Malalignment of the lower limb and anterior cruciate ligament (ACL) deficiency are separate, common conditions that compromise knee joint function and contribute to osteoarthritis (OA) (7,16,29,45). When malalignment and ACL deficiency coexist, unbalanced loads are placed on an unstable knee, and disease progression is hastened (14). Combined (staged or simultaneous) opening wedge high tibial osteotomy (HTO) and ACL reconstruction enables realignment of the knee, in both the coronal and the sagittal planes, and restoration of ligamentous support during one operation (14). In addition to redistributing weight-bearing loads away from the diseased medial compartment, the resulting improvements in bony and ligamentous stability after the combined surgery are thought to improve overall knee joint function, although the effects of these surgeries on the progression of OA remain unclear (8,29).
Several authors have demonstrated the importance of the coronal plane knee moment to medial compartment knee OA (3,30,44). During the stance phase of walking, the dynamic line of action of the ground reaction force passes medial to the knee, resulting in an external adduction moment about the joint. Although an indirect measure, an abundance of evidence now exists suggesting that the knee adduction moment is a valid and reliable proxy for the load on the knee medial compartment (6,53). The normal predominance of a knee adduction moment during walking places larger loads on the medial relative to lateral tibiofemoral joint and is hypothesized to contribute to articular cartilage degeneration in the medial compartment (37,44). The degeneration of articular cartilage and resulting reduction in medial joint space can produce further varus alignment and even greater adduction moment about the knee. A central goal of medial opening wedge HTO is therefore to correct malalignment and to decrease the knee adduction moment in an attempt to break this perpetuating cycle (14).
Several authors have also proposed that sagittal plane knee moments play an important role in controlling anterior tibial translation. Berchuck et al. (5) reported that patients with ACL deficiency exhibit an abnormally sustained external knee extension moment throughout the stance phase of walking, or lack of an external knee flexion moment, which they defined as "quadriceps avoidance" gait. Since that report, several researchers have examined sagittal knee moments in patients with ACL deficiency and have observed inconsistent and contradicting results (18,43). Although "quadriceps avoidance" has also been noted in those with knee OA (19,34), others have reported similar or even increased flexion moments (2,33). Changes in knee sagittal plane moments after ACL reconstruction have also been inconsistent (11,18,51).
Alterations in quadriceps, hamstring, and gastrocnemius activity have been observed in patients with knee pathology. These muscles are the primary muscles that span the knee and play an important role in knee stability and in reducing strain on the ACL (20,32). During walking, muscles of the lower extremity contract in an alternating pattern to create a coordinated movement pattern. When compared with healthy, matched controls, increased muscle coactivation has been observed in patients with medial compartment knee OA (12,30) and patients with ACL deficiency (13,22), leading to the suggestion that coactivation is a compensatory mechanism adopted in an attempt to increase joint stability. These patient groups also exhibit altered timing of muscles in the lower extremity (2,4,12,31).
Noyes et al. (37) have published the only study examining three-dimensional gait kinetics in patients with varus malalignment, medial compartment knee OA, and ACL deficiency. Those patients exhibited higher external knee adduction and extension moments and a lower external knee flexion moment than control subjects. Although muscle activity was not measured, the authors suggested that their findings were consistent with increased knee flexion muscle forces (hamstring activity) and decreased knee extension forces (quadriceps activity). Despite its potential to alter joint moments and muscle activity, the effects of simultaneous HTO and ACL reconstruction surgery have not been previously investigated. Therefore, our objectives were to evaluate the effects of simultaneous HTO and ACL reconstruction on 1) the external knee adduction moment, 2) the external knee flexion and exension moments, and 3) the quadriceps, hamstrings, and gastrocnemius muscle activity during walking. We hypothesized that significantly lower external knee adduction and extension moments, increased external knee flexion moments, and decreased muscle coactivation would be observed after surgery.
Participants' demographics and clinical characteristics are reported in Table 1. Patients with medial compartment knee OA and chronic ACL insufficiency were screened for study eligibility. To be included, patients had to have mechanical varus alignment and knee OA according to the Altman classification (1), with greatest severity in the medial compartment of the tibiofemoral joint and ACL deficiency greater than 1 yr. Patients with grade IV degenerative changes in two or more knee compartments and who were ≥60 yr were excluded because they were considered better candidates for arthroplasty. Other exclusion criteria included inflammatory or infectious arthritis of the knee, end stage disease in the patellofemoral joint prior HTO on the contralateral limb, multiligamentous instability, major neurological deficit that would affect gait, major medical illness with life expectancy <2 yr or with unacceptably high operative risk, pregnancy, unable to speak or read English, and psychiatric illness that limited informed consent. Lower limb alignment (mechanical axis angle) (47) and Kellgren and Lawrence (24) grade of OA severity were determined from double-limb standing hip-to-ankle anterior-posterior (AP) radiographs (48). The mechanical axis angle of the lower limb was defined as the included angle formed between a line drawn from the center of the hip to the center of the knee and a line drawn from the center of the ankle to the center of the knee (9). Positive values corresponded to valgus alignment whereas negative values indicated varus alignment of the lower limb. Knee stability was assessed through the pivot shift and Lachman tests completed by the orthopedic surgeon. The study was approved by the institution's Research Ethics Board for Health Sciences Research Involving Human Subjects, and all patients provided informed written consent.
