Skeletal stabilization with a multiplane external fixation device. Biomechanical evaluation and finite element model

The general question of the influence of fracture stability on bone healing remains unanswered and has important bearing on fracture stabilization by external fixation. The stiffness of an external fixator is dependent on pin placement and frame orientation. These parameters are under the surgeon's control, within limits set by soft tissue injury and fixator design. Fixator configuration parameters include common fragment pin separation, common fragment pin angle, common fragment pin number, effective pin length, use of transfixing pins versus half-pins, and use of two versus three connecting rods. Each configuration parameter was varied independently under compressive, bending, and torsional load to determine the influence of that parameter on fracture stability under such loads. The interaction between these configuration parameters is complex. In general terms, fracture site stability may be increased by increasing common fragment pin separation, placing common fragment pins orthogonal to one another, increasing common fragment pin number, decreasing effective pin length, using transfixing rather than half-pins, and using as many connecting rods as possible. The difference between fracture site stability attainable with transfixing pins and with half-pins may be minimal if implementation of the other parameters combined with half-pins is optimized.     

Conventional bicortical pin substitution with a novel unicortical pin in external fixation: A biomechanical study

IntroductionAs most patients with polytrauma or open fractures are converted from temporary external fixation to definite stabilization, the prevention of complications such as infection is especially important. To overcome the high risk of infection associated with the use of the conventional bicortical pin for temporary external fixation, the authors developed a novel unicortical pin and analyzed it in a biomechanical study.MethodsThe unicortical pin consisted of an inner screw, purchasing the cortical bone, and an outer sleeve with 6 spikes. A bicortical pin was used for the purpose of comparison. A fracture gap model was stabilized using a monoplanar configuration. Both the unicortical pins (Uni group) and bicortical pins (Bi group) underwent axial compressive and torsional load testing using a servo-hydraulic testing machine. Stiffness, load to failure, and mode of failure were documented.ResultsStiffness and load to failure of the Uni group (average, 40.5 N/mm and 1098.4 N, respectively) were greater than that of the Bi group (average, 33.7 N/mm and 968.6 N, respectively) in the axial compressive load test (P = 0.008 and 0.032). Stiffness and load to failure of the Uni group (average, 1.2 Nm/degree and 1.7 Nm, respectively) were also significantly higher than those of the Bi group (average, 0.8 Nm/degree and 0.6 Nm, respectively) in the torsional load test (P = 0.008 and 0.016). All pins in the Bi group were bent at the pin-synthetic bone interface without synthetic bone failure. Contrarily, the Uni group did not show any pin bending or failure. However, in the axial compression test, partial cracks in the synthetic bone were found at the interface with spikes in the outer shell. In addition, in the torsion test, incomplete fractures were seen through the inner screws' holes.ConclusionCompared with the conventional bicortical pin, the newly designed unicortical pin significantly increased fracture stability under both axial compressive and torsional loads. The unicortical pin can be considered an alternative biomechanical solution to obtain adequate stability when performing external fixation of fractures.     

Bone properties affect loosening of half-pin external fixators at the pin-bone interface

IntroductionLocal bone yielding at the pin–bone interface of external fixation half-pins has been known to initiate fixator loosening. Deterioration of bone properties due to ageing and disease can lead to an increase in the risk of pin loosening. This study determines the extent, locations and mechanics of bone yielding for unilateral external fixation systems at the tibial midshaft with changes in age-related bone structure and properties. The study also evaluates the effect of the number of pins used in the fixation system and use of titanium pins (in place of steel) on bone yielding.MethodsWe employ nonlinear finite element (FE) simulations. Strain-based plasticity is used to simulate bone yielding within FE analyses. Our analyses also incorporate contact behaviour at pin–bone interfaces, orthotropic elasticity and periosteal–endosteal variation of bone properties.ResultsThe results show that peri-implant yielded bone volume increases by three times from young to old-aged cases. The use of three, rather than two half-pins (on either side of the fracture), reduces the volume of yielded bone by 80% in all age groups. The use of titanium half-pins resulted in approximately 60–65% greater volumes of yielded bone.ConclusionsWe successfully simulate half-pin loosening at the bone–implant interface which has been found to occur clinically. Yielding across the full cortical thickness may explain the poor performance of these devices for old-aged cases. The models are able to identify patients particularly at risk of half-pin loosening, who may benefit from alternative fixator configurations or techniques such as those using pre-tensioned fine wires.     

