IL-39: Dissolution rate (ASTM F1926) and solubility product (ISO 13779-6) of calcium phosphates

Calcium phosphate ceramics are widely used for the repair of bone defects and as coatings for orthopaedic implants. In order to match the growth rate of new bone, the resorption properties of these materials must be carefully controlled. In particular, the resorption rate may be affected by foreign crystalline or amorphous phases or chemical contamination. In vitro testing of the dissolution kinetics as well as the (equilibrium) solubility of calcium phosphates provides an indication of the stability of the material in the human body. Therefore, dissolution testing is required in ASTM F1926 and the determination of the solubility product is recommended in ISO 13779-6.

RMS Foundation 16.09.2024 6 min. Lesezeit

A dissolution test according to ASTM F1926 was recently developed at the RMS Foundation and validated according to the requirements of our ISO 17025 accreditation. In this test, a sample is placed in a buffer solution (MES or TRIS) at 37 °C for 24 hours and the Ca concentration is measured continuously with a Ca-ion selective electrode (see figure). In addition, the pH value of the solution is measured at the beginning and at the end of the test to exclude pH drift, i.e. to ensure sufficient buffer capacity. The test can be carried out on bone substitutes of any form as well as on coated samples or powder ground off from a coating. Since a direct correlation between the dissolution rate in vitro and the resorption rate in vivo is not possible, parallel tests on a historical control or a certified reference material are recommended. The solubility product (K_SP) is a material property that describes a chemical equilibrium between a solid substance and a solution of the same substance. We have developed and validated a test method to determine the K_SP of hydroxyapatite raw material powders according to ISO 13779-6. This method consists of soaking a sample in dilute phosphoric acid at 37 °C for 60 days and then measuring the pH, Ca and P concentration using inductively coupled plasma mass spectrometry (ICP-MS). The K_SP is then calculated using an equation described in ISO 13779-6 (Annex A.8). For comparison, the CSP of a pure, fully crystallized reference material (e.g. NIST calcium hydroxyapatite, SRM2910) can be determined in parallel. Depending on the purity of the sample, a significantly shorter test duration can be selected.

IL-39_TestgefaessPuffer

Figure: Test vessel filled with buffer solution (ASTM F1926), A) Ca-ion-selective electrode, B) purge gas tube (N2), C) stirrer (for powder, granulate or molded samples), D) temperature sensor, E) pH electrode, F) sample material.

Sample requirements:
Dissolution test (ASTM F1926): 150 mg powder/granules, shaped specimens, powder abraded from coatings or coated test coupons
Solubility product (ISO 13779-6): 400 mg powder (6 replicates each)

Equipment:
Polymer membrane ca-ion selective electrode (Metrohm Titrando), pH electrodes, Agilent 7700x ICP-MS

Other dissolution tests in the range of services:
- Dissolution and pH change of calcium phosphates according to ISO 13175-3
- Ceramic degradation products (biological evaluation) according to ISO 10993-14