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. Author manuscript; available in PMC: 2021 Sep 1.
Published in final edited form as: J Orthop Sports Phys Ther. 2020 Aug 1;50(9):523–530. doi: 10.2519/jospt.2020.9407

Secondary injury prevention program may decrease contralateral ACL injuries in female athletes: 2-year injury rates in the ACL-SPORTS randomized control trial

Jessica L Johnson 1,2, Jacob J Capin 2,3,4, Amelia JH Arundale 2,5, Ryan Zarzycki 2,6, Angela H Smith 2, Lynn Snyder-Mackler 1,2
PMCID: PMC7484246  NIHMSID: NIHMS1619233  PMID: 32741328

Abstract

Objective:

To determine if the addition of perturbation training to a secondary injury prevention program reduced the rate of second anterior cruciate ligament (ACL) injuries compared to the prevention program alone.

Design:

single-blinded randomized control trial

Methods:

Thirty-nine female athletes who intended to return to cutting/pivoting sports were enrolled 3-9 months after primary ACLR. Athletes were randomized into either progressive strengthening, agility, plyometric, and prevention (SAPP) or SAPP plus perturbation training (SAPP+PERT) groups; each had ten sessions over 5 weeks. Occurrence and side of second ACL injury were recorded for two years after primary ACLR.

Results:

There were nine second ACL injuries in the two years after ACLR. There was no statistically significant difference in rate or side of second ACL injury between the SAPP+PERT and SAPP groups.

Conclusion:

Adding perturbation training to a secondary ACL injury prevention program did not affect the rate of second ACL injuries in female athletes

Keywords: ACL, second injury prevention, female athletes, young athletes, perturbation, return to sport

INTRODUCTION

The incidence of primary ACL reconstruction (ACLR) is on the rise, with a 77% increase for women and 19% increase for men over a twelve year period.9 Female athletes have a higher incidence of ACL injuries in the comparable sports of basketball, soccer, and lacrosse2 compared to male athletes. Athletes who return to cutting and pivoting sports after ACLR have increased odds of graft rupture and contralateral injury compared to those who return to less strenuous sports.47 Up to one in three athletes who return to sport may sustain a second ACL injury, nearly half of those within two months of returning to sport19. Female athletes have a higher contralateral injury rate compared to males31,34,46, with the reported risk of a contralateral ACL injury as high as six times more likely compared to male athletes (26% versus 5% respectively)31.

While younger athletes are more likely to return to their pre-injury levels of sport4,21,47, athletes under 20 years-old have six times increased odds for a graft rupture and three times increased odds for a contralateral tear compared to older athletes.47 A systematic review of athletes aged 6-19 years undergoing ACLR found an overall second ACL injury rate of 27%21. Young female athletes have an even higher rate of second ACL injury40, up to 32%46.

When an important marker of success (return to their previous level of sport) is also a key risk factor for second ACL injury, clearly there is a need for targeted secondary ACL injury prevention and return to sport (RTS) training.. Current clinical practice guidelines for primary prevention of knee and ACL injuries5 recommend preventative training programs include a combination of neuromuscular training, strengthening, balance, and proximal control exercises44. The most effective post-operative training programs for returning to pre-injury level of function and reducing the risk of reinjury include quadriceps strengthening and neuromuscular training for 9-12 months18,22,27. Neuromuscular training techniques, such as perturbation training, designed to induce compensatory changes in muscle activation patterns and facilitate dynamic joint stability38, improve self-reported knee function more than strength training alone in the first six months after ACLR37. Perturbation training improves knee stability through adaptations in neuromuscular control with potential destabilizing activities about the knee15. It is unclear how female athletes respond to post-operative perturbation training.

The Anterior Cruciate Ligament-Specialized Post-Operative Return to Sports (ACL-SPORTS) training program is a sport-specific secondary ACL injury prevention program.50 ACL-SPORTS included progressive strengthening, agility, plyometric, and prevention (SAPP) exercises. The program is effective for preventing secondary ACL injury in men, with only one graft rupture in 40 male athletes6. However, the second ACL injury prevention effects in women have yet to be explored.

