the use of roentgen stereophotogrammetry to study micromotion of

The use of Roentgen stereophotogrammetry to study

micromotion of orthopaedic implants

Edward R. Valstar a,*, Rob G.H.H. Nelissen a, Johan H.C. Reiber b, Piet M. Rozing a

aDepartment of Orthopaedics, Leiden University Medical Center, P.O. Box 9600, 2300 RC Leiden, The NetherlandsbDivision of Image Processing, Department of Radiology, Leiden University Medical Center, P.O. Box 9600, 2300 RC Leiden, The Netherlands

Received 31 October 2001; accepted 3 May 2002

Abstract

Roentgen stereophotogrammetry is the most accurate Roentgen technique for three-dimensional assessment of micromotion

of orthopaedic implants. The reported accuracy of Roentgen Stereophotogrammetric Analysis (RSA) ranges between 0.05 and

0.5 mm for translations and between 0.15j and 1.15j for rotations. Because of the high accuracy of RSA, small patient groups

are in general sufficient to study the effect on prosthetic fixation due to changes in implant design, addition of coatings, or new

bone cements. By assessing micromotion of a prosthesis in a short-term (i.e. 2 years) clinical RSA study, a prediction can be

made on the chance of long-term (i.e. 10 years) loosening of the prosthesis. Therefore, RSA is an important measurement tool to

screen new developments in prosthetic design, and to prevent large groups of patients from being exposed to potentially inferior

designs. In this article, the basics of the RSA technique are explained, and the importance of clinical RSA studies is illustrated

with two examples of clinical RSA studies which RSA delivered very valuable information. Thereafter, two recent

developments in RSA that have been implemented at Leiden University Medical Center are presented: digital automated

measurements in RSA radiographs and model-based RSA.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Roentgen stereophotogrammetry; model-based RSA; motion; orthopaedic implants; X-ray; medical imaging

1. Introduction

Artificial joint replacement is a common treatment

for joints that have been affected by trauma, artrosis,

or rheumatoid arthritis. Worldwide, 1 million total hip

replacements and 500,000 total knee replacements are

performed every year. The maximum lifespan of a

prosthesis will be about 15 to 20 years.

Unfortunately, some prostheses have to be revised

before the expected maximum lifespan. This may be

necessary when the prosthesis has worn out, or when

the prosthesis has loosened with respect to the sur-

rounding bone.

In general, loosening starts with progressive micro-

motion, in the range of 0.2–1 mm, of the prosthesis

relative to the surrounding bone. Once it has started, it

is a continuous process that will destroy bone, and as

a result, the prosthesis will start to migrate over larger

distances. At present, this process of prosthetic loos-

ening and bone destruction can only be stopped by

revision of the prosthesis.

0924-2716/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0924-2716 (02 )00064 -3

* Corresponding author. Tel.: +31-71-526-2975; fax: +31-71-

526-6743.

E-mail address: [email protected] (E.R. Valstar).

www.elsevier.com/locate/isprsjprs

ISPRS Journal of Photogrammetry & Remote Sensing 56 (2002) 376–389

A revision operation is much more demanding for

the patient than the implantation of a primary pros-

thesis, and the results are inferior to those of primary

prostheses: many patients suffer from pain, a

decreased range of motion, and the loosening rate of

revision prostheses is much higher than for primary

prostheses.

Therefore, it is of utmost importance to develop

prostheses that have a long lifespan. Since loosening

of a prosthesis starts with micromotion, knowledge

about micromotion is important, as it may predict

future loosening (Karrholm et al., 1994; Ryd et al.,

1995). By studying micromotion, one can obtain

insight in the loosening process of prostheses, which

can be used to improve prostheses.

In clinical practice, loosening of prostheses is

assessed indirectly at successive radiographs by meas-

uring radiolucent—dark—lines around the prosthesis

and assessing positional differences of the prosthesis

relative to the bone. Radiolucent lines indicate the

presence of a fibrous layer around the prosthesis that

is always present when a prosthesis is loose. In Fig. 1

some basic measurements in conventional radiographs

for assessment of prosthesis position and orientation

are indicated.

