Retraction and force analysis of transporting bone segment during Ilizarov bone transport

Background: Previous research rarely reported the cause and relevant factors of retraction of transporting bone segment (TBS). The purpose of this study is to analyze the force and cause of retraction of TBS during Ilizarov bone transport after removal of its xator. Methods: 37 cases with tibial bone defect treated by Ilizarov bone transport, in whom the xator of TBS was removed before mature of mineralization of the distraction callus or union of the docking site, were analyzed retrospectively. Bivariate correlation was used to analyze relationship between retraction distance of TBS and age, gender, disease course, length of bone defect, times of pre-operation, size of TBS, transport distance, cause of removal, timing and time interval of removal of TBS xator. Risk factors with signicant level were further identied using multivariate linear regression. Results: Bivariate correlation analysis showed the timing of removal was negatively correlated while the time interval, cause of removal, transport distance and size of TBS were positively correlated with the transport distance (cid:0) all p<0.05), however the age, gender, disease course, length of bone defect and times of pre-operation were not correlated with the transport distance (cid:0) p>0.05). Multivariate linear regression analysis showed the timing of removal, transport distance and size of TBS were signicant risk factors for the retraction distance (p<0.05), of which, the timing of removal had the greatest impact, followed by the transport distance and size of TBS, however the cause of removal and time interval were not signicant for the retraction (p>0.05). Conclusion: The traction force of the TBS endured from soft tissue , not from the distraction callus, is elastic and can induce a retraction of TBS when its xator is removed in advance. The retraction distance is related to the size of TBS, transport distance and timing of removal, especially the timing of removal is an independent risk factor.


Introduction
Callus distraction (distraction osteogenesis) is to perform slow bone transport or lengthening using external distraction system or intramedullary distraction system after osteotomy by Ilizarov's method has been an effective technique to reconstruct large bone defect and correct discrepancies of limbs [1][2][3], of which, external distraction system is more common in clinic. Removal of external xator is usually performed after mature of mineralization of the distraction callus and union of the docking site in conventional Ilizarov bone transport. However, conventional Ilizarov bone transport presents high rates of delayed union and nonunion of the docking site and pin-track infection or loosening, inconvenience for rehabilitation and nursing, and psychological disorder induced by long-term external xation [3][4][5][6]. These complications and defects have become the bottleneck for the development of this technique. Some scholars [7][8][9] reported modi ed Ilizarov bone transport to avoid di cult healing of the docking site, in which the external xator was removed when the docking site is closing and then plate or intramedullary nail was used as a relay internal xation (bone graft may be performed at the same time). Sometimes, the external xator must be removed in advance in situations of pin-track problem (pin-track infection or associated with loosening) is not effective for conservative treatment or patient can't tolerate long-term external xation, then a relay internal xation was used. In these cases, 1-2 weeks or more time are needed for the healing of the pin-track before implantation of relay internal xation. During the period between removal of total external xator or TBS xator and new implantation of relay internal xation, retraction of transporting bone segment (TBS) may occur even if plaster cast was used. The retraction of TBS has adverse effect on the healing of the docking site. Previous literatures paid more attention to the bone healing and complications in distraction osteogenesis, rarely reported the cause and relevant factors of retraction of TBS [10][11][12]. The clinical data of 37 patients with removal of TBS xator in advance during bone transport process in our hospital from January 2009 to December 2018 were analyzed retrospectively to investigate the force and cause of retraction of TBS.

Inclusion and exclusion criteria
The inclusion criteria were Patients with tibial defect were treated by Ilizarov bone transport; TBS fixator or total external fixator was removed before mature of mineralization of the distraction callus or union of the docking site. The exclusion criteria were Patients with incomplete radiographic data; Age less than 15 years old;3Another external xator was used after TBS fixator or total external fixator was removed. This study was approved by the ethics committee of Wuxi No.9 People's Hospital and Shuyang People's Hospital, and written informed consents were obtained from all participants.

Patients
The present retrospective study was approved by the Ethics committee of Wuxi Ninth
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][11][12][13][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][11][12][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][11][12][13]. Horas et al. [11] conducted a experiment using eight cadaveric thigh specimens with 60 mm bone defect at the middle femur to assess the traction force required for 40mm 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 [12]; 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 [11].
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 signi cant 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. [7] reported on patients with large tibia bone defect treated by modi ed 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 xator 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 signi cant 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 xation 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 ndings 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 xator 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 xator, especially the timing of removal is an independent risk factor.

Con ict of interest
The authors declare that they have no con ict of interest.

Funding
None.

Figure 1
Scatter diagram of the regression model standardized predicted value of retraction distance