The goal of knee stabilization through ACLR is to restore knee function and optimize patient activity level. The MOON cohort was used to evaluate activity level 2-years after ACLR and to model predictive factors for a higher activity level. In addition, return to play for the two most common sports associated with a torn ACL [10]—soccer and American football—were evaluated at between 1 and 2-years after ACLR. The reviewed studies represent the highest level of evidence for activity level and return to sports after ACLR, which is useful information for surgeons and patients.

The Marx Activity Scale was chosen as the primary outcome measure for activity level in the MOON cohort. This validated scale was designed as a self-reported measure of specific functions that are potentially challenging for ACL deficient persons. Other advantages of the Marx scale over other measures (e.g., IKDC and Tegner) include a lack of ceiling effect and the incorporation of activity frequency. Nonetheless, our data demonstrated a high correlation (Spearman p = 0.63; P < 0.001) between reported activity level from IKDC and the Marx scale at 2-years [6, 7] in MOON patients.

At 2-years after ACLR [6, 7] in the original MOON 2002 cohort (n = 393, 88 % follow-up) there was a significant decrease in Marx activity level from a baseline score of 12 (range 8–16) to 9 (range 3–13). Only 45 % of patients achieved the same or a higher level of activity 2-years after ACLR by this score. When controlling for age, marital status, student status, sport, competition level, associated articular/meniscal injuries and the status of the contralateral knee, only two factors were associated with a higher activity level at 2-years: a high baseline activity score and lower body mass index. In contrast, female sex, smoking and an index ACLR which was a revision (8 % of this cohort subset) all contributed to a decrease odds of maintained activity level. It is important to note that despite common belief, concurrent injuries (meniscus, cartilage) did not influence activity level at 2-years. The MOON group hypothesized that psychosocial factors, including a fear of re-injury or a change in circumstance (e.g., graduate from school, loss of interest) may also play a considerable role in activity level post-ACLR.

McCullough et al. [20] examined return to American football in 147 MOON cohort patients from 2002–2003. This included 68 high school and 26 collegiate level players. Overall return to football was 70 %, including 43 % who felt they returned to play at the pre-injury level. Among those who did not return to play, two thirds cited “other interests” as a significant contributor and half cited “fear of re-injury or further damage.” Furthermore, at 2-years after ACLR clinically and statistically significant differences were seen in favor of those who returned to football for KOOS knee-related QoL and Marx Activity Scale. No differences were seen by player position. Athletes who primarily play soccer can expect similar return to play results [21]. At a mean of 12 months, 72 % returned to soccer including 85 % at the pre-injury level or higher.

A MOON subset, consisting of 235 patients with a normal EUA and IKDC score of the contralateral knee and without history of prior contralateral surgery, were followed for 2 years to examine rates of revision and contralateral primary ACLR [22]. A rate of 3.0 % ipsilateral revision, and 3.0 % contralateral primary ACLR were found. With the expansion of this cohort to include all patients registered in MOON from 2002–2003 (N = 980), the 2-year ipsilateral reoperation rates were 4.8 % revision ACLR, 5.0 % repeat meniscal surgery, and 1.8 % subsequent chondral procedure [23].

Hettrich et al. [23] also reported on complications of ACLR that required reoperation at 2-years. Among 980 patients, 4.1 % returned to the operating room for anterior debridement, manipulation under anesthesia or synovectomy after a diagnosis of arthrofibrosis. Hardware removal from the tibia was less common at 0.6 %. Five deep infections were reported (0.5 %), including two which occurred 3 weeks. These data included 91 % primary ACLR and 9 % revision ACLR index events; however, revision was not found to be a significant predictor of reoperation in regression modeling. In multivariate analysis, only increasing age reduced the odds of reoperation (34-year-old vs. 17-year-old; OR = 0.47), while the use of allograft (OR = 2.33) significantly increased the odds.

The success of meniscal repair is higher when performed concurrently with ACLR [24]. Among the original MOON cohort, the rate of successful repair using a variety of surgical techniques and implants was 96 % for 82 meniscal repairs performed concurrently with ACLR [25]. Only three patients (3.7 %) underwent reoperation for failed repair at 2-years.

The relationship between graft choice, specifically autograft and allograft, and the risk of revision was elucidated in MOON by Kaeding et al. [26]—Fig. 20.1. The highest percentage of failures occurred in the age 10–19 year old category, and for those who underwent allograft (8.9 %) compared to autograft (3.5 %) reconstruction. When controlling for age in the model, allograft was still a significant risk factor for revision (Odds ratio = 4). From this data a clinical prediction algorithm for age and graft choice was developed, the results of which were illustrated by two clinical scenarios. In a 14-year-old the risk of ACL re-tear is 22.0 % for allograft and 6.6 % for autograft ACLR. In a 40-year old the risk of ACL re-tear is 2.6 % for allograft and 0.6 % for autograft ACLR. A surgeon and older patient may together decide that a 2.6 % risk of revision is acceptable and thus proceed with allograft ACLR, but clearly a 22 % risk of failure in a young patient is not tolerable (Fig. 20.1).

This section will review the available MOON data concerning return to play, patient reported outcomes, and reoperation at between 3 and 8 years after primary ACLR. In MOON, most commonly the 6-year follow-up point was chosen for the investigation of intermediate outcomes.