- Email: [email protected]
Socket wear assessment
The Journal of Arthroplasty Vol. 8 No. 4 1993
Socket Wear Assessment A Comparison of Three Different Radiographic Methods Acke Ohlin, MD,* and G6ran
S e l v i k , MD-[-:[:
Abstract: Three different methods to measure wear of total hip sockets from plain radiographs were compared with the resuhs obtained by direct assessment of the internal deformity 0f28 retrieved sockets by means of a coordinate measuring machine. The values obtained by the uni-radiographic, the duo-radiographic, and the radiographic methods based on the change of distance between the centers of the socket and the prosthetic head (CEN) all correlated significantly with the standard obtained by the coordinate measuring machine, provided the sockets were not loose. The most accurate rad!ographic measurements were obtained with the CEN method. A prerequisite for a reliable estimation of socket wear rate is that it be based on observations of radiographically intact sockets. Key words: hip, total hip arthroplasty, socket loosening, socket wear, roentgen stereophotogrammetric analysis, socket deformation.
arthroplasty with those obtained immediately prior to revision, we classified the sockets as loose or intact. Loosening was defined as a migration of 2 m m or m o r e or a n y other change of position, for example, rotation. From these radiographs socket w e a r was calculated according to uni- and duo-radiographic techniques as described by Charnley and Halley 6 (Fig. 1). By calculating the w e a r in 10 cases of nonmigrated sockets twice, the duo-radiographic technique revealed a variation of 20% between measurements. In l 1 of the cases, w e a r was also measured from pairs of radiographs, obtained during the postoperative period and prior to the exchange operation, using a subroutine of the roentgen stereophotogrammetric analysis (RSA), ~3 supplemented with techniques for calculating the center of circular objects. 2 At least 20 dots were marked on the circular or semicircular metal ring of the socket and on the contour of the head (Fig. 2). The coordinates of these dots were digitized on a Wild A8 (lleerbrugg, Switzerland) cartograph m e a s u r e m e n t table supplied with a television camera and m o n i t o r for magnification. The centers of the projected ball a n d socket wire were calculated by m e a n s of a c o m p u t e r program. Wear was calculated as the change in distance between the
Clinical studies on socket wear, based solely on radiographic assessments, 3"s'6 have been criticized for being unreliable. 7"12 The aim of this study was to compare the results obtained with two w e l l - k n o w n and one previously reported, but seldom used, radiographic m e t h o d s - - t h e standard being rite w e a r rate measured directly on the s a m e sockets retrieved at revision.
Patients and Methods Twenty-eight total hip sockets were retrieved dtiring exchange operations for aseptic loosening. There were 22 sockets m a d e from high-density polyethylene and 6 of the Christiansen type, m a d e from polyo x y m e t h y l e n e (Delrin, DuPont, Wilmington, DE). By comparing anteroposterior radiographs of the hips and pelvis obtained shortly after the primary *From Department of Orthopaedics, ]Hahn5 General llospital, Mabn6, Sweden. [From the Department of Anatomy, Lurid University, Lurid, Sweden. eDeceased.
Supported by the Swedish National Board for Technical Development. Reprint requests: Acke Ohlin, MD, Department of Orthopaedics, Maim6 General ltospital, S-214 01 Maim6, Sweden.
427
428
The Journal of Arthroplasty Vol. 8 No. 4 August 1993 b
A
a
b
B Fig. 1. Wear assessment according to the (A) uni-radiographic and (B) duo-radiographic methods. (A) Wear = (a - b)/2 (no correction for magnification). (B) Wear = a - b (correction for often different magnifications ).
Fig. 2. Markings on the circular or semicircular metal wire on the socket and prosthetic ball.
