Individuals with TKA did undergo an expected recovery 3 to 12 months with improvements in the involved knee's ROM and quadriceps strength, functional performance capacity, and perceived function questionnaire scores. While their functional scores are similar to the age- and body mass -matched healthy individuals, their operated quadriceps strength and self-selected gait patterns continued to differ substantially from the comparison group. Persistent quadriceps weakness is of particular concern as quadriceps strength had a strong relationship to functional performance test results at both test points. When the surgical leg is still quite weak at the 3 month test, symmetry in quadriceps strength had a relationship with knee kinematics during gait, but when strength was improved at 12 months the relationship substantially weakened. The TKA group's self-selected gait was slow paced with limited knee flexion excursions during weight acceptance that persisted over time. The TKA group's gait kinetics during weight acceptance is characterized by a large contribution from the hip joint and a small contribution from the knee. The atypical distribution of joint torques actually worsened over time despite often perceived and measured improvements.
While patients should experience functional improvement after surgery, achieving functional outcomes that can match healthy subjects is a challenging goal. Some investigators question whether it is possible to expect patients with TKA to improve to the point that they match their healthy peers (Finch et al. 1998
; Heck et al. 1998
). The subjects in the current study performed exceptionally well and actually did achieve comparable functional performance to the matched healthy cohort. Their good functional results may be, in part, explained by the inclusion criteria of the study and is a potential limitation in the application of our findings. But even with similar functional outcome scores between groups, there were substantial differences in gait between groups with the TKA group walking more slowly with less knee flexion excursion during weight acceptance, and a hip dominated lower extremity support moment distribution. These differences are consistent with findings of past studies of TKA gait patterns (Bolanos et al. 1998
; Chen et al. 1991
). Thus, the performance outcomes and gait analysis provide conflicting messages when trying to determine if the functional ability of our TKA group can be defined as normal.
The apparent conflict in measures of walking ability could be a result of the differences in methodology between the measures. While the subjects were asked to walk at a self-selected speed in the gait analysis, the clinical functional outcomes asked them to perform the tasks “as quickly” as they felt safe and comfortable (Kennedy et al. 2006
; Podsiadlo and Richardson 1991
). Those encouragements can influence performance (Ikai and Steinhaus 1961
) and they may have helped raise the performance of the TKA group to nearing the capability of the healthy state on those tests. When the TKA group is pressed to perform they can achieved near healthy equivalency, but when allowed to perform at their typical pace they perform below the healthy standard. We expected to observe the opposite finding based on the work of Noble et al. with a greater likeness between groups with less demanding tasks and greater differences between groups when the functional measure was more demanding (Noble et al. 2005
). Perhaps, the physical performance outcome measures are more reflective of potential functional capacity and the gait analysis results may be a better reflection of the actual patient performance during typical daily activity.
By the three month test, patients with TKA typically have recovered from the acute pain and swelling experienced after surgery and yet they continue to exhibit pronounced quadriceps weakness (66% of the non-operated strength). In their weakened state, there was a greater reliance on the strength of the non-operated limb to perform functional tasks with higher correlations between non-operated quadriceps strength and performance measures of TUG, SCT, and 6MW as compared to the operated. Subjects in the TKA group maintained a limp during their self-selected gait pattern with the operated limb exhibiting less v-GRF, stance time, and knee flexion excursion during weight acceptance when compared to the non-operated leg. Quadriceps strength did play a moderate role in gait patterns at this time point as asymmetry in quadriceps strength related to asymmetry in knee flexion excursion.
When strength improved at the 12 month point (87% of non-operated), there was a reduced reliance on the non-operated limb for functional performance. The relationship between the non-operated quadriceps strength and the performance tests remained strong, but the relationship had a general weakening between tests. Concurrently, the relationship between functional performance and operated quadriceps strength increased from the 3 month test. With increased strength during recovery, the TKA group achieved better functional performance with an increased utilization of the operated limb's strength. It appears that during the physically challenging tasks of physical performance tests, patients in the TKA group needed and used their involved limb's strength gains over time and had subsequent better scores on functional tasks.
