Positive evidence.

The number of studies documenting a significant association between prognostic factors and pain intensity outweighs the number of studies with no significant association by three or more.

Preliminary positive evidence.

The number of studies with a significant association outweighs the number of studies with no significant association by two.

Preliminary negative evidence.

The number of studies with no significant association between predictors and pain intensity outweighs the number of studies with a significant association by two.

Negative evidence.

The number of studies with no significant association outweighs the number of studies with a significant association by three or more.

Methodological quality.

The assessment of the studies’ methodological quality by the two reviewers yielded the following results. Fourteen studies were of fair quality of evidence and seven showed poor quality. All of the included studies were prospective cohort studies involving surgical intervention, which precluded blinding and randomization of the patients. This resulted in low scores for internal validity. Furthermore, none of the studies discussed power or if the pain measurement indicated a clinically meaningful effect.

Sample characteristics.

Taken together, the studies comprise 2,581 patients undergoing surgery for lumbar disc herniation. With the exception of one paper [34], all studies documented a higher proportion of males. This is most likely due to the fact that men show higher rates of disc herniation and surgery [35]. The mean age of the participating patients ranged from 35 to 46 years, while the absolute age range was 12 to 82 years of age.

Study designs and follow-up times.

All studies are prospective cohort studies with a preoperative baseline assessment of pain and at least one follow-up assessment. Nine studies included a short-term assessment [11,32,36–42], seven studies assessed pain over the medium-term [11,38,39,42–45] and fifteen studies provided a long-term follow-up [11,15,16,32,34,36–38,42,46–51], ranging from 1 [36] to 7 years [15,16] after surgery.

Surgical Procedure.

All studies reported that they applied conventional surgical procedures. In ten studies the patients underwent microdiscectomy [11,36–38,41–43,45,47,50]. Six studies documented that the patients were treated with standard open discectomy [32,34,40,44,48,49]. Two studies [16,51] included patients who were operated both with and without a microscope. In the study by Graver et al. [15] traditional surgical techniques were performed. Folman et al. [46] reported that the patients underwent state-of-the-art surgery. Likewise, den Boer et al. [39] stated that the decision for surgery was based on national guidelines. In this review, no pattern could be identified showing that outcome was dependent on type of surgery.

Diagnostic instruments.

All studies used a Visual Analogue Scale (VAS) ranging from 0 to 10 or 0 to 100. In most studies, these rating scales assessed the patients' current pain intensity, but some studies used diverging instructions [16,39,41,44]. Silverplats et al. [16] reported a composite score of three rating scales for back pain and leg pain, asking for pain when most severe, pain when least severe and current pain. Basler & Zimmer [44] applied a pain diary in which the patients had to rate their pain on a VAS over a time span of two weeks. Den Boer [39] asked for the average back and leg pain in the past week. Hegarty & Shorten [41] assessed average pain intensity in the past two weeks.

Twelve studies investigated leg and back pain on separate scales, one study assessed back and leg pain with one scale and five studies assessed the patients' overall pain intensity. In two studies, only leg pain was assessed [34,46] and one study examined back pain only [48].

Pain intensity at baseline.

Back pain intensity at baseline was moderate to severe across studies. Eight studies showed higher leg pain intensity compared to back pain intensity. Two studies showed equal pain levels for back and leg pain before surgery [37,42]. Back pain ranged from 4.1 [51] to 7.0 [47]. Leg pain ranged from 5.6 [50] to 7.9 [43]. Studies assessing both back and leg pain or overall pain with one scale yielded scores of 4.7 [39] to 8.0 [38].

Short-term results.

Eight out of nine studies reported a significant reduction of pain in the short-term [11,32,36–40,42], indicating that the majority of patients benefited from surgical performance. Pain scores were predominantly mild across studies. Mean scores ranged from 2.0 to 3.0 for back pain [11,42], 0.0 to 2.0 for leg pain [11,42] and 1.8 to 4.0 for overall pain [36,38,39]. The percentual improvement ranged from 50 to 81% [11,40] for back pain, 71 to 100% for leg pain [11] and 39 to 64% for overall pain [36,39]. Hegarty & Shorten [41] documented that more than half of the patients reported an overall pain relief of at least 70% according to the VAS-10.