Fig. 3. The probe of the measuring machine (Coordimet 1200, C. E. Johansson, Sweden) attached to the u n w o m socket surface. The ridge between the worn and unworn surface, respectively, is marked.
two centers in the first and last films, corrected for magnification. One standard deviation in this m e a suring process equaled 0.1 ram. This latter radiographic m e t h o d is abbreviated CEN. Deformation of the concave surfaces of the retrieved sockets was assessed with a coordinate m e a suring m a c h i n e (Coordimet 1200, G. E. J o h a n s s o n , Sweden) that has a resolution of 5 p.m (Fig. 3). The socket was placed on modeling clay in a position that allowed the probe to orbit the inner surface of the socket. Ten arbitrary points of both the w o r n and u n w o r n area of this inner surface were measured. T w o partial spheres were thus obtained, and the distance b e t w e e n these two centers denoted the true w e a r (COORD, values obtained with the coordinate measuring m a c h i n e ) - - o u r standard. Sockets in which m o r e t h a n two partial spheres w e r e found, a p h e n o m e n o n probably due to altered positions during service, were excluded from further analysis. Also, the correlation of w e a r with time of service was s t u d i e d . The radiographical m e a s u r e m e n t s (UNI and DUO) were m a d e by the first author (A. O.) in collaboration with a radiologist, whereas the CEN calculations were performed independently by the second author (G. S.). The COORD m e a s u r e m e n t s were performed at the FFV Aerotech, Link6ping, Sweden. Linear correlation coefficients w e r e calculated and covariance analyses were performed.
Socket Table
Wear Assessment
1. S o c k e t W e a r A s s e s s e d b y T h r e e R a d i o g r a p h i c
9
Ohlin and Selvik
429
Methods
P atien t No.
Prosthesis*
Position ( l , U n c h a n g e d ; 2, C h a n g e d )
T i m e of I m p l a n t U s a ge (mo)
UNI
DUO
CEN
COORD
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
3 3 2 1 2 3 3 3 3 2 2 1 2 2 2 2 2 2 2 1 2 2 2 2 2 2 2 2
2 l 2 I 1 2 1 1 2 1 2 1 2 1 2 2 2 2 2 1 1 2 2 1 1 1 2 1
60 96 112 88 126 96 78 74 88 57 65 99 155 176 174 172 169 107 96 168 144 144 142 84 165 100 149 145
4.0 3.5 3.0 0.5 0.5 2.0 3.0 2.5 4.5 0.5 0.5 2.0 2.5 1.5 2.5 1.0 1.0 1.0 6.0 1.0 1.0 1.5 3.0 1.0 1.5 1.0 1.0 --
3.2 3.5 2.3 0.4 0.3 1.5 1.5 2.0 2.3 0.9 !.7 2.0 1.8 1.3 1.5 1.0 1.2 0.8 6.0 0.8 2.0 1.6 3.3 0.0 1.6 0.5 1.7 2.5
2.15 2.22 0.21 0.5 0.76 1.28 1.84 1.36 4.31 0.24 0.58 ------------------
3.40 3.28 2.10 0.90 2.23 1.87 2.92 2.24 2.97 0.87 2.06 1.29 1.35 !.99 !.74 1.64 2.30 1.99 1.70 2.02 2.72 1.53 4.25 0.69 1.74 !.13 2.24 2.38
* 1, Charnley; 2, Lubinus, Bruuswik, C o m p u t e r Assisted Design; 3, Christiansen. UNI, uni-radiographic technique; DUO, d u o - r a d i o g r a p h i c technique; CEN, calculations based on ball a nd socket centers; COORD, values obtained with the coordinate m e a s u r i n g machine.
Results Data on patients, prostheses, time of usage, and values obtained from the different measurements are presented in Table 1. There was a significant but only fair correlation between the COORD values and the values obtained with the uni- and duo-radiographic methods, whereas the CEN values correlated somewhat better (Table 2). ttowever, considering only stable sockets,
T a b l e 2. C o r i ' e l a i i o n V a l u e s B e t w e e n t h e V a r i o u s M e l h o d s f o r W e a r Assessment:=All S o c k e l s I n c l u d e d UNI DUO No. of cases p CEN No. of cases P COORD No. of cases P
0.858 27 <.001 0.789 ll <.01 0.458 27 <.05
Abbreviations same as in Table 1.
