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Ground reaction forces on stairs
Gait & Posture 26 (2007) 48–58 www.elsevier.com/locate/gaitpost
Ground reaction forces on stairs Part II: Knee implant patients versus normals Alex Stacoff a,*, Ine`s A. Kramers-de Quervain a,b, Gerhard Luder a, Renate List a, Edgar Stu¨ssi a a
Institute for Biomechanics, ETH Zu¨rich, Schlieren, Switzerland b Schulthess Clinic, Zu¨rich, Switzerland
Received 15 December 2005; received in revised form 11 July 2006; accepted 27 July 2006
Abstract The goal of this study was to compare selected parameters of vertical ground reaction forces (GRF) of good outcome patients with different prosthesis designs with a matched control group during level walking, stair ascent and descent. Forty subjects, 29 with three main implant designs (including four subjects with a passive knee flexion restriction), and 11 healthy controls were measured with 8–10 repetitions. Vertical ground reaction forces were measured during two consecutive steps with force plates embedded in the walkway and the staircase. Defined parameters of the force signals were used to compare the results of the test groups. The results show, that, postoperatively, good outcome patients produce gait patterns of the vertical ground reaction force which are comparable to normal healthy subjects with the exception of a few distinct differences: a significant reduction ( p < 0.05) in the vertical loading on the operated side during level walking at take-off, at weight acceptance and take-off during stair ascent of the normal stair. During stair descent, the patients did not reduce load on the operated side, but increased load variation and side-to-side asymmetry; thus, the mechanical loads on the implants were high, which may be important information with respect to loading protocols of knee implant simulators. No systematic differences in any of the test parameters were found between posterior cruciate-retaining (LCS MB and Innex CR) versus non-retaining (LCS RP and Innex UCOR) implant designs. The restricted group showed significant reductions ( p < 0.05) of several loading parameters as well as an increased side-to-side asymmetry. About one third of the force parameters of the good outcome patients showed a side-to-side asymmetry between two consecutive steps, which was over a proposed level of acceptance. # 2006 Elsevier B.V. All rights reserved. Keywords: Gait analysis; Knee prosthesis; Biomechanics
1. Introduction Currently, about 400,000 total knee arthroplasties (TKA) are implanted worldwide each year. With respect to the wear of these implants, walking has been identified as the most important activity [1]. Howver, little biomechanical information is available about the loading at the lower extremities in more demanding daily activities such as stair ascent and descent [2–4]. Thus, the * Corresponding author at: Institute for Biomechanics, Department of Materials, ETH Ho¨nggerberg, HCI 365.1, 8093 Zu¨rich, Switzerland. Tel.: +41 44 633 62 18. E-mail address: [email protected] (A. Stacoff). 0966-6362/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.gaitpost.2006.07.015
question of interest is whether patients with implants increase or decrease loading during stair ambulation compared to normals, and whether they show increased variability and/or side-to-side asymmetry. Such information would be valuable not only from the orthopaedic point of view but also with respect to loading protocols of simulators, which approximate the forces during walking and stair ambulation [1,5]. Furthermore, for patients with a restricted passive range of motion after surgery only limited information is available with respect to loading patterns. Using stairs has been recognised as a considerably more demanding task than level walking [1,2,6,7]. On the other hand, the number of instrumented stair cases
A. Stacoff et al. / Gait & Posture 26 (2007) 48–58
equipped with two force plates which allow the analyses of external loads during two consecutive steps [2–4,7–12] is still limited. Whereas during level gait ground reaction force (GRF) values reach about 1.1–1.2 body weight (BW) [13], values for stair ascent reach 1.4 BW and for stair descent 1.8 BW [2,3,11]. In a previous study of the same institution [11] it was shown that age and stair inclination influence vertical ground reaction forces and it was suggested to consider them when comparing normal subjects with patients. Whether patients with total knee arthroplasties alter the loading of the lower extremities in favour of the nonoperated side or whether they allow asymmetrical loading during stair ambulation remains controversial in the literature. Older studies [10,14,15] have demonstrated that patients with total knee arthroplasty show a deficit compared to normals while ascending stairs. In particular, those with cruciate-sacrificing prostheses have been associated with femoral roll-back during flexion [16] (although not confirmed by fluoroscopy [17]). In a recent study, Catani et al. [16] showed that the cruciate-retaining group behaved like cruciate-sacrificing knees and it was suggested that the antero-posterior constraint structures (ligamentous or mechanical by the implant) are important to maintain more physiological knee kinematics. Keeping this controversy in mind, suggestions with respect to an acceptable level of variability and asymmetry
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in ground reaction force parameters are difficult to establish and are not readily available (see Table 1). Only few authors have reported vertical ground reaction force parameters of two consecutive steps during level walking, fewer still have made suggestions with respect to acceptable levels of variability and asymmetry, and only one has reported test parameters on stairs (Table 2). Recently, Buzzi et al. [18] suggested that elderly subjects show increased local instability or inability to compensate for the natural stride-to-stride variations, i.e. an increased variability of kinematic parameters during gait. A similar increase in variability with age was found by Stacoff et al. [11] with kinetic parameters. Thus, a guideline or level of acceptance would be helpful for the interpretation of patient data. Thus, the purpose of this study was to test whether the vertical ground reaction forces of two consecutive steps of subjects with total knee arthroplasty differ from normal subjects during level gait and stair ambulation, considering selected force parameters and their respective variability and side-to-side asymmetry. The underlying objective was to test whether vertical ground reaction forces contain enough information to distinguish test groups with different knee conditions. For that reason not only good outcome patients were chosen with a total knee arthroplasty, but also patients with a unicondylar implant, as well as patients with a restricted range of motion.
Table 1 Adapted values from previous publications of coefficient of variation and absolute symmetry index during level walking Author
Subject description age in years
Parameter
Borden et al. [33]
13 patients 68 (7) 9 normal subjects 68 (6)
Force Force
Giakas and Baltzopoulos [24]
10 normal males 23.3 (2.4)
Force
Herzog et al. [23]a
62 normals: 33 males (mean 27) 29 females (mean 25)
Force Loading rate Unloading rate
Masani et al. [34]
10 normal males 28.8 (5.2)
Force: 3 km /h Force: 8 km /h
McCrory et al. [25]a
35 normals 27.5 (5.7)
Force Loading rate Unloading rate Force Loading rate Unloading rate
27 subjects with hip arthroplasty 59.7 (13.8)
Coefficient of variation
Absolute symmetry index
Reported (%)
Reported (%)
Suggested level of acceptance (%)
Suggested level of acceptance (%)
1–2 1–2 2–3
<10
3–5
<10
1 to +1 4 0 2 3–7 0–1 1 1 1–4 20 3
Stacoff et al. [11]
20 normals 58.3 (9.9 years)
Force Loading rate Unloading rate
2–5 8–10 5
White et al. [22]
15 normal children 11 cerebral palsy children
Force Force
7–15 10–22
3–5 12 7 <12.5
The acceptable levels are suggestions made by the corresponding authors. a Calculated as symmetry index (after Herzog et al. [23]), which allows for positive and negative values.
5–14 13–20
<10
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Table 2 Adapted values from a previous publication of the coefficients of variation and absolute symmetry index during stair ascent and descent including subjects of younger age (below 40 years) Author
Subject description age in years
Parameter
Coefficient of variation reported (%)
Absolute symmetry index reported (%)
Stacoff et al. [11]
20 healthy subjects 58.3 (19.9)
Force Loading rate Unloading rate
4–10 10–15 8–11
5–15 13–20 10–15
It was not the intention of this study to compare preoperative with postoperative measurements.
2. Methods 2.1. Subjects and implants Written consent was given by 40 subjects, 25 good outcome patients with different implants, four patients with restricted (<908 knee flexion) passive range of motion (RoM), and 11 control subjects (normals) (Table 3). Preoperatively, all patients suffered from advanced osteoarthritis and were (due to pain) not capable of performing alternating stair ambulation nor walking without support or a severe limp; thus, preoperative measurements were not considered a meaningful baseline for comparison and were not collected in this study. Approval was obtained from the local ethic committee. The normal subjects are a subset of a previous study [11] and were matched in age and gender to the patient groups of the present study. Surgery was performed 13–38 months prior to the enrolment in this study. The selection criteria for the good outcome subjects were: (i) comparable age, height and weight to the normal group (see Table 3), (ii) fully recovered and rehabilitated, (iii) postoperative and radiological results rated as excellent (except for the restricted knee group), (iv) Knee Society Score over 90 (Table 3, after Insall et al. [19]), passive flexion well over 1008, (v) free from any relevant functional pathology in all other joints of the lower extremities and (vi) the operated and the contra lateral knees free of pain during testing. All subjects stated
that they could walk without restrictions for several hours and that they were able to climb and descend stairs without the use of a handrail (exception: restricted group). Subjects with a leg length discrepancy of more than 1 cm were not included in the study. The restricted group consisted of subjects who had developed adhesions and their knee flexion was reduced below 908 after rehabilitation; they had previous surgery and complications, were all males and were larger and heavier in comparison. Because stair ambulation is difficult with restricted knee flexion this test group was not easy to recruit. The following implant types were used: (a) unicondylar implant (Allegretto, Centerpulse Orthopedics Ltd., Winterthur, Switzerland; a Zimmer company) for patients with a unicompartimental disease, whereby the medial condyle was replaced and the cruciate ligaments were retained; (b) Low-Contact-Stress implant LCS1 (DePuy, a Johnson & Johnson company, Warsaw, USA) with a Rotating Platform (RP) design, both cruciate ligaments sacrificed; (c) LCS1 with Meniscal Bearings (MB), posterior cruciate ligament retaining; (d) INNEXTM (Centerpulse Orthopedics Ltd., Winterthur, Switzerland; a Zimmer company) with cruciate ligament retaining CR; and (e) INNEXTM with cruciate ligaments sacrificed (UCOR). Groups (b–e) are total mobile bearing knee arthroplasties and are summarised as the ‘‘total’’ group in Tables 4–6 (with 20 subjects). The restricted group had LCS1 RP prostheses. The rationale for the choice of the implants was: (i) the severity of osteoarthritis (total versus unicondylar) and (ii) change in design (LCS older than Innex). All patients were operated at the same orthopaedic hospital in Zurich, Switzerland.