The purpose of this study was to determine if adding perturbation training to a second injury prevention program was more effective than the prevention program alone in reducing second ACL injury rates in female athletes after ACLR. We hypothesized that female athletes who received perturbation training in addition to the second injury prevention program would have fewer graft ruptures and fewer contralateral ACL injuries compared to those who received the prevention program alone.

METHODS

White et al50 previously published the methods of the ACL-SPORTS single-blinded randomized controlled trial which was approved by the University of Delaware Institutional Review Board and registered at clinicaltrials.gov (NCT01773317), with funding provided by the National Institute of Child Health and Human Development (NICHD) AR048212. This analysis is part of the a priori secondary outcomes for this trial. Prior to enrollment, all athletes gave written consent (assent if under 18 years with parent/guardian consent). The CONSORT diagram is in Figure 1. This analysis achieves objective (a) in patient/athlete/public partner involvement in the research: addressing outcomes deemed important by patients.

FIGURE 1.

FIGURE 1.

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CONSORT flow diagram for ACL-SPORTS female athletes

Participants

Participants were recruited from the local community through physician and physical therapist referral, newspaper and flyer advertisements, and word of mouth, with 40 female athletes enrolled from December 2011 through January 2017 from 17 surgeons. Selection criteria were: age 13-55 years, participated in and planning to return to a cutting/pivoting/jumping sport more than 50 hours per year, no previous ACL injury, and no history of other major lower extremity injury/surgery. Participants must have had a unilateral ACLR with no grade III concomitant ligament injuries or cartilage defects larger than 1cm2.

Surgical technique, graft choice, and rehabilitation prior to enrollment were not controlled. At enrollment, participants were screened by a physical therapist, and had no knee pain, minimal to no knee effusion, and full knee range of motion. They were less than nine months after ACLR, had ≥80% quadriceps index (QI), initiated a running progression, and not yet returned to Level I/II sport. Athletes were randomized to SAPP or SAPP plus perturbation (SAPP+PERT) using a random number generator by a research coordinator (MC). All researchers performing data collection were blinded.

All participants completed training. However, the researchers received information that one athlete (SAPP+PERT group) may not have had an intact ACL graft at enrollment. Therefore, we excluded her data from all analyses. All participants were required to pass objective return to sport criteria19,49. Participants returned to the clinic at 1- and 2-years after surgery for functional and clinical testing and patient reported outcomes. Those who were unable to return in person at two years (n=3) were contacted by phone. Self-reported second ACL injury status was collected for all 39 participants, as well as time from surgery to RTS, time from surgery to second ACL injury, and time from RTS to second ACL injury. Additionally, 100% of participants returned to sport by two years, 87% at their pre-injury level of sport.11

Training

Training occurred twice a week for five weeks under the supervision of a physical therapist at the University of Delaware Physical Therapy Clinic. Perturbation exercises used a platform/roller board combination, unilateral stance on a roller board, and unilateral stance on a tilt board, each with therapist perturbations in multiple planes (Figure 2); a full list and description of all training exercises can be found in White et al50. Training also included education and cuing for correct technique of all exercises especially avoiding valgus collapse during landings; progression was determined according to soreness and effusion guidelines1,13,50. All participants were required to pass the following RTS criteria before beginning return to sport: ≥90% QI and four single legged hop limb symmetry index (LSI), ≥90% scores on the Knee Outcomes Survey-Activities of Daily Living Subscale (KOS-ADLS) and a single item global gating of perceived knee function (GRS), and obtain surgeon approval.

FIGURE 2:

FIGURE 2:

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Perturbation exercises performed by the SAPP+PERT group

A: Platform and roller board, B: Unilateral roller board, C: Unilateral tilt board

Age

Since younger age at primary ACL injury increases the risk of a second injury21,40,46,47, we divided our athletes as those under 25, those under 20, and those under 18.

Statistics

We compared rate and side of second ACL injury between the two groups using chi-square tests of proportions and time from primary surgery to second ACL injury using independent t-tests (alpha = 0.05) with SPSS (IBM Corp, Armonk NY). To compare to previously published literature, we categorized the rate and side of second ACL injury by age, independent of group assignment and calculated chi-squared tests of proportions for each age category. Power was calculated a priori for the primary outcomes of the trial (biomechanical and clinical and functional outcomes), and the study was adequately powered50.

