Improved function from progressive strengthening interventions after total knee arthroplasty: A randomized clinical trial with an imbedded prospective cohort

Abstract

Objective

To determine the effectiveness of progressive quadriceps strengthening with or without neuromuscular electrical stimulation (NMES) on quadriceps strength, activation, and functional recovery after total knee arthroplasty (TKA), and to compare progressive strengthening with conventional rehabilitation.

Methods

A randomized controlled trial was conducted between July 2000 and November 2005 in an academic outpatient physical therapy clinic. Two hundred patients who had undergone primary, unilateral TKA for knee osteoarthritis were randomized to 1 of 2 interventions 4 weeks after surgery, and 41 patients eligible for enrollment who did not participate in the intervention were tested 12 months after surgery (standard of care group). All randomized patients received 6 weeks of outpatient physical therapy 2 or 3 times per week through 1 of 2 intervention protocols: an exercise group (volitional strength training) or an exercise-NMES group (volitional strength training and NMES). Treatment effects were evaluated by a burst superimposition test to assess quadriceps strength and volitional activation 3 and 12 months postoperatively. The Medical Outcomes Study Short Form 36 and Knee Outcome Survey were completed. Knee range of motion, Timed Up and Go, Stair-Climbing Test, and 6-Minute Walk were also measured.

Results

Strength, activation, and function were similar between the exercise and exercise-NMES groups at 3 and 12 months. The standard of care group was weaker and exhibited worse function at 12 months compared with both treatment groups.

Conclusion

Progressive quadriceps strengthening with or without NMES enhances clinical improvement after TKA, achieving similar short- and long-term functional recovery and approaching the functional level of healthy older adults. Conventional rehabilitation does not yield similar outcomes.

INTRODUCTION

Knee osteoarthritis (OA) results in persistent pain, limited function, and poor quality of life (1). In the US, nearly 500,000 total knee arthroplasties (TKAs) are performed each year for severe knee OA (2). Candidates for this procedure have radiographic evidence of joint damage, moderate to severe persistent pain, and clinically significant functional limitations that diminish quality of life (3).

TKA reliably alleviates pain and improves self-reported function, yet patients continue to exhibit marked impairments in quadriceps strength, voluntary muscle activation, and functional performance (e.g., walking, stair climbing) (4, 5). A 60% reduction in quadriceps strength is evident 1 month after surgery; volitional muscle activation, explaining more of the quadriceps strength loss than cross-sectional area, is reduced by 17% (6). Functional performance is reported to worsen by 20–25% 1 month after TKA (7). These deficits in strength and function do not resolve spontaneously. Most recover to preoperative status; however, impairments in strength and function remain below healthy age-matched populations for years after TKA (4, 8).

Progressive rehabilitation targeting these deficits has not been systematically studied and is not routinely prescribed. A recent meta-analysis found short-term benefit but no long-term advantage of structured rehabilitation; however, only 6 studies included primary outcomes of pain, range of motion (ROM), and self-reported function (9). In addition, a National Institutes of Health–sponsored consensus development conference on TKA concluded that “the use of rehabilitation services is one of the most understudied aspects of the perioperative management of patients following total knee replacement” and “there is no evidence supporting the generalized use of any specific preoperative or postoperative rehabilitation interventions” (10). Numbers of primary TKAs per year in the US (478,000 in 2004, the last year for which the National Hospital Discharge Survey data are available) (2) have exceeded predictions of just 5 years ago for the year 2020. Since the consensus conference, there have been no studies that have investigated vigorous therapeutic exercise regimens and documented intensity, frequency, and functional- and impairment-based outcomes.

