Osseointegration is regarded as the process of direct bone apposition on implant surfaces where the bone in contact with the implant later undergoes structural adaptation for mechanical load. It is defined as the direct structural and functional connection between ordered, living bone and the surface of a load carrying implant under a light microscope. The histological appearance resembles a functional ankylosis with no intervention of fibrous or connective tissue between bone and implant surface.
Osseointegration is also a measure of implant stability, which occurs at two stages-primary and secondary. Primary stability has been found to be essential for obtaining successful secondary stability. Therefore, a feature of osseointegration is the transition of the implant from the primary (physical) stability to secondary (biological) stability.We first need to understand, how to achieve primary stability? It is obtained mainly in the cortical bone compartment by press fitting an implant (attained by placing a slightly oversized implant in a surgically prepared implant bed or osteotomy site).This causes plastic deformation of bone resulting in fissure and microcracks. The blood vessels are also compressed or ruptured, thereby having a zone of avascular necrosis. This avascular bone activates remodeling and with time the avascular bone is replaced by vital bone.Therefore, primary stability of an implant comes from the mechanical engagement of the implant with bone when it is placed surgically while the secondary stability is through the process of osseointegration and involves bone regeneration and remodeling around the implant
Osseointegration is also a measure of implant stability, which occurs at two stages-primary and secondary. Primary stability has been found to be essential for obtaining successful secondary stability. Therefore, a feature of osseointegration is the transition of the implant from the primary (physical) stability to secondary (biological) stability.We first need to understand, how to achieve primary stability? It is obtained mainly in the cortical bone compartment by press fitting an implant (attained by placing a slightly oversized implant in a surgically prepared implant bed or osteotomy site).This causes plastic deformation of bone resulting in fissure and microcracks. The blood vessels are also compressed or ruptured, thereby having a zone of avascular necrosis. This avascular bone activates remodeling and with time the avascular bone is replaced by vital bone.Therefore, primary stability of an implant comes from the mechanical engagement of the implant with bone when it is placed surgically while the secondary stability is through the process of osseointegration and involves bone regeneration and remodeling around the implant
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| a and b : Histologic appearance of an osseointegrated implant |
Theories of Osseointegration
Various concepts have been proposed to explain this integration with the difference being in the interpositional tissue between the implant and the recipient bone.
Fibro-osseous Retention V/S Osseointegration
Fibro-osseous retention is the tissue to implant contact interposition of the healthy, dense collagenous tissue between the implant and the bone which is a concept based on the development of an intermediate ligamental system. While in Osseointegration load transfer to surrounding bone through an implant depends on the development of an ankylotic like relationship between the implant and the bone. The direct bone – to – implant interface without intervening connective tissue was proposed as early as 1939 by Strock and described by Branemark.
Branemark’s Theory of Osseointegration
In 1952, Swedish orthopedic surgeon, Branemark, conducted microscopic studies done in situ of the bone marrow in rabbit’s fibula with in vivo vital microscopic technique. Implanted titanium chambers containing an optical system for transillumination were used. However, these titanium chambers were not retrievable as they had fused with the surrounding bone. Further study revealed that titanium metal was biocompatible and had actually integrated with the bone and thus he called this process as osseointegration.
Branemark proposed that implants integrate such that the bone is laid very close to the implant without any intervening connective tissue. The titanium oxide permanently fuses with the bone. Branemark also stated that the implant should not be loaded and left out of function during the healing period for osseous integration to occur. Weiss’s Theory of Fibroosseous Fixation Weiss’ theory stated that there is a fibro-osseous ligament formed between the implant and the bone and this ligament can be considered as equivalent to the periodontal ligament found in the gomphosis. He defended the presence of collagen fibers at the bone-implant interface. He interpreted it as the peri-implantal ligament with an osteogenic effect and advocate the early loading of the implant.He believed that the collagen fibers invest the implant originating at the trabaculae of cancellous bone on one side, weaving around the implant, and reinserting into a trabaculae on the other side. When functional forces are exerted on the implant, deformation on the aspect of the implant alveolus closest to the implant surface exhibits compression which creates a negative charge, while tension at the outer aspect of the deformed trabeculae creates a positive charge there. The difference between the inner aspect and the outer aspect (tension) results in a bioelectric current (a piezoelectric effect) which induces a differentiation of the pleuripotential cells in the vicinity into components associated with bone maintenance around the implant.
