Journal of Sport Rehabilitation, 2015, 24, 36-46 http://dx.doi.org/10.1123/jsr.2013-0110 © 2015 Human Kinetics, Inc.
www.JSR-Journal.com ORIGINAL RESEARCH REPORT
Clinical Thresholds for Quadriceps Assessment After Anterior Cruciate Ligament Reconstruction Christopher Kuenze, Jay Hertel, Susan Saliba, David R. Diduch, Arthur Weltman, and Joseph M. Hart Context: Normal, symmetrical quadriceps strength is a common clinical goal after anterior cruciate ligament reconstruction (ACLR). Currently, the clinical thresholds for acceptable unilateral quadriceps function and symmetry associated with positive outcomes after return to activity are unclear. Objective: To establish quadriceps-activation and knee-extension-torque cutoffs for clinical assessment after return to activity after ACLR. Design: Descriptive laboratory study. Setting: Laboratory. Patients: 22 (10 female, 12 male; age = 22.5 ± 5.0 y, height = 172.9 ± 7.1 cm, mass = 74.1 ± 15.5 kg, months since surgery = 31.5 ± 23.5) recreationally active persons with a history of unilateral, primary ACLR at least 6 months prior and 24 (12 female/12 male, age = 21.7 ± 3.6 y, height = 168.0 ± 8.8 cm, mass = 69.3 ± 13.6 kg) recreationally active healthy participants. Main Outcome Measures: Patient-reported measures of pain, knee-related function, and physical activity level were recorded for all participants. Normalized knee-extension maximum-voluntary-isometric-contraction (MVIC) torque (Nm/kg) and quadriceps central-activation ratio (CAR, %) were measured bilaterally in all participants. Receiver-operator-characteristic (ROC) curves were used to establish thresholds for unilateral measures of normalized knee-extension MVIC torque and quadriceps CAR, as well as limb-symmetry indices (LSI). ROC curves then established clinical thresholds for normalized knee-extension MVIC torque and quadriceps CAR LSIs associated with healthy knee-related function. Results: Involved-quadriceps CAR above 89.3% was the strongest unilateral indicator of healthy-group membership, while quadriceps CAR LSI above 0.996 and knee-extension MVIC torque above 0.940 were the strongest overall indicators. Unilateral normalized knee-extension MVIC torque above 3.00 Nm/kg and quadriceps CAR LSI above 0.992 were the best indicators of good patient-reported knee-related outcomes. Conclusions: Threshold values established in this study may provide a guide for clinicians when making return-to-activity decisions after ACLR. Normalized knee-extension MVIC torque (>3.00 Nm/kg) and quadriceps CAR symmetry (>99.6%) are both strong indicators of good patient-reported outcomes after ACLR. Keywords: knee-extension torque, central-activation ratio, ACLR rehabilitation Normal quadriceps function is a common yet frustrating clinical goal after anterior cruciate ligament (ACL) reconstruction (ACLR).1–3 Force-based measures of quadriceps strength have commonly been used as indicators of functional improvement and readiness for a return to recreational activity.4 It is often difficult for clinicians to confidently estimate return to full function due to a lack of preinjury data and clearly established thresholds related to positive patient-reported outcomes and long-term joint health in the active population. In most cases, assessments of quadriceps function are compared between the injured and noninjured limb, which limits the clinician’s ability Kuenze is with the Dept of Kinesiology and Sport Sciences, University of Miami, Coral Gables, FL. Hertel, Saliba. Weltman, and Hart are with the Dept of Human Services, and Diduch, the Dept of Orthopaedic Surgery, University of Virginia, Charlottesville, VA. Address author correspondence to Christopher Kuenze at [email protected]. 36
to asses unilateral quadriceps dysfunction due to the fact that the uninvolved limb may also be experiencing quadriceps dysfunction after ACLR.5,6 These between-limbs comparisons or comparisons with established normative values for quadriceps function have been effective in identifying individuals with a history of injury; however, a better understanding of threshold values for quadriceps strength and activation that are associated with positive outcomes may be helpful for clinical decision making.1,7 Establishing criteria for positive outcomes after ACLR is an evolving multifactorial area of research that includes physiologic and patient-reported components.8–11 Clinicians commonly focus on a return to preinjury levels of neuromuscular function, physical performance, and level of physical activity to determine whether a patient has achieved a positive outcome at the time of return to activity.12 Patient-reported measures such as the Knee Osteoarthritis Outcome Score (KOOS) have been most often used to track patient outcomes over the course of rehabilitation, as well as many years after return
Clinical Quadriceps-Function Thresholds After ACLR 37
to activity.13 The KOOS is a simple patient-completed questionnaire that has been shown to correlate well with measures of physical function,13 as well as knee-joint health.14 However, it remains unclear if common clinical measures such as quadriceps strength and symmetry can indicate which patients may be more likely to report positive knee-related function via measures such as the KOOS. Previous reports have established a unilateral quadriceps activation of 95.0%7 and a limb-symmetry index (LSI) of 80.0%15 as indicative of acceptable quadriceps function after ACL injury. These thresholds have been supported in the literature, but it remains unclear whether a level of quadriceps activation and symmetry can be predictive of good patient-reported outcomes after ACLR.7,15,16 In addition, most investigations in this area have focused on laboratory measures of quadriceps activation without establishing specific thresholds for more clinically available measures of quadriceps strength, such as isometric knee-extension torque. Clear clinical thresholds for quadriceps strength and activation may enable clinicians to make informed and evidence-based decisions regarding rehabilitation progress and clearance for return to activity after ACLR. Therefore, the primary purpose of
this study was to establish clinically relevant thresholds for quadriceps strength and activation that are indicative of positive patient-reported outcomes after ACLR.