DUO
0.527 11 NS 0.489 28 <.01
all correlations increased considerably and the correlation coefficient for the CEN/COORD vo.lues reached 0.94. No significant relationship was found between any of the radiographic values and the COORD values in loosened sockets (Tables 3, 4). In an analysis of covariance between the UNI and COORD values, a similar pattern was seen, with tile exception that the UNI values were relatively higher in loosened sockets (P = .02) (Fig. 4).
T a b l e 3. C o r r e l a t i o n V a l u e s B e t w e e n t h e V a r i o u s M e t h o d s for W e a r A s s e s s m e n t : R a d i o g r a p h i c a l l y Slable Sockets
CEN
0.709 II <.05
UNI DUO No. of cases p CEN No. of cases P COORD No. of cases P
0.802 14 < .001 0.968 6 <.01 0.642 14 <.05
Abbreviations s a me as in Table 1.
DUO
CEN
0.844 <.05 0.692 15 <.01
0.942 6 <.01
430
The Journal of Arthroplasty Vol. 8 No. 4 August 1993
Table 4. Correlation Values Between the Various Methods for Wear Assessment: Radiographically Loosened Sockets
DUO No. of cases P CEN No. of cases P COORD No. of cases P
UNI
DUO
CEN
0.849 13 <.00l 0.724 5 NS 0.297 13 NS
0.327 5 NS 0.289 13 NS
0.685 5 NS
Abbreviations same as in Table 1.
By studying the correlation o f " t r u e w e a r " according to the COORD m e t h o d with time, a good correlation was found regarding the all stable Charnley sockets (r = 0.98; n = 3). Among the polyethylene sockets worn by larger balls (prosthesis types, 2; Table 1) a poor correlation was found (r = 0.34; n = 17), and among the Christiansen sockets (n = 6) no correlation was observed. Howe')er, considering only radiographically stable sockets, the correlations of wear with time increased considerably to 0.74 (n = 8) for polyethylene sockets with a large inner diameter and to 0.84 (n = 3) for the Christiansen type. According to the COORD measurements, the annual wear rate for the radiographically intact sockets of the Christiansen type was 0.43 m m (n = 3), for the Charnley socket it was 0.14 mm (n = 3), and for the large ball polyethylene sockets it was 0.17 m m (n = 8).
Discussion In a theoretical and experimental study Clark et al. 7 concluded that the reliable wear m e a s u r e m e n t cannot be made from plain clinical r a d i o g r a p h s - - t h e main reason being that a poorly defined external
X
9 .5
. 1
.
, 1.5
.
. 2
.
,2.5
.
, 3
-
. 3.5
-
, 4
. 4.5
coord
Fig. 4. The relationship between loosening and wear measuring methods, UNI versus COORD. e, unchanged position; X, changed position of sQcket. UNI, uni-radiographic method; COORD, values obtained with the coordinate measuring machine.
edge of the socket and a varying socket orientation affect the measurements considerably. Also, difficulties in conceiving the wear axis contributed negatively. Some of these difficulties are overcome using the CEN method, but a divergence between the wear axis and the projection plane can, in fact, never be ruled out from plain radiographs. Hence, a good correlation was found between the CEN and COORD methods, the results obtained with the latter method representing " t r u e w e a r " if creep is disregarded. We found that radiographic socket wear could only be measured accurately if the sockets were stable, and also that the values obtained in loose sockets were not only less exact, but were usually overestimated. Unexpectedly, the COORD wear values also were poorly correlated with the time o f service w h e n the whole sample of implants was analyzed. However, the correlation values increased significantly w h e n only stable sockets were taken into consideration. The most plausible explanation is that a variable a m o u n t of b o n e - c e m e n t particles are released into the joint cavity w h e n socket loosening has occurred, resulting in a three-body wear process k n o w n to increase the socket wear rate to a varying extent. ~ The time of service between socket loosening and revision was impossible to determine in each case. Thus, all radiographic socket wear rate calculations are unreliable if radiographically loosened sockets are ineluded. As expected, the uni- and duo-radiographic techniques correlated well with each .other. However, they only showed a fair correlation with the COORD t e c h n i q u e - - o u r s t a n d a r d - - t h e correlations for stable sockets being 0.6 and 0.7, respectively. In an investigation comparing the "true" penetration depth of the ball into its socket with the radiographically measured w e a r (DUO), Wroblewski 14 found an extremely good correlation. However; the standard used was based on an indirect method for assessing wear from acrylic casts by means of a shadow-graph technique. Also, Rimnac et al. ~] considered socket wear assessment (DUO) from plain radiographs, even of loosened sockets, to be justified. The penetration depth of the ball was measured with a Vernier caliper o n slices of the explanted sockets. Afifi and Jacob, 1 utilizing serial radiographs, measured the penetration of the ball relative to the midpoint of the long axis of the ellipse formed by the equatorial metal wire of the socket. In three cases such assessments correlated very well with the n~easurements on silastic casts. The annual wear rate was, admittedly based on low numbers, high for the polyoxymethyline Christiansen socket, which is in line with our previous
