Dr Sailee Bhogwekar, Dr Praveen Badwaik, Dr Sangeeta Yadav, Dr Shruti Gill, Dr Arshad Idrisi, Dr Bhavna Ahuja
Department of Prosthodontics, Crown & Bridge.,
T.P.C. T’s Terna Dental College, Navi Mumbai
Osseointegration is defined as the apparent direct attachment or connection of osseous tissue to an inert, alloplastic material without intervening ﬁbrous connective tissue. Implant tissue interface is considered to be an extremely dynamic region of interaction. Various physical and chemical interactions are seen between the implant surface and bone. In the processes of osseointegration, there is initial interlocking between bone and the implant body, which is then followed by continuous bone apposition and remodeling toward the implant leading to biologic fixation of the implant. The process itself is quite complicated and there are various factors that influence the formation and maintenance of bone at the implant surface. Osseointegration can only be achieved and maintained by a gentle surgical implant placement technique, an extended healing time and a correct stress distribution when in function. The objective of this present review is to analyze the modern understanding of clinical assessments and factors that regulate the success and failure of Osseointegrated dental implants.
Keywords: Bone–Metal interface, Endosseous implants, Mechanical interlock, Implant stability.
Citations: Bhogwekar S, Badwaik P, Yadav S, Gill S, Idrisi A, Ahuja B. Osseointegration: Union Between Biology and Technology. J Prosthodont Dent Mater 2021;2(1): 38-48.
A dental implant is a synthetic device made of a biocompatible material that is surgically placed into the alveolar bone and supports a prosthodontic appliance.1 Ideally, there needs to be a uninterrupted physical, chemical, and biological interface between the human tissues and the implant material. This interaction between the alveolar bone and the implant surface was termed as osseointegration. The clinical success of dental implants is based on their good initial primary stability and early osseointegration. Histologically the interface between the osseous tissue and implant resembled as a functional ankylosis without any intervening fibrous connective tissue.
The concept of Osseointegration was discovered by Per- Ingvar Branemark, Professor at the institute for Applied Biotechnology, University of Goteborg, Sweden and his co-worker and, has had a dramatic influence on clinical treatment of oral implants. Branemark et al. first described the process of osseointegration more than 65 years ago. The main research of his group was to understand the process of bone healing and bone response to the thermal, mechanical and chemical injuries by using vital microscopy. Their work launched a new era of research on shapes and materials of dental implants. Dr. P. Branemark introduced a two-stage threaded titanium root-form implant; he developed and tested a system using pure titanium screws which he termed fixtures. These were first placed in his patients in 1965 and were the first to be well-documented and the most well-maintained dental implants thus far. The surface of original osseointegrated implants were moderately smooth. They were called machined or turned implants. But it wasn't until the last few years that the focus of biomedical research shifted from implant geometry to the osteoinductive potential of implant surfaces.
Mechanism of osseointegration:
The trauma caused during the surgical procedure and the interlocking of the implant to the hard and soft tissues initiate the process of healing.
The implant site wound healing depends on the:
- Presence of adequate cells
- Their adequate nutrition
- Adequate stimulus for bone
The three main phases of bone healing which are necessary for osseointegration are:
Biological process of integration:
Various studies of titanium implants have shown that the process of healing can be divided in three phases: osteophilic, osteoconductive and osteoadaptive.
Osteophilic phase: (Fig.1)
When an implant is placed into the cancellous marrow space blood is initially present between implant and bone. At this time only a small amount of bone is in contact with the implant surface; the rest is exposed to extracellular fluids. Due to this a generalized inflammatory response occurs in response to the surgical insult. By the end of first week, inflammatory cells are responding to foreign antigens. Vascular ingrowth from the surrounding tissues begins by third day. A mature vascular network forms by 3 weeks. Ossification also begins during the first week and the initial response observed is the migration of osteoblasts from the trabecular bone which can be due to the release of BMP’s. The osteophyllic phase lasts about 1 month.
