R. Wenz, S. Lee, J. Meyer, S. Marcinek, A. Mehta, T. Slater (Sunnyvale, CA, USA. Rosbach, Germany)
Balloon kyphoplasty is a minimally invasive surgical procedure for treatment of osteoporotic vertebral compression fractures. The vertebral body is accessed transpedicularly using a cannula through which an inflatable bone tamp (IBT) is inserted and inflated to reduce the fracture and create a void; this IBT is then removed and a viscous fixation material is injected to fill the void and stabilize the fracture. This material is usually either non-resorbable polymethylmethacrylate (PMMA) bone cement or resorbable calcium phosphate (CaP) bone substitute. Implantation of PMMA can result in fibrous tissue formation, but osseointegration (direct apposition of bone on the cement surface) is considered desirable. A new composite bone cement, ActivOs™ Bone Cement with Hydroxyapatite, has been formulated, combining hydroxyapatite (HA) particles with PMMA powder to produce a PMMA-HA material possessing the handling characteristics and mechanical strength of PMMA and the osteoconductive properties of HA. Product development included evaluation of handling, visibility, mechanical and biological properties.
The powder and liquid components of PMMA-HA were mixed for 30 seconds and transferred to specialized devices for dispensing (KyphX Bone Filler Devices, Kyphon, Sunnyvale, CA). Dough time (as judged by the “sticky glove test”) and working period (time from dough time until it becomes difficult to dispense) were measured. Injection into a cadaver vertebral body was viewed under live fluoroscopy to verify its visibility. Cylindrical specimens (6mm diameter x 12mm length) were molded and mechanical testing was performed according to ASTM F451 and ISO 5833. A sheep study was performed to evaluate the biological response. PMMA-HA and PMMA were implanted into 8mm cylindrical defects in the lumbar vertebrae of skeletally mature sheep. After 1, 3 and 6 months, sheep were sacrificed, the vertebrae harvested and processed into stained, histological sections. Qualitative interpretation and quantitative histomorphometry analyses were performed to assess the extent of direct bony apposition on the perimeter of the cement surface.
The cement reaches a doughy consistency in 4-6 minutes, and can be dispensed until 13-18 minutes, for a working period of up to 7-14 minutes. The cement was confirmed to be sufficiently radiopaque such that extravasation out of the vertebral body could be visualized on fluoroscopy. The average compressive strength was 84 ± 3 MPa, above the 70 MPa minimum. PMMA-HA was observed to exhibit frequent direct apposition (mean: 40.5%, range: 11-98%) compared to infrequently (mean: 8.1%, range: 2-12%) for the PMMA control, a statistically significant (p < 0.05) five-fold increase.
A composite PMMA-HA bone cement formulation has been developed for kyphoplasty. This formulation exhibits appropriate handling properties for delivery, sufficient radiopacity for fluoroscopic visualization, good compressive strength, and greater osseointegration than a PMMA control in a sheep model. This PMMA-HA cement provides the kyphoplasty surgeon with another choice of material for vertebral body augmentation.
ActivOs™ Bone Cement with Hydroxyapatite is CE-marked by Medtronic.