IN AN IN VITRO MODEL CAN NTX BE USED AS A MARKER OF WEAR PARTICULATE INDUCE OSTEOLYSIS INSTEAD OF THE ESTABLISHED RADIOLABELLED BONE MODEL.

Background: Aseptic loosening of total joint arthroplasty is characterised by osteolysis, which is caused by osteoclasts and macrophages. Collagen cross link molecules N Telopeptides (NTx) are released during osteolysis and represents a highly specific marker for bone resorption. NTx could be a potential marker in the laboratory investigation of aseptic loosening with the advantage of being cheaper and easier to perform compared to present established markers. The aim of this study was to show that NTx generated during osteolysis by cells extracted from human interface membranes of aseptically loosened hips correlates with the established radiolabelled 45Ca bone resorption model. Methods: Cells from human interface membranes of aseptic loosened hip joints were cultured with dead radiolabelled (45Ca) mice calvaria discs and in the control culture no cells were added to the culture system. Calvaria discs used in each experiment comparison were from the same parietal bone. The supernatant culture medium were extracted on day 3,7,10 & 14 and assayed for NTx and by scintillation counting. On day 14 the remaining culture medium and cells were assayed by scintillation counting and the total remaining 45Ca in the bone were measured. Results: All results were expressed as the ratio of bone exposed to cells (BC)/bone only (B). Supernatant samples for 45Ca and NTx showed a rise in BC/B ratio with time. These were 0.83, 0.88, 0.97 & 1.08 (p=0.0001) and 1.06, 1.21, 1.41 &1.40 (p = 0.03) respectively. In the bottom sampling for 45Ca the mean ratio of BC/B was 1.8 (p=0.0001) and the mean BC/B ratio for the remaining 45Ca in the bone at the end of the culture was 0.81 (p=0.0007). There was a strong correlation between 45Ca and NTx (r = 0.88). Discussion: The initial drop of calcium levels can be explained by calcium uptake by the cells. We believe this is the first time human interface membrane cells have been shown to release NTx during osteolysis in an in vitro model. Replacing 45Ca radilabelled bone with NTx as a marker represents an important step towards simplifying and reducing the cost of an in in vitro model of particle induced osteolysis.