Total hip arthroplasty (THA) is a successful surgical technique that can be used for the effective treatment of fractured neck of femur, osteoarthritis, tumours, avascular necrosis, failed internal fixation, developmental dysplasia and rheumatoid arthritis. Revision surgery is necessary if loosening allows relative motion between the femoral stem and femur, causing pain and mechanical instability of the THA. The large number of revision operations undertaken each year as a result of implant failure emphasises the need for better biomechanical understanding of the femoral implant system. During 2001-02 in Australia 26,689 hip replacement operations were performed, with 3,710 of these being revision operations. The Exeter stem is the most commonly used cemented stem for primary and revision hip replacement in Australia. It is therefore very important to understand the mechanics of this clinically successful implant. Few studies have presented a through investigation into the mechanics of the Exeter stem from a fundamental perspective. To address these issues, mechanical and finite element (FE) methods were used to conduct experiments and numerical investigations into the mechanics of the Exeter stem. The femur geometry, for both the experimental and FE studies, was based upon the Sawbones model 3303 medium left third generation femur. The stem orientation for all specimens of the study was replicated from the orientation achieved by the senior surgeon implanting into the Sawbones femur. Test rigs were designed specifically to constrain the femur for the purposes of loading and stability measurements. The experimental investigation was used to investigate the torsional mechanical stability of the stem and to monitor this stability following periods of cyclic loading, using a resultant hip contact force, while monitoring the distal migration of the stem. The experimental investigation was also able to provide data for the validation of the finite element model. The resultant hip contact force was represented experimentally by a cyclic load of 1Hz applied to the head of the implant. The specimen was tested for four days. The loading regime for the initially implanted specimen involved the application of load for 6 hours a day, allowing the specimen to relax under no load for 18 hours a day. The mechanical stability of the initially implanted specimen was tested prior to the application of the cyclic load and immediately after the loading periods, prior to relaxation. Further tests were undertaken to assess the mechanical stability of the stem following the removal and reimplantation of the same stem without the use of additional bone cement (a procedure used surgically when only the acetabular component requires replacement). The reimplanted specimens were tested for a further two days following reimplantation. The six hours of loading for the reimplanted specimen was achieved using three, two hour loading periods. The stability of the reimplaned stem was assessed following each loading period. Initial studies found that the material properties of the Sawbones femurs were highly temperature dependent. If the temperature of the short glass fibre reinforced (SGFR) epoxy used for the cortical bone analogue was increased from room temperature to body temperature there was a reduction in the Young's modulus of up to 37 percent. This finding led to further investigation into the strain state of the femur for varus and neutral stem orientations to reduce femur failure during cyclic loading. The strains of the varus stem orientation were found to be higher than the strains of the neutral stem. The experiments investigating the mechanical stability under cyclic loading continued using the neutral stem orientation. For the neutral stem orientation it was found that there was no perceivable variation in the torsional stiffness of the initially implanted system during the cyclic loading period even though distal migration was observed. Torsional stiffness was observed to be compromised immediately after reimplantation. However, the torsional stiffness of the reimplanted specimen was recovered within the first two hour loading period. No perceivable variation in the torsional stiffness was observed between the initially implanted specimens and the reimplanted specimens following the first two hours of loading. The finite element model (FEM) found good agreement with the experimental investigation in terms of measured strain at two of three rosette positions and failure of the cortical bone. Trends for the stress-strain state of the stem showed good agreement with the clinical findings of failure and wear of the stem. The stress-strain state of the cement predicted the expected compressive and hoop stresses once debonding of the stem-cement interface had progressed. Strain on the surface of the femur was well predicted for pure torsional loading. The FEM has provided a valuable tool for future investigation of the effect of factors such as implant positioning on femoral component mechanics. The experimental and finite element models developed within the scope of this project have provided a powerful analysis tool for the investigation of the femoral component mechanics in THA. Application of the model to clinically relevant problems has given valuable insight into the mechanisms behind the success of this particular implant type. Models such as this will provide information on implant failure modes that will further lead to an increased implant life expectancy and a reduction in the number of revision operations performed.

