A biomechanical study comparing polyaxial locking screw mechanisms

Objective

Locking plates have become ubiquitous in modern fracture surgery. Recently, manufacturers have developed locking plates with polyaxial screw capabilities in order to optimise screw placement. It has already been demonstrated that inserting uniaxial locking screws off axis results in weaker loads to failure. Our hypothesis was that even implants specifically designed for polyaxial insertion would experience a drop-off in resistance when using non-perpendicular screws.

Methods

Four different types (one monoaxial and three polyaxial locking plates) of readily available small fragment plates were tested. A biomechanical model was developed to test the screws until failure (defined as breakage and rapid loss of >50% force). Screws were inserted at 0, 10 and 15°.

Results

The standard monoaxial locking mechanism sustained saw a 60% reduction in force (332 N vs. 134 N) when screws were inserted cross-threaded at 10°. Two polyaxial systems saw similar significant reductions in force of 45% and 34%, respectively at 15°. A third system utilizing an end cap locking mechanism showed highly variable results with large standard deviations. Polyaxial screws showed on average only limited reduction at 10 degrees of insertion angle.

Conclusion

Newer designs of locking plates have attractive properties to allow more surgical options during fixation. However this freedom comes at the price of reduced force. Our results show that the safe zone for inserting these screws is closer to 20°, rather than the 30° indicated by the manufacturers. Also, the various polyaxial locking mechanisms seem to influence the overall resistance of the screws.     

Superior gluteal artery injury during iliosacral screw placement

Percutaneous fixation of an unstable pelvic ring injury is becoming a popular method of pelvic stabilization. As posterior pelvic percutaneous techniques become more common, the possibility of iatrogenic complications increases. This case report describes an injury to the superior gluteal artery during percutaneous iliosacral screw insertion and the treatment of this potentially devastating injury.     

Arthroscopic-assisted percutaneous reduction and screw fixation of displaced scaphoid fractures

Many scaphoid fractures can be treated with percutaneous screw insertion, but fracture displacement usually necessitates open reduction. Two surgeons treated 20 consecutive patients with displaced fractures of the scaphoid using arthroscopic-assisted percutaneous screw fixation. Thirteen patients had dorsal (antegrade) and seven had palmar (retrograde) percutaneous screw insertion. At an average follow-up of 18 (range 6-48) months, all of the fractures were healed and there were no implant problems. The early results of arthroscopic-assisted percutaneous screw fixation of displaced fractures of the scaphoid suggest that union can be obtained and good to excellent function achieved predictably without the need for open exposure. Avoidance of an open exposure limits wrist ligament injury and may preserve blood supply. Further evaluation of this procedure is merited.     

Radiographic predictors of symptomatic locking screw removal after treatment of tibial fractures with intramedullary nails

IntroductionThe purpose of this study was to determine the radiographic parameters associated with symptomatic locking screw removal after intramedullary tibial nail insertion. Our hypothesis was that locking screws located closer to joints and those extending longer than the width of the bone result in more symptomatic implant removal.MethodsWe conducted a retrospective cohort study at our Level I trauma center. Seventy-five patients underwent surgical removal of symptomatic locking screws from 2007 to 2014 and were compared with a control group of 122 patients from the same time period who did not undergo symptomatic locking screw removal. Our main outcome measures were radiographic and demographic factors associated with implant removal.ResultsMultivariable regression indicated that a proximal locking screw that started anterolateral and was directed posteromedial was the strongest radiographic predictor of symptomatic removal (odds ratio [OR], 2.83; p = 0.03). An Injury Severity Score <11 (OR, 3.10; p < 0.001) and a body mass index <25 kg/m² (OR, 2.15; p = 0.02) were also associated with locking screw removal. The final prediction model discriminated patients requiring symptomatic locking screw removal with moderate accuracy (area under the receiver operating characteristic curve = 0.73).ConclusionsThe strongest radiographic predictor for symptomatic locking screw removal after tibial nail insertion was the direction of the most proximal locking screw. In contrast to previous research on retrograde femoral nails, tibial locking screws that were closer to the joints were not associated with an increased likelihood of symptomatic screw removal. Clinicians can use these data to help counsel patients regarding the likelihood of symptomatic screws and perhaps to help guide screw placement in cases with multiple options.     

