Magnetic resonance diagnosis of posterior horn tears of the lateral meniscus using a thin axial plane: the zip sign--a preliminary study.

OBJECTIVE: The "zip" sign is a newly described form of meniscal tear progressing from the distal insertion of menisco-femoral ligaments (MFLs) through the lateral meniscal wall; the tear occurs during anterior cruciate ligament (ACL) rupture. The purpose of this study was to evaluate the zip sign on knee MRI within the context of ACL injuries.

METHODS: From a series of 261 MR examinations for acute knee injury, we selected 97 patients with both MR and arthroscopic data for a retrospective blinded review. The zip sign was defined on axial thin MR sections as a straight line from the distal insertion of MFLs in association with five sagittal images lateral to the posterior cruciate ligament (PCL) where the MFLs were identified. Sensitivity and specificity in detecting lateral meniscal tears before and after having defined the zip sign were calculated.

RESULTS: Sensitivity in detecting the tears of the posterior horn of the lateral meniscus (PHLM) reached 87.5% (CI 0.68-0.97) after zip sign criteria were defined. The zip sign has excellent inter-observer agreement, К>0.90.

CONCLUSION: The zip sign indicates a lesion at the insertion site of MFLs into the PHLM on thin axial images associated with sagittal MR sections that may improve MR sensitivity in detecting PHLM tears.