Universal Spine System Research Articles

Comparison of the biomechanical performance of three spinal implants for treating the wedge-shaped burst fractures

Spinal fracture is a serious problem impairing life quality, associating with low back pain and many other chronic diseases. Among all the spinal fractures, the rate of thoracolumbar fractures is the highest and accounts for approximately 90%. Although surgical treatment is an effective approach, it is still unclear which treatment method performs the best. The aim of the present study was to investigate the biomechanical performance of three spinal implants for treating the thoracolumbar wedge-shaped burst fractures using the finite element (FE) method. FE model of the T12/L1/L2 spinal segment was created from CT images and the thoracolumbar wedge-shaped burst fractures were created by removing some elements in the anterior part of L1. The FE models of the traditional system, the universal spine system (USS) and the cortical bone trajectory (CBT) system were created and their biomechanical performances were evaluated. The results revealed that among the three fixation systems, the highest von-Mises stress occurred in the CBT system. Under all the loading scenarios except for the lateral bending, the maximal von-Mises stress was higher when the USS system rather than the traditional system was applied. The average displacement around the fracture site was the highest in the CBT system. Except for the lateral bending, the average displacement around the fracture site was higher when the USS system rather than the traditional system was applied. For all the fixation approaches, the highest von-Mises stress always occurred at the screw junctions. The present study provided important data for the treatment of thoracolumbar wedge-shaped burst fractures. For example, the traditional spinal system is preferentially selected for the thoracolumbar wedge-shaped burst fracture of L1.

The feasibility of short-segment Schanz screw implanted in an oblique downward direction for the treatment of lumbar 1 burst fracture: a finite element analysis

BackgroundTo evaluate the biomechanical properties of short-segment Schanz screw implanted in an oblique downward direction for the treatment of lumbar 1 burst fracture using a finite element analysis.MethodsThe Universal Spine System (USS) fixation model for adjacent upper and lower vertebrae (T12 and L2) of lumbar 1 vertebra burst fracture was established. During flexion/extension, lateral bending, and rotation, the screw stress and the displacement of bone defect area of the injured vertebrae were evaluated when the downward inserted angle between the long axis of the screws and superior endplate of the adjacent vertebrae was set to 0° (group A), 5° (group B), 10° (group C), and 15°(group D). There were 6 models in each group.ResultsThere were no significant differences in the maximum screw stress among all the groups during flexion/extension, lateral bending, and rotation (P > 0.05). There were no significant differences in the maximum displacement of the bone defect area of the injured vertebrae among all the groups during flexion/extension, lateral bending, and rotation (P > 0.05).ConclusionShort-segment Schanz screw implanted in an oblique downward direction with different angles (0°/parellel, 5°, 10°, and 15°) did not change the maximum stress of the screws, and there was a lower risk of screw breakage in all groups during flexion/extension, lateral bending, and rotation. In addition, the displacement of the injured vertebra defect area had no significant changes with the change of angles.

A comparative study between the Universal Spinal System® (USS) and the CD Horizon® Legacy™ (CDH) in the management of thoracolumbar fractures.

Introduction: For the treatment of unstable non-osteoporotic thoracolumbar fractures, the clinical and radiological outcome of short-segment fixation with the USS™ – Universal Spine System (DePuy Orthopedics, Inc., Warsaw, IN, USA) and the CD HORIZON® LEGACY™ 5.5 Spinal System, (Medtronic Sofamor Danek USA, Inc., Memphis, TN, USA) were compared.Methods: From March 2015 to January 2016, 40 consecutive patients with unstable traumatic thoracolumbar fractures who met our inclusion criteria were treated with either the USS system or CDH Legacy system. Segmental kyphosis angle (SKA) and anterior body height (ABH) of fractured vertebrae, and ASIA Impairment Scale (AIS) were evaluated. Radiological fusion was confirmed with plain X-rays and when indicated with computerized tomography (CT).Results: The mean immediate kyphotic angle correction was 16.6° for the Schanz and 6.4 for the Legacy system, and the immediate mean anterior vertebral body height correction was 0.92 cm for the Schanz and 0.51 cm for the Legacy system. Our study shows a significant statistical difference between Schanz and Legacy systems regarding post-operative segmental kyphosis and height correction immediately postoperatively, at 6 months and at one-year follow-up (p-value < 0.005). The degree of pain reduction and neurological improvement was not influenced by the screw system.Conclusion: Usage of USS in thoracolumbar fracture as a short-segment fixation led to a near anatomical reduction when compared to the Legacy system. However, there was no advantage regarding pain reduction and neurological outcome.

