A Systems Biology Approach to the Study of Contact Osteogenesis on Endosseous Implants
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A Systems Biology Approach to the Study of
Contact Osteogenesis on Endosseous Implants
Jason Chau, MASc. student, IBBME
Abstract—Contact osteogenesis provides early endosseous
implant stability by forming bone directly on the implant surface,
and is critical in determining the clinical success of the dental
implant. This process is influenced by early blood-implant
interactions in the endosseous wound site, and can be modulated
with the use of different implant materials and microtextured
surface topographies. This paper reviews the key stages of contact
osteogenesis – hemostasis, osteoconduction, angiogenesis, and de
novo bone formation – and describes how systems biology can be
applied at each stage to improve our understanding of contact
osteogenesis, implant integration, and implant design, or to
provide clinically relevant applications.
Index Terms—De novo bone formation, endosseous implant,
hemostasis, osteogenesis, peri-implant healing, platelet biology,
systems biology
I. INTRODUCTION
P
ROSTHODONTICS,
or the branch of dentistry that deals with
the replacement of missing teeth or related jaw structures
using artificial devices, is becoming an increasingly successful
therapeutic modality with the advent of better endosseous (inbone) implant designs that shorten post-operation recovery
time. It has been estimated that approximately one million
implant procedures are now performed annually, worldwide
[1]. Of central importance to the design of better implants, is a
solid understanding of the cellular and molecular interactions
occurring at the tissue-implant interface during the postoperation healing phase, and the mechanisms and factors that
regulate these interactions.
To this end, two types of peri-implant bone regeneration
have been identified in the literature. The first type, distance
osteogenesis, occurs when new bone formation is initiated at
the old bone surface surrounding, but at a distance from, the
implant. This causes new bone to slowly ‘grow’ towards the
implant surface and results in the eventual approximation of
the implant surface shape by the newly formed bone. Because
distance osteogenesis occurs from a distance, there will always
be an intervening layer of non-bone cells between the bone and
implant, and can therefore be considered inferior from both a
mechanical and integrative standpoint. Distance osteogenesis
is reviewed more extensively elsewhere [2]. The second type,
contact osteogenesis, occurs when osteogenic cells migrate to
the implant surface, differentiate into osteoblasts, and begin
laying down new bone directly onto the implant. This results in
the apposition of newly secreted matrix to the implant surface,
and is important for providing early implant stability in the
endosseous site when the surrounding bone is mostly
cancellous, as is the case in the posterior maxilla. Because the
requirement of a stable union between endosseous implants
and bone is paramount in the field of skeletal surgery [3],
contact osteogenesis can be considered to be a clinically
superior mode of healing. Hence this paper will focus on the
application of systems biology to contact osteogenesis.
Since, by definition, an implant has neither bone matrix nor
cells on its surface upon implantation, the bone mass that
subsequently appears on implant surfaces in contact
osteogenesis must be deposited by osteogenic cells that have
migrated to the implant surface and begun secretory activities.
Like all biological systems, the endosseous healing site is a
complex network of interacting components including cells,
cytokines, growth factors, and other molecular species.
Although recent research has yielded much information
regarding contact osteogenesis in the peri-implant
compartment, many questions still remain. For example, how
do protein-protein interactions affect platelet activation in the
extravasated blood of the peri-implant compartment? What is
the bone-bonding mechanism, and how do different implant
surface materials and designs affect the bone-bonding
interface? The recent application of proteomics to the study of
platelet biology may yield answers to some of these questions.
[4] Clearly, a systems biology approach would be useful for
modeling and analyzing the large quantities of empirical data
that could be derived from such a system.
As a whole, bone-healing research has heretofore been
primarily a hypothesis-driven science (i.e. employing a topdown approach), and hence based on observable phenomena
and researchers’ intuitions. A data-driven approach such as
that emphasized in systems biology could therefore lead,
naturally, to a more comprehensive understanding of the
system and its underlying mechanisms. Furthermore, bone
implant surface designs and materials comprise environmental
and chemical perturbations to the biological system.