Table 3 Subject specifications Subjects TM
a
INNEX CR INNEXTM UCORa LCS1 MBa LCS1 RPa Unicondylar Restricted Normal
Gender
Age
2male 2f 3m 1f 2m 4f 2m 4f 3m 2f 4m 6m 5f
65.3 72.5 63.5 67.8 67.2 66.8 69.5
(6.2) (4.7) (8.6) (7.8) (3.9) (7.4) (6.0)
Leg length (m)
Height (m)
Weight (kg)
Passive Flexion ROM (8)
Knee Society Scores
0.91 0.91 0.86 0.89 0.90 0.95 0.88
1.71 1.75 1.64 1.69 1.76 1.79 1.68
76.4 87.7 75.8 76.2 77.7 85.2 72.4
118; 110–130 126; 120–135 118; 110–125 115; 100–125 127; 115–140 84; 75–90 (8) 136; 130–150
97.75; (97–99) (0.96) 99.25; (97–100) (1.5) 96.83; (95–100) (1.94) 97.5; (92–100) (2.95) 95.6 (91–100) (3.58) 81.25 (52–94) (19.65) 100
(0.05) (0.05) (0.04) (0.07) (0.06) (0.02) (0.04)
(0.10) (0.10) (0.06) (0.11) (0.08) (0.06) (0.06)
(15.1) (13.4) (16.3) (13.2) (5.6) (11.2) (8.7) 1
(9) (8) (5) (10) (10) (7) 1
All listed subjects were tested during level gait and on the standard stair. On the steep stair, only 3 LCS MB and 3 LCS RP were available. The Knee Society Scores are based on Insall et al. [19]. The selection of normal subjects is based on previous work (Stacoff et al. [11]). Abbreviations—MB: meniscal bearing, posterior cruciate retaining; RP: rotating platform (non-retaining); CR: cruciate retaining; UCOR: ultra congruent, only rotating (non-retaining). a Indicates that the INNEXTM and LCS1 groups were summarised in one group denoted as ‘‘total’’ (see Tables 4–6).
Table 4 Group means (S.D.) of the four test groups Parameters Left/Prosthesis
Right/Contra lateral
Patient group v (m/s)
T (m/s)
Fz2 (BW)
Fz3 (BW)
Fz4 (BW)
Level gait
Normal ‘‘Total’’ Unicondylar Restricted
1.30 1.31 1.35 1.29
(0.13) (0.18) (0.14) (0.15)
673.6 665.3 681.0 700.9
1.18 1.15 1.11 1.09
0.72 0.74 0.77 0.78
(0.08) (0.07) (0.05) (0.06)
1.15a,2,3 (0.05) 1.10 (0.06) 1.062 (0.05) 1.063 (0.03)
7.57 7.66 6.44 7.26
Stand up
Normal ‘‘Total’’ Unicondylar Restricted
0.61 0.57 0.50 0.51
(0.08) (0.07) (0.12) (0.10)
683.9a,2 (115.1) 775.7 (120.5) 911.72 (265.1) 856.6 (143.0)
1.11a,2 (0.08) 1.04 (0.07) 0.982 (0.05) 1.04 (0.05)
0.72 0.71 0.79 0.78
(0.05) (0.09) (0.08) (0.10)
1.21 1.18 1.17 1.14
(0.11) (0.08) (0.13) (0.09)
5.52a (1.07) 10.87a (1.25) 4.37 (1.42) 9.60 (1.50) 3.79 (1.73) 9.36 (2.58) 4.38 (1.25) 8.85 (0.62)
Steep up
Normal ‘‘Total’’ Unicondylar Restricted
0.51 0.50 0.46 0.43
(0.06) (0.07) (0.07) (0.09)
745.3 802.1 875.9 915.8
(115.2) (116.8) (147.0) (177.2)
1.11 1.08 1.01 1.06
(0.06) (0.10) (0.06) (0.05)
0.71 0.67 0.77 0.78
(0.09) (0.10) (0.07) (0.13)
1.25 1.23 1.16 1.19
(0.16) (0.10) (0.14) (0.10)
4.84 4.30 3.41 3.85
(0.90) (1.42) (0.85) (0.68)
11.12 10.06 9.31 9.02
Stand down Normal ‘‘Total’’ Unicondylar Restricted
0.67 0.65 0.58 0.53
(0.08) (0.10) (0.12) (0.14)
588.1 663.7 752.5 812.7
(116.6) (120.6) (159.2) (181.6)
1.51 1.55 1.40 1.48
(0.15) (0.16) (0.13) (0.15)
– – – –
– – – –
12.86 14.27 11.75 13.44
(2.55) (4.78) (4.96) (2.99)
– – – –
Steep down Normal ‘‘Total’’ Unicondylar Restricted
0.62 0.58 0.51 0.49
(0.08) (0.10) (0.05) (0.12)
626.4 665.2 747.6 781.3
(107.6) (109.4) (88.9) (189.6)
1.47 1.65 1.53 1.33
(0.20) (0.26) (0.25) (0.38)
– – – –
– – – –
12.95 17.01 17.07 14.26
(3.59) (5.96) (11.35) (5.44)
– – – –
(42.9) (59.4) (58.2) (53.4)
(0.12) (0.12) (0.06) (0.04)
bn (BW/s) (1.63) (2.00) (1.18) (0.76)
en (BW/s) 9.01a (0.98) 8.76 (1.05) 8.02 (0.65) 7.59 (0.60)
(1.22) (1.48) (2.24) (1.05)
T (m/s)
Fz2 (BW)
Fz3 (BW)
Fz4 (BW)
678.9 667.1 689.0 723.1
1.19 1.17 1.16 1.18
(0.13) (0.12) (0.06) (0.12)
0.72 0.73 0.72 0.73
(0.09) (0.09) (0.07) (0.10)
1.14 1.13 1.13 1.10
(0.07) (0.06) (0.06) (0.02)
7.30 8.01 7.29 7.74
(1.50) (2.92) (1.19) (2.08)
9.20 9.12 8.60 8.01
(1.06) (1.07) (1.16) (0.41)
686.4a,2 (116.6) 772.7 (108.9) 889.02 (221.5) 889.8 (171.8)
1.11 1.08 1.04 1.11
(0.07) (0.09) (0.05) (0.09)
0.68 0.70 0.78 0.67
(0.03) (0.08) (0.08) (0.13)
1.20 1.26 1.29 1.20
(0.10) (0.10) (0.14) (0.17)
5.57 4.99 4.14 4.88
(0.94) (1.49) (1.48) (0.76)
10.80 10.34 10.45 8.44
(1.41) (1.24) (2.76) (2.69)
747.2 801.4 875.5 949.1
1.10 1.11 1.07 1.12
(0.10) (0.08) (0.04) (0.03)
0.65 0.67 0.75 0.62
(0.07) (0.08) (0.04) (0.15)
1.21 1.32 1.28 1.22
(0.15) (0.15) (0.17) (0.24)
4.92 4.76 4.13 4.41
(0.99) (1.49) (1.16) (0.38)
10.69 10.93 10.20 8.21
(1.58) (1.21) (2.54) (3.51)
601.3a,3 (125.1) 661.535 (105.6) 763.5 (138.6) 877.73,5 (260.2)
1.53 1.66 1.60 1.44
(0.16) (0.27) (0.21) (0.12)
– – – –
– – – –
12.61 15.40 13.40 11.04
(2.77) (6.62) (6.10) (2.53)
– – – –
644.7a (102.4) 653.2 (90.3) 751.7 (64.8) 853.4 (280.5)
1.53 1.71 1.68 1.32
(0.21) (0.36) (0.25) (0.41)
– – – –
– – – –
13.37 17.45 17.73 11.90
(3.80) (7.29) (11.04) (5.99)
– – – –
(41.5) (57.3) (52.7) (63.5)
(121.2) (109.7) (143.8) (196.1)
bn (BW/s)
en (BW/s)
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Test
‘‘Total’’ stands for all INNEX and LCS subjects combined. Stand, standard stair. Left/right refers to the control group. The following numbers indicate significant differences at p < 0.05: 1, normal vs. ‘‘total’’; 2, normal vs. unicondylar; 3, normal vs. restricted; 4, ‘‘total’’ vs. unicondylar; 5, ‘‘total’’ vs. restricted; a, over all groups.
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Table 5 Mean values (S.D.) of the coefficient of variation of all four test groups and test conditions
‘‘Total’’ stands for all INNEX and LCS subjects combined. Stand, standard stair. Left/right refers to the control group. The shaded areas highlight values, which are above the suggested level of acceptance (see Table 7). The following numbers indicate significant differences at p < 0.05: 1, normal vs. ‘‘total’’; 2, normal vs. unicondylar; 3, normal vs. restricted; 4, ‘‘total’’ vs. unicondylar; 5, ‘‘total’’ vs. restricted; a, over all groups.