These measurements are not very accurate: radio-

lucency may occur in areas that are overprojected by

the metal of the implant and thus might not be

observed: as a result, the amount of radiolucency may

be underestimated (Nelissen, 1995; Reading et al.,

1999). Migration of the prosthesis is assessed by

measuring changes in the relative positions of its

prosthetic landmarks and the bony landmarks over

time. However, bony landmarks are not sufficiently

distinctive and are therefore difficult to measure in a

reproducible manner. For these reasons, measurements

on plain radiographs are not accurate. In total hip

arthroplasty, for example, migration measurements

may have an accuracy between 5 and 12 mm (95%

Fig. 1. (a) A conventional radiograph of a pelvis with two hip

prostheses. These hip prostheses consist of a femoral stem and a cup.

The circles indicate the head of the femoral stem. The cup on the right

hand is still in place. The cup on the left hand side, however, has

penetrated the pelvis over a distance d, a distance of approximately 20

mm. The angle of inclination of the cup is indicated by a. This angleshould ideally be 45j; in this patient, the angle has decreased to 30j.This cup will have to be revised. (b) A conventional radiograph of the

femur of the same patient as in (a). The femoral stem of the prosthesis

has subsided over distance d, approximately 18 mm. The two

approximate vertical lines in the image indicate the central lines of the

femur and the prosthesis. Ideally, the angle between these two lines, a,should be 0j. In this case, the angle is 20j. This prosthesis component

will also have to be revised.

E.R. Valstar et al. / ISPRS Journal of Photogrammetry & Remote Sensing 56 (2002) 376–389 377

confidence interval), depending upon the choice of

bony landmarks (Malchau et al., 1995).

Several attempts have been made to increase the

accuracy of measurements in plain radiographs.

Improvements have been made by the standardisation

of the position of the patient, by the use of additional

landmarks, and by the use of software that performs

the measurements in a reproducible and objective

manner (Hardinge et al., 1991). A measurement

technique that combines these three improvements is

the Einzel Bild Roentgen Analyse that has an accuracy

of 1 mm (Krismer et al., 1995). However, to measure

sub-millimetre micromotion of implants during the

first half-year after implantation or to detect differ-

ences between migration patterns of small groups of

implants, this accuracy will not be sufficient.

Therefore, in 1974, Selvik (1989) developed a

highly accurate technique for the assessment of

three-dimensional migration of prostheses, Roentgen

Stereophotogrammetric Analysis (RSA). The reported

accuracy of RSA ranges between 0.05 and 0.5 mm for

translation and between 0.15j and 1.15j for rotations

(95% confidence interval; Karrholm, 1989).

In this article, the basics of the RSA technique will

be explained, and the importance of clinical RSA

studies is illustrated with two examples of clinical

RSA studies in which RSA delivered very valuable

information. Thereafter, two recent developments in

RSA that have been implemented at Leiden Univer-

sity Medical Center will be introduced: one concern-

ing digital automated measurements in RSA

radiographs and one concerning model-based RSA.

Finally, conclusions are drawn and some recommen-

dations for future research are given.

2. Basics

RSA is the most accurate Roentgen technique for

the assessment of micromotion of orthopaedic

implants. To each that accuracy, however, several

steps have to be taken that make the technique rather

complicated in a clinical setting.

2.1. Bone and prosthesis markers

To accurately measure migration in RSA radio-

graphs, bony landmarks are not sufficiently distinc-

tive. In order to obtain well-defined measurement

points, tantalum beads are inserted into the bone with

a special insertion instrument. These beads have a

diameter of 0.5, 0.8, or 1 mm. Due to their small size

and spherical shape, their projection will not be

influenced by changes in patient position or Roentgen

focus position. Therefore, the position of these

markers can be measured with great accuracy. Since

most prostheses do not have landmarks that can be

measured in a reproducible manner either, they have

to be marked with at least three non-collinear markers

(Fig. 2).

2.2. Roentgen set-up

In RSA, two synchronised Roentgen tubes are used

to obtain two projections of an area of interest of a

patient. Using the information in these two projec-

tions, it is possible to reconstruct the three-dimen-

sional position of markers in that area. The two

Roentgen tubes are positioned at approximately 1.60

m above the Roentgen cassette at a 20j-angle to the

Fig. 2. The Interax total knee prosthesis consists of two components:

one component that is inserted in the femur and one that is inserted in

the tibia. Three 2-mm markers have been attached to the tibial

component, and seven markers have been inserted in the tibia.

