1) Description of Measurement

Side-bending films quantify coronal plane flexibility of scoliotic curves by measuring the residual Cobb angle during maximal right and left lateral bending. This measurement is used to determine whether a curve is structural or compensatory, to assess curve stiffness, and to guide operative planning, including the selection of fusion levels.

Coronal flexibility reflects the ability of a spinal curve to correct with active bending and correlates with disc, facet, and ligamentous elasticity.

2) Instructions to Measure

• Obtain three full-length standing radiographs:

• Erect AP

• Maximal right lateral bending

• Maximal left lateral bending
  
  

• On the erect AP film, identify the major curve and measure the baseline Cobb angle using the standard end-vertebra method.

• On the corresponding side-bending film (right or left, depending on curve convexity):

• Identify the same superior and inferior end vertebrae used for the erect Cobb angle.

• Measure the residual Cobb angle while the patient is maximally bent.
  
  

•  Calculate Coronal Flexibility (%):

Flexibility (%) = Erect Cobb - Bending CobbErect Cobb100

• Record the lowest residual Cobb angle obtained during bending, as this represents maximal achievable correction.

3) Normal vs. Pathologic Ranges

• High flexibility curve: > 50%

• Moderately flexible: 30 - 50%

• Rigid curve: < 30%

• Structural, stiff deformity: Minimal/no change

Key points:

• Curves with < 30% flexibility are typically considered structural
  
  

• Highly flexible curves may not require inclusion in fusion constructs

4) Important References

Ohrt-Nissen S, Shigematsu H, Cheung JPY, et al. Predictability of Coronal Curve Flexibility in Postoperative Curve Correction in Adolescent Idiopathic Scoliosis: The Effect of the Sagittal Profile. Global Spine J. 2020 May;10(3):303-311. doi: 10.1177/2192568219877862.

Lenke LG, Edwards CC 2nd, Bridwell KH. The Lenke classification of adolescent idiopathic scoliosis: how it organizes curve patterns as a template to perform selective fusions of the spine. Spine (Phila Pa 1976). 2003 Oct 15;28(20):S199-207. doi: 10.1097/01.BRS.0000092216.16155.33.

King HA, Moe JH, Bradford DS, Winter RB. The selection of fusion levels in thoracic idiopathic scoliosis. J Bone Joint Surg Am. 1983 Dec;65(9):1302-13.

Luk KD, Cheung KM, Lu DS, Leong JC. Assessment of scoliosis correction in relation to flexibility using the fulcrum bending correction index. Spine (Phila Pa 1976). 1998 Nov 1;23(21):2303-7. doi: 10.1097/00007632-199811010-00011.

5) Other info....

Side-bending films are critical for:

• Distinguishing structural vs non-structural curves

• Determining lowest instrumented vertebra (LIV)

• Predicting postoperative coronal balance

Should be interpreted in conjunction with:

• Standing AP Cobb angles

• Sagittal profile

• Clinical flexibility and rotation

In patients unable to actively bend, supine traction or push-prone films may be used as alternatives

Residual Cobb angle on bending is often more predictive of surgical correction potential than the baseline erect Cobb angle alone