Anatomical Safety of the U.S.S. Pedicular Hook-Screw in the Adult Thoracic Spine

Introduction Pedicle hooks have been extensively used in spine surgery. One of the major concerns of such instrumentation is the pullout of the implants. Recently, a screw that anchors the pedicle was added to secure the hook, and to improve both pullout strength as well as purchase. An anatomical study was performed on human cadavers to assess the anatomical safety of these implants. From this study, we can conclude that the use of screws to hold the pedicle hooks is generally safe. Further studies should be done to assess this in deformed spines. Materials &amp; Methods: Eleven embalmed human cadavers with a mean age of 76 years (6 male bodies and 5 female bodies) were used for this study. The same staff spine surgeon performed all surgical procedures. The thoracic pedicles were instrumented bilaterally starting from T1 down to T12 with an AO Universal Spine System Pedicular Hook Screw. Abandoning the procedures was permitted whenever the safety of any neural structures was jeopardized or the implant could not be placed correctly. The surgeon's comments were documented. For subsequent analyses after instrumentation, the spines were harvested en-bloc and later dissected into individual vertebrae. Two independent blinded observers did a visual inspection to grade the hook and screw positions. All specimens were then radiographed individually in three dimensions (antero-posterior, medio-lateral &amp; cranio-caudal). Two blinded observers independently evaluated the hook and screw positions following the visual classification system. Only the antero-posterior and lateral radiographs were provided, because only these would be available in a clinical situation. Results Thirty-five implant placements (ten in T11, eighteen in T12 and seven in all other levels) were abandoned prior to the surgical procedure. Six hooks were requested to be repositioned. Four implant placements were abandoned after drilling the screw hole (three screws with a medial deviation, and one screw with a lateral deviation). Of the remaining 225 implants, 29 were lost during harvesting and preparation, leaving 196 implants for further evaluation. One hundred and eighty three implants were considered safe by the surgeon at the end of the surgical procedure, 13 were believed not to be safe and were removed. No screws were found to be disrupting the integrity of the neural structures in the canal or in the foramen. A comparison between the visual and radiological evaluations of both the screw and hook positions was performed. The visual evaluation showed an inter-observer agreement of 85% for the hooks and 78% for the screws. The radiographic evaluation showed an agreement of 75% for the hooks, and of 43% for the screw between observers, a value considerably lower than expected. When comparing each of the two radiological evaluations to the “gold standard,” only 62% to 67% of the hooks and 42% to 48% of the screws had an agreement. Conclusion Overall, the use of U.S.S. pedicle hook-screws is suggested to be safer than either the conventional pedicle hook, which is often malpositioned inside the canal, or the classical pedicle screw, which is too large for the thoracic pedicles.

Effets biomécaniques de la vertébroplastie dans les fractures thoraco-lombaires explosées traitées par ostéosynthèse trans-pédiculaire : une analyse par modèle d’éléments finis

ObjectiveTo investigate the biomechanical effects of augmentation of the fractured vertebrae after posterior instrumentation. MethodsBy simulating internal fixation plus augmentation with cement, eight tridimensional, anatomically detailed finite element models of the T11-L1 functional spinal junction were developed. Two kinds of models for mimicking different severity of the fracture were established according to the Denis’ classification. Augmentation with cement was conducted after reduction with posterior fixation using an universal spine system. These models assumed a three-column loading configuration as follows: compression, anteflexion, extension, lateroflexion and axial rotation. Stress of the implants and spine was evaluated. ResultsData showed that for severely fractured models, augmentation apparently decreased the von Mises stresses by 50% for the rods and 40% for the screws, about 40% for the inferior endplate of T11, and 50% for the superior endplate of L1 in vertical compression and other load situations. ConclusionWe should only apply vertebroplasty to prevent correction loss and implants failure based on the fact that it could significantly decrease stress of the instrumentations and spine when the vertebrae are severely fractured. Level of evidenceLevel IV. Biomechanical study.

Biomechanical effects of vertebroplasty on thoracolumbar burst fracture with transpedicular fixation: A finite element model analysis

ObjectiveTo investigate the biomechanical effects of augmentation of the fractured vertebrae after posterior instrumentation. MethodsBy simulating internal fixation plus augmentation with cement, eight tridimensional, anatomically detailed finite element models of the T11-L1 functional spinal junction were developed. Two kinds of models for mimicking different severity of the fracture were established according to the Denis’ classification. Augmentation with cement was conducted after reduction with posterior fixation using a universal spine system. These models assumed a three-column loading configuration as follows: compression, anteflexion, extension, lateroflexion and axial rotation. Stress of the implants and spine was evaluated. ResultsData showed that for severely fractured models, augmentation apparently decreased the von Mises stresses by 50% for the rods and 40% for the screws, about 40% for the inferior endplate of T11, and 50% for the superior endplate of L1 in vertical compression and other load situations. ConclusionWe should only apply vertebroplasty to prevent correction loss and implants failure based on the fact that it could significantly decrease stress of the instrumentations and spine when the vertebrae are severely fractured. Level of evidenceLevel IV, biomechanical study.

