• C Delaunay and A I Kapandji.
• Clinique de l'Yvette, Longjumeau.
• Rev Chir Orthop Reparatrice Appar Mot. 1994 Jan 1; 80 (5): 388-402.

IntroductionThe authors review and analyze a series of patients treated by total hip arthroplasty with a Karl Zweymüller threaded cup.MethodsIn a group of 115 consecutive primary total hip arthroplasties with the Karl Zweymüller cementless prosthesis, 100 acetabular screw rings have been retrospectively studied with a 5.8 years average follow-up (range: 40 to 96 months).ResultsThe clinical results were graded excellent and good in 97 per cent, fair in one and poor in two (2 per cent). Only one single threaded component failed. Radiologically, the majority 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 cumulative rates were of 98.21 +/- 5.61 per cent (standard deviation) for the current series and of 99.18 +/- 3.18 per cent for the 300 primary arthroplasties having been performed with the Zweymüller screw ring since 1986.DiscussionOnce bone integration apparently occurred, the authors have never observed any secondary migration indicating failure of the bone anchorage phenomenon. These mid-term results are a) significantly better than those of all first generation screw rings with no micro-porous surfaces; b) better than those of the PCA acetabular cup, c) slightly better than those of the Charnley cemented sockets for which loosening rates increase with time; and d) with equivalent follow-up, similar to those of the Harris-Galante cup. In the authors opinion, the main reason leading to failure and abandoning of the first generation smooth screw rings is not the screw-in concept of their primary fixation but the lack of any surface micro-porosity providing no support for osteointegration.ConclusionDue to its biomecanical characteristics, excellent primary stability, thinness of its pure titanium micropitted surface and low module of elasticity, the Zweymüller screw ring achieves a quite successful compromise between the different conditions necessary not only to obtain but above all to maintain osteointegration at least at mid term.

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