Subject: Effect of Anatomic Realignment on Muscle Function During Gait in Patients With Medial Compartment Knee Osteoarthritis
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Copyright notice and Disclaimer Effect of Anatomic Realignment on Muscle Function During Gait in Patients With Medial Compartment Knee Osteoarthritis Dan K. Ramsey, PhD, Lynn Snyder-Mackler, ScD, PT, SCS, Michael Lewek, PhD, PT, William Newcomb, MD, and Katherine S. Rudolph, PhD, PT Research University of Delaware, Newark Address correspondence to Katherine S. Rudolph, PhD, PT, 301 McKinly Lab, University of Delaware, Newark, DE 19716. E-mail: krudolph@UDel.Edu AUTHOR CONTRIBUTIONS Dr. Ramsey 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. Snyder-Mackler, Rudolph. Acquisition of data. Ramsey, Lewek. Analysis and interpretation of data. Ramsey, Snyder-Mackler, Rudolph. Manuscript preparation. Ramsey, Snyder-Mackler, Newcomb, Rudolph. Statistical analysis. Ramsey, Snyder-Mackler, Rudolph The publisher's final edited version of this article is available free at Arthritis Rheum. |
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Abstract Objective Individuals with medial compartment knee osteoarthritis (OA) and genu varum use different movement and muscle activation patterns to increase joint stability during gait. The purpose of this study was to ascertain whether opening-wedge high-tibial osteotomy (OW-HTO) corrected pathomechancial abnormalities associated with the progression of knee OA. Methods Fifteen patients diagnosed with medial knee OA and genu varum who were scheduled for OW-HTO were tested prior to and 1 year following OW-HTO. Fifteen age- and sex-matched controls were also tested. Frontal plane laxity was measured from stress radiographs. All participants underwent quadriceps strength testing with a burst superimposition technique and gait analysis with surface electromyography to calculate knee joint kinematics and kinetics and muscle co-contraction during the stance phase of gait. Participants rated their knee function and instability using a self-report questionnaire. Results Static alignment improved following the surgery. Medial laxity (P = 0.003) and instability (P = 0.002) significantly improved, and statistical reductions in the adduction moment resulted in lower levels of vastus medialis-medial gastrocnemius muscle co-contractions (P = 0.089). Despite improvements in global rating of knee function (P = 0.001), the OA group’s ratings remained significantly lower than those of the healthy controls (P = 0.001). Quadriceps strength deficits and knee flexion impairments persisted. Conclusion Persistent quadriceps weakness and impaired knee kinematics after realignment suggest that the movement strategy may perpetuate joint destruction and impede the long-term success of realignment. Rehabilitation should focus on quadriceps strength and improving joint mobility to improve the long-term function of individuals with medial knee OA. Keywords: Knee osteoarthritis, Osteotomy, Gait, Co-contraction |
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INTRODUCTION Knee osteoarthritis (OA) is the most common cause of functional disability, affecting ~6% of US adults over the age of 30 (1,2). The goal of contemporary management is to control pain and improve knee function and quality of life. When nonoperative management fails, high-tibial osteotomies (HTO) are commonly prescribed to realign the knee and reduce symptoms (3,4). However, mechanisms other than malalignment that exacerbate cartilage destruction may persist. Excessive frontal plane joint laxity, quadriceps weakness, and high muscle co-contraction are also related to knee OA and all have been implicated in contributing to functional deficits and the pathogenesis of knee OA (5-11). Recently, we reported that frontal plane laxity was localized to the medial compartment and that greater medial laxity was related to increased muscle activity (12). Persons with medial knee OA commonly experience knee instability, the sensation of buckling, shifting, and giving way, which was related to higher muscle co-contraction in muscles that cross the knee (12). Co-contraction may be a response to weak quadriceps muscles. Individuals with weak quadriceps walk with less knee flexion during weight acceptance, which can interfere with the knee’s ability to dissipate loads (6,11,13). Individuals with medial knee OA demonstrate reduced knee flexion excursions during weight acceptance and increased lower extremity muscle co-contraction during gait (12,13). Higher co-contraction can lead to high joint contact forces that could be detrimental to articular cartilage. However, how alignment of the knee influences muscle co-contraction remains unknown. Although HTO outcomes are generally good, with reported improvements in pain and function, Machner et al (14) and Oberg and Oberg (15) found quadriceps strength deficits 1 year after