RESULTS

Thirty-nine female athletes were enrolled between December 2011 and January 2017. There were no differences in any demographics between groups at enrollment (Table 1). Summary data relevant to this study are included in this article, with full data available upon request from the study primary investigator, Lynn Snyder-Mackler (smack@udel.edu). Please include how proposed data will be used.

TABLE 1:

Demographics of participants at enrollment by group

Demographic SAPP (n=20) SAPP+PERT (n=19) p value
Age at primary surgery
Mean ± SD, range (years)		 18.9 ± 5.8
(14.0-53.7)		 19.0 ± 8.8
(12.7**-54.0)		 0.99
Height at enrollment (m) 1.65 ± 0.06 1.65 ± 0.08 0.82
Weight at enrollment (kg) 68.8 ± 10.9 67.9 ± 14.3 0.83
Graft type 8 patellar tendon autografts
8 hamstring autografts
4 allografts		 8 patellar tendon autografts
10 hamstring autografts
1 allografts		 0.32
Time from surgery to passing RTS criteria (weeks)
mean ± SD (range)		 37.0 ± 11.4
(18.4-63.0)		 37.0 ± 12.1
(20.3-54.0)		 0.99
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No comparisons were significant at the p≤0.05.

**

Subject was 13.5 years at enrollment

Second ACL injury

There were 9 second ACL injuries within two years of ACLR in the women of the ACL-SPORTS trial: 4 graft ruptures and 5 contralateral injuries, for an overall second ACL injury rate of 23% (Table 2). All second ACL injuries occurred in athletes with a hamstring autograft. There were no group differences in rate (p=0.77) or side (p=0.25) of second ACL injury, thus the groups were collapsed for additional comparisons. Post-hoc analysis revealed an effect size of w=0.047 for power of (1-β)=0.059.

TABLE 2:

By group comparisons for SAPP versus SAPP+PERT

SAPP (n=20) SAPP+PERT (n=19) p value
Second ACL injuries 5 (25%) 4 (21%) 0.77
Side of second ACL injury 4 contralateral (20%)
1 graft rupture (5%)		 1 contralateral (5%)
3 graft ruptures (16%)		 0.25
Mechanism of second ACL injury 3 non-contact
(2 contralateral, 1 graft rupture)
1 direct contact (contralateral injury)
1 contact to body (contralateral injury)		 4 non-contact
(1 contralateral, 3 graft ruptures)
Time from surgery to second ACL injury (weeks)
mean ± SD (range)		 50.3 ± 6.6
(42.3-56.6)		 69.9 ± 24.8
(34.7-87.7)		 0.13
Time from passing RTS criteria to second ACL injury (weeks)
mean ± SD (range)		 19.4 ± 4.45
(14.14-21.72)		 40.9±24.7
(14.14-62.14)		 0.09
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No comparisons were significant at p≤0.05

Age

The second ACL injuries by age are in Table 3. Eight of the second ACL injuries occurred in women under 18 at primary surgery; all 9 occurred in those under 20 years at primary surgery. However, there was not a statistically significant difference in rate of second ACL injury by age category. Results by age, with comparison to previous literature, are in Table 4

TABLE 3:

Rates of second ACL injury by age

Age group Graft ruptures Contralateral ACL Overall p value
<25 (n=35) 4 (11.4%) 5 (14.2%) 9 (25.7%) 0.25
<20 (n=32) 4 (12.5%) 5 (15.6%) 9 (28.1%) 0.11
<18 (n=26) 3 (11.5%) 5 (19.2%) 8 (30.8%) 0.11
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TABLE 4:

Comparison between rates of second injury for female athletes of the ACL-SPORTS trial matched by age to previous literature.

