This register-based study among 353 patients undergoing lumbar microdiscectomy investigated the effects of gender, age and preoperative pain on postoperative change in back and leg pain severity during a two-year follow-up. Back and leg pain demonstrated similar developmental trajectories – a steep improvement during the first postoperative months and then mild worsening towards the end of follow-up. Disregarding that later worsening, pain level remained at much lower level after two years after the surgery compared with a baseline. Gender or age did not significantly affect the trajectories of pain changes. However, a statistically insignificant trend could be seen for the effect of age – back pain was worsening after a one-year follow-up among younger patients but not among the older ones. Experiencing preoperative pain over a year worsened significantly the outcome of surgery measured by leg pain severity. Severity of preoperative pain affected significantly the trajectory of pain development after surgery – severe preoperative pain predicted worse outcome.
The generalizability of the findings might be limited due to several issues. Only a few demographic variables were available for the analysis. Thus, it is possible that there might be several important factors, other than age, gender or preoperative pain, which might affect the changes in pain severity after the surgery. However, two out of four main factors (preoperative pain, anxiety, age, and type of surgery) related to alleviating pain, according to a previous review on postoperative pain, were available [19]. The study was set at a university surgery clinic, which is a highly specialized unit, and therefore, the results might be different in lower-level units.
Surgery due to intervertebral disc displacement focuses primarily on relieving sciatic leg pain [9]. There is uncertainty on how much microdiscectomy affects back pain [10]. Only a few previous research has suggested that also back pain may be substantially relieved by the procedure in question [8]. This is in line with the present results, which did not only suggest simultaneous improvement in both back and leg pain, but observed essentially the same magnitude of these changes. While the severity of leg pain was a little worse than back pain at the baseline, they both decreased to the same level in two-year follow-up.
Several speculations could be introduced to explain the similarity between postoperative changes in back and leg pain. Additionally to mechanical compression, intervertebral disc displacement causes local inflammation. When burst out prolapse mass is dissolved by surgery or by natural course over time, inflammation starts to ease, which affects both local and radicular pain. Also, patients with intervertebral disc displacement may often consider pain in buttocks as “back pain” and not as “radicular pain”, which it in fact is. This may affect the interpretation of surgery outcome when attempting to distinguish local back pain and radicular leg pain. Pain overall and irritated by prolapse nerve tissue may affect the mechanics of low back structures. It might be difficult to achieve a painless position and e.g., sitting is often more painful than standing. When radicular pain is gone, also low back mechanics are normalized.
The analgetic effect of surgery lasted at least to the end of two-year follow-up. Certainly, controls would be required in order to draw strong conclusions, but, nevertheless, these results are encouraging – most of the patients experienced pain relief in a relatively long run after the surgery.
In line with previous research, no significant effect of gender or age on the magnitude of postoperative pain relief was found [7, 13, 15]. Few previous studies from the same research team have suggested that older age and female gender might predict worse outcome of microdiscectomy [12].
The only clearly statistically significant findings in this study were the worsening predictive effect of prolonged and severe preoperative pain. This is in line with previous discussions on the “right timing” for discectomy [17]. It is possible that pain is not completely related to a present prolapse situation, but, instead, partially caused by other reasons, usually degenerative spinal changes. These additional reasons are not cured by a discectomy and pain may stay. Also, prolonged pain may cause psychological or social effect on the functioning of patients. Due to this effect, pain may continue even if the morphological cause is cured.
The present results should be confirmed by additional research conducted in different settings, diverse populations and wider sets of available independent variables.