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Gait affecs tibial component fixation
The Journal of Arthroplasty Vol. 14 No. 5 1999
Gait Affects Tibial C o m p o n e n t F i x a t i o n M a r i a B. H i l d i n g , M D , P h D , * L e i f R y d , M D , P h D , t S 6 r e n T o k s v i g - L a r s e n , A l i s o n M a n n , RPT, t a n d A n d e r s S t e n s t r 6 m , M D , PhD-[-
M D , P h D , l-
Abstract: The prognosis of fixation in patients with total knee arthroplasty was classified as either good or poor based on migration results over 4 to 8 years using roentgen stereophotogrammetry. Gait analysis with the Vicon system was performed in 27 asymptomatic patients (1 bilateral), selected according to gender, implant design, age, and fixation principle to form two equivalent groups with different prognoses. The pour prognosis group walked with a predominantly flexing moment and higher moment peaks in the sagittal plane compared with the good prognosis group, in which moments were abnormally small. We conclude that individual gait patterns and subsequent differences in joint loading affect tibial component fixation. Key words: knee prosthesis, loosening, gait analysis, knee moment, load, roentgen stereophotogrammetry.
Migration data (principally roentgen stereophotogrammetric analysis [RSA]) indicate that symptomatic loosening of implants is a late manifestation of early events [1-5]. RSA can be used as a tool to predict final o u t c o m e in terms of fixation and thereby study factors of possible importance to loosening. One such factor is the a m o u n t of loading to which the implant is exposed. The activity level is usually considered, reflecting the a m o u n t of steps performed, roughly between 700,000 and 2 million a year [6,7]. Each step can be executed in m a n y different ways, however, causing varying joint loading, which can be studied by gait analysis. Several gait studies have found large variations in knee joint m o m e n t s before and after knee arthroplasty [8-141.
Differences in joint loading may be assumed to have an effect on implant fixation. Previously the significance of different gait patterns and m o m e n t s has been difficult to interpret in the absence of final o u t c o m e results. By use of RSA predictions of tibial c o m p o n e n t fixation, results in a prior study have indicated a possible association between risk of loosening and joint loading [15]. To corroborate this finding, knee arthroplasty patients already monitored by RSA were selected to form two equivalent groups with different prognoses: stable or continuously migrating implants. Gait analysis was then performed, and the knee joint m o m e n t s in the two groups were compared.
From the *Department q#Orthopaedics, Central Hospital of V~ister~s, Viister~s; and l-Department of Orthopaedics, University Hospital of Lund, Lund, Sweden.
Materials and Methods Patients monitored by RSA after total knee arthroplasty at the Department of Orthopaedics, Lund University Hospital, Sweden, were selected according to the following criteria: i) preoperative diagnosis of osteoarthritis, ii) insignificant and nonprogressing pain from the operated knee, iii) ability to walk without assistance, and iv) complete and highquality RSA data with at least 4 years of follow-up. The RSA m e t h o d has been thoroughly described
Submitted March 3, 1998; accepted September 9, 1998. Funds were received from lhe Greta och J o h a n Kocks Stiftelse, Stiftelsen Bist~nd ~t van['6ra i Sk~ne, K o n u n g Gustav V's 80-~rs fond, Altrcd Osterlunds Stiftelse, NUTEK, The Medical Facuhy of Lund University, and the Medical Research Council Project 09509 in support of the research material described in this article. Reprint requests: Maria B. Hilding, MD, PhD, D e p a r t m e n t ot Orthopaedics, Central Hospital of V~ster~s, S-721 89 V~ster~s, Sweden. Copyright © 1999 by Churchill Livingstone ® 0883- 5403/99/1405-00 12510.00/0
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previously [4,16,17]. Classification into two different prognosis groups was done both by application of the previously identified threshold of 2 0 0 - ~ m migration ( m a x i m u m total point motion) b e t w e e n 1 and 2 years and by consideration of segment m o t i o n in 6 ° freedom, with 0 . 2 - r a m translation and 0.3 ° rotation as the limit of significant motions during 1 year (migration) [4]. Fifteen cases with continuous migration were m a t c h e d to 15 cases w i t h o u t significant migration, according to gender, prosthesis design (all n o n c o n s t r a i n e d , cruciateretaining), fixation technique (with or w i t h o u t cement), and age ( < 1 0 years of difference). Activity level was graded from 1 to 10 on a scale developed for hip replacements [ 18]. The 30 patients selected provided informed consent to participate in gait analysis. The gait laboratory, D e p a r t m e n t of Orthopaedics and Scandinavian Laboratory of Prosthetics and Orthotics, Lund University Hospital, Sweden, was equipped with a Kistler (Kistler I n s t r u m e n t e AG, Winterthur, Swizerland) force plate (type 9284) and a Vicon (Oxford Metrics, Oxford, UK) three-dimensional m o t i o n analysis system (370 version 1.2), including five infrared video cameras. Before m e a s u r e m e n t s , the system was calibrated, and patient weight, height, and other a n t h r o p o m e t r i c data were registered according to the instructions from the manufacturer. Patients changed clothes into tight-fitting g y m shorts and bare legs but w o r e their o w n comfortable shoes. After palpation of a n a t o m i c landmarks by two e x p e r i e n c e d physiotherapists, passive reflective m a r k e r s were fixed by adhesive tape to the skin at the locations defined by the manufacturer. The m a r k e r at the anterosuperior iliac spine was difficult to place adequately because of obesity in m a n y patients. This was considered acceptable because the location of this m a r k e r is of no i m p o r t a n c e in the calculation of knee joint m o m e n t s , the m a i n outcome variable. After trial walking, intended for patient adaptation and speed stabilization, the recordings were done on level ground with self-selected walking speed. The force plate was hidden u n d e r a m a t to avoid targeting. Three gait cycles with force data from each left and right leg were recorded. Knee joint angles, m o m e n t s , and powers in all three planes were obtained by Vicon software by use of an inverse dynamics model, in which gravitational and inertial forces are taken into account. Results were received both as curves, graphically representing the course of stance and swing, and as data files with numerical values with 20-ms intervals (50 Hz). M o m e n t s are normalized to body weight in the Vicon system and defined as muscular m o m e n t s .
