A pedicle screw system and a lamina hook system provide similar primary and long-term stability: a biomechanical in vitro study with quasi-static and dynamic loading conditions

Abstract

Purpose For the stabilization of the thoracolumbar spine area, various stabilization techniques have been developed in recent decades. The aim of these techniques is to immobilize the treated segment to repositioning or correct the spine and guaranty long-term stability to achieve a reliable fusion. The aim of this study was to simulate in an in vitro experiment the postoperative long-term situation in elderly osteoporotic patients to compare two different stabilization principles; a pedicle screw system and a lamina hook system. Methods Two comparable groups with respect to age and bone mineral density with each n = 6 fresh-frozen human, bi-segmental thoracolumbar spine specimens (T11-L1) were used. Antero-posterior and lateral radiographs were taken before the test, to assess the spinal status. Then the intact specimens were biomechanically characterized with pure moments in the three anatomical planes in different states in terms of range of motion and neutral zone. After implantation of either, a pedicle screw system or a lamina hook system, the primary stability was determined under the same conditions. Subsequently the specimens were cyclically loaded under complex loading, using a custommade set-up in a dynamic materials testing machine with increasing moments from 3 to 66 Nm until 100,000 cycles or until one of the three defined ''failure'' criteria was reached.(1) A failure of a bony structure. (2) Exceeding of the threefold ROM of the primary stability after implantation in flexion plus extension. (3) Reaching of the ROM based on the intact state before implantation both in flexion plus extension.Results The results showed that the ROM was strongly reduced after instrumentation similar for both implant systems in all motion planes. The highest stabilization was found in flexion/extension. During cyclic loading with increasing moments, the ROM increased continuously for both systems. The number of load cycles until one of the failure criteria was reached varied only slightly between the two groups. In the pedicle screw group 30,000 (median) loading cycles (range 5000-80,000) with a corresponding moment of 24 Nm (range 9-54) could be reached. In the lamina hook group 32,500 load cycles (range 20,000-45,000) could be achieved with a corresponding moment of 25.5 Nm (range 18-33). There was a slight trend that the pedicle screw system is influenced more by bone mineral density. Conclusion Both implant systems provide similar primary stability and similar long-term stability. In the pedicle screw group, there was a stronger correlation between bone mineral density and the reached number of load cycles.

Pedicle screw anchorage of carbon fiber-reinforced PEEK screws under cyclic loading

“…The shear component, however, is neutralized by the translation of the x-y table. In contrast to these single pedicle screw loading tests allowing for pairwise left-right comparisons under comparable conditions, Wilke et al [38] used entire assemblies of bisegmentally instrumented spinal motion segments. While this test setup tests the whole clinically applied instrumentation, it does not allow for control of numerous confounding factors related to specimen variation (e.g., BMD, vertebrae morphology, pedicle morphology).…”

Paravertebral foramen screw fixation for posterior cervical spine fusion: biomechanical study and description of a novel technique

“…6,16,18 Even more relevant may be tests of segments that are instrumented with entire assemblies of implants with 6 degrees of freedom loading. 26 Nevertheless, we used a pullout test because it is a relatively easy and consistent test with which to assess the new screw method, and because a standardized testing method has been established (American Society for Testing and Materials, F543). Second, because of the limited availability of cadaver specimens, relatively small numbers of specimens were used, narrowing the statistical power of the study and allowing interspecimen variability, which leads to large standard deviations in BMD, and results in a relatively large standard deviation in pullout strength of the screws.…”

Cortical threaded pedicle screw improves fatigue strength in decreased bone quality

“…The biomechanical cadaver model is not able to replicate in vivo conditions with respect to screw ingrowth and multidirectional loading. In this in vitro experiment, the load was applied through the screw head, which does not exactly reflect the complex in vivo situation where multiaxial load is also applied through the vertebral body [21,26]. However, the most relevant loading direction (according to Rohlmann et al [21]) was applied in a standardized and reproducible manner.…”