REVIEW Open Access

Feasibility of designing, manufacturing anddelivering 3D printed ankle-foot orthoses: asystematic reviewElizabeth Wojciechowski1,2* , Angela Y. Chang1, Daniel Balassone2, Jacqueline Ford2, Tegan L. Cheng1,2,David Little1,2, Manoj P. Menezes1,2, Sean Hogan2 and Joshua Burns1,2

Abstract

Background: Ankle-foot orthoses (AFO) are prescribed to manage difficulty walking due to foot drop, bonyfoot deformities and poor balance. Traditional AFOs are handmade using thermoplastic vacuum formingwhich provides limited design options, is labour-intensive and associated with long wait times. 3D printinghas the potential to transform AFO production and health service delivery. The aim of this systematic reviewwas to determine the feasibility of designing, manufacturing and delivering customised 3D printed AFOs byevaluating the biomechanical outcomes, mechanical properties and fit of 3D printed compared to traditionallymanufactured AFOs.

Method: Electronic databases were searched from January 1985 to June 2018 according to terms related to 3D printingand AFOs. Studies of any design from healthy or pathological populations of any age were eligible for inclusion. Studiesmust have investigated the effect of customised 3D printed AFOs using any 3D printing technique on outcomes relatedto walking ability, biomechanical function, mechanical properties, patient comfort, pain and disability. Any other orthotictype or AFOs without a 3D printed calf and foot section were excluded. The quality of evidence was assessed using theGRADE process.

Results: Eleven studies met the eligibility criteria evaluating 3D printed AFOs in healthy adults, and adults and childrenwith unilateral foot drop from a variety of conditions. 3D printing was used to replicate traditional AFOs and developnovel designs to optimise the stiffness properties or reduce the weight and improve the ease of use of the AFO. 3Dprinted custom AFOs were found to be comparable to traditional custom AFOs and prefabricated AFOs in terms oftemporal-spatial parameters. The mechanical stiffness and energy dissipation of 3D printed AFOs were found to besimilar to prefabricated carbon-fibre AFOs. However, the sample sizes were small (n = 1 to 8) and study quality wasgenerally low.

Conclusion: The biomechanical effects and mechanical properties of 3D printed AFOs were comparable to traditionallymanufactured AFOs. Developing novel AFO designs using 3D printing has many potential benefits including stiffnessand weight optimisation to improve biomechanical function and comfort.

Keywords: 3D printing, Additive manufacturing, Ankle foot orthoses, AFO

* Correspondence: elizabeth.wojciechowski@sydney.edu.au1The University of Sydney, Sydney, New South Wales, Australia2Paediatric Gait Analysis Service of NSW, The Children’s Hospital atWestmead, Cnr Hawkesbury Road and Hainsworth Street, Locked Bag 4001,Westmead, NSW 2145, Australia

© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link tothe Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver(http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Wojciechowski et al. Journal of Foot and Ankle Research (2019) 12:11 https://doi.org/10.1186/s13047-019-0321-6

BackgroundAnkle-foot orthoses (AFO) are externally worn medicaldevices that support the foot, ankle and lower leg tomanage lower limb impairments such as foot drop, bonyfoot deformities and poor balance. They are routinelyprescribed to improve the walking ability of children andadults with neurological disorders such as cerebral palsy,Charcot-Marie-Tooth disease, cerebrovascular accident(stroke) and multiple sclerosis [1]. Customised AFOs areoften prescribed to prevent trips and falls resulting fromfoot drop, alleviate chronic pain associated with joint de-formity, and control the ground reaction force duringthe stance phase of gait to reduce fatigue. Many users ofAFOs experience poor fit, pain and discomfort, dislikethe aesthetics of the devices and are limited by thechoice of design and associated footwear [1–3]. In par-ticular children and adolescents, females and people liv-ing alone are reported to be the most dissatisfied usersof AFOs [1]. Hence, many children and adults with mus-culoskeletal and neuromuscular disorders don’t weartheir prescribed AFOs and instead utilise compensatorystrategies during gait despite these being physiologicallyinefficient and potentially injurious [4]. Therefore, manyusers only choose to wear their devices when their con-dition becomes severe even though earlier AFO usemight have significant clinical benefits [4].AFOs are usually handmade from plaster of Paris cast-

ing of the patient’s lower legs [5]. Once set, this negativeimpression is removed and filled with liquid plaster toform a positive model. The positive model is then modi-fied through manual addition or removal of plaster,followed by thermoplastic vacuum forming over the posi-tive model with polypropylene. Removal of undesired orexcess polypropylene and smoothing occur prior topatient fitting. This traditional approach is labour inten-sive, provides limited design options, can be costly andoften associated with long wait times [6]. 3D printing is amanufacturing method whereby materials are joined, layerby layer, to fabricate an object from a digital source. It hasthe potential to eliminate several steps associated withtraditional methods of AFO manufacture [6]. 3D printingenables design freedom by facilitating deviation fromtraditional design paradigms and hence allows the devel-opment of patient-specific AFOs. These AFOs can beoptimised to individual biomechanical requirements toprovide improved function, better fit and enhanced aes-thetics. Novel patient-specific 3D printed AFOs are likelyto have a dramatic effect on patient satisfaction, adherenceto AFO usage and overall health related outcomes. Theaim of this systematic review of the literature was to inves-tigate the feasibility of 3D printing for the design, manu-facture and delivery of AFOs by evaluating biomechanicaleffects, mechanical properties and fit of 3D printed com-pared to traditionally manufactured AFOs.