E.R. Valstar et al. / ISPRS Journal of Photogrammetry & Remote Sensing 56 (2002) 376–389378

vertical (Fig. 3a and b). A calibration box is used to

calibrate the Roentgen set-up. The calibration box that

we mainly use in our studies has two planes that hold

tantalum markers of which the three-dimensional

position is accurately known. The markers in the

plane close to the radiographic film are denoted

fiducial markers, the markers in the plane distant from

the radiographic film are denoted control markers.

The fiducial markers define the three-dimensional

fiducial coordinate system and the control markers

are used to assess the position of the Roentgen foci.

2.3. Measurement of two dimensional marker projec-

tions

After the RSA radiograph has been taken (Fig. 4),

the coordinates of the bone and prosthesis markers

have to be measured accurately. In conventional RSA

research, this was done using a manually operated

measuring table with an accuracy of 0.02 mm. Manual

digitisation of marker coordinates is a rather tedious

task that might take up to 45 min per film. Therefore,

software has been developed that automates this task,

and drastically reduces analysis time. This digitally

automated approach to RSAwill be described in more

detail in Section 4.

2.4. Calibration

2.4.1. Transformation from radiographic coordinates

to fiducial coordinates

In order to be able to calculate the three-dimen-

sional positions of markers and landmarks on the

implant, the measured coordinates have to be trans-

formed to the lower plane of the calibration box

(fiducial plane). This is done with these two equa-

tions:

xfid;i ¼l1xrad;i þ l2yrad;i þ l3

l7xrad;i þ l8yrad;i þ 1i ¼ 1; . . . ; n; ð1Þ

yfid;i ¼l4xrad;i þ l5yrad;i þ l6

l7xrad;i þ l8yrad;i þ 1i ¼ 1; . . . ; n; ð2Þ

where, (xfid,i,yfid,i) are two-dimensional fiducial coor-

dinates of a point, (xrad,i,yrad,i) are two-dimensional

Fig. 3. (a) The RSA set-up. Two synchronised Roentgen tubes are positioned above a calibration box. (b) The joint of interest is positioned at the

intersection of both Roentgen bundles.


radiographic coordinates of a point, and n is the

number of points. The l-parameters are assessed by

using the known positions of the fiducial markers on

the calibration box and their measured projections on

the radiograph. At least four non-linear fiducial

markers and their projection are needed to calculate

the l-parameters.

The parameters are first estimated in a linear

manner. Therefore, Eqs. (1) and (2) are rewritten, so

that they are linear for the l-parameters:

xfid ¼ l1xrad þ l2yrad þ l3 � l7xradxfid � l8yradxfid; ð3Þ

yfid ¼ l4xrad þ l5yrad þ l6 � l7xrad yfid � l8yrad yfid: ð4Þ

The solution of this problem may be found by

performing a QR-decomposition (Golub and Van

Loan, 1989).

This linear estimate is used as a start point for

a non-linear Newton–Gauss algorithm (Dennis

and Schnabel, 1983). The cost function that is used

is:

J ¼ ete; ð5Þ

where, e is an error vector that may be written as:

e ¼

l1xrad;1þl2yrad;1þl3l7xrad;1þl8yrad;1þ1

� xfid;1


� xfid;2

]

l1xrad;nþl2yrad;nþl3l7xrad;nþl8yrad;nþ1

� xfid;n


� yfid;1


� yfid;2

]

l4xrad;nþl5yrad;nþl6l7xrad;nþl8yrad;nþ1

� yfid;n

266666666666666666666666664

377777777777777777777777775

: ð6Þ

Fig. 4. An RSA radiograph of a total knee prosthesis from two synchronised Roentgen tubes. Note the markers in the bone and the markers

attached to the prosthesis (see Fig. 2). Markers that are located outside the bone of the patient are calibration box markers that are used to define

a three-dimensional laboratory coordinate system, and to assess the Roentgen foci positions.


