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.     

Healing patterns of transverse and oblique osteotomies in the canine tibia under external fixation

Healing patterns were compared between transverse and 60 degrees oblique osteotomies in canine tibiae stabilized under external fixation. Under similar in vitro testing conditions (osteotomy ends in contact), the axial stiffness of the oblique osteotomy under external fixation was 45% of the paired transverse osteotomy. Under torsion and bending, the two sides exhibited similar fixation rigidity. The animals put significantly less weight on the oblique side than on the transverse side in the early phases of bone healing (p less than 0.05 for static and p = 0.024 for dynamic weight bearing). The osteotomy bending stiffness at 60 days (in vivo) and the torsional stiffness of osteotomy union measured after sacrifice at 90 days were significantly higher on the transverse side (p = 0.013 and p = 0.016, respectively). Intracortical new bone formation was significantly higher on the transverse side (p less than 0.01), indicating a difference in the rate of cortical healing and remodeling. The average pin removal torque was significantly lower on the oblique side (p less than 0.05), a sign of increased pin loosening. In the clinical situation, unstable oblique or spiral fractures should be protected from weight bearing even under stable external fixation due to the delayed recovery of bone union stiffness property as well as increased pin tract problems.     

Anodic plasma chemical treatment of titanium Schanz screws reduces pin loosening

OBJECTIVE: This study was designed to assess the benefits of a new Anodic Plasma Chemical calcium-phosphate (APC-CaP) surface treatment on reducing pin track infection and pin loosening in comparison to anodized titanium (Ti) during external fracture fixation. METHODS: A tibial midshaft, transverse, 6-mm gap osteotomy was created in 17 adult female Swiss alpine sheep. The tibia was stabilized with an external fixator and 4 Schanz screws of Ti or APC-CaP-treated Ti. The sheep were examined during a 12-week observation period. Infection was assessed with weekly clinical pin track grading and microbiologic assessment at sacrifice. Pin loosening was assessed by grading for radiolucency on biweekly radiographs and by measuring extraction torque on pin removal. In vivo bending stiffness measurements were performed to determine gap healing. A qualitative histologic assessment of the tissue adjacent to pin sites was also performed. RESULTS: A trend (P = 0.056) for less infection around APC-CaP pins was found at 6 weeks, but the strength of this difference diminished with time. Significantly more radiolucency was found around Ti pins after 8 (P = 0.011) and 12 (P < 0.001) weeks. At all pin sites, the extraction torque for APC-CaP pins was higher than for Ti pins (P = 0.007). No difference in the progression of gap healing was found. Histology showed bone growth at the implant surface in the form of distance osteogenesis for Ti and contact osteogenesis for APC-CaP. CONCLUSIONS: This study has shown that the APC-CaP surface improves the clinical performance of Ti pins with respect to pin loosening and pin track infection.     

Biomechanical analysis of pin placement and pin size for external fixation of distal radius fractures

A series of biomechanical analyses were performed to explain the recent reduction in treatment-related complications of external fixation of distal radius fractures using a limited open approach for pin placement and larger 4-mm self-tapping half pins. A comparison of pull-out strength, stress concentration effect, and inherent bending strength of 3- and 4-mm half pins was performed. The effect of proximal pin placement in the radius or in the ulna and the effect of distal pin placement in four, six, or eight metacarpal cortices were determined. These analyses demonstrate that the 4-mm self-tapping half pins result in a significantly higher pull-out strength and only a small decrease in torsional load strength of the bone. They also demonstrate that proximal pin fixation in the radius produces the most stable fixation and that distal pin fixation into six metacarpal cortices produces a strong configuration that does not violate the interosseous muscles of the second intrinsic compartment. The rate of treatment-related complications in the external fixation of distal radius fractures (specifically, pin loosening, bending and breakage, fracture through pin sites, collapse at the fracture site, and intrinsic contracture) are addressed in this study. Such complications can be minimized by using 4-mm pins after central predrilling, with proximal placement in the radius and distal placement through six cortices of the bases of the second and third metacarpals.     

