ASTM F2996-20

Standard Practice for Finite Element Analysis (FEA) of Non-Modular Metallic Orthopaedic Hip Femoral Stems

SIGNIFICANCE AND USE
3.1 This practice is applicable to the calculation of stresses seen on a femoral hip stem when loaded in a manner described in ISO 7206-4 (2010). This method can be used to establish the worst-case size for a particular implant. When stresses calculated using this practice were compared to the stresses measured from physical strain gauging techniques performed at two laboratories using two different methods, the results correlated to within 8 %.  
3.2 This test method can be used to estimate the effects of design variables on the stress and strain of metallic hip femoral stems in a set-up mimicking that described in ISO 7206-4 (2010).
SCOPE
1.1 This practice establishes requirements and considerations for the numerical simulation of non-modular (that is, limited to monolithic stems with only a femoral head/trunnion taper interface) metallic orthopaedic hip stems using Finite Element Analysis (FEA) techniques for the estimation of stresses and strains. This standard is only applicable to stresses below the yield strength, as provided in the material certification.  
1.2 Purpose—This practice establishes requirements and considerations for the development of finite element models to be used in the evaluation of non-modular metallic orthopaedic hip stem designs for the purpose of prediction of the static implant stresses and strains. This procedure can be used for worst-case assessment within a series of different sizes of the same implant design to reduce the physical test burden. Recommended procedures for performing model checks and verification are provided to help determine if the analysis follows recommended guidelines. Finally, the recommended content of an engineering report covering the mechanical simulation is presented.  
1.3 Limits—This practice is limited in discussion to the static structural analysis of non-modular metallic orthopaedic hip stems (which excludes the prediction of fatigue strength).  
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 14-Jun-2020
• ICS: 11.040.40 - Implants for surgery, prosthetics and orthotics
• Technical Committee: F04 - Medical and Surgical Materials and Devices
• Drafting Committee: F04.22 - Arthroplasty

Relations

• Replaces: ASTM F2996-13 - Standard Practice for Finite Element Analysis (FEA) of Non-Modular Metallic Orthopaedic Hip Femoral Stems
• Effective Date: 15-Jun-2020
• Referred By: ASTM F2068-15 - Standard Specification for Femoral Prostheses—Metallic Implants (Withdrawn 2023)
• Effective Date: 15-Jun-2020

Overview

ASTM F2996-20 specifies the standard practice for finite element analysis (FEA) of non-modular metallic orthopaedic hip femoral stems. Managed by ASTM Committee F04 on Medical and Surgical Materials and Devices, this standard enables manufacturers and researchers to simulate and estimate mechanical stresses and strains in monolithic hip stem designs. By utilizing numerical simulation aligned with ISO 7206-4 (2010) loading protocols, ASTM F2996-20 provides a consistent approach to evaluating implant behavior under static conditions, supporting worst-case assessments and potential reduction of physical testing requirements.

Key Topics

• Applicability: Focuses on non-modular metallic hip stems (single-piece stems with only femoral head/trunnion taper interface).
• Scope of Analysis: Covers static structural FEA for stress and strain prediction, excluding fatigue life assessments.
• Model Development: Describes requirements for geometric representation, material property consistency (using modulus of elasticity and Poisson’s ratio), mesh convergence practices, and suitable element types.
• Loading and Boundary Conditions: Outlines the application of loading and constraints as per ISO 7206-4, emphasizing replication of worst-case scenarios in implant geometry and positioning.
• Results Reporting: Recommends essential reporting criteria, such as device description, methodology, geometry simplifications, model validation, mesh convergence, and summary of key findings.

Applications

This standard is widely utilized in:

• Orthopaedic Implant Design Validation: Enables manufacturers to evaluate new or modified hip stem designs via computer simulations, supporting regulatory submissions or internal risk assessments.
• Worst-Case Scenario Analysis: Assists in determining the most critical implant size or configuration, ensuring comprehensive coverage within a family of similar designs and reducing redundant physical testing.
• Model Credibility: Offers guidance on model verification and validation, including checks against experimental data, which enhances confidence in simulation results.
• Engineering Reporting and Documentation: Standardizes the structure of FEA results for communication with regulatory agencies (such as the FDA), facilitating compliance and transparency.
• Sensitivity Studies: Supports investigation into the effects of design variables (geometry, material properties, loading direction) on implant performance, helping to optimize product safety and longevity.