Avoiding iatrogenic vascular injury in tibial external fixation with half pins. An in-vivo study based on CT angiography

BackgroundExternal fixation is an important tool in the management of variety of tibial fractures. Appropriate half pin insertion is important, to provide stable fixation without compromising the surgical field for definitive surgical procedures, and avoiding further damage to the important structures of the traumatized limb. There is paucity of literature about the optimal trajectories and safe corridors for half pins insertion based on in vivo studies. The available studies are based on anatomic atlases, cadaveric studies or half pin related complications.The aim of the current study is to present the findings of CT angiograms, in patients with external fixation of tibia, to enhance our understanding of optimal trajectories in safe corridors for half pins insertion.Material and methodsWe performed a retrospective study of patients with external fixators on the tibia, who had undergone CT angiogram as part of pre-operative planning for orthoplastic reconstructive procedures. The relationship between the tips of the fixator half pins and named vessels of the leg were analyzed, pins within 5 mm of a named vessel were considered to be a risk of causing iatrogenic injury.ResultsA total 51 patients, with in situ temporizing external fixators, with 134 half pins in different segments of the tibia were analyzed. More than 5 mm of penetration beyond the far cortex was noted in 47%, while in another 16% of pins penetration was more than 10 mm beyond the cortex. A tip to vessel distance (TVD) of 5 mm or less was noted in 28/134 (21%) of the pins, which highlights potential risk to the neurovascular bundles of the leg.ConclusionRisk of iatrogenic injury to neurovascular structures from half pin insertion can be reduced by meticulous use of fluoroscopy, by avoiding penetration beyond the far cortex, and avoiding exiting with half pins on the lateral surface in the distal 1/3rd of segment II of tibia. Moreover observing optimal trajectories and safe corridors for pin insertion, and selection of appropriate type of half pin can mitigate the risk to these structures.     

Alendronate improves screw fixation in osteoporotic bone

BACKGROUND: Animal studies have demonstrated the efficacy of the use of bisphosphonates to enhance screw fixation in bone. In this prospective, randomized study of pertrochanteric fractures treated with external fixation, we tested whether systemic administration of bisphosphonates would improve the fixation of hydroxyapatite-coated screws implanted in osteoporotic bone. METHODS: Sixteen consecutive patients with a pertrochanteric fracture were selected. Inclusion criteria were female gender, an age of at least sixty-five years, and a bone mineral density T-score of less than -2.5 standard deviations. Exclusion criteria included bisphosphonate treatment during the two-year period prior to the fracture. Fractures were fixed with a pertrochanteric fixator and four hydroxyapatite-coated pins. Two pins were implanted in the femoral head (pin positions 1 and 2), and two were placed in the femoral diaphysis (pin positions 3 and 4). The patients were randomized either to therapy with alendronate for a three-month postoperative period (Group A) or to no therapy (Group B). The Group-A patients received an oral dose of 70 mg of alendronate per week. The fixators were removed after three months. RESULTS: All of the fractures healed, and no loss of reduction, nonunion, or delayed union was observed. The combined mean extraction torque (and standard deviation) of the pins implanted at positions 1 and 2 (cancellous bone) was 2558 +/- 1103 N/mm in Group A and 1171 +/- 480 N/mm in Group B (p < 0.0005). The combined mean extraction torque of the pins implanted at positions 3 and 4 (cortical bone) was 4327 +/- 1720 N/mm in Group A and 4075 +/- 1022 N/mm in Group B. CONCLUSIONS: These data show that weekly systemic administration of alendronate improves pin fixation in cancellous bone in elderly female patients with osteoporosis. We observed a twofold increase in extraction torque with the pins implanted in cancellous bone. These results support the use of alendronate in the treatment of osteoporotic pertrochanteric fractures to improve screw fixation in the femoral head.     

Comparative theoretical study of various external fixation devices for the stabilization of distal radius fractures

In the treatment of comminuted Colles' fractures different types of external fixation devices are used. Three main types are compared: (1) Fixation with two pins in each plane: Ace-Colles' type; (2) fixation with four or more parallel pins in one plane: Wagner/Hoffmann type; (3) fixation with four pins in one plane, two pins on each side of the fracture forming an angle of 60 degrees: ASIF-type fixator. These three types are compared with reference to the different forces that have to be neutralized by the seating of the pin in the bone. To minimize these forces with the aim of preventing pin-loosening, the theoretical results are used as the basis of practical surgical advice: (1) The distance between skin and fixator should be as short as possible. (2) The fixator should be fixed as close as possible to the fracture. (3) The diameter of the pins should be as great as possible. (4) If more than two pins per plane are used (Hoffmann/Wagner type), the pins should be wide apart. (5) If only two pins per plane are used (Ace-Colles Type) the bending stress on each pin is high (6) If more than two parallel pins per plane are used (Hoffmann/Wagner type), the axial forces on each pin are high. (7) The pins should be fixed at right angles to the fractured bone. Pins at other angles do more harm than good.     

