Lumbar spinal arthrodesis is an accepted surgical option in the treatment of spondylosis to achieve stability and to eliminate pain. The indications for lumbar fusion include isthmic or degenerative spondylolisthesis, spinal stenosis with segmental instability, facet joint arthropathy, discogenic pain, iatrogenic destabilization, recurrent disc herniation, and spinal deformity.5,26 There is broad discretion for each surgeon as to the specific approach, technique, and instrumentation to be used, despite large-scale randomized clinical trials.27,28 With the advent of biologics and techniques, the fusion rates have increased;6,15,16 however, a proportional improvement of the clinical outcome is unproven and remains anecdotal.19
A pedicle screw–based dynamic stabilization system aims to alter the mechanical loading of lumbar spinal motion segments. By unloading the disc and allowing a limited range of motion, the system theoretically provides needed segmental stability. Beneficial clinical outcomes associated with the Dynesys Dynamic Stabilization System (Zimmer Spine) have only been reported in small series.8,12,21,24,25,29,30 Furthermore, the long-term outcomes and protective effects on adjacent-segment disease remains uncertain.11,13,17
One of the major concerns in dynamic stabilization is the incidence of screw loosening. Pedicle screws inserted without bone fusion may not hold against the weight-bearing forces of the human lumbar spine. Unlike arthrodesis, the durability of these dynamic constructs depends on both the anchoring of the pedicle screws and the material strength. Therefore, the long-term follow-up of and management strategies for screw loosening in patients in whom a dynamic stabilization has been used deserve more attention than conventional spinal fusion procedures.
In the present study we collected data in consecutive patients treated with the Dynesys Dynamic Stabilization System; the mean follow-up duration exceeded 3 years (37 months). The incidence of screw loosening and breakage was computed. Additionally, the risk factors and their effects on clinical outcomes were analyzed. This is the largest series in the literature of dynamic stabilization achieved with pedicle screws (a total of 658 screws) focusing on the management and outcome of loosened or broken pedicle screws.
Methods
Enrollment of Patients
The cases of consecutive patients who underwent posterior decompression and placement of a dynamic stabilization (Dynesys) system between September 2006 and May 2009 were retrospectively reviewed.
The surgical indications included symptomatic lumbar spinal stenosis, spondylolisthesis lower than Grade 1, recurrent disc herniation, and degenerative disc disease with discogenic pain associated with intractable radicular pain, back pain, or neurogenic claudication. The patients were considered candidates for surgery only if they had persistent symptoms refractory to conservative treatment for more than 4 months. This study was approved by the institutional ethics committee.
Surgical Technique
All surgeries were performed after induction of general anesthesia and with patients in a prone position of neutral lumbar lordosis with adequate cushioning. Two senior authors in the current study (H.C. and W.C.H.) performed all the procedures. Midline skin and fascia incisions were made for a standard laminectomy and foraminotomy. A medial partial facetectomy was performed in severe cases of foraminal stenosis to ensure that the exiting nerve root was adequately decompressed. Bilateral fascia incisions were then made for dissection through the intermuscular (Wiltse) plan. Under the guidance of intraoperative fluoroscopy, the Dynesys screws (closed-head, nonhydroxyapatite-coated, titanium alloy) were placed transpedicularly via the insertion point at the base of the transverse process without additional facet destruction. Special caution was taken to avoid pullback or reinsertion of the screws. The length and diameter of the screws were determined by preoperative CT scanning and confirmed intraoperatively. The polycarbonate urethane spacers were tailored for only slight distraction of the facet joints with independent measurement using the pedicle distance gauge. The spacers were then inserted together with the polyester cords following the standard technique. Two Exudrains were applied and maintained for 3 days.
Clinical Evaluation
The data were prospectively collected and retrospectively reviewed. Clinical outcome was assessed preoperatively and approximately 3, 12, and 24 months postoperatively. The assessment was performed by 2 special nursing assistants using the VAS for back and leg pain, as well as the ODI, under the supervision of the physicians.
Radiological Evaluation
Every patient underwent preoperative imaging, including plain and dynamic radiography, MR imaging, and CT scanning. Follow-up imaging included standard anteroposterior, lateral, and flexion-extension radiography at approximately 1, 2, 3, 6, 12, and 24 months after surgery. An initial halo sign (radiolucent line around the implant > 1 mm wide) followed by a double halo sign on later radiographs or CT scans was defined as screw loosening.2,12,22 The measurement was performed using quantitative measurement analysis software (SmartIris, Taiwan Electronic Data Processing Co.). Radiographic interpretations, including halo sign and osseous integration, were made by independent radiologists and two neurosurgeons. For any ambiguity, CT scans were used for the final determination of halo sign and discussion to attain consensus.
