Anterior cruciate ligament (ACL) injuries are common among individuals involved in athletics and other high level activities. To return successfully to the same activity level after injury, early surgical stabilization often is recommended. This can be problematic for athletes vying for scholarships and championship games and for seasonal laborers in the midst of the busy season; these individuals may choose to delay surgery for a temporary return to their activities. However, the risk of extending the damage to the knee and the unpredictable functional outcome after ACL injury make it difficult for clinicians to determine who safely can attempt a temporary return to high-level activities without surgical stabilization. Reports show that 70–80% of patients with ACL rupture will experience knee instability, or a “full episode of giving way,” during high-level activities, and some even have knee instability with daily activities (13). However, there are certain individuals who, despite their ACL rupture, do not experience knee instability even with high-level activities. These individuals have been able to “adapt” successfully to the injury. The differences in functional outcome suggest that the knee stabilization strategies used by patients with ACL deficiency are not homogenous. Despite this, most studies group all subjects with ACL rupture together or create groups based on the time from injury to investigate knee stabilization strategies. This can conceal differences that become apparent when patients are grouped according to their potential for dynamic knee stabilization.
Because there can be vast differences in functional outcome after ACL injury, we developed a classification scheme to improve studies of knee stabilization strategies in patients after ACL rupture. Initially, a retrospective classification was used. Those patients with ACL rupture who returned to activities involving cutting, jumping, or pivoting for a minimum of 1 yr and had not experienced knee instability were classified as “copers,” and those who experienced episodes of giving way were classified as “noncopers” (5). The classification scheme has since been expanded and improved by the addition of a screening examination used to classify patients prospectively soon after injury on the basis of the potential for dynamic knee stability (6). Those patients who fail the screening examination have a high risk for experiencing knee instability and are categorized as noncopers; those who pass the screening examination have good potential for maintaining dynamic knee stability and are classified as potential copers. Fitzgerald et al. (6) reported that 42% of the patients who underwent the screening examination were classified as potential copers. Once a potential coper returns to preinjury high-level activities for a minimum of 1 yr without experiencing episodes of giving way, the potential coper is classified as a coper. We have conducted research to delineate the knee stabilization strategies of copers, potential copers, and noncopers. In addition, we have investigated the use of a neuromuscular rehabilitation program that includes the perturbation of support surfaces to improve dynamic knee stability for a temporary return to high-level activities.
Our recent research has illuminated the short-term differential response to acute ACL injuries that we believe is based on the potential for reorganizing neuromuscular responses soon after injury to stabilize the knee. We hypothesized that 1) we can identify those with better potential to stabilize their knees, 2) those individuals who adapt to the injury better than others employ different muscular mechanisms than those who do not, and 3) they can be trained to be even more stable with appropriate intervention. The purpose of this review therefore was to describe the use of a classification system based on functional outcome after ACL injury, and the screening examination that is used for classification. Use of this classification system has enabled researchers to study and understand better the knee stabilization strategies that are adopted by patients with a ruptured ACL. Furthermore, a rehabilitation program that includes the perturbation of support surfaces is described. This rehabilitation program is intended to allow appropriately chosen individuals (potential copers) to attempt a temporary return to high-level activities without additional episodes of giving way that could extend the damage to the knee. The research that has shown the effectiveness of this program in improving dynamic knee stability and the adaptations induced by the training also are described.
COPERS AND NONCOPERS
Although joint laxity is not different between copers and noncopers, many differences in functional performance and movement patterns can be observed (5,15). Clinical tests show higher quadriceps strength, hop testing, and self-report of function scores in copers compared with noncopers (5). Although quadriceps femoris strength is greater in groups of copers, there are still noncopers who have good quadriceps strength yet cannot successfully stabilize the knee. This suggests that the ability to generate large quadriceps muscle torques is important, yet by itself is insufficient to dynamically stabilize the knee joint (5,11). Therefore, classification as a coper or as a noncoper is independent of quadriceps strength. An analysis of movement patterns during walking and jogging shows that copers have sagittal plane knee motions and moments during weight acceptance that are similar to uninjured individuals (see Fig. 1) (2,11,12). Conversely, noncopers reduce sagittal plane knee excursions and moments, using a characteristic “stiffening strategy” to try to maintain knee stability (see Fig. 1) (2,11,12). We speculate that this stiffening strategy bodes poorly for long-term joint integrity and that it could contribute to the high incidence of early-onset knee osteoarthritis in individuals after ACL rupture. Rudolph et al. (11) hypothesized that this reduced knee motion and internal knee extensor moment during weight acceptance, which has been observed by other researchers (1), is the hallmark of a noncoper. Based on functional outcomes and analysis of movement patterns, we define dynamic knee stability as the ability for a patient with ACL deficiency to perform high-level activities without episodes of giving way while maintaining normal movement patterns.
