Effect of implant design and endplate preparation on the compressive strength of interbody fusion constructs

Resorbable cages for spinal fusion: an experimental goat model

A biomechanical cadaver study and an in vivo monosegmental spinal fusion study were performed to evaluate a novel bioresorbable poly-L-lactic acid (PLLA) cage. The yield strength of a spinal segment was chosen as the main design parameter for the resorbable cages to be used in a goat model. A 3-year in vivo study revealed a significantly faster and more complete fusion using PLLA cages as compared to titanium cages with the same dimensions. In the PLLA group, the intervertebral grafting height did not change and bone remodeling within the cage was completed 2 years after implantation. In terms of degradation of the PLLA, similar features were observed in vivo and in vitro. Degradation was almost completed 3 years after implantation. Tissue reaction was mild during the 3-year period.

Load transfer mechanisms in cylindrical interbody cage constructs

STUDY DESIGN

A finite-element model was created to study parametrically the load transfer characteristics of a lower lumbar motion segment implanted with dual anteroposterior cylindrical interbody cages.

OBJECTIVES

To describe the frontal plane bone-implant interface stresses acting on a generic cylindrical interbody cage, to evaluate the effect of implant material properties on these stresses, and to determine the associated load transfer mechanisms.

SUMMARY OF BACKGROUND DATA

From a biomechanical perspective, the long-term success of an interbody cage fusion depends on effective load transfer. The cage must stress the graft sufficiently to promote fusion, while keeping bone-implant interface stresses in a range that will prevent implant subsidence or loosening. At this writing, no published study has described interface stresses or load transfer mechanisms for these devices.

METHODS

A planar finite-element model was used to simulate uniform compression loading of the implanted segment. Material properties of the interbody cage were varied to simulate cortical bone, titanium, and stainless steel implants. Normal and shear interface stresses were output along the length of the interface.

RESULTS

Magnitudes of both the normal and shear interface stresses were substantially higher at the medial and lateral sides of the interface than in the center. Interface stresses were largely independent of implant material.

CONCLUSIONS

Cylindrical interbody implants have inherent limitations, including stress concentrations at the bone-implant interface and possible stress shielding of the graft. The results from the current study suggest that implants made of cortical bone have substantially the same load transfer characteristics as metal devices of similar geometry.

The effects of bone density and disc degeneration on the structural property distributions in the lower lumbar vertebral endplates

In this study, we hypothesized that vertebral bone density and disc degeneration would affect the structural property distributions of the lower lumbar vertebral endplates (L3-L5). The results may have implications for improving interbody implant designs to better resist subsidence. A 3 mm diameter hemispherical indenter was used to perform indentation tests at 0.2 mm/s to a depth of 3 mm at 27 standardized locations in 55 bony endplates of intact human lumbar vertebrae (L3-L5). The resulting load-displacement curves were used to extract the failure load and stiffness of each test site. Bone density was measured using lateral DEXA scans. Disc condition was determined using a four-point grading scale. Three-way analyses of variance were used to analyze the relationships between the data. The overall failure load decreased with bone mineral density (BMD) in the superior (p < 0.0001) and inferior (p = 0.011) lumbar endplates. In both endplates, the posterolateral regions were significantly stronger than more central regions. With increasing BMD, this difference became more pronounced in the superior endplates only (p = 0.005). Increased disc degeneration was associated with an overall failure load decrease in the inferior lumbar endplates (p = 0.002). The strength in the central regions of the superior endplates was reduced with increasing degeneration, but this was not observed peripherally (p = 0.001). Stiffness magnitude or distribution was not significantly affected by BMD or disc degeneration. The locations of the strongest regions of the endplate did not change with either bone density or disc degeneration. This implies that implant shapes designed using the basic structural property maps for the L3-L5 endplates are appropriate for use in patients with a wide range of pathologies, even though overall failure loads are generally lower in patients with reduced bone density and greater degrees of disc degeneration.

Bone graft incorporation in radiographically successful human intervertebral body fusion cages

STUDY DESIGN

Biopsies were obtained from within radiographically successful human intervertebral body fusion cages to document the histology of remodeling bone graft.

OBJECTIVES

The purpose of this study is to describe the tissue within successful human interbody cages with special reference to the viability of bone and the presence or absence of debris particles.

SUMMARY OF BACKGROUND DATA

The use of interbody fusion cages is gaining rapid acceptance, but there is little histologic documentation of the nature of tissue within successful human interbody fusion cages.

METHODS

Needle biopsies were obtained of tissue within radiographically successful intervertebral body fusion cages at the time of pedicle screw removal for back pain or fusion of adjacent spinal level in nine spinal levels of eight patients. Preoperative diagnoses of these eight adult patients included disease conditions in the sagittal plane: spondylosis (5), degenerative disc disease (6), failed laminectomy and discectomy (2), radiculopathy (1), and spondylolisthesis (1). In all cases the cages had been packed with autograft (iliac crest 7, local 1) at the time of insertion. Cage implantation was performed with anterior (anterior lumbar interbody fusion 4, corpectomy and plate fixation 1), and posterior (posterior lumbar interbody fusion 4), segmental instrumentation (plate 1, or pedicle screws 8). All cases except one cervical case had posterolateral fusion or bilateral facet fusion. The cages were composed of carbon fiber-reinforced polymer (Brantigan cage; DePuy AcroMed, Raynham, MA, n = 5) or titanium mesh (Harms Cage; DePuy AcroMed, Raynham, MA, n = 4). Cages had been in situ from 8 to 72 months (mean 28 months). All nine biopsies from eight patients were obtained from within the center of the cages. Specimens were decalcified, routinely embedded in paraffin, stained with hematoxylin and eosin, and viewed qualitatively with transmitted and polarized light.

