Psychosocial factors 3-months after anterior cruciate ligament reconstruction predict 6-month subjective and objective knee outcomes

2.1 Subjects

Recreational and competitive-level athletes who sustained a unilateral ACL injury and underwent ACLR were eligible for participation. Inclusion criteria included a unilateral ACLR, age between 15 and 45 years, time from injury to surgery of 4 months or less, and pre-injury Tegner Activity Rating of 5 or greater indicating participation in sports. Potential subjects were excluded if they had a history of a previous ACL injury/reconstruction or other lower extremity surgery, a total knee dislocation at the time of injury, or concomitant ligamentous injury requiring surgical repair. However, a concurrent meniscus repair or meniscectomy may have been performed at the time of ACLR.

All surgical procedures were performed by two board-certified orthopedic surgeons from the same orthopedic practice (D. L. J. or M. L. I.). Rehabilitation was not controlled in this study as subjects were referred to community-based physical therapists. However, all subjects were issued the same post-operative protocol that allowed for immediate weight-bearing with an assistive device and knee motion as tolerated. Following a protocol approved by the University of Kentucky Institutional Review Board, adult subjects gave written informed consent before participating in the study, whereas minor subjects gave written assent and the parent or guardian gave written informed consent.

2.2 Testing protocol

Testing time points were 3 months and 6 months post-operatively. To qualify to be tested at 6 months, subjects were required to be cleared by the orthopedic surgeon to return to running and to have initiated a running progression without increased symptoms. Demographic variables were collected 3 months after surgery and included age, sex, body mass index, pre-injury Tegner Activity Rating, and graft type. The psychosocial response measures were collected 3 months after surgery and the self-reported functional outcomes, quadriceps strength assessment, and three-dimensional running gait analysis were collected 6 months after surgery.

2.3 Psychosocial response measures

At 3 months after ACLR, psychosocial response was evaluated using the ACL Return to Sport after Injury (ACL-RSI) and Knee Self-Efficacy Scale (K-SES). The ACL-RSI and K-SES were chosen for this study as these psychosocial response measures are knee-specific instruments that were specifically designed for patients with an ACL injury.^{29, 30} The ACL-RSI is a 12-item scale that measures psychological readiness to return to sport with questions regarding emotions, confidence, and risk appraisal. Scores for each item are summed and then averaged for a total score between 0 and 100 with a higher total score indicating greater psychological readiness for sport.²⁹ The K-SES is a 22-item scale that measures self-efficacy, which is the belief in one's abilities to perform/execute an action, in the domains of daily activities, sports activities, physical function tasks, and knee function in the future. Scores for each item are summed and then averaged for a total score between 0 and 10 with a higher total score indicating greater knee self-efficacy.³⁰ Both the ACL-RSI^{29, 31, 32} and K-SES^{30, 33} have shown to be valid and reliable for ACL injury and reconstruction.

2.4 Self-reported functional outcomes

At 6 months after ACLR, self-reported functional outcomes were evaluated using the International Knee Documentation Committee (IKDC) and the Knee Injury and Osteoarthritis Outcome Score (KOOS) sports/recreational activities (KOOS_Sport) and quality of life (KOOS_QoL) subscales. The IKDC and KOOS are two of the most frequently used knee-specific outcome measures and contain items that have been shown to be frequently experienced and perceived as important by patients who have had an ACL injury.³⁴ The IKDC is a 10-item scale related to knee symptoms, physical function, and activity. The score ranges from 0 to 100 with higher scores indicating higher self-reported knee function and lower self-reported symptoms.³⁵ The KOOS_Sport is a 5-item scale related to physical function during activities including squatting, running, jumping, and turning/twisting, whereas the KOOS_QoL is a 4-item scale related to the impact of knee injury on daily function and quality of life. Compared to the other KOOS subscales, the items within the KOOS_Sport and KOOS_QoL subscales have been shown to be more meaningful to patients who have had an ACL injury.³⁴ Both subscales have a score range from 0 to 100 with higher scores indicating better function in sport/recreational activities and better knee-related quality of life, respectively.³⁶ The IKDC^{35, 37, 38} and KOOS^{34, 36} subscales have shown to be valid and reliable outcome measures for ACL injury and reconstruction.

