Knee Joint Movements in Subjects Without Knee Pathology and ...

Research Report

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Knee Joint Movements in Subjects Without Knee Pathology and Subjects With Injured Anterior Cruciate Ligaments ўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўўў

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Background and Purpose. Although weight-bearing (WB) exercise and increased hamstring muscle activity may contribute to knee joint stability in knees with an injured anterior cruciate ligament (ACL), the relationship among ACL integrity, muscle activity, and joint surface motion is not fully understood. The purpose of this study was to investigate whether knee joint rolling and gliding movements and electromyographic (EMG) activity differed between subjects with injured ACLs and subjects without knee pathology. Subjects. Fifteen subjects with injured ACLs (9 men and 6 women; mean age⫽26 years, SD⫽7, range⫽18 –36) and 15 age- and sex-matched subjects without knee pathology (9 men and 6 women; mean age⫽25 years, SD⫽6, range⫽18 –36) participated in the study. Methods. Sagittal-plane knee joint rolling and gliding movements and lower-extremity EMG activity were measured during non–weight-bearing (NWB) and WB movements. Mixed-model analyses of variance were conducted to analyze rolling and gliding and EMG data. Results. During NWB knee extension, greater joint surface gliding occurred in knees with injured ACLs at full knee extension. During WB knee extension, greater gliding occurred in knees with injured ACLs throughout the range of motion tested. No differences in EMG activity occurred between groups. Discussion and Conclusion. The results suggest that, in the absence of increased hamstring muscle activity, anterior tibial displacement is not reduced in knees with injured ACLs during WB movement. [Hollman JH, Deusinger RH, Van Dillen LR, Matava MJ. Knee joint movements in subjects without knee pathology and subjects with injured anterior cruciate ligaments. Phys Ther. 2002;82:960 –972.]

Key Words: Anterior cruciate ligament, Biomechanics, Electromyography, Instantaneous center of rotation, Knee.

960

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John H Hollman, Robert H Deusinger, Linda R Van Dillen, Matthew J Matava

Physical Therapy . Volume 82 . Number 10 . October 2002

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elative amounts of joint surface rolling and gliding may be inferred when there is movement at the knee through the application of the concept of the path of instantaneous centers of rotation (PICR).1– 4 For example, when the PICR is located close to the point of contact between joint surfaces, more rolling than gliding occurs.1,2 Conversely, when the PICR is located far from the point of contact, less rolling than gliding occurs. Based on their visual observations, Frankel et al1 and Gerber and Matter3 reported that PICR patterns in human knees with injured anterior cruciate ligaments (ACLs) differed from the patterns in knees without pathology. Similarly, researchers investigating PICR in canine stifles with injured ACLs reported observable deviations from PICR patterns in stifles without pathology.4 In these studies, however, the observed differences in PICR patterns were not quantified. Nevertheless, it is reasonable to hypoth-

esize that changes in intrinsic joint surface rolling and gliding movements, as suggested by altered PICR patterns, may occur in knees with injured ACLs. Frankel et al,1 Gerber and Matter,3 and Mitton et al4 did not examine PICR or measure the amount of joint surface rolling and gliding under weight-bearing (WB) conditions. Tibiofemoral joint compression forces are greater during WB knee extension than during non– weight-bearing (NWB) knee extension. During WB knee extension, the knee joint is loaded through body weight as well as through muscle activity. This is more loading than occurs during NWB knee extension, when the leg is unobstructed as it moves through space and the knee is loaded primarily through muscle activity.5,6 As joint compression increases, joint surface geometry regulates relative joint surface rolling and gliding.2,6 As joint compression decreases, the articular ligaments become

JH Hollman, PT, PhD, is Assistant Professor, Department of Physical Therapy, Clarke College, 1550 Clarke Dr, Dubuque, IA 52001-3198 (USA) ( [email protected]). Address all correspondence to Dr Hollman. RH Deusinger, PT, PhD, is Assistant Professor, Departments of Internal Medicine and Biomedical Engineering, Washington University School of Medicine, St Louis, Mo. LR Van Dillen, PT, PhD, is Assistant Professor, Program in Physical Therapy, Washington University School of Medicine. MJ Matava, MD, is Assistant Professor, Department of Orthopaedic Surgery, Washington University School of Medicine. Dr Hollman, Dr Deusinger, and Dr Van Dillen provided concept/research design and writing. Dr Hollman provided data collection. Dr Hollman and Dr Van Dillen provided data analysis. Dr Hollman and Dr Deusinger provided project management. Dr Matava provided subjects. Dr Deusinger provided facilities/equipment and institutional liaisons. Dr Deusinger, Dr Van Dillen, and Dr Matava provided consultation (including review of manuscript before submission). The authors thank Shirley Sahrmann, PT, PhD, FAPTA, Scott Minor, PT, PhD, Jack Engsberg, PhD, Kevin Truman, PhD, and Dequan Zou, DSc, for reviewing portions of the manuscript and Janice Loudon, PT, PhD, ATC, for laying the groundwork for this research. The Washington University School of Medicine Institutional Review Board approved the protocol for this study. The results of this study were presented in a poster at the 47th Annual Meeting of the Orthopaedic Research Society, February 25–28, 2001, San Francisco, Calif, and in an abstract in the Transactions of the 47th Annual Meeting, Orthopaedic Research Society. This project was funded in part by NIH-NCMMR training grant #5T32HDO743405. This article was submitted September 21, 2001, and was accepted March 28, 2002.