We used the oppositely directed external m o m e n t , simply by reversing sign of the m o m e n t s , to facilitate comparisons with previous results. From the three available graphical recordings on each knee, the most representative curve was chosen by one of the physiotherapists. This choice was m a d e with the physiotherapist blinded regarding the RSA results. Data reduction was p e r f o r m e d to avoid statistical comparison of curve shapes. The following values, all concerning the knee joint, were chosen for statistical analysis: the peak extension m o m e n t from initial contact (or heel-strike), the peak flexion m o m e n t at loading response, the difference bet w e e n the two f o r m e r (a potentially stressful rocking force), the area (by integral) u n d e r the m o m e n t curve directed in flexion and extension, the peak adduction m o m e n t , and the m a x i m a l total joint power. The m o m e n t s in the transverse plane, acting in rotation, were not analyzed because of their small values and supposedly insignificant contribution to knee joint loading.
Statistics The statistical analysis was p e r f o r m e d by the i n d e p e n d e n t t test because quantitative data from two i n d e p e n d e n t groups, with different prognoses, were compared. P values < .05 were considered significant.
Results Gait analysis was p e r f o r m e d in 27 patients out of the 30 planned. Two canceled their a p p o i n t m e n t s late because of illness. One was deleted after analysis because of t e m p o r a r y malfunction of the force plate. Twenty-eight cases refers to 27 patients and 28 knees. The p o o r prognosis group consisted of 14 cases, 6 m e n and 7 w o m e n , with 8 Duracon s t e m m e d prostheses, 5 p o r o u s - c o a t e d a n a t o m i c (PCA) Mark II, and 1 PCA primary, out of which 4 were cem e n t e d and 10 cementless, with m e a n age 74 years (range, 6 4 - 8 4 years). The good prognosis group consisted of 14 cases, 8 m e n and 6 w o m e n , with 8 Duracon s t e m m e d and 6 PCA MK II prostheses, 2 c e m e n t e d and 12 cementless, with m e a n age 76 years (range, 69-85 years). In both groups, gait analysis was p e r f o r m e d with a m e a n follow-up time of 6 years postoperative (range, 5-9 years), and RSA was p e r f o r m e d with a m e a n follow-up time of 5 years (range, 4 - 8 years). The activity level was similar in the two groups, with m e d i a n value 6 (range, 4-9). The two prognosis groups were thus considered comparable, but the original intention to use m a t c h e d pairs was not fully achieved.
Gait Affects Tibial Component Fixation
Mean peak flexion m o m e n t s in Nm/kg were 0.35 (standard deviation [SD] 0.11) and 0.22 (SD 0.11) in the poor and good prognosis groups (P = .005). Mean peak extension m o m e n t s were 0.37 (SD 0.16) in the poor and 0.23 (SD 0.1) in the good prognosis groups ( P = .01). The m e a n difference between these two peaks was 0.60 (SD 0.13) and 0.38 (SD 0.21) (P = .003). The poor prognosis group had a p r e d o m i n a n t l y flexing m o m e n t , with most of the area u n d e r the sagittal plane m o m e n t curve (71%) on the flexing side of 0. The good prognosis group showed generally smaller oscillations of the flexion and extension m o m e n t curve a r o u n d 0. The area difference of the flexing m o m e n t between the groups was significant (P = .05). Most of the patients, however, showed substantial deviations from normal curves, often more p r o n o u n c e d in the good prognosis group, in which small sagittal plane moments were produced. In the frontal plane, the peak adduction m o m e n t was not different in the poor prognosis group, 0.42 (SD 0.14), and in the good prognosis group, 0.43 (SD 0.16) (P = .9). The total knee joint power calculations did not reveal any difference between the prognosis groups (poor, 0.7 [SD 0.3]; good, 0.6 [SD 0.3]; P = .4). Gait speed in m/s was 1.03 (SD 0.3) in the poor prognosis group and 0.85 (SD 0.3) in the good prognosis group, a nonsignificant difference (P = .09).