2.2. Test set-up of the instrumented gait analysis The gait analysis system at the Institute for Biomechanics at the ETH Zu¨rich, Switzerland allows collecting kinematic, kinetic and EMG data simultaneously while the test subjects walk over a 25 m walkway or climb a set of
stairs [20]. Stair ambulation was performed on two sets of stairs, the standard inclination with a 29 cm run and a 17 cm rise (i.e. an inclination of 30.58) and the steep inclination with a 23 cm run and a 20 cm rise (inclination of 418). Two force plates (KISTLER 9068A, Winterthur, Switzerland), sampling at a rate of 2000 Hz was embedded
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Table 6 Group means (S.D.) of the asymmetry index of the four test groups
‘‘Total’’ stands for all INNEX and LCS subjects combined. Stand, standard stair. The shaded areas highlight values, which are above the suggested level of acceptance (see Table 7). The following numbers indicate significant differences at p < 0.05: 1, normal vs. ‘‘total’’; 2, normal vs. unicondylar; 3, normal vs. restricted; 4, ‘‘total’’ vs. unicondylar, 5, ‘‘total’’ vs. restricted; a, over all groups.
into the third and fourth step from the bottom and the entire construction was mechanically stiff such that highly reproducible results of vertical ground reaction forces could be obtained [11]. Each subject was asked to walk or climb the stairs at his or her comfortable speed, thus, at a self-selected free gait velocity, 8–10 times (Fig. 1). Handrails were available (left and right) but the subjects were asked not to use them unless they felt unstable or insecure on the steps. Test trials with the use of handrails were omitted in all test groups except for the restricted group; these patients were allowed to use the handrails during testing. 2.3. Parameterization of vertical ground reaction forces The test parameters of the vertical ground reaction force were detected after the routine described by Stu¨ssi and Debrunner ([21]; see Fig. 2) and described in Stacoff et al. [11]. The force values were normalised to body weight. The first peak, often denoted as impact peak or Fz1, was not detected in the present study. Between Fz2 and Fz4, the
minimum (Fz3) was detected, representing unloading during midstance. The loading rate at touchdown ‘‘bn’’ and the unloading rate at take-off ‘‘en’’ describes the ‘‘intensity’’ with which the force develops or ceases (units: BW/s) and is represented best using the 80% value of Fz2 and Fz4, respectively (Fig. 2) after Stu¨ssi and Debrunner [21]. This definition has the advantage that no extreme slopes are included in the data, which would produce overestimated or underestimated values. In the present study, the parameterization of the test parameters, the measurement of the time of ground contact (T; from force data), and the average walking velocity (v) over two consecutive strides (from kinematic data) was performed for all test subjects under all five test conditions [11]. 2.4. Coefficient of variation and gait symmetry The coefficient of variation was used to determine the trial-to-trial variability of the test parameters. It is standard practice to define this coefficient as standard deviation divided by the mean times 100% [22].
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Fig. 1. Examples of ground reaction force curves in level walking and on stairs of one subject with a unicondylar implant. Stair ascent: first maximum slightly decreased with a less pronounced loading rate; stair descent: dominant first maximum at touchdown, second often not present.
The symmetry index of the vertical ground reaction force parameters was examined using the definition after Herzog et al. [23] and adapted to the absolute symmetry index after Giakas and Baltzopoulos [24] and White et al.
[22] (Eq. (1)). The advantage of the absolute symmetry index over the symmetry index is that when averaging the symmetry indices (over several test trials or over several subjects), positive and negative values do not produce a
Fig. 2. Definition and detection of selected parameters of the vertical ground reaction force measured during level walking (after Stu¨ssi and Debrunner [21]).
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Table 7 Suggested level of acceptance applied to the data of this paper
3.2. Test parameters
Test condition
Parameter
Coefficient of variation
Absolute symmetry index
Table 4 shows the comparison of the test parameters of the five test conditions and the four test groups.
Level gait
Force Loading/unloading rate
<5 <12.5
<5 <15
Force Loading/unloading rate
<12.5 <17.5
<15 <20
Stair
null value. absolute symmetry indexð%Þ X prosthesis X contralateral 100 ¼ ðX prosthesis þ X contralateral Þ 0:5
(1)
Searching for a borderline between pathological and normal gait a number of authors have suggested different levels of acceptance for the coefficient of variation and side-to-side symmetry (Tables 1 and 2). It has been argued that these levels depend on the measurement accuracy which was found to be higher for the vertical ground reaction force compared to loading rates (Herzog et al. [23]). Taking this into account for the present study, different levels of acceptance were suggested for the loading rate and vertical ground reaction force (Table 7). The study relied on the availability of the patients, which lead to inhomogeneous test groups. Consequently, all parameters were tested with a conservative non-parametric test for small samples (Kruskal–Wallis) at p < 0.05. To test the differences between the test groups, a post hoc test (ANOVA, Bonferroni) was applied.
3.2.1. Level gait Generally, only few test parameters were found to be significantly different between the patient groups and the normal group. On the prosthesis side all patient groups showed a significantly reduced ground reaction force during take-off (Fz4: between 1.06 and 1.10 BW) compared to the normal group (1.15 BW), but not on the contra lateral side (Table 4). During midstance the values of Fz3 of the normal group had the tendency to be smaller (0.72 BW) indicating a more dynamic gait pattern compared to the patient groups (between 0.74 and 0.78 BW). Over all groups the unloading rate ‘‘en’’ showed significant differences on the operated side indicating that patients unloaded slower (between 7.59 and 8.76 BW/s) during take-off than the normal subjects (9.01 BW/s). 3.2.2. Stair ascent Generally, the characteristics of the vertical ground reaction force curve of the patient groups showed a decrease at load acceptance on the operated side (Fz2: between 0.98 and 1.08 BW versus normals 1.11 BW; bn: between 3.41 and 4.84 BW/s versus normals 5.52 BW/s). During take-off Fz4 showed the tendency to be lower in all patient groups compared to the normals and en was reduced significantly on the standard stair (between 8.85 and 9.60 versus normals 10.87). Specifically, compared to the normals, the unicondylar group showed significant differences on the prosthesis side (Fz2: 0.98 BW) compared to normals (1.11 BW) on the standard stair even though they walked at about the same reduced speed as the restricted group which did not show this tendency.
3. Results 3.1. General observations and gait velocity The vertical ground reaction force curves showed considerable variations between the test conditions (Fig. 1). During stair ascent, curves showed a double waveform similar to the one known from level gait, but with the second maximum less dominant than what was expected from normal subjects [11]. In descending the stairs, the curves exhibited large variations after the first maximum with and without a second maximum (Fz4). As expected, gait velocity dropped significantly from level walking (between 1.29 and 1.35 m/s) to stair ascent (between 0.43 and 0.61 m/s) and descent (between 0.49 and 0.67 m/s), that of stair decent being between that of level gait and stair ascent (Table 4). The patient groups had the tendency to walk slower during stair ascent and descent than the normals, but the differences were not significant.
3.2.3. Stair descent Generally, contrary to what would be expected, the patient groups did not significantly decrease vertical loading (Fz2: between 1.33 and 1.55 BW) on the prosthesis side compared to normals (between 1.47 and 1.51 BW on both stairs). The ‘‘total’’ and unicondylar groups showed a tendency to load the contra lateral side more than the operated side compared to the normals (see asymmetry). The restricted group showed a reduced vertical loading (Fz2) because they were allowed to support their body weight on the railing. 3.3. Coefficient of variation The shaded areas in Table 5 indicate where the coefficient of variation went over the proposed level of acceptance. During level gait and stair ascent the patient groups increased variation significantly in one parameter compared to the normal group (en on the contra lateral side). Between patient groups, the ‘‘total’’ group had a reduced unloading
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rate (en) compared to the unicondylar and the restricted groups on the contra lateral side. During stair descent significant differences between the patient groups and the normals were only found on the steep stair. Particularly, the loading rate (bn) and Fz2 were increased in patients more than in normals (Table 5). Thus, the patient groups showed more difficulty in consistently repeating stair descent force patterns than during level gait and stair ascent. 3.4. Asymmetry The shaded areas in Table 6 indicate that side-to-side asymmetry of the patient groups went over the proposed level of acceptance in 25 out of 76 cases. During level walking the patient groups showed an increase in most parameters compared to the normals (not significant). During stair ascent and descent two comparisons were found significantly increased in the patient groups (bn on the standard stair up and Fz2 on the steep stair down). Patient groups loaded their prosthesis side less than the contra lateral side particularly during weight acceptance (Fz2 and bn, Table 4). When comparing asymmetry within each of the five test conditions, the largest was often found in the restricted group. 3.5. Comparison of cruciate-retaining versus nonretaining groups The comparison between the cruciate-retaining (ret) and the non-retaining (n-ret) group focused on the LCS-MB and Innex-CR (n = 10) and the LCS-RP and Innex-UCOR (n = 10) subjects. Of all comparisons and all parameters, only the asymmetry of the loading rate bn on the standard stair up was significantly larger in the non-retaining group, p < 0.034. In relation to other findings of this study this is surprisingly little indicating that force differences in the vertical direction between retaining and non-retaining patients were basically not present.