Development of a variable stiffness external fixation system for stabilization of segmental defects of the tibia

The mechanical properties of a variable stiffness external fixation system were explored. Initial testing of a unilateral fixator configuration demonstrated that system rigidity could be increased by maximizing pin separation distance in the fracture component and the number of pins used while minimizing pin separation distance across the fracture site and the sidebar offset distance from bone. A triangulated system composed of half pin frames mounted anteriorly and medially on the tibial aspects and linked by crossbars was devised. Progressive disassembly of the frame was shown to result in progressive decreases in fixator rigidity in all planes.     

Development of a variable stiffness external fixation system for stabilization of segmental defects of the tibia

The mechanical properties of a variable stiffness external fixation system were explored. Initial testing of a unilateral fixator configuration demonstrated that system rigidity could be increased by maximizing pin separation distance in the fracture component and the number of pins used while minimizing pin separation distance across the fracture site and the sidebar offset distance from bone. A triangulated system composed of half pin frames mounted anteriorly and medially on the tibial aspects and linked by crossbars was devised. Progressive disassembly of the frame was shown to result in progressive decreases in fixator rigidity in all planes.     

Intramedullary pin fixation in clavicular fractures: A study comparing the use of small and large pins

The S-shaped clavicle poses a problem for intramedullary pin fixation. Stability of fracture fixation is closely related to the length of intramedullary pin engagement. This study was carried out to determine the engagement length of intramedullary pins into clavicular fractures using a small and a large pin. Seven pairs of fresh cadaveric clavicles were prepared and arranged into Group 1 and Group 2 for paired study. A mid-third clavicular fracture was created at the junction of the two curves of the clavicle. In Group 1, a 3.2 mm diameter threaded Steinman pin was introduced into the medullary canal of the clavicle by retrograde technique and the medial fragment of the fracture was drilled until the pin perforated the bone cortex. In Group 2, a 4 mm diameter threaded Steinman pin was used in the same manner. The results showed that Group 1 had an average engagement of pin into the clavicle of 9.11 cm with a ratio to total length of the clavicle of 0.59. In Group 2, the average engagement length into the clavicle was 7.17 cm with a ratio of 0.47. The difference was significant, with the smaller pin providing better fixation. The pins in both groups perforated the lateral fragment at the posterosuperior aspect and the medial fragment at the anterior aspect of the clavicle. The angle that the pin made with the long axis of the clavicle in Group 1 was 22.43 degrees and in Group 2, 26.57 degrees. Although the 3.2 mm diameter pin was more aligned to the long axis of the clavicle than the 4 mm diameter pin, the difference was not significant.     

Behavior of an external fixation frame incorporating an angular separation of the fixator pins. A finite element approach

A finite element model has been developed to simulate the deformation that occurs at the fracture site of an externally fixed bone as a result of applied bending, compression, and torsional loads. The pin configuration in this model is constructed to allow an angular separation of the fixator pins. The mechanical effect of this angular separation and of the distribution of the pins along the fixator bar is examined. The model shows that an angular separation of the pins provides a more symmetric deformation of the fracture site when a bending load is applied in different directions to the bone and thereby protects a fracture from excessive movement in any direction. The torsional stability of an external fixation frame is considerably increased by incorporating an angular separation of the pins. The model also shows that the most stable configuration for the fixator uses a wide separation of the pins along the fixator bar.     

The role of lateral-entry Steinmann pins in the treatment of pediatric supracondylar humerus fractures

Purpose  

Loss of pin fixation in supracondylar fractures can occur with failure to achieve bicortical fixation. Bicortical fixation may be challenging for those pins that attempt to penetrate the diaphyseal cortex, where the bone is thick. Lateral-entry Steinmann pins may allow for better penetration through cortical bone because they are more rigid than typical Kirschner wires.     

The effect of radial preload on the implant-bone interface: a cadaveric study

The most common complication of external fixation is pin loosening. Preloading the implant-bone interface is believed to retard this process. Radial preload, in particular, may be useful, as it allows loading in more than one direction. To investigate the effect of varying degrees of radial preload on the pin-bone interface, 30 freshly thawed human cadaveric tibiae were sectioned into 4-cm segments. Uniform drill holes were produced in the anterior tibial ridge of all segments and custom experimental bolts, oversized in diameter by as much as 1 mm, were pressed into each specimen. Macroscopic surface fractures were noted at the time of bolt insertion for misfits greater than 0.2 mm. Following histologic preparation, the implant-bone interface was evaluated microscopically based on the appearance of osteonal compression, lamellar distortion, and microfractures. Insertion of external fixator pins with misfits of greater than 0.4 mm resulted in significant microscopic structural damage to the bone surrounding the pin. High degrees of radial preload, exceeding the elastic limit of cortical bone, may be produced around pin holes by a small misfit. The use of oversized pins or screws must therefore be questioned.     

