New Evidence That ACL Graft Maturation Progresses in a Predictable Pattern

by Elizabeth Hofheinz, M.P.H., M.Ed.

An ongoing hot potato in the sports medicine world, the timing of return-to-play has gotten another serious look in the literature lately. The work, “Quantitative Assessment of In Vivo Human Anterior Cruciate Ligament Autograft Remodeling: A 3-Dimensional UTE-T2* Imaging Study,” appears in the September 11, 2020 edition of The American Journal of Sports Medicine.

Co-author Ryan J. Warth, M.D. is assistant professor and director of clinical research operations of orthopedic surgery at McGovern Medical School at UTHealth in Houston. He told OSN, “The timing of return to play after ACL reconstruction is still one of the most heavily debated topics in the realm of sports medicine, and early return to play after is thought to be one of the most important risk factors for graft rupture, repeat surgery, and downstream knee osteoarthritis. However, it is still difficult to define what ‘early return to play’ actually means, even in the scientific literature.”

Imperfect tools

“There does not exist a widespread, clinically feasible method designed to determine whether or not an ACL graft has matured. As a result, clinicians must rely on time- and function-based criteria to decide whether patients can safely return to play. Time-based criteria are based primarily on data from controlled animal studies, and function-based criteria focus on neuromuscular coordination and strength. Although generally grounded in science, neither of these approaches provide any assurance that the graft itself has healed.”

The authors on the current study used ultra-short echo T2* (UTE-T2*) magnetic resonance imaging (MRI) to assess the status of in vivo ACL graft remodeling on 12 patients ranging in age from 14-45 years who underwent primary ACL reconstruction with semitendinosus tendon or bone–patellar tendon–bone autograft (with or without meniscal repair).

“…Patients returned for UTE MRI at 1, 3, 6, 9, and 12 months after ACL reconstruction,” wrote the authors. “Imaging at 1 month included the contralateral knee. MRI pulse sequences included high-resolution 3-dimensional gradient echo sequence and a 4-echo T2-UTE sequence (slice thickness, 1 mm; repetition time, 20 ms; echo time, 0.3, 3.3, 6.3, and 9.3 ms). All slices containing the intra-articular ACL were segmented from high-resolution sequences to generate volumetric regions of interest (ROIs). ROIs were divided into proximal/distal and core/peripheral sub-ROIs using standardized methods, followed by voxel-to-voxel registration to generate T2* maps at each time point. This process was repeated by a second reviewer for interobserver reliability…”

Ten patients were ultimately included in the analysis, with the authors saying, “…(n = 7, semitendinosus tendon; n = 3, bone–patellar tendon–bone). T2* values increased from 5.5 ± 2.1 ms at 1 month to 10.0 ± 2.9 ms at 6 months, followed by a decline to 8.1 ± 2.0 ms at 12 months. Similarly, mean T2*_inj/T2*_intact ratios increased from 62.8% ± 22.9% at 1 month to 111.1% ± 23.9% at 6 months, followed by a decline to 92.8% ± 29.8% at 12 months. Sub-ROIs exhibited similar increases in T2* until reaching a peak at 6 months, followed by a gradual decline until the 12-month time point. There were no statistically significant differences among the sub-ROIs.”

First to quantify the pattern

Dr. Warth: “ACL graft maturation appears to progress in a predictable pattern from 1-12 months after surgery. Although often theorized, actual quantification of this pattern in human patients through serial imaging has not been demonstrated until now. The degree of region-dependent variations in T2* measurements was quite interesting and may generate new research questions involving the effects different surgical techniques, postoperative rehab protocols, and biologic treatments on the shapes of these curves.”

“We hope these exciting results serve as a springboard for future work in optimizing and refining these new graft imaging techniques, while also striving to establish clinical utility and efficacy. Additionally, our results provide a foundation to study the effects of biologic treatments, such as PRP [platelet-rich plasms] and BMAC [bone marrow aspirate concentrate], on the progression of ACL remodeling in a meaningful, objective manner.”

“When clinicians are able to non-invasively assess the maturation status of an ACL graft in a manner that is reliable and cost-effective, the term ‘early return to play’ will become a contraindication rather than a topic of reflection. We hope that our study provides the framework for subsequent imaging advancements in the area of return to play after ACL reconstruction – our patients need and deserve our attention on this matter.”

“This work would not have been possible without an All-Star collaborative team of MR physicists, engineers, research coordinators, and faculty including our Department Chair, Dr. Walter Lowe.”