Thesis (PhD)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: The knee is the largest, most complicated and incongruent joint in the human body. It sustains very high forces and is susceptible to injury and disease. Osteoarthritis is a common disease prevalent among the elderly and causes softening or degradation of the cartilage and subcondral bone in the joint, which leads to a loss of function and pain. This problem can be alleviated through a surgical intervention commonly termed a “knee replacement”. The aim of a knee replacement procedure is to relieve pain and restore normal function. Ideally, the knee replacement prosthesis should have an articulating geometry similar to that of the patient’s healthy knee, and must allow for normal motion. Unfortunately, this is often problematic since knee prostheses are supplied in standard sizes from a variety of manufacturers and each one has a slightly different design. Furthermore, commercial prostheses are not always able to restore the complex geometry of an individual patient’s original articulating surfaces. This dissertation shows that there is a significant variation between knee geometries, regardless of gender and race. This research aims to resolve the problem in two parts: Firstly by presenting a method for preoperatively selecting the optimal knee prosthesis type and size for a specific patient, and secondly by presenting a design procedure for designing and manufacturing patient-specific unicompartmental knee replacements. The design procedure uses mathematical modelling and an artificial neural network to estimate the original and healthy articulating surfaces of a patient’s knee. The models are combined with medical images from the patient to create a knee prosthesis that is patient-specific. These patient-specific implants are then compared to conventional implants with respect to contact stresses and kinematics. The dissertation concludes that patient-specific implants can have characteristics that are comparable to or better than conventional prostheses. The unique design methodology presented in this dissertation introduces a significant advancement in knee replacement technology, with the potential to dramatically improve clinical outcomes of knee replacement surgery. / AFRIKAANSE OPSOMMING: Die knie is die grootste, mees komplekse en mees ongelyksoortige gewrig in die liggaam. Osteoarthritis is ’n siekte wat algemeen by bejaardes voorkom en die versagting of agteruitgang van die kraakbeen en subchondrale bene in die gewrig tot gevolg het, wat tot ’n verlies van funksionering en pyn lei. Hierdie probleem kan verlig word deur ’n chirurgiese ingryping wat algemeen as ’n “knievervanging” bekend staan. Die doel van ’n knievervangingsprosedure is om pyn te verlig en normale funksionering te herstel. Ideaal gesproke behoort die knievervangingsprostese ’n gewrigsgeometrie te hê wat soortgelyk aan die pasiënt se gesonde knie is, en normale beweging moontlik maak. Ongelukkig is dit dikwels problematies aangesien knieprosteses in standaardgroottes en deur ’n verskeidenheid vervaardigers verskaf word, wat elkeen se ontwerp effens anders maak. Verder kan kommersiële prosteses nie altyd die komplekse geometrie van ’n individuele pasiënt se oorspronklike gewrigsoppervlakke vervang nie. Hierdie proefskrif wys dat daar ’n betekenisvolle variasie tussen knieafmetings is, afgesien van geslag en ras. Hierdie navorsing is daarop gemik om die problem op tweërlei wyse te benader: Eerstens deur ’n metode aan te bied om die optimal knieprostesetipe en -grootte vir ’n spesifieke pasiënt voor die operasie uit te soek, en tweedens om ’n ontwerpprosedure aan te bied vir die ontwerp en vervaardiging van pasiëntspesifieke unikompartementele knievervangings. Die ontwerpprosedure gebruik wiskundige modellering en ’n kunsmatige neurale netwerk om die oorspronklike en gesonde gewrigsoppervlakke van ’n pasiënt se knie te bepaal. Die modelle word met mediese beelde van die pasiënt gekombineer om ’n knieprostese te skep wat pasiëntspesifiek is. Hierdie pasiëntspesifieke inplantings word dan met konvensionele inplantings vergelyk wat kontakstres en kinematika betref. Daar word tot die slotsom gekom dat die pasiëntspesifieke inplantings oor eienskappe kan beskik wat vergelykbaar is met of selfs beter is as dié van konvensionele prosteses. Die unieke ontwerpmetodologie wat in hierdie proefskrif aangebied word, stel beduidende vordering in knievervangingstegnologie bekend, met die potensiaal om die kliniese uitkomste van knievervangingsoperasies dramaties te verbeter.

This work describes advanced fabrication of custom orthopaedic implants using unconventional additive manufacturing technology - Selective Laser Melting (SLM). There was a main focus on custom knee replacement and certainly on its femoral component. The study investigated three general issues within the domain of the usage of additive manufacturing technology in medical application. First, there was an evaluation of process parameters influences of SLM fabrication method on surface and mechanical properties of titanium Ti6Al4V ELI specimens. This material was used because of its biocompatibility and its wide use within implant fabrication. Then, a proposal of the manufacturing strategy was carried out and the fabrication of customized knee femoral component prototype by SLM technology was done. The elaboration of the numerical chain prior the SLM implant fabrication, from patient's CT knee scan to final femoral replacement model, was also done. Then, a proposal of different 3-axis and 5-axis strategies of machining of the fitting femoral surface of bone prototype (3D printed from the powder) using CNC machines FV 25 CNC and TAJMAC ZPS MCV 1210 was projected and also, the 3-axis spiral machining was realised. The individual machining tool paths were generated by software Power Mill from Delcam group. Finally, these machining strategies were generated as a prior step before a machining of real patient’s bone, therefore the machining tests of cartilage and bone were done.

京都大学 / 0048 / 新制・課程博士 / 博士(医学) / 甲第22346号 / 医博第4587号 / 新制||医||1042(附属図書館) / 京都大学大学院医学研究科医学専攻 / (主査)教授 安達 泰治, 教授 森本 尚樹, 教授 福原 俊一 / 学位規則第4条第1項該当 / Doctor of Medical Science / Kyoto University / DFAM