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The Influence of Quadriceps Strength Asymmetry on Patient-Reported Function at Time of Return to Sport After Anterior Cruciate Ligament Reconstruction Christin Zwolski, Laura C. Schmitt, Catherine Quatman-Yates, Staci Thomas, Timothy E. Hewett and Mark V. Paterno Am J Sports Med published online July 16, 2015 DOI: 10.1177/0363546515591258 The online version of this article can be found at: http://ajs.sagepub.com/content/early/2015/07/16/0363546515591258
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The Influence of Quadriceps Strength Asymmetry on Patient-Reported Function at Time of Return to Sport After Anterior Cruciate Ligament Reconstruction Christin Zwolski,*yz§ PT, OCS, Laura C. Schmitt,yz§|| PT, PhD, Catherine Quatman-Yates,yz§{ PT, PhD, Staci Thomas,yz MS, Timothy E. Hewett,yz{#** PhD, FACSM, and Mark V. Paterno,yz§{ PT, PhD, SCS, ATC Investigation performed at Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio, USA Background: An objective assessment of quadriceps strength after anterior cruciate ligament reconstruction (ACLR) is an important clinical measure to determine readiness to return to sport (RTS). Not all clinicians are equipped with the means to objectively quantify quadriceps strength limb symmetry indices (Q-LSIs) via lower extremity isokinetic dynamometers, as recommended by previous studies. Purpose/Hypothesis: The purpose of this study was to determine whether the International Knee Documentation Committee 2000 Subjective Knee Form (IKDC) score at time of RTS was a predictor of quadriceps strength in a young, athletic population after ACLR. Two hypotheses were tested: (1) Individuals with higher self-reports of function would demonstrate better quadriceps strength of the involved limb than individuals with lower self-reports of function at the time of RTS, and (2) individuals with higher self-reports of function would have normal quadriceps strength limb symmetry. Study Design: Cohort study (diagnosis); Level of evidence, 2. Methods: At time of RTS, 139 subjects who had undergone ACLR completed the IKDC. In addition, an isometric quadriceps strength test (Biodex dynamometer) was performed on both lower extremities. Peak torques were calculated, as was the Q-LSI, determined by the formula (involved limb peak torque/uninvolved limb peak torque) 3 100%. Participants were dichotomized based on IKDC scores: high IKDC (IKDC 90) and low IKDC (IKDC \90). Two-way analysis of variance was used to determine the effect of limb (involved vs uninvolved) and group (high vs low IKDC) on isometric quadriceps strength. Chi-square and logistic regression analyses were then performed to determine whether IKDC scores could predict Q-LSI. Results: At time of RTS, a significant correlation between IKDC scores and (1) peak isometric torque (r = 0.282, P \ .001) and (2) Q-LSI (r = 0.357, P \ .001) was observed. Individuals with IKDC scores 90 were 3 times (OR = 3.4; 95% CI, 1.71-6.93) more likely to demonstrate higher Q-LSI (90%). An IKDC score 94.8 predicted Q-LSI 90% with high sensitivity (0.813) and moderate specificity (0.493). Conclusion: Participants with higher IKDC scores demonstrated an increased likelihood of presenting with greater involved limb quadriceps strength and better Q-LSI. Based on the results of this study, a patient-reported outcome measure, such as the IKDC, may be able to serve as a valuable screening tool for the identification of quadriceps strength deficits in this population; however, it should not be considered an accurate surrogate for isokinetic dynamometry. Furthermore, a score of 94.8 on the IKDC is likely to indicate that a patient’s quadriceps strength is at an acceptable RTS level. Keywords: ACL reconstruction; patient-reported outcomes; quadriceps strength; IKDC; return to sport
Epidemiological studies report that as many as 250,000 anterior cruciate ligament (ACL) knee injuries occur each year in the United States, with an estimated 50% occurring in young athletes ages 15 to 25 years.7,17,20 Many individuals who suffer ACL injuries require surgical reconstruction
of the ACL (ACLR) to return to sports participation, which often results in significant deficits in range of motion,8,11 strength,43,55 neuromuscular control,57 and functional abilities41,46 in the early months after surgery. Although return to sports is a common goal for many of these patients, less than 50% of athletes are able to return to their preinjury level of sports and physical activity within the first 12 months.4 For young athletes who are able to successfully return to their respective sport, it is approximated that 20% to 30% will go on to sustain a second ACL injury,31,40,47,58 and more than
The American Journal of Sports Medicine, Vol. XX, No. X DOI: 10.1177/0363546515591258 Ó 2015 The Author(s)
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50% will likely develop knee osteoarthritis within 10 years after primary injury.34,35 Therefore, it is important for sports medicine clinicians to facilitate recovery in patients with ACLR in a way that optimizes their ability to participate in physical activity and to minimize their risk for subsequent injuries and future health complications. The application of widely accessible clinical measures that can identify individuals at high risk for poorer outcomes is a crucial aim for sports medicine practitioners. One crucial decision in the management of patients who have undergone ACLR is to identify when they can safely return to sport (RTS). Although RTS criteria that clinicians use to make this determination are highly variable,36 and current literature contains inconsistent results regarding the influence of strength on functional outcomes,9 quadriceps strength of the involved limb is consistently used as one of the key variables associated with ACLR outcomes and risk factors for reinjury.1,5,19,37,39,56 Quadriceps weakness of the involved limb, particularly when compared with that of the uninvolved knee, has been shown to be associated with asymmetric limb-loading strategies and decreased functional stability of the knee.12,13,23,42,57 This deficit, commonly found at both short- and long-term follow-ups in patients who have been released to play sports again,38,50,54,55 may be due to a lack of consensus and standardization regarding quadriceps strength critical values to minimize poorer outcomes and increased risk of subsequent injuries and complications.36 The quadriceps limb symmetry index (Q-LSI), which is a measure that compares involved limb quadriceps strength relative to the uninvolved limb quadriceps strength, has been commonly used in many ACLR postoperative function and outcomes studies as an assessment of both isometric and isokinetic strength.1,4,5,37,54 Current evidence has shown that a 10% Q-LSI deficit can be expected in the normal population.48 Likewise, results from a recent survey of international sports medicine professionals identified good quadriceps strength, operationalized by a score of 90% Q-LSI, as 1 of the 6 important measures of successful outcome after ACLR along with hamstring strength limb symmetry index .90%, the absence of giving way, returning to sports, no more than mild knee joint effusion, and high patient-reported outcome scores.36 Researchers have traditionally promoted the isokinetic dynamometer as the best tool for the quantification of isometric quadriceps limb symmetry.24,25,29,30,45,50,52 As isokinetic dynamometers are relatively expensive and require additional skills and time to administer, the practicality
of obtaining a valid Q-LSI score may be low in many clinical settings. To increase the feasibility of approximation of quadriceps strength, it would be valuable to identify or create a screening assessment tool that could easily be administered in any setting and is strongly related to a 90% Q-LSI score. Patient-reported outcome assessment tools specific to the knee joint, such as the International Knee Documentation Committee 2000 Subjective Knee Form (IKDC), have also traditionally been regarded as an important measure of successful outcome after ACLR36 and are often used in conjunction with other RTS objective measures.1,5,19 The IKDC contains items highly relevant to patients with various knee conditions, including those with reconstructed ACLs.18 It has been shown to have high validity and consistency in both the adult population and individuals age 6 to 18 with knee injuries.49 This tool is easy to administer in most clinical settings,14,15,53 including pediatric sports medicine, where the incidence of ACL injury is high. Additionally, published normative data that include individuals younger than 24 years are available for reference3; therefore, we chose to analyze the IKDC for the purposes of this study. If a widely accessible measure of self-reported knee function could be used to predict which individuals are at higher risk of strength deficits after ACLR, it can be used as an important screening tool to inform RTS decision making and direct interventions to improve probability for successful return to previous level of sports participation and minimal risk for future injury. Therefore, the purpose of this study was to investigate whether IKDC score at the time of RTS can help to determine appropriateness and necessity of quadriceps limb symmetry assessment using lower extremity dynamometry during RTS decision making in a young, athletic population after ACLR. We hypothesized that (1) individuals with higher self-reports of function would demonstrate better quadriceps strength of the involved limb than individuals with lower self-reports of function at the time of RTS, and (2) individuals with higher self-reports of function would have normal quadriceps strength limb symmetry of 90%.
