Andersen, Lonnie Ulrich

En fuldstændig hofte artroplastik er en af de mest udbredte ortopædkirurgiske operationer, og det er den væsentligste behandling imod fremtræden slidgigt i hofteleddet. Det er blevet estimeret at mellem 0.5-1% af befolkningen i den vestlige verden, skal have foretaget en hofte artroplastik i løbet af deres liv. Under operationen svækkes hoftens naturlige stabilisatorer, hvilket gør patienten sårbar overfor dislokation i løbet af de første 3-6 måneder. Studier har vist, at ca 3-5% af patienterne oplever en dislokation på et eller andet tidspunkt. Det er endnu ikke fuldstændig klarlagt, hvilke faktorer der disponerer nogle patienter overfor dislokation, hvilket gør det svært at tage præventative tiltag til at undgå det. Formålet med denne Phd var at designe et implanterbart, bionedbrydeligt implantat, der kan modvirke dislokation i de første 3-6 måneder.Implantatet skal være let at implantere og skal kunne fastgøres på de fleste hofteimplantater, uden at det ændrer det tilstedeværende design af hofteimplantatet. Formålet er at have en struktur der putter en begrænsning på det nuværende hofteimplantat når det bevæger sig ud i de ekstreme positioner, der er associeret med dislokation. Samtidig må det ikke begrænse hoftens bevægelse, mere end hvad det nuværende hofteimplantat gør. Dette betyder, at belastning tøjnings profilen for for implantatet skal have et tøjningsområde i begyndelsen, hvor belastningen forbliver lav selvom tøjningen stiger. Når hoften bevæger sig ud i ekstreme positioner, skal belastningen stige drastisk og låse, hvorved dislokation forhindres. For at opnå dette skal implantatet være et kegleformet net, der omkranser hofteimplantatet.Ud fra det nuværende hofteimplantats geometri blev der udviklet en simpel model, der kan beskrive den mest almindelige bevægelse associeret med dislokation. Ud fra denne model kunne tøjnings- og belastningskravene beregnes for de forskellige designmuligheder. Analysen blev lavet ved at se på to forskellige implanteringsmuligheder og to forskellige låsescenarioer. Resultaterne fra modellen viste, at det initiale tøjningsområde skulle være mindst 30%, og at nettet skulle kunne modstå belastninger på mellem 1700 N og 5000 N. Desuden viste analysen, at begge implanteringmuligheder og låsescenarioer ville være mulige set ud fra et teoretisk standpunkt. Valget er afhængigt af afstanden fra acetabular koppen, til det sted på hofteskålen hvor nettet blev implanteret.Implantatet skal være virksomt under de første 3-6 måneder, imens leddet er mest sårbart overfor dislokation, og derefter skal det langsomt nedbrydes således, at leddet kan blive stærkere. Ud fra de resultater der blev fundet i litteraturen blev det besluttet, at poly-L- lactic acid ville være et egnet materiale. For at karakterisere materialet, i forhold til dets funktion, blev det nedbrudt i fosfat bufferet saltvand i 6 måneder. Hver måned blev der kørt en række test, der bestod af uniaksielle træktest ved to forskellige deforma- tionshastigheder, stress/belastning relaksationstests og krybe (creep) tests. Resultaterne fra de uniaksielle trækprøver viste, at der var meget lidt forskel på det elastiske modulus, yield stress, og gennemsnitlig tøjning ved brud under de første 5 til 6 måneder. Der blev dog set et signifikant fald i elastisk modulus imellem 5. og 6. måned for begge tøjningshastigheder, hvilket stemmer godt overens med nedbrydningsprofilen for L-PLA. Der blev desuden set et gradvis fald i belastningen ved brud, hen over de 6 måneder, for begge tøjningshastigheder. Tøjningshastigheden i sig selv, blev desuden, fundet signifikant i forhold til alle faktorer undtagen den gennemsnitlige tøjning ved brud. Både stress relaksations testene og krybetestene viste, at der er en ændring i de viskoelastiske egenskaber i de første par måneder. Dette kunne indikere, at materialet udviser mindre relaxation og krybning efter en måned i fosfat bufferet saltvand. Det er dog svært at drage endegyldige konklusioner ang dette, da der er stor variation i data, og for få datapunkter at gøre godt med. Dette taget i betragtning blev det generelt fundet, at de 5-6 måneder i fosfat bufferet saltvand, ikke havde en nævneværdig effekt på de viskoelastiske egenskaber, og at L-PLA trådene derfor vil bibeholde deres integritet, igennem hele den kritiske periode hvor implantatet skal være funktionelt.L-PLA trådenes materialeegenskaber blev brugt, til at analysere hvordan et vævet net ville opføre sig under forskellige typer deformation. Analysen blev lavet ved at bruge en allerede eksisterende model, og så modificere denne til projektets problemstilling. Resultaterne fra modellen sammenlignet med de grænseværdier der blev fundet i tøjningsanalysen, viste at hvis man skulle opretholde hoftens normale bevægelighed, så skulle nettet orienteres i en 45° vinkel i forhold til deformationsretningen. Den initielle tøjningsregion blev beregnet til at ligge imellem 35-40%, og det blev fundet at stoffet kunne modstå en belastning på mellem 2000 og 5000 N og stadig være i den elastiske region. Ud fra analysen of materialetestene blev det fundet, at hvis man bruger et simpelt vævet net i L-PLA, ville man kunne opnår den styrke og fleksibilitet, der er nødvendig i forhold til et implantat der kan modvirke dislokation.Total hip arthroplasty is the most common surgical procedure performed in the orthopaedic field, and it is considered the cornerstone in the treatment of osteoarthritis in the hip joint. It has been estimated that between 0.5-1% of the population in most Western Countries requires a total hip arthroplasty at some point in