Biomechanical analysis of differing pedicle screw insertion angles
Introduction
Pedicle screw fixation has become the mainstay of fixation for stabilization of the posterior lumbar spine. Originally described by Boucher in 1959, Roy-Camille popularized this technique in Europe in the 1960s, and his spinal plating system has been called the “predecessor of most modern pedicular screw–plate fixation systems” (Boucher, 1959, Roy-Camille et al., 1976, Roy-Camille, 1992). Pedicle screw fixation is now readily accepted for treatment of fractures, tumors, and degenerative disease. Loosening due to fatigue loading and screw breakage are commonly cited reasons for failure, and numerous studies have been conducted to determine which factors are most important in determining biomechanical stability of the pedicle screw (Esses and Bednar, 1989, Willet et al., 1993, Zdeblick et al., 1993). To date, biomechanical studies have for the most part examined pullout failure of the screw as the endpoint to determine stability (Barber et al., 1998, Law et al., 1993, Yerby et al., 1997). Even those few reports that used a cyclic loading model utilized a displacement control rather than load control mechanism to determine relative stability (Barber et al., 1998, Law et al., 1993, Soshi et al., 1991). While parameters studied included using bigger screws, drilling or probing the pilot hole, tapped and untapped screws, coupling, angular insertion, and augmentation with bushings and polymethylmethacrylate, fatigue failure based on the clinical scenario has rarely been reported.
Morphometric anatomic studies have determined that pedicles flare out laterally from the upper to lower lumbar spine (Ruland et al., 1991, Zindrick, 1991). Transverse pedicle angles of the lower lumbar spine range from 8.0–23.5° at L3 (mean 14.4°) to 19.0–44.0° at L5 (mean 29.8°) (Zindrick et al., 1987). Some studies have suggested that convergent screws are a stronger construct, and have recommended that screws be inserted axially within the lumbar pedicle (Barber et al., 1998). On the basis of these studies and those testing pullout strength (Law et al., 1993, Yerby et al., 1997), screw insertion technique along the axis of the pedicle has been described as superior, with increasing angular distance from the vertebral midline at lower levels of the lumbar spine (Cook et al., 2000). However, the patient population subjected to surgery includes a significant number of obese or morbidly obese individuals who present a challenge in exposure for pedicular insertion of screws. The incisions are deep and approaching the axial pedicle along its axis is difficult. As an alternative to pedicular insertion, in the technique described by Roy-Camille in 1976 and 1992, pedicle screws are inserted in a vertical fashion, crossing the axis of the pedicle rather than proceeding in line with it. While attempts have been made to determine the stability of these screws, most studies utilize a displacement control to determine pullout strength at the bone–screw interface, rather than examining them dynamically at sub-failure forces to determine relative stability based on amount of screw toggle acquired during fatigue testing (Barber et al., 1998, Brantley et al., 1994, Soshi et al., 1991).
This study was designed to determine the stability of pedicle screws that were inserted by both straight and angled techniques. Cyclic, sub-failure load control was used to simulate in vivo loading. The displacement of each screw was measured and compared with the contralateral screw that was inserted by the differing technique. Based on this work, the fatigue stability of the two different types of screw insertion technique was examined by answering two research questions: (1) is the rate of damage different between the two screw insertion methods? (2) is there a difference in stiffness and creep damage between the two methods?
Section snippets
Specimen preparation and screw implantation
Use of human tissue was approved by our hospital Institutional Review Board. Five fresh frozen cadaveric spines were obtained from a tissue bank. These specimens were procured from T6 to the sacrum with minimal soft tissue attachment and were stored at −32 °C. None had a history of metastatic disease. Fourteen total vertebral bodies were tested in this protocol. There were 4 males and 1 female with an average age of 67 years (range of 42–82 yrs). The vertebral bodies of L3–L5 were dissected
Results
Preliminary testing revealed that a 200 N force would cause traumatic fracture of the pedicle within the first and second cycles. Further testing at 50 N demonstrated the first and second phases of the standard three-phase response of fatigue could be obtained. A specimen was tested at 25 N but revealed no measurable damage, and the decision was made to proceed with testing the remainder of the specimens at 50 N. At these small loads, no preliminary or experimental specimen was cycled to failure
Discussion
Multiple studies have been conducted to examine stability characteristics of lumbar pedicle screw systems that have been developed for posterior fixation. Zindrick et al. (1986) performed a thorough biomechanical study performing axial pullout and cyclic loading modes (displacement control) with multiple screw designs at various depths. The construct was assumed failed when 50% of the initial force was required to displace a total of 6 mm (3 mm caudad, 3 mm cephalad). Further, screws were inserted
Conclusions
This study showed that straight screw insertion results in a pedicle-screw construct that has a better fatigue performance. From a clinical perspective, insertion of the pedicle-screws in a straight fashion is certainly more practical as it does not require extensive dissection, retraction, or excision of paraspinal musculature to achieve screw insertion along transverse pedicle angles that can range up to 38° from the midline. Further, this technique, though with less support from the
Acknowledgements
This publication was, in part, made possible by Grant Number AR049343 from the National Institutes of Health. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the NIH. The authors gratefully acknowledge the late Mr. James Sapp for his contribution to design and building of the loading fixtures.
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