Finite element analysis of an additional implant for intramedullary nailing]

In the surgical treatment of fractured femurs, the fracture is bridged by a medullary nail fixed in the bone with interlocking screws. Failure of bone substance in the region of the interlocking screws is the most common complication in the treatment of osteoporotic bone. With the aim of preventing this complication, an additional implant was developed. A finite element analysis of an ideal bone/implant system was carried out to investigate the role of the additional implant. Three defined finite element models were generated, and the associated stress situations compared. The first model is a standard fixation without the additional implant. In the second model, the additional implant is integrated within the bone/implant system. The third model uses a modified form of the additional implant. The results show that both additional implants reduce the stresses occurring, both in the bone substance and at the screws. The modified form of the additional implant proved to be the most favorable version. In the case of the original additional implant, the negative effect of the sharp edges of the thread was demonstrable.     

Numerical investigation of mechanical effects caused by various fixation positions on a new radius intramedullary nail

Fracture of the radius diaphysis is an unusual injury in adults. Open reduction and plate osteosynthesis has been recommended by most of the authors. However, this trend has started to change with the recent introduction of newly designed interlocking intramedullary (IM) nails. New generation of IM nails are developed in order to utilise the advantages of IM nails against plates. Because of its anatomical structure, the radius bone has a complex geometry. Therefore, the callus structure, which forms during the healing period, should not be affected from external effects, such as excessive loads or motion. In this study, effects of radial styloid process (RSP), dorsal side and ulnar notch edge fixations of a new design radius IM nail on the healing period were numerically investigated. A three-dimensional solid model of radius was obtained from computed tomography images of a volunteer and callus structure model, was placed accordingly and different fixations of implants were performed. The models were analysed under axial loads transferring from the wrist to the radius bone using finite element method. As a result of the analysis, fixation of IM nail from RSP was found to be beneficial on healing period in terms of both callus motion and emerging stresses.     

The fixion proximal femur nailing system: biomechanical properties of the nail and a cadaveric study

The treatment of choice for early mobilization of hip fracture is surgery, which traditionally employs side plates and screws or intramedullary nails. We examined the biomechanical properties of a new proximal femoral nail system. The new expandable Fixion proximal femur nailing (PFN) system, made of stainless-steel alloy, consists of a nail, a peg and an anti-rotation pin. Upon positioning, the nail and peg are expanded to their maximal diameter. The current biomechanical study investigated: nail bending strength and stiffness, fatigue properties and hip peg strength. A cadaveric study that determined the effect of the expandable peg on the femoral head included subsidence testing, pull and torsion testing and intra-osseous pressure (IOP) measurements before and after expansion. Biomechanical properties of the new nail met ASTM F384 guideline requirements. The cadaver study yielded equivalent results for the pullout test between the peg and the hip screw, but found the peg superior in the torsion strength test. IOP during peg insertion and expansion was substantially lower than the threshold pressure that causes avascular necrosis. The biomechanical tests found the new system to be safe and able to provide good abutment of the nail to the bone. We conclude that the Fixion PFN system proved to be an effective proximal femur fracture fixation device.     

A Comparative Analysis of Different Treatments for Distal Femur Fractures using the Finite Element Method

The main objective of this work is the evaluation, by means of the finite element method (FEM) of the mechanical stability and long-term microstructural modifications in bone induced to three different kinds of fractures of the distal femur by three types of implants: the Condyle Plate, the less invasive stabilization system plate (LISS) and the distal femur nail (DFN). The displacement and the stress distributions both in bone and implants and the internal bone remodelling process after fracture and fixation are obtained and analysed by computational simulation. The main conclusions of this work are that distal femoral fractures can be treated correctly with the Condyle Plate, the LISS plate and the DFN. The stresses both in LISS and DFN implant are high especially around the screws. When respect to remodelling, the LISS produces an important resorption in the fractured region, while the other two implants do not strongly modify bone tissue microstructure.     