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more involved in regulating joint surface rolling and gliding.2,6 Consequently, several researchers have shown that the knee cruciate ligaments are loaded in the opposite manner from each other during movement and are loaded differently when in WB and NWB.7–11 Anterior cruciate ligament strain is greater than posterior cruciate ligament (PCL) strain during NWB knee extension, whereas PCL strain is greater than ACL strain during WB knee extension. Presumably, this is because greater anterior displacement of the tibia with respect to the femur is believed to occur during NWB.9,10 Collectively, the results of these studies7–11 suggest that knee joint surface rolling and gliding movements, and therefore PICR, may differ between WB and NWB movements. Hollman et al,12 using PICR to estimate knee joint surface rolling and gliding in subjects without knee joint pathology, reported that greater joint surface gliding occurs during NWB knee extension. These data, however, have not been published in a peer-reviewed publication. Given that the ACL provides the primary restraint to anterior tibial displacement,13 we hypothesized that joint surface gliding may be exaggerated in knees with injured ACLs during WB and NWB. Electromyography (EMG) can provide some insight into variations in muscle activity. In EMG studies, increased muscle activity about a joint is associated with increased joint compression.9 We believe, therefore, that it is reasonable to expect that altered muscle activity may also influence the relative amount of rolling and gliding and the PICR. Evidence suggests the hamstring muscles contribute to knee joint stability.9,10 In the absence of the structural integrity of the ACL, hamstring muscle co-contraction is thought to provide stability during knee extension either by virtue of its posteriorly directed force, its contribution to increased joint compression, or a combination of both mechanisms.5,9,14 –16 Data from several studies indicate that people with an injured ACL or reconstructed ACL demonstrate greater hamstring muscle EMG activity or altered hamstring muscle timing during functional activities than do subjects without knee pathology.17–21 In our opinion, this suggests that a change in muscle coordination may be necessary to improve stability in knees with injured ACLs. We contend that an increase in hamstring muscle activity may be sufficient to counteract the increased gliding that might otherwise be present. Electromyographic analyses that accompany the measurement of joint surface rolling and gliding movements, in our opinion, can determine whether differences in muscle activity are associated with changes in joint surface rolling and gliding.

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The primary purpose of our study was to compare joint surface rolling and gliding between subjects without knee pathology and subjects with injured ACLs during NWB and WB movements. A secondary purpose was to determine whether EMG activity of selected lowerextremity muscles, particularly the hamstring muscles, differed between these groups. We hypothesized that greater joint surface gliding would occur in knees with injured ACLs than in knees without pathology, assuming that an increase in hamstring muscle activity would not sufficiently counteract the hypothesized change in rolling and gliding movements. If a change in rolling or gliding did not occur, we hypothesized that greater hamstring muscle activity would be present in subjects with injured ACLs. Methods Subjects Fifteen adult subjects with injured ACLs (9 men and 6 women; mean age⫽26 years, SD⫽7, range⫽18 –36; mean height⫽173 cm, SD⫽8, range⫽160 –185; mean body mass⫽81 kg, SD⫽20, range⫽54 –113) and 15 ageand sex-matched subjects without knee pathology (9 men and 6 women; mean age⫽25 years, SD⫽6, range⫽18 –36; mean height⫽172 cm, SD⫽11, range⫽155–193; mean body mass⫽69 kg, SD⫽9, range⫽52–91) participated in this study. The number of subjects satisfied a statistical power (1 – ␤) greater than 0.80, based on an analysis of joint surface rolling among initial subgroups of 7 subjects with injured ACLs and 11 subjects without knee pathology. The effect size was calculated to be 0.29 using procedures described by Cohen and Cohen.22 We determined the effect size in order to make our findings more likely to be statistically significant. We did not determine potential effects based on any claim of clinical or biomechanical meaningfulness. Subjects with injured ACLs were recruited from: (1) the Department of Orthopaedic Surgery, Washington University School of Medicine, St Louis, Mo, (2) the undergraduate and graduate student populations of Washington University, St Louis, Mo, and (3) the Athletics Department, Clarke College, Dubuque, Iowa (Tab. 1). Data from subjects with injured ACLs were included only if an orthopedic surgeon diagnosed the ACL injury (confirmed by magnetic resonance imaging or by surgery following data collection for this study), either in isolation or with an associated injury to the medial collateral ligament (MCL), lateral collateral ligament (LCL), or meniscus. The MCL, LCL, and meniscus are secondary anterior stabilizers of the knee joint and provide stabilization in the absence of an intact ACL.13 Combined, these structures contribute only up to 14% of the total resistance to anterior tibial displacement13;

Physical Therapy . Volume 82 . Number 10 . October 2002

ўўўўўўўўўўўўўўўўўўўўўўўў Table 1. Characteristics of Subjects With Injured Anterior Cruciate Ligaments

Subject No.

