Differential accumulation of lead and zinc in double-tidemarks of articular cartilage

The trace elements Lead (Pb) and Zinc (Zn) are of great biological impact. Long-term exposure to increased Pb concentrations is associated with several chronic diseases whereas Zn is essential for numerous enzymes. In a recent study we found remarkably elevated concentrations of Pb and Zn in the tidemark (TM), which is a mineralization front of human articular cartilage. Duplication or multiplication of TMs occurs with advancing age or degeneration. We hypothesized that trace elements accumulate in TMs as a function of time. Thus, in cases of double TMs, the inner (older) TM should have higher Pb and Zn levels than the outer (younger) TM. Undecalcified tissue from articular cartilage and subchondral bone of femoral heads and patellae was examined by synchrotron radiation induced confocal micro X-ray fluorescence analysis (SR µ-XRF) and by quantitative backscattered electron imaging to determine the local distribution of Ca, Zn and Pb in this tissue. The evaluation of x-ray fluoresence intensities in double TMs revealed in average a 2.6 fold higher Pb-level in the inner TM compared to the outer TM while the Zn concentrations were simmilar. The biochemical composition of the TM seems to selectively accumulate Pb from the interstitial fluid of articular cartilage. In contrast, Zn is supposed to be a fixed constituent of the TM. The results might be of clinical importance, when during degenerative processes in osteoarthritis the tidemark region is destroyed and remarkable concentrations of Pb (up to the 62 fold of the bone level) are released into the body fluid