Lower Hounsfield Units on CT Are Associated with Cage Subsidence After Anterior Cervical Discectomy and Fusion

CLINICAL ARTICLE J Neurosurg Spine 33:425–432, 2020 Lower Hounsfield units on CT are associated with cage subsidence after anterior cervical discectomy and fusion Minghao Wang, MD, PhD,1,2 Praveen V. Mummaneni, MD,1 Zhuo Xi, MD,1 Chih-Chang Chang, MD,1 Joshua Rivera, BA,1 Jeremy Guinn, BS,1 Rory Mayer, MD,1 and Dean Chou, MD1 1Department of Neurological Surgery, University of California, San Francisco, California; and 2Department of Neurosurgery, First Affiliated Hospital of China Medical University, Shenyang, China OBJECTIVE A consequence of anterior cervical discectomy and fusion (ACDF) is graft subsidence, potentially leading to kyphosis, nonunion, foraminal stenosis, and recurrent pain. Bone density, as measured in Hounsfield units (HUs) on CT, may be associated with subsidence. The authors evaluated the association between HUs and subsidence rates after ACDF. METHODS A retrospective study of patients treated with single-level ACDF at the University of California, San Fran- cisco, from 2008 to 2017 was performed. HU values were measured according to previously published methods. Only patients with preoperative CT, minimum 1-year follow-up, and single-level ACDF were included. Patients with posterior surgery, tumor, infection, trauma, deformity, or osteoporosis treatment were excluded. Changes in segmental height were measured at 1-year follow-up compared with immediate postoperative radiographs. Subsidence was defined as segmental height loss of more than 2 mm. RESULTS A total of 91 patients met inclusion criteria. There was no significant difference in age or sex between the subsidence and nonsubsidence groups. Mean HU values in the subsidence group (320.8 ± 23.9, n = 8) were significantly lower than those of the nonsubsidence group (389.1 ± 53.7, n = 83, p < 0.01, t-test). There was a negative correlation between the HU values and segmental height loss (Pearson’s coefficient −0.735, p = 0.01). Using receiver operating characteristic curves, the area under the curve was 0.89, and the most appropriate threshold of HU value was 343.7 (sensitivity 77.1%, specificity 87.5%). A preoperative lower HU is a risk factor for postoperative subsidence (binary logistic regression, p < 0.05). The subsidence rate and distance between allograft and polyetheretherketone (PEEK) materials were not significantly different (PEEK 0.9 ± 0.7 mm, allograft 1.0 ± 0.7 mm; p > 0.05). CONCLUSIONS Lower preoperative CT HU values are associated with cage subsidence in single-level ACDF. Preop- erative measurement of HUs may be useful in predicting outcomes after ACDF. https://thejns.org/doi/abs/10.3171/2020.3.SPINE2035 KEYWORDS anterior cervical discectomy and fusion; ACDF; Hounsfield units; HU; subsidence NTERBODY graft or cage subsidence after anterior cer- Dual-energy x-ray absorptiometry (DEXA) is the most vical discectomy and fusion (ACDF)1 may lead to widely used way to assess BMD to diagnose osteopenia nonunion, loss of foraminal height, postoperative ky- or osteoporosis. However, DEXA scans may not be rou- Iphosis, adjacent-segment degeneration secondary to plate tinely performed before spinal fusion because of cost, migration, and recurrent pain.2,3 Previous studies have re- denials from insurance companies, and variability from ported that bone mineral density (BMD) is one of the fac- one part of the skeleton to another.11 Because of the rela- tors that affects graft subsidence after ACDF.4–7 Other fac- tively minimal corrective forces applied to the spine, and tors affecting subsidence include age, cervical alignment, because of the relatively high rates of good outcomes in integrity of the endplate, use of plate fixation, number of ACDF, DEXA scans are not often performed as part of the treated levels, and properties of the interbody graft.5,8–10 routine preoperative workup for ACDF. However, many Thus, assessment of bone health in ACDF patients is an reports have shown that the Hounsfield units (HUs) mea- important part of the perioperative evaluation. sured on CT scans are reflective of BMD as measured by ABBREVIATIONS ACDF = anterior cervical discectomy and fusion; AUC = area under the ROC curve; BMD = bone mineral density; DEXA = dual-energy x-ray absorptiometry; HU = Hounsfield unit; PEEK = polyetheretherketone; ROC = receiver operating characteristic; ROI = region of interest. SUBMITTED January 9, 2020. ACCEPTED March 30, 2020. INCLUDE WHEN CITING Published online June 5, 2020; DOI: 10.3171/2020.3.SPINE2035. ©AANS 2020, except where prohibited by US copyright law J Neurosurg Spine Volume 33 • October 2020 425 Unauthenticated | Downloaded 10/01/21 02:19 AM UTC Wang et al. DEXA.12–15 Furthermore, lower HU values in the lumbar using the radiology patient archiving and communication spine are associated with pseudarthrosis and graft subsid- system. The ROI was expanded to be as large as possible ence, reflecting lower bone density.16–18 To our knowledge, within the vertebral body, but it did not include any corti- there is no report on the