Despite significant progress in spinal fusion surgery over the past several years, the pursuit of minimally invasive spine surgery is ongoing. Lumbar spine fusion remains an efficient surgical method for treating various lumbar degenerative diseases, including spondylolisthesis and spinal stenosis (1–3). The percutaneous endoscopic transforaminal lumbar interbody fusion technique, also known as Endo-TLIF, has emerged significantly in recent years, drawing the attention of spine surgeons owing to its benefits, such as reduced trauma, less bleeding, conservation of soft tissues, quicker recovery rate, short duration of hospital stay, and better clinical results (4–6). In 2000, Sehat et al. were the first to describe the notion of hidden blood loss (HBL). They discovered that it comprised 26% and 49% of total blood loss (TBL) following total knee and hip replacement operations, respectively (7). Subsequent studies have shown that HBL contributes to increased TBL and risk of postoperative blood transfusion if not properly managed (8, 9). Furthermore, elevated HBL levels can result in extended periods of hospital stay and recuperation, slow progress in wound recovery, heightened susceptibility to infections, and increased risk of cardiovascular and cerebrovascular accidents due to augmented bleeding volume (10, 11). Given the growing concern among spinal surgeons regarding HBL, an increasing number of researchers have investigated this issue (12–14). However, despite the expanding literature on HBL and minimally invasive spine surgery, no study has yet been conducted to quantify HBL and analyze its risk factors specifically in the context of Endo-TLIF. Ignorance of hidden blood loss after Endo-TLIF can lead to several complications in patients due to increased blood loss, such as anemia, increased rates of postoperative infections, prolonged hospital stays, and slow incision recovery. To address this, we carried out a retrospective study to determine the amount of HBL in Endo-TLIF procedures and investigate the associated risk factors. Our aim was to provide theoretical guidance to spinal surgeons performing Endo-TLIF to prevent related complications.

In our study, 83 patients (42 males and 41 females) underwent Endo-TLIF surgery. The average age was 58.6 ± 9.9 years (35–79) and average BMI was 24.5 ± 2.8 (18.5–31.3). The demographic information is presented in Table 1. The Hct and Hb preoperative readings were 0.412 ± 0.04 and 140 ± 15 g/L respectively, whereas the postoperative readings were 0.338(P < 0.001) and 110 ± 16(P < 0.001), respectively. Meanwhile, 58 patients developed anemia after surgery (P < 0.001, Table 2). The findings of the single-factor correlation analyses are presented in Table 3. Hypertension (i.e., blood pressure ≥140/90 mmHg) (P = 0.003), fusion levels (P = 0.003), and surgery time (P < 0.001) were significantly correlated with HBL through single-factor correlation analysis. However, the factors such as sex (P = 0.597), age (P = 0.420), BMI (P = 0.860), diabetes (i.e., fasting blood-glucose ≥6.1 mmol/L) (P = 0.809), osteoporosis (i.e., Dual-emission x-ray absorptiometry, T value ≤–2.5) (P = 0.219), smoking (P = 0.103), drinking (P = 0.978), Pfirrmann grades (P = 0.150), duration of symptoms (P = 0.795), preoperative lumbar subcutaneous fat tissue thickness (P = 0.867), preoperative FIB (P = 0.666), APTT (P = 0.778), TT (P = 0.650), PT (P = 0.820), and Plts (P = 0.373) were not correlated with HBL. Multiple linear regression analysis revealed a significant correlation between fusion level (P < 0.001) and surgery time (P < 0.001) with HBL (Table 4).

Degenerative lumbar spine disease is common in older patients and frequently causes symptoms such as lower back pain, shooting pain in the lower limbs, and periodic limping. These can lead to decreased movement, possible disability, and substantial reduction in the patient's quality of life. An efficient method to treat such degenerative conditions of the lumbar spine is through a procedure known as lumbar interbody fusion. It directly decompresses the nerve roots, corrects lumbar lordosis, and indirectly restores the height of the lumbar spinal space, ultimately achieving cure.

While traditional open procedures, such as PLIF and TLIF, yield positive results, they are often poorly tolerated by many older patients with underlying health conditions. Open surgery requires large incisions, muscle stripping, and pulling, increasing the risk of complications such as excessive blood loss, extended postoperative recovery time, and higher chances of postoperative infections. Recently, more spine surgeons have turned to Endo-TLIF surgery, a less invasive operation that causes less bleeding and involves small incisions with a cannula-protected operation, offering improved protection for the soft tissues, reducing postoperative infection risk, and promoting quick recovery. Existing literature suggests that Endo-TLIF effectively alleviates pain in older patients with poor underlying conditions who struggle with open surgery, significantly improving their quality of life (14, 20–22).

HBL is a condition where bleeding is undetectable during surgery. Traditionally, physicians have relied on estimates and formula calculations to determine blood loss. However, research conducted by Sehat et al. indicated that this technique is susceptible to a considerable number of errors. He demonstrated how postoperative blood can seep into muscle gaps and underlying compartments, leading to hidden bleeding that cannot be measured accurately, referred to as HBL (7).

HBL poses serious risks to patient recovery and safety after surgery. Underestimating blood loss can result in postoperative anemia, which in turn can weaken the patient's strength and immunity and increase the risk of infection and complications. Moreover, the healing process of wounds may be hindered by covert blood loss, which contributes to leakage of blood into tissue areas, consequently increasing the possibility of infection and development of scars. Furthermore, it can lead to prolonged postoperative hospitalization and increased utilization of healthcare resources.