Application triangular osteosynthesis for vertical unstable sacral fractures

The aim of this study was to explore the operative technique and effectiveness of triangular osteosynthesis for vertically unstable sacral fractures. From January 2009 to December 2010, 25 vertical unstable sacral fractures in 22 patients were treated with triangular osteosynthesis using the combination of universal spine system and iliosacral screw in our institution. Patients were followed up prospectively with routine post-operation visits for clinical and radiographic examination. The reduction quality was evaluated according to Matta criterion. Clinical function outcome, and nerve function outcome was evaluated by Majeed and Gibbons criterion. All patients were followed up for an average of 14 months (range 8-26 months). Local infection was seen in two patients, and both were healed with debridement and antibiotics. Patients experienced early weight-bearing ability, no fracture reduction loss, and hardware loosening, while all fractures appeared as bone union at the final follow-up. According to Matta criterion for fracture reduction, the results were excellent in 18 sides, good in 6 sides, and fair in 1 side. According to Majeed functional scoring at last follow-up, the results were excellent in 13 cases, good in 6 cases, fair in 2 cases, and poor in 1 case. Neurologic impairment was noted in 13 patients pre-operative, six patients achieved complete recovery, six patients achieved partial improvement, one patient did not have any improvement at the last follow-up. Triangular osteosynthesis is a relatively new fixation for vertical unstable sacral fractures, the fixation is rigid, permits early full weight-bearing, and nerve decompression can be performed, which facilitates function recovery.

Stress Distribution on the Screws in Posterior Lumbar Fusion of Isthmic Spondylolisthesis with 2- or 3-Vertebra Fixation Techniques: A Biomechanical Cadaveric Study

Two- or three-vertebra fixation techniques are both used in the treatment of spondylolisthesis. However, the number of spinal segments that should be implanted in spondylolisthesis reduction and fixation is still controversial, and there are no published reports on stress distribution on the screws with 2- or 3-vertebra fixation techniques. Understanding stress distribution in screws would be of potential great clinical importance and supply more biomechanical evidence in surgery. The aim of this study was to compare and quantitatively analyze the stress distribution on the screws in 2- or 3-vertebra fixation techniques in cadaveric models of spondylolisthesis. Sixteen fresh specimens of human lumbar spines were used in this study. The spondylolisthesis model was generated by Panjabi method and fixed with the SINO universal spine system by 2- (group A) or 3-vertebra (group B) fixation technique. Rectangular electrical resistance strain gauges were fixed at upper and lower surface of the root of screws bilaterally. The samples were tested under flexion/extension, left/right lateral bending, and axial compression loading. Stress on the screws was measured by strain gauge monitor, respectively. Under the five different loading conditions, the stress could be compressive stress or tensile stress. Under the compression, flexion, and bending loading condition, the stress in reduction screws in group A is higher than in group B (P < 0.01). However, under the extension loading condition, stress of lower surface in reduction screws in group A is 49% lower than in group B. With regard to the anchor screws, under flexion and lateral bending conditions, stress in group A is lower than in group B (P < 0.05). Under compression and extension loading conditions, stress in group A is slightly higher than in group B, but no significant difference is detected. In most loading conditions, stress in reduction screws in 2-vertebra fixation technique was higher than in 3-vertebra fixation technique. The 3-vertebra fixation technique might effectively reduce stress on the reduction screws, and decrease the probability of fatigue fractures of the screws.