To have knee stability while maintaining normal movement patterns, it is likely that adaptations must occur in muscle activation. Evaluating muscle function during gait in individuals with an ACL injury who have been separated based on functional outcome (copers vs noncopers) provides insight into why the knee stabilization strategy may or may not be successful. Boerboom et al. (2) demonstrated that a small group of individuals with a torn ACL who had returned to preinjury activity levels had atypical hamstring activity during gait when compared with a group of healthy control subjects. The analysis, however, was not capable of differentiating at what point in the gait cycle differences occurred. Rudolph et al. (11) conducted an extensive analysis of muscle activity during walking and found that successful stabilization in copers appeared to be related to idiosyncratic alterations in muscle activity (see Fig. 2). Essentially, the copers discovered their own way to adapt to the injury. The reduced knee motion of the noncopers appears to be related to earlier activation and to longer duration of activity in the gastrocnemius and hamstring muscles (11). Both muscles also had delayed onset to peak times during the gait analysis (11). The use of a validated functional classification scheme allowed for the delineation of successful muscle adaptations in the copers.
The goal of the screening examination, described by Fitzgerald et al. (7), is to identify individuals prospectively, shortly after ACL injury, with the potential to return temporarily without knee instability, to high-level preinjury activities. Not every patient with an ACL rupture is a candidate for participation in the screening exam (see Fig. 3). Those with bilateral injury are excluded because there is no “control” limb for comparison, and those with multiple ligament damage are excluded because the additional knee laxity from other ruptured ligaments is more difficult to control through dynamic means. All patients undergo magnetic resonance imaging (MRI) and are excluded if repairable meniscal damage or full-thickness cartilage defects are present because there is the potential for further damage with nonoperative management. Participation in the screening examination is therefore reserved for patients with a complete isolated unilateral ACL rupture. All others are recommended for early surgical stabilization.
Before participating in the screening examination, all patients receive rehabilitation to resolve impairments. Patients must have full knee range of motion, no pain or joint effusion, be able to tolerate hopping on the injured limb, and have involved side isometric quadriceps strength ≥70% of the uninvolved side before they can proceed to the screening examination. Adequate quadriceps strength is a prerequisite for performing the screening examination; however, quadriceps strength testing is independent of the classification protocol. Evaluation of the potential for coping or noncoping strategies occurs only after quadriceps strength is at a satisfactory level to allow for hop testing. Administering the screening examination before the resolution of impairments would provide an inaccurate assessment of the patient’s performance. We perform an isometric quadriceps strength test using a burst-superimposition technique with the knee fixed at 90° (14). The burst-superimposition technique consists of superimposing a supramaximal tetanic train of electrical stimulation on a maximal isometric effort for the purposes of quantifying voluntary activation.
