A magnesium calcium phosphate-based cement as a bone adhesive: characterization and biomechanical evaluation

Usually, comminuted fractures contain fragments that are too small for fixation with Kirschner (K)-wires or screws. For those bony or osteochondral fragments, a bone adhesive would be desirable to, for example, enable easy anatomic reduction, avoid discarding of the fragments, and enable temporary fixation to visualize reduction before definitive osteosynthesis is performed. Most of the currently available bone adhesives have shortcomings, such as cytotoxicity, lack of resorbability, and inadequate mechanical properties. Thus, there is room for improved bone adhesives. The present work involves synthesis, characterization, and biomechanical evaluation of three variants of a novel magnesium calcium phostphate-based cement that may be used as a bone adhesive.

Three novel experimental formulations of a magnesium calcium phosphate-based cement and a commercially-available cyanoacrylate bone adhesive (Glubran^® 2) were used. The formulations were a magnesium phosphate (Mg₃PO₄ + MgO + phytic acid) (MPC_25), a magnesium calcium phosphate (Mg_2.75Ca_0.25PO₄ + MgO + phytic acid) (MPCa_22.5), and a magnesium phosphate that had undergone modified temperature stages during sintering (Mg₃O₈P₂ * x H₂O) (HT-MPC). In vitro quasi-static compression tests were conducted using cuboid specimens. Split fractures of the lateral tibial plateau were created in dissected porcine tibiae. The lateral fracture fragments were glued onto the condyles. Load was applied on the glued fracture fragments via the femoral component of a knee hemiarthroplasty. Cyclic loading tests with increasing load levels, load-to-failure tests, and torque tests were conducted using this biomechanical model.

Among the experimental cement formulations, HT-MPC had the highest compressive strength (26.8 ± 9.5 MPa), MPCa_22.5 had the highest cyclic increasing load-to-failure (162 ± 40 N) and the highest load-to-failure (295 ± 84 N), while the highest calculated shear strength was obtained with both MPC_25 and MPCa_22.5 (0.27 ± 0.12 and 0.26 ± 0.06 MPa, respectively), and the highest torque-to-failure was obtained with both MPCa_22.5 and HT-MPC (2.2 ± 0.8 and 2.1 ± 1.2 Nm, respectively). The calculated shear strength for the experimental cement formulations (0.13-0.38 MPa) is above the minimum that has been suggested to be required for a bone adhesive to be used in clinical practice (0.2 MPa). Relative to the experimental cement formulations, the compressive strength of Glubran^® 2 was significantly lower, but for each of the other four biomechanical parameters, values were significantly higher.

Each of the synthesized novel magnesium calcium phosphate-based cement formulations has adequate compressive strength, shear strength and resistance to fatigue failure. Thus, each merits further study for use in intraoperative fixation of small bone fragments.