Biomechanical stability of pedicle versus cortical screws in the lumbar spine

Abstract

Introduction: For posterior lumbar spine fusion, instrumentation is usually performed with pedicle screws (PS). However, there is a trend for minimally invasive techniques such as cortical screws (CS) to reduce the degree of muscle dissection needed. Previous cadaveric biomechanical cyclic testing of the stability of PS and CS has reported that CS required an increased force to cause the same amount of displacement as PS on the same specimen. However, implant failure complications in CS is not low (8.9%). Furthermore, the load failure mechanisms of PS and CS remain unclear. Thus, this study aims to investigate the load failure mechanisms of PS and CS and compare the stability of these two fixation methods. Method: Three vertebral cadaveric specimens (two L4 and one L5) from an aged group (>65yrs) were collected and prepared by removing all associated muscles and ligaments. For each specimen, a PS (diameter:5.5mm; length:40mm) and a CS (diameter:5.5mm; length:35mm) was inserted. Each screw underwent displacement controlled (10mm) vertical loading. The force-displacement curve features were compared between these two fixation methods. Results: For all specimens, it was found that vertical loading of the screws induced no cut out of the screws. The comparative trend of the force-displacement curves between PS and CS generated from three specimens were consistent and for each of the specimen, three phases of force-displacement behavior featuring different PS and CS stabilities were discovered. During the initial phase (with the displacement of the screws on specimen 1: <1mm displacement; specimen 2: <2.5mm displacement; specimen 3: <3.5mm displacement), the forces required to migrate the PS were larger than the forces required to migrate the CS. However, a snap-through buckling effect was discovered at the second phase of the force-displacement curves (with the displacement of the screws on specimen 1: 1-4mm; specimen 2: 2.5-7mm displacement; specimen 3: 3.5-6 mm displacement) whereas CS demanded larger forces to cause the same amount of displacement as PS. Interestingly, at the third phase (with the displacement of the screws on specimen 1: >4mm; specimen 2: >7mm displacement; specimen 3: >6 mm displacement), while the CS did not require increased force to cause increased displacement, PS demanded increasing force. Discussion: A three-phases-force-displacement behavior was discovered when comparing PS and CS, with PS being more stable at phase 1 and phase 3 and the CS performing superiorly at phase 2. Under loading, the overall performance of CS is predictable, but PS fails faster than CS until the stable point is reached. This phenomenon is possibly associated with the material complexity surrounding PS including both cortical and trabecular bone, whereas for CS the surrounding material is mainly cortical bone. In conclusion, under large displacement, PS can withstand increased force than CS.