Osteoconductive phase: (Fig.2)
Once the osteoblasts reach the implant, the bone cells spread along the metal surface laying down osteoid. Initially this is an immature connective tissue matrix and bone deposited is a thin layer of woven bone called foot plate. Fibro-cartilaginous callus is eventually remodeled into bone callus. This process occurs during the next 3 months. After 4 months of implant placement the maximum surface area is covered by bone.
Osteoadaptive phase: (Fig.3)
The final phase begins after 4 months of implant placement. Once loaded, implants do not gain or lose bone contact but the foot plates thicken in response and some reorientation of the vascular pattern may be visible.
Factors that influence osseointegration
There are three important factors which influence osseointegration:
1. Implant related factors
2. Surgical factors
3. Patient related factors
Implant related factors
Various implant related factors like Implant Biomaterial (Biocompatibility), Implant Design, Implant Surface Topography (Surface roughness), State of host bed and Loading protocols affect osseointegration.
Implant Biocompatibility: Biocompatibility is defined as the capacity of a material to exist in harmony with the surrounding biologic environment; not having toxic or injurious effects on biologic functions.
Key factors that influence the benefits and maintenance of biocompatibility:
- Corrosion resistance
- Cytotoxicity of corrosion products
- Metal contamination
Due to its high biocompatibility, good resistance to corrosion, and no toxicity on macrophages or fibroblasts, lack of inflammatory response in peri-implant tissues and its ability to repair itself by reoxidation when damaged, commercially pure titanium (Cp Ti) is widely used as an implant material. Another material used for implants; Titanium -6 Aluminum-4 Vanadium (TI-6AL-4 V) alloy exhibits soft tissue reactions very similar to those reported to Cp Ti. Thanks to the extensive research work and advancements within the field of biomaterials, newer materials like zirconia, roxolid, surface modified titanium implants came into use. These materials not only fulfil the functional requirements but also are esthetically pleasing.
Following numerous experimental studies, zirconia has earned its place as a possible substitute for titanium in implantology. Currently, tetragonal zirconia polycrystal, particularly 3 mol% yttrium oxide (yttria) -stabilized zirconia, is the ceramic of choice for dental implants.
Advantages of zirconia :
- Better esthetics
- No corrosion of the zirconia
- No piezo-electric currents between dissimilar metal in the mouth
- Thermally non-conductive
- Roxolid is a high-performance implant biomaterial that offers higher strength than cpTi (commercially pure titanium). It is a homogenous titanium and zirconium alloy containing about 13–18% zirconium.
Advantages of Roxolid :
- Stronger than titanium grade 4
- High mechanical strength
- Excellent osteoconductivity
- Minimally invasive treatment approach required.
Guillermo Manzano in his systematic review stated that the values for the bone implant contact and removal torque of zirconia implants in most of the studies analyzed did not show statistical differences compared with titanium implants. He also stated that modified-surface zirconia may have potential as a candidate for a successful implant material, although further clinical studies are necessary.
Implant Design: (Fig. 4) The dental implant applications dictate a need for a thread shape optimized for long term function, load transmission under occlusal, intrusive and shear loading. The type of force which is generated depends on the shape of the implant thread. Misch et al. (2008) suggested that V and reverse buttress thread have 300 and 150 angles respectively, whereas square thread may be perpendicular to long axis (00). Hence V-shape threads generate higher shear force than both reverse buttress and square thread, with square thread generating the least shear force. In squared and buttress threads, the axial load of these implants are mostly dissipated through compressive force, while V-shaped and reverse buttress-threaded implants transmit axial force through a combination of compressive, tensile and shear forces. A shear force in a V – thread and reverse buttress thread is 10 times greater than the shear force on a square thread. The reduction in shear loading at the interface between thread and bone provides for more compressive load transfer, which is particularly important in compromised bone density, short implant lengths, or higher force magnitude.
Implant Surface Characteristics: The extent of bone implant interface is positively correlated with an increasing roughness of implant surface.
Roughened surface causes :
- At histological level we can see greater bone to implant contact
- Micro irregularities - cellular adhesion.
