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methods for volumetric imaging, especially magnetic resonancemethods. SXC T scanning of the extremities avoids exposure ofreproductive organs and is considered a low-risk, noninvasivetechnique.The lower limb residuum soft tissue envelope is subject toboth short- and long-term changes because of edema, venouspooling, exercise, weight gain/loss, muscle contraction, and at-rophy. In addition, this soft tissue envelope has a heterogeneouscomposition and is continuously deformable. Even thoughlower extremity volumetry methods have been developed toquantify and help understand the effects of edema an d atro-phy, ~6-2~ each of these methods is s till lim ited in its asses smen tcapabilities. We measured residua volumes with optical surfaceand SXCT volumetry.22 We found optical surface scanner vol-umetry gave quantifiable measurement results but is lirnited inits ability to assess prosthetic fit because of the lack of subsur-face information. SXCT allows the socket-residuum interfaceto be seen simultaneously with the bone r emnant location forvisualization and measureme nt of the residuum with the pros-thesis in situ, so soft tissue envelope change during fitting canbe determined.Before SXCT and optical surface scanning could be used formeasuring the shape of the residuum, we needed to know theprecision and repeatability of these devices. Therefore, wetested the 3Space electromagnetic digitizer,a optical surfacescanner (OSS), b and SXCT (Somatom Plus S) c on multiple sub-jects and plaster casts taken of their residua and compared thesemeasurements to physical caliper measurements. We sought toanswer 3 questions regarding the absolute and relative precisionof measurements made directly on subjects and their casts inthis study: (1) What is the precision and repeatability of thelimb measurements using the various measuring devices? Arethere differences in precision and repeatability among measure-ment methods? (2) Are there differences between measurementstaken directly on subjects and comparable measurements takenon casts? (3) Are there differences between measurements madewith the different devices?

M E T H O D SPopulation

Thirteen below-knee adult amputee subjects were recruitedand informed consent was obtained (as approved by our Institu-tional Review Board) to participate in our study. The studygroup consisted of 9 men and 4 wome n ranging in age from 31years to 76 years (average age 49.15 years, median age 49).The first 9 of the 13 subjects were measured twice on twoseparate dates, or measurement sessions. The population of met-ropolitan SL Louis is predominantly Caucasian and Black.Therefore, we included only these two racial groups in ourst ud y- -2 Caucasian women, 8 Caucasian men, 2 African Amer-ican wome n and 1 African American man. Ten to 15 subjectsare sufficient to identify major sources of error in a complexmeas ureme nt protocol such as the one we used. 23Optical Surface Scanning

The OSS system used in this study employs a stationarymultiple-sensor fixed geometry adapted from an optical humanhead scanner.24 Four camera/projector sensors are conf igured toview the residual limb in extension with the subject seated ora horizontal ly oriented plaster cast surface (fig 1). By utiliz ingmultiple sensors that digitize overlapping surface segments,360 coverage is achieved. Th e multiple sensors allow flexibilityin their positioning to cover the entire surface including thedistal end not viewable by other optical scanning methods. The

/ .84cm 3 0 c m, / . - / - . , ,~ -- I i/ 27 .,/ V - - ' ~ / I C 1 3 0 e r a [ /

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I ~ 2 8 4 c m ~ [I

F i g 1 . L i n e d r a w i n g o f t h e s u r f a c e sc a n n in g s t r u c t u r e w i t h c a m e r a ( C )a n d p r o j e c t o r ( P ) c o n f i g u r a t io n . C a m e r a 4 ( C 4 ) lo c a t e d d i r e c t l y i n f r o n to f t h e r e s i d u u m c a p t u r e s t h e d i s t a l e n d o f t h e r e s i d u u m . T h r e e p a i r s o fc a m e r a s ( C l - C 3 ) a n d p r o j e c t o r s (P 1 - P3 ) a r e l o c a t e d a t t h e b a s e a n d t o po f t h e s t r u c t u r e .

number of sensors chosen (four in this case) was based on thesurface complexity, thus matching the system to the problem.The residuum OSS consists of 4 cameras and 3 projectors,which were affixed to a metal frame (fig 1) to capture an approx-imate 38cm cubic volume enveloping the subjec t's below-kneeresiduum. The instrument and its operation are similar to themedical human head scanner and are described in detail inprevious publications23'25-28 and the head scanner had beenproven to be accurate an d repeatabl e in a previous study. 23Co m puted To m o g ra phy

3D CT scanning of isolated human bones has been describedand 3D reconstruction images have been shown to be accuratein osseous anthropometric applications.29 Subjects lie su pine i nthe SXCT scanner with their residual limb held fully extended.We generated 3D reconstructions of the residual limb from anisotropic data volume synthesized from continuous 2D spiralCT slices (fig 2). Validation of distance and volume measure-ments from SXCT data of lower extremity residua were per-formed to determine the applicability of SXCT in prostheticfitting.Electromagnetic D ig i tizer

An electromagnetic digitizer has been used in our laboratoryand proven to be accurate when measuring landmarks on humanskulls29 and faces 23 as compared to caliper measurements. Th edigitizer consists of a hand-held wand that captures informationwith six degrees of freedom. The tip of the wand is placed ontoa point and a pedal is pressed to digitize the point location onthe subject who is seated in front of the digitizer with theirresidual limb held horizontally in space. We tested the digi-tizer's ability to capture the re siduum soft tissue surface pointlocations as compared to the OSS, SXCT, and calipers.Experimental Design

The experimental design is shown in figure 3 for one of twomeasurement sessions. At a single measurement session, the

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F i g 2 . V o l u m e t r i c S X C T s l ic ed a t a r e n d e r e d a s d e p i c t i o n o f t h eb e l o w - k n e e s k i n s u r f a c e a l o n gw i t h s e v e r a l o f t h e d o t l o c a t i o n s .T h e s l i c e s c a n a l s o b e v i e w e da l o n g e i t h e r t h e s h o r t o r l o n ga x is , a l l o w i n g o n e t o s e e t h e i n -t e r n a l p r o x i m i ty o f t h e f e m u r ,t i b i a , a n d f i b u l a r e l a t i v e t o t h eski n sur f ace .

subject' s residuum (Measurement Object) was measured twicewith each measure ment device. The caliper~ measurements weretaken twice and digitizer measurements were taken four timesdirectly from the subje ct's residuum. The OSS and SXCT mea-surements could only be obtained from the scan data, so twoscans each were taken using the OSS and SXCT scanner. Foreach scan, measurements were taken twice directly on the com-puter displayed image using software programs. At a later date,the subject returned for a second measurement session (thelength of time between visits ranged from 1.5 to 4 months, withthe average being 2.1 months).On e nrolling a subject to participate in the study and obtaininginformed consent, the following experimental protocol was usedand measurements taken in the order described during a singlemeasurement session: (1) Dots were placed on the residuumand the residuum was measured using water displacement andcalipers; (2) the residuum was measured using a digitizer; (3)the residuum was scanned using 3D OSS; (4) a negative plastercast was made of the subject's residuum; and (5) the residuumwas scanned using the SXCT scanner. The steps in the protocolwere repeated on each subject at a second measurement session.MeasurementObject

