Distraction osteogenesis is to perform slow bone transport or lengthening using external distraction system or intramedullary distraction system after osteotomy. The resistance force of the TBS suffered (equal to the traction force) comes from two aspects during bone transport: one is generated from the distraction of the soft tissues of TBS, and the other is generated from callus distraction at the lengthening site, which come from different sites or directions, and have different properties.
Although the periosteal connection was cut off after osteotomy, there were still adherent structure of the TBS such as fascia, tendon or muscle, nerve, vessels, skin, tendons, ligaments and the connections among them. The magnitude of the traction force from soft tissues reported differs by different authors [10–13], which is mainly related to the transport distance, site and size of TBS. The thicker the skeleton, or the longer the TBS and transport distance, the greater the force[10–13]. Horas et al.  used eight cadaveric thigh specimens to make a 60 mm bone defect at the middle femur, and then assessed the traction force required for 40-mm and 60-mm long of TBS using a novel type of intramedullary distraction system. The results showed that the traction force generated by soft tissue was linearly correlated with the transport distance; after a period of sharply increase in force at 0–10 mm transport distance, a relatively slow increase in force at 10–50 mm distance, whereas it again increased rapidly up to a maximum of 444.5 N at 50–60 mm transport distance; the traction force required for 60-mm long of TBS was higher than that for 40-mm long of TBS. The study indicated that the TBS size and transport distance were closely related to the magnitude of traction force generated by its adjacent soft tissues.
The whole distraction osteogenesis process is divided into three phases: 1–2 weeks of latency period, then about 3–4 months of distraction period, and at last another 3–4 months of consolidation period [12, 15]. The callus distraction gradual appears at distraction period, then gradual become dense and mature of mineralization at consolidation period. Mature of mineralization (complete consolidation) of the callus distraction can prevent the retraction of TBS.
There were still different opinions on the main traction force of the TBS endured during bone transport [10–12]. Aronson et al.  concluded that with the increase of transport length, the traction force generated by callus distraction gradually increases, which is greater than the traction force generated by soft tissues. However, Wolfson et al.  considered that the soft tissues play a decisive role in traction force generation. We believe that two kinds of traction forces of the TBS change dynamically during bone transport. In the early stage (within 3 months after bone transport), the traction force from the soft tissues is great than that from the callus distraction and becomes an important role; in the middle stage (3–6 months after bone transport), the former reaches a peak and the latter gradually increases; in the late stage (> 6 months after bone transport), the former become small, the latter gradually increases and becomes an important role. The former has elastic properties, whereas the latter does not have elastic properties and has anti-retraction properties, which can prevent the retraction of TBS [10–14]. Therefore, the retraction of TBS is induced by soft tissue, rather than callus distraction. Juzheng H et al.  reported on patients with large tibia bone defect treated by bone transport using external distraction system and relay plate internal fixation, slight retraction after bone transport in 8 months still observed in their study. In this study, we observed that in patients with delayed mineralization, there was still a slight retraction after the removal of TBS fixator in 10 months postoperatively. All patients with retraction of TBS in whom the callus distraction were immature of mineralization.
The retraction distance is mainly affected by the magnitude of distraction force and time interval of TBS removal. Theoretically, the longer the time interval, the more the retraction. However, the data of time interval in this study is concentrated and not normal distribution, so it is not correlated with the transport distance. Our study showed that the transport distance and the size of TBS, especially the timing had the greatest impact; the cause of removal is another important factor affecting the retraction because it reflects the timing of removal as refractory pin-track problem was usually at early phase whereas healing difficult of the docking site was usually at late phase. In the typical case 1 of this study, the time interval was longer (because of pin-track infection and loosening, internal fixation and bone grafting were not performed until 4 weeks after removal of TBS fixator), the timing of TBS removal was earlier (3.5 months), the TBS size was larger (15.2 cm), the transport distance was longer (10.5 cm), which resulted in great retraction (3 cm), showing that the retraction distance of TBS is mainly related with the timing of removal, TBS size, transport distance and time interval of removal.