2.5 Quadriceps strength assessment

At 6 months after ACLR, quadriceps strength was assessed bilaterally via maximum voluntary isometric contractions with a Biodex Multi-Joint System 4 Isokinetic Dynamometer (Biodex Medical Systems). Isometric quadriceps strength testing was chosen for this study as a prior review indicates that deficits are more apparent with isometric testing compared to isokinetic testing, especially at higher velocities.³⁹ Each subject performed a 5-second maximum isometric hold with the subject's knee placed at 90 degrees of flexion. Subjects performed one practice trial to allow for familiarization with the task, followed by four test trials. Maximum verbal encouragement was provided. Subjects were given 30 seconds rest between trials and 5 minutes rest between limbs. The non-operative limb was tested before the operative limb. For each limb, the peak torque was averaged from the four test trials and normalized to body weight. Quadriceps strength limb symmetry was then calculated as: ([operative limb/non-operative limb] × 100).

2.6 Three-dimensional gait analysis

At 6 months after ACLR, knee flexion excursion was assessed during a three-dimensional motion analysis of running. Running was chosen for this study since it is a demanding activity that requires single limb control and involves greater knee joint excursions than walking. Knee flexion excursion was of particular interest given the role of the quadriceps muscle in controlling knee motion during the stance phase of the gait cycle.⁴⁰ Furthermore, reduced knee flexion range of motion has been associated with reduced functional performance after ACLR.⁴¹

Using a previously reported marker set, 53 reflective markers were placed on the subject.⁴² Thirty-one of these markers were placed on anatomical landmarks, including sternal notch, spinous process of C7, bilateral superior acromion processes, posterior L5/S1 vertebral joint, bilateral greater trochanters, bilateral iliac crests, bilateral anterior superior iliac spines, bilateral posterior superior iliac spines, bilateral medial and lateral distal femurs, bilateral medial and lateral proximal tibias, bilateral medial and lateral malleoli, bilateral first and fifth metatarsal heads, and bilateral distal foot. Twenty-two tracking markers were attached including four rigid plates secured to bilateral thigh and shank with four markers on each plate, as well as three tracking markers identifying proximal, distal, and lateral heels were secured to the rear foot of each shoe. To minimize the influence of footwear, all subjects wore neutral running shoes (New Balance WR662; New Balance Athletic Shoe Inc.).

After allowing a 5 minute warm-up, the subjects ran at a self-selected test speed (2.44 ± 0.38 m/s) for 2 minutes on an instrumented treadmill (Bertec). Marker trajectories were collected over 10 second trials during the latter half of the running segment with a 10 camera motion analysis system (Motion Analysis Corp) at 200 Hz. Filtering and joint angle calculations were performed using Visual3D software (C-motion). Marker trajectories were filtered at 8 Hz using a fourth-order, low-pass, zero-lag Butterworth filter. The knee angle was calculated using Cardan X–Y–Z angle rotation and referencing the shank coordinate system to the thigh coordinate system. Knee flexion excursion, defined as the difference between the knee flexion angle at initial contact and peak knee flexion angle during stance phase, was extracted using custom LabView code (National Instruments). Knee flexion excursion, averaged from five individual strides, was analyzed during the stance phase for the operative and non-operative limbs in each subject. Knee flexion excursion limb symmetry was then calculated as: ([operative limb/non-operative limb] × 100).

2.7 Statistical analyses

Statistical analysis was performed with SPSS Statistics Version 25.0 (IBM Corp). Descriptive statistics were generated for subject demographic data, 3-month psychosocial response variables (ACL-RSI and K-SES), and 6-month knee function variables (IKDC, KOOS_Sport, KOOS_QoL, quadriceps strength limb symmetry, and knee flexion excursion limb symmetry).