The error vector e and the cost function J are a

function of the l-parameters. In order to determine the

search direction for the optimisation, the Jacobean has

to be determined:

Jac ¼ BJ

Be

Be

Bl: ð7Þ

The expression for the Newton–Gauss optimisation

is:

lnew ¼ lold � ðJactJacÞ�1ðJacteÞ; ð8Þ

where, lnew holds the l-parameters that are the result

of the current optimisation step and lold is the result

of the previous optimisation step. After the l-

parameters have been assessed, points that have

been measured in the radiograph are transformed

to the fiducial coordinate system by means of Eqs.

(1) and (2).

The quality of this transformation is expressed as

the distance between the fiducial markers and their

transformed projections.

2.4.2. Calculation of foci positions

The next calibration step is the assessment of

position of both Roentgen foci. For this assessment,

the markers in the upper plane of the calibration

box, the control markers, are used. Through these

markers (ci) and their transformed projections (ciV),projection lines are determined:

riðaiÞ ¼ ci þ aiðci � ciVÞ�l < ai < l i ¼ 1; . . . ; n; ð9Þ

where n indicates the number of markers that are

used. In the ideal situation, these lines will intersect

in one point. However, measurement errors occur

and the position of the focus f has to be determined

by solving the least squares problem (Soderkvist,

1990):

minf ;ai

N

ðc1V� c1Þ 0 . . . 0 I3

0 ðc2V� c2Þ ] I3

] O 0 ]

0 . . . 0 ðcnV� cnÞ I3

2666666664

3777777775

�

a1

a2

]

an

f

26666666666664

37777777777775

�

c1

c2

]

cn

2666666664

3777777775N; ð10Þ

that is solved by a QR-decomposition (Golub and

Van Loan, 1989).

2.4.3. Calculation of three-dimensional marker posi-

tions

The determination of the three-dimensional posi-

tion of the tantalum markers is similar to the deter-

mination of the focus position. The projections of the

marker in the two images are denoted by t1 and t2. The

position of the two Roentgen foci is denoted by f1 and

f2. The equations of the projection lines that connect

the transformed projections and the corresponding

foci are:

l1ðaÞ ¼ f1þ að f

1� t1Þ �l < a < l;

l2ðbÞ ¼ f1þ bð f

2� t2Þ �l < b < l: ð11Þ

The three-dimensional position of the marker is

the position where these two lines intersect. How-

ever, due to measurement errors, the lines will not

intersect but they will cross each other at a short

distance. The three-dimensional position of the

marker p is assumed to be at the middle of the

shortest line connecting the two projection lines.

The length of this shortest line is denoted crossing

linear error. The three-dimensional position of p is


the solution of the least squares problem (Soderkvist,

1990):

minp;a;b

N

ðt1 � f1Þ 0 I3

0 ðt2 � f2Þ I3

264

375

a

b

p

26666664

37777775�

f1

f2

264

375N;

ð12Þ

that is dissolved by a QR-decomposition (Golub

and Van Loan, 1989).

2.5. Motion of rigid bodies

After the three dimensional position of the markers

has been assessed, the relative motion of the prosthe-

sis with respect to the bone can be calculated. The

bone markers function as a reference rigid body

relative to which the motion of the second rigid body,

the prosthesis, is calculated. Since patients cannot be

positioned in exactly the same position and orientation

between follow-ups, the position and orientation of

this reference rigid body change between follow-ups.

In order to obtain the same position and orientation of

the bone, the bone markers in the first radiograph and

in the follow-up radiographs are matched onto each

other; thereafter, the relative motion of the prosthesis

with respect to the bone can be calculated. The results

from the motion calculations are a rotation matrix and

a translation vector.

2.5.1. Calculation of rotation matrix and translation

vector

Assume that we have n points in a rigid body and

let a1,. . .,an be the positions of these points on

instance one and let b1,. . .,bn be the positions on

instance two. In order to assess a rotation matrix M

and a translation vector d, the following equation has

to be solved:

minM ;d

Xni¼1

NMai þ d � biN2; ð13Þ

so that M is an orthogonal matrix.

