Spine Fusion Begins with the Endplate in MindA new fusion treatment for chronic low back pain uses titanium cages inserted between the vertebrae. The disc material is taken out, and the cage is put into the same space. The cage gives the spine a strong support while fusion occurs. As with any new treatment, different cage designs are being tested.
The success of the fusion depends on the bone graft. Bits and pieces of bone are placed inside the cage. New bone growth around the area requires bleeding bone next to the graft. Usually the endplate is removed to get to the bleeding bone. The endplate is a flat piece of cartilage between the disc and the vertebral bone. The endplate keeps the disc tissue from pushing into the vertebral bone. It also keeps the bone away from the bone-filled cage. How important is the endplate? If it's removed, will it put the cage at risk for failure?
This is the subject of a study by scientists in Switzerland. They inserted cages between the vertebrae of cadavers (spines saved after death for study). They measured the strength of the endplate and changes in the load on the endplate with cage insertion.
They used two different models of endplates. One was "soft" and equal to having part of the middle of the endplate taken out. The second model was a "hard" endplate with the center of the endplate equal in strength to the outer ring. The soft endplate mimics conditions when the endplate is removed. The hard endplate is more like the normal, live human endplate.
They found less stress on the bone when harder endplates are used. Increasing the stiffness of the endplate shifts the load to the center of the vertebral bone. Overall stress on the bone is less with a stiffer endplate.
The authors also report that cage insertion affects the bone more than changing the endplates. And they found that the strength of the place where the bone and cage come together is also affected by the condition of the bone.
Two common problems leading to cage failure are endplate fracture and the cage's sinking into the bone. Based on the results of this study, these researchers suggest making a cage that doesn't put pressure on the center of the endplate or bone. Contact should be with the outer edge or ring of the endplate. This design would also allow for more contact between the graft and bleeding bone in the middle.
Finally, doctors should check to make sure the bone is strong enough to support the load. This is important with the changes in load transfer that occur when the cage is inserted and before the fusion is complete.
Anne Polikeit, et al. The Importance of the Endplate for Interbody Cages in the Lumbar Spine. In European Spine Journal. December 2003. Vol. 12. No. 6. Pp. 556-561.
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