Gustilo type IIIC fractures of the tibia might result in nonunion, surgical site infection, osteomyelitis, and amputation . Various factors which may influence the union of open tibial fractures have been proposed in the literature [8, 10]. However, this is the first retrospective study dedicated to the prognosis of type IIIC tibial fractures regarding union time, osteomyelitis, and amputation.
We found a 5.2% rate of nonunion after a two-year follow-up, comparable with findings of 7.4% and 6.8% nonunion in all tibial fractures reported previously [14, 15].
In our cohort, a significantly lower number of cases with multiple trauma or triple arterial injury achieved union within 2 years compared to those without either condition. One patient had multiple trauma and triple arterial injury simultaneously. Although the result was not statistically significant, a slightly longer mean union time was noted for distal third tibial fractures compared to fractures that did not involve the distal tibia. The length of the bone defect significantly affected the union time in univariate analysis and the multivariate model, and the probability of union within 2 years decreased with an increase in the bone defect length.
Distal third tibial fractures have been proposed as an adverse factor in healing of open tibial fractures [16–18]. Stranix et al.  proposed that these fractures are commonly complicated by damage to the PTA and peroneal artery, which usually implies a more serious impact during trauma than injuries to the ATA at the middle third tibia. A study of 18 patients with type IIIC fractures found that PTA injury was associated with an increased rate of nonunion . The PTA supplies blood to the inner two-thirds of the diaphyseal cortex of the tibia, while the outer third of the diaphyseal cortex is supplied by the periosteum. Therefore, PTA injury, with severance of the posterior envelope in distal fractures, may compromise tibial union [18, 19]. Fractures with PTA injury were more inclined to unite after 2 years or require amputation compared to those without PTA injury; however, these results were not statistically significant.
Six patients with ISS ≥ 16 points, which was a risk factor for nonunion in this study, had hypovolemic shock associated with the trauma, and the reconstructions were deferred until the patient’s vital signs had stabilized. Bundkirchen et al.  evaluated the healing process after hemorrhagic trauma in a mouse model, which revealed a reduced callus density. Subsequently, they concluded that healing is impeded when severe hemorrhagic shock occurs.
A bone gap > 40 mm was associated with delayed union or nonunion/amputation in the multivariate model, while fractures with a bone gap of less than 30 mm healed within 2 years. A randomized controlled trial of tibial fractures proposed 1 cm or 50% of the circumference of tibial diaphyseal defect as a critical cut-off for bone defect size . However, a systematic review by Azi et al.  found no direct relationship between the size of the bone defect and the union rate when NVBG was applied in open fractures of long bones. Although 12 cases with VBG or Masquelet had a longer bone gap than the remaining patients in our cohort, the difference in the union time was not statistically significant, which supports our theory that the bone gap might not be the most important factor affecting union time.
Osteomyelitis and related predicting factors
Stepwise modelling of multivariate logistic regression of nine cases of osteomyelitis revealed that a time from injury to definitive soft tissue coverage that was longer than 22 days, which was the third interquartile in our data, significantly increased the risk of osteomyelitis (Fig. 2).
Despite the agreement that soft tissue coverage within 1 week benefits the outcome of severe tibial fractures [9, 23], soft tissue coverage might be delayed for many reasons . In our cohort, a 22 day cut-off represented the third interquartile range of time for definitive soft tissue coverage. Of the fractures with osteomyelitis, 44.4% received definitive soft tissue coverage more than 22 days after the initial trauma—these fractures were complicated by multiple traumas which delayed the timing of soft tissue coverage. Our results were consistent with a previous study, which proposed that severely traumatized tissue commonly requires more debridement to obtain a clear margin for better wound healing potential, which often delays the definitive coverage to > 2 weeks, and a longer time for wound coverage independently predicts infection . In contrast, Stranix et al. found that the timing from injury to flap coverage was not associated with increased major complications in their study; however, their study focused more on flap failure rate rather than infection .
Amputation and related predicting factors
There were five cases of early amputation (amputated within 1 d) and seven delayed amputations (amputated after 1 d). Among the 12 amputations, nine were amputated despite initial revascularization attempts, and three did not receive revascularization due to extensive soft tissue loss or avulsion injury. Multivariate logistic regression showed that diabetes, bone gap > 50 mm, lower BMI, and triple arterial injury increased the risk of amputation in IIIC tibial fractures, despite attempts at revascularization (Fig. 3).
There was a 20.7% amputation rate in our cohort, which was similar to the findings of 19.4% and 21% in previous studies [18, 26]. Although the injured arterial structures were not predictive of the union time, triple arterial injury was a risk factor for amputation in multivariate analysis. Three of six triple arterial injury cases received immediate amputation in our cohort.
A low BMI was noted as a significant risk factor for amputation in multivariate analysis. Literature regarding multiple trauma has shown an “obesity paradox,” in which the overall survival seemed to be higher in obese patients than in underweight patients [27, 28]. One study found that a BMI < 18.4 kg/m2 was associated with an increased risk of mortality after multiple trauma . We surmised that a lower BMI indicated less subcutaneous fat coverage to cushion or protect neurovascular and bony structures.
Diabetes mellitus (DM) has been proposed to impair healing and cause chronic wounds . A significantly higher amputation rate was noted in patients with DM compared to those without DM (4.3%). Piwnica–Worms et al.  found a significant relationship between DM and the amputation rate in 129 patients with lower extremity trauma who required free flap reconstruction, which corroborated with our findings.
The limitations of our study stem from its retrospective nature. We were limited to existing medical records, and cases with missing information were excluded. Although more patients were enrolled as compared to that in previous studies of type IIIC tibial fractures, the principles of management might differ between surgeons, which could not be controlled for. Outcomes regarding the quality of life should also be evaluated and analyzed in future prospective studies.