Measurements Most Frequently Used in Orthopedic Imaging

Cobb Angle

Although other methods exist, the Scoliosis Research Society chose the Cobb angle as the standard system for measuring scoliosis. The Cobb angle is measured on an anteroposterior radiograph of the full spine. A strict, standardized technique is key to reduce measurement errors. The radiograph should be obtained with the patient in standing position, 180 cm from the radiation source, with feet and knees together, and centered on the dorsolumbar hinge. The radiograph must include the cervical spine and the cranium superiorly and both femoral heads inferiorly in the same radiograph ( Fig. 1A ). On lateral images, the patient should be placed with the arms over the shoulders so that the upper extremities are not superimposed over the spine. Use of shields is recommended to reduce the radiation dose, especially in breast and pelvis.

To measure the Cobb angle, the most tilted vertebrae above and below the apex of the curve (terminal vertebrae) should be identified first. The upper limit is the vertebra above the apex of the curve whose upper end plate is more tilted toward the concavity of the curve to be measured. The vertebra at the lower end is the one below the apex whose lower end plate is more tilted toward the curve. The Cobb angle is formed by the lines that run parallel to the surface of the upper end plate of the vertebra at the upper end and the lower end plate of the vertebra at the lower end. However, to make measurement easier, a complementary angle (i.e., the one formed by the angle that is perpendicular to those two lines) is generally used (see Fig. 1B ). If end plates are difficult to visualize, the borders of the pedicles may be used.

The main drawback of the Cobb angle is its intraobserver and interobserver variability (between 2.8 and 10 degrees). This may cause difficulties when assessing the progress of scoliosis in a patient. To minimize the interobserver error, the radiologist should measure the Cobb angle on the current study and also in the previous studies. A patient may not show significant variations compared with the last follow-up, but a significant curvature progression can be observed when compared with previous studies, changing the management of the patient. Thus, it is important to review and measure the full range of radiographs of every patient. Moreover, supine radiographs underestimate the deformity Cobb angle, so it should always be measured on upright radiographs.

Adolescent spinal scoliosis is diagnosed when a 10-degree curvature is measured in the anteroposterior upright radiograph. Curves of less than 10 degrees are termed curvatures and 10 degrees or over, scoliosis. A 5-degree increase in curvature over a 12-month interval represents progression. The Cobb angle can also be used as a way to determine the prognosis. So, scoliosis in skeletally immature patients progresses in 65% of those with 20- to 30-degree angles and in nearly all patients with more than 30 degrees. In adults, thoracic curvatures more than 30 degrees tend to progress. Thoracic curvatures more than 100 degrees affect lung function.

The Cobb angle is also used to decide management in idiopathic scoliosis: Surgery is indicated in skeletally immature patients with a progressing 40-degree scoliosis or in mature patients with painful or progressive 45-degree scoliosis. Also, a reduction in the Cobb angle of 50% after wearing a brace foretells a successful bracing treatment. When planning surgical treatment, side-bending radiographs are used to decide whether minor scoliotic curves should be included in the surgical fusion. When these lesser curves decrease to less than 25 degrees, they are considered nonstructural, and otherwise, structural.

The Cobb angle is also used to evaluate thoracic kyphosis and cervical and lumbar lordosis. In these cases, it is measured in standing lateral radiographs. In cervical lordosis, the range of normal values, measured from the upper C1 end plate to the lower C7 end plate, is 40 degrees (±9.7 degrees). Thoracic kyphosis measurement is performed between the upper T1 end plate and the lower T12 end plate. Normal thoracic kyphosis in healthy individuals ranges from 20 to 50 degrees. Because of the difficulty in viewing the first dorsal vertebrae in lateral radiographs, the T4 upper end plate can be used as an option to perform the measurement.

Measurement of lumbar lordosis is performed between the L1 upper end plate and the S1 upper end plate. Normal values range from 31 to 79 degrees, depending, in part, on the measuring technique used. This is because, occasionally, the difficulty of correctly identifying S1 causes the lower L5 end plate to be used to draw the angle.

Sagittal and Coronal Balance

Sagittal balance measures the relationship of the positions of head and pelvis in the standing lateral view of the spine. It is measured by drawing a vertical line perpendicular to the floor, downward from the center of the C7 vertebral body. Sagittal balance is the distance between this line and the posterior aspect of S1 (see Fig. 1C ). When the line is anterior to the posterior aspect of S1, sagittal balance is termed positive, and negative when it is posterior. Values of −2 to +2 cm are considered normal. The sagittal balance correlates with functional impairment in scoliosis, and restoration of a normal sagittal balance should be one of the aims of surgery.