A study of external skeletal fixation systems for unstable pelvic fractures

This study was undertaken to determine the feasibility of constructing an anterior pelvic external fixator capable of resisting displacement of vertical shear fractures. Newly designed tapered thread pins for cancellous bone had better bone pin fixation and their greater 6-mm diameter provided more than twice the stiffness of Hoffmann 4-mm pins when tested in clusters. Using these pins and a more rigid anterior frame as one model, various pelvic fixators were tested to determine their resistance to vertical shear forces. These tests determined that this new fixator was 16 times stiffer than a Bonnel single anterior frame and five times stiffer than a double anterior frame (both constructed of Hoffmann components). Extrapolation from the data shows that 13 mm of posterior fracture displacement would occur with loads of one half body weight using the new fixator. Additionally it was noted that Hoffmann frames constructed with two 5-mm pins performed as well as those using three 4-mm pins.     

Safe zone of pin insertion for nonbridging external fixators in distal radial fractures: MRI analysis

BackgroundOf the anatomical reduction and fixation methods used to treat distal radius fracture, non-bridging external fixation has the advantage of enabling early wrist motion. The surgical technique relies on successful placement of the pin in individual fracture fragments. The present study aimed to identify the safe zone of pin insertion for a non-bridging external fixator into the distal radius that avoids metal impingement of extensor tendons.MethodsThe width and length of the septal attachments of the extensor retinaculum were measured on axial MR images of 62 wrists.ResultsThe 2–3 septum was the widest and longest, with a width of 2–7 mm and a location 0–36 mm proximal to the wrist joint. The width of the 1–2 septum was 2–6 mm, and was widest at 10 mm proximal to the joint. The 1–2 septum was triangular-shaped, while the 2–3 septum was oval-shaped. The 3–4 and 4–5 septa had narrow attachments and were adequate for pin insertion (with a pin 1–2 mm in width) at a position less than 8 mm proximal to the wrist. The width of the 1 R septum (radial to the 1st septum) was 2–6 mm at the radiovolar aspect of the wrist.ConclusionsThere were two safe pin insertion sites; the first was safe at the distal aspect only (8–10 mm proximal to the wrist) and included the 1–2, 3–4, and 4–5 septa, while the second was safe from 0 mm to 32–38 mm proximal to the wrist and included the 1 R and the 2–3 septa. The 1 R septum had adequate size for use as a new pin insertion site that aligns in the internervous plane and has minimal risk of superficial radial nerve injury.     

External fixation of the pelvic ring: An experimental study on the role of pin diameter,pin position,and parasymphyseal fixator pins

Background?The mechanical properties of current external fixator systems for unstable (type C) pelvic ring fractures are inferior to internal fixation, and are not optimal for definitive treatment. We explored methods to increase stability of external fixator constructs.

Methods?An experimental model was used for load tests. The same pelvic fixator was used while different pin diameters, pin positions, and modes of pubic symphysis fixation were tested.

Results?Changing of the pin diameter of the unthreaded part from 6 to 8?mm resulted in an increase in stiffness of 20%. An increase in stiffness by a factor of 1.9 was found by placing a pin on the iliac crest and one supra-acetabular. An additional increase by a factor of 3.6 was obtained by adding pubic symphysis plate fixation. Parasymphyseal pin fixation instead reduced stiffness, but not so much as when parasymphyseal pins were connected to the external fixator of the pelvic ring. The final configuration was at least 6 times stiffer than the initial configuration.

Interpretation?The new concept of parasymphyseal pin fixation connected to an external fixator of the pelvic ring produces a considerable increase in stability for the treatment of type C pelvic ring injuries, as does an increase in pin diameter and alternative pin positioning.     

