The synovial-like membrane at the bone interface in loose total hip replacements contains high levels of extracellular group II phospholipase A2.

Progressive lysis of bone in loose total hip replacement has been ascribed to the capacity of the synovial-like membrane present at the bone interface to produce prostaglandin E2 and cytokines such as tumor necrosis factor (TNF alpha). Phospholipase A2 (PLA2) produces rate limiting precursor i.e. arachidonic acid in the biosynthesis of various types of biologically active lipids including prostaglandins. It has been shown that extracellular human group II phospholipase A2 is present 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 been also shown to induce in an experimental animal model to cause disruption of a synovial like membrane without increasing prostaglandin production. Here we have evaluated PLA2 and TNF alpha levels in the supernatant of homogenate of the synovial-like membrane present at the bone 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 phospholipase A2 (2616 +/- 862 ng/ml). These findings suggest that extracellular PLA2 may play a major role in the process that cause disruption of the membrane at the bone-prosthesis interface.