To the best of our knowledge, this is the first study to examine the relationship between PLR, SII levels and the severity of TPFs. Our study found that patients with severe TPFs were found to have higher PLR and SII compared with patients with mild to moderate TPFs. These results demonstrate that PLR and SII, as economical, simple, and easily measured laboratory parameters, can reflect the radiographic severity of TPFs in young and middle-aged adults, and simultaneously can also be used as effective predictors of the severity of TPFs.
The tibial plateau fracture is a common intraarticular fracture in which the fracture lines usually involving the proximal end of the tibia. These fractures account for 1%-2% of all fractures and often result in knee joint mobility disorders and instability. The commonly used Schatzker classification, which is based on two-dimensional imaging can effectively reflect the severity of TPFs and reveal more details of TPFs. The six basic types of TPFs have been proven and generally accepted as a practical classification criterion. However, the more details the classification presents, the higher the likelihood of disagreement in an inter-and intra-observer basis , therefore, the Schatzker classification is easily influenced by doctors' experience and familiarity with the Schatzker classification.
The PLR and SII indicators have been widely studied in recent years as immunoinflammatory indicators as they are easy to obtain and cost-effective in clinical application. The PLR has great value in predicting the severity and mortality in many diseases such as hip fracture, inflammatory diseases, malignancy, systemic lupus erythematosus, and cardiovascular disease [10–12, 15]. Moreover, the SII has been associated with the prognosis of malignancies, post-traumatic thrombosis, and fractures following osteoporosis [16–18]. Platelets contain a large amount of soluble and cell-related immunomodulatory molecules that enhance the immune response when the body is damaged . Therefore, platelets not only serve as important hemostatic cells but also bind to leukocytes and vascular cells to regulate cellular responses which play an important role in immune defense and inflammatory responses and lead to the activation of inflammation or apoptosis of cells [20, 21]. Multiple studies have shown that various stress events such as tissue damage, severe trauma, and major surgery can stimulate an increase in circulating white blood cells, which are characterized by an increase in neutrophils . For example, Morell et al. found a positive correlation between white blood cell count and the injury severity score . Moreover, Neutrophils can promote tissue repair after injury by removing tissue debris at the site of injury and secreting growth factors or pro-angiogenic factors. Therefore, neutrophils play a similar role in immune defense as platelets and can be triggered by trauma or tissue inflammation [23, 24]. In addition, Numerous stressful events can lead to lymphocyte depletion and further reflect the body's immune system's resistance and adaptability . Therefore, the induction of immune defense and inflammatory responses in the body after a violent injury, such as TPFs, can lead to an increase in platelets and neutrophils and a decrease in lymphocytes, which supports the use of PLR and SII to predict the severity of TPFs.
There are a variety of factors can affect PLR and SII levels. This study tried to exclude the influence of confounding factors. The PLR is an effective predictor of cardiovascular and rheumatic diseases as has been demonstrated in many studies [13, 15]. In addition, PLR has been reported to be associated with the prognosis of many severe injuries, dementia, and malignancies in recent years [10, 16, 26]. For example, Wang et al. found that patients with a high PLR (≥ 189) had a higher one-year mortality rate than patients with low PLR (< 189) . Mao et al. found that PLR > 193.55 in patients indicates malnutrition and more advanced cancer stage . Many studies have reported SII to be closely related to fracture after osteoporosis. For example, Fang et al. found that SII ≥ 834.89 was identified as a significant risk factor for postmenopausal osteoporotic fracture . Therefore, in the screening stage of TPFs cases, we tried to exclude the interference of the above confounding disease factors.
Tibial plateau fractures are usually caused by high-energy injuries and usually involve the lateral plateau, which is prone to serious knee ligament disorders and neurovascular injuries. More importantly, Bicondylar tibial plateau fractures (Schatzker types V-VI)  are often accompanied by severe intramedullary fractures, comminuted fractures, and extensive intramedullary hemorrhage, and may lead to greater latent blood loss . However, Schatzker classification based on two-dimensional imaging system is still not satisfactory in displaying the above TPFs-related injuries, so PLR and SII have unique advantages in reflecting the severity of local TPFs. Moreover, there is often a concentration of body fluids, loss of hemoglobin, and redistribution of red blood cells in patients with severe TPFs, which can lead to changes in HGB, and electrolyte concentration. Therefore, these indicators cannot be used to represent the severity of TPFs[27, 28].
This study also has some limitations. First, the PLR and SII values were only collected at admission; their dynamic changes were not monitored over time after admission. Second, it was a single-center retrospective study with a relatively small sample size and a multi-center large sample retrospective study is needed to confirm our findings. Third, this study failed to elaborate the detailed physiological mechanism of the elevation of PLR and SII in patients with severe TPFs, and there was a lack of clinical evidence to guide surgical treatment. Finally, some elderly patients with low-energy impairments were excluded from the study which may result in selection bias.