OFFSET LAMINAR HOOKS DECREASE BENDING MOMENTS OF PEDICLE SCREWS DURING IN-SITU CONTOURING

Study Design. A biomechanical study was conducted using cadaver Spines to determine the influence of supplemental offset laminar hooks on pe dicle screw bending moments and migration during in situ contouring of short-segment pedicle instrumentation. Objectives. To determine the e ffects of offset laminar hooks on short-segment pedicle instrumentatio n constructs during in situ contouring. It was hypothesized that the s crew bending moments and screw migration would decrease when offset la minar hooks were used with short-segment pedicle instrumentation. Summ ary of Background Data. Clinical studies have implicated screw bending or breakage at the screw hub as failure mechanisms in short-segment p edicle instrumentation constructs used to stabilize thoracolumbar frac tures, particularly when rods are contoured in situ. Methods. Cadaver spines were instrumented using short-segment pedicle instrumentation o r short-segment pedicle instrumentation with supplemental offset lamin ar hooks. The instrumentation was contoured in situ, and screw bending moments were measured at the hub of the screws. Screw migration was m easured from lateral radiographs. Comparisons of screw bending moments and migration were made between the two instrumentation configuration s. Results. The addition of offset laminar hooks significantly reduced screw bending moments and screw migration during in situ contouring. The mean screw bending moments decreased approximately 30% at the maxi mum bending angle of 30 degrees (P < 0.05), and the mean screw migrati on during contouring decreased from 8 degrees to 2 degrees (P<0.05). C onclusions. Addition of offset laminar books to short-segment pedicle instrumentation decreases screw bending moments and migration of the s crews during in situ contouring of the rod. The authors speculate that decrease in loading of the screw will improve durability of the const ructs clinically.