Upright MRI measurement of mechanical axis and frontal plane alignment as a new technique: a comparative study with weight bearing full length radiographs.

OBJECTIVE

The purpose of this prospective study was to investigate the practicality, accuracy, and reliability of upright MR imaging as a new radiation-free technique for the measurement of mechanical axis.

METHODS

We used upright MRI in 15 consecutive patients (30 limbs, 44.7 ± 20.6 years old) to measure mechanical axis deviation (MAD), hip-knee-ankle (HKA) angle, leg length, and all remaining angles of the frontal plane alignment according to Paley (mLPFA, mLDTA, mMPTA, mLDTA, JLCA). The measurements were compared to weight bearing full length radiographs, which are considered to be the standard of reference for planning corrective surgery. FDA-approved medical planning software (MediCAD) was used for the above measurements. Intra- and inter-observer reproducibility using mean absolute differences was also calculated for both methods.

RESULTS

The correlation coefficient between angles determined with upright MRI and weight bearing full length radiographs was high for mLPFA, mLDTA, mMPTA, mLDTA, and the HKA angle (r > 0.70). Mean interobserver and intraobserver agreements for upright MRI were also very high (r > 0.89). The leg length and the MAD were significantly underestimated by MRI (-3.2 ± 2.2 cm, p < 0.001 and -6.2 ± 4.4 mm, p = 0.006, respectively).

CONCLUSIONS

With the exception of underestimation of leg length and MAD, upright MR imaging measurements of the frontal plane angles are precise and produce reliable, reproducible results.