The screening examination is designed to be easy to administer and inexpensive to conduct. It consists only of a series of functional hop tests, a self-report of knee function survey, a global rating of knee function, and the assessment of the number of episodes of giving way since the initial injury. Previous investigators had demonstrated that a series of one-legged hop tests could be used to determine successfully the presence of asymmetry between limbs in individuals with ACL deficiency (10). Therefore, a series of four specific hop tests are included in our screening examination, as follows: 1) a single hop for distance; 2) a cross-over hop for distance, in which the patient crosses a 15-cm wide tape for three consecutive hops; 3) a straight triple hop for distance; and 4) a timed 6-m hop test (10). Patients perform each hop test on the uninvolved limb first, followed by the involved limb, and wear a functional brace on the involved limb during testing. Patients perform two practice trials, and then two trials that are averaged to gain a representative value for that limb. For the single, cross-over, and triple hops, the involved side average score is divided by the uninvolved side average score and multiplied by 100. For the timed hop, the uninvolved side average score is divided by the involved side average score and multiplied by 100. After hop testing, patients complete the Activities of Daily Living Scale portion of the Knee Outcome Survey (KOS-ADLS), which is a self-report measure of knee function (8). The KOS-ADLS contains questions regarding the severity of symptoms experienced during activities of daily living and the patient’s ability to perform various activities. A score of 100% equates to no symptoms and no difficulty during activities of daily living. Patients also complete a global rating of knee function from 0–100%, with 100% corresponding to preinjury levels. Finally, all patients are asked how many times they have experienced giving way episodes since the initial injury. Fitzgerald et al. (6) defined an episode of giving way as a “buckling, or subluxation, of the tibiofemoral joint that results in pain and joint effusion.”
Subjects who meet all of the following criteria (see Table 1) are classified as potential copers: 1) timed hop test ≥80%, 2) KOS-ADLS ≥80%, 3) global rating ≥60%, and (4) not more than one episode of giving way after the initial injury until the time of the screening examination (6). Failure to meet any of the above criteria results in classification as a noncoper. All noncopers are referred back to the physician for operative stabilization.
Fitzgerald et al. (6) demonstrated the ability of the screening examination to delineate those with better potential for dynamic knee stabilization through a clinical study. Of the 93 consecutive patients who participated in the screening examination, 39 (42%) met the criteria for potential copers (6). Only 28 of the 39 potential copers wished to attempt a temporary return to high-level activities, suggesting that most patients (65 of the original 93, or 70% of the patients who underwent the screening examination) are either at risk for knee instability or are unwilling to attempt to return without operative stabilization. Of the 28 potential copers who attempted a temporary return, 22 (79%) did not have a giving way episode over the 6-month follow-up. This success rate is higher than the 25–30% rate that has been reported in studies of unclassified or self-selected patients (13). The screening examination therefore appears to be more successful at selecting appropriate individuals for short-term nonoperative management. In addition, none of the patients who failed nonoperative management extended the knee injury, as determined by MRI and subsequent arthroscopic surgery (6). However, no long-term data exists on the incidence of early onset of knee osteoarthritis in potential copers who have chosen to resume high-level activities. In addition, we have not completed a long-term follow-up to assess how many potential copers actually become copers.
Chmielewski et al. (3) investigated the ability of the screening examination to distinguish successfully the patient’s potential for dynamic knee stability based on movement patterns during walking and jogging. A group of potential copers were compared with a group of noncopers and a group of uninjured subjects. The potential copers had knee angles during weight acceptance that resembled the noncopers, yet maintained knee joint moments that resembled those of the uninjured group. If a reduced knee angle and internal knee extensor moment during weight acceptance is the hallmark of a noncoper, then the potential copers’ movement patterns are intermediate between those of the noncopers and uninjured subjects. This biomechanical data implies that the potential copers are in the midst of developing a stabilization strategy that is more effective than that of the noncopers (3). The existence of differences in movement patterns between potential copers and uninjured subjects suggests that the stabilization strategy is incomplete and that rehabilitation may be required before pursuing a return to high-level activities.
PERTURBATION-AUGMENTED REHABILITATION PROTOCOL
The perturbation augmented rehabilitation program, described by Fitzgerald et al. (7), can be used to prepare potential copers successfully for a temporary return to high-level activities. The program consists of 10 treatment sessions of rehabilitation that include perturbation of support surfaces (perturbation training), muscle strengthening exercises for the lower extremity, cardiovascular training, agility training, and sport-specific skill training. The perturbation-training portion of the program is based on a “force-feedback” concept and is designed to influence an inhibitory response in the muscles that could destabilize the joint while facilitating activity in those muscles that can restrain any undesired translational motion (9). It is important that the rehabilitation program be flexible enough to allow each individual to adopt his or her own strategy for successful stabilization of the knee.
