A Dynamic Interbody Cage Improves Bone Formation in Anterior Cervical Surgery: A Porcine Biomechanical Study

Abstract

Background: Anterior cervical discectomy and fusion (ACDF) with a rigid interbody spacer is commonly used in the treatment of cervical degenerative disc disease. Although ACDF relieves clinical symptoms, it is associated with several complications such as pseudoarthrosis and adjacent segment degeneration. The concept of dynamic fusion has been proposed to enhance fusion and reduce implant subsidence rate and post-fusion stiffness; this pilot preclinical animal study was conducted to begin to compare rigid and dynamic fusion in ACDF.

Questions/purposes: Using a pig model, we asked, is there (1) decreased subsidence, (2) reduced axial stiffness in compression, and (3) improved likelihood of bone growth with a dynamic interbody cage compared with a rigid interbody cage in ACDF?

Methods: ACDF was performed at two levels, C3/4 and C5/6, in 10 pigs weighing 48 to 55 kg at the age of 14 to 18 months (the pigs were skeletally mature). One level was implanted with a conventional rigid interbody cage, and the other level was implanted with a dynamic interbody cage. The conventional rigid interbody cage was implanted in the upper level in the first five pigs and in the lower level in the next five pigs. Both types of interbody cages were implanted with artificial hydroxyapatite and tricalcium phosphate bone grafts. To assess subsidence, we took radiographs at 0, 7, and 14 weeks postoperatively. Subsidence less than 10% of the disc height was considered as no radiologic abnormality. The animals were euthanized at 14 weeks, and each operated-on motion segment was harvested. Five specimens from each group were biomechanically tested under axial compression loading to determine stiffness. The other five specimens from each group were used for microCT evaluation of bone ingrowth and ongrowth and histologic investigation of bone formation. Sample size was determined based on 80% power and an α of 0.05 to detect a between-group difference of successful bone formation of 15%.

Results: With the numbers available, there was no difference in subsidence between the two groups. Seven of 10 operated-on levels with rigid cages had subsidence on a follow-up radiograph at 14 weeks, and subsidence occurred in two of 10 operated-on levels with dynamic cages (Fisher exact test; p = 0.07). The stiffness of the unimplanted rigid interbody cages was higher than the unimplanted dynamic interbody cages. After harvesting, the median (range) stiffness of the motion segments fused with dynamic interbody cages (531 N/mm [372 to 802]) was less than that of motion segments fused with rigid interbody cages (1042 N/mm [905 to 1249]; p = 0.002). Via microCT, we observed bone trabecular formation in both groups. The median (range) proportions of specimens showing bone ongrowth (88% [85% to 92%]) and bone volume fraction (87% [72% to 100%]) were higher in the dynamic interbody cage group than bone ongrowth (79% [71% to 81%]; p < 0.001) and bone volume fraction (66% [51% to 78%]; p < 0.001) in the rigid interbody cage group. The percentage of the cage with bone ingrowth was higher in the dynamic interbody cage group (74% [64% to 90%]) than in the rigid interbody cage group (56% [32% to 63%]; p < 0.001), and the residual bone graft percentage was lower (6% [5% to 8%] versus 13% [10% to 20%]; p < 0.001). In the dynamic interbody cage group, more bone formation was qualitatively observed inside the cages than in the rigid interbody cage group, with a smaller area of fibrotic tissue under histologic investigation.

Conclusion: The dynamic interbody cage provided satisfactory stabilization and percentage of bone ongrowth in this in vivo model of ACDF in pigs, with lower stiffness after bone ongrowth and no difference in subsidence.

Clinical relevance: The dynamic interbody cage appears to be worthy of further investigation. An animal study with larger numbers, with longer observation time, with multilevel surgery, and perhaps in the lumbar spine should be considered.

Conflict of interest statement

All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research® editors and board members are on file with the publication and can be viewed on request.

Copyright © 2021 by the Association of Bone and Joint Surgeons.

Figures

Fig. 1.

Rigid cages have a common bulk design, and dynamic cages have a characteristic “Z”-shaped configuration in the lateral view, permitting micromovement under cervical physiologic loads.

Fig. 2.

Flow diagram of the animal experiment.

Fig. 3.

Intraoperative fluoroscopy confirmed that the implanted cages are centrally positioned in the disc space.

Fig. 4.

A-B (A) An illustration and (B) image demonstrate how an axial load via compression (up to 500 N) is applied to the sample, a fused motion segment with cages in its disc space. Moreover, according to ASTM 2077, three rigid interbody cages and three dynamic interbody cages were tested to obtain stiffness values for comparison.

Fig. 5.

A microCT image showing the measurements used for quantitative bone analyses. BV: bone volume; TV: total volume of interest; GV: bone graft volume; a: total cage bone growth length (mm/cut); b and c: local bone growth length (mm/cut). (1) Bone ongrowth was defined as the ratio of local bone growth length on the upper and lower endplate of the vertebrae and total cage bone graft length at five randomly selected sagittal and coronal cut planes ([b + c] / 2a × 100%); (2) bone volume fraction of the boundary trabecular bone was defined as the ratio of the boundary bone volume (BVB) and the total volume of interest (TV) with dimensions of 3 × 3 mm2 ([BVB / TV] × 100%); (3) bone ingrowth in the cage was defined as the proportion of the ingrowth bone volume (BVI) and the total volume of interest (TVI) within the cage ([BVI / TVI] × 100%); and (4) the amount of residual bone graft (GV), measured by software on the microCT, was used to determine the ratio of GV to total volume of interest (TVI) within the cage ([GV / TVI] × 100%).

Fig. 6.

A-B Unimplanted rigid interbody cages and dynamic interbody cages were tested first. (A) The median (range) stiffness of the rigid interbody cages was up to 19,900 N/mm (14,713 to 24,698), and the dynamic interbody cages were less stiff, at 337 N/mm (201 to 504; p < 0.001 ). Even after fusion, stiffness of motion segments fused with dynamic interbody cages (531 N/mm [372 to 802]) was less than that with rigid interbody cages (1042 N/mm [905 to 1249]; p = 0.002). (B) Regarding the displacement amount under the same load, compared with fused motion segments with rigid cages, the characteristics of fused motion segments with dynamic cages were more similar to those of unimplanted dynamic cages. The plot of load versus displacement for the dynamic cages was less linear, especially at larger displacement, which was caused by the limited space inside the shape structure of dynamic cages.

Fig. 7.

A-B Bone formation could be observed under microCT in both groups. (A) In the fused rigid cage group, trabecular bone formation could be observed within the cage. In spite of metallic artifact noted around the titanium alloy cage, there was a dark halo space (blue arrow) without any bone growth. (B) In the fused dynamic cage group, there was no dark halo area (white arrow) noted around the titanium alloy cage, and trabecular bone formation could be observed clearly.

Fig. 8.

Regarding the boundary area, the median (range) bone ongrowth (88% [85% to 92%]) and bone volume fraction (87% [72% to 100%]) were higher in the dynamic interbody cage group than the bone ongrowth (79% [71% to 81%]; p

Fig. 9.

A-C A histologic examination qualitatively shows many areas of apparent bone ingrowth in the porous bone graft with localized fibrous tissue. (A) Compared with the rigid interbody cage group, more bone formation (dark brown) was observed inside the (B) dynamic interbody cage group with a smaller area of fibrotic tissue (pink). (C) In addition, bone formation by direct ossification could be observed between the grooves at the lateral aspect of the dynamic interbody cage.