Scoliosis x ray

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Rohan A. Bhimani, M.B.B.S., D.N.B., M.Ch.[2]

Overview

X-rays are vital for initial assessment and to determine progression of the curve. Various assessments are done on the x-ray such as measuring the cobb angle and pedicular rotation.

X-Ray

• Full-length standing spine x-rays are the standard method for evaluating the severity and progression of the scoliosis, and whether it is congenital or idiopathic in nature. In growing individuals, serial radiographs are obtained at 3-12 month intervals to follow curve progression.
• Various measurements are done on the x-ray.

Identification of the Apex and Significant Vertebrae

• Identification of the curve apex and significant vertebrae is crucial for denoting the curve type, selecting the surgical approach and instrumentation system, and determining the optimal level for fusion.

Apex Vertebra

• Apex is the vertebra or disc with the greatest rotation or farthest deviation from the center of the vertebral column.

End Vertebrae

• End vertebrae are those with the maximal tilt toward the apex of the curve, and they are used to measure the Cobb angle.

Neutral Vertebrae

• Neutral vertebrae are those that show no evidence of rotation on standing frontal (either posteroanterior [PA] or anteroposterior [AP]) radiographs; their pedicles are in the normal, symmetric positions.

Stable Vertebrae

• Stable vertebrae are the vertebrae farthest cephaled that are bisected or nearly bisected by the central sacral vertical line (CSVL) at a level below the end vertebra of the distal curve.
• The CSVL is a roughly vertical line that is drawn perpendicular to an imaginary tangential line drawn across the top of the iliac crests on radiographs. It bisects the sacrum.

Measurement of the Cobb Angle

Cobb angle measurement of a dextroscoliosis.

• The Cobb angle of a scoliotic curve is the angle formed by the intersection of two lines, one parallel to the endplate of the superior end vertebra and the other parallel to the endplate of the inferior end vertebra (Image).
• The angle may be plotted manually or digitally.
• It is the main standard for diagnosis, monitoring, therapeutic planning, and epidemiologic analysis of scoliosis.
• A progressive curve that requires management is defined by a Cobb angle increase of 5° or more between consecutive radiographic examinations.

Pitfalls of the Cobb Angle

• It is performed by using a two-dimensional radiographic image of a 3-D deformity and does not take vertebral rotation into account.
• Cobb angle measurement may be inherently difficult.
• A diurnal variation of 5° has been observed in Cobb angle measurements of the same curve over the course of a single day, with an angular increase occurring in the afternoon.
• Because of the vertebral rotation associated with scoliosis, it may be difficult to position the patient so as to obtain an accurate frontal view, and the actual Cobb angle might be 20% greater than that plotted on radiographs.
• During surgery, cobb angle decrease due to prone positioning and anesthesia and sometimes followed by a postoperative rebound effect, with a loss of correction when the patient returns to the standing position.

Identification of Primary and Secondary Curves

• Major curves, also called primary curves, are the largest abnormal curves in the scoliotic spine and the first to develop.
• Minor curves, also called secondary curves, are smaller and are considered to develop afterward, to compensate for the perturbation of balance that accompanies the progression of major curves by repositioning the head and trunk over the pelvis to maintain balance.
• The terms major curve and minor curve are sometimes used as synonyms for structural curve and nonstructural curve respectively.

Assessment of Vertebral Alignment and Balance

The saggital and coronal balance are both within the normal range Source: Case courtesy of Dr Balint Botz , Radiopaedia.org, rID: 63126

• The central sacral vertical line(CSVL) drawn on radiographs serves as a reference for identifying stable vertebrae, evaluating coronal balance, and determining the curve type.
• The plumb line is a vertical line drawn downward from the center of the C7 vertebral body, parallel to the lateral edges of the radiograph.
• Plumb line is used to evaluate coronal balance on standing frontal radiographs and sagittal balance on standing lateral radiographs.
• Coronal balance is assessed by measuring the distance between the CSVL and the plumb line.
• Sagittal balance is evaluated by measuring the distance between the posterosuperior aspect of the S1 vertebral body and the plumb line.
• For both coronal and sagittal measurements, balance is considered abnormal if the distance is greater than 2 cm.
• For measurements of coronal balance, a plumb line located to the right of the CSVL is considered to reflect positive coronal balance.
• On the other hand, a plumb line located to the left of the CSVL is considered to reflect negative coronal balance.
• Similarly, for measurements of sagittal balance, a plumb line that is anterior to the posterosuperior aspect of the S1 body is considered to reflect positive sagittal balance.
• However, a plumb line that is posterior to the posterosuperior aspect of the S1 body is considered to represent negative sagittal balance.

Measurement of Vertebral Rotation

• Modern instrumentation systems has led to a marked increase in the importance of measuring vertebral rotation in the scoliotic spine.

The shortcomings of the Cobb angle for describing vertebral rotation are partly overcome by the so-called Nash-Moe method.

• In Nash-Moe method, the pedicle location on frontal radiographs is used as an indicator of the extent of vertebral rotation.
• The half vertebra on the convex side of curvature is divided into three segments, and rotation is quantified on the basis of the pedicle location in regard to the segments (Grade 1-4).

Rib-Vertebral Angle Difference (RVAD) of Mehta

• For patients with infantile idiopathic scoliosis, the likelihood of curve progression is determined radiographically by measuring the rib-vertebral angle difference (RVAD) of Mehta.
• An RVAD of 20 degrees or less indicates that the curve is unlikely to progress, whereas an RVAD of 20 degrees or more indicates a curve is likely to progress.
• An additional method of predicting curve progression also described by Mehta is the relationship of the convex rib head with the apical vertebra body (phase of the rib head).
• In phase-1 rib, there is no overlap of the rib head of the convex rib of the apical vertebra with the vertebral body; such curves have a low risk of progression.
• However, in phase-2 rib, there is an overlap; hence, there is a high risk of progression.

Skeletal Maturity

• Skeletal maturity is assessed by the Risser sign (maturity of iliac crest apophysis) and open versus closed triradiate cartilage of the acetabulum.
• These features are used in predicting the growth remaining, hence, the curve progression, which influences the choice of treatment.