Response to Letter to the Editor for “Are Variable Screw Angle Change and Screw-to-Vertebral Body Ratio Associated With Radiographic Subsidence Following Anterior Cervical Discectomy and Fusion?”

Dear editor,

We welcome the opportunity to respond to the “Letter to the Editor” regarding our article entitled “Are Variable Screw Angle Change and Screw-to-Vertebral Body Ratio Associated With Radiographic Subsidence Following Anterior Cervical Discectomy and Fusion?” The authors of this letter bring up some concerns regarding the method by which the screw angle change which we observed could occur and question whether these differing etiologies may have differing clinical significance or might make assessment of variable screw angle change less applicable to the general spine surgical community. Their primary concern is a lack of clarity about whether the variable-angle screws and the cervical plate are fully locked following implantation. As mentioned in the article, the 3 spine surgeons who performed the procedures which make up the study group all utilize a similar technique, with variable-angle screws placed at the cephalad and caudal vertebral bodies and fixed-angle screws placed into any intervening vertebral bodies, ie, in a 2- or 3-level fusion. The instrumentation system utilized by all 3 surgeons was the same and has screws which are described as “variable-angle” and which have a rounded inferior surface of the screw head which intersects with the plate or “fixed-angle” with a more squared-off surface along the bottom of the screw head. The variable angle screws are designed to allow for an angular change in order to compensate for placement of lordotic grafts. In all cases a rotational flange integral to the plate is used to secure the screws to the plate and prevent backout. Therefore, we would answer “yes, the plate and screws are locked following implantation” but there may still be some capability for angular change.

The authors of the “letter to the editor” propose 3 possible scenarios that could explain post-operative screw angle change. Our study demonstrated that the angular screw change was observed primarily in the caudal vertebral body and was also associated with a decrease in the prepared disc space height between the treated vertebrae which we used as a marker of subsidence. Given that the cervical plate has a fixed length, if the graft subsides into the endplate of either the superior or inferior vertebral body, and if there is a subsequent change in the caudal screw angle change, it is our best understanding that this change is arising partly from a loss of angulation between the interface of the screw and the cervical plate combined with slight movement of the screw within the vertebral body. We did not observe significant screw pullout or loss of fixation to suggest significant translational motion of the screw-plate construct in relation to the vertebral body, and would therefore suggest that our finding corresponds to “scenario 3” identified by the authors. We want to make clear as well that we are not proposing that the angular change leads to subsidence or vice versa. Rather, given that subsidence is sometimes difficult to determine on post-operative x-rays, if a surgeon notes a significant angular change on postoperative X-rays, especially in the caudal screws of a construct, there should be a higher suspicion for a subsidence of the graft at this level. Our finding that levels with a higher screw-vertebral body ratio were less likely to demonstrate an angular screw change also suggests that longer screws may have more structural integrity and could protect against subsidence, a finding which has been demonstrated in previous published reports.

We agree with the authors that inclusion of some pictorial examples of cases demonstrating angular screw change might have been useful, and will be sure to rectify this oversight in any future publications on this or related topics.