gms | German Medical Science

Objectives: In absence of deformity or injury of the contralateral leg, the contralateral leg length is used to plan limb lengthening. Length variability on long-leg weight-bearing radiographs (LLR) can lead to inaccurate deformity correction. The aim of the study was to (1) examine the variability of the measured limb length on LLR and (2) to examine the influence of the position of the magnification device.

Methods: The limb lengths of 38 patients during deformity correction with a taylor-spatial-frame were measured retrospectively on 7.3±2.6 (4-13) LLR per patient. The measured length of the untreated limb between LLR were used to determine length variability between LLR in each patient.

The following statistical analyses were performed: The absolute values of the length of the untreated limb were used. The range (maximum-minimum) of the measured limb lengths were calculated for each patient independently and were used to describe the variability of measured limb length within a patient.

To answer the secondary aim, we took LLR from a 90cm validation distance. A magnification device was placed in different positions: at the middle of the 90cm distance (z-position), 5cm anterior and 5cm posterior from the z-position, at the bottom and top of the validation distance as well as 5cm medial and 15cm lateral from the z-position. All measurements were performed twice by two observers. The intra- and interrater reliability between were determined by calculating the intra-class correlation coefficient (ICC (3,1)) for measurements of the primary and secondary study aim.

Results: The measured length ranged between LLR within a patient from 10 to 50mm. 76% of patients had a measured limb length difference of more than 2cm between taken LLR.

The ICC showed excellent intra- and interobserver reliability with small measurement differences between observers (4.17±3.16; 3.75±2.97mm) and between observations (2.78±2.01; 4.07±3.28 mm):

Compared to length measurement of the 90cm test object with the magnification device in the z-position (90.1cm), positioning the device 5cm anterior led to smaller (88.6cm) and 5cm posterior led to larger measurements (91.7cm). The measured length with the magnification device at the bottom, top, medial or lateral (90.4; 89.9; 90.2; 89.8cm) to the object differed not relevantly.

Conclusions: High variability of limb length ranging from 1 cm to 5 cm between different LLR within one patient were observed. The high variability can be explained by different positions of the magnification device in the sagittal plane. Placing the device 5 cm anterior or posterior of the limb, the measured length changed signficantly on LLR. Surgeons, radiologists and radiology assistants have to place the magnification device in the correct and in a reproducible position. This should be considered for all length and size measurements on radiographs.