MethodThe systematic review protocol was developed in accord-ance with the Preferred Reporting Items for SystematicReviews and Meta-Analyses (PRISMA) statement [7].

Search strategyElectronic database searches were performed in June2018 in MEDLINE (via OvidSP), EMBASE, AMED (viaOvidSP), CINAHL (via EBSCO), Web of Science, Sco-pus, Cochrane and ProQuest Central according tosearch terms related to 3D printing (3D print*, three di-mensional print*, additive manufactur*, rapid prototype*,additive fabricat*, additive process*, additive technique*,freeform fabricat*, selective laser sinter*, sterolithograpy,fusion deposition model*, laminated object manufactur*,layer manufactur*) combined with terms related toAFOs (ankle foot ortho*, AFO, static ankle foot ortho*,fixed ankle foot ortho*, solid ankle foot ortho*, groundreaction ankle foot ortho*, floor reaction ankle foot or-tho*, dynamic ankle foot ortho*, hinged ankle foot or-tho*, articulation ankle foot ortho*, passive ankle footortho*). 3D printing commenced in the mid-1980s [8],therefore searches were from January 1985. The searchstrategy was developed for MEDLINE and modified foruse in other databases (Additional file 1).

Eligibility criteriaStudies of human participants of any sample size were eli-gible, and there were no age, sex, cultural or ethnicityrestrictions. Participants were either from healthy or anyclinical populations. Studies must have investigated theeffect of customised 3D printed AFOs using any 3Dprinting technique [stereolithography (SLA), fused depos-ition modelling (FDM), laminated object manufacturing(LOM), selective laser sintering (SLS), selective laser melt-ing (SLM)] on outcomes related to walking ability orbiomechanical function, mechanical properties, patientcomfort, pain and disability. Any other orthotic type (e.g.below ankle, University of California Berkeley Laboratoryorthoses, supermalleolar, knee-ankle-foot orthoses) ororthoses not used for walking (e.g. night use) or any othermanufacturing technique (e.g. CAD/CAM where the ma-chine carves a block to form an orthosis based on digitalmodel milling) other than 3D printing, or AFOs where thecalf and foot section were not 3D printed were excluded.All study designs were included, except for narrative andsystematic reviews. Although reviews were excluded, thereference lists were examined for any additional relevantreferences. Unpublished data and data from studies withno full-length publication were excluded.

Study selection, data extraction and study qualityFollowing the deletion of duplicates, titles and abstractsfrom the search results were screened by two authors

Wojciechowski et al. Journal of Foot and Ankle Research (2019) 12:11 Page 2 of 12

(E.W. and D.B.) using the predetermined eligibility cri-teria. Full-text articles were retrieved for the remainingarticles and independently reviewed by two authors(E.W. and D. B.) for inclusion. Data extraction andevaluation was then completed on the remaining articlesby two authors (E.W. and J. F.) independently. Wherethere was disagreement at any stage an additional re-viewer (J.B.) was consulted. Data extraction includedstudy design, sample size, participant characteristics,orthotic details and outcome measures.The Oxford Centre for Evidence-Based Medicine 2011

Levels of Evidence (OCEBM Levels) was used to deter-mine a level of evidence for each study based on the de-sign [9]. The American Academy for Cerebral Palsy andDevelopmental Medicine (AACPDM) conduct ratingwas used to critique the individual studies during thedata extraction data process [10]. The overall quality ofevidence was then assessed using the Grading of Recom-mendations Assessment, Development and Evaluation(GRADE) process (GRADEpro software) [11, 12].

ResultsDescription of included studiesThe initial electronic database search resulted in a totalof 128 articles, leaving 73 articles after the removal ofduplicates. No additional articles were identified follow-ing a hand search of reference lists. Upon completion ofthe title and abstract screening, 21 were selected for pos-sible inclusion in the review and full-text articles wereretrieved. Following the screening of the full text, 11studies met the inclusion criteria and were included inthis review [13–23]. A flow diagram of the search historyand selection process is shown in Fig. 1. The 11 studiesincluded one case-control trial [13], eight case studies[14–18, 21–23], one prototyping study [19] and one fi-nite element analysis study [20]. A description of theAFO details included in the studies can be found inTable 1.

Quality of included studiesNine of the studies were OCEBM level 4 and two level5 [Table 1]. The conduct rating of the studies using theAACPDM was moderate for two studies [13, 23] andweak for all other studies (Table 1 and Additionalfile 2). The outcomes assessed using the GRADEprocess included walking ability, patient satisfaction,patient-perceived comfort, bending stiffness, energydissipation, destructive testing, dimensional accuracybetween the CAD model and printed AFO, durabilityand material displacement [Table 2]. The GRADEoverall quality evidence for all these outcomes wasconsidered very low.

