We appreciate the authors for their interest in our paper and raising pertinent questions for further discussions on facet tropism and lumbar disc prolapse.

Firstly, we agree that the definition of the value of tropism should depend on the population being studied. Some authors have defined facet tropism as the standard deviation of the facet joint angles [1]. Our morphometric study demonstrated standard deviations of 6.38° and 6.67° at L4–L5 and L5–S1, respectively, and also showed similar values of facet joint angles compared with previous studies [2]. Similarly, our current study also demonstrated that facet tropism as small as 6° increases the likelihood of lumbar disc prolapse at L4–L5 and L5–S1 [3]. Therefore, it does not seem unreasonable to use the definition of facet tropism as provided by Vanharanta et al. [4].