**1. Introduction**

The distraction osteogenesis is a method of bone regeneration and fixation that was establish‐ ed by Ilizarov around the 1950s, causing revolution in the reconstruction and deformity correction surgeries [1]. The technique consists of a controlled bone fracture (osteotomy) and, after the formation of the soft callus (latency period), continuous traction of bone fragments in opposite directions, using an external fixator (lengthening phase), until the desired extension is reached, then fixing it until consolidation (neutral fixation/consolidation phase). This process does not use grafting and allows to cover extensive bone defects with newly formed bone (regenerated) or to remodel malformed structures. Nowadays it is applied in multiple ortho‐

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pedic and maxillofacial surgeries. However, it is a long-duration therapy and the patient may remain in use of an external fixator for a period of up to 20 weeks.

The treatment duration generates great physical and psychological morbidity. Its importance was recognized by Ilizarov himself when he postulated: "keeping the apparatus for longer than necessary is as damaging as removing early" [2]. Therefore, the optimization of therapy is fundamental to reduce the complications and ensure its success. For that reason, the distraction osteogenesis monitoring must be realized. Several methods are available, with some being more suited to each phase. The histological characteristics of the regenerated tissue in each phase are fundamental for the comprehension of the monitoring methods, as we see later.

#### **1.1. Latency period**

Latency period consists in the physiological tissue repairing response initiated immediately after the osteotomy. The lesion unleashes an inflammatory reaction with clot formation

**Figure 1.** Distraction osteogenesis scheme. (A) Osteotomy, (B) gap increase with ossification centers (black dots), (C) progressive gap increase, confluence of ossification centers, and peripheral bone formation and (D) hard callous bridg‐ ing the gap with mild corticalized margins and central invagination.

between and around the osteotomy segment and cytokine activation, growth factors, and mesenchymal cellular aggregation. A granulation tissue is formed, consisting of extracellular matrix rich in fibrin, collagen, neutrophils, fibroblasts, and cells with osteogenic potential [3]. This set is called soft callus (see **Figure 1**).

The goal of this phase is the formation and maturation of the soft callus, with sufficient neovascularization to stimulate ossification. Classically, its duration is established around 5– 7 days. There is no need for monitoring in this phase.

#### **1.2. Lengthening phase**

pedic and maxillofacial surgeries. However, it is a long-duration therapy and the patient may

The treatment duration generates great physical and psychological morbidity. Its importance was recognized by Ilizarov himself when he postulated: "keeping the apparatus for longer than necessary is as damaging as removing early" [2]. Therefore, the optimization of therapy is fundamental to reduce the complications and ensure its success. For that reason, the distraction osteogenesis monitoring must be realized. Several methods are available, with some being more suited to each phase. The histological characteristics of the regenerated tissue in each phase are fundamental for the comprehension of the monitoring methods, as we see

Latency period consists in the physiological tissue repairing response initiated immediately after the osteotomy. The lesion unleashes an inflammatory reaction with clot formation

**Figure 1.** Distraction osteogenesis scheme. (A) Osteotomy, (B) gap increase with ossification centers (black dots), (C) progressive gap increase, confluence of ossification centers, and peripheral bone formation and (D) hard callous bridg‐

ing the gap with mild corticalized margins and central invagination.

remain in use of an external fixator for a period of up to 20 weeks.

later.

**1.1. Latency period**

84 Advanced Techniques in Bone Regeneration

After the latency phase, the distraction apparatus is activated and the soft callus is strained initiating several osteogenic processes. The straining produced in the longitudinal axis alters the cellular expression in the soft callus. At the gap center, high strain forces inhibit cellular differentiation and stimulate its proliferation. In the extremities, strain forces are milder, thus allowing cellular differentiation. Therefore, the callus center has high cellular density com‐ posed of precursor mesenchymal cells (fibroblasts and pre-osteoblasts), while the extremities are paucicellular and composed mostly of osteoblasts. Hence, the osteogenesis process occurs from the deeper region of the extremities toward the center and surface [3, 4].

The formation of bone tissue is modulated by the rate of distraction and its rhythm. The first is the total length gain in a day, which varies from 0.5 to 1.0 mm/day; the second refers to the number of daily activations to reach the desired length, varying from two to four activations per day. Excessive traction tends to stimulate the formation of cysts and fibrous tissue, reducing the resistance of the regenerated tissue; on the other hand, less traction leads to precocious consolidation, making it impossible to achieve the desired length [5].

#### **1.3. Consolidation phase (neutral fixation)**

Reaching the desired length, the external fixator is locked, fixing the bone extremities and ensuring proper support during the consolidation period. The apparatus must remain in position until the regenerated tissue is sufficiently consolidated to avoid complications such as deformations or post-distraction fractures. The duration of this period is usually defined as the number of the days of the lengthening phase multiplied by two (lengthening phase × 2). This rule, however, does not avoid the occurrence of complications, making other parameters of evaluation necessary.