Participant characteristicsOut of the 11 studies a total of 32 adult participantsparticipated (17 male, 6 female, 9 sex not specified). Theaverage age was 39 ± 17 years and ranged from 21 to 68years, however three studies did not report the age ofthe participants [19–21]. Sample sizes were n = 1 [14, 15,18, 20, 22], n = 2 [16, 17], n = 7 [21] and n = 8 [13, 23].Four out of the 11 studies were conducted on patientpopulations including unilateral foot drop due to dorsi-flexor weakness from stroke, cerebral palsy, L5 hernia,carbon monoxide intoxication and mechanical trauma[13], post-polio syndrome [14], trauma and cerebralpalsy [21] and an embolectomy [22]. Six studies re-cruited healthy participants [15–18, 20, 23] and onestudy did not report or evaluate the AFO in any partici-pants (bench testing only) [19].

Orthotic detailsType of AFOTen of the 11 studies evaluated the feasibility of using3D printing to produce a type of dynamic passive AFO,which relies on the material properties and physicalfeatures to establish functional characteristics such asbending or rotational stiffness. Specifically, two studiesreplicated a posterior leaf spring AFO [13, 15], one studyreplicated the geometry and design characteristics of aprefabricated carbon-fibre AFO (Dynamic Brace, Dy-namic Bracing Solutions, Inc., San Diego, CA) [14] andsix studies developed novel AFOs [16–19, 21, 22]. Thenovel designs included the development of a parame-terised AFO using computer aided modelling [16, 17].One study integrated a 3D printed component withoff-the-shelf bearings and gas springs to produce an ad-justable stiffness AFO [18]. Another study manufactureda segmented AFO consisting of a 3D printed calf sectionand foot section and a central interchangeable carbonfibre spring [19]. Similarly, another study integrated acommercially available metal hinge also capable ofadjusting the stiffness of the AFO into a 3D printed ar-ticulated AFO [23]. Other designs also included an AFOwith a 3D printed 3mm calf and foot section connectedwith two carbon fibre rods [21] and a 3D printed devicesupporting the ankle and foot and secured with laces[24]. The only study that didn’t produce a dynamic pas-sive AFO used 3D printing to produce a rigid (solid)AFO, however no testing was performed on the manu-factured AFO [20].

Comparative AFOThree studies compared 3D printed AFOs to tradition-ally manufactured custom AFOs [13, 21, 22]. One studycompared 3D printed AFOs to the Dynamic Brace car-bon fibre AFO [14] and one study made comparisons toprefabricated injection moulded AFOs [15]. Six studies

Wojciechowski et al. Journal of Foot and Ankle Research (2019) 12:11 Page 3 of 12

did not compare 3D printed AFOs to any other trad-itionally dispensed AFOs [16–20, 23].

3D printing method and materialSix studies used selective laser sintering (SLS) to produceAFOs [13, 14, 16, 18, 21, 22], one study used stereolithog-raphy (SLA) [15] and four studies used fused depositionmodelling (FDM) [17, 19, 20, 25]. The types of materialused for fabrication of the AFOs varied considerably.Three studies used Nylon 12 (PA2201, DuraForm PA,PA2200) [13, 14, 18]. A range of materials were used in

the remaining studies including Rilsan D80 (Nylon 11)[14], DuraForm GF (Glass filled Nylon 12) [14], Accura 40resin [15], DSM Somos 9120 Exposy Photopolymer [15],DuraFrom Ex [16], medical-grade polycarbonate [17],acrylonitrile butadiene styrene (ABS) [19, 20], polyamide12 (PA12) [21], polyurethane [22] and polylactide (PLA)[23]. Two studies compared different types of 3D printingmaterials by printing the same AFO in more than one ma-terial, one study compared Rilsan D80, DuraForm PA andDuraForm GF [14] and one study compared Accura 40resin and DSM Somos 9120 Epoxy Photopolymer [15].

Fig. 1 Flow diagram of the search history and selection process

Wojciechowski et al. Journal of Foot and Ankle Research (2019) 12:11 Page 4 of 12

Table 1 Participant characteristics, 3D printed orthotic details and outcomes of includes studies

Reference AACPDM levelof evidence &conduct rating

Participants’Characteristics

Orthotic Details Outcomes and Results

Study Design N Condition Intervention vs controlcondition

3D printingmethod andmaterial

Outcomes Main Results and Authorsconclusions

OCEBMlevel

Aydin etal., 2018[20]

V (1/7)Computinganalysis andprototyping

1 Healthyparticipant

Customised FDM AFO vsno control

FDMABS

FEA: Materialdisplacement

Material displacement ofthe AFO model usingmechanical propertiesfrom 3D tested specimenswas higher compared tothe using mechanicalproperties from suppliedwith the FEA software.