Balloon Kyphoplasty Combined With Posterior Instrumentation for the Treatment of Burst Fractures of the Spine-1-Year Results

Clinical and radiologic follow-up of patients with osteoporosis with painful, traumatic A3-type vertebral compression fractures in the thoracic and lumbar spine. Prospective study over the period of 1 year. Level I Trauma Center. Twenty-five patients with osteoporosis (14 women and 11 men) and painful, traumatic A3-type fractures were treated with Balloon Kyphoplasty in combination with a short-segment posterior fixation (using the Universal Spine System). Twenty-one patients returned for their 1-year follow-up visit. X-rays were taken preoperatively, postoperatively, and at 3, 6, and 12 months' follow-up to evaluate vertebral height and local Cobb angle. Pain was measured using the self-reporting Visual Analogue pain Scale (VAS). Disability was measured using the Oswestry Disability questionnaire (ODI). The mean pain score (VAS) improved significantly from pretreatment to posttreatment from 7.8 +/- 2.2 (5.6-10) to 4.9 +/- 2.1 (2.8-8.0) (P < 0.001). It improved further to 2.9 +/- 1.4 (2.1-4.3) at 3 months and increased slightly after 1 year (3.8 +/- 1.9) (1.9-4.7). Limitation of daily activities (ODI) improved significantly from 88% (78-100) to 35% (15-48) at 3 months (P < 0.05). Improvement was maintained at 1 year at 36.5% (10-42). At 1-year follow-up, maintenance of the height restoration and kyphotic deformity correction was found. Balloon Kyphoplasty in combination with short-segment posterior instrumentation in traumatic A3 vertebral fractures led to a significant reduction in pain and disability. The combination of both surgical techniques was able to restore and maintain vertebral body height and correction of angular deformity. This technique might offer important safety advantages over an invasive anterior-posterior approach.

Are Surgeonsʼ Preferences for Instrumentation Related to Patient Outcomes?

Although many techniques for the surgical treatment of scoliosis have been described, we are not aware of any randomized trials that have compared implant systems. The relationship between surgeons' preferences for implants and patient outcomes is unknown. The purpose of the present study was to compare quality of life and curve correction associated with use of the Moss Miami system and the Universal Spine System for spinal fusion in patients with adolescent idiopathic scoliosis. The present study was a double-blind, randomized clinical trial. All adolescent patients with idiopathic scoliosis who were scheduled for posterior instrumentation and arthrodesis with or without anterior release were screened for eligibility. Patients were randomly allocated to treatment with either the Moss Miami system or the Universal Spine System. The primary outcome measure for this trial was Quality of Life Profile for Spinal Disorders. Of the 129 subjects who were enrolled in the trial, sixty (95%) of sixty-three from the Universal Spine System group and sixty (91%) of sixty-six from the Moss Miami group were included in the final analysis. Two years postoperatively, the total Quality of Life score did not differ by the clinically important value of 5.5 between the two groups (difference, 1.07; 95% confidence interval, -3.67 to 5.82; p = 0.66). The percentage of Cobb angle correction was not significantly different for thoracic curves (55.1% +/- 18.3% for the Moss Miami system group, compared with 54.1% +/- 18.7% for the Universal Spine System group) (difference, -1%; 95% confidence interval, -7% to 5%; p = 0.77) or lumbar curves (45.4% +/- 24.6% for the Moss Miami system group, compared with 41.9% +/- 26.8% for the Universal Spine System group) (difference, -4%; 95% confidence interval, -16% to 11%; p = 0.57). Although surgeons were more satisfied with the Universal Spine System (difference, 42%; 95% confidence interval, 29% to 55%; p < 0.0001), satisfaction ratings were not related to any surgical outcomes. The Moss Miami system and the Universal Spine Systems provided similar quality of life and curve correction. Surgeon preference may be an unreliable means of selecting implants from the patient outcomes perspective.

Fixed lumbar apical vertebral rotation predicts spinal decompensation in lenke type 3c adolescent idiopathic scoliosis after selective posterior thoracic correction and fusion

Retrospective radiographic review of surgically treated double major curves (Lenke type 3C) in adolescent idiopathic scoliosis. To evaluate the role of selective posterior thoracic correction and fusion in double major curves with third generation instrumentation and to identify preoperative radiographic parameters that predict postoperative coronal spinal decompensation. Traditionally the surgical treatment of double major curves consists of fusion of both the thoracic and the lumbar curve. Few attempt to perform selective thoracic fusion in this curve pattern because of the potential to create spinal imbalance. Thirty-six patients with Lenke type 3C curves underwent a selective posterior thoracic correction and fusion with either Cotrel-Dubousset instrumentation or the Universal Spine System. Radiographs were evaluated to assess coronal and sagittal balance, curve flexibility, and curve correction at a minimum follow up of 2 years. Postoperative coronal spinal decompensation was investigated with respect to preoperative radiographic parameters on standing anteroposterior (AP), standing lateral radiographs, thoracic and lumbar supine side-bending radiographs. Coronal spinal decompensation was defined as plumbline deviation of C7 of more than 2 cm with respect to the centre sacral vertical line (CSVL) within 2 years of surgery. Twenty-six patients (72%) showed satisfactory frontal plane alignment patients (28%) showed coronal spinal decompensation. Significant group differences, however, were identified for lumbar apical vertebral rotation, measured according to Perdriolle (La scoliose. Son êtude tridimensionnelle. Maloine, Paris, pp 179, 1979) (A 16 degrees , B 22 degrees , P = 0.02), percentage correction (derotation) of lumbar apical vertebrae in lumbar supine side-bending films in comparison to standing AP radiographs (A 49%, B 27%, P = 0.002) and thoracic curve flexibility (A 43%, B 25%, P = 0.03). High correlation was noted between postoperative decompensation and derotation of lumbar apical vertebrae in pre-operative lumbar supine side-bending films with a critical value of 40% (Pearson correlation coefficient; P = 0.62, P < 0.001). Ten of 36 patients (28%) with Lenke type 3C adolescent idiopathic scoliosis showed coronal spinal decompensation of more than 2 cm after selective posterior thoracic correction and fusion. Lumbar apical vertebral derotation of less than 40% provided the radiographic prediction of postoperative coronal spinal imbalance. We advise close scrutiny of the transverse plane in the lumbar supine bending film when planning surgical strategy.

