Poster Session - Implant Fixation - Hall E 47th Annual Meeting, Orthopaedic Research Society, February 25 - 28, 2001, San Francisco, California 1039

BONE INGROWTH AND RESORPTION IN A CALCIUM SULFATE/RESORBABLE POROUS CERAMIC COMPOSITE +*Walsh, W (A-Intepore-Cross International); *Chapman-Sheath, P (A-Interpore Cross International); *Rusell, J; *Cain, S; **Debes, J (E-Interpore Cross

International); *Bruce, W; *Svehla, M +*University of New South Wales, Orthopaedic Research Laboratories, Prince of Wales Hospital, Randwick, NSW 2031, Australia. Orthopaedic Research

Laboratories, Prince of Wales Hospital, Randwick, NSW 2031, Australia, 61 2 9382 2657, Fax: 61 2 9382 2660, W.Walsh@unsw.edu.au INTRODUCTION: Calcium sulfate (CS) bone graft substitutes have used for over 100 years. The limitations of CS include rapid resorption and lack of a three dimensional porous structure for bone infiltration. To overcome this, Nadkarni et al [1] added particulate hydroxyapatite (HA) to form a composite with the CS which increased bone formation in-vivo. However, HA resorbs extremely slowly and may be considered a permanent implant. The use of a Resorbable Porous Ceramic (RPC) in lieu of HA in CS composites may offer advantages of increased bone formation into a porous matrix while allowing ultimately for complete resorption of the implant and replacement with bone. METHODS AND MATERIALS: Bilateral defects (5 mm wide x 15 mm long) were created 3 mm below the joint line in the proximal tibia in 15 skeletally mature New Zealand white rabbits following ethical approval. Marker pins were placed at the proximal and distal defect margins to identify the central portion. The defects were filled with a calcium sulfate (CS) slurry (BonePlast, Interpore-Cross International, Irvine, CA) or in combination with a resorbable porous ceramic (RPC) (Pro Osteon 200R, Interpore-Cross International, Irvine, CA) (group B). CS in powder form was mixed in a sterile fashion at the time of surgery and allowed to partially set prior to implantation. The CS and 200 R implant were mixed prior to implantation. Empty defects (negative control) or defects filled with morcellised bone autograft from the defect sites (positive control) were also performed. The tibiae were harvested at 3, 6 and 12 weeks (group B only), radiographed (standard x-rays and faxitron) in the AP and lateral planes. Tibias were processed for torsional testing and quantitative histomorphometry using back scattering SEM. Four additional rabbits (8 tibias) were sacrificed at time zero (n=6 tibias) to determine intact tibia mechanical properties and implant and void percentages at time zero (n=2). Torsional mechanical testing was performed using a MTS 858 Bionix testing machine. The tibiae were potted in woods metal using an alignment jig and subjected to cyclic torsional loading (+/- 2° for ten cycles at a rate of 0.7° per second) followed by loading to failure at a rate of 0.7° per second. The samples were fixed in phosphate buffered saline, embedded in polymethlymethacrylate and cut longitudinally at the marker pins to expose the central portion of the defect. Backscatter electron imaging was performed on a Cambridge Instruments S360 scanning electron microscope at 10x to provide an overview of the defect as well as 6 sites at 40x magnification within the defects (3 cortex and 3 canal) for quantification. The percentages of bone ingrowth, remaining implant material and soft tissue voids at each time point were determined using a threshold technique using Global Labview Image Data was analyzed using a 2-way analysis of variance. RESULTS: Radiographic analysis revealed progressive CS and 200 R resorption with time. New woven bone was noted by 6 weeks in both groups. Torsional properties remained inferior to controls at 3 and 6 weeks and approached control levels by 12 weeks (Group B). Fractures initiated in a spiral fashion at the distal defect margin. Scanning electron microscopy revealed residual calcium sulfate in the central portion of the defects in both groups (figures 1 and 2). Immature woven bone was noted at the defect margins as well as within the central portion of the defect but no cortex at 6 weeks in defects filled with the calcium sulfate slurry (group A) alone or in combination with 200 R (group B). New woven bone ongrowth and ingrowth within the 3 dimensional ultrastructure of the 200 R bone graft as well as some resorption of the calcium carbonate core was observed at 6 and 12 weeks (Group B). Calcium sulfate was not detected at 12 weeks for group B while continued resorption of the calcium carbonate core and new bone formation with new cortical development was observed.

Figure 1: Group A CaSO4 slurry at 6 weeks (10x, 40x).

Figure 2: Group B, CaSO4 slurry + 200 R at 6 weeks (10x, 40x). Quantitative SEM revealed progressive implant resorption, increased bone formation and void versus time for both groups (figure 3). A significant increase in new bone was noted at 6 weeks in group A (p< 0.05) and group B by 12 weeks (p< 0.05). Void decreased with time but was not significant.

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Figure 3: Quantitative histomorphometry based on backscatter SEM. Discussion CS alone does not provide a 3D matrix for new bone ingrowth or ongrowth. This study examined CS alone or with a resorbable porous ceramic (200 R). CS had nearly completely resorbed by 6 weeks in both groups. New bone formation was noted with CS alone but the defect remained open without cortical closure. Bone ingrowth and ongrowth in the porous coral (200R) was observed at all time points. Resorption of the RPC was followed by new bone formation in the interior as well as on the HA layer. The combination with CS vastly improved the handling properties of the RPC at the time of surgery. The material fully sets in-situ and was able to be molded and packed into the defect. These improved handling properties may provide an additional benefit along with the resorption of CS and calcium carbonate which can provide a local ion pool for new bone formation. The 3D porous nature similar to cancellous bone provides a matrix for cellular infiltration and proliferation. The composite nature of this combination may also provide some early load bearing capacity. The CS contributes to rapid early bone formation and is virtually completely resorbed by six weeks, whereas the RPC persists significantly longer continuing to provide a 3-D scaffold for continued bone formation. [1] Nadkarni et al., Trans ORS 2000 p 683. **Interpore Cross International, Irvine, CA.