5

Deckers etal., 2018[21]

V (1/7) Case-studies

7 Trauma,neuro-muscular dis-order andcerebral palsy(3 children, 4adults)

Customised SLS AFO witha 3 mm thick calf andfoot section connectedwith 2 carbon fibre rods(6 weeks) vs traditionallymanufactured AFOs (6weeks)

SLSPolyamide 12(PA12)

Observationafter 6-weektrial

No noticeable failure orwear with the traditionallymanufactured AFOs after6 weeks. 5/7 SLS AFObroke during the 6-weekperiod, 1 SLS AFO showedsigns of cracking and 1did not fail.

4

Cha et al.,2017 [22]

V (1/7) Casestudy

1 Right sidefoot dropafterembolectomy(female, 68yrs)

Novel customised SLSAFO vs traditionalpolypropylene AFO(altered wear over 2months)

FDMPolyurethane

Durability testof 300,000cyclesQUEST after 2months3DGA: temporalspatialparameters,anklekinematics

No crack, shape orstiffness change followingthe durability test. Theparticipant was moresatisfied with 3D printedAFO in terms of weightand ease of use. Temporalspatial parameters weresimilar between AFOshowever ankle dorsiflexionin swing was less with the3D printed AFO comparedto the traditional AFO.

4

Choi et al.,2017 [23]

IV (3/7) Case-studies

8 Healthyparticipants (4male, 4female 25.3SD 4.5 yrs)

Customised articulatedFDM AFO with a metalhinged joint and 2 elasticpolymer bands at 4 levelsof stiffness and noresistance vs no control

FDMPLA

3DGA:kinematicsUltrasoundMusculoskeletalmodelling

Increasing AFO stiffnessincreased peak ankledorsiflexion moment anddecreased peak kneeextension and peak ankledorsiflexion. The methodmay assist AFO designand prescription toimprove gait.

4

Creylman,et al., 2013[13]

IV (3/7) Case-control

8 Unilateraldrop foot dueto dorsiflexorweaknessfrom multipleconditions(male 46.6 yrs.SD 12.5)

Customised SLS AFO vstraditionally manufacturedpolypropylene AFO vsbarefoot

SLSNylon 12 (PA2201)

3DGA: temporalspatialparameters andkinematics.

No statistically significantdifferences between thetraditionally manufacturedAFO and of SLS AFO interms of temporal spatialgait parameters, ankleangle at initial contactand maximum ankleplantarflexion duringswing. Significantdifferences were noted inankle range of motion.Authors attribute this todifferences in materialstiffness.

4

Faustini, etal., 2008[14]

V (1/7) Case-study

1 Post-PolioSyndrome(male 66 yrs)

SLS PD-AFO vs DynamicBrace CF-AFO

SLSNylon 11(Rilsan D80),Nylon 12(DuraForm PA)and glass-filledNylon 12

Rotationalstiffness, energydissipation &destructivetesting.

Nylon 11 exhibited theleast amount ofmechanical damping andwas the only material towithstand the destructivetesting

4

Wojciechowski et al. Journal of Foot and Ankle Research (2019) 12:11 Page 5 of 12

OutcomesThe reported outcomes included walking ability assessedusing 3D gait analysis [13, 15, 18, 23, 26], patient satis-faction assessed using the Quebec User Evaluation ofSatisfaction with Assistive Technology (QUEST) [22],patient-perceived comfort assessed by interview [15, 16],dimensional accuracy between CAD model and 3Dprinted AFO [16, 17] and mechanical properties of theAFO. The reported mechanical properties included stiff-ness [14, 17, 19], energy dissipation [14], destructivetesting [14] and cyclic-fatigue loading [22] assessed usingbench testing equipment and material displacementusing finite element analysis (FEA) [20]. No studies re-ported pain or disability reduction as an outcome.

Walking abilityFive studies evaluated the effect of 3D printed AFOs onwalking ability [13, 15, 18, 23, 26]. Creylman and

colleagues [13], compared SLS AFOs and traditionallymanufactured AFOs to barefoot walking. They foundthat use of either AFO showed significant benefit interms of stride length and stance phase duration of theaffected limb and ankle kinematics compared to barefootwalking. No statistically significant differences in tem-poral spatial parameters (stride duration, stride lengthand stance phase duration of both affected and un-affected limb), ankle angle at initial contact and max-imum ankle plantarflexion during swing were foundbetween the traditionally manufactured AFOs and SLSAFOs. However, significant differences were noted inankle range of motion over the whole gait cycle betweenthe traditionally manufactured AFOs and SLS AFOs,with the SLS AFO showing a smaller range of motion.The authors attributed these differences to differences inmaterial stiffness between the AFOs. Similarly, Mavroi-dis and colleagues [15], found that a prefabricated AFO

Table 1 Participant characteristics, 3D printed orthotic details and outcomes of includes studies (Continued)

Reference AACPDM levelof evidence &conduct rating

Participants’Characteristics

Orthotic Details Outcomes and Results

Study Design N Condition Intervention vs controlcondition

3D printingmethod andmaterial

Outcomes Main Results and Authorsconclusions

OCEBMlevel

(DuraForm GF)

Mavroidis,et al., 2011[15]

V (2/7) Case-study

1 Healthyparticipant

Customised SLA AFO(rigid & flexible) vsprefabricated injectionmoulded polypropyleneAFO vs shod only

SLAAccura 40 resinand DSMSomos 9120EpoxyPhotopolymer

3DGA: temporalspatialparameters,kinematics andkinetics.Patientperceived fit.