4. Discussion 4.1. Limitations of the study Although relatively simple to perform, studies providing ground reaction force data on normal subjects and patients performing two consecutive steps during level gait and on stairs are still rare. One possible reason for this is the difficulty in subject selection. Stair ambulation is a task, which can be mastered more easily by taller patients and those with good outcome compared to smaller patients with restrictions. Thus, the selection of patients in general and for this study in particular is influenced by the patient’s compliance; a fact which has to be kept in mind for the interpretation of the results. Nevertheless, the study results show that only by the use of the vertical ground reaction force parameters loading differences and thus, side-to-side
asymmetries can be detected between test groups with different knee conditions. However, for a more detailed analysis, i.e. a comparison of different knee prostheses, further information using kinematic measurements and muscle activity is necessary. 4.2. Level gait Good outcome patients were able to produce a normal walking speed, thus, the temporal parameters (v, T) did not differ between the test groups. However, on the operated side all patient groups showed a significantly reduced ground reaction force during take-off (Fz4; p < 0.05) indicating that they had difficulties to produce the same amount of push-off force compared to the normals and compared to the healthy side. McCrory et al. [25] found a significantly greater loading rate on the unaffected side with hip arthroplasty patients (not found in the present study); but used patients from 2 months postoperatively (present study: 13 months postoperatively). In most patient groups the decreased loading on the operated side produced an increase in asymmetry, which could be discriminated from normals using the proposed level of asymmetry. Whether these differences are substantial over time is currently not known. 4.3. Gait velocity Ground reaction force parameters of level walking are dependent on walking speed [26]. Whether this is also true for stair ambulation has not yet been thoroughly investigated. It has been speculated that differences in swing and stance phase proportions make this relationship less clear [2,6,7]. Since elderly subjects tend to walk slower than younger subjects it was suggested to use age matched subject groups when comparing patients with normals [11]. This was applied in the present study. The present results show that during level walking the patients walked at the same speed, thus, the differences in loading could not be attributed to differences in walking speed. During stair ascent and descent the patients were slightly, but not significantly slower than the normals. Thus, it was concluded that the reported reductions of the ground reaction forces were not due to speed differences but actual reductions in vertical loading. 4.4. Stair ascent and descent In general, good outcome patients were seen to climb stairs quite well but showed significant reduction of vertical loading on the operated side during weight acceptance ( p < 0.05). When going down the stairs all patients loaded the operated side up to almost two times body weight. It is possible that both of these findings are related to a muscular deficit on the operated side (i.e. quadriceps weakness) both concentrically (stair ascent) and eccentrically (stair descent). This result confirms that the external load on the prostheses
A. Stacoff et al. / Gait & Posture 26 (2007) 48–58
can be high, and is likely to be even higher internally due to muscular forces. 4.5. Comparisons between patient groups 4.5.1. The restricted group During level walking this group showed a significantly ( p < 0.05) reduced loading during take-off (Fz4) on the operated side. On the stairs the tendency was observed to reduce vertical loading in comparison to the other test groups, which may be explained by the fact that these patients were allowed to use the handrails. Side-to-side asymmetry was increased above the proposed level of acceptance and showed significant differences (bn: standard stair up and Fz2: steep stair down). During steep stair descent, the coefficient of variation was particularly increased in the loading rate bn, which may be interpreted as a hesitation of these patients in consistently repeating this difficult task. 4.5.2. Unicondylar group The unicondylar group showed no systematic difference to either other patient group indicating that the different operating procedure did not produce a systematic variation in the vertical loading patterns. It is possible, however, that kinematic differences could be detected by using a comprehensive gait analysis [27] or by using video fluoroscopy whereby the internal movement of the implant during gait is described [28,29]. 4.5.3. Retaining versus non-retaining Only one significant difference was found between these two patient groups: asymmetry in the loading rate (bn) was larger in the non-retaining compared to the retaining group on the standard stair up. It may be argued that expected differences would have to be in the for-aft direction [10,14,15], a finding which has been challenged recently by Catani et al. [16] using modern cruciate-sacrificing total knee arthroplasties (as in the present study). Kowalk et al. [30] concluded that shearing forces are probably minimised in stair climbing, possibly reducing the need for quadriceps avoidance. Thus, the fact that under the present test conditions no systematic discrepancy was found, shows, that the design differences (retaining versus non-retaining) did not have an effect on the vertical ground reaction force characteristics. This is supported by Benedetti et al. [31] who stated that kinetic gait alterations seem to be independent from the peculiarities of the geometrical implant design. On the other hand, fluoroscopic studies [29] have shown that design differences can produce considerable internal kinematic differences, which seem not to be detectable externally using force plates.
57
slight, but measurable side-to-side asymmetry. About one third of the force parameters were over the proposed level of acceptance, indicating that such levels may be a useful tool to highlight special cases. Andriacchi [32] hypothesised that individuals with a joint pathology adapt their gait, reprogramming movement patterns. Whether the present good outcome patients were still on a learning curve after their operation or not, is not known (surgery was performed at least 13 months prior to the gait study, thus, rehabilitation was considered to be completed). In order to get more insight into an ongoing adaptation or reprogramming future studies would have to investigate recovery as a function of time (see McCrory et al. [25]). 4.7. Summary and conclusions The present investigation shows, that overall, the loading patterns of the vertical ground reaction force curve of the good outcome group looked similar to that of the normal group with few distinct differences. The findings include: During level gait, a significant reduction ( p < 0.05) in the vertical loading on the operated side was found at take-off (indicated by two parameters Fz4 and en), and at weight acceptance (Fz2) and take-off (en) during stair ascent of the standard stair. During stair descent, contrary to what would be expected, the patients did not reduce load on the operated side, but increased load variation and side-to-side asymmetry. Thus, the mechanical loads on the implants were high which may be important information with respect to loading protocols of knee implant simulators. Only few but no systematic significant differences were found between the tested prostheses (Innex CR, UCOR; LCS MB, RP; unicondylar). No systematic difference was found between posterior cruciate-retaining (LCS MB and Innex CR) versus nonretaining (LCS RP and Innex UCOR) implant designs. The restricted group showed significant reductions ( p < 0.05) of several loading parameters as well as an increased side-to-side asymmetry during level walking and stair ambulation. In this study, levels of acceptance were proposed which indicate that about one third of the force parameters of good outcome patients showed a side-to-side asymmetry during level gait and stair ambulation of two consecutive steps. Thus, levels of acceptance may be a useful tool in future studies to discriminate between patient groups and controls.
Acknowledgments 4.6. Asymmetry The present study confirms that good outcome patients with total knee arthroplasties walk and climb stairs with a
The authors are grateful for the support of the Swiss National Foundation (Grant No: 3200-055651.98); Centerpulse, SULZER Medica, Orthopedics Ltd., a Zimmer
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company, Winterthur, Switzerland; Swiss Life Foundation, Zu¨rich, Switzerland and Synos Foundation, Mu¨nsingenBern, Switzerland for their support of this investigation. Many thanks go also to Bruno Leisinger, Martin Bossert and Thomas Bichsel for their help during the data analysis of the present work.
References [1] Benson LC, DesJardins JD, Harman MK, LaBerge M. Effect of stair descent loading on ultra-high molecular weight polyethylene wear in a force-controlled knee simulator. Proc Inst Mech Eng [H] 2002;216: 409–18. [2] Christina KA, Cavanagh PR. Ground reaction forces and frictional demands during stair descent: effects of age and illumination. Gait Posture 2002;15:153–8. [3] Lobo da Costa PH, Amadio AC. Ground reaction force factors and electromyographic patterns during level and stair walking in children. In: Proceedings of the, XV, congress of the international society of biomechanics; 1995. p. 564–5. [4] Riener R, Rabuffetti M, Frigo C. Stair ascent and descent at different inclinations. Gait Posture 2002;15:32–44. [5] Heinlein B, Boss S. Fatigue testing of ultracongruent polyethylene inserts in TKA. In: Proceedings of the XIX congress of the international society of biomechanics; 2003. [6] Archea JC. Environmental factors associated with stair accidents by the elderly. Clin Geriatr Med 1985;1:555–69. [7] Yu B, Kienbacher T, Growney ES, Johnson ME, An KN. Reproducibility of the kinematics and kinetics of the lower extremity during normal stair-climbing. J Orthop Res 1997;15:348–52. [8] Besser MP, Kowalk DL, Vaughan CL. Mouting and calibration of stairs on piezoelectric force platforms. Gait Posture 1993;231–5. [9] Kaufman KR, Hughes C, Morrey BF, Morrey M, An KN. Gait characteristics of patients with knee osteoarthritis. J Biomech 2001; 34:907–15. [10] Kelman GJ, Biden EN, Wyatt MP, Ritter MA, Colwell Jr CW. Gait laboratory analysis of a posterior cruciate-sparing total knee arthroplasty in stair ascent and descent. Clin Orthop 1989;21–25. discussion 25–26. [11] Stacoff A, Diezi C, Luder G, Stussi E, Kramers-de Quervain IA. Ground reaction forces on stairs: effects of stair inclination and age. Gait Posture 2005;21:24–38. [12] Zachazewski JE, Riley PO, Krebs DE. Biomechanical analysis of body mass transfer during stair ascent and descent of healthy subjects. J Rehabil Res Dev 1993;30:412–22. [13] Chao EY, Laughman RK, Schneider E, Stauffer RN. Normative data of knee joint motion and ground reaction forces in adult level walking. J Biomech 1983;16:219–33. [14] Andriacchi TP, Galante JO, Fermier RW. The influence of total kneereplacement design on walking and stair-climbing. J Bone Joint Surg Am 1982;64:1328–35. [15] Dorr LD, Ochsner JL, Gronley J, Perry J. Functional comparison of posterior cruciate-retained versus cruciate-sacrificed total knee arthroplasty. Clin Orthop 1988;36–43.