Socket Wear Assessment
observations 9"~~ and in accordance with reports for the Charnley socket, 4"~4 and s o m e w h a t higher for the polyethylene sockets with a large inner diameter. The v o l u m e of w o r n p o l y m e r per millimeter of penetration depth increases with bali diameter and therefore the Charnley socket seems preferable from this point of view. We conclude that w e a r rate calculations m a d e by any method are justified only on the condition that they are based on radiographically intact sockets. Uni- and duo-radiographic techniques are acceptable for rough estimations of w e a r from plain radiographs. Wear assessment based on the CEN method, that is, changes in the distance between the center of the ball and the center of the socket seems to be the most accurate noninvasive method. Admittedly, w e a r is calculated only in the planes of the radiograph, but clinical observations on retrieved sockets indicate that this will reveal the most p r o n o u n c e d wear.
Acknowledgments
,:
The authors would like to thank Dr. C.-F. Gentz, Department of Diagnostic Radiology, M a i m 6 General tlospital, Malm6, Sweden for assistance in evaluating the radiographs, and Ake Blomquist, FFV Aerotech, Link6ping, Sweden for technical assistance.
References I. Afifi KF, Jacob HAC: Verschliessmessung bei Hiift-Totalendoprothesen mit poly~'thenpfanne (RCH-IO00) und hartverchromten Protasul-lO Kopf. Z Orthop 119:157, 1981
9
Ohlin and Selvik
431
2. Baldursson tt, Egund N, llansson LI, Selvik G: Instability and wear of total hip endoprostheses determined with roentgen stereophotogrammetry. Arch Orthop Trauma Surg 95:257, 1979 3. Buchhorn U, Willert HG, Semlitsch M, Weber 1I: Dimensionverfinderungen der polyfithen-ttiiftpfannen bei MiJller Hfiftendoprothesen. Z Orthop 122:127, 1984 4. Charnley J: Low friction arthroplasty of the hip. Springer-Verlag, New York, 1979 5. Charnley J, Cupic Z: Tile nine and ten year results of the low-friction arthroplasty of tile hip. Clin Orthop 95:9, 1973 6. Charnley J, Halley DK: Rate of wear in total hip replacement. Clin Orthop 112:170, 1975 7. Clarke IC, Black K, Rennie BS, Amstutz tIC: Can wear in hip total arthroplasties be assessed from radiographs? Clin Orthop 121:126, 1976 8. Dumbleton J: Tribology of natural and artificial joints. Elsevier, Amsterdam, 1981 9. Mathiesen E, Lindgren U, Reinholt F, Sudmann E: Wear of the acetabular socket. Acta Orthop Scand 57: 193, 1986 10. Ohlin A, Persson PG: Christiansen total hip arthroplasty: a radiographic and histologic study. J Arthroplasty 4:207, 1989 I 1. Rimnac CM, Wilson PD, Fuchs MD, Wright TM: Acetabular cup wear in total hip arthroplasty. Orthop Clin North Am 19:631, 1988 12. Rose RM, Radin EL: Wear of polyethylene ill total hip prostheses. Clin Orthop 170:107, 1982 13. Selvik G: A roentgen stereophotogrammetric method for the study of kinematics of the skeletal system. PhD Thesis, Lund University, Lund, Swedc~, 1974 14. Wroblewski BM: Direction and rate of socket wear in Charnley low-friction arthroplasty. J Bone Joint Surg 67B:757, 1985
Socket Wear Assessment A Comparison of Three Different Radiographic Methods Acke Ohlin, MD,* and G6ran
S e l v i k , MD-[-:[:
Abstract: Three different methods to measure wear of total hip sockets from plain radiographs were compared with the resuhs obtained by direct assessment of the internal deformity 0f28 retrieved sockets by means of a coordinate measuring machine. The values obtained by the uni-radiographic, the duo-radiographic, and the radiographic methods based on the change of distance between the centers of the socket and the prosthetic head (CEN) all correlated significantly with the standard obtained by the coordinate measuring machine, provided the sockets were not loose. The most accurate rad!ographic measurements were obtained with the CEN method. A prerequisite for a reliable estimation of socket wear rate is that it be based on observations of radiographically intact sockets. Key words: hip, total hip arthroplasty, socket loosening, socket wear, roentgen stereophotogrammetric analysis, socket deformation.