A comparative analysis of different treatments for distal femur fractures using the finite element method

The main objective of this work is the evaluation, by means of the finite element method (FEM) of the mechanical stability and long-term microstructural modifications in bone induced to three different kinds of fractures of the distal femur by three types of implants: the Condyle Plate, the less invasive stabilization system plate (LISS) and the distal femur nail (DFN). The displacement and the stress distributions both in bone and implants and the internal bone remodelling process after fracture and fixation are obtained and analysed by computational simulation. The main conclusions of this work are that distal femoral fractures can be treated correctly with the Condyle Plate, the LISS plate and the DFN. The stresses both in LISS and DFN implant are high especially around the screws. When respect to remodelling, the LISS produces an important resorption in the fractured region, while the other two implants do not strongly modify bone tissue microstructure.     

A comparison of stress distributions for different surgical procedures, screw dimensions and orientations for a Temporomandibular joint implant

Finite element analysis is a useful analytical tool for the design of biomedical implants. The aim of this study was to investigate the behavior of temporomandibular joint implants with multiple design variables of the screws used for fixation of the implant. A commercially available implant with full mandible was analyzed using a finite element software package. The effects of different design variables such as orientation, diameter and stem length of the screws on the stress distribution in bone for two different surgical procedures were investigated. Considering the microstrain in bone as a principal factor, the acceptable ranges for screw diameter and length were determined. Parallel orientation of the screws performed better from a stress point of view when compared to the zig-zag orientation. Sufficient contact between the implant collar and mandibular condyle was shown to reduce the peak stresses which may lead to long term success. The distance between screw holes in the parallel orientation was much closer when compared to the zig-zag orientation. However, the stresses in bone near the screw hole area for the parallel orientation were within acceptable limits.     

逆行交锁髓内钉联合单侧骨皮质锁定钢板固定治疗股骨髁上骨不连

目的:探讨逆行交锁髓内钉联合单侧骨皮质钢板固定治疗股骨髁上骨不连的临床疗效。方法:对25例股骨髁上骨不连,均采用逆行交锁髓内钉联合单侧骨皮质钢板固定加自体髂骨植骨治疗。结果:25例获12～24个月随访,平均12个月。4～8个月内均获骨性愈合。结论:应用逆行交锁髓内钉联合单侧骨皮质钢板固定后骨折端可获得坚强内固定,手术操作简便、安全,可早期进行膝关节和股四头肌功能锻炼,是一种治疗股骨髁上骨不连的有效方法。     

Bone remodelling analysis of a bovine femur for a veterinary implant design

The response of bovine bone to the presence of an implant is analysed with the aim of simulating bone remodelling in a developing model of a polymeric intramedullary interlocking nail for veterinary use. A 3-D finite element model of the femur diaphysis is built based on computed tomography images and using a CAD-based modelling pipeline. The bone remodelling process after the surgery is analysed and compared with the healthy bone. The remodelling law assumes that bone adapts to the mechanical environment. For the analyses a consistent set of loads is determined for the bovine walk cycle. The remodelling results reproduce the morphologic features of bone and provide evidence of the difference on the bone behaviour when comparing metallic and polymeric nails. Our findings indicate that an intramedullary polymeric nail has the advantage over the metallic one of improving long-term bone healing and possibly avoiding the need of the implant removal.     

The discrete nature of trabecular bone microarchitecture affects implant stability

Small endosseous implants, such as screws, are important components of modern orthopedics and dentistry. Hence they have to reliably fulfill a variety of requirements, which makes the development of such implants challenging. Finite element analysis is a widely used computational tool used to analyze and optimize implant stability in bone. For these purposes, bone is generally modeled as a continuum material. However, bone failure and bone adaptation processes are occurring at the discrete level of individual trabeculae; hence the assessment of stresses and strains at this level is relevant. Therefore, the aim of the present study was to investigate how peri-implant strain distribution and load transfer between implant and bone are affected by the continuum assumption. We performed a computational study in which cancellous screws were inserted in continuum and discrete models of trabecular bone; axial loading was simulated. We found strong differences in bone-implant stiffness between the discrete and continuum bone model. They depended on bone density and applied boundary conditions. Furthermore, load transfer from the screw to the surrounding bone differed strongly between the continuum and discrete models, especially for low-density bone. Based on our findings we conclude that continuum bone models are of limited use for finite element analysis of peri-implant mechanical loading in trabecular bone when a precise quantification of peri-implant stresses and strains is required. Therefore, for the assessment and improvement of trabecular bone implants, finite element models which accurately represent trabecular microarchitecture should be used.     