Discussion Total knee arthroplasty is generally a successful operation with pain relief and improved function. The durability of the results can be h a m p e r e d by loosening. According to the accumulated damage scenario [19], one factor out of m a n y suspected to promote loosening is high activity level, by repetitive overloading of the implant. Not only different activity levels, but also the a m o u n t of loading at each step during gait can cause overloading [201, which is supported by unexpected findings in a previous prospective study [15]. Patients with degenerative arthritis, whose tibial components showed large migration by RSA and consequently were at risk of loosening [41, walked with increased knee joint flexion moments, whereas patients with stable implants walked with significantly less flexion moments, both before and after the operation. We interpreted these m o m e n t differences as indicators of loading differences, which m a y well have affected tibial c o m p o n e n t fixation. The aim of this study was to corroborate these findings in a study designed a priori to focus on this aspect. The predictions made from approximately 5 years of monitoring by RSA in this study are more reliable than those we made
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previously from 2 years of follow-up. The results were affirmative; also in this study, the group of patients with tibial c o m p o n e n t s at risk of symptomatic loosening walked with larger flexion moments. ~vvo principal patterns of early migration have been identified by RSA: Either the implant is anchored securely, with little migration that ceases after the first postoperative year, or the implant migrates continuously and may later develop symptomatic loosening [4]. Thus, it seems as if it is determined in the postoperative period if the fixalion of the implant stays in an equilibrium, w h e r e it is able to sustain the forces it is exposed to, or if a gradual deterioration of the fixation occurs. The forces acting during normal gait have generally been considered to be of little variation [21-24]. The forces cannot be estimated without cumbersome calculations in which simplifying assumptions in models are used [21 ], but the knee joint m o m e n t s can be obtained by gait analysis with reasonable accuracy [25,26]. Moments can be used as an indicator of joint loading because they are essentially proportionate to the total joint force [23]. The sagittal plane m o m e n t curve during normal-level gait has been investigated in several independent studies [8,9,12,26[, which all show a repeatable pattern of four peaks, also found in a normal reference material from the gait laboratory in Lund (Fig. 1). At initial contact (or heel-strike), an external extension m o m e n t of short duration appears, then a fast change into flexion, which continues during loading response, followed by a more slowly developing extension m o m e n t during midstance, again changed into flexion at push-off. This normal pattern was seldom found in this study. In the poor prognosis group, a flexing m o m e n t t h r o u g h o u t stance was often found after a small extending peak at heel-strike. The good prognosis group showed even larger deviations from normal, small amplitudes, and sometimes complete absence of peaks, a pattern that causes unloading of the knee joint. The more protective gait style found in the good prognosis group would thus be advantageous to implant longevity, even if it is far from normal. Previous gait studies on knee prosthesis patients have identified different sagittal plane patterns: predominantly flexion, predominantly extension, and the normal biphasic pattern [8-14]. The implications of these findings have been obscure in the absence of longterm results. It has been assumed in m a n y gait studies that the goal after knee prosthesis surgery is to recover normal function. Persistent gait abnormalities have b e e n considered undesirable and used as arguments against certain implant designs [8,10,11]. This outlook m a y have to be
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The Journal of Arthroplasty Vol. 14 No. 5 August 1999
Flexion Nm/kg O.5 I
O,3~ ~ 0 -0,11 0,4
stance
push off
!
swing
0,2 0,1
-~
-0,2
,
References
-0,3 -0,4 -0,5
r e c r u i t m o n o c y t e s , t h e p r e c u r s o r s of m a c r o p h a g e s , from the bloodstream, and consequently the interface m a y b e p r o t e c t e d f r o m t h e a c t i o n of w e a r p a r t i c l e s [311. P a t i e n t s w i t h t o t a l k n e e a r t h r o p l a s t y s h o w vario u s p a t h o l o g i c m o m e n t p a t t e r n s in t h e sagittal p l a n e , w h i c h r e s u l t in c o n s i d e r a b l e d i f f e r e n c e s in knee joint loading. We propose that repetitive loading c o n t r i b u t e s at a n e a r l y stage to t h e p r o c e s s of tibial c o m p o n e n t l o o s e n i n g .