Age (y)

Sex

Knee

Pathologya

Month After Injury

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

21 18 18 18 36 31 33 32 24 32 34 27 34 20 19

Female Male Female Female Female Male Male Male Female Male Male Female Male Male Male

Right Right Right Left Right Left Left Left Left Left Left Left Left Left Right

ACL/meniscus ACL/MCL/meniscus Failed ACL repair ACL ACL ACL ACL ACL ACL ACL ACL/meniscus ACL ACL/LCL/meniscus ACL ACL

1 1.5 1.5 24 6 96 2 120 1.5 1.5 84 1 3 10 7

a ACL⫽anterior cruciate ligament, meniscus⫽medial meniscus, MCL⫽medial collateral ligament, LCL⫽lateral collateral ligament.

therefore, we included subjects with injuries to these structures in the study. Additional requirements were that each subject have, at minimum, active range of motion of the knee of ⱖ10 degrees to 90 degrees of knee flexion (relative to a 0°–180° notation system23) and an active straight leg raise of 60 degrees or greater in a supine position. This position was chosen in an effort to reduce potential effects of short hamstring muscles on knee joint movements. All subjects with injured ACLs had at least 120 degrees of knee flexion, although 3 subjects with acute injuries had mild knee joint effusion at the time of testing and had deficits in full extension of up to 5 degrees. We made a judgment that those with mild effusion, assuming they could move through the knee range of motion required of the study, would not have substantially adverse effects on the results and were therefore included in the study. Potential subjects with injured ACLs were excluded if they had any history of neurological or neuromuscular pathology. Comparison subjects without knee pathology were recruited from a population of convenience—the undergraduate and graduate student populations at Washington University in St Louis. Potential subjects were excluded from the study if they had any history of knee injury or neurological or neuromuscular pathology. Comparison subjects were allowed to take part only if they satisfied the following criteria: (1) active range of motion of the knee of 0 to 130 degrees of flexion or greater, (2) an active straight leg raise in a supine position of 60 degrees or greater, (3) grade 5 force production capability of the right quadriceps femoris and hamstring muscles based on manual muscle testing techniques,24 and (4) negative findings on the LachPhysical Therapy . Volume 82 . Number 10 . October 2002

man’s test, posterior drawer test, valgus and varus ligament stability tests, and McMurray’s meniscus test.25 We did not examine the reliability of these measurements. Instrumentation Movements were recorded with 2 Panasonic AG-455P SVHS video cameras.* Kinematic data were processed with Ariel Performance Analysis System (APAS) software, APAS99 version 3.5.† The APAS software provides accurate measurements of linear (mean error less than 2 mm) and angular (mean error less than 0.3°) standards26 and reliable measurements of angular velocity data, with intraclass correlation coefficients exceeding .85 at various speeds of movement.27 Coordinate transformations and instantaneous center of rotation (ICR) calculations are described elsewhere28 and were performed using custom-written programs in Microsoft Excel 97, version SR-1.‡ Electromyographic signals were collected with bipolar surface electrodes having an on-site pre-amplification gain of 310.§ Each active electrode pair consisted of two 8-mm– diameter silver–silver chloride electrodes with an interelectrode distance of 35 mm. The on-site preamplifiers had common mode rejection ratio ratings of approximately 105 dB at 60 Hz with high direct current input impedance of 100,000 M⍀ and bandwidth frequencies of 8 Hz to 31 kHz. The EMG signals were processed with APAS† software, and normalization procedures were performed with custom-written programs in Microsoft Excel.‡ A Bertec force platform㛳 was used as a video synchronization and EMG triggering device. Subjects made contact with the force platform while initiating their movement, which simultaneously turned on an external light source that was visible in the video fields of both video cameras and initiated EMG sampling. This triggering mechanism enabled videographic images and EMG data to be synchronized. Procedure All subjects signed an informed consent form approved by the Washington University School of Medicine Institutional Review Board. The primary investigator ( JHH) conducted a brief physical examination to assess knee joint range of motion and assessed muscle force and knee stability in subjects without knee pathology. Active range of motion of the knee was measured with a universal goniometer using techniques described by

* Panasonic Broadcast & Television Systems Co, Franklin Park, IL 60131. † Ariel Dynamics Inc, 6 Aliconte St, Trabuco Canyon, CA 92679. ‡ Microsoft Corp, One Microsoft Way, Redmond, WA 98052-6399. § Motion Control, Division of Iomed Inc, 1290 West 2320 South, Ste A, Salt Lake City, UT 84119. 㛳 Bertec Corp, 819 Loch Lomond Ln, Worthington, OH 43085.

