Design of patient specific bone stiffness mimicking scaffold.

The difference in stiffness of a patient's bone and bone implant causes stress shielding. Thus, implants which match the stiffness of bone of the patient result in better bone growth and osseointegration. Variation in porosity is one of the methods to obtain implants with different stiffness values. This study proposes a novel method to design biomimetic bone graft implant based on computed tomography (CT) scan data, that creates similar pre- and post-implant mechanical environment on peri-implant bone. The design methodology is demonstrated by taking three different sections of human femur bone, greater trochanter, diaphysis and epicondyle, with two different implant materials, Ti-6Al-4V and Ti-Mg. Bones from these three sections were replaced with porous implants of effective stiffness of replaced bone, as would have been required after a resection surgery. Models were simulated with physiological loading condition using finite element (FE) method. Variation of maximum von Mises stress and average strain on peri-prosthetic bone were found to be in the range of -6% to 10.7% and -7% to -17.9% for porous implants and 26% to 50% and -36% to -59% for solid implant respectively compared to natural bone. The results revealed that the porous implants, which have been designed based on CT scan data, can effectively produce mechanical response at peri-implant bone, which is very close to pre-implanted condition. Following this methodology, more osseointegration friendly mechanical environment can be achieved at peri-implant bone for any anatomical location independent of implant materials.

Design of Patient Specific Spinal Implant (Pedicle Screw Fixation) using FE Analysis and Soft Computing Techniques.

BACKGROUND: This work uses genetic algorithm (GA) for optimum design of patient specific spinal implants (pedicle screw) with varying implant diameter and bone condition. The optimum pedicle screw fixation in terms of implant diameter is on the basis of minimum strain difference from intact (natural) to implantation at peri-prosthetic bone for the considered six different peri-implant positions. METHODS: This design problem is expressed as an optimization problem using the desirability function, where the data generated by finite element analysis is converted into an artificial neural network (ANN) model. The finite element model is generated from CT scan data. Thereafter all the ANN predictions of the microstrain in six positions are converted to unitless desirability value varying between 0 and 1, which is then combined to form the composite desirability. Maximization of the composite desirability is done using GA where composite desirability should be made to go up as close as possible to 1. If the composite desirability is 1, then all 'strain difference values in 6 positions' are 0. RESULTS: The optimum solutions obtained can easily be used for making patient-specific spinal implants.

Measurement of strain in the rod for lumbar pedicle screw fixation: An experimental and finite element study.

Spinal fusion with pedicle-screw-rod is being used widely for treating spinal deformities diseases. Several biomechanical studies on screw rod based implant failure through screw pullout, bending, screw breakage have been performed. But few studies are available regarding the effect of strain for breakage of rod. So, the purpose of the present study is to observe strain at the rod connected with the pedicle screw for different loading condition. The strain in stainless steel (SS) connecting rods for pedicle screw fixation were measured using strain gauge. In order to investigate the bio-mechanical response of lumbar spine with reference to strain in the rod, a simple experimental setup was developed using a specimen of L1-S spine segment. SS rods were used for pedicle screw implant on prototyped lumbar Spine. Prior to testing with pedicle screw, the lumbar spine specimen was also compared with FE results. The strain measured using strain gauges at L3-L4 level on SS rod were within a range of 85 to 310 microstrain under 6, 8, 10 Nm flexion and extension, and for L4-L5 level, these values were within a range of 95 to 440 microstrain. It was found that FE result was higher than the strain gauge result and the error varied between 10.5% to 33% with average error of 22.8%. However similar stain behavior was observed by the FE analysis. The proposed method, as well as the qualitative data, might be helpful for the researchers to understand biomechanical behavior of pedicle-screw implanted spine.

Effect of two-level pedicle-screw fixation with different rod materials on lumbar spine: A finite element study.

BACKGROUND: Pedicle-screw-rod fixation system is very popular surgical remedy for degenerative disc disease. It is important to observe load vs. spinal motion characteristic for better understanding of clinical problems and treatment of spinal instability associated with low-back pain. OBJECTIVE: The objective of this study is to understand the effect [range of motion (ROM) and intervertebral foramen height] of pedicle-screw fixation with three rod materials on lumbar spine under three physiological loading conditions. METHOD: A three-dimensional finite element (FE) model of lumbar to sacrum (L1-S) vertebrae with pedicle-screw-rod fixation at L3-L5 level is developed. Three rod materials [titanium alloy (Ti6Al4V), ultra-high molecular weight poly ethylene (UHMWPE) and poly-ether-ether-ketone (PEEK)] are used for two-level fixation and the FE models are simulated for axial rotation, lateral bending and flexion-extension under ±10 Nm moment and 500 N compressive load and compared with the intact (natural) model. RESULT & DISCUSSION: For axial rotation, lateral bending and flexion, ROM increased 2.8, 4.5 and 1.83 times respectively for UHMWPE, and 3.7, 7.2 and 2.15 times respectively for PEEK in comparison to Ti6Al4V. As ROM is 49, 29 and 31% of the intact model during axial rotation, lateral bending and flexion respectively, PEEK rod produced better motion flexibility than Ti6Al4V and UHMWPE rod. Foramen height increased insignificantly by 2.21% for the PEEK rod with respect to the intact spine during flexion. For the PEEK rod, maximum stress of 40 MPa during axial rotation is much below the yield stress of 98 MPa. CONCLUSION: Ti6Al4V pedicle-screw-rod fixation system highly restricted the ROM of the spine, which is improved by using UHMWPE and PEEK, having lower stiffness. The foramen height did not vary significantly for any implant materials. In terms of ROM and maximum stress, PEEK rod may be considered for a better implant design to get better ROM and thus mobility.