**2. Monitoring methods**

Several complementary methods can be used to follow the evolution of the procedure, each one with advantages and disadvantages, depending on the phase of the process.

#### **2.1. Ionizing radiation**

X-ray, dual energy X-ray absorptiometry (DEXA) and computed tomography can be utilized for the evaluation of the bone callus. However, the necessity for a serialized evaluation, exposure to ionizing radiation, metallic artifacts susceptibility, and the high cost and more restricted availability of computed tomography and DEXA limit the utilization of these methods.

The X-ray is the most used, because it allows proper evaluation of the bone extremities and distraction distance (gap). Using the "three cortical rule," where the visualization of at least three corticals with 2 mm thickness on orthogonal views is necessary, the removal of the external fixator is indicated [6, 7]. Yet, it was observed that this method is subject to great observer variation, not being more accurate than random chance [8]. Other limitations are related to the initial stages where radiography is incapable of evaluating soft tissue.

Studies valued the role of DEXA in the evaluation of the regenerated tissue, from the length‐ ening stages until its attempt to objectively define the best moment to remove the external fixator. Some research parameters include the relation between the regenerated bone mineral density and contralateral limb, and also the percentage of the weekly increase of bone mineral density [9, 10]. Despite promising results, there is still no standardization of these parameters.

The quantitative computed tomography (QCT) sums the quantitative evaluation with highresolution images of regenerating bones, presenting better correlation than the DEXA and allowing a global evaluation to the assistant doctor. However, high cost, little availability, and great exposure to radiation (more than the other methods) limit its application [1].

#### **2.2. Ultrasonography**

Recently, the role of ultrasonography in the monitoring and distraction has been target of several studies. Several characteristics make this a method of interest, as it does not use ionizing radiation, it is widespread, and it is not subject to artifacts related to external fixators. The top advantage is the possibility of characterization of soft tissue and precocious detection of complications like cysts and collections.

For the ultrasonographic evaluation, linear transducers for high resolution must be used (5– 12 MHz). The osteotomy is evaluated with beams perpendicular to the bone corticals, longi‐ tudinally and transversely along the bone axis. In the initial evaluation, the ultrasonography identifies the osteotomy corticals as hyperechoic surfaces, with posterior acoustic shadow and acute margins. Between them is located the soft callus, defined as a hypoechoic area with great penetration of the ultrasonographic beam [11]. (see **Figure 1**)

In the first weeks, the appearance of echogenic outbreaks longitudinally oriented in the interior of the "gap" is noticed. In the cross-sectional assessment, there is a "cut wire" aspect. Between 2 and 4 weeks, the first individualized ossification center starts to be identified. Over time, there is an increase in number and size of those centers with the tendency to confluence on the longitudinal axis. Gradually, there is loss of penetration power in the callus and rounding of the edges of osteotomies. After 6–8 weeks, it is defined a cortical margin with mild central invagination and thickness markedly reduced in relation to the osteotomies corticals [12].

With the progression of the corticalization, the ultrasound beams are gradually more reflected, thus losing the correlation between the method and characterization of changes in the stiffness and strength of the regenerated tissue (attenuation × reflection) [13]. Eventually, the beams' reflection will be full, preventing the evaluation continuity with this method. Therefore, ultrasound assessment is limited in the final stages of consolidation and there are no studies in literature investigating its role establishing enough bone healing for the apparatus removal [1].

The ultrasound appearance during the entire process was compared with tissue density measurement with computerized tomography, evidencing an exponential increase in bone density through the consolidation time [14], but no objective ultrasound parameter was proposed as a follow-up measure. Ultrasound evaluation itself is mainly subjective and prone to multiple variables as transducer position and measurement site [15]. Troulis et al. proposed the use of through-transmission method, in which the beam penetration depth in the soft callus is used as the stiffness indicator. But in his work throught transmission was compared with the surgeon's intraoperative bone stiffness evaluation, which was also subjective [16]. The lack of objective parameters to assess bone mass index limits its application as a monitoring method.

Quantitative acoustic parameters (ultrasonometry) and its correlation with soft callus prop‐ erties have been studied. Velocity propagation across the gap correlates with bone healing [17], while speed of sound, acoustic reflection, and attenuation correlate with trabecular bone mass index [18] and acoustic backscattering relates to trabecular microstructure [19]. Strong association between the penetration depth within the gap and maximum load and torsional stiffness of the consolidated callus has been shown. With noninvasive, radiation-free, objective data, follow-up protocols can be studied and early therapy modifications (distraction rate, medications) may optimize the distraction and consolidation processes.