closing-wedge osteotomies. Whether realignment improves knee stability or affects movement and muscle activation patterns is unknown. The purpose of the present study therefore was to examine the effect of anatomic tibial realignment on abnormal knee joint kinetics, kinematics, and muscle function during gait among patients with medial knee OA and moderate genu varum. We postulated that quadriceps strength, joint moments, movement patterns, and muscle activity patterns would improve significantly from preoperative levels and would be similar to those of healthy age-matched controls following opening-wedge high-tibial osteotomy (OW-HTO). Moreover, knee instability and mediolateral joint laxity, as measured by patient self-report questionnaires and stress radiography, would be normalized. |
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PATIENTS AND METHODS Participants Fifteen patients (9 men: mean ± SD age 53.1 ± 6.4 years, mean ± SD body mass index [BMI] 30.4 ± 4.4 kg/m2 and 6 women: mean ± SD age 49.3 ± 7.9 years, mean ± SD BMI 29.2 ± 4.3 kg/m2) diagnosed with medial knee OA and genu varum were referred from a local orthopedic practice. Patients were being treated for pain and were scheduled for OW-HTO. Inclusion criteria were based on clinical history, physical examination, severity of malalignment, Kellgren and Lawrence radiographic grade 2 or higher, joint space narrowing as observed from standing posteroanterior radiographs with the knee flexed 30° (16), and difficulty with at least 1 item of the American College of Rheumatology clinical and radiographic criteria for the classification and reporting of knee OA (17). Skeletal alignment was measured from bilateral long cassette radiographs from the hip joint to the feet in the anteroposterior standing position. The weight-bearing line (center of the femoral head to the center of the ankle mortise) was <35%, calculated as the perpendicular distance from the weight-bearing line to the medial edge of the proximal tibia (18). A ratio <50% denotes varus angulation. Persons with history of ligament deficiency; cardiovascular disease; diabetes; neurologic impairment; impaired balance; rheumatoid arthritis; total knee replacement in either knee; other orthopedic problems in the hips, ankles, or spine; or a BMI ≥40.0 were excluded. Patients were tested prior to OW-HTO (preoperative) and 12 months later (postoperative). Fifteen healthy age-and sex-matched controls (9 men: mean ± SD age 52.0 ± 5.2 years, mean ± SD BMI 28.8 ± 2.4 kg/m2 and 6 women: mean ± SD age 47.5 ± 5.6 years, mean ± SD BMI 29.8 ± 7.7 kg/m2) with neutral alignment (norm) and no knee pain or radiographic evidence of knee OA were recruited from the community. All participants signed an informed consent, which was approved by the Human Subjects Review Board. Measurement of joint laxity Joint laxity was measured using stress radiographs. With participants lying supine and the knee flexed 20°, a 150-newton varus and valgus force was applied to the knee using a TELOS stress device (Austin & Associates, Fallston, MD). Joint space was measured with precision calipers (0.05-mm accuracy) at the narrowest points on the medial and lateral compartments for both varus and valgus stresses. Medial and lateral joint laxity were computed as follows (19): medial laxity = medial joint opening minus medial joint closing; and lateral laxity = lateral joint opening minus lateral joint closing. This method has excellent reliability, as demonstrated by intraclass correlation coefficients (ICCs) (3, 1) of 0.95 and 0.97 for lateral and medial laxity, respectively, using repeated measurements on 8 healthy participants (12). Surgery All participants underwent OW-HTO, performed by 1 experienced orthopedic surgeon (WN), a mean of 25 days (range 2–62 days) after preoperative testing. Patients used crutches and were non–weight bearing for 4 weeks and subsequently began slow progression from partial to full weight bearing over the ensuing 4 weeks. One participant received formal physical therapy. Measurement of pain and functional status Functional status and the influence of pain were assessed using the Knee Outcome Survey Activities of Daily Living Scale (KOS-ADLS) (20). Instability was assessed using the question, “To what degree does giving way, buckling or shifting of the knee affect your level of daily activity?” which was taken from the KOS-ADLS. Reliability of the self-reported measure of knee instability question demonstrated moderate test–retest reliability, with an ICC (2, 1) of 0.72 (11). Participants also rated their global knee function on a scale from 0% to 100% where 100% indicates no disability. Quadriceps femoris strength and central activation ratio Quadriceps strength was assessed with the participant seated in an isokinetic dynamometer (KINCOM; Chattanooga Group, Chattanooga, TN) with the hips and knees flexed to 90° and the back supported. The axes of