Consequently, surgeons have begun to focus on addressing concealed blood loss. An increasing number of scholars have discovered that even in minimally invasive surgery, HBL cannot be ignored (Zhou et al. noted a substantial HBL (488.4 ± 294.0 ml, 52.5% of TBL) post MIS-TLIF treatment (12). Similarly, an average hidden blood loss of (317 ± 156) ml was discovered in 125 osteoporotic vertebral compression fracture patients treated with percutaneous vertebroplasty (23). Wang et al. reported notable HBL of approximately 469.5 ± 195.3 ml following UBE surgery, constituting 57.6% of TBL (13). Additionally, patients who underwent anterior cervical discectomy fusion experienced a substantial HBL of approximately 487.98 ± 255.96 ml, making up 67.61 ± 5.20% of the TBL (24). However, there is currently a lack of research on HBL in Endo-TLIF, preventing the calculation of HBL and analysis of risk factors. This knowledge gap hinders the effective management of patients undergoing Endo-TLIF. To resolve this problem, we conducted a retrospective study on HBL and its risk factors in patients who underwent Endo-TLIF. Through this, we intended to offer theoretical assistance for postoperative care and minimize the complications caused by excessive blood loss. This study included various risk factors such as sex, age, BMI, hypertension, diabetes, osteoporosis, smoking, drinking, Pfirrmann grades, fusion segments, symptom duration, preoperative lumbar subcutaneous fat tissue thickness, operation time, preoperative FIB, PT, APTT, TT, and Plts. In this study, we found that the HBL in patients treated with Endo-TLIF was 819.8 ± 204.9 ml. Single-factor correlation analysis revealed that increased blood pressure, operation time, and fusion segment were associated with high HBL. Multiple linear regression analysis confirmed that the duration of operations and fusion segments independently contributed to the risk of HBL, consistent with the findings of Zhang (25). Long operation times may result in increased HBL, possibly owing to prolonged washing time, extended wear time of the wound surface, and more severe inflammatory reactions, leading to increased hemolysis. This finding is consistent with the conclusions of Wang et al. and Lei et al (13, 26). Prolonged surgery may involve greater tissue cutting and incision widening, leading to increased blood vessel damage and HBL. It can also cause localized tissue edema, further increasing the risk of HBL (26). Analysis of the increase in HBL associated with the fusion segments revealed that an increase in the fusion segments led to greater soft tissue damage and bone destruction. With more fusion segments, additional tissue spaces and surgical cavities were created, increasing the blood storage capacity of HBL. Furthermore, bone debris from multi-level decompression and intervertebral space treatment may enter the bloodstream, causing abnormal capillary bed opening and blood extravasation, thereby increasing HBL. Removing the joints and ligaments located between the disc and vertebral body affects the blood flow to the fused area, reducing blood supply and leading to ischemia and concealed hemorrhage within the fused section. This finding aligns with the conclusions of Yu et al. (27). In our study, we discovered that factors such as sex, age, BMI, hypertension, diabetes, osteoporosis, smoking or drinking habits, Pfirrmann grades, symptom duration, prior lumbar subcutaneous fat tissue density, preoperative FIB, PT, APTT, TT, and Plts examinations had no substantial correlation with HBL. These results contradict those of the previous studies. Wang et al. identified age and preoperative FIB as risk factors in patients undergoing UBE surgery (13), while Zhou et al. revealed that age, muscle thickness, and preoperative FIB were independent risk factors for HBL in MIS-TLIF (12). Dai found that diabetes and hypertension are risk factors for HBL, which may be due to varying surgical procedures. Our Endo-TLIF procedure involves four small incisions of approximately 1 cm for decompression, fusion, and screw placement. This approach minimizes soft tissue damage and handles cartilage endplates under direct endoscopic visualization to reduce bone damage, thus decreasing HBL and mitigating risk factors associated with it. Moreover, the variety of patient populations with differing types and severity of conditions that various studies may incorporate can cause disparities in outcomes. In our study, the TBL of Endo-TLIF was 880.7 ± 219.8 ml. Some scholars have suggested that the use of new bone materials, such as biologics or allografts, may reduce TBL (28). Nevertheless, some studies have suggested that autologous iliac bone is the preferred graft for spinal fusion surgery. Consequently, we continued to extract the iliac bone during the surgery. However, this procedure can lead to concealed blood loss, and an overall increase in blood loss (29–31). Therefore, it is crucial to use bone wax and other techniques to effectively control bleeding after bone removal.

Our study has several limitations. Primarily, this was a retrospective study that focused on a single center and involved a small number of patients. The included risk factors are not comprehensive enough. To mitigate the risk of false positives and bias, we need to execute a prospective, multicenter study with a large sample size in the future to enhance the accuracy of the results. These factors likely influenced our findings. Secondly, we used Hct levels measured 2–3 d postoperatively, but it was still not possible to determine the balance of blood circulation, potentially causing errors in the amount of HBL calculated using the formula. Lastly, all our surgical cases were performed during the platform stage of the surgeons’ operations, ignoring the surgery-related situation of spine surgeons at the rising stage of the learning curve; therefore, the correlation between surgical experience and HBL was not addressed.

To summarize, a significant percentage of the TBL in patients with lumbar degenerative diseases treated with Endo-TLIF can be attributed to HBL. An increased number of fusion segments and prolonged operation time are risk factors for increased perioperative HBL during Endo-TLIF. Attention to HBL is critical for ensuring patient safety during perioperative care.