relationship between graft subsid- cal bone, such as the lateral walls or endplates. The final ence and HU values in ACDF. Therefore, in this study, we HU value was calculated as the mean of all 3 measure- measured the association between subsidence after single- ments (Fig. 1A). level ACDF19 and HU values, based on preoperative CT The segmental height was measured as the distance with a minimum 1-year follow-up.20,21 from the midpoint of the superior endplate of the upper vertebral body to the midpoint of the inferior endplate Methods of the lower vertebral body spanning the fusion. These Patient Cohort measurements were recorded at the initial postoperative radiograph (within 2 weeks after surgery) and at 1-year We retrospectively reviewed all single-level ACDF follow-up (Fig. 1B). Subsidence was defined as a segmen- cases from 2008 to 2017 performed by two attending neu- tal height loss of more than 2 mm at the 1-year follow-up, rosurgeons at the University of California, San Francisco. or interbody graft migration into either endplate with or All data were collected and assessed independently by without segmental height loss of more than 2 mm (Fig. two spine surgeons (D.C., P.V.M.). Inclusion criteria were 1B). Graft and cage type were also recorded and calcu- adults undergoing a single-level ACDF for degenerative lated for possible differences. spinal conditions (no prior anterior or posterior surgery was performed at the studied level), a preoperative CT Statistical Methods scan, minimum 1-year follow-up with imaging, and anterior plate fixation. Exclusion criteria were patients with Mean values were expressed as means ± standard de- infection, tumor, trauma, additional posterior cervical viations. Demographics and HU differences between the surgery (staged or not), cervical kyphosis > 5°, or the use subsidence and nonsubsidence groups were analyzed with of standalone integrated screw and cage devices. Patients the Pearson chi-square test (or Fisher exact test according who had cages with integrated fixation (either blades or to the sample size and expected values) and the Student integrated screws obviating the need for a plate) were all t-test. After using the Kolmogorov-Smirnov test to check excluded; only patents with anterior cervical plating were whether HU values and segmental height loss conform to included in this study. Patients who had been treated for a normal distribution, a Pearson correlation test was uti- osteoporosis with medical therapy were also excluded. lized to test whether these two sets of data were correlat- Data collected included demographic variables, interbody ed. A receiver operating characteristic (ROC) curve was graft or cage type, the presence of previous fusion at other computed to establish separation criteria between the sub- levels, segmental height loss at 1-year follow-up, and the sidence group and nonsubsidence group. The areas under presence or absence of graft or cage subsidence. the ROC curve (AUCs) were calculated for segmental HU value assessment. The most appropriate threshold (cutoff Surgical Procedure value) of HUs with a higher sensitivity and specificity was also established using the ROC curve. In addition, a bina- A standard Smith-Robinson anterior transcervical ap- ry logistic regression analysis was performed to examine proach was performed in all patients without major differ- whether the HU value was a risk factor for postoperative ences between the two surgeons in this study. After expo- graft subsidence. All statistical analysis was processed us- sure, the discectomy was performed in the standard fash- ing IBM SPSS Statistics (version 21.0, IBM Corp.), and p ion, and the posterior longitudinal ligament was excised values < 0.05 were considered significant. for complete decompression of the spinal cord and exiting nerve roots with the microscope. The cartilaginous endplates were removed, and the bony endplates were decorti- Results cated. An interbody cage with local autograft or structural A total of 91 single-level ACDF patients met inclu- allograft with local autograft was inserted. Anterior plate sion criteria (Fig. 2). All 91 patients had a preoperative fixation was performed in all patients, and no standalone CT scan, an immediate postoperative radiograph within 2 integrated screw-cage devices were used. weeks, and a radiograph at 1-year follow-up (mean 11.7 ± 1.0 months). Of the 91 patients, 8 patients developed graft Radiographic Evaluation subsidence, while 83 were classified into the nonsubsid- HU values were measured at the vertebrae above and ence group. There were no significant differences in age, below the graft placement (e.g., C5–6 ACDF had C5 and sex, smoking status, and BMI between the subsidence and C6 vertebral bodies measured for HUs). To evaluate HU nonsubsidence groups (Table 1). In addition, there was no values of each vertebral body and minimize the mea- difference in the subsidence rates between the two sur- surement error, each vertebral body was measured three geons (3/38 and 5/53, p > 0.05).

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