Perturbation training consists of three successive techniques performed during each session. In the rockerboard technique, the patient stands in unilateral stance on a rockerboard and perturbations are applied in an anterior–posterior and medial–lateral direction. Second, in the rollerboard–platform technique, the patient stands with one foot on a rollerboard and the contralateral limb on a stationary platform set at the same height as the rollerboard, while multidirectional perturbations are applied to the rollerboard. Finally, in the rollerboard technique, multidirectional perturbations are given with the patient standing in unilateral stance on a rollerboard. The patient stands with the knee slightly flexed and is instructed to regain a balanced position after the application of a perturbation for the rockerboard and rollerboard techniques. For the rollerboard–platform technique, the patient is instructed to respond to the force applied to the rollerboard to match both the direction and magnitude of the force without rigid co-contraction. Initially, perturbations are provided in the sagittal plane, in a predictable direction, with verbal cues to indicate the beginning of the movement. As sway decreases (rockerboard and rollerboard techniques) or the patient responds quickly to the force without rigid co-contraction (rollerboard–platform technique), perturbations are progressed by application in random directions, with greater force and speed, and finally in combination with sport-specific drills. Fitzgerald et al. (7) detail the specifics of the program progression.
The perturbation-augmented rehabilitation program has been shown to be significantly better at successfully returning patients temporarily to high-level activities, compared with a standard rehabilitation program (7). Fitzgerald et al. (7) randomly assigned 26 potential copers into either a standard rehabilitation program or a program that included the same standard rehabilitation in conjunction with the perturbation of support surfaces as described above. Of the 12 potential copers in the perturbation group, 11 (92%) successfully returned to high-level activities, whereas only 7 of the 14 (50%) in the standard group had a successful return. Success was defined as a complete return to preinjury activity levels for the duration of the season, without experiencing episodes of giving way or reducing activity levels. A χ2 analysis indicated that the perturbation group had a significantly greater frequency of successful return to high-level activities (P = 0.028). A positive likelihood ratio of 4.88 revealed that patients who received perturbation training were almost five times more likely to have a successful return to high-level physical activities. Previous rehabilitation protocols have focused on improving lower extremity muscular strength and endurance, agility training, modifying activity levels, and bracing the injured knee. Patients with a deficient ACL must learn to reorganize the lower extremity muscle activity patterns to provide better stability to the knee joint to return to potentially destabilizing activities without experiencing giving way. Translating support surfaces can challenge dynamic knee stability in a controlled environment and has the potential to induce compensatory alterations in muscle activity patterns in patients with an ACL rupture.
The success of the perturbation training protocol suggests that some neuromuscular adaptations are occurring as a result of the training. Examining how patients stabilize their knee during a functional task can provide insight into the adaptations resulting from training. Chmielewski et al. (4) tested a group of potential copers before and after perturbation training to determine the presence of altered muscle activity patterns during walking. After the perturbation training, quadriceps activity during the early stance phase of walking increased by 41% relative to prerehabilitation values. The potential for an eccentric quadriceps contraction to induce instability at the tibiofemoral joint suggests that other muscles had to alter their activity patterns (timing, magnitude, or both) to compensate for the increased quadriceps activity. Specifically, the hamstrings can provide a direct posterior pull on the tibia, and the gastrocnemius can increase knee stiffness. During gait, soleus activity typically occurs after loading response. This activity may limit the forward progression of the tibia and may assist with knee extension, countering the potentially destabilizing effect of the quadriceps. As suspected, quadriceps timing variables (onset, time to peak amplitude, and termination) and magnitude variables (peak value and integral of activity during weight acceptance) were significantly predicted after training by combinations of hamstring, gastrocnemius, and soleus muscle activation variables (see Table 2) (4). No such relationships existed before training, suggesting that these adaptations were not a result of the injury itself, but rather a result of participation in the rehabilitation program.
The functional classification system has helped determine differences in muscle activation, movement patterns, and functional outcomes in individuals with an ACL rupture. A screening examination can be used to determine prospectively the potential for dynamic knee stability in individuals with ACL rupture who were previously active in high-level activities. The results of the screening examination can then be used to guide patient management. Although the long-term consequences of classification on the development of knee osteoarthritis remain unknown, it appears that short-term functional outcomes can be improved significantly through the selection of appropriate individuals and the inclusion of the perturbation of support surfaces to a training regimen.