METHODS Participants These data represent a subset of a larger prospective, longitudinal study on ACLR outcomes. A total of 139 young athletes (90 female, 49 male) were included in this analysis. All
*Address correspondence to Christin Zwolski, PT, OCS, Cincinnati Children’s Hospital Medical Center–Liberty Campus, 7777 Yankee Road, MLC 16016, Liberty Township, OH 45044, USA (email: [email protected]). y Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio, USA. z Sports Medicine Biodynamics Center and Human Performance Laboratory, Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio, USA. § Division of Occupational Therapy and Physical Therapy, Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio, USA. || Division of Physical Therapy, School of Health and Rehabilitation Sciences, Ohio State University, Columbus, Ohio, USA. { Department of Pediatrics, College of Medicine, University of Cincinnati, Cincinnati, Ohio, USA. # Department of Orthopaedic Surgery, College of Medicine, University of Cincinnati, Cincinnati, Ohio, USA. **Departments of Biomedical Engineering and Rehabilitation Sciences, the Ohio State University, Columbus, Ohio, USA. One or more of the authors has declared the following potential conflict of interest or source of funding: This work was funded in part by support from the National Institutes of Health (grant F32-AR055844) and the National Football League Charities Medical Research grants (2007, 2008, 2009, 2011).
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participants (1) were between 9 and 25 years old, (2) had recently sustained a primary ACL injury treated with surgical reconstruction, (3) had completed rehabilitation, and (4) were released by both their physician and physical therapist or athletic trainer to return to their preinjury participation level in pivoting or cutting (level 1 or 2)10 sport for at least 50 hours per year. We did not control the rehabilitation plan of care or RTS criteria for these study subjects. Athletes were not included if they had a history of an ipsilateral or contralateral ACL injury or had suffered a lower extremity injury or low back injury during the previous 12 months. The study protocol was approved by an institutional review board, and informed consent was obtained from all participants and guardians (if applicable) before beginning the testing protocol.
Data Collection Data collection occurred at a single testing session within 4 weeks of the patient’s medical clearance to return to sport. Demographic and anthropometric data (age, height, and weight), knee strength evaluations, and IKDC scores were collected for all participants.
Isometric Strength Isometric quadriceps strength was assessed using a Biodex isokinetic dynamometer (Biodex Medical Systems Inc). The athletes were seated in the chair with their hips flexed to 90°. The knee joint axis was aligned with the axis of rotation of the dynamometer, with the thigh, waist, and distal tibia stabilized with straps to secure the athlete in the testing position. After execution of gravity correction, the leg was placed in 60° of flexion. The athletes were instructed to place their arms across their chest and perform maximal effort isometric knee extension by ‘‘kicking out as hard as you can for 5 seconds.’’ After each 5-second contraction, the athletes were given a 10-second rest; they performed the test a total of 3 times on each leg. The uninvolved side was tested first for each athlete, followed by the involved side. Consistent, real-time visual and verbal feedback was provided during the test to ensure maximum effort by the participant. This procedure has been used to quantify quadriceps torque in individuals with ACL injury and reconstruction and has yielded reliable measurements.27 Recent studies that analyze isometric quadriceps strength in this manner have found an association between higher Q-LSI and better performance on functional tests used for RTS decision making.49,50
Patient-Reported Knee Function Each subject completed the International Knee Documentation Committee 2000 Subjective Knee Form (IKDC). The IKDC is scored on a 0-to-100 scale, with 100 representing higher knee function.21 The items in the IKDC are highly relevant to individuals after ACLR (eg, symptoms, sports activity, and function), making it a commonly used clinical tool for assessment of self-reported knee function
in this patient population.18,21,22 The IKDC has high validity and reliability,51 and normative data have been identified for individuals who use the IKDC after knee injury,21 allowing for a standardization of successful patient-reported outcomes.3
Statistical Analysis To test the hypotheses, we conducted several analyses. First, we measured the maximum peak torque across the 3 trials, which was recorded in foot-pounds (ft-lb) and converted to newton meters per kilogram (Nm/kg). These values were then used to calculate Q-LSI using the following formula: (involved limb peak torque/uninvolved limb peak torque) 3 100%. Next, Pearson product moment correlation coefficients were used to assess the relationship between IKDC score and peak isometric torque of the involved limb (hypothesis 1) and Q-LSI (hypothesis 2). As both peak isometric torque of the involved limb and Q-LSI were significantly correlated with IKDC, both variables were investigated with additional analyses. The study participants were then dichotomized into 2 groups: those with a high patient-reported function, operationalized by an IKDC score 90 (high IKDC group), and those with a low patient-reported function, operationalized by an IKDC score \90 (low IKDC group). To account for the normal variance in IKDC scores, previous studies have operationally defined self-reported knee function within normal ranges as IKDC scores greater than or equal to the age- and sex-matched population 15th percentile.33 Therefore, the cutoff score of 90 for the IKDC was chosen for this study based on previously established normative values for individuals aged 24 years and younger.3,32 To test hypothesis 1, a repeated-measures 2-way analysis of variance (ANOVA) was performed to identify the effect of limb (involved vs uninvolved) and self-reported function (high IKDC vs low IKDC) on isometric peak torque. To test hypothesis 2, study participants were also dichotomized into a high quadriceps strength group operationalized by a Q-LSI 90% (high Q-LSI) and a low quadriceps strength group operationalized by a Q-LSI \90% (low Q-LSI). A cutoff score of 90% was based on studies which indicate that a side-to-side difference in peak quadriceps force output of more than 10% may reflect significant differences in muscle performance with a low chance of measurement error.48 In addition, studies have used similar quadriceps strength group values in individuals after ACLR,30,47 and a Q-LSI 90% is a recommended criterion for determination of safe to return to sport.1,26,44 Once subjects were classified as having clinically acceptable scores for high function (IKDC 90) and good quadriceps strength (Q-LSI 90%), chi-square analyses and logistic regression analyses were used to determine the odds of a patient with high self-reported function on the IKDC also having clinically acceptable quadriceps strength. Odds ratio, sensitivity, and specificity were calculated to assess the efficacy of the IKDC as a screening tool and/or diagnostic indicator of quadriceps strength symmetry after ACLR.
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TABLE 1 Participant Demographic Dataa
Age, y Height, cm Weight, kg BMI, kg/m2 Time from surgery to testing, mo Graft type (HS/BTB/allograft), n
All Patients (N = 139)
High IKDC Group (n = 68)
Low IKDC Group (n = 71)
P (High vs Low IKDC Group)b
16.7 6 2.7 167.7 6 10.9 67.4 6 16.7 23.8 6 4.9 8.2 6 2.4 71/57/11
15.8 6 2.4 166.4 6 11.1 65.0 6 15.3 23.1 6 4.1 8.5 6 2.6 50/17/1
17.8 6 2.7 169.0 6 10.5 69.7 6 17.8 24.4 6 5.6 8.0 6 2.2 21/40/10
\.001 .16 .10 .13 .16 \.001
a Results are reported as mean 6 SD unless otherwise indicated. BMI, body mass index; BTB, bone–patellar tendon–bone graft; HS, hamstring graft; IKDC, International Knee Documentation Committee 2000 Subjective Knee Form. b Independent-samples t test.