MeasurementDevice

Repeated Scans

Repeated Measurements

Residua/Cast

Caliper 3SpacerM/OSS/SXCT

1 2

A A1 2 1A IB 2A 2BF i g 3 . C a l i p e r a n d d i g i ti z e r m e a s u r e m e n t s a r e t a ke n 2 a n d 4 t i m e s , r e -s p e c t i v e l y , s i n c e t h e f i d u c i a l s ( e g , d o t s ) c a n b e m e a s u r e d d i r e c t l y . F o rt h e O S S a n d S X C T s c a n n e r s , r e p e a t s c a n n e d i m a g e s w e r e m e a s u r e dt w i c e t o a s s i s t i n i s o l a t i n g e r r o r r e s u l t i n g f r o m t h e o p e r a t o r ' s c h o i c e o fp o i n t s a n d th e m e a s u r e m e n t d e v ic e .

The subj ect's plaster positive casts (one from each measure-ment session) were measured using the same protocol as per-formed on the subjects themselves (steps 1, 2, 3, and 5 weredone). We evaluated the positive plaster casts as an independentstandard to reduce the error associated with subject motionduring scans. Each dot on the su bject' s resid uum was transferredto the negative cast when molded by the prosthetist and couldbe seen in the positive cast made from the negative cast. Theplaster positives allowed us to eliminate variables associatedwith subject motion and soft tissue deformation, and representsthe best possible precision when using precision digital calipers,digitizer, OSS, and SXCT scanner.B e l o w - K n e e R e s i d u a T e s t i n g

To determine the measurement locations for the below-kneeresiduum, the subject's residuum was positioned in a water bathand three proximal marks were placed on the residuum usinga perman ent marker at the water level to allow quick, repeatable,and simple volume measurements. The first dot was markednear or on the patellar tendon (dot 3 shown in fig 4), with twoadditional dots (dots 1 and 2 shown in fig 4) placed approxi-mately 120 degrees on either side of the first, one medial andone lateral. After marking the three dots, the subject's leg waslowered into the water bucket until the dots intersected thewater line. The water displaced by the subject's residuum wasmeasured (the water bucket was sitting on a scale) using theArchimedes principle to determine its volume. We found theArchimedes principle of volume measureme nt to be more rapid(4 measurements could be taken within 1 minute or less), pre-cise, and repeatable than a water displacement measurementmethod where the water displaced was captured and measured.After the subject removed his or her limb from the water bath,one additional marker (dot 6 shown in fig 4) was placed nearthe distal end of the tibia, and two additional markers (dots 4and 5 shown in fig 4) placed approximately 120 degrees oneither side of the distal end tibia marker, one medial and onelateral.

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Prox iA n t eD o l

P r o x i m a lL a t e r a lD o t 2

P r o x i m a lM e d i a lD o t I

D i s t a lL a t e r ~D o t 5D ilA n K , . . . .D o t 6

D i s t a ld e d i a lD o t 4

F i g 4 . A n t e r i o r v ie w o f a r ig h t b e l o w - k n e e a m p u t e e . T h e d o t lo c a t i o n sw e r e n o t p l a c e d o n b o n y l a n d m a r k s b e c a u s e o f t h e h a r d w a r e d e s i g n o ft h e O S S a n d t h e w a t e r v o l u m e m e a s u r e m e n t m e t h o d . T h e O S S r e q ui re dd o t s 3 a n d 6 t o b e p l a c e d a l o n g t h e a n t e r i o r f a c e o f t h e t i b i a , a n d f o u ra d d i t i o n a l d o t s w e r e p l a c e d a t 1 2 0 - d e g r e e i n c r e m e n t s ( m e d i a l a n d l a t -e r a l ) b e c a u s e t h e O S S c a m e r a s w e r e l o c a t e d a t 1 2 0 - d e g r e e i n t e r v a l sa b o u t t h e s c a n n e r s t r u c t u r e . D o t 3 w a s p l a c e d n e a r o r o n t h e p a t e l l a rt e n d o n .

Distance measurements were repeated twice, using calipers,between the various pairwise combinations of the six markercenters located on the subject. The repeated measurements pro-duced two distance measurement sets of 15 measurements each.The digitizer was used to digitize each marker location onceand was repeated a total of four times. The 3D coordinates fromthe digitizer were saved into a file and the distance measure-ments automatically calculated. The repeated measurements ofthe marker locations on the resid uum were required to estimatethe measurement error introduced by the operator. This mea-surement process was repeated for each plaster positive.After measuring the r esiduum using the calipers and digitizer,.635cm black colored dots were applied to the permanentmarker locations for making measurements with the 3D opticalsurface scanner. Subjects were scanned twice using the pros-thetic 3D optical surface scanner. The 3D surface data for eachof the two scans were processed and rendered on a SiliconGraphics, Inc., 4D/340 VGX graphics workstatione to deter mineif movement had occurred and to verify that all six dots had beencaptured. The surface data was read, displayed as a cylindricallyunwrapped image, and measured using a program written withthe PV-Wave3 command language. Measurements were per-formed twice to determine error introduced during dot digitiza-tion for each surface data set.On completion of the surface scans, conventional radiogra-phy/CT markers containi ng 1.5-mm diameter lead spheres werecentered on the colored .635cm dots so measurements could betaken between them. The prosthetist then placed a thin sock onthe subject and marked the sphere locations on the sock with a

water transferable pencil. The negative cast was made withminimal application of pressure except in the patellar tendonregion, which assisted in cast orientation during subsequentmeasurements. When the positive plaster cast was produced,the pencil marks and the sphere locations were visible.Subjects were transported to the SXCT scanner and scannedwith radiographic CT markers in place. Subjects were placedon the sc anner table foot/resid uum first. S tarting below the distalend of the residuum, approximately 256mm of the subject'sresiduum was scanned using 8mm/sec table feed over a continu-ous 32-second scan period. Scans were repeated to aid in errordetermination. The SXCT scanning parameters were collima-tion of 8mm, table feed of 8mm/sec, 120kVp, 210mAs, gantrytilt of 0 , and maxim um scan time of 32 seconds. The raw 3Dimage data were stored on an optical disk and processed on aSiemens CT satellite evaluation console,c The 3D voxel imageswere evaluated using ANALYZE31'32 software, f The 1.5ramsphere locations were identified and the distance between thesepoints were measured using ANALYZE and the distances calcu-lated.The plaster positive casts of the subjects were measured andscanned using the same procedure as described above. The castwas digitized or scanned twice using each measureme nt modal-ity, without moving it, to ensure the two data sets were automat i-cally superimposed.Statistical Me t h o d s