•-,,--normal curve .,-=--poor prognosis ~ g o o d
prognosis
-0.6 heel strike
-0.7 Extension Nm/kg
Fig. 1o Average sagittal plane moments of three groups from the gait laboratory in Lund. A negative sign indicates extension, and a positive sign indicates flexion because moments are defined as external. The moments are measured in Nm/kg (body weight) as a function of the gait cycle, in which the stance phase starts by heel-strike and finishes by push-off. Normal curve shows a normal reference material (9 men, 9 women, 25-55 years). Poor prognosis is a group of 13 patients (14 knees) with continuous migration of their tibial components and thus at risk of future loosening. Good prognosis is a group of 14 patients with stable implants.
m o d i f i e d o n t h e basis of o u r findings. A conflict m a y exist b e t w e e n n o r m a l f u n c t i o n a n d p r o s t h e t i c l o n gevity. N o r m a l gait is p r o b a b l y t h e m o s t efficient w a y to p r o p a g a t e . E n e r g y cost a n d m u s c l e u s e can be a s s u m e d to b e o p t i m i z e d . A n i n t e g r a l p a r t of this c o s t - e f f e c t i v e n o r m a l gait is a c e r t a i n a m o u n t of j o i n t l o a d i n g . It m a y be h y p o t h e s i z e d t h a t p a t i e n t s u n l o a d i n g t h e i r k n e e s w a l k less efficiently, m o r e slowly, a n d w i t h a g r e a t e r e n e r g y cost b u t save t h e i r prostheses. I n c r e a s i n g i n t e r e s t h a s b e e n f o c u s e d o n t h e inf l u e n c e of m e c h a n i c a l forces o n tissue d i f f e r e n t i a t i o n [ 2 7 - 2 9 ] . I n this c o n t e x t , it m a y b e i n t e r e s t i n g to s p e c u l a t e o n w h a t effect a p r o t e c t i v e w a l k i n g p a t t e r n m a y h a v e . S m a l l m o m e n t s give s m a l l o v e r a l l forces, a n d , d e p e n d i n g o n t h e b i p h a s i c p r o p e r t i e s of t h e i n t e r f a c e , this gives s m a l l e r i n t e r n a l s t r a i n f r o m fluid flow. U s i n g b i p h a s i c c o d e s in finite e l e m e n t m o d e l a n a l y s e s , it h a s b e e n s u g g e s t e d t h a t f i b r o c a r t i lage is m o r e likely to d e v e l o p u n d e r t h e tibial c o m p o n e n t u n d e r s u c h c i r c u m s t a n c e s [30]. Cartilage is d e v o i d of b l o o d vessels a n d t h u s c a n n o t
1. Freeman MAR, Plante-Bordeneuve P: Early migration and late aseptic failure of proximal femoral prostheses. J Bone Joint Surg Br 76:432, 1994 2. Krismer M, Bauer R, Fischer M: Prediction of the final outcome of hip arthroplasty by measurement of migration. J Bone Joint Surg Br (Suppl II) 77:163, 1995 3. K~rrholm J, Borss~n B, L~wenhielm B, Snorrason F: Does early micromotion of femoral stem prostheses matter? 4-7 year stereoradiographic follow-up of 84 cemented prostheses. J Bone Joint Surg Br 76:912, 1994 4. Ryd L, Albrektsson BE J, Carlsson L, et al: Roentgen stereophotogrammetric analysis (RSA) as a predictor of mechanical loosening. J Bone Joint Surg Br 77: 377, 1995 5. Walker PS, Mai SE Cobb AG, et al: Prediction of clinical outcome of THR from migration measurements on standard radiographs: a study of cemented Charnley and Stanmore femoral stems. J Bone Joint Surg Br 77:705, 1995 6. Blunn GW, Walker PS, Joshi A, Hardinge K: The dominance of cyclic sliding in producing wear in total knee replacements. Clin Orthop 273:253, 1991 7. Northfield MR, Schmalzried TP, Belcher G, Amstutz HC: Quantitative assessment of activity in joint replacement patients. Transactions 40th annual meeting Orthopaedic Research Society, New Orleans, 1994, p 680 8. Andriacchi TP, Galante JO, Fermier RW: The influence of total knee-replacement design on walking and stairclimbing. J Bone Joint Surg Am 64:1328, 1982 9. Brugioni D J, Andriacchi TP, Galante JO: A functional and radiographic analysis of the total condylar knee arthroplasty. J Arthroplasty 5:173, 1990 10. Chassin EP, Mikosz RP, Andriacchi TP, Rosenberg AG: Functional analysis of cemented medial unicompartmental knee arthroplasty. J Arthroplasty 11:553, 1996 11. Dorr LD, Ochsner JL, Gronley J, Perry J: Functional comparison of posterior cruciate-retained versus cruclare-sacrificed total knee arthroplasty. Clin Orthop 236:36, 1988 12. Simon SR, Trieshmann HW, Burden RG, et al: Quantitative gait analysis after total knee arthroplasty for
Gait Affects Tibial Component Fixation
13.