Surgical treatment for scoliosis extending to main thoracic spine by keyvertebral-screws technique (KVST)

Objective To introduce a key-vertebral-screws technique (KVST) in the surgical treatment of scoliosis extending to main thoracic levels, and to find the role of fulcrum bending in predicting the result of surgical treatment for scoliosis by this technique. Methods Seventeen consecutive patients with scoliosis extending to main thoracic spine, who underwent pure posterior fusion without anterior or posterior release by KVST between January 2004 and July 2005 were evaluated for fulcrum bending flexibility, surgical correction rate, fulcrum bending correction index (FBCI) in main thoracic curves. Universal Spine System (USS) instrumentation was used in 15 cases, Monarch in another 2 cases. The severity of the curves was measured by Cobb's method using RadWork 6.0 software. Preoperative standing AP radiographs, preoperative fulcrum bending anterioposterior (AP) radiographs, postoperative standing AP radiographs, and most recent follow-up standing AP radiographs for spine were measured and recorded. All the data were analyzed with two-sample paired t-test by Origin 7.0 software. Results Infection and neurological complications were not noted. No major complications were found. Just one case had some axial back pain, which got a full recovery from physiotherapy for 2 weeks. In the X-ray, there was an average correction of 71.5% of the fused main thoracic curves, which had no significant lose of correction in final follow-up. For the whole fused main thoracic curves, the fulcrum bending flexibility were lower to operation correction rate ( P = 0.013). The average FBCI was 123%. From the data, the more rigid curves (especially fulcrum bending flexibility <50%), the more correction rate operation could get, compared with fulcrum bending flexibility. Conclusion (1) KVST is a good method in the surgical treatment of thoracic scoliosis, which can get satisfying result with lower medical cost. (2) Fulcrum bending flexibility is lower than operative correction rate by KVST in main thoracic curves ( P<0.05). In the more rigid curves assessed by fulcrum-bending radiograph, the operative corrective could be gained, especially in the curves which FBCI is lower than 50%.

Pullout strength of anterior spinal instrumentation: a product comparison of seven screws in calf vertebral bodies

A lot of new implant devices for spine surgery are coming onto the market, in which vertebral screws play a fundamental role. The new screws developed for surgery of spine deformities have to be compared to established systems. A biomechanical in vitro study was designed to assess the bone-screw interface fixation strength of seven different screws used for correction of scoliosis in spine surgery. The objectives of the current study were twofold: (1) to evaluate the initial strength at the bone-screw interface of newly developed vertebral screws (Universal Spine System II) compared to established systems (product comparison) and (2) to evaluate the influence of screw design, screw diameter, screw length and bone mineral density on pullout strength. Fifty-six calf vertebral bodies were instrumented with seven different screws (USS II anterior 8.0 mm, USS II posterior 6.2 mm, KASS 6.25 mm, USS II anterior 6.2 mm, USS II posterior 5.2 mm, USS 6.0 mm, USS 5.0 mm). Bone mineral density (BMD) was determined by quantitative computed tomography (QCT). Failure in axial pullout was tested using a displacement-controlled universal test machine. USS II anterior 8.0 mm showed higher pullout strength than all other screws. The difference constituted a tendency (P = 0.108) when compared to USS II posterior 6.2 mm (+19%) and was significant in comparison to the other screws (+30 to +55%, P < 0.002). USS II posterior 6.2 mm showed significantly higher pullout strength than USS 5.0 mm (+30%, P = 0.014). The other screws did not differ significantly in pullout strength. Pullout strength correlated significantly with BMD (P = 0.0015) and vertebral body width/screw length (P < 0.001). The newly developed screws for spine surgery (USS II) show higher pullout strength when compared to established systems. Screw design had no significant influence on pullout force in vertebral body screws, but outer diameter of the screw, screw length and BMD are good predictors of pullout resistance.

