Effect of Insertional Direction of Pedicle Screw on Screw Loosening: A Biomechanical Study on Synthetic Bone Vertebra under a Physiology-like Load

OBJECTIVES

It is now understood that pedicle screw loosening at the implant-bone interface can lead to poor screw-bone interface purchase and decreased fixation stability. Previous biomechanical tests used cadaveric vertebrae and pull-out or torque loads to assess the effect of the insertional direction of pedicle screws on screw loosening. However, these tests faced challenges in matching biomechanical differences among specimens and simulating in vivo loads applied on pedicle screws. This study aimed to evaluate the effect of the insertional direction of pedicle screws on screw loosening using tension-compression-bending loads and synthetic bone vertebrae.

METHODS

Polyaxial pedicle screws were inserted into nine synthetic bone vertebrae in three directions (three samples per group): cranial, parallel, and caudad (-10°, 0°, +10° of the pedicle screw rod to the upper plane of the vertebra, respectively). Pedicle screws in the vertebrae were loaded using a polyethylene block connected to a material testing machine. Tension-compression-bending loads (100N-250N) with 30,000 cycles were applied to the pedicle screws, and displacements were recorded and then cycle-displacement curve was drawn based on cycle number. Micro-CT scans were performed on the vertebrae after removing the pedicle screws to obtain images of the screw hole, and the screw hole volume was measured using imaging analysis software. Direct comparison of displacements was conducted via cycle-displacement curve. Screw hole volume was analyzed using analysis of variance. The correlation between the displacement, screw hole volume and the direction of pedicle screw was assessed by Spearman correlation analysis.

RESULTS

The smallest displacements were observed in the caudad group, followed by the parallel and cranial groups. The caudad group had the smallest screw hole volume (p < 0.001 and p = 0.009 compared to the cranial and parallel groups, respectively), while the volume in the parallel group was greater than that in the cranial group (p = 0.003). Correlation analysis revealed that the insertional direction of the pedicle screw was associated with the displacement (p = -0.949, p < 0.001) and screw hole volume (p = -0.944, p < 0.001).

CONCLUSION

Strong correlations were found between the insertional direction of the pedicle screw and relevant parameters, including displacement and screw hole volume. Pedicle screw insertion in the caudad direction resulted in the least pedicle screw loosening.

Forearm bone mineral density predicts screw loosening after lumbar fusion similar to lumbar Hounsfield unit value in patients with lumbar spondylolisthesis

Preoperative bone density assessment is necessary to predict screw loosening. The forearm BMD is a useful predictor of BMD-related complications after lumbar operation. Our results show that the forearm BMD is as effective a predictor of screw loosening as the lumbar average HU value. Measurement of the forearm BMD may be a useful adjunct in predicting screw loosening following lumbar fusion.

PURPOSE

To determine the relationship between forearm bone mineral density (BMD) and the risk of pedicle screw loosening in patients with lumbar spondylolisthesis.

METHODS

We retrospectively evaluated 270 patients who underwent posterior lumbar interbody fusion for lumbar spondylolisthesis. The patients were divided into two groups on the basis of the with or without loose screws: the loosening group and the non-loosening group. The patient's gender, age, BMI, smoking and diabetes histories, and the operative segment were recorded as the basic information. The Hounsfield unit (HU) value for the BMD of the L1-4 lumbar was measured using computed tomography. The patient's distal one-third of the length of the radius and ulna of the non-dominant forearm was chosen as the site for dual-energy X-ray (DXA) bone density testing.

RESULTS

The rate of screw loosening was 13% at a minimum 12 months follow-up. Average forearm BMD (0.461 ± 0.1 vs 0.577 ± 0.1, p < 0.001) and mean HU value (L1-4) (121.1 ± 27.3 vs 155.6 ± 32.2, p < 0.001) were lower in the screw loosening group than those in the non-loosening group. In multivariate logistic regression analysis, the forearm BMD (OR 0.840; 95%CI 0.797-0.886) and HU value (L1-4) (OR 0.952; 95%CI 0.935-0.969) were independent risk factor for screw loosening. The area under the curve (AUC) for the forearm BMD and HU value for prediction of pedicle screw loosening was 0.802 and 0.811. The forearm BMD cut-off for predicting pedicle screw loosening was 0.543 (sensitivity, 0.800; specificity, 0.864).