An <Emphasis Type="Italic">in Vivo</Emphasis> Experimental Comparison of Stainless Steel and Titanium Schanz Screws for External Fixation

Abstract Objective: To compare the clinical benefits of stainless steel (SS) to titanium (Ti) on reducing pin track irritation/infection and pin loosening during external fracture fixation. Methods: A tibial gap osteotomy was created in 17 sheep and stabilized with four Schanz screws of either SS or Ti and an external fixation frame. Over the 12 week observation period, pin loosening was assessed by grading the radiolucency around the pins and measuring the extraction torque on pin removal at sacrifice. Irritation/infection was assessed with weekly clinical pin track grading. A histological analysis of the tissue adjacent to the pin site was made to assess biocompatibility. Results: A statistically non-significant trend for less bone resorption around Ti pins was found during the early observation period. However, at sacrifice, there was no difference between the two materials. Also, there was no difference in the extraction torque, and there was similar remodeling and apposition of the bone around the pins. A statistically non-significant trend for more infection about SS pins at sacrifice was found. Histology showed a slightly higher prevalence of reactionary cells in SS samples, but was otherwise not much different than around Ti pins. Conclusions: There is no clinically relevant substantial advantage in using either SS or Ti pins on reducing pin loosening or pin track irritation/infection.     

Parametric analyses of pin-bone stresses in external fracture fixation devices

Problems occurring in the use of external fixators for bone fractures include pin-bone interface necrosis, infection, and loosening. These may be initiated and enhanced by pin-bone interface stress levels. Based on stress data from finite element method (FEM) models, an analytical "closed-form" model of the local pin-bone configuration in long-bone fracture fixation is developed. This model is relatively simple and useful for routine applications in combination with clinical studies and animal experiments. Although approximate, the most significant stress predictions in the pin-bone structure compare well with more sophisticated FEM analysis results. The analytical model is used for extensive parametric analyses, investigating the effects on the pin-bone interface stress distribution of frame configuration parameters, pin diameter and modulus, bone dimensions, and elastic characteristics. The results indicate that these stresses may reach very high levels under unfavorable circumstances but can be drastically reduced by increasing the bending rigidity of the pin, reducing the side-bar separation and applying full-pin configuration in favor of half-pins.     

Hydroxyapatite-coated Schanz pins in external fixators used for distraction osteogenesis : a randomized,controlled trial

BACKGROUND: Complications of external fixation include loosening of the fixation pins and pin-track infection. Laboratory studies and clinical trials have suggested that hydroxyapatite coating improves the osteointegration of various orthopaedic implants. The purpose of this study was to determine whether the prevalence of pin-related complications can be reduced by the use of hydroxyapatite-coated pins in fixators applied for distraction osteogenesis. METHODS: Forty-six consecutive patients undergoing segmental transport or lengthening of the tibia were randomized to the use of either standard titanium Schanz pins or hydroxyapatite-coated stainless-steel Schanz pins. The fixators were used for an average of thirty-eight weeks (range, fourteen to seventy-two weeks). All patients were closely monitored for pin loosening and infection, and digitized radiographs were assessed for bone resorption around all pins. When the external fixator was removed, the torque required to extract the pins was determined with use of an electronic torque wrench. RESULTS: In the control group (titanium pins), twenty-two pins (13%) loosened and an infection occurred at the site of twenty pins. An extensive infection of the canal developed in one patient. Twenty-two pins were removed or replaced because of these complications. In the hydroxyapatite group, no clinical or radiographic signs of pin loosening or infection were observed and no pins required early removal or exchange. The mean torque (and standard deviation) required to remove the hydroxyapatite-coated pins was 0.43 0.18 N-m compared with 0.10 0.09 N-m for the uncoated pins (p < 0.001). CONCLUSION: Coating pins with hydroxyapatite increases their fixation to bone and reduces the rate of infection and loosening during external fixation for distraction osteogenesis. Use of hydroxyapatite-coated pins should be considered in clinical situations requiring prolonged external fixation.     

