This is the best view to determine if there is curvature in the spine and may also help identify any metastasis. Evaluate for proper spinous process and vertebral body alignment (Fig 3). When viewing the curvature, be aware that a curvature in the thoracic spine may exist when viewing the lumbar spine and this may be a compensatory curve. Most adolescent scoliosis is levoscoliotic or convex left. Degenerative curvature can be in any direction. The convexity opens the neural foramina and the concavity narrows the open foramina . The concave part of the scoliosis can close the neural foramen and pinch the nerve. While looking at the AP, be sure to visualize the pedicles at every level (Fig 4). A “winking owl” sign may herald a destructive process like metastatic cancer in the pedicle (Fig 5) .
The Cobb angle is the most widely used measurement to quantify the magnitude of spinal deformities . The reader must decide which vertebrae are the ends of the curvature or terminal vertebrae. Lines are drawn along the endplates, and the angle between the two lines, where they intersect, measured. Most PACS will have a dedicated angle tool to measure this without needing the lines to intersect or add the lines at right angles. The Cobb angles can help measure or monitor progression and can aid in treatment decisions (Fig 6).
Adjacent soft tissue, including the crura of the diaphragm (seen adjacent to the upper lumbar spine) and the bulges of the psoas and quadratus lumborum muscles seen lower down, should also be assessed. The kidneys, gallbladder, female genital tract, aorta, and other structures may contain calcification, and nephrocalcinosis (and renal stones), gallstones, a calcified uterine fibroid, and a calcified aorta may be diagnosed as ‘incidental findings’ .
The lateral view is best for showing loss of disc space, degenerative changes or subluxation. One should first check for alignment of the vertebral bodies (Fig 7). Each vertebral body, disc spaces, posterior elements and transverse processes (Fi. 8). Disc spaces should gradually increase height from superior to inferior in the lumbar spine with the exception for the L5-S1 space sometimes being less than L4-5. The pedicles gradually become wider from superior to inferior. Each exam should look closely for signs of compression fractures such as loss of disc height. These fractures are more likely in the lower thoracic or upper lumbar vertebrae and can be acute or chronic . The marginal or posterior vertebral line should be evaluated, looking for the subluxation or spondylolisthesis. Adjacent soft tissue should be assessed. As mentioned previously, a calcified aorta is commonly seen on the lateral view.
The oblique view is useful for finding a pars fracture or defect. The defect of the pars interarticularis is commonly known as the Scottie Dog sign. Each vertebrae should be evaluated for the Scottie dog and a crack through the neck is a fractured pars interarticularis (Fig. 9). The Scottie Dog’s leg and ear are the superior and inferior articulating processes respectively. In patients with facet joint degeneration, changes will be seen in the Scottie Dog’s ear and leg. If Scottie Dog’s neck is missing, the patient may have a traumatic injury to the pars interarticularis, a congenital defect or both .
The flexion and extension views may be used if the provider suspects instability of the time or sees spondylolisthesis on lateral x-rays, which is defined as anterior displacement of one vertebrae upon another (Fig. 10). In an unstable, mobile, spondylolisthesis, the displacement will be greater in flexion, and less in extension. In other words, the displacement may reduce in extension (Fig. 11, 12). In any case, a mobile spondylolisthesis can cause severe mechanical back pain, with leg pain if a nerve is pinched in the intervertebral foramen. Spondylolisthesis may affect up to 10 percent of the population and measurements are made to aid in treatment .
Wedge fractures are usually stable injuries caused by excessive flexion, which results in a collapse of the anterior vertebral body (Fig. 13). On the lateral view, there is decreased height of the anterior wall of the vertebral body with the posterior wall intact and the spinal canal is not involved. If there is marked compression (greater than 40 percent) of the anterior two-thirds of the vertebral body or if the fracture extends across the entire vertebrae, the examiner may need to do flexion or extension views or suspect an unstable fracture . Flexion and extension views would show an increase in the interspinous space (between two adjacent spinous processes).
While compression fractures are generally viewed as stable, there are other fractures of the lumbar spine that are viewed as unstable injuries. Chance fractures, also known as lap seatbelt fractures, usually occur at L2 or L3. These are usually seen on AP or lateral views (Fig. 14). Horizontal splitting or vertebrae and rupture of ligaments which may include the interspinous ligament, posterior longitudinal ligament, ligamentum flavum, facet joint capsule, and intervertebral disc. They are associated with intra-abdominal injury in up to 50 percent of cases [8,9]
Burst fractures caused from falls in which the heels of buttocks strike the ground first and may cause damage to the upper lumbar vertebrae, though burst fractures as low as L5 have been reported. Burst fractures result in collapse of the entire vertebral body and this is seen best on lateral x-rays (Fig. 15). On the AP view, the interpedicular distance is increased. Signs of unstable burst fractures include acute neurologic impairment, kyphosis greater than 20 degrees, acute subluxation and greater than 50 percent or greater encroachment on the spinal canal [1,10]. If a burst fracture is identified, further imaging (CT scan or MRI) is commonly ordered.
In conclusion, many orthopedic or sports medicine practices will experience patients with back pain. Lumbar radiographs are commonly ordered for both acute and chronic back pain and AP and lateral films are a mainstay for most conditions. Oblique views can help find spondylolysis and flexion and extension views can help quantify spondylolisthesis. Providers should also be aware of characteristics of both stable and unstable fractures.
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