Titanium Implants Induce Expression of Matrix Metalloproteinases in Bone During Osseointegration

Titanium Implants Induce Expression of Matrix Metalloproteinases in Bone During Osseointegration
Saturday, 5 February 2005
RedNova News
Abstract-
Implanted pure titanium fixtures are able to completely integrate with bone, in part because of the formation of a strong extracellular matrix (ECM) bond at the titanium-bone interface. In this study, we used a rodent femur model of intramedullary osseointegration to analyze the changes in immunoreactivity of ECM- controlling matrix metalloproteinases (MMPs), tissue inhibitor of metalloproteinase-3 (TIMP-3), and tumor necrosis factor alpha (TNF- alpha) during osseointegration. We observed dramatic increases in MMP-2, MMP-9, MMP-7, TIMP-3, and TNF-alpha in osteocytes, osteoclasts, haversian canals, and the interface matrix in bone ipsilateral to the titanium implant. An increase in TIMP-3, MMP-9, and MMP-7 in hypertrophied chondrocytes and the vascular component of the epiphysial growth plate was also observed in experimental bone. These findings were not seen in contralateral or sham- operated bone, where the titanium fixtures were threaded into the femur and immediately removed. Our data link titanium-induced bone remodeling to changes in expression and distribution of MMPs.
Key words: bone, endosteal, histopathology, intramedullary, matrix, MMP, osseointegration, TIMP, titanium, TNF.
Abbreviations: BSA = bovine serum albumin, DAB = diaminobenzidine, ECM = extracellular matrix, MMP = matrix metalloproteinases, NIH = National Institutes of Health, OD = optical density, PBS = phosphate buffered saline, TIMP = tissue inhibitors of metalloproteinases, TNF = tumor necrosis factor, VA = Department of Veterans Affairs.
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INTRODUCTION
The remarkable success of endosteal titanium implants in dental, cranial-maxillary facial reconstruction, and orthopedic applications [1] can be attributed to the capability of pure titanium implants to become permanently integrated with living bone, a phenomenon defined as osseointegration [2]. Direct contact between living bone and the surface of a load-carrying titanium implant forms a strong structural and functional extracellular matrix (ECM) bond at the interface that is composed of proteoglycans, glycoproteins, and adhesion molecules [3-12]. This matrix bond increases in strength over time [1,4], promoting reparative osteogenesis at the interface that results in clinical fixation of the implant [6].
Bone matrix turnover is regulated by the extracellular zinc- dependent enzyme family of matrix metalloproteinases (MMPs) comprising collagenases, gelatinases, stromelysins and membrane- type MMPs [13]. Bone development and remodeling requires activity of MMPs for matrix maintenance and repair, bone rsorption, and the coupling to bone formation [12-14]. MMP-9 (gelatinase B) is thought to be important in controlling osteoclast differentiation and recruitment into remodeling bone [14-18]. Both MMP-9 and MMP-2 (gelatinase A) have been implicated in bone resorption that results in the loosening of prostheses [19-20]. MMP-7 (matrilysin) degrades proteoglycans [21], the key structural substrates for adhesion of titanium fixture to bone [3-4,10-11]. This potent proteoglycanase has also been shown to regulate macrophage migration during bone resorption through the release of the immunomodulatory cytokine tumor necrosis factor alpha (TNF-a) [22]. A number of MMPs release soluble TNF-a from its transmembrane precursor form [23], and TNF-a in turn, induces MMP gene expression, including MMP-9 [24]. TNF-a promotes bone resorption [25-26] and governs signaling mechanisms between osteoblasts and osteoclasts through the regulation of endocrine stimulants of bone resorption, such as parathyroid hormone [14].
With the use of a transverse fracture model in TNF-a receptor knockout mice, TNF-a has been shown to be critical for osteoprogenitor cell recruitment and intramembranous bone formation [27]. MMP function is regulated through expression, activation from a proenzyme form, and importantly, interaction with naturally occurring tissue inhibitors of metalloproteinases (TIMPs). Within the TIMP family, TIMP-3 is a multifunctional protein exclusively localized in ECM [28] and is a potent modulator of angiogenesis [29], a process considered fundamental in coupling bone rsorption and formation [30]. In vitro studies have shown that titanium particles induced MMP-2 and TNF-a in cultured macrophages [31- 33] and TNF-a in osteoblastlike cells [34], while MMP-2 and MMP- 9 were stimulated by titanium substrates in fibroblasts [35] and by pure titanium discs in primary human osteoblasts [36].
To better understand the in vivo molecular mechanisms of osseointegration, we previously developed a rat femur model of intramedullary osseointegration and observed dynamic changes in neural-immune activity in bone and protein gene product 9.5 (PGP 9.5) and calcitonin gene-related peptide-positive (CGRP-positive) sensory nerve fibers at the titanium-bone interface [37]. We are now reporting that pure titanium threaded rods implanted in the medullary cavity of rat femurs generate increases in immunoreactive MMP-2, MMP-7, MMP-9, TNF-a, and TIMP-3 that correlate with the structural and functional remodeling in bone, resulting in osseointegration.
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