When fixing a mid or distal periprosthetic femoral fracture with an existing hip replacement, creation of a stress-riser is a significant concern. Our aim was to identify the degree of overlap required to minimise the risk of future fracture between plate and stem.

Each fixation scenario was tested using 4th generation composite femoral Sawbones®. Each sawbone was implanted with a collarless polished cemented stem with polymethyl methacrylate bone cement and cement restrictor. 4.5mm broad Peri-loc™ plates were positioned at positions ½, 1 and 2 shaft diameters (SD) proximal and distal to the tip of the femoral stem. Uni-axial strain gauges (medial and lateral longitudinal gauges, anterior and posterior torsional gauges) measured microstrain at tip of the femoral stem with a standard load of 500N in axial, 3-point lateral and composite torsion/posterior loading using an Instron machine.

With axial loading fixation with 2SD proximal resulted in the least amount of strain, in both tension & compression, at the tip of the femoral stem. Fixation with 4 unicortical screws was significantly better than 2 alternating unicortical screws (mean microstrain difference 3.9 to 15.3, p<0.0001). With lateral 3-point loading fixation with 2SD proximal overlap and 2 alternating unicortical screws resulted in the least amount of strain, in both tension and compression, at the tip of the femoral stem (p<0.0001). With torsion & posterior displacement 2SD proximal fixation resulted in the least amount of rotational strain. There was no significant difference between 4 unicortical screws compared to 2 alternating unicortical screws (p>0.05 in 3 of 4 gauges).

Fixation of midshaft or distal femoral fractures with a well-fixed total hip arthroplasty should have at least 2 shaft diameters of proximal overlap with a 4.5mm broad plate. It is not clear if 4 unicortical screws or 2 alternating screws are optimal.

Introduction: The viscosity of bone cement used in total joint arthroplasty is an important for determining the proper handling characteristics of the cement and its interlock with bone. The degree of penetration and, therefore, the integrity of the arthroplasty are dependent on the viscosity of the bone cement system. As yet there is still no standard measurement of the efficacy of each bone/cement system with regard to the ability of the cement to penetrate the interstices of the bone.

Aim: To quantify the rheological properties of bone cement systems with the view to assisting in cement selection for orthopaedic purposes

Material & Methods: The rheological properties of a variety of current bone cements were determined using a novel apparatus developed at the CSIRO called the Micro Fourier Rheometer (MFR). This device measures the complex viscosity and complex modulus by subjecting a sample to small amplitude oscillatory squeezing between two parallel plates. The force transmitted through the sample is detected by a dynamic load cell and the complete signal spectrum is then analysed using Fourier Techniques. The bone cement is mixed according to manufacturers’ instructions and placed between the plates and is then subjected to a random displacement. Subsequent Fourier analysis lends itself to rheological parameters such as real and imaginary modulus, viscosity and phase (1–100 Hz).

Results: Consistent with earlier studies, it was found that the viscosity increased with time in an almost linear manner due to the progression of the polymerisation reaction of the cement. Thereupon the cement mass began its exothermic phase and the viscosity increased exponentially until fully set. The complex modulus at this time, when extrapolated to zero frequency, corresponded to the static modulus (as in conventional mechanical testing). The viscosity was highly dependent upon the shear rate (or frequency). As the cement was sheared the viscosity reduced, establishing the pseudo-plastic or shear-thinning nature of these materials. The phase provided an accurate measure of the setting and working time of the cement brands corresponding with studies by Krause (1982) and Ferracane (1981).

Conclusions: The results supported the conclusion that rapid insertion of the prosthesis is recommended, creating high shear stresses, thus decreasing the cement’s viscosity and allowing better cement penetration and mechanical interlock. The study highlights the differences between the major brands of bone cement.