Statistical Analysis
All statistical analyses were performed using the SPSS software (SPSS Inc.). Independent t-tests and paired t-tests were used for continuous variables, and the Fisher exact test was applied for categorical data. A p value of 0.05 was considered statistically significant.
Results
Of the 160 consecutive patients who underwent 1- or 2-level dynamic stabilization with Dynesys system, 126 patients (78.8%), in whom 658 screws were placed, completed the clinical and radiological evaluations for at least 2 years postoperatively. There were 72 men (57.1%) and 54 women (42.9%) whose mean age was 60.4 ± 11.8 years at the time of surgery (Table 1).
Clinical and demographic characteristics*
| Characteristic | Value (%) |
|---|---|
| no. of patients | 126 |
| sex | |
| male | 72 (57.1) |
| female | 54 (42.9) |
| mean age (yrs) | 60.4 ± 11.8 |
| mean follow-up (mos) | |
| imaging | 26.6 ± 7.4 |
| clinical | 37.0 ± 7.1 |
| no. of instrumented levels | |
| 1 | 50 (39.7) |
| 2 | 76 (60.3) |
* Mean values are presented ± SD.
The mean clinical follow-up duration was 37.0 ± 7.1 months, and the mean radiological follow-up duration was 26.6 ± 7.4 months. In the 126 patients, 50 (39.7%) underwent a 1-level and 76 (60.3%) underwent a 2-level stabilization. The distributions of index levels were as follows: 1 (0.8%) at L2–3; 4 (3.2%) at L3–4; 35 (27.8%) at L4–5; 10 (7.9%) at L5–S1; 3 (2.4%) at L2–4; 58 (46.0%) at L3–5; and 15 (11.9%) at L4–S1 (Table 2).
Distribution of treated levels
| Instrumentation | Level | No. of Patients (%) |
|---|---|---|
| 1 level* | L2–3 | 1 (0.8) |
| L3–4 | 4 (3.2) | |
| L4–5 | 35 (27.8) | |
| L5–S1 | 10 (7.9) | |
| 2 levels† | L2–4 | 3 (2.4) |
| L3–5 | 58 (46.0) | |
| L4–S1 | 15 (11.9) |
* One-level surgery was performed in 50 patients (39.7%).
† Two-level surgery was performed in 76 patients (60.3%).
Screw Loosening
Among the 658 screws, radiologically evident loosening (identified by the presence of a halo sign and confirmed by double halo sign or CT scanning) was found in 31 screws (4.7%) in 25 (19.8%) patients.
The mean age of patients with screw loosening was significantly greater than that in younger individuals (64.8 ± 8.8 vs 59.3 ± 12.2, respectively; p = 0.036). The sex composition and the number of levels stabilized were not significantly different (p = 0.897 and p = 0.674, respectively) between patients with and without screw loosening. In patients with Type 2 diabetes mellitus, the rate of screw loosening was significantly higher than that in euglycemic patients (36.0% vs 15.8%, respectively; p = 0.024) (Table 3).
Comparison of data loosened screw and intact group*
| Variable | Total | Screw Loosening (%) | p Value | |
|---|---|---|---|---|
| Yes | No | |||
| no. of patients | 126 | 25 (19.8) | 101 (80.2) | |
| age (years) | 60.4 ± 11.8 | 64.8 ± 8.8 | 59.3 ± 12.2 | 0.036 |
| sex | 0.897 | |||
| male | 72 | 14 (19.4) | 58 (80.6) | |
| female | 54 | 11 (20.4) | 43 (79.6) | |
| level | 0.674 | |||
| 1 | 50 | 9 (18.0) | 41 (82.0) | |
| 2 | 76 | 16 (21.1) | 60 (78.9) | |
| serum glucose status | 0.024 | |||
| Type 2 DM | 25 | 9 (36.0) | 16 (64.0) | |
| euglycemia | 101 | 16 (15.8) | 85 (84.2) | |
| mean preop scores | ||||
| VAS back pain | 6.3 ± 3.1 | 6.1 ± 2.7 | 6.4 ± 3.2 | 0.709 |
| VAS leg pain | 7.1 ± 2.7 | 7.0 ± 2.6 | 7.1 ± 2.7 | 0.868 |
| ODI (%) | 52.4 ± 18.4 | 50.8 ± 17.7 | 52.8 ± 18.7 | 0.625 |
| mean 24-mo postop scores | ||||
| VAS back pain | 2.5 ± 2.7 | 3.4 ± 2.9 | 2.3 ± 2.6 | 0.088 |
| VAS leg pain | 2.1 ± 2.9 | 2.1 ± 3.1 | 2.1 ± 2.8 | 0.962 |
| ODI (%) | 22.3 ± 20.6 | 24.9 ± 19.5 | 21.7 ± 20.9 | 0.519 |
* Mean values are presented ± SD. Bolded values indicate statistical significance. Abbreviation: DM = diabetes mellitus.
