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Biological fixation of cementless femoral implants requires primary stability by optimal fit in the proximal femur. The anatomy of the bone must then be known precisely. We analysed in vitro the accuracy of bone measurements of 32 femurs and compared the dimensions obtained from radiographs and CT scans with the true anatomical dimensions. Standard radiographs gave only a rough approximation of femoral geometry (mean difference: 2.4 +/- 1.4 mm) insufficiently accurate to allow selection of the best fitting prosthesis from a range of sizes and altogether inadequate to design a custom-made prosthesis. CT scans give greater accuracy (mean difference: 0.8 +/- 0.7 mm) in our experimental conditions, but in clinical practice additional sources of error exist.

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Traction tests were performed on the bovine anterior cruciate ligament-bone complex at seven strain rates (0.1, 1, 5, 10, 20, 30, 40%/s). Corresponding stress-strain curves showed that, for a given strain level, the stress increased with the augmentation of the strain rate. This phenomenon was important since the stress increased by a factor of three between the tests performed at the lowest and highest strain rates. The influence of the strain rate was quantified with a new variable called the "supplemental stress". This variable represented the percentage of total stress due to the effect of strain rate. It was observed that at a strain rate of 40%/s, more than 70% of the stress in the ligament was due to the strain rate effect. In fact, the strain rate strongly affected the toe region, but did not influence the linear part of the stress-strain curves. The use of the linear tangent moduli was then not adequate to describe the strain rate effect in the anterior cruciate ligament-bone complex. This study showed that the "supplemental stress" was a synthetic and convenient variable to quantify the effect of the strain rate on the entire stress-strain curves. This quantification is especially important when comparing the mechanical behavior between anterior cruciate ligament and tissues used as ligament graft.

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Traction tests on soft tissues show that the shape of the stress-strain curves depends on the strain rate at which the tests are performed. Many of the constitutive models that have been proposed fail to properly consider the effect of the strain rate when large deformations are encountered. In the present study, a framework based on elastic and viscous potentials is developed. The resulting constitutive law is valid for large deformations and satisfies the principles of thermodynamics. Three parameters -two for the elasticity and one for the viscosity -were enough to precisely fit the non-linear stress-strain curves obtained at different strain rates with human cruciate ligaments and patellar tendons. The identification results then in a realistic, three-dimensional viscoelastic constitutive law. The developed constitutive law can be used regardless of the strain or rotation values. It can be incorporated into a finite element program to model the viscoelastic behavior of ligaments and tendons under dynamic situations.

Venous thromboembolism after total hip replacement continues to be a serious problem. We conducted a study to determine whether adjustment of the dose of subcutaneous heparin to yield partial thromboplastin times in the high-normal range results in a greater reduction of postoperative deep-vein thrombosis than fixed doses of heparin. Seventy-nine patients undergoing elective hip arthroplasty were randomly divided into two groups two days before surgery. Group 1 (41 patients) received a fixed dose of 3500 IU of heparin subcutaneously ever eight hours; 16 of the 41 (39 per cent) had deep-vein thrombosis diagnosed by venography. Group 2 (38 patients) was started on the same dose, which was then adjusted to keep the activated partial thromboplastin time between 31.5 and 36 seconds. From the day of operation to the eighth postoperative day these patients needed progressively more heparin to maintain the activated partial thromboplastin time in the prescribed range. Only 5 of the 38 (13 per cent) had deep-vein thrombosis (P less than 0.01), and the number of thrombi in proximal veins was also lower in this group (P = 0.003). The number of units of blood transfused, the frequency of postoperative wound hematomas, and the drop in hemoglobin levels were identical in the two groups. Adjusted low-dose heparin prophylaxis appears to be a safe and efficacious method to reduce the frequency of deep-vein thrombosis in patients undergoing total hip replacement.