SPINE Volume 31, Number 1, pp 000 – 000 ©2006, Lippincott Williams & Wilkins, Inc. Ex Vivo Experiments of a New Injection Cannula for Vertebroplasty Gamal M. Baroud, PhD,* Pierre-Luc Martin,* and Francois Cabana, MD† Study Design. An experimental study using cadaveric vertebrae. Objectives. To measure the injection pressure in an ex vivo model and determine whether the redesigned can- nula, featuring 2 distinct sections of different diameters, significantly reduces the injection pressure. Summary of Background Data. An important limita- tion of vertebroplasty is the excessive pressure required to inject sufficient cement. The redesigned cannula ac- counts for the “bottleneck” of the injection previously identified. It has a distal section that is adapted to the pedicles and a proximal section with a larger diameter. Methods. Two cannulas were tested, a conventional 8-gauge and the redesigned cannula. There were 3 mL of cement injected in small strokes of 0.75 mL into the left and right hemivertebrae. A custom-made injection device monitored the injection pressure and cement volume. Results. Average injection pressure was 2.3 MPa, of- ten approaching physical human limits and resulting in insufficient filling. The average pressure of the redesigned cannula was 44% lower than that of the conventional cannula, a highly significant result (P  0.001). Conclusion. The new cannula may improve vertebro- plasty by significantly easing cement injection. It is cost effective and can be easily integrated into the existing procedure. Key words: injection pressure, vertebroplasty, can- nula, bone cement augmentation, cement injection pro- cess. Spine 2006;31:000 – 000 During vertebroplasty, the pressure to inject bone ce- ment into a vertebral body is often excessive. 1–3 It has been reported that with a 5-mL syringe, the pressure applied on the plunger of the syringe during an actual vertebroplasty can often exceed 1500 kPa, which ap- proaches human physical limits. 4–6 This rate is equiva- lent to approximately 170 N of force, or 17 kg. When the pressure requirements are excessive, the procedure has to be aborted. 4,7 The early termination may result in insuf- ficient cement filling, which may impact the success of the procedure. 7 Mainly 2 methods of overcoming the pressure prob- lem have emerged: (1) to increase the pressure applied, 1,2 and (2) to lower the cement viscosity. 3 There are cur- rently at least 7 devices available on the market to in- crease the pressure applied to the cement. The main lim- itations of these devices are that under excessive pressure, the liquid may separate from the suspen- sion, 4,8 and that they reduce the tactile feedback to the physician. 9 The viscosity of bone cement can be lowered in a variety of ways. The liquid-to-powder ratio can be in- creased during mixing, but this may result in complica- tions, such as toxicity 10 and reduced cement strength. 11 Another popular method is to delay the polymerization process. 3 When the refrigerated components are mixed, the resultant mixture may not be uniform, which in- creases the likelihood of phase separation during injec- tion. 12 A new method to lower viscosity by improving the cement mixing has recently been introduced in the literature. 13,14 However, the disadvantage of lowering cement viscosity is that the risk of leakage may in- crease. 12,15 We have adopted a different approach to lowering the injection pressure, which is the total pressure required to inject the cement into a vertebral body, and to bringing it within human physical limits. We altered the geometric configurations of an 8-gauge cannula. The new cannula features 2 distinct sections with 2 different internal and external diameters (Figure 1). 16 The geometry of the dis- tal section one third that enters the pedicles is governed by the anatomic limits and, therefore, has a diameter identical to a conventional 8-gauge cannula. The diame- ter of the proximal two thirds, which, in part, passes through the soft tissue, is 2 times larger than the distal diameter. Although the basic idea behind the redesigned can- nula seems intuitive, it was arrived at through a thorough understanding of the underlying mechanisms of the in- jection process. 4,17 This understanding enabled us to iso- late and examine if there is a particular component of the injection pressure that acts as the pressure bottleneck. Specifically, does the resistance occur primarily in the cannula or in the vertebral body, or are both internal and external components equally responsible? The unanticipated results of a theoretical model 4 and an experimental follow-up study 17 were that about 90% of the injection pressure is required to overcome the flow resistance in the cannula, and only about 10% is re- From the *Laboratoire de biome ´canique, Ge ´nie me ´canique, Universite ´ de Sherbrooke, and †Centre Hospitalier Universitaire de Sherbrooke (CHUS), Sherbrooke, Que ´bec, Canada. Acknowledgment date: November 22, 2004; First revision date: De- cember 22, 2004; Acceptance date: February 21, 2005. This work has been supported by the Canadian Institute of Health Research (CIHR) Grant number MOP 57835, the Natural Science and Engineering Research Council (NSERC), and the Fonds Que ´becois de recherche sur la nature et les technologies (FQRNT). The manuscript submitted does not contain information about medical device(s)/drug(s). Federal funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. Address correspondence and reprint requests to Gamal Baroud, PhD, Laboratoire de biome ´canique, De ´partement de ge ´nie me ´canique, Fac- ulte ´ de ge ´nie, Universite ´ de Sherbrooke, Sherbrooke, Que ´bec, Canada J1K 2R1; E-mail: Gamal.Baroud@USherbrooke.ca balt6/zsp-spine/zsp-spine/zsp00106/zsp5528-06z xppws S1 11/11/05 5:24 Art: BRS200278 Input-dms 1 <zjs;Biomechanics> • <zjss;Biomechanics> • <zdoi;10.1097/01.brs.0000192722.30052.a4> F1