Abstract
During orthopaedic procedures such as total knee arthroplasty (TKA), total hip arthroplasty (THA), and intramedullary nailing, it is necessary to hammer implants into the intramedullary canal of long bones. This hammering action can generate a high intramedullary pressure, leading to the release of bone marrow fat globules into the cardiovascular system, and ultimately, the possible development of fat embolism syndrome. In the present study, the effect of parameters such as implant tip geometry, peak impact force, hammer tip material, bone to implant radial gap, and marrow viscosity, on the resulting intramedullary pressure generated when hammering implants into a simulated femur analogue was examined. The bone analogue consisted of a porous plastic cylinder, having similar porosity and pore size to human femoral bone, with bone marrow being represented by a paraffin wax/petroleum jelly mixture. It was found that intramedullary pressure is only slightly lowered by a change in implant tip geometry, and that the use of a steel tipped (as opposed to rubber) hammer resulted in an increase in average pressure in the proximal portion of the bone, but a decrease distally. A lower implant insertion speed, lower hammering force, and a larger bone to implant radial gap were found to significantly reduce the intramedullary pressure. The number of hammer strikes required to insert an implant was found to increase significantly with marrow viscosity, but the average intramedullary pressure was found to decrease with increasing viscosity. Numerical modelling was also found to offer great promise for analysing hammering procedures for orthopaedic research into fat embolism syndrome. Numerical and experimental results were matched with approximately a 20% deviation.
Highlights
1.1 M otivation Fat embolism (FE) refers to the presence of fat globules within the peripheral circulation, and the associated clinical symptoms are collectively termed Fat embolism syndrome (FES).* In many orthopaedic procedures, it is necessary to breach the intramedullary canal of long bones, so that prosthetic stems, intramedullary nails, and other devices can be inserted
Though its etiology is not completely understood, it is generally accepted that FES results from the release of the fat from the intramedullary contents into the bloodstream, due to elevated pressure within the intramedullary canal.^ The reported incidence and mortality of FES varies from study to study; based on a recent 10 year retrospective study, the incidence of FES was identified in 0.9% of cases with a mortality of 7%
To meet the primary objective, three secondary objectives will be explored; (i) The creation of a synthetic bone analogue model that can be used in hammering and reaming experiments to mimic the fluid flow in a human cadaveric femur, (ii) To use the bone analogue to perform experiments aimed at verifying numerical (FIDAP computational fluid dynamics and ANSYS finite element) models of fluid flow and elevated pressure in bone, developed by other members of the research team, (iii) To perform an experimental parametric study aimed at establishing the process parameters that most affect intramedullary pressure, so that an optimal operative procedure which reduces the risk of FES can be established
Summary
1.1 M otivationFat embolism (FE) refers to the presence of fat globules within the peripheral circulation, and the associated clinical symptoms are collectively termed Fat embolism syndrome (FES).* In many orthopaedic procedures, it is necessary to breach the intramedullary canal of long bones, so that prosthetic stems, intramedullary nails, and other devices can be inserted. Due to the aging population, there is an increasing demand for orthopaedic procedures such as total hip and total knee arthroplasty. With this great demand the incidence of FES is becoming more frequent, and it is very important that operative techniques and equipment be modified to reduce the risk o f this complication. The main objective of the present thesis is to examine the factors that lead to elevated intramedullary (i.e., the canal inside long bones in which the marrow resides) pressure, and provide suggestions for alternate process parameters and operative techniques aimed at decreasing the occurrence of the syndrome in orthopaedic procedures that require hammering of an implant into the intramedullary canal. To meet the primary objective, three secondary objectives will be explored; (i) The creation of a synthetic bone analogue model that can be used in hammering and reaming experiments to mimic the fluid flow in a human cadaveric femur, (ii) To use the bone analogue to perform experiments aimed at verifying numerical (FIDAP computational fluid dynamics and ANSYS finite element) models of fluid flow and elevated pressure in bone, developed by other members of the research team, (iii) To perform an experimental parametric study aimed at establishing the process parameters that most affect intramedullary pressure, so that an optimal operative procedure which reduces the risk of FES can be established
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