Neuromuscular electrical stimulation (NMES) has been used in other populations to successfully target quadriceps dysfunction (11–13). Several case studies have demonstrated the potential of NMES to enhance the recovery of muscle strength and muscle function in persons after TKA (14–16). It has been hypothesized that NMES preferentially targets the larger force-producing type II muscle fibers, resulting in greater strength gains and reversal of activation deficits (17–19). We investigated whether the addition of NMES to a progressive volitional strength training program would improve the recovery of quadriceps strength, volitional muscle activation, and function following TKA in a single-blind, randomized clinical trial (RCT). Our primary hypothesis was that NMES combined with a progressive volitional strength training program would yield greater gains in strength, activation, and function 3 and 12 months after TKA than a progressive volitional strength training program alone. We also hypothesized that patients treated with a combination of NMES and progressive strength training would achieve better self-report of function, ROM, and pain scores than would patients participating in progressive strength training alone. In an imbedded prospective cohort study, our primary hypothesis was that patients participating in progressive strength training (with or without NMES) would have greater muscle strength, activation, and function, but similar ROM, pain, and self-report scores 12 months after TKA than a cohort receiving the standard of care in the community. We believe that a rehabilitation program specifically designed to counter age, OA, and postsurgical-related changes to the neuromuscular system will improve impairment and functionally based outcomes after TKA.

PATIENTS AND METHODS

Patients.

Individuals ages 50–85 years scheduled to undergo unilateral TKA by 3 experienced local orthopedic surgeons were considered candidates for study inclusion and were mailed study information. A total of 1,093 individuals were scheduled for TKA between July 2000 and November 2005. A telephone interview to determine eligibility was conducted if potential candidates responded to the mailings and expressed interest in participation. Patients were excluded if they had 1) uncontrolled hypertension, 2) diabetes, 3) body mass index (BMI) >40 kg/m² (20), 4) symptomatic OA in the contralateral knee (defined as self-reported knee pain >4 on a 10-point verbal analog scale), 5) other lower extremity orthopedic problems limiting function, 6) neurologic impairment, or 7) a residence outside of a 20-mile radius of the clinic. All patients underwent a tricompartmental, cemented TKA with a medial parapatellar surgical approach and received inpatient rehabilitation and home physical therapy prior to enrollment. Patients of 1 referring surgeon (LR) who met all criteria but were unable to participate in the RCT and agreed to be tested 12 months after TKA were recruited to represent the standard of care in the community (standard of care group) as a comparison cohort to the referring surgeon's (LR) RCT cohort.

This study was approved by the Human Subject Review Board at the University of Delaware and was in accordance with the Declaration of Helsinki. All participants provided written informed consent prior to participation. This study was conducted at the University of Delaware Physical Therapy Clinic, a public, not-for-profit institution.

Intervention.

Treatment began 3–4 weeks after TKA. Eligible participants completed a baseline study evaluation and a comprehensive clinical evaluation by 2 independent licensed physical therapists. Following the clinical evaluation, patients were enrolled into outpatient rehabilitation at the University of Delaware Physical Therapy Clinic, group assignment was revealed to the participant, and the treatment intervention was initiated. One hundred participants were randomized to a progressive volitional strength training program (exercise group) and 100 participants were randomized to a combined NMES and volitional strength training program (exercise-NMES group) (Figure 1 and Table 1).

Figure 1. Consolidated Standards of Reporting Trials group diagram. BMI = body mass index; NMES = neuromuscular electrical stimulation.

Table 1. Baseline demographics of the RCT population and 1-year characteristics of the standard of care and RCT cohorts*

Characteristic	RCT comparison			Cohort comparison
	Exercise (n = 100)	Exercise-NMES (n = 100)	P	RCT (n = 41)	Standard of care (n = 41)	P
* Values are the mean ± SD unless otherwise indicated. RCT = randomized clinical trial; NMES = neuromuscular electrical stimulation; BMI = body mass index; SF-36 = Short Form 36; PCS = physical component score; MCS = mental component score. † Significant by t-test or chi-square test.
Women, %	45	47	0.777	58	66	0.470
Age at surgery, years	65.2 ± 8.5	65.3 ± 8.3	0.886	65.4 ± 7.8	65.92 ± 9.49	0.913
Height, meters	1.71 ± 0.10	1.71 ± 0.12	0.670	1.70 ± 0.10	1.67 ± 0.11	0.270
Weight, kg	88.6 ± 15.3	86.8 ± 17.2	0.413	88.8 ± 15.5	92.7 ± 20.3	0.334
BMI, kg/m^2	29.99 ± 3.90	29.67 ± 4.85	0.604	30.60 ± 4.14	33.11 ± 7.12	0.069
Quality of life (SF-36)
PCS	29.59 ± 6.57	30.76 ± 7.75	0.253	46.71 ± 8.74	43.89 ± 8.57	0.152
MCS	50.40 ± 11.98	51.43 ± 10.69	0.522	57.42 ± 5.59	54.06 ± 10.15	0.083
Physical therapy visits	16.7 ± 1.8	17.0 ± 1.2	0.813	16.9 ± 1.1	22.8 ± 8.9	0.004†

