Abstract
Introduction
The assessment of leg length is essential for planning the correction of deformities and for the compensation of length discrepancy, especially after hip or knee arthroplasty. CT scan measures the “anatomical” lengths but does not evaluate the “functional” length experienced by the patients in standing position. Functional length integrates frontal orientation, flexion or hyperextension. EOS system provides simultaneously AP and lateral measures in standing position and thus provides anatomical and functional evaluations of the lower limb lengths.
The objective of this study was to measure 2D and 3D anatomical and functional lengths, to verify whether these measures are different and to evaluate the parameters significantly influencing these potential differences
Material and Methods
70 patients without previous surgery of the lower limbs (140 lower extremities) were evaluated on EOS images obtained in bipodal standing position according to a previously described protocol.
We used the following definitions:
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anatomical femoral length between the center of the femoral head (A) and center of the trochlea (B)
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anatomical tibial length between the center tibial spine (intercondylar eminence) (C) and the center of the ankle joint (D)
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functional length is AD
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global anatomical length is AB + CD
Other parameters measured are HKA, HKS, femoral and tibial mechanical angles (FMA, TMA), angles of flexion or hyperextension of the knee, femoral and tibial torsion, femoro-tibial torsion in the knee, and cumulative torsional index (CTI). All 2D et3D measures were evaluated and compared for their repeatability.
Results
Regarding repeatability, an ICC> 0.95 was found for all measurements except for the tibial mechanical angle (0.91 for 2D, 3D 0.92 for 3D). We observed 54/140 lower limbs with Flessum/Recurvatum angles (FRA) >10°.
2D results (mean, SD) were
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41,8mm(2,9) for femoral anatomical length
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36,1mm(2,8) for tibial anatomical length
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78,0mm(5,4) for global anatomical length
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78,5 mm(5,5) for functional length
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7,4°(12,0) for Flessum/Recurvatum angle
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−1,5°(6,4) for HKA
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4,9°(2,0) for HKS
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92,1°(3,4) for FMA
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87,1°(3,4) for TMA
3D results (mean, SD) were
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42,4mm (2,8) for femoral anatomical length
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36,6mm (2,8) for tibial anatomical length
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79,0mm (5,4) for global anatomical length
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78,9mm (5,5) for functional length
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7,2°(12,0) for Flessum/Recurvatum angle
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−1,0°(5,9) for HKA
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4,9°(1,5) for HKS
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92,7°(2,7) for FMA
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87,9°(3,9) for TMA
The 2D/3D measurements of functional lengths were statistically significant (p <0.0001. Student's test). For anatomical lengths.
2D/3D measurements were also statistically significant (p <0.0001. Student's test for femoral tibial and global anatomical lengths)
Some parameters significantly influenced 2D/3D differences:
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for the global anatomical length: FRA P<0,0001, TMA P=0,0173, HKA P=0,0259 and femoro-tibial torsion P=0,0026
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for the functional length FRA P=0,0065
Discussion and conclusion
EOS imaging allows to accurately assess the anatomical and functional length experienced by the patient. These new data open new perspectives for planning length or axis corrections and for an optimized evaluation in some medico legal issues after joint replacement or posttraumatic sequelae. This study points out the importance of 3D measurements in outliers cases (varus or valgus cases, flessum or recurvatum of the knee)