Related Standards

• ISO 7206-4:2010 - Implants for Surgery – Partial and Total Hip Joint Prostheses – Part 4: Determination of Endurance Properties and Performance of Stemmed Femoral Components. This is the key reference for loading protocols and test set-up within ASTM F2996-20.
• ASME V&V 40-2018 - Provides framework for verification and validation of computational modeling in medical devices.
• FDA Guidance on Computational Modeling Studies - Offers recommendations for reporting and credibility assessment of modeling in premarket submissions.

Practical Value

ASTM F2996-20 delivers practical benefits for the orthopaedic industry by:

• Promoting consistency in FEA methodology for hip femoral stems.
• Enabling efficient regulatory pathways through recognized simulation practices.
• Facilitating reduced physical testing by using validated simulation to predict worst-case static stress and strain.
• Supporting safe and effective design optimization of hip implants.

Keywords

Finite element analysis (FEA), non-modular metallic orthopaedic hip stems, static simulation, hip implant stress analysis, model verification, ISO 7206-4, ASTM F2996, mesh convergence, material properties, computational modeling, orthopaedic implant validation.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: F2996 − 20
Standard Practice for
Finite Element Analysis (FEA) of Non-Modular Metallic
Orthopaedic Hip Femoral Stems
This standard is issued under the fixed designation F2996; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
1.1 This practice establishes requirements and consider-
Barriers to Trade (TBT) Committee.
ations for the numerical simulation of non-modular (that is,
limited to monolithic stems with only a femoral head/trunnion
2. Referenced Documents
taper interface) metallic orthopaedic hip stems using Finite
Element Analysis (FEA) techniques for the estimation of 2
2.1 ISO Standards:
stresses and strains.This standard is only applicable to stresses
ISO 7206-4 (2010) Implants for Surgery—Partial and Total
below the yield strength, as provided in the material certifica-
HipJointProstheses—Part4:DeterminationofEndurance
tion.
Properties and Performance of Stemmed Femoral Com-
1.2 Purpose—This practice establishes requirements and ponents
considerations for the development of finite element models to
be used in the evaluation of non-modular metallic orthopaedic
3. Significance and Use
hip stem designs for the purpose of prediction of the static
3.1 This practice is applicable to the calculation of stresses
implant stresses and strains. This procedure can be used for
seen on a femoral hip stem when loaded in a manner described
worst-case assessment within a series of different sizes of the
inISO7206-4(2010).Thismethodcanbeusedtoestablishthe
same implant design to reduce the physical test burden.
worst-case size for a particular implant. When stresses calcu-
Recommended procedures for performing model checks and
lated using this practice were compared to the stresses mea-
verification are provided to help determine if the analysis
sured from physical strain gauging techniques performed at
follows recommended guidelines. Finally, the recommended
two laboratories using two different methods, the results
content of an engineering report covering the mechanical
correlated to within 8 %.
simulation is presented.
3.2 This test method can be used to estimate the effects of
1.3 Limits—This practice is limited in discussion to the
design variables on the stress and strain of metallic hip femoral
static structural analysis of non-modular metallic orthopaedic
stems in a set-up mimicking that described in ISO 7206-4
hip stems (which excludes the prediction of fatigue strength).
(2010).
1.4 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this 4. Geometric Data
standard.
4.1 Finite element models are based on a geometric repre-
1.5 This standard does not purport to address all of the
sentation of the device being studied. The source of the
safety concerns, if any, associated with its use. It is the
geometricdetailscanbeobtainedfromdrawings,solidmodels,
responsibility of the user of this standard to establish appro-
preliminary sketches, or any other source consistent with
priate safety, health, and environmental practices and deter-
defining the model geometry. In building the finite element
mine the applicability of regulatory limitations prior to use.
model, certain geometric details may be omitted from the
1.6 This international standard was developed in accor-
orthopaedic implant geometry shown in the computer-aided
dance with internationally recognized principles on standard-
design (CAD) model if it is determined that they are not
ization established in the Decision on Principles for the
relevant to the intended analysis. Engineering judgment shall
be exercised to establish the extent of model simplification and
1 shall be justified.
ThispracticeisunderthejurisdictionofASTMCommitteeF04onMedicaland