Among the 25 patients in whom screws loosened, 22 (88%) were identified within 6.6 months of surgery, whereas 3 were found at 9.5, 10.0, and 48.0 months. In 18 (72%) of the patients, the loose screws were located in the inferior level of construct, whereas in the other 7 (28%) the loose screws were seen in the superior level of the construct. None of the loose screws were found in the middle portion of construct in cases of 2-level stabilization.
Clinical Outcome
The overall clinical outcomes, measured by VAS (back and leg pain scores) and ODI, were significantly improved after surgery at 3, 12, and 24 months after surgery compared with status preoperatively. Furthermore, patients had significant improvement in VAS back, VAS leg, and ODI scores regardless of whether screw loosening was present or absent (Fig. 1).
There was significant clinical improvement at each postoperative evaluation time point compared with preoperative status. There were no statistical differences between patients with and without screw loosening at all evaluation time points: VAS for back pain (A); VAS for leg pain (B); and ODI (C).
At the 24-months follow-up interval, the patients with screw loosening had significantly improved clinical scores of 3.4 ± 2.9, 2.1 ± 3.1, and 24.9 ± 19.5 (VAS back, VAS leg, and ODI, respectively) compared with preoperatively (6.1 ± 2.7, 7.0 ± 2.6, and 50.8 ± 17.7; p = 0.001, p < 0.001, and p < 0.001, respectively). Likewise, at 24 months postoperatively patients without screw loosening had significantly improved scores of 2.3 ± 2.6, 2.1 ± 2.8, and 21.7 ± 20.9 compared with preoperatively (6.4 ± 3.2, 7.1 ± 2.7, and 52.8 ± 18.7; p < 0.001, p < 0.001, and p < 0.001, respectively) (Table 3). The improvements were also significant at 3 and 12 months postoperatively (p = 0.001 and p < 0.001, respectively). There were no significant differences between the 2 groups at all evaluation time points (that is, preoperatively, and at 3, 12, and 24 months postoperatively; all p > 0.05) (Fig. 1).
Management of Loose and Broken Screws
There were 31 (4.7%) loose screws in 25 patients (19.8%) (Figs. 2 and 3) and 3 broken screws (0.46%) in 3 patients (2.38%) (Figs. 4 and 5). The comparison of patients with these problematic screw subgroups is demonstrated in Table 4. Due to the small numbers, statistical analysis was not performed.
Summary of data in 26 patients with problematic screws
| Variables | Initial Radiographic Halo Sign* | Initially Solid | ||
|---|---|---|---|---|
| Osseous Integration | Persistent Halo Sign | Broken Screw | Broken Screw† | |
| no. of patients | 6 | 17 | 2 | 1 |
| age (yrs) | 64.5 | 64.0 | 73.0 | 34.0 |
| sex | ||||
| male | 3 | 9 | 2 | 1 |
| female | 3 | 8 | 0 | 0 |
| instrumented levels | ||||
| 1 | 4 | 5 | 0 | 0 |
| 2 | 2 | 12 | 2 | 1 |
| serum glucose status | ||||
| Type 2 DM | 2 | 7 | 0 | 0 |
| euglycemia | 4 | 10 | 2 | 1 |
| screw location | ||||
| top | 1 | 6 | 0 | 0 |
| bottom | 5 | 11 | 2 | 1 |
| 1st identified (mos) | 4.1 | 4.6 | 3.3 | 24.5 |
| imaging follow-up (mos) | 40.0 | 24.8 | 27.9 | 24.5 |
* Indicates screw loosening.