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Norkin and White,23 and muscle force was measured manually using techniques described by Kendall et al.24 Only the involved side was tested in subjects with injured ACLs. In subjects without knee pathology, the side tested was selected in a random manner to equalize the distribution of right and left knees between the groups. For 11 subjects without knee pathology, we randomly selected whether the right or left knee would be tested. After we determined that 15 subjects would be tested per group, the side tested in the group without pathology was assigned so that there would be an equal number of right and left knees tested in the study. Surface EMG electrodes were taped over the mid– muscle belly of each subject’s vastus lateralis, medial hamstring (ie, the semitendinosus), medial gastrocnemius, and gluteus maximus muscles. Electrodes were placed in a manner that we believe was in parallel with the line of action of the muscle. To facilitate signal conduction, each electrode site was rubbed with an alcohol wipe prior to electrode attachment. Electrode locations were found by following the description of Ericson et al,29 and specific locations were identified by palpating the respective muscle bellies. Once electrodes were applied, EMG signals were sampled during maximal voluntary isometric contractions (MVICs) for each muscle group. Photoreflective markers, which were 2 cm in diameter, were placed on the lateral thigh and lateral leg to provide rigid-body representation of the thigh and leg segments. The markers were placed on the thigh 10 cm distal to the greater trochanter and 5 cm proximal to the lateral epicondyle and on the leg 1 to 2 cm distal to the fibular head and 1 to 2 cm proximal to the lateral malleolus, which minimized skin movement artifact.30 We did not examine the reliability of placing these markers. Subjects sat on either a 35.6-cm (14-in) or 40.6-cm (16-in) wooden chair, depending on the subject’s height and lower-extremity limb length that allowed the knee to be in approximately 100 degrees of flexion in its resting position. Subjects performed 5 repetitions each of the NWB and WB movements. To do the NWB movement, subjects extended their leg to a position of maximal knee extension. Subjects did the WB movement by executing a 2-legged sit-to-stand movement. Subjects initiated both testing conditions by exerting pressure on the force platform, which allowed us to synchronize the kinematic and EMG data. Because joint rotation at slow angular speeds (⬍30°/s) decreases PICR measurement accuracy,31 we attempted to control the speed of movement by instructing subjects to perform their movements over a period of approximately 1 second.

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Data Processing Reflective marker spatial locations and instantaneous center of rotation (ICR) calculations. All spatial location data were recorded at 60 Hz, the sampling frequency of the video cameras. Images were captured, compressed, and stored on the computer hard drive with APAS software.† Subsequently, the video files were transformed via direct linear transformation.32 Three-dimensional spatial locations of reflective markers defining the thigh and leg were autodigitized and smoothed between 3 and 6 Hz using a second-order Butterworth digital filter. Cutoff frequencies for each marker were selected by using spectral power analyses in the “Filter” module of APAS software.† Spatial location data were transformed to a local reference system fixed in the thigh segment with its origin located at the lateral femoral epicondyle. The PICR data were calculated in a manner originally described by Winter31 and detailed by Hollman and Deusinger28 and Loudon.33 The joint’s estimated instantaneous angular velocity (␻) and the leg segment’s estimated instantaneous tangential linear velocity (V) in the sagittal plane of motion were used to calculate locations of the ICR by identifying the distance (R) of the ICR from the lateral malleolus marker by V ⫽ ␻ ⫻ R. In Cartesian coordinates represented by a right-handed coordinate system, the location of the ICR is calculated as the distance (Rx , Ry) from the lateral malleolus as follows: ⫺ Ry ⫽

Vx ␻

and Rx ⫽

Vy . ␻

The accuracy and precision of ICR measurement that we used is based on data obtained with a model from which the ICR was a stationary and known entity. Calculations yielded a mean error magnitude of less than 1 mm and a standard deviation of 4.3 mm.28 The reliability coefficient for the method when applied to human knee joint PICR measurement exceeds .80.33 The ICR coordinate locations calculated for the first 3 digitizable trials from each movement condition were used for obtaining mean condition-dependent PICR patterns for each subject. Intrinsic knee joint surface movements model. Coordinates of the ICR were obtained for each participant through the range of motion and extracted at 10-degree

Physical Therapy . Volume 82 . Number 10 . October 2002

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Based on the selected femoral contact point locations and the ICR data obtained experimentally, the relative proportion of rolling to gliding was calculated using the slip ratio described by O’Connor and Zavatsky.37 The slip ratio is defined by slip ratio ⫽

sm sf

where sm is the displacement between successive contact points on the convex femoral surface and sf is the displacement between successive contact points on the flat tibial surface. The variables sm and sf are approximated by

Figure 1. Distal femoral portion of mathematical knee model. A quarter circle having a radius of 2 cm represents the posterior aspect of the condyle (second quadrant), and a quarter circle having a radius of 4 cm represents the distal-anterior aspect of the condyle (fourth quadrant). An elliptic curve (third quadrant) forms a transition between the 2 circular components. Contact point locations represent femorotibial contact points in 10-degree increments from 90 degrees of knee flexion (the point most posterior) to 0 degrees of knee flexion (the point most distal, moving anteriorly).