TABLE 2 Quadriceps Femoris Strengtha
Normalized quadriceps femoris peak torque, Nm/kg Uninvolved limb Involved limb P value Q-LSI, %
All Patients (N = 139)
High IKDC Group (n = 68)
Low IKDC Group (n = 71)
2.6 6 0.5 2.3 6 0.5 \.001 88.7 6 17.5
2.6 6 0.5 2.5 6 0.5 .001 94.8 6 15.0
2.7 6 0.5 2.2 6 0.5 \.001 82.9 6 17.9
P (High vs Low IKDC Group)
.704 .002 \.001
a
Results are reported as mean 6 SD. IKDC, International Knee Documentation Committee 2000 Subjective Knee Form; Q-LSI, quadriceps strength limb symmetry index.
RESULTS Independent-samples t test revealed no significant difference (P . .05) in height or mass between the high and low IKDC groups. The high IKDC group was younger (15.8 6 2.4 years) than the low IKDC group (17.8 6 2.7 years) (P \ .001) (Table 1). Pearson product moment correlation analysis revealed a significant correlation between the peak isometric quadriceps torque of the involved limb and IKDC score (r = 0.282, P \ .001) and a significant correlation (r = 0.357, P \ .001) between Q-LSI and IKDC scores at time of RTS (Figure 1). Two-way ANOVA showed a significant limb 3 group interaction (P \ .0001). Specifically, post hoc analysis inclusive of paired and independent sample t tests revealed that the peak isometric quadriceps torque of the involved limb of the low IKDC group was significantly lower (2.2 6 0.5 Nm/kg) than that of the uninvolved limb (2.7 6 0.5 Nm/kg) in the low IKDC group and lower than those of both the involved (2.5 6 0.5 Nm/kg) and the uninvolved limb (2.6 6 0.5 Nm/kg) in the high IKDC group (Table 2 and Figure 2). After subdivision into high and low Q-LSI groups, chisquare analysis revealed that individuals with IKDC scores 90 were 3 times (OR 3.4; 95% CI, 1.71-6.93) more likely to demonstrate Q-LSI 90%. In further analyses, using logistic regression controlled for age, with IKDC score as a continuous variable, we determined that an IKDC score 94.8 predicted whether the patient would possess a Q-LSI 90% with high sensitivity (0.813) and
moderate specificity (0.493) (P = .005). With a cutoff score of 95, post hoc analyses revealed that the peak isometric quadriceps torque of the involved limb of the low IKDC group was again significantly lower than that of the uninvolved limb in the low IKDC group and lower than those of both the involved and the uninvolved limb in the high IKDC group (Figure 3).
DISCUSSION The purpose of this study was to determine whether patient-reported outcome for knee function at time of RTS could be used as a screening tool for quadriceps strength deficits in a young, athletic population. In accordance with our hypotheses, patients with higher scores on the IKDC demonstrated an increased likelihood of presenting with greater involved limb quadriceps strength and better limb symmetry compared with patients with lower knee function scores after ACLR. Regression analyses revealed that IKDC scores 94.8 had a sensitivity of greater than 81% when the tool was used as a predictor of strength. Although this study was not designed to specifically delineate a criterion value for IKDC score and acceptable quadriceps strength for RTS, the results indicate that younger athletes who have regained appropriate quadriceps strength limb symmetry after ACLR may be identified by IKDC score 94.8. These findings introduce the potential utility of the IKDC as a screening tool for
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Figure 1. Scatter plot of quadriceps limb symmetry index (Q-LSI; in percentages) relative to score on International Knee Documentation Committee 2000 Subjective Knee Form (IKDC).
Figure 2. Isometric quadriceps peak torque relative to IKDC group (cutoff score of 90): involved versus uninvolved limbs. Error bars represent 95% CI. *Peak torque for the involved limb of the low IKDC group was significantly lower compared with the uninvolved limb of low IKDC group and both limbs of the high IKDC group (P \ .001). IKDC, International Knee Documentation Committee 2000 Subjective Knee Form. residual strength deficits at time of RTS after ACLR. Furthermore, a score of 95 on the IKDC may identify athletes who are ready for strength testing by means of isokinetic dynamometer. Varying methods of objectifying quadriceps strength at time of RTS are used in ACLR protocols; however, the use of isokinetic lower extremity dynamometers has
Figure 3. Isometric quadriceps peak torque of self-reported function groups relative to IKDC group (cutoff score of 95): involved vs uninvolved limbs. Error bars represent 95% CI. *Peak torque for the involved limb of the low IKDC group was significantly lower compared with the uninvolved limb of low IKDC group and both limbs of the high IKDC group (P \ .0001). IKDC, International Knee Documentation Committee 2000 Subjective Knee Form. traditionally been the preferred practice due to high reliability in this population.2,16,25,30,38 Unfortunately, not all clinicians have the means to perform this test, which may be attributed to equipment cost and feasibility. The results of this study indicate that a patient-reported outcome, such as the IKDC, may be a valuable screening tool to identify quadriceps strength deficits in this population, however, it should not be considered an accurate surrogate for isokinetic dynamometry. Within a cohort of athletes 13 to 60 years of age, Logerstedt and colleagues32 found lower IKDC scores to be more indicative of failure on a battery of return-to-activity criteria tests, which included isometric quadriceps strength testing, single-legged hop testing, and patient-reported outcome measures. However, to our knowledge, no previous work has sought to identify the relationships between single performance-based measures, such as quadriceps strength limb symmetry indices, and self-reported function at time of RTS after ACLR. According to IKDC normative data, the IKDC score of 94.8 found through logistic regression in this study is very similar to the mean IKDC scores for healthy individuals younger than 25 years, for both male (95.5 6 8.2) and female (93.4 6 9.5) patients.3 Also comparable with the predictive value of the IKDC determined in this study, Lentz and colleagues28 determined a score of 93 on the IKDC as the cutoff in their calculations of the highest positive likelihood ratio for RTS readiness. Although the population studied by Lentz et al varied
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from that used in this study, these results may collectively suggest that higher IKDC scores are associated with the ability to meet RTS criteria and successfully return to previous level of activity. In this study, interestingly, participants in the high IKDC group were on average 2 years younger than those in the low IKDC group. This may be due in part to the lifestyle changes occurring during the rapid growth years of adolescence and young adulthood, as older participants may have found returning to a high level of sports participation more difficult due to the transition from high school to collegiate athletics. Additionally, as individuals progress into adulthood, they take on new functional tasks due to work duties and lifestyle changes, which may redefine the meaning of ‘‘function’’ seen throughout the items of the IKDC. A focus of further research should be the consideration of lower patient-reported outcome scores in relation to lifestyle and activity level changes in this young, athletic population intending to return to high-level sports. Despite this study’s finding that the IKDC predicts quadriceps strength with high sensitivity, it is important to emphasize that the IKDC should not be considered a valid replacement for the objective assessment of quadriceps strength. Instead, the schema of high IKDC versus low IKDC might prove useful as a screening tool to identify individuals who may require further isokinetic strength testing to confirm RTS decision making, particularly in the case of a clinician who does not have access to lower extremity dynamometry. Based on the findings from this study, a score of 94.8 on the IKDC is likely to indicate that a patient’s quadriceps strength is at an acceptable RTS level. Concurrently, an IKDC score of \95 may be a sensitive indicator of quadriceps strength limb symmetry less than 90%. However, because the specificity of the IKDC relative to quadriceps strength was not very high, a score of 94.8 on the IKDC should not exempt an athlete from quadriceps strength assessment by means of lower extremity dynamometry. An IKDC score 95 may instead signify the athlete is ready and appropriate for strength testing. While no specific IKDC score has been established as the ideal criterion value for RTS readiness in a population of young athletes after ACLR, the findings of this study indicate that scores higher than 94.8 may be associated with quadriceps strength limb symmetry values that meet current RTS recommendations. Should an athlete self-report an IKDC score lower than 90, the athlete will likely also demonstrate unacceptable quadriceps strength limb symmetry for successful RTS. We strongly advocate limb symmetry testing to determine whether strength deficits are contributing to lower self-reports of knee function. Due to the moderate specificity of the IKDC found in this study, a score higher than 94.8 does not discernibly indicate that the young athlete will meet current quadriceps strength limb symmetry RTS criteria. Similar conclusions were made by Logerstedt and colleagues,32 who found that among athletes with IKDC scores within normal limits at 12 months after ACLR, only 61.8% were able to successfully pass a battery of RTS tests. The results of this study prompt further investigation regarding the ability of the IKDC to be used as a screening tool for additional variables used in RTS decision making. Future
research should explore the interaction of the IKDC with other commonly used objective RTS assessments such as postural stability,6,42 functional hop testing,1 and landing biomechanics.11 Furthermore, clinical decision making may be enhanced by examination of the specificity of the individual IKDC items, in addition to those from other knee-specific patient-reported outcomes, as predictors of quadriceps strength. To better predict quadriceps strength, clinicians may benefit from a patient-reported outcome tool developed from items found to have the highest sensitivity and specificity. It is important to identify limitations for the results of this study. First, the study sample consisted of young, active athletes due to the high prevalence of ACL injury seen in this population, including some skeletally immature individuals who may have undergone unique surgical management. A consequence of focus on this population may include low generalizability of results to other demographics, especially older individuals or those not returning to cutting and pivoting sports. Second, a modified form of the IKDC, the pedi-IKDC, has been developed specifically for a young demographic. This study did not investigate the predictive abilities of the pedi-IKDC since research has indicated the IKDC is a valid and consistent knee-specific patient-reported outcome in a population of active individuals aged 6 to 18 years,51 and it is used more consistently in clinical settings. However, future research may be helpful to determine the associations between pedi-IKDC scores and quadriceps strength performance at RTS. Third, the dichotomization of participants into high and low IKDC groups resulted in a significant difference in mean age between groups. Further investigation of the relationship between age and self-reported function may provide beneficial insight for future studies of this population.
