Polymethylmethacrylate (PMMA) bone cement is strong in compression, however it tends to fail under torsion. Sufficient pressurisation and subsequent interdigitation between cement and bone are critical for the mechanical interlock of cemented orthopaedic implants, and an irregular surface on the acetabular cup is necessary for reasonable fixation at the cup-cement interface. There is limited literature investigating discrepancies in the failure mechanisms of cemented all-polyethylene acetabular cups with and without cement spacers, under torsional loading. In vitro experimental comparison of three groups of polyethylene acetabular prosthesis (PAP) cemented into prepared sawbone hemipelvises: * PAP without PMMA spacers maintaining an equal cement mantle circumferentially. (Group 1 n=3) * PAP without PMMA spacers cemented deliberately ‘bottoming-out’ the implant within the acetabulum. (Group 2 n=3) * PAP with PMMA spacers. (Group 3 n=3) The constructs were tested to torstional failure on a custom designed setup, and statistical analysis done by a one-way ANOVA and Tukey-Welsh test. Group 3 demonstrated superior torsional resistance with a statistically significant torque of 145Nm (SD±12Nm) at failure, compared to group 2 (109Nm, SD±7Nm) and group 1 (99Nm, SD±8Nm). Group 3 experienced failure predominantly at the bone-cement interface, in contrast, Groups 1 and 2 exhibited failure predominantly at the cup-cement interface. There was no significant difference between Group 1 and 2. Qualitative analysis of the failure mode indicates the efficient redistribution of stress throughout the cement mantle, consistent with the greater uniformity of cement. PMMA spacers increase the resistance to torsional failure at the implant-cement interface. Acetabular components without spacers (Groups 1 and 2) failed at the implant-cement interface before the cement-bone interface, at a statistically significantly lower level of torque to failure. Although the PMMA spacers may reduce cement interdigitation at the cement-bone interface the torsional forces required to fail are likely supraphysiological.
Bone stress fracture triggers a rapid increase in blood flow in association with mast cell production of inducible nitric oxide synthase (iNOS). NOS inhibition blocks the increase in blood flow and reduces woven bone formation needed for stress fracture healing. Vascular-bone interactions are critical in skeletal development and fracture healing. We recently showed that angiogenesis is required for stress fracture healing. However, the changes in vascularity that occur in the first 72 hours after stress fracture can not be explained by angiogenesis. Here, we evaulated early changes in blood flow and vasodilation after either damaging (stress fracture) or non-damaging mechanical loading in rats.Summary Statement
Introduction
Modern microsurgery has allowed severed digits to be salvaged by replantation. A retrospective case review was undertaken of all patients undergoing digital replantation at Middlemore Hospital between February 2004 and February 2009. 48 digits from 28 patients underwent digital replantation during this period. The aim of the analysis was to determine what factors were predictive for survival of the replants. Secondary outcomes of interest included subjective functional recovery, pain and further procedures. Digital replantation over the review period was subject to a 75% survival rate. Smoking and male gender were identified as significant negative prognostic factors (p=0.02). 69% of patients reported post operative stiffness, chronic pain or cold intolerance. The majority of replanted digits underwent secondary procedures. Patients should be counseled prior to digital replantation that while the procedure is subject to a high rate of digit survival, they should expect stiffness and discomfort and are likely to undergo secondary procedures.