This problem may be solved in several ways

(Spoor and Veldpaus, 1980; Veldpaus et al., 1988),

but the most elegant way to solve this problem has

been described by Soderkvist (1990). This method

that is based on singular value decomposition is

presented in this section. M is the orthogonal rotation

matrix and d is the translation vector. An expression

for d is Soderkvist, 1990:

d ¼ 1

n

Xni¼1

ðbi �MaiÞ ¼ b�Ma: ð14Þ

When this expression is substituted in expression (13),

the only unknown remains M:

minM

Xni¼1

NMðai � aÞ � ðbi � bÞN2: ð15Þ

When we define A=[a1�a,. . .,an�a] and B=[b1�b,. . .,bn�b], the problem may be written as:

minM

NMA� BN; ð16Þ

so that M is an orthogonal matrix.

The solution of the rotation matrix is:

M ¼ UV t; ð17Þ

in which

BAt ¼ UAV t ð18Þ

is the singular value decomposition. The solution of d

is found when M is substituted in Eq. (14).

2.5.2. Relative motion

In clinical RSA studies, one is often interested in

the motion of an implant with respect to the surround-

ing bone. This means that we are interested in the

relative motion between two rigid bodies. When these

rigid bodies are denoted A and B and the relative

motion is calculated between time t0 and t1, the

relative motion may be calculated as (Soderkvist,

1990):

At1cMAAt0 þ dAut and Bt1cMBBt0 þ dBu

t; ð19Þ


where, ut=[1,. . .,1]. The relative rotation may be

expressed as:

Mrel ¼ MtAMB: ð20Þ

After the origin o is positioned in the geometrical

centre of At0:

o ¼ 1

nA

XnAi¼1

At0;i; ð21Þ

the relative translation may be expressed as:

drel ¼ MtAðdb � dAÞ þ ðMrel � I3Þo; ð22Þ

where, I3 is a unity matrix. The translation vector drelthat has been calculated with Eq. (22) might be

difficult to understand for the clinician. Therefore,

the difference in position of the geometrical centre of

a rigid body at two points in time is often presented

instead of drel.

3. Clinical studies

Because of the high accuracy of RSA, small patient

groups are in general sufficient to study the effect on

prosthetic fixation due to changes in implant design,

addition of coatings to the prosthesis, or new bone

cements. RSA has been applied in many studies that

were primarily conducted in Sweden and in the other

Scandinavian countries. More than 3000 patients have

been included in several studies and more than 150

scientific papers have been published (overview in:

Karrholm, 1989; Ryd, 1992). Topics that have been

studied by RSA are: prosthetic fixation, joint stability

and joint kinematics, fracture stability, skeletal

growth, vertebral motions, and spinal fusion.

3.1. The Boneloc cement disaster

The importance of evaluating new developments in

small patient groups before a mass introduction on the

market can be illustrated by the introduction of Bone-

loc cement (Biomet, Warsaw, IN, USA) in 1991.

Cement is used to fixate a prosthesis in the bone.

Bone cement is a polymer that is made by mixing a

liquid monomer and a powder. During the polymer-

isation process that follows mixing, the polymer is

formed and heat is created: the advantage of Boneloc

cement was the lower polymerisation temperature, 43

jC instead of 80 jC in traditional cements. This lower

temperature was expected to reduce local cell death

and a better bone-cement interlock would be obtained,

thus improving the fixation of prostheses.

However, in clinical practice, a deterioration in

prosthetic fixation was observed: several clinics

reported loosening of implants after using Boneloc

cement. As a consequence, two clinical RSA studies

were started: a total knee study with 19 patients

(Nilsson and Dalen, 1998) and a total hip study with

11 patients (Thanner et al., 1995). These RSA studies

supported the clinical observations: implants fixated

with Boneloc migrated significantly more than

implants fixated with conventional cement. Therefore,

Boneloc cement leads to an increased risk for revision

caused by aseptic loosening.

Unfortunately, at that point in time, Boneloc had

been used in more than 1000 cases in Norway alone.

After a period of 4.5 years, the revision rate of the

prostheses was 14 times higher than for prostheses

fixed with conventional cement (Furnes et al., 1997).

This might have been prevented by a pre-marketing

clinical test of the fixation with an adequate RSA

study.

