Distraction osteogenesis is used to perform slow bone transport or lengthening using external distraction system or intramedullary distraction system after osteotomy. The resistance force (traction force) of the TBS suffered comes from two aspects during bone transport: one is generated from the distraction of the soft tissues around TBS, and the other is generated from the distracted callus at the lengthening site.
Although the periosteal connection was cut off after osteotomy, TBS still has adherent structure such as fascia, tendon or muscle, nerve, vessels, skin, tendons, ligaments and the connections among them. The magnitude of the traction force from soft tissue reported differs by different authors [10-13], which is mainly related to the transport distance, site and size of TBS. In general, 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. The force increased sharply at 0-10 mm transport distance, then slowly increased at 10-50 mm distance, sharply increased again to the 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 related to the magnitude of traction force generated by its adjacent soft tissues. However, there is no report on the effect of the timing of removal of external fixator on the retraction distance
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 [12,15,16]. The distraction callus gradually appears at distraction period, then gradually becomes dense, and finally matures at consolidation period. Full consolidation (maturation of consolidation) of the distracted callus can prevent the retraction of TBS.
There were still different opinions on which is the main force and which is the force causing retraction of TBS [10-12]. Aronson et al.  concluded that with the increase of transport distance, the traction force generated by distraction callus gradually increases, which is greater than that generated by soft tissues. However, Wolfson et al.  considered that soft tissue plays a decisive role in the generation of traction force. We believe that two kinds of traction forces of TBS endured change dynamically during bone transport, and have different properties. In the early stage (within 3 months after bone transport), the traction force from the soft tissues is greater than that from the distraction callus 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 gradually decreases while the latter gradually reaches its peak, which becomes an important role. The traction forces of TBS endured from the soft tissues and the unconsolidated distracted callus have elastic properties (as the tissue in the unconsolidated distracted callus is collagen aligned in linear bundles that can shorten like a spring until calcified) [10-14], which can make retraction of TBS. However, the traction force from the consolidated distracted callus has anti-retraction properties, which can prevent the retraction of TBS. The earlier the timing of removal, the larger the elastic traction forces, so the greater the retraction distance, while the later the timing of removal, the smaller the elastic traction forces, so the lesser the retraction distance, that is, the timing of removal is closely related to the elastic forces or the retraction distance of TBS. Juzheng H et al  reported an improved bone transport through conversion of external fixation to internal fixation after closure of the docking site, certain degree of retraction was observed in their study. In our study, most patients were within 8 months after operation, only 3 patients were in more than 10 months after operation due to delayed consolidation; the distracted callus in all patients with retraction of TBS was not fully consolidated.
Beside traction force, time interval is another important factor influencing the 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 large retraction distance (30 mm). Our study showed that the timing of removal and time interval are the main factors of the retraction distance, especially the timing of removal had the greatest impact, followed by the time interval, but the transport distance and size of TBS are not the main factors.
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; In the case of early removal, another external fixation should be performed or quickly converted to internal fixation to avoid the adverse effect of more retraction on the healing of the docking site. Otherwise, more bone grafts are 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. Those findings are helpful to understand the retraction of TBS, improve prognosis and reduce complications of bone transport in the treatment of bone defect.