L imb m e a s u r e m e n t p r e c i s i o n a n d r e p e at a b il i ty . Precisionand repeatability is obtained from a nested analysis of variance(ANOVA) with scan nested within session, which in turn isnested within subject. The error term is due to the repeatedmeasures of a single scan. Caliper measurements were onlyperformed twice at each session, so the lowest level of thenested ANOVA (SCAN) is absent from the analysis. The modelto address Question One (What is the precision and repeatabilityof the limb measurements using the various measuring devices?Are there differences in precision and repeatability among mea-surement methods?) is: Yijkl = # + ID~ + SESSIONj(IDi) +SCANk(SESSIONj(ID~)) + e0k~, where Y~ju is meas uremen t 1of scan k (SCANk) taken at session j (SESSION j) on subject i(IDa). The expected mean squares for each level are given bySokal and Rohlf. 33 Preci sion (meas uremen t error) is evalua ted interms of standard deviations (precision of a ny one mea surementdevice given in units of length or volume) and coefficients ofvariation (precision relative to the magnitude of the distancemeasured expressed in percent). The standard deviations mea-sure the square root of the average squared difference betweenclasses within a factor. Repeatability (precision of the measure-ment relative to the difference between subjects being mea-sured) is the proportion of the total variation which occursbetween subjects, differences among repeated measurementswithin subjects typically being considered as measurementerror.S u b j e c t v e r s u s c a s t m e a s u r e m e n t s . Question Two (Arethere differences between measurements taken directly on sub-jects and comparable measurements taken on casts?) has twoaspects. First, is there a bias in measurements made on casts,such as, are they typically smaller or larger than measurementstaken directly on subjects? This was tested by an ANOV A withcast as the factor. Separate analys es were performed for eachmeasuring device. Second, even when there is no bias, it ispossible that the casting process results in considerable randomerror, the cast being larger or smaller than the subject dependingon which subject is examined. These aspects to Question Twowere analyzed by a nested ANO VA with the cast factor nestedwith in sess ion and then furthe r nes ted with in subject: Y~jk~ = /Z



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T a b l e 1 : E r r o r S t a n d a r d D e v i a t i o n f o r E r ro r R e s u l t i n g F r o m R e p e a t M e a s u r e m e n t s o f a S i n g l e S ca n a n d E r r o r R e s u l t i n gF r o m R e p e a t S c a n s C o m b i n e d ( f o r S ub j ec t s)

4 8 1

Error Standard Deviation (SD) (mm) Coefficient of Variation (%)DIG OSS SXCT PHY DIG OSS SXCT PHY

DistancesMa ximu m 1.94 3.11 1.53 1.06 1.92 2.96 1.30 1.05Minimum .78 .75 .59 ,41 .54 .69 .48 .46Median 1.31 1,62 .86 .83 1.01 1.52 .62 .58Avera ge SD 1.34 1.79 .88 .74 1.14 1.50 .75 .61Vo lum e SD (cc) N/A 32.44 8.66 11.22 N/A 2.67 ,71 .92

+ 1D ~ + S E S S I O N j ( I D ~ ) + C S k ( S E S S I O N j ( I D 0 ) + e ij kl , w h e r eY~jk~ s m e a s u r e m e n t 1 o f c a s t o r s u b j e c t k ( C S k ) t a k e n a t s e s s i o nj o n s u b j e c t i. W e w i l l c o n s i d e r t h e s t a n d a r d d e v i a t i o n d u e t od i f f e r e n c e s b e t w e e n c a s t o r s u b j e c t ( C S ) a s a m e a s u r e o f t h ea v e r a g e d i f f e r e n c e b e t w e e n c a s t s a n d t h e s u b j e c t s t h e y r e p r e -s e n t .Modality differences. B o t h b i a s a n d r a n d o m e r r o r c o n t r i b -u t e t o m e a s u r e m e n t d i f f e r e n c e s o b t a i n e d b y d i f f e r e n t m e t h o d s .F o r s a k e o f c o n v e n t i o n , th e c a l i p e r m e a s u r e m e n t s w i l l b e t a k e na s t h e s t a n d a r d a n d m e a s u r e m e n t s b a s e d o n o t h e r d e v i c e s c o m -p a r e d to t h e m . B i a s w i l l b e a n a l y z e d b y a n A N O V A w i t h m e a -s u r e m e n t t y p e a s t h e f a c t o r . R a n d o m e r r o r i s m e a s u r e d w i t h an e s t e d A N O V A m o d e l t o a d d r e s s Q u e s t i o n T h r e e ( A r e t h e r ed i f f e r e n c e s b e t w e e n m e a s u r e m e n t s m a d e w i t h t h e d i f f e r e n t d e -v i c e s ? ) : Y ijk l : ~ + I D i q- S E S S I O N j ( I D i ) + T Y P E k ( S E S S I O N -j ( I D i ) ) + e ij k~ , w h e r e y~ jk~ i s m e a s u r e m e n t 1 o f m e a s u r e m e n tm o d a l i t y k ( T Y P E k ) t a k e n a t s e s s i o n j o n s u b j e c t i. W e w i l lc o n s i d e r t h e s t a n d a r d d e v i a t i o n d u e t o t y p e ( T Y P E ) a s a m e a s u r eo f t h e a v e r a g e d i f f e r e n c e b e t w e e n m e a s u r e m e n t s t a k e n w i t hc a l i p e r s a n d t h o s e t a k e n b y t h e d e v i c e s p e c i f i e d . E a c h d e v i c ei s c o m p a r e d t o c a l i p e r m e a s u r e m e n t s s e p a r a t e l y . T h e a n a l y s i si s a l s o p e r f o r m e d s e p a r a t e l y f o r d i r e c t s u b j e c t m e a s u r e m e n t sa n d t h o s e t a k e n o n c a s t s .

R E S U L T SL i m b M e a s u r e m e n t P r e c i s i o n a n d R e p e a t a b i l i t y

I n t e r m s o f p r e c i s i o n o r m e a s u r e m e n t e r ro r , i t i s a ls o i n s t r u c -t i v e t o c o n s i d e r t h e m a g n i t u d e o f e r r o r b e t w e e n a n d w i t h i nr e p e a t e d s c a n s . T h e e r r o r s t a n d a r d d e v i a t i o n f o r e r r o r c a u s e db y r e p e a t m e a s u r e m e n t s o f a s i n g l e s c a n a n d e r r o r c a u s e d b yr e p e a t s c a n s c o m b i n e d i s ty p i c a l l y b e t w e e n 1 . 3 a n d 1 . S m m f o rd i r e c t s u b j e c t m e a s u r e m e n t s t a k e n w i t h t h e d i g i t i z e r o r O S Sa n d b e t w e e n 0 . 7 a n d 0 . 9 m m f o r d i r e c t m e a s u r e m e n t s w i t h c a li -p e r s o r w i t h S X C T ( t a b le 1 ). T h e a s s o c i a t e d c o e f fi c i e n ts o fv a r i a t i o n a r e b e t w e e n 1 . 1 % a n d 1 . 5 % a n d b e t w e e n 0 . 6 % a n d. 7 5 % o f t h e m e a n v a l u e s , r e s p e c t i v e l y . M e a s u r e m e n t s o f t h ec a s t s c a n b e m a d e e v e n m o r e a c c u r a t e l y w i t h s t a n d a r d d e v i a -t i o n s o f o n l y 0 . 3 m m f o r c a l i p e r a n d d i g i t i z e r m e a s u r e m e n t sa n d 0 . 6 r a m f o r O S S a n d S X C T m e a s u r e m e n t s . T h e r ef o re , t h e s ed e v i c e s , o n a v e r a g e , c a n r e p e a t m e a s u r e m e n t s t o w i t h i n 0 . 7 t o1 . 8 m m w h e n m a d e d i r e c t l y o n s u b j e c t s a n d t o w i t h i n 0 . 3 t o0 . 6 m m w h e n m a d e o n c a s t s ( t a b l e 2 ) . E r r o r i n l i m b v o l u m e s