Surgical Materials and Devices and is the direct responsibility of Subcommittee
F04.22 on Arthroplasty.
Current edition approved June 15, 2020. Published August 2020. Originally
approved in 2013. Last previous edition approved in 2013 as F2996 – 13. DOI: Available from International Organization for Standardization (ISO), 1, ch. de
10.1520/F2996-20. la Voie-Creuse, CP 56, CH-1211 Geneva 20, Switzerland, http://www.iso.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F2996 − 20
4.2 It is most appropriate to consider the “worst-case” stress 6.2 The load shall be applied such that the magnitude and
condition for the orthopaedic implant being simulated. The direction are identical to those defined in ISO 7206-4 (2010).
“worst-case” shall be determined from all relevant engineering The point of load application shall produce a statically equiva-
considerations, such as stem geometry, dimensions, and head lent bending moment to a load applied through the head center
offset. If finite element analysis is being used for determining
with its head offset.
the worst case, then the worst case head offset may not be
6.2.1 The load in the model will be applied to the end
known. It may be necessary to run several variants of head
circular face of the hip stem trunnion or in a justifiably
offset to determine this.
equivalent manner. The trunnion may be extended or truncated
to approximate the loading conditions that simulate the worst-
5. Material Properties
case head offset, which may be determined via an iterative
5.1 The required material properties for input into an FEA
process. This approximation should be reported if performed.
model for the calculation of strains and displacement are
Alternatively, a rigid couple can be used to tie the load point to
modulus of elasticity (E) and Poisson’s ratio (ν). These values
the trunnion end circular face. Refer to Fig. 1.
can be obtained from material certification data. It should be
6.2.2 It is recognized that the loading conditions in this
noted that as ISO 7206-4 (2010) is run under load control, the
practice are not identical to that of ISO 7206-4 (2010).
FEA should also be run under load control. When the FEA is
However, the differences in loading conditions (for example,
run under load control, the modulus of elasticity will not affect
load applied to surface of head versus face of stem trunnion;
thestresscalculationsundersmalldisplacementtheorybutwill
potting level differences; use of bone cement which is not
affect displacement and strain. The influence of Poisson’s ratio
modeled in FEA) should not significantly affect identification
on the stress calculations is negligible.
of the “worst-case” stress condition, which is the primary
5.2 Ensure that material property units are consistent with
objective of this practice. When subsequent physical fatigue
geometric units in the CAD model. SI units are the preferred testing per ISO 7206-4 (2010) is performed, comparison of the
units of measure.
physical test results (that is, location of origin of distal stem
fracture) should be compared to the FEA test results to
6. Loading
determine if there were any significant differences. If so, the
6.1 The loading and orientation of the hip stem shall be reason for these differences shall be evaluated, necessary
guided by ISO 7206-4 (2010) standard. The areas of particular adjustments shall be made to the physical test fixtures or finite
interest are the stresses in the neck region, driver hole region, element model, and, depending on the result of the analysis,
potting level, and other design-specific critical regions. testing of additional components may be necessary.
FIG. 1 Load Application
NOTE 1—Generating the statically equivalent maximum bending moment by (a) an offset node tied rigidly to the circular trunnion face, or (b)a
cylindricalextension(ortruncationofcirculartrunnionfacewhichequalsthemaximumfemoralheadoffset(whichisanapproximationoftheoffsetnode
method, to be documented if utilized).As an example, the modeling of a +8 mm femoral head offset is shown here. Figures are for illustration purposes
only.
NOTE 2—Boundary condition is located at the distal face/cut region of the stem.
F2996 − 20
6.3 Ensure that load units are consistent with material size, and the shape of the elements used must be able to
property units. represent the expected behavior without significant numerical
limitation or complication. Most FEApackages have a built-in
7. Boundary Conditions
program which checks the shape of the element for the type of
analysis selected. If this tool is not available, then additional
7.1 The hip stem shall first be sectioned at a distance from
checks are needed.
the center of the head as described in ISO 7206-4 (2010) with
the worst-case head/neck offset. This sectioned region repre-
8.3 Thenumberandspacingofnodes(thatis,meshdensity)
sents the potting level to which stresses and strains shall be
shouldbeconsistentwiththetypeofelementusedandthetype