Methods Participants Twenty-two (10 female, 12 male) ACL-reconstructed and 24 (12 female, 12 male) healthy volunteers participated in this study (Table 1). Participants were included if they were between the ages of 18 and 40 years, had a body-mass index less than 35, and reported that they were recreationally active via the Godin Leisure-Time Exercise Questionnaire in accordance with American College of Sports Medicine guidelines (exercised at least 3–5 times a week at a moderate intensity for no less than 30 min).17 Participants were excluded if they had a history of lower-extremity injury resulting in altered activity level or evaluation by a medical professional within the last 6 weeks. Participants were also excluded if they had any history of diagnosed neurological disorder. In addition, participants in the healthy control group were excluded if they had a history of lower-extremity surgery or neu-
Table 1 Participant Demographics Healthy
ACL reconstruction
P
Age (y)
21.7 ± 3.6
22.5 ± 5.0
.58
Gender
12 male, 12 female
12 male, 10 female
.97
Height (cm)
168.0 ± 8.8
172.9 ± 7.1
.05
Weight (kg)
69.3 ± 13.6
74.1 ± 15.5
.27
Body-mass index
24.3 ± 3.2
24.6 ± 4.0
.81
Visual analog scale for pain while squatting (cm)
0.1 ± 0.3
0.3 ± 0.5
.23
KOOS pain
99.4 ± 1.4
92.6 ± 7.7
www.JSR-Journal.com ORIGINAL RESEARCH REPORT
Clinical Thresholds for Quadriceps Assessment After Anterior Cruciate Ligament Reconstruction Christopher Kuenze, Jay Hertel, Susan Saliba, David R. Diduch, Arthur Weltman, and Joseph M. Hart Context: Normal, symmetrical quadriceps strength is a common clinical goal after anterior cruciate ligament reconstruction (ACLR). Currently, the clinical thresholds for acceptable unilateral quadriceps function and symmetry associated with positive outcomes after return to activity are unclear. Objective: To establish quadriceps-activation and knee-extension-torque cutoffs for clinical assessment after return to activity after ACLR. Design: Descriptive laboratory study. Setting: Laboratory. Patients: 22 (10 female, 12 male; age = 22.5 ± 5.0 y, height = 172.9 ± 7.1 cm, mass = 74.1 ± 15.5 kg, months since surgery = 31.5 ± 23.5) recreationally active persons with a history of unilateral, primary ACLR at least 6 months prior and 24 (12 female/12 male, age = 21.7 ± 3.6 y, height = 168.0 ± 8.8 cm, mass = 69.3 ± 13.6 kg) recreationally active healthy participants. Main Outcome Measures: Patient-reported measures of pain, knee-related function, and physical activity level were recorded for all participants. Normalized knee-extension maximum-voluntary-isometric-contraction (MVIC) torque (Nm/kg) and quadriceps central-activation ratio (CAR, %) were measured bilaterally in all participants. Receiver-operator-characteristic (ROC) curves were used to establish thresholds for unilateral measures of normalized knee-extension MVIC torque and quadriceps CAR, as well as limb-symmetry indices (LSI). ROC curves then established clinical thresholds for normalized knee-extension MVIC torque and quadriceps CAR LSIs associated with healthy knee-related function. Results: Involved-quadriceps CAR above 89.3% was the strongest unilateral indicator of healthy-group membership, while quadriceps CAR LSI above 0.996 and knee-extension MVIC torque above 0.940 were the strongest overall indicators. Unilateral normalized knee-extension MVIC torque above 3.00 Nm/kg and quadriceps CAR LSI above 0.992 were the best indicators of good patient-reported knee-related outcomes. Conclusions: Threshold values established in this study may provide a guide for clinicians when making return-to-activity decisions after ACLR. Normalized knee-extension MVIC torque (>3.00 Nm/kg) and quadriceps CAR symmetry (>99.6%) are both strong indicators of good patient-reported outcomes after ACLR. Keywords: knee-extension torque, central-activation ratio, ACLR rehabilitation Normal quadriceps function is a common yet frustrating clinical goal after anterior cruciate ligament (ACL) reconstruction (ACLR).1–3 Force-based measures of quadriceps strength have commonly been used as indicators of functional improvement and readiness for a return to recreational activity.4 It is often difficult for clinicians to confidently estimate return to full function due to a lack of preinjury data and clearly established thresholds related to positive patient-reported outcomes and long-term joint health in the active population. In most cases, assessments of quadriceps function are compared between the injured and noninjured limb, which limits the clinician’s ability Kuenze is with the Dept of Kinesiology and Sport Sciences, University of Miami, Coral Gables, FL. Hertel, Saliba. Weltman, and Hart are with the Dept of Human Services, and Diduch, the Dept of Orthopaedic Surgery, University of Virginia, Charlottesville, VA. Address author correspondence to Christopher Kuenze at [email protected]. 36