rotation of the knee and dynamometer were aligned and the distal shank was affixed to the arm 5.0 cm proximal to the lateral malleolus. Velcro straps secured both the thigh and waist for stabilization. The thigh was cleansed with isopropyl rubbing alcohol. Two pre-gelled, 3 × 5–inch self-adhesive electrodes (ConMed Corporation, Utica, NY) were placed over the proximal vastus lateralis and distal vastus medialis to deliver a supramaximal train of electrical stimulation. Participants performed 2 submaximal and 1 near-maximal voluntary isometric contraction (MVIC) lasting 2–3 seconds for familiarization and to potentiate the muscle. Visual feedback was used to set the target force during warm up. If the target was surpassed, force limits were increased incrementally. After 5 minutes of rest, participants were instructed to maximally contract the quadriceps femoris muscle for ~4 seconds. Intense verbal encouragement and visual feedback of force output were used to motivate the participants to produce an MVIC. Approximately 2 seconds into the test, the supramaximal burst (100 pulses/second, 600-μsecond pulse duration, 10 pulse tetanic train, 130 volts; Grass S88 stimulator, Grass Technologies, West Warwick, RI) was delivered to fully activate the quadriceps. Knee extension force was measured and evaluated at 200 Hz using custom-written software (Labview; National Instruments, Austin, TX). If a maximal voluntary force output was achieved (no increase in torque during the stimulation), the quadriceps was considered fully activated and testing ceased. The test was repeated up to 3 times, with 5-minute rest intervals between tests to minimize fatigue, if the participant did not generate a volitional force within 95% of the electrically elicited force. The volitional force that immediately preceded the electrical stimulus was normalized to the participant’s BMI (kg/m2) to allow for comparison between participants and groups. The highest volitional force achieved during the 3 attempts was used for analysis. Testing was performed on the involved limbs of the OA group and the test limb randomly assigned for the control group. As a measure of activation, the central activation ratio (CAR) was calculated according to the following equation:
where Fvolitional is the volitional force produced by the quadriceps immediately prior to the electrical stimulus, and Felectrical is the peak force produced when the electrical stimulus was superimposed on the volitional effort. A CAR of 0.95 was considered complete activation of the muscle (no inhibition). Motion capture Participants underwent 3-dimensional gait analysis with simultaneous surface electromyographic measurement. Kinematic data were collected at 120 Hz using a passive 6-camera system (VICON 512; Vicon, Oxford, UK) and ground reaction force data were recorded at 1,800 Hz from 2 Bertec force platforms (Bertec Corp., Worthington, OH). Motion and kinetic recordings were synchronized for simultaneous collection. Joint centers of the lower limb were defined using 15-mm retroreflective markers placed bilaterally over the iliac crests, greater trochanters, lateral femoral condyles, lateral malleolus, and fifth metatarsal heads. Rigid thermoplastic shells affixed with 4 markers were attached to an elastic underwrap (SuperWrap; Fabrifoam, Exton, PA) surrounding the thigh and shank. Both shank and thigh shells were placed posterolaterally and overwrapped to minimize movement (21). A marker triplet placed on the sacrum and 2 additional markers on the participant’s heel counter along with the marker on the fifth metatarsal head were used to track pelvis and foot movement, respectively. Muscle activity was recorded concurrently at 1,800 Hz using a 16-channel system (Motion Lab Systems, Baton Rouge, LA) and the signals were bandpass filtered in the hardware between 20 and 1,000 Hz prior to sampling. Preamplified surface electrodes (20-mm interelectrode distance, 12-mm disk diameter) were placed over the mid-muscle belly of the semitendinosis, biceps femoris, vastus medialis, vastus lateralis, and medial and lateral heads of the gastrocnemius (22). To ensure correct electrode placement, maximum voluntary isometric contractions were performed. An MVIC and a resting baseline were recorded for each muscle for normalization. Participants walked along a 10-meter walkway at a self-selected pace, as measured by 2 photoelectric cells spaced 286.5 cm apart. Ten trials were collected whereby participants contacted opposing force platforms with each foot, without targeting. Walking velocity was maintained to within 5%, as measured during practice trials with the photoelectric cells. Data management and processing Marker trajectories and ground reaction force data were low-pass filtered (Butterworth fourth order, phase lag) at 6 and 40 Hz, respectively, using custom-written software (Labview; National Instruments, Austin, TX). Lower limb kinematics were