Search

The complex biomechanics of the lumbar spine and its associated structures have been studied using finite element (FE) analysis. Several FE studies used simplified approaches to model the spinal ligaments, assuming that the significant effect of spinal ligaments is on the ranges of motion (ROM) rather than stress–strain distributions on the vertebral body. A comparison of different ligament configurations (tension-only and tension & compression) and their effects on ROM and stress–strain distribution is necessary to verify the above assumption. In this study, an FE model of the L1-L5 lumbar spine was developed and analyzed for three different cases of physiological movements (flexion, extension, and lateral bending). It was found that the spinal flexibility was almost equally constrained by both ligament configurations. The variation in ROM observed for tension-only ligament and tension & compression ligament model ranged between 0.5∘ and 4∘ under all loading cases. However, no considerable changes were observed in stress–strain distributions. The findings of this study indicate that assuming the impact of ligaments only in restricting the ROM is reliable and two-node tension-only or tension & compression elements are useful to model the spinal ligaments in FE studies. [ABSTRACT FROM AUTHOR]

The use of uncemented stems in hip arthroplasty has been increasing, even in osteoporotic patients. The major concerns of uncemented hip-stems, however, are peri-prosthetic fracture, thigh pain, and proximal femoral stress-/strain-shielding. In this study, a novel design of uncemented hip-stem is proposed that will reduce such concerns, improve osseointegration, and benefit both osteoporotic and arthritic patients. The stem has a central titanium alloy core surrounded by a set of radial buttresses that are partly porous titanium, as is the stem tip. The aim of the study was to investigate the mechanical behaviour of the proposed partly-porous design, examining load transfer in the short-term, and comparing its strain-shielding behaviour with a solid metal implant. The long-term effect of implant-induced bone remodelling was also simulated. Computed tomography based three-dimensional finite element models of an intact proximal femur, and the same femur implanted with the proposed design, were developed. Peak hip contact and major muscle forces corresponding to level-walking and stair climbing were applied. The proposed partly-porous design had approximately 50% lower strain-shielding than the solid-metal counterpart. Results of bone remodelling simulation indicated that only 16% of the total bone volume is subjected to reduction of bone density. Strain concentrations were observed in the bone around the stem-tip for both solid and porous implants; however, it was less prominent for the porous design. Lower strain-shielding and reduced bone resorption are advantageous for long-term fixation, and the reduced strain concentration around the stem-tip indicates a lower risk of peri-prosthetic fracture. [ABSTRACT FROM AUTHOR]

Solid metallic hip implants have much higher stiffness than the femur bone, causing stress-shielding and subsequent implant loosening. The development of low-stiff implants using metallic porous structures has been reported in the literature. Ti6Al4V alloy is a commonly used biomaterial for hip implants. In this work, Body-Center-Cubic (BCC), Cubic, and Spherical porous structures of four different porosities (82%, 76%, 70%, and 67%) were investigated to establish the range of ideal porosities of Ti6Al4V porous structures that can match the stiffness of the femur bone. The effective mechanical properties have been determined through Finite Element Analysis (FEA) under uniaxial compressive displacement of 0.32 mm. FEA predictions were validated with the analytical calculations obtained using Gibson and Ashby method. The effective mechanical properties of 82%, 76%, 70%, and 67% porous BCC and Cubic structures were found to match the mechanical properties of cortical bone closely. They were also well comparable to the Gibson-Ashby method-based calculations. BCC and Cubic porous structures with 67–82% porosity can mimic the stiffness of the femur bone and are suitable for low-stiff hip implant applications.