Percutaneous cannulated screw coracoclavicular fixation for acute acromioclavicular dislocations

In 1941, Bosworth introduced a new method of repairing acute complete acromioclavicular dislocations in which a noncannulated coracoclavicular lag-screw was inserted by a blind technique. The author reintroduces the percutaneous coracoclavicular fixation concept. A cannulated screw was specially designed, and the technique of percutaneous insertion under fluoroscopic image control was developed. Fifty-three acromioclavicular dislocations were treated by this method. There were 40 Type III, five Type IV, and three Type V dislocations with distal clavicle fractures in conjunction with complete coracoclavicular ligament tears. Technical failures, which occurred in 17 of 53 patients (32%) included: failed percutaneous insertion in two; early screw pullout in three; late screw pullout in four; subluxation after screw removal in six; and malreduction of Type IV dislocation in two. There was no screw breakage or evidence of migration. Serous drainage occurred in two patients.     

Vertebral rotation during pedicle screw insertion in patients with cervical injury

Background  

Cervical pedicle screws, when misplaced, tend to perforate laterally. One of the reasons for lateral perforation is vertebral rotation during screw insertion. However, actual vertebral rotation during pedicle screw insertion is unknown. In this study, we measured vertebral rotation during pedicle screw insertion in patients with cervical injury.     

Pseudoaneurysm of the superior gluteal artery during iliosacral screw fixation

We present a case of a pseudoaneurysm of the superior gluteal artery following placement of an iliosacral screw in a patient with an unstable pelvic ring fracture. Percutaneous fixation of posterior pelvic ring injuries is becoming a popular method for pelvic stabilization. Several techniques are available t achieve stability and allow early mobilization in patients with displaced pelvic fractures. Percutaneous screw fixation of sacral fractures or sacroiliac joint disruptions should reduce operative and anaesthesia times as well as blood loss, while lowering the risk of surgical wound problems are decreased. The risks with this technique are generally iatrogenic, related with surgeon's lack of experience or imaging difficulties at the time of screw placement. The present case report describes an injury to the superior gluteal arterial system during percutaneous iliosacral screw insertion.     

Maneuvering the retrograde medullary screw in pubic ramus fractures

This report describes an improved percutaneous insertion technique of a retrograde medullary screw in pubic ramus fractures. This improved technique enables the use of a relatively thick retrograde medullary screw in difficult anatomical variations such as narrow or curved pubic ramus. Maneuvering of the retrograde medullary screw gives control of the proximal fragment and can assist in fracture reduction.     

Accuracy of atlantoaxial transarticular screw insertion

STUDY DESIGN: The accuracy and safety of atlantoaxial transarticular screw insertion were evaluated in clinical cases. OBJECTIVES: To evaluate the accuracy and safety of atlantoaxial transarticular screw insertion under lateral fluoroscopic monitoring without opening the joint. SUMMARY OF BACKGROUND DATA: Atlantoaxial transarticular screw fixation has been reported to be biomechanically superior to posterior atlantoaxial wiring techniques. Several clinical series have been reported in the literature. In some reports, the risk of screw insertion in this technique has been pointed out. MATERIALS AND METHODS: Fifty-six consecutive patients with atlantoaxial instability were treated by transarticular screw fixation. One hundred twelve screw insertions in these 56 patients were assessed by surgical record and computed tomographic examination. One screw could not be inserted because of the difficulty of adequate placement during operation; 111 screws were therefore inserted. Adequate position was defined as when the screw perforated the lateral atlantoaxial joint. RESULTS: In this series, neither vertebral artery injury nor spinal cord injury was experienced clinically. One guide wire was broken during drilling with a cannulated drill. Computed tomographic examination demonstrated that 106 screws perforated the atlantoaxial joint. Therefore, 95.5% of screws were adequately positioned. There were two screws positioned lateral to the joint, two medially, and one anteroinferiorly to the joint. CONCLUSIONS: Atlantoaxial transarticular screw insertion using image intensifier without opening the lateral joint was performed safely, but not accurately, in all cases.     

