Orbital Stress Analysis, Part IV: Use of a "Stiffness-Graded" Biodegradable Implants to Repair Orbital Blow-Out Fracture
ABSTRACT The purpose of this study was to develop a finite element model (FEM) of a human orbit, of 1 patient, who had an orbital blow-out fracture, to study the effect of using a "stiffness-graded" (SG) biodegradable implant on the biomechanics of bone-fracture repair.
An FEM of the orbit and the globe, of 1 patient who had an orbital blow-out fracture and was treated with biodegradable poly-L/DL-lactide [P(L/DL)LA 70/30], was generated based on computed tomography scan images. Simulations were performed with a computer using a commercially available finite element software. The FEM was then used to study the effect of using an SG biodegradable implant on the stress distribution in the fractured bone. This was compared with the stress distribution at the fracture interface and at the bone-implant interface, when using P(L/DL)LA implant with a uniform stiffness.
The use of SG implants caused less stress shielding to the fractured bone. At 50% of the bone healing stage, stress at the fracture interface was compressive in nature, that is, 0.2 MPa for the uniform implant, whereas SG implants resulted in tensile stress of 0.2 MPa. The result was that SG implants allowed the 50% healed bone to participate in loadings. Stiffness-graded implants are more flexible and hence permit more bending of the fractured bone. This results in higher compressive stresses, induced at the fractured faces, to accelerate bone healing. However, away from the fracture interface, the reduced stiffness and elastic modulus of the implant cause the neutral axis of the composite structure to be lowered into the bone, resulting in the higher tensile stress in the bone layer underneath the implant.
The use of SG implants induced significant changes in the stress patterns at the fracture interface and at the bone-implant interface. Stiffness-graded biodegradable implants offered less stress shielding to the bone, providing higher compressive stress at the fractured surface, to induce accelerated bone healing, as well as higher tensile stress in the intact portion of the bone. It seems that this is the first reported study, in the literature, on the use of SG biodegradable implants to repair and promote bone healing at the fracture site of the inferior orbital wall bone defect.
SourceAvailable from: Jehad Al Sukhun[Show abstract] [Hide abstract]
ABSTRACT: The purpose of this study was to develop a three-dimensional finite-element model (FEM) of the human orbit, containing the globe, to predict orbital deformation in subjects following a blunt injury. This study investigated the hypothesis that such deformation could be modelled using finite-element techniques. One patient who had CT-scan examination to the maxillofacial skeleton including the orbits, as part of her treatment, was selected for this study. A FEM of one of the orbits containing the globe was constructed, based on CT-scan images. Simulations were performed with a computer using the finite-element software NISA (EMRC, Troy, USA). The orbit was subjected to a blunt injury of a 0.5 kg missile with 30 ms(-1) velocity. The FEM was then used to predict principal and shear stresses or strains at each node position. Two types of orbital deformation were predicted during different impact simulations: (i) horizontal distortion and (ii) rotational distortion. Stress values ranged from 213.4 to 363.3 MPa for the maximum principal stress, from -327.8 to -653.1 MPa for the minimum principal stress, and from 212.3 to 444.3 MPa for the maximum shear stress. This is the first finite-element study, which demonstrates different and concurrent patterns of orbital deformation in a subject following a blunt injury. Finite element modelling is a powerful and invaluable tool to study the multifaceted phenomenon of orbital deformation.Journal of The Royal Society Interface 05/2006; 3(7):255-62. DOI:10.1098/rsif.2005.0084 · 3.86 Impact Factor
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ABSTRACT: The purpose of this study was to develop a finite element model (FEM) of a human orbit, who experienced a pure orbital blowout fracture, to study the effect of the geometrical mismatch-induced stresses on the orbital floor/graft interface and how to improve the graft design when restoring the orbital floor. A FEM of the orbit and the globe of 1 patient who experienced pure orbital blowout fracture and treated with autogenous bone graft was generated based on computed tomographic scans. Simulations were performed with a computer using a commercially available finite element software NISA (EMRC, Troy, MI). The FEM was then used to study the effects of changing the geometry, position, material properties, and method of fixation of the autogenous bone graft on its predictions. The factors that had the biggest impact on the predicted principal strain magnitudes were absence of cancellous bone (up to 60%) and bony support of the graft (up to 50%). Applying rigid fixation reduced stresses by 30% posteriorly and by almost 100% anteriorly. Alterations to the geometry of the bone graft, such as an increase in its thickness, increased principal strain magnitudes (up to 42%). Applying rigid fixation reduced principal stresses significantly. The role of rigid fixation becomes more prominent when there is no bony support posteriorly and/or medially. This study also highlights the importance of preserving cancellous bone, when harvesting and preparing the autogenous bone graft to reconstruct the orbital floor. The possibility that absence of cancellous bone and the resulting stresses may be a source of graft resorption and/or failure cannot be excluded.The Journal of craniofacial surgery 07/2011; 22(4):1294-8. DOI:10.1097/SCS.0b013e31821c6afd · 0.68 Impact Factor
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ABSTRACT: The purpose was to study the biomechanics of bone fracture repair, of the orbital floor, using osteosynthetic bioresorbable implant and how to improve the implant design. A finite element model of the orbit and the globe of 1 patient who experienced orbital blowout fracture and treated with bioresorbable poly-L/DL-lactide (P[L/DL]LA 70:30) implant (PolyMax; Synthes, Oberdorf, Switzerland) was generated based on computed tomographic scans. Simulations were performed with a computer using a commercially available finite element software. The effects of changing the geometry, bony support, and method of fixation of the implant on the finite element model predictions were investigated. The factor that had the biggest impact on the predicted principal strain magnitudes was absence of bony support of the implant (up to 65%). Applying elastic fixation reduced stresses (up to 40%) posteriorly. The principal stresses inside the bone and the implant were evenly distributed when elastic fixation was applied to the implant. Applying rigid fixation increased stresses (up to 50% and 80% anteriorly and posteriorly, respectively). The resulting stress values indicated a likely rapid failure of the osteosynthetic implant when rigid fixation was applied. Applying rigid fixation induced a significant increase in stress patterns. Principal stresses were reduced remarkably when elastic fixation was applied to the implant. The role of fixation becomes more prominent when there is no bony support posteriorly and/or medially. It is recommended to avoid rigid fixation and to apply elastic fixation when using bioresorbable P(L/DL)LA 70:30 implants to reconstruct inferior orbital wall bony defects.The Journal of craniofacial surgery 07/2011; 22(4):1299-303. DOI:10.1097/SCS.0b013e31821c6ae9 · 0.68 Impact Factor