In a society whereby the incidence of obesity is increasing and medico-legal implications of treatment failure are more frequently ending with the consulting doctor, clarity is required as to any restrictions placed on common orthopaedic implants by manufacturing companies. The aim of this study was to identify any restrictions placed on the commonly used femoral stem implants in total hip replacement (THR) surgery, by the manufacturers, based on patient weight. The United Kingdom (UK) National Joint Registry (NJR) was used to identify the five most commonly used cemented and uncemented femoral stem implants during 2012. The manufacturing companies responsible for these implants were asked to provide details of any weight restrictions placed on these implants. The Corail size 6 stem is the only implant to have a weight restriction (60Kg). All other stems, both cemented and uncemented, were free of any restrictions. Fatigue fracture of the femoral stem has been well documented in the literature, particularly involving the high nitrogen stainless steel cemented femoral stems and to a lesser extent the cemented cobalt chrome and uncemented femoral stems. In all cases excessive patient weight leading to increased cantilever bending of the femoral stem was thought to be a major factor contributing to the failure mechanism. From the current literature there is clearly an association between excessive patient weight and fatigue failure of the femoral stem. We suggest avoiding, where possible, the insertion of small stems (particularly cemented stems) and large offset stems (particularly those with a modular neck) in overweight patients

Introduction. The fatigue strength of ultrahigh molecular weight polyethylene (UHMWPE) in total joint implants is crucial to its long term success in high demand applications, such as in the knee, and is typically determined by measuring the crack propagation resistance in razor-notched specimens under cyclic load [1]. This only tells part of the story: that is, how well the material resists crack propagation once a crack is present. A second, equally important component of fatigue strength is how well the material resists crack formation. Previous studies cyclically loaded a cantilevered post until failure [2], postulating that the post would break very quickly after crack initiation. Parran et. al. proposed a novel method to measure the crack initiation time by holding a sample in constant tension until a crack was visually observed [3]. We hypothesize that the crack initiation times of various UHMWPEs will follow similar trends as the more omnipresent crack propagation resistance tests. Materials and Methods. The following UHMWPE formulations were tested: (i) virgin, (ii) gamma sterilized in vacuum, (iii) 91 kGy gamma irradiated, and (iv) 91 kGy gamma irradiated and subsequently melted. GUR1020 and GUR1050 bar stock of varying irradiation doses were machined into compact tension specimens [4] with a notch depth of 17 mm and a blunt notch root radius of 0.25 mm, mimicking a geometry of a joint replacement component. Specimens were held in constant tension until failure; 3 to 5 different loads between 1 kN and 2.25 kN (n=3 samples per load per material) were tested. A video camera was focused on the face of the notch and took a picture every 10 seconds. The photos were reviewed to manually determine the crack initiation time (Fig 1). The time it took for the sample to completely fail – that is, shear into two separate pieces – was also recorded. Results. For all materials tested, the crack initiation time (Fig 2a,b) and the time to failure (Fig 2c,d) decreased as the applied load increased. The crack initiation time increased for the gamma sterilized materials when compared to the virgin materials while the time to failure decreased. The highly crosslinked, 91 kGy materials had crack initiation times and times to failure that were less than that of the virgin material. Post irradiation melting greatly diminished the fatigue strength of the material, yielding the lowest crack initiation time and time to failure. Discussion. The test yielded results consistent with current knowledge: that is, high-dose irradiation yields a slight drop in fatigue strength, and post-irradiation melting greatly reduces strength. This test was simple to set up and run and can be a good tool to determine the relative fatigue strengths of UHMWPE formulations for orthopaedic applications. For figures/tables, please contact authors directly.

Aims

The timing of when to remove a circular frame is crucial; early removal results in refracture or deformity, while late removal increases the patient morbidity and delay in return to work. This study was designed to assess the effectiveness of a staged reloading protocol. We report the incidence of mechanical failure following both single-stage and two stage reloading protocols and analyze the associated risk factors.