Osteoporosis is a systemic skeletal disease characterized by low bone mass and deterioration of the micro-architecture of bone tissue and can lead to bone fragility and an increased risk of fracture. It is common in postmenopausal women and elderly men.  Clinically, degenerative neck conditions are often complicated by osteoporosis. Osteoporosis can negatively affect cervical spine surgery and postoperative rehabilitation. In patients undergoing surgery due to degenerative cervical spine disorders, osteoporosis patients are more likely to receive posterior cervical fusion surgery than non osteoporosis patients; also, they tend to have more postoperative bleeding, higher probability of revision surgery, longer hospital stay, and higher hospitalization expenditure .
Clinically, the bone quality of cervical vertebrae is evaluated according to the T-score and BMD measured by DXA; however, whether the total BMD T-score of L1–L4 can reflect the actual bone quality of cervical vertebrae remains unclear. At present, a good option for evaluating the bone quality of cervical spine is the quantitative CT (QCT), which can accurately select a specific site for BMD measurement. Compared with DXA, QCT has higher diagnostic performance and can measure the BMD of cervical vertebrae more accurately [10, 11]. However, QCT is expensive, exposes patients to a large amount of radiation, and is available in only a small number of hospitals, which limit its wider application in the clinical settings. In recent years, some authors have attempted to use HU value to assess the condition of lumbar vertebrae and proposed corresponding threshold values for diagnosing osteoporosis. CT is a routine examination before spine surgery. and the acquisition of HU value is simple and convenient. The HU value can be used to evaluate bone quality with no additional examinations being required.
4.1 Measurement of HU values of the cervical vertebrae
To our knowledge, no literature has described the application of HU value in evaluating the bone quality of the cervical spine. Only a few studies have used QCT to investigate the BMD of cervical vertebrae, mainly in young volunteers. A study using QCT to measure the BMD of spine in healthy adult males showed that the BMD of cervical vertebrae was 256.0 ± 48.1 mg/cm3, and it gradually decreased from the cervical vertebrae to the lumbar vertebrae . CT scans on the cervical vertebrae of eight fresh cadavers revealed the overall average density of the cervical vertebral body trabecular bone was 270 ± 74 mg/cm3. In 88 patients undergoing anterior cervical surgery, the range of the BMD of the cervical vertebrae measured by QCT was 235.5–302.0 mg/cm3 before surgery, which gradually decreased in a craniocaudal direction . Another report used QCT to measure the BMD of cervical vertebrae at different anatomical locations and found that the BMD peaked at C5 and decreased in the directions of C3 and C7 (mean central vertebral body BMD for C3–C7, respectively: 336, 354, 360, 311, and 248 mg/cm3) . In a recent study, the C1-T1 BMD of 194 ACDF patients was measured by QCT. The results showed that the trabecular BMD was highest in the mid-cervical spine (C4) and decreased in the caudal direction, with an average of C1 = 253.3 mg / cm3, C2 = 276.6 mg / cm3, C3 = 272.2 mg / cm3, C4 = 283.5 mg / cm3, C5 = 265.1 mg / cm3, C6 = 235.3 mg / cm3, C7 = 216.8 mg / cm3, T1 = 184.4 mg / cm3.While the varied BMD of cervical vertebrae in different studies may be related to the differences in subjects and measuring instruments, there is a consistent trend that the BMD of cervical vertebrae gradually decreases in a craniocaudal direction.
Similar results were obtained in our current study: the C2–C7 alignments were negatively correlated with the HU value of the cervical vertebrae; as the cervical alignments decreased, the HU value of the cervical vertebrae gradually decreased. In addition, the HU values of C6 and C7 were significantly lower than those of C2–C5; therefore, the fixation strength must be considered when fixing C6 and C7 vertebrae with screws. Meanwhile, the anterior cervical instruments involving C6 and C7 also need further improvements.