Design and testing of external fixator bone screws

In external fixation, bone screw loosening still presents a major clinical problem. For this study, the design factors influencing the mechanics of the bone-screw interface were analysed and various experimental screws designed with the intention of maximizing the strength and stiffness of the inserted screw. Push-in, pull-out and bending tests were then carried out on the three experimental screws, and on two commercially available screws in both a synthetic material and in cadaveric bone; photoelastic tests on different screw threadforms were also performed. The results of the push-in and pull-out tests indicate that both the screw threadform and cutting head have a significant effect on the holding strength of the screw. The photoelastic tests show that most of the applied load is distributed over the first few threads closest to the load, and that the area between the thread crests is subjected to high shear stresses.     

Nail position has an influence on anterior knee pain after tibial intramedullary nailing

Our aim was to determine the possible relationship between anterior knee pain (AKP) and nail position marked as a distance from tip of nail to tibial plateau (NP) and to the tuberositas tibiae (NT). Nail position has an influence on anterior knee pain after tibial intramedullary nailing. We evaluated postoperative outcome results of 50 patients in the last 3 years with healed fractures initially treated with intramedullary (IM) reamed nails with 2 or 3 interlocking screws on both parts of the nail and with the use of medial paratendinous incision for nail entry portal. Patients marked a point on the visual analog scale (VAS) that corresponded to the level of postoperative AKP felt. Two groups of patients were formed on the basis of AKP (pain level was neglected): groups A and B, with and without pain, respectively. The difference between the two groups concerning NP measurements was statistically significant (p < 0.05), but not concerning NT measurements at the p < 0.05 level. Patients were classified by pain with high accuracy (98%) according to a classification tree. Symptoms of AKP did not appear if the tip of the nail position was more than 6.0 mm from the NP and more than 2.6 mm from the NT. However, for better evaluation of these results it will be necessary to examine a larger number of postoperative patients with AKP.     

Experimental and probabilistic analysis of distal femoral periprosthetic fracture: a comparison of locking plate and intramedullary nail fixation. Part B: probabilistic investigation

The following is Part B of a two-part study. Part A evaluated, biomechanically, intramedullary (IM) nails versus locking plates for fixation of an extra-articular, metaphyseal wedge fracture in synthetic osteoporotic bone. Part B of this study introduces deterministic finite element (FE) models of each construct type in synthetic osteoporotic bone and investigates the probability of periprosthetic fracture of the locking plate compared with the retrograde IM nail using Monte Carlo simulation. Deterministic FE models of the fractured femur implanted with IM nail and locking plate, respectively, were developed and validated using experimental data presented in Part A of this study. The models were validated by comparing the load-displacement curve of the experimental data with the load-displacement curve of the FE simulation with a root-mean square error of less than 3?mm. The validated FE models were then modified by defining the cortical and cancellous bone modulus of elasticity as uncertain variables that could be assumed to vary randomly. Monte Carlo simulation was used to evaluate the probability of fracture (POF) of each fixation. The POF represents the cumulative probability that the predicted shear stresses in the cortical bone will exceed the expected shear strength of the cortical bone. This investigation provides information regarding the significance of post-operative damage accumulation on the POF of the implanted bones when the two fixations are used. The probabilistic analysis found the locking plate fixation to have a higher POF than the IM nail fixation under the applied loading conditions (locking plate 21.8% versus IM nail 0.019%).     

Stress analysis of a centrally fractured rib fixated by an intramedullary screw

The stress on an intramedullary screw rib fixation device holding together a centrally fractured human rib under in vivo force loadings was studied using finite element analysis (FEA). Validation of the FEA modelling using pullout from porcine ribs proved FEA to be suitable for assessing the structural integrity of screw/bone systems such as rib fixated by a screw. In the human rib fixation investigation, it was found that intramedullary bioresorbable Bioretec screws can fixate centrally fractured human ribs under normal breathing conditions. However, under coughing conditions, simulation showed Bioretec fixating screws to bend substantially. High stresses in the screw are mainly the result of flexion induced by the force loading, and are restricted to thin regions on the outside of the screw shaft. Stiffer screws result in less locally intense stress concentrations in bone, indicating that bone failure in the bone/screw contact regions can be averted with improvements in screw stiffness.     