CONCLUSIONS

The forearm BMD was an independent risk factor for loosening of the lumbar pedicle screws. The forearm BMD was a valid predictor of pedicle screw loosening in patients undergoing lumbar fusion, as was the CT HU value.

Can You Feel it? - Correlation Between Intraoperatively Perceived Bone Quality and Objectively Measured Bone Mineral Density

STUDY DESIGN

clinical study.

OBJECTIVES

Loosening of pedicle screws is a frequent complication in patients with osteoporosis. The indication for additional stabilization, such as cement augmentation, is more often based on the subjective intraoperative feeling of the surgeon than on a preoperative bone mineral density (BMD) measurement. Aim was to evaluate the correlation of the intraoperative perceived bone quality in comparison to the objectively measured BMD.

METHODS

A total of 62 patients undergoing dorsal stabilization using pedicle screws at a level-1 trauma center were analyzed. The preoperative CT scan measured each instrumented vertebra's pedicle size and BMD. During the surgery, the perceived screw stability was graded by the respective surgeon for each screw.

RESULTS

204 vertebral bodies were evaluated. Looking at all implanted screws a significant correlation between the measured BMD and the perceived screw stability was found (Resident r = .450; R2 = .202; P < .001/Attending r = .364; R2 = .133; P < .001), but there was no significant correlation in the osteoporotic patients (Resident P = .148 / Attending P = .907). The evaluation of the screws implanted in osteoporotic vertebrae showed that the surgeons considered a total of 31% of these screws to be sufficiently stable.

CONCLUSIONS

There was no significant correlation between the measured BMD and the perceived pedicle screw stability in the group with osteopenic / osteoporotic bone (<100 mg/cm³). The results indicate that it is not possible to reliably determine the bone quality and the resulting screw stability in patients with reduced BMD. The preoperative measurement of the BMD should become a crucial part of preoperative planning.

Dynamic cord compression induced by proximal junctional failure and loose pedicle screws after thoracolumbar fusion surgery: a case report

BACKGROUND

One of the common mechanical complications following spinal fusion surgery is proximal junctional failure (PJF). The incidence of neurological deficit associated with PJF has been poorly described in the literature. Here, we report a case in which numbness in the lower extremities was recognized as the first symptom, but the discrepancy in the imaging findings made PJF difficult to diagnose.

METHODS

A 71-year-old female underwent corrective fusion surgery. Three weeks later, she complained of persistent right leg numbness. Standing X-ray showed the back-out of the pedicle screws (PSs) in the upper instrumented vertebra (UIV), but there was no obvious evidence of cord compression on computed tomography (CT), which caused the delay of diagnosis. Five weeks later, magnetic resonance image (MRI) did not show cord compression on an axial view, but there were signal changes in the spinal cord.

RESULTS

The first reason for the delayed diagnosis was the lack of awareness that leg numbness could occur as the first symptom of PJF. The second problem was the lack of evidence for spinal cord compression in various imaging tests. Loosened PSs were dislocated on standing, but were back to their original position on supine position. In our case, these contradictory images led to a delay in diagnosis.

CONCLUSION

Loosened PSs caused dynamic cord compression due to repeated deviation and reduction. Supine and standing radiographs may be an important tool in the diagnosis of PJF induced by dynamic cord compression.

Pedicle screw augmentation in posterior constructs of the thoracolumbar spine: How many pedicle screws should be augmented?

BACKGROUNDS

To evaluate the effects of different pedicle screw augmentation strategies on screw loosening and adjacent segment collapse at the proximal end of long-segment instrumentation.

METHODS

Eighteen osteoporotic (9 male, 9 female donors; mean age: 74.7 ± 10.9 [SD] years) thoracolumbar multi-segmental motion segments (Th11 - L1) were assigned as follows: control, one-level augmented screws (marginally), and two-level augmented screws (fully augmented) groups (3 × 6). Pedicle screw placement was performed in Th12 and L1. Cyclic loading in flexion started with 100-500 N (4 Hz) and was increased by 5 N every 500 cycles. Standardized lateral fluoroscopy images with 7.5 Nm loading were obtained periodically during loading. The global alignment angle was measured to evaluate the overall alignment and proximal junctional kyphosis. The intra-instrumental angle was used to evaluate screw fixation.