† One patient had a broken screw that had no halo sign of loosening on the initial radiographs until 24.5 months postoperation.
Representative case of a 67-year-old woman who underwent dynamic stabilization at L4–5, and in whom a loosened right L-5 pedicle screw (B, arrowhead) was documented 5 months postoperatively. There was no clinical deterioration. A–D: Radiographs obtained at 1, 5, 17, and 33 months after surgery, respectively. Osseous integration and filling of the previously loosening tract around the screw were demonstrated on radiographs at 17 (C) and 33 (D) months postoperatively. E–G: Reconstructed CT scans acquired 33 months postoperatively (sagittal view, L-4 level; axial views, L-5 level).
Representative case of a 70-year-old man who underwent dynamic stabilization at L4–5 and in whom screw loosening occurred bilaterally at the L-5 pedicles (B–E, arrowheads) 3.5 months after surgery. These screws remained loose at 18 months postoperatively and asymptomatic. A–C: Radiographs acquired 1, 3.5, and 18 months postoperatively, respectively. D and E: Sagittal and axial CT scans acquired at 18 months after surgery.
Representative case of a 78-year-old man who underwent placement of a 2-level (L4–S1) stabilization system and in whom screw loosening occurred at bilaterally at the S-1 pedicles (B, C, E, and G, arrowheads) 4 months after surgery. The screws remained loose and the left S-1 screw was found to be broken (C, arrow) 28 months postoperatively, although the patient experienced no symptoms. At the same time, bilateral L-4 pedicle screws were also noted to be loose (arrowheads). A–C: Radiographs acquired 2, 4, and 28 months after surgery, respectively. D–G: Reconstructed CT scans obtained 28 months postoperatively (coronal, L-4; axial, L-5; axial, S-1).
Representative case of a 34-year-old man who underwent placement of a 2-level (L3–5) stabilization system and in whom the left L-5 pedicle screw broke (B, arrow) 24.5 months after surgery without clinical symptoms. A and B: Radiographs acquired 1 and 24.5 months postoperatively, respectively.
In the 25 patients with postoperative screw loosening, it was regular follow-up radiography that identified the problem; there were no clinically relevant complaints. Because adverse effects were not apparent, none of the loose screws were removed. Continuous follow-up without intervention was exercised, and no further migration of the screws was documented. Surprisingly, 6 (24%) of the patients had remarkable radiographic improvement on later evaluations. The radiographic disappearance of a halo sign on later follow-up examinations was confirmed by CT, and therefore, osseous integration was indicated (Fig. 2). These 6 patients remained asymptomatic in a longer mean follow-up of 40 months. Conservative management was planned for patients in whom postoperative radiographs demonstrated a halo sign, unless further migration of screws or clinical deterioration indicated surgery.
The 3 patients in whom there was screw breakage have all been asymptomatic to date and continue to be followed up. Two of the broken screws were found in patients with screw loosening (Fig. 4), and the other one was noted in a patient without radiographically identified loosening (Fig. 5).
No revision surgery was performed for screw misplacement or neurological deterioration. However, 1 diabetic patient (0.79% in 126) suffered from wound infection and underwent surgery for debridement and removal of implants 28 months after the primary surgery.
Diabetes Mellitus and Screw Loosening
There were 25 diabetic patients (19.8%), all with Type 2 disease, in this series. Patients with diabetes had a significantly higher rate (36.0%) of screw loosening than patients with normal serum glucose (15.8%) (p = 0.024) (Table 3). For diabetic patients who underwent dynamic stabilization, the preoperative serum HbA1c (glycated hemoglobin) was significantly higher in cases involving loose screws than in those without screw loosening (8.92 ± 1.78 vs 7.31 ± 1.19, p = 0.018). Given an HbA1c of less than or equal to 8.0% as the cutoff value,10,20 patients with well-controlled diabetes had a significantly lower chance of screw loosening (28.6% vs 71.4%, p = 0.021) than those with poorly controlled diabetes (HbA1c ≥ 8.1%) (Table 5).
Screw loosening in 25 patients with diabetes*
| HbA1c at Op | Screw Loosening | Loosening Rate | p Value | |
|---|---|---|---|---|
| Yes | No | |||
| no. of patients | 9 | 16 | ||
| ≥8.1 % | 5 | 2 | 71.4% | 0.021 |
| ≤8.0 % | 4 | 14 | 28.6% | |
| mean ± SD | 8.92 ± 1.78 | 7.31 ± 1.19 | 0.018 | |
* Mean values are presented ± SD. Bolded values indicate statistical significance.
