Answer to the Letter to the Editor of Jianqiang Ni et al. concerning “ALIF and total disc replacement versus 2-level circumferential fusion with TLIF: a prospective, randomized, clinical and radiological trial” by Hoff EK, Strube P, Pumberger M, et al. (2015) Eur Spine J. doi:10.1007/s00586-015-3852-y