This study aimed to collect and assess the postoperative results and distribution of complications related to bone transport using a unilateral external rail fixator in treating femoral bone defects caused by an infection in our hospital from January 2008 to December 2019. Radical debridement combined with antibiotic spacer was necessary to ensure an infection-free limb. Bone transport using an external rail fixator was a pragmatic method to reconstruct the femoral defects and resolve the limb deformity, such as shortening, angular, sclerosis, and muscle atrophy caused by the initial injury. In this study, the rate of total bone union was 100%, and the per-patient complication was 0.82. The excellent and good rate of bone and function of the proximal group was better than the others. Pin tract infection, muscle contracture, axial deviation, and soft tissue incarceration were more likely to occur in the intermediate group, and the distal group was at high risk of joint stiffness, delayed union, or nonunion.
It’s generally recognized that the defect and deformity can be resolved simultaneously by the bone transport using an external fixator, allowing the patient to utilize the limb function earlier to prevent joint stiffness or arthritis [1,2,3,4, 17], However, this technique is also a great challenge for surgeons since the long EFT and tricky complications [18, 19]. Some interventions have been applied to shorten the EFT, such as fixation combined with an internal and external fixator, cyclic distraction and compression technique at the consolidation stage, or injection of biological agents that promote bone healing in the distraction area, etc. [7, 11, 20]. For instance, Gupta et al.  reported a consecutive series of 14 patients with tibia nonunion, and satisfactorily managed by bone transport using a monorail fixator combined with locking plates. All patients had a mean defect size of 6.4 cm, a mean external fixator index of 21.2 days/cm, and a per-patient complication incidence of 0.5. Furthermore, the study published by Gulabi et al.  presented a series of five patients with tibia nonunion successfully treated by bone transport using Ilizarov fixator combined with an intramedullary nail. The above-improved methods indeed possessed advantages in shortening the EFT, but the heavy financial burden and complex surgical procedures make them difficult to utilized widely. Hence, what’s far more important for the application of bifocal bone transport is to describe the characteristic of different locations’ bone transport and the distribution of the complications.
The high density of muscles, blood vessels, and soft tissue are distributed around the femur, especially the proximal, which facilitates bone regeneration and mineralization [1, 3, 23]. But the potential risks are kept company with these advantages. As for the surgical procedure of bone transport, the subcutaneous tissue and blood vessels are easy to be involved when inserting screws, which results in tissue necrosis and infection. In addition, the greater counterforce caused by the plentiful muscle is also left to the screws, which hinders the sliding of the transport bone segment. It has also been noticed that the distraction gap may be filled by soft tissue before the end of the distraction stage. For these, the use of a screw sleeve was recommended to plan the screws’ inserting position preoperatively and assist the insertion intraoperatively, which can reduce the probability of damaging the blood vessels and soft tissues. The periosteal formation induced by the antibiotic bone spacer after radical debridement had been suggested by the recent study, which may reduce the occurrence of soft tissue incarceration [19, 24]. And the utilization of hydroxyapatite-coated screws in the metaphysis and cortical screws in the intermediate may increase the holding force and reduce the risk of axial deviation [19, 21, 24]. Besides, the satisfactory bone union of the proximal group may be related to the selection of osteotomy line in the metaphysis of the femur, which provides fruitful blood supply for avoiding the docking site nonunion.
The autogenous bone grafting at the docking site was recommended to perform when the distraction stage finished [3, 23, 25,26,27]. Wan et al.  showed a series of 28 femoral bone defects treated by bone transport using a monolateral external fixator combined with bone grafting at the docking site, and obtained a good bone healing rate of 92.8%. Furthermore, the study published by Yin et al.  presented a cohort of 110 patients with bone defects of the lower extremity were treated successfully by bone transport, and the bone grafting at the docking site was applied as well. In our cohort, the bone grafting was managed at the end of the distraction stage, and satisfactory bone healing results were obtained. However, there was still one patient with nonunion and three patients with the delayed union, which occurred in intermediate and distal femoral bone transport. The delightful results were obtained with the application of bone grafting, but there was still one patient with nonunion in the intermediate group and three patients with the delayed union in the distal group. As far as we were considering, the distal site of the femur adjacent to the attachment point of the muscle resulted in poor blood supply. Fortunately, bone union was received finally after adjustment of the external fixator to compress the regenerate tissue zone until the regenerate bony parts were contacted.
Pin tract infection (38.1%) cannot be ignored, which occurred in the intermediate group and the proximal group mostly [24, 29, 30]. The reason for this phenomenon may be related to the developed musculature, rich soft tissue, and blood supply of the intermediate. This also explained why the more axial deviations and muscle contractures took place here. Previous studies have found that the occurrence of pin tract infection is related to gender, obesity, smoking, and steroid use [31, 32]. Although there is no difference in the comparison of statistical factors among the three groups in our cohort, quitting smoking and maintaining healthy living habits during treatment are conducive to the prevention of pin tract infection. Concurrently, regular X-ray examination of the affected limb was also essential to promptly prevent the axial deviation, especially applying the unilateral fixator. As for joint stiffness, the quadriceps plasty methods and their modifications may be a resolution to knee stiffness [18, 31,32,33]. But, complications may be brought out by the above method, including scar contracture, skin necrosis, wound dehiscence, edema, and severe pain. In our cohort, quadriceps plasty was not performed. Quadriceps plasty may be an alternation to manage knee stiffness, but the value of it is difficult to decide since the complex complications. Conversely, knee arthroplasty was a salvage approach that may be considered for the patient’s joint with poor range of motion and persistent pain.
Via published articles [11, 16, 34, 35], the application of bone transport in the treatment of infected femoral bone defects is more satisfactory than in the tibia, especially in the grade of bone healing. For instance, Sen et al.  showed a mean EFI of 31.8 days/cm in a cohort of 17 patients treated by the modified technique of acute shortening and re-lengthening. Additionally, Chou et al.  presented a mean EFI of 0.97 months/cm in a series of six infected femoral nonunions managed by the staged protocol of Ilizarov distraction osteogenesis and intramedullary nailing. In this study, however, the EFT and EFI of our cohort were higher than in the previous study. For this reason, we concluded that the intractable bone destruction was caused by the long duration of infection combined with previous multiple surgical interventions. Thus, detailed preoperative planning, skillful mastery of the external fixation framework, and methodical postoperative management should be prepared for patients with a long duration of infection. The novel combined technique described above can be a workable choice to shorten the EFT and EFI. But the risk of multiple operations’ complications cannot be underestimated, such as long incisions and extensive scars.
Last, several limitations were in this study, which was associated with its retrospective and single-center design nature. First of all, there is no mature algorithm for the resolution of complications in different bone transport locations of the femur. In addition, prospective case series with infected femoral nonunion treated by bone transport were rare. Therefore, a prospective multi-center study of large samples is crucial for clinical guidance.