An examination under anesthesia and a diagnostic tricompartmental arthroscopy with management of meniscal pathology as required were carried out on all patients. Medial opening wedge HTO was performed using the technique described by Fowler et al. (21). The desired correction for the osteotomy was calculated using a method similar to that described by Dugdale et al. (17), with the goal of moving the weight-bearing line laterally to a maximum position of 62.5% of the medial-to-lateral width of the tibial plateau. Placement of fixation was confirmed by fluoroscopy. Autograft or allograft bone was used in osteotomies greater than 7.5 mm. ACL reconstruction was carried out after completion of the HTO. Gracilis and semitendinosus tendons were harvested through the osteotomy incision and prepared into a four-bundle construct that was then sized to determine appropriate tunnel diameters. The Endobutton™ was used for femoral fixation, and tibial fixation was achieved using multiple low profile staples in a belt-buckle fashion with or without interference screws with the knee in extension and the graft under tension. Postoperative management included a hinged knee brace and feather-touch weight bearing for 6 wk. With clinical and radiographic evidence of healing of the osteotomy site, partial weight bearing was permitted at 6 wk and full weight bearing at 12 wk. Patients participated in a supervised physiotherapy program for range of motion, maintenance of surrounding joint strength, and function in the brace until healing of the osteotomy had occurred. Once the osteotomy had healed, ACL rehabilitation was carried out following a standardized postoperative ACL rehabilitation protocol that emphasized progressive proprioception and strengthening exercises with a focus on functional activities and return to strenuous activities by 9-12 months postoperatively.
Gait analysis and EMG data collection
Patients underwent three-dimensional gait analysis preoperatively and 12 months postoperatively. Gait analysis was completed using an eight-camera motion capture system (Motion Analysis Corporation, Santa Rosa, CA) synchronized with a single, floor-mounted force platform (Advanced Mechanical Technology Inc., Watertown, MA) and an eight-channel telemetric EMG system (Telemyo; Noraxon USA Inc., Scottsdale, AZ). Passive-reflective markers were placed on the patient using a 22-marker, modified Helen Hayes marker set (23). Extra markers were placed bilaterally over the medial knee joint line and the medial malleolus during an initial static standing trial on the force platform to determine body mass, marker orientation, and positions of joint centers of rotation for the knee and the ankle. These four additional markers were removed before gait testing. Surface electrodes (Kendall® Medi-trace 200, Ag/AgCl; Tyko Healthcare Group LP, Mansfield, MA) were placed over the midbelly of the rectus femoris, the medial hamstrings, and the lateral head of the gastrocnemius of the operative limb (39). The electrode locations were shaved and cleaned with isopropyl alcohol to minimize noise in the electrical signal.
During the gait analysis, patients were instructed to walk across the laboratory at their typical walking speed while kinetic (sampled at 1200 Hz), electromyographic (sampled at 1200 Hz with an analog to digital conversion and band-pass filtered at 16-500 Hz), and kinematic data (sampled at 60 Hz) were collected during the middle of several strides. Because different footwear has been shown to alter gait biomechanics (25-28), we chose to eliminate this potential confounder and increased variability across subjects by having patients walk barefoot pre- and postoperatively. A total of five trials were obtained for the operative limb.
External coronal and sagittal moments about the knee were calculated from the kinematic and kinetic data using commercial software (Orthotrak 6.0; Motion Analysis Corporation) and custom postprocessing and data reduction techniques. Each lower limb segment (thigh, shank, and foot) was modeled as a rigid body with a local coordinate system that coincided with anatomic axes. Inertial properties of each limb segment were approximated anthropometrically, and translations and rotations of each segment were calculated relative to neutral positions defined during the initial standing static trial. Walking speed was calculated as the average walking velocity between successive foot contacts of the tested limb. The peak adduction moment during stance, the peak flexion moment during early stance, and the peak extension moment during late stance were identified for each trial and were averaged across the five trials. Values were then normalized to body size (%BW×Ht).