Discussion
This study analyzed 658 screws used to place the Dynesys for dynamic stabilization in 126 patients with lumbar spondylosis. In a mean follow-up period of 3 years, 31 screws (4.7%) in 25 (19.8%) patients became loose. All the patients with loose screws were asymptomatic and identified radiographically during regular follow-up. Older age and diabetes were risk factors of screw loosening. The clinical outcomes were equally improved regardless of screw loosening. Due to the clinical silence, these patients were managed conservatively with continuous follow-up. In 6 (24%) of these patients with loose screws improvement of osseous reintegration was demonstrated on later radiographic studies during a mean follow-up period of 40 months. Therefore, nonsurgical management of these loose screws is a reasonable option in dynamic stabilization. This inference of management is opposed to a common belief underpinning the management of loose screws in a fusion construct, but it requires further study to corroborate. However, this report is the largest series to date of its type, and the radiographic improvement with osseous reintegration of these loose screws is reported for the first time in the literature of dynamic stabilization.
Screw loosening is an inherent issue in dynamic stabilization for lumbar spondylosis. In rigid fixation constructs the pedicle screws need to provide stabilizing strength only until bone fusion is achieved. A dynamic construct coupled with the bony interface, however, requires more long-term durability because no bone graft incorporation ever occurs. Therefore, the longer the follow-up period, theoretically the higher the rate of screw loosening will be. Furthermore, screw loosening in static constructs is often related to pseudarthrosis requiring revision. Excess loading in a rigid fixation without bone fusion may cause implant breakage. Dynamic constructs, however, are designed to shift rather than bear the entire loading of the lumbar spine. The cost of this flexibility is the need for more material strength and anchoring force at the interface. In the present study, we have demonstrated that loose screws in a dynamic stabilization construct can be asymptomatic and there is a chance of bone regrowth.
There are several series involving the Dynesys system with different incidences of screw loosening. Stoll et al.25 reported on 10 loose screws (3.6%) in 8 (11%) of 73 patients at a mean follow-up of 38 months, and one patient underwent revision surgery at 14.5 months postoperatively. Grob et al.8 reported on screw loosening in 4 (13%) of 31 patients at the 2-year follow-up, and 2 of these patients underwent reoperation. Welch et al.29 documented 1 loose screw in 101 patients at the 1-year follow-up. Würgler-Hauri and colleagues30 noted 2 loose screws and 4 broken screws in 36 patients at the 1-year follow-up. Schnake et al.24 reported 4 (16. 7%) of 24 patients with 4 loose screws (4.2%), including one that was broken, at the 2-year follow-up. Schaeren and coworkers23 observed no new case of implant failure between 2 and 4 years in the continuous follow-up of 19 patients in the same series. In a mean follow-up period of 54 months, Di Silvestre et al.3 reported 4 (13.8%) of 29 patients treated for degenerative scoliosis with asymptomatic screw loosening. Our group reported a screw loosening rate of 19.7% (14 of 71 patients) and 4.6% (17 of 368 screws).12 Most of the rates reported for screw loosening range from 10% to 20% of the patients and seldom require revision surgery. This feature of dynamic stabilization is distinct from arthrodesis series, in which screw loosening occurs at a lower rate but often causes pseudarthrosis warranting reoperation.4,18,31
The etiological and risk factors of the screw loosening in dynamic stabilization are not extensively addressed in the literature. In a previous report, age greater than 55 years has been proposed as a risk factor without adverse effects on clinical outcomes.12 In the present study, the mean age of patients with loosened screws was 5.5 years older than that in patients without screw loosening (64.8 ± 8.8 vs 59.3 ± 12.2, respectively; p = 0.036). Differences in sex and 1- or 2-level constructs were not significant between cases involving loose screws and those not involving them. Because patients with inadequate bone density should not be considered candidates for dynamic stabilization, patient age may be of importance as it might correlate with decreased bone mineral density.
