading:
Immediate loading refers to implant-based surgical technique in which the “implant supported restoration is placed into occlusal loading within atleast 48 hours after implant placement.” Immediate loading of a dental implant not only includes a non-submerged, one stage surgery but also actually loads the implant with a provisional restoration at the same appointment or shortly thereafter1. It is indicated when there is adequate bone quality (type I, II and III), sufficient bone height of approximately 12mm for a minimum length of 10 mm implant, sufficient bone width of approximately 6 mm and the ability to achieve an adequate antero-posterior(AP) spread between the implants. A poor antero-posterior spread decreases the mechanical advantage gained by splinting and the ability to cantilever the restoration. The contraindications include poor systemic health,severe parafunctional habits, bone of poor quality (e.g. type IV), less bone height, less Bone width and inability to achieve an adequate AP spread.
According to Gapski et al2, several factors influence the results of immediate implant loading which could be divided into the following four categories:
| 1. Surgery-related factors |
|
| 2. Host-related factors |
|
| 3. Implant-related factors |
|
| 4. Occlusion-related factors |
|
The immediate implant-loading concept challenges the conventional healing time of 3 to 6 months of no loading before the restoration of the implant. Often the risks of this procedure are perceived to be during the first week after the implant insertion surgery. In reality, the bone interface is stronger on the day of implant placement compared with 3 monthslater. The surgical process of the implant osteotomy preparation and implant insertion cause a regional accelerator phenomenon of bone repair around the implant interface. As a result of the surgical placement, organized, mineralized lamellar bone in the preparation site becomes unorganized, less-mineralized, woven bone of repair next to the implant.
The implant-bone interface is weakest and most at risk of overload at 3 to 6 weeks after surgical insertion because the surgical trauma causes bone remodeling at the interface that is least mineralized and unorganized during this time frame. However, this has proved to be sufficient in most bone types and clinical situations for two-stage healing and delayed implant loading. One method to decrease the risk of immediate occlusal overload is to decrease the surgical trauma and amount of initial bone remodeling at implant placement. A protocol for immediate load is to tighten the implant within the bone to 45 to 60 N-cm. Although this concept helps to ensure that the implant has rigid fixation and is in good quality bone, the additional torque used to secure or evaluate fixation of an implant in bone actually may increase the strain at the interface and therefore increase the amount of remodeling, which decreases the strength of the bone-implant interface. Hence, it is prudent to minimize factors related to thermal injury and surgical trauma when considering immediate load to the implant interface.
Rationale for immediate loading is not only to reduce the risk of fibrous tissue formation which may result in clinical failure, but also to minimize woven bone formation and promote lamellar bone maturation to sustain occlusal load.
The Branemark novum concept3:
The Novum Conceptwas conceived in 1980. Branemark Novum, an essentially new treatment modality is based on with the Branemark Classic osseointegration procedure, a two-step surgical approach with varying time intervals between the steps. The distinctive feature of Novum is that it requires only 6 to 8 hours for the entire reconstruction and thus gives the patient a third dentition in just 1 day. Novum system is a one-day treatment of the edentulous or periodontally hopeless mandibular dentition. There are four drill templates and eight drill guides that precisely position three implants which are totally parallel and level. A prefabricated lower bar is placed on the three implants, and an upper bar fits on the lower bar. The restorative dentist has previously selected denture teeth and recorded the vertical dimension of occlusion. The case is waxed up, adjusted, processed and fit and insertion done on the same day.
The advantage of this procedure is completion of the surgery and reconstruction in one day, with rigid stabilization at the time of implant placement and also the reduced cost. Disadvantages include appearance of the lower bar when the patient pulls down his or her lower lip, surgical procedure is very labour-intensive and much more demanding than routine implant surgery, limited patient selection due to anatomic limitations and also the surgical template might not fit all mandibles.
Factors that reduce the risk of immediate loading:
- Bone microstrain
- Increased Surface area
- Decreased Force conditions
- Mechanical properties of bone
Early loading:
Early loading refers to an implant supported restoration that is in occlusion between 2 weeks and 3 months after implant placement. A fundamental goal of early loading is improving bone formation in order to support occlusal loading at two months.
