Resistance of the Lumbar Spine Against Axial Compression Forces after Implantation of Three Different Posterior Lumbar Interbody Cages

Summary.

Background:

The aim of using interbody fusion cages is to distract the degeneratively decreased disc height to decompress the neural structures in the intervertebral foramina and allow bony fusion. Prerequisite for a successful fusion therapy is a high resistance against subsidence and breakage.

Method:

Three types of implants, a cylindrical threaded titanium cage (Ray^TM) (1c), a bullet shaped PEEK cage (Stryker^TM) (1a) and a rectangular titanium cage with an endplate anchorage device (Marquardt^TM) (1b) were implanted in eight monosegmental lumbar spine specimens (L 2/3 and L 4/5). Each specimen underwent a cyclic loading test with 40000 cycles at a rate of 5 Hz. A cyclic axial compression force ranging from 200 Newton [N] to 1000 N was applied and the axial translation recorded simultaneously to determine the subsidence tendency. After this procedure the specimens were tested with a progressive axial force until breakage.

Findings:

There were only small differences in the subsidence tendency for the three cage designs. The height reduction due to cyclic loading ranged between 0.9 mm (Marquardt^TM), 1.2 mm (Stryker^TM) and 1.4 mm (Ray^TM). The median break force ranged from 5486 N (Marquardt^TM), 8359 N (Stryker^TM) to 8413 N (Ray^TM). No correlation between bone mineral density and failure load could be detected.

Interpretation:

Endplate preparation and cage design of the tested implants do not seem to influence the resistance of the segment against cyclic axial compression. The compression with a continuously increasing load revealed that an implant-bone failure is not to be expected in physiological limits for all three cage types.