Anterior cervical interbody constructs: effect of a repetitive compressive force on the endplate

Abstract

Graft subsidence following anterior cervical reconstruction can result in the loss of sagittal balance and recurring foraminal stenosis. This study examined the implant-endplate interface using a cyclic fatigue loading protocol in an attempt to model the subsidence seen in vivo. The superior endplate from 30 cervical vertebrae (C3 to T1) were harvested and biomechanically tested in axial compression with one of three implants: Fibular allograft; titanium mesh cage packed with cancellous chips; and trabecular metal. Each construct was cyclically loaded from 50 to 250 N for 10,000 cycles. Nondestructive cyclic loading of the cervical endplate-implant construct resulted in a stiffer construct independent of the type of the interbody implant tested. The trabecular metal construct demonstrated significantly more axial stability and significantly less subsidence in comparison to the titanium mesh construct. Although the allograft construct resulted in more subsidence than the trabecular metal construct, the difference was not significant and no difference was found when comparing axial stability. For all constructs, the majority of the subsidence during the cyclic testing occurred during the first 500 cycles and was followed by a more gradual settling in the remaining 9,500 cycles. Keywords: cervical endplate; fatigue; cyclic testing; biomechanics; subsidence Anterior cervical fusion has traditionally been a predictable and reliable surgical technique for treatment of multiple spinal pathologies. [1][2][3] The success has been due to factors that are addressed by the surgical procedure: the removal of the pathologic process; the decompression of the neural elements; the correction of sagittal balance; and the stabilization of the motion segment. Maintenance of sagittal balance and continued stabilization of the motion segment following anterior reconstruction is essential in obtaining a successful clinical outcome. Subsidence of an interbody graft or construct prior to bony fusion can be detrimental to sagittal balance and/or stabilization of the motion segment resulting in recurrent foraminal stenosis.Options available for reconstructing the anterior cervical column include a structural autograft, a structural allograft, a carbon fiber cage, a PEEK cage, a titanium mesh cage, or a trabecular metal spacer. Material properties and interface geometries vary, but the devices function similarly by providing mechanical integrity and stability to the construct while bony fusion takes place. Independent of the device, settling occurs following anterior interbody fusion, 4-6 the amount of which is affected by intraoperative endplate injury or removal, osteoporosis, and the cross-sectional area of the implant. 7,8 Obtaining contact between the endplate and implant surfaces is technically challenging, and a certain amount of settling occurs early post-operatively. With bony implants, additional settling occurs as the graft is resorbed prior to bone fusion. The amount of settling is often <1 mm. Other causes of post-op...

Comparison of anterior cervical fusion by titanium mesh cage versus nano-hydroxyapatite/polyamide cage following single-level corpectomy

“…In the n-HA/PA66 group, the final rate of severe cage subsidence was 6 %, which was significantly lower than the TMC group. The risk factors related to the development of subsidence of the cages are varied, including not only intraoperative end-plate preparation and osteoporosis but also cage material and cage shape [14,21,22]. The titanium mesh cage has sharp footprints designed to anchor the cage into the adjacent endplates and avoid a translation; however, the sharp footprints lead to a smaller cage-endplate interface contact area, increasing the cutting and penetration from the cage into the vertebra.…”

Effects of Titanium Mesh Cage End Structures on the Compressive Load at the Endplate Interface: A Cadaveric Biomechanical Study

“…Biomechanical studies have shown that adding an end ring onto the conventional TMC could be beneficial by expanding the contact area between the endplate and the TMC, which can decrease the stress concentration and distribute the load from the cephalad more evenly across the endplate [16,21,24]. However, high rates of TMC subsidence were still observed in long-term follow-up studies of ACCF despite the enlarged contact area of the end surface [4,28,29], suggesting that a simple, flat end ring could not effectively prevent subsidence.…”

“…However, high rates of TMC subsidence were still observed in long-term follow-up studies of ACCF despite the enlarged contact area of the end surface [4,28,29], suggesting that a simple, flat end ring could not effectively prevent subsidence. When performing compression tests, previous biomechanical works assumed that the endplate was parallel to the end of the TMC and did not consider that the endplate has a gradient under physiological conditions [16,21,24]. Therefore, these studies may artificially increase the true contact area and exaggerate the effect of a flat end ring on the compressive load.…”

“…A TMC with a larger diameter is recommended due to the advantages of relying on the strong posterolateral region of the endplate, a larger TMC-endplate interface contact area, and a larger bone graft volume [17,18,21,23]. In addition, adding an end ring onto the conventional TMC is considered effective for increasing the compressive load because it alleviates the stress concentration of the interface by evenly distributing the stress [16,21,24]. …”

“…Following ACCF, a flat-ended TMC only contacts the superior endplate in the posterior region, which leads to a TMC-endplate interface with a very small contact area [4,5,9]. When the endplate is placed flat for testing [16,21,24], the contact area of the interface is artificially expanded, which might result in an exaggerated maximum load. Thus, it is imperative to place the endplate at a physiological gradient during compression testing to evaluate the true mechanical behavior of the TMC-endplate interface.…”