Abstract
A large body of the orthopaedic literature clearly indicates that the cement mantle surrounding the femoral component of a cemented total hip arthroplasty should be at least 2 mm thick. In the early 1970s, another concept was introduced and is still in use in France consisting of implanting a canal filling femoral component line-to-line associated with a thin cement mantle. This principle has been named the “French paradox”. An explanation to this phenomenon has been provided by in-vitro studies demonstrating that a thin cement mantle in conjunction with a canal filling stem was supported mainly by cortical bone and was subjected to low stresses. We carried out a study to evaluate the in-vivo migration patterns of 164 primary consecutive Charnley-Kerboull total hip replacements. All prosthesis in the current series combined an all-polyethylene socket and a 22.2 mm stainless steel femoral head. The monobloc double tapered (5.9 degrees) femoral component was made of 316L stainless steel with a highly polished surface (Ra = 0.04 μm), a quadrangular section, and a neck-stem angle of 130 degrees. The stem was available in six sizes with a stem length (shoulder to tip) ranging from 110 mm to 160 mm, and a neck length ranging from 24 mm to 56 mm. For each size, the femoral component was available in two to four different diameters to adapt the implant to the medullary canal. Hence the whole range comprised a total of 18 standard femoral components. The femoral preparation included removal of diaphyseal cancellous bone to obtain primary rotational and varus/valgus stability of the stem prior to the line-to-line cementation. We used the Ein Bild Roentgen Analyse Femoral Component (EBRA-FCA) method to assess the subsidence of the femoral component. At the minimum 15-year follow-up, 73 patients were still alive and had not been revised at a mean of 17.3 years, 8 patients had been revised, 66 patients were deceased, and 8 patients were lost to follow-up. The mean subsidence of the entire series was 0.63 ± 0.49 mm (0 – 1.94 mm). When using a 1.5 mm threshold, only four stems were considered to have subsided. With revision of either component for any reason as the endpoint, the cumulative survival rate at 17 years was 90.5 ± 3.2% (95% CI, 84.2% to 96.8%). With radiological loosening of the femoral component as the endpoint, the cumulative survival rate at 17 years was 96.8 ± 3.1% (95% CI, 93.2% to 100%). This study demonstrated that, in most cases, a highly polished double tapered stem cemented line-to-line does not subside up to 18-year follow-up.