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monarticular degenerative arthritis. J Bone Joint Surg Am 65:605, 1983 Strickland AB, Andriacchi TP, Galante JO: Compensatory mechanisms for gait abnormalities following total knee replacement. Transactions 29th annual meeting Orthopaedic Research Society, Anaheim, CA, 1983; p 156 Wilson SA, McCann PD, Gotlin RS, et al: Comprehensive gait analysis in posterior-stabilized knee arthroplasty. J Arthroplasty 11:359, 1996 Hilding MB, Lanshammar H, Ryd L: Knee joint loading and tibial component loosening: RSA and gait analysis in 45 osteoarthritic patients before and after TKA. J Bone Joint Surg Br 78:66, 1996 Ryd L: Micromotion in knee arthroplasty: a roentgen stereophotogrammetric analysis of tibial component fixation. Acta Orthop Scand (Suppl 220) 57:1, 1986 Selvik G: Roentgen stereophotogrammetry: a method for the study of the kinematics of the skeletal system. Acta Orthop Scand (Supp1232) 60:1, 1989 Amstutz HC, Thomas BJ, Jinnah R, et al: Treatment of primary osteoarthritis of the hip: a comparison of total joint and surface replacement arthroplasty. J Bone Joint Surg Am 66:228, 1984 Huiskes R: Failed innovation in total hip replacement. Acta Orthop Scand 64:699, 1993 Kerrigan DC, Todd MK, Riley PO: Knee osteoarthritis and high-heeled shoes. Lancet 351 : 1399, 1998 Collins J J: The redundant nature of locomotor optimization laws. J Biomech 28:251, 1995 Harrington IJ: A bioengineering analysis of force actions at the knee in normal and pathological gait. Biomed Eng 11:167, 1976
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23. Kuster MS, Wood GA, Stachowiak GW, G~ichter A: Joint load considerations in total knee replacement. J Bone Joint Surg Br 79:109, 1997 24. Morrison JB: The mechanics of the knee joint in relation to normal walking. J Biomech 3:51, 1970 25. Andrews M, Noyes FR, Hewett TE, Andriacchi TP: Lower limb alignments are related to stance phase knee adduction in normal subjects: a critical analysis of the reliability of gait analysis data. J Orthop Res 14:289, 1996 26. Kadaba MP, Ramakrishnan HK, Wootten ME, et al: Repeatability of kinematic, kinetic and electromyographic data in normal adult gait. J Orthop Res 7:849, 1989 27. Carter DR: Mechanical loading history and skeletal biology. J Biomech 20:1095, 1987 28. Giori NJ, Ryd L, Carter DR: Mechanical influences on tissue differentiation at bone-cement interfaces. J Arthroplasty 10:514, 1995 29. Prendergast PJ, Huiskes R, S~balle K: Biophysical stimuli on cells during tissue differentiation at implant interfaces. J Biomech 30:539, 1997 30. Yuan X, Ryd L, Huiskes R: Deformation and fluid flow in bone-prosthesis interface tissue under dynamic loading conditions. Presented at the European Society of Biomechanics, Leuven, Belgium, August, 1996 31. Hansson U, Blunn G, Ryd L: Reactions to particulate wear debris in the nonreplaced knee compartment. Presented at the European Orthopaedic Research Society, Barcelona, Spain, April 1997
Gait Affects Tibial C o m p o n e n t F i x a t i o n M a r i a B. H i l d i n g , M D , P h D , * L e i f R y d , M D , P h D , t S 6 r e n T o k s v i g - L a r s e n , A l i s o n M a n n , RPT, t a n d A n d e r s S t e n s t r 6 m , M D , PhD-[-
M D , P h D , l-
Abstract: The prognosis of fixation in patients with total knee arthroplasty was classified as either good or poor based on migration results over 4 to 8 years using roentgen stereophotogrammetry. Gait analysis with the Vicon system was performed in 27 asymptomatic patients (1 bilateral), selected according to gender, implant design, age, and fixation principle to form two equivalent groups with different prognoses. The pour prognosis group walked with a predominantly flexing moment and higher moment peaks in the sagittal plane compared with the good prognosis group, in which moments were abnormally small. We conclude that individual gait patterns and subsequent differences in joint loading affect tibial component fixation. Key words: knee prosthesis, loosening, gait analysis, knee moment, load, roentgen stereophotogrammetry.
Migration data (principally roentgen stereophotogrammetric analysis [RSA]) indicate that symptomatic loosening of implants is a late manifestation of early events [1-5]. RSA can be used as a tool to predict final o u t c o m e in terms of fixation and thereby study factors of possible importance to loosening. One such factor is the a m o u n t of loading to which the implant is exposed. The activity level is usually considered, reflecting the a m o u n t of steps performed, roughly between 700,000 and 2 million a year [6,7]. Each step can be executed in m a n y different ways, however, causing varying joint loading, which can be studied by gait analysis. Several gait studies have found large variations in knee joint m o m e n t s before and after knee arthroplasty [8-141.
Differences in joint loading may be assumed to have an effect on implant fixation. Previously the significance of different gait patterns and m o m e n t s has been difficult to interpret in the absence of final o u t c o m e results. By use of RSA predictions of tibial c o m p o n e n t fixation, results in a prior study have indicated a possible association between risk of loosening and joint loading [15]. To corroborate this finding, knee arthroplasty patients already monitored by RSA were selected to form two equivalent groups with different prognoses: stable or continuously migrating implants. Gait analysis was then performed, and the knee joint m o m e n t s in the two groups were compared.
