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Modified cemented stem fixation in hip arthroplasty with computer based analysis – Experimental results

27 cemented stems were implanted in fresh‐frozen human cadaver femora with third generation antegrade cementing technique and with prechilled vacuum‐mixed and pre‐pressurized Palacos R bone cement. 3 groups with different implantation techniques were compared according to the achieved cement mantle thickness distribution. Group 1 (modified stem fixation): 7 canulated stems inserted over guidetubes that were fixed in distal synthetic plugs (3 Willert CF‐30 and 4 flanged Charnley stems). The guidetubes served simultaneously for stem centralization and evacuation of the intramedullary cavity. Group 2: 12 stems with distal centralizers with fins (4 flanged Charnley, 8 anatomic cemented AC), and Group 3: 8 stems without centralizer (4 Willert CF‐30 and 4 flanged Charnley). For analysis, each femur was cut into mean 25 cross‐sections (range, 22 ‐ 31). Contact radiographs were taken and digitized with a sampling distance of 0.12mm (204.8 dpi). Using a custom‐made computer program (FemStat 1.001), the cement mantle thickness was determined at mean 650 implant surface points per cross section. Results: The percentage of critical cement mantle thickness was lower for the canulated stems with guidetube. Here 14.1 % of the measurements were smaller than 2mm. For the stems with distal centralizer and without centralizer this was 28.3 % and 28.2 % (p = 0.003 and p = 0.002, U‐test). Cement mantles smaller than 1mm were also reduced for group 1 (2.8 % compared with 4.7 % and 7.5 %) but this was not significant (p = 0.384 and p = 0.094). The stems without centralizers had only slightly inferior cement mantles if compared to stems with distal centralizers with fins (for < 2mm p = 0.571, for < 1mm p = 0.305). Comparing different cementing techniques for one constant type of prosthesis (Charnley) the canulated stems with guide tubes provided a significantly better cement mantle thickness than the stems with distal centralizers (for < 2mm 10.3 % compared with 26.6 %, p = 0.029; for < 1mm 1.5 % compared with 3.4 %, p = 0.343). Distal centralizers failed, as the intramedullary cavity is elliptic in cross section and the gaps between the wings caused malposition. For all stems in group 3 improvement of the cement mantle thickness was predominantly achieved in the distal and middle third of the femur. Conclusion: The implantation technique with canulated stems inserted over a guidetube allows superior stem positioning leading to a more favourable cement mantle.