evaluated. A second parallel cut shall then be made 10 mm
of result desired. This may be demonstrated with a mesh
below the first sectioned region. The hip stem shall be
densitystudy,wherebyaseriesofmodelswithincreasingmesh
constrained in all directions on all faces distal to the second
refinement in the critical stress regions is used to demonstrate
cut. Constraining the stem in this manner ensures that exces-
solution convergence. This allows the error associated with
sive erroneous stresses are not generated at the region of
subsequent models to be estimated. The method used to
interest due to the influence of rigid fixation. Refer to Fig. 2,
demonstrate mesh convergence, in analysis cases where it is
Fig. 3, and Fig. 4, which present three stem length variants
not performed directly onto the model being analyzed, shall be
provided in ISO 7206-4 (2010). The use of other stress
documented in the FEA report. It is recommended that a
evaluationlevelsorconstraintlevels,orboth,shallbejustified.
minimum of three levels of mesh refinement be performed and
a model convergence of ≤5 % be demonstrated on the quantity
8. Analysis
of interest (see 8.6) and at all regions of interest. A stress
8.1 The analysis and modeling system, programs, or soft-
convergence of >5 % shall be justified based on the context of
ware used for the finite element model creation and analysis use.
should be capable of fully developing the geometric features
8.4 The choice of element type is left to the analyst;
andidealizingtheloadingandboundaryconditionenvironment
however, it is recommended for analysis of a hip stem that
of the orthopaedic implant. An engineering justification shall
tetrahedral or hexahedral elements be used. If tetrahedral
be provided to support any assumptions or simplifications.
elements are considered, use of 4-noded elements should be
8.2 The finite element mesh can be created using automatic avoided to prevent stress and strain incompatibilities across
meshing, manual meshing, or a combination of the two elements.Additionally, the linear, 4-noded tetrahedron element
techniques. The overriding consideration is that the type, the is a constant strain element. This means that displacement
FIG. 2 Boundary Condition Location for Hip Stem Length#120 mm
NOTE 1—CT is the distance between center of the head and the most distal point of the stem.
NOTE 2—Boundary condition is located at the distal face/cut region of the stem.
F2996 − 20
FIG. 3 Boundary Condition Location for Hip Stem Length of 120 mm < CT# 250 mm
NOTE 1—CT is the distance between center of the head and the most distal point of the stem.
NOTE 2—Boundary condition is located at the distal face/cut region of the stem.
interpolation is linear and the corresponding stresses and evaluation is not being performed on the final design of the
strains are constant within any element. Therefore, a very device or if there are other significant assumptions that may
refined mesh is required around locations where high stress/ limit the use of the results, this must be clearly stated.
strain gradients are present when utilizing these elements. (2) A description of boundary constraints, loads, and the
When elements which are not directly identified in the guide material properties of Young’s Modulus and Poisson’s ratio as
are used, documentation shall be provided in the FEA report a minimum. The source of the material property data utilized
which demonstrates their validity.
should be referenced.
(3) Asummary of the finite element modeling and analysis
8.5 The finite element results should be examined to ensure
systemusedfortheanalysis.Ifcurrentversionsofwidelyused,
that the geometrical models of the implant, boundary
commerciallyavailablesoftwareareused,thissummarycanbe
conditions, and applied loads have been appropriately defined
by name and reference to the version used. For non-
in the analysis and properly represent the behavior being
commercially available, proprietary tools, or customer user
analyzed.
modification of commercially available software, sufficient
st
8.6 The primary measure of interest is the Maximum (1 )
technical background and results of test problems should be
Principal Stress. Refer to Fig. 5. A secondary measure of
provided to demonstrate the utility, verification, applicability,
interest is the von Mises stress at the location of maximum
and limitations of the software tool.
principalstress.Ifotherstressvaluesareused,theirvalidityfor
(4) A description of the procedure used to convert the
use should be documented.
geometric or CAD representation of the device to the finite
element model.Any geometry simplifications should be docu-
9. Report
mented.
9.1 The finite element analysis for the evaluation of an (5) A description of the finite element model and its
orthopaedic implant should be fully documented in an engi- relation to the device being evaluated. The number of nodes
neering report. The actual format of the report should include, and elements (or the degrees of freedom in the model), the
