Carol Ann Toth

3:57 Transpedicular lumbar interbody fusion by means of an osteogenic protein–1 (bone morphogenic protein–7)–enhanced hydroxyapatite cement

Abstract Purpose of study: In a sheep model, we developed a posterior approach method to treat burst fractures, providing primary stability for axial loading in a sheep model. Using this model and method we tested the efficacy of the osteogenic protein (OP)-1–enhanced hydroxyapatite cement to form an interbody fusion. Methods used: In 36 sheep, L4–L6 were instrumented posteriorly, intervertebral disc L4–L5 was removed under transpedicular endoscopic control by means of a bilateral transpedicular approach and end plates L4–L5 were decorticated. In 12 sheep, the empty disc space was filled transpedicularly with an OP-1–enhanced hydroxyapatite cement. Another 12 sheep were treated with the hydroxyapatite cement without OP-1. The remaining 12 animals were treated with autograft. Animals were euthanized, and specimens were evaluated by means of radiological (X-ray and computed tomography), histological and histomorphometric analysis with fluorochrome labeling. of findings: Radiologically, in 10 of 12 cases of the OP-1–enhanced hydroxyapatite cement–treated group, a solid block of OP-1–enhanced cement was seen reaching from end plate to end plate with one consistent horizontally running crack line in the area of the former disc space. Histologically, the OP-1–enhanced cement was fully integrated at the site of the bone interface and entirely sheathed by newly formed callus. No signs of fragmentation or gross resorption of the OP-1–enhanced cement were seen in these animals. No general or local signs of inflammatory reactions were noted. For the group treated with the hydroxyapatite cement without OP-1, bony fusion of the motion segment was seen in only 2 of 12 animals. In the fused cases, there was a solid block of cement reaching from end plate to end plate and showing one horizontal fracture line. The bone–cement integration was seen histologically at the site of the interface. The remaining 10 sheep showed heavy fragmentation of the cement, with gross resorption and without signs of osseous integration. Four of these animals showed severe signs of aseptic inflammatory reactions at the implantation site. In the autograft-treated group, only 10 of 12 sheep could be evaluated. In 6 of the 10 remaining sheep, a contact between the autograft and the upper end plate was seen radiologically. However, only 1 of 10 featured solid unilateral interbody fusion both radiologically and histologically. No general or local signs of inflammatory reactions were seen in these animals. Relationship between findings and existing knowledge: Autograft in this model led only to a 1 of 10 interbody fusion rate. Hydroxyapatite cement without OP-1 (interbody fusion rate 2 of 12) does not seem to be superior to autograft, despite enabling primary stability for axial loading of the motion segment under investigation. Shear forces caused early fracture of the cement and subsequent fragmentation along with gross resorption initiating severe inflammatory reactions in 4 of 12 cases. The OP-1–enhanced hydroxyapatite cement cannot prevent the early horizontal fracture in the cement mass resulting from shear forces. However, the osteoinductive effects of OP-1–enhanced hydroxyapatite cement accelerated the process of biointegration, resulting in full osseous integration and callus sheathing in 10 of 12 cases. Overall significance of findings: If compared with both control groups, transpedicular lumbar interbody fusion was successful in 10 of 12 cases at 8 weeks after surgery when the OP-1–enhanced hydroxyapatite cement was implanted (p=.0016). The morbidity of harvesting autograft, as well as the additional anterior surgical approach, were avoided. Disclosures: Device or drug: osteogenic protein–1 (OP-1). Status: investigational. Conflict of interest: Thomas Blattert, grant research support.