3.2. The effect of hydroxyapatite on fixation of knee

prostheses

There are two approaches for fixing prostheses in

bone. In the first approach, cement is used to form a

strong interdigitation between the bone and the pros-

thesis. In the second approach, the prosthesis is intro-

duced into a preformed cavity in the bone that exactly

matches the surface of the prosthesis; bone ingrowth

into the prosthetic surface will provide fixation. Bone

ingrowth can be stimulated with special surface coat-

ings that are sprayed onto the prosthetic surface. One of

these coatings is hydroxyapatite.

At Leiden University Medical Center, a prospec-

tive, randomised, double-blind study was executed to

evaluate three different means of fixing tibial (lower

leg bone) components to the bone in knee replacement

(Nelissen et al., 1998). The aim was to study the effect

of the addition of hydroxyapatite on the fixation of


uncemented knee prostheses. Eleven prostheses fixed

with cement, 10 hydroxyapatite-coated prostheses

fixed without cement, and 10 non-coated prostheses

fixed without cement were studied. RSA was used to

assess micromotion of the components during a 2-year

follow-up period.

With this small cohort of patients and within the

short follow-up period of 2 years, cemented tibial

components and hydroxyapatite-coated tibial com-

ponents fixed without cement were found to have far

less micromotion along the three orthogonal axes

as compared to non-coated tibial components fixed

without cement: At the 2-year follow-up evaluation,

the subsidence of the non-coated components was

�0.73F0.924 mm, the subsidence of the cemented

components was�0.05F0.109mm, and of the hydrox-

yapatite-coated components �0.06F0.169 mm. So,

after 2-year follow-up, the subsidence of the non-

coated components was significantly larger than the

subsidence of the other two groups. Because of its

small size, this difference could not have been detected

with conventional Roentgen measurement.

In conclusion, micromotion of hydroxyapatite-

coated tibial components fixed without cement was

similar to that of tibial components fixed with cement.

Therefore, hydroxyapatite, a biological mediator, may

be necessary for the adequate fixation of tibial com-

ponents when cement is not used. The orthopaedic

company selling the prosthesis took the outcome of

the study very seriously; the promotion of the non-

coated cementless prosthesis was terminated.

4. Recent developments in RSA

4.1. Digital RSA

A disadvantage of conventional film-based RSA is

that it requires a lot of user interactions. In each

radiograph measurement, points have to be labelled.

Subsequently, the coordinates of all of these points

have to be measured manually using a highly accurate

measuring table.

In order to reduce the total analysis time of RSA

radiographs, a software package has been developed

that is able to perform the measurements of the

coordinates automatically in digital RSA images

(RSA Clinical Measurement System (RSA-CMS),

firm Medical Imaging Systems (MEDIS), Leiden,

The Netherlands). RSA-CMS (Vrooman et al., 1998;

MEDIS, 2000) can handle scanned conventional

radiographs or direct digital radiographs Digital Imag-

ing and Communications in Medicine in (DICOM)

format: a worldwide standard for digital images in

medicine. This software package runs on a PC with

the Windows NT operating system (Fig. 5).

RSA-CMS utilises image processing algorithms

developed specifically for the automatic detection

and identification of RSA markers, i.e. the calibration

markers, the bone markers, and the prosthesis markers

in RSA radiogrphs. The positions of the marker

centres are automatically determined by using an

extension and improvement of the circle finding

algorithm described by Duda and Hart (1972). After-

wards, the marker positions are enhanced to sub-pixel

accuracy by estimating a paraboloid through the grey

scale profile of the projected markers (Vrooman et al.,

1998). So, not only information of the contour pixels

of the marker is used, but the information of pixels in

the projected area is used for an optimal result.

By means of a fitting algorithm, the calibration

markers—fiducial markers and control markers—are

extracted from the total group of markers and auto-

matically labelled. Furthermore, the software mat-

ches markers in the two radiographs, which make a

stereo pair, reconstructs the spatial coordinates of the

markers, and finally calculates the micromotion of

the endoprosthesis. With RSA-CMS, the RSA pro-

cedure can be performed fully automatically. If

necessary, the user can interactively correct inter-

mediate results for misdetections such as artefacts in

the film surface, numbers in the patient tag, wires,

and screws.

