Effect of callus development on the deformation of external fixation frames.

PURPOSE: We designed a sensor that measures the bending moments at the articulations and the torque of the rod of a Hoffmann II® external fixation. We considered the effect of the callus formation in the stabilisation of a "fracture-fixation system." METHODS: Four Hoffmann II® frame configurations were mechanically tested. Two carbon fibre tubes represent the bone fragments (length 180 mm, outer diameter 25 mm, inner diameter 19 mm). The callus is represented by the interposition of springs of different rigidity (10-405 N/mm) in the fracture gap between the tubes. RESULTS: The deformation of the frame is in inverse proportion to the stiffness of the callus; the slope of the curve drops rapidly during early development of the callus, to reach a plateau after some 50 % of recovery of the normal mechanical characteristics of the bone. This simulation supports the theoretical approach, i.e. the external frame resists larger stresses at the start of the fracture healing. Over a callus stiffness of some 200 N/mm the pattern of the curves remains similar, regardless of the frame configuration. CONCLUSION: An optimisation of the frame is possible, adapted to the actual mechanical situation of the callus. A monitoring system is deemed reliable after making sure that the elementary components behave the same way in the clinical condition as in the laboratory. In an experimental set up we confirmed its reliability in a clinical-like situation.

Histopathology of aseptic necrosis of the femoral head in sickle cell disease.

This study compares the histopathology of bone biopsies from patients suffering from sickle cell anaemia (homozygote SS) to heterozygote patients (SA) and homozygotes with aseptic osteonecrosis (AA). The sensitivity to bacterial infection of sickle cell patients raises the question of the aetiology of sepsis in the onset of the necrosis. To our knowledge this study is the first to analyse the histopathology of osteonecrosis of the femoral head, at its early stages, in sickle cell anaemia. At the University Hospital of Pointe-à-Pitre, from 1994 to 2007, 38 bone biopsies were obtained from adult patients with avascular necrosis of the femoral head at the time of a core decompression procedure (SS, SC: 27; AS: 5; AA: 6). The histology of the biopsies confirmed the necrosis; all bacteriological cultures were negative. Patients displaying one S gene (SS, SC, AS) compared to homozygote subjects (AA) showed a significant increase of a nonspecific inflammatory granulomatosis (p = 0.003). No relationship was observed between the radiological stages and the histology whatever the genotype (p = 0.1). Inflammatory histopathology without sepsis or advanced alteration characterises the early stages of sickle cell necrosis. This inflammatory process is absent in idiopathic avascular necrosis.