␪2

sm ⫽

冕

R 䡠 d␪

␪1

and ␪2

intervals from 90 degrees to 10 degrees of knee flexion. Knee joint surface movements were examined on 2 levels. First, a mean PICR for each movement condition was plotted and analyzed visually to examine observable differences between the subject groups and between the WB and NWB movements. Second, participants’ ICR coordinate data were applied to a planar mathematical knee joint model (Fig. 1) that was used to calculate relative proportions of knee joint surface rolling and gliding. Dimensions of the model were adapted from morphology studies of the distal femoral articular surface34 –36 to formulate a geometrical configuration of the femur in the sagittal plane. Three geometrical components constituted the model. One component was a circle having a 2-cm radius that represents the posterior condylar curvature. A second circle with a 4-cm radius represented the distal-anterior condylar surface curvature. The third geometrical component of the model was an ellipse fitted between the 2-cm and 4-cm curves that created a smooth transition between the 2 circles to represent the continuous condylar surface of the femur. The proximal tibia was modeled as a linear straight-line surface. Coordinates for the femoral surface contact points were calculated at 10-degree intervals along the elliptical surface of the model.

Physical Therapy . Volume 82 . Number 10 . October 2002

sf ⫽

冕

共R ⫺ r icr兲 䡠 d ␪

␪1

where R is the radius of curvature of the model, ricr is the radius from an ICR location to its respective femoral contact point, and ␪ is the fixed rotation angle (10°) between successive contact points. Ultimately, percent rolling (% rolling), defined by % rolling ⫽

冋

册

sm 䡠 100 s m ⫹ 共s m ⫺ s f兲

and percent gliding (% gliding), defined by % gliding ⫽ 100 ⫺ % rolling were the potential outcome measures of rolling and gliding movements. We used % gliding for statistical procedures in this study. There are no reliability data published in peer-reviewed journals for these measurements. With unpublished data obtained in our laboratory from samples consisting of 11 subjects without knee pathology and 7 subjects with injured ACLs, the withinsession reliability coefficients (specifically, the relative generalizability coefficients38) for calculating % rolling in knees without pathology is .83 and in knees with injured ACLs is .92.

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EMG. The EMG data were sampled at 1,000 Hz. We used this sampling rate to satisfy the Nyquist theorem, which states that the sampling rate must be greater than or equal to 2 times the highest frequency component in the analog signal. Therefore, we were able to sample muscle fiber firing frequencies up to 500 Hz. All raw signals were full-wave rectified and subsequently processed through a linear envelope with a 50-millisecond floating window. The processed EMG data obtained from the test conditions then were normalized by dividing the magnitude of a muscle’s EMG activity by its peak MVIC magnitude and multiplying by 100, and the normalized EMG values were expressed as a percentage of MVIC (% MVIC). The normalized EMG values, which were synchronized with knee angles, were extracted at respective 10-degree knee angle positions and used for data analysis. Because we consider lower-extremity EMG data to be highly reliable within testing sessions for movements similar to those used in this study,39 data from one repetition in each movement condition was processed for each subject.

between groups (␣⫽.05). We chose this analysis to determine whether differences in EMG activity occurred between subjects with injured ACLs and subjects without knee pathology. If the group main effect or interactions involving the group factor were significant, then a mixed-model ANOVA was performed on each muscle to determine which muscle or muscles differed between groups. Results Movements The PICR patterns and % gliding data for both groups are presented in Figures 2 and 3, respectively. Within each movement condition, the PICR for subjects with injured ACLs is located further from the joint surface than for subjects without knee pathology (Fig. 2), suggesting that greater joint surface gliding occurs in knees with injured ACLs. This is most evident in the last 20 degrees of knee extension in the NWB movement.

Movements. A 3-way mixed model analysis of variance (ANOVA) with ␣⫽.05 was conducted to test the null hypothesis that no difference occurred in rolling and gliding movements between subjects without knee pathology and subjects with injured ACLs. The dependent variable was % gliding. The between-subjects factor was group (subjects without knee pathology and subjects with injured ACLs). The within-subjects factors were movement (NWB and WB) and knee angle (90°, 80°, . . ., 10°). We tested the main effects of group and movement and the group interactions with movement and knee angle to determine in which condition or conditions and at which knee angle or angles rolling and gliding movements differed between groups. If either a group ⫻ movement or group ⫻ movement ⫻ knee angle interaction existed, we then conducted a mixed-model ANOVA on each movement to determine whether differences occurred between groups within a movement. If a group ⫻ knee angle interaction existed, we conducted post hoc t tests with a Bonferroni-adjusted alpha to determine the knee angle or angles at which % gliding differed. The main effect of movement was tested to determine whether joint surface rolling and gliding differed between the WB and NWB movements across groups.