The data in this study warranted serum glucose control prior to implantation of the dynamic stabilization system. Diabetic patients are at higher risk of screw loosening (36.0% vs 15.8%, respectively; p = 0.024) than nondiabetic patients. Furthermore, patients with better-controlled diabetes (HbA1c ≤ 8.0%) preoperatively had a significantly lower rate of screw loosening (28.6% vs 71.4%, respectively; p = 0.021). Additionally, disease status in the 2 diabetic patients with loose screws and osseous reintegration was also well controlled (Table 4). Hyperglycemia in patients is reported to cause reduced bone formation and diminished mechanical strength of newly formed bone.1,7,9,14 Osseous integration between implant and host bone interfaces has been extensively studied in dental surgery but seldom addressed in the spine literature.10,20
More loose screws were identified in the lower vertebral body of the construct than the upper (72% vs 28%, respectively). In particular, in 2-level stabilization constructs, none of the loose screws were found in the middle vertebral body. This finding is consistent with several other reports of the Dynesys system. Stoll et al.25 reported that in all cases with loose screws, the most cranial and/or the most caudal screws were involved. Di Silvestre et al.3 noted that asymptomatic radiolucent lines found in 4 patients were all around the S-1 screws. This is indirect biomechanical evidence to suggest that marginal segments of the dynamic construct share more load during movement. The lower end of the construct is at higher risk of screw pullout, which deserves more caution in surgical planning and execution.
The actual timing of the occurrence of screw loosening is uncertain. The majority (88%) of the loose screws in the present study were found within 6.6 months of surgery, and the remaining 3 cases were at 9.5, 10.0, and 48.0 months. Similar findings have been reported by Stoll et al.,25 in whose series most loose screws appeared in the early postoperative period (< 6 months), and none appeared later than 1 year. In a 4-year follow-up study of Dynesys system–treated patients, Schaeren and colleagues23 found no new case of screw loosening between 2 and 4 years. In the current series, we identified only 1 case of delayed-onset loosening (bilateral L-4 pedicle screws) in addition to the preexisting bilateral S-1 pedicle screws mentioned earlier. The frequent early-onset screw loosening may indicate that poor implant–host bone osseous incorporation is caused more by medical conditions (for example, age, osteopenia, cigarette smoking, diabetes, or steroid use). Additionally, loosening of the implant can depend on surgical techniques of screw insertion. However, screw loosening is multifactorial and these inferences need future investigation for corroboratation.
The indication for dynamic stabilization is a caveat of this series. The authors still advocate static constructs with interbody bone grafts for patients with obvious spinal instability causing mechanical back pain. Any patient with spondylolisthesis of Grade 1 or higher is not considered a candidate for the dynamic stabilization. We tend to use the dynamic construct for younger patients who have undergone repeated discectomies or wide partial facetectomies, but there is a gray zone between arthrodesis and decompression alone of the lumbar spine. At times, restriction rather than elimination of the motion of the lumbar spine may be sustainable. Although most of the patients in the present series had satisfactory outcomes, our study does not provide sufficient data to support the optimal indications of the dynamic stabilization device. Future studies with stratification of each specific indication for the dynamic stabilization device are necessary to clarify its optimal application.
There are limitations to this study. This cohort was composed of patients with lumbar spondylosis of various kinds, including unstable stenosis, degenerative disc disease, mild spondylolisthesis, recurrent disc herniation, and iatrogenic instability. The surgical indications for dynamic stabilization in the study were not uniform, which might contribute differently to screw loosening. The length and diameter of screws were not analyzed. Cigarette smoking, bone quality, and body mass index may also have been confounding factors. The risk factors were not fully disclosed. Moreover, the actual time of occurrence of the screw loosening was earlier than that caught on imaging studies due to clinical silence. The true effect on clinical outcomes can only be determined by longer follow-up. The in vivo elasticity and durability of the dynamic constructs are uncertain.
Conclusions
Screw loosening in dynamic stabilization is not uncommon (4.7% of screws in 19.8% patients). Older age patients or those with diabetes have higher rates of screw loosening. Screw loosening can be asymptomatic and have a chance of osseous integration on later follow-up. Although there were few adverse effects on clinical outcomes, longer-term follow-up is required to examine the impact of loosened screws.
Disclosure
The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.
Author contributions to the study and manuscript preparation include the following. Conception and design: JC Wu, Tu. Acquisition of data: JC Wu. Analysis and interpretation of data: Tsai, CL Wu. Drafting the article: Tsai, CL Wu. Critically revising the article: Ko, Tu. Reviewed submitted version of manuscript: Ko, JC Wu, Tu. Approved the final version of the manuscript on behalf of all authors: Ko. Statistical analysis: Tsai. Administrative/technical/material support: Huang, Cheng. Study supervision: Huang, Cheng.
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