CONCLUSION The findings of this study present the clinical utility of the IKDC as a simple and accessible screening tool for residual quadriceps strength asymmetries at time of RTS after ACLR. While we recommend further research to support and strengthen our findings, this study provides early evidence of the predictive abilities of the IKDC on quadriceps strength and limb symmetry after ACLR.
ACKNOWLEDGMENT The authors acknowledge the ongoing support of the ACLR Outcome Project from the faculty and staff within the Division of Sports Medicine and the Division of Occupational Therapy and Physical Therapy at Cincinnati Children’s Hospital Medical Center. REFERENCES 1. Adams D, Logerstedt DS, Hunter-Giordano A, Axe MJ, SnyderMackler L. Current concepts for anterior cruciate ligament reconstruction: a criterion-based rehabilitation progression. J Orthop Sports Phys Ther. 2012;42(7):601-614.
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2. Almosnino S, Dvir Z, Bardana DD, Diaconescu ED, Stevenson JM. Ascertaining maximal voluntary effort production during isokinetic knee strength testing of anterior cruciate ligament-reconstructed patients. Am J Phys Med Rehabil. 2014;93(2):169-181. 3. Anderson AF, Irrgang JJ, Kocher MS, Mann BJ, Harrast JJ. The International Knee Documentation Committee Subjective Knee Evaluation Form: normative data. Am J Sports Med. 2006;34(1):128-135. 4. Ardern CL, Webster KE, Taylor NF, Feller JA. Return to the preinjury level of competitive sport after anterior cruciate ligament reconstruction surgery: two-thirds of patients have not returned by 12 months after surgery. Am J Sports Med. 2011;39(3):538-543. 5. Barber-Westin SD, Noyes FR. Objective criteria for return to athletics after anterior cruciate ligament reconstruction and subsequent reinjury rates: a systematic review. Phys Sportsmed. 2011;39(3):100-110. 6. Beard DJ, Kyberd PJ, Dodd CA, Simpson AH, O’Connor JJ. Proprioception in the knee. J Bone Joint Surg Br. 1994;76(6):992-993. 7. Beynnon BD, Vacek PM, Newell MK, et al. The effects of level of competition, sport, and sex on the incidence of first-time noncontact anterior cruciate ligament injury. Am J Sports Med. 2014;42(8):18061812. 8. Button K, Roos PE, van Deursen RW. Activity progression for anterior cruciate ligament injured individuals. Clin Biomech. 2014;29(2):206212. 9. Czuppon S, Racette BA, Klein SE, Harris-Hayes M. Variables associated with return to sport following anterior cruciate ligament reconstruction: a systematic review. Br J Sports Med. 2014;48(5):356-364. 10. Daniel DM, Stone ML, Dobson BE, Fithian DC, Rossman DJ, Kaufman KR. Fate of the ACL-injured patient: a prospective outcome study. Am J Sports Med. 1994;22(5):632-644. 11. de Fontenay BP, Argaud S, Blache Y, Monteil K. Motion alterations after anterior cruciate ligament reconstruction: comparison of the injured and uninjured lower limbs during a single-legged jump. J Athl Train. 2014;49(3):311-316. 12. Decker MJ, Torry MR, Noonan TJ, Riviere A, Sterett WI. Landing adaptations after ACL reconstruction. Med Sci Sports Exerc. 2002;34(9):1408-1413. 13. Di Stasi SL, Logerstedt D, Gardinier ES, Snyder-Mackler L. Gait patterns differ between ACL-reconstructed athletes who pass return-tosport criteria and those who fail. Am J Sports Med. 2013;41(6):13101318. 14. Dunn WR, Spindler KP. Predictors of activity level 2 years after anterior cruciate ligament reconstruction (ACLR): a Multicenter Orthopaedic Outcomes Network (MOON) ACLR cohort study. Am J Sports Med. 2010;38(10):2040-2050. 15. Granan LP, Bahr R, Steindal K, Furnes O, Engebretsen L. Development of a national cruciate ligament surgery registry: the Norwegian National Knee Ligament Registry. Am J Sports Med. 2008;36(2):308315. 16. Greenberg EM, Greenberg ET, Ganley TJ, Lawrence JT. Strength and functional performance recovery after anterior cruciate ligament reconstruction in preadolescent athletes. Sports Health. 2014;6(4):309-312. 17. Griffin LY, Albohm MJ, Arendt EA, et al. Understanding and preventing noncontact anterior cruciate ligament injuries: a review of the Hunt Valley II meeting, January 2005. Am J Sports Med. 2006;34(9):1512-1532. 18. Hambly K, Griva K. IKDC or KOOS: which one captures symptoms and disabilities most important to patients who have undergone initial anterior cruciate ligament reconstruction? Am J Sports Med. 2010;38(7):1395-1404. 19. Harris JD, Abrams GD, Bach BR, et al. Return to sport after ACL reconstruction. Orthopedics. 2014;37(2):e103-108. 20. Hewett TE, Di Stasi SL, Myer GD. Current concepts for injury prevention in athletes after anterior cruciate ligament reconstruction. Am J Sports Med. 2013;41(1):216-224. 21. Irrgang JJ, Anderson AF, Boland AL, et al. Development and validation of the International Knee Documentation Committee subjective knee form. Am J Sports Med. 2001;29(5):600-613.