From the *Department q#Orthopaedics, Central Hospital of V~ister~s, Viister~s; and l-Department of Orthopaedics, University Hospital of Lund, Lund, Sweden.
Materials and Methods Patients monitored by RSA after total knee arthroplasty at the Department of Orthopaedics, Lund University Hospital, Sweden, were selected according to the following criteria: i) preoperative diagnosis of osteoarthritis, ii) insignificant and nonprogressing pain from the operated knee, iii) ability to walk without assistance, and iv) complete and highquality RSA data with at least 4 years of follow-up. The RSA m e t h o d has been thoroughly described
Submitted March 3, 1998; accepted September 9, 1998. Funds were received from lhe Greta och J o h a n Kocks Stiftelse, Stiftelsen Bist~nd ~t van['6ra i Sk~ne, K o n u n g Gustav V's 80-~rs fond, Altrcd Osterlunds Stiftelse, NUTEK, The Medical Facuhy of Lund University, and the Medical Research Council Project 09509 in support of the research material described in this article. Reprint requests: Maria B. Hilding, MD, PhD, D e p a r t m e n t ot Orthopaedics, Central Hospital of V~ster~s, S-721 89 V~ster~s, Sweden. Copyright © 1999 by Churchill Livingstone ® 0883- 5403/99/1405-00 12510.00/0
589
590
The Journal of Arthroplasty Vol. 14 No. 5 August 1999
previously [4,16,17]. Classification into two different prognosis groups was done both by application of the previously identified threshold of 2 0 0 - ~ m migration ( m a x i m u m total point motion) b e t w e e n 1 and 2 years and by consideration of segment m o t i o n in 6 ° freedom, with 0 . 2 - r a m translation and 0.3 ° rotation as the limit of significant motions during 1 year (migration) [4]. Fifteen cases with continuous migration were m a t c h e d to 15 cases w i t h o u t significant migration, according to gender, prosthesis design (all n o n c o n s t r a i n e d , cruciateretaining), fixation technique (with or w i t h o u t cement), and age ( < 1 0 years of difference). Activity level was graded from 1 to 10 on a scale developed for hip replacements [ 18]. The 30 patients selected provided informed consent to participate in gait analysis. The gait laboratory, D e p a r t m e n t of Orthopaedics and Scandinavian Laboratory of Prosthetics and Orthotics, Lund University Hospital, Sweden, was equipped with a Kistler (Kistler I n s t r u m e n t e AG, Winterthur, Swizerland) force plate (type 9284) and a Vicon (Oxford Metrics, Oxford, UK) three-dimensional m o t i o n analysis system (370 version 1.2), including five infrared video cameras. Before m e a s u r e m e n t s , the system was calibrated, and patient weight, height, and other a n t h r o p o m e t r i c data were registered according to the instructions from the manufacturer. Patients changed clothes into tight-fitting g y m shorts and bare legs but w o r e their o w n comfortable shoes. After palpation of a n a t o m i c landmarks by two e x p e r i e n c e d physiotherapists, passive reflective m a r k e r s were fixed by adhesive tape to the skin at the locations defined by the manufacturer. The m a r k e r at the anterosuperior iliac spine was difficult to place adequately because of obesity in m a n y patients. This was considered acceptable because the location of this m a r k e r is of no i m p o r t a n c e in the calculation of knee joint m o m e n t s , the m a i n outcome variable. After trial walking, intended for patient adaptation and speed stabilization, the recordings were done on level ground with self-selected walking speed. The force plate was hidden u n d e r a m a t to avoid targeting. Three gait cycles with force data from each left and right leg were recorded. Knee joint angles, m o m e n t s , and powers in all three planes were obtained by Vicon software by use of an inverse dynamics model, in which gravitational and inertial forces are taken into account. Results were received both as curves, graphically representing the course of stance and swing, and as data files with numerical values with 20-ms intervals (50 Hz). M o m e n t s are normalized to body weight in the Vicon system and defined as muscular m o m e n t s .
We used the oppositely directed external m o m e n t , simply by reversing sign of the m o m e n t s , to facilitate comparisons with previous results. From the three available graphical recordings on each knee, the most representative curve was chosen by one of the physiotherapists. This choice was m a d e with the physiotherapist blinded regarding the RSA results. Data reduction was p e r f o r m e d to avoid statistical comparison of curve shapes. The following values, all concerning the knee joint, were chosen for statistical analysis: the peak extension m o m e n t from initial contact (or heel-strike), the peak flexion m o m e n t at loading response, the difference bet w e e n the two f o r m e r (a potentially stressful rocking force), the area (by integral) u n d e r the m o m e n t curve directed in flexion and extension, the peak adduction m o m e n t , and the m a x i m a l total joint power. The m o m e n t s in the transverse plane, acting in rotation, were not analyzed because of their small values and supposedly insignificant contribution to knee joint loading.