Medium-term results.

Five out of seven studies comparing preoperative pain with medium-term pain found a significant improvement [11,38,39,42,45]. The remaining two studies also showed a relief of pain, but did not test the statistical significance. Back, leg and overall pain scores were in the mild range. Back pain scores were between 0.6 and 2.0 [11,43] and yielded a percentual improvement of 67 to 83%. Leg pain scores were between 0.5 and 1.5 [11,43] with a percentual change between 79 and 93%. Overall pain scores were between 1.9 and 2.6 [39,44] and mean improvement ranged from 62 to 75%.

Long-term results.

All 15 studies comparing baseline pain with long-term pain reported an improvement. Of these, 10 studies reported statistically significant decreases of pain intensity. Back, leg and overall pain scores were in the mild to moderate range. Back pain ranged from 1.4 to 3.6 [49,50] with a percentual improvement of 28 to 78%. Leg pain scores were between 1.2 and 3.4 [46,47] with a percentual change of 53 to 83%. Overall pain scores ranged from 2.0 to 3.1 and showed an improvement of 52 to 75% [36,38]. Two studies compared short-term outcome with long-term outcome, showing no further significant changes of back, leg or overall pain [32,36]. Likewise, the comparison of medium- and long-term pain yielded no significant differences of overall pain scores [38]. These findings indicate that the short-term benefits from surgery could be maintained over the long-term.

Pain intensity over the course of time.

All studies in this review, comprising 2,581 patients undergoing surgery for lumbar disc herniation, found a reduction of back, leg or overall pain intensity after surgery. The majority of studies tested for statistical significance of mean pain relief, showing a significant improvement of presurgical pain intensity compared to follow-up assessments in the short-, medium- and long-term. While average pain scores were moderate to severe before surgery, they were only mild to moderate after surgery. In addition, reductions of pain that were observed in the short-term were maintained in the long-term. These findings indicate that surgery was successful in terms of pain relief over a long period of time, which is in line with the Spine Patient Outcomes Research Trial (SPORT) [52] and the Maine Lumbar Spine Study (MLSS) [53], two large prospective studies on the outcome of surgically and non-surgically treated patients with lumbar disc herniation. The SPORT [52] showed significant improvements of bodily pain and physical function using the SF-36 [54] in a cohort of operated patients over two years. The 10-year results from the MLSS [53] showed that 69% of lumbar disc surgery patients reported improvement in their predominant symptom, namely back or leg pain. Considering leg and back pain as two separate outcome parameters, this review shows greater reductions of leg pain than back pain in short-, medium- and long-term follow up assessments.

Despite predominantly positive findings, several studies in this review indicated that in some patients surgery was not successful in terms of pain relief [11,16,32,41,48]. Furthermore, there was a lack of studies examining the course of postsurgical pain. Pre-post comparisons are important to assess the surgical success, but they are not sufficient when it comes to identifying typical processes of pain chronification and pain fluctuation.

Factors associated with pain intensity over time.

Interestingly, none of the studies mentioned surgical complications as a determinant for ongoing pain. However, studies investigating other surgical procedures such as amputations or thoracotomy stated that iatrogenic nerve damage may be the most important cause of long-term postsurgical pain [55,56]. Hence, these researchers recommend applying surgical techniques minimizing the risk of nerve injury. In this review, the experience of postsurgical pain was determined by a variety of socio-demographic, medical and psychological factors.

