Pain intensity and passive joint mobility assessments
The average pre-surgery Hospital for Special Surgery (HSS) score was 59.1 (+/- 10.15) with a maximum of 100. The mean pain Visual Analog Scale (VAS) value was 49 mm (+/- 9) (VAS; worst pain ever 100 mm, no pain 0 mm). The mean post-surgery HSS score was 80.8 (+/- 8.4) and the VAS value was 7 (+/- 9). The VAS value vas obtained just after stepping-down task. The passive mobility of the knee joint was tested for all patients. The average pre- and post-surgery mobilities were 113 degrees (+/- 21) and 105° (+/- 18) respectively, whereas the mobility of the sound knee was 128° (+/- 11).
Anticipatory Postural Adjustments
The osteoarthritis patients exhibited an increase of the duration of the postural phase (T1-Tbal; Fig. ) after surgery. This effect was however not statistically significant [F(1,4) = 5,67; p = 0.075]. On average, the total duration of the postural phase was longer (835 ms +/- 207) in the post-surgery session, than in the pre-surgery session (652 ms +/-143) and no side-effect was observed within patients. Post-surgery patients did not recover a duration similar (p < 0.001) to that observed in the control group (543 ms +/- 107).
By contrast, the onset of this phase in terms of "thrust" exerted onto the ground (Fig. ) was not different in patients before and after surgery [F(1,4) = 0.038; P = 0.85]. The A/P and M/L peaks remained synchronized before (22 ms+/-60) and after surgery (16 ms +/- 17). These events were tightly coupled in patients after surgery as in the control group (1 ms +/- 26). In addition, the M/L peak amplitude was not different in patients between pre- (270 mm +/-45) and post-surgery sessions (257 mm +/-37). After surgery, the M/L thrust was close to that observed in the control group (258 mm +/-35; p = 0.88).
Figure 2 Schema of the horizontal shift of the center of mass (CM) and associated center of pressure (CP) (left part) and description of the M/L and A/P CP curves (right part). The dotted lines show the time-relationships between each component. Note that the (more ...)
The total duration of the movement phase (T2–T3) was not different in patients [F(1,4) = 1.80; p = 0.24] (Pre-surgery: 1694 ms +/- 355 ; Post-surgery 1502 ms +/- 230). The movement duration in post-surgery was similar (p = 0.066) to that observed in the control group (1402 ms +/- 193).
The maximal flexion reached by the leading leg did not differ statistically in patients before and after surgery [F(1,6) = 5.44; p = 0.058] (see table. ) and no side-effect was observed within patients. After surgery, the maximal flexion was close to that observed in the control group (p = 0.37). By contrast, considering the flexion of the leg that was previously the supporting leg, after surgery, patients decreased the leg flexion [F(1,5) = 19,8; p = 0.006] (see table. ) and no side-effect was observed. However, in post-surgery session, the maximal flexion of the supporting leg remained reduced compared to the control group (p = 0.0017).
Maximal knee joint angle reached during the stepping-down performance of the leading leg and of the supporting leg during the swing phase.
Time-relationships between APAs and movement initiation
The stepping down movement of the leading leg was triggered while the unloading phase (peak-Tbal, Fig. ) was being performed, before the M/L CP shift was completed. The time-relationships between unloading (Tbal) and stepping down initiation (T2) differed in patients before and after surgery [F(1,4) = 15.53; p = 0.016]. Before surgery, in patients who used the arthritis limb as the supporting limb (unusual strategy), the movement initiation was delayed and coincided (-64 ms +/-452) with the end of the lateral unloading. This result, however, varied widely, as shown by the high standard deviation. Post-surgery, stepping down is triggered largely before the unloading is completed (sound supporting leg: -514 ms +/-60; operated supporting leg: -492 ms +/-176). Post-surgery patients did not recover an anticipation similar to that observed in the control group (-214 ms +/-40) (p < 0.001).
The delayed movement initiation (T2) when supporting on the arthritis leg before surgery, might be aimed at shortening the duration of the supporting phase for the painful leg. This was not the case, however, because there was no clear side-effect [F(1,4) = 6.33; p = 0.065] on the duration of the monopodal stance. In addition, this duration was even longer [F(1,4) = 19.8; p = 0.011] before than after surgery (797 ms+/-197 and 681 ms+/-156, respectively). The post-surgery duration decreased to a value close (p = 0.38) to that observed in the control group (644 ms +/-49).
The adaptation of the weight acceptance is illustrated in Fig. . The ground impact, defined as the maximal value of the vertical ground reaction force and normalized to the body weight, did not differ in patients before and after surgery [F(1,4) = 3.37; p = 0.14]. However, in patients landing on the sound leg (i.e. using the arthritis leg as the supporting leg) before surgery, the ground impact increased (142 % +/-36) [side-effect F(1,4) = 7.59; p = 0.05] compared to those landing on the arthritis leg (118 %+/-37) (Fig. ). This result indicated a reduced breaking capacity of the supporting knee joint during the monopodal stance, which enhanced the forthcoming ground impact. After surgery, the ground impact decreased to a value close to that observed in the control group (p = 0.78) (Fig. ).
Schema of the vertical ground reaction force recorded on the landing force platform. Weight acceptance was from the ground contact to the peak and was calculated in percentage relative to the body weight to normalize the data for all the subjects.
There was no significant difference in "time to peak" of the vertical force (weight acceptance velocity) for both sides in patients before and after surgery [F(1,4) = 3.37; p = 0.14] (Fig ).
EMG activities associated with ground contact
The comparison between kinetic events and associated EMG pattern points out some differences. First, during the swing phase, the moving limb exhibited a pre-activation of the VL before the ground contact. The leading VL pre-activation was correlated with increasing activity of the VL on the supporting side (Fig. ). The onset of the pre-activation of the VL muscle of the leading limb did not differ in patients before and after surgery [F(1,5) = 1.63; p = 0.25]. However, in pre-surgery session, the pre-activation occurred earlier [side-effect F(1,5) = 11.84; p = 0.018] when landing on the arthritis leg (-414 ms+/-90) than when landing on the sound leg (-335 ms +/-90). This was also observed for the post-surgery sessions (345 ms +/-67 and -298 ms +/-74, respectively).
Kinetic and rectified EMG patterns recording with one control subject. The EMGs were recorded at a proximal level (VL, Vastus lateralis) for both sides. Note the supporting and leading VL activity prior to the ground contact.
VL muscle activation was not statistically different in patients before and after surgery [F(1,5) = 0.5; p = 0.50] (Fig. ). The pre-activation increased [window-effect F(3,15) = 14.36; p < 0.001] from the first window (-300 ms to -150 ms) to the second (-150 ms to ground contact) and to the third (ground contact to 150 ms). However, when landing on the sound leg, the activity of the leading VL strongly increased before the ground contact (-150 ms to ground contact) [interaction side*window [F(3,15) = 5.13; p = 0.012]. Note that in this latter case, the VL of the leg to be stepped down supported 140% of the body weight. No such increase was observed in patients landing on the arthritis leg. Post-surgery (Fig. ), this enhanced activity no longer exhibited differences compared to the control group (P = 0.39).
Dynamic profiles of VL activation recorded on the forthcoming landing leg before and after surgery. EMG data are windowed each 150 ms from 300 ms before ground contact to 300 ms after ground contact (Arbitrary Units, AU).