Statistics The statistical analysis was p e r f o r m e d by the i n d e p e n d e n t t test because quantitative data from two i n d e p e n d e n t groups, with different prognoses, were compared. P values < .05 were considered significant.
Results Gait analysis was p e r f o r m e d in 27 patients out of the 30 planned. Two canceled their a p p o i n t m e n t s late because of illness. One was deleted after analysis because of t e m p o r a r y malfunction of the force plate. Twenty-eight cases refers to 27 patients and 28 knees. The p o o r prognosis group consisted of 14 cases, 6 m e n and 7 w o m e n , with 8 Duracon s t e m m e d prostheses, 5 p o r o u s - c o a t e d a n a t o m i c (PCA) Mark II, and 1 PCA primary, out of which 4 were cem e n t e d and 10 cementless, with m e a n age 74 years (range, 6 4 - 8 4 years). The good prognosis group consisted of 14 cases, 8 m e n and 6 w o m e n , with 8 Duracon s t e m m e d and 6 PCA MK II prostheses, 2 c e m e n t e d and 12 cementless, with m e a n age 76 years (range, 69-85 years). In both groups, gait analysis was p e r f o r m e d with a m e a n follow-up time of 6 years postoperative (range, 5-9 years), and RSA was p e r f o r m e d with a m e a n follow-up time of 5 years (range, 4 - 8 years). The activity level was similar in the two groups, with m e d i a n value 6 (range, 4-9). The two prognosis groups were thus considered comparable, but the original intention to use m a t c h e d pairs was not fully achieved.
Gait Affects Tibial Component Fixation
Mean peak flexion m o m e n t s in Nm/kg were 0.35 (standard deviation [SD] 0.11) and 0.22 (SD 0.11) in the poor and good prognosis groups (P = .005). Mean peak extension m o m e n t s were 0.37 (SD 0.16) in the poor and 0.23 (SD 0.1) in the good prognosis groups ( P = .01). The m e a n difference between these two peaks was 0.60 (SD 0.13) and 0.38 (SD 0.21) (P = .003). The poor prognosis group had a p r e d o m i n a n t l y flexing m o m e n t , with most of the area u n d e r the sagittal plane m o m e n t curve (71%) on the flexing side of 0. The good prognosis group showed generally smaller oscillations of the flexion and extension m o m e n t curve a r o u n d 0. The area difference of the flexing m o m e n t between the groups was significant (P = .05). Most of the patients, however, showed substantial deviations from normal curves, often more p r o n o u n c e d in the good prognosis group, in which small sagittal plane moments were produced. In the frontal plane, the peak adduction m o m e n t was not different in the poor prognosis group, 0.42 (SD 0.14), and in the good prognosis group, 0.43 (SD 0.16) (P = .9). The total knee joint power calculations did not reveal any difference between the prognosis groups (poor, 0.7 [SD 0.3]; good, 0.6 [SD 0.3]; P = .4). Gait speed in m/s was 1.03 (SD 0.3) in the poor prognosis group and 0.85 (SD 0.3) in the good prognosis group, a nonsignificant difference (P = .09).
Discussion Total knee arthroplasty is generally a successful operation with pain relief and improved function. The durability of the results can be h a m p e r e d by loosening. According to the accumulated damage scenario [19], one factor out of m a n y suspected to promote loosening is high activity level, by repetitive overloading of the implant. Not only different activity levels, but also the a m o u n t of loading at each step during gait can cause overloading [201, which is supported by unexpected findings in a previous prospective study [15]. Patients with degenerative arthritis, whose tibial components showed large migration by RSA and consequently were at risk of loosening [41, walked with increased knee joint flexion moments, whereas patients with stable implants walked with significantly less flexion moments, both before and after the operation. We interpreted these m o m e n t differences as indicators of loading differences, which m a y well have affected tibial c o m p o n e n t fixation. The aim of this study was to corroborate these findings in a study designed a priori to focus on this aspect. The predictions made from approximately 5 years of monitoring by RSA in this study are more reliable than those we made
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previously from 2 years of follow-up. The results were affirmative; also in this study, the group of patients with tibial c o m p o n e n t s at risk of symptomatic loosening walked with larger flexion moments. ~vvo principal patterns of early migration have been identified by RSA: Either the implant is anchored securely, with little migration that ceases after the first postoperative year, or the implant migrates continuously and may later develop symptomatic loosening [4]. Thus, it seems as if it is determined in the postoperative period if the fixalion of the implant stays in an equilibrium, w h e r e it is able to sustain the forces it is exposed to, or if a gradual deterioration of the fixation occurs. The forces acting during normal gait have generally been considered to be of little variation [21-24]. The forces cannot be estimated without cumbersome calculations in which simplifying assumptions in models are used [21 ], but the knee joint m o m e n t s can be obtained by gait analysis with reasonable accuracy [25,26]. Moments can be used as an indicator of joint loading because they are essentially proportionate to the total joint force [23]. The sagittal plane m o m e n t curve during normal-level gait has been investigated in several independent studies [8,9,12,26[, which all show a repeatable pattern of four peaks, also found in a normal reference material from the gait laboratory in Lund (Fig. 1). At initial contact (or heel-strike), an external extension m o m e n t of short duration appears, then a fast change into flexion, which continues during loading response, followed by a more slowly developing extension m o m e n t during midstance, again changed into flexion at push-off. This normal pattern was seldom found in this study. In the poor prognosis group, a flexing m o m e n t t h r o u g h o u t stance was often found after a small extending peak at heel-strike. The good prognosis group showed even larger deviations from normal, small amplitudes, and sometimes complete absence of peaks, a pattern that causes unloading of the knee joint. The more protective gait style found in the good prognosis group would thus be advantageous to implant longevity, even if it is far from normal. Previous gait studies on knee prosthesis patients have identified different sagittal plane patterns: predominantly flexion, predominantly extension, and the normal biphasic pattern [8-14]. The implications of these findings have been obscure in the absence of longterm results. It has been assumed in m a n y gait studies that the goal after knee prosthesis surgery is to recover normal function. Persistent gait abnormalities have b e e n considered undesirable and used as arguments against certain implant designs [8,10,11]. This outlook m a y have to be
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Flexion Nm/kg O.5 I
O,3~ ~ 0 -0,11 0,4
stance
push off
!