Graphical abstract Finite element (FE) analysis has been used in studying the biomechanics of the lumbar spine. While some FE studies used a follower load technique that intended to replace the compressing effect of local muscle force, other studies considered satisfying the relationship between the human body's posture and the centre of gravity (CG) for investigating spine biomechanics. However, the above studies did not reveal the importance of a coordinate system that gratifies the posture-CG relation and follower load techniques. The present FE study compares the variation in ranges of motion (ROM) and stress-strain distributions due to the application of loading via different coordinate systems, follower (FCS) and global (GCS). A subject-specific computed tomography scan-based intact spine (L1-L5) FE model was developed and simulated for physiological movements. The FE results indicated a minimum deviation in ROM of 2.7° for the L1-L5 full model at all physiological activities between the defined coordinate systems. The observed variation for the L3-L4 functional spinal unit was between 4.7° and 19°. The von Mises strain in the vertebrae was between 0.0007 and 0.003 for the FCS case. In contrast, the peak von Mises strain for the GCS case was above the compressive yield strain limit of cancellous bone by 38.5%. The GCS model transferred the load unsymmetrically, whereas the distribution was symmetrical for the FCS case, without any potential risk of bone failure. These observations clearly indicate that the selection of the appropriate loading coordinate system is as crucial as the magnitude of loading. [ABSTRACT FROM AUTHOR]

AbstractPorous implants reduce stress-shielding within implanted bones. Body-centered-cubic (BCC) and cubic cellular structures were investigated in this study to mimic bone properties. For this purpose, CAD models (7 BCC and 7 cubic) with 50% to 80% porosity were designed and 3D-printed. Morphological analysis revealed minor dimensional deviations between the CAD and the 3D-printed models. Using finite element analysis, experimental approach, and regression analysis, an ideal porosity range to mimic the mechanical properties of bone has been established. Lattices with a porosity of 60%–65% and 50%–55% for cubic and BCC, respectively, can emulate the mechanical properties of cortical bones. [ABSTRACT FROM AUTHOR]

ABSTRACT: Proximal femoral fractures can be categorized into two main types: Neck and intertrochanteric fractures accounting for 53% and 43% of all proximal femoral fractures, respectively. The possibility to predict the type of fracture a specific patient is predisposed to would allow drug and exercise therapies, hip protector design, and prophylactic surgery to be better targeted for this patient rendering fracture preventing strategies more effective. This study hypothesized that the type of fracture is closely related to the patient‐specific femoral structure and predictable by finite element (FE) methods. Fourteen femora were DXA scanned, CT scanned, and mechanically tested to fracture. FE‐predicted fracture patterns were compared to experimentally observed fracture patterns. Measurements of strain patterns to explain neck and intertrochanteric fracture patterns were performed using a digital volume correlation (DVC) technique and compared to FE‐predicted strains and experimentally observed fracture patterns. Although loaded identically, the femora exhibited different fracture types (six neck and eight intertrochanteric fractures). CT‐based FE models matched the experimental observations well (86%) demonstrating that the fracture type can be predicted. DVC‐measured and FE‐predicted strains showed obvious consistency. Neither DXA‐based BMD nor any morphologic characteristics such as neck diameter, femoral neck length, or neck shaft angle were associated with fracture type. In conclusion, patient‐specific femoral structure correlates with fracture type and FE analyses were able to predict these fracture types. Also, the demonstration of FE and DVC as metrics of the strains in bones may be of substantial clinical value, informing treatment strategies and device selection and design. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:993–1001, 2018. [ABSTRACT FROM AUTHOR]

An appropriate method of application of the hip-joint force and stress analysis of the pelvic bone, in particular the acetabulum, is necessary to investigate the changes in load transfer due to implantation and to calculate the reference stimulus for bone remodelling simulations. The purpose of the study is to develop a realistic 3D finite element (FE) model of the hemi-pelvis and to assess stress and strain distribution during a gait cycle. The FE modelling approach of the pelvic bone was based on CT scan data and image segmentation of cortical and cancellous bone boundaries. Application of hip-joint force through an anatomical femoral head having a cartilage layer was found to be more appropriate than a perfectly spherical head, thereby leading to more accurate stress-strain distribution in the acetabulum. Within the acetabulum, equivalent strains varied between 0.1% and 0.7% strain in the cancellous bone. High compressive (15-30 MPa) and low tensile (0-5 MPa) stresses were generated within the acetabulum. The hip-joint force is predominantly transferred from the acetabulum through the lateral cortex to the sacroiliac joint and the pubic symphysis. The study is useful to understand the load transfer within the acetabulum and for further investigations on acetabular prosthesis.