Biomechanical stability of cortical versus pedicle screws in the lumbar spine with relevance of bone density

Dr Cheung, Jason Pui Yin   (Principal Investigator (PI))

Professor Lu William Weijia   (Co-Investigator)

Emeritus Professor Luk Keith Dip Kei   (Co-Investigator)

24

2017-06-30

2019-06-29

55400

Biomechanical stability of cortical versus pedicle screws in the lumbar spine with relevance of bone density

Biomechanics, Cortical screw, Lumbar, Pedicle screw, Spine

Orthopaedics/Traumatology

201611159073

Seed Fund for PI Research – Basic Research

2016

Completed

Lumbar spinal fusion is one of the most common spine surgeries done worldwide with previous reports of over 120000 done in the US every year.[1] In posterior lumbar fusions, fixation is usually performed with pedicle screw fixation due its stability as a construct. A growing amount of surgeons are developing minimally invasive techniques to reduce the degree of muscle dissection and larger skin incisions. As a ""minimally-invasive"" alternative to pedicle screws, cortical screws can be used to stabilize the spinal segment. As compared to pedicle screws where the insertion in the lumbar spine is placed at the confluence of the superior articular process, transverse process and mammillary process, these cortical screws are inserted at the junction of the superior articular process and the pars interarticularis. Hence, the muscle dissection only has to be limited to the level of the facet joints with no further lateral dissection necessary for screw insertion. Due to the potential reduction in surgical trauma, some surgeons have even successfully discharged these patients on the day of surgery.[2] It is not uncommon to use this type of construct for lumbar fusions for spondylolisthesis. Due to the deformity and slip of the vertebrae anteriorly, lateral dissection for exposure of transverse processes for pedicle screw insertion and fusion is usually difficult. Cortical screws can provide an easier access for spinal fixation. As an adjunct fixation with lumbar interbody fusion, cortical screws have been shown to be on par with pedicle screws in terms of fusion rates, symptom improvement and functional status.[3, 4] Despite comparable outcomes with traditional pedicle screw fixation, it is not unusual to see a complication rate up to 8.9% including implant failure with cortical screws.[5, 6] One failure method of cortical bone screws causing screw loosening and pars with pedicle fracture have been illustrated in one cadaveric study.[7] In osteoporotic bone, insertion of the screws may cause fracture at the lateral border of the pars which extends obliquely in the sagittal plane through the medial aspect of the superior facet, through the pedicle and exits at its lateral border. The head of the screw can be impinged medially against the base of the spinous process and lamina and thus lead to lateral deviation of the tapped trajectory and finally loosening or fracture of the pars and pedicle. A case study from the investigators’ experience can illustrate another problem with this fixation method. A 71-year-old woman with osteoporosis presenting with back pain and left L5 radicular pain was managed. Imaging showed L4/5 spondylolisthesis and spinal stenosis compatible with her clinical symptoms. A minimally-invasive approach was utilized for stabilization of the segment to relieve her of pain. A lateral approach insertion of a interbody cage to the L4/5 was performed with a posterior approach to insert cortical screws to fix the L4/5 spinal segment. Despite excellent relief of symptoms in first 12 days after surgery, recurrence of symptoms occurred and radiographs showed a collapsed L5 with subsidence of the cage and loosening of the L5 cortical screws in the absence of any trauma. Revision surgery with extension of the construct to S1 and exchange of all cortical screws to pedicle screws were required for fixation. Intraoperatively, the cortical screws were noted to have pivoted and caused lamina fracture. We postulate that during insertion of the cage, the endplate was violated and due to the osteoporotic bone, the cage subsided and the construct lost its anterior support. Without the anterior support, the posterior instrumentation became a tension device which placed the cortical screws at a biomechanical disadvantage. With further pistoning as the patient mobilized, fatigue failure occurred at the screw-bone junction and the screws pivoted and led to lamina fracture and screw loosening, and subsequently construct failure with loss of height, cage subsidence and recurrence of symptoms. This is a failure mechanism that should be tested in the laboratory to determine the ability of the cortical screws to resist to fatigue loads especially if the anterior column is deficit. Although there have been several biomechanical studies testing cortical screws, most have tested the pullout strength of pedicle versus cortical screws and finding them to be similar even for osteoporotic bone.[8-12] However, the load failure mechanism that is identified in the clinical scenario has not been tested. Hence our study objectives include: 1) To determine the fatigue load to failure between cortical and pedicle screws in a stable anterior column model. 2) To determine the fatigue load to failure between cortical and pedicle screws in an anterior column deficient model. 3) To determine the difference in load resistance by strategic placement of cortical screws in areas of better bone density.