Osseointegration Vs. Biointegration
In 1985, dePutter observed that there are two ways of implant anchorage or retention: mechanical and bioactive. Mechanical retention refers to the metallic substrate systems such as titanium alloy. The retention is based on the undercut forms such as vents, slots, dimples, screws etc and involves direct contact between the dioxide layer on the base metal and bone with no chemical bonding. Bioactive retention is achievedwith bioactive materials such as hydroxyapatite (HA), which bond directly to the bone, similar to ankylosis of natural teeth. Bone matrix is deposited on the HA layer as a result of physicochemical interaction between the collagen of bone and the HA crystals of the implant.
Various concepts have been proposed to explain this integration with the difference being in the interpositional tissue between the implant and the recipient bone.
Fibro-osseous Retention V/S Osseointegration
Fibro-osseous retention is the tissue to implant contact interposition of the healthy, dense collagenous tissue between the implant and the bone which is a concept based on the development of an intermediate ligamental system. While in Osseointegration load transfer to surrounding bone through an implant depends on the development of an ankylotic like relationship between the implant and the bone. The direct bone – to – implant interface without intervening connective tissue was proposed as early as 1939 by Strock and described by Branemark.
Branemark’s Theory of Osseointegration
In 1952, Swedish orthopedic surgeon, Branemark, conducted microscopic studies done in situ of the bone marrow in rabbit’s fibula with in vivo vital microscopic technique. Implanted titanium chambers containing an optical system for transillumination were used. However, these titanium chambers were not retrievable as they had fused with the surrounding bone. Further study revealed that titanium metal was biocompatible and had actually integrated with the bone and thus he called this process as osseointegration.
Branemark proposed that implants integrate such that the bone is laid very close to the implant without any intervening connective tissue. The titanium oxide permanently fuses with the bone. Branemark also stated that the implant should not be loaded and left out of function during the healing period for osseous integration to occur. Weiss’s Theory of Fibroosseous Fixation Weiss’ theory stated that there is a fibro-osseous ligament formed between the implant and the bone and this ligament can be considered as equivalent to the periodontal ligament found in the gomphosis. He defended the presence of collagen fibers at the bone-implant interface. He interpreted it as the peri-implantal ligament with an osteogenic effect and advocate the early loading of the implant.He believed that the collagen fibers invest the implant originating at the trabaculae of cancellous bone on one side, weaving around the implant, and reinserting into a trabaculae on the other side. When functional forces are exerted on the implant, deformation on the aspect of the implant alveolus closest to the implant surface exhibits compression which creates a negative charge, while tension at the outer aspect of the deformed trabeculae creates a positive charge there. The difference between the inner aspect and the outer aspect (tension) results in a bioelectric current (a piezoelectric effect) which induces a differentiation of the pleuripotential cells in the vicinity into components associated with bone maintenance around the implant.
Osseointegration Vs. Biointegration
In 1985, dePutter observed that there are two ways of implant anchorage or retention: mechanical and bioactive. Mechanical retention refers to the metallic substrate systems such as titanium alloy. The retention is based on the undercut forms such as vents, slots, dimples, screws etc and involves direct contact between the dioxide layer on the base metal and bone with no chemical bonding. Bioactive retention is achievedwith bioactive materials such as hydroxyapatite (HA), which bond directly to the bone, similar to ankylosis of natural teeth. Bone matrix is deposited on the HA layer as a result of physicochemical interaction between the collagen of bone and the HA crystals of the implant.
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