FINDINGS

Considering screw fixation as a failure criterion, the failure loads of the control (683 N), and marginally (858 N) and fully augmented (1050 N) constructs were significantly different (ANOVA p = 0.032).Taking the overall specimen alignment as failure criteria, failure loads of the three groups (control 933 ± 271.4 N, marginally 858 N ± 196 N, and full 933 ± 246.3 N were in the same range and did not show any significance (p = 0.825).

INTERPRETATION

Global failure loads were comparable among the three groups and unchanged with augmentation because the adjacent segment and not the instrumentation failed first. Augmentation of all screws showed significant improved in screw anchorage.

A prospective study to identify preoperative serum parameters for spinal implant infection detected by sonication fluid culture

PURPOSE

Low-virulent microorganisms identified on pedicle screws by sonication fluid culture (SFC) are an important cause of implant loosening. While sonication of explanted material improves the detection rate, the risk of contamination exists and no standardized diagnostic criteria for chronic low-grade spinal implant-related infection (CLGSII) are stablished. Besides, the role of serum C-reactive protein (CRP) and procalcitonin (PCT) in CLGSII has not been adequately investigated.

METHODS

Blood samples were collected prior to implant removal. To increase sensitivity, the explanted screws were sonicated and processed separately. Patients exhibiting at least one positive SFC were classified in the infection group (loose criteria). To increase specificity, the strict criteria only considered multiple positive SFC (≥ 3 implants and/or ≥ 50% of explanted devices) as meaningful for CLGSII. Factors which might promote implant infection were also recorded.

RESULTS

Thirty-six patients and 200 screws were included. Among them, 18 (50%) patients had any positive SFCs (loose criteria), whereas 11 (31%) patients fulfilled the strict criteria for CLGSII. Higher serum protein level was the most accurate marker for the preoperative detection of CLGSSI, exhibiting an area under the curve of 0.702 (loose criteria) and 0.819 (strict criteria) for the diagnosis of CLGSII. CRP only exhibited a modest accuracy, whereas PCT was not a reliable biomarker. Patient history (spinal trauma, ICU hospitalization and/or previous wound-related complications) increased the likelihood of CLGSII.

CONCLUSION

Markers of systemic inflammation (serum protein level) and patient history should be employed to stratify preoperative risk of CLGSII and decide the best treatment strategy.

Prediction of Pedicle Screw Loosening Using an MRI-Based Vertebral Bone Quality Score in Patients with Lumbar Degenerative Disease

OBJECTIVE

To explore a new magnetic resonance imaging (MRI)-based bone quality assessment method for predicting pedicle screw loosening in the lumbar spine.

METHODS

We reviewed 174 patients aged ≥50 years who were treated for lumbar degenerative diseases using posterior lumbar interbody fusion. All patients were followed-up for ≥12 months. Based on the presence of radiolucent areas on follow-up MRI images, the patients were divided into loosening and nonloosening groups. The vertebral bone quality (VBQ) score was calculated using T1-weighted MRI images. Demographic data, health history, and radiological parameters were also recorded and compared between the 2 groups. Logistic regression analysis was used to predict the independent risk factors affecting screw loosening.

RESULTS

Screw loosening occurred in 29.88% (52/174) of patients. A total of 83 screws (9.18%, 83/904) were loosened. There were differences in the age, fixation length, fixation at S1, preoperative and postoperative PI-LL, PT, preoperative LL, lowest bone mineral density (BMD), and VBQ scores (P < 0.05) between the nonloosening and loosening groups. In the logistic regression, the VBQ score (OR = 1.02 per point; 95% CI: 1.01-1.03; P < 0.001) was identified as an independent factor influencing screw loosening.

CONCLUSIONS

As an independent risk factor for screw loosening after lumbar spine fusion, the VBQ score provides a new noninvasive protocol for assessing bone quality during surgical planning.

Percutaneous Sacroplasty for Symptomatic Sacral Pedicle Screw Loosening

Background

This study aimed to evaluate the efficacy of fluoroscopy-guided percutaneous sacroplasty in patients with sacral pedicle screws loosening after instrumented spinal fusion.

Methods

We retrospectively reviewed the medical records of 18 patients who underwent percutaneous sacroplasty to treat sacral pedicle screws loosening from January 2016 to December 2019. Imaging studies, visual analog scale (VAS), length of hospital stay, and complications were recorded. The clinical outcomes based on the Oswestry disability index (ODI) and the modified Brodsky's criteria (MBC) were also evaluated to determine the efficacy of percutaneous sacroplasty.