First, univariate analyses were performed using Pearson product moment correlation coefficients to assess the relationships between the 3-month psychosocial response variables and the 6-month knee function variables. Multivariate analyses were then performed using forward linear regression to determine if both of the 3-month psychosocial response variables were significant in multivariate models for the 6-month knee function variables. Statistical significance was defined as p ≤ 0.05 for all analyses.

4.1 Psychosocial factors and self-reported functional outcomes

To date, there have been inconclusive findings as to the association between psychosocial factors and self-reported function early in the rehabilitation process.^{18, 43-45} The lack of consistency could be due to the psychosocial variable studied, as well as the time point selected. For instance, Chmielewski et al.⁴⁴ found that fear of movement/re-injury was associated with IKDC scores between 6 and 12 months after surgery, but not within the first 3 months or between 3 and 6 months after surgery. By comparison, other work has found that increased self-efficacy and decreased fear of movement are associated with increased self-reported knee function within the first 3 months after surgery.⁴⁵ However, studies have shown that baseline psychosocial response is not predictive of 3-month⁴⁵ or 6-month⁴³ outcomes. Our findings add to this body of literature by showing that psychological readiness for sport and knee self-efficacy assessed 3 months after surgery were positively associated with the IKDC, KOOS_Sport, and KOOS_QoL at 6 months after surgery. It is important to note that in the multivariate model, both the ACL-RSI and K-SES were significant predictors of the IKDC; however, only the ACL-RSI was found to be predictive of the KOOS_Sport and KOOS_QoL. Depending on the time frame studied, different psychosocial constructs may play a larger role than others when it comes to an athletes' self-reported function.

The research to date has highlighted that post-surgery psychosocial response is not static.^{43, 46, 47} Our data indicates that one potential important time point for assessing psychosocial response is 3 months after surgery. This time point marks a transition that can often be met with fear, nervousness, and lack of confidence as it is when athletes are typically able to commence impact drills, such as double limb jumping tasks and running. However, further investigations are needed with comprehensive longitudinal assessments of several psychosocial constructs throughout the rehabilitation process to evaluate which, and when, these factors have a significant role in an athletes' self-reported function.

4.2 Psychosocial factors and quadriceps strength

Most of the research to date on the relationship between psychosocial response and quadriceps strength after ACLR has focused on later time points (i.e., 6–12 months post-surgery). These studies have identified that lower fear of re-injury,^{21, 26} greater psychological readiness to return to sport,⁴⁸ and higher levels knee self-efficacy⁴⁹ are associated with greater quadriceps strength at the time of return to sport. The results of this study expand upon these findings by indicating that this relationship exists much earlier in the rehabilitation process (i.e., 3 months post-surgery). However, when comparing our findings to studies assessing the relationship between psychosocial response and quadriceps strength early in the rehabilitation process, there are conflicting results. For example, Chmielewski et al.⁵⁰ reported a weak relationship between greater kinesiophobia 1 week after surgery and greater quadriceps strength symmetry 3 months after surgery. On the other hand, Beischer et al.⁵¹ reported that patients with higher knee self-efficacy at 4 months after surgery had increased odds for symmetrical muscle function at 1 year after surgery. Our results are more in line with the work of Beischer et al.,⁵¹ as we found that greater knee self-efficacy at 3 months after surgery was moderately associated with greater quadriceps strength limb symmetry at 6 months post-surgery.

It is important to note that when knee self-efficacy was combined with psychological readiness for sport in the multivariate model, only psychological readiness for sport was found to be a significant predictor of quadriceps strength symmetry. While the K-SES includes a variety of items pertaining to one's belief in being able to perform various tasks, activities, and knee function, the ACL-RSI includes items pertaining to an athlete's emotions, such as fear, nervousness, and frustration, in addition to confidence in oneself. Potentially, a more holistic psychosocial response measure may better assist clinicians to identify athletes at risk of having impaired physical recovery. Future work is needed to evaluate both athletes’ self-reports of psychosocial response and objective measures of strength throughout the rehabilitation process (early after surgery to time of return to sport) to more fully understand the interplay between psychosocial and physical recovery. Such work is needed to better inform clinical practice guidelines and to design more targeted post-operative interventions.