ACL-SPORTS Paterno 2012 Paterno 2014 ACL-SPORTS Webster 2016 ACL-SPORTS Webster 2016
Age category <25 years old <20 years old <18 years old
Sample size (female only) 35 42 59 32 116 26 85ǂ
Overall Second injury 22.8% 33.3% 32.2% 28.1% 35% 30.8% 31.8%
Graft rupture 11.4% 7.1% 8.5% 12.5% 12% 11.5% 12.9%
Contralateral rupture 14.2% (11.4% without contact injuries) 26.2% 23.7% 15.6% 17% 19.2% 18.8%
Surgeon/PT clearance for RTS? YES YES YES YES Not reported YES Not reported
Objective RTS criteria? YES NO NO YES Variable YES Variable
Follow-up time 2 years after ACLR 1 year after RTS 2 year after RTS (approx. 32.3 months) 2 years after ACLR Mean 5 years after ACLR (3-10 years) 2 years after ACLR Mean 5 years after ACLR (3-10 years)
Surgery to RTS (mean) 8.8±2.6 months Not reported 8.3±2.0 months 8.8±2.6 months Not reported 8.7±2.7 months Not reported
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ǂ

Personal communication from Kate Webster, November 5, 2019.

Results from this analysis are in grey columns, other published literature is in white.

DISCUSSION

The purpose of this secondary outcomes analysis was to determine if adding perturbation training to a second injury prevention program was more effective than the prevention program alone in reducing second ACL injury rates in female athletes after ACLR. There was not a statistically significant difference in rate or side of second ACL injury between those who received SAPP+PERT and those who received SAPP alone, so we collapsed the groups to determine any differences in outcomes with our injury prevention program compared to the existing literature.

GRAFT RUPTURE

The graft rupture rate in our study is comparable, or slightly higher than previous research (see Table 4 for comparisons). There are many risk factors for graft rupture, including younger age at primary injury21,40,46,47, return to a cutting/pivoting/jumping sport45,47, and graft type26,29,35. Almost half the athletes in our study had a hamstring autograft, and all graft ruptures occurred in those with a hamstring graft; Paterno et al did not report graft types31,34. Hamstring grafts have slightly higher rates of failure than bone-patellar tendon-bone (BPTB) grafts35,36,39. Athletes who had ACLR with hamstring autografts achieved impairment resolution earlier and returned to sports on average 4 months earlier than those with BPTB autografts. Therefore, biological healing may have played a role in the graft failure41. Because age, time to return to sport, and rate of return to cutting/pivoting sports were comparable, possible differences in graft selection may account for the differences between our athletes and those reported by Paterno et al.

CONTRALATERAL INJURY

The contralateral ACL injury rate in our study is lower or comparable to previous research (see Table 4 for comparisons). The lower rates of contralateral ACL injuries may be due to the bilateral training in the ACL-SPORTS training program: all agility drills and a majority of the plyometric and strengthening activities were performed in both limbs. With similar altered movement patterns and impairments predicting primary and secondary ACL injuries8,20,32, if poor mechanics and movement patterns were at fault in primary ACL injury, similar mechanics and movement patterns may exist in the contralateral limb. Additionally, the uninvolved limb may also develop altered mechanics as compensation for the injured limb10,33,51. Neuromuscular training can improve impairments7,38 and movement patterns12,28. The ACL-SPORTS training emphasis on proper landing technique and movement patterns during agilities, plyometric and performance activities bilaterally50 may explain the lower contralateral injury rate in our study.

RETURN TO SPORT

Because athletes in our study had the highest rate of return to sport (100% returned to sport, 87% to preinjury level11) reported in the literature4,47, they also had greater sports exposure and subsequently, higher risk of any second ACL injury45,47. Yet, the rate of second ACL injury in our study was not higher than previous research46,47. Webster et al did not report the rate of returning to cutting/pivoting sports46,47 and neither Paterno et al31,34 nor Webster et al46,47 reported any control of their participants’ rehabilitation. Thus, the similarities in injury rates across different studies may reflect a positive effect of our intervention.

POST-SURGICAL FOLLOW-UP

We registered new injuries in the first 2 years after ACL reconstruction. Over half of all second injuries occur in the first year after ACLR46,47, and more than three quarters occur within 2 years of surgery46. There were minimal differences in rates of second injuries from 1- to 2-years after return to sport31,34, with a mean time from RTS to second injury of 7.0 months34. In the female athletes of the ACL-SPORTS trial, average time from surgery to RTS was 8.5 months, giving an average follow-up of 15.5 months after RTS. Therefore, our 2-year registration period should be sufficient to capture most second ACL injuries in our cohort.

