Context The Femur Basic Model Comparison Objectives Results Application Structural Analysis of the Femur: A Collaborative Tool for Surgeons and Engineers Alice Younge IStructE YRC 15 th March 2012 Supervisors:Dr ATM Phillips Prof AA Amis

Context The Femur Basic Model Comparison Objectives Results Application OVERVIEW Introduction –The Femur –Context –Objectives Beam Theory Modelling –Basic Model –Comparison to finite element Model –Results of comparison to finite element model Applications

Context The Femur Basic Model Comparison Objectives Results Application INTRODUCTION Longest and strongest bone in the human body Two main functions: Support structure Facilitates movement Proximal end  Hip Joint Distal end  Knee Joint The Femur:

Context The Femur Basic Model Comparison Objectives Results Application INTRODUCTION The Femur: Consists of two types of bone: - Cortical Thin layer on the outside of the bone – Strong, stiff, low porosity shell like structure - Trabecular Continuous within the inner surface of the cortical shell – Bony plates and struts, high porosity sponge like structure

Context The Femur Basic Model Comparison Objectives Results Application INTRODUCTION The joints are surrounded by: Ligaments - Connect bone to bone Provide stability for the joint they surround The Femur:

Context The Femur Basic Model Comparison Objectives Results Application INTRODUCTION The joints are surrounded by: Muscles - Support and stabilise the joint Provide the power for locomotion The Femur:

Context The Femur Basic Model Comparison Objectives Results Application From April 2010 to April 2011 the NHS performed over 70,000 hip replacements in England We are all expecting to live longer and with a better quality of life A replacement hip joint lasts for ~ 15 years Additionally increased incidence of patients with musculoskeletal disorders – About 1 in 500 babies born in the UK have Cerebral Palsy Advances in design of replacement joint and surgical procedure are essential CONTEXT

Context The Femur Basic Model Comparison Objectives Results Application Ultimate objective: Build joint replacements that outlast the patient Interim objective: To create a beam theory model which could be used to - Assess the femur in natural condition and following arthroplasty Assess various clinical procedures and inform the choice of surgical approach Predict the effect of changes at one joint, hip or knee, to the other Inform new and existing designs of artificial hip or knee joint Assess the effects of muscle damage through disease, injury or surgery OBJECTIVES

Context The Femur Basic Model Comparison Objectives Results Application BEAM THEORY Background within biomechanics: Popular in early to mid 1900s Models crude and calculations carried out by hand By 1970s finite element method considered to be superior Little development since Advancement of computational power and with modern surgical planning time constraints – beam theory modelling is again becoming a viable option

Context The Femur Basic Model Comparison Objectives Results Application BEAM THEORY Basic model definition: 1) Medium left fourth generation Sawbones femur scanned using a computerised tomography (CT) scan at intervals of 0.75 mm. -Series of x-ray beams scan the bone -Create detailed images of the structure 2) CT data converted to contour geometry using Mimics. - Converts 2D image data to 3D model

Context The Femur Basic Model Comparison Objectives Results Application BEAM THEORY 3) Using Rhino, a CAD package, and the composite femur model a neutral axis was estimated through the head region and down the shaft of the femur. An altered axis was then included to ensure smooth loading through the neck region.

Context The Femur Basic Model Comparison Objectives Results Application BEAM THEORY 4) Using Rhincerous, a CAD package, section cuts were taken perpendicular to the axis. - Head region, 2mm intervals  23 sections - Neck region, equally spaced in an arc  30 sections - Shaft region, 5mm intervals  65 sections 5) For each section, cortical and trabecular, line plots in the form of.dxf files were exported.

Context The Femur Basic Model Comparison Objectives Results Application BEAM THEORY 6) The.dxf files were imported in to Oasys GSA as perimeter sections. Section properties were calculated and assigned for all cortical and trabecular parts. The section properties included the: - Area, - Centroid, - Second moments of area, Custom script written to calculate: - Torsion constant

Context The Femur Basic Model Comparison Objectives Results Application BEAM THEORY 7) The mid-point of the cortical centroid values were calculated and nodes were plotted at these points. - Ensured even distribution of the element section properties between the nodes. The resulting node path 8) Trabecular sections aligned coincident to cortical sections -Parallel axis theorem used to calculate new section properties

Context The Femur Basic Model Comparison Objectives Results Application BEAM THEORY 9) Element assigned between each node with individual section property – cortical and trabecular Image shows section definition plot of cortical (right) and trabecular (left) parts 10) Muscle origination and insertion nodes (surface nodes) plotted - Surface nodes connected to centroid nodes via stiff beam elements to the femur  direct transfer of force

Context The Femur Basic Model Comparison Objectives Results Application BEAM THEORY Now have the basic beam theory model Using the basic model one can: Rotate the model to any position/stance Apply any chosen muscle ligament or joint reaction force Alter the position of the muscle origination or insertion site Change the material properties of the bone, cortical or trabecular Add an additional component such as an artificial joint

Context The Femur Basic Model Comparison Objectives Results Application Femur position at 12° of adduction & 7° of flexion – representative of single leg stance Twenty six muscles & seven ligaments included as cable elements with specific stiffness’s Each muscle had a defined force-displacement relationship Force-displacement curve based on the muscles peak contractile force and stiffness value Note: Stiffness acted in and tension not in compression Compare Beam Theory Model to Finite Element Model: BEAM THEORY

Context The Femur Basic Model Comparison Objectives Results Application BEAM THEORY An equivalent acetabular and condylar structure was defined and connected using stiff beam elements Muscles originating on the pelvis connected to the acetabular structure Muscles inserting into the tibia were fully constrained Force of 835 N applied to L5S1 node Compare Beam Theory Model to Finite Element Model:

Context The Femur Basic Model Comparison Objectives Results Application Acetabular structure Muscle origination point Stiff beam element Cable element representing muscle Tibial plateau BEAM THEORY

Context The Femur Basic Model Comparison Objectives Results Application BEAM THEORY Beam theory line plot Beam theory section plot Finite element model

Context The Femur Basic Model Comparison Objectives Results Application BEAM THEORY Processing Results: Bending moment output Myy Section definition and properties Stress calculated using moment, position and section properties Strain calculated using stress and Young’s modulus

Context The Femur Basic Model Comparison Objectives Results Application BEAM THEORY FE shown in black Beam theory shown in red Time to analyse model: Beam theory: 1 second Finite element: 1937 seconds (super computer) Results – Comparison to Finite Element model:

Context The Femur Basic Model Comparison Objectives Results Application BEAM THEORY FE lateral surface shown in black FE medial surface shown in blue Beam theory centroid line shown in red Reaction forces at condyles Deflection of femoral head

Context The Femur Basic Model Comparison Objectives Results Application APPLICATIONS Beam theory model main advantages: Analysis time – very fast Ease with which parameters can be changed - muscle origination site - loading condition Beam theory model applications: Rapidly assess the effects of muscle damage through, disease injury or surgery - changes in muscle attachment etc Assess femur in natural condition and following arthroplasty. To inform a new design of artificial hip or knee joint Predict the effect of changes at one joint, hip or knee, to the other

Context The Femur Basic Model Comparison Objectives Results Application THANK YOU ANY QUESTIONS Contact: