Sagittal Spine Length Measurement: A Novel Technique to Assess Growth of the Spine.

Study Design: The accuracy and repeatability of a novel sagittal spine length (SSL) radiographic measurement was examined using photographic and radiographic imaging.

Objectives: To validate the new SSL technique for measuring growth in early-onset scoliosis (EOS) patients.

Summary Of Background Data: Current assessment of patient growth undergoing growth-friendly surgical treatment for EOS is the use of serial vertical spine height measurements (VH) on coronal radiographs. Spine-based distraction implants are able to control the coronal plane deformity of scoliosis, but exhibit a "law of diminishing returns" in the impact of each follow-up lengthening surgery. As these treatments are kyphogenic, we hypothesize that the increase in kyphosis is, in fact, growth out of plane, not captured by standard spine height measurements.

Methods: Measurement accuracy was assessed using 6 spine model alignments and clinical radiographs of 23 retrospective EOS patients. Inter- and intrarater reliabilities were assessed using interclass coefficient (ICC) analyses. The discrepancy between the VH and SSL was also investigated.

Results: The model assessment showed excellent accuracy, with a 1.54 mm (SD: 1.07, range: 0.03-3.14, p = .226) mean error and mean ICCs of 0.999. As the kyphosis increased, a progressive difference between the phantom VH and SSL was observed. Interrater reliability ICCs of the clinical radiographs averaged 0.981 and 0.804, whereas intrarater reliabilities averaged 0.966 and 0.826, for the coronal and sagittal radiographs, respectively. Mean clinical SSLs were 177.5 mm (SD: 28.5, range: 114.3-250.3), whereas the VH averaged 161.6 mm (SD: 31.8, range: 58.5-243.0), resulting in a 16.0-mm (SD: 16.7, range: 0.3-90.3, p < .0001) difference between the two measurements with a progressive difference as the kyphosis increased.

Conclusions: The novel SSL measurement is accurate, repeatable, and complements the current growth assessments for EOS treatments. Until sagittal spine lengths are taken into consideration, the "law of diminishing returns" should be interpreted with caution.

Level Of Evidence: Level II - Prospective Comparative Study.