Developing a system profile for endosseous wound sites would
allow researchers to systematically investigate the effects of
perturbation in an iterative manner, and improve our
understanding of both the system and implant’s design. Hence
the study of contact osteogenesis in endosseous wound sites
falls naturally within the framework of a systems biology
approach. There is also currently debate as to whether the
A Systems Biology Approach to the Study of Contact Osteogenesis on Endosseous Implants
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presence of an implant induces a different mode of healing
than that seen in fracture healing [5], [6]. A systems biology
understanding of contact osteogenesis would help to answer
that question.
II. HEMOSTASIS
Hemostasis is the stoppage of bleeding or hemorrhage, and
is achieved by the formation of a blood clot in endosseous
wound sites. Given the surgical trauma associated with the
implantation of endosseous implants, and the vascular nature
of bone, blood is guaranteed to be the first tissue to come into
contact with the implant surface [7], [8]. Since the migration of
osteogenic cells to the implant surface is believed to be
dependent on the activation of blood platelets and the
establishment of a temporary fibrin scaffold within the periimplant compartment [9], early blood-implant interactions can
be assumed to be of critical importance in determining later
cellular responses that occur in the wound healing cascade,
and ultimately the successful integration of the implant with
the surrounding tissue. Although hemostasis involves many
more cellular and molecular interactions than those that are
described here, the current discussion will focus on cascades
induced by specific platelet releasates for reasons of simplicity
and immediate relevance. Anderson [10], and Gemmell and
Park [7], provide a more complete discussion of the cellular
cascades of wound healing and hemostasis in the presence of
an implant, respectively.
In particular, the activation of platelets causes the platelets
to change shape and release many different cytokines, growth
factors, and microparticles, some of which create chemotactic
gradients for osteogenic cells, stimulate osteoblast
proliferation, and act as enzymes to catalyze coagulation
reactions. Implant surface-adsorbed fibrinogen, a soluble
protein in blood, mediates the initial adhesion of platelets by
binding to the GPIIb/IIIa integrin platelet surface receptor
[11]. The amount of fibrinogen adsorbed on the implant
surface, [8] as well as platelet activation levels [12] – [15],
have both been shown empirically to increase on surfacemicrotextured implants as compared to smooth-surfaced
implants. The increase in fibrinogen adsorption has been
postulated as being a result of the increased surface area over
which adsorption may occur, as well as the presence of surface
features upon which fibrinogen may become entangled. Since
platelet activation catalyzes the conversion of prothrombin to
thrombin, which cleaves fibrinogen to fibrin, it follows that
there should be more fibrin attached to implants with
microtextured surfaces than implants with smooth surfaces.
This simplified summary of events at the blood-implant
interface is illustrated in Fig. 1. Good anchorage of the fibrin
scaffold to an implant surface is important because migrating
osteogenic cells have been shown to exert a contractile force
of approximately 3 nN/cell [16], and the arrival of these
migrating cells to the implant surface in contact osteogenesis is
critically dependent on the retention of the fibrin scaffold to
Fig. 1. Simplified representation of hemostatic events occurring at the
endosseous implant surface. Events include fibrinogen adsorption, platelet
binding, platelet degranulation, prothrombin to thrombin conversion,
fibrinogen cleavage, and fibrin adsorption.
the implant. Hence there can be little doubt that platelet
activation plays a critical role in blood clot formation as well
as the regulation of contact osteogenesis on endosseous
implants.
What is not well known is how the implant surface
chemistry and rugosity modulates the number, and degree of
activation, of platelets. In a more general sense, knowledge of
material ‘thrombogenecity,’ or the ability of different materials
to induce the formation of a blood clot [17], at the molecular
level, is currently limited. Gemmell and Park [7] provide a
detailed review of the biochemical species and reactions
involved in the blood coagulation cascade, and identify that,
“how the physical or chemical composition of the implant
surface impacts on these reactions is largely unknown”.
