Article

Development of a patient-specific bone analog for the biomechanical evaluation of custom implants

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Abstract

Purpose – The aim of this study is to describe an improved experimental substrate for the mechanical testing of patient-specific implants fabricated using direct metal additive manufacturing processes. This method reduces variability and sample size requirements and addresses the importance of geometry at the bone/implant interface. Design/methodology/approach – Short-fiber glass/resin materials for cortical bone and polyurethane foam materials for cancellous bone were evaluated using standard tensile coupons. A method for fabricating bone analogs with patient-specific geometries using rapid tooling is presented. Bone analogs of a canine radius were fabricated and compared to cadaveric specimens in several biomechanical tests as validation. Findings – The analog materials exhibit a tensile modulus that falls within the range of expected values for cortical and cancellous bone. The tensile properties of the cortical bone analog vary with fiber loading. The canine radius models exhibited similar mechanical properties to the cadaveric specimens with a reduced variability. Research limitations/implications – Additional replications involving different bone geometries, types of bone and/or implants are required for a full validation. Further, the materials used here are only intended to mimic the mechanical properties of bone on a macro scale within a relatively narrow range. These analog models have not been shown to address the complex microscopic or viscoelastic behavior of bone in the present study. Originality/value – Scientific data on the formulation and fabrication of bone analogs are absent from the literature. The literature also lacks an experimental platform that matches patient-specific implant/bone geometries at the bone implant interface.

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... Some papers show the need and methods developed for this purpose for CAD/CAM-based or 3D printing-based fabrication of patient-specific substitute models based on imaging data, but each of them consists of only one material and thus cannot be fully adjusted to the individual properties [10][11][12]. Horn et al. demonstrates a process for fabricating an anatomical model with cancellous and cortical regions based on rapid tooling [13]. ...
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... Additive manufacturing is used to develop and manufacture scaffolds for tissue engineering as per the various requirements including porosity, biodegradability and biocompatibility at the exact shape. 6,9 The printing of bone regeneration scaffolds which are accurately made up of calcium phosphate and collagen. It is used for the construction of vascularised cell-laden and cartilaginous tissues that contains hyaluronic acid and chondrocytes. ...
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Basic research in canine mechanics must be performed to better understand the forces and moments the appendicular skeleton must withstand. This type of research may allow surgeons to make substantial advances in total joint replacement and fracture fixation design and may enhance our understanding of bone remodeling and fracture occurrence in relation to exercise and trauma. In our study, craniocaudal bending stiffness, mediolateral bending stiffness, axial compressive stiffness, and torsional stiffness of the humerus, femur, radius, and tibia of dogs was determined, using nondestructive bending, compression, and torsional tests. Entire diaphyseal and middiaphyseal properties of these long bones were evaluated. Bones also were tested to failure in torsion to quantify the failure properties of these long bones. Left to right variability was examined to validate the use of contralateral limbs as the control condition for experimental studies. There were no significant right to left differences in entire diaphyseal mechanical properties for any of the long bones, except for compressive stiffness of femurs. Homotypic differences in entire diaphyseal mechanical properties, if present, ranged from 8.0 to 35% for the 4 long bones (power = 0.8). For middiaphyseal mechanical properties, there were no significant right to left differences in the 4 long bones, except for craniocaudal bending stiffness of tibias. The homotypic differences in middiaphyseal mechanical properties, if present, ranged from 7.2 to 62% for the 4 long bones (power = 0.8). In all bones and loading modes, middiaphyseal stiffness was greater than entire diaphyseal stiffness (P < 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)