3D printed AFOs providedgood fit to theparticipant’s anatomy andwere comparably to theprefabricated AFO duringgait

4

SchrankandStanhope.,2011 [16]

V (0/7)Case-studies

2 Healthyparticipants(male 48 yrs.;female 21 yrs)

4 half scale PD-AFO andtwo full-scale PD-AFO vsno control

SLSNylon(DuraForm EXNatural Plastic)

Dimensionalaccuracy.Patientperceived fit.

Dimension discrepancieswere well under a 2 mmtolerance for the four half-scale orthoses. Subjectiveevaluations of the full-scale PD-AFOs followinguse in gait were positive

4

Schrank, etal., 2013[15]

V (0/7) Case-studies

2 Healthyparticipants(male 25 yrs.;female 24 yrs)

2 sets of stiffness tunedPD-AFOs vs no control

FDMmedical-gradepolycarbonate(PC-ISO).

Dimensionalaccuracy,manufacturingprecision andbendingstiffnesspredictionaccuracy.

The virtual functionalprototyping had excellentdimensional accuracy,good manufacturingprecision and strongpredication accuracy withthe derived modulus

4

Telfer, etal., 2012[18]

V (1/7) Case-study

1 Healthyparticipant(male 29 yrs)

Customised SLS AFO attwo different stiffnesslevels vs shod only

SLSNylon-12(PA2200)

3DGA:kinematics andkinetics

The AFO had distincteffects on anklekinematics which couldbe varied by adjusting thestiffness level of thedevice

4

Walburn,et al., 2016[19]

V (0/7)Prototyping

0 None A novel segmented 3Dprinted and CFRP AFO vsno control

FDMABS

Linear stiffnesscoefficient

A novel segmented 3Dprinted and CFRP AFO hasbeen proposed.

5

AACPDM American Academy for Cerebral Palsy and Developmental Medicine, OCEBM Oxford Centre for Evidence-Based Medicine Levels of Evidence, AFO Ankle-foot orthoses, PD-AFO Passive-dynamic ankle foot orthoses, CFRP carbon fibre reinforce spring, SLS Selective laser sintering, SLA Stereolithography, FDM Fuseddeposition modelling, 3DGA Three-dimensional gait analysis, FEA Finite element analysis, QUEST Quebec User Evaluation of Satisfaction with Assistive Technology

Wojciechowski et al. Journal of Foot and Ankle Research (2019) 12:11 Page 6 of 12

Table

2GRA

DEeviden

ceprofile

Qualityassessmen

t№

ofpatients

Effect

Quality

Impo

rtance

№of

stud

ies

Stud

yde

sign

Risk

ofbias

Inconsistency

Indirectne

ssIm

precision

Other

considerations

AFO

smanufacturedusing3D

printin

gtechniqu

esTradition

ally

manufacturedAFO

sRelative

(95%

CI)

Absolute

(95%

CI)

Walking

ability

(assessedby

3Dgaitanalysis)

5ob

servational

stud

ies1,2,3,4,5

serio

usa,b,c

notserio

usserio

us1,5,6,7a,b

serio

usd

none

2011

––

⨁◯◯

◯VERY

LOW

IMPO

RTANT

Patient

perceivedcomfort(assessedby

interview)

2ob

servational

stud

ies2,8

very

serio

use

notserio

usserio

usa

serio

usd

none

Interview

afteruseof

AFO

durin

ggait

⨁◯◯

◯VERY

LOW

IMPO

RTANT

Patient

satisfaction(assessedwith

theQUEST)

2ob

servational

stud

ies4

very

serio

usf

notserio

usno

tserio

usserio

usd

none

11

––

⨁◯◯

◯VERY

LOW

IMPO

RTANT

Bend

ingstiffne

ss(assessedby

benchtestingusingcustom

stiffne

sstestingde

vice)

3ob

servational

stud

ies6,9,10

serio

usa,c,g

notserio

usserio

usa

serio

usd

none

31

––

⨁◯◯

◯VERY

LOW

IMPO

RTANT

Energy

Dissipatio

n(assessedby

bend

ingtestingandanalysingtheresulting

acceleratio

n-tim

etrajectory)

1ob

servational

stud

ies6

not

serio

usno

tserio

usserio

usa

serio

usd

none

11

––

⨁◯◯

◯VERY

LOW

IMPO

RTANT

DestructiveTesting(assessedby

benching

testingusingahydraulic

axialloadcell)

1ob

servational

stud

ies4

not

serio

usno

tserio

usserio

usa

serio

usd

none

11

––

⨁◯◯

◯VERY

LOW

IMPO

RTANT

Dim

ension

alaccuracy

betw

eenCADmod

elandprintedAFO

(assessedby

theFaroArm

,fitwith

a3mm

sphe

ricaltip

)

