Sara Parashin, of the Orthopaedic Innovation Centre in Canada, looks at ways to improve the outcome of spinal fusion using pedicle screw fixation
Many people in North America suffer from severe back pain. To remedy, patients may undergo surgical intervention to immobilise the affected areas by means of spinal fusion. In spinal fusion, screws are implanted into neighbouring vertebrae and joined by rods to create a rigid construct, thereby promoting intervertebral fusion. Unfortunately, a reasonable portion of these procedures suffer from fusion failure. Common causes for fusion failure are screw loosening and pull-out from bone as a result of poor fixation as well as fracture of the hardware (screws or rods). Screw fixation may depend on thread pitch, purchase depth, and bone quality. Hardware durability may depend on the depth of screw engagement that is chosen by the surgeon.
To improve the success rate of spinal fusion, the Orthopaedic Innovation Centre (Winnipeg, Canada) has been working with orthopaedic surgeons and residents from the Health Sciences Centre (Winnipeg, Canada) to study pedicle screw fixation in lumbar spinal fusion. Phantom bone models have been used to study three main factors relating to pedicle screw fixation: screw design, bone quality, and screw purchase depth. The purpose of this research was to investigate the relationship between these contributing factors and screw fixation. Additionally, to determine the effect of post-operative screw loosening from physiological loading.
Pedicle screw fixation was determined by performing mechanical axial pull-out failure tests of the screws in custom-made artificial bone models (figure 1). Bone models were created to mimic pedicle screw trajectory, consisting of the pedicle channel (cortical bone) and vertebral body (cancellous bone). Material characteristics were selected based on cadaveric bone densities described by researchers at the Food and Drug Administration (Nagaraja and Palepu, J Biomech Eng 2016). Testing included three screw designs, four levels of bone quality, and three purchase depths. The following describes each level per factor:
Screw design (figure 2):
Cylindrical shank with coarse thread pitch
Conical shank with fine thread pitch
Conical shank with square-edged coarse thread pitch
Bone quality (figure 3):
Screw purchase depth:
One revolution backed out
Two revolutions backed out
The effect of pedicle screw loosening was measured by simulating clinically relevant load conditions on the test models to create a loosened channel. This included cantilever cyclic loading controlled by load frame equipment (figure 4). Cyclic loads represented the average force a patient may endure on their lower back during post-operative daily movements such as laying down, sitting-up, and standing. Cyclic tests ran for the equivalent of time these movements would occur during the average recovery period after lumbar spinal fusion (six months). Loosened screws were then tested for screw fixation by axial pull-out tests. Pull-out strengths were compared between loosened and non-loosened screws.
It was hypothesised that: 1) screw fixation between screw designs is not equal, 2) screw fixation in osteoporotic bone is less than screw fixation in normal bone, 3) screw fixation is reduced when backed out of bone, and 4) repeated cyclic loading decreases screw fixation.
Results showed that screws with a conical fine pitch design had the strongest screw fixation for nearly all bone qualities and screw purchase depths. At full insertion in severe osteoporosis, the conical screws with square-edged coarse thread pitch showed the greatest fixation. Overall, the cylindrical screws with coarse thread pitch expressed the least amount of fixation. At full purchase depth, the differences in pull-out strengths between designs were largest in normal and mild osteoporotic bone and lowest in moderate and severe osteoporotic bone.
Screw fixation showed a strong linear relationship between bone quality and axial pull-out strength for all screw designs. In mild, moderate, and severe osteoporotic bone, strengths were reduced by approximately 15 per cent, 50 per cent, and 80 per cent, respectively, compared to normal bone. Despite a significant loss in fixation when tested in severe osteoporotic condition, pull-out strength values remained above physiological load thresholds.
Induced screw loosening showed no significant effect on fixation in normal bone. However, fixation was reduced in cases of osteoporosis after cyclic loading. A <10 per cent reduction in strength was seen in mild osteoporotic bone, 5-40 per cent reduction in strength was seen in moderate osteoporotic bone, with the highest reduction in strength of 20-70 per cent seen in severe osteoporotic bone after cyclic loading. The conical with square-edged coarse thread pitch screws suffered the largest loss in fixation as a result of screw loosening, dropping below physiological loading conditions in severe osteoporotic bone. As a result, repeated flexion-extension of the spine in severe osteoporotic bone could lead to fusion failure caused by weak screw fixation.
Backed out screws tested for pull-out strength without cyclic loosening did not show a large difference in fixation when compared to screws in full purchase. However, backed out screws suffered a greater fixation loss after cyclic loading than screws in full purchase. In fusion procedures, screws may be backed out to attain appropriate anatomical form and function. Although backing out screws may improve the overall spine construct, our results show it may lead to early fusion failure as a result of screw loosening.
When surgical intervention is taken to relieve back pain, it is important to consider the severity of osteoporosis, screw design, degree of screw purchase, and the physiological loads endured post-operatively. Patients suffering from severe osteoporosis have the highest risk of fusion failure due to significant reduction in screw fixation. Conical screws that feature a fine thread pitch have been shown to achieve the greatest amount of strength in artificial bone. Caution should be taken when backing out pedicle screws during fusion procedures, as fixation may be sacrificed.
It is important to note that the testing from this study was limited to individual screw pull-out strengths in artificial bone constructs. Typical two- and three-level lumbar spinal fusion procedures involve multiple screws in addition to connecting rods which will enhance clinical screw fixation compared to that of artificial bone modelling.
Continued biomechanical research is underway to further investigate the cause of fusion failure. Next, looking at the effect of bone cement augmentation in cannulated and fenestrated screws. Specifically, how this may enhance fixation in severe osteoporotic cases. Subsequent research will investigate screw fixation in the cervical region using various surgical techniques for bi-cortical and uni-cortical purchase depths to improve mechanical strength of the bone-screw interface.
The Orthopaedic Innovation Centre would like to acknowledge the Alexander Gibson Fund at the University of Manitoba for funding this research. The researchers have no relevant industry disclosures.
Nagaraja, S., Palepu, V. Comparisons of anterior plate screw pullout strength between polyurethane foams and thoracolumbar cadaveric vertebrae. J Biomech Eng, 2016. Vol. 138: p. 104505-1-6.
Sara Parashin is a biomedical engineer at the Orthopaedic Innovation Centre (Winnipeg, Manitoba) who dedicates her time to orthopaedic research studies, engineering testing and explant retrieval analysis. Sara began working for the company in 2015 and now leads the biomechanical testing and clinical research departments related to the spine.