22. Irrgang JJ, Anderson AF, Boland AL, et al. Responsiveness of the International Knee Documentation Committee subjective knee form. Am J Sports Med. 2006;34(10):1567-1573. 23. Keays SL, Bullock-Saxton JE, Newcombe P, Keays AC. The relationship between knee strength and functional stability before and after anterior cruciate ligament reconstruction. J Orthop Res. 2003; 21(2):231-237. 24. Knezevic OM, Mirkov DM, Kadija M, Milovanovic D, Jaric S. Evaluation of isokinetic and isometric strength measures for monitoring muscle function recovery after anterior cruciate ligament reconstruction. J Strength Cond Res. 2014;28(6):1722-1731. 25. Krishnan C, Williams GN. Factors explaining chronic knee extensor strength deficits after ACL reconstruction. J Orthop Res. 2011;29(5): 633-640. 26. Kvist J. Rehabilitation following anterior cruciate ligament injury: current recommendations for sports participation. Sports Med. 2004; 34(4):269-280. 27. Larsen JB, Farup J, Lind M, Dalgas U. Muscle strength and functional performance is markedly impaired at the recommended time point for sport return after anterior cruciate ligament reconstruction in recreational athletes. Hum Movement Sci. 2014;39c:73-87. 28. Lentz TA, Zeppieri G Jr, Tillman SM, et al. Return to preinjury sports participation following anterior cruciate ligament reconstruction: contributions of demographic, knee impairment, and self-report measures. J Orthop Sports Phys Ther. 2012;42(11):893-901. 29. Lepley AS, Ericksen HM, Sohn DH, Pietrosimone BG. Contributions of neural excitability and voluntary activation to quadriceps muscle strength following anterior cruciate ligament reconstruction. Knee. 2014;21(3):736-742. 30. Lewek M, Rudolph K, Axe M, Snyder-Mackler L. The effect of insufficient quadriceps strength on gait after anterior cruciate ligament reconstruction. Clin Biomech. 2002;17(1):56-63. 31. Leys T, Salmon L, Waller A, Linklater J, Pinczewski L. Clinical results and risk factors for reinjury 15 years after anterior cruciate ligament reconstruction: a prospective study of hamstring and patellar tendon grafts. Am J Sports Med. 2012;40(3):595-605. 32. Logerstedt D, Di Stasi S, Grindem H, et al. Self-reported knee function can identify athletes who fail return-to-activity criteria up to 1 year after anterior cruciate ligament reconstruction: a DelawareOslo ACL cohort study. J Orthop Sports Phys Ther. 2014;44(12): 914-923. 33. Logerstedt D, Grindem H, Lynch A, et al. Single-legged hop tests as predictors of self-reported knee function after anterior cruciate ligament reconstruction: the Delaware-Oslo ACL cohort study. Am J Sports Med. 2012;40(10):2348-2356. 34. Lohmander LS, Englund PM, Dahl LL, Roos EM. The long-term consequence of anterior cruciate ligament and meniscus injuries: osteoarthritis. Am J Sports Med. 2007;35(10):1756-1769. 35. Lohmander LS, Ostenberg A, Englund M, Roos H. High prevalence of knee osteoarthritis, pain, and functional limitations in female soccer players twelve years after anterior cruciate ligament injury. Arthritis Rheum. 2004;50(10):3145-3152. 36. Lynch AD, Logerstedt DS, Grindem H, et al. Consensus criteria for defining ‘‘successful outcome’’ after ACL injury and reconstruction: a Delaware-Oslo ACL cohort investigation. Br J Sports Med. 2015;49(5):335-342. 37. Myer GD, Paterno MV, Ford KR, Quatman CE, Hewett TE. Rehabilitation after anterior cruciate ligament reconstruction: criteria-based progression through the return-to-sport phase. J Orthop Sports Phys Ther. 2006;36(6):385-402. 38. Otzel DM, Chow JW, Tillman MD. Long-term deficits in quadriceps strength and activation following anterior cruciate ligament reconstruction. Phys Ther Sport. 2015;16(1):22-28. 39. Palmieri-Smith RM, Thomas AC, Wojtys EM. Maximizing quadriceps strength after ACL reconstruction. Clin Sports Med. 2008;27(3):405424, vii-ix. 40. Paterno MV, Rauh MJ, Schmitt LC, Ford KR, Hewett TE. Incidence of contralateral and ipsilateral anterior cruciate ligament (ACL) injury
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after primary ACL reconstruction and return to sport. Clin J Sport Med. 2012;22(2):116-121. Paterno MV, Schmitt LC, Ford KR, Rauh MJ, Myer GD, Hewett TE. Effects of sex on compensatory landing strategies upon return to sport after anterior cruciate ligament reconstruction. J Orthop Sports Phys Ther. 2011;41(8):553-559. Paterno MV, Schmitt LC, Ford KR, et al. Biomechanical measures during landing and postural stability predict second anterior cruciate ligament injury after anterior cruciate ligament reconstruction and return to sport. Am J Sports Med. 2010;38(10):1968-1978. Risberg MA, Holm I, Tjomsland O, Ljunggren E, Ekeland A. Prospective study of changes in impairments and disabilities after anterior cruciate ligament reconstruction. J Orthop Sports Phys Ther. 1999;29(7):400-412. Roi GS, Creta D, Nanni G, Marcacci M, Zaffagnini S, Snyder-Mackler L. Return to official Italian First Division soccer games within 90 days after anterior cruciate ligament reconstruction: a case report. J Orthop Sports Phys Ther. 2005;35(2):52-61; discussion 61-56. Rudolph KS, Axe MJ, Snyder-Mackler L. Dynamic stability after ACL injury: who can hop? Knee Surg Sports Traumatol Arthrosc. 2000;8(5):262-269. Salem GJ, Salinas R, Harding FV. Bilateral kinematic and kinetic analysis of the squat exercise after anterior cruciate ligament reconstruction. Arch Phys Med Rehabil. 2003;84(8):1211-1216. Salmon L, Russell V, Musgrove T, Pinczewski L, Refshauge K. Incidence and risk factors for graft rupture and contralateral rupture after anterior cruciate ligament reconstruction. Arthroscopy. 2005;21(8): 948-957. Sapega AA. Muscle performance evaluation in orthopaedic practice. J Bone Joint Surg Am. 1990;72(10):1562-1574. Schmitt LC, Paterno MV, Ford KR, Myer GD, Hewett TE. Strength asymmetry and landing mechanics at return to sport after ACL
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The Influence of Quadriceps Strength Asymmetry on Patient-Reported Function at Time of Return to Sport After Anterior Cruciate Ligament Reconstruction Christin Zwolski, Laura C. Schmitt, Catherine Quatman-Yates, Staci Thomas, Timothy E. Hewett and Mark V. Paterno Am J Sports Med published online July 16, 2015 DOI: 10.1177/0363546515591258 The online version of this article can be found at: http://ajs.sagepub.com/content/early/2015/07/16/0363546515591258
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The Influence of Quadriceps Strength Asymmetry on Patient-Reported Function at Time of Return to Sport After Anterior Cruciate Ligament Reconstruction Christin Zwolski,*yz§ PT, OCS, Laura C. Schmitt,yz§|| PT, PhD, Catherine Quatman-Yates,yz§{ PT, PhD, Staci Thomas,yz MS, Timothy E. Hewett,yz{#** PhD, FACSM, and Mark V. Paterno,yz§{ PT, PhD, SCS, ATC Investigation performed at Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio, USA Background: An objective assessment of quadriceps strength after anterior cruciate ligament reconstruction (ACLR) is an important clinical measure to determine readiness to return to sport (RTS). Not all clinicians are equipped with the means to objectively quantify quadriceps strength limb symmetry indices (Q-LSIs) via lower extremity isokinetic dynamometers, as recommended by previous studies. Purpose/Hypothesis: The purpose of this study was to determine whether the International Knee Documentation Committee 2000 Subjective Knee Form (IKDC) score at time of RTS was a predictor of quadriceps strength in a young, athletic population after ACLR. Two hypotheses were tested: (1) Individuals with higher self-reports of function would demonstrate better quadriceps strength of the involved limb than individuals with lower self-reports of function at the time of RTS, and (2) individuals with higher self-reports of function would have normal quadriceps strength limb symmetry. Study Design: Cohort study (diagnosis); Level of evidence, 2. Methods: At time of RTS, 139 subjects who had undergone ACLR completed the IKDC. In addition, an isometric quadriceps strength test (Biodex dynamometer) was performed on both lower extremities. Peak torques were calculated, as was the Q-LSI, determined by the formula (involved limb peak torque/uninvolved limb peak torque) 3 100%. Participants were dichotomized based on IKDC scores: high IKDC (IKDC 90) and low IKDC (IKDC \90). Two-way analysis of variance was used to determine the effect of limb (involved vs uninvolved) and group (high vs low IKDC) on isometric quadriceps strength. Chi-square and logistic regression analyses were then performed to determine whether IKDC scores could predict Q-LSI. Results: At time of RTS, a significant correlation between IKDC scores and (1) peak isometric torque (r = 0.282, P \ .001) and (2) Q-LSI (r = 0.357, P \ .001) was observed. Individuals with IKDC scores 90 were 3 times (OR = 3.4; 95% CI, 1.71-6.93) more likely to demonstrate higher Q-LSI (90%). An IKDC score 94.8 predicted Q-LSI 90% with high sensitivity (0.813) and moderate specificity (0.493). Conclusion: Participants with higher IKDC scores demonstrated an increased likelihood of presenting with greater involved limb quadriceps strength and better Q-LSI. Based on the results of this study, a patient-reported outcome measure, such as the IKDC, may be able to serve as a valuable screening tool for the identification of quadriceps strength deficits in this population; however, it should not be considered an accurate surrogate for isokinetic dynamometry. Furthermore, a score of 94.8 on the IKDC is likely to indicate that a patient’s quadriceps strength is at an acceptable RTS level. Keywords: ACL reconstruction; patient-reported outcomes; quadriceps strength; IKDC; return to sport