Both groups received outpatient physical therapy 2 or 3 times per week for 6 weeks with a minimum requirement of 12 therapy visits. The rehabilitation protocol was centered on an impairment-based model described by Stevens et al (15). The novel components of the intervention were the progressive nature of the strengthening exercises and the addition of NMES. Interventions targeting knee extension and flexion ROM, patellar mobility, quadriceps strength, pain control, and gait were included in both programs.

The volitional strength training program specifically targeted the quadriceps femoris muscle group. Intensity and type of strengthening exercises were based on the individual's initial clinical evaluation and followup assessments. In addition to the quadriceps, exercises targeted the hamstrings, gastrocnemius, soleus, hip abductors, and hip flexors (15). Exercises were initiated with 2 sets of 10 repetitions and then progressed to 3 sets of 10 repetitions. Weights were increased to maintain a 10-repetition maximum targeted intensity level.

The NMES component consisted of 10 electrically elicited contractions of the quadriceps femoris muscle. Patients were seated in an electromechanical dynamometer (KinCom; Chattanooga Corporation, Chattanooga, TN) with the knee stabilized at 60° of knee flexion. Two 7.62 × 12.70–cm self-adhesive electrodes (ConMed, Utica, NY) were placed over the rectus femoris muscle belly proximally and the vastus medialis muscle belly distally. Stimulation was characterized by a 2,500-Hz, sinusoidal, alternating waveform current at 50 bursts per second for 10 seconds, plus a 2-second ramp on time with an 80-second rest period between contractions (Versastim 380; Electro Med Health Industries, Miami Beach, FL) (21). Current amplitude was raised to the patient's maximum tolerance. The target intensity was a minimum electrically-elicited force output of 30% of the patient's daily maximal volitional isometric contraction (MVIC) force. Patients did not assist or volitionally contract their muscle during the electrical stimulation.

Procedural reliability.

Procedural reliability was assessed for each patient once during the course of rehabilitation by a physical therapist not involved in the patient's care. The number of steps correctly completed in the protocol was divided by the total number of protocol steps and multiplied by 100. Mean ± SD procedural reliability for the exercise group was 92% ± 8% and for the exercise-NMES group was 93% ± 9%.

Testing.

All followup assessments were performed 3 months and 12 months after TKA by investigators blinded to treatment group assignment. Baseline and followup study evaluations were completed during a 90-minute session that included health questionnaires, active knee ROM, Timed Up and Go (TUG), Stair-Climbing Test (SCT), quadriceps strength and activation testing, and 6-Minute Walk (6MW). Subjects completed functional and health questionnaires prior to strength and functional testing. The standard of care group was assessed once 12 months postoperatively.

Primary outcome measures.

Quadriceps strength and activation testing.

Quadriceps strength and volitional muscle activation were measured using a burst superimposition technique (22), which is a validated quadriceps strength assessment widely used in a variety of populations with and without knee pathologies (6, 23–26). Briefly, subjects' knees were stabilized in 75° of flexion on a dynamometer (KinCom). A supramaximal burst of electrical stimulus was administered during a 3–5-second MVIC (Grass 8800; Grass Instruments, Warwick, RI). The testing procedure was repeated a maximum of 3 times for each leg if incomplete recruitment was evident.