1ob

servational

stud

ies8

serio

ush

notserio

usserio

usa

serio

usd

none

2–

––

⨁◯◯

◯VERY

LOW

IMPO

RTANT

Durability

(assessedby

mechanicalstresstestof

300,000cycles)

1ob

servational

stud

ies4

serio

usa

notserio

usserio

usa

serio

usd

none

2–

––

⨁◯◯

◯VERY

LOW

IMPO

RTANT

Durability

(follow

up:6

weeks;assessedby

observation)

1ob

servational

stud

ies11

serio

usd,e

notserio

usno

tserio

usserio

usd

none

77

––

⨁◯◯

◯VERY

LOW

IMPO

RTANT

Materiald

isplacem

ent(assessedby

finite

elem

entanalysis)

Wojciechowski et al. Journal of Foot and Ankle Research (2019) 12:11 Page 7 of 12

Table

2GRA

DEeviden

ceprofile

(Con

tinued)

Qualityassessmen

t№

ofpatients

Effect

Quality

Impo

rtance

№of

stud

ies

Stud

yde

sign

Risk

ofbias

Inconsistency

Indirectne

ssIm

precision

Other

considerations

AFO

smanufacturedusing3D

printin

gtechniqu

esTradition

ally

manufacturedAFO

sRelative

(95%

CI)

Absolute

(95%

CI)

1ob

servational

stud

ies7

serio

ush,i

notserio

usserio

usa

serio

usd

none

1–

––

⨁◯◯

◯VERY

LOW

IMPO

RTANT

CICon

fiden

ceinterval

1Telfe

r,et

al.,20

12,2Mavroidis,e

tal.,20

11,3Creylman

,etal.,20

13,4Cha

,etal.,20

17,5Cho

ietal.2

017.,6Faustin

i,et

al.,20

08,7Ayd

inet

al.2

018

8Schran

kan

dStan

hope

,201

1,9Schran

k,et

al.2

013,

10Walbran

,etal.,

2016

,12Deckers,e

tal.,20

18Explan

ations:a

Not

allstudies

compa

redto

trad

ition

ally

man

ufacturedAFO

s,bDifferen

cesin

stud

ypo

pulatio

ns,c

Differen

cesin

type

ofAFO

assessed

,dPa

rticipan

ts/nu

mbe

rof

AFO

sassessed

low,e

Noqu

antitativeassessmen

t,fNoblinding

ofAFO

s,gMetho

dused

toassess

outcom

edifferen

tacross

stud

ies,

hInflu

ence

ofob

lique

build

orientations

andpo

sitio

nsan

ddifferen

tsizedan

dshap

edorthoses

nottested

,iNum

berof

samples

tested

low

Wojciechowski et al. Journal of Foot and Ankle Research (2019) 12:11 Page 8 of 12

allowed more ankle range of motion compared to twoSLA AFOs (rigid and flexible) possibly due to greatercompliance of the polypropylene material. However,overall ankle kinematics and kinetics of the three AFOswere similar. In contrast, Telfer and colleagues [18],evaluated the effect of a SLS AFO set at two differentstiffness levels in the sagittal plane to shoe only walking.The stiffness of the SLS AFO was controlled by adjust-ing pressure in gas springs. Significantly reduced plan-tarflexion during the early stance phase was observedbetween stiffness conditions, with the higher stiffnesssetting allowing the minimal amount of plantarflexion.They suggested that tailoring the stiffness of SLS AFOsmay provide support to suit different activities in a waythat traditional AFOs are unable to offer. Similarly, Choiand colleagues [23] describe a method of evaluating theimpact of AFO stiffness on the Achilles tendon andgastrocnemius function using an articulated FDM AFOwith a metal hinge and two elastic polymer bands. TheAFO was set at no resistance and 4 levels of stiffness. In-creased AFO stiffness resulted in increased peak ankledorsiflexion moment and decreased peak knee extensionand peak ankle dorsiflexion angles. The authors proposethat this method may assist in AFO design and prescrip-tion to improve gait and function.Cha and colleagues [22] also used 3D gait analysis to

compare an FDM polyurethane AFO tightened withshoe laces to a traditionally manufactured rigid polypro-pylene AFO in one participant. They found that bothAFOs similarly improved temporal spatial parameterscompared to barefoot walking. However, ankle kinematicdata showed that the traditional AFO was more effectivein supporting ankle dorsiflexion during swing comparedto the 3D printed AFO. The authors suggested that this3D printed AFO needed to be designed in more dorsi-flexion to compensate for stretching of the AFO duringwear. However, this is likely due to the design of the 3Dprinted AFO which was developed to be more like asupramalleolar orthosis rather than an AFO as thedevice only surrounded the ankle and hindfoot and notthe lower leg.