A New Stand-Alone Anterior Lumbar Interbody Fusion Device: Biomechanical Comparison with Established Fixation Techniques

Established lumbar fixation methods were assessed biomechanically, and a comparison was made with a new stand-alone anterior lumbar interbody cage device incorporating integrated anterior fixation. To compare the stability of a new stand-alone anterior implant (Test-device) with established fixation methods to assess its suitability for clinical use. Our hypothesis being that the Test-device would provide stability comparable to that provided by an anterior cage when supplemented with posterior pedicle screw fixation. It is accepted that the use of rigid pedicle screw instrumentation increases the chance of achieving a solid fusion, but its use may be associated with a significant increase in postoperative morbidity caused by disruption of the posterior musculature. It is also evident that this increased fusion rate is generally not associated with increased clinical success. This dilemma has led to a search for a solution and to the development of the Test-device anterior lumbar interbody device. The kinematic properties of either the L3-L4 or L4-L5 lumbar motion segment of 8 cadaveric lumbar spines have been tested using the following sequence of fixation: intact, Test-device, Test-device and translaminar facet screws (TS), Cage and TS, Cage and Universal Spine System (USS), and Cage and small stature USS. All fixation techniques except the cage and TS decreased (P < 0.05) range of motion (ROM), neutral zone (NZ), and elastic zone (EZ), and increased (P < 0.05) stiffness in comparison to the intact motion segment in all test modes. There was a significant increase (P < 0.01) in the ROM, NZ, and EZ, and decrease in the stiffness of the cage and TS group in comparison to all other stabilization techniques in flexion and rotation. There was no significant difference in the ROM, NZ, EZ, and stiffness between the Test-device and cage and USS groups in flexion, extension, and bending. The Test-device resulted in a significantly lower EZ (P < 0.05) and a significantly higher stiffness (P < 0.05) in rotation than all other fixation methods. The Test-device alone provided similar and the Test-device and TS higher stability than the pedicle screw constructs evaluated. These results support progression to clinical trials using the Test-device as a stand-alone implant.

Biomechanical evaluation of an expansive pedicle screw in calf vertebrae

The main objective of the present study is to evaluate biomechanically a newly designed expansive pedicle screw (EPS) using fresh pedicles from calf lumber vertebrae in comparison with conventional pedicle screws, (CDH) CD Horizon, Universal Spine System pedicle screw (USS) and Tenor (Sofamor Denek). Pull-out and turning-back tests were performed on these pedicle screws to compare their holding strength. Additionally, revision tests were undertaken to evaluate the mechanical properties of EPS as a "rescue" revision screw. A fatigue simulation test using a perpendicular load up to 1,500,000 cycles was also carried out. The results showed that the turning back torque (Tmax) and pull-out force (Fmax) of EPS screws were significantly greater than those of USS, Tenor and CDH screws (6.5x40 mm). In revision tests, the Fmax of both types of EPS screws (6.5x40 mm; 7.0x40 mm) were significantly greater than that of CDH, USS, and Tenor screws (P<0.05). Furthermore, no screws were broken or bent at the end of fatigue tests. The findings from the current study suggest that expansive pedicle screws can significantly improve the bone purchase and the pull-out strength compared to USS, Tenor and CDH screws of similar dimensions before and after a failure simulation.