Comparison of compression and torque measurements of self-tapping and pretapped screws

The choice of an internal fixation system for maxillofacial surgery is made difficult because of lack of information with respect to functional load. This study attempted to clarify some of the controversy with respect to maxillofacial use of these implants. Maximal compressive force to torque values were measured in standardized bone thicknesses of 1, 2, 3, and 4 mm. The screws tested were pretapped AO 1.5-, 2.0-, 2.7-, and 3.5-mm rescue screws and self-tapping Luhr, Champy, and AO 1.5- and 2.0-mm screws. Ten measurements were made for each screw type/bone thickness combination using a piezoelectric washer and torque screwdriver. It was apparent that for 1- and 2-mm bone thicknesses the use of self-tapping screws resulted in the highest compression values. In 3- and 4-mm bone thicknesses, pretapped screws offered the highest compression values. As expected, self-tapping screws had the highest torque values on insertion owing to torque loss in cutting the screw threads. The 2.7-mm screw offered no advantage over the 2.0-mm screws in 1- and 2-mm bone thicknesses but resulted in higher compression values in 3- and 4-mm bone thicknesses.     

Effect of material damping on bone remodelling

This paper considers the effect of internal material damping on the stresses, strains, and surface and internal remodelling behaviour in a section of axisymmetrical bone with a force-fitted axially oriented medullary pin. The bone response to several loading situations is modelled using visco-elastic equations. An approximate method is developed to analyse the proposed mathematical model. By considering a numerical example, the effect of material damping on the remodelling stresses is quantified.     

Evaluation of threshold stress for bone resorption around screws based on in vivo strain measurement of miniplate

The purpose of this study is to investigate the critical threshold stress causing bone resorption evaluated from strain measurement in vivo, comparing the various finite element models. In this study strains of miniplates used for mandibular fractures were measured once a week until the strains reduced. The maximum bite force for each patient was applied in the incisal, right molar and left molar region. The strains increased and reached a peak level at 2-4 weeks, whereas the bite forces increased during the period of measurements. A 3-D osteosynthesis model using finite element method showed that the compressive stresses of the bone surrounding screws ranged within approximately -40 MPa under the condition generating the same amounts of strains measured in the miniplates. Furthermore, various finite element models simulating mandibular reconstruction using the fibular graft were constructed. The models for reconstruction using single strut fibula showed distinct stress concentration in the cortical bone surrounding screws, and the peak stress levels were 2 to 3 times as strong as that of the fracture model. We conclude that critical threshold for bone resorption should be approximately -50 MPa (3600 micro strain).     

Calcium phosphate cement augmentation of cancellous bone screws can compensate for the absence of cortical fixation

An obvious means to improve the fixation of a cancellous bone screw is to augment the surrounding bone with cement. Previous studies have shown that bone augmentation with Calcium Phosphate (CaP) cement significantly improves screw fixation. Nevertheless, quantitative data about the optimal distribution of CaP cement is not available. The present study aims to show the effect of cement distribution on the screw fixation strength for various cortical thicknesses and to determine the conditions at which cement augmentation can compensate for the absence of cortical fixation in osteoporotic bone. In this study, artificial bone materials were used to mimic osteoporotic cancellous bone and cortical bone of varying thickness. These bone constructs were used to test the fixation strength of cancellous bone screws in different cortical thicknesses and different cement augmentation depths. The cement distribution was measured with microCT. The maximum pullout force was measured experimentally. The microCT analysis revealed a pseudo-conic shape distribution of the cement around the screws. While the maximum pullout strength of the screws in the artificial bone only was 30±7 N, it could increase up to approximately 1000 N under optimal conditions. Cement augmentation significantly increased pullout force in all cases. The effect of cortical thickness on pullout force was reduced with increased cement augmentation depth. Indeed, cement augmentation without cortical fixation increased pullout forces over that of screws without cement augmentation but with cortical fixation. Since cement augmentation significantly increased pullout force in all cases, we conclude that the loss of cortical fixation can be compensated by cement augmentation.     