Epidemiological studies report that as many as 250,000 anterior cruciate ligament (ACL) knee injuries occur each year in the United States, with an estimated 50% occurring in young athletes ages 15 to 25 years.7,17,20 Many individuals who suffer ACL injuries require surgical reconstruction
of the ACL (ACLR) to return to sports participation, which often results in significant deficits in range of motion,8,11 strength,43,55 neuromuscular control,57 and functional abilities41,46 in the early months after surgery. Although return to sports is a common goal for many of these patients, less than 50% of athletes are able to return to their preinjury level of sports and physical activity within the first 12 months.4 For young athletes who are able to successfully return to their respective sport, it is approximated that 20% to 30% will go on to sustain a second ACL injury,31,40,47,58 and more than
The American Journal of Sports Medicine, Vol. XX, No. X DOI: 10.1177/0363546515591258 Ó 2015 The Author(s)
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50% will likely develop knee osteoarthritis within 10 years after primary injury.34,35 Therefore, it is important for sports medicine clinicians to facilitate recovery in patients with ACLR in a way that optimizes their ability to participate in physical activity and to minimize their risk for subsequent injuries and future health complications. The application of widely accessible clinical measures that can identify individuals at high risk for poorer outcomes is a crucial aim for sports medicine practitioners. One crucial decision in the management of patients who have undergone ACLR is to identify when they can safely return to sport (RTS). Although RTS criteria that clinicians use to make this determination are highly variable,36 and current literature contains inconsistent results regarding the influence of strength on functional outcomes,9 quadriceps strength of the involved limb is consistently used as one of the key variables associated with ACLR outcomes and risk factors for reinjury.1,5,19,37,39,56 Quadriceps weakness of the involved limb, particularly when compared with that of the uninvolved knee, has been shown to be associated with asymmetric limb-loading strategies and decreased functional stability of the knee.12,13,23,42,57 This deficit, commonly found at both short- and long-term follow-ups in patients who have been released to play sports again,38,50,54,55 may be due to a lack of consensus and standardization regarding quadriceps strength critical values to minimize poorer outcomes and increased risk of subsequent injuries and complications.36 The quadriceps limb symmetry index (Q-LSI), which is a measure that compares involved limb quadriceps strength relative to the uninvolved limb quadriceps strength, has been commonly used in many ACLR postoperative function and outcomes studies as an assessment of both isometric and isokinetic strength.1,4,5,37,54 Current evidence has shown that a 10% Q-LSI deficit can be expected in the normal population.48 Likewise, results from a recent survey of international sports medicine professionals identified good quadriceps strength, operationalized by a score of 90% Q-LSI, as 1 of the 6 important measures of successful outcome after ACLR along with hamstring strength limb symmetry index .90%, the absence of giving way, returning to sports, no more than mild knee joint effusion, and high patient-reported outcome scores.36 Researchers have traditionally promoted the isokinetic dynamometer as the best tool for the quantification of isometric quadriceps limb symmetry.24,25,29,30,45,50,52 As isokinetic dynamometers are relatively expensive and require additional skills and time to administer, the practicality
of obtaining a valid Q-LSI score may be low in many clinical settings. To increase the feasibility of approximation of quadriceps strength, it would be valuable to identify or create a screening assessment tool that could easily be administered in any setting and is strongly related to a 90% Q-LSI score. Patient-reported outcome assessment tools specific to the knee joint, such as the International Knee Documentation Committee 2000 Subjective Knee Form (IKDC), have also traditionally been regarded as an important measure of successful outcome after ACLR36 and are often used in conjunction with other RTS objective measures.1,5,19 The IKDC contains items highly relevant to patients with various knee conditions, including those with reconstructed ACLs.18 It has been shown to have high validity and consistency in both the adult population and individuals age 6 to 18 with knee injuries.49 This tool is easy to administer in most clinical settings,14,15,53 including pediatric sports medicine, where the incidence of ACL injury is high. Additionally, published normative data that include individuals younger than 24 years are available for reference3; therefore, we chose to analyze the IKDC for the purposes of this study. If a widely accessible measure of self-reported knee function could be used to predict which individuals are at higher risk of strength deficits after ACLR, it can be used as an important screening tool to inform RTS decision making and direct interventions to improve probability for successful return to previous level of sports participation and minimal risk for future injury. Therefore, the purpose of this study was to investigate whether IKDC score at the time of RTS can help to determine appropriateness and necessity of quadriceps limb symmetry assessment using lower extremity dynamometry during RTS decision making in a young, athletic population after ACLR. We hypothesized that (1) individuals with higher self-reports of function would demonstrate better quadriceps strength of the involved limb than individuals with lower self-reports of function at the time of RTS, and (2) individuals with higher self-reports of function would have normal quadriceps strength limb symmetry of 90%.
METHODS Participants These data represent a subset of a larger prospective, longitudinal study on ACLR outcomes. A total of 139 young athletes (90 female, 49 male) were included in this analysis. All
*Address correspondence to Christin Zwolski, PT, OCS, Cincinnati Children’s Hospital Medical Center–Liberty Campus, 7777 Yankee Road, MLC 16016, Liberty Township, OH 45044, USA (email: [email protected]). y Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio, USA. z Sports Medicine Biodynamics Center and Human Performance Laboratory, Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio, USA. § Division of Occupational Therapy and Physical Therapy, Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio, USA. || Division of Physical Therapy, School of Health and Rehabilitation Sciences, Ohio State University, Columbus, Ohio, USA. { Department of Pediatrics, College of Medicine, University of Cincinnati, Cincinnati, Ohio, USA. # Department of Orthopaedic Surgery, College of Medicine, University of Cincinnati, Cincinnati, Ohio, USA. **Departments of Biomedical Engineering and Rehabilitation Sciences, the Ohio State University, Columbus, Ohio, USA. One or more of the authors has declared the following potential conflict of interest or source of funding: This work was funded in part by support from the National Institutes of Health (grant F32-AR055844) and the National Football League Charities Medical Research grants (2007, 2008, 2009, 2011).
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participants (1) were between 9 and 25 years old, (2) had recently sustained a primary ACL injury treated with surgical reconstruction, (3) had completed rehabilitation, and (4) were released by both their physician and physical therapist or athletic trainer to return to their preinjury participation level in pivoting or cutting (level 1 or 2)10 sport for at least 50 hours per year. We did not control the rehabilitation plan of care or RTS criteria for these study subjects. Athletes were not included if they had a history of an ipsilateral or contralateral ACL injury or had suffered a lower extremity injury or low back injury during the previous 12 months. The study protocol was approved by an institutional review board, and informed consent was obtained from all participants and guardians (if applicable) before beginning the testing protocol.
Data Collection Data collection occurred at a single testing session within 4 weeks of the patient’s medical clearance to return to sport. Demographic and anthropometric data (age, height, and weight), knee strength evaluations, and IKDC scores were collected for all participants.