Data were collected and analyzed using custom written software (LabView; National Instruments, Austin, TX). Muscle activation was calculated using a modification of the central activation ratio (CAR) method that accounts for the tendency of the CAR to overestimate activation (22, 27). The CAR is calculated by dividing MVIC by the electrically-augmented force. The trial with the largest quadriceps MVIC force was normalized to BMI (NMVIC; newtons/BMI) for data analysis.

Functional measures.

The TUG, SCT, and 6MW were used to assess functional performance. The TUG measures the time to rise from a seated position in an armed chair (seat height 46 cm), walk 3 minutes, turn around, and return to a seated position in the chair (R_intrarater = 0.95, R_interrater = 0.98) (28, 29). The SCT measures the time to ascend and descend 12 steps (height 7.9 cm; R = 0.90) (30, 31). For both the TUG and SCT, the average of 2 trials was analyzed. The 6MW measures the distance a person can walk in 6 minutes (R = 0.94) (31–33). Participants were allowed use of an assistive device and were instructed to move as quickly as they felt safe and comfortable.

Secondary outcome measures.

Self-assessment questionnaires.

The Medical Outcomes Study Short Form 36 (SF-36) (34) and the Knee Outcome Survey Activities of Daily Living scale (KOS ADLS) (35) were administered to measure perceived functional ability (R = 0.92) (35). Both the mental and physical component scores of the SF-36, which are reliable measures of health status in patients with OA (36), were computed.

Knee ROM.

Active knee flexion and extension ROM were measured in the supine position with a long axis goniometer (37). Positive values indicate positions of knee flexion and negative values indicate positions of knee hyperextension (R_c flexion = 0.96, R_c extension = 0.81) (38).

Knee pain.

Knee pain (pain KOS) was measured with a question on the KOS ADLS: “How does pain affect the function of your knee during daily activities?” Scores ranged from 0 (pain prevents me from all activities) to 5 (pain has no effect on daily activities).

Statistical analysis.

Data were processed using SPSS statistical software, version 15.0 (SPSS, Chicago, IL). Independent-sample t-tests were used to compare the randomized groups at baseline. Primary and secondary outcome variables were analyzed using a repeated-measures analysis of covariance with time as the repeated-measures factor (3 and 12 months), rehabilitation group (exercise versus exercise-NMES) as the between-groups factor, and baseline score as the covariate. An intent-to-treat approach was adopted (39).

Independent-sample t-tests were used to compare the RCT cohort and the standard of care group at 12 months. Effect size was quantified using Cohen's d (mean difference between groups divided by the pooled SD) (40). The modified Hochberg procedure, which has the highest power among accepted modified Bonferroni techniques, was used to adjust our probability level of 0.05 for multiple comparisons (41).

Hierarchical linear regression analysis was used to determine the predictors (independent variables: NMVIC, pain KOS, flexion ROM, extension ROM) of functional performance (dependent variables: TUG, SCT, 6MW) at 12 months. Four models were considered: model 1 included only NMVIC; model 2 for TUG and SCT included NMVIC and knee flexion ROM, and model 2 for 6MW included NMVIC and knee extension ROM; model 3 included pain KOS in addition to the model 2 predictors; and model 4 included all 4 independent variables. The F test was used to analyze the significance of the resultant change in R² with the addition of each independent variable to the regression model. Clinical reasoning was used to determine order of variable inclusion into the models based on each variable's importance to the functional task.

RESULTS

No adverse events were related to the exercise intervention. Only 1 patient reported feeling dizzy and lightheaded following the first NMES treatment. Three subjects in the exercise group and 16 patients in the exercise-NMES group did not complete treatment (Figure 1). There was no significant difference between the exercise group and the exercise-NMES group in total number of treatment visits (P = 0.25) (Table 1). The groups attended a pooled mean ± SD of 16.9 ± 1.3 visits (range 12–18); only 11 patients completed <15 visits.

Comparisons between the randomized groups.

There were no significant differences between the exercise and exercise-NMES groups on any outcome measure at 3 or 12 months (P > 0.08 for all after adjustment for baseline values) (Table 2). Both groups significantly improved on all measures from baseline to 3 months and from 3–12 months (P < 0.001 for all) with the exception of the mental component score of the SF-36, which only improved from 0–3 months (Table 2).

