Local Regulatory T-cell Failure as a Potential Mechanism of the Halo Sign in Spinal Instrumentation: A Mechanistic Hypothesis.

The radiolucent halo seen on computed tomography (CT) around a loosening pedicle screw is usually interpreted as a mechanical sign, explained by micromotion, wear debris, and particle disease. Those explanations come from hip and knee arthroplasty. A pedicle screw is not an arthroplasty component; it is driven into the vertebral body, so part of its surface lies within the bone marrow from the moment of insertion. This article asks whether that difference has immunological consequences. The proposal is that metal-derived products may be presented by bone marrow antigen-presenting cells (APCs) to local T cells, supporting an in situ type IV immune response. That response may stay clinically silent while local regulation holds. Loss of that regulation may then permit receptor activator of nuclear factor-κB ligand (RANKL)-driven osteoclastogenesis along the screw. The provisional term "Sakaguchi-type regulatory failure" is used for that proposed local loss of regulatory T-cell (Treg)-mediated restraint. It is not a recognized pathological or radiological entity. If the model has merit, patients whose regulatory capacity is already impaired should show more implant-related failure, and the spinal literature is compatible with that. The reported excess, however, is in reoperation rather than the halo sign and is confounded by bone density, corticosteroid use, and low-grade infection. Spine-specific evidence further constrains the model, and the three findings that do so are set out alongside the case for it. No direct human spinal evidence exists either in support of or against the hypothesis. Predictions that would refute the hypothesis are also presented. This is a hypothesis article, not a systematic review.