Removal of a broken locking screw from the intramedullary canal using a sliding knot

A locking screw lost in the intramedullary canal during extraction of an intramedullary nail is not an uncommon problem. This report describes a simple procedure using sliding wire knot and an Ender nail for removing a broken locking screw from the intramedullary canal. This method allows the easy removal of a broken locking screw from the intramedullary canal without a special instrument.     

Cortex of the pedicle of the vertebral arch. Part I: Deformation characteristics during screw insertion

OBJECT: Elastic deformation has been proposed as a mechanism by which vertebral pedicles can maintain pullout strength when conical screws are backed out from full insertion. The response to the insertion technique may influence both the extent of deformation and the risk of acute fracture during screw placement. The aim of this study was to determine the deformation characteristics of the lumbar pedicle cortex during screw placement. METHODS: Lumbar pedicles with linear strain gauges attached at the lateral and medial cortices were instrumented using 7.5-mm pedicle screws with or without preconditioning by insertion and removal of 6.5-mm screws. The strains and elastic recoveries of the medial and lateral cortices were determined. RESULTS: Mean medial wall strains tended to be lower than mean lateral wall strains when the 6.5-mm and 7.5-mm screw data were pooled (p = 0.07). After the screws had been removed, 71 to 79% of the deformation at the lateral cortex and 70 to 96% of the deformation at the medial cortex recovered. When inserted first, the 7.5-mm screw caused more plastic deformation at the cortex than it did when inserted after the 6.5-mm screw. Occasional idiosyncratic strain patterns were observed. No gross fracture was observed during screw placement. CONCLUSIONS: Screw insertion generated plastic deformation at the pedicle cortex even though the screw did not directly contact the cortex. The lateral and medial cortices responded differently to screw insertion. The technique of screw insertion affected the deformation behavior of the lumbar pedicles. With myriad options for screw selection and placement available, further study is needed before optimal placement parameters can be verified.     

Working length and proximal screw constructs in plate osteosynthesis of distal femur fractures

BackgroundThe study purpose is to evaluate the working length, proximal screw density, and diaphyseal fixation mode and the correlation to fracture union after locking plate osteosynthesis of distal femoral fractures using bridge-plating technique.MethodsA four-year retrospective review was performed to identify patients undergoing operative fixation of distal femur fractures with a distal femoral locking plate using bridge-plating technique for the metadiaphyseal region. Primary variables included fracture union, secondary surgery for union, plate working length, and diaphyseal screw technique and configuration. Multiple secondary variables including plate metallurgy and coronal plane fracture alignment were also collected.ResultsNinety-six patients with distal femur fractures with a mean age 60 years met inclusion criteria. None of the clinical parameters were statistically significant indicators of union. Likewise, none of the following surgical technique parameters were associated with fracture union: plate metallurgy, the mean working length, screw density and number of proximal screws and screw cortices. However, diaphyseal screw technique did show statistical significance. Hybrid technique had a statistically significant higher chance of union when compared to locking (p = 0.02). All proximal locking screw constructs were 2.9 times more likely to lead to nonunion.ConclusionsPlating constructs with all locking screws used in the diaphysis when bridge-plating distal femur locking plates were 2.9 times more likely to incur a nonunion. However, other factors associated with more flexible fixation constructs such as increased working length, decreased proximal screw number, and decreased proximal screw density were not significantly associated with union in this study.     