Coronal balance is measured in a standing anteroposterior view. A vertical line is drawn from the center of the C7 vertebral body. Coronal balance is the distance between this line and the central sacral vertical line. When the line is on the right of the sacrum, balance is termed as positive and negative when it is on the left. Values of −2 to +2 cm are considered normal.

Vertebral Rotation

Scoliosis always is associated with rotation on the vertebral axis. This rotation has a high prognostic value. Although there are several methods to assess it, the Nash-Moe technique is most commonly used ( Fig. 2 ). It is evaluated on an anteroposterior standing radiograph of the spine. The Nash-Moe technique classifies vertebral rotation into five grades, depending on the position of the pedicles in relation to the lateral borders and center of the vertebra. It is considered normal (grade 0) when both pedicles are symmetric and at an equal distance from the center and the lateral borders of the vertebra. The pedicle shift is classified as grade 1 if the pedicle of the side of the concavity has not yet made contact with the vertebra's lateral border. In grade 2, the pedicle of the concavity has almost disappeared along the lateral border of the vertebra, and the pedicle of the convexity is near the center of the vertebra. In grade 3, the pedicle of the concavity has disappeared and that of the convexity is in the middle of the vertebral body. Finally, in grade 4, the pedicle of the convexity has overreached the center of the vertebra.

Rib-Vertebral Angle Difference

Described by Mehta, the rib-vertebral angle difference (RVAD) enables the diagnosis of resolving or progressive infantile scoliosis (in children 3 to 5 years old). It is measured on a posteroanterior spinal radiograph, on the vertebra at the apex of the curve (apical vertebra). It is the difference between the angles formed by the apical vertebra upper end plate and the ribs adjacent on the convex and the concave sides of the curve. When the difference between the two angles is greater than 20 degrees, the scoliosis is considered to be progressive, and in that case, 85% of patients will suffer an increase of deformity over time. If it is less than 20 degrees, the scoliosis is resolving and tends to resolve itself spontaneously.

Axial Angle of the Shoulder

The axial angle of the shoulder is measured in the anteroposterior radiograph of the shoulder, obtained with the arm in external rotation. This angle ( Fig. 3 ) is formed by the line that follows the long axis of the humeral shaft and the line drawn between the apex of the lesser tuberosity and the lower border of the articular surface of the humerus (where the cortical image goes from being a band to being a line). On average, this angle measures 60 degrees in men and 62 degrees in women. When it is less than 40 degrees, it is called humerus varus. It can change as a result of congenital defects, deficiency diseases, hypoparathyroidism, fractures of the humeral head, and varus consolidation of a fracture.

Glenoid Version

In most humans, the scapula is not perpendicular to the axial plane of the scapula. Although glenoid version has been studied using plain radiography, CT appears superior for evaluation of the glenoid fossa. Glenoid version is measured in the axial scan corresponding to the midglenoid level. To measure the glenoid version, a line is drawn following the axis of the scapula, from midpoint of the glenoid fossa to the medial end of the scapula. The glenoid version is the angle between a line perpendicular to this axial line and a line traced through anterior and posterior margins of the glenoid ( Fig. 4 ). Normal glenoid is in slight anteversion (2 ± 5 degrees).

Excessive retroversion has been linked to posterior instability of the shoulder. Measurement of the glenoid version is also important for effective surgical planning in total shoulder arthroplasty. The main problem with this technique of measurement is that the results are dependent on scanning orientation.

Measurement on Shoulder Bony Stock

Accurate placement and adequate osseous support for the glenoid component are important to achieve a successful result in total shoulder replacement. Only a small volume of bone is available for fixation of the prosthesis in patients with osteoarthritis in whom operative procedures are being contemplated, especially when extensive destruction by the disease has occurred.

The measurements used to calculate bony stock before total shoulder replacement are taken on axial CT images of the shoulder at three levels:

• Upper, through the base of the coracoid ( Fig. 5 ).

  

• Middle, through the middle of the glenoid cavity (see Fig. 5 ).

• Lower, 1 cm below

The measurements taken are :

• Maximum anteroposterior diameter of the glenoid.

• Position and extent of unsupported bone anteriorly and posteriorly.

• Position and extent of supported bone.

• Medial displacement. This is a measure of the depth of the glenoid. It is the distance from the base of the coracoid to the joint surface recorded at upper, middle, and lower levels.