Comparison of treatment of fracture midshaft clavicle in adults by external fixator with conservative treatment

PurposeHigh rate of malunion and non union in displaced fracture clavicle treated conservatively lead to use of different types of internal fixation methods which also were found to be associated with various complications. Moreover their superiority over conservative treatment has not been established. This study was designed to compare clinical outcome of conservative treatment with external fixator in cases with displaced midshaft clavicle fractures.MethodsFifty adult consenting cases of acute midshaft fracture clavicle, displaced >15 mm were included. Twenty five cases were allotted to conservative (group A) and external fixator (group B) each. In group A treatment was given in form of clavicle brace. In group B schanz pins were inserted obliquely between supero-inferior and anterior-posterior direction and connected with rod. The outcome was measured by Constant score, union time and complications.ResultsMean radiographic union time in group A was 23.45 ± 1.40 weeks (with 8% non union and 80% malunion) and in group B it was 9.36 ± 1.49 weeks. Mean Constant score at 6 months in group A was 78.28 ± 6.45 and in group B 92.72 ± 1.48. Mean shortening at 6 months in group A was 19.36 mm. In group B shortening at 6 months was noticed in three cases (6, 5, 6 mm).ConclusionClose reduction of acute fracture mid clavicle and application of external fixator is a simple procedure providing the benefits of rigid fixation and undisturbed fracture environment. Pain relief is faster, union time is shorter and there are no hardware related problems.     

Antimicrobial efficacy of external fixator pins coated with a lipid stabilized hydroxyapatite/chlorhexidine complex to prevent pin tract infection in a goat model.

BACKGROUND: Pin tract infection is a common complication of external fixation. An antiinfective external fixator pin might help to reduce the incidence of pin tract infection and improve pin fixation. METHODS: Stainless steel and titanium external fixator pins, with and without a lipid stabilized hydroxyapatite/chlorhexidine coating, were evaluated in a goat model. Two pins contaminated with an identifiable Staphylococcus aureus strain were inserted into each tibia of 12 goats. The pin sites were examined daily. On day 14, the animals were killed, and the pin tips cultured. Insertion and extraction torques were measured. RESULTS: Infection developed in 100% of uncoated pins, whereas coated pins demonstrated 4.2% infected, 12.5% colonized, and the remainder, 83.3%, had no growth (p < 0.01). Pin coating decreased the percent loss of fixation torque over uncoated pins (p = 0.04). CONCLUSION: These results demonstrate that the lipid stabilized hydroxyapatite/chlorhexidine coating was successful in decreasing infection and improving fixation of external fixator pins.     

Cortical bone pegs in the treatment of osteochondral fracture of the knee

Cortical bone pegs have been used in osteochondritis dissecans to fix the loosened or loose fragment. In osteochondral fractures of the knee the fragment has either been removed or reattached usually with headless pins or ASIF screws and only occasionally with cortical bone pegs. We report on four patients with osteochondral fractures of the knee, where cortical bone pegs were used as the means of fixation, with good results.     

Stiffness of small-bone external fixation methods: an experimental study

A variety of small-bone external fixation methods were evaluated to determine bending and torsional stiffness. Several methods of external pin stabilization with bone cement and with a commercial device were used. Among experimental variables examined were: the number of pins, pin diameter, pin length, pin spacing, and pin threading. The most rigid fixation was achieved with four pins held with a wire-reinforced bone cement fixator. Pin diameter was the most significant variable in the determination of stiffness with this configuration.     

儿童肱骨髁上骨折模型克氏针固定构型的三维有限元分析

目的:建立带骺软骨儿童肱骨远端新力学模型,模拟肱骨髁上骨折并进行三维有限元分析,研究克氏针的数量、针道、出口高度及穿针构型对固定稳定性的影响。方法:于图片存档及通信系统(picture archiving and communications system,PACS)中挑选1名6岁男孩的正常肱骨远端三维CT,将图像数据导入Simpleware及SolidWorks 2016软件,构建1个含有肱骨小头骨化中心及远端骺软骨的儿童肱骨远端模型。制作常见伸直型儿童肱骨髁上骨折模型,并模拟交叉克氏针及单纯外侧克氏针的多种固定构型,在屈伸、内外翻方向施加30N,内外旋方向施加5 N的力,通过远折端的位移情况分析其稳定性。结果:2枚克氏针固定构型中,出口在骨干-干骺端交界区(metaphyseal-diaphyseal junction,MDJ)上缘的2枚交叉针最稳定,可对抗超过2 585 Nmm/°的旋转应力;而在对抗屈伸及内外翻应力测试中,低位针经过肱骨小头骨化中心的2枚外侧针最稳定,分别可对抗接近45 N/mm及190 N/mm的应力。在最稳定的外侧2枚克氏针构型中加入第3根针,所有方向上的稳定性显著增加,其中3枚交叉针最稳定,屈伸、内外翻及内外旋时分别可对抗198 N/mm、395 N/mm及6 251 Nmm/°的应力。结论:2枚外侧分散针可为儿童肱骨髁上骨折提供足够的稳定性。在2枚交叉针构型中,出口高度在MDJ区域上缘的交叉针最稳定。3枚交叉针构型在所有研究构型中最稳定。