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To compare the axial mechanical stability of 3 circular external fixators systems with and without hemispheric washers. Experimental study. The axial stiffness and load necessary to produce 0.5 and 1 mm of displacement of 10 circular external fixator constructs from 3 manufacturers were tested on a materials testing machine. The constructs tested included the Small Bone fixator (SBF; Hofmann S.a.S., Monza, Italy), the IMEX ring fixator (IMEX Inc., Longview, TX), and the Multiplanar C-Fix (MCF; PanVet Distribuzione, Seriate, Italy). Five configurations were tested for each construct: (1) conventional nut fixation, (2) hemispheric washer fixation with connecting rods offset by 0, (3) 1, and (4) 2 holes, and (5) with a ring placed at maximum angulation. The loads resisted at 0.5 and 1 mm of displacement did not differ when frame configurations were compared (P =.25733 and.33769, respectively). The linear stiffness of the following configurations were decreasingly stiff: standard constructs, hemispheric washers with connecting rods perpendicular to rings, hemispheric washers with connecting rods offset by 1 hole, hemispheric washers with connecting rods offset by 2 holes, and ring offset in relation to bone model. The SBF constructs tested were 34% and 41% more rigid than the IMEX and MCF constructs tested despite the larger diameter of the connecting rods for the IMEX frames (6 mm) compared with the SBF frames (5 mm). The IMEX constructs tested were 6% more rigid than the MCF constructs tested. Adding hemispheric washers and angling connecting rods in relation to rings did not influence the loads resisted at 0.5 and 1 mm displacement but decreased construct stiffness. The use of hemispheric washers had minor effects on the biomechanical performance of fixator frames tested in this study when used to angle a ring in relation to connecting rods for circular external fixators.
Book
GENERAL CONSIDERATIONS Basic Composition and Structure of Bone -E. Bonucci Basic Concepts of Mechanical Property Measurement and Bone Biomechanics -Y. H. An and W. R. Barfield and R. A. Draughn Mechanical Properties of Bone -Y.H. An Factors Affecting Mechanical Properties of Bone -P. Zioupos, C. W. Smith, and Y. H. An Basic Facilities and Instruments for Mechanical Testing of Bone -C.V. Bensen and Y.H. An Methods of Evaluation for Bone Dimensions, Densities, , Morphology, and Structures -Y.H. An, W. R. Barfield and I. Knets General Considerations of Mechanical Testing -Y.H. An and C.V. Bensen A Hierarchical Approach to Exploring Bone Mechanical Properties -C. E. Hoffler, B. R. McCreadie, E. A. Smith, and S. A. Goldstein Nondestructive Mechanical Testing of Cancellous Bone -F. Linde and I. Hvid Synthetic Materials and Structures Used as Models for Bone -J. A. Szivek METHODS OF MECHANICAL TESTING OF BONE Tensile and Compression Testing of Bone -T. S. Keller and M. A. Liebschner Bending Tests of Bone -M. J. Lopez and M. D. Markel Torsional Testing of Bone -B. Furman and S. Saha Indentation Testing of Bone -B. E. McKoy, Q. Kang and Y. H. An Penetration Testing of Bone Using an Osteopenetrometer -I. Hvid and F. Linde Microhardness Testing of Bone -S. S. Huja, T. R. Katona, and W. E. Roberts Nanoindentation Testing of Bone -J. Y. Rho and G. M. Pharr Single Osteon Micromechanical Testing -M. G. Ascenzi, A. Benvenuti, and A. Ascenzi Micromechanical Testing of Single Trabeculae -P. L. Mente Strain Gauge Measurements from Bone Surfaces -J. A. Szivek and V. M. Gharpuray Screw Pullout Test for Evaluating Mechanical Properties of Bone -M. S. Crum, F. A. Young, Jr., and Y. H. An Viscoelastic Properties of Bone and Testing