Patients Undergoing Lumbar Spine Fusion has Low Trunk Muscle Strength and Imbalance between Flexor and Extensor Muscles Preoperatively and still 1 Year after the Surgery

Abstract

Introduction Lumbar spine fusion (LSF) may be performed in the treatment of several spinal disorders if conservative treatment fails. Decreased physical activity due to pain together with spinal fusion may cause muscle atrophy and therefore decrease the trunk muscle strength. Previously, the average trunk muscle strength in flexion and in extension were reported to be 564 and 629 N, respectively, in control population compared with those of 375 and 404 N in patients 2 months after disc herniation surgery (Häkkinen et al, 2003). In healthy people trunk extensor strength has been reported to be greater than flexor strength, that is, extensor/flexor strength ratio is more than 1 (Yahia et al, 2011). Objective The aim of the present study was to evaluate the influence of lumbar spine fusion to the flexion and extension strength of the trunk muscles at 1 year follow-up after the surgery. Material and Methods Altogether 195 patients (66% females, mean [SD] age 61 [12] years and body weight 78 [15] kg) undergoing instrumented LSF attended the present study. Indications for surgery were degenerative spondylolisthesis, spondylolysis, spinal stenosis, disc herniation or degeneration, postoperative conditions or scoliosis. The median (IQR) duration of preoperative back pain was 31 (18, 66) months. Isometric flexion and extension strength of the trunk muscles was measured using a strain-gauge dynamometer and back pain was measured using visual analogue scale (VAS 0–100 mm) preoperatively and at 1 year after surgery. Results Mean (SD) preoperative trunk flexion and extension strength levels were 295 (172) and 205 (144) N, respectively. A year after the surgery, mean (95% CI) trunk flexion strength increased by 69 (53–85) N ( p < 0.001) and extension strength by 53 (37–70) N ( p < 0.001). Mean (SD) preoperative extension/flexion strength ratio was 0.75 (0.38) and did not change during the follow-up. Flexion strength/body weight ratio increased from preoperative 0.38 (0.20) by 0.09 (0.07–0.11) and extension strength/body weight ratio from 0.27 (0.18) by 0.07 (0.05–0.09) ( p < 0.001) 1 year after the surgery. Mean (SD) back pain VAS decreased from preoperative 63 (27) to 25 (26) mm at 1 year and pain intensity during flexion and extension strength measurements decreased from 41 (29) to 11(21) mm and from 55 (29) to 14 (25) mm, respectively ( p < 0.001). No correlation between strength measurements and back pain was found at either time point. Conclusion The trunk muscle strength in patients with chronic back pain undergoing LSF is low. After LSF some improvement can be found, however, still 1 year after the surgery the trunk muscle strength remains low. Moreover, the flexion strength is better than extension strength, which is in contrast to results of healthy people in previous studies. Therefore, after the spinal fusion surgery the rehabilitation should aim at improving muscle strength and special attention should be paid to correct the imbalance between trunk flexion and extension muscles.