Isometric Strength Isometric quadriceps strength was assessed using a Biodex isokinetic dynamometer (Biodex Medical Systems Inc). The athletes were seated in the chair with their hips flexed to 90°. The knee joint axis was aligned with the axis of rotation of the dynamometer, with the thigh, waist, and distal tibia stabilized with straps to secure the athlete in the testing position. After execution of gravity correction, the leg was placed in 60° of flexion. The athletes were instructed to place their arms across their chest and perform maximal effort isometric knee extension by ‘‘kicking out as hard as you can for 5 seconds.’’ After each 5-second contraction, the athletes were given a 10-second rest; they performed the test a total of 3 times on each leg. The uninvolved side was tested first for each athlete, followed by the involved side. Consistent, real-time visual and verbal feedback was provided during the test to ensure maximum effort by the participant. This procedure has been used to quantify quadriceps torque in individuals with ACL injury and reconstruction and has yielded reliable measurements.27 Recent studies that analyze isometric quadriceps strength in this manner have found an association between higher Q-LSI and better performance on functional tests used for RTS decision making.49,50
Patient-Reported Knee Function Each subject completed the International Knee Documentation Committee 2000 Subjective Knee Form (IKDC). The IKDC is scored on a 0-to-100 scale, with 100 representing higher knee function.21 The items in the IKDC are highly relevant to individuals after ACLR (eg, symptoms, sports activity, and function), making it a commonly used clinical tool for assessment of self-reported knee function
in this patient population.18,21,22 The IKDC has high validity and reliability,51 and normative data have been identified for individuals who use the IKDC after knee injury,21 allowing for a standardization of successful patient-reported outcomes.3
Statistical Analysis To test the hypotheses, we conducted several analyses. First, we measured the maximum peak torque across the 3 trials, which was recorded in foot-pounds (ft-lb) and converted to newton meters per kilogram (Nm/kg). These values were then used to calculate Q-LSI using the following formula: (involved limb peak torque/uninvolved limb peak torque) 3 100%. Next, Pearson product moment correlation coefficients were used to assess the relationship between IKDC score and peak isometric torque of the involved limb (hypothesis 1) and Q-LSI (hypothesis 2). As both peak isometric torque of the involved limb and Q-LSI were significantly correlated with IKDC, both variables were investigated with additional analyses. The study participants were then dichotomized into 2 groups: those with a high patient-reported function, operationalized by an IKDC score 90 (high IKDC group), and those with a low patient-reported function, operationalized by an IKDC score \90 (low IKDC group). To account for the normal variance in IKDC scores, previous studies have operationally defined self-reported knee function within normal ranges as IKDC scores greater than or equal to the age- and sex-matched population 15th percentile.33 Therefore, the cutoff score of 90 for the IKDC was chosen for this study based on previously established normative values for individuals aged 24 years and younger.3,32 To test hypothesis 1, a repeated-measures 2-way analysis of variance (ANOVA) was performed to identify the effect of limb (involved vs uninvolved) and self-reported function (high IKDC vs low IKDC) on isometric peak torque. To test hypothesis 2, study participants were also dichotomized into a high quadriceps strength group operationalized by a Q-LSI 90% (high Q-LSI) and a low quadriceps strength group operationalized by a Q-LSI \90% (low Q-LSI). A cutoff score of 90% was based on studies which indicate that a side-to-side difference in peak quadriceps force output of more than 10% may reflect significant differences in muscle performance with a low chance of measurement error.48 In addition, studies have used similar quadriceps strength group values in individuals after ACLR,30,47 and a Q-LSI 90% is a recommended criterion for determination of safe to return to sport.1,26,44 Once subjects were classified as having clinically acceptable scores for high function (IKDC 90) and good quadriceps strength (Q-LSI 90%), chi-square analyses and logistic regression analyses were used to determine the odds of a patient with high self-reported function on the IKDC also having clinically acceptable quadriceps strength. Odds ratio, sensitivity, and specificity were calculated to assess the efficacy of the IKDC as a screening tool and/or diagnostic indicator of quadriceps strength symmetry after ACLR.
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TABLE 1 Participant Demographic Dataa
Age, y Height, cm Weight, kg BMI, kg/m2 Time from surgery to testing, mo Graft type (HS/BTB/allograft), n
All Patients (N = 139)
High IKDC Group (n = 68)
Low IKDC Group (n = 71)
P (High vs Low IKDC Group)b
16.7 6 2.7 167.7 6 10.9 67.4 6 16.7 23.8 6 4.9 8.2 6 2.4 71/57/11
15.8 6 2.4 166.4 6 11.1 65.0 6 15.3 23.1 6 4.1 8.5 6 2.6 50/17/1
17.8 6 2.7 169.0 6 10.5 69.7 6 17.8 24.4 6 5.6 8.0 6 2.2 21/40/10
\.001 .16 .10 .13 .16 \.001
a Results are reported as mean 6 SD unless otherwise indicated. BMI, body mass index; BTB, bone–patellar tendon–bone graft; HS, hamstring graft; IKDC, International Knee Documentation Committee 2000 Subjective Knee Form. b Independent-samples t test.
TABLE 2 Quadriceps Femoris Strengtha
Normalized quadriceps femoris peak torque, Nm/kg Uninvolved limb Involved limb P value Q-LSI, %
All Patients (N = 139)
High IKDC Group (n = 68)
Low IKDC Group (n = 71)
2.6 6 0.5 2.3 6 0.5 \.001 88.7 6 17.5
2.6 6 0.5 2.5 6 0.5 .001 94.8 6 15.0
2.7 6 0.5 2.2 6 0.5 \.001 82.9 6 17.9
P (High vs Low IKDC Group)
.704 .002 \.001
a
Results are reported as mean 6 SD. IKDC, International Knee Documentation Committee 2000 Subjective Knee Form; Q-LSI, quadriceps strength limb symmetry index.
RESULTS Independent-samples t test revealed no significant difference (P . .05) in height or mass between the high and low IKDC groups. The high IKDC group was younger (15.8 6 2.4 years) than the low IKDC group (17.8 6 2.7 years) (P \ .001) (Table 1). Pearson product moment correlation analysis revealed a significant correlation between the peak isometric quadriceps torque of the involved limb and IKDC score (r = 0.282, P \ .001) and a significant correlation (r = 0.357, P \ .001) between Q-LSI and IKDC scores at time of RTS (Figure 1). Two-way ANOVA showed a significant limb 3 group interaction (P \ .0001). Specifically, post hoc analysis inclusive of paired and independent sample t tests revealed that the peak isometric quadriceps torque of the involved limb of the low IKDC group was significantly lower (2.2 6 0.5 Nm/kg) than that of the uninvolved limb (2.7 6 0.5 Nm/kg) in the low IKDC group and lower than those of both the involved (2.5 6 0.5 Nm/kg) and the uninvolved limb (2.6 6 0.5 Nm/kg) in the high IKDC group (Table 2 and Figure 2). After subdivision into high and low Q-LSI groups, chisquare analysis revealed that individuals with IKDC scores 90 were 3 times (OR 3.4; 95% CI, 1.71-6.93) more likely to demonstrate Q-LSI 90%. In further analyses, using logistic regression controlled for age, with IKDC score as a continuous variable, we determined that an IKDC score 94.8 predicted whether the patient would possess a Q-LSI 90% with high sensitivity (0.813) and
moderate specificity (0.493) (P = .005). With a cutoff score of 95, post hoc analyses revealed that the peak isometric quadriceps torque of the involved limb of the low IKDC group was again significantly lower than that of the uninvolved limb in the low IKDC group and lower than those of both the involved and the uninvolved limb in the high IKDC group (Figure 3).