Table 2. Mean of primary and secondary outcome measures and percent change in assessment scores in the randomized groups*

Variables	Exercise group						Exercise-NMES group
	0 months (n = 100)	3 months (n = 92)	12 months (n = 81)	% change, 0–3 months	% change, 3–12 months	% change, 0–12 months	0 months (n = 100)	3 months (n = 76)	12 months (n = 68)	% change, 0–3 months	% change, 3–12 months	% change, 0–12 months
* Both groups significantly improved from 0–3 months and from 3–12 months on all measures. There were no significant differences between the exercise group and the exercise-NMES group at any time point. NMES = neuromuscular electrical stimulation; SF-36 = Short Form 36; PCS = physical component score; MCS = mental component score; KOS ADLS = Knee Outcome Survey Activities of Daily Living scale; NMVIC = normalized maximum voluntary isometric contraction; BMI = body mass index; CAR = central activation ratio; ROM = range of motion. † Not significantly different from 3 months.
SF-36 PCS	29.59	44.64	46.74	51	5	58	30.76	44.45	46.05	45	4	50
SF-36 MCS	50.40	56.77	57.16†	13	1	13	51.43	57.17	56.63†	11	−1	10
KOS ADLS	0.57	0.81	0.86	42	6	51	0.59	0.80	0.85	36	6	44
Pain KOS	2.44	1.08	0.89	−56	−18	−64	2.24	1.11	0.82	−50	−26	−63
Timed Up and Go, seconds	12.04	8.02	7.68	−33	−4	−36	12.10	8.29	8.07	−31	−3	−33
Stair-Climbing Test, seconds	25.76	12.78	11.75	−50	−8	−54	27.51	14.28	13.62	−48	−5	−50
Six-Minute Walk, meters	401	535	554	33	4	38	401	530	545	32	3	36
NMVIC, newtons/BMI	10.58	17.35	20.60	64	19	95	10.42	19.05	22.64	83	19	117
CAR	0.75	0.78	0.82	4	5	9	0.78	0.82	0.89	5	9	14
Flexion ROM, degrees	97.6	114.7	119.0	18	4	22	100.1	115.2	120.9	15	5	21
Extension ROM, degrees	6.4	1.8	0.4	−72	−78	−94	5.8	2.0	0.3	−66	−85	−95

Cohort comparison.

The exercise and exercise-NMES groups (RCT cohort) were not significantly different in any primary and secondary outcome measure at 12 months, and their baseline demographics were similar. Therefore, the data from all of the referring surgeon's (LR) patients in the RCT cohort (n = 41) were compared with the cohort of his patients representing the standard of care in the community (n = 41). The standard of care group had more physical therapy sessions than the RCT cohort (P < 0.001) (Table 1).

At 12 months, the RCT cohort was significantly stronger than the standard of care group (P = 0.007). Mean NMVIC was 21% less in the standard of care cohort compared with the RCT cohort. The standard of care group also exhibited worse functional performance at 12 months. The standard of care group took 24% longer on the TUG (P = 0.004), 44% longer to complete the SCT (P < 0.001), and walked a 15% shorter distance on the 6MW (P = 0.003) (Figure 2). There was no significant difference between the standard of care group and the RCT cohort on any of the secondary outcome measures of the SF-36 physical component score, KOS score, pain KOS score, knee flexion ROM, knee extension ROM, or voluntary muscle activation (P > 0.01 for all).

Figure 2. Comparison between randomized clinical trial (RCT) cohort and standard of care cohort. A, Timed Up and Go (TUG) and Stair-Climbing Test (SCT) performance. B, 6-Minute Walk (6MW) distance. C, Normalized maximal volitional isometric contraction (NMVIC). The standard of care group performed significantly worse than the RCT cohort on all performance measures. Error bars represent the SD. * P < 0.01. m = meters.

Progressive strength training following TKA had a moderate effect on quadriceps strength (effect size d = 0.63; 95% confidence interval [95% CI] −1.26, 7.50), TUG performance (effect size d = 0.69; 95% CI −3.04, −0.58), and 6MW performance (effect size d = 0.70; 95% CI 93, 442), and had a large effect on SCT performance (effect size d = 0.86; 95% CI −8.06, −2.44).