Patient-perceived comfortOnly one study used a patient satisfaction questionnaire(QUEST) [22] to compare a traditionally manufacturedAFO to a 3D printed AFO. This study found that theparticipant was more satisfied with 3D printed AFOs interms of weight and ease of use. The traditional AFOwas difficult to wear due to the thickness. Two studiesassessed patient-perceived comfort through interview.Participants reported greater comfort whilst wearing the3D printed AFO [15] and positive feedback during gaitafter 1 h of walking [16]. However, comparisons werenot made to traditionally manufactured AFOs and no

quantitative assessment of patient comfort or fit wasperformed in these studies.

Dimensional accuracy between CAD model and 3D printedAFOTwo studies assessed the dimensional accuracy betweenthe CAD model of the AFO and 3D printed AFOs bymeasuring 3D inter-dimple distances on the foot plate,strut and cuff of the 3D printed AFO using a FaroArmfit with a 3 mm spherical tip [16, 17]. Dimensional dis-crepancies were < 2 mm tolerances for four half-scaledAFOs [16]. However, the influence of build orientation,device size or shape were not tested.

Mechanical propertiesThree studies evaluated the bending or rotational stiffnessof 3D printed AFOs [14, 17, 19] using custom experimen-tal stiffness testing devices. Two studies measured stiff-ness to validated finite element analysis (FEA) [14, 17].Faustini and colleagues [14] measured rotational stiffnessby clamping the footplate of the AFO against a verticalbase and applying an increasing vertical force to the cuffand found that the stiffness measures of the SLS AFOsmanufactured in Rilsan D80 (Nylon 11), DuraForm PA(Nylon 12) and DuraFrom GF (glass-filled Nylon 12) werewithin in 5% of the targeted carbon fibre AFO stiffnessvalues. Similarly, Schrank and colleagues [17] measuredbending stiffness using a custom experimental stiffnesstesting device and found that the stiffness predication ac-curacy of the FDM AFO printed in polycarbonate wasstrong (average 0.20+/− 0.14 Nm/deg).FEA has also been used to determine the material dis-

placement of an AFO computer model [20]. Aydin andcolleagues [20] derived the mechanical properties from3D printed test specimens manufactured with differentinfill densities. The material displacement of the AFOmodel was higher when mechanical properties werederived from test specimens compared to when mechan-ical properties were supplied from FEA software (Solid-Works 2016). This illustrates the need to use mechanicalproperties from 3D printed test specimens rather thandefault material properties.In addition to measuring stiffness, Faustini and col-

leagues [14] also measured energy dissipation and per-formed destructive testing on the 3D printed AFOs. Theyestablished that the AFO fabricated with Rilsan D80(Nylon 11) exhibited the least amount of mechanicaldamping and was the only material to withstand destruc-tive testing compared to AFOs fabricated in DuraForm PA(Nylon 12) and DuraForm GF (glass-filled Nylon 12) [14].The only study to conduct durability testing of a 3D

printed AFO performed 300,000 cycles of mechanicalstress testing and found there was no damage, shape orstiffness change of the AFO printed in polyurethane [22].

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DiscussionThe main finding of this systematic review was that 3Dprinted AFOs were found to be comparable to tradition-ally manufactured custom AFOs, prefabricated AFOsand prefabricated carbon-fibre AFOs in terms of tem-poral spatial parameters, ankle kinematics, mechanicalstiffness, and energy dissipation. However, only five ofthe 11 studies compared 3D printed AFOs to tradition-ally manufactured AFOs. Populations studied were pri-marily healthy adults and patients with foot drop. Onlyone study evaluated 3D printed AFOs in children. Thesample sizes of the studies retrieved were small (n = 1 to8) and study quality was generally low.The studies retrieved evaluated the feasibility of 3D

printing a posterior leaf spring AFO [13, 15], a carbon-fibre AFO [14], a rigid AFO [20] and various novel AFOdesigns [16–19, 21–23]. None of the studies assessed thefeasibility of 3D printing a traditional hinged AFO with aplantarflexion stop. The novel AFO approaches maxi-mised the design freedom of 3D printing to either opti-mise the stiffness properties of the AFO [16–19, 23] orreduced the weight and ease of use [22]. Telfer and col-leagues [18] integrated 3D printed components (shanksection, strut, foot section and slider) with off-the-shelfbearings and gas springs to produce an adjustable stiffnessAFO [18]. The combination of off-the-shelf and 3Dprinted components allows the user to lower the stiffnessand tailor the support provided depending on theircurrent activity. Similarly, Schrank and colleagues [17]optimised the stiffness of 3D printed AFOs using CADand FEA to tune and predict the properties of a parame-terised AFO. Optimising AFO stiffness is likely to improvelower limb biomechanics such as ankle power at push off,make tasks such as ascending and descending stairs moreefficient, and reduce trips, fatigue and pain. However,none of the novel AFOs were tested in specific patientpopulations or were designed with a particular patientgroup in mind.Deckers and colleagues [21] and Cha and colleagues

