The effects of age, pathology, and fusion on cervical neural foramen area

Abstract

Cervical radiculopathy is a relatively common neurological disorder, often resulting from mechanical compression of the nerve root within the neural foramen. Anterior cervical discectomy and fusion (ACDF) is a common treatment for radicular symptoms that do not resolve after conservative treatment. One mechanism by which ACDF is believed to resolve symptoms is by replacing degenerated disc tissue with bone graft to increase the neural foramen area, however in vivo evidence demonstrating this is lacking. The aim of this study was to evaluate the effects of age, pathology, and fusion on bony neural foramen area. Participants included 30 young adult controls (<35 years old), 23 middle-aged controls (36 to 60 years old), and 36 cervical arthrodesis patients tested before and after ACDF surgery. Participants' cervical spines were imaged in the neutral, full flexion, and full extension positions while seated within a biplane radiography system. A validated model-based tracking technique determined three-dimensional vertebral position and orientation and automated software identified the neural foramen area in each head position. The neural foramen area decreased throughout the entire sub-axial cervical spine with age and pathology, however, no changes in neural foramen area were observed due solely to replacing degenerated disc tissue with bone graft. The neural foramen area was not associated with disc height in young adult controls, but moderate to strong associations were observed in middle-aged controls. The results provide evidence to inform the debate regarding localized versus systemic spinal degeneration and provide novel insight into the mechanism of pain relief after ACDF.

Keywords: ACDF; arthrodesis; arthrokinematics; cervical spine; kinematics; neural foramen.

Figure 1.. Data Workflow

(A) Participants performed static flexion/extension while (B) synchronized biplane radiographs were collected. (C) C1 to C7 CT scans were collected and (D) used to create 3D bone models. (E) 3D vertebral kinematics were determined using a validated CT model-based tracking process. (F) 6 DOF kinematics were calculated for each position.

Figure 2.. Bone-specific anatomic coordinate systems and foramen identification locations.

(A) Eight markers on the anterior, posterior, left and right edges of the superior and inferior vertebral body (red spheres) defined the medial-lateral (red arrow), superior-inferior (green arrow), and anterior-posterior (blue arrow) axes of the anatomic coordinate system for each vertebra. (B) Neural foramen markers (red spheres) were located at the (1) superior-medial edge, (2) middle superior pedicle surface, (3) superior-lateral edge, and (4) middle inferior pedicle surface on the left foramen. Similar points can be seen on the right foramen.

Figure 3.. Bony neural foramen opening identification.

(A) Neural foramen markers on the superior surface of C4 and inferior surface of C3 were used to establish a (B) best-fit plane through the neural foramen. (C) The intersection of this plane with the superior vertebra (red line) and inferior vertebra (green line) was identified, then the intersection points that were closest to the medial and lateral markers were connected (black lines). (D) Anterior view of C3 and C4 showing left and right foramen openings (blue) that were then used to calculate area.

Figure 4.. Shape changes in the neural foramen from flexion to neutral to extension in 3 young controls, 3 middle-aged controls, and 3 arthrodesis patients.

The outline of the neural foramen is shown for 3 representative subjects from each group; blue in flexion, red in neutral, and green in extension.

Figure 5.. Pre-surgery symptomatic versus asymptomatic neural foramen openings in 28 C5/C6 arthrodesis patients in the neutral head position.

Solid foramen outlines represent the asymptomatic side while dotted lines represent the symptomatic side.

Figure 6.. Bony neural foramen areas for all sub-axial cervical motion segments in young and middle-aged controls.

Blue bars correspond to flexion, red correspond to neutral, and green correspond to extension. The solid bars correspond to the middle-aged controls and the dashed bars correspond to the younger controls. Error bars represent ±1 SD.

Figure 7.. Bony neural foramen areas for all sub-axial cervical motion segments in middle-aged controls and arthrodesis patients before surgery.

Blue bars correspond to flexion, red correspond to neutral, and green correspond to extension. The solid bars correspond to the middle-aged controls and the dashed bars correspond to the arthrodesis patients. Error bars represent ±1 SD.

Figure 8.. Neural foramen areas in flexion, neutral, and extension at the superior adjacent motion segment, the arthrodesis motion segment, and the inferior adjacent motion segment before and after surgery.

Dashed bars correspond to foramen before surgery while vertically striped correspond to after surgery. Blue bars correspond to flexion, red correspond to neutral, and green correspond to extension. Error bars represent ±1 SD.

Figure 9.. Correlation between disc height and neural foramen area in neutral head position.

Red triangles represent young controls, blue circles represent middle-aged controls, and each groups’ correlation between disc height and foramen area is represented with a line matching that color.