Predictors of function at 12 months.

Timed Up and Go.

The predictive model that included NMVIC, flexion ROM, pain KOS score, and extension ROM explained 28% of the variability in TUG performance (R² = 0.28, P < 0.001). Quadriceps strength was the strongest predictor of TUG performance (R² = 0.23, P < 0.001). Flexion ROM significantly added to the predictive model (R² = 0.27, P < 0.001), whereas neither pain nor extension ROM significantly added to the model (Table 3).

Table 3. Hierarchical regression models used to predict functional performance 12 months after total knee arthroplasty*

Model	R	R^2	R^2 change	F change	P
* ROM = range of motion; KOS = Knee Outcome Survey Activity of Daily Living scale. † P < 0.01.
Timed Up and Go
Quadriceps strength†	0.477	0.228	0.228	42.521	< 0.001
Quadriceps strength + flexion ROM†	0.523	0.273	0.045	8.914	0.003
Quadriceps strength + flexion ROM + pain KOS	0.531	0.282	0.008	1.650	0.201
Quadriceps strength + flexion ROM + pain KOS + extension ROM	0.533	0.284	0.002	0.421	0.518
Stair-Climbing Test
Quadriceps strength†	0.474	0.225	0.225	41.833	< 0.001
Quadriceps strength + flexion ROM	0.493	0.243	0.018	3.33	0.07
Quadriceps strength + flexion ROM + pain KOS	0.507	0.257	0.014	2.747	0.1
Quadriceps strength + flexion ROM + pain KOS + extension ROM	0.508	0.258	0.001	0.204	0.653
Six-Minute Walk
Quadriceps strength†	0.593	0.352	0.352	54.851	< 0.001
Quadriceps strength + extension ROM	0.604	0.365	0.013	2.004	0.16
Quadriceps strength + extension ROM + pain KOS	0.608	0.369	0.005	0.755	0.387
Quadriceps strength + extension ROM + pain KOS + flexion ROM	0.610	0.372	0.003	0.393	0.532

Stair-Climbing Test.

The predictive model that included NMVIC, flexion ROM, pain KOS score, and extension ROM explained 26% of the variability in SCT performance (R² = 0.26, P < 0.001). Quadriceps strength was the single strongest predictor of SCT performance (R² = 0.23, P < 0.001). The addition of flexion ROM, pain, and extension ROM did not contribute significantly to the predictive model (Table 3).

Six-Minute Walk.

The predictive model that included NMVIC, extension ROM, pain KOS score, and flexion ROM explained 37% of the variability in 6MW performance (R² = 0.37, P < 0.001). Quadriceps strength was the single strongest predictor of 6MW performance (R² = 0.35, P < 0.001). The addition of flexion ROM, pain, and extension ROM did not contribute significantly to the model (Table 3).

DISCUSSION

This RCT investigated the use of 2 rehabilitation protocols, a progressive strength training program and a combined NMES and progressive strength training program, in the postoperative management of individuals following primary, unilateral TKA for knee OA. Results were also compared with a cohort of individuals who received the standard of care for rehabilitation after TKA. The individuals in the study were typical of the population of individuals undergoing TKA, exhibiting profound impairments in quadriceps strength and volitional muscle activation following surgery (6, 42, 43). There were no significant differences on any outcome variable between the arms of the RCT, but both the exercise group and the exercise-NMES group demonstrated significant improvements in quadriceps strength and muscle activation, functional performance, and self-report of function during the rehabilitation period and 12 months after surgery. Importantly, both the exercise and exercise-NMES groups in the single surgeon prospective cohort comparison demonstrated substantially greater quadriceps strength and functional performance 12 months postoperatively than the standard of care group.