The statistical analysis of % gliding revealed that overall, there was no group main effect (F⫽2.44, df⫽1,28, P⫽.13, 1⫺␤⫽0.36), but there were interactions in each effect containing the group factor (group ⫻ movement: F⫽9.95, df⫽1,28, P⫽.004; group ⫻ knee angle: F⫽2.51, df⫽8,224, P⫽.013; and group ⫻ movement ⫻ knee angle: F⫽3.01, df⫽8,224, P⫽.003). In the NWB movement, there was no group main effect (% gliding: subjects without knee pathology⫽43.3%⫾1.8% [mean⫾SEM], subjects with injured ACLs⫽45.0%⫾2.2%; F⫽0.27, df⫽1,8, P⫽.608). There was, however, a group ⫻ knee angle interaction (F⫽5.55, df⫽8,224, P ⬍.001) in the NWB movement. Post hoc t tests revealed that, at 10 degrees of knee flexion, greater joint surface gliding occurred in knees with injured ACLs than in knees without pathology (% gliding⫽51.1%⫾1.3% and 43.0%⫾1.7%, respectively; t28⫽⫺3.44, P⫽.004). In the WB movement, there was a group main effect (% gliding: subjects without knee pathology⫽40.8%⫾2.3%, subjects with injured ACLs⫽44.8%⫾2.4%; F⫽5.51, df⫽1,8, P⫽.026), indicating that greater joint surface gliding occurred in knees with injured ACLs than in knees without pathology throughout the knee range of motion tested. In summary, joint surface gliding was greater in knees with injured ACLs than in knees without pathology at full knee extension (10° of knee flexion) in the NWB movement and throughout the range of motion (10°– 90°) in the WB movement.

EMG. A 4-way mixed-model ANOVA with the same between-subject factor (group) and 3 within-subjects factors—including movement, knee angle, and muscle (vastus lateralis, medial hamstring, medial gastrocnemius, gluteus maximus)—was conducted to test the null hypothesis that no difference existed in % MVIC

There was a main effect for movement (F⫽9.67, df⫽1,28, P⫽.004). More joint surface gliding occurred in the NWB movement than in the WB movement (% gliding: WB⫽42.8%⫾1.7%, NWB⫽44.2%⫾1.4%) across both groups. A movement ⫻ knee angle interaction (F⫽12.11, df⫽8,224, P ⬍.001) revealed that the differ-

Data Analysis

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ўўўўўўўўўўўўўўўўўўўўўўўў Figure 2. Mean paths of instantaneous centers of rotation (PICR) in the (A) non–weight-bearing (NWB) and (B) weight-bearing (WB) movements.

ence in % gliding between movements was particularly apparent at full knee extension (% gliding: WB⫽33.7%⫾1.3%, NWB⫽47.1%⫾2.6%; t29⫽5.44, P⫽.001). More joint surface gliding occurred in the NWB movement than in the WB movement among subjects in both groups, particularly at full knee extension. EMG The EMG data are illustrated in Figure 4. Overall, EMG activity was greater in the WB movement than in the NWB movement (F⫽81.56, df⫽1,28, P⫽.001). Electromyographic activity also differed among muscles (F⫽39.10, df⫽4,112, P⫽.001) and across knee angles (F⫽7.47, df⫽8,224, P⫽.001). There was no group main effect, however, nor were there any interactions involving the group factor (Tab. 2). Because EMG activity did not differ between subjects without knee pathology and subjects with injured ACLs (F⫽2.00, df⫽1,28, P⫽.169, 1⫺␤⫽0.36), no additional analysis of EMG data was conducted. Discussion Knee Joint Surface Movements We found that PICR patterns during NWB knee extension (Fig. 2A) in subjects with injured ACLs moved farther from the joint surface as the knee approached full extension. We found that % gliding was greater than 50% (Fig. 3A) at the 10-degree knee flexion angle, indicating that, at full knee extension, more gliding than Physical Therapy . Volume 82 . Number 10 . October 2002

