Osteoprogenitor Cells from Bone Marrow to Augment Spinal Fusions

Traditionally, to achieve a spinal fusion, bone harvested from the iliac crest (pelvic bone) is transplanted into the fusion site. This bone is considered the gold standard. The autograft (patient's bone) consists of mineralized tissue, which provides an osteoconductive (forms a scaffold for new bone) and osteoinductive (induces formation of new bone) matrix as well as a bone marrow component replete with osteogenic (bone formation) precursor cells.

In the USA approximately 200,000 autologous bone grafts are harvested from the iliac crest for lumbar spine fusion procedures every year. Even with this gold standard creating a structurally stable and solid fusion remains a persistent orthopaedic challenge with fusion rates varying from 60% to over 90%.

Additionally, the harvesting of bone graft from patients may result in significant donor site morbidity, with approximately 30% of patients reporting chronic hip pain following surgery. These donor site complications can slow or delay patient mobilization and rehabilitation, which may increase the length of hospitalization and associated health care costs. For this reason, there continues to be an impetus to develop and validate alternative graft materials, capable of replicating the performance of iliac crest autograft, while reducing or eliminating the attendant complications.

The effectiveness of any bone graft depends on its inherent characteristics and cellular constituents, particularly the subset of stem cells that are capable of growing new bone. It has long been recognized that bone marrow has inherent osteogenic (bone growth) potential due to the presence of osteoprogenitor cells (a type of cell that helps make new bone). If delivered in an appropriate matrix, these cells have been shown to contribute substantially to bone formation at the grafted site. To date, many studies have been undertaken to evaluate methods by which to concentrate specific cells, such as centrifugation and culture-expansion, but the results have been inconsistent and highly variable.

A new method of osteoprogenitor cell selection and concentration from a bone marrow aspirate has recently been developed and clinically tested at the Cleveland Clinic. This novel approach allows the "selective retention" of the important bone forming cells, which can then be combined with banked bone to replicate the components of autogenous bone and be used to generate a spinal fusion. By virtue of its percutaneous (through the skin) minimally invasive approach this technique eliminates the morbidity associated with open bone graft harvesting.

The osteoprogenitor cells are harvested by first aspirating bone marrow from the iliac crest with a standard bone biopsy needle. The bone marrow is then subsequently combined with a specific bone matrix using a patented technology. The processing method builds upon the natural "attachment" tendency of bone-forming cells. Relying on the principle of an affinity column (chemistry, a binding force), the technology facilitates the retention of osteoprogenitor cells within the matrix, while discouraging retention of other non-bone forming cells.

In a pilot study just recently completed, 24 patients undergoing a spinal fusion with the use of osteoprogenitor cells reported no significant complications as a result of the bone marrow aspiration. In these patients the early results show that all progressed very rapidly towards a solid spinal fusion as determined by serial x-rays and high resolution CT scans. This group of patients will be continuously monitored for a minimum of two years to fully evaluate their clinical outcome. The initial impression has been as good if not better clinical outcomes than with traditional bone graft harvesting.

Many new methods and products are available to augment or stimulate bone to form a fusion. The majority of these products are triggers of osteogenic (bone forming) activity. Only the osteoprogenitor cells themselves are capable of truly forming bone on their own. Thus it is intuitive that to develop a predictable method of generating a fusion one would have to provide the machinery to form bone, in this case the osteoprogenitor cells, as opposed to the triggers of osteogenic activity, which would be ineffective in the absence of the osteoprogenitor cells. This technique of harvesting and concentrating osteoprogenitor cells provides the machinery needed to predictably form bone directly at the fusion site. It obviates the need for an invasive bone graft harvest, and because it is harvested from the patient issues of biocompatibility do not exist.

This technique is also cost effective compared to many other new methods and products as the material necessary to harvest the bone marrow and the matrix components are readily available. This method of osteoprogenitor cell harvesting and utilization to generate a spinal fusion ushers in a whole new era in tissue engineering, which will no doubt lead to other advances soft tissue and solid organ regeneration.