Abstract
Objectives:
In young athletes who undergo an anterior cruciate ligament reconstruction (ACLR), up to one-third will suffer a second injury. Sport and activity level, quadriceps strength, ability to single-leg hop, biomechanical movement patterns, and time to return-to-sport (RTS) clearance are among the known risk factors associated with ACL reinjury. Passing a battery of RTS testing has shown to reduce second injury rate by up to 84%. However, the feasibility of validated RTS batteries in most orthopedic clinics is poor, though recent steps have been made toward investigating more clinic-friendly testing methods. The purpose of this study is to compare muscle performance and drop vertical jump mechanics in a matched cohort of young athletes who, after passing a clinic-based RTS battery after primary ACLR, returned to sport with or without sustaining a second injury.
Methods:
This is a secondary analysis of 22 young athletes (age 16.2 ± 2.4 years, BMI 24.6 ± 4.7 kg/m2, 16 male, 6 female) selected from part of a prospective cohort study with 69 athletes. All 69 athletes from the original trial underwent RTS testing 5-15 months after primary ACLR, had no prior history of injury or surgery to either knee and were planning to return to >50 hours/year of cutting and pivoting sports. 53 of these athletes passed a clinic-based RTS battery of >90% limb symmetry during isometric quadriceps strength using a Klau crane scale at 90° knee flexion, 1-rep max knee extension (from 90° to 0°), and 4 single-leg hop tests (single, crossover, triple for distance, 6-meter timed), as well as >90% on the Global Rating Scale and International Knee Documentation Committee (IKDC) 2000 Subjective Knee Form. These athletes then underwent formal lab testing within 14 days of passing clinic testing. Lab testing included isometric quadriceps strength (90° knee flexion) and isokinetic concentric quadriceps and hamstring strength (60°/sec) using an electromechanical dynamometer (Biodex System 4 Pro, Shirley, NY). Additionally, biomechanical testing was performed with an 8-camera Qualisys system (240 Hz; Goteborg, Sweden) and 2 embedded Bertec force plates (2160 Hz; Columbus, OH) to measure knee joint angles and external moments normalized to body mass and height during 5 bilateral drop vertical jumps (BDVJ) and 5 unilateral drop vertical jumps (UDVJ). All athletes were followed for 1 year after passing clinic-based RTS testing for reinjury and RTS outcomes.
Nine athletes who returned to their preinjury level of sport within the first year after passing RTS testing sustained a second ACL injury to their ipsilateral (N=6) or contralateral (N=3) knee. Two additional athletes sustained a non-ACL second injury to their ipsilateral meniscus. The mean time to reinjury was 15.5 ± 4.9 months after ACLR. These 11 athletes were matched by sex, graft type, age, sport, competition level, and meniscus repair status to 11 athletes who returned to their preinjury level of sport without sustaining a second injury. Muscle performance variables of interest between groups included peak isometric quadriceps torque and rate of torque development (RTD) and peak isokinetic quadriceps and hamstring torque. BDVJ and UDVJ variables of interest during the first landing period included the knee abduction angle (KAA) and moment (KAM) at initial contact, peak knee flexion moment (pKFM) and KFM loading rate during landing, and knee flexion power (KFP) during the propulsion phase. Independent t-tests and chi-squared tests were used to compare demographics and baseline characteristics between groups. Muscle performance and DVJ biomechanics between limb and group were analyzed using 2x2 ANOVAs (a=0.05).
Results:
Demographics and baseline characteristics did not differ between matched groups (Table 1). Time to pass RTS testing (p=0.810) and return to preinjury sport level (p=0.203) did not differ between groups. There were no significant limb*group interaction effects for isometric peak quadriceps torque (p=0.985), isokinetic peak quadriceps torque (p=0.811) or isokinetic peak hamstring torque (p=0.472) (Table 2). However, athletes who sustained a second injury had a higher and more symmetric isometric quadriceps RTD from 0-100ms (p=0.017) (Table 2, Figure 1) but not from 100-200ms (p=0.096). During DVJ analysis, athletes who sustained a second injury had more asymmetric KAM at initial contact of the BDVJ (p=0.032). However, there were no significant interaction effects for KAA at initial contact (BDVJ: p=0.399; UDVJ: p=0.338), pKFM during landing (BDVJ: p=0.798; UDVJ: p=0.474), KFM loading rate during landing (BDVJ: p=0.345; UDVJ: p=0.726), or KFP during propulsion (BDVJ: p=0.905; UDVJ: p=0.562).
Conclusions:
This study found that more asymmetric frontal plane loading at initial contact during BDVJ, as well more symmetric and higher early quadriceps RTD, were associated with second knee injury. Asymmetries in knee frontal kinetics have previously shown to predict primary ACL injury. Our finding of higher early quadriceps RTD symmetry being associated with second injury does not have a clear explanation; however, one possible explanation may be that athletes with a higher early quadriceps RTD placed higher forces and demands through their injured knee upon returning to sport. Future work is needed to identify factors outside of standard clinical and laboratory assessments that contribute to second injury risk after ACLR.