DISCUSSION The purpose of this study was to determine whether patient-reported outcome for knee function at time of RTS could be used as a screening tool for quadriceps strength deficits in a young, athletic population. In accordance with our hypotheses, patients with higher scores on the IKDC demonstrated an increased likelihood of presenting with greater involved limb quadriceps strength and better limb symmetry compared with patients with lower knee function scores after ACLR. Regression analyses revealed that IKDC scores 94.8 had a sensitivity of greater than 81% when the tool was used as a predictor of strength. Although this study was not designed to specifically delineate a criterion value for IKDC score and acceptable quadriceps strength for RTS, the results indicate that younger athletes who have regained appropriate quadriceps strength limb symmetry after ACLR may be identified by IKDC score 94.8. These findings introduce the potential utility of the IKDC as a screening tool for
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Figure 1. Scatter plot of quadriceps limb symmetry index (Q-LSI; in percentages) relative to score on International Knee Documentation Committee 2000 Subjective Knee Form (IKDC).
Figure 2. Isometric quadriceps peak torque relative to IKDC group (cutoff score of 90): involved versus uninvolved limbs. Error bars represent 95% CI. *Peak torque for the involved limb of the low IKDC group was significantly lower compared with the uninvolved limb of low IKDC group and both limbs of the high IKDC group (P \ .001). IKDC, International Knee Documentation Committee 2000 Subjective Knee Form. residual strength deficits at time of RTS after ACLR. Furthermore, a score of 95 on the IKDC may identify athletes who are ready for strength testing by means of isokinetic dynamometer. Varying methods of objectifying quadriceps strength at time of RTS are used in ACLR protocols; however, the use of isokinetic lower extremity dynamometers has
Figure 3. Isometric quadriceps peak torque of self-reported function groups relative to IKDC group (cutoff score of 95): involved vs uninvolved limbs. Error bars represent 95% CI. *Peak torque for the involved limb of the low IKDC group was significantly lower compared with the uninvolved limb of low IKDC group and both limbs of the high IKDC group (P \ .0001). IKDC, International Knee Documentation Committee 2000 Subjective Knee Form. traditionally been the preferred practice due to high reliability in this population.2,16,25,30,38 Unfortunately, not all clinicians have the means to perform this test, which may be attributed to equipment cost and feasibility. The results of this study indicate that a patient-reported outcome, such as the IKDC, may be a valuable screening tool to identify quadriceps strength deficits in this population, however, it should not be considered an accurate surrogate for isokinetic dynamometry. Within a cohort of athletes 13 to 60 years of age, Logerstedt and colleagues32 found lower IKDC scores to be more indicative of failure on a battery of return-to-activity criteria tests, which included isometric quadriceps strength testing, single-legged hop testing, and patient-reported outcome measures. However, to our knowledge, no previous work has sought to identify the relationships between single performance-based measures, such as quadriceps strength limb symmetry indices, and self-reported function at time of RTS after ACLR. According to IKDC normative data, the IKDC score of 94.8 found through logistic regression in this study is very similar to the mean IKDC scores for healthy individuals younger than 25 years, for both male (95.5 6 8.2) and female (93.4 6 9.5) patients.3 Also comparable with the predictive value of the IKDC determined in this study, Lentz and colleagues28 determined a score of 93 on the IKDC as the cutoff in their calculations of the highest positive likelihood ratio for RTS readiness. Although the population studied by Lentz et al varied
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from that used in this study, these results may collectively suggest that higher IKDC scores are associated with the ability to meet RTS criteria and successfully return to previous level of activity. In this study, interestingly, participants in the high IKDC group were on average 2 years younger than those in the low IKDC group. This may be due in part to the lifestyle changes occurring during the rapid growth years of adolescence and young adulthood, as older participants may have found returning to a high level of sports participation more difficult due to the transition from high school to collegiate athletics. Additionally, as individuals progress into adulthood, they take on new functional tasks due to work duties and lifestyle changes, which may redefine the meaning of ‘‘function’’ seen throughout the items of the IKDC. A focus of further research should be the consideration of lower patient-reported outcome scores in relation to lifestyle and activity level changes in this young, athletic population intending to return to high-level sports. Despite this study’s finding that the IKDC predicts quadriceps strength with high sensitivity, it is important to emphasize that the IKDC should not be considered a valid replacement for the objective assessment of quadriceps strength. Instead, the schema of high IKDC versus low IKDC might prove useful as a screening tool to identify individuals who may require further isokinetic strength testing to confirm RTS decision making, particularly in the case of a clinician who does not have access to lower extremity dynamometry. Based on the findings from this study, a score of 94.8 on the IKDC is likely to indicate that a patient’s quadriceps strength is at an acceptable RTS level. Concurrently, an IKDC score of \95 may be a sensitive indicator of quadriceps strength limb symmetry less than 90%. However, because the specificity of the IKDC relative to quadriceps strength was not very high, a score of 94.8 on the IKDC should not exempt an athlete from quadriceps strength assessment by means of lower extremity dynamometry. An IKDC score 95 may instead signify the athlete is ready and appropriate for strength testing. While no specific IKDC score has been established as the ideal criterion value for RTS readiness in a population of young athletes after ACLR, the findings of this study indicate that scores higher than 94.8 may be associated with quadriceps strength limb symmetry values that meet current RTS recommendations. Should an athlete self-report an IKDC score lower than 90, the athlete will likely also demonstrate unacceptable quadriceps strength limb symmetry for successful RTS. We strongly advocate limb symmetry testing to determine whether strength deficits are contributing to lower self-reports of knee function. Due to the moderate specificity of the IKDC found in this study, a score higher than 94.8 does not discernibly indicate that the young athlete will meet current quadriceps strength limb symmetry RTS criteria. Similar conclusions were made by Logerstedt and colleagues,32 who found that among athletes with IKDC scores within normal limits at 12 months after ACLR, only 61.8% were able to successfully pass a battery of RTS tests. The results of this study prompt further investigation regarding the ability of the IKDC to be used as a screening tool for additional variables used in RTS decision making. Future
research should explore the interaction of the IKDC with other commonly used objective RTS assessments such as postural stability,6,42 functional hop testing,1 and landing biomechanics.11 Furthermore, clinical decision making may be enhanced by examination of the specificity of the individual IKDC items, in addition to those from other knee-specific patient-reported outcomes, as predictors of quadriceps strength. To better predict quadriceps strength, clinicians may benefit from a patient-reported outcome tool developed from items found to have the highest sensitivity and specificity. It is important to identify limitations for the results of this study. First, the study sample consisted of young, active athletes due to the high prevalence of ACL injury seen in this population, including some skeletally immature individuals who may have undergone unique surgical management. A consequence of focus on this population may include low generalizability of results to other demographics, especially older individuals or those not returning to cutting and pivoting sports. Second, a modified form of the IKDC, the pedi-IKDC, has been developed specifically for a young demographic. This study did not investigate the predictive abilities of the pedi-IKDC since research has indicated the IKDC is a valid and consistent knee-specific patient-reported outcome in a population of active individuals aged 6 to 18 years,51 and it is used more consistently in clinical settings. However, future research may be helpful to determine the associations between pedi-IKDC scores and quadriceps strength performance at RTS. Third, the dichotomization of participants into high and low IKDC groups resulted in a significant difference in mean age between groups. Further investigation of the relationship between age and self-reported function may provide beneficial insight for future studies of this population.
CONCLUSION The findings of this study present the clinical utility of the IKDC as a simple and accessible screening tool for residual quadriceps strength asymmetries at time of RTS after ACLR. While we recommend further research to support and strengthen our findings, this study provides early evidence of the predictive abilities of the IKDC on quadriceps strength and limb symmetry after ACLR.
ACKNOWLEDGMENT The authors acknowledge the ongoing support of the ACLR Outcome Project from the faculty and staff within the Division of Sports Medicine and the Division of Occupational Therapy and Physical Therapy at Cincinnati Children’s Hospital Medical Center. REFERENCES 1. Adams D, Logerstedt DS, Hunter-Giordano A, Axe MJ, SnyderMackler L. Current concepts for anterior cruciate ligament reconstruction: a criterion-based rehabilitation progression. J Orthop Sports Phys Ther. 2012;42(7):601-614.
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