The phenomenon of increased dependence on the non-operated leg for performance in the subacute phase evolving into more of a shared utilization between limbs at the end of recovery is also supported from the gait kinematic results, but the findings are more subtle. As mentioned previously, the TKA group has significant differences between legs in multiple variables of interest during 3 month analysis which can be general grouped into a finding of limping on the operated limb. Most of the differences in kinematics present at 3 months are not significantly different by the 12 month test. While it is expected that individuals would strive towards coordinating their limbs to enhance gait symmetry (Reisman et al. 2005
), the expectation was for greater utilization of the operated limb with increased gait speed. The data suggest that the lack of difference in peak knee flexion between limbs at a year stem from a non-significant yet potentially clinically meaningful increase in knee motion in the operated and a tendency towards reduced motion in the non-operated knee. We hypothesize that when the operated limb has a very weak quadriceps, like many patients at the 3 months after TKA, then patients depend on the non-operated side to compensate by placing greater forces on the non-operated limb and using more knee excursion during weight acceptance. As strength recovers, then the limbs moved closer to balance in kinematics and GRF rather than a simple improvement in the involved.
While the TKA group takes advantage of their operated quadriceps strength with better performance scores over time, they don't seem to maximize the potential of their greater strength during the gait analysis measures. Some of the differences in relationship between quadriceps strength and the performance tests versus self selected walking could be attributed to diminished physical demand during self-pace walking. Unlike the findings in the performance tests, the correlation between symmetry in quadriceps strength and symmetry in knee flexion excursion actually weakened over time. The original hypothesis was that as there was expected improvement in quadriceps strength over time then there would be a coincidental improvement in knee flexion excursion during weight acceptance. While involved quadriceps strength made gains, there was no statistically significant improvement in corresponding knee flexion excursion. A reduced knee excursion during limb loading in gait is often associated with low knee internal extension moments which minimally challenge the knee extensor musculature. Thus, as the TKA group maintained a stiff legged gait pattern between test sessions, it is reasonable to accept that there would be a reduction in the correlation between quadriceps strength symmetry and knee flexion symmetry. Perhaps there is a threshold when quadriceps strength can impact knee kinematics during self-selected gait that is surpassed once symmetry reached 87% at the year test. Combined, these findings suggest that improving quadriceps strength outcomes may not be enough to reach the full potential gains in function possible from TKA surgery. The addition of a gait retraining program that encourages patients with TKA to utilize improvements in quadriceps strength with greater knee excursions during stance may be efficacious.
The comparison to normal helps keep perspective beyond just symmetry between limbs. This is especially evident when interpreting the kinetic results. Self-selected gait speed failed to make a customary improvement from 3 to 12 months after surgery. Patients with TKA coupled their slow gait speed with a stiff knee movement with a greater dependence on the torque at the hip to contribute to the support moment of the limb during the end of weight acceptance. The hip extension dominated support moment present at 3 months actually is accentuated at 12 months with even less contribution of the total support moment coming from the knee extension moment and an even greater contribution from the hip. This compensatory mechanism between hip and knee extension moment is often present in patients with knee OA and perhaps the TKA group's distribution of torque is a remnant of preoperative gait habits (McGibbon and Krebs 2002
). Since individuals after TKA place greater reliance on the hip extension moment during weight acceptance there is diminished stimulation to the quadriceps muscle during limb loading. An insufficient mechanical stimulus could negatively effect muscle strength gains over time (Mueller and Maluf 2002
) and may help to explain why the operated quadriceps strength is less than the normal cohort.
The current study does have limitations that deserved to be considered when interpreting our data. The TKA group did have exceptional functional outcomes and may represent more of a “best case” scenario in terms of what can currently be expected after surgery. While this may be a factor that could limit our external validity, it may also strengthen the weight of the message in that even patients with some of the best outcomes do not adopt normal gait patterns. Comparisons with the non-operated leg must also be considered since many with unilateral TKA eventually go on to have the other knee replaced due to degenerative changes and pain. It is not difficult to argue that the non-operated knee may be at least in the early stages of OA or may require surgery. The gamut is wide and can cause considerable variability in the findings, especially with a relatively limited sample size. While we have a well matched healthy comparison group, our sample may predispose the project towards type II error in reporting no significance differences which may have actually achieved statistical significance with a larger sample.