w a s a p p r o x i m a t e l y 5 to 1 0 c c ( 0 . 5 % t o 1 % o f t h e m e a n v a l u e )e x c e p t f o r d i r e c t s u b j e c t m e a s u r e m e n t s t a k e n o n s u b j e c t s w i t ht h e o p t i c al s u r f a c e s c a n n e r , w h i c h h a d a s t a n d a r d d e v i a t i o n o f3 2 c c , r e p r e s e n t i n g 2 . 6 % o f t h e m e a n v a l u e .W e c a n c o m p a r e t h e e r r o r v a r ia n c e s c a u s e d b y t h e v a r io u s m e a -s u r e m e n t d e v i c e s t o d e t e r m i n e w h e t h e r o n e i s s i g n i f i c a n tl y b e t t e rt h a n a n o t h e r i n i t s p re c i s i o n w h e n u s e d t o m e a s u r e r e s id u a . T h i s i sd o n e u s i n g a r a t i o o f t h e i r e r r o r v a r i a n c e s a n d t e s t i n g f o r s t a t i s t i c a ls i g n i fi c a n c e w i t h a n F r a ti o . E r r o r i n c l u d e s t h a t c a u s e d b y s c a n a n dr e p e a t m e a s u r e m e n t w i t h i n s c a n . I n c o m p a r i n g c a l i p e r m e a s u r e -m e n t s w i t h t h o se o b t a i n e d b y o t h e r m e a n s , t h e r e w e r e 2 2 d e g r e eso f f r e e d o m f o r c a l ip e r m e a s u r e m e n t s a n d t h e r e w e r e 6 6 d e g r e e s o ff r e e d o m f o r t h e o t h e r m o d a f i t i e s .R e p e a t a b i l i t y f o r li n e a r d i m e n s i o n s w a s l o w f o r a l l m e a s u r e -m e n t m e t h o d s b e c a u s e o f th e r e l a t i v e l y l a r g e v a r ia t i o n b e t w e e nm e a s u r e m e n t s t a k e n a t d i f f e r e n t s e s s i o n s . F o r a l l m e a s u r e m e n td e v i c e s a n d f o r b o t h d i r e c t m e a s u r e m e n t s m a d e o n s u b j e c t s a n dc a s ts , a p p r o x i m a t e l y 7 0 % o f t h e v a r i a t io n i s b e t w e e n s u b j e c t s ,a s c a n b e s e e n i n t h e w i d e v a r i a t i o n i n s h a p e a n d s i z e o f t h ev a r i o u s s u b j e c ts s c a n n e d w i t h t h e S X C T ( f i g 5 ) , 3 0 % b e t w e e ns e s s i o n s , a n d a v e r y s m a l l p e r c e n t a g e b e t w e e n s c a n s a n d b e -t w e e n m e a s u r e m e n t s o f a s i n g l e s c a n ( t a b l e 3 ) . E v e n t h o u g ht h e p e r c e n t a g e w a s s m a l l , m o s t o f t h e b e t w e e n - s c a n v a r i a n c e sf o r a l l m o d a l i t i e s w e r e s i g n i f i c a n t l y g r e a t e r t h a n z e r o f o r d i r e c ts u b j e c t m e a s u r e m e n t s . R e p e a t a b i l i t y f o r l i m b v o l u m e w a s v e r yh i g h , t y p i c a ll y a r o u n d 9 3 % w i t h o n l y 7 % b e t w e e n s e s s i o n s( t a b le 4 ). N o n e o f t h e e r r o r v a r i a n c e s w i t h i n s e s s i o n s a c c o u n t e df o r m o r e t h a n 1 % o f t h e t o t a l f o r a n y m e a s u r e m e n t .S t a n d a r d d e v i a t i o n s d u e t o m e a s u r e m e n t s e s s i o n s w e r e a p -p r o x i m a t e l y 1 0 m m f o r l i n e a r d i s t a n c e s a n d 1 0 0 c c f o r v o l u m ew i t h c o e f f i c i e n t s o f v a r i a t i o n ( C O V ) a t 8 % o f t h e m e a n f o rd i s t a n c e s a n d 8 % t o 1 3 % o f t h e m e a n f o r v o l u m e s ( t a b le 5 ) .

F o r d i r e c t l i n e a r m e a s u r e m e n t s m a d e o n s u b j e c t s , c a li p e r s a r em o r e r e p e a t a b l e t h a n m e a s u r e m e n t s t a k e n w i t h t h e d i g i t i z e r o rt h e O S S ( s i g n i f i c a n t l y l e s s e r r o r fo r c a l i p e r m e a s u r e m e n t s i n 1 2a n d 1 4 o f 1 5 c a s e s , re s p e c t i v e l y ) . H o w e v e r , c a l i p e r m e a s u r e m e n t sa r e s i g n i fi c a n tl y m o r e r e p e a ta b l e t h a n s p i r a l C T - b a s e d d a t a f o ro n l y 5 o f 1 5 l in e a r d i m e n s i o n s . S X C T m e a s u r e m e n t s a r e s i g n if i -c a n t l y m o r e r e p e at a b l e i n 1 0 o u t o f 1 5 m e a s u r e m e n t s c o m p a r e dt o t h e d i g it i z e r a n d f o r 1 3 o f 1 5 m e a s u r e m e n t s c o m p a r e d t o t h eO S S . F i n a l l y , t h e d i g i t i z e r h a s s i g n i f i c a n t l y l e s s e r r o r t h a n t h eO S S f o r 8 o f t h e 1 5 m e a s u r e m e n t s , t h e r e v e r s e h o l d i n g m a e fo ra s i n g l e m e a s u r e m e n t . F o r m e a s u r e m e n t s o f v o l u m e , b o t h w a t e r

T a b l e 2 : E r r o r S t a n d a r d D e v i a t i o n f o r E rr o r R e s u l t i n g F r o m R e p e a t M e a s u r e m e n t s o f a S i n g l e S c a n a n d E r r o r R e s u l t in gF r o m R e p e a t S c a n s C o m b i n e d ( f o r C a s ts )Casts' Error Standard Deviation (ram) Casts' Coefficient of Variation (%)