A promising recent development in platelet biology research
that should improve our understanding of platelet activation
mechanisms, and lend itself to the study of endosseous implant
designs using a systems biology approach, is the application of
proteomics to the study of platelets. Platelet proteomics is the
study of the protein complement of the part of the genome that
encodes for all platelet biology [4]. Indeed, all studies of
platelet activation up until now have had a reductionist
approach, using only one protein at a time [12]. The advent of
platelet proteomics will allow researchers to take a data-driven
approach, and address, for example, the question of what
happens when platelets are exposed to a mixture of proteins.
The availability of such a comprehensive knowledge base
will be invaluable for the re-evaluation of the blood
coagulation cascade under a systems biology framework, and
enable us to gain a more detailed understanding of how
different implant materials and surface designs affect implant
thrombogenecity. Consequently, better endosseous implant
designs could be developed.
III. PERI-IMPLANT ANGIOGENESIS
Angiogenesis is the growth of blood vessels occurring in an
adult through the migration and proliferation of endothelial
cells [18], and is important for revascularization of the
A Systems Biology Approach to the Study of Contact Osteogenesis on Endosseous Implants
endosseous wound site after tissue has regenerated. Following
the destruction of the necrotic tissue by a vanguard of
macrophages in the peri-implant compartment (i.e. another
biological cascade initiated in part by platelet degranulation,
but not described above), a zone of migratory fibroblasts and
capillary sprouts, followed by another zone of secretorily
active fibroblasts and functioning capillary loops is seen to
occur [19]. These two latter zones represent the phenomena of
angiogenesis and early tissue matrix regeneration respectively
[18]. The resulting dense capillary network that is created is
required because remodeling tissues have higher metabolic
needs. It is usually resorbed once healing is complete. From
this observation alone, it can be expected that angiogenesis is a
complex phenomenon regulated by a complex cascade of
biochemical signals.
Transforming growth factor beta (TGF-) and plateletderived growth factor (PDGF) are two growth factors that are
released upon platelet activation and play important roles in
angiogenesis. The effects of TGF-, in particular, are difficult
to characterize because markedly different results have been
observed in vitro depending on the concentration, the assay
used, and the cofactors present [19]. Furthermore, little work
has been done to understand how angiogenesis may be
affected by environmental perturbations such as the presence,
or surface design, of an endosseous implant [18].
Despite the many hurdles that need to be overcome to gain a
comprehensive understanding of angiogenesis, with a
sufficient amount of empirical information and a data-driven
approach, the dose and time responses of relevant cytokines
and growth factors, and ultimately angiogenesis itself, could be
modeled and simulated accurately as mathematical equations.
The angiogenic response to environmental perturbations such
as endosseous implants could then also be studied from this
systems biology framework.
IV. CONTACT OSTEOGENESIS
A. Osteoconduction
Following the establishment of a chemotactic gradient by
blood platelet activation, and the formation of a fibrin clot
during hemostasis, osteogenic cells will migrate towards the
endosseous implant through the three-dimensional protein
meshwork of the clot. This migratory activity is known as
‘osteoconduction,’ and is distinct, in this context, from the
common use of the term in biomaterials literature to mean the
apparent growth of bone tissue “along” an implant surface
[19]. As previously mentioned, if the contractile forces of the
osteogenic cell migration exceed the adhesive forces of the
transient fibrin scaffold over which the cells are migrating,
then the scaffold will detach from the implant surface. This
would cause the osteogenic cells to stop migrating,
differentiate into osteoblasts, and begin secreting new bone
matrix away from the implant surface, forming bony spicules
in the peri-implant compartment [9].
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Osteoconduction and the influences of factors such as
chemotactic gradient strength and the amount of fibrin present
at each location in space could easily be modeled with a set of
partial differential equations to gain a better understanding of
possible osteogenic cell fates in the peri-implant compartment.