For each trial, the raw EMG data were high-pass filtered (cutoff 30 Hz) and rectified. The data were then normalized to the maximum EMG value for the specific muscle during the trial. Using the following formula described by Rudoph et al. (42), coactivation values for quadriceps-hamstrings and quadriceps-gastrocnemius were calculated:
This formula determines the sample-by-sample estimate of muscle coactivation, and the end value provides an estimate of the magnitude of coactivation for the period of 100 ms before heel strike to first peak knee adduction moment. For each trial, the time intervals were then normalized to 100 data points, and a single coactivation value was determined for each muscle pair. Average coactivation values were then calculated for the five trials. Figure 1 provides a representative sample of a subject's preoperative data and illustrates the method of calculating the coactivation index.
Muscle timing (onset and cessation) was determined by visual inspection. A period of 200 ms where EMG activity was minimal was selected to determine a baseline mean of muscle activity. The quadriceps, hamstrings, and gastrocnemius were considered active when the muscle's activity exceeded two SD of the baseline mean. For each trial, the EMG data were time normalized to 100% of gait cycle (toe off to toe off). Onsets and cessation of muscle activity were expressed as a percentage of gait cycle relative to heel strike. Therefore, heel strike equals 0% of the gait cycle, and a negative percentage indicates the onset of activity occurred before heel strike. Duration of muscle activation was determined based on the muscle's onset and cessation percentages. Average onset and duration percentages were determined for the five trials.
All statistical analyses were completed using SPSS 15 (SPSS Inc., Chicago, IL). Knee moments, muscle coactivation, and muscle onset and duration were compared before and after surgery using paired t-tests. The 95% confidence intervals (CI) around mean changes were calculated. Sample size calculation was based on our primary objective of evaluating changes in the knee adduction moment after surgery. Based on a within-subject comparison (i.e., operative limb pre- and postsurgery), 21 subjects provided 85% power to detect a large effect size (Cohen's d = 0.7, two-sided α = 0.05).
The mean increase in mechanical axis angle after surgery was 6.36° (95% CI = 4.43°-8.29°), indicating that malalignment was improved from the preoperative varus angulation to approximately neutral. All patients exhibited instability preoperatively with most patients having Lachman and pivot shift scores of 2+ or 3+ (see Table 1). Postoperatively, there was a significant improvement in stability with scores on the Lachman and the pivot shift all decreasing to equal or 1+. Consistent with patients with knee OA, all patients had degenerative tears to the menisci observed with arthroscopy. During surgery, 15 patients underwent partial meniscectomy and three patients underwent meniscal repair with sutures.
There were significant increases in step length (66.14 ± 8.8 vs 70.52 ± 6.59 cm, P < 0.01) and cadence (100.18 ± 13.61 vs 105.14 ± 9.46 steps per minute, P < 0.05), which resulted in a small but significant (P < 0.01) increase in gait speed from 1.13 ± 0.20 to 1.22 ± 0.15 m·s−1. Despite the increase in speed, there were significant (P < 0.001) decreases in both peak knee adduction and flexion moments and a significant (P < 0.01) increase in peak extension moment (Table 2). Figures 2 and 3 illustrate the ensemble curves for the coronal and the sagittal plane moments before and after surgery.
There were no significant changes in muscle coactivation between the quadriceps-hamstrings (P = 0.140) and the quadriceps-gastrocnemius (P = 0.987). Table 2 provides descriptive details of the coactivation. There were also no significant differences in muscle onset or duration for the quadriceps, hamstrings, or gastrocnemius. Figure 4 illustrates the temporal patterns of the quadriceps, hamstrings, and gastrocnemius before and after surgery.
The present findings suggest that improving knee alignment and instability significantly alters external knee moments during walking. Most notably, despite an increase in walking speed, there was a consistent and large reduction in the knee adduction moment. Without exception, the adduction moment decreased in all patients, and the vast majority of patients (n = 14, 67%) experienced a decrease greater than the suggested minimum detectable change of 1%BW×Ht (6). This finding is consistent with our hypothesis and with results from previous studies evaluating HTO (36,40,52). Because the knee adduction moment is a surrogate measure of dynamic medial knee joint loading (50), this finding suggests that HTO significantly reduces the load on the diseased medial tibiofemoral compartment. Importantly, the knee adduction moment also has been shown to predict radiographic disease progression (35). Although longitudinal research is required, the present findings are also consistent with the goal of delaying progression of OA in these patients.