The accuracy of the digital RSA system was

compared to the accuracy of a manually operated

RSA system from Sweden (Win-RSA, Tilly, Lund,

Sweden). For this purpose, we used radiographs of a

phantom and radiographs of patients that were pro-

vided by the Lund University Hospital, Lund, Sweden

(Valstar et al., 2000). In a phantom experiment, the

manually operated system produced significantly bet-

ter results than the digital system, although the max-

imum difference between the median values of the

manually operated system and the digital system was

as small as 0.013 mm for translation and 0.033j for

rotations. These slightly less accurate results were


probably caused by the film digitiser that was used. In

radiographs of patients, a better scanner was used and

the manually operated system and the digital system

produced equally accurate results: no significant dif-

ferences were found. Again, it was demonstrated that

digital RSA provides a high accuracy.

During the course of our project, digital RSA

systems were also under development at several

other institutions. The first results of digital RSA

were published by Østgaard et al. (1997). In this

semi-automatic system, the positions of markers have

to be indicated manually and the software refines

those positions. Only one validation study was

carried out with this system, but it was never used

in a clinical setting. In Oxford, a digital RSA-system

was developed (Gill et al., 1998) that has been used

in a clinical setting (Alfaro-Adrian et al., 1999). The

Oxford system is also able to assess the position and

orientation of hip implants by using prosthetic land-

marks (Turner-Smith and Bulstrode, 1993). Another

development is the digital measurement module that

was created for the commercially available UmRSA

system (RSA Biomedical Innovations, Umea, Swe-

den). An increase of the accuracy of this digital

module was reported in a comparative study with the

manually operated implementation (Borlin, 1997).

For another commercially available RSA system,

the WinRSA-system (Tilly Medical Products, Lund,

Sweden), a digital measurement module was recently

developed.

Fig. 5. Screen layout of the RSA-CMS software with an analysed RSA radiograph of a hip prosthesis. Note the different labels for fiducial,

control, bone, and prosthesis markers and the lines that connect the corresponding markers in the two radiographs.


No results of this system have yet been published.

Both systems—UmRSA and WinRSA—operate in a

semi-automatic manner, whereas RSA-CMS has the

advantage that it is fully automatic, i.e. the software

finds the markers without any user interaction. The

development of all these digital RSA-systems dem-

onstrates that there is a definite need for RSA-systems

that are faster and easier to use than conventional RSA

systems.

4.2. Model-based RSA

Attaching tantalum markers to prostheses is a

prerequisite for RSA but it may be difficult and is

sometimes even impossible. Furthermore, marking of

implants is an expensive procedure and in some

countries only allowed by the regulatory bodies after

extensive testing and comprehensive documentation.

Therefore, we developed a model-based RSA

method that uses a triangulated surface model of

the prosthesis (Fig. 6). A projected contour of this

model is calculated and this calculated model con-

tour is matched onto the detected contour of the

actual implant (Fig. 7a) in the RSA radiograph (Fig.

Fig. 6. The solid model of the Interax femoral component together

with its meshed representation.

Fig. 7. The optimisation process for model-based RSA illustrated with the interax tibial component: (a) After the region of interest has been

determined by the observer, the contour of the prosthesis is automatically detected by means of the Canny operator. (b) The first estimation of the

position and orientation of the prosthesis model is projected onto the radiograph. (c) An intermediate result of the optimisation procedure: the

overlap of both contours is increasing. (d) The final result of the optimisation procedure: an optimum overlap of both contours has been obtained.


7b). The difference between the two contours is

minimized by variation of the position and orienta-

tion of the model (Fig. 7c). When a minimal differ-

ence between the contours is found, an optimal

position and orientation of the model has been

obtained (Fig. 7d).

The method was validated by means of a phantom

experiment (Valstar et al., 2001). The phantom con-

sisted of a Plexiglas cylinder with 12 1-min spherical

markers embedded in its surface. Three prosthesis

components were used in this experiment: the fem-

oral and tibial component of an Interax total knee

prosthesis (Stryker Howmedica Osteonics, Ruther-

fort, USA) and the femoral component of a Profix

total knee prosthesis (Smith and Nephew, Memphis,

USA). For each experiment, one of the components

was rigidly attached to the base plane of this

cylinder. The calculated model position and orienta-

tion were compared to the position and orientation of

the cylinder. Since the actual motion between the

prosthesis and cylinder was zero, any change in

relative position indicates an error in the assessment

of the micromotion parameters by the model-based

RSA method.