RTS CRITERIA

Passing RTS criteria can reduce risk of second ACL injuries14,19,24, but there is conflicting evidence about the efficacy and impact of these criteria25,30. All athletes in the ACL-SPORTS trial were required to pass objective criteria and have surgeon approval for RTS: ≥90% QI and four single legged hop limb symmetry index (LSI), ≥90% scores on the Knee Outcomes Survey-Activities of Daily Living Subscale (KOS-ADLS) and a single item global gating of perceived knee function (GRS). In the cohorts reported by Paterno et al, athletes were required to have surgeon and physical therapist approval for RTS but did not have to meet any objective criteria before release31,34. The study cohort by Webster et al reported criteria including running and squatting, but did not provide objective thresholds for passing46.

While we cannot separate the impact of our RTS criteria on rate of second ACL injury from the impact of the training program, these findings suggest that the ACL-SPORTS training program with objective RTS criteria may reduce risk of second ACL injury in female athletes, however, it is not enough to reduce the risk of second ACL injury in our youngest female athletes.

CLINICAL IMPLICATIONS

The reduction in contralateral ACL injury rate in our female athletes compared to other published research is promising, especially with an easy to implement training program. However, our overall second injury rate of 26% is still much too high to believe we have addressed the needs of our athletes. While the ACL-SPORTS training was highly effective in reducing second ACL injury rates in male athletes (1 second injury in 40 athletes)6, it was not as effective in female athletes. Current rehabilitation programs are not meeting all the needs of female athletes, particularly those under 18 years. High compliance to a neuromuscular training program is associated with a lower rate of ACL injuries in female athletes42. Higher volume and more frequent, longer duration sessions are effective for primary knee injury prevention43. However, ten sessions over five weeks, as in our study, may not maximize the benefits of training for female athletes. Additional, research on longer, higher intensity, more frequent secondary prevention programs for female athletes are needed, as well as research on the influence of types of feedback16,17 and psychological readiness to return to sport3,23,48.

STRENGTHS AND LIMITATIONS

A strength of this study is our sample. We recruited from a variety of surgeons and participants had post-operative rehabilitation at multiple physical therapy clinics, making our results generalizable. Reasonable enrollment criteria ensured that all participants entered the study at an appropriate point to begin the return to sport progression, but were not overly burdensome.

Athletes self-reported return to sport and level of participation and we did not assess number of athletic exposures (practice/games). Because all athletes were required to meet RTS criteria, we are unable to separate the effects of the training program from the RTS criteria. Additionally, while a majority of second ACL injuries occur within two years of ACLR, 2 years may not be sufficient follow-up to capture the true rate of contralateral ACL injuries, which may occur later after ACLR.

While our sample of n=39 is small, Type II error is unlikely. Post-hoc analysis revealed an effect size of w=0.047 for power of (1-β)=0.059. We would have needed more than 3000 participants to be adequately powered to detect a between group difference.

CONCLUSIONS

While the addition of perturbation training to a secondary injury prevention program does not seem to have benefits for female athletes, the participants in the ACL-SPORTS training program report fewer contralateral injuries compared to previously published results.

KEY POINTS.

Findings

The addition of perturbation training did not affect the rate of second ACL injuries in female athletes.

Implications

The common core elements of the ACL-SPORTS training program (progressive strength, agility, plyometrics, and performance) may reduce the risk of contralateral ACL injury in female athletes as part of an easily implemented return-to-sports phase training program.

Caution

We had a small sample and assessed second injury up to 2 years after primary ACLR, which may not be long enough to capture all secondary injuries. Additionally, we did not evaluate or control for athletic exposures.

Acknowledgments:

Thank you to Martha Callahan and the Delaware Rehabilitation Institute for their assistance with patient recruitment, scheduling, and data management. Thank you to Kathleen White Cummer for all her work on this study. Thank you to the University of Delaware Physical Therapy Clinic, Celeste Dix, PT, DPT, and our other undergraduate and graduate lab assistants.

The study was approved by the University of Delaware Institutional Review Board and registered at clinicaltrials.gov (NCT01773317), with funding provided by NICHD R01-AR048212. JJC received funding from NIH F30-HD096830 and Foundation for Physical Therapy Research Promotion of Doctoral Studies (PODS) Level I&II scholarships. JJC’s postdoctoral training is funded by an Advanced Geriatrics Fellowship from the Eastern Colorado Veterans Affairs Geriatric Research Education and Clinical Center.

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