Results

All patients had undergone at least 1 year of follow-up in our institute (range, 12-24 months). The average VAS score was 5.6 (range, 4-7) before surgery and decreased to 1.7 (range, 1-3) at the final visit. All patients were discharged on the next day after surgery. No patients experienced complications, such as cement leakage, deep infection, or neurologic deterioration. All patients achieved good or excellent outcomes based on the MBC. The ODI scores improved from 51.8 preoperatively to 25.6 postoperatively.

Conclusion

Percutaneous sacroplasty was an effective treatment approach for relieving the patient's symptoms caused by sacral pedicle screws loosening and could be a valuable treatment alternative to extensive revision surgery.

Level of clinical evidence

IV.

Novel method for selecting slices of the same cross-sectional view from digital tomosynthesis for monitoring posterior spinal instrumentation

BACKGROUND

A method to evaluate pedicle screw loosening on digital tomosynthesis images is yet to be established owing to lack of methods for selecting slices of the same cross-sectional view. We aimed to develop an objective method for selecting slices of the same cross-sectional view on digital tomosynthesis images.

METHODS

First, an objective method of pixel selection was developed by measuring the size of glass disk and titanium alloy screw on digital tomosynthesis images followed by comparison with the actual sizes. Second, a method for selecting slices of the same cross-sectional view was explored on a bone model with posterior spinal instrumentation using the screw centerline and rod curvature as indicators of the same cross section. The angle between the screw centerline and rod was calculated to verify the accuracy in obtaining the same cross-sectional view. Third, the method for selecting slices of the same cross-sectional view was applied to six patients after posterior lumbar spinal instrumentation.

RESULTS

The pixel selection method enabled objective determination of a pixel on the peripheral lines of objects with an error as low as 200 μm in distance measurements on titanium alloy and glass. The mean differences of rod-screw angles between two slices were less than 1° and were not statistically significant in the bone model and patient images.

CONCLUSION

A method for selecting slices of the same cross-sectional view on digital tomosynthesis images was successfully developed. This method can enable objective and quantitative evaluations of pedicle screw loosening.

Impact of Screw Diameter on Pedicle Screw Fatigue Strength-A Biomechanical Evaluation

OBJECTIVE

Loosening of pedicle screws is a frequently observed complication in spinal surgery. Because additional stabilization procedures such as cement augmentation or lengthening of the instrumentation involve relevant risks, optimal stability of the primarily implanted pedicle screw is of essential importance. The aim of the present study was to investigate the effect of increasing the screw diameter on pedicle screw stability.

METHODS

A total of 10 human cadaveric vertebral bodies (L4) were included in the present study. The bone mineral density was evaluated using quantitative computed tomography and the pedicle diameter using computed tomography. The vertebrae underwent instrumentation using 6.0-mm × 45-mm pedicle screws on 1 side and screws with the largest possible diameter (8-10-mm × 45-mm) on the other side. Fatigue testing was performed by applying a cyclic loading (craniocaudal sinusoidal 0.5 Hz) with increasing peak force (100 N + 0.1 N/cycle) until screw head displacement of 5.4 mm was reached.

RESULTS

The mean fatigue load was 334 N for the 6-mm diameter screws and was increased significantly to 454 N (+36%) for the largest possible diameter screws (P < 0.001). With an increase in the fatigue load by 52%, this effect was even more pronounced in vertebrae with reduced bone density (bone mineral density <120 mg/cm³; n = 7; P < 0.001). The stiffness of the construct was significantly greater in the largest diameter screw group compared with the standard screw group during the entire testing period (start, P < 0.001; middle, P < 0.001; end, P = 0.009).

CONCLUSIONS

Increasing the pedicle screw diameter from a standard 6-mm screw to the largest possible diameter (8-10 mm) led to a significantly greater fatigue load.

Potential contribution of pedicle screw design to loosening rate in patients with degenerative diseases of the lumbar spine: An observational study

BACKGROUND

The majority of published data report the results of biomechanical tests of various design pedicle screw performance. The clinical relevance and relative contribution of screw design to instrumentation stability have been insufficiently studied.

AIM

To estimate the contribution of screw design to rate of pedicle screw loosening in patients with degenerative diseases of the lumbar spine.

