Background: The natural history of Kaplan fiber (KF) injuries after acute primary anterior cruciate ligament (ACL) reconstruction (ACLR) remains unknown.
Purpose/hypothesis: The purpose of this study was to evaluate the temporal change in the magnetic resonance imaging (MRI) appearance of the KF complex after acute primary ACLR. It was hypothesized that KF injuries would resolve with time.
Study design: Case series; Level of evidence, 4.
Methods: A retrospective MRI analysis was conducted on 89 patients with ACL-injured knees to assess the change in the radiological appearance of KFs after primary ACLR. Patients who had undergone index MRI and ACLR within 90 days of the injury and further MRI at 9 months after surgery were included. Diagnostic criteria to identify radiological evidence of a KF injury and its subsequent resolution were applied, including the presence of high signal intensity on fluid-sensitive sequences, which is indicative of a pathological process radiologically. The proximity of KFs to the femoral cortical suspensory device (CSD) was noted on MRI scans and quantified in millimeters.
Results: A KF injury was identified in 30.3% (27/89) of patients, with isolated high signal intensity observed in an additional 18.0% (16/89). At 9 months, MRI evidence of the reconstitution of the KF complex was found in 51.9% (14/27) of patients, with persistent discontinuity in the remaining patients (13/27). All patients (16/16) with isolated high signal intensity had complete resolution on repeat MRI scans. KF thickening was observed in 26.1% (12/46) of patients with previously healthy KFs and in 25.0% (4/16) with isolated high signal intensity. The CSD was positioned in close proximity (≤6 mm) to the center of the KF attachment in 61.8% (55/89) of patients, and this was associated with increased rates of KF thickening.
Conclusion: The KF injury resolved radiologically in over half of the patients at 9 months after acute primary ACLR. High signal intensity in the region of the KFs on index MRI scans resolved in all cases, with evidence of residual KF thickening in only one-quarter of cases on repeat MRI scans, equivalent to the rate in those with healthy KFs. As such, it is not advisable to use high signal intensity on preoperative MRI scans as the sole criterion for the diagnosis of a KF injury. The position of the CSD after ACLR was intimately related to the KF attachment in the majority of patients, which was associated with KF thickening on postoperative MRI scans.
Keywords: Kaplan fiber; MRI; anterior cruciate ligament; cortical suspensory device; natural history.