Intracranial pressure-alterations during controlled intramedullary reaming of femoral fractures: an animal study.

Reaming during intramedullary nailing increases the intramedullary pressure (IMP) and can cause fat intravasation and subsequently lead to pulmonary dysfunction, especially in polytraumatized patients with lung contusion as well as increases in intracranial pressure (ICP).ICP changes were measured in two groups consisting of 12 sheep each with either a fractured or intact femur. The animals were exposed to haemorrhagic shock and were resuscitated by autoinfusion. Transverse midshaft fractures were created bilaterally in one group before reaming of both femoral shafts. Controlled reaming was performed at 15 and 50 mm/s driving speed (DS) and at 150 and 450 revolutions per minute (RPM) with a reaming control system using AO-Reamers. Fat intravasation was measured by transoesophageal sonography (TES) and Gurd test. ICP monitoring was performed with a piezo-electric epidural catheter. Haemodynamic and respiratory parameters, ICP, and cerebral perfusion pressure were measured continuously. High DS and low RPM caused higher IMP and ICP increases in both groups. Significantly higher microemboli were observed in TES in the fractured group as compared to the unfractured group with the same reaming parameters (P=0.021). However, the existence of a femoral fracture did not show a significant influence on changes in ICP (P-value=0.057). Reaming should be performed at a low DS and high RPM to minimize the risk of fat intravasation and the subsequent ICP increase.

Loading of pedicle screws within the vertebra.

We studied cadaveric motion segments instrumented with unique pedicle screw transducers and loaded in a corpectomy model. We hypothesized that the pedicle screw bending moments could be characterized using a mathematical model. Previous studies have estimated the loading characteristics of pedicle screws either by finite element analysis or by experimentally measuring the screw bending strains external to the lamina and pedicle. In our study, the L4 vertebra was instrumented with modified pedicle screws and fixation rods, and loaded axially. The screws were instrumented to measure bending moments at three locations along the threaded shaft of the screw. The recorded bending moments were maximum near the screw hub and decreased in a non-linear manner toward the screw tip. A mathematical model was fit to the bending moment data and accurately described the loading response of a pedicle screw within a vertebra. This model validates previously unsubstantiated analytical models and provides a tool for predicting which construct design variables contribute to pedicle screw failure. In addition, this experimental model should prove useful in validating finite element models designed to investigate vertebral loading of pedicle screws.

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 pedicle screw bending moments and migration during in situ contouring of short-segment pedicle instrumentation. OBJECTIVES: To determine the effects of offset laminar hooks on short-segment pedicle instrumentation constructs during in situ contouring. It was hypothesized that the screw bending moments and screw migration would decrease when offset laminar hooks were used with short-segment pedicle instrumentation. SUMMARY OF BACKGROUND DATA: Clinical studies have implicated screw bending or breakage at the screw hub as failure mechanisms in short-segment pedicle instrumentation constructs used to stabilize thoracolumbar fractures, particularly when rods are contoured in situ. METHODS: Cadaver spines were instrumented using short-segment pedicle instrumentation or short-segment pedicle instrumentation with supplemental offset laminar hooks. The instrumentation was contoured in situ, and screw bending moments were measured at the hub of the screws. Screw migration was measured from lateral radiographs. Comparisons of screw bending moments and migration were made between the two instrumentation configurations. 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 maximum bending angle of 30 degrees (P < 0.05), and the mean screw migration during contouring decreased from 8 degrees to 2 degrees (P < 0.05). CONCLUSIONS: Addition of offset laminar hooks to short-segment pedicle instrumentation decreases screw bending moments and migration of the screws during in situ contouring of the rod. The authors speculate that decrease in loading of the screw will improve durability of the constructs clinically.