Analysis of bone cement distribution around fenestrated pedicle screws in low bone quality lumbosacral vertebrae.

PURPOSE: To study the exact distribution of bone cement around augmented fenestrated pedicle screws in both lumbar and sacral vertebrae of patients with low bone quality. METHODS: A total of 37 patients with instrumented lumbar fusion were investigated. 3D computed tomography virtual models of the injected cement and screws were obtained. The models were computed for their centroid (i.e. their average mass centre point), and their coordinates (x, y, z) were projected on their respective screw-transversal and screw-longitudinal planes for further analysis. RESULTS: The results showed better bone cement homogeneous distribution around the screws in lumbar (L4 and L5) than in sacral (S1) vertebrae. In the lumbar region, the centroids were transversally projected near the transversal centre of symmetry of the screws. On the other hand, in the sacral region, the cement flowed preferentially outside the centre of symmetry of the screws, into the sacral ala. CONCLUSIONS: The results confirm the different flow behaviours of bone cement in lumbar versus sacra vertebrae. The computer methodology followed in this study helps to understand the clinical monitoring observations and lays the foundations for better positioning of the screws and specific vertebrae-oriented screw designs.

Advantageous new conic cannula for spine cement injection.

STUDY DESIGN: Experimental study to characterize the influence of the cannula geometry on both, the pressure drop and the cement flow velocity established along the cannula. OBJECTIVE: To investigate how the new experimental geometry of cannulas can affect the extravertebral injection pressure and the velocity profiles established along the cannula during the injection process. SUMMARY OF BACKGROUND DATA: Vertebroplasty procedure is being used to treat vertebral compression fractures. Vertebra infiltration is favored by the use of suitable: (1) syringes or injector devices; (2) polymer or ceramic bone cements; and (3) cannulas. However, the clinical use of ceramic bone cement has been limited due to press-filtering problems. Thus, new approaches concerning the cannula geometry are needed to minimize the press-filtering of calcium phosphate-based bone cements and thereby broaden its possible applications. METHODS: Straight, conic, and combined conic-straight new cannulas with different proximal and distal both length and diameter ratios were drawn with computer-assisted design software. The new geometries were theoretically analyzed by: (1) Hagen-Poisseuille law; and (2) computational fluid dynamics. Some experimental models were manufactured and tested for extrusion in order to confirm and further advance the theoretical results. RESULTS: The results confirm that the totally conic cannula model, having proximal to distal diameter ratio equal 2, requires the lowest injection pressure. Furthermore, its velocity profile showed no discontinuity at all along the cannula length, compared with other known combined proximal and distal straight cannulas, where discontinuity was produced at the proximal-distal transition zone. CONCLUSION: The conclusion is that the conic cannulas: (a) further reduced the extravertebral pressure during the injection process; (b) showed optimum fluid flow velocity profiles to minimize filter-pressing problems, especially when ceramic cements are used; and (c) can be easily manufactured. In this sense, the new conic cannulas should favor the use of calcium phosphate bone cements in the spine. LEVEL OF EVIDENCE: N/A.