rolling occurs in knees with injured ACLs than in knees without pathology. These results disagree with those of Gerber and Matter,3 who reported that the PICR shifts anteriorly and inferiorly (ie, toward the joint surface) within the femoral condyle during NWB, particularly in the range of 20 to 40 degrees of knee flexion. Our findings support the conclusion that the PICR in knees with injured ACLs moves away from the joint surface rather than toward the joint surface and that increased joint surface gliding occurs (Figs. 2A and 3A). Because the knee is extending, the increased gliding motion of the tibia in an anterior direction means, in our opinion, that greater anterior tibial displacement occurs in knees with injured ACLs than in knees without pathology during full knee extension in NWB. Our results agree with those of Vergis et al40 and Lysholm and Messner,41 who reported that more anterior tibial displacement occurs in knees with injured ACLs during both passive40 and active40,41 knee extension conditions. A second finding of our study is that, although less joint surface gliding occurred in the WB movement than in the NWB movement for both groups throughout the WB motion, greater joint surface gliding did occur in knees with injured ACLs than in knees without pathology (Fig. 3B). Researchers9,42,43 have found greater increases in anterior tibial displacement, anterior shearing forces, and ACL strain during knee extension in NWB than during knee extension in WB. For example, Jenkins et al44 demonstrated with knee arthrometer measurements that isometric quadriceps femoris muscle contractions Hollman et al . 967

produced greater anterior tibial displacement during a NWB movement than during a WB movement. Jenkins et al reported intraclass correlation coefficients ranging from .74 in the WB condition to .96 in the NWB condition. We did not examine reliability as part of our study. Results of such studies have led some clinicians to use WB exercise for many lower extremity disorders, particularly for the rehabilitation of patients with injured or reconstructed ACLs.45,46 Our findings, however, suggest that increased joint surface gliding may still be prevalent in knees with injured ACLs during WB movements. EMG Several researchers17–21 have indicated that muscle activity during the stance phase of the gait cycle differs between individuals without knee pathology and individuals with injured ACLs. People with injured ACLs are thought to produce more or earlier hamstring muscle activity in the gait cycle than people without knee pathology. Therefore, we hypothesized that subjects with injured ACLs may have exhibited increased hamstring muscle activity, particularly during WB movement. We found no differences in muscle activity, however, between subjects without knee pathol- Figure 3. Mean and standard deviation values of joint surface gliding (% gliding) in (A) non–weightogy and subjects with injured ACLs. bearing (NWB) and (B) weight-bearing (WB) movements. In the NWB movement, more joint The lack of increased hamstring muscle surface gliding occurred in knees with injured ACLs at the 10-degree knee flexion angle activity may partially account for the (t28⫽⫺3.44, P⫽.004). In the WB condition, more joint surface gliding occurred in knees with increase in joint surface gliding seen in injured ACLs throughout the knee range of motion tested (F⫽5.51, df⫽1,8, P⫽.026). the knees with injured ACLs, in particular throughout the WB movement. Renstrom et al16 and Solomonow et al47 reported that Clinical Relevance hamstring muscle activity reduces anterior tibial disMeasurement of PICR may be used to examine joint placement. Our data suggest that, in the absence of surface rolling and gliding movements and may provide increased hamstring muscle activity in individuals with a basis for quantifying knee joint dysfunction or instabilinjured ACLs, anterior tibial displacement may not be ity. Additional research incorporating PICR measurereduced to a level equivalent to the displacement that ments and joint surface rolling and gliding movements occurs in knees without pathology. may be useful. For example, some people with injured ACLs are able to perform functional tasks, including The power analysis for our study was based on rolling athletic activities, without the knee instability or pain and gliding data only. Muscle activity effects may have often associated with ACL injury.48 Factors that differenbeen present, but they were not detected because of low tiate people who are able to cope with their injured ACL statistical power of the EMG portion of our study from those who are not able to cope have not been fully (1–␤⫽0.36). The effect size of differences in EMG elucidated.48,49 An ability to quantify rolling and gliding magnitude between groups in our study was equal to movements may provide insight into why some people 0.27, which would have required 60 subjects to obtain a function better with damaged ACLs than others. statistical power of 0.80.

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ўўўўўўўўўўўўўўўўўўўўўўўў Figure 4. Mean and standard deviation values of electromyographic (EMG) data expressed as a percentage of maximal voluntary isometric contraction (% MVIC) during non–weight-bearing (NWB) and weight-bearing (WB) movements for the (A) vastus lateralis, (B) medial hamstring, (C) medial gastrocnemius, and (D) gluteus maximus muscles.