[16] Catani F, Benedetti MG, De Felice R, Buzzi R, Giannini S, Aglietti P. Mobile and fixed bearing total knee prosthesis functional comparison during stair climbing. Clin Biomech (Bristol Avon) 2003; 18:410–8. [17] Banks SA, Markovich GD, Hodge WA. In vivo kinematics of cruciateretaining and -substituting knee arthroplasties. J Arthroplasty 1997;12: 297–304. [18] Buzzi UH, Stergiou N, Kurz MJ, Hageman PA, Heidel J. Nonlinear dynamics indicates aging affects variability during gait. Clin Biomech (Bristol Avon) 2003;18:435–43. [19] Insall JN, Dorr LD, Scott RD, Scott WN. Rationale of the Knee Society clinical rating system. Clin Orthop 1989;13–4. [20] Kramers-de Quervain IA, Stu¨ssi E, Mu¨ller R, Drobny T, Munzinger U, Gschwend N. Quantitative gait analysis after bilateral total knee arthroplasty with two different systems within each subject. J Arthroplasty 1997;12:168–79. [21] Stu¨ssi E, Debrunner HU. Parameter-analyse des menschlichen ganges. Biomed Tech (Berl) 1980;25:222–4. [22] White R, Agouris I, Selbie RD, Kirkpatrick M. The variability of force platform data in normal and cerebral palsy gait. Clin Biomech (Bristol Avon) 1999;14:185–92. [23] Herzog W, Nigg BM, Read LJ, Olsson E. Asymmetries in ground reaction force patterns in normal human gait. Med Sci Sports Exerc 1989;21:110–4. [24] Giakas G, Baltzopoulos V. Time and frequency domain analysis of ground reaction forces during walking: an investigation of variability and symmetry. Gait Posture 1997;5:189–97. [25] McCrory JL, White SC, Lifeso RM. Vertical ground reaction forces: objective measures of gait following hip arthroplasty. Gait Posture 2001;14:104–9. [26] Andriacchi TP, Ogle JA, Galante JO. Walking speed as a basis for normal and abnormal gait measurements. J Biomech 1977;10:261–8. [27] Kramers-de Quervain IA, Tunesi R, Luder G, Stu¨ssi E, Stacoff A. Pattern of stair ambulation in individuals with restricted knee motion after total knee arthroplasty (TKA) in comparison to highly functional arthroplasty subjects and to a healthy control group. In: Proceedings of the XVIIIth international congress of biomechanics, xviiith international congress of biomechanics; 2001.p. A816. [28] Banks SA, Fregly BJ, Boniforti F, Reinschmidt C, Romagnoli S. Comparing in vivo kinematics of unicondylar and bi-unicondylar knee replacements. Knee Surg Sports Traumatol Arthrosc 2005;13:551–6. [29] Banks SA. Understanding knee arthroplasty kinematics: news you can use. In: Insall JN, Scott RD, editors. Surgery of the knee. Elsevier; 2006 [chapter 11]. [30] Kowalk DL, Duncan JA, McCue 3rd FC, Vaughan CL. Anterior cruciate ligament reconstruction and joint dynamics during stair climbing. Med Sci Sports Exerc 1997;29:1406–13. [31] Benedetti MG, Catani F, Bilotta TW, Marcacci M, Mariani E, Giannini S. Muscle activation pattern and gait biomechanics after total knee replacement. Clin Biomech (Bristol Avon) 2003;18:871–6. [32] Andriacchi TP. Dynamics of pathological motion: applied to the anterior cruciate deficient knee. J Biomech 1990;23(Suppl. 1):99–105. [33] Borden LS, Perry JE, Davis BL, Owings TM, Grabiner MD. A biomechanical evaluation of one-stage vs two-stage bilateral knee arthroplasty patients. Gait Posture 1999;9(1):24–30. [34] Masani K, Kouzaki M, Fukunaga T. Variability of ground reaction forces during treadmill walking. J Appl Physiol 2002;92:1885–90.
Ground reaction forces on stairs Part II: Knee implant patients versus normals Alex Stacoff a,*, Ine`s A. Kramers-de Quervain a,b, Gerhard Luder a, Renate List a, Edgar Stu¨ssi a a
Institute for Biomechanics, ETH Zu¨rich, Schlieren, Switzerland b Schulthess Clinic, Zu¨rich, Switzerland
Received 15 December 2005; received in revised form 11 July 2006; accepted 27 July 2006
Abstract The goal of this study was to compare selected parameters of vertical ground reaction forces (GRF) of good outcome patients with different prosthesis designs with a matched control group during level walking, stair ascent and descent. Forty subjects, 29 with three main implant designs (including four subjects with a passive knee flexion restriction), and 11 healthy controls were measured with 8–10 repetitions. Vertical ground reaction forces were measured during two consecutive steps with force plates embedded in the walkway and the staircase. Defined parameters of the force signals were used to compare the results of the test groups. The results show, that, postoperatively, good outcome patients produce gait patterns of the vertical ground reaction force which are comparable to normal healthy subjects with the exception of a few distinct differences: a significant reduction ( p < 0.05) in the vertical loading on the operated side during level walking at take-off, at weight acceptance and take-off during stair ascent of the normal stair. During stair descent, the patients did not reduce load on the operated side, but increased load variation and side-to-side asymmetry; thus, the mechanical loads on the implants were high, which may be important information with respect to loading protocols of knee implant simulators. No systematic differences in any of the test parameters were found between posterior cruciate-retaining (LCS MB and Innex CR) versus non-retaining (LCS RP and Innex UCOR) implant designs. The restricted group showed significant reductions ( p < 0.05) of several loading parameters as well as an increased side-to-side asymmetry. About one third of the force parameters of the good outcome patients showed a side-to-side asymmetry between two consecutive steps, which was over a proposed level of acceptance. # 2006 Elsevier B.V. All rights reserved. Keywords: Gait analysis; Knee prosthesis; Biomechanics
1. Introduction Currently, about 400,000 total knee arthroplasties (TKA) are implanted worldwide each year. With respect to the wear of these implants, walking has been identified as the most important activity [1]. Howver, little biomechanical information is available about the loading at the lower extremities in more demanding daily activities such as stair ascent and descent [2–4]. Thus, the * Corresponding author at: Institute for Biomechanics, Department of Materials, ETH Ho¨nggerberg, HCI 365.1, 8093 Zu¨rich, Switzerland. Tel.: +41 44 633 62 18. E-mail address: [email protected] (A. Stacoff). 0966-6362/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.gaitpost.2006.07.015
question of interest is whether patients with implants increase or decrease loading during stair ambulation compared to normals, and whether they show increased variability and/or side-to-side asymmetry. Such information would be valuable not only from the orthopaedic point of view but also with respect to loading protocols of simulators, which approximate the forces during walking and stair ambulation [1,5]. Furthermore, for patients with a restricted passive range of motion after surgery only limited information is available with respect to loading patterns. Using stairs has been recognised as a considerably more demanding task than level walking [1,2,6,7]. On the other hand, the number of instrumented stair cases
A. Stacoff et al. / Gait & Posture 26 (2007) 48–58
equipped with two force plates which allow the analyses of external loads during two consecutive steps [2–4,7–12] is still limited. Whereas during level gait ground reaction force (GRF) values reach about 1.1–1.2 body weight (BW) [13], values for stair ascent reach 1.4 BW and for stair descent 1.8 BW [2,3,11]. In a previous study of the same institution [11] it was shown that age and stair inclination influence vertical ground reaction forces and it was suggested to consider them when comparing normal subjects with patients. Whether patients with total knee arthroplasties alter the loading of the lower extremities in favour of the nonoperated side or whether they allow asymmetrical loading during stair ambulation remains controversial in the literature. Older studies [10,14,15] have demonstrated that patients with total knee arthroplasty show a deficit compared to normals while ascending stairs. In particular, those with cruciate-sacrificing prostheses have been associated with femoral roll-back during flexion [16] (although not confirmed by fluoroscopy [17]). In a recent study, Catani et al. [16] showed that the cruciate-retaining group behaved like cruciate-sacrificing knees and it was suggested that the antero-posterior constraint structures (ligamentous or mechanical by the implant) are important to maintain more physiological knee kinematics. Keeping this controversy in mind, suggestions with respect to an acceptable level of variability and asymmetry
49
in ground reaction force parameters are difficult to establish and are not readily available (see Table 1). Only few authors have reported vertical ground reaction force parameters of two consecutive steps during level walking, fewer still have made suggestions with respect to acceptable levels of variability and asymmetry, and only one has reported test parameters on stairs (Table 2). Recently, Buzzi et al. [18] suggested that elderly subjects show increased local instability or inability to compensate for the natural stride-to-stride variations, i.e. an increased variability of kinematic parameters during gait. A similar increase in variability with age was found by Stacoff et al. [11] with kinetic parameters. Thus, a guideline or level of acceptance would be helpful for the interpretation of patient data. Thus, the purpose of this study was to test whether the vertical ground reaction forces of two consecutive steps of subjects with total knee arthroplasty differ from normal subjects during level gait and stair ambulation, considering selected force parameters and their respective variability and side-to-side asymmetry. The underlying objective was to test whether vertical ground reaction forces contain enough information to distinguish test groups with different knee conditions. For that reason not only good outcome patients were chosen with a total knee arthroplasty, but also patients with a unicondylar implant, as well as patients with a restricted range of motion.
Table 1 Adapted values from previous publications of coefficient of variation and absolute symmetry index during level walking Author
Subject description age in years
Parameter
Borden et al. [33]
13 patients 68 (7) 9 normal subjects 68 (6)
Force Force
Giakas and Baltzopoulos [24]
10 normal males 23.3 (2.4)
Force
Herzog et al. [23]a
62 normals: 33 males (mean 27) 29 females (mean 25)
Force Loading rate Unloading rate
Masani et al. [34]
10 normal males 28.8 (5.2)
Force: 3 km /h Force: 8 km /h
McCrory et al. [25]a
35 normals 27.5 (5.7)
Force Loading rate Unloading rate Force Loading rate Unloading rate
27 subjects with hip arthroplasty 59.7 (13.8)
Coefficient of variation
Absolute symmetry index
Reported (%)
Reported (%)
Suggested level of acceptance (%)
Suggested level of acceptance (%)
1–2 1–2 2–3
<10
3–5
<10
1 to +1 4 0 2 3–7 0–1 1 1 1–4 20 3
Stacoff et al. [11]
20 normals 58.3 (9.9 years)
Force Loading rate Unloading rate
2–5 8–10 5
White et al. [22]
15 normal children 11 cerebral palsy children
Force Force
7–15 10–22
3–5 12 7 <12.5
The acceptable levels are suggestions made by the corresponding authors. a Calculated as symmetry index (after Herzog et al. [23]), which allows for positive and negative values.
