Mechanical response of cervical vertebral endplates to axial loading

After anterior cervical discectomy the implantation of a spacer is common practice. The majority of these spacers are trapezoid titanium cages. During the development of a height-adjustable cervical implant we needed to establish the testing limits for this device. A known phenomenon is subsidence of the cage into the vertebral endplates, which leads to a decrease in height and/or angulation of the cervical spinal segment. In contrast to the thoracic and lumbar spines, there are only limited data concerning the load-bearing ability of cervical endplates. The aim of our investigation was to obtain these data.

Bone density of 16 cervical vertebrae was estimated by quantitative computed tomography. After embedding of the vertebrae into PMMA, each endplate was slowly compressed until failure using a metal indenter resembling the form of a newly developed cervical implant. A fixed protocol with increasing loading cycles was followed. Endpoint was breakage of the endplate as established by failure to resist the increasing loading forces produced by the testing machine.

The mean bone density of the 16 cervical vertebrae was 204 with a standard deviation of 52 mg Ca-HA/mL (range 130-281). The endplates failed with a mean loading of 1084 N +/- 314 (range 340-1550 N). The maximum load correlates with the bone density (R2 = 0.7347). With the 97.79 mm2 load bearing surface of the cage we calculate a mean cervical endplate break strength of 10.47 MPa and a 95 % confidence interval of 12.66-9.51 MPa. An initial settling produced by resting of the anchoring teeth in the cervical endplates was observed in 8 vertebrae at a load of 113 N (range 50-250 N).

In contrast to the thoracic and lumbar spines, cervical endplates show a lower resistance against axial forces. The data are important to understand postoperative cage subsidence and to establish testing limits for the development of new implant designs.