Bad accounts system

My hip replace was done in May 2016. My medical aid only allows R27960 for a prostheses. I requested from Johnson & Johnson a quote for a steel head pennical quote as the medical aid will not cover the ceramic head. The quote was R27000 after discount. Both me and my husband are disabled. A few months later we received the shocked account of R11822.87 from Pinehaven Hospital where they confirmed that the medical aid did not paid everything for the hip replacement, To our surprise we discovered that they charged for a ceramic head and not for a steel head as per approved quotation. I have the proof of months correspondence between ourselves, the hospital accounts department and Johnson & Johnson where we requested them that they charged for the wrong head. No success. Yesterday we received a summons for the outstanding amount. How can the hospital keep us liable if Johnson & Johnson send them the incorrect invoice? I do have the quotation it differs.