1, 8, 13, 14, 17 HU values of lumbar vertebrae from L1 to L5 have excellent reliability, significant correlation with DXA T-scores, and good performance in diagnosing osteoporosis. Since 2011, the method of using Hounsfield units (HUs) from clinical CT scanning has been widely recommended to assess BMD and screen for osteoporosis. 19 Moreover, DXA can overestimate the BMD of the lumbar spine and cause a missed diagnosis of osteoporosis in lumbar degenerative disease, especially for degenerative lumbar scoliosis, which is a more common disease needing fusion to S1. 4 Based on the positive correlation between the BMD of different sites in the same patient, the BMD of the S1 body can usually only be estimated via DXA rather than measured directly. 9, 18 Dual-energy x-ray absorptiometry (DXA) is a widely used method to evaluate BMD and diagnose osteoporosis, but it is routinely used to measure BMD of the first to fourth lumbar vertebrae (L1–4) and hips. 18, 19 Therefore, preoperative measurement of the bone mineral density (BMD) of the S1 body is essential for surgical planning, which helps surgeons decide whether or not to use other techniques to prevent failure at S1. 7, 19Īn S1 body with poor bone stock can cause screw loosening in the bone-implant interface, leading to the failure of fixation. 3 However, fusion to S1 can also have a high rate of complications such as pedicle screw loosening or pseudarthrosis, especially for older patients with osteoporosis. 15 In addition, long fusion to S1 for lumbar deformity is associated with better maintenance of sagittal alignment and a lower rate of subsequent disc degeneration of L5–S1. T he first sacral vertebra (S1) is always chosen as the caudal level for spinal fusion with instrumentation for lumbar degenerative diseases because the L5–S1 level has the highest prevalence of disc degeneration in the lumbar spine, which may require decompression and fusion.
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