Summary

PET/MRI is an unprecedented modality for evaluating the functional biologic activity within ACL graft reconstructions. PET data demonstrates elevated generalized activity at 1 year post-op and statistically significant decreases in activity within both tunnels at specific time points. Increases in activity within the intra-articular graft is seen between the 3 to 6 month period.

Abstract

ACL reconstruction is a common orthopedic procedure following injury, however little is known regarding the functional biologic activity of the post-operative graft. No study to date has attempted to evaluate the biologic activity of the ACL graft using PET/MRI over the first post-operative year.

18 patients were recruited in this IRB-approved prospective pilot study. All patients underwent Graftlink® semitendinosus autograft hamstring ACL reconstruction by one sports fellowship trained orthopedic surgeon. A suspensory cortical fixation device (ACL Tightrope®) was chosen to allow for improved radiologic evaluation of graft/tunnel integration without foreign body fixation. Patients were followed clinically with validated self-reported surveys (KOOS and SF-12). Patients underwent simultaneous 18F-FDG PET and MRI (Siemens Biograph mMR) of both the operative and non-operative knee (for control) at 3 months, 6 months and 12 months post operatively. Whole-knee ROIs were placed on both knees to characterize organ scale changes. MRI images were re-sliced to align each tunnels perpendicular to the image plane and circular ROIs were manually drawn on each plane with diameter equal to the tunnel diameter. This same method was applied to PET images. Average PET uptake was normalized to injected dose and body weight (SUV) or to the non-operative knee. MRI images were then reviewed by a musculoskeletal fellowship trained radiologist for morphological changes.

Preliminary results demonstrate increased PET activity within the operative knee at all time periods as compared with the non-operative knee. Activity decreases over time, but still reveals considerable generalized activity even at 1 year post-operatively. Averaged data across all patients exhibit decreasing PET activity (SUV) in the whole knee from 3 months to 12 months and also when normalized against the non-operative knee. Decreased PET activity within the tunnels over the study period is statistically significant; within the tibial tunnel at 3 to 6 months and 3 to 12 months (P = 0.043 and P = 0.001) and the femoral tunnel between 6 to 12 months and 3 to 12 months (P = 0.031 and P = 0.033). There is an increase in PET activity within the intra-articular ACL graft between the 3 and 6 month period, with a peak at 6 months, but without statistical significance. An increase in generalized PET activity is seen within the normal knee at 6 months, likely representing a return to increased activity. No complication was seen on the MRI portion of the study. All patients had a satisfactory clinical outcome.

Post-operative PET/MRI evaluations of ACL graft reconstructions demonstrate evolving biologic activity within the graft as well as both tunnels. Although activity decreases over time, there is considerable generalized activity at 1 year. Focal decreases in activity within the tunnels may be indicative of ligamento-osseous morphologic changes. An increase in the intra-articular graft biologic activity in the 3 to 6 month time period appear to be consistent with prior investigations. Preliminary results suggest that graft incorporation continues beyond one year post-operatively. This information will continue to enhance our understanding of graft integration in the setting of post-operative rehabilitation protocols.