calculated using rigid body analysis and Euler angles and were referenced to the coordinate system from the standing calibration, and joint moments were derived using inverse dynamics (Visual 3D; C-Motion, Rockville, MD) and were normalized to body mass and height. Stance was time normalized to 100% and averaged across the 10 trials for each participant, condition, and group. Variables of interest were knee flexion excursion from initial contact to peak knee flexion and to the peak external knee adduction moment during early stance. The raw electromyographic (EMG) data were initially filtered with a 350-Hz low-pass Butterworth filter, then were full-wave rectified and filtered again with a phase corrected eighth-order, 20-Hz low-pass Butterworth filter to generate the linear envelope (Labview; National Instruments). Linear envelopes were normalized to peak EMG from the MVIC. Co-contraction indices (CCIs), which we defined as the simultaneous activation of antagonist muscles, were derived for the following muscle pairs: vastus medialis-medial hamstrings (VMMH), vastus medialis-medial gastrocnemius (VMMG), vastus lateralis-lateral hamstrings, and vastus lateralis-lateral gastrocnemius. Muscle responses were analyzed from 100 msec prior to initial contact (to account for electromechanical delay [23]) to when the first peak knee adduction moment occurred. This time interval was normalized to 100 data points. CCIs for each pair were derived using the following equation (24):
The ratio of the lower EMGi (the level of activity in the less active muscle) and the higher EMGi (the level of activity in the more active muscle [to avoid, divide by zero errors]) was multiplied by the sum of the activity found in the 2 muscle groups. This provided a sample-by-sample estimate of the relative activation of the pair of muscles as well as the magnitude of the co-contraction. The resulting values were averaged to arrive at a single value representing the magnitude of co-contraction between the 2 muscles, then the mean for the 10 trials was calculated. Statistical analysis Statistical analysis was performed using SPSS 13 (SPSS, Chicago, IL). Within-group comparisons (preoperative and postoperative) were assessed using paired t-tests to determine differences in joint angles and moments, quadriceps strength, joint laxity and alignment, and KOS-ADLS scores. Independent t-tests were used for group comparisons (OA and norm). A within-participant repeated-measures analysis of variance was used to compare co-contraction values for both preoperative and postoperative settings. Post hoc t-tests were performed when a significant main effect for group was identified. Hierarchical regression analyses were used to assess the influence of medial laxity, quadriceps strength, and functional knee instability on the predicted self-reported ratings of overall knee function (KOS-ADLS, global knee rating score). These variables were chosen because each has been reported to be associated with disease progression. Independent variables were entered in a stepwise manner. Self-reported rating of knee instability was entered last so that its effect on knee function could be determined after controlling for the other variables. Pearson’s product moment correlations were also performed to assist in interpreting the results. Statistical significance was set at P less than 0.05 except for muscle co-contraction indices. An alpha level of P less than 0.1 was established in an effort to avoid a type I error given the highly variable nature of EMG data (25). |
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RESULTS Radiographic data are depicted in Table 1. The weight-bearing line of the knee was transferred from the medial compartment to the lateral compartment (mean ± SD 20.6% ± 11.6% versus 72.7% ± 9.6%; P < 0.001). Medial laxity was significantly reduced (P = 0.003) and was equivalent to that of healthy controls (P = 0.372). Lateral compartment laxity was significantly lower in the preoperative group than in the control group (P = 0.035), but no differences were observed between postoperative patients and controls (P = 0.313).
The OA group had significant improvements in global rating of knee function (P = 0.001) but remained significantly impaired compared with healthy controls (P = 0.001) (Figures 1A and 1B). Dynamic knee stability was also significantly improved (P = 0.002) in the OA group and was equivalent to that of controls (P = 0.177) (Figure 1C). Twelve of 15 patients (80%) reported the presence of instability preoperatively (Table 2). Eight (53%) of the 12 patients who had symptoms of instability indicated that the instability affected their ability to perform activities of daily living. Postoperatively, 4 (26%) reported the symptom of instability, but only 1 of these individuals indicated that instability affected activities of daily living. No one in the control group reported instability.