[22] were the only studies to test their novel AFOdesigns in patient populations and over time. Deckersand colleagues [21] tested an AFO design with a 3Dprinted calf and foot section connected by two carbonfibre rods in children and adolescents with pathologiesfrom trauma, neuromuscular disorder (unspecified) andcerebral palsy over a 6-week period. However, only oneout of the seven 3D printed AFOs did not break or failduring the 6-week period. This highlights the need forappropriate design and validation of devices in a safelaboratory environment to ensure that the basic per-formance of the novel device is demonstrated. Validationof novel designs could be achieved through bench test-ing, computer modelling and testing with patient popu-lations in a controlled laboratory setting before any user

validation in the natural environment over longer pe-riods of time is performed.Consideration should also be given to the age, gender,

footwear and lifestyle of the user when designing anddeveloping new devices. Out of the 11 studies retrieved,only one study collected a patient satisfaction question-naire, two reported subjective feedback on comfort andfit of the device and none evaluated pain or disability re-duction. Poor AFO satisfaction has been identified inchildren and adolescents, females and people livingalone due to experiences with poor fit, pain and dislikeof the aesthetics of the devices [1–3]. Aspects of the de-vice that users want improved are the size, weight, ad-justability, finish and durability of their AFOs [1]. Chaand colleagues [22] showed improvement in patient sat-isfaction in terms of weight and ease of use for theirnovel AFO design, however the device did not preventfoot drop as much as the traditionally manufacturedAFO [22]. In the future, evaluation of novel 3D printeddesigns in specific groups, such as children, shouldexplore comfort and fit, pain and disability as well asoverall satisfaction and adherence and the biomechanicalfunction of the device.The 3D printing method and materials varied mark-

edly between studies. Faustini and colleagues [14] werethe only study reporting some of the mechanical proper-ties of AFOs manufactured using SLS in more than onematerial (Nylon 11, Nylon 12 and glass-filled Nylon 12).They concluded that Nylon 11 was the optimum mater-ial in terms of mechanical damping and deformation.Only one study evaluated the durability of the AFOthrough cyclic loading tests [19] however no studiesevaluated the lifespan of the AFO or performed long-term follow up of greater than two months. The 3Dprinting methods reported were SLS, SLA and FDM.SLA is a method of 3D printing that utilises one or moreultraviolet lasers to cure a liquid photopolymer resin.However, the structural, functional and aesthetic integ-rity of an SLA product may be compromised with pro-longed UV exposure, where the material becomes brittleor discoloured [27, 28]. Therefore, SLA devices may re-quire further material science studies or post-processingstrategies to mitigate exposure to UV. SLS is similar toSLA however instead of using a laser to cure a liquid, alaser is used to trace and fuse or melt a powder substrateto fabricate an object, layer by layer. SLS manufacturedAFOs also require lengthy ‘warm up’ and ‘cool down’ pe-riods for optimal fabrication and safe removal. Indeed,one study reported a ‘cool down’ time of 6 h for the printbed to drop below 50°C prior to retrieval of the fabri-cated AFO [14]. FDM describes the process of deposit-ing melted material through a heated extruder onto aplatform. After the extruder has deposited a singlecross-sectional layer of material the build platform is

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lowered to allow for printing of the next layer. Walbranand colleagues [19], used FDM to manufacture a seg-mented AFO. Segmentation of the calf and foot sectioninto two parts allows optimal orientation for maximumstrength during FDM manufacturing, whereas currentlya single piece AFO cannot be orientated in a way wherethe strength is maximised in all directions [19]. Furtherresearch is required to determine the ideal printing ma-terial, printing method, print orientation and post pro-cessing for optimum durability and long-term usage.

ConclusionThe use of 3D printing to manufacture AFOs seems tohave many potential benefits over traditional methods,including the development of novel designs that opti-mise stiffness and energy dissipation, improve walkingbiomechanics, comfort and fit. The feasibility of using3D printing to manufacture AFOs is dependent on theAFO design and printing method and therefore add-itional research is needed before 3D printed AFOs canbe integrated into clinical practice. Further research isrequired to evaluate 3D printing AFOs in paediatricpopulations, and to determine the most appropriateprinting technique and optimal materials to improvewalking ability, patient satisfaction and long-term usageand durability.

Additional files

Additional file 1: Search strategy for Medline, modified for otherdatabases. (DOCX 26 kb)

Additional file 2: Conduct of group design studies. (DOCX 26 kb)

AcknowledgementsThe authors would like to thank Dr. Paula Bray for providing advice on studyevaluation and assessment method.

FundingEW has a PhD Stipend through the Research Training Program (RTP) fromCommonwealth Government of Australia. This research project received noother funding.

Availability of data and materialNot applicable.

Authors’ contributionsConception and design of the systematic review was completed by EW, JB,TLC, DL, MM. EW conducted the search strategies and drafted and revisedthe manuscript. Article inclusion, data extraction and analysis wereundertaken by EW, AC, DB and JF. JB, TLC, MPM, DL, SH were responsible forcontent expertise. All authors contributed equally in the preparation of thismanuscript and approved of the final manuscript.

Ethics approval and consent to participateNot applicable.

Consent for publicationNot applicable.

Competing interestsThe authors declare that there are no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.

Received: 4 October 2018 Accepted: 30 January 2019

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