Spine

Pedicle screw designs in spinal surgery: much ado about nothing?

Pedicle screw designs have evolved with the aim of improving the primary pull-out strength, and one of the major innovations has been varying thread patterns. Little is published about the revision pull-out strength of pedicle screws when they are replaced in vivo. The authors of this study from Singapore have compared three common thread types: a single thread with a single helical thread; a dual thread with a double helical thread along the entire screw length; and a mixed thread that has a double helical thread over the proximal core of the screw, with a single helix distally.¹ Screws of each design were inserted into a polyurethane foam in a set sequence. First, a 6.5 mm screw was inserted into the block and the primary pull-out strength was measured. Second, a 6.5 mm screw was inserted, removed, and reinserted, then the revision pull-out strength was measured. Third, a 6.5 mm screw was inserted and removed, then a 7.5 mm screw of the same design was inserted and the revision pull-out strength was measured. As would be expected, dual-thread screws had a significantly greater pull-out strength compared with mixed and single-thread screws. Mixed screws had a significantly lower revision pull-out strength compared with both single-thread and dual-thread screws. When revision screws were upsized with a 7.5 mm screw, there was no significant difference between the screw types, although the dual-thread screws still had the greatest pull-out strength. The study has limitations; the high-density synthetic bone does not have the same bone structure as vertebral bodies and is not representative of osteoporotic bone. Mixed-thread screws were designed to improve insertional torque and thus mechanical stability in the pedicle, which is responsible for 60% of the pull-out strength of the pedicle screw. This study, however, effectively shows that mixed screw designs had the poorest pull-out strength of all designs and revision screws needed upsizing to give equivalent strength.

Fusing to the pelvis: usually better than not in neuromuscular scoliosis

Achieving the best outcomes following fusion for lumbar degenerative conditions

The authors of this study from Louisville, Kentucky (USA) have investigated which prognostic factors predict the best outcome following a fusion for lumbar degenerative conditions.³ As we all know, there are a few patients who achieve complete symptom remission following any operation. However, in degenerative conditions, the chances of making a complete recovery are usually poorer than in other pathological processes. These investigators built on previous studies that defined a best outcome as a postoperative Oswestry Disability Index (ODI) of 20 or less and pain numerical rating scale (NRS) of 2 or less following surgery as thresholds, equating to patients being unlikely to seek additional medical care or require additional healthcare resources. In this longitudinal cohort study, 396 patients were included from a single hospital. The variables analyzed including age, sex, body mass index, American Society of Anethesiologists (ASA) grade, number of surgical levels, surgical time, preoperative ODI, preoperative back and leg pain, worker’s compensation status, surgical approach, smoking, and the principal diagnosis. At one year postoperatively, 74 patients (19%) were assigned to a ‘best outcomes’ group, as classified by the authors. This group was compared with the remaining patients. The patients who achieved the minimal symptom level were older, had a lower preoperative ODI, and had lower preoperative back pain. They were more likely to have a diagnosis of spondylolisthesis or disc herniation, and were less likely to have a diagnosis of adjacent segment disease or mechanical disc collapse. In terms of intraoperative factors, they underwent fewer surgical levels and had shorter operative times. Stepwise forward regression analysis indicated diagnosis, age, baseline ODI, and number of levels as predictive variables. This study is useful for surgeons when selecting good surgical candidates or counselling patients on the effectiveness of surgery and the chance of symptom resolution and postoperative satisfaction. It also highlights the difficulties in achieving an excellent result when there is such a collection of symptoms and diagnoses.

Sonication and spinal infections

Lordosis is key during TLIF

When kyphosis is tackled surgically, often in the setting of degenerative disease, the restoration of proper sagittal balance is critical in avoiding symptoms caused by the abnormal biomechanics applied to the thoracic and cervical spine, and to the lower limbs. When planning correction, it is difficult to determine the amount of lumbar lordosis that needs to be built into the applied construct. We do not yet have an accurate understanding on how the amount of lordosis applied to the construct influences medium- to late-term outcomes. Transforaminal lumbar interbody fusion (TLIF) is a common technique used to restore lordosis in degenerative deformity correction and fusion procedures. A group from Ljubljana (Slovenia) have carried out a prospective cohort study looking at the functional and radiological outcomes of patients undergoing single level fusion with TLIF in addition to a posterior construct for degenerative spine disease at five years postoperatively.⁵ The group reviewed the outcomes of 57 patients undergoing the procedure at a single centre and examined the relationship between their radiological outcomes and their Oswestry Disability Index (ODI) scores at five years postoperatively. The authors found that an increase in sagittal vertical axis was common and correlated with an elevated ODI score. Patients with a higher pelvic incidence (PI) were found to have a greater difference between ideal and measured lumbar lordosis than those with a lower PI. However, patients with low PI showed higher ODI scores when lumbar lordosis was not corrected to the ideal position, in contrast with those patients with a higher PI at five years postoperatively. This study is limited by being unable to account for the biomechanics of the cervical spine or lower limbs in both compensating for imbalance and contributing to disability, and so these conclusions are unlikely to represent the whole story. The results are perhaps counterintuitive, but certainly this study highlights the major potential pitfall of interbody fusion when lordosis is incompletely considered. As a result, the clinical message from the authors is that anteriorizing TLIF cages is key to preventing these suboptimal outcomes.

Comparing the efficacy of methods for immobilizing the cervical spine

Cervical immobilization is a controversial topic, with some trauma systems continuing to employ triple immobilization, and others choosing to use alternative systems for caring for a potential cervical injury. However, in-line stabilization continues to be commonplace, and a variety of devices are employed in establishing this. In this innovative study from Iowa City, Iowa (USA), the authors used a simulated motion platform to assess the efficacy of a selection of immobilization systems in preventing cervical movement during emergency transfer.⁶ The authors measured the motion that an urgently transferred patient would be subjected to in both a road ambulance and helicopter in various conditions using sensors in the vehicles during real journeys. These data were then fed into a motion simulator, which was able to replicate the motions and vibrations in six directions. The group then chose four different immobilization systems and applied them to 16 healthy volunteers, and used inertial measurement devices to see which system most effectively immobilized the cervical spine. The group found that a simple collar, a vacuum mattress with collar, and a long spinal board (LSB) were all better than a simple trolley and straps in limiting cervical spine rotation. Lateral flexion was most effectively limited by a LSB; however, flexion and extension were not significantly limited by any immobilization system when compared with a simple trolley and straps. The group noted that the test subjects were fit and healthy average-sized male patients, and that results might not be generalizable; however, they suggest that a LSB might be the most practical solution for cervical immobilization, given its speed of application and ease of use. This study does not contribute to the debate around whether immobilization has an impact on patient outcomes, but does show that where this philosophy is followed, LSBs still have a role.

Predicting SSI after instrumented thoracolumbar spine surgery in adults

Surgical site infection (SSI) can be a catastrophic event in the surgical management of spine patients. Any predictive model that facilitates establishing the likelihood of infection will be useful in the care of these complex patients and would help to identify modifiable factors that could be addressed. In this topical study from Maastricht (The Netherlands), the authors collected data regarding potential predictors of SSI in 898 adult patients, all of whom underwent instrumented posterior fusion of the thoracolumbar spine.⁷ Logistic regression of these data was used to develop and validate a model for prediction of SSI using a range of potential covariates after instrumented spine surgery. A total of 60 patients were diagnosed with SSI. After testing each of the potential covariates, the authors developed a predictive model that relied on age, body mass index, American Society of Anesthesiologists (ASA) score, degenerative or revision surgery, and non-steroidal anti-inflammatory drug (NSAID) use. All of these appeared to be independent predictor variables for the risk of SSI. Subsequent statistical testing shows that the model demonstrates good discriminative ability. The use of a patient-specific model will allow individualized risk assessment and counselling, and may guide practical interventions to reduce SSI in this patient cohort.