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Table 2. Summary of Mixed-Model Analysis of Variance for Electromyographic Data Source of Variance

a

df

SS

MS

F

P

Between Subjects Group Error

1 28

2313.54 32419.80

2313.54 1157.85

2.00

Within Subjects Movement Group ⫻ movement Error

1 1 28

25327.46 28.83 8694.84

25327.46 28.83 310.53

81.56 0.09

.001a .763

Muscle Group ⫻ muscle Error

4 4 112

60248.32 1594.44 43142.40

15062.08 398.61 385.20

39.10 1.03

.001a .393

Knee Angle Group ⫻ knee angle Error

8 8 224

1635.20 369.36 6130.88

204.40 46.17 27.37

7.47 1.69

.001a .103

Movement ⫻ muscle Group ⫻ movement ⫻ muscle Error

4 4 112

10116.32 564.40 16692.48

2529.08 141.10 149.04

16.97 0.95

.001a .440

Movement ⫻ knee angle Group ⫻ movement ⫻ knee angle Error

8 8 224

8401.20 181.76 6348.16

1050.15 22.72 28.34

37.06 0.80

.001a .601

Muscle ⫻ knee angle Group ⫻ muscle ⫻ knee angle Error

32 32 896

3152.32 611.20 18099.20

98.51 19.10 20.20

4.88 0.95

.001a .555

Movement ⫻ muscle ⫻ knee angle Group ⫻ movement ⫻ muscle ⫻ knee angle Error

32 32 896

9841.92 636.80 19765.76

307.56 19.90 22.06

13.94 0.90

.001a .625

.169

Indicates significance, ␣⫽.05.

Limitations The methods and model used in this study were based on several assumptions that may differ to varying degrees among people. We believe, therefore, that the application of PICR is probably not yet appropriate for clinical decision making. The method for measuring PICR and our model for calculating % gliding were based on the assumption that the knee joint is primarily a joint with a single degree of freedom and a joint that rotates about a flexion/extension axis (represented 2-dimensionally by the PICR) and translates in the sagittal plane. The knee is actually a 3-dimensional joint that incorporates secondary rotations in the frontal (abduction/adduction) and transverse (axial rotation) planes of motion. The assumption that knee movements can be represented by general plane motion discounts potential effects of axial rotation (the “screw home mechanism”) on the calculation of rolling and gliding. Changes in the screw home mechanism may account for the differences in rolling and gliding movements we observed. The evidence of whether differences occur in the screw home mechanism during NWB and WB movements, however, is conflicting.50,51 Our mathematical knee model for estimating joint surface rolling and gliding may also be limited because the 970 . Hollman et al

geometric components of the model represent average joint surface geometry obtained from a limited number of subjects in 3 morphological studies,34 –36 and we did not distinguish between the medial and lateral femoral condyles. The size and radii of curvature of the bony segments and joint surfaces comprising the knee joint vary among people.34 –36 Nevertheless, several researchers52–54 have found that the radii of the posterior portions of the medial and lateral condyles that articulate with the tibia are not meaningfully different. Because movement and group interaction effects were present in our study, we believe our model was appropriate for addressing our goal of comparing average rolling and gliding movements between groups. Some of our other measurements also contribute to limitations. Although skin markers are assumed to represent rigid body motion of underlying skeletal segments, it is well known that a violation of the rigid body assumption occurs with the use of skin markers in motion analysis.30,55,56 Less error, however, is typically introduced for sagittal-plane measurements at the knee than either frontal- or transverse-plane measurements, and marker placement methods do exist that minimize skin or soft tissue movement error.30 We attempted to minimize soft tissue movement error by modifying typiPhysical Therapy . Volume 82 . Number 10 . October 2002

ўўўўўўўўўўўўўўўўўўўўўўўў

cal marker positions as recommended by Cappozzo et al.30 Furthermore, previous research33 has demonstrated high correlations (Pearson product moment coefficients [r] ranging from .78 to .88) in cadaver knees between ICR locations determined through use of skin markers with videographic motion analysis and through the use of bone markers with fluoroscopy. We therefore believe our methods were sound.

9 Lutz GE, Palmitier RA, An KN, Chao EYS. Comparison of tibiofemoral joint forces during open-kinetic-chain and closed-kinetic-chain exercises. J Bone Joint Surg Am. 1993;75:732–739.

Conclusion Knee joint surface rolling and gliding movements differ between subjects with injured ACLs and subjects without knee pathology. More joint surface gliding occurs in knees with injured ACLs at full extension during NWB knee extension and throughout the range of motion during WB knee extension. No differences in EMG activity occurred between the 2 groups in the vastus lateralis, medial hamstring, medial gastrocnemius, and gluteus maximus muscles. Because the ACL accounts for up to 86% of resistance to anterior tibial displacement in the knee13 and was the primary variable of interest that differed between groups, we conclude that the ACL has a role in regulating joint surface rolling and gliding movements. We believe that the absence of a structurally intact ACL results in the excess joint surface gliding associated with increased anterior tibial displacement during both NWB and WB knee extension. Our results also suggest that anterior tibial displacement may not be completely reduced as a function of WB movement. We believe further investigation into the efficacy of WB exercise and potential effects of WB movement on knees with injured ACLs is warranted.

12 Hollman JH, Deusinger RH, Zou D, Matava MJ. Estimation of knee joint surface rolling/gliding kinematics via instant center of rotation measurement. In: Proceedings of the American Society of Biomechanics Annual Meeting. Chicago, Ill: American Society of Biomechanics; July 19 –22, 2000.

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