Vascularized Bone Grafts for Spinal Fusion-Part 1: The Iliac Crest

BACKGROUND

Iliac crest autograft has been the gold standard for harvest of fusion materials in spine surgery. The benefits of a vascularized version of this bone graft-including delivery of stem cells, ability to deliver antibiotics to the fusion bed, and relative ease of harvest-make this technique superior to free bone transfer in the achievement of augmented spinal fusion.

OBJECTIVE

To present a brief summary of similar existing concepts before describing the novel technique of this vascularized posterior iliac crest bone graft.

METHODS

Vascularized posterior iliac crest bone graft can be harvested from the same midline lumbar incision used for thoracolumbar spinal fusion, through lateral dissection around the paraspinals to the iliac crest. Recipient sites in the posterolateral bony spinal gutters may be as rostral as T12 and caudal as the sacrum. The ability to cover multiple lumbar levels can be achieved with desired lengths of the donor iliac crest.

RESULTS

Over 14 vascularized iliac crest bone grafts have been performed to augment lumbar fusion for salvage after pseudoarthrosis. Operative time and bleeding are reduced compared to free flap procedures, and no patients have experienced any complications related to these grafts. Indocyanine green (ICG) angiography has been utilized in a novel way to ensure the vascularity of the bone graft prior to arthrodesis.

CONCLUSION

While long-term follow-up will be required to fully characterize fusion rates and patient morbidity, this innovative surgical option augments spinal fusion in patients with, or at increased risk for, pseudoarthrosis.

Failure in Lumbar Spinal Fusion and Current Management Modalities

Lumbar spinal fusion is a commonly performed procedure to stabilize the spine, and the frequency with which this operation is performed is increasing. Multiple factors are involved in achieving successful arthrodesis. Systemic factors include patient medical comorbidities-such as rheumatoid arthritis and osteoporosis-and smoking status. Surgical site factors include choice of bone graft material, number of fusion levels, location of fusion bed, adequate preparation of fusion site, and biomechanical properties of the fusion construct. Rates of successful fusion can vary from 65 to 100%, depending on the aforementioned factors. Diagnosis of pseudoarthrosis is confirmed by imaging studies, often a combination of static and dynamic radiographs and computed tomography. Once pseudoarthrosis is identified, patient factors should be optimized whenever possible and a surgical plan implemented to provide the best chance of successful revision arthrodesis with the least amount of surgical risk.

Cytosolic degradation of T-cell receptor alpha chains by the proteasome

The T-cell antigen receptor (TCR) is an hetero-oligomeric membrane complex composed of at least seven transmembrane polypeptide chains that has served as a model for the assembly and degradation of integral membrane proteins in the endoplasmic reticulum (ER). Unassembled TCRalpha chains fail to mature to the Golgi apparatus and are rapidly degraded by a non-lysosomal "ER degradation" pathway that has been proposed to be autonomous to the ER. In these studies we show that the degradation of core-glycosylated TCRalpha is blocked by N-acetyl-L-leucyl-L-leucyl-L-norleucinal (ALLN) and lactacystin, implicating the proteasome in ER degradation. Either acute or chronic treatment of TCRalpha-transfected cells with proteasome inhibitors cause the core-glycosylated TCRalpha chains to progressively shift to an approximately 28-kDa form that lacks N-linked oligosaccharides and the N-terminal signal peptide. The susceptibility of this 28-kDa species to extravesicular protease indicates that it is not protected by the ER membrane and, hence, cytoplasmic. These data suggest a model in which TCRalpha chains that are translocated across the membrane, core-glycosylated, but fail to assemble are dislocated back to the cytoplasm for degradation by cytoplasmic proteasomes. Our data also suggest that covalent modification of TCRalpha with ubiquitin is not required for its degradation.