arthroplasty with those obtained immediately prior to revision, we classified the sockets as loose or intact. Loosening was defined as a migration of 2 m m or m o r e or a n y other change of position, for example, rotation. From these radiographs socket w e a r was calculated according to uni- and duo-radiographic techniques as described by Charnley and Halley 6 (Fig. 1). By calculating the w e a r in 10 cases of nonmigrated sockets twice, the duo-radiographic technique revealed a variation of 20% between measurements. In l 1 of the cases, w e a r was also measured from pairs of radiographs, obtained during the postoperative period and prior to the exchange operation, using a subroutine of the roentgen stereophotogrammetric analysis (RSA), ~3 supplemented with techniques for calculating the center of circular objects. 2 At least 20 dots were marked on the circular or semicircular metal ring of the socket and on the contour of the head (Fig. 2). The coordinates of these dots were digitized on a Wild A8 (lleerbrugg, Switzerland) cartograph m e a s u r e m e n t table supplied with a television camera and m o n i t o r for magnification. The centers of the projected ball a n d socket wire were calculated by m e a n s of a c o m p u t e r program. Wear was calculated as the change in distance between the
Clinical studies on socket wear, based solely on radiographic assessments, 3"s'6 have been criticized for being unreliable. 7"12 The aim of this study was to compare the results obtained with two w e l l - k n o w n and one previously reported, but seldom used, radiographic m e t h o d s - - t h e standard being rite w e a r rate measured directly on the s a m e sockets retrieved at revision.
Patients and Methods Twenty-eight total hip sockets were retrieved dtiring exchange operations for aseptic loosening. There were 22 sockets m a d e from high-density polyethylene and 6 of the Christiansen type, m a d e from polyo x y m e t h y l e n e (Delrin, DuPont, Wilmington, DE). By comparing anteroposterior radiographs of the hips and pelvis obtained shortly after the primary *From Department of Orthopaedics, ]Hahn5 General llospital, Mabn6, Sweden. [From the Department of Anatomy, Lurid University, Lurid, Sweden. eDeceased.
Supported by the Swedish National Board for Technical Development. Reprint requests: Acke Ohlin, MD, Department of Orthopaedics, Maim6 General ltospital, S-214 01 Maim6, Sweden.
427
428
The Journal of Arthroplasty Vol. 8 No. 4 August 1993 b
A
a
b
B Fig. 1. Wear assessment according to the (A) uni-radiographic and (B) duo-radiographic methods. (A) Wear = (a - b)/2 (no correction for magnification). (B) Wear = a - b (correction for often different magnifications ).
Fig. 2. Markings on the circular or semicircular metal wire on the socket and prosthetic ball.