Hydroxyapatite-coated external-fixation pins. The effect on pin loosening and pin-track infection in leg lengthening for short stature

Pin loosening and infection are inherent complications of external fixation. This study deals with their effects of using either hydroxyapatite (HA)-coated or uncoated external fixation pins in leg-lengthening procedures on patients of short stature. We used HA-coated pins on one side and uncoated pins on the other (randomly determined) in 28 bilateral lengthenings undertaken in 23 patients. A total of 322 pins was used. The mean implantation time was 530 days and the mean lengthening achieved was 78% of initial bone length. Mean extraction torque was 7611.6 Nmm degree(-1) for HA-coated and 85.4 Nmm degree(-1) for uncoated pins (p < 0.001). The rate of pin loosening was 4% (7/161) for HA-coated and 80% (129/161) for uncoated pins (p < 0.001). There was no statistically significant difference in the incidence of pin-track infection between the two groups. The use of HA coating appears to be an effective method of reducing the incidence of pin loosening in external fixation with a long implantation time and for mechanically highly stressed procedures such as leg lengthening for short stature.     

The effect of rigidity on fracture healing in external fixation

Knowledge of the basic biomechanics of external fixation is necessary to obtain the full benefits of the technique for bone fracture treatment. The rigidity of external fixation, including pin-bone interface stresses, is discussed and bone healing and remodeling under different fixation stiffnesses and fracture gap conditions are described. The rigidity of fixation ultimately depends on the biomechanical characteristics of the fracture, the accuracy of reduction, and the amount of physiologic loading. Comparative experiments using a canine tibial fracture model have suggested that fixation rigidity is important in early bone healing and in the prevention of pin loosening. Bone union can be achieved under external fixation through different pathways, ranging from callus-free gap healing under a rigid neutralization configuration to direct-contact healing with periosteal new bone formation under axially dynamized stable fixation. Cortical reconstruction by secondary osteons seems to be important for the ultimate strength of the bone union.     

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.     

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.     

Fluid accumulation and the rapid spread of bacteria in the pathogenesis of external fixator pin track infection

An animal model was used to study the pathogenesis of pin track infection. The roles of fluid accumulation around the pin/bone interface and mechanical loosening of the pin were specifically studied. In addition, the spread of bacteria in relation to the clinical appearance of the pin track was assessed. This study demonstrated that fluid accumulation around the interface is an important factor in the spread of infection from the superficial wound track to the medulla of the bone. Spread occurs despite the absence of mechanical pin loosening, and can occur before any clinical features of infection are apparent.     

LCP钢板与外固定架治疗C型桡骨远端骨折的疗效比较

  目的 比较LCP钢板与外固定架治疗C型桡骨远端骨折的疗效。方法 回顾性分析2007年10月至2010年10月分别采用LCP钢板和外固定架治疗C型桡骨远端骨折的病例。其中LCP组38例,男13例,女25例;年龄23~65岁,平均45.1岁;按AO/OTA分型：C1型15例,C2型18例,C3型5例。外固定组26例,男12例,女14例;年龄25~60岁,平均47.1岁;按AO/OTA分型：C1型11例,C2型13例,C3型2例。比较两组术后影像学指标、功能指标,并采用Gartland-Werley功能评分标准对术后疗效进行评估。结果 所有患者均获得随访,随访时间LCP组12~36个月,平均23.8个月;外固定组12~44个月,平均31.2个月。X线片均显示两组骨折全部愈合。两组患者尺偏角、桡骨高度、腕关节背伸、掌屈、尺偏、桡偏、旋前、旋后及Gartland-Werley评分均无显著性差异。LCP组掌倾角11.71°±3.02°,外固定组掌倾角7.15°±2.98°,LCP组优于外固定组。LCP组伤口感染2例,正中神经损伤1例,固定松动1例;外固定组钉道感染3例,固定松动1例,腕关节僵硬2例。结论 两种固定方式治疗C型桡骨远端骨折均可达到较为满意的临床效果,LCP钢板术后影像学结果稍好,固定可靠。临床可根据患者具体情况选择治疗方式。     

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.     

A skeletal traction stirrup with self-aligning low friction joints to reduce pin loosening

Skeletal traction may be complicated by loosening of the pin transfixing the bone, especially if the bone is osteoporotic. A loose pin predisposes to infection of the pin track.A skeletal traction stirrup with self-