Brian Saunders and Tony Freemont at the University of Manchester, UK, discuss a new approach for treating back pain using injectable microgel implants.  

Keeping our backs, and especially the intervertebral discs, healthy is essential for our well-being. With at least 40% of lower back pain being caused by intervertebral disc degeneration, the annual costs of this condition to health care and lost productivity to economies are enormous. Costs to the US alone were estimated at over $40 billion in 2004.

The intervertebral disc must be flexible and capable of absorbing and distributing loads that far exceed those of the body's weight. The load-absorbing part of the intervertebral disc is the nucleus pulposus, which consists of a natural hydrogel with a high water content. Unfortunately the water content of the nucleus pulposus decreases with age and when affected by some diseases, reducing its ability to distribute load. Degeneration also results in the formation of interconnected voids in the disc, which causes a decrease in height. This adversely alters the biomechanics in the spine, and the altered biomechanical load distribution accelerates void formation. 

Current therapies for treating the degeneration of intervertebral discs include spinal fusion or disc replacement. Both approaches involve complex operations taking considerable surgical time, and resulting in the patient being in hospital for days and off work for months. They also treat the symptoms and not the cause. A minimally invasive method for restoring disc height and biomechanical load distributions is urgently needed. One such approach being developed involves injecting dispersions of pH-responsive microgel particles into the spine. These particles are like nanometre-sized polymer sponges which swell when the pH approaches values present in the nucleus pulposus. This changes the dispersion from a fluid into a stiff, load-bearing gel. The pH triggered fluid-to-gel transition is essential for developing a minimally-invasive approach to delivering load-bearing implants. Injectable fluids have the added advantage of filling irregularly shaped voids. Additionally the particles are pre-prepared outside the body, meaning the need to perform chemical reactions in the body is avoided. The gels also have mechanical properties that can be tuned.

To assess the potential of this approach, models of degenerated intervertebral discs containing the gels have been studied. The treated discs were placed within a compression testing rig to test their mechanical properties, and exposed to loads similar to those experienced by human intervertebral discs during exercise. These tests showed it is possible to restore the mechanical properties of degenerated intervertebral discs to normal values using the responsive microgel. Another hopeful sign for this technique are preliminary experiments that have shown good biocompatibility of the microgel with intervertebral discs cells. 

A future aim for this responsive microgel approach is to mix the dispersions with biological species that encourages the creation of a biomechanical environment suitable for the regeneration of disc tissue within the nucleus pulposus. 
There are some important challenges that researchers need to overcome in order to develop this technology into a new treatment, including establishing an interparticle bonding approach capable of preventing migration of the particles within the nucleus pulposus under load. Also, the particles may need to be engineered to biodegrade at controllable rates. 

This new approach for treating back pain has considerable potential for providing an injectable implant targeted at degeneration of the intervertebral discs. Importantly, the approach does not exclude other approaches, such as spinal fusion, should revision be necessary. The versatility in particle design of microgels will assist this process greatly and could enable the future application of this technique to other soft tissue types within the body.

Read Tony Freemont and Brian Saunders' feature article 'pH-Responsive microgel dispersions for repairing damaged load-bearing soft tissue' in issue 5, 2008 of Soft Matter.