Clinical evaluation and computed tomography scan analysis of screw tracts after percutaneous insertion of pedicle screws in the lumbar spine

STUDY DESIGN: An examination of the accuracy of percutaneous pedicle screw placement in the lumbar spine. Using computed tomography scan analysis after implant removal, the screw tracts could be analyzed regarding the degree and direction of screw dislocation. OBJECTIVES: To investigate the misplacement rate and related clinical complications of percutaneous pedicle screw insertion in the lumbar spine. SUMMARY OF BACKGROUND DATA: The feasibility of the external fixation test has been investigated in several studies. Although pedicle screw misplacement has been reported as one of the main complications, there are no reliable data on the misplacement rate for this difficult surgical procedure. METHODS: In this study, 51 consecutive patients with suspected segmental instability were investigated after external transpedicular screw insertion for the external fixation test. Computed tomography scans of all instrumented pedicles from L2 to S1 were performed after screw removal. The screw tracts were analyzed, and the direction and degree of the pedicle violations were noted. In addition, the screw and pedicle angles were measured. RESULTS: Of 408 percutaneously inserted pedicle screws, only 27 screws (6.6%) were misplaced. There were 19 medial pedicle violations, 6 lateral cortical defects, and only 1 cranial and 1 caudal displacement. With respect to the spinal level, S1 showed the highest misplacement rate, with 11 screw dislocations (12%). After surgery, found two nerve root injuries were found. Only one of the injuries (L4) was related to the malposition of a screw. CONCLUSIONS: This study has shown that percutaneous insertion of pedicle screws in the lumbar spine is a safe and reliable technique. Despite the low misplacement rate of only 6.6%, it should be kept in mind that the surgical procedure is technically demanding and should be performed only by experienced spine surgeons.     

Syndesmosis transfixation screw

Instability of the distal tibia-fibular joint necessitates the implant of a fibular tibial transfixation screw. The screw should be placed 2 cm above the anterior syndesmosis. The angle of insertion is 30 degrees upwards from dorsal in relation to the frontal plane. If tibiofibular stability cannot be maintained following anatomical reconstruction of the fibula the transfixation screw must engage the medial tibial cortex. This is imperative, since the transfixation screw cannot withstand the biomechanical forces during motion and bearing of load if the screw penetrates only three corticals.     

C臂机导航下经皮椎弓根钉内固定联合对侧微创TLIF治疗腰椎管狭窄症

目的 探讨导航辅助下经皮椎弓根钉内固定,联合对侧微创经椎间孔椎体间融合术(Transforaminal Lumbar Interbody Fusion,TLIF),治疗腰椎管狭窄症的临床应用.方法 2010年6月至2012年6月,对47例腰椎管狭窄症患者行导航下经皮椎弓根钉内固定,联合对侧微创TLIF术治疗,观察手术时间、出血量以及手术前后的VAS和ODI评分,并将经皮钉侧和开放置钉侧进行配伍对照比较.结果 平均出血量为(420±45) mL;术前VAS评分为(6.85±1.03),术后1月VAS评分为(1.88±0.79);术前ODI评分为(31.6±3.05),术后1月ODI评分为(43.1±3.23).配伍对照研究结果显示,经皮钉组A级101枚、B级9枚、C级2枚,开放置钉组A级86枚、B级21枚、C级4枚、D级1枚.经皮钉组平均置钉时间为(11.25±4.33) min,透视次数为(2.73±0.42)次,开放置钉组平均置钉时间为(15.43±5.65) min,透视次数为(4.12±0.85)次.差异均有统计学意义.结论 导航辅助下经皮椎弓根钉内固定,联合对侧微创TLIF治疗腰椎管狭窄症疗,效果显著,导航辅助下经皮钉组置钉术的准确度优于开放置钉组,透视次数和置钉时间均少于开放组.     

Flexible intramedullary nail removal using a broken screw removal set

The use of flexible titanium intramedullary nails for management of pediatric long bone fractures and some adult fractures has become common. Nail removal after union can be challenging and often requires a larger exposure than nail placement to allow the insertion of grasping devices, such as pliers. A percutaneous technique for the removal of flexible intramedullary nails using extraction bolts from a broken screw removal set is presented. The technique is especially useful when the end of the nail is not prominent and where a hollow reamer from the same set can expose the nail end for the extraction bolt to be used.     

Difficulty in locking head screw removal

Background

Locking plates are an internal fixation material useful in the treatment of bone fractures, which provides effective stabilization between the plate and locking head screws (LHSs) via the locking mechanism. However, difficulty in removing LHSs is relatively common, and such cases can require long surgical procedures or use of special removal equipment. The purpose of this study was to report the incidence and risk factors for difficult removal of LHSs.

