An association can be found between hounsfield units and success of lumbar spine fusion

Abstract

Background: Measuring Hounsfield units (HUs) from computed tomography (CT) scans has recently been proposed as a tool for assessing vertebral bone quality, as it has been associated with bone mineral density, compressive strength, and fracture risk. Vertebral bone quality is believed to be an important determinant of outcome and complication rates following spine surgery and potentially influences success of interbody spinal fusion.

Questions/purposes: The purpose of this study was to investigate the association between HU on CT scans and fusion success in patients with lateral transpsoas surgery for lumbar interbody fusion (LIF).

Methods: The CT scans of 28 patients with a combined 52 levels of stand-alone LIF were evaluated at a minimum of 12 weeks postoperatively. Coronal and sagittal images were evaluated for evidence of fusion, and HU values were collected from axial images. HU measurements were also taken from vertebral bodies proximal to the construct to evaluate global bone quality.

Results: Of the 52 LIF levels, 73% were assessed as fused and 27% were nonunited at the time of evaluation. The successful fusion levels had significantly higher HU measurements than the nonunion levels (203.3 vs. 139.8, p < 0.001). Patients with successful fusion constructs also had higher global bone density when vertebral bodies proximal to the construct were compared (133.7 vs. 107.3, p < 0.05).

Conclusion: With the aging population and increasing prevalence of osteoporosis, preoperative assessment of bone quality prior to spinal fusion deserves special consideration. We found that a successful lumbar fusion was associated with patients with higher bone density, as assessed with HU, both globally and within the fusion construct, as compared to patients with CT evidence of nonunion.

Keywords: Hounsfield units; fusion; osteoporosis.

Fig. 1

Technique of obtaining Hounsfield units (HU) values is demonstrated. a A successful lateral lumbar interbody fusion construct is shown to the left. Bridging bone is seen between L4 and L5 on both coronal and sagittal CT images (a (1, 2)). HU readings were made just caudal to the superior end plate, mid-body, and just cranial to the inferior end plate (a (3–5)). The mean of the three readings produced the vertebral body HU score. The mean of L4 and L5 was used to assess the construct bone density. Also apparent is the lack of artifact produced by the interbody PEEK cage. The bone density is visibly increased with higher HU values. b A nonunion patient showing a lack of bridging bone between L4 and L5 is shown to the right. Lower HU values both proximal to and within the construct are observed, and the subsidence of the PEEK cage is apparent. Axial images (b (3–5)) show radiographic signs of osteoporosis including lower attenuation, decreased trabecular density, and apparent cortical thinning.

Fig. 2

Mean Hounsfield units (with standard error bars) of vertebral bodies in the nonunion (139.8) and fused (203.3) groups (p < 0.001) along with individual data points are shown.

Fig. 3

Hounsfield units measurements (with standard error bars) of vertebral bodies proximal to the fusion construct compared with HU values within the construct. Both the fusion and nonunion groups showed significantly higher HU values within the BMP-2-augmented LIF construct as compared to proximal levels (p < 0.0001 and p < 0.01, respectively). The global BMD, as assessed by proximal vertebrae HU, was higher in patients with a fused construct as compared to those with a nonunited construct (p < 0.05).