Methods -N. Sasaki Observation of Material Failure Mode Using a SEM with a Built-in Mechanical Testing Device -R. M. Wang and Y. H. An Ultrasonic Methods for Evaluating Mechanical Properties of Bone -J. Y. Rho Evaluating Mechanical Properties of Bone Using Scanning Acoustic Microscopy -C. H. Turner and J. L. Katz Peripheral Quantitative Computed Tomography for Evaluating Structural and Mechanical Properties of Small Bone -J. L. Ferretti Computer Modeling for Evaluating Trabecular Bone Mechanics -R. Saxena and T. S. Keller METHODS OF MECHANICAL TESTING OF THE BONE-IMPLANT INTERFACE Factors Affecting the Strength of the Bone-Implant Interface -B. E. McKoy, Y. H. An and R. J. Friedman Implant Pushout and Pullout Test -A. Berzins and D. R. Sumner The Validity of a Single Pushout Test -W. J. A. Dhert and J. A. Jansen Tensile Testing of Bone-Implant Interface -T. Nakamura and S. Nishiguchi Fracture Toughness Tests of the Bone-Implant Interface -X. Wang, K. A. Athanasiou, and C. M. Agrawal In Vitro Measurements of Implant Stability -A. Berzins and D. Sumner In Vitro Testing of the Stability of Acetabular Components -J. R. Davis, R. A. Lofthouse, and R. H. Jinnah In Vitro Testing of the Stability of Femoral Components -S. H. Naidu, F. M. Khoury and J. M. Cuckler Screw Pullout Test -L. A. Ferrara and T. C. Ryken Finite Element Analysis for Evaluating Mechanical Properties of the Bone-Implant Interface -K. R. Williams Fatigue Testing of Bioabsorbable Screws in a Synthetic Bone Substrate -W. S. Pietrzak, D. R. Sarver, and D. H. Kohn Testing Intervertebral Stability after Spinal Fixation -K. S. James and A. U. Daniels
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Objectives— To compare the pullout properties of 3.5‐mm AO/ASIF self‐tapping screws (STS) to corresponding standard cortex screws (CS) in a uniform synthetic test material and in canine femoral bone. The influence of screw‐insertion technique, test material, and test‐material thickness were also assessed. Study Design— In vitro experimental study. Sample Population— Two independent studies: a uniform synthetic test material and paired femurs from mature dogs. Methods— Mechanical testing was performed in accordance with standards established by the American Society for Testing and Materials for determination of axial pullout strength of medical bone screws. Completely inserted STS, completely inserted CS, and incompletely inserted STS were tested in 3 groups of 10 test specimens each in 4.96‐mm and 6.8‐mm thick sheets of synthetic material. In the bone study, group 1 consisted of 24 completely inserted STS compared with 24 completely inserted CS, and group 2 consisted of 24 incompletely inserted STS versus 24 completely inserted CS. Comparisons were made between paired femurs at corresponding insertion sites. Pullout data were normalized, thereby eliminating the effect of test‐material thickness on pullout properties. Mean values were compared using 2‐way ANOVA. Statistical significance was set at P < .05 . Results— In both the 4.96‐mm and 6.8‐mm synthetic material, pullout testing of the completely inserted STS demonstrated significantly greater yield strength and ultimate strength than completely inserted CS. There was no significant difference between incompletely inserted STS and completely inserted STS. The 6.8‐mm test material significantly increased yield strength and ultimate strength for all test groups compared with the 4.96‐mm test material. In canine bone, there was no significant difference in yield strength of completely inserted STS and completely inserted CS. Yield strength of completely inserted STS and completely inserted CS were significantly greater than incompletely inserted STS. Conclusions— Pullout properties of completely inserted STS were significantly greater than corresponding CS in a uniform test material. In canine bone, the pullout strength of STS and CS were not different. Incomplete STS insertion resulted in an 18% reduction in holding power as compared with completely inserted CS and STS in canine bone. Clinical Relevance— The length of STS used in canine bone should be such that the cutting flutes extend beyond the trans cortex to maximize pullout strength.