Methods

During the 5-year-6-month period from April 2006 to September 2011, 83 locking plates containing a total of 482 LHSs were removed in 80 patients at our institution. Out of 482 LHSs, there were 118 LHSs with a 2.4–2.7 mm diameter, 308 LHSs with a 3.5 mm diameter and 56 LHSs with a 5.0 mm diameter. The incidence of removal difficulty was examined on the basis of screw diameter. In addition, the risk factors were assessed in only LHSs with a 3.5 mm diameter. LHSs with a 3.5 mm diameter were divided into 2 groups, the difficult removal group and the easy removal group, and the data were examined based on age, sex, time between insertion and removal, and screw position. The incidence of removal difficulty in LHSs with a 3.5 mm diameter was examined every 6 months between insertion and removal.

Results

Difficulty in removal was encountered in none (0 %) of 118 LHSs with a 2.4–2.7 mm diameter, 15 (4.9 %) of 308 LHSs with a 3.5 mm diameter, and none (0 %) of 56 LHSs with a 5.0 mm diameter. In only LHSs with a 3.5 mm diameter, the mean ages of the patients in the difficult removal group and the easy removal group were 32.1 and 45.6 years, respectively. The average time between insertion and removal in the difficult removal group and the easy removal group was 529.2 and 389.2 days, respectively. There was a statistically significant difference in age and time between insertion and removal. Removal was difficult in 15 (9.1 %) of 165 LHSs with a 3.5 mm diameter in those with >1 year between insertion and removal.

Conclusion

This study suggests that (1) the use of LHSs with a 3.5 mm diameter is a necessary condition for difficulty in screw removal, and that (2) longer time from internal fixation to removal, and (3) younger age, are risk factors for it. When removing LHSs with a 3.5 mm diameter, appropriate instruments and sufficient training are necessary.     

Standard multiplanar fluoroscopy versus a fluoroscopically based navigation system for the percutaneous insertion of iliosacral screws: a cadaver model

OBJECTIVES: To compare the safety and efficiency of standard multiplanar fluoroscopy (StdFluoro) and virtual fluoroscopy (VirtualFluoro) for use in the percutaneous insertion of iliosacral screws. DESIGN:: Human cadaver study comparing 2 imaging modalities during iliosacral screw insertion; imaging randomized from side to side. SETTING: Bioskills laboratory in a medical school. PARTICIPANTS: Twenty-nine embalmed whole human cadavers without prior hip or pelvic surgery. INTERVENTION: Iliosacral screws were inserted into the S1 bodies using a percutaneous insertion technique. Screws were inserted on one side using StdFluoro, and on the other side, screws were placed using VirtualFluoro. MAIN OUTCOME MEASUREMENTS: Time necessary for imaging preparation, screw insertion, and actual fluoroscopy were recorded. Accuracy and safety of screw placement was assessed using computed tomography and an anatomic dissection of the pelvis. RESULTS:: Fifty-six of 58 iliosacral screws were placed within the desired bony corridor of the posterior pelvis. One screw placed using each method was inserted erroneously, but both were relatively minor deviations. There were no obvious injuries to major vessels or nerve roots. The total surgical time required for preparation of imaging and screw insertion averaged 7.3 minutes using StdFluoro and 6.7 minutes using VirtualFluoro (P = 0.4). Although the time necessary for screw insertion using VirtualFluoro averaged only 3.5 minutes, compared to 7.0 minutes for StdFluoro (P < 0.05), this time savings was offset by that required for application and calibration of tracking devices when using VirtualFluoro. The average fluoroscopy time using StdFluoro method was 26 seconds, whereas that for the VirtualFluoro was only 6 seconds (P < 0.01). CONCLUSIONS: Most of the percutaneous iliosacral screws were safely inserted using StdFluoro and VirtualFluoro, and total surgical times were similar using both methods. As VirtualFluoro continues to evolve, improved efficiency in operative times may be expected. Currently, the most beneficial aspect of using VirtualFluoro during the insertion of percutaneous iliosacral screws appears to be significantly decreased use of fluoroscopy when compared to StdFluoro.     

