Numerical simulation of bone screw induced pretension: the cases of under-tapping and conical profile.

Even though screw induced pretension impacts the holding strength of bone screws, its implementation into the numerical simulation of the pullout phenomenon remains a problem with no apparent solution. The present study aims at developing a new methodology to simulate screw induced pretension for the cases of: (a) cylindrical screws inserted with under-tapping and (b) conical screws. For this purpose pullout was studied experimentally using synthetic bone and then simulated numerically. Synthetic bone failure was simulated using a bilinear cohesive zone material model. Pretension generation was simulated by allowing the screw to expand inside a hole with smaller dimensions or different shape than the screw itself. The finite element models developed here were validated against experimental results and then utilized to investigate the impact of under-tapping and conical angle. The results indicated that pretension can indeed increase a screw's pullout force but only up to a certain degree. Under-tapping increased cylindrical screws' pullout force up to 12%, 15% and 17% for synthetic bones of density equal to 0.08 g cm(-3), 0.16 g cm(-3) and 0.28 g cm(-3), respectively. Inserting a conical screw into a cylindrical hole increased pullout force up to 11%. In any case an optimum level of screw induced pretension exists.

A novel liver parenchyma transection technique using locking straight rigid ties. An experimental study in pigs.

INTRODUCTION: Technological advances have led to the development of many devices used in liver resections. However, no single transection tool is uniformly considered to be better than the others. This study aimed to develop an effective, fast, and cost-efficient technique for hepatic parenchymal transection. MATERIALS AND METHODS: A liver parenchyma compression device in the form of a locking straight rigid tie (LoStRiT) was newly developed. Twelve pigs were distributed into two groups. The control group ( n = 6) comprised animals that underwent hepatectomy using the standard Kelly-clysis technique. The study group (n = 6) comprised animals that underwent hepatectomy using sequential LoStRiT mechanisms. The transection speed, blood loss, and biloma formation were recorded. RESULTS: The mean parenchymal transection speed was 1.27 ± 0.27 cm(2)/min for the control group and 2.39 ± 0.56 cm(2)/min for the LoStRiT group ( p = .003). The mean blood loss per kilogram of body weight was 9.8 ± 5.2 ml/kg for the control group and 3.9 ± 0.9 ml/kg for the LoStRiT group ( p = .040). No bilomas were identified. CONCLUSION: LoStRiT hepatectomy appears to be effective, fast, and reproducible in a porcine model of liver resection. Further development of this novel and potentially cost-efficient technique includes construction of the device using absorbable materials.

Rotator cuff repair: a biomechanical ex vivo ovine study.

The purpose of this study was to assess load to failure of sutures, suturing techniques, and suture anchors used in rotator cuff surgery in order to explore their weaknesses. Ten types of sutures (absorbable and nonabsorbable), four types of suturing techniques, and eight types of suture anchors (bioabsorbable and metallic) were tested. Material Testing Machine and attached load cell were used to test the biomaterials in ex vivo ovine tissues. The results show that the mean load to failure and stiffness were higher in nonabsorbable sutures. Massive cuff tear and modified Mason-Allen suturing techniques had higher failure strength and stiffness when compared to simple and mattress techniques, but there was no substantial difference between them. Metallic suture anchors had higher failure strength when compared to bioabsorbable ones. Often either in metallic or in bioabsorbable anchors, the eyelet fails first. The failure sequence in ovine tissues is found to be in the following order: simple and mattress suturing techniques, nonabsorbable sutures, massive cuff tear and modified Mason-Allen suturing techniques, bioabsorbable anchors, and metallic anchors. Thus, it is concluded that metallic anchors mounted with modern nonabsorbable sutures will fail last. If we use simple and mattress techniques, the tendon-suture level will fail first, but it will improve substantially using more sophisticated suturing techniques (massive cuff tear or modified Mason-Allen).

Comparison of four reconstructive methods for diaphyseal defects of the humerus after tumor resection.

The objective of the current study was to compare quantitative data on the biomechanical analysis of different techniques for fixation of intercalary bone defects of the humerus, by means of consistently applied methodology on composite models. A total of 25 humeral specimens of composite models were used. An intercalary defect was created and reconstructed using plates, intramedullary nails, external fixators and segmental prosthetic implants. The specimens were loaded under axial compression, four-point bending and torsion within the linear elastic region. Modular segmental implants and intramedullary nails were able to compensate significantly greater amounts of compressive loads compared to locking plates and external fixators. However, in flexion and torsion, the modular segmental implants and the external fixators were significantly better load-bearing devices compared to the intramedullary nails and plates. Early mobilization of the upper limb in patients with diaphyseal bone defects of the humerus could probably be more safe and tolerable when reconstructed with modular segmental implants.