4.3 Psychosocial factors and knee mechanics

Limited previous work has evaluated the relationship between psychosocial response and mechanics following ACLR. Drawing from literature in other orthopedic patient populations, associations between psychosocial response and joint mechanics have been inconsistent.^52-54 This literature, however, has examined pain-related fear, fear of movement, and knee self-efficacy, suggesting that the result may depend on the psychosocial factor studied.

It has been well established that patients exhibit impaired knee mechanics during walking and running after ACL injury and reconstruction,^{4, 55, 56} but only recently have investigators begun to explore how psychosocial factors may influence these gait mechanics. Zarzycki et al.⁵⁷ examined knee mechanics during walking in patients 6 months after ACLR and found that patients with lower psychological readiness for sport presented with significantly less knee flexion at initial contact and significantly less peak knee flexion in the surgical limb. This relationship, though, was weak with less than 10% of the variance being explained by the ACL-RSI. We build upon this study by showing that there was no significant relationship between the ACL-RSI at 3 months after ACLR and knee flexion excursion limb symmetry while running at 6 months after ACLR. However, there was a moderate significant relationship between the K-SES at 3 months after ACLR. These results suggest that early recovery of knee self-efficacy, not psychological readiness for sport, is more related to knee flexion excursion during running. An athlete's perceived self-efficacy has been shown to be an important factor for rehabilitation adherence and clinical outcome after ACL injury and reconstruction.^{43, 58, 59} Therefore, we speculate that greater self-efficacy early in rehabilitation may translate to increased trust and confidence in the knee itself, contributing to improved resolution of gait impairments. It is important to note that although there was a moderate relationship, only 17.3% of the variance in knee flexion excursion limb symmetry was explained by the K-SES and a cause–effect relationship cannot be established. Thus, future research needs to elucidate the relationship between psychosocial response and knee mechanics to determine whether addressing psychosocial factors leads to improved symmetry during gait, or whether addressing gait asymmetries leads to changes in psychosocial response.

4.4 Clinical implications

The finding that psychosocial response at 3 months after surgery is significantly associated with self-reported function and knee impairment outcomes at 6 months after surgery suggests that psychosocial factors, such as fear of re-injury, confidence in performance, risk appraisal to return to sport, and knee self-efficacy, may be of clinical importance during rehabilitation. Psychosocial factors are often taken into account at time points later in rehabilitation, typically when athletes commence sport-specific training at 6–8 months and return to sport at 9–12 months after ACLR. However, the results of this study indicate that 3 months after surgery, which typically coincides with the initiation of more high-demand activities, may be another applicable time to monitor psychosocial response. As athletes are likely to be still receiving supervised rehabilitation visits at this stage, there is potential to intervene and/or establish a collaborative care team so psychosocial recovery from injury can occur in parallel with physical recovery. It must also be noted that although this study indicates that psychosocial response influences subjective and objective knee function, it is possible that knee function may also influence psychosocial response.

4.5 Limitations

We recognize there are several limitations to our study that should be noted. This study does not provide a long-term component to evaluate if and how the outcome variables may change beyond 6 months after ACLR. Thus, we are unable to provide further insights into the associations and predictive value of psychosocial response on return to sport participation. Furthermore, rehabilitation after ACLR was not controlled with patients being treated by a variety of community-based physical therapists and athletic trainers. Therefore, it is unknown if patients received any additional treatment or psychosocial counseling during their rehabilitation. We also acknowledge that several outcome measures in addition to the ACL-RSI and K-SES have been used in the assessment of psychosocial response after ACLR. Due to this wide variation in outcome measures, it is difficult to compare between studies. In addition, other non-psychosocial factors such as environmental factors, physical factors, and social determinants of health, which were not measured, may have important influences on knee function. Since these other variables were not included, the associations in this study may be inflated. Finally, since this was an observational study, causality cannot be determined based on the associations. Future studies are needed to determine whether interventions that target psychosocial factors early in the rehabilitation process can lead to positive changes in knee function at later time points in the rehabilitation process.