Retrograde (volar) scaphoid screw insertion-a quantitative computed tomographic analysis

PURPOSE: The benefit of placing the screw tip in the center of the proximal pole of the scaphoid is supported by clinical and biomechanical data. In this investigation we attempted to quantify guidelines for optimal screw insertion into the scaphoid through a volar percutaneous approach using measurements from computed tomography images. METHODS: The parameters of safe insertion of a 3.0-mm cannulated-headed screw (Synthes, Paoli, PA) using a volar (retrograde) insertion technique were measured using quantitative computer analysis of computed tomography images of 15 unfractured scaphoids. In the coronal plane the average screw length for the most radial, most ulnar, and intermediate paths for safe screw insertion and the radial clearance of the trapezium were measured. In the sagittal planes defined by these screw paths the length of the screw, the length and depth of the concavity on the volar surface of the scaphoid, and the distance between a line corresponding to the center of the screw path and the volar surface of the trapezium were measured. RESULTS: Significant differences (1-way analysis of variance) were measured for the average screw lengths for different screw insertion paths in both the coronal and sagittal planes (with the shortest screw lengths observed for the most ulnar starting points) and for the volar clearance of the trapezium, which was greater for a radial screw path. The intermediate screw path-a measure of optimal screw insertion-passed radial to the radial edge of the trapezium in 10 of 15 patients (67%) with an average clearance of 3.9 mm (range, 1.1-7.9 mm) in those patients. The depth of the concavity on the volar surface of the scaphoid averaged 1.6, 2.0, and 2.5 mm in the 3 sagittal planes. CONCLUSIONS: Screw fixation of the scaphoid through a volar approach is hindered by the trapezium, risks cutout through the concavity in the volar surface of the scaphoid, and is most likely to violate the dorsoulnar aspect of the radiocarpal articular surface of the scaphoid if the screw is too long. A relatively radial starting point facilitates placement of the screw tip in the center of the proximal pole and helps avoid the trapezium but drilling or partial excision of the trapezium often may be necessary for optimal screw placement.     

Iliosacral screw insertion using CT-3D-fluoroscopy matching navigation

Background

Percutaneous iliosacral screw insertion requires substantial experience and detailed anatomical knowledge to find the proper entry point and trajectory even with the use of a navigation system. Our hypothesis was that three-dimensional (3D) fluoroscopic navigation combined with a preoperative computed tomography (CT)-based plan could enable surgeons to perform safe and reliable iliosacral screw insertion. The purpose of the current study is two-fold: (1) to demonstrate the navigation accuracy for sacral fractures and sacroiliac dislocations on widely displaced cadaveric pelves; and (2) to report the technical and clinical aspects of percutaneous iliosacral screw insertion using the CT-3D-fluoroscopy matching navigation system.

Methods

We simulated three types of posterior pelvic ring disruptions with vertical displacements of 0, 1, 2 and 3 cm using cadaveric pelvic rings. A total of six fiducial markers were fixed to the anterior surface of the sacrum. Target registration error over the sacrum was assessed with the fluoroscopic imaging centre on the second sacral vertebral body. Six patients with pelvic ring fractures underwent percutaneous iliosacral screw placement using the CT-3D-fluoroscopy matching navigation. Three pelvic ring fractures were classified as type B2 and three were classified as type C1 according to the AO-OTA classification. Iliosacral screws for the S1 and S2 vertebra were inserted.

Results

The mean target registration error over the sacrum was 1.2 mm (0.5–1.9 mm) in the experimental study. Fracture type and amount of vertical displacement did not affect the target registration error. All 12 screws were positioned correctly in the clinical series. There were no postoperative complications including nerve palsy. The mean deviation between the planned and the inserted screw position was 2.5 mm at the screw entry point, 1.8 mm at the area around the nerve root tunnels and 2.2 mm at the tip of the screw.

Conclusion

The CT-3D-fluoroscopy matching navigation system was accurate and robust regardless of pelvic ring fracture type and fragment displacement. Percutaneous iliosacral screw insertion with the navigation system is clinically feasible.