Introduction: Rotation is becoming an increasingly important consideration in the management of scoliosis yet it is difficult to measure reliably. The Perdriolle technique is a widely used and validated technique for estimating the rotation of the apical vertebra. The landmarks required to measure vertebral rotation using this technique are frequently obscured following instrumentation and the application of bone graft. We propose that the Perdriolle technique cannot be applied reliably in the presence of pedicle screw constructs.

Method: This was a manual radiographic measurement analysis comparing intraobserver and interobserver reliability of the Perdriolle “Torsiometre” and the Cobb angle measurement in scoliosis prior to and after pedicle screw instrumentation.

Results: Mean difference and 95% limits of agreement between pre-operative intra-observer readings was 2.5° (−15° and 20°). This suggests on average there was little systematic disagreement between the two readings (2.5° on average). There were large discrepancies between individual pairs of readings.

29.6% of post-operative films (17%–39%) were judged to have sufficient landmarks visible to enable measurement of vertebral rotation compared to 10% of pre-operative films.

Marked increase in systematic bias between consultants with post-operative radiographs to pre-operative films was observed.

Conclusion: We question the validity in measuring the rotation of the curve using the Perdriolle technique on post-operative films following pedicle screw instrumentation. The predominant factors for the obscuration of landmarks include the presence of bone graft, pedicle screws and rods.