Cervical subtotal discectomy prosthesis validated in non-human primate model: A novel artificial cervical disc replacement concept?

Abstract

Objective: To evaluate the biological function of cervical subtotal discectomy prosthesis (CSDP) implantation in a non-human primate model. Methods: A CSDP was tested for cytocompatibility and osseointegration capacity before implantation in non-human primates. Subsequently, the CSDP was improved based on three-dimensional CT measurements of the non-human primate cervical spine. Eight cynomolgus monkeys were selected for removal of the intervertebral disc and lower endplate of the C5/6 segment to complete the model construction for CSDP implantation. In 18-month follow-up, physiological indices, radiology, and kinematics were assessed to estimate the biological function of the CSDP in non-human primates, including biosafety, osseointegration, and biomechanics. Results: Co-cultured with the CSDP constituent titanium alloy (Ti6Al4V-AO), the mouse embryo osteoblast precursor cell MC3T3-E1 obtained extended adhesion, remarkable viability status, and cell proliferation. After implantation in the mouse femur for 28 days, the surface of Ti6Al4V-AO was covered by a large amount of new cancellous bone, which formed further connections with the femur cortical bone, and no toxicity was detected by blood physiology indices or histopathology. After completing implantation in primate models, no infection or osteolysis was observed, nor was any subsidence or displacement of the CSDP observed in CT scans in the 18-month follow-up. In particular, the interior of the cervical vertebra fixation structure was gradually filled with new trabecular bone, and the CSDP had achieved fixation and bony fusion in the vertebral body at 1 year post-operation. Meanwhile, no signs of inflammation, spinal cord compression, adjacent segment degeneration, or force line changes were observed in subsequent MRI observations. Moreover, there were no pathological changes of the joint trajectory, joint motion range, stride length, or the stance phase ratio revealed in the kinematics analysis at 3, 6, 12, or 18 months after CSDP implantation. Conclusion: We successfully designed a new cervical subtotal discectomy prosthesis and constructed an excellent non-human primate implantation model for the evaluation of subtotal disc replacement arthroplasty. Furthermore, we demonstrated that CSDP had outstanding safety, osseointegration capacity, and biomechanical stability in a non-human primate model, which might be a new choice in the treatment of cervical disc diseases and potentially change future outcomes of degenerative cervical diseases.

Keywords: artificial disc; biomechanics; cervical arthroplasty; cervical artificial disc replacement; primate model; prosthesis.

FIGURE 1

CSDP design, improvement, and implantation. (A) The cervical subtotal discectomy prosthesis (CSDP) design. Four components: CDP structure, link structure, CVF structure, and locking screw. (B) Cervical spine micro-CT anatomical measurements in transverse, sagittal, and coronal planes. IAEW, intervertebral anterior edge width. IPEW, intervertebral posterior edge width. ILEW, intervertebral lateral edge width. IH, intervertebral height. VH, vertebral height. VL, vertebral length. (C) Improved CSDP for non-human primate. (D) Construction of non-human primate model of CSDP implantation.

FIGURE 2

Biocompatibility detection of CSDP in vitro. (A) LIVE/DEAD cell viability observation. The cell viability of MC3T3-E1 cells co-cultured with Ti6Al4V-AO and Ti6Al4V for 1, 3, and 7 days. (B) Cell adhesion morphology scanning electron microscopy observation. The adhesive cell morphology of MC3T3-E1 cells co-cultured with Ti6Al4V-AO and Ti6Al4V for 1, 3, and 7 days. (C) CCK-8 cell proliferation rate detection. The proliferation rate of MC3T3-E1 cells co-cultured with Ti6Al4V-AO and Ti6Al4V for 1, 3, and 7 days. (n = 4). *p < 0.05.

FIGURE 3

Evaluation of biocompatibility and osteogenesis of CSDP in mouse model. (A) Hematoxylin and eosin (HE) staining of mouse organs at 14 and 28 days after surgery. (B) Osteogenesis morphological analysis of micro-CT in mouse model. (C) HE staining of bone tissue sections around implants. (D) Mouse blood biochemical test. The complete blood data, including white blood cells (WBCs), red blood cells (RBCs), and platelets (PLTs). Blood levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) as liver function markers. Blood urea nitrogen (BUN) as kidney function marker. *p < 0.05, **p < 0.01, ***p < 0.001, n.s., no statistical difference (n = 12).

FIGURE 4

Blood biochemical evaluation of CSDP implanted in non-human primate model. The blood routine includes red blood cells (RBCs), hematocrit (HCT), white blood cells (WBCs), hemoglobin (HGB), mean corpuscular hemoglobin concentration (MCHC), platelets (PLTs), lymphocytes (lymphs), neutrophils (OTHRs), and eosinophils (EOs). Blood levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) used as liver function markers. Blood urea nitrogen (BUN) used as kidney function marker. n.s., no statistical difference (n = 8).

FIGURE 5

Radiological observation of CSDP in non-human primates. (A) CT and MRI 1 month before and 3, 12, and 18 months after CSDP implantation. (B) X-ray imaging 1 month before and 3, 12, and 18 months after CSDP implantation. (C) The trabecular bone gradually grew into the cervical vertebra fixation (CVF) structure through the tunnel at 3 months post-operation. Increased quantity and density of trabecular bone grew into the CVF structure, and the CSDP obtained intravertebral fusion at 1 year, then strengthened at 18 months post-surgery.

FIGURE 6

Kinematic analysis of limbs and joint trajectory situation in non-human primates. (A) The joint trajectory and joint motion range analysis at 1 month before surgery and 3, 12, and 18 months after surgery. (B) The kinematic analysis 3D-stick plot models pre and post-operation, describing the basic situation of the joint trajectory.

FIGURE 7

The foundation date of kinematic analysis in non-human primates. (A) Quantitative detections of stance phase ratio, stride length, and limb range of motion of elbow, hip, knee, and ankle. (B) The basic statistics of changes in joint trajectory values at 1 month before surgery and 3, 6, 12, and 18 months after surgery. (C) Visualized strip-based data of forelimb and hindlimb stance and swing phases. n.s., no statistical difference. (n = 8).