The use of twin-ring Ilizarov external fixator constructs: application and biomechanical proof-of principle with possible clinical indications.

BACKGROUND: In peri- or intra-articular fractures of the tibia or femur, the presence of short metaphyseal bone fragments may make the application of an Ilizarov external fixator (IEF) challenging. In such cases, it may be necessary to bridge the adjacent joint in order to ensure stable fixation. The twin-ring (TR) module of circular external fixation is proposed as an alternative method that avoids joint bridging, without compromising stability of fixation. The aim of this study is to present the experimental tests performed to compare the biomechanical characteristics of the single- and TR IEF modules. The clinical application of the TR module in select patients is also presented and the merits of this technique are discussed. METHODS: In this experimental study, the passive stiffness and stability of the single-ring (SR) and twin-ring (TR) IEF modules were tested under axial and shear loading conditions. In each module, two perpendicular wires on the upper surface and another two wires on the lower surface of the rings were used for fixation of the rings on plastic acetal cylinders simulating long bones. RESULTS: In axial loading, the main outcome measure was stiffness and the SR module proved stiffer than the TR. In shear loading, the main outcome measure was stability, the TR module proving more stable than the SR. DISCUSSION: The TR configuration, being stiffer in shear loading, may make joint bridging unnecessary when an IEF is applied. If it is still required, TR frames allow for an earlier discontinuation of bridging; either case is in favour of a successful final outcome. CONCLUSION: The application of the TR module has led to satisfactory clinical outcomes and should be considered as an alternative in select trauma patients treated with an IEF. Biomechanically, the TR module possesses features which enhance fracture healing and at the same time obviate the need for bridging adjacent joints, thereby significantly reducing patient morbidity.

Intrinsic passive stiffness of 2 constructs of varus proximal femoral osteotomy: external fixator or blade plate.

BACKGROUND: Despite the published clinical evidence of the implementation of external fixation of proximal femoral osteotomies, there is lack of specific laboratory justification. METHODS: Two groups of 5 adult composite femur constructs of varus proximal femoral osteotomy were tested under incremental and cyclical loading of up to 600N along the mechanical axis. Five were fixed with a blade plate and another 5 with a monolateral external fixator. Load versus displacement curves were produced, and passive stiffness of all constructs was calculated. The described loading regime aimed to simulate the initial postoperative state and provide data for the assessment of vertical intrinsic passive stiffness in partial weight-bearing conditions. RESULTS: Although the blade plate constructs showed higher average stiffness, this was not statistically significant [F(1,8)=1.712, P=0.23]. No construct failed. No failure or plastic deformation was observed under the described loading regime. CONCLUSIONS: Vertical intrinsic passive stiffness in partial weight-bearing conditions during the initial postoperative period can be considered satisfactory subsequent to unilateral external fixation of a varus intertrochanteric osteotomy. CLINICAL RELEVANCE: The results support the hypothesis that external fixation is a biomechanically sound alternative to internal fixation of varus intertrochanteric osteotomies, in selected patients.

A parametric study of cylindrical pedicle screw design implications on the pullout performance using an experimentally validated finite-element model.

The present study aims to the design of a finite-element model simulating accurately the pullout behaviour of cylindrical pedicle screws and predicting their pullout force. Three commercial pedicle screws, subjected to pure pullout from synthetic bone, were studied experimentally. The results were used for the design, calibration and validation of a finite-element model. Special attention was paid to the accurate simulation of the failure inside the host material under shear. For this purpose, a bilinear cohesive zone material model was adopted, controlling the mode-II debonding of neighbouring elements in the vicinity of the screw. Comparison between experimental and numerical results proved that the implementation of this approach can significantly enhance the accuracy of the numerical simulation of a screw's mechanical behaviour under pure pullout loads. The numerical model was used for the parametric study of various factors affecting the pullout performance of a cylindrical pedicle screw. It was concluded that the major parameter influencing the pullout force is the outer radius (increasing its value by 36% increases the pullout force by 34%). The influence of the purchase length of the screw is of similar quantitative nature. The respective dependence on the thread inclination, depth and pitch was significantly weaker.

Comparison of fixation methods of double-bundle double-tibial tunnel ACL reconstruction and double-bundle single-tunnel technique.

An experimental study was conducted in order to evaluate biomechanical methods of single-bundle reconstruction in ACL and compare it with a new double-bundle double-tibial tunnel technique. Twenty-four porcine cadaver knees, divided into 4 groups of 6 knees each and 48 proper extensors of the fourth toe tendons, were used for the fixation techniques. In groups A and B, a double-bundle technique with a single femoral and tibial tunnel was used, fixed to a femoral and tibial post with screws and with buttons, respectively. In groups C and D, a double-bundle technique (technique Delta) with two separate tibial tunnels was used, fixed to a femoral and tibial post with screws and with buttons, respectively. A material testing system (Instron) was used for anteriorly translating the tibia until failure. The femoral and tibial post as a fixation method is superior to the conventional buttons technique. The more anatomical double-bundle reconstruction technique provided significantly higher structural properties and smaller loss of fixation compared with the single-bundle reconstruction technique. The comparison of the two techniques gave superior results to the femoral and tibial screws over the buttons. The double-bundle technique attempts to restore the anterior stability of the knee joint.