- High surface energy - improved cellular attachment
The different methods used for increasing surface roughness or applying osteoconductive coatings to dental implants are:
- Sand Blasting: Titanium implants are blasted with hard ceramic particles which are projected through a nozzle at high velocity by means of compressed air.
- Chemical etching: Etching with strong acids such as HCl, H2SO4, HNO3 and HF is used for roughening dental implants.
- Porous surfaces: Spherical powder of the metallic/ceramic material becomes a coherent mass within the metallic core of the implant body.
- Plasma-sprayed surfaces: The plasma sprayed coating is produced by argon gas flame spraying powdered titanium at I5000⁰C onto a core not heated above 220°C.
- Anodized surface: Thicker oxidized layer is generally produced on titanium implants by heat treatment or by placing the implant as an anode in a galvanic cell with a suitable electrolyte.
State of the Host Bed: Ideal host bed for implant placement is one which is Healthy and with an adequate bone stock, Bone height, Bone width, Bone length and Bone density. Osseointegration of implants were found to be more successful in D2 type of bone. Whereas implants placed in D4 type of bone showed more failure rate.
The use of implants, in combination with free grafts as an immediate procedure, is a viable treatment, providing the graft bone survives and the fixture/ bone interface remains uncontaminated. The need to achieve primary stability both of the graft bone and the implant is self-evident, as is the advantage of a threaded implant design in achieving fixation of immediate grafts to the underlying bone. When implants were placed in grafted sites the time required for osseointegration increased.
Loading Conditions: The main factor for success at the time of placement is achieving primary stability. Any micromotion during initial phases of bone healing will hamper the process of integration. Failure to osseointegrate is most often caused by overloading due to transmucosal forces of removable appliance over the implant site.
Any measure to keep a patient functioning with fixed provisional restoration during the healing phases of treatment, will allow for easier patient management. If immediate loading at the time of implant placement is to be considered, not only should the primary stability be extremely tight, but control of the occlusion on the temporary interim restoration must be adjusted and monitored carefully through the initial healing period.
Rationale of immediate loading:
- Reduces the risk of fibrous tissue formation
- Minimizes woven bone formation
- Promotes lamellar bone maturation to sustain occlusal load
- Enhances bone remodelling
- Increased bone density
Surgical Technique: Minimal tissue violence at surgery is important for osseointegration. This purpose depends on continuous and careful cooling while surgical drilling is performed at low speed. If surgical technique is too violent, frictional heat will cause a temperature rise in the bone and the cells that are responsible for bone repair will be destroyed. However, the critical time/temperature relationship for bone tissue necrosis is around 47 °C when applied for 1 min.
Protocol for drilling in high-density bone
In this sequence first a fine Round Burr is used to mark the position of the implant site. As a second step, the pilot hole is enlarged to the initial implant site with the Pilot Drill. Preparation continues with the Final Drills, depending on final implant diameter. Then screw tap is used which leads to expansion of the approximating bone as well as the gradual condensing of the surrounding bone (Fig 4-8a).
Protocol for drilling in normal bone
All burs except the screw tap are used in sequence (Fig 4-8b)
Protocol for drilling in low-density bone
In this sequence, the 2-mm bur establishes the depth of the site; it is followed by the first bur corresponding to the implant diameter. The goal in this instance is to underprepare the implant site, without using the final bur and the screw tap (Fig 4-8c). If, during placement of the implant, frictional resistance is elevated and requires torque of more than 50 Ncm to overcome, the implant may be temporarily removed. The site preparation is then finished with the next bur in sequence and the implant is placed, still without use of the screw tap. (Fig 5)
Improvements of the surgical technique seem to be a reliable method of increasing oral implant success. Recently a new surgical technique has been developed for placement of implants that does not require removal of bone to place the implant and leads to enhanced stability and increased bone density. This technique is known as Osseo densification.