swing
0,2 0,1
-~
-0,2
,
References
-0,3 -0,4 -0,5
r e c r u i t m o n o c y t e s , t h e p r e c u r s o r s of m a c r o p h a g e s , from the bloodstream, and consequently the interface m a y b e p r o t e c t e d f r o m t h e a c t i o n of w e a r p a r t i c l e s [311. P a t i e n t s w i t h t o t a l k n e e a r t h r o p l a s t y s h o w vario u s p a t h o l o g i c m o m e n t p a t t e r n s in t h e sagittal p l a n e , w h i c h r e s u l t in c o n s i d e r a b l e d i f f e r e n c e s in knee joint loading. We propose that repetitive loading c o n t r i b u t e s at a n e a r l y stage to t h e p r o c e s s of tibial c o m p o n e n t l o o s e n i n g .
•-,,--normal curve .,-=--poor prognosis ~ g o o d
prognosis
-0.6 heel strike
-0.7 Extension Nm/kg
Fig. 1o Average sagittal plane moments of three groups from the gait laboratory in Lund. A negative sign indicates extension, and a positive sign indicates flexion because moments are defined as external. The moments are measured in Nm/kg (body weight) as a function of the gait cycle, in which the stance phase starts by heel-strike and finishes by push-off. Normal curve shows a normal reference material (9 men, 9 women, 25-55 years). Poor prognosis is a group of 13 patients (14 knees) with continuous migration of their tibial components and thus at risk of future loosening. Good prognosis is a group of 14 patients with stable implants.
m o d i f i e d o n t h e basis of o u r findings. A conflict m a y exist b e t w e e n n o r m a l f u n c t i o n a n d p r o s t h e t i c l o n gevity. N o r m a l gait is p r o b a b l y t h e m o s t efficient w a y to p r o p a g a t e . E n e r g y cost a n d m u s c l e u s e can be a s s u m e d to b e o p t i m i z e d . A n i n t e g r a l p a r t of this c o s t - e f f e c t i v e n o r m a l gait is a c e r t a i n a m o u n t of j o i n t l o a d i n g . It m a y be h y p o t h e s i z e d t h a t p a t i e n t s u n l o a d i n g t h e i r k n e e s w a l k less efficiently, m o r e slowly, a n d w i t h a g r e a t e r e n e r g y cost b u t save t h e i r prostheses. I n c r e a s i n g i n t e r e s t h a s b e e n f o c u s e d o n t h e inf l u e n c e of m e c h a n i c a l forces o n tissue d i f f e r e n t i a t i o n [ 2 7 - 2 9 ] . I n this c o n t e x t , it m a y b e i n t e r e s t i n g to s p e c u l a t e o n w h a t effect a p r o t e c t i v e w a l k i n g p a t t e r n m a y h a v e . S m a l l m o m e n t s give s m a l l o v e r a l l forces, a n d , d e p e n d i n g o n t h e b i p h a s i c p r o p e r t i e s of t h e i n t e r f a c e , this gives s m a l l e r i n t e r n a l s t r a i n f r o m fluid flow. U s i n g b i p h a s i c c o d e s in finite e l e m e n t m o d e l a n a l y s e s , it h a s b e e n s u g g e s t e d t h a t f i b r o c a r t i lage is m o r e likely to d e v e l o p u n d e r t h e tibial c o m p o n e n t u n d e r s u c h c i r c u m s t a n c e s [30]. Cartilage is d e v o i d of b l o o d vessels a n d t h u s c a n n o t
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