Medial knee laxity did not predict self-reported instability preoperatively (r2 = 0.127, P = 0.332) or postoperatively (r2 = −0.262, P = 0.173). Knee instability was significantly related to self-report ratings of global knee function (r = 0.636, P = 0.011) after realignment, with higher knee function scores being positively associated with greater knee stability. Quadriceps weakness and diminished knee flexion excursions during stance persisted after surgery (Figures 2A and 2B). When tested postoperatively, quadriceps strength was not statistically different from preoperative strength (P = 0.626), but the OA group had significantly lower quadriceps strength than controls (preoperative: P = 0.036, postoperative: P = 0.024). Using our operational definition of 0.950 for the determination of full quadriceps activation, no differences in muscle inhibition were evident, as demonstrated by similar CAR values, preoperatively and postoperatively (mean ± SD CAR 0.937 ± 0.052 and 0.940 ± 0.028, respectively; P = 0.839) or for the control group (0.933 ± 0.066). The hierarchical regression analyses indicated that only the addition of knee stability accounted for a significant increase in the amount of variance explained (r2 = 0.487, r2 change = 0.481, F = 10.319, P = 0.008); 48% of the variance in the KOS global knee rating score was predicted by instability. Both medial laxity (r2 = 0.000, P = 0.998) and quadriceps strength (r2 = 0.006, P = 0.795) did not account for the explained variance.
As expected, knee adduction angles and moments were significantly reduced throughout stance (Figure 3). Knee flexion excursions during weight acceptance remained unchanged (Figure 2B and Figure 3) (P = 0.130) but were significantly lower than those in the control group (preoperative: P = 0.004, postoperative: P = 0.043).
Co-contraction indices, as shown in Figure 4, revealed that the level of VMMG co-contractions for the OA group was significantly reduced following OW-HTO (mean ± SD preoperative: 14.2 ± 7.7, mean ± SD postoperative: 11.1 ± 5.3; P = 0.089). Comparing VMMG co-contractions with those of the healthy group (mean ± SD 7.2 ± 5.6; P = 0.01), preoperative co-contraction indices were significantly higher, but magnitudes after surgery were not different from controls (P = 0.135). VMMH co-contraction indices were not statistically different across conditions (mean ± SD preoperative: 14.5 ± 5.7, mean ± SD postoperative: 13.3 ± 9.3; P = 0.660) or across groups (mean ± SD norm: 12.6 ± 8.7; P = 0.486 and P = 0.755, respectively). Regression analysis revealed a negative association between quadriceps strength and medial muscle co-contraction before surgery (VMMG: r = −0.599, P = 0.024; VMMH: r = −0.688, P = 0.006). Weaker quadriceps resulted in higher levels of medial muscle co-contraction. Strength and medial muscle co-contractions were not correlated after surgery for the patient or the control group.
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DISCUSSION The goal of this study was to determine whether anatomic tibial realignment via OW-HTO corrected knee pathomechanics and muscle function associated with poor knee function and the progression of OA. The factors that have been shown to influence disease progression include high knee adduction moments, excessive frontal plane laxity, and quadriceps weakness (5-11,26,27). Less knee flexion during weight acceptance and high muscle co-contraction may exacerbate joint destruction by increased impact loads (28) and increased joint contract pressures (13,29), respectively. In spite of improvements in alignment, adduction moments, and joint laxity, realignment did not affect quadriceps strength and knee movement patterns, and only VMMG muscle co-contractions were reduced. This finding suggests that some of the factors that are likely to be involved in joint degeneration persist and could worsen patient outcomes over time. Postoperatively, along with the expected improvement in frontal plane knee alignment and lower knee adduction moments, self-reported knee instability and medial joint laxity improved to levels comparable with controls. The mechanism by which medial laxity improved is not entirely understood; however, we suggest that the anterior superficial fibers of the medial collateral ligament, which are released during the surgery, heal in a shortened position. This explanation is consistent with the work of Naudie et al (30), who demonstrated that a medial opening-wedge osteotomy may tighten the capsuloligamentous structures around the knee when used to correct hyperex-tension-varus thrust. Had the medial structures remained slack, the only mechanism by which medial laxity could be reduced is via increased medial joint width during closing (valgus loading), which is unlikely because cartilage does not regenerate. It is logical to presume that lateral laxity would increase with OW-HTO, and greater lateral laxity was observed after surgery; however, the difference was not statistically significant. Normal ligament length and tension could increase joint integrity by improving mechanical restraints and mediate joint stability via reflexive muscle responses elicited by mechanoreceptors originating in the periarticular joint structures (31-34). Self-reported knee instability was markedly improved after realignment and was largely responsible for improvements in self-reported knee function. The findings that medial knee laxity did not predict self-reported knee instability and that knee instability (not medial laxity or quadriceps strength) predicted postoperative knee function underscore the importance of knee instability in this population. Lower adduction moments, less knee instability, and less medial laxity all bode well for joint integrity, but the factors that were not improved after realignment are cause for concern. Quadriceps weakness is a very important factor in persons with knee OA. Although the cause remains unclear, it appears that inhibition does not substantially contribute to reductions in strength. Quadriceps weakness has been related to worsening knee function (7,8,35-37), the development of knee OA (9,38-41), and knee OA progression (8,42). Persistent quadriceps weakness found at 12 months postoperatively is consistent with other studies that have demonstrated long-term weakness after knee realignment (14,15). Quadriceps weakness may explain why the global rating of knee function remained lower than in controls. In addition to its effect on joint function, quadriceps weakness also has important implications to joint integrity. Quadriceps weakness is known to accompany reduced knee flexion during weight acceptance, particularly in individuals who experience knee instability (43). The reduced knee flexion excursion seen postoperatively may allow higher impact loads to persist, which exacerbate continued cartilage destruction and eventually negate the positive outcome of the surgery. Another factor that can influence joint contact forces is muscle co-contraction. Despite valgus knee realignment and reduced VMMG muscle co-contractions, VMMG muscle co-contraction remained higher than in healthy controls. By transferring the mechanical axis laterally, it is possible that the higher magnitude of VMMG co-contraction serves to lessen lateral compartment loading after the OW-HTO. This explanation is consistent with the work of Schipplein and Andriacchi who proposed that the lateral muscles may prevent lateral condylar lift off in persons with varus alignment (29). It is plausible that medial co-contraction might lessen load on the lateral compartment; however, it could also cause increased joint pressure on the diseased medial compartment, thereby contributing to continued medial compartment destruction. Overall, realignment improved the quality of life for these patients by increasing knee function and mitigating some of the factors that might lead to joint destruction, but the knee stiffening strategy (less knee motion, higher muscle co-contraction) might have been improved had these patients performed quadriceps strengthening activities during rehabilitation, which was conspicuously absent. Postoperative rehabilitation following HTO is uncommon. Presumably because it is thought that when alignment is restored and pain is relieved, increased use of the limb will lead to the return of normal strength and movement patterns. However, our data clearly show that this is not the case. Given the importance of quadriceps strength in mitigating OA progression, resistance training to restore quadriceps strength 3–6 months after surgery seems vital to maintain the improvements caused by HTO. Although quadriceps strengthening activities would likely improve knee movement patterns and function in postoperative patients, we believe that strength training alone would be inadequate to address all of the impairments that persisted in this group. Recently, quadriceps strength was found to increase the likelihood of knee OA progression (42) in individuals with high frontal plane joint laxity or varus or valgus malalignment. Although the frontal plane laxity in this group was diminished, the average alignment indicated a shift from varus to valgus malalignment. It is possible that postoperative quadriceps strength training would lead to more OA progression despite lower load on the medial compartment. Knee instability may play a role in whether strong quadriceps muscles lead to an increase or decrease in knee OA progression. When strong quadriceps that co-contract with antagonists (resulting in high joint compression) are coupled with high shear forces and concomitant knee instability, the cumulative effect could be highly detrimental to articular cartilage. Therefore, we propose that rehabilitation to strengthen the quadriceps be accompanied by neuromuscular training to reduce knee instability so that the cycle of destruction might be slowed or stopped. |
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Acknowledgments The authors thank Laurie Andrews, RTR, for assisting with the radiographs. Supported by National Center for Research Resources grant P20-RR-016458 and NIH grants T32-HD-007490, R01-HD-037985, and R01-AR-048212. |
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Footnotes The contents of this report are the sole responsibility of the authors and do not necessarily represent the official views of the National Center for Research Resources or the NIH. |
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