This type of information could be used, for example, in the
pre-surgical assessment of whether or not a patient has a high
enough concentration of blood coagulation components (e.g.
platelets) to ensure successful endosseous integration of the
implant.
B. De novo bone formation
If the migrating osteogenic cells reach the implant surface,
they will undergo differentiation into osteoblasts, become
sessile, and begin secreting new bone matrix onto the implant
surface. This process of initial matrix secretion by newly
differentiated osteogenic cells is known as ‘de novo bone
formation,’ and is distinguished from continued appositional
growth by previously differentiated osteoblasts, by the
presence of a non-collagenous mineralized ‘cement line’
matrix of approximately 0.5 m thickness at the bone-implant
interface, that is not seen in the latter. De novo bone formation
has been described as a four-stage process comprising the
initial adsorption of the non-collagenous bone proteins
osteopontin (OPN) and bone sialoprotein (BSP) to the implant
surface, the initiation of mineralization (i.e. nucleation of the
cement-line matrix) by the adsorbed proteins, continued
mineralization resulting from crystal growth, and the assembly
of a mineralized collagenous matrix overlying the cement-line
matrix [19] – [21]. Because this interfacial cement line matrix
bonds to both the bone and endosseous implant surfaces, it is
of particular interest for studying the phenomenon of ‘bone
bonding,’ the exact meaning and perceived mechanisms of
which have engendered much debate [9]. Hosseini et al. have
recently shown that the cement line matrix exhibits a
heterogeneous morphology and consists of several organic as
well as inorganic components, and that this matrix is formed
by cells of a specialized osteogenic phenotype prior to the
expression of BSP [22].
Employing a systems biology approach to develop a system
profile by studying the physical and chemical characteristics of
the cement line matrix boundary with many different implant
surfaces (i.e. both material- and topography-wise) would not
only yield clues as to the exact nature of the bone bonding
mechanism, but could also allow us to forge an understanding
of how to use different implant materials and surface designs
to control the biological response of the cement line matrix
generation for therapeutic purposes.
V. IMPLANT MATERIALS
Titanium and titanium alloy are currently the most widely
used metals in the manufacture of endosseous implants
because of their biocompatibility, lightness, mechanical
strength, and high corrosion resistance [23]. However,
titanium and titanium alloys are generally not considered to be
A Systems Biology Approach to the Study of Contact Osteogenesis on Endosseous Implants
bone bonding [9]. Calcium phosphate materials such as
hydroxyapatite (HA) are considered to be bone bonding [24].
The two dominant paradigms of thought on the nature of the
bone bonding mechanism are a chemical bonding interaction at
the tissue-implant interface, and a micromechanical
interlocking of the two surfaces. Since bone bonding
represents a macroscopically seamless integration with the
surrounding bony tissue regardless of the underlying
mechanism, it is a desirable for endosseous implant surfaces to
be bone bonding. To this end, calcium phosphate surface
coatings have been applied to titanium and titanium alloy
implants to produce biocompatible implants that are
mechanically strong, corrosion resistant, and bone bonding.
As mentioned above, a data-driven approach to studying the
cement line matrix formation on different implant materials
could help elucidate the true mechanism of bone bonding.
VI. CONCLUSION
A systems biology approach to the study of contact
osteogenesis in endosseous wound sites would provide
researchers with a more complete understanding of the events
that occur in the peri-implant healing compartment, as well as
how they are modulated by novel implant surface designs.
Moreover, due to the multidisciplinary nature of peri-implant
endosseous healing research, a data-driven integrative
approach would be better suited to the task than traditional
hypothesis-driven methods. As new high-throughput analysis
tools are developed, new areas of research, such as platelet
proteomics, will be established to complement the systems
biology approach proposed here, and the benefits to our
understanding of tissue-implant interactions as well as clinical
protocols will become increasingly evident.
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