Cement augmentation of lag screws: an investigation on biomechanical advantages

Abstract

In trauma surgery, lag screws are commonly used. However, in osteoporotic bone, anchorage can be considerably compromised. This study investigates the biomechanical potential of cement augmentation in terms of improved fixation. 36 Surrogate osteoporotic bone specimens were utilised in three biomechanical experiments, each comparing 6 augmented with 6 non-augmented samples. Standard partially-threaded lag screws (Synthes) were placed following surgical standard. For the augmented groups, 0.4ml of polymethylmethacrylate was injected into the pre-drilled hole prior to screw placement. Interfragmentary compression was determined using a cannulated ring compression sensor. Maximum torque was recorded with a torque wrench. Compressive relaxation after 24h, relaxation after loosening and re-tightening the screw as well as maximum compression and torque at failure were measured. Mean relaxation was significantly lower for the augmented group (p<0.01). After 24h, a remaining fragmental compression of 62% for the augmented and 52% for the non-augmented specimens was found. Loosening and re-tightening of the screw did not affect the compressive relaxation when augmentation was applied (p=0.529), compared to an increased relaxation after re-tightening in the non-augmented group (p=0.04). The mean maximum compression and torque until failure were significantly higher for the augmented group (p<0.001). Cement augmentation of lag screws can improve fixation stability in terms of installing and maintaining interfragmentary compression. Effects of relaxation can be reduced and re-tightening of screws is possible without compromising the fixation. Particularly in reduced bone mass, augmentation of lag screws can markedly increase the security of the technique.