Patient Related Factors
Patients Age, smoking habit, Radiation therapy, compromised oral hygiene, Vitamin C deficiency, Uncontrolled Diabetes, Bone density and available bone have effect on osseointegration. The rate of osseointegration is affected due to all this factors. Trans et al in his systematic review had stated that there was no sufficient data to conclude that diabetes affected the ability of dental implants to osseointegrate. Also, no evidence was found on impaired dental implant osseointegration in osteoporosis patients. Implant survival rate was estimated to be 98% in osteoporotic patients and it was 97% in diabetic patients. Also, it is reasonable to assume that if bisphosphonate therapy, which is the most common medical treatment for osteoporosis significantly reduce bone turnover, they could influence dental implant osseointegration. Javier Ata-Ali in his systematic review stated that the placement of dental implants in patients treated with bisphosphonates does not affect osseointegration.
For many years, it has been considered that immunosuppression is an absolute contraindication to undertake the treatment with the use of dental implants. Radzewski R in his article mentioned that Immunosuppressive medications administered to the patients after organ transplants (kidney, pancreas, and liver) do not have any impact on the osseointegration of dental implants.
The use of dental implants in young patients isn't limited, but multidisciplinary treatment planning is directly connected with skeletal maturation. It’s evident that jaw growth is important for dental implants insertion. By adolescent patients we can insert dental implant in the jaw bone only if the growth and skeleton maturation is finished. If implant insertion is planned in a growing child, we must accept the fact that osseointegration forms ankyloses and implants do not follow the spontaneous and continues eruption of the natural dentition (Papez et al., 2015).
Methods of Evaluation of Osseointegration :
Stability is a required characteristic of osseointegration. When an implant is placed surgically, primary stability is a function of the bone quality, implant design and surgical technique. Greater implant stability can be interpreted as an increase in the strength of osseointegration. Implant which is placed in the dense cortical bone should have higher initial stability than the one placed in a weak cancellous bone. This implant stability can be evaluated using following methods:
I. Invasive methods :
1. Histomorphometric analysis: This is obtained by calculating the peri-implant bone quantity and bone-implant contact (BIC) from a dyed specimen of the implant and peri-implant bone.
2. Tensional test: It was earlier measured by detaching the implant plate from the supporting bone. It was later modified by Bränemark by applying the lateral load to the implant fixture.
3. Push out/Pull out tests: Push-out/pull-out test investigates the healing capabilities at the bone implant interface. It measures interfacial shear strength by applying load parallel to the implant-bone interface.
4. Removal torque analysis: Removal torque analysis implant is considered stable if the reverse or unscrewing torque was >20 Ncm. However, the disadvantage is that at the time of abutment connection implant surface in the process of osseointegration may fracture under the applied torque stress.
Historically, microscopic or histologic examination has been considered as the gold standard method to evaluate the degree of osseointegration. However, because of the invasiveness of this method and related ethical issues, other different methods of analysis have been proposed.
II. Non-invasive methods :
1. Percussion test: A well osseointegrated implant makes a ringing sound on percussion whereas a dull sound is produced when an implant has undergone fibrous integration.
2. Imaging techniques: Imaging techniques are widely used to assess both quantity and quality of the jawbone. Following the surgery, imaging methods are used to assess the health of the implant, evaluating the bone quantity and quality changes, and estimating the crestal bone loss, which is a consequence of the osseointegration process.
3. Reverse torque test: A reverse or unscrewing torque is applied to assess implant stability at the time of abutment connection. If implants rotate under the applied torque, then they are considered failures and are then removed.
4. Periotest (Siemens AG, Benshein, Germany): It is a device which is electrically operated and electronically monitored tapping head that percusses the implant a total of 16 times in about 4 sec.
5. Resonance frequency analysis (Osstell): It measures implant stability and bone density at different point of time using vibration and structural principal analysis. Classically, the implant stability quotient (ISQ) has been found to differ between 40 and 80, the higher the ISQ, the higher the implant stability. It is inversely proportional to the resonance frequency.
The successful replacement of lost natural tooth by means of tissue-integrated implants represents a crucial advance in clinical treatment. Detailed understanding and application of factors affecting the osseointegration and biological process of osseointegration in clinical practice is the key factor for the success of dental implants.
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