Non-optimal clinical alignment of components in total hip replacements (THRs) may lead to edge loading of the acetabular cup liner. This has the potential to cause changes to the liner rim not accounted for in standard wear models. A greater understanding of the material behaviours could be beneficial to design and surgical guidance for THR devices. The aim of this research was to combine finite element (FE) modelling and experimental simulation with microstructural assessment to examine material behaviour changes during edge loading. A dynamic deformable FE model, matching the experimental conditions, was created to simulate the stress strain environment within liners. Five liners were tested for 4Mc (million cycles) of standard loading (ISO14242:1) followed by 3Mc of edge loading with dynamic separation (ISO14242:4) in a hip simulator. Microstructural measurements by Raman spectroscopy were taken at unloaded and highly loaded rim locations informed by FE results. Gravimetric and geometric measurements were taken every 1Mc cycles. Under edge loading, peak Mises stress and plastic deformation occur below the surface of the rim during heel strike. After 7Mc, microstructural analysis determined edge loaded regions had an increased crystalline mass fraction compared to unloaded regions (p<0.05). Gravimetric wear rates of 12.5mm3/Mc and 22.3mm3/Mc were measured for standard and edge loading respectively. A liner penetration of 0.37mm was measured after 7Mc. Edge loading led to an increase in gravimetric wear rate indicating a different wear mechanism is occurring. FE and Raman results suggest that changes to material behaviour at the rim could be possible. These methods will now be used to assess more liners and over a larger number of cycles. They have potential to explore the impact of edge loading on different surgical and patient variables.
The purpose of this study was to determine the prevalence of non-accidental injury in toddlers (birth to two years old) presenting with a femur fracture to a single institution within a Western Canadian population. There are currently no Canadian studies that investigate this association in the medical literature. A retrospective case-control study was performed for children less than or equal to two years of age with a diagnosis of femur fracture who presented to a single institution from 1994 to 2005. Patients with associated pathological or metabolic bone disorders were excluded. The primary outcome measure was the prevalence of non-accidental injury associated with femur fracture. Secondary outcome measures included: patient demographics, injury characteristics, radiologic and other work-up, and suspicion of abuse. Statistical analysis was performed using tests of analysis of variance and multivariate analysis to identify independent and significant risk factors associated with non-accidental injury. One hundred twenty-seven children that met the inclusion criteria were identified. The overall prevalence of non-accidental injury was 11% (14/127 patients) and 17% (10/60 patients) in children under twelve months. Mean age was twelve months in the abused group and sixteen months in the accidental group. Forty-three percent of the abused group had delayed presentation compared to 8% of the accidental group. In the abused group, 43% had unknown, unwitnessed, inconsistent histories versus 4% in the accidental group. No specific fracture pattern or location suggestive for non-accidental injury was identified. Forty-three percent (6/14 patients) of the abused group and 12% (13/113 patients) of the accidental group had other injuries. Multivariate analysis determined that delayed presentation (p=0.001), mechanism of injury (p=0.02) and other associated injuries (p=0.003) were the only independently significant risk factors for abuse. Toddlers who present with femoral fracture are at risk for associated non-accidental injury though perhaps this risk is not as high as previously thought. Regardless, a high index of suspicion is mandatory when these children are encountered and careful screening with a thorough history, physical examination, and other investigations where indicated, are warranted to rule out associated child abuse.
