Maintenance of Derotation in Adolescent Idiopathic Scoliosis: a Novel Technique Measuring Postoperative Vertebral Rotation by Pedicle Screw Position

Abstract

Background: Current methods of measuring vertebral rotation by plain radiographs rely on anatomic landmarks that are not present in the postoperative spine or require advanced imaging. Furthermore, there are few studies on the incidence of crankshaft with modern pedicle instrumentation. Questions/Purposes: We sought to (1) describe and validate a method of vertebral rotation measurement using plain radiographs and (2) measure postoperative rotation in a series of patients treated for adolescent idiopathic scoliosis. Methods: Patients with adolescent idiopathic scoliosis treated with surgery over a 6-year period were reviewed. Patients with computed tomography (CT) scans and radiographs within 60 days of another were included. Vertebral rotation was calculated by radiographic measurements and measured directly by CT scan. As an internal control, patients with two apical pedicle screws on all radiographs were analyzed. Rotation was measured for all patients with at least 1 year of radiographic follow-up. Results: Three thousand five hundred fifty-two instrumented spinal levels in 308 consecutive patients were reviewed. Ten patients with 93 screws were analyzed by CT and radiographs. The average discrepancy between computed tomography (CT) and radiographs was 3.3±1.9°, with 81.7% (76/93) within 5°. Intra-and inter-rater reliabilities for measured axial rotation were excellent (intra-class correlation coefficient (ICC)=0.879 and 0.900, respectively). One hundred swventy-eight patients were eligible with an average followup of 2.3±1.2 years; 84.8% (151/178) had screw(s) visible on all images at the major curve apex. The average postsurgical rotation was 3.5±2.9°; 19.2% (29/151) were measured to have a rotation over 5°, and 4.0% (6/151) demonstrated a rotation over 10°. Only 4.6% (7/151) of patients demonstrated a postoperative Cobb angle change over 10°. Conclusions: Most major curves have apical pedicle screw instrumentation that can be followed by radiographs alone to measure rotation. Vertebral rotation measurement requires only plain radiographs and is a more sensitive determination for subtle postoperative crankshaft than change in Cobb angle.

Measurement of Postoperative Apical Vertebral Rotation Using Radiographic Images in Adolescent Idiopathic Scoliosis-Quantitative Evaluation Using Inverse Trigonometric Functions

“…Another study using the separation between the rods on a lateral radiograph to estimate the rotation using a trigonometric function also yielded results similar to those obtained with CT (r=0.84) 22) , although the distance between the rods is difficult to measure at the PS level due to the screw heads, and a consistent rod width is assumed. In contrast, Bjerke et al 8) reported that the average difference between the axial rotation measured on CT and that measured using radiographs was 3.3±1.9°, which is similar to the difference reported in the current study in which no assumptions regarding screw length or rod width are made. This method is accurate and reproducible in patients in whom a PS is inserted.…”

“…A previous study reported that the evaluation of vertebral rotation after posterior fusion using the inverse trigonometric function method is a more sensitive determination of CSP than changes in the Cobb angle 8) . This previous study defined CSP as a Cobb angle >10°, which occurred in 4.6% of patients and a vertebral rotation >5° occurred in 19.2% of patients.…”

“…In the lateral view, an acceptable alignment was determined using the distance between the most superior aspects of the acetabula in the sagittal plane. If this distance was <10 mm, the radiograph was acceptable 8) . Furthermore, we ensured that the spinous processes of the L5 vertebra were positioned in the midline of both pedicles in the radiograph.…”

“…The postoperative rotation angle was calculated using the inverse trigonometric function method 8) . The lateral to medial and posterior to the anterior lengths of the PS were measured from the center of the rod to the tip of the PS, parallel to the floor on frontal and lateral radiographs.…”

“…Bjerke et al 8) reported a method to evaluate axial rotation using frontal and lateral radiographs and an inverse trigonometric function based on the length of the pedicle screw (PS). This method evaluates the degree to which the PS is inclined toward the vertebrae and provides an accurate, continuous variable, and reproducible method for measuring postoperative vertebral rotation.…”