Regarding overall pain intensity, positive evidence was found for preoperative pain and preliminary positive evidence was found for age, anxiety and coping. The fact that stronger pain before surgery was significantly related to stronger postsurgical pain is supported by a review by Kehlet and colleagues [55] and may at least partly be explained by neuroplastic changes in the processing of pain leading to persisting nociception [41]. However, it may also result from neuroplastic changes induced by surgery or lack of analgesics [55]. Concerning age, it is assumed that older patients may do worse because they have fewer biophysical resources to recuperate from surgical procedures [20,41]. Higher levels of anxiety [38,41] were related to stronger short-, medium- and long-term postoperative pain. Likewise, a systematic review and meta-analysis emphasizes the association of preoperative anxiety and catastrophizing with postsurgical pain [57]. In line with this, a study by von Korff et al. [58] showed that mental comorbidity had a negative impact on the pain experience, indicating that spine pain can be seen as a construct depending on other health conditions and should be treated with a broadly based approach. D’Angelo et al. [38] indicate that residual pain may be related to a predisposition of an anxious reaction that evokes muscular tension and might cause a lower pain tolerance [59]. Lebow et al. [42] highlight the question of whether depression, anxiety and poor mental health predispose a patient to failure of pain relief, signifying that psychological factors in and of themselves lead to stronger pain, or if it is the disc-related pain that leads to psychological vulnerability. Looking at the studies included in this review it seems to be the interaction between mental health and pain that is responsible for chronic pain rather than psychological factors per se. For example, Lebow and colleagues argued that patients with poor mental health before surgery showed an improved mental well-being after surgery, but this improvement occurred months after observed reductions of pain. Moreover, persisting pain after surgery may be due to an association between psychological factors and pain that was already existent before surgery. Therefore, various authors suggested to introduce psychological screenings to identify patients at risk for pain chronification [15,36,38,39,42,48] and expand preventative approaches [55], as well as psychological interventions, in addition to routine surgical treatment [21,22,38,39,42,48]. A strong association between coping behavior and depressive symptoms was found by three independent studies [39,41,48]. Herda et al. [22] assume that persistent complaints, despite successful outcome according to orthopaedic criteria, can often be attributed to cognitive behavioral factors. They found that presurgical pain-related cognition such as catastrophizing, helplessness and passive pain coping seems to be relatively stable over time, hindering the patient from finding ways to actively control their pain after surgery. Therefore, they suggest that cognitive psychological pain therapy should teach patients how to control dysfunctional thoughts and develop inner monologues that are adaptive for pain processing.

Factors that were associated with ongoing back or leg pain were mainly the same. Positive evidence was found for depression and disability and preliminary positive evidence was shown for somatization and mental well-being. The evidence for an association between back pain and depression was better than for leg pain, but when looking at all types of pain together, depression was the most salient factor. This finding is in line with other studies and reviews in this field [60,61]. Freidl et al. [61] showed that patients with somatic illnesses suffered from pain syndromes more often when they had comorbid depression. Depression is also a major factor in a biopsychosocial model of disc-related pain chronification by Hasenbring [17]. In this model, it is assumed that the genetic disposition for depression contributes to a higher risk of pain chronification. Moreover, appraisal strategies undergoing complex interactions with somatic, cognitive and emotional reactions typical for depression can lead to maladaptive coping and more sensitive pain perception. Considering related literature, the interaction between psychosocial factors and postsurgical pain seems to apply not only to disc-related pain but also to long-term phantom pain in amputees [62].

An assessment of predictors considering the different times of follow-up yielded no clear pattern of factors associated with acute short-term pain or chronified long-term pain. Instead, considering psychological factors, such as anxiety, depression, somatization and mental well-being, showed associations with postsurgical pain over all periods of follow-up. Accordingly, Kehlet el al. [55] argue that both acute and chronic pain interact with biopsychosocial mechanisms. For example, in their meta-analysis, Theunissen et al. [57] showed that anxiety and catastrophizing were not only related to acute postsurgical pain but also to chronic pain.

The influence of work-related factors on postsurgical pain intensity remains relatively unclear. However, some studies indicate that occupational aspects do have an impact on the patients’ sensation of pain. In an earlier study of this work group, a negative subjective prognosis of employment and depression were the most important risk factors for postsurgical pain in lumbar and cervical disc surgery patients [60].