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To evaluate the fatigue life of stacked and single, veterinary cuttable plates (VCP) and small, limited contact, dynamic compression plates (LC-DCP). In vitro biomechanical study. Fracture models (constructs; n = 8) were assembled for each of 6 groups all with 8-hole plates: 2.0 mm LC-DCP; 2.4 mm LC-DCP; single 1.5/2.0 mm VCP; stacked 1.5/2.0 mm VCP; single 2.0/2.7 mm VCP; and stacked 2.0/2.7 mm VCP. Plate(s) were secured to 2 polyvinylchloride pipe lengths, mounted in a testing system with a custom jig, and subjected to axial loading (10-100 N) for 1,000,000 cycles at 10 Hz or until failure. Differences in number of cycles to failure among groups were compared. Failure mode was determined. All LC-DCP and single VCP constructs failed before 1,000,000 cycles. Stacked 2.0/2.7 mm VCP constructs withstood 1,000,000 cycles without failure. ANOVA and Fisher's least significant difference tests demonstrated significantly more cycles to failure for the stacked 1.5/2.0 mm VCP and stacked 2.0/2.7 mm VCP compared with the single 1.5/2.0 mm VCP, single 2.0/2.7 mm VCP, 2.0 mm LC-DCP, or 2.4 mm LC-DCP. Constructs that failed did so through a screw hole adjacent to the gap. Stacked VCP constructs have greater fatigue lives than comparably sized LC-DCP or single VCP constructs. Plates with 2.4 mm screws were not significantly different from the comparable construct with 2.0 mm screws. Although these data reveal that stacked VCP create a superior construct with respect to cyclic fatigue, surgeons must decide whether this is a clinical advantage on a case-by-case basis.
Standard Specification for Rigid Polyurethane Foam for Use as a Standard Material for Testing Orthopaedic Devices and Instruments Standard Test Method for Tensile Properties of Plastics Properties of Ti-6Al-4V non-stochastic lattice structures fabricated via electron beam melting
  • References Astm Standards
References ASTM Standards (2008a), Standard Specification for Rigid Polyurethane Foam for Use as a Standard Material for Testing Orthopaedic Devices and Instruments, 1839-08, ASTM International, West Conshohocken, PA. ASTM Standards (2008b), Standard Test Method for Tensile Properties of Plastics, D638-08, ASTM International, West Conshohocken, PA. Cansizoglu, O., Harrysson, O.L.A., Cormier, D.R., West, H.A. II and Mahale, T. (2008), " Properties of Ti-6Al-4V non-stochastic lattice structures fabricated via electron beam melting ", Materials Science & Engineering A, Vol. 492, pp. 468-474.
Cortical screw purchase in synthetic and human femurs Are circular external fixators weakened by the use of hemispheric washers?
  • R Zdero
  • K Elfallah
  • M Olsen
  • E H Schmetisch
  • Fl Boca Raton
  • D J Marcellin-Little
  • S C Roe
  • G L Rovesti
  • A Bosio
  • A Ferretti
Zdero, R., Elfallah, K., Olsen, M. and Schmetisch, E.H. (2009), " Cortical screw purchase in synthetic and human femurs ", Journal of Biomechanical Engineering, Vol. 131, pp. 1-7. Further reading Cowin, S.C. (2001), Bone Mechanics Handbook, CRC Press, Boca Raton, FL. Marcellin-Little, D.J., Roe, S.C., Rovesti, G.L., Bosio, A. and Ferretti, A. (2002), " Are circular external fixators weakened by the use of hemispheric washers? ", Veterinary Surgery, Vol. 31, pp. 367-374.
Structural properties of a novel design of composite analog humeri models”
  • J T Dunlop
  • A C M Chong
  • G L Lucas
  • F W Cooke
Standard Specification for Rigid Polyurethane Foam for Use as a Standard Material for Testing Orthopaedic Devices and Instruments
  • Astm Standards
Direct fabrication of metal orthopedic implants using electron beam melting technology”, paper presented at Solid Freeform Fabrication Symposium
  • O L A Harrysson
  • D R Cormier
  • D Marcellin-Little
  • K Jajal
Standard Test Method for Tensile Properties of Plastics
  • Astm Standards