THE SYNOVIAL-LIKE MEMBRANE AT THE BONE INTERFACE IN LOOSE TOTAL HIP REPLACEMENTS CONTAINS HIGH-LEVELS OF EXTRACELLULAR GROUP-II PHOSPHOLIPASE A(2)

Progressive lysis of bone in loose total hip replacement has been ascr ibed to the capacity of the synovial-like membrane present at the bone interface to produce prostaglandin E(2) and cytokines such as tumor n ecrosis factor (TNF alpha). Phospholipase A(2) (PLA(2)) produces rate limiting precursor i.e. arachidonic acid in the biosynthesis of variou s types of biologically active lipids including prostaglandins. It has been shown that extracellular human group II phospholipase A(2) is pr esent in large amount in synovial fluid of patients with synovitis and that the expression of this enzyme is under the control of cytokines such as TNF alpha. Furthermore, the human extracellular enzyme has bee n also shown to induce in an experimental animal model to cause disrup tion of a synovial like membrane without increasing prostaglandin prod uction. Here we have evaluated PLA(2) and TNF alpha levels in the supe rnatant of homogenate of the synovial-like membrane present at the bon e interface retrieved from six patients with a loose non septic failed total hip replacement. In all the membranes examined were found high levels of both TNF alpha (856 +/- 211 units/ml) and extracellular phos pholipase A(2) (2616 +/- 862 ng/ml). These findings suggest that extra cellular PLA(2) may play a major role in the process that cause disrup tion of the membrane at the bone-prosthesis interface.