5–14 13–20
<10
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A. Stacoff et al. / Gait & Posture 26 (2007) 48–58
Table 2 Adapted values from a previous publication of the coefficients of variation and absolute symmetry index during stair ascent and descent including subjects of younger age (below 40 years) Author
Subject description age in years
Parameter
Coefficient of variation reported (%)
Absolute symmetry index reported (%)
Stacoff et al. [11]
20 healthy subjects 58.3 (19.9)
Force Loading rate Unloading rate
4–10 10–15 8–11
5–15 13–20 10–15
It was not the intention of this study to compare preoperative with postoperative measurements.
2. Methods 2.1. Subjects and implants Written consent was given by 40 subjects, 25 good outcome patients with different implants, four patients with restricted (<908 knee flexion) passive range of motion (RoM), and 11 control subjects (normals) (Table 3). Preoperatively, all patients suffered from advanced osteoarthritis and were (due to pain) not capable of performing alternating stair ambulation nor walking without support or a severe limp; thus, preoperative measurements were not considered a meaningful baseline for comparison and were not collected in this study. Approval was obtained from the local ethic committee. The normal subjects are a subset of a previous study [11] and were matched in age and gender to the patient groups of the present study. Surgery was performed 13–38 months prior to the enrolment in this study. The selection criteria for the good outcome subjects were: (i) comparable age, height and weight to the normal group (see Table 3), (ii) fully recovered and rehabilitated, (iii) postoperative and radiological results rated as excellent (except for the restricted knee group), (iv) Knee Society Score over 90 (Table 3, after Insall et al. [19]), passive flexion well over 1008, (v) free from any relevant functional pathology in all other joints of the lower extremities and (vi) the operated and the contra lateral knees free of pain during testing. All subjects stated
that they could walk without restrictions for several hours and that they were able to climb and descend stairs without the use of a handrail (exception: restricted group). Subjects with a leg length discrepancy of more than 1 cm were not included in the study. The restricted group consisted of subjects who had developed adhesions and their knee flexion was reduced below 908 after rehabilitation; they had previous surgery and complications, were all males and were larger and heavier in comparison. Because stair ambulation is difficult with restricted knee flexion this test group was not easy to recruit. The following implant types were used: (a) unicondylar implant (Allegretto, Centerpulse Orthopedics Ltd., Winterthur, Switzerland; a Zimmer company) for patients with a unicompartimental disease, whereby the medial condyle was replaced and the cruciate ligaments were retained; (b) Low-Contact-Stress implant LCS1 (DePuy, a Johnson & Johnson company, Warsaw, USA) with a Rotating Platform (RP) design, both cruciate ligaments sacrificed; (c) LCS1 with Meniscal Bearings (MB), posterior cruciate ligament retaining; (d) INNEXTM (Centerpulse Orthopedics Ltd., Winterthur, Switzerland; a Zimmer company) with cruciate ligament retaining CR; and (e) INNEXTM with cruciate ligaments sacrificed (UCOR). Groups (b–e) are total mobile bearing knee arthroplasties and are summarised as the ‘‘total’’ group in Tables 4–6 (with 20 subjects). The restricted group had LCS1 RP prostheses. The rationale for the choice of the implants was: (i) the severity of osteoarthritis (total versus unicondylar) and (ii) change in design (LCS older than Innex). All patients were operated at the same orthopaedic hospital in Zurich, Switzerland.
Table 3 Subject specifications Subjects TM
a
INNEX CR INNEXTM UCORa LCS1 MBa LCS1 RPa Unicondylar Restricted Normal
Gender
Age
2male 2f 3m 1f 2m 4f 2m 4f 3m 2f 4m 6m 5f
65.3 72.5 63.5 67.8 67.2 66.8 69.5
(6.2) (4.7) (8.6) (7.8) (3.9) (7.4) (6.0)
Leg length (m)
Height (m)
Weight (kg)
Passive Flexion ROM (8)
Knee Society Scores
0.91 0.91 0.86 0.89 0.90 0.95 0.88
1.71 1.75 1.64 1.69 1.76 1.79 1.68
76.4 87.7 75.8 76.2 77.7 85.2 72.4
118; 110–130 126; 120–135 118; 110–125 115; 100–125 127; 115–140 84; 75–90 (8) 136; 130–150
97.75; (97–99) (0.96) 99.25; (97–100) (1.5) 96.83; (95–100) (1.94) 97.5; (92–100) (2.95) 95.6 (91–100) (3.58) 81.25 (52–94) (19.65) 100
(0.05) (0.05) (0.04) (0.07) (0.06) (0.02) (0.04)
(0.10) (0.10) (0.06) (0.11) (0.08) (0.06) (0.06)
(15.1) (13.4) (16.3) (13.2) (5.6) (11.2) (8.7) 1
(9) (8) (5) (10) (10) (7) 1
All listed subjects were tested during level gait and on the standard stair. On the steep stair, only 3 LCS MB and 3 LCS RP were available. The Knee Society Scores are based on Insall et al. [19]. The selection of normal subjects is based on previous work (Stacoff et al. [11]). Abbreviations—MB: meniscal bearing, posterior cruciate retaining; RP: rotating platform (non-retaining); CR: cruciate retaining; UCOR: ultra congruent, only rotating (non-retaining). a Indicates that the INNEXTM and LCS1 groups were summarised in one group denoted as ‘‘total’’ (see Tables 4–6).
Table 4 Group means (S.D.) of the four test groups Parameters Left/Prosthesis
Right/Contra lateral
Patient group v (m/s)
T (m/s)
Fz2 (BW)
Fz3 (BW)
Fz4 (BW)
Level gait
Normal ‘‘Total’’ Unicondylar Restricted
1.30 1.31 1.35 1.29
(0.13) (0.18) (0.14) (0.15)
673.6 665.3 681.0 700.9
1.18 1.15 1.11 1.09
0.72 0.74 0.77 0.78
(0.08) (0.07) (0.05) (0.06)
1.15a,2,3 (0.05) 1.10 (0.06) 1.062 (0.05) 1.063 (0.03)
7.57 7.66 6.44 7.26
Stand up
Normal ‘‘Total’’ Unicondylar Restricted
0.61 0.57 0.50 0.51
(0.08) (0.07) (0.12) (0.10)
683.9a,2 (115.1) 775.7 (120.5) 911.72 (265.1) 856.6 (143.0)
1.11a,2 (0.08) 1.04 (0.07) 0.982 (0.05) 1.04 (0.05)
0.72 0.71 0.79 0.78
(0.05) (0.09) (0.08) (0.10)
1.21 1.18 1.17 1.14
(0.11) (0.08) (0.13) (0.09)
5.52a (1.07) 10.87a (1.25) 4.37 (1.42) 9.60 (1.50) 3.79 (1.73) 9.36 (2.58) 4.38 (1.25) 8.85 (0.62)
Steep up
Normal ‘‘Total’’ Unicondylar Restricted
0.51 0.50 0.46 0.43
(0.06) (0.07) (0.07) (0.09)
745.3 802.1 875.9 915.8
(115.2) (116.8) (147.0) (177.2)
1.11 1.08 1.01 1.06
(0.06) (0.10) (0.06) (0.05)
0.71 0.67 0.77 0.78
(0.09) (0.10) (0.07) (0.13)
1.25 1.23 1.16 1.19
(0.16) (0.10) (0.14) (0.10)
4.84 4.30 3.41 3.85
(0.90) (1.42) (0.85) (0.68)
11.12 10.06 9.31 9.02
Stand down Normal ‘‘Total’’ Unicondylar Restricted
0.67 0.65 0.58 0.53
(0.08) (0.10) (0.12) (0.14)
588.1 663.7 752.5 812.7
(116.6) (120.6) (159.2) (181.6)
1.51 1.55 1.40 1.48
(0.15) (0.16) (0.13) (0.15)
– – – –
– – – –
12.86 14.27 11.75 13.44
(2.55) (4.78) (4.96) (2.99)
– – – –
Steep down Normal ‘‘Total’’ Unicondylar Restricted
0.62 0.58 0.51 0.49
(0.08) (0.10) (0.05) (0.12)
626.4 665.2 747.6 781.3
(107.6) (109.4) (88.9) (189.6)
1.47 1.65 1.53 1.33
(0.20) (0.26) (0.25) (0.38)
– – – –
– – – –
12.95 17.01 17.07 14.26
(3.59) (5.96) (11.35) (5.44)
– – – –
(42.9) (59.4) (58.2) (53.4)
(0.12) (0.12) (0.06) (0.04)
bn (BW/s) (1.63) (2.00) (1.18) (0.76)
en (BW/s) 9.01a (0.98) 8.76 (1.05) 8.02 (0.65) 7.59 (0.60)
(1.22) (1.48) (2.24) (1.05)
T (m/s)
Fz2 (BW)
Fz3 (BW)
Fz4 (BW)
678.9 667.1 689.0 723.1
1.19 1.17 1.16 1.18
(0.13) (0.12) (0.06) (0.12)
0.72 0.73 0.72 0.73
(0.09) (0.09) (0.07) (0.10)
1.14 1.13 1.13 1.10
(0.07) (0.06) (0.06) (0.02)
7.30 8.01 7.29 7.74
(1.50) (2.92) (1.19) (2.08)
9.20 9.12 8.60 8.01
(1.06) (1.07) (1.16) (0.41)
686.4a,2 (116.6) 772.7 (108.9) 889.02 (221.5) 889.8 (171.8)
1.11 1.08 1.04 1.11
(0.07) (0.09) (0.05) (0.09)
0.68 0.70 0.78 0.67
(0.03) (0.08) (0.08) (0.13)
1.20 1.26 1.29 1.20
(0.10) (0.10) (0.14) (0.17)
5.57 4.99 4.14 4.88
(0.94) (1.49) (1.48) (0.76)
10.80 10.34 10.45 8.44
(1.41) (1.24) (2.76) (2.69)
747.2 801.4 875.5 949.1
1.10 1.11 1.07 1.12
(0.10) (0.08) (0.04) (0.03)
0.65 0.67 0.75 0.62
(0.07) (0.08) (0.04) (0.15)
1.21 1.32 1.28 1.22
(0.15) (0.15) (0.17) (0.24)
4.92 4.76 4.13 4.41
(0.99) (1.49) (1.16) (0.38)
10.69 10.93 10.20 8.21
(1.58) (1.21) (2.54) (3.51)
601.3a,3 (125.1) 661.535 (105.6) 763.5 (138.6) 877.73,5 (260.2)
1.53 1.66 1.60 1.44
(0.16) (0.27) (0.21) (0.12)
– – – –
– – – –
12.61 15.40 13.40 11.04
(2.77) (6.62) (6.10) (2.53)
– – – –
644.7a (102.4) 653.2 (90.3) 751.7 (64.8) 853.4 (280.5)
1.53 1.71 1.68 1.32
(0.21) (0.36) (0.25) (0.41)
– – – –
– – – –
13.37 17.45 17.73 11.90
(3.80) (7.29) (11.04) (5.99)
– – – –
(41.5) (57.3) (52.7) (63.5)
(121.2) (109.7) (143.8) (196.1)
bn (BW/s)
en (BW/s)
A. Stacoff et al. / Gait & Posture 26 (2007) 48–58
Test
‘‘Total’’ stands for all INNEX and LCS subjects combined. Stand, standard stair. Left/right refers to the control group. The following numbers indicate significant differences at p < 0.05: 1, normal vs. ‘‘total’’; 2, normal vs. unicondylar; 3, normal vs. restricted; 4, ‘‘total’’ vs. unicondylar; 5, ‘‘total’’ vs. restricted; a, over all groups.
