Synthetic hydroxyapatite (HAP) implants are common in craniofacial surgery; the major problem with HAP implants is fixation instability. An ultrasound-assisted pinned resorbable fixation system has recently been introduced. The authors hypothesized that this resorbable system could be used to fix HAP implants. The authors evaluated the optimal HAP porosity and resorbable pin using the pull-out test and microscopic analysis. Hydroxyapatite blocks were synthesized with 0% (dense type), 35% (fine pore type), and 50% (large pore type) porosity. Bioabsorbable pins of 4 sizes were tested: diameters of 1.6 or 2.1mm and lengths of 4 or 7mm. Among the 1.6mm diameter pins, the 7-mm length pins showed higher pull-out strength (large pore, 6.5±1.2 kgf; fine pore, 4.1±0.7 kgf; dense, 2.4±0.4 kgf) than the 4-mm length pins (large pore, 2.1±1.0 kgf; fine pore, 1.3±0.4 kgf; dense, 1.5±0.9 kgf) with all 3 HAP types (P<0.01). Among 2.1-mm diameter pins, all were removed from the large and fine pore HAP types without any resistance. Only three of seven 2.1×7mm pins inserted into the dense type HAP showed pull-out strength (3.9±2.1 kgf). Among all conditions, the 7-mm length, 1.6-mm diameter pin in large pore type HAP showed the highest pull-out strength. Microscopically, the large pore type HAP showed the liquefying pin flowing into the large pore adjacent to the pilot hole. By contrast, the fine pore and dense pore types of HAP did not allow flow, and the pins did not change in structure 3-dimensionally. The authors found that pull-out strength depends on HAP porosity, and pin length and diameter.
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