Background A high fibular location on plain radiographs has been considered suggestive of a discoid lateral meniscus (DLM). We sought to determine whether the high location of the fibular head observed in DLM patients represents a true pathologic finding.
Methods Eighty-two patients with symptomatic DLM who underwent arthroscopic treatment constituted the study group (DLM group). The same number of age-matched patients without DLM (control group) were recruited. The DLM group was further divided according to the presence or absence of a knee extension block (EB) of more than 10 degrees into EB(+) and EB(-) subgroups. We defined the compensated distance to the fibular head (CDF) as the distance from the lateral joint line to the tip of the fibular head divided by the femoral condylar width. CDF values measured on anteroposterior (AP) plain radiographs and sagittal MR images were compared between the DLM and control groups and between the EB(+) and EB(-) subgroups.
Results There was no significant difference in the mean preoperative CDF value on either AP plain radiographs or sagittal MR images between the DLM and control groups. However, the mean CDF value on AP plain radiographs was significantly lower in the EB(+) subgroup than in the EB(-) subgroup ( p =0.011). The knee EB disappeared in all patients after arthroscopic treatment when the CDF on AP plain radiographs showed no significant difference between subgroups ( p >0.05). Linear regression analysis revealed that the CDF on AP plain radiographs decreased significantly when knee flexion was increased from 0 to 30 degrees ( p =0.003).
Conclusions The high fibular location observed on AP plain radiography in DLM patients was associated with limited knee extension. Altered X-ray beam projection to the flexed knee creates the false appearance of a high fibular location on AP plain radiography.