Comparing outcomes of this RCT with published outcomes after TKA and healthy older adults highlights the superior results achieved by both treatment arms (4, 44, 45) (Table 4). Walsh et al measured stair-climbing time (ascent and descent of 10 stairs) in 29 individuals 1 year after TKA (4). Mean ± SD stair-climbing time reported was 27.2 ± 1.4 seconds, 46% slower than age-matched controls (12.6 ± 0.39 seconds). Mean stair-climbing time of RCT participants (ascent and descent of 12 stairs) was 12.69 seconds 12 months postoperatively, equivalent to the control subjects in the study by Walsh et al, and 47% faster than their subjects with TKA. Furthermore, the number of individuals in our study that used an assistive device during the performance assessments decreased; no subjects used an assistive device on either the TUG or SCT at 3 or 12 months, and only 1 used an assistive device at 3 and 12 months on the 6MW.

Table 4. Comparison of function measures with published data of controls and patients who underwent total knee arthroplasty*

	3 months	12 months	Control
* NMES = neuromuscular electrical stimulation.
Knee flexion range of motion, degrees
Kramer et al, 2003 (44)	100	110
Kumar et al, 1996 (50)	115
Rajan et al, 2004 (51)	95	98
Ranawat et al, 2003 (52)		119
Walsh et al, 1998 (4), F/M		114/110	143/142
Current study
Exercise	115	119
Exercise-NMES	115	120
Standard of care		120
Timed Up and Go, seconds
Steffen et al, 2002 (46)
Age 60–69 years			8.00
Age 70–79 years			9.00
Age 80–85 years			10.00
Stratford et al, 2003 (53)			12.3
Current study
Exercise	8.02	7.68
Exercise-NMES	8.29	8.07
Standard of care		9.01
Stair-Climbing Test, seconds
Stratford et al, 2003 (53)			23.5
Walsh et al, 1998 (10 steps) (4), F/M		31.1/23.33	13.45/11.81
Current study
Exercise	12.78	11.75
Exercise-NMES	14.28	13.62
Standard of care		16.24
Six-Minute Walk, meters
Enright and Sherrill, 1998 (54), F/M			494/576
Kramer et al, 2003 (44)	320	400
Moffet et al, 2004 (45)
Usual care	350	375	450
Functional rehabilitation	380	405
Steffen et al, 2002 (46), F/M
Age 60–69 years			538/572
Age 70–79 years			471/527
Age 80–89 years			392/417
Current study
Exercise	535	554
Exercise-NMES	530	545
Standard of care		462

Few studies have investigated the effectiveness of rehabilitation after TKA (5, 44). Kramer et al compared the outcomes of a home-based program and a clinic program over a 1-year period (44). Although there was no difference between the 2 treatment groups, the recovery of patients in their study was much slower than our patients. In the 6MW, subjects in our trial walked 212 meters further 3 months postoperatively and 150 meters further 12 months postoperatively. Our subjects had 10° more knee flexion at 12 months (Table 4). Similarly, Moffet et al compared a functional rehabilitation program with customary care (5). Their results support our argument that intensive rehabilitation improves function after TKA. However, their program with similar patients was not as successful; subjects in their study walked ∼145 meters less on the 6MW 12 months after TKA (Table 4). We demonstrated that quadriceps strength was the strongest predictor of function at 1 year. Moffet et al did not target quadriceps strength in their intervention. The addition of progressive exercises and NMES specifically aimed at strengthening the quadriceps femoris may have resulted in greater functional ability in our study cohort.

A review of the literature and comparison with our prospective standard of care cohort illustrates the lack of consensus regarding rehabilitation after TKA even within a single surgeon's practice. The standard of care group attended more outpatient physical therapy visits (range 0–46 visits). By the standards typically used to describe the success of TKA (pain, knee flexion ROM, and patient self-report), the standard of care group did as well as the RCT intervention groups and were similar to patient outcomes reported in the literature; however, in the areas of strength and function, the standard of care group performed much more poorly and their functional performance was well below reported norms of healthy individuals (4, 5, 46). The standard of care group took 1.5 seconds longer to complete the TUG, 4.5 seconds longer to complete the SCT, and walked 90 meters less on the 6MW than healthy older adults in 2 other studies (46, 47). Review of the physical therapy records of the standard of care cohort revealed a primary focus on ROM exercise, stationary cycling, and various straight-leg raising exercises without weights. The inconsistency of care in both the number of visits and nature of exercises most likely contributes to the difference in outcomes observed. Our data suggest that individuals who do not undertake an intensive rehabilitation program following TKA are clearly at a disadvantage.

