Biochemical Mechanism of Titanium Fixation into Living Bone: Acid Soluble Phosphoproteins in Bone Binds with Titanium and Induced Endochondral Ossification in vivo

SYNOPSIS In 2014, we discovered that bone phosphoproteins, which are collectively called SIBULING family of proteins, are equipped with titanium-binding ability. Further-more, the titanium implant devices which were coated with titanium-bound SIBLING induced more than 100 times faster bone formation of early stage when implanted into rat calvaria. These findings led us to an explanation why titanium implants could be fixed into living bone. Several other phosphoproteins including, phosvitin, caseins and phosphophoryn (a dentin phosphoprotein) were also found to bind with titanium by use of a chromatographic column packed with titanium beads. In this study we demonstrated that a typical phosphoprotein, phosvitin lost its titanium-binding ability in a time-dependent manner by the reaction with λ -protein phosphatase. The fact confirmed that certain specific phosphoserines λ residues in this protein were responsible for the titanium-protein interaction. For an additional confirmation of SIBLING-titanium interaction, we extracted bone and dentin proteins with a new and simple method of acidic condition and applied them to the chromatographic column packed with titanium beads. The results showed that definite portions of the acid soluble proteins from both bone and dentin were retained in the column. Electrophoretic analysis showed the retained fractions were Stains-all positive, indicating that both bone and dentin contain multiple phosphoproteins which have affinity with titanium. The titanium-bound fraction of acid extract was again coated on the titanium device and implanted into rat calvaria. After one week, histology showed that in addition to definite pattern of bone formation, process of endochondral ossification was clearly observed. In the control implant of uncoated titanium device, only collagenous tissues were observed, without any cartilage nor bone formation. Based upon these findings we reconfirmed that the core biochemical mechanisms underlying the strong bond between the titanium and living bone is based upon the interaction between the implanted titanium and multiple bone phosphoproteins in the host tissue.