Final answer:
Knees are bent during an AP lumbar spine x-ray to reduce the lumbar curvature for a clearer view of the vertebrae, aiding in the diagnosis of conditions affecting spinal alignment.
Step-by-step explanation:
When x-raying an AP lumbar spine, the knees are bent to assist in reducing the lumbar curvature by flattening the lower back, thus providing a clearer view of the vertebrae. The positioning helps in minimizing the effects of spinal curvatures, like kyphosis or lordosis, and compensates for the natural curvature that could obscure the details in an X-ray image. This is important in diagnosing conditions such as osteoporosis, which can lead to compression fractures and alterations in spinal curvature. Additionally, variations in pelvic structure, like the Q-angle mentioned, affect the alignment of the hips and femur, which can further impact how the spine appears on an X-ray.