Answer to the Letter to the Editor of Koen Van Boxem et al. concerning manuscript “Injection therapy and denervation procedures for chronic low-back pain: a systematic review” by Henschke N, Kuijpers T, Rubinstein SM, van Middelkoop M, Ostelo R, Verhagen A, Koes BW, van Tulder MW, Eur Spine J, 2010