but not be limited to, the following: finite element type selec
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: F2996 − 13 F2996 − 20
Standard Practice for
Finite Element Analysis (FEA) of Non-Modular Metallic
Orthopaedic Hip Femoral Stems
This standard is issued under the fixed designation F2996; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This practice establishes requirements and considerations for the numerical simulation of non-modular (that is, limited to
monolithic stems with only a femoral head / trunnion head/trunnion taper interface) metallic orthopaedic hip stems using Finite
Element Analysis (FEA) techniques for the estimation of stresses and strains. This standard is only applicable to stresses below
the yield strength, as provided in the material certification.
1.2 Purpose—This practice establishes requirements and considerations for the development of finite element models to be used
in the evaluation of non-modular metallic orthopaedic hip stem designs for the purpose of prediction of the static implant stresses
and strains. This procedure can be used for worst case worst-case assessment within a familyseries of implant sizes to provide
efficiencies in the amount of physical testing to be conducted. different sizes of the same implant design to reduce the physical test
burden. Recommended procedures for performing model checks and verification are provided to help determine if the analysis
follows recommended guidelines. Finally, the recommended content of an engineering report covering the mechanical simulation
is presented.
1.3 Limits—This practice is limited in discussion to the static structural analysis of non-modular metallic orthopaedic hip stems
(which excludes the prediction of fatigue strength).
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ISO Standards:
ISO 7206-4 (2010) Implants for Surgery—Partial and Total Hip Joint Prostheses—Part 4: Determination of Endurance
Properties and Performance of Stemmed Femoral Components
This practice is under the jurisdiction of ASTM Committee F04 on Medical and Surgical Materials and Devices and is the direct responsibility of Subcommittee F04.22
on Arthroplasty.
Current edition approved July 15, 2013June 15, 2020. Published August 2013August 2020. Originally approved in 2013. Last previous edition approved in 2013 as
F2996 – 13. DOI: 10.1520/F2996-13.10.1520/F2996-20.
Available from International Organization for Standardization (ISO), 1, ch. de la Voie-Creuse, CP 56, CH-1211 Geneva 20, Switzerland, http://www.iso.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F2996 − 20
3. Significance and Use
3.1 This practice is applicable to the calculation of stresses seen on a femoral hip stem when loaded in a manner described in ISO
7206-4 (2010). This method can be used to establish the worst case worst-case size for a particular implant. When stresses
calculated using this practice were compared to the stresses measured from physical strain gauging techniques performed at two
laboratories using two different methods, the results correlated to within 8 %.
3.2 This test method can be used to estimate the effects of design variables on the stress and strain of metallic hip femoral stems
in a set-up mimicking that described in ISO 7206-4 (2010).
4. Geometric Data
4.1 Finite element models are based on a geometric representation of the device being studied. The source of the geometric details
can be obtained from drawings, solid models, preliminary sketches, or any other source consistent with defining the model
geometry. In building the finite element model, certain geometric details may be omitted from the orthopaedic implant geometry
shown in the CAD computer-aided design (CAD) model if it is determined that they are not relevant to the intended analysis.
Engineering judgment shall be exercised to establish the extent of model simplification and shall be justified.
4.2 It is most appropriate to consider the “worst case” “worst-case” stress condition for the orthopaedic implant being simulated.
The “worst case” “worst-case” shall be determined from all relevant engineering considerations, such as stem geometry,
dimensions, and head offset. If finite element analysis is being used for determining the worst case, then the worst case head offset
may not be known. It may be necessary to run several variants of head offset, in order offset to determine this.
5. Material Properties
5.1 The required material properties for input into an FEA model for the calculation of strains and displacement are modulus of
elasticity (E) and Poisson’s ratio (ν). These values can be obtained from material certification data. It should be noted that as ISO
7206-4 (2010) is run under load control, the FEA should also be run under load control. When the FEA is run under load control,
the modulus of elasticity will not affect the stress calculations under small displacement theory but will affect displacement and
