CT density measurement of bone graft within an intervertebral lumbar cage: increase of hounsfield units as an indicator for increasing bone mineral content.

Radiographic assessment of lumbar interbody fusion is difficult, especially in the presence of cages and metal artifacts. The purpose of this study is to assess whether impacted bone chips in intervertebral lumbar cages show a tendency toward increased attenuation on postoperative computed tomography (CT) scans as a sign of ongoing revascularization and increased bone mineral content inside the cage. Twenty-one patients with single- or double-level lumbar fusion underwent CT scanning 1-44 months postoperatively. CT attenuation of bone inside the cage was measured with Hounsfield units. Intra- and interobserver variability were evaluated. Regression analysis showed an increase of 7.5 HU/month postoperatively for an initial value of 615 HU. Intra- and interobserver variability showed an interclass coefficient of 0.97. CT attenuation of bone graft inside an intervertebral cage increases in the postoperative period. Hounsfield unit measurement can be performed with a high degree of accuracy and reproducibility and may in the future provide a useful tool in studying cage contents in individual patients.