Identification of a Subpopulation of Marrow MSC-Derived Medullary Adipocytes That Express Osteoclast-Regulating Molecules: Marrow Adipocytes Express Osteoclast Mediators

Increased marrow medullary adipogenesis and an associated decrease in bone mineral density, usually observed in elderly individuals, is a common characteristic in senile osteoporosis. In this study we investigated whether cells of the medullary adipocyte lineage have the potential to directly support the formation of osteoclasts, whose activity in bone leads to bone degradation. An in vitro mesenchymal stem cell (MSC)-derived medullary adipocyte lineage culture model was used to study the expression of the important osteoclast mediators RANKL, M-CSF, SDF-1, and OPG. We further assessed whether adipocytes at a specific developmental stage were capable of supporting osteoclast-like cell formation in culture. In vitro MSC-derived medullary adipocytes showed an mRNA and protein expression profile of M-CSF, RANKL, and OPG that was dependent on its developmental/metabolic stage. Furthermore, RANKL expression was observed in MSC-derived adipocytes that were at a distinct lineage stage and these cells were also capable of supporting osteoclast-like cell formation in co-cultures with peripheral blood mononuclear cells. These results suggest a connection between medullary adipocytes and osteoclast formation in vivo and may have major significance in regards to the mechanisms of decreased bone density in senile osteoporosis.     

交锁髓内钉与经皮锁定钢板治疗胫骨骨折的临床对照研究

目的:探讨交锁髓内钉与经皮锁定钢板治疗胫骨骨折的疗效。方法:选取了100例胫骨骨折患者,按住院单双号分为两组,对照组(48例)给予交锁髓内钉,观察组(52例)给予经皮锁定钢板治疗。通过观察并记录疗效,围手术期指标及随访3个月的并发症发生情况,评价交锁髓内钉与经皮锁定钢板治疗胫骨骨折的疗效。结果:采用经皮锁定钢板内固定和交锁髓内钉治疗胫骨骨折,两种手术方法所需手术时间无统计学差异(P0.05),经皮锁定钢板内固定术术中出血量更少,术后观察组患者住院时间和骨折愈合时间更短(P0.05),术后3个月,观察组患者有效率明显高于对照组(P0.05),随访3个月期间,观察组并发症5例,发生率9.6%,对照组并发症12例,发生率25.0%,观察组并发症发生率明显低于对照组(P0.05)。结论:经皮锁定钢板对胫骨骨折具有较好的疗效,手术安全性高,并发症少,能促进骨折愈合,使患者尽早康复,值得临床推广使用。     

Are allogenic or xenogenic screws and plates a reasonable alternative to alloplastic material for osteosynthesis—A histomorphological analysis in a dynamic system

Despite invention of titanium and resorbable screws and plates, still, one of the main challenges in bone fixation is the search for an ideal osteosynthetic material. Biomechanical properties, biocompatibility, and also cost effectiveness and clinical practicability are factors for the selection of a particular material. A promising alternative seems to be screws and plates made of bone. Recently, xenogenic bone pins and screws have been invented for use in joint surgery.In this study, screws made of allogenic sheep and xenogenic human bone were analyzed in a vital and dynamic sheep-model and compared to conventional titanium screws over a standard period of bone healing of 56 days with a constant applied extrusion force. Biomechanical analysis and histomorphological evaluation were performed.After 56 days of insertion xenogenic screws made of human bone showed significantly larger distance of extrusion of on average 173.8 μm compared to allogenic screws made of sheep bone of on average 27.8 and 29.95 μm of the titanium control group. Severe resorption processes with connective tissue interposition were found in the histomorphological analysis of the xenogenic screws in contrast to new bone formation and centripetal vascularization of the allogenic bone screw, as well as in processes of incorporation of the titanium control group.The study showed allogenic cortical bone screws as a substantial alternative to titanium screws with good biomechanical properties. In contrast to other reports a different result was shown for the xenogenic bone screws. They showed insufficient holding strength with confirmative histomorphological signs of degradation and insufficient osseointegration. Before common clinical use of xenogenic osteosynthetic material, further evaluation should be performed.