to asses unilateral quadriceps dysfunction due to the fact that the uninvolved limb may also be experiencing quadriceps dysfunction after ACLR.5,6 These between-limbs comparisons or comparisons with established normative values for quadriceps function have been effective in identifying individuals with a history of injury; however, a better understanding of threshold values for quadriceps strength and activation that are associated with positive outcomes may be helpful for clinical decision making.1,7 Establishing criteria for positive outcomes after ACLR is an evolving multifactorial area of research that includes physiologic and patient-reported components.8–11 Clinicians commonly focus on a return to preinjury levels of neuromuscular function, physical performance, and level of physical activity to determine whether a patient has achieved a positive outcome at the time of return to activity.12 Patient-reported measures such as the Knee Osteoarthritis Outcome Score (KOOS) have been most often used to track patient outcomes over the course of rehabilitation, as well as many years after return
Clinical Quadriceps-Function Thresholds After ACLR 37
to activity.13 The KOOS is a simple patient-completed questionnaire that has been shown to correlate well with measures of physical function,13 as well as knee-joint health.14 However, it remains unclear if common clinical measures such as quadriceps strength and symmetry can indicate which patients may be more likely to report positive knee-related function via measures such as the KOOS. Previous reports have established a unilateral quadriceps activation of 95.0%7 and a limb-symmetry index (LSI) of 80.0%15 as indicative of acceptable quadriceps function after ACL injury. These thresholds have been supported in the literature, but it remains unclear whether a level of quadriceps activation and symmetry can be predictive of good patient-reported outcomes after ACLR.7,15,16 In addition, most investigations in this area have focused on laboratory measures of quadriceps activation without establishing specific thresholds for more clinically available measures of quadriceps strength, such as isometric knee-extension torque. Clear clinical thresholds for quadriceps strength and activation may enable clinicians to make informed and evidence-based decisions regarding rehabilitation progress and clearance for return to activity after ACLR. Therefore, the primary purpose of
this study was to establish clinically relevant thresholds for quadriceps strength and activation that are indicative of positive patient-reported outcomes after ACLR.
Methods Participants Twenty-two (10 female, 12 male) ACL-reconstructed and 24 (12 female, 12 male) healthy volunteers participated in this study (Table 1). Participants were included if they were between the ages of 18 and 40 years, had a body-mass index less than 35, and reported that they were recreationally active via the Godin Leisure-Time Exercise Questionnaire in accordance with American College of Sports Medicine guidelines (exercised at least 3–5 times a week at a moderate intensity for no less than 30 min).17 Participants were excluded if they had a history of lower-extremity injury resulting in altered activity level or evaluation by a medical professional within the last 6 weeks. Participants were also excluded if they had any history of diagnosed neurological disorder. In addition, participants in the healthy control group were excluded if they had a history of lower-extremity surgery or neu-
Table 1 Participant Demographics Healthy
ACL reconstruction
P
Age (y)
21.7 ± 3.6
22.5 ± 5.0
.58
Gender
12 male, 12 female
12 male, 10 female
.97
Height (cm)
168.0 ± 8.8
172.9 ± 7.1
.05
Weight (kg)
69.3 ± 13.6
74.1 ± 15.5
.27
Body-mass index
24.3 ± 3.2
24.6 ± 4.0
.81
Visual analog scale for pain while squatting (cm)
0.1 ± 0.3
0.3 ± 0.5
.23
KOOS pain
99.4 ± 1.4
92.6 ± 7.7
