Background

Residual rotatory knee laxity following ACL reconstruction portends suboptimal clinical outcomes. When found in conjunction with ACL tear, KF injuries may be evidence of high-grade rotatory knee laxity. Previous investigations have been underpowered to conclusively define the prevalence of KF injuries. The accurate identification of KF injuries is vital for devising an optimal surgical tactic that restores knee stability and minimizes the likelihood of clinical failure.

OBJECTIVES:
1. To define the prevalence of KF injury in the setting of a complete ACL tear on MRI and 2. To examine the relationship between KF injuries and injuries to structures commonly found in conjunction with ACL tears.

Methods

After institutional review board approval, a database of patients with a complete ACL tear confirmed on MRI within 90 days of injury was created. All MRI were read by two, fellowship-trained musculoskeletal radiologists. KF injury was evaluated using a previously described grading scheme: Grade 0, normal; Grade I, periligamentous edema; Grade 2, partial tear; and Grade 3, complete tear. In instances where there was complete disagreement between reviewers regarding the presence of KF injury, MRI were read by a third reviewer. Using previously published KF injury prevalence rates among smaller series, an a priori sample size estimate determined that 120 MRI would be needed to yield an inter-rater reliability of 0.9 with 80% and an alpha level of 0.05. Weighted kappa of inter-rater agreement was determined for all MRI examinations. Wilxcon’s Kruskal Wallis test was performed to assess for associations between KF injury and magnet strength (1.5T v. 3.0 T), patient age, patient gender, the presence of medial and/or lateral meniscal tears, and/or posterolateral tibial bone bruise.

Results

Between 2014 and 2020, 131 patients (94 males, 37 females) with a complete ACL tear underwent MRI examination within 3 months of injury. The mean age of the cohort was 27.8 +/- 6.8 years. Sixty-five MRI (49.6%) were completed with a 1.5T magnet and sixty-six MRI (50.4%) with a 3T magnet. KF injuries were identified in 51 of 131 (38.9%) knees (Grade 1: 28 knees, Grade 2: 5 knees, and Grade 3: 5 knees.) Inter-rater agreement for KF for across all MRI was fair (kappa =0.28 ) with forty-three MRI requiring third reviewer adjudication. There were no significant associations between KF injury and patient gender (P = 0.26), magnet strength (P = 0.98) medial meniscal tears (P = 0.45), lateral meniscal tears (P = 0.33), or the presence of a posterolateral tibial bone bruise (P = 0.16)

Conclusion

The prevalence of KF injuries was 38.9% of cases, which is comparable to previously described rates. The presence of KF injury was not associated with other injuries commonly observed in conjunction with ACL tear. However, KF injuries were not consistently detected among multiple, independent reviewers. An alternate diagnostic technique may be required to accurately assess for injury patterns suggestive of high-grade rotatory knee laxity in the setting of ACL tear.