51
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A. Stacoff et al. / Gait & Posture 26 (2007) 48–58
Table 5 Mean values (S.D.) of the coefficient of variation of all four test groups and test conditions
‘‘Total’’ stands for all INNEX and LCS subjects combined. Stand, standard stair. Left/right refers to the control group. The shaded areas highlight values, which are above the suggested level of acceptance (see Table 7). The following numbers indicate significant differences at p < 0.05: 1, normal vs. ‘‘total’’; 2, normal vs. unicondylar; 3, normal vs. restricted; 4, ‘‘total’’ vs. unicondylar; 5, ‘‘total’’ vs. restricted; a, over all groups.
2.2. Test set-up of the instrumented gait analysis The gait analysis system at the Institute for Biomechanics at the ETH Zu¨rich, Switzerland allows collecting kinematic, kinetic and EMG data simultaneously while the test subjects walk over a 25 m walkway or climb a set of
stairs [20]. Stair ambulation was performed on two sets of stairs, the standard inclination with a 29 cm run and a 17 cm rise (i.e. an inclination of 30.58) and the steep inclination with a 23 cm run and a 20 cm rise (inclination of 418). Two force plates (KISTLER 9068A, Winterthur, Switzerland), sampling at a rate of 2000 Hz was embedded
A. Stacoff et al. / Gait & Posture 26 (2007) 48–58
53
Table 6 Group means (S.D.) of the asymmetry index of the four test groups
‘‘Total’’ stands for all INNEX and LCS subjects combined. Stand, standard stair. The shaded areas highlight values, which are above the suggested level of acceptance (see Table 7). The following numbers indicate significant differences at p < 0.05: 1, normal vs. ‘‘total’’; 2, normal vs. unicondylar; 3, normal vs. restricted; 4, ‘‘total’’ vs. unicondylar, 5, ‘‘total’’ vs. restricted; a, over all groups.
into the third and fourth step from the bottom and the entire construction was mechanically stiff such that highly reproducible results of vertical ground reaction forces could be obtained [11]. Each subject was asked to walk or climb the stairs at his or her comfortable speed, thus, at a self-selected free gait velocity, 8–10 times (Fig. 1). Handrails were available (left and right) but the subjects were asked not to use them unless they felt unstable or insecure on the steps. Test trials with the use of handrails were omitted in all test groups except for the restricted group; these patients were allowed to use the handrails during testing. 2.3. Parameterization of vertical ground reaction forces The test parameters of the vertical ground reaction force were detected after the routine described by Stu¨ssi and Debrunner ([21]; see Fig. 2) and described in Stacoff et al. [11]. The force values were normalised to body weight. The first peak, often denoted as impact peak or Fz1, was not detected in the present study. Between Fz2 and Fz4, the
minimum (Fz3) was detected, representing unloading during midstance. The loading rate at touchdown ‘‘bn’’ and the unloading rate at take-off ‘‘en’’ describes the ‘‘intensity’’ with which the force develops or ceases (units: BW/s) and is represented best using the 80% value of Fz2 and Fz4, respectively (Fig. 2) after Stu¨ssi and Debrunner [21]. This definition has the advantage that no extreme slopes are included in the data, which would produce overestimated or underestimated values. In the present study, the parameterization of the test parameters, the measurement of the time of ground contact (T; from force data), and the average walking velocity (v) over two consecutive strides (from kinematic data) was performed for all test subjects under all five test conditions [11]. 2.4. Coefficient of variation and gait symmetry The coefficient of variation was used to determine the trial-to-trial variability of the test parameters. It is standard practice to define this coefficient as standard deviation divided by the mean times 100% [22].
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A. Stacoff et al. / Gait & Posture 26 (2007) 48–58
Fig. 1. Examples of ground reaction force curves in level walking and on stairs of one subject with a unicondylar implant. Stair ascent: first maximum slightly decreased with a less pronounced loading rate; stair descent: dominant first maximum at touchdown, second often not present.
The symmetry index of the vertical ground reaction force parameters was examined using the definition after Herzog et al. [23] and adapted to the absolute symmetry index after Giakas and Baltzopoulos [24] and White et al.
[22] (Eq. (1)). The advantage of the absolute symmetry index over the symmetry index is that when averaging the symmetry indices (over several test trials or over several subjects), positive and negative values do not produce a
Fig. 2. Definition and detection of selected parameters of the vertical ground reaction force measured during level walking (after Stu¨ssi and Debrunner [21]).
A. Stacoff et al. / Gait & Posture 26 (2007) 48–58
55
Table 7 Suggested level of acceptance applied to the data of this paper
3.2. Test parameters
Test condition
Parameter
Coefficient of variation
Absolute symmetry index
Table 4 shows the comparison of the test parameters of the five test conditions and the four test groups.
Level gait
Force Loading/unloading rate
<5 <12.5
<5 <15
Force Loading/unloading rate
<12.5 <17.5
<15 <20
Stair
null value. absolute symmetry indexð%Þ X prosthesis X contralateral 100 ¼ ðX prosthesis þ X contralateral Þ 0:5
(1)
Searching for a borderline between pathological and normal gait a number of authors have suggested different levels of acceptance for the coefficient of variation and side-to-side symmetry (Tables 1 and 2). It has been argued that these levels depend on the measurement accuracy which was found to be higher for the vertical ground reaction force compared to loading rates (Herzog et al. [23]). Taking this into account for the present study, different levels of acceptance were suggested for the loading rate and vertical ground reaction force (Table 7). The study relied on the availability of the patients, which lead to inhomogeneous test groups. Consequently, all parameters were tested with a conservative non-parametric test for small samples (Kruskal–Wallis) at p < 0.05. To test the differences between the test groups, a post hoc test (ANOVA, Bonferroni) was applied.
3.2.1. Level gait Generally, only few test parameters were found to be significantly different between the patient groups and the normal group. On the prosthesis side all patient groups showed a significantly reduced ground reaction force during take-off (Fz4: between 1.06 and 1.10 BW) compared to the normal group (1.15 BW), but not on the contra lateral side (Table 4). During midstance the values of Fz3 of the normal group had the tendency to be smaller (0.72 BW) indicating a more dynamic gait pattern compared to the patient groups (between 0.74 and 0.78 BW). Over all groups the unloading rate ‘‘en’’ showed significant differences on the operated side indicating that patients unloaded slower (between 7.59 and 8.76 BW/s) during take-off than the normal subjects (9.01 BW/s). 3.2.2. Stair ascent Generally, the characteristics of the vertical ground reaction force curve of the patient groups showed a decrease at load acceptance on the operated side (Fz2: between 0.98 and 1.08 BW versus normals 1.11 BW; bn: between 3.41 and 4.84 BW/s versus normals 5.52 BW/s). During take-off Fz4 showed the tendency to be lower in all patient groups compared to the normals and en was reduced significantly on the standard stair (between 8.85 and 9.60 versus normals 10.87). Specifically, compared to the normals, the unicondylar group showed significant differences on the prosthesis side (Fz2: 0.98 BW) compared to normals (1.11 BW) on the standard stair even though they walked at about the same reduced speed as the restricted group which did not show this tendency.