Delayed loading:
It refers to implant prosthesis with an occlusal load after more than 3 months after implant placement. The delayed occlusal loading approach may use either a two-stage surgical procedure that covers the implants with soft tissue or a one-stage approach that exposes a portion of the implant at the initial surgery.
The rationale behind this approach is that premature loading of implants would lead to implant micro movement, caused by functional force, around the bone-implant interface during wound healing and may induce fibrous tissue formation rather than bone contact, leading to clinical failure. In addition, coverage of an implant has also been thought to prevent infection and epithelial down growth. Initial exposure or biomechanical stimuli often induced a fibrous connective tissue interface between implants and bone. Hence the submerged implants were preferable for initial rigid fixation4.
Progressive loading:
Branemark first proposed the concept of progressive or gradual bone loading during prosthetic reconstruction to decrease crestal bone loss and early implant failure of endosteal implants in 19805.
Factors affecting Progressive loading of implants:
- Bone Density
- Bone-Implant Interface
A review of the literature of in vivo and in vitro studies6,7 has shown that dynamic or cyclic loading is necessary to cause a significant metabolic change in the bone cell population. The greater the rate of change of applied strain in bone, the more bone formation is increased. The effect of applied strains on bone is dictated not only by the rate of the applied load but also by the magnitude and duration. Cyclic loading is necessary to cause a significant metabolic change in bone cell population. Lower-magnitude loads applied for many cycles can cause the same anabolic effects of larger loads applied for a limited number of cycles. Therefore a range of clinical conditions may equate to an increase in bone density.
The bone strength is related directly to density, with Division Dl bone being 10 times stronger than D4 bone to stresses that cause micro-fracture8. Therefore increasing bone density around an implant increases the strength of bone, which in turn can help avoid crestal bone loss and implant failure.
Elements of progressive loading protocol :
 |
Discussion:
Marginal bone resorption around dental implants can jeopardize the stability of peri-implant tissue which may lead to peri-implantitis or unesthetic implant restorations.
Certain studies have also evaluated the effect of loading on the success of dental implants. Henry and Rosenberg9 used Branemark implants with bicortical anchorage. After a time period of 6-7 weeks before loading the implants, a success rate of 100% was obtained whereas Salama et al.10 found no difference in success rate between the randomly applied immediate and delayed loading. Scortecci 11 also studied the immediate loading of implants with bicortical anchorage. They demonstrated that bicortical anchorage and the placement of a rigid prosthesis allows the immediate loading of implants, with a predictable outcome.
Horiuchi et al.12 also studied the immediate loading of Branemark implants and suggested it was as predictable as delayed loading in the placement of overdentures, both in the maxilla and mandible. Jo et al.13 concluded that the main factor influencing the success of immediate loading is the primary stability of the implants at the time of the loading. Vercruyssen and Quirynen showed in their long-term study, that some factors such as smoking, guided bone regeneration, the presence of dehiscence and bone quantity clearly showed a significant impact on the marginal bone loss around the dental implants14.
With the trend of shortening treatment time and reducing patient discomfort, immediate loading implants has emerged. However, meticulous selection is needed to integrate this treatment into daily practice. Regular maintenance may be another factor to ensure the long-term success of immediately loaded implants. In addition, factors that may influence the outcome of this approach such as surgery related, host factors, implant and occlusion-related factors should be considered and analyzed prior to initiation of treatment.