Pull-out strength of the suprapedicle claw construct: a biomechanical study

The pull-out of the superior screw is a well recognized problem in anterior instrumentation of the spine for scoliosis. A biomechanical pull-out study of anterior vertebral body screw in cadaveric thoracic spine was therefore designed to investigate and compare the pull-out strength of three different anterior vertebral body fixations using the AO Universal Spine System: simple bicortical screw, bicortical screw with an opposite washer (sometimes called pull-out resistant nut), and a new construct made of a bicortical screw with the addition of a suprapedicular hook on the same vertebra (or claw construct). The T4 to T9 vertebral bodies from six human cadavers (total of 36 specimens) were instrumented with three different instrumentation constructs after measuring the bone mineral density of each individual vertebra. After stabilization of the vertebral bodies, the screws were extracted employing a material testing system using axial pull-out. The maximum axial forces were recorded at the time of the construct failure. The mean ultimate fixation strength (UFS) values after being adjusted for bone mineral density and vertebral body diameter were 631, 711, and 1244 N for the three different constructs, respectively (screw alone, screw with an opposite washer, and screw with a suprapedicle claw). The difference in UFS was not significant for the first two constructs tested (screw alone and screw with an opposite washer). However, the difference in ultimate fixation strength between the claw and the other constructs was highly significant (P<0.0001). Specimens with low BMD did not benefit as much from claw construct as the ones did with a normal BMD. The failure mode of each construct was described, but was in neither case judged dangerous for the spinal cord. This study shows that the suprapedicle claw construct improves the pull-out strength of an anterior vertebral body screw by 80%, and changes the mode of failure so as not to rely only on the screw characteristics or solely on the vertebral body. By adding a suprapedicle hook in a claw configuration, one may prevent superior screw pull-out in anterior spine surgery for scoliosis.

Does Gender Affect Outcome of Surgery in Adolescent Idiopathic Scoliosis?

A retrospective follow-up study comparing 2 matched groups. To compare long-term results of operative treatment of adolescent idiopathic scoliosis (AIS) between males and females matched for age, curve magnitude and type, and the instrumentation used. It has been suggested that in AIS, the spine is more rigid in males than in females, because brace treatment fails to halt progression of the curves in males. However, there are no studies comparing the results of surgical treatment of AIS between males and females. Thirty male and female pairs were matched for age (+/-1 year), major curve magnitude (+/-5 degrees), curve type according to the King classification, and the instrumentation used (Harrington in 11 pairs, Cotrel-Dubousset in 9 pairs, and Universal Spine System in 10 pairs). The mean age at the time of operation was 16.2 +/- 2.6 years for the males and 15.5 +/- 2.3 years for the females. The mean follow-up times were 14.3 years (range 6.7-23.0 years) and 14.1 year (range 6.4-23.7 years), respectively. Radiographs were obtained presurgery and after surgery, at the 2-year follow-up assessment, and at the final follow-up visit. In addition, a physical examination was performed, and the Scoliosis Research Society (SRS) questionnaire was completed. Spine mobility and nondynamometric trunk strength were measured at the final follow-up visit. Before surgery, the mean Cobb angle of the thoracic curve was 55 degrees (range 42-83 degrees) in the males and 56 degrees (range 43-80 degrees) in the females. In bending radiographs, the major curve reduced significantly more in females than in males (P = 0.027). The mean angles of the thoracic and lumbar curves were similar at all follow-up visits. The final correction of the thoracic curves was 30% (range -19-65%) in the males and 33% (range -7-71%) in the females (not significant). No significant difference was observed in thoracic kyphosis or lumbar lordosis between the study groups at the final follow-up. The total SRS questionnaire score averaged 96 (range 66-113) in the males and 95 (range 75-108) in the females. None of the males and 2 of the females reported back pain often or very often in the questionnaire. Nondynamometric trunk strength measurements corresponded with reference values, on average, and were similar in both groups. Abnormal trunk side bending tended to be more common in the males than in the females (43% vs. 23%, P = 0.10, not significant). Curves in males appear to be more rigid than in females with AIS. However, posterior surgery for AIS provides similar short and long-term results in males and females.

Accuracy of Pedicle Screw Placement in Thoracic Spine Fractures

Transpedicular instrumentation in the thoracic spine is technically difficult because of small pedicle diameters. There are not many studies assessing in vivo accuracy, complications, and the revision rate of transpedicular screws in the thoracic spine. In this retrospective study 278 thoracic pedicle screws of 43 patients were assessed using a scoring system and postoperative computed tomography (CT) scans. 43 consecutive patients with traumatic thoracic spine fractures underwent posterior transpedicular reduction and fixation. The universal spine system was used for placement of 278 pedicle screws under fluoroscopic control. Fractures were classified according to the Magerl classification by plain radiographs and CT scans. Evaluation of pedicle screw placement was assessed by a scoring system from grade I to grade III. Revision rate of pedicle screws as well as pre- and postoperative neurologic status were analyzed. In the thoracic spine, vertebral body T7 was mainly fractured followed by T6 and T5. In 37 (86.0%) of 43 patients type C fractures and in three patients each type A and B fractures were determined, respectively. Of all 278 pedicle screws, 78.1% were inserted in pedicles T4–T9. The rate of optimally placed grade I screws was 84.5%. Grade II screws were scored in 6.8% and grade III screws in 8.6%. The revision rate of all pedicle screws was 5.0%, whereas 64.3% of these were grade III. None of the patients suffered neurologic or vascular structure compromise postoperatively. Although most pedicle screws were placed in pedicles of the middle thoracic spine where the pedicle width is smallest, most screws were optimally placed with grade I. The revision rate of pedicle screws was acceptable and associated with the grade of pedicle screw placement.