• Glenoid slope. This is the angle between the sagittal plane and a line joining the anterior and posterior margins of the glenoid.

• Glenoid version.

• Width of the scapular neck. Measured at the upper level.

Measurements in Hill-Sachs Lesions

Hills-Sachs lesions are impression fractures resulting from impaction of the posterolateral aspect of the humeral head with the anterior glenoid during shoulder dislocation. Engagement of the Hill-Sachs lesion can affect postoperative recurrence of anterior shoulder instability. On CT, the engaging Hill-Sachs lesions were larger and more horizontally oriented to the humeral shaft than nonengaging lesions. Thus, precise preoperative judgment of the Hill-Sachs lesion and prediction of its engagement using CT findings is useful in planning additional procedures to treat a significant bone defect on the humeral head.

Three measurements ( Fig. 6 ) can be used to predict engagement :

• Width and depth of the Hill-Sachs lesion. To measure them, a circle is drawn on an axial CT image around the articular surface of the humeral head. Width is the distance between both ends of the lesion where the bone defect leaves the circle. Depth is the longest distance between the circle and the bottom of the lesion.

• Bicipital angle. This is the angle between a line connecting the bicipital groove to the center of the humeral head and a line connecting the center of the humeral head to the center of the Hill-Sachs lesion. This angle determines the location of the lesion.

• Hill-Sachs angle. Measured on 3D CT, it determines the orientation of the lesion. It is the angle between a line following the deepest groove of the lesion and the longitudinal axis of the humerus.

Shoulder Greater Tuberosity Displacement in Isolated Fracture

Clinical outcome of isolated fractures of the greater tuberosity of the proximal humerus depends on the magnitude of the displacement. Although diagnosis and classification of these fractures have been based on standard plain radiographs, CT measurement is more accurate. On CT or MRI, measurements should be performed in reconstructed coronal and sagittal images. The information that must be provided is:

• Direction of displacement: anterior, posterior, cranial, or caudal

• Distance between the upper surface of the humeral head and upper margin of the main fragment or distance between the outer surface of the humeral head and the outer margin of the main displaced fragment

More than 5 mm displacement in the general population or more than 3 mm in active patients with frequent overhead activity is an indication for surgical fixation.

Bony Bankart Measurement

The successful arthroscopic treatment of recurrent anterior shoulder instability requires optimal patient selection. Postoperative failure is higher in patients with glenoid bone loss. Thus, it is important to determine the extent of such loss. CT has become the most accurate technique for this measurement.

To evaluate the glenoid rim, 3D CT images with the humeral head eliminated should be obtained. The configuration of the glenoid rim is evaluated on en face views. Measurements of the surface area of the osseous fragments at the anteroinferior portion of the glenoid are compared with the surface area of a circle that fits the glenoid. The size of the fragment is classified as large (more than 25% of the glenoid fossa), medium (12.5% to 25%), or small (less than 12.5%).

Ulnar Variance

Ulnar variance is the measurement of the differences in the apparent length of the ulna and radius at wrist level. It is measured on a neutral dorsovolar radiograph of the wrist, obtained with the elbow and shoulder at 90 degrees and without beam angulation. Ulnar variance is defined as negative (minus) if the distal articular surface of the ulna lies proximal to the distal articular surface of the radius, as positive (plus) if it is proximal, or as neutral if both articular surfaces are aligned.

There are two methods for measuring ulnar variance. The simplest is the method of perpendiculars, which requires only the aid of a ruler. The long axis of the radius is drawn, and a line perpendicular to it is traced through the ulnar corner of the articular surface of the radius. The ulnar variance is the distance between that line and the most distal point of the dome of the ulna ( Fig. 8 ). If the ulna is extended distal to that line, the variance will be positive; if the opposite occurs, it will be negative.

Studies carried out on normal individuals show a broad variability of the parameter in nonsymptomatic individuals, as well as variations between ethnic groups, both genders, and different age groups.

Ulnar variance is important for its possible association with several disorders of the wrist. Thus, Kienböck disease and acute carpal instability are frequently associated with a negative variance, whereas a positive variance may lead to ulnocarpal impaction. Ulnar variance is also useful in predicting functional recovery after wrist fracture consolidation. A residual positive variance of more than 5 mm after a Colles fracture significantly increases the probability of a poor functional outcome. In Galeazzi fractures, a positive variance of more than 10 mm implies a broken interosseous membrane and, as a result, instability of the distal radioulnar joint.