ACL reconstruction and the implication of its tibial attachment for stability of the joint: anthropometric and biomechanical study.

The planar topography of the anterior cruciate ligament (ACL) insertion was investigated and correlated to the use of the double-bundle/double tibial tunnel ACL reconstruction technique within the ACL tibial insertion area. The anteroposterior and mediolateral length of the tibial ACL attachment and the distances of the tibial insertion area from the anterior and posterior tibial borders were measured and the stability of the joint was tested using the double-bundle/double tibial tunnel ACL reconstruction technique. The anteroposterior length, 19.54 mm in men and 17.36 mm in women, of the ACL insertion, averaged approximately 40% of the total intercondylar anteroposterior dimension of the plateau. This broad distribution of insertion fibres ensures ligament tension and hence joint stability. The reported anteroposterior broad insertion of ACL fibres to the tibia is not sufficiently reproduced by the use of one or more bundles having a common tibial tunnel for the ACL reconstruction. In our view, this might be better achieved with two different bundles, with separate tunnels, and independent tensioning in different knee angles. This technique might achieve better results in human knee stability as opposed to other reported techniques.

Biomechanical evaluation of conventional internal contemporary spinal fixation techniques used for stabilization of complete sacroiliac joint separation: a 3-dimensional unilaterally isolated experimental stiffness study.

STUDY DESIGN: Comparative 3-dimensional biomechanical testing. OBJECTIVE: To compare 5 fixation techniques, 3 using screws or screw and plates and 2 spinal, used for stabilization of complete unilateral sacroiliac dislocation in composite models. SUMMARY OF BACKGROUND DATA: Harrington compression rods have been used for posterior iliosacral stabilization. Recently, the use of compact spinal instrumentation has been introduced for stabilization of iliosacral joint separation to achieve immediate and permanent stability, allowing early mobilization. To the authors' knowledge, no comparative mechanical studies between commonly used internal fixation techniques and contemporary spinal instrumentation have been performed. METHODS: Fifteen identical composite models of the left hemipelvis and sacrum were used to simulate consistently the "worst-case scenario" of complete unilateral sacroiliac dislocation. Subgroups of 3 models each were used to apply 5 (A-E) alternative fixation iliosacral joint fixation techniques: 1 multiaxial 7.5 mm Cotrel-Dubousset screw inserted in the posterior superior iliac spine and connected with a long Cotrel-Dubousset horizontal rod with 6.5 mm multiaxial Cotrel-Dubousset screws inserted bilaterally in the S1 pedicles (technique A); 1 multiaxial 7.5 mm Cotrel-Dubousset titanium pedicle screw inserted in the posterior superior iliac spine and connected with a short horizontal Cotrel-Dubousset-rod to a 6.5 mm multiaxial Cotrel-Dubousset-screw inserted to the ipsilateral S1 pedicle (technique B); 1, 6.5 mm cancellous AO-screw (technique C); 2, 6.5 mm cancellous AO screws (technique D); and 2 dynamic stainless steel compression plates (technique E) placed anteriorly. Constructs were biomechanically tested. The ilium was unilaterally rigidly fixed, the sacrum was put horizontal in the mediolateral direction with a forward tilt of 30 degrees (close to physiologic conditions) in the sagittal plane, and a vertical quasi-static compressive load ranging from 0 to 500 N was applied on the endplate of S1, reproducing a "worst case" loading scenario. Construct stiffness, frontal plus sagittal kinematics, and iliosacral joint gap size for all 5 techniques were measured. RESULTS: The construct stiffness (N/mm +/- standard deviation) ranged for model: A, 121 +/- 18; B, 78 +/- 10; C, 168 +/- 13; D, 193 +/- 42; and E, 145 +/- 4. All other parameters exhibited minor variations between the different techniques of fixation: at the 400 N load level, the maximum iliosacral gap globally ranged 0.9-2.8 mm, the maximum mediolateral sacral tilt ranged 1.3-2.4 degrees, and the maximum anteroposterior sacral tilt ranged 0.6-3.0 degrees. CONCLUSIONS: The iliosacral fixation with 2 6.5 mm AO-cancellous screws for complete sacroiliac dislocation demonstrated the highest stiffness and the short spinal instrumentation the poorest stiffness. All other fixation techniques could be generally considered of equivalent stability value.