**10. Future design implications**

A number of other implant designs have been proposed albeit with little clinical implementation. The hydrogel Prosthetic Disc Nucleus (PDN) is a hydrogel core in a polyethylene shell or jacket meant to only replace the nucleus in the lumbar spine while preserving the annulus fibrosis. This technique relied on the compressed core to be inserted and absorb fluid over the first four or five days allowing it to expand and restore disc height. In the trend toward minimally invasive, there is great potential to become percutaneous. While stem cells have proven useful in osteobiologics, there is still a great need for their development in cartilage and disc replacement. Clearly, the future lies in cellular-based disc repair and reconstruction but for now, that hope is elusive.

*Minimally Invasive Spine Surgery - Advances and Innovations*
