Surgery with disc prosthesis versus rehabilitation in patients with low back pain and degenerative disc: two year follow-up of randomised study

Dear Editor,

We would like to congratulate the authors on conducting a randomised
controlled trial (RCT) comparing a surgical intervention to rehabilitation
for patients with chronic low back pain.(1) They reported a statistically
significant difference of less than 10 points in the Oswestry disability
index at 2 years between groups and concluded that it "did not clearly
exceed the pre-specified minimally important clinical difference", the
value used in the sample size calculation. It is important to note that
the use of such a value in the sample size calculation does not make it
the minimally important clinical difference (MCID) as acknowledged by the
authors. The basis of the 10 points difference was not justified other
than to reference another trial (2) which used the same difference though
in turn provided no justification for using this value. We support the
view of others that the reporting of how the sample size was determined
requires greater clarity and transparency and indeed acknowledgement of
the discussion which takes place in the design of trial.(3)

As a step in this direction clear guidance is required on robust
methods to determine what an important difference is and what trial size
is needed to detect such a difference. Such guidance must recognise the
different requirements of commissioners of RCTs, reviewers of both grant
applications and subsequent reports of trial results, and consumers of
research.
The Difference ELicitation in TriAls (DELTA) project (4), funded by
Medical Research Council UK, is investigating methods for determining the
target difference. This includes a systematic review of methods, a survey
of current trial practice and development of a guidance document. We hope
this project will be a step in the right direction and facilitate
discussion in the trial community and progress in this vital, yet
neglected, aspect of trial design.

References

1. Hellum C, Gunnar Johnsen L, Strorheim K, Nygaard OP, Brox JI,
Rossvoll I, Ro M, Sandvik L, Grundnes O, and the Norwegian Spine study
group. Surgery with disc prosthesis versus rehabilitation in patients with
lowback pain and degenerative disc: two year follow-up of randomised
study. BMJ 2011;342:d2786.

2. Brox JI, Reikeras O, Nygaard O, Sorensen R, Indahl A, Holm I, et
al. Lumbar instrumented fusion compared with cognitive intervention and
exercises in patients with chronic back pain after previous surgery for
disc herniation: a prospective randomized controlled study. Pain
2006;122:145-55.

3. Charles P, Giraudeau B, Dechartres A, Baron G, Ravaud P. Reporting
of sample size calculation in randomised controlled trials: review. BMJ
2009;338:b1732.

4.
http://www.abdn.ac.uk/hsru/research/assessment/methodological/delta/