Reduced bone density based on Hounsfield units following long-segment spinal fusion with Harrington rods.

Long-segment instrumentation, such as Harrington rods, offload vertebrae within the construct, which may result in significant stress shielding of the fused segments. The present study aimed to determine the effects of spinal fusion on bone density by measuring Hounsfield units (HU) throughout the spine in patients with a history of Harrington rod fusion. Patients with a history of Harrington rod fusion treated at a single academic institution were identified. Mean HU were calculated at five spinal segments for each patient-cranial adjacent mobile segment, cranial fused segment, mid-construct fused segment, caudal fused segment, and caudal adjacent mobile segment. Mean HU for each level were compared using paired-samples t-test with statistical significance defined by p<0.05. Hierarchical multiple regression, including age, gender, body mass index, and time since original fusion, was used to determine predictors of mid-fused segment HUs. One hundred patients were included (mean age 55 ±12 years, 62% female). Mean HU for the mid-construct fused segment (110, 95%CI [100-121]) were significantly lower than both the cranial and caudal fused segments (150 and 118, respectively; both p<0.05), as well as both the cranial and caudal adjacent mobile segments (210 and 130, respectively; both p<0.001). Multivariable regression showed mid-construct HU were predicted only by patient age (-2.6HU/year; 95%CI [-3.4, -1.9]; p<0.001) and time since original surgery (-1.4HU/year; 95%CI [-2.6, -0.2]; p=0.02). HU were significantly decreased in the middle of prior long-segment fusion constructs, suggesting multilevel fusion constructs lead to vertebral bone density loss within the construct, potentially from stress shielding.

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