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Acknowledgements:
The authors thank Leandro Nuñez, investigator from AOSPINE Latin America, for his
assistance in statictics and manuscript preparation.
Editorial staff perspective
This is a class of evidence III prognostic study.
The idea behind this article—to compare screw placement accuracy between two different
patho-entities is laudable and clever. The paper also reintroduces the readership
to a more systematic form of recording pedicle screw malpositions and reviews response
possibilities.
There were a number of questions raised which could not be readily resolved. They
are listed here to promote further deliberation on the part of readers.
1. Selection bias? Reviewers noted a difference of the number of screws listed in
the thoracic group for trauma (2.3 compared to the scoliosis group 3 and 3.7 in the
lumbar group for trauma versus 7.6 in the scoliosis group. The reasons for these differences
may lie in physician preferences. However, they also may reflect the influence of
physician ‘wisdom’ or experience. What was the indication for screw placement as opposed
to hooks and when and why was the decision made to not use fixation? If there are
24 patients in the scoliosis group, then if all patients had screws bilaterally, there
would be a total of 48 screws listed in Table 5. A cursory review of this suggests
that there are multiple uninstrumented pedicles in this series. This is discussed
in the web appendix (at www.aospine.org/ebsj) to a certain extent, but there appears
to be a bias as to when to use screws and when not to use screws that is not explained.
2. Systematic error? Although this would require quite a bit more work, it would be
interesting to know the size of the pedicles instrumented and the size of screws placed
in relation to pedicle size. One might suspect that the trauma group has larger ‘targets’
than the scoliosis group. This may be a factor to account for differences in accuracy
and may also be a factor in where screws were ‘avoided’.
3. Methods: Factors which may have influenced placement accuracy such as BMI and were
not evaluated in this study. Significant differences in patient age and gender between
the two groups of patients may be surrogates for factors such as osteoporosis which
may influence screw placement. It is unclear whether the differential timing in performance
of postoperative CT between the trauma patients (immediate) and those with scoliosis
(variable timing) may influence evaluation of placement.
These are important considerations, which again show the limitations of retrospective
studies. Despite best intentions and a creative idea for identifying a comparison
group, the attempt of reinterpreting previously made clinical decisions in the context
of a retrospective study is very complex and may be contaminated with wrongful assumptions.
