**5.4 Midline placement**

During a cervical hybrid arthroplasty the C-ADR implant should routinely be placed first so no adjacent plate or hardware obstructs any anatomic or fluoroscopic visualization. In order to secure proper midline positioning during intraoperative placement some arthroplasty implants have instrumentation designed to help verify

### **Figure 2.**

*(a) The cervical spine is supported here with a foam cushioned pillow, often used by the anesthesiologists when placing the patients in a prone position. In this case the foam cushion supports the neck by acting as a counter force to any horizontal translational or shear forces at play during final implant tapping and placement. Here a cervical bite block is used and a 10 lb. weight allows for axial traction through a neck holster. During surgery this same neck holster can be pulled by the anesthesiologist to allow indirect distraction of the interspace and therefore ease placement of the prosthesis and fusion spacers intraoperatively. The C-arm, pictured here, is left in the lateral position for the majority of the case. (b) Coincidentally the cushion also provides a stable surface where needles can be placed to confirm the length of the surgical incision for a longitudinal skin incision preoperatively.*

### **Figure 3.**

*(a) A standard ruler is cut to 16 mm and this ruler can then be placed within the interspace to evaluate the depth of the trial, and therefore implant needed. (b) This ruler can then be placed within the interspace to evaluate the width of the trial, and therefore implant needed.*

the ideal location. This is important because minimizing prosthetic deviation to within 1.2 mm of the ideal center midline position, has been shown to ensure no detrimental clinical outcomes or long term repercussions [44]. In order to secure proper midline positioning first a collinear Anterior Posterior (AP) fluoroscopic view must be secured in line with the intended interspace (**Figure 4**). Once an appropriate image has been obtained, accurate midline positioning of the prosthesis can be confirmed (**Figure 5a-c**). In order to confirm proper midline positioning attention should be made towards discrete morphological and anatomical landmarks. First with visual inspection, confirming equidistant placement of the trial in regards to the longus coli. On fluoroscopy the spinous processes should lie en face and midline with respect to their corresponding vertebral bodies. The edges of the trial should lie equidistant with respect to each of the ascending bilateral uncinate joints. Final midline placement can be confirmed with fluoroscopic visualization (**Figure 5a**)**.**

### **5.5 Measuring fluoroscopic depth**

Most implant trials come with a drill, chisel, or similar device used to cut grooves in the vertebral body for insertion of the final implant. In order to confirm final implant placement the final imaging obtained while trialing can be compared with the spinal implant to confirm final and accurate positioning **(Figure 6a-c).**

### **5.6 Final implantation**

After midline confirmation under fluoroscopy of the arthroplasty attention should be made towards sealing any exposed cancellous surfaces with bone wax in

**Figure 4.**

*Ferguson view is an AP view of the cervical disc space taken with opening of collimation and caudal tilt angulation of the x-ray tube 30° to 35°.*

order to prevent heterotopic ossification. Next retractors can be repositioned at the adjacent level for placement of the fusion implant in standard fashion. If using a plate attention should be paid towards the proximity of the plate in regards to the adjacent disc space as this has be found to be the critical determinant of adjacent level heterotopic ossification [45, 46].

### **Figure 5.**

*(a) Proper midline positioning of the prosthesis can be confirmed fluoroscopically with placement of a nerve hook within the instrumentation until a center center "field goal" view is obtained. The nerve hook is clearly bisecting the flanges of the trial. Drilling in this orientation will lock in an appropriately midline positioned implant. (b) Improper midline placement: Here the nerve hook is no longer bisecting the flanges of the implant which confirms the implant is malrotated towards the right, the retractor needs to be loosened and repositioned so that the trial can be repositioned accordingly. (c) Improper midline placement: Here the nerve hook is no longer bisecting the flanges of the implant which confirms the implant is malrotated towards the left and needs to be repositioned accordingly.*

### **Figure 6.**

*(a) Prior to removal of the trial, discrete measurements can be taken to confirm the exact depth of the insertion of the drill bit. Measurements are obtained from the tip of the drill to the posterior margin of each vertebral body. (b) This depth can then be compared to the implant positioning as tapping occurs to confirm final placement of the arthroplasty. Measurements are obtained from the tip of the implant to the posterior margin of each vertebral body. (c) A final xray and measurements can be obtained to ensure the final implant has not moved after removal of the instrumentation. This will help confirm if any final tamping needs to be performed.*