METHODS

This study is a prospective evaluation of 175 patients with degenerative diseases and instability of the lumbar spine segments. Participants underwent spinal instrumentation employing pedicle screws with posterior only or transforaminal interbody fusion. Follow-up was for 18 mo. Patients with signs of pedicle screw loosening on computed tomography were registered; logistic regression analysis was used to identify the factors that influenced the rate of loosening.

RESULTS

Parameters included in the analysis were screw geometry, type of thread, external and internal screw diameter and helical pitch, bone density in Hounsfield units, number of levels fused, instrumentation without anterior support, laminectomy, and unilateral and bilateral total facet joint resection. The rate of screw loosening decreased with the increment in outer diameter, decrease in core diameter and helical pitch. The rate of screw loosening correlated positively with the number of fused levels and decreasing bone density. Bilateral facet joint removal significantly favored pedicle screw loosening. The influence of other factors was insignificant.

CONCLUSION

Screw parameters had a significant impact on the loosening rate along with bone quality characteristics, the number of levels fused and the extensiveness of decompression. The significance of the influence of screw parameters was comparable to those of patient- and surgery-related factors. Pedicle screw loosening was influenced by helical pitch, inner and outer diameter, but screw geometry and thread type were insignificant factors.

Can cavity-based pedicle screw augmentation decrease screw loosening? A biomechanical in vitro study

PURPOSE

In an osteoporotic vertebral body, cement-augmented pedicle screw fixation could possibly be optimized by the creation of an initial cavity. The aim of this study is to compare three test groups with regard to their loosening characteristics under cyclic loading.

METHODS

Eighteen human, osteoporotic spine segments were divided in three groups. Flexibility tests and cyclic loading tests were performed with an internal fixator. The screws were fixed after creation a cavity and with cement (cavity-augmented group), without cavity and with cement (augmented group), and without cavity and without cement (control group). Cyclic loading up to 100,000 cycles was applied with a complex loading protocol. Screw loosening was measured with flexibility tests after implantation and after cyclic loading. Cement distribution was visualized from CT scans.

RESULTS

In all groups, range of motion increased during cyclic loading, representing significant screw loosening after 100,000 cycles. In both augmented groups, screw loosening was less pronounced than in the control group. The cavity-augmented group showed only a slight tendency of screw loosening, but with smaller variations compared to both other groups. This may be explained with a trend for a more equal and homogeneous cement volume around each tip for the cavity-augmented group.

CONCLUSION

This study demonstrated that creating a cavity may allow a more equal fixation of all pedicle screws with slight reduction of loosening. However, augmentation only through a cannulated screw is almost equivalent, if care is taken that enough cement volume can be pushed out around the tip of the screw.

Biomechanical effects of posterior pedicle screw-based instrumentation using titanium versus carbon fiber reinforced PEEK in an osteoporotic spine human cadaver model

BACKGROUND

Aim of this biomechanical investigation was to compare the biomechanical effects of a carbon fiber reinforced PEEK and titanium pedicle screw/rod device in osteoporotic human cadaveric spine.

METHODS

Ten human fresh-frozen cadaveric lumbar spines (L1-L5) have been used and were randomized into two groups according to the bone mineral density. A monosegmental posterior instrumentation (L3-L4) using titanium pedicle screws and rods was carried out in group A and using carbon fiber reinforced PEEK in group B. A cyclic loading test was performed at a frequency of 3 Hz, starting with a peak of 500 N for the first 2000 cycles, up to 950 N for 100,000 cycles under a general preload with 100 N. All specimens were evaluated with regard to a potential collapse of the implanted pedicle screws. A CT supported digital measurement of cavities around the pedicle at 3 defined measuring points was performed. Finally, the maximum zero-time failure load of all specimens was determined using a universal testing machine (80% F_max).

FINDINGS

Regarding maximum axial force (group A: 2835 N, group B: 3006 N, p = 0.595) and maximum compression (group A: 11.67 mm, group B: 15.15 mm, p = 0.174) no statistical difference could be shown between the two groups. However, significant smaller cavity formation around the pedicle screws could be observed in group B (p = 0.007), especially around the screw tip (p < 0.001).

INTERPRETATION

Carbon fiber reinforced PEEK devices seem to be advantageous in terms of microscopic screw loosening compared to titanium devices.

