Since 2002, the Multicenter Orthopaedic Outcomes Network (MOON) has enrolled and followed a population cohort of patients who underwent anterior cruciate ligament (ACL) reconstructive surgery. Its objective is to establish patient-specific predictive models of clinically important outcomes.

The prospective multicenter cohort study aims to identify short- and long-term prognosis and predictors of sports function, activity level, and general health through validated patient-reported outcomes. It also seeks to quantify the incidence of ACL reconstruction graft and/or contralateral ACL failures and additional surgical procedures.

The brainchild of Richard D. Parker, MD; Kurt P. Spindler, MD; and Jack T. Andrish, MD, MOON now includes seven institutions, a roster of 17 surgeons, and a registry of more than 4,400 ACL reconstructions—the largest prospective longitudinal ACL reconstruction cohort in the United States.

An update and summary of findings from MOON appears in the March issue of the Journal of the AAOS (JAAOS). Recently, AAOS Now spoke with two of the authors—Dr. Spindler and T. Sean Lynch, MD.

AAOS Now: How does MOON differ from a registry?

Dr. Spindler: A registry collects good baseline information but doesn’t necessarily have any follow-up. MOON captures a richer database, because it has a mechanism for follow-up and the resources to capture data at a more granular level. MOON can achieve 80 percent follow-up at 2 years, 6 years, and 10 years. It can also bring people back for specific questions, such as younger people whose only injury was the ACL injury to determine who has normal knees and who gets early arthritis. It fundamentally analyzes the question better.

MOON is a prospective longitudinal cohort, similar to the Framingham cohort. There is clearly a plan and variables deemed to be important. Researchers can lay them out, capture data in a rigorous manner, and spend a lot of time and effort maintaining follow-up.

AAOS Now: Can you summarize your findings regarding knee pain and other factors, such as graft type and age, as predictors?

Dr. Lynch: Initially, a bone bruise was thought to be associated with a more difficult rehabilitation and pain after surgery; however, early studies proved that this was not the case. MOON has proven that poorer outcomes are associated with smoking, increased body mass index (BMI), and lower education level. Younger individuals have been found to be at an increased risk for failure. Patients with grade III or IV articular cartilage damage in certain areas of the knee have poorer outcomes.

We’re continuing to focus on the role of meniscal injuries in association with ACL tears and their impact on outcomes. Current studies are showing a meniscal repair failure rate of about 14 percent, while meniscal tears left alone are doing fairly well at the same follow-up. This may mean we are overtreating these concomitant injuries.

Dr. Spindler: We learned that the failure rate is age-dependent. The younger and more active patients are, the higher the failure rate, regardless of graft type. We learned that allografts have three times the risk of failure than autografts in the high school and college age group. That changed clinical practice, at least in academic centers. No one in the MOON group will put an allograft in a high school or college kid.

We also learned that reinjury rates don’t differ between males and females. Females are at much higher risk for an initial tear, but all patients with a once-torn ACL are in a high-risk group and sex doesn’t matter. That’s why a strategy to prevent reinjury is so important.

Randomized trials can’t determine these findings, because they don’t exist. Cohort data—level I prognostic and level II therapeutic—are necessary for an answer.

AAOS Now: What are some of the clinical implications of these findings in terms of rehabilitation?

Dr. Spindler: Most MOON surgeons believe that neuromuscular prevention techniques work and will reduce the risk between 40 percent and 60 percent. All young ACL patients should be offered and encouraged to participate in training programs before they return to normal activity. An 18-year-old patient with autograft tissue has a 12 percent risk of tearing either the same side or the other in 5 or 6 years; if allograft tissue was used, the risk of another tear is 18 percent.

Dr. Lynch: Many patients think they can return to sport after 6 to 9 months, but they may still have neuromuscular deficiencies. Surgeons need to advise them that they are not out of the woods, that some fine tuning is necessary before they can get back onto the field.

AAOS Now: What accounts for the high injury rate in the contralateral knee?

Dr. Spindler: Both MOON data and meta-analyses show that the other knee is at twice the risk of the injured knee. I think the reason is that individuals who originally tear their ACL are high-risk patients. So when those individuals return to play, the risk factors—whether genetics or how aggressively they play or their neuromuscular coordination—haven’t changed. This makes sense given that men and women have similar injury rates after the original injury.

AAOS Now: What are your findings regarding tunnel placement?

Dr. Lynch: We learned the importance of placing the tunnels anatomically both on the femoral and the tibial side. Studies showed the MOON surgeons were fairly consistent in where they were putting their tunnels, generally right where they should be.

AAOS Now: Where do we stand in terms of predicting arthritis?

Dr. Spindler: Structural changes that are consistent with osteoarthritis (OA), as detected on radiographs, are distinct from OA symptoms such as pain. More than 10 percent of patients with ACL reconstruction will have pain consistent with symptomatic OA at 6 years. Radiographic changes are noticeable at around 2.5 years, which is consistent with early degeneration, but patients are still functioning fine. I think it will take a 10-year follow-up study to confirm that the 2-year findings are real and to assess clinical outcomes.

Patients with ACL tears also have a high prevalence of meniscal tears. Meniscal repairs are a necessary component of the ACL reconstruction procedure to help minimize the potential for posttraumatic osteoarthritis.

AAOS Now: What are the priorities now for the MOON group?

Dr. Spindler: The next 4 years in collecting data will be critical. To make solid conclusions and to really know what happens takes a decade and a half. Some people with an ACL reconstruction do great at 10 years. We should be able to predict which patients will do well at 10 years and which will get some early arthritis—and to identify that latter group when the OA is modifiable.

MOON has been a wonderful team effort by surgeons, scientists, biostatisticians, and others. When we work together to answer questions as a team, we can accomplish some significant improvements.

Dr. Lynch: The MOON group took an individual activity and made it a team sport. The questions being asked require many patients and numerous surgeons to quickly find answers. It requires a multidisciplinary approach and buy-in from all contributing members. The MOON approach to research has been a blueprint for other multicenter studies, including revision ACL, shoulder (rotator cuff and instability), knee osteochondritis dissecans, and a soon-to-be meniscal study group.

MOON was supported in part by the National Institutes of Health’s National Institute of Arthritis and Musculoskeletal and Skin Diseases.

Terry Stanton is a senior science writer for AAOS Now. He can be reached at tstanton@aaos.org