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 traction force of the TBS suffered at distraction and consolidation periods comes from two aspects: one is generated from the distraction of the adherent soft tissue of TBS, and the other is generated from the distraction callus at the lengthening site. They come from different sites or directions and have different properties and functions. The former is elastic and make at retraction of TBS, while the latter isn’t elastic and has anti-retraction properties [10-14].
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 traction force from soft tissue begins with transporting of TBS at latency period, gradually become greater at distraction period and reach its peak at the end of distraction period. The magnitude of the traction force from soft tissues is mainly related to the transport distance, site and size of TBS [10-13], i.e The thicker the skeleton, the longer of the transport distance, or the greater the size of TBS, the greater the traction force[10-13]. Horas et al.  conducted a experiment using eight cadaveric thigh specimens with 60 mm bone defect at the middle femur to assess the traction force required for 40-mm and 60-mm long of TBS using a novel type of intramedullary distraction system, conclusion found that the traction force generated by soft tissue was linearly correlated with the transport distance; after a period of sharply increased in force at 0-10 mm transport distance, a relatively slow increased 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 the TBS size and transport distance were closely related to the magnitude of the traction force generated by its adjacent soft tissues.
The distraction callus appears at the early stage of distraction period, then gradually become dense and maturing of mineralization at consolidation period. The distraction force from the distraction callus appears with the appearance of the distraction callus, and gradually increases with the maturity of the distraction callus, i.e it has to do with time[15,16].
Therefore, the two kinds of retraction force of the TBS suffered change dynamically during bone transport. In the early stage (within 3 months after bone transport), the retraction force from the soft tissues is great than that from the distraction callus 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 retraction distance is mainly affected by the distraction force from soft tissue and timing of removal. The greater the retraction force and earlier the timing of removal, the greater the retraction distance. Our study showed that the timing of removal, transport distance and size of TBS were significant risk factors for the retraction distance, especially the timing of removal had the greatest impact. In the typical case 1 of this study, mainly because the timing of TBS removal was earlier (3.5 months), meanwhile the TBS size was larger (15.2 cm) and the transport distance was longer (10.5 cm), which resulted in greater retraction (3cm). Juzheng H et al.  reported on patients with large tibia bone defect treated by modified Ilizarov bone transport using external distraction system, slight retraction after bone transport in 8 months still observed in their study. In this study, we also observed a slight retraction in patients with delayed mineralization after removal of TBS fixator in 10 months postoperatively; all patients with retraction of TBS in whom the distraction callus were immature of mineralization. After statistical analysis combined with the actual situation of the research object, we concluded that the timing of removal is an independent risk factor. Theortically, the time interval may affect the retraction distance. However, it was not a significant risk factor in the study.
In order to avoid the adverse effect on the healing of the docking site caused by retraction of TBS, temporary external fixation should be considered, especially when removal is performed at early stage. This study explored the retraction and force of TBS during Ilizarov bone transport. The findings of this study are helpful to understand the related factors of retraction of TBS, improve prognosis and reduce complications of bone transport in the treatment of bone defect. However, the present study had certain limitations. Firstly, the number of cases is small. Secondly, there is sampling error or bias. Therefore, more clinical data and multicenter studies or experimental research are needed to investigate the relevant factors of retraction of TBS.
In conclusion, when the fixator of TBS is removed before mature of mineralization of the distraction callus or union of the docking site during Ilizarov bone transport, the distraction force from the adherent soft tissue, not from the distraction callus, is elastic and can make a retraction of TBS. The retraction distance is related to the size of TBS, transport distance and timing of removal of TBS fixator, especially the timing of removal is an independent risk factor.