BRUSSELS, BELGIUM -- UC San Francisco researchers have shown in studies of mice that high loads on the spine alter the physical structure and biologic activity of discs -- the gel-filled cushions between bones in the spine.
"The findings should lead to improved treatments for one cause of low back pain, disc degeneration -- a condition in which discs become dehydrated and inefficient at supporting forces in the spine," according to Jeffrey Lotz, PhD, UCSF assistant professor and director of the orthopaedic bioengineering laboratory, department of orthopaedic surgery. "In addition, the research may allow us to define what is an appropriate or inappropriate load for the back to better protect workers from occupationally-related injury."
Lotz presented his findings today (June 12) at the annual meeting of the International Society for the Study of the Lumbar Spine where he received the 1998 Volvo Award For Low Back Pain Research.
The researchers conducted their study on the mouse tail, which is an extension of the mouse's spine and consists of discs analogous to those in a human back. This is the first model, said Lotz, that has allowed researchers to control precisely the forces on the discs and to examine in detail how discs respond to these forces.
"The relationship between physical activity and back pain has been an enigma. Historically, clinicians thought excessive force on the spine could rupture the discs acutely, leading to low back pain," said Lotz. "But our studies suggest the process is more complex and insidious."
The study, said Lotz, sheds light on why a majority of humans develop significant disc degeneration and may not be aware of it until it is too late. In the current study, researchers examined the biological and biomechanical consequences of excessive loading to understand the subtle relationship between forces on the back and disc degeneration.
Researchers applied three levels of force (low, medium, high) to mouse discs for a week -- levels comparable to lifting activities experienced by humans. Additional groups of mice were allowed to recuperate for four weeks after the loads were removed before examining the discs.
Study findings demonstrated that the higher the loading the greater the degree of cell death. The high loading inhibited the natural process by which cells maintain and repair the disc. Over time this diminishes the disc's ability to attract water and "re-inflate" itself during periods of rest, Lotz said.
"When discs lose the ability to attract water, they become underinflated in a way similar to a flat tire," he said. "The force which is usually supported by the pressure in the middle, is now being supported by the tissue on the outside of the disc, leading to unstable movement of the spine." Researchers found discs with high loads did not recover because of excessive cell death and a lost ability to attract water. Cells on the outside of these discs responded to the chronic underpressurization and began to produce fibrocartilage, which normally is not present in healthy discs but is a common finding in degenerated human discs, he said.
"This is the first study to show that heavy loads on the spine lead to remodeling of the disc -- that is, a change in its architecture in response to altered stresses," said Lotz.
These findings are relevant to human back pain, he said. Chemicals made by cartilage cells, now close to the edge, may irritate nerve roots, according to Lotz, and bulging of the discs may also push on nearby nerve roots. These research findings have implications for biological repair strategies to correct disc degeneration, he said. For example, UCSF researchers are injecting growth factors directly in the mouse disc in attempts to stimulate disc cells to restore the correct disc tissue architecture. Ultimately, this strategy may decrease the need for spinal surgery or fusion of back bones, traditional treatments for degenerated discs.
In addition, UCSF researchers are using this model to identify the limits of healthy and unhealthy back forces, which ultimately may allow them to develop improved occupational guidelines for protecting workers.
The Volvo Car Company of Gotenborg Sweden sponsors an annual international competition to foster research on low back pain. Each year, three papers are selected by a committee of scientists who are members of the International Society for the Study of the Lumbar Spine. The papers are in one of three areas: clinical studies; bioengineering studies; or studies in other basic science areas.
The co-investigators of the study were Olivier Colliou, BS, graduate student in the UCSF/UC Berkeley Bioengineering Graduate Group; Jennie Chin, BA, research specialist, UCSF department of orthopaedic surgery; Neil Duncan, PhD, assistant professor, department of civil engineering, University of Calgary; and Ellen Lieberg, BA, research assistant, UCSF department of orthopaedic surgery.