The influence of anatomy (normal versus scoliosis) on the free-hand placement of pedicle screws: Is misplacement more frequent in patients with anatomical deformity?

ABSTRACT

Study design: Retrospective prognostic study

Objective: To evaluate whether patients with anatomical deformity due to scoliosis have a higher frequency of inaccurate pedicle screw insertion and related complications using the free-hand technique compared with those whose normal anatomy had been impacted by trauma.

Methods: Consecutively treated trauma patients with otherwise normal anatomy (48 patients instrumented with 291 screws, group A) and scoliosis patients (24 patients instrumented with 287 screws, group B) were evaluated. Screw position on CT was evaluated using the classification by Gertzbein and Robbins with modification by Karagoz Guzey. (See web appendix at www.aospine.org/ebsj for complete classification description.) Images were examined by two fellows and one junior staff member none of whom participated in patient management. Screw position was determined by consensus.

Results: In group A, five (1.7 %) out of 289 screws were severely misplaced and 26 (9 %) screws caused either medial (3.8 %) or lateral (5.2 %) cortical breeches. The other 258 (89.3 %) screws were fully contained within the cortical boundaries of the pedicle. In group B, seven (2.8 %) out of 256 screws were severely misplaced. Thirty-three (13 %) screws caused cortical breeches, either medial (9 %), lateral (2 %), or anterior (2 %), and 216 (84.3 %) screws were fully contained within the cortical boundaries of the pedicle and the vertebra. Neurological complications were reported in one patient with scoliosis. No vascular complications were reported in either group.

Conclusions: The percentage of incorrectly placed screws was similar in both groups, trauma and deformity patients. The presence of vertebral anatomical changes related to adult scoliosis was not associated with an increase in the screw-related neurological or vascular complications.

 

STUDY RATIONALE AND CONTEXT Pedicle screw fixation affords multidimensional control, greater rigidity, and may increase the fusion rates 1 2 3 4 5 compared with other options making it the method of choice for most surgeons. However, accurate insertion relies heavily on anatomical landmark identification. Distortion of anatomy and spatial orientation, which occurs in adult scoliosis with spondylosis for example, may make landmark identification difficult. When inserted incorrectly, pedicle screws can cause neurological or vascular injuries. Understanding which patient groups may be at higher risk for screw misplacement is therefore important. OBJECTIVES To evaluate the frequency of pedicle screw misplacement and complications in patients with severe anatomical distortion (adult scoliosis) compared to those with normal anatomy (trauma patients) following posterior instrumentation using the free-hand technique. Our hypothesis was that a higher frequency of screw misplacement would occur in patients with severe anatomical distortion.

References

• 1 Belmont Jr P J, Klemme W R, Dhawan A. et al . In vivo accuracy of thoracic pedicle screws.  Spine. 2001;  26(21) 2340-2346
• 2 Gaines Jr R W. The use of pedicle screw internal fixation for the operative treatment of spinal disorders.  J Bone Joint Surg Am. 2000;  82 1458-1476
• 3 Gertzbein S D, Robbins S E. Accuracy of pedicular screw placement in vivo.  Spine. 1990;  15(1) 11-14
• 4 Hartl R, Theodore N, Dickman C A. et al . Technique of thoracic pedicle screw fixation for trauma.  Operative Techniques Neurosurg. 2004;  7 22-30
• 5 O’Brien M F, Lenke L G, Mardjetko S. et al . Pedicle morphology in thoracic adolescent idiopathic scoliosis: Is pedicle fixation an anatomically viable technique?.  Spine. 2000;  25(18) 2285-2293
• 6 Rajasekaran S, Vidyadhara S, Ramesh P. et al . Randomized clinical study to compare the accuracy of navigated and non-navigated thoracic pedicle screws in deformity correction surgeries.  Spine. 2007;  32(2) E56-64
• 7 Guzey F K, Emel E, Seyithanoglu Hakan M. et al . Accuracy of pedicle screw placement for upper and middle thoracic pathologies without coronal plane spinal deformity using conventional methods.  J Spinal Disord Tech. 2006;  19(6) 436-441
• 8 Kosmopoulos V, Schizas C. Pedicle screw placement accuracy: a meta-analysis.  Spine. 2007;  32(3) E111-120
• 9 Youkilis A S, Quint D J, McGillicuddy J E. et al . Stereotactic navigation for placement of pedicle screws in the thoracic spine.  Neurosurgery. 2001;  48(4) 771-779
• 10 Luque E R. Interpeduncular segmental fixation.  Clin Orthop Relat Res. 1986;  (203) 54-57
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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.