DIG OSS SXCT PHY DIG OSS SXCT PHYDistancesMa ximu m .43 .94 .74 .33 .37 .75 .81 .31Minimu m .20 .36 .41 .15 .15 .32 .32 .09Medi an ,30 .55 .60 .21 .23 .40 .44 .19Avera ge SD ,30 .58 .57 .22 .25 .49 .48 .19Vo lum e SD (cc) N/A 7.03 7.23 4.15 N/A .58 ,60 .34



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4 8 2 B E L O W - K N E E R E S I D U A M E A S U R E M E N T S , C o m m e a n

F i g 5 . V o l u m e t r i c S X C T r e n d e r -i n g o f s k in s u r f a c e o f 1 2 b e l o w -k n e e a m p u t e e p a t i e n t s . T h e s k i ns u r f a c e i s d e p i c t e d w i t h t h e f i -d u c i a l m a r k e r l o c a t i o n s u s e d f o rc o m p a r a t i v e m e a s u r e m e n t a n a l -y s i s . V o l u m e s w e r e c o m p u t e db y d e l i m i ti n g t h e d a t a s e t w i t h ap l a n e p a s s i n g t h r o u g h t h r e epro ximal f iduc ia ls .

d i s p l a c e m e n t a n d S X C T m e a s u r e s a r e s i g n i f i ca n t l y m o r e r e p e a t -a b l e t h a n O S S m e a s u r e m e n t s , b u t t h e y a r e n o t s i g n i fi c a n t ly d if f er -e n t f r o m o n e a n o t h e r . I n s u m m a r y , c a l i p e r m e a s u r e m e n t s a r e t h em o s t r e p e a ta b l e , f o l lo w e d c l o s e l y b y S X C T m e a s u r e m e n t s , t h e nd i g i ti z e r m e a s u r e m e n t s , a n d , f i n al l y, O S S m e a s u r e m e n t s .F o r m e a s u r e m e n t s t a k e n o n c a s t s , t h e r e l a t i v e r e p e a t a b i l i t yi s d i f fe r e n t . C a l i p e r m e a s u r e m e n t s s t i l l h a v e t h e h i g h e s t r e p e a t -a b i l i ty , b u t d i g i t i z e r m e a s u r e m e n t s a r e o n l y s l i g h t l y le s s r e p e a t -a b l e . S X C T a n d O S S r e p e a t a b i l i ty is i n d i s t i n g u i s h a b l e a n d i n

e a c h c a s e i s s i g n i f i c a n t l y le s s t h a n c a l i p e r o r d i g i t i z e r m e a s u r e -m e n t s . F o r m e a s u r e s o f c a s t v o l u m e , b o t h S X C T a n d O S S m e a -s u r e s a r e l e s s r e p e a t a b l e t h a n w a t e r d i s p l a c e m e n t b u t a r e n o ts i g n i f ic a n t l y d i ff e r e n t f r o m e a c h o t h e r.F o r a l l m e a s u r e m e n t m e t h o ds , r e p e a t a b i li t y o f m e a s u r e m e n t so n c a s t s w a s s i g n i f i c a n t ly b e t t e r t h a n f o r d i r e c t m e a s u r e m e n t so n s u b j e c t s ( t a b le 6 ) . H o w e v e r , t h i s w a s o n l y m a r g i n a l l y t ru ef o r m e a s u r e m e n t s t a k e n w i t h S X C T . A l t h o u g h t h i s m a y i n d i c a tea p r e f e r e n c e f o r m e a s u r i n g c a s t s r a t h e r t h a n t a k i n g d i r e c t s u b j e c t

T a b l e 3 : A v e r a g e R e p e a t a b i l i t i e s o f t h e 1 5 L i n e a r D i s t a n c e s F r o m R e s i d u a M e a s u r e m e n t sDIG OSS SXCT PHY Units

SubjectsIndi vidua ls 70.89 68.58 68,58 66.63 %Sessions 28.24 30.25 31.12 33.15 %Scans .28 .88 .18 N/A %Dire ct Error .59 .29 .12 .22 %Total Varia nce 289.67 302.14 311.55 301.28 mm 2CastsIndiv idual s 70.10 69.16 69.47 69.46 %Sess ions 29.87 30.72 30.42 30.52 %Scans .01 .03 .03 N/A %Direct Error .02 .09 .09 .02 %Total Varian ce 335.17 327.36 333.13 326.56 mm 2



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B E L O W - K N E E R E S I D U A M E A S U R E M E N T S , C o m m e a n 4 8 3T a b l e 4 : R e s i d u a V o l u m e R e p e a t a b il it ie s f o r O S S , S X C T , a n d P H Y

OSS SXCT PHY UnitsS u b j e c t sI n d i v i d u a l s 9 3 . 1 3 5 9 2 . 9 7 5 9 4 . 8 2 %S e s s i o n s 6 . 1 7 6 . 9 8 5 . 1 0 %Scans .67 ,00 N/A %Di rec t E r ro r .625 .045 .08 %T o t a lV a r i a n c e 1 5 1 , 5 5 4 1 5 5 , 4 3 8 1 5 9 , 9 2 4 c c 2C a s t sI n d i v i d u a l s 8 3 . 4 0 9 3 . 2 5 9 4 . 4 5 %S e s s i o n s 1 6 . 5 6 6 , 7 2 5 .5 4 %S c a n s . 0 0 . 0 0 N / A %Di rec t E r ror .04 ,03 .01 %T o t a lV a r i a n c e 1 5 7 , 8 9 7 1 8 6 , 7 6 4 1 7 9 , 6 0 7 c c2

measurements, it must also be considered that casts may beimperfect representations of the limb stumps themselves.S u b j e c t V e r s u s C a s t D i f f e r en c e s

There was no significant bias of casts relative to subjects inthe caliper measurements. Only two measures were significantlylarger in casts when measure d with the digitizer, measurementnumber 7 was on average 3.43mm longer and measurementnumber 8 was 2.48 mm longer on average. Measurement number10 was 3.22mm shorter, on average, for measurements of caststaken with OSS. For SXCT measures, measurement number 4is 3.72turn shorter, measurement number 5 is 2.21mm longer,measurement number 11 is 3.40mm longer, measurement num-ber 14 is 4.30mm shorter, and measurement number 15 is3.50mm longer on casts than on subjects. Given the one statisti-cally significant bias expected by chance and the lack of consis-tency in the results in terms of the direction of bias, it appearsthat there is no serious bias toward measurements being largeror smaller when taken from subjects than when taken fromcasts. No bias amo ng the measurem ent methods was detectedbetween volume of casts or subjects.Random error was statistically significant in all cases. Differ-ences between direct measurements and measurements on castswere approximately 3.75mm or 3.3% of the mean for caliper,digitizer, and OSS measurements. Differences were greater forSXCT measures, approximately 4.90mm, or 4.3% of the mean.Volume measures differed by 54cc (4.4% of mean), 72cc (6%of mean) and 47cc (4.3% of mean) for water displacement,OSS, and SXCT, respectively (table 7).M o d a l i t y D i f f e r e n ce s

