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|NI FEATURE: FACING ADVERSITY…TOMORROW IS ANOTHER DAY! - LETTER TO EDITOR
|Year : 2017 | Volume
| Issue : 2 | Page : 382-385
Pedicle screw loosening – reassessing short segment fixation in dorsolumbar junctional fractures
Prasad Krishnan1, Rajaraman Kartikueyan1, Sachinkumar M Patel1, Subhasis Deb2
1 Department of Neurosurgery, National Neurosciences Centre, Kolkata, West Bengal, India
2 Department of Orthopedics, Peerless Hospital and B K Roy Research Centre, Kolkata, West Bengal, India
|Date of Web Publication||10-Mar-2017|
Department of Neurosurgery, National Neurosciences Centre, Garia, Kolkata - 700094, West Bengal
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Krishnan P, Kartikueyan R, Patel SM, Deb S. Pedicle screw loosening – reassessing short segment fixation in dorsolumbar junctional fractures. Neurol India 2017;65:382-5
|How to cite this URL:|
Krishnan P, Kartikueyan R, Patel SM, Deb S. Pedicle screw loosening – reassessing short segment fixation in dorsolumbar junctional fractures. Neurol India [serial online] 2017 [cited 2017 Mar 26];65:382-5. Available from: http://www.neurologyindia.com/text.asp?2017/65/2/382/201820
Short segment posterior spinal instrumentation (fixation from one level above to one level below the injured vertebra ) is an accepted  motion preserving technique in the treatment of thoracic and lumbar spinal injuries. Though traditional long segment fixation (two levels above and two levels below) was shown to be rigid enough to stabilize unstable dorsolumbar fractures, short segment instrumentation (SSI) had the advantage of less soft tissue dissection, less blood loss, decreased operating time, and fewer implants. Parker et al., (in 2000) reported excellent fusion of fractured vertebrae with SSI in 46 patients who were followed-up for a mean duration of four and a half years.
Initial studies focused on screw fractures as the major delayed implant related complication, and not much attention was paid to screw loosening because the criteria for defining it were unclear. A recent review in 2015, however, places the rate of loosening between 1–15% in nonosteoporotic spines and upto 60% in osteoporotic spines  for rigid fixation systems.
| » Case 1|| |
A 16-year-old boy presented with bilateral lower limb weakness and urinary retention (for which he was catheterized) following a fall from height. X-rays and magnetic resonance imaging (MRI) revealed a D12 fracture with compression of the lower end of the spinal cord [Figure 1]a and [Figure 1]b. He underwent D11 to L1 fixation with pedicular screw and rod construct with partial reduction of the internal kyphus by ligamentotaxis with D12 laminectomy and posterolateral fusion (intertransverse and interfacet). Over time, he recovered neurologically and began to ambulate. He presented 3 years later with discomfort at the operative site and said that he could feel the implants protruding into his skin. X-ray showed loosening of both the upper screws with implant back out [Figure 1]c. Clinical examination also revealed a visible paraspinal mass [Figure 1]d that was hard to touch. His implants were removed, and as the fractured vertebra showed sclerosis, no refixation was done.
|Figure 1: (a) T2 sagittal magnetic resonance imaging sequence showing D12 fracture with thecal sac and cord compression with signal changes in the latter; (b) preoperative X-ray; (c) postoperative X-ray showing back out of implants with increase in kyphus as compared to the preoperative image; and, (d) clinical photograph showing the implant protruding below the skin (green arrow)|
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| » Case 2|| |
A 23-year-old lady presented with lower limb dysasthesias and weakness with difficulty in passing urine following fall from a height. MRI showed an L1 fracture with retropulsed fragments causing thecal sac compression [Figure 2]a. She underwent D12 to L2 fixation with pedicle screw and rod construct [Figure 2]b. Here too, ligamentotaxis was employed to reduce the internal kyphus and reconstitute the body height followed by L1 laminectomy, bilateral transpedicular decompression and posterolateral fusion. She presented 1year later with complaints of the construct protruding into her skin, which increased during forward bending. X-ray showed gross back out of both the upper screws with loss of height of the fractured body [Figure 2]c. Clinical examination showed the construct lifting the skin in the paraspinal area bilaterally [Figure 2]d. Her implants were revised using longer screws of greater diameter, incorporation of one level more superiorly, and application of a cross connector.
|Figure 2: (a) T2 sagittal magnetic resonance imaging sequence showing L1 fracture with retropulsed fragments and thecal compression; (b) immediate postoperative X-ray; (c) postoperative X-ray 1 year later showing back out of upper screws; and, (d) clinical photograph showing the construct protruding below the skin|
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Pedicle screw loosening is diagnosed by the “halo sign” – a radiolucent area 1 mm wider than the screw diameter on X-rays. However, this is both observer dependent and is also influenced by the quality of the radiograph (penetration as well as exposure time). Sanden et al., have mentioned the sensitivity of this sign as being only 64%, although its specificity has been reported to be 100%. Aghayev et al., using X-rays, reported that change in the angle between the screw axis and the cranial endplate of >1.9 degrees between the immediate and the 6 month postoperative X-ray films is 75% sensitive and 89% specific for diagnosing screw loosening. Galbusera et al., mentioned that the development of a low dose CT scanning protocol would be the best way to assess for pedicle screw loosening.
Loosening of well-placed screws in a nonosteoporotic bone is reported to be due to factors such as stress shielding, suboptimal initial position of the screw, an increased strain at screw- bone interface due to inadequate anterior support,, and finally osteolysis induced by the implant itself. Though intuitively, one would think that osteoporotic bone predisposes to a higher incidence of screw loosening and decreased screw stability, loosening has been not been found to correlate with greater insertional torque – a measure of bone density. Both our cases were also young and had no evidence of osteoporosis.
In both our cases, posterolateral fusion was done. However, a systematic review  stated that this has no protective role in preventing recurrence of kyphosis or implant failure due to the “egg shell phenomenon” – i.e., lack of bone anteriorly following distraction done to reduce the kyphosis (as was done in both our cases). Anterior augmentation done either through a separate approach or by transpedicular bone grafting along with SSI to augment the anterior column may decrease stress on the implant and give better outcomes, as shown in a series by Ramani et al.
Our single surgeon series consisted of 37 cases of isolated dorsolumbar junctional fractures without anterior column augmentation (either D12 or L1) operated from 2004 to 2014. In all of them, a follow up of more than 1 year was available. Twelve patients underwent a short segment fixation. A further 16 patients underwent fixation at two levels above and one level below (a 2a+1b construct), and 9 patients underwent fixation at two levels above and two levels below (a 2a+2b construct) the fracture. None of these patients underwent an anterior stabilization. There were 2 cases of screw loosening as described and both occurred in patients who underwent a short segment fixation (an incidence of 16.66%).
The dorsolumbar junction is an area where the natural dorsal kyphosis changes to a lumbar lordosis. We hypothesize that the lack of anterior augmentation in a “posterior only” approach in this region would cause the reconstituted (by distraction) anterior column to collapse on load bearing. Early ambulation in the postoperative period when accompanied by this loss of correction would shift the onus of weight transmission posteriorly and increase the stress at the bone–screw interface at the upper level causing it to loosen out – a phenomenon described as “cephalo-caudad toggling.” After encountering these complications, we have stopped performing only SSI and posterolateral fusion for dorsolumbar junctional injuries, even though we feel it is still a good option for selected cases of upper dorsal injuries.
SSI alone by a posterior alone approach even with posterolateral fusion is probably not a suitable technique in dorsolumbar junctional fractures due to the risks of implant loosening, particularly in patients who can be ambulated early. The rate of implant loosening is dependent on the length of follow-up, imaging protocol, and the type of instrumentation used. Newer screw designs such as expandable screws and cannulated screws that allow for augmentation with bone cement may decrease the incidence of this complication, However, these need to be evaluated in the long term., Alternatively, including one more level superiorly or adding an interpedicular bone grafting might obviate this problem. More reports from larger series on the results of SSI can elucidate the magnitude of the problem and help in establishing guidelines for the optimal number of segments to be included in the construct.
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Conflicts of interest
There are no conflicts of interest.
| » References|| |
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[Figure 1], [Figure 2]