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 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. They come from different sites or directions, and have different properties.
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 gradually appears at distraction period, then gradually becomes dense, and finally it matures at consolidation period. Complete mineralization of the callus distraction (complete consolidation) can prevent the retraction of TBS.
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 transport distance and the size of TBS were closely related to the magnitude of traction force generated by its adjacent soft tissues. However, the timing of removal was not considered in this study.
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 its peak and the latter gradually increases; in the late stage (>6 months after bone transport), the former becomes 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, certain degree of retraction often observed in their study. In this study, most patients with retraction of TBS were within 8 months postoperatively, 3 patients were in more than 10 months postoperatively due to delayed mineralization; the callus distraction in all patients with retraction of TBS was incompletely mineralized, i.e. the timing of removal is closely related to the magnitude of traction force.
Beside distraction force, time interval is another important factor influencing retraction distance of TBS. The longer the time interval, the more the retraction. In the typical case 1 of this study, the timing of TBS removal was earlier (3.5 months), the time interval was longer (25 days) , which resulted in great retraction (30 mm). Our study showed that the timing of removal and time interval are the key factors influencing the retraction of TBS, especially the timing had the greatest impact, followed by the time interval, but the transport distance and size of TBS are not key factors influencing the retraction of TBS.
Understanding the force and retraction phenomenon of TBS during bone transport is helpful to take corresponding measures to avoid adverse effect or complications. For example, the ends should be pressurized for 2 ~ 3 weeks when the docking site is closed in traditional bone transport; the earlier the time of removal of fixator, the longer the time interval, the larger the retraction distance. In this situation, another external fixation method should be used to avoid the adverse effect of more retraction of TBS on the healing of the the docking site. Otherwise more bone graft is needed because it is difficult to complete the reduction and closure.
This study explored the causes and relevant factors of the retraction of TBS during Ilizarov bone transport. The findings of this study are useful for understand of the retraction of TBS, improving prognosis and reducing complications of bone transport in the treatment of bone defect.