No bias was detected in either direct subject measurementsor cast measurements for any of the measureme nt types relativeto caliper measurements.Tab le 5 : S tan dard Deviat ions (P rec is ion) and Coef f ic ientso f V a r i a t i o n ( C O V ) ( P r e ci s io n R e l a t iv e t o L e n g t h o r V o l u m e )f o r M e a s u r e m e n t S e s si o ns

DIG OSS SX CT PHYA v e r a g e S D o f d i st a n c e s ( m m )S u b j e c t s 8 . 8 5 9 . 4 0Casts 9 .63 9 .69S D o f v o l u m e s ( cc )Sub jec ts N /A 96.71Casts N/A 161.72A v e r a g e C O V f o r d i s t a n ce s ( % )S u b j e c t s 7 . 6 4 8 . 1 7Casts 8 .17 8 .18C O V f o r v o l u m e s ( % )S u b j e c t s N / A 7 . 9 7Casts N/A 13.32

9.47 9 .809.65 9 .611 0 4 . 1 4 9 0 . 3 3112.01 99.76

8.14 8 .108.08 8 .409 . 2 3 8 . 1 08.57 7 .44

Measurements taken directly on subjects have random differ-ences from caliper measurements of approximately 2.0mm(1.6% of the mean) for digitizer and OSS, and 3.5mm (3.0%of the mean) for SXCT (table 8). Casts show much smallerrandom deviations of approximately .75mm (0.6% of the mean)differences from the caliper measurements for all three alternatemeasurement methods (table 9). Cast measurements taken usingdigitizer, OSS, and SXCT are almost always more similar tocaliper measurements than measurements made directly on thesubject.When deviations from caliper measures were compared, therewere 18 degrees of freedom for caliper measurements. Thedigitizer and OSS show a similar degree of random deviationfrom caliper measurements; however, each is often significantlycloser to the caliper measure ments (8 of 15 linear measurementsfor the digitizer and 13 of 15 linear measurements for OSS)than are the SXCT measures.

DISCUSSIONL i m b M e a s u r e m e n t P r e c is i o n a n d R e p e a t a b i li ty

Measurements taken at any one session were remarkably re-peatable, differing only about lmm or 1% on average of themean values for measurements taken directly on subjects andonly about .25 to .50mm for measurements taken on casts. Ev enthough error was small for all devices, it was least for calipermeasurements. Measurements taken directly on subjects fromSXCT scans were nearly as repeatable as caliper measurements,whereas digitizer and OSS measurements were less repeatable.For measurements taken on casts, digitizer measures werenearly as repeatable as those taken by calipers, whereas SXCTand OSS measures were slightly less repeatable. Casts weremeasured with greater repeatability than direct measures onsubjects with all devices, although this difference was muchreduced for SXCT measurements.With the protocol used here, measures of limb stumps cannotbe very reliably measured at different sessions using any of themeasurement devices directly on subjects or on casts. On aver-age, linear measurements differ by about 10mm or 8% of themean. Approximately 30% of the variance may be caused byerror in repeatedly placing the dots near the same position onthe residuum from one subject visit (measurement session) tothe next visit. This error may also be caused by the subject'sresiduum changing size between sessions (the length of timebetween visit ranged from 1.5 to 4 months with the averagebeing 2.1 months). Eight of the nine subjects who participatedin two measurement sessions wore permanent prostheses. Theoptical surface scann er design dictated the placem ent of the dotsat 120-degree intervals about the subject's residuum becausethe OSS sensors were placed at 120-degree intervals about thestructure. Also, it was necessary to mark the patellar tendon dotwhile the residuum was immersed in the water bath, whichmade it difficult to identify the patellar tendon or the tibial

T a b l e 6 : R a t i o s o f E r ro r S t a n d a r d D e v ia t io n s f o r S u b j e c t sRelat ive to CastsStandard Deviat ion of Scan + Direct MeasurementError (ram)DIG OSS SXCT PHY

D i s t a n c e sM a x i m u m 8 . 0 2 5 . 5 8 2 . 2 6 5 . 9 6M i n im um 1.82 1 .49 1 .06 1 .95Me dian 4 .57 2 .55 1 .45 3 .10A v e r a g e S D 4 . 7 6 3 . 2 0 1 .5 6 3 . 4 0V o l u m e S D ( cc ) N / A 4 . 6 2 1 .2 0 2 . 7 0



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4 8 4 BELOW-KNEE RESIDUA MEASUREMENTS, C o m m e a nT a b l e 7 : C o m p a r i s o n o f D is t a n c e M e a s u r e m e n t s o n S u b je c ts a n d C a s t s

Standard Deviation (ram) Coefficient of Variation (%)DIG OSS SXCT PHY DIG OSS SXCT PHY Mean Distances (mm)

DistancesMax imu m 5.56 6.48 8.76 12.00Min imu m 2.60 2.40 3.16 2.16Med ian 3.65 3.76 4.03 3.33Avera ge SD 3.90 3.68 4.90 3.87Vo lum e SD (cc) N/A 72.17 47.42 53.85

7.64 8.90 12.03 7.76 166.82.19 1.88 2.42 1.50 72.83.04 2.66 3.70 2.71 119.33.34 3.39 4.30 3.32 122.7N/A 5.94 3.91 4.44 1214.0

tuberosity. Therefore, all six dots were not placed on bonylandmark locations, so repeatedly and precisely finding the sixdot locations from one visit (session) to another was not pos-sible.If measur ement error between sessions is important in fittingprostheses, using bony landmarks would aid in more accuratelylocating dots from one session to the next. In our original con-ceptualized experimental protocol, we thought that finding bonylandmarks on the BK re siduum would be used for making mea-surements, but due to the OSS design and water displacementmeasurements, we were unabl e to use bony landmark locations.Therefore, we anticipated that finding the same dot placementlocations from one session to the ne xt session would be difficultand our results indicate this fact.S u b je c t V e r su s C a s t D i f f e r e n c e s

There are o nly occasional small biases in measures made oncasts relative to those made on subjects. However, there israndom error introduced by casting, on the order of 4mm. Al-though casts can be measured more repeatably, their randomdeviation from the subject is greater than the gain i n accuracyobtained in measuring them. The random deviation may becaused by two variables: (1) two different prosthetists per-formed the castings on different subjects, and (2) when the dotlocations transferred from the sub ject's residuum to the negativecast, their location was not always easy to identify in the positiveplaster cast. Placeme nt of a blue pencil mark o n the thin sockworn by the subject over each dot to indicate their locationsbefore the negative plastering process did not always remedythe identification problem. In fact, in a numb er of cases the bluemarks would be shifted up to approximately 10mm from theimpression left by the lead sphere markers. Also, the two pros-thetists did not make negative casts of the same individual todetermine if there was a difference in their casting methods.M o d a l i ty D i f f e r e n c e s

