Bioresorbable pins represent, nowadays, a valid alternative to conventional methods for fixations of small bones in orthopaedics surgeries (i.e., Kirschner wires). As they have the need to get in contact with the body, they are generally made out of materials that meet specific requirements, such as being nontoxic, biocompatible and having adequate biomechanical properties and physical structure, which means that they have to support adequate mechanical strength and to be designed in a specific geometry that allows surface-specific reactions promoting degradation when put within the body [2]. In this sense, biodegradable polymers (e.g., polylactic acid) represent a valid and versatile solution, as they exhibit at once most of the aforementioned characteristics. On the other hand, the mechanical limitations of the involved polymers, not only once in place, but also when properly installed into the patient’s bone, can still represent an issue and slow down this solution in the overcoming of the traditional ones. Two major directions can be identified for optimization: the first one is surely the composition of the polymer, as this affects the initial mechanical properties at installation and their evolution through degradation over time. The second is instead the actual shape of the pin and, as a consequence, its ability to properly act as a fixation mean and its response to axial and tangential loads over time. As a matter of fact, an optimized pin would be extremely beneficial in the development of a less invasive approach to orthopaedical surgeries, as this would avoid a second intervention for the removal of the fixation tools. Starting from four commercially available biodegradable polymers and aiming at identifying an optimal shape for the cross section of the pin and an optimal composition of the polymer, assuming 6-8 weeks degradation, a model for both the degradation and mechanical behaviours of the pin have been developed in COMSOL Multiphysics. Specifically, the polymer degradation has been simulated through a mathematical model considering that the degradation occurs mostly through hydrolysis, whereas the mechanical model has been developed thanks to FEM analysis, by properly loading the tested device. Once the final best-case has been established, pins having the desired specification have been produced via injection moulding, treated with β-sterilization (25-30 kGy irradiation) and finally tested in human femoral head for laboratory use. Moreover, the optimized pin has been compared with the devices already available on the market, after being subjected to a chemical characterization and performance analysis.
I dispositivi convenzionali di fissazione per la chirurgia ortopedica (ad esempio i fili di Kirschner) potrebbero essere sostituiti da soluzioni meno invasive e sistemi riassorbili realizzati, ad esempio, con materiali biodegradabili. Ovviamente, questi dispositivi sono in contatto con il corpo e devono, pertanto, soddisfare specifici requisiti: essere non tossici, biocompatibili e con adeguate proprietà biomeccaniche e fisiche. Tra i possibili materiali, sicuramente i polimeri, tra cui l’acido polilattico, offrono una valida alternativa in quanto sono biodegradabili, bioriassorbibili e non tossici per il corpo. Affinché tale soluzione possa prevalere sui sistemi già esistenti, sono necessari ulteriori progressi e sviluppi al fine di superare le limitazioni meccaniche da cui sono affetti i polimeri. Quest’ultime, difatti, si verificano non solo in situ, ma anche nel momento in cui il dispositivo di fissazione viene impiantato all’interno dell’osso umano. Per tali motivi, l’ottimizzazione svolta in questo lavoro di tesi si è concentrata su due principali aspetti: il primo, ovviamente, riguarda la composizione del polimero, in quanto essa influenza le proprietà meccaniche del dispositivo, sia durante l’impianto, sia durante il tempo di guarigione, essendo soggetta a degrado. Il secondo parametro, invece, riguarda la forma del pin e, di conseguenza, la sua abilità di agire come un vero e proprio sistema di fissazione e di rispondere ai carichi assiali e tangenziali. Allo stato dell’arte quindi, un pin, ottimizzato secondo i parametri citati, potrebbe essere una soluzione estremamente favorevole per lo sviluppo di un approccio meno invasivo in chirurgia ortopedica. Lo scopo di questo progetto è quello di identificare un’adeguata geometria del dispositivo e un’ottima composizione del polimero, assumendo 6-8 settimane di degrado. Il degrado chimico e il comportamento meccanico sono stati sviluppati in un modello creato tramite il software COMSOL Multiphysics: il degrado del polimero sarà simulato attraverso un modello matematico basato su leggi di conservazione che permettono di valutare la cinetica del fenomeno. Una volta stabilita la combinazione ottima tra i due parametri citati, il pin con le opportune specifiche sarà prodotto per injection molding da una compagnia in Svizzera, sterilizzato tramite raggi β e testato su teste di femore. Inoltre, il pin scelto sarà confrontato con i dispositivi già disponibili sul mercato dopo essere stato sottoposto ad una completa caratterizzazione chimica (GPC) e ad un’analisi delle perfomance.
Design of a bioresorbable fixation system for small bone segments
CIGNOLI, ELEONORA
2017/2018
Abstract
Bioresorbable pins represent, nowadays, a valid alternative to conventional methods for fixations of small bones in orthopaedics surgeries (i.e., Kirschner wires). As they have the need to get in contact with the body, they are generally made out of materials that meet specific requirements, such as being nontoxic, biocompatible and having adequate biomechanical properties and physical structure, which means that they have to support adequate mechanical strength and to be designed in a specific geometry that allows surface-specific reactions promoting degradation when put within the body [2]. In this sense, biodegradable polymers (e.g., polylactic acid) represent a valid and versatile solution, as they exhibit at once most of the aforementioned characteristics. On the other hand, the mechanical limitations of the involved polymers, not only once in place, but also when properly installed into the patient’s bone, can still represent an issue and slow down this solution in the overcoming of the traditional ones. Two major directions can be identified for optimization: the first one is surely the composition of the polymer, as this affects the initial mechanical properties at installation and their evolution through degradation over time. The second is instead the actual shape of the pin and, as a consequence, its ability to properly act as a fixation mean and its response to axial and tangential loads over time. As a matter of fact, an optimized pin would be extremely beneficial in the development of a less invasive approach to orthopaedical surgeries, as this would avoid a second intervention for the removal of the fixation tools. Starting from four commercially available biodegradable polymers and aiming at identifying an optimal shape for the cross section of the pin and an optimal composition of the polymer, assuming 6-8 weeks degradation, a model for both the degradation and mechanical behaviours of the pin have been developed in COMSOL Multiphysics. Specifically, the polymer degradation has been simulated through a mathematical model considering that the degradation occurs mostly through hydrolysis, whereas the mechanical model has been developed thanks to FEM analysis, by properly loading the tested device. Once the final best-case has been established, pins having the desired specification have been produced via injection moulding, treated with β-sterilization (25-30 kGy irradiation) and finally tested in human femoral head for laboratory use. Moreover, the optimized pin has been compared with the devices already available on the market, after being subjected to a chemical characterization and performance analysis.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/145130