gms | German Medical Science

Objective: Inadequate compression of bone fragments caused by insufficient compression force or screw pull-out of the spongiosa screw is a possible cause for pseudarthrosis after compression screw osteosynthesis. A new developed system, consisting of a spongiosa screw with a fine pitch thread at the shaft and a sleeve nut generating the compression, is compared to two established systems.

Method: Three types of screws in four groups were compared. A compression screw pre-drilled with 1.25 mm (CS 1.25) and 2.5 mm K-wire (CS 2.5), a double threaded screw (DTS) and a screw with a sleeve nut (SLN). For the experiments an artificial bone made of polyurethane foam in four different densities (0.16 g/cm³ – 0.48 g/cm³) was used. As a substitute for the cortical bone a 1 mm thick epoxy sheet was laminated onto each foam block. The blocks were cut into two pieces and pre-drilled according to the instructions of the manufacturer. Four thin-film force sensors were placed between the block fragments to measure the compression force during screw placement. The applied insertion torque was continuously measured with a torque sensor. Five measurements for each group of screws and foam densities were done (80 measurements in total).

Results: The compression force was significantly different in all four groups (p<0.05). The compression force was found to be very low for DTS, followed by CS 1.25 and CS 2.5 (105 N, 142 N and 189 N at 0.32 g/cm³ foam density). The SLN provided the highest compression force (283 N at 0.32 g/cm³). There was a correlation between applied insertion torque and compression force for CS 1.25, CS 2.5 and SLN. No relation was found for DTS.

Conclusions: The three types of screws exhibit different characteristics. The DTS provides the smallest amount of compression but produces a relatively long plateau between beginning of the compression and screw pull-out. The insertion torque gives no indication regarding the compression effect. The CS 2.5 was more effective in providing a higher maximum compression force, but also exhibits a very small plateau between maximum compression and screw pull-out. The increase of the insertion torque could be used as an indicator for the achieved compression force. The CS 1.25 demonstrated lower values in compression and torque compared to the CS 2.5. The high compression of the SLN arises from the large caliber of the spongiosa screw. The long plateau and the good correlation between insertion torque and compression minimizes the risk of screw pull-outs.