The loading concepts in osseointegrated supported prosthesis along with their relative indications, contraindications, advantages and disadvantages can be summarised as follows:
LOADING CONCEPTS |
INDICATIONS |
CONTRAINDICATIONS |
ADVANTAGES |
DISADVANTAGES |
|
IMMEDIATE LOADING |
- Adequate bone quality - Sufficient bone height and width - Ability to achieve an adequate antero posterior spread between the implants. |
- Poor systemic health - Severe parafunctional habits - Inability to achieve an adequate AP spread |
- Eliminates the need for and maintenance of a removable provisional prosthesis - Reduced treatment time and Cost effective |
- Cannot be applied to every implant patient - Requires more chair side time |
|
EARLY LOADING |
- Sufficient bone quality |
- Smokers with uncontrolled DM - History of failed implants - Large deviation in saggital/vertical bite relation |
- Reduced treatment time - Cost effective |
- Crestal bone loss - primary stability compromised |
|
DELAYED LOADING |
- Can be done for all prosthesis |
- No absolute contraindication |
- Reduced risk of bacterial infection - Prevents apical migration of oral epithelium along implant body |
- Time consuming |
|
PROGRESSIVE LOADING |
- Few implants planned - Softer bone types |
- No absolute contraindication but most critical in D4 type bone |
- Decreased crestal bone loss - No early implant failure |
- Time consuming |
Conclusion :
Various loading protocols have been applied over the past few decades. Immediate loading has achieved similar success rated when compared to other loading protocols. Primary implant stability is a key factor to be considered before attempting immediate implant loading along with other factors like patient’s medical and psychological condition, and the team experience to decide the type of loading protocol that will be used for a particular situation. A proper diagnosis, treatment planning and analysis of bone quantity and quality as well as careful selection and application of loading concept are necessary for long term success of Osseointegrated supported prosthesis.
References:
- F. Lobbezoo, J. E. I. G. Brouwers, M.S. Cune & M. Naeije. Dental implants in patients with bruxing habits. Journal of Oral Rehabilitation 2006 33; 152–159
- L. Schropp & F. Isidor Timing of implant placement relative to tooth extraction. Journal of Oral Rehabilitation 2008 35 (Suppl. 1); 33–43
- Engstrand P, Nannmark U, Mårtensson L, Galéus I, Brånemark PI. Brånemark Novum: prosthodontic and dental laboratory procedures for fabrication of a fixed prosthesis on the day of surgery. Int J Prosthodont. 2001 Jul-Aug;14(4):303-9.
- Appleton RS, Nummikoski PV, Pigno MA, Cronin RJ, Chung K-H. A radiographic assessment of progressive loading on bone around single osseointegrated implants in the posterior maxilla. Clin. Oral Impl. Res. 16, 2005; 161–167.
- Glossary of Prosthodontic Terms. Journal of Prosthetic Dentistry. July 2005
- Misch CE. Stress factors: Influence on treatment planning. In: Misch CE (ed). Dental Implant Prosthetics St Louis: Elsevier Mosby, 2005:71-90
- Misch CE. Force factors related to the patient. In: Misch CE (ed). Dental Implant Prosthetics St Louis: Elsevier Mosby, 2005:91-104 Schwarz, Melvyn S. Mechanical complications of dental implants. Clin Oral Implants Res. 2000;11 Suppl 1:156-8
- Henry PJ, Rosenberg I. Single-stage surgery for rehabilitation of the edentulous mandible: Preliminary results. Practical and Periodontics Aesthetic Dent 1994;6:15-22.
- Salama H, Rose LF, Salama M, Betts NJ. Immediate loading of bilaterally splinted titanium root form implants in fixed prosthodontics: A technique reexamined. Two case reports. Int J Periodontics Restorative Dent 1994;15: 345-61.
- Scortecci G. Inmediate function of cortically anchored disk-design implants without bone augmentation in moderately to severely resorbed completely edentulous maxillae. J Oral Implant 1999;25:70-9.
- Horiuchi K, Uchida H, Yamamoto K, Sugimura M. Immediate loading of Brånemark system implants following placement in edentulous patients: a clinical report. Int J Oral Maxillofac Implants 2000;15:824-30.
- Jo HY, Hobo PK, Hobo S. Freestanding and multiunit immediate loading of the expandable implant: an up-to-40-month prospective survival study. J Prosthet Dent 2001;85:148-55.
- Vercruyssen M, Quirynen M. Long-term, retrospective evaluation (implant and patient-centred outcome) of the two-implant- supported overdenture in the mandible. Part 2: marginal bone loss. Clin Oral Implants Res 2010;21:466-72.
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