The spinal column consists of 33-34 vertebrae, it provides the main support of the body and protects the spinal cord from injury. It has two fundamental mechanical requirements: the stiffness to ensure stability and the flexibility to allow movement. Nowadays, the main causes leading to spinal surgery are mainly linked to the onset of degenerative disease, deformity, trauma, or pathologic fracture. The gold standard for their treatment is characterized by spinal fixation which is commonly achieved through rods and pedicle screws which lead to spine stability and a reduction of pain of the patient, but they produce the loss of physiological capacity of the range of motion due to the fusion of the vertebral bodies. The biomaterials used in spinal surgery have to satisfy mechanical and biocompatibility requirements: the most commonly used are stainless-steel, cobalt-chromium alloy, titanium alloy, and more recently, PEEK (Polyether-ether-ketone), a thermoplastic polymer. In the clinical setting, the spinal rods are not used in their undeformed and straight shape but they have to be countered in such a way that they fit the natural curvature of the spine. This operation can be achieved through the French bender, a specific tool used during the surgery, or by means of cold bending on an industrial level. The bending process introduces residual stresses on the devices which can influence the fatigue behaviour of the rod. Despite this, although several implant failures are still recorded in clinical setting due to the phenomenon of mechanical fatigue, few studies are aimed at investigating the causes. In this context, the goal of this thesis is to evaluate the effects of the bending of the spinal rods, obtained both through uniform contouring and with French bender, on the fatigue resistance of the spinal rods made of different materials (Ti6Al4V, Co-Cr, PEEK) The evaluation was performed exploiting numerical tools. In order to prepare numerical models, it was first necessary to create a CAD model of the rod, which, was later discretized following an accurate of mesh sensitivity analysis. A study was then carried out considering the two bending methods and the correspondent introduced residual stresses. In particular, the rods were contoured by means of a model of French bender (FB) and a model of four-points bending (FP), which allows achieving a more homogeneous bending. To compare the different bending methods, it was decided to obtain in both cases a global rod curvature which can be described by a radius of 150 mm, a value consistent with industrial preforming values. Finally, numerical fatigue analyses were performed in a four-point bending configuration. The tests were performed on five different configurations of rods: straight rods (S), French bender contoured rods tested in lordotic configuration (FBL), French bender contoured rods tested in kyphotic configuration (FBK), four-points bending contoured rods tested in lordotic configuration (FPL), four-points bending contoured rods tested in kyphotic configuration (FPK). After one peak/valley cycle simulation of the rod, the stress tensors were extracted and processed in order to compute Sines equivalent stress and to represent the diagrams with constant life at 1 million cycles (called “Haigh diagrams”).
La colonna vertebrale umana è composta da 33-34 vertebre, fornisce il supporto principale del corpo e protegge il midollo spinale da lesioni. Possiede due requisiti meccanici fondamentali: la rigidezza per garantire stabilità e la flessibilità per consentire il movimento. Oggigiorno le principali cause che portano alla chirurgia spinale sono legate all’insorgenza di patologie degenerative, deformità, traumi o fratture patologiche. Il gold standard per il loro trattamento è caratterizzato dalla fissazione spinale, che consiste nell’impianto di barre spinali e viti peduncolari le quali portano stabilità del rachide e una diminuzione del dolore del paziente ma causando la perdita della capacità fisiologica dell’intervallo di movimento per la fusione dei corpi vertebrali. I biomateriali utilizzati nella chirurgia spinale devono soddisfare requisiti meccanici e di biocompatibilità: i maggiormente impiegati sono l’acciaio inossidabile, la lega di cobalto-cromo, la lega di titanio, e più recentemente, il PEEK (Polietere-etere-chetone), un polimero termoplastico. In ambito clinico le barre spinali non vengono utilizzate nella loro forma indeformata e dritta ma devono essere piegate in modo tale che si adattino alla curva naturale della colonna vertebrale. Questa operazione può essere ottenuta per mezzo di un apposito strumento in sede operatoria: il French bender, oppure per mezzo di piegatura a freddo a livello industriale. Il processo di piegatura introduce sforzi residui nel dispositivo che possono influenzare il comportamento a fatica della barra. Nonostante ciò, sebbene in clinica si registrino tutt’ora numerosi fallimenti degli impianti a causa del fenomeno della fatica meccanica, pochi studi sono indirizzati ad indagarne le cause. È in questo contesto che si pone l’obiettivo di questo lavoro di tesi, ovvero è quello di valutare l’effetto della piegatura a freddo e con French bender sulla resistenza a fatica di barre spinali di diverso materiale (Ti6Al4V, Co-Cr, PEEK) sfruttando