A combined partially threaded cancellous lag screw for achieving maximum compressive force without compromising pullout strength

OBJECTIVE: The partially threaded cancellous lag screw (PTLS) could not provide maximum compressive force (CMAX) for compression due to compromised pullout strength (POS). The combined partially threaded cancellous lag screw (CPTLS) could provide higher CMAX than PTLS. However, the change of POS at the point of CMAX when using CPTLS for compression has never been explored. The aim of this study was to determine whether POS decreased at the point of CMAX during CPTLS compression for different bone mineral densities (BMDs).

MATERIALS AND METHODS: Three synthetic cancellous bone blocks were used for this study, and the BMDs were 0.12 g/cm3, 0.16 g/cm3, and 0.20 g/cm3, respectively. 20 pilot holes with 3.2 mm diameters were prepared equably in each block. A CPTLS was inserted through the custom-designed measuring device into a pilot hole manually until failure for measuring CMAX, and the pullout test was done with the identical CPTLS for measuring POS.

RESULTS: The CMAX and POS of the CPTLS were not significantly different in the three specimens, and the ratios of the mean CMAX to the mean POS were very similar in the three specimens (0.98 in the 0.12 g/cc specimen, 1.01 in the 0.16 g/cc specimen and 0.98 in the 0.20 g/cc specimen).

CONCLUSIONS: CMAX is achieved without a decrease in POS during CPTLS compression independent of the BMD.

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