Supported by the National Institutes of Health (grant nos. 5R01HD037985–02 and 1P20RR016458–010003), the Foundation for Physical Therapy Promotion of Doctoral Studies Program, and the Norwegian Research Council.
1. Berchuck, M., Andriacchi, T. P. Bach, B. R. and Reider. B. Gait
adaptations by patients who have a deficient anterior cruciate ligament. J. Bone Joint Surg. Am. 72: 871–877, 1990.
2. Boerboom, A. L., Hof, A. L. Halbertsma, J. P. van Raaij, J. J. Schenk, W. Diercks, R. L. and van Horn. J. R. Atypical hamstrings electromyographic activity as a compensatory mechanism in anterior cruciate ligament deficiency. Knee Surg. Sports Traumatol. Arthrosc. 9: 211–216, 2001.
3. Chmielewski, T. L., Rudolph, K. S. Fitzgerald, G. K. Axe, M. J. and Snyder-Mackler. L. Biomechanical evidence supporting a differential response to acute ACL injury
. Clin. Biomech. (Bristol, Avon). 16: 586–591, 2001.
4. Chmielewski, T. L., Rudolph, K. S. and Snyder-Mackler. L. Development of dynamic knee stability after acute ACL injury
. J. Electromyogr. Kinesiol. 12: 267–274, 2002.
5. Eastlack, M. E., Axe, M. J. and Snyder-Mackler. L. Laxity, instability, and functional outcome after ACL injury
: copers versus noncopers. Med. Sci. Sports Exerc. 31: 210–215, 1999.
6. Fitzgerald, G. K., Axe, M. J. and Snyder-Mackler. L. A decision-making scheme for returning patients to high-level activity with nonoperative treatment after anterior cruciate ligament rupture. Knee Surg. Sports Traumatol. Arthrosc. 8: 76–82, 2000.
7. Fitzgerald, G. K., Axe, M. J. and Snyder-Mackler. L. The efficacy of perturbation training in nonoperative anterior cruciate ligament rehabilitation
programs for physical active individuals. Phys. Ther. 80: 128–140, 2000.
8. Irrgang, J. J., Snyder-Mackler, L. Wainner, R. S. Fu, F. H. and Harner. C. D. Development of a patient-reported measure of function of the knee. J. Bone Joint Surg. Am. 80: 1132–1145, 1998.
9. Nichols, T. R. A biomechanical perspective on spinal mechanisms of coordinated muscular action: an architecture principle. Acta Anat. 151: 1–13, 1994.
10. Noyes, F. R., Barber, S. D. and Mangine. R. E. Abnormal lower limb symmetry determined by function hop tests after anterior cruciate ligament rupture. Am. J. Sports Med. 19: 513–518, 1991.
11. Rudolph, K. S., Axe, M. J. Buchanan, T. S. Scholz, J. P. and Snyder-Mackler. L. Dynamic stability in the anterior cruciate ligament deficient knee. Knee Surg. Sports Traumatol. Arthrosc. 9: 62–71, 2001.
12. Rudolph, K. S., Eastlack, M. E. Axe, M. J. and Snyder-Mackler. L. 1998 Basmajian Student Award Paper: movement patterns after anterior cruciate ligament injury: a comparison of patients who compensate well for the injury and those who require operative stabilization. J. Electromyogr. Kinesiol. 8: 349–362, 1998.
13. Shelton, W. R., Barrett, G. R. and Dukes. A. Early season anterior cruciate ligament tears. A treatment dilemma. Am. J. Sports Med. 25: 656–658, 1997.
14. Snyder-Mackler, L., Delitto, A. Stralka, S. W. and Bailey. S. L. Use of electrical stimulation to enhance recovery of quadriceps femoris muscle force production in patients following anterior cruciate ligament reconstruction. Phys. Ther. 74: 901–907, 1994.
15. Snyder-Mackler, L., Fitzgerald, G. K. Bartolozzi3rd, A. R. and Ciccotti. M. G. The relationship between passive joint laxity and functional outcome after anterior cruciate ligament injury. Am. J. Sports Med. 25: 191–195, 1997.