In this prospective observational study, consecutive patients with clinically and radiologically defined LSS underwent surgical treatment (decompression with/without posterolateral or transforaminal lumbar interbody fusion) between October 2015 and April 2018. The exclusion criteria were: (1) medical conditions that the investigator judged to affect the surgery or that might result in an abnormal postoperative course, such as stroke, neuromuscular disease, cancer, high risk of infection, or cardiovascular disease; (2) dementia or inability to answer the questionnaire independently; (3) previous lumbar spine surgery; and (4) inability to walk. Participant inclusion/exclusion criteria were similar to those of previous studies [13, 14]. Written informed consent was obtained from all participants. This study was approved by the Ethics Committee of the Tottori University Faculty of Medicine on August 31, 2015 (No. 1508B013).
Demographic and clinical information
The age, sex, height, weight, body mass index (BMI), current smoking habits, employment status, symptom duration, comorbidities (hypertension, dyslipidemia, and diabetes), and type and level of surgery were collected from medical records. Leg numbness was assessed using the visual analogue scale . Lower extremity muscle strength was assessed using manual muscle testing (MMT) . Tibialis anterior, extensor hallucis longus, extensor digitorum longus, flexor hallucis longus, flexor digitorum longus, and gastrocnemius were targeted. Participants with an MMT grade of less than 5 (normal) were judged to have muscle weakness. Walking distance was used to assess intermittent claudication. Participants were measured for walking distance by walking a 90-m path until they could no longer walk due to pain or numbness. The maximum distance for this measurement was 500 m.
Pain catastrophizing was assessed using the pain catastrophizing scale (PCS) . The Japanese version of the PCS used in this study has also been shown to be reliable and valid . Participants responded to 13 questions on a scale from 0 ("not at all") to 4 ("always"). The final score ranged from 0 to 52. The higher the PCS score, the more severe the catastrophic thoughts of pain. In this study, the PCS overall score and subscale scores for helplessness, magnification, and rumination were used.
Walking speed was evaluated using a 10-m walking test. Walking speed was calculated from the time required to walk 10 m at a normal speed. Normal walking speed is a highly reliable metric .
Pain and clinical outcomes
The intensity of leg and back pain due to LSS was assessed using the numerical rating scale (NRS) . Clinical outcomes were assessed using the Japanese Orthopaedic Association (JOA) score for lower back pain . The JOA score was calculated with a range of 0–29 points, with lower scores indicating a more severe disease.
Data collection schedule
During the preoperative evaluation, which took place between admission and surgery, demographic data were collected, and the aforementioned PCS and NRS questionnaires and walking test were implemented. The results of the preoperative evaluation were used as the baseline data. The participants underwent general rehabilitation, including aerobic exercise, stretching, and strength training, for 2 weeks after surgery. The postoperative evaluation was performed at 3, 6, and 12 months postoperatively using the same aforementioned questionnaire and functional tests. The data collection schedule of this study was the same as that of a previous study .
Data are presented as mean and standard deviation or median and interquartile range. Paired t-tests were used to compare the baseline and 12-month postoperative outcomes. In addition, Pearson correlation coefficients were used to assess the association between PCS and each variable at each evaluation time point and the association between the change in PCS and the change in each variable from baseline to 12 months postoperatively. In addition, the mixed-effect model for repeated measurements  was used to evaluate the association between PCS and walking speed at three time points during the follow-up period (3, 6, and 12 months postoperatively), adjusting for potential biases caused by missing data. In the multivariate analysis model, we modeled the following potential confounding factors as explanatory variables: age, sex, BMI, symptom duration, surgical treatment (decompression: 0, decompression with fusion: 1), NRS for leg and back pains, and preoperative PCS. We modeled the correlation matrix of outcome variables as the unstructured structure. All data were analyzed using SPSS version 24 for Windows (IBM Corporation, Armonk, NY, USA) and SAS version 9.4 (SAS Institute, Inc., Cary, NC). All P-values were two-sided, and the significance level was set at 0.05.