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General Spine News

Titanium Cages Don't Share the Load Evenly

Doctors have found a new way to fuse the low back. Surgical fusion cages can be implanted between the vertebrae to be fused. Made of bone, graphite, or titanium, these cages are placed between the vertebral bones of the spine. The surgeon first removes the disc that sits between the two vertebrae and then inserts the cages. Usually, one cage goes on the left side and one on the right.

Some experts feel the cages work best when placed in pairs. However, researchers havn't known if the two cages share the loads that are placed on the spine. This study was done to help find the answer.

These researchers placed tiny sensors inside titanium cages before implanting them in the spine of nine human cadavers. (A cadaver is a human body used after death for scientific study.) Once the cages were in place, the spine segments were tested through 1500 cycles of movement.

A special testing system compared forces and motion from one side to the other and from front to back. The authors found that the load sharing wasn't even.

At first, there was a higher load on the back of the vertebra. Later, the load shifted from the back to the front of the cage in all spines. There was also a measured difference in load from side to side. It's unclear if this uneven load sharing will cause any problems in the adult patient who has a lumbar fusion using cages.

The authors also saw that when the ligament ring around the disc (called the annulus) is cut in the front, there's more motion when the spine is bent backward (extension). Since the fusion is supposed to hold the spine still, this could be a problem. The authors suggest using a brace to restrict extension when the fusion procedure using cages is done from the front (anterior approach).

Stephane Lavoie, MD, et al. Load Sharing and Kinematics of Threaded Cages for Lumbar Interbody Fusion. In Clinical Orthopaedics and Related Research. March 2003. Vol. 408. Pp. 174-179.


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