3. Results 3.1. General observations and gait velocity The vertical ground reaction force curves showed considerable variations between the test conditions (Fig. 1). During stair ascent, curves showed a double waveform similar to the one known from level gait, but with the second maximum less dominant than what was expected from normal subjects [11]. In descending the stairs, the curves exhibited large variations after the first maximum with and without a second maximum (Fz4). As expected, gait velocity dropped significantly from level walking (between 1.29 and 1.35 m/s) to stair ascent (between 0.43 and 0.61 m/s) and descent (between 0.49 and 0.67 m/s), that of stair decent being between that of level gait and stair ascent (Table 4). The patient groups had the tendency to walk slower during stair ascent and descent than the normals, but the differences were not significant.
3.2.3. Stair descent Generally, contrary to what would be expected, the patient groups did not significantly decrease vertical loading (Fz2: between 1.33 and 1.55 BW) on the prosthesis side compared to normals (between 1.47 and 1.51 BW on both stairs). The ‘‘total’’ and unicondylar groups showed a tendency to load the contra lateral side more than the operated side compared to the normals (see asymmetry). The restricted group showed a reduced vertical loading (Fz2) because they were allowed to support their body weight on the railing. 3.3. Coefficient of variation The shaded areas in Table 5 indicate where the coefficient of variation went over the proposed level of acceptance. During level gait and stair ascent the patient groups increased variation significantly in one parameter compared to the normal group (en on the contra lateral side). Between patient groups, the ‘‘total’’ group had a reduced unloading
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A. Stacoff et al. / Gait & Posture 26 (2007) 48–58
rate (en) compared to the unicondylar and the restricted groups on the contra lateral side. During stair descent significant differences between the patient groups and the normals were only found on the steep stair. Particularly, the loading rate (bn) and Fz2 were increased in patients more than in normals (Table 5). Thus, the patient groups showed more difficulty in consistently repeating stair descent force patterns than during level gait and stair ascent. 3.4. Asymmetry The shaded areas in Table 6 indicate that side-to-side asymmetry of the patient groups went over the proposed level of acceptance in 25 out of 76 cases. During level walking the patient groups showed an increase in most parameters compared to the normals (not significant). During stair ascent and descent two comparisons were found significantly increased in the patient groups (bn on the standard stair up and Fz2 on the steep stair down). Patient groups loaded their prosthesis side less than the contra lateral side particularly during weight acceptance (Fz2 and bn, Table 4). When comparing asymmetry within each of the five test conditions, the largest was often found in the restricted group. 3.5. Comparison of cruciate-retaining versus nonretaining groups The comparison between the cruciate-retaining (ret) and the non-retaining (n-ret) group focused on the LCS-MB and Innex-CR (n = 10) and the LCS-RP and Innex-UCOR (n = 10) subjects. Of all comparisons and all parameters, only the asymmetry of the loading rate bn on the standard stair up was significantly larger in the non-retaining group, p < 0.034. In relation to other findings of this study this is surprisingly little indicating that force differences in the vertical direction between retaining and non-retaining patients were basically not present.
4. Discussion 4.1. Limitations of the study Although relatively simple to perform, studies providing ground reaction force data on normal subjects and patients performing two consecutive steps during level gait and on stairs are still rare. One possible reason for this is the difficulty in subject selection. Stair ambulation is a task, which can be mastered more easily by taller patients and those with good outcome compared to smaller patients with restrictions. Thus, the selection of patients in general and for this study in particular is influenced by the patient’s compliance; a fact which has to be kept in mind for the interpretation of the results. Nevertheless, the study results show that only by the use of the vertical ground reaction force parameters loading differences and thus, side-to-side
asymmetries can be detected between test groups with different knee conditions. However, for a more detailed analysis, i.e. a comparison of different knee prostheses, further information using kinematic measurements and muscle activity is necessary. 4.2. Level gait Good outcome patients were able to produce a normal walking speed, thus, the temporal parameters (v, T) did not differ between the test groups. However, on the operated side all patient groups showed a significantly reduced ground reaction force during take-off (Fz4; p < 0.05) indicating that they had difficulties to produce the same amount of push-off force compared to the normals and compared to the healthy side. McCrory et al. [25] found a significantly greater loading rate on the unaffected side with hip arthroplasty patients (not found in the present study); but used patients from 2 months postoperatively (present study: 13 months postoperatively). In most patient groups the decreased loading on the operated side produced an increase in asymmetry, which could be discriminated from normals using the proposed level of asymmetry. Whether these differences are substantial over time is currently not known. 4.3. Gait velocity Ground reaction force parameters of level walking are dependent on walking speed [26]. Whether this is also true for stair ambulation has not yet been thoroughly investigated. It has been speculated that differences in swing and stance phase proportions make this relationship less clear [2,6,7]. Since elderly subjects tend to walk slower than younger subjects it was suggested to use age matched subject groups when comparing patients with normals [11]. This was applied in the present study. The present results show that during level walking the patients walked at the same speed, thus, the differences in loading could not be attributed to differences in walking speed. During stair ascent and descent the patients were slightly, but not significantly slower than the normals. Thus, it was concluded that the reported reductions of the ground reaction forces were not due to speed differences but actual reductions in vertical loading. 4.4. Stair ascent and descent In general, good outcome patients were seen to climb stairs quite well but showed significant reduction of vertical loading on the operated side during weight acceptance ( p < 0.05). When going down the stairs all patients loaded the operated side up to almost two times body weight. It is possible that both of these findings are related to a muscular deficit on the operated side (i.e. quadriceps weakness) both concentrically (stair ascent) and eccentrically (stair descent). This result confirms that the external load on the prostheses
A. Stacoff et al. / Gait & Posture 26 (2007) 48–58
can be high, and is likely to be even higher internally due to muscular forces. 4.5. Comparisons between patient groups 4.5.1. The restricted group During level walking this group showed a significantly ( p < 0.05) reduced loading during take-off (Fz4) on the operated side. On the stairs the tendency was observed to reduce vertical loading in comparison to the other test groups, which may be explained by the fact that these patients were allowed to use the handrails. Side-to-side asymmetry was increased above the proposed level of acceptance and showed significant differences (bn: standard stair up and Fz2: steep stair down). During steep stair descent, the coefficient of variation was particularly increased in the loading rate bn, which may be interpreted as a hesitation of these patients in consistently repeating this difficult task. 4.5.2. Unicondylar group The unicondylar group showed no systematic difference to either other patient group indicating that the different operating procedure did not produce a systematic variation in the vertical loading patterns. It is possible, however, that kinematic differences could be detected by using a comprehensive gait analysis [27] or by using video fluoroscopy whereby the internal movement of the implant during gait is described [28,29]. 4.5.3. Retaining versus non-retaining Only one significant difference was found between these two patient groups: asymmetry in the loading rate (bn) was larger in the non-retaining compared to the retaining group on the standard stair up. It may be argued that expected differences would have to be in the for-aft direction [10,14,15], a finding which has been challenged recently by Catani et al. [16] using modern cruciate-sacrificing total knee arthroplasties (as in the present study). Kowalk et al. [30] concluded that shearing forces are probably minimised in stair climbing, possibly reducing the need for quadriceps avoidance. Thus, the fact that under the present test conditions no systematic discrepancy was found, shows, that the design differences (retaining versus non-retaining) did not have an effect on the vertical ground reaction force characteristics. This is supported by Benedetti et al. [31] who stated that kinetic gait alterations seem to be independent from the peculiarities of the geometrical implant design. On the other hand, fluoroscopic studies [29] have shown that design differences can produce considerable internal kinematic differences, which seem not to be detectable externally using force plates.
57
slight, but measurable side-to-side asymmetry. About one third of the force parameters were over the proposed level of acceptance, indicating that such levels may be a useful tool to highlight special cases. Andriacchi [32] hypothesised that individuals with a joint pathology adapt their gait, reprogramming movement patterns. Whether the present good outcome patients were still on a learning curve after their operation or not, is not known (surgery was performed at least 13 months prior to the gait study, thus, rehabilitation was considered to be completed). In order to get more insight into an ongoing adaptation or reprogramming future studies would have to investigate recovery as a function of time (see McCrory et al. [25]). 4.7. Summary and conclusions The present investigation shows, that overall, the loading patterns of the vertical ground reaction force curve of the good outcome group looked similar to that of the normal group with few distinct differences. The findings include: During level gait, a significant reduction ( p < 0.05) in the vertical loading on the operated side was found at take-off (indicated by two parameters Fz4 and en), and at weight acceptance (Fz2) and take-off (en) during stair ascent of the standard stair. During stair descent, contrary to what would be expected, the patients did not reduce load on the operated side, but increased load variation and side-to-side asymmetry. Thus, the mechanical loads on the implants were high which may be important information with respect to loading protocols of knee implant simulators. Only few but no systematic significant differences were found between the tested prostheses (Innex CR, UCOR; LCS MB, RP; unicondylar). No systematic difference was found between posterior cruciate-retaining (LCS MB and Innex CR) versus nonretaining (LCS RP and Innex UCOR) implant designs. The restricted group showed significant reductions ( p < 0.05) of several loading parameters as well as an increased side-to-side asymmetry during level walking and stair ambulation. In this study, levels of acceptance were proposed which indicate that about one third of the force parameters of good outcome patients showed a side-to-side asymmetry during level gait and stair ambulation of two consecutive steps. Thus, levels of acceptance may be a useful tool in future studies to discriminate between patient groups and controls.
Acknowledgments 4.6. Asymmetry The present study confirms that good outcome patients with total knee arthroplasties walk and climb stairs with a
The authors are grateful for the support of the Swiss National Foundation (Grant No: 3200-055651.98); Centerpulse, SULZER Medica, Orthopedics Ltd., a Zimmer
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company, Winterthur, Switzerland; Swiss Life Foundation, Zu¨rich, Switzerland and Synos Foundation, Mu¨nsingenBern, Switzerland for their support of this investigation. Many thanks go also to Bruno Leisinger, Martin Bossert and Thomas Bichsel for their help during the data analysis of the present work.
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