ACETABULAR SCREW RINGS AND SURFACE TREATMENTS - A SERIES OF 115 CONSECUTIVE PRIMARY ACETABULAR ARTHROPLASTIES WITH THE ZWEYMULLER,KARL THREADED CUP WITH A 5.8 YEARS AVERAGE FOLLOW-UP

Introduction The authors review and analyze a series of patients treat ed by total hip arthroplasty with a Karl Zweymuller threaded cup. Meth ods In a group of 115 consecutive primary total hip arthroplasties wit h the Karl Zweymuller cementless prosthesis, 100 acetabular screw ring s have been retrospectively studied with a 5.8 years average follow-up (range: 40 to 96 months). Results The clinical results were graded ex cellent and good in 97 per cent, fair in one and poor in two (2 per ce nt). Only one single threaded component failed. Radiologically, the ma jority of bone to implant gaps appeared filled. Four screw rings were not initially stable. One of them was due to a technical error and was revised ten days post surgery. Two migrated three degrees within the two first years but remained stable since with a good clinical result. The fourth never stabilized and represents the single failure of this series. At last follow-up, 99 per cent of components were stable and 96 per cent of bone-implant interfaces showed no radiolucent line. At the 7-8 years interval, the survivorship with radiographic stability c umulative rates were of 98,21 +/- 5,61 per cent (standard deviation) f or the current series and of 99,18 +/- 3,18 per cent for the 300 prima ry arthroplasties having been performed with the Zweymuller screw ring since 1986. Discussion Once bone integration apparently occurred, the authors have never observed any secondary migration indicating failur e of the bone anchorage phenomenon. These mid-term results are a) sign ificantly better than those of all first generation screw rings with n o micro-porous surfaces; b) better than those of the PCA acetabular cu p, c) slightly better than those of the Charnley cemented sockets for which loosening rates increase with time; and d) with equivalent follo w-up, similar to those of the Harris-Galante cup. In the authors opini on, the main reason leading to failure and abandoning of the first gen eration smooth screw rings is not the screw-in concept of their primar y fixation but the lack of any surface micro-porosity providing no sup port for osteointegration. Conclusion Due to its biomecanical characte ristics, excellent primary stability, thinness of its pure titanium mi cropitted surface and low module of elasticity, the Zweymuller screw r ing achieves a quite successful compromise between the different condi tions necessary not only to obtain but above all to maintain osteointe gration at least at mid term.