For the prosthetic components used in this study,

the accuracy of the model-based method was found to

be lower than the accuracy of traditional RSA. For the

Interax femoral and tibial components, significant

dimensional tolerances were found that were probably

caused by the casting process and manual polishing of

the components surfaces. For these components, sys-

tematic errors were found for the translations and the

rotations. The largest standard deviation for any trans-

lation was 0.19 mm and for any rotation it was 0.52j.For the Profix femoral component that had no large

dimensional tolerances, the largest standard deviation

for any translation was 0.22 mm and for any rotation it

was 0.22j.From this pilot study, we may conclude that the

accuracy of the current model-based RSA method is

sensitive to dimensional tolerances of the implant. We

aim at improvement of this model-based RSA method

so that it will be insensitive to large dimensional

tolerance and that it will provide an accuracy that is

comparable to the accuracy of traditional RSA. Cur-

rently, these improvements to the model-based RSA

method are subject of research that is carried out at

our department.

5. Conclusion

RSA is a highly accurate, but rather complicated

measurement technique for the assessment of micro-

motion of prostheses. Since progressive micromotion

is an important indicator for inadequate fixation (i.e.

loosening) of prostheses, and extensive short-term

micromotion might indicate a future revision opera-

tion of the prosthesis, clinical RSA studies are impor-

tant. This is illustrated by the outcomes of clinical

RSA studies after the introduction of Boneloc cement,

and in the clinical RSA study in which we studied the

effect of hydroxyapatite on the fixation of knee

prostheses. With conventional Roentgen techniques,

the negative results found in these studies could not

have been obtained in such a short evaluation period,

and with such a small number of patients involved. By

performing clinical RSA studies, a lot of unnecessary

suffering may be prevented.

At Leiden University Medical Centre (LUMC),

simplification of the RSA technique by automation

of the measurements and the introduction of model-

based RSA are two important research topics. Vali-

dation test proved that the novel automated RSA

system has a high accuracy, and subsequently, it is

now used in several clinical RSA studies. However,

when comparing model-based RSA to conventional

RSA, this new technique was less accurate when

implants with large dimensional tolerances were

used. The model-based RSA method needs to be

modified in order to obtain a high accuracy for these

implants. These modifications are currently subject

of research.

6. Future research

Although the current RSA-software functions

properly, its functionality should be extended. With

the current RSA-system, markers are detected with

sub-pixel accuracy. This is done by estimating of a

paraboloid through the grey-scale distribution of a

projected marker. This sub-pixel accuracy can only be

obtained when markers have a uniform background.

Since these requirements are not always fulfilled in

clinical practice, the development of a model for

markers with a non-uniform background needs to be

pursued (Borlin, 1997).


A better integration of RSA-CMS with Picture

Archiving and Communication Systems (PACS)

should be considered in order to take full-advantage

of the ongoing digitisation and integration of digital

radiology. Currently, there are several DICOM stand-

ards (i.e. for angiography, computed tomography,

magnetic resonance imaging), but a DICOM standard

for RSA still has to be defined. Such a DICOM

standard will make the exchange of RSA-radiographs

between hospitals feasible.

As said before, the model-based RSA-method

needs to be improved to become an alternative for

conventional RSA. One could consider a technique

that omits badly scaled parts of the implant from the

optimisation procedure or a technique that uses addi-

tional three-dimensional measurements on the surface

of each implant in order to scale the model to match

the individual implant. Currently, these modifications

are subject of a study that is being carried out at our

department.

Although in two clinical RSA studies, a correlation

between the short-term micromotion results and the

long-term revision rate caused by aseptic loosening

was demonstrated (Karrholm et al., 1994; Ryd et al.,

1995), more research is needed to fully understand

this relation. Therefore, it is important to extend the

follow-up of patients that have been included in short-

term RSA-studies. Only then, the importance of

performing small clinical RSA-studies can be vali-

dated and can an extrapolation of short-term (i.e. 2

years) RSA micromotion results to long-term (i.e. > 10

years) clinical results be fully justifies. In this per-

spective, we would recommend that a database is set-

up with results of all clinical RSA studies. This

database could be used as register for objective results

on prosthesis fixation.

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