strain. The influence of Poisson’s ratio on the stress calculations is negligible.
5.2 Ensure that material property units are consistent with geometric units in the CAD model. SI units are the preferred units of
measure.
6. Loading
6.1 The loading and orientation of the hip stem shall be guided by the ISO 7206-4 (2010) standard. The areas of particular interest
are the stresses in the neck region, driver hole region, potting level, and other design-specific critical regions.
6.2 The load shall be applied such that the magnitude and direction are identical to thatthose defined in ISO 7206-4 (2010). The
point of load application shall produce a statically equivalent bending moment to a load applied through the head center with its
worst case head offset.
6.2.1 The load in the model will be applied to the end circular face of the hip stem trunnion or in a justifiably equivalent manner.
The trunnion may be extended or truncated to approximate the loading conditions that simulate the worst case worst-case head
offset, which may be determined via an iterative process. This approximation should be reported if performed. Alternatively, a rigid
couple can be used to tie the load point to the trunnion end circular face. Refer to Fig. 1.
6.2.2 It is recognized that the loading conditions in this practice are not identical to that of ISO 7206-4 (2010). However, the
differencedifferences in loading conditions (for example, load applied to surface of head versus face of stem trunnion; potting level
differences; use of bone cement which is not modeled in FEA) doesshould not significantly affect identification of the “worst case”
stress condition and construct for subsequent bench testing, “worst-case” stress condition, which is the primary objective of this
practice. When subsequent physical fatigue testing per ISO 7206-4 (2010) is performed, comparison of the physical test results
(that is, location of origin of distal stem fracture) should be compared to the FEA test results to determine if there were any
significant differences. If so, the reason for these differences shall be evaluated, necessary adjustments shall be made to the physical
test fixtures or finite element model, and, depending on the result of the analysis, testing of additional components may be
necessary.
F2996 − 20
FIG. 1 Load Application
NOTE 1—Generating the statically equivalent maximum bending moment by (a) an offset node tied rigidly to the circular trunnion face, or (b) a cylindrical
extension (or truncation of circular trunnion face which equals the maximum femoral head offset (which is an approximation of the offset node method,
to be documented if utilized). As an example, the modeling of a +8 mm femoral head offset is shown here. Figures are for illustration purposes only.
NOTE 2—Boundary condition is located at the distal face/cut region of the stem.
6.3 Ensure that load units are consistent with material property units.
7. Boundary Conditions
7.1 The hip stem willshall first be cutsectioned at a distance from the center of the head as described in ISO 7206-4 (2010) with
the worst case worst-case head/neck offset. This cut sectioned region represents the potting level to which stresses and strains shall
be evaluated. A second parallel cut shall then be made 10 mm below the first cut. sectioned region. The hip stem shall be
constrained in all directions on all faces distal to the second cut. Constraining the stem in this manner ensures that excessive
erroneous stresses are not generated at the region of interest due to the influence of rigid fixation. Refer to Fig. 2, Fig. 3, and Fig.
4, which present three stem length variants provided in ISO 7206-4 (2010). The use of other stress evaluation levels or constraint
levels, or both, shall be justified.
8. Analysis
8.1 The analysis and modeling system, programs, or software used for the finite element model creation and analysis should be
capable of fully developing the geometric features and idealizing the loading and boundary condition environment of the
orthopaedic implant. An engineering justification shall be provided to support any assumptions or simplifications, or
both.simplifications.
8.2 The finite element mesh can be created using automatic meshing, manual meshing, or a combination of the two techniques.
The overriding consideration is that the type, the size, and the shape of the elements used must be able to represent the expected
behavior without significant numerical limitation or complication. Most FEA packages have a built-in program which checks the
shape of the element for the type of analysis selected. If this tool is not available, then additional checks are needed.
F2996 − 20
FIG. 2 Boundary Condition Location for Hip Stem Length #120 mm
NOTE 1—CT: Distance between center of the head and the most distal point of the stem.
NOTE 1—CT is the distance between center of the head and the most distal point of the stem.
NOTE 2—Boundary condition is located at the distal face/cut region of the stem.
