Deterioration of the mechanical properties of calcium phosphate cements with Poly (γ-glutamic acid) and its strontium salt after in vitro degradation

The mechanical reliability of calcium phosphate cements has restricted their clinical application in load-bearing locations. Although their mechanical strength can be improved using a variety of strategies, their fatigue properties are still unclear, especially after degradation. The evolutions of uniaxial compressive properties and the fatigue behavior of calcium phosphate cements incorporating poly (gamma-glutanlic acid) and its strontium salt after different in vitro degradation times were investigated in the present study. Compressive strength decreased from the 61.2 +/- 5.4 MPa of the original specimen, to 51.1 +/- 4.4, 42.2 +/- 3.8, 36.8 +/- 2.4 and 28.9 +/- 3.2 MPa following degradation for one, two, three and four weeks, respectively. Fatigue life under same loading condition also decreased with increasing degradation time. The original specimens remained intact for one million cycles (run-out) under a maximum stress of 30 MPa. After degradation for one to four weeks, the specimens were able to withstand maximum stress of 20, 15, 10 and 10 MPa, respectively until run-out. Defect volume fraction within the specimens increased from 0.19 +/- 0.021% of the original specimen to 0.60 +/- 0.19%, 1.09 +/- 0.04%, 2.68 +/- 0.64% and 7.18 +/- 0.34% at degradation time of one, two, three and four weeks, respectively. Therefore, we can infer that the primary cause of the deterioration of the mechanical properties was an increasing in micro defects induced by degradation, which promoted crack initiation and propagation, accelerating the final mechanical failure of the bone cement. This study provided the data required for enhancing the mechanical reliability of the calcium phosphate cements after different degradation times, which will be significant for the modification of load-bearing biodegradable bone cements to match clinical application.