Although there was a statistically significant decrease in the knee flexion moment and increase in the extension moment, changes were considerably more variable than the adduction moment in both direction and magnitude. This is also consistent with previous research examining sagittal moments after ACL reconstruction (10,11,18,51). We did not observe "quadriceps avoidance" gait pre- or postoperatively because all patients demonstrated an external knee flexion moment during early stance. Further inspection of kinematic (knee joint angles) and temporal spatial data (cadence, step length, and walking speed) suggested that the participants did not exhibit "quadriceps avoidance" or stiff legged gait 12 months after surgery. Schipplein and Andriacchi (44) and Messier et al. (33) found that those with knee OA had greater external flexion moments that were accompanied by increase in knee compressive forces. They speculated that this increase in flexion moment may be an adaptive mechanism that increases compressive forces to help maintain stability (44). It is possible that because the participants in the present study experienced an improvement in stability after surgery, they were able to decrease their flexion moment and subsequent compressive forces.
Despite the observed changes in knee moments and anterior-posterior (AP) stability, concomitant changes in muscle coactivation or onset and duration were not observed. Like the results for the sagittal knee moment, changes in the measures of muscular activity were inconsistent. Because the quadriceps, hamstrings, and gastrocnemius muscles operate primarily in the sagittal plane, it is possible that some of the variability in the sagittal moment is related to the large variability in the muscle activations. Although somewhat speculative, it is also possible that potential learned, protective adaptations to gait, such as increased coactivation, remain for prolonged periods of time despite improvements in knee stability. Similarly, it is possible that even with improved AP stability, more subtle medial-lateral and rotational instabilities may remain (38) and may encourage continued coactivation. Ramsey et al. (41) observed a tendency (P = 0.089) for medial quadriceps and gastrocnemius coactivation to decrease and medial quadriceps and hamstrings coactivation to remain unchanged after HTO in patients with varus gonarthrosis. No study to our knowledge has examined changes muscle coactivation after ACL reconstruction. In the current study, we did not differentiate between medial and lateral musculature like Ramsey et al. (41). Although difference may be due to the different patient populations, it may be beneficial to examine different electrode placements (i.e., include medial and lateral locations).
After surgery, there were no significant changes in muscle activation onset or duration. Previous research on patients with knee OA has suggested prolonged quadriceps, hamstring, and gastrocnemius activity during gait (2,12). Studies examining activation increases in patients with ACL deficiency have noted similar findings related to hamstrings and gastrocnemius activity but no differences in quadriceps activation (4,31). After ACL reconstruction, the muscle activation patterns during gait have been reported to be similar to that of healthy adults (10). We are unaware of previous research examining these muscle activation patterns after HTO or HTO-ACL reconstruction.
The Adult Muscle Activity Chart published by Shriner's Hospital (San Francisco, CA) has illustrated that the quadriceps and the hamstrings are activated approximately 10% and 15% of the gait cycle before heel strike, respectively, whereas the gastrocnemius is activated approximately 15% after heel strike. The quadriceps, hamstrings, and gastrocnemius remain active until approximately 5%, 10%, and 50% after heel strike. The onset of the quadriceps and hamstrings activity for our patients was similar to those published norms and therefore suggests that normal onset timing is present pre- and postoperatively. However, pre- and postoperatively, the gastrocnemius was activated earlier, and the quadriceps and the hamstrings remain active for a longer period than the published norms resulting in prolonged muscle activation. These alterations may be a learned response to stabilize the knee. If so, this response appears to remain after corrective surgery.
Limitations of the present study are summarized below. Although an important goal of simultaneous medial opening wedge HTO and ACL reconstruction is to limit the progression of the degenerative changes already present in the medial compartment of the tibiofemoral joint, the success of this goal cannot be evaluated based on our findings. Although decreases in the knee adduction moment and improvements in knee stability are consistent with this goal, their effect on the OA disease process is currently unclear. The effect of ACL reconstruction alone on the initial development of knee OA is particularly controversial (29) and may relate to many factors, including inadequate restoration of more subtle rotational kinematics (38,49) that were beyond the scope of the present study. The majority of patients involved in this study were males (n = 19). Although this is representative of our clinical population who undergo the simultaneous HTO and ACL reconstruction, it may limit the generalizability of the results. The influence of electrode placement on our findings also deserves consideration. We followed the anatomical landmarking guidelines suggested by Perroto et al. (39) and confirmed appropriate electrode placement by having the patients perform various muscle contractions. Although the same trained investigator applied the electrodes pre- and postoperatively, slight variations in placement may have occurred. Additionally, we examined only one site per muscle group due to the potential for cross-talk (15,46). Future studies examining medial and lateral activation of the quadriceps, hamstrings, and gastrocnemius may yield different results.
Overall, the present findings suggest that simultaneous medial opening wedge HTO and ACL reconstruction improves radiographic lower limb alignment, increases clinical measures of knee stability, and reduces load in the medial compartment of the knee during gait without apparent alterations in muscle group activity.
This research was undertaken thanks to funding from the Canada Research Chairs Program (TBB), the Canadian Institutes of Health Research, and Arthrex Inc.
The results of the present study do not constitute endorsement by ACSM.
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