For the vast majority of patients with low back pain, the pathoanatomical cause cannot be currently determined(14). Malignancy, vertebral fracture, or axial spondyloarthritis are serious causes for low back pain, but only account for a very small proportion of cases(15). It has been investigated that intervertebral disc, facet joint, and vertebral endplates contributing to low back pain is not possible. Among asymptomatic individuals, imaging findings of spine degeneration (disc bulge, disc protrusion) is prevalent(16). What is more, the rate of spontaneous regression was found to be 96% for disc sequestration, 70% for disc extrusion, 41% for disc protrusion and 13% for disc bulging(17). Thus, identifying the pathoanatomical basis of low back pain is in urgent need to open up new approaches for people with disabling low back pain worldwide.
Disorder of hip and lumber have overlapping presentation and symptoms. Heidi et al. found that positive hip physical examination is prevalent in patients with low back pain, patients with low back pain and positive hip examination findings have more pain and worse function compared with patients without hip examination findings(18). It is estimated that about 60.4% patients with hip osteoarthritis also complain of low back pain, and after total hip arthroplasty, significant improvement of low back pain can be observed one-year postoperative(19). According to a systematic review of John et al., patients with low back pain frequently accompany limited hip range of motion, and these patients routinely improve after surgical intervention for hip diseases(5). Hip diseases may closely relate with low back pain, but by what means are they related is still unknown. The iliopsoas musculotendinous unit is a powerful hip flexor which has important function in femoral external rotation and with lateral bending, flexion, and stabilizing the spine in the frontal plane(20, 21). It originates from the outer surfaces of the vertebral bodies of T12-L5 and inserts on the lesser trochanter of the femur (Fig. 2). We hypothesis that the pathology of iliopsoas musculotendinous unit may contributes to low back pain caused by hip disorders.
Iliopsoas tendinopathy is closely related with the repetitive pathologic movement of the tendon. In our study, we found that iliopsoas tendinopathy can be observed in most patients with low back pain and significant relieved low back pain was observed after local injection of anesthetic and steroid into the iliopsoas tendon, indicating iliopsoas tendinopathy is a plausible source of low back pain. Altogether, we believe that iliopsoas tendon is a structure closely related with the generation of low back pain. But further investigation is needed to further confirm the relation between iliopsoas tendinopathy and low back pain. Besides, why does iliopsoas tendinopathy generate low back pain and how does intraarticular pathologies corelate with iliopsoas tendinopathy and contribute to low back pain is still unknown.
To our knowledge, our study is the first study to report low back pain as a major presentation for iliopsoas tendinopathy and investigate the clinical effect of ultrasound-guided local injection on it. Our results showed that ultrasound-guided injection of anesthetic and steroid into the iliopsoas tendon can improve low back and groin pain significantly for patients with iliopsoas tendinopathy. Significant mid-term outcome was also observed in our study. 84% (38 of 45 patients) presented absolute pain relief and an improved HHS. In patients with low back pain and/or groin pain and local tenderness over the iliopsoas tendon, iliopsoas tendinopathy should be considered. CT and MRI can be helpful in excluding other diagnosis including osteoarthritis and femoral acetabular impingement. Local injection of anesthetic and steroid can be applied as both therapy and a way of confirming the diagnosis.
In consistent with previous researches, our result also demonstrated that local injection has therapeutic effect on iliopsoas pathology. In patients with painful snapping hip, injection can be beneficial, 16 of 18 patients in the study had a good response at an average of 4 months(22). Adler et al. performed sonography-guided for the presumed diagnosis of iliopsoas tendinosis, 44% of these patients had continued relief at 1 year(10). Agten et al. reported an improvement at 1 month of the patients with iliopsoas tendinopathy after fluoroscopy-guided iliopsoas bursa injection(11). For patients with iliopsoas impingement after total hip arthroplasty, nonoperative management is reported to lead to groin pain resolution in 50% of patients(23). Nevertheless, a systematic review showed that nonsurgical treatment including injection was only successful in 39% patients with iliopsoas impingement after total hip arthroplasty(24).
Although we demonstrated promising therapeutic effect of local injection on low back pain caused by iliopsoas tendinopathy, however, there are several limitations of our study. We included a relatively small number of patients with iliopsoas tendinopathy, and long-term follow-up is needed to further confirm our results. Additionally, we had no control group for comparison, and we present our study in a single center. We will further amplify the sample size and put randomized controlled trials into practice to confirm our study. Another limitation of our study is that the use of VAS and HHS to assess the outcome are subjective, the sense of pain can vary from person to person.