There is no bias in measurements taken with the differentdevices used in this study. However, there is statistically sig-nificant random error in that particular digitizer, OSS, andSXCT measurements may be larger or smaller than the calipermeasurements. These deviations from the caliper measurementsare on the order of 2.0 to 3.5mm. The SXCT measures are most

T a b l e 8 : C o m p a r i s o n o f C a l i p e r M e a s u r e m e n t s o f S u b j e c t st o O t h e r M e a s u r e m e n t T y p e sSubjects' Standard Subjects' Coefficient ofDeviations (mm) Variation (%)

DIG OSS SXCT DIG OSS SXCTDistancesMaxi mu m 3.21 2.60 5.38 2.69 2,17 6.27Minimum .70 .43 1.46 .72 .41 1.39Med ian 2.31 1.85 3.73 1.93 1.63 2.61Ave rag e SD 2.11 1.86 3.48 1.77 1.56 3.02Volu me SD (cc) N/A 59.90 58.76 N/A 4.93 4.84

divergent from the caliper measures. On further analysis of theSXCT data, we found that because the subje ct's residuum waslying relaxed on the SXC T table (instead of being tensed whilesuspended in air as when the caliper, digitizer, and OSS mea-surements were taken) the shape of the residuum had changed,causing 12 of the 15 measurements to be much more divergentthan the remaining three measurements (SXCT MeasurementNumber 3, 8, and 12 values were 2.37mm, 3.28mm, and1.46mm, respectively, and can be compared to the values intable 8) for all subjects. These three measurements had similarrandom errors on the order of 2mm when compared to thecalipers, digitizer, and OSS. These three measurements werebetween dots 1 to 4, 2 to 5, and 3 to 6, which were longitudinalmeasurements (proximal to distal) and would have the leastamount of change when a subject's residuum was lying relaxedon the table as compared to the other 12 measurements.G e n e r a l D i sc u ss io n

The precision and repeatability error of the digitizer, OSS,and SXCT being greater on subjects as compared to the plastercasts may be attributed to several variables observed during theexperimental protocol. First, while making measurements withthe calipers, OSS, and digitizer, the subject's residuum was notrigidly fixed in position, allowing the subject's leg to movewhile measurements were taken; and for the individual takingthe measurements with calipers and digitizer, no hand/arm restwas available, which allowed their hands/measurement deviceto move slightly while taking measuremen ts. For casts, the mea-surement device could be held firmly against the cast dot loca-tion, unlike with the subjects, where steadying the measureme ntdevice against the dot would cause the subject's skin surfaceto deform. Second, a single measurement session would lastfrom 3 to 5 hours. Dur ing this time period, the subjec t' s prosthe-sis would be removed for the entire time period. Several subjectsindicated that at the beg inning of the session they had swellingproblems. So we repeatedly measured the volume of their resid-uum at different times during the session. We found that onthese subjects the volume measurements changed from approxi-mately 5% up to 15% over the course of the measurementsession. Therefore, the order in which the measuring deviceswere used to measure the su bject's residuum could be important.For practical logistical reasons, especially access to the SXCT

T a b l e 9 : C o m p a r i s o n o f C a l ip e r M e a s u r e m e n t s o f C a s ts t o O t h e rM e a s u r e m e n t T y p e sCasts' Standard Deviations Casts' Coefficient of(mm) Variation (%)DIG OSS SXCT DIG OSS SXCT

DistancesMa ximum 1.33 1.46 1.46 1.23 1.01 1.25Min imu m A1 .53 .15 .09 .34 .18Medi an .68 .78 .55 .52 .59 .46Avera ge SD .71 .81 .66 .62 .68 .53Volu me SD (cc) N/A 70.23 17 .4 4 N/A 5.79 1.44

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B E L O W - K N E E R E S I D U A M E A S U R E M E N T S , C o m m e a n 4 8 5s c a n n e r ( a v e r y b u s y i n s t r u m e n t l o c a t e d in B a r n e s H o s p i t a l, S t .L o u i s , M O , w h i c h i s a p p r o x i m a t e l y 1 5 0 0f t . f r o m t h e O S S a n dd i g it i z er ) , w e u s e d n o n r a n d o m o r d e r i n g o f s c a n n i n g t es t s. W eh a d n o w o r k a b l e a l t e r n a t iv e t h a t w o u l d a l l o w u s t o r a n d o m l yo r d e r t h e t e st s a n d c o m p l e t e t h e p r o t o c o l. A l s o , i f th e r e w a s at e s t o r d e r b i a s , i t w a s c o n s i s t e n t f r o m s u b j e c t t o s u b j e c t. S w e l l -i n g m a y h a v e c a u s e d t h e m e a s u r e m e n t s t o b e a f f e c t e d d u e t ot h e c h a n g e i n t h e s u b j e c t ' s r e s i d u u m v o l u m e . T h e p l a s te r c a s t in gp r o c e s s w a s p e r f o r m e d a f t e r t h e s u b j e c t ' s p ro s t h e s is h a d b e e nr e m o v e d f o r 1 t o 2 h ou r s , w h i c h m a y a l s o p a r t ia l l y a c c o u n t fo rt h e d i f f e r e n c e s in t h e c a s ts v e r s u s s u b je c t s. T h e S X C T s c a n sw e r e t a k e n a p p r o x i m a t e l y 2 . 5 t o 4 . 5 h o u r s f r o m t h e s t a r t o f t h es e s s i o n . B e c a u s e n o b i a s w a s d e t e c t e d i n t h e S X C T d a t a , i tw o u l d a p p e a r t h a t n o s w e l l i n g h a d o c c u r r e d , b u t i t al s o m u s tb e r e m e m b e r e d t h at t h e r e s i d u u m h a d c h a n g e d s h a p e d u r i n gt h e S X C T s c a n , w h i c h c o u l d a i d in m a s k i n g a b ia s . F o r e x a m p l e ,l e t u s a s s u m e t h e r e s i d u u m t o b e c y l i n d r ic a l in s h a p e , u s i n g t h ea v e r a g e m e a n d i s t a n c e ( 1 2 2 . 7 3 m m ) f r o m t a b l e 7 f o r b o t h t h ec y l i n d e r ' s h e i g h t a n d d i a m e t e r , w i t h t h e h e i g h t b e i n g c o n s t a n ta n d t h e d i a m e t e r v a r y i n g , a 2 - m m d i a m e t e r i n c r e a s e t a k e n f r o mt a b l e 8 ( 1 . 6 % i n c r e a s e i n d i s t a n c e ) w o u l d r e s u l t i n a p p r o x i -m a t e l y a 3 % i n c r e a s e i n v o l u m e s i z e ( 4 8 c c ) , a n d a 3 . 5 - m md i a m e t e r i n c r e a s e t a k e n f r o m t a b l e 8 ( 2 . 9 % i n c r e a s e i n d i s ta n c e )w o u l d r e s u lt i n a n a p p r o x i m a t e i n c r e a s e o f 6 % i n v o l u m e s i z e( 8 3 c c ), s i m i l a r t o t h e a v e r a g e v o l u m e s h o w n i n ta b l e 8 . S o e v e na s m a l l a m o u n t o f s w e l l i n g ( 5 % o r l e s s ) f r o m t h e b e g i n n i n g t ot h e e n d o f a m e a s u r e m e n t s e s s i o n o n a l l s ub j e c ts c o u l d e a s i l yh a v e c a u s e d t h e c a l i p e r m e a s u r e m e n t s t o b e d i f f e r e n t f r o m t h ed i g i ti z e r , O S S , a n d S X C T . I f t he d i s ta n c e a n d v o l u m e m e a s u r e -m e n t s w e r e t a k e n o n a l l th e s u b j e c ts a t t he e n d o f t h e m e a s u r e -m e n t s e s s io n , t h i s a s s u m p t i o n c o u l d h a v e b e e n p r o v e n .