modelli numerici. Al fine di preparare i modelli numerici, è stato necessario dapprima realizzare un modello CAD della barra, che, successivamente è stato discretizzato seguendo un’attenta analisi di sensitività della mesh. A seguire è stato realizzato uno studio su due metodi di piegatura e sulle tensioni residue introdotte. In particolare, le barre sono state piegate per mezzo di un modello di French bender (FB) e di un modello di flessione a quattro punti (FP), che consente di ottenere una curvatura più omogenea. Per poter confrontare i diversi metodi di piegatura, è stato scelto di ottenere in entrambi i casi una curvatura globale della barra descrivibile da un raggio di 150 mm, valore coerente con i valori di preformatura industriale. Infine, sono state svolte valutazioni numeriche sulla resistenza a fatica in configurazione di flessione a quattro punti. I test sono stati svolti su cinque configurazioni diverse delle barre: barre dritte (S), barre piegate tramite French bender, montate in configurazione lordotica (FBL), barre piegate tramite French bender, montate in configurazione cifotica (FBK), barre piegate tramite flessione a quattro punti, montate in configurazione lordotica (FPL), barre piegate tramite flessione a quattro punti, montate in configurazione cifotica (FPK). Dopo la simulazione di un ciclo di carico/scarico della barra, i tensori degli sforzi sono stati estratti ed elaborati al fine di calcolare i valori dello sforzo equivalente di Sines e rappresentare i diagrammi a vita costante a 1 milione di cicli (detti “diagrammi di Haigh”).
Studio dell'effetto della piegatura sulla resistenza a fatica di barre spinali in cobalto-cromo, titanio e Peek
Siino, Federico
2020/2021
Abstract
The spinal column consists of 33-34 vertebrae, it provides the main support of the body and protects the spinal cord from injury. It has two fundamental mechanical requirements: the stiffness to ensure stability and the flexibility to allow movement. Nowadays, the main causes leading to spinal surgery are mainly linked to the onset of degenerative disease, deformity, trauma, or pathologic fracture. The gold standard for their treatment is characterized by spinal fixation which is commonly achieved through rods and pedicle screws which lead to spine stability and a reduction of pain of the patient, but they produce the loss of physiological capacity of the range of motion due to the fusion of the vertebral bodies. The biomaterials used in spinal surgery have to satisfy mechanical and biocompatibility requirements: the most commonly used are stainless-steel, cobalt-chromium alloy, titanium alloy, and more recently, PEEK (Polyether-ether-ketone), a thermoplastic polymer. In the clinical setting, the spinal rods are not used in their undeformed and straight shape but they have to be countered in such a way that they fit the natural curvature of the spine. This operation can be achieved through the French bender, a specific tool used during the surgery, or by means of cold bending on an industrial level. The bending process introduces residual stresses on the devices which can influence the fatigue behaviour of the rod. Despite this, although several implant failures are still recorded in clinical setting due to the phenomenon of mechanical fatigue, few studies are aimed at investigating the causes. In this context, the goal of this thesis is to evaluate the effects of the bending of the spinal rods, obtained both through uniform contouring and with French bender, on the fatigue resistance of the spinal rods made of different materials (Ti6Al4V, Co-Cr, PEEK) The evaluation was performed exploiting numerical tools. In order to prepare numerical models, it was first necessary to create a CAD model of the rod, which, was later discretized following an accurate of mesh sensitivity analysis. A study was then carried out considering the two bending methods and the correspondent introduced residual stresses. In particular, the rods were contoured by means of a model of French bender (FB) and a model of four-points bending (FP), which allows achieving a more homogeneous bending. To compare the different bending methods, it was decided to obtain in both cases a global rod curvature which can be described by a radius of 150 mm, a value consistent with industrial preforming values. Finally, numerical fatigue analyses were performed in a four-point bending configuration. The tests were performed on five different configurations of rods: straight rods (S), French bender contoured rods tested in lordotic configuration (FBL), French bender contoured rods tested in kyphotic configuration (FBK), four-points bending contoured rods tested in lordotic configuration (FPL), four-points bending contoured rods tested in kyphotic configuration (FPK). After one peak/valley cycle simulation of the rod, the stress tensors were extracted and processed in order to compute Sines equivalent stress and to represent the diagrams with constant life at 1 million cycles (called “Haigh diagrams”).| File | Dimensione | Formato | |
|---|---|---|---|
|
Tesi_Siino.pdf
non accessibile
Dimensione
4.18 MB
Formato
Adobe PDF
|
4.18 MB | Adobe PDF | Visualizza/Apri |
I documenti in POLITesi sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
https://hdl.handle.net/10589/175901