Intra- and interobserver reliability analysis of pediatric lower limb parameters on digital long leg radiographs

BACKGROUND: Malalignments of the lower extremity are common reasons for orthopedic consultation because it may lead to osteoarthritis in adulthood. An accurate and reliable radiological assessment of lower limb alignment in children and adolescents is essential for clinical decision-making on treatment of limb deformities and for regular control after a surgical intervention. OBJECTIVE: First, does the analysis of full-length standing anteroposterior radiographs show a good intra- and interobserver reliability? Second, which parameter is most susceptible to observer-dependent errors? Third, what is the Standard Error of Measurement (SEM(95%)) of the absolute femoral and tibial length? METHODS: Two observers evaluated digital radiographs of 144 legs from 36 children and adolescents with pathological valgus alignment before a temporary hemiepiphysiodesis and before implant removal. Parameters included Mechanical Femorotibial Angle (MFA), Mechanical Axis Deviation (MAD), mechanical Lateral Distal Femoral Angle (mLDFA), mechanical Medial Proximal Tibial Angle (mMPTA), mechanical Lateral Proximal Femoral Angle (mLPFA), mechanical Lateral Distal Tibial Angle (mLDTA), Joint Line Convergence Angle (JLCA), femur length, tibial length. Intra- and interobserver reliability (ICC(2,1)), SEM(95%) and proportional errors were calculated. RESULTS: The intra- and interobserver reliability for almost all measurements was found to be good to excellent (Intra-ICC(2,1): 0.849–0.999; Inter-ICC(2,1): 0.864–0.996). The SEM(95%) of both observers was found to be ± 1.39° (MFA), ± 3.31 mm (MAD), ± 1.06° (mLDFA) and ± 1.29° (mMPTA). The proportional error of MAD and MFA is comparable (47.29% vs. 46.33%). The relevant knee joint surface angles show a lower proportional error for mLDFA (42.40%) than for mMPTA (51.60%). JLCA has a proportional error of 138%. Furthermore, the SEM(95%) for the absolute values of the femoral and tibial length was 4.53 mm for the femur and 3.12 mm for the tibia. CONCLUSIONS: In conclusion, a precise malalignment measurement and the knowledge about SEM(95%) of the respective parameters are crucial for correct surgical or nonsurgical treatment. The susceptibility to error must be considered when interpreting malalignment analysis and must be considered when planning a surgical intervention. The results of the present study elucidate that MAD and MFA are equally susceptible to observer-dependent errors. This study shows good to excellent intra- and interobserver ICCs for all leg alignment parameters and joint surface angles, except for JLCA. Trial registration: This study was registered with DRKS (German Clinical Trials Register) under the number DRKS00015053. LEVEL OF EVIDENCE: I, Diagnostic Study.