P r a c t i c a l ly s p e a k i n g , i t a p p e a rs t h at a n y o f t h e m e a s u r e m e n tm e t h o d s i s m o r e p r e c i s e t h a n t h e c h a n g e s i n r e s i d u u m s i z e t h a to c c u r b o t h d u r i n g a n d b e t w e e n s e s s i o n s , a n d d u e t o s u b j e c t ' sr e l a x a t i o n . A c c o r d i n g l y , p r o s t h e t i c f i t ( a n d c o m f o r t ) i s u n l i k e l yt o b e a f f e c t e d b y m e a s u r e m e n t m e t h o d s , i e , a l l a re l i k e l y tob e e q u a l l y g o o d i n p r a c ti c e . T e n m i l l im e t e r s o f u n c e r ta i n t y i nm e a s u r e s i s f a r l a r g e r t h a n t h e c a l i br a t e d p r e c i s i o n o f a n y o f t h em e a s u r i n g i n s t r u m e n t s, a n d a l s o s e e m s t o in d i c a t e th a t s e v e r a lm e a s u r e m e n t s a n d a d j u s t m e n t s m a y o f t e n b e n e c e s s a ry b e f o r ea p r o s t h e s i s e x h i b i t s g o o d f i t .

C O N C L U S I O NW e t e s t e d t h e a b i li t y o f a n o p ti c a l s u r f a c e s c a n n e r ( O S S ) a n d

s p i r al x - r a y C T s c a n n e r ( S X C T ) t o p r e c is e l y a n d r e p e a t a b lym e a s u r e r e s i d u a i n b e l o w - k n e e v o l u n t e e r s . T h e r e s u l t s d e m o n -s t r a t e t h a t t h e d i g i t i z e r , O S S , a n d S X C T p r e c i s i o n a n d r e p e a t -a b i l i t y a r e s u f f i c i e n t f o r q u a n t i t a t i v e s t u d i e s . W e c o m p a r e d d i s -t a n c e m e a s u r e m e n t s f r o m c a l i p e r s w i t h t h e d i g i t i z e r , O S S , a n d3 D S X C T s c a n n e r m e a s u r e m e n t s a n d f o u n d su b s t an t i al e q u i v a -l e n c e o f t h e s e m e t h o d s .

6A s i n d i c a t e d b y R u b i n a n d a s s o c i a te s , t h e r e s i d u a l l i m b p h y s -i c a l c h a r a c t e r i s t i c s v a r y w i d e l y , a s s h o w n i n f i g u r e 5 , m a k i n ga r i g i d a p p r o a c h t o p r o s t h e t i c f i t t in g im p r a c t i c a l . O S S a n d S X C Tw i l l n o t o n l y a l l o w t h e p r o s t h e t i s t t o s e e t h e w i d e v a r i a t i o n i ns k i n s u r f a c e g e o m e t r y , b u t S X C T a l l o w s t h e p r o s t h e t i s t t o s e et h e i n t e r n a l s t r u c tu r e o f t h e r e s i d u u m . T h e t i b i a l a n d f i b u l a rg e o m e t r y a l o n g w i t h t h e i r p r o x i m i t y t o t he s k i n s u r f a c e w i l l a i di n o v e r c o m i n g b l i n d f i tt i ng ( n o t k n o w i n g w h a t i s l o c a t e d a n dh o w f a r t h e b o n e i s l o c a t e d b e n e a t h t h e s k i n s u r f a c e) b y t h ep r o s th e t is t , a n d i m p r o v e t h e c u s t o m f i tt i ng p r o c e s s w i t h S X C T ' sa b i l i t y t o c a p t u r e i n t e r n a l 3 D m o r p h o l o g y .

Ac k n o w l ed g m en t s: Th e o rig i n al d esi g n co n cep t o f t h e C en cit , In c . ,h ead o p t i ca l su rface scan n i n g sy st em i s c red i t ed t o Dr. Jo h n Gri n d o n .Th i s wo rk was su p p o rt ed b y t h e Nat i o n al In st i t u t es o f Heal t h / Nat i o n al

Cen ter for Me dical Rehabili tation Research grant RO1 HD30169. W ewi sh t o t h an k Un i v ersa l Vi si o n Part n ers , o wn ers o f t h e C en ci t t ech n o l -o g y , fo r t h e i r su p p o rt . Th e ANALYZE so ft ware sy st em was p ro v i d edb y Dr. R i ch ard R o b b an d Den n i s Han so n o f t h e M ay o B i o med i calIm ag i n g R eso u rce i n R o ch ester , M N . W e ap p reci a te t h e co o p era t i o n o fJames W eb er , Presid en t, Gi l C h ris t ley , C PO , an d M arsh a Kl u n k , C PO ,of the Orthotic and Prosthetic L aboratories, Inc. (St . Louis, MO ) forexper t advice, referral of subjects, and for perform ing the plaster casting.Re f e r e n c e s1 . Wal sh NE, Lan cast er JL, Fau l k n er V, R o g ers WE. A co mp u t eri zedsystem to manufacture prostheses for amputees in develop ing coun-tries. J Prosth O rthot 1989; 1(3):165-81.2. Niels en CC. A survey of amputees: functional le vel and l ife satisfac-tion, information needs, and the pros thetist 's role. J Prosth O rthot1991;3(3):125-9.3 . Ni ch o l as JJ , R o b i n so n LR , Sch u l z R , B l a i r C , Al i o t a R , Hai rs t o nG. P ro b l ems ex p eri en ced an d p ercei v e d b y p rost h et ic p a ti en ts . JProsth Orthot 1993;5(1):36-9.4 . M i l l s t e i n S, B ai n D, Hu n t er GA. A rev i ew o f emp l o y ee p at t ern sof industrial am pu tee s- -F act ors influencing rehabili tation. ProsthOrthot Int 1985;9:69-78.5. Chadderton H C. Cons um er concerns in prosthetics. Prosth O rthotInt 1983;7:15-6.6 . R u b i n G, Fi sch er E , D i x o n M . Prescri p ti o n o f ab o v e-k n ee an d b e-

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