RESULTS

All needle biopsies were obtained from within the center of the cages, and no patient developed spinal instability after the biopsy. All nine biopsies showed small fragments of necrotic bone associated with viable bone and restoration of hematopoietic bone marrow. Numerous cement lines demarcated the edges of previous cycles of remodeling. The ratio of necrotic to viable bone varied greatly among cases. Small particles of debris were associated with four of the five carbon-fiber cages and one of the four specimens from titanium cages, but there was no visible bone resorption or inflammation.

CONCLUSIONS

Autogenous bone graft was incorporated in these radiographically successful human intervertebral body fusion cages. A few debris particles were observed, but there was no histologic evidence of particle-induced bone resorption or inflammation.

Radiographic spinal profile changes induced by cage design after posterior lumbar interbody fusion preliminary report of a study with wedged implants

STUDY DESIGN

A prospective comparative radiographic study between two geometrically varying implants utilized in single-level posterior lumbar interbody fusion (PLIF) was performed.

OBJECTIVE

The aim of this study was to determine the influence of 4 degrees wedged cages on postoperative lateral lumbar spinal profile.

SUMMARY OF BACKGROUND DATA

The biomechanical and clinical importance of realigning the sagittal lumbar profile in surgical management of spinal instabilities is known. Wedged cages are therefore increasingly attaining popularity in PLIF. As yet the significance of wedged implants on postoperative sagittal spinal profile has not been assessed.

METHODS

Forty patients were randomly assigned to two equal-sized groups. In one group rectangular cages and in the second group cages with a wedged design and an inclination of 4 degrees were implanted. Quantitative assessment of the lumbar spinal profile on standing neutral lateral radiographs was performed before surgery as well as 6 weeks and 12 months after surgery. Results were statistically compared.

RESULTS

A significant improvement of lumbar sagittal profile after use of 4 degrees wedged compared with nonwedged cages was not found. The greatest effect on lumbar profile and segmental lordosis was observed in fusion of segment L4-L5 with 4 degrees wedged cages.

CONCLUSIONS

These results show that normal sagittal alignment after single-level lumbar fusion can be achieved with rectangular and 4 degrees wedged cages. Although results after utilization of 4 degrees wedged cages do not significantly differ, these implants offer the surgeon one more sizing variation with which physiologic lumbar lordosis may be attained. The biomechanical implications of wedged implants on the rigidity of a fused segment remain to be analyzed.

Segmental stability and compressive strength of posterior lumbar interbody fusion implants

STUDY DESIGN

Human cadaveric study on initial segmental stability and compressive strength of posterior lumbar interbody fusion implants.

OBJECTIVES

To compare the initial segmental stability and compressive strength of a posterior lumbar interbody fusion construct using a new cortical bone spacer machined from allograft to that of titanium threaded and nonthreaded posterior lumbar interbody fusion cages, tested as stand-alone and with supplemental pedicle screw fixation.

SUMMARY OF BACKGROUND DATA

Cages were introduced to overcome the limitations of conventional allografts. Radiodense cage materials impede radiographic assessment of the fusion, however, and may cause stress shielding of the graft.

METHODS

Multisegmental specimens were tested intact, with posterior lumbar interbody fusion implants inserted into the L4/L5 interbody space and with supplemental pedicle screw fixation. Three posterior lumbar interbody fusion implant constructs (Ray Threaded Fusion Cage, Contact Fusion Cage, and PLIF Allograft Spacer) were tested nondestructively in axial rotation, flexion-extension, and lateral bending. The implant-specimen constructs then were isolated and compressed to failure. Changes in the neutral zone, range of motion, yield strength, and ultimate compressive strength were analyzed.

RESULTS

None of the stand-alone implant constructs reduced the neutral zone. Supplemental pedicle screw fixation decreased the neutral zone in flexion-extension and lateral bending. Stand-alone implant constructs decreased the range of motion in flexion and lateral bending. Differences in the range of motion between stand-alone cage constructs were found in flexion and extension (marginally significant). Supplemental posterior fixation further decreased the range of motion in all loading directions with no differences between implant constructs. The Contact Fusion Cage and PLIF Allograft Spacer constructs had a higher ultimate compressive strength than the Ray Threaded Fusion Cage.

CONCLUSIONS

The biomechanical data did not suggest any implant construct to behave superiorly either as a stand-alone or with supplemental posterior fixation. The PLIF Allograph Spacer is biomechanically equivalent to titanium cages but is devoid of the deficiencies associated with other cage technologies. Therefore, the PLIF Allograft Spacer is a valid alternative to conventional cages.