Probe versus drill: A biomechanical evaluation of two different pedicle preparation techniques for pedicle screw fixation in human cadaveric osteoporotic spine

BACKGROUND

Aim of this biomechanical study was to investigate the anchorage of pedicle screws in osteoporotic vertebrae using two different preparation techniques (probe versus drill-assisted).

METHODS

Twelve thoracic vertebrae were used for the study. The right and left pedicles of the vertebra were prepared with a thoracic probe or a 3.2 mm drill bit and divided into two groups. A standard titanium (diameter: 5.5 mm, length: 45 mm) pedicle screw was then inserted. All pedicle screws were initially loaded with -25 N to +25 N in the cranio-caudal direction. The load was increased by 5 N every 500 cycles up to a maximum load of 10,000 cycles. Loosening was defined as a displacement of the pedicle screw head of >5 mm. The two groups were compared in terms of maximum number of cycles and maximum force until loosening.

FINDINGS

The pedicle screws prepared with the thoracic probe failed on average after 3819 cycles (SD 3281) and the pedicle screws prepared with the 3.2 mm drill after 3335 cycles (SD 3477). There was no significant difference between the two preparation techniques (P = .797). With regard to the maximum force until loosening, there was also no significant difference between the two techniques (thoracic probe: 61 N (SD 33), 3.2 mm drill bit: 56 N (SD 34), P = .791).

INTERPRETATION

Preparation of the pedicle screw hole either with a probe or drill bit doesn't seem to have an influence on pedicle screw loosening rates in the osteoporotic spine.

Hounsfield units value is a better predictor of pedicle screw loosening than the T-score of DXA in patients with lumbar degenerative diseases

PURPOSE

To compare the performance of using Hounsfield units (HU) value derived from computed tomography and T-score of dual-energy X-ray absorptiometry (DXA) to predict pedicle screw loosening.

METHODS

We reviewed 253 patients aged ≥ 50 years undergoing pedicle screw fixation for lumbar degenerative diseases (LDD). The evaluation of screw loosening: radiolucent zones of ≥ 1 mm thick in X-ray. The criterion for osteoporosis: the lowest T-score ≤ - 2.5. The average HU value of L1-L4 was used to represent lumbar bone mineral density (BMD). The area under receiver operating characteristics curve (AUC) was used to evaluate the performance of predicting screw loosening.

RESULTS

One patient underwent reoperation for screw loosening at 9 months follow-up. At 12 months follow-up, the loosening rate was 30.6% (77/252) in the remaining 252 patients. Osteoporotic patients had higher loosening rate than non-osteoporotic patients (39.3% vs. 25.8%, P = 0.026). The T-score showed no significant difference between loosening group and non-loosening group (- 2.1 ± 1.5 vs. - 1.7 ± 1.6, P = 0.074), and so is the lowest lumbar BMD of DXA (0.83 ± 0.16 g/cm² vs. 0.88 ± 0.19 g/cm², P = 0.054). The HU value was lower in the loosening group (106.8 ± 34.4 vs. 129.8 ± 45.7, P < 0.001). The HU value (OR, 0.980; 95%CI 0.968-0.993; P = 0.002) was the independent influencing factor of screw loosening. The AUC of predicting screw loosening was 0.666 (P < 0.001) for HU value and 0.574 (P = 0.062) for T-score.

CONCLUSIONS

HU value is a better predictor of pedicle screw loosening than T-score of DXA in patients aged ≥ 50 years with LDD. We should not only focus on the DXA measurements when making surgical plans concerning lumbar fixation. These slides can be retrieved under Electronic Supplementary Material.

Ultrasound melted polymer sleeve for improved primary pedicle screw anchorage: A novel augmentation technique

BACKGROUND

Cement augmentation of pedicle screws to prevent screw loosening is associated with significant complications, such as cement leakage or bone necrosis. Therefore, an alternative strategy to improve pedicle screw anchorage has been recently developed: Polymer reinforcement of pedicle screws uses an in situ melted polymer sleeve in order to enhance screw anchorage. This biomechanical study evaluated the effect of polymer-reinforcement by comparing polymer-reinforced pedicle screws to non-augmented as well as cement-augmented screws under cyclic loading.

