We reaffirmed that TBI have accelerated callus formation and fracture healing in patients with tibia fractures. Time to bridging callus formation and callus ratio were significantly superior in Group 1 despite the lower calcium level. Leukocytosis and lymphocytosis were predominant and RBC profiles including hemoglobin and hematocrit were lower in Group 1. Open fracture, gender, GCS, and presence of concomitant fracture did not show significant differences in radiological outcomes. Increasing age and number of intracranial hemorrhagic lesion were negatively correlated with callus ratio. Higher Marshall classification category showed very strongly positive correlation with time to callus formation.
Garland and Dowling reported absent correlation between TBI and accelerated tibial fracture healing as a pioneer study.  However, it contained mixed cohort of patients including severe open fractures and various surgical or conservative treatment modalities that could have made it inconclusive. Following clinical studies showed an obvious osteogenic effect of TBI from long bones to flat bones. [1–3, 22, 23] However, few factors have been investigated as candidate substances to explain the phenomenon such as runt-related transcription factor 2, serine protease 7, cathepsin K, and hFOB1.19 cell line. [1, 22] Recently, Morioka et al. reported a neuroinflammatory response in polytrauma with TBI in rodent model.  They showed that hematoma formation inferred from systemic lymphocytes, RBCs, hemoglobin and hematocrit was strongly positively related with fracture healing using multivariate principal component analysis. The first stage of bone healing is the inflammatory phase. The inflammatory phase is mainly mediated by fracture hematoma consisted of blood cells, mesenchymal stem cells, fibroblasts and etc., and can last for about 5 days. [4, 7] These cells promote gathering of inflammatory cells via release of pro-inflammatory cytokines such as tumor necrosis factor-alpha, interleukins 1 and 6, and subsequent growth factors. [9, 10, 24] A closer look of the Morioka’s study reveals that TBI additional to tibia fracture showed increased WBCs, monocytes, and lymphocytes and decreased RBC profiles compared to fracture only after 5 days from injury.  These changes gradually recovered to normal range after 15 days from injury. Our results are consistent with results of the previous studies in that Group 1 showed significant elevation of WBC and lymphocyte (P ≤ 0.028) and decrease of RBC, hemoglobin, and hematocrit (P < 0.001) at admission although there was lack of further laboratory tests. Larger hematomas in Group 1 might have accelerated the proinflammatory response to secondary bone healing.
Moderate to severe TBI (GCS ≤ 12) is well known to cause pituitary or hypothalamic dysfunction.  Yang et al. showed promoted callus formation in the fracture with TBI group as well. However, when they subdivided the TBI group to GCS ≤ 8 and GCS > 8, there was no significant differences in time to callus formation and callus thickness (P = 0.521, P = 0.153).  Several hormones such as leptin, prolactin, calcitonin-gene-related peptide from cerebral dysfunction and damage to blood brain barrier are believed to be the possible factors of accelerated bone healing in TBI despite that accurate mechanisms remain uncertain. [4, 26–35] Morioka et al. revealed inverse correlation of fracture callus with brain lesion by analyzing total lesion volume and gross lesion area in TBI.  Cadosch et al. reported a negative linear relationship between GCS and callus ratio.  They showed that GCS was correlated with callus ratio (P < 0.05), time to union (P = 0.04), and proliferation rate of hFOB cells after 6 hours from injury (P = 0.03). Similarly, this study also showed a significantly negative correlation between number of intracranial hemorrhagic lesion and callus ratio (Spearman’s rho = -0.772, P = 0.003), and a negative correlation tendency between GCS and callus ratio (Spearman’s rho = -0.508, P = 0.076). Interestingly, Marshall classification showed a very highly positive correlation with time to callus formation despite its negative correlation with calcium and phosphate levels (Spearman’s rho = 0.939, P < 0.001). Marshall classification places patients into one of six categories of increasing severity based on the findings on non-contrast brain CT scan.  It is primarily concerned with degree of swelling and presence and size of hemorrhage. Higher categories have worse prognosis and survival. Thus, it might be a more accurate assessment of suppression of brain function; therefore, it could be more related to callus formation. Following study of relationship between Marshall classification and bone healing in TBI with larger cohort would help assess the role of brain and estimate the accelerated fracture healing in TBI.
Aging showed a significantly negative correlation with callus ratio in this study (Spearman’s rho = -0.458, P = 0.001). Increasing age has been well known to negatively affect the cellular and molecular processes of fracture healing throughout all phases. [36, 37] Intrinsic changes in stem cell population and microenvironmental changes that alter the biological activity of progenitor cells are the two aspects to potentially affect tissue regeneration.
The two radiological outcomes of time to callus formation and callus ratio did not significantly correlate with each other (Spearman’s rho = -0.089, P = 0.549). Interestingly, time to callus formation was positively correlated with higher RBC profiles (Spearman’s rho = 0.441–0.465, P ≤ 0.002). Rapid callus formation in general could be more related to the number of RBCs capable of exchanging oxygen and waste despite the low RBC profile in group 1 in this study. These conflicting results might indicate that several mechanisms including accelerated hematoma and brain dysfunction have blended effects in promoting fracture healing. Further study with serial laboratory tests would be helpful in distinguishing their effects. Male gender showed significantly superior monocytes, lymphocytes, RBC profiles, and ALP compared to female gender. However, there was no significant differences in the two radiological outcomes between each other. It is comparable to the results of a previous study in which there was no gender difference in fracture healing.  Presence of open fracture below Gustilo type II did not significantly affect any radiological and laboratory tests.
This study has several limitations. First, despite that all patients in both groups were evaluated with the same postoperative follow-up protocol and were intended to be involved thoroughly, this study was a retrospective study which might have resulted in a possible selection bias. Second, the study population was relatively small, which could have decreased the statistical power of the results. We could have derived a more significant categories that might have correlated with each other such as GCS. Third, all patients lacked pre-trauma laboratory tests which could be the reference points for analyzing the lower level of hemoglobin. In addition, there is a weak point that low level of hemoglobin contributed exclusively to the formation of fracture hematoma. However, factors which might have affected the difference of hemoglobin level such as aging and gender were not significantly different between the groups. A previous preclinical study has treated the low level of hemoglobin as the hematoma formation.  Following study regarding these factors can derive further relationship between proinflammatory response and accelerated fracture healing in TBI.