Functional outcome in patients with thoracolumbar burst fractures treated with dorsal instrumentation and transpedicular cancellous bone grafting.

The aim of the study was to develop an insight into the impairments in spinal fracture patients, operatively treated with an internal fixator, and also into their ability to participate in daily living, return to work and quality of life as defined by the World Health Organization. Nineteen patients operated for a type A fracture of the thoracolumbar spine (T9-L4) between 1993 and 1998 in the University Hospital Groningen, the Netherlands, aged between 18 and 60 years, without neurological deficit were included in the study. Operative treatment consisted of fracture reduction and internal fixation using the Universal Spine System, combined with transpedicular cancellous bone grafting and dorsal spondylodesis. No ventral fusion operations, laminectomies or discectomies were done. Restrictions in body function and structure were measured on radiographs and in functional capacity tests, such as lifting tests and ergometry. Restrictions in activities were studied with the Visual Analogue Scale (VAS) Spine Score and the Roland Morris Disability Questionnaire (RMDQ). Restrictions in participation/quality of life were analysed with the Short Form 36 (SF36) and described in the return to work status. The radiological results are comparable to the literature. The reduction of the anterior wedge angle was followed by a gradual partial loss of intervertebral angle and regional angle. The maximum oxygen uptake (VO2-max) was reduced in only 8.3% of the patients. Arm and trunk lift was within the normal range in 87% and 80% of the patients respectively, but only 53% of the patients were able to perform a leg lift within the normal range. A mean RMDQ score of 4.0 positive items (SD 6.0) was found, and the mean VAS Spinal Score was 79.4 (SD 25.0), both better than in other series. No significant differences compared to the values of a comparable (healthy) age group could be identified in any variable of the SF36. A high correlation was seen between RMDQ, VAS Spine Score and the SF36 categories. No correlation was found between the anterior wedge angle and the regional angle on the one hand, and functional capacity tests or questionnaire scores on the other. Of the patients in paid employment before the trauma, 87% had returned to work at follow-up. About 50% of the patients had been obliged to change the intensity of their work or the kind of work they performed after the injury and treatment. In this matter, leg (muscle) performance seems a more important factor than overall condition (VO2-max). The results of the study indicate that patients with thoracolumbar spinal fractures without neurological deficit, treated with dorsal instrumentation, perform like healthy people 3-8 years after injury, according to the RMDQ, VAS Spine Score and SF36 results. Physical capacity tests reveal that leg (muscle) performance seems a more important factor in impairment than arm lift or overall condition.

Safety of supplemental endplate screws in thoracic pedicle hook fixation.

The AO Universal Spine System thoracic pedicle hook design includes a fixation screw that passes obliquely through the inferior facet into the pedicle to engage in the posterior portion of the superior vertebral body endplate. This endplate screw provides additional purchase at the hook-bone interface. To determine the safety of this fixation system the authors reviewed the operative notes, radiographs, and outcomes of patients who underwent placement of endplate screws. Thirty-six patients (16 male and 20 female patients) who required posterior thoracic instrumentation for spinal deformity (11 cases), neoplasm (15 cases), and traumatic injury (10 cases) were included in this study. One hundred sixty-four endplate screws were placed (mean 4.3/patient) to augment pedicle hooks for posterior thoracic instrumentation. The number of instrumented levels ranged from seven to 16. The positions of the screws in relation to the pedicle, neural foramen, spinal canal, and endplate were evaluated by assessing plain radiographs and computerized tomography scans (10 cases). Eighty-two screws (56%) were in ideal position. Lateral pedicle wall perforation occurred with 51 screws (35%). Three screws violated the medial wall and nine screws violated the superior or inferior walls of the pedicle. There were no clinical sequelae associated with any of the malpositioned screws. Adequate follow-up radiographic data were not available in five patients. The mean follow-up duration was 19.8 months (range 3-61 months). Two patients required revision surgery at 3 months and 18 months, respectively, because of hook/endplate screw displacement. There was also one case of an endplate screw fracture without hook displacement that was discovered during subsequent revision surgery. The remainder of the endplate screws and associated pedicles hooks maintained their original positions. There was no case of spinal cord, nerve root, pulmonary, or vascular injury. The placement of supplemental endplate screws in conjunction with thoracic pedicle hooks can be conducted safely.