Formation of macropores in calcium phosphate cement implants

A calcium phosphate cement (CPC) was shown to harden at ambient temperatures and form hydroxyapatite as the only end-product. Animal study results showed that CPC resorbed slowly and was replaced by new bone. For some clinical applications, it would be desirable to have macropores built into the CPC implant to obtain a more rapid resorption and concomitant osseointegration of the implant. The present study investigated the feasibility of a new method for producing macropores in CPC. Sucrose granules, NaHCO3, and Na2HPO4 were sieved to obtain particle sizes in the range of 125 microm to 250 microm. The following mixtures of CPC powder (an equimolar mixture of tetracalcium phosphate, Ca4(PO4)2O, and dicalcium phosphate anhydrous, CaHPO4) and one of the above additive granules were prepared: control-no additive; mixture A-0.25 mass fraction of sucrose; mixture B-0.25 mass fraction of NaHCO3; mixture C-0.25 mass fraction of Na2HPO4, and mixture D-0.33 mass fraction of Na2HPO4. Cement samples were prepared by mixing 0.3 g of the above mixtures with 0.075 ml of the cement liquid (1 mol/l Na2HPO4). After hardening, the specimens were placed in water for 20 h at about 60 degrees C to completely dissolve the additive crystals. Well-formed macropores in the shapes of the entrapped crystals were observed by scanning electron microscope (SEM). The macroporosities (mean+/-standard deviation; n = 6) expressed as volume fraction in % were 0, 18.9 +/- 1.7, 26.9 +/- 1.6, 38.3 +/- 4.4 and 50.3 +/- 2.7 for the control, A, B, C and D, respectively. The diametral tensile strengths (mean+/-standard deviation; n = 3) expressed in MPa were 10.1 +/- 0.7, 3.7 +/- 0.3, 2.4 +/- 0.2, 1.5 +/- 0.5 and 0.4 +/- 0.1, respectively, for the five groups. The results showed that macropores can readily be formed in CPC implants with the use of water-soluble crystals. The mechanical strength of CPC decreased with increasing macroporosity.