8.3 The number and spacing of nodes (that is, mesh density) should be consistent with the type of element used and the type of
result desired. This may be demonstrated with a mesh density study, whereby a series of models with increasing mesh refinement
in the critical stress regions is used to demonstrate solution convergence. This allows the error associated with subsequent models
to be estimated. The method used to demonstrate mesh convergence, in analysis cases where it is not performed directly onto the
model being analyzed, shall be documented in the FEA report. It is recommended that a minimum of three levels of mesh
refinement be performed and a model convergence of ≤5 % is ≤5 % be demonstrated on all measures and regions the quantity of
interest (see 8.6footnote 3).) and at all regions of interest. A stress convergence of >5 % shall be justified based on the context of
use.
8.4 The choice of element type is left to the analyst; however, it is recommended for analysis of a hip stem that tetrahedral or
hexahedral elements be used. If tetrahedral elements are considered, use of 4-noded elements should be avoided to prevent stress
and strain incompatibilities across elements. Additionally, the linear, 4-noded tetrahedron element is a constant strain element. This
means that displacement interpolation is linear and the corresponding stresses and strains are constant within any element.
Therefore, a very refined mesh is required around locations where high stress/strain gradients are present when utilizing these
elements. When elements are used which are not directly identified in the guide, guide are used, documentation shall be provided
in the FEA report which demonstrates their validity.
8.5 The finite element results should be examined to ensure that the geometrical models of the implant, boundary conditions, and
applied loads have been appropriately defined in the analysis and properly represent the behavior being analyzed.
st
8.6 The primary measure of interest is the Maximum (1 ) Principal Stress. Refer to Fig. 5. A secondary measure of interest is the
von Mises stress at the location of maximum principal stress. If other stress values are used, their validity for use should be
documented.
F2996 − 20
FIG. 3 Boundary Condition Location for Hip Stem Length of 120 mm < CT # 250 mm
NOTE 1—CT: Distance between center of the head and the most distal point of the stem.
NOTE 1—CT is the distance between center of the head and the most distal point of the stem.
NOTE 2—Boundary condition is located at the distal face/cut region of the stem.
9. Report
9.1 The finite element analysis for the evaluation of an orthopaedic implant should be fully documented in an engineering report.
The actual format of the report should comply with any acceptable proprietary or non-proprietary engineering report format;
however, the report shall include, but is not include, but not be limited to, the following:
(1) A complete description of the device being analyzed, including detailed dimensions. Report The report can reference a
source CAD geometry file by name and revision number. If the evaluation is not being performed on the final design of the device
or if there are other significant assumptions that may limit the use of the results, this must be clearly stated.
(2) A description of boundary constraints, loads, and material properties. the material properties of Young’s Modulus and
Poisson’s ratio as a minimum. The source of the material property data utilized should be referenced.
(3) A summary of the finite element modeling and analysis system used for the analysis. If current versions of widely used,
commercially available software are used, this summary can be by name and reference to the version used. For non-commercially
available, proprietary tools, or customer user modification of commercially available software, sufficient technical background and
results of test problems should be provided to demonstrate the utility, verification, applicability, and limitations of the software tool.
(4) A description of the procedure used to convert the geometric or CAD representation of the device to the finite element
model. Any geometry simplifications should be documented.
(5) A description of the finite element model and its relation to the device being evaluated. The number of nodes and elements
(or the degrees of freedom in the model), the finite element type selected including(including its capabilities,capabilities), and any
st
special considerations involved in the model should be included. For each region of interest, the maximum (1 ) principal stress
st
and von Mises stress at the location of maximum (1 ) principal stress shall be reported.
(6) A description of mesh convergence considerations and how they were applied to the analysis.
(7) A description of any numerical considerations or convergence criterioncriteria assoc
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