The synovial - like membrane at the bone interface in loose total hip replacements contains high levels of extracellular group II phospholipase A 2

Abstract

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 E 2 and cytokines such as tumor necrosis factor (TNFα). Phospholipase A 2 (PLA 2) 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 A 2 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 TNFα. 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 PLA 2 and TNFα 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 TNFα (856 ± 211 units ml ) and extracellular phospholipase A 2 (2616 ± 862 ng ml ). These findings suggest that extracellular PLA 2 may play a major role in the process that cause disruption of the membrane at the bone-prosthesis interface.