4.2 Utility of the HU values of cervical vertebrae in assessing BMD
In our current study, the mean HU value of cervical vertebrae was positively correlated with the total BMD T-score of the lumbar vertebrae (Figure 2). We therefore speculate that the bone quality of the cervical vertebrae is parallel with that of the lumbar vertebrae, and the total T-score of the lumbar vertebrae can, to a certain extent, reflect the bone quality of the cervical vertebrae. In clinical practice, DXA is required to assess the bone quality of cervical spine before a cervical spine surgery. If lumbar spine DXA reveals a high total T-score of the lumbar vertebrae, the quality of cervical vertebrae can also be good.
In one study on the lumbar vertebrae, HU values had a strong positive correlation with T-score in the non-degenerative group, exhibiting correlation coefficients (r) greater than 0.7; in the degenerative group, in contrast, r was 0.6 or more . In our current study, the correlation coefficient (r = 0.487) between the average HU value of the cervical vertebrae and the T-score of the lumbar vertebrae was lower than the correlation coefficient between the HU value and the T-score of the lumbar vertebrae. The reason for this may be that the HU values and T-scores of the lumbar vertebrae in the above-mentioned study  were obtained from the same lumbar vertebrae, whereas the T-score in our current study was based on the total BMD T-score of L1–L4, which represents the overall bone quality of the lumbar spine.
During the measurement of the HU values of the cervical vertebrae, only the delineated vertebral trabecular bones were measured; in contrast, measurement of the T-score of the lumbar vertebrae covered all the bone mass (including the cortical bone, trabecular bone, attached structures, bone spurs, and even calcified arteries) per unit area of the lumbar vertebrae. Therefore, compared with the T-score of the lumbar vertebrae, the HU values of the cervical vertebrae can more accurately reflect the bone quality of cervical vertebrae. Thus, HU measurement can be used as an auxiliary diagnostic method to avoid missed diagnoses of cervical osteoporosis.
4.3 Utility of the HU values of cervical vertebrae in assessing bone quality before surgery
According to the World Health Organization (WHO) criteria, patients should be diagnosed with osteoporosis when the minimum T-score is ≤ -2.5, with osteopenia if the T-score is between -1 and -2.5, and as normal if the T-score is ≥ -1 [1,17]. Research by other colleagues in our center found that the threshold matching T-scores of -2.5 were 110, 100, 85, and 80 HU for L1, L2, L3, and L4, respectively, and the authors believed these HU thresholds could be applied as a complementary method to lower the rate of missed diagnoses of osteoporosis in patients with degenerative diseases of the lumbar spine . Some authors have provided HU thresholds at different sites to help diagnose osteoporosis. For example, patients with an average glenoid neck HU measurement of less than 197 had a 97% chance of having abnormal BMD, while patients with an HU measurement over 257 had a 100% chance of having normal BMD . Similarly, patients with a skull HU value of <610 as determined via brain CT may be considered for further evaluation for possible osteoporosis .
In our current study, the BMD T-score was used as the standard for diagnosing osteopenia and osteoporosis. The HU threshold corresponding to osteopenia was 327HU, which was close to the average HU values (322.52 ± 89.27 HU) of C2–C7 in 939 patients. In fact, all the patients in this group underwent cervical spine surgery due to cervical degenerative conditions, with a mean age of 59 years and an average T-score of -0.73. Some patients had osteopenia or osteoporosis. Clinicians can use the mean HU value of cervical vertebrae to assist the T-score–based diagnosis of osteopenia or osteoporosis in the lumbar spine. Osteoporosis is suspected in patients with an average HU value of less than 269 HU in C2–C7, and this threshold had a sensitivity of 63.8% and a specificity of 80.8%. In our current study, the HU values of C6 and C7 remarkably decreased; thus, if the HU value of any C2–C5 vertebrae is below 269 HU, the possibility of osteoporosis is extremely high.
A possible limitation of this study was that all our subjects were inpatients undergoing cervical spine surgery; although a healthy population was not used as a control group, in clinical settings, spine surgeons are more concerned with the bone quality of patients scheduled for surgery. This study only describes the correlation between CT value of cervical spine and DXA bone mineral density. Future longitudinal studies are needed to quantify the correlation between CT value of cervical vertebra and hardware failure, interbody subsidence and pseudoarthrosis in anterior cervical surgery.