Fig. 3. The probe of the measuring machine (Coordimet 1200, C. E. Johansson, Sweden) attached to the u n w o m socket surface. The ridge between the worn and unworn surface, respectively, is marked.
two centers in the first and last films, corrected for magnification. One standard deviation in this m e a suring process equaled 0.1 ram. This latter radiographic m e t h o d is abbreviated CEN. Deformation of the concave surfaces of the retrieved sockets was assessed with a coordinate m e a suring m a c h i n e (Coordimet 1200, G. E. J o h a n s s o n , Sweden) that has a resolution of 5 p.m (Fig. 3). The socket was placed on modeling clay in a position that allowed the probe to orbit the inner surface of the socket. Ten arbitrary points of both the w o r n and u n w o r n area of this inner surface were measured. T w o partial spheres were thus obtained, and the distance b e t w e e n these two centers denoted the true w e a r (COORD, values obtained with the coordinate measuring m a c h i n e ) - - o u r standard. Sockets in which m o r e t h a n two partial spheres w e r e found, a p h e n o m e n o n probably due to altered positions during service, were excluded from further analysis. Also, the correlation of w e a r with time of service was s t u d i e d . The radiographical m e a s u r e m e n t s (UNI and DUO) were m a d e by the first author (A. O.) in collaboration with a radiologist, whereas the CEN calculations were performed independently by the second author (G. S.). The COORD m e a s u r e m e n t s were performed at the FFV Aerotech, Link6ping, Sweden. Linear correlation coefficients w e r e calculated and covariance analyses were performed.
Socket Table
Wear Assessment
1. S o c k e t W e a r A s s e s s e d b y T h r e e R a d i o g r a p h i c
9
Ohlin and Selvik
429
Methods
P atien t No.
Prosthesis*
Position ( l , U n c h a n g e d ; 2, C h a n g e d )
T i m e of I m p l a n t U s a ge (mo)
UNI
DUO
CEN
COORD
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
3 3 2 1 2 3 3 3 3 2 2 1 2 2 2 2 2 2 2 1 2 2 2 2 2 2 2 2
2 l 2 I 1 2 1 1 2 1 2 1 2 1 2 2 2 2 2 1 1 2 2 1 1 1 2 1
60 96 112 88 126 96 78 74 88 57 65 99 155 176 174 172 169 107 96 168 144 144 142 84 165 100 149 145
4.0 3.5 3.0 0.5 0.5 2.0 3.0 2.5 4.5 0.5 0.5 2.0 2.5 1.5 2.5 1.0 1.0 1.0 6.0 1.0 1.0 1.5 3.0 1.0 1.5 1.0 1.0 --
3.2 3.5 2.3 0.4 0.3 1.5 1.5 2.0 2.3 0.9 !.7 2.0 1.8 1.3 1.5 1.0 1.2 0.8 6.0 0.8 2.0 1.6 3.3 0.0 1.6 0.5 1.7 2.5
2.15 2.22 0.21 0.5 0.76 1.28 1.84 1.36 4.31 0.24 0.58 ------------------
3.40 3.28 2.10 0.90 2.23 1.87 2.92 2.24 2.97 0.87 2.06 1.29 1.35 !.99 !.74 1.64 2.30 1.99 1.70 2.02 2.72 1.53 4.25 0.69 1.74 !.13 2.24 2.38
* 1, Charnley; 2, Lubinus, Bruuswik, C o m p u t e r Assisted Design; 3, Christiansen. UNI, uni-radiographic technique; DUO, d u o - r a d i o g r a p h i c technique; CEN, calculations based on ball a nd socket centers; COORD, values obtained with the coordinate m e a s u r i n g machine.
Results Data on patients, prostheses, time of usage, and values obtained from the different measurements are presented in Table 1. There was a significant but only fair correlation between the COORD values and the values obtained with the uni- and duo-radiographic methods, whereas the CEN values correlated somewhat better (Table 2). ttowever, considering only stable sockets,
T a b l e 2. C o r i ' e l a i i o n V a l u e s B e t w e e n t h e V a r i o u s M e l h o d s f o r W e a r Assessment:=All S o c k e l s I n c l u d e d UNI DUO No. of cases p CEN No. of cases P COORD No. of cases P
0.858 27 <.001 0.789 ll <.01 0.458 27 <.05
Abbreviations same as in Table 1.
DUO
0.527 11 NS 0.489 28 <.01
all correlations increased considerably and the correlation coefficient for the CEN/COORD vo.lues reached 0.94. No significant relationship was found between any of the radiographic values and the COORD values in loosened sockets (Tables 3, 4). In an analysis of covariance between the UNI and COORD values, a similar pattern was seen, with tile exception that the UNI values were relatively higher in loosened sockets (P = .02) (Fig. 4).
