Response to: ‘A dose–response relationship between severity of disc degeneration and intervertebral disc height in the lumbosacral spine’—authors’ reply

We welcome the comments from Emanuel et al. [1]. In a systematic review, Pfirrmann’s method [2] was endorsed as a valid and reliable ‘gold-standard’ for assessing intervertebral disc degeneration [3]. Nevertheless, the Pfirrmann system uses qualitative descriptors such as ‘disc height normal to slightly decreased’. This descriptor is highly subjective with confounding influences of gender, age and body habitus and the ambiguity of what represents a ‘normal’ disc height. There is no quantitative measure of disc height in the Pfirrmann grading system. This prompted us to validate disc height against the ‘gold-standard’. In our work [4], we validate that for every one-grade increase in the Pfirrmann score, there is a reduction in disc height. For example, there was a 1.60 mm reduction (95 % confidence interval –2.37 to –0.83 mm) in disc height for each Pfirrmann grade increase at the level of L3/4, independent of age, gender, body mass index and smoking history. While Emanuel et al. highlight that disc height has diurnal and joint loading variation [1], the Pfirrmann Score has the same inherent limitations [2]. Any extraneous variability from such measures would have served to cause misclassification and reduced our chances of demonstrating significant results in the current study. Moreover, contemporaneous assessment of disc height and the Pfirrmann grade was made, mitigating extraneous variability. While there was large