PELD is a typical minimally invasive discectomy procedure that avoids the destruction of posterior spinal tissue and can be a feasible alternative to the conventional posterior approach for LDH. China has made the most contributions, based on the number of publications, to the field of fully endoscopic spine surgery . Meanwhile, increasing attention has been given to the surgical complications of PELD. rLDH is a common complication of PELD and the most common reason for an unsatisfactory outcome following PELD for LDH. However, there exists debate on the risk factors for rLDH after PELD, and it is very difficult to define these risk factors due to the involvement of various complicated factors. In our study, we found that 57 (6.05%) of 942 patients who underwent PELD experienced rLDH. Chan Hong Park et al.  suggested that the recurrence rate of LDH after PTED was 11%. However, Du et al.  did not observe the recurrence of LDH after PELD in their study. Therefore, there is no consensus on the recurrence of LDH after PELD. However, the rate of rLDH after PELD may not have any guiding significance for patients with LDH. Meanwhile, we further found that age, sex, BMI, occupational lifting, facet joint degeneration, and time to ambulation were significantly correlated with the incidence of rLDH.
Most studies have reported age as one of the most important risk factors for recurrence after PELD [8, 9]. Yao et al.  found that an age over 50 years was closely associated with rLDH after successful PELD. In our study, we found that the average age of the 57 patients with rLDH was 56.7 ± 8.7 years. According to a retrospective cohort study, patients who were 57 years or older showed a higher reoperation rate after PELD than those younger than 57 years . Recurrence after PELD is the main factor of reoperation . In general, the older the patient is, the higher the probability of recurrence; therefore, PELD may not be an appropriate option for elderly people.
Generally, spinal tissue degeneration becomes increasingly severe with age. Marinelli et al.  reported that the stage of disc degeneration was associated with ageing. Meanwhile, patients with rLDH have been reported to have significantly more severe disc degeneration before surgery than patients with no rLDH . However, we did not find a significant difference in the classification of intervertebral lumbar disc degeneration between the recurrence and nonrecurrence groups; lumbar disc degeneration was not associated with recurrence after PELD. Intervertebral lumbar disc degeneration is commonly accompanied by Modic changes , and severe intervertebral lumbar disc degeneration is significantly more common in patients with Modic changes . Additionally, Modic changes are significantly correlated with an increased rate of rLDH . However, our study found no difference in Modic changes between the recurrence and nonrecurrence groups. This further shows that rLDH may be the result of multiple factors. Lumbar disc degeneration may not be an independent risk factor for recurrence after PELD.
In our study, there was a significant difference in facet joint degeneration between the recurrence and nonrecurrence groups, indicating that facet joint degeneration is a significant risk factor for recurrence after PELD. A study of the anatomy of the lumbar facet joints demonstrated a significant role in shear load–carrying capacity . Meanwhile, it was concluded that the posterior elements of the lumbar spine were more efficient in resisting anterior and posterior shear loads . Therefore, we considered that degeneration of the posterior rather than anterior elements has a greater impact on spinal stability. Spinal instability has been found to be related to the biomechanical stress on the affected disc, which may be related to rLDH . In the present study, we found that rLDH after PELD was more likely in those with grade II facet joint degeneration. This is the first time that an association between facet joint degeneration and rLDH after PELD has been proposed. However, there were no differences in facet joint parameters (FO and FT) between the two groups. Therefore, further biomechanical studies of facet joints in rLDH need to be performed.
The BMI is a statistical index determined using a person's weight and height to provide an estimate of body fat in males and females of any age. Kim et al.  reported higher rates of recurrence after successful PELD among patients with higher BMIs. Yao et al.  suggested that BMI ≥ 25 was the most robust risk factor responsible for recurrence after PELD. A meta-analysis also showed that the prevalence of rLDH in obese patients (BMI ≥ 25) was significantly higher than that in patients with a normal BMI . However, some studies have failed to demonstrate such a correlation between obesity and recurrence following lumbar microdiscectomy . In our study, we found that patients with rLDH had higher BMIs than those without rLDH (25.3 ± 2.9 vs. 23.6 ± 3.5, P < 0.001). BMI is one of the most important risk factors for postoperative rLDH. Our results are consistent with those reported by Yao et al. , who considered that obesity was the most important risk factor for recurrence after PELD. While we certainly agree, we also believe that strict bed rest is very important to prevent postoperative recurrence. Meanwhile, we considered various explanations for the differences in the research results, as follows. First, rLDH is an adverse outcome caused by multiple factors, and different risk factors have different effects on recurrence. Second, individual differences among patients could lead to different results from various studies. Third, there are many kinds of minimally invasive discectomy procedures, such as PELD, microendoscopic discectomy (MED), and microdiscectomy, and each surgeon has a different learning history and experience level. These two factors play important roles in recurrence. Meanwhile, we found a significant correlation between occupational lifting and rLDH. Li et al.  also reported that heavy work increased the risk of recurrence after discectomy, and clinical research reported by Kong et al.  demonstrated that a high BMI and heavy physical load intensity increased the possibility of rLDH. Furthermore, Sohrab et al.  suggested that the force required for LDH in the human spine was inversely correlated with the degree of disc degeneration. However, if the disc is subjected to higher pressures, it will become more degenerated and therefore more likely to herniate through an annular defect. Therefore, we consider that the overloading of damaged intervertebral discs might lead to rLDH. Above all, weight loss is very important in the perioperative period for patients with LDH, and long-term heavy work should be avoided after lumbar discectomy.
There are no clear standards for the duration of bedrest after PELD. Some scholars have considered that early ambulation should be carried out after lumbar discectomy, as it is conducive to promoting the recovery of bodily functions [2, 3]. However, Kim et al.  suggested that because early ambulation would increase the load on the injured intervertebral disc and easily lead to rLDH, the time of ambulation should be delayed. Qin et al.  also reported that the time to first ambulation was an important factor affecting recurrence after PELD. The recurrence rate among patients who ambulated within the first day after the operation was significantly higher than that among other patients . According to the characteristics of scarring and wound healing of fibrous soft tissue, we suggest that patients should stay in bed for at least 3 weeks. However, many patients often fail to do this. Therefore, we strictly require patients to stay in bed within the first day after PELD. Aside from eating or defecating, patients should rest in bed as much as possible. When patients get out of bed, they must wear lumbar support to reduce the load on the intervertebral disc and limit the duration of upright activities as much as possible. At the same time, patients should perform functional exercises of the back muscles and strength training of the limbs in bed. Although the outcomes of our study are consistent with those reported by Qin et al. , there are some differences between the two studies. First, there are differences in the number of cases, follow-up time and surgeon experience level between their study and our study. In particular, the number of cases in their study was small. Second, the definition of the time to ambulation after PELD is different; they defined the time to ambulation as the time until the first walk after the operation , while we defined the time to ambulation as the beginning of a long period of upright activity. In our study, we found that the average time to ambulation in patients with rLDH was 17 days, while the average time to ambulation in patients without recurrence was 24 days. The time to ambulation was closely associated with rLDH after successful PELD. Therefore, we suggest that later ambulation is helpful to reduce the rate of rLDH after PELD.