MRI criteria for patella alta and baja.

OBJECTIVE: To determine the range of the patellar tendon length to patellar length ratio on magnetic resonance imaging (MRI) of the knee in order to aid in the establishment of MRI criteria for patella alta and baja. PATIENTS: Two hundred and forty-five patients ages 6-85 (mean 44) years who went through 262 consecutive 1.5 MRI studies of the knee performed during November 2000 through February 2001 were evaluated, regardless of their clinical symptoms.
DESIGN: Patellar length (PL) and patellar tendon length (TL) were measured by a single musculoskeletal radiologist on sagittal images by a line connecting the superior and inferior patellar poles and the shortest length of the inner margin of the tendon respectively. TL/PL ratio was subsequently calculated. The distribution of ratios was evaluated; the extreme 2.5% at each end of the distribution was defined as patella alta and baja.
RESULTS: The TL/PL ratio ranged between 0.56 and 1.71 (mean 1.05). After plotting the ratios, we noted an asymmetric curve skewed to the left. Based upon calculation of the extreme 2.5% of the ratio at each end of the plot, we determined that the MRI definition of patella alta and baja is a ratio of TL/PL of more than 1.50 and less than 0.74 respectively. We found that females had significantly higher TL/PL ratio than males (1.0878 and 1.0032 respectively). Ratios defined for patella alta and baja were 1.52 and 0.79 respectively in females and 1.32 and 0.74 respectively in males ( p<0.0001).
CONCLUSION: Patella alta and baja are determined as TL/PL of more than 1.50 and less than 0.74 respectively, somewhat different than traditionally quoted radiographic and previously described MRI criteria.