Quadriceps strength is related to functional performance (7, 30), and it was the single greatest predictor of function in our sample (rising from a chair, stair climbing, and walking distance). Functional performance peaks ∼3 years after surgery and slowly declines in the subsequent 10-year period (8). Therefore, inadequate strength recovery following TKA holds several implications. Moderate to strong effect sizes for all functional measures and strength favored the intervention groups. The largest effect of the intervention was on the SCT, which places the highest demand on the quadriceps. Stair climbing is the single largest residual dysfunction after TKA and the ability to climb stairs worsens over time (8). Failing to obtain adequate functional recovery may accelerate functional decline and predispose these individuals to an early loss of functional independence as they age.

Both the exercise and exercise-NMES groups exceeded the best previously reported functional performance scores at 1 year (4, 45) (Table 4). The difference in our intervention was the addition of a progressive quadriceps strengthening program. Functional decline may be delayed with adequate quadriceps strength training; followup of these patients continues to discern the long-term impact of the implemented strengthening interventions. In addition, many of these studies used knee ROM as an outcome measure; however, evidence suggests that ROM does not strongly correlate with function (30). On the other hand, performance measures accurately portray functional status and are sensitive to changes with postoperative recovery (31). Quadriceps strength is also more strongly correlated with functional performance (7); therefore, we would recommend that both strength and function be used to assess rehabilitation outcomes.

The primary etiology of early quadriceps strength loss after TKA is voluntary activation failure (48). We predicted that the addition of NMES would result in better quadriceps strength, activation, and function than a progressive exercise program alone, but this hypothesis was not supported. Both programs resulted in improved activation, and consequently strength improved over time, translating to better function. Similar findings were reported by Hurley and Newham, who demonstrated improvement in voluntary muscle activation in persons with knee OA using isokinetic and isometric strengthening exercises (49). In conjunction with Hurley and Newham, our results suggest that activation deficits do not undermine the effectiveness of progressive volitional strength training.

There were limitations to the present study. Although the dropout rate may raise concerns, we planned for a 20% dropout rate and our a priori power analysis indicated that 30 subjects would be needed in each treatment arm to demonstrate a treatment effect. The NMES treatment was uncomfortable for some and accounted for the higher dropout rate in this group. Only 1 subject in the exercise group dropped out because of poor tolerance for the exercise regimen, whereas 11 of the 16 participants who dropped out of the exercise-NMES group did not complete their interventions because they reported the NMES treatment to be too uncomfortable. Second, the standard of care group was not randomized, but rather was comprised of individuals who agreed to complete the 12-month assessment and gave permission for their rehabilitation records to be reviewed. We used this protocol in order to compare patients treated by the same surgeon using the same technique; the standard of care group was referred to various local facilities for outpatient physical therapy.

In summary, there were no significant or clinically meaningful differences between the 2 interventions. Both 6-week rehabilitation programs after TKA emphasized progressive quadriceps strengthening and resulted in better functional outcomes than a cohort that received the standard of care. Furthermore, outcomes exceeded those previously reported in the literature, approaching the function of age-matched healthy older adults.

AUTHOR CONTRIBUTIONS

Dr. Snyder-Mackler had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study design. Petterson, Mizner, Stevens, Raisis, Bodenstab, Newcomb, Snyder-Mackler.

Acquisition of data. Petterson, Mizner, Stevens, Snyder-Mackler.

Analysis and interpretation of data. Petterson, Mizner, Stevens, Snyder-Mackler.

Manuscript preparation. Petterson, Mizner, Stevens, Raisis, Bodenstab, Newcomb, Snyder-Mackler.

Statistical analysis. Petterson, Snyder-Mackler.

Acknowledgements

We wish to acknowledge the expertise of the physical therapists at the University of Delaware Physical Therapy Clinic.