their life. The weakening of the natural stabilizers in the hip joint during the surgery, makes the patients susceptible to dislocation, in the first 3-6 months after the surgery. Studies show that approximately 3-5% of patients experience a dislocation at some stage. The factors predisposing for early dislocation have not been completely established, making it difficult to take successful preventative measures. The objective of this PhD thesis was to design an implantable, biodegradable device to guard against these dislocations.The hip dislocation preventer should allow for easy adaptation, and mounting onto most types of hip implants, without changing the basic design of the present implant. The objective is to have a structure, which will put a restrain on the artificial hip implant as it moves into the extreme positions associated with dislocation, without further affecting the normal movement relative to the hip implant. Therefore, the stress strain profile of the device, would have to include an initial strain region, where the stress remain low, after which the stress should increase rapidly as the devise locks, preventing the dislocation. To achieve this, the design of the hip dislocation preventer should be a cone shaped mesh, encapsulating the hip implant.Using the the basic geometry of the hip implant, a simple model was developed, describing the most common movement pattern associated with hip dislocation. The requirements concerning the initial strain region, was determined from the model. The analysis was done using two different attachment solutions, and two different locking scenarios. The results showed, that the initial strain region of the hip dislocation preventer would have to be at least 30%, and that the mesh should be able to withstand loads between 1700 N and 5000 N. Furthermore, the analysis show, that if the devise is designed to allowing the hip some degree of subluxation, the range of movement of the hip would be increased by approximately 15-19°, relative to the solution without subluxation. However, this also increases the requirements of the initial strain region by 8-13%. The conclusion to the requirement analysis, was that both attachment solutions proposed in the project, are possible in theory, depending on the distance of the mounting point on the modified acetabular cup.The hip dislocation preventer should work as a restrictive force during the first 3-6 months, when the joint is most vulnerable to dislocations. After this period, it should slowly degrade, enabling the joint to become stronger. From the results found in the literature, poly-L-lactic acid (L-PLA) was chosen as a suitable material. In order to characterize the material, L-PLA yarns were degraded in phosphate buffered saline for 6 months. Each month, a set of test consisting of, uniaxial tensile tests, at two different deformation rates, stress relaxation tests, and creep tests were performed. The uniaxial tensile test generally show very little change in the elastic modulus, yield stress, and mean strain at break, during the first 5 to 6 months. A significant drop in the elastic modulus was observed between month 5 and 6 at both loading rates, which corresponds well with the degradation period of L-PLA. A larger number of experiments, and a longer degradation period is required, in order to determine if the small fluctuations in the properties, is a general property of the material. The stress at break was found to gradually decrease during the 6 months, and the deformations rate was found to have a significant effect on the yield stress and the stress at break, but not on the elastic modulus and strain at break. Both the stress relaxation, and the creep test show that there is a change in the material in the initial degradation period, and the results could indicate, that the material experiences less relaxation and creep, a month after the in vitro degradation period. However, there is a large variance in the data, and more tests are needed to determine if this observation is correct. The overall conclusion of the material tests of the L-PLA yarns is, that the six month degradation in vitro, did not effect the tensile properties in a way that would significantly effect their functionality. The L-PLA yarns would therefore maintain their integrity during the critical period, where the hip dislocation preventer should be functional.The tensile properties of the L-PLA yarns were used to analyse, how a plain weave mesh would behave during different types of elongations. A model proposed by Kawabate et al in 1973 was modified, and used to analyse the problem, using different weave densities. Comparing the results found using the model, with the limits found during the strain analysis, showed that in order to maintain the full range of motion of the hip, the weave would have to be oriented in a 45° angle to the direction of deformation. From the model the initial strain region was predicted to lie between 35-40%, and the tensile force that the fabric can withstand, without going into plastic deformation was between 2000-5000 N. From the analysis and the material tests it was found that using a plain weaved LPLA mesh, the strength and flexibility needed of the hip dislocation preventer would be attainable

Online Research Database In Technology

Performance of Biodegradable Polymers used in Mechanically Loaded Implants

http://orbit.dtu.dk/ws/files/74585611/Final..PDF

Performance of Biodegradable Polymers used in Mechanically Loaded Implants

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Online Research Database In Technology