Letter re: “Are Variable Screw Angle Change and Screw-To-Vertebral Body Ratio Associated with Radiographic Subsidence Following Anterior Cervical Discectomy and Fusion?”

We have read with great interest the recent article by Louie PK et al titled “Are Variable Screw Angle Change and Screw-to-Vertebral Body Ratio Associated With Radiographic Subsidence Following Anterior Cervical Discectomy and Fusion? ¹ ”, which introduces a novel early measurement method for early subsidence following an Anterior cervical discectomy and fusion called screw angle change. We commend the authors for proposing a new approach to evaluating cage subsidence following ACDF. However, we hope to record some of our opinions and seek clarification from the author in order to better popularize the results of this study.

The primary concern is that the article does not clearly specify the source of the screw angle change. Specifically, it is not explained whether the change in screw angle results from movement between the screw and the plate, or from changes within the vertebral body itself. To our knowledge, the variable-angle system allows for the insertion of screws at different angles, enabling the surgeon to adjust the angle and direction of the screws based on the specific circumstances. ² However, the authors didn’t mention whether variable-angle screws and the corresponding plates are fully locked following implantation in this article. These factors could influence the measurement of screw angle change and its accuracy in assessing postoperative subsidence.

Based on our understanding, we believe there are 3 possible scenarios that could explain postoperative screw angle change:

(1) Displacement between the screw and plate: In this case, the screw itself remains fixed in the vertebral body, but the plate and screw angle change due to movement between the 2 components.

(2) Movement of the screw within the vertebral body: The screw and plate remain locked, but the screw moves within the vertebra, altering the screw angle.

(3) Combination of displacement between the screw and plate and movement of the screw within the vertebral body: In this scenario, both the screw and plate, as well as the screw and vertebra, experience movement, which could complicate the interpretation of screw angle change.

Unfortunately, the article does not provide detailed information on the specific type of screws and plates used, nor does it include case photographs of significant screw angle change changes. As a result, the exact cause of the observed screw angle change remains unclear.

We recommend that the authors include a measurement of the angle change between the screw and the plate, and compare this with the screw angle change measured using the endplates and screws. This comparison would help clarify the origin of the observed changes.

If the first scenario (screw-plate displacement) is the primary cause, we anticipate that the measurements of the screw angle change from the original article will correspond closely with the screw angle change measured using the endplates and screws. Should this consistency be observed, we would like the authors to assess which of the 2 methods has the smaller potential measurement error and is more suitable for popularization.

In the second scenario, where screw displacement within the vertebral body occurs, this may suggest microfractures in the bone surrounding the screw, leading to screw loosening. This aspect requires closer scrutiny, as it could affect the long-term stability of the fixation.

Finally, if both interfaces (screw-plate and screw-vertebra) are subject to movement, the measurement of screw angle change would be influenced by multiple factors, making this method of measuring subsidence less reliable.

Some previous studies have focused on the parameter of screw angulation,^3-5 but there are few studies that address the changes in screw angle. We commend the authors for proposing a novel parameter for subsidence, but we also wish for the authors to elucidate the issues mentioned above. A more detailed explanation of the methodology and additional case examples would be greatly appreciated to improve the clinical applicability of this promising measurement technique.