T a b l e 3. C o r r e l a t i o n V a l u e s B e t w e e n t h e V a r i o u s M e t h o d s for W e a r A s s e s s m e n t : R a d i o g r a p h i c a l l y Slable Sockets
CEN
0.709 II <.05
UNI DUO No. of cases p CEN No. of cases P COORD No. of cases P
0.802 14 < .001 0.968 6 <.01 0.642 14 <.05
Abbreviations s a me as in Table 1.
DUO
CEN
0.844 <.05 0.692 15 <.01
0.942 6 <.01
430
The Journal of Arthroplasty Vol. 8 No. 4 August 1993
Table 4. Correlation Values Between the Various Methods for Wear Assessment: Radiographically Loosened Sockets
DUO No. of cases P CEN No. of cases P COORD No. of cases P
UNI
DUO
CEN
0.849 13 <.00l 0.724 5 NS 0.297 13 NS
0.327 5 NS 0.289 13 NS
0.685 5 NS
Abbreviations same as in Table 1.
By studying the correlation o f " t r u e w e a r " according to the COORD m e t h o d with time, a good correlation was found regarding the all stable Charnley sockets (r = 0.98; n = 3). Among the polyethylene sockets worn by larger balls (prosthesis types, 2; Table 1) a poor correlation was found (r = 0.34; n = 17), and among the Christiansen sockets (n = 6) no correlation was observed. Howe')er, considering only radiographically stable sockets, the correlations of wear with time increased considerably to 0.74 (n = 8) for polyethylene sockets with a large inner diameter and to 0.84 (n = 3) for the Christiansen type. According to the COORD measurements, the annual wear rate for the radiographically intact sockets of the Christiansen type was 0.43 m m (n = 3), for the Charnley socket it was 0.14 mm (n = 3), and for the large ball polyethylene sockets it was 0.17 m m (n = 8).
Discussion In a theoretical and experimental study Clark et al. 7 concluded that the reliable wear m e a s u r e m e n t cannot be made from plain clinical r a d i o g r a p h s - - t h e main reason being that a poorly defined external
X
9 .5
. 1
.
, 1.5
.
. 2
.
,2.5
.
, 3
-
. 3.5
-
, 4
. 4.5
coord
Fig. 4. The relationship between loosening and wear measuring methods, UNI versus COORD. e, unchanged position; X, changed position of sQcket. UNI, uni-radiographic method; COORD, values obtained with the coordinate measuring machine.
edge of the socket and a varying socket orientation affect the measurements considerably. Also, difficulties in conceiving the wear axis contributed negatively. Some of these difficulties are overcome using the CEN method, but a divergence between the wear axis and the projection plane can, in fact, never be ruled out from plain radiographs. Hence, a good correlation was found between the CEN and COORD methods, the results obtained with the latter method representing " t r u e w e a r " if creep is disregarded. We found that radiographic socket wear could only be measured accurately if the sockets were stable, and also that the values obtained in loose sockets were not only less exact, but were usually overestimated. Unexpectedly, the COORD wear values also were poorly correlated with the time o f service w h e n the whole sample of implants was analyzed. However, the correlation values increased significantly w h e n only stable sockets were taken into consideration. The most plausible explanation is that a variable a m o u n t of b o n e - c e m e n t particles are released into the joint cavity w h e n socket loosening has occurred, resulting in a three-body wear process k n o w n to increase the socket wear rate to a varying extent. ~ The time of service between socket loosening and revision was impossible to determine in each case. Thus, all radiographic socket wear rate calculations are unreliable if radiographically loosened sockets are ineluded. As expected, the uni- and duo-radiographic techniques correlated well with each .other. However, they only showed a fair correlation with the COORD t e c h n i q u e - - o u r s t a n d a r d - - t h e correlations for stable sockets being 0.6 and 0.7, respectively. In an investigation comparing the "true" penetration depth of the ball into its socket with the radiographically measured w e a r (DUO), Wroblewski 14 found an extremely good correlation. However; the standard used was based on an indirect method for assessing wear from acrylic casts by means of a shadow-graph technique. Also, Rimnac et al. ~] considered socket wear assessment (DUO) from plain radiographs, even of loosened sockets, to be justified. The penetration depth of the ball was measured with a Vernier caliper o n slices of the explanted sockets. Afifi and Jacob, 1 utilizing serial radiographs, measured the penetration of the ball relative to the midpoint of the long axis of the ellipse formed by the equatorial metal wire of the socket. In three cases such assessments correlated very well with the n~easurements on silastic casts. The annual wear rate was, admittedly based on low numbers, high for the polyoxymethyline Christiansen socket, which is in line with our previous
Socket Wear Assessment
observations 9"~~ and in accordance with reports for the Charnley socket, 4"~4 and s o m e w h a t higher for the polyethylene sockets with a large inner diameter. The v o l u m e of w o r n p o l y m e r per millimeter of penetration depth increases with bali diameter and therefore the Charnley socket seems preferable from this point of view. We conclude that w e a r rate calculations m a d e by any method are justified only on the condition that they are based on radiographically intact sockets. Uni- and duo-radiographic techniques are acceptable for rough estimations of w e a r from plain radiographs. Wear assessment based on the CEN method, that is, changes in the distance between the center of the ball and the center of the socket seems to be the most accurate noninvasive method. Admittedly, w e a r is calculated only in the planes of the radiograph, but clinical observations on retrieved sockets indicate that this will reveal the most p r o n o u n c e d wear.
Acknowledgments
,:
The authors would like to thank Dr. C.-F. Gentz, Department of Diagnostic Radiology, M a i m 6 General tlospital, Malm6, Sweden for assistance in evaluating the radiographs, and Ake Blomquist, FFV Aerotech, Link6ping, Sweden for technical assistance.
References I. Afifi KF, Jacob HAC: Verschliessmessung bei Hiift-Totalendoprothesen mit poly~'thenpfanne (RCH-IO00) und hartverchromten Protasul-lO Kopf. Z Orthop 119:157, 1981
9
Ohlin and Selvik
431
2. Baldursson tt, Egund N, llansson LI, Selvik G: Instability and wear of total hip endoprostheses determined with roentgen stereophotogrammetry. Arch Orthop Trauma Surg 95:257, 1979 3. Buchhorn U, Willert HG, Semlitsch M, Weber 1I: Dimensionverfinderungen der polyfithen-ttiiftpfannen bei MiJller Hfiftendoprothesen. Z Orthop 122:127, 1984 4. Charnley J: Low friction arthroplasty of the hip. Springer-Verlag, New York, 1979 5. Charnley J, Cupic Z: Tile nine and ten year results of the low-friction arthroplasty of tile hip. Clin Orthop 95:9, 1973 6. Charnley J, Halley DK: Rate of wear in total hip replacement. Clin Orthop 112:170, 1975 7. Clarke IC, Black K, Rennie BS, Amstutz tIC: Can wear in hip total arthroplasties be assessed from radiographs? Clin Orthop 121:126, 1976 8. Dumbleton J: Tribology of natural and artificial joints. Elsevier, Amsterdam, 1981 9. Mathiesen E, Lindgren U, Reinholt F, Sudmann E: Wear of the acetabular socket. Acta Orthop Scand 57: 193, 1986 10. Ohlin A, Persson PG: Christiansen total hip arthroplasty: a radiographic and histologic study. J Arthroplasty 4:207, 1989 I 1. Rimnac CM, Wilson PD, Fuchs MD, Wright TM: Acetabular cup wear in total hip arthroplasty. Orthop Clin North Am 19:631, 1988 12. Rose RM, Radin EL: Wear of polyethylene ill total hip prostheses. Clin Orthop 170:107, 1982 13. Selvik G: A roentgen stereophotogrammetric method for the study of kinematics of the skeletal system. PhD Thesis, Lund University, Lund, Swedc~, 1974 14. Wroblewski BM: Direction and rate of socket wear in Charnley low-friction arthroplasty. J Bone Joint Surg 67B:757, 1985