Cushioning the blow of annulus fibrosus defect
Damage of the intervertebral discs cushioning the spine is a leading cause of lower back pain, but to date there is no effective treatment. Catherine Le Visage and her team from the University of Nantes in France want to change that. They have developed an implant that prompts tissue to regenerate in a highly specific structure, mimicking the native tissue.
“Intervertebral discs have a major role in spine mobility,” explains Le Visage, “but they get damaged over time, be it from aging or from trauma.” The discs owe their cushioning effect to their soft jelly interior called the nucleus pulposus, and tough exterior capsule called the annulus fibrosus (AF).
The AF is a highly structured multilayer tissue of aligned collagen fibres with a cross-ply structure of 60 degrees to and from the vertical axis of the spine. “It is very impressive if you think about how it’s designed to resist pressure,” says Le Visage, “for example when you’re standing or sitting the hydrogel [in the discs] is compressed and some of this pressure goes into the anulus fibrosis. it can be seen as a car tyre – when you press on it, it can slightly deform but still maintain the structure.”
Damage to the AF lets the interior gel squeeze out of the disc, causing painful herniation. Surgery to remove the bulge can help, but this doesn’t repair the AF. “The problem is that AF tissue does not heal spontaneously,” says Le Visage, “Instead, a thick tissue develops, you get new blood vessels and new nerves - all of this can create additional pain.” Current approaches to healing torn AF tissue, using sutures and even glue, have had limited success.
Regenerating the AF
The unmet need for AF repair inspired Le Visage and her regenerative medicine and skeleton research team. “We said, why don’t we try to design a new material that could be implanted into an AF defect to promote tissue regeneration,” says Le Visage. A bioscaffold to promote AF repair would need to prompt tissue to regenerate in the same criss-cross alignment, to achieve the same shock absorber effect as native tissue.
The team decided to create a biodegradable scaffold using polycaprolactone (PCL), a well known material used in many other implants. With the help of colleagues at Nanyang Technological University, Singapore, the PCL was drawn into fibres using electrospinning. This technique gives control over the fibres created. “We can prepare random fibres or aligned fibres,” says Le Visage, “we can adjust the diameter of the fibres and check that they’re aligned.”
In preliminary tests (1), AF cells were isolated from the intervertebral discs of euthanised sheep and grown on monolayer PCL scaffolds in lab dishes. After 28 days of culture, the cells had colonised the scaffold. “What was really impressive was that the cells aligned along the fibres and they were able to synthesize collagen,” says Le Visage, “and the collagen was also aligned along the fibres. So it looks like the fibres could guide the alignment of the cells and the extracellular matrix that they are synthesizing.”
THE CELLS ALIGNED ALONG THE FIBRES AND THEY WERE ABLE TO SYNTHESIZE COLLAGEN
The encouraging lab results led to an animal study in two sheep, performed with the help of veterinary colleagues at the University of Nantes. “We selected the ovine model since the sheep spine shares many similarities with the human spine,” says Le Visage. This large animal experiment was the biggest challenge in the project, says Le Visage, noting that the expertise of her veterinary colleagues in animal models and especially in imaging was crucial.
For the sheep studies, the team created a multilayer scaffold, stacking layers of fibres onto each other with the 60° cross-ply angle to mimic native AF tissue. “During surgery, we removed part of the anulus fibrosis, creating a rectangular defect, and then we implanted the polymer scaffold”, says Le Visage. After one month, the surface and interior of the implant was colonised by cells, suggesting good integration into the surrounding tissue. As in the lab studies, the cells were aligned along the fibres of the implant and the cells had even synthesized collagen in the same orientation. “It was a big surprise for us that it went so fast,” says Le Visage, “that we could obtain a tissue that was quite close to the native AF tissue. That was really exciting for us.”
Despite the excitement, the work is far from over. Although the new tissue looks like native AF tissue under the microscope, it’s not yet clear whether the tissue is “mechanically compliant”, that is whether it has the same cushioning effect as normal AF tissue. Le Visage also wants to extend the implantation time to 6 months in sheep experiments. This will check that the unexpectedly speedy tissue regeneration doesn’t outstrip biodegradation of the polymer scaffold. “The repair went pretty fast, so we might have to envision using another polymer that will degrade faster,” says Le Visage, “if the polymer doesn’t degrade fast enough, it could limit tissue repair.”
Although the implant is not ready for clinical use, the team already works with clinicians from the Nantes hospital. “We benefit from their input on the clinical needs,” says Le Visage, “they have patients where they don’t know how to treat this type of anulus fibrosis defect.” This expertise feeds into the project alongside many other scientific disciplines. “In our project, we have people with expertise and skills related to material design, chemical analysis, biological integration, and surgery,” says Le Visage, “I definitely recommend assembling a team of people with interdisciplinary expertise, so that most, if not all, aspects of the development of a novel medical device are taken into consideration.”
In vitro and in vivo evaluation of an electrospun-aligned microfibrous implant for Annulus fibrosus repair. 2019 Maude Gluais, Johann Clouet, Marion Fusellier et al. Biomaterials, 205, 81.