METHODS

For each of the two comparisons (polymer-reinforced vs. non-augmented screws and polymer-reinforced vs. cement-augmented screws), polymer-reinforced screws and control screws were placed into the left and right pedicle of seven vertebrae (mean age: 74.0 (SD 9.3) years) to allow for pairwise left-right comparisons. Each screw was subjected to cyclic cranio-caudal loading with an initial load ranging from -50 N to +50 N and with stepwise increasing compressive loads (5 N every 100 cycles) until screw loosening.

FINDINGS

Polymer-reinforced pedicle screws resisted a higher number of load cycles until loosening than the contralateral non-augmented control screws (4300 SD 2018 vs. 2457 SD 1116 load cycles, p = 0.015). Screw anchorage of polymer-reinforced pedicle screws was comparable to that of cement augmented control screws (3857 (SD2085) vs. 4300 (SD1257) load cycles until failure, p = 0.64).

INTERPRETATION

Our findings indicate that polymer-reinforcement significantly enhances pedicle screw anchorage in low quality bone and that its effect is similar in size than that of cement augmentation.

Exploratory analysis of predictors of revision surgery for proximal junctional kyphosis or additional postoperative vertebral fracture following adult spinal deformity surgery in elderly patients: a retrospective cohort study

Background

Proximal junctional kyphosis (PJK) following adult spinal deformity (ASD) surgery in elderly patients is markedly influenced by osteoporosis causing additional vertebral fracture and loosening of pedicle screws (PS). This study aimed to investigate the association between mean bone density represented in Hounsfield units (HU) on spinal computed tomography (CT) and revision surgery for PJK or postoperative additional vertebral fracture following ASD surgery in elderly patients.

Methods

The subjects were 54 ASD patients aged 65 years or older who were treated with correction and fusion surgery of four or more levels and could be followed for 2 years or longer. Bone density was measured before surgery using lumbar dual-energy X-ray absorptiometry (DXA) and spinal CT in all patients. The patients were divided into group A (n = 14) in which revision surgery was required for PJK or additional vertebral fracture and group B (n = 40) in which revision surgery was not required. We retrospectively investigated incidences of PJK, additional vertebral fracture, and PS loosening, perioperative parameters, radiographic parameters before and after surgery, and osteoporosis treatment administration rate.

Results

No significant difference was noted in young adult mean (YAM) on DXA between groups A and B, respectively (P = 0.62), but the mean bone densities represented in HU of the T8 (P = 0.002) and T9 (P = 0.01) vertebral bodies on spinal CT were significantly lower in group A, whereas those of the L4 (P = 0.002) and L5 (P = 0.01) vertebral bodies were significantly higher in group A. The incidence of PJK was not significantly different (P = 0.07), but the incidence of additional vertebral fracture was significantly higher in group A (P < 0.001). The incidences of uppermost PS loosening within 3 months after surgery were 71% and 40% in groups A and B, respectively (P = 0.04).

Conclusions

In elderly patients who required revision surgery, the mean bone densities of vertebral bodies at T8 and T9 were significantly lower. The mean bone density represented in HU on spinal CT may be useful for risk assessment of and countermeasures against revision surgery after ASD surgery in elderly patients.

A case of pedicle screw loosening treated by modified transpedicular screw augmentation with polymethylmethacrylate

We report a case of pedicle screw loosening treated by modified transpedicular screw augmentation technique using polymethylmethacrylate (PMMA), which used the anchoring effect of hardened PMMA. A 56-year-old man who had an L3/4/5 fusion operation 3 years ago complained of continuous low back pain after this operation. The computerized tomography showed a radiolucent halo around the pedicle screw at L5. We augmented the L5 pedicle screw with modified pedicle screw augmentation technique using PMMA and performed an L3/4/5 pedicle screw fixation without hook or operation field extension. This modified technique is a kind of transpedicular stiffness augmentation using PMMA for the dead space around the loosed screw. After filling the dead space with 1-2 cc of PMMA, we inserted a small screw. Once the PMMA hardened, we removed the small screw and inserted a thicker screw along the existing screw threading to improve the pedicle screws' pullout strength. At 10 months' follow-up, x-ray showed strong fusion of L3/4/5. The visual analogue scale (VAS) of his back pain was improved from 9 to 5. This modified transpedicular screw augmentation with PMMA using anchoring effect is a simple and effective surgical technique for pedicle screw loosening. However, clinical analyses of long-term follow-up and biomechanical studies are needed.