Session II - Spine

Observations on the Effect of Surgically Implanted Metallic Bullet Fragments on the Spinal Cord and Intervertebral Disc: An Animal Model

Nathaniel Tindel, MD, Mitchell Reiter, MD, Alexander Marcillo, MD, Richard Bunge, MD, Frank J. Eismont, MD

University of Miami, Miami, Florida, USA

Purpose: There have been many clinical studies regarding bullet injuries to the spine, but very little basic research has been done concerning the local effects of the metallic bullet fragments on the spinal cord and intervertebral discs and concerning the systemic effects regarding the cerebrospinal fluid, liver and kidneys. This study was performed in two different animal models using three types of bullet fragments in order to answer these questions.

Methods: Fragments from three types of commercially available bullets were studied: uncovered lead core bullets, the copper jacket from a lead core bullet, and the "Silvertip" jacket (an aluminum alloy). Eighteen New Zealand rabbits underwent atraumatic implantation of these metallic fragments within the spinal canal. Half were placed in the extradural space and half were placed in the intradural space. A second set of experiments was done placing these same types of bullet fragments within the disc spaces of six pure bred dogs. The animals in both groups were observed for radiographic changes and serological evidence of heavy metal toxicity for up to 12 months. At the time of sacrifice, radiographs, blood analysis, cerebrospinal fluid analysis, and liver and kidney biopsies were performed in order to assess lead and copper content. Histological analysis of the spinal cords and intervertebral discs was then performed.

Results: No animal developed paralysis or heavy metal toxicity. There were notable radiographic changes in the bone adjacent to the intradiscal fragments. Blood analysis revealed statistically significant elevation for copper but not for lead fragments when compared to controls. Cerebrospinal fluid and liver analysis showed no statistically significant differences in copper or lead levels. Extradural or intradiscal placement of metallic fragments did not affect the spinal cord histologically regardless of the type of metal. Intradural copper fragments produced tremendous histologic signs of spinal cord destruction, intradural lead fragments produced moderate destruction, and intradural aluminum produced minimal changes. The intradiscal fragments produced the same gradation of histologic changes of the local disc tissue, depending on the type of metal.

Discussion: Whether bullets lodged in the spine should be removed or not removed is a clinical decision based on numerous factors including neurologic status, level of the spine (C, T, or L), the occurrence of infection, etc. These animal models show that some fragments, especially copper fragments, can cause significant histologic damage to the spinal cord when located in the intradural space and that intraspinal and intradiscal copper fragments can also cause a significant elevation of serum copper levels. These are additional factors to be considered in taking care of patients with gunshot wounds to the spine.

Conclusions: This animal study shows that both the spinal cord and intervertebral disc in the animal model can have profoundly different reactions to metal fragments from commercially available bullets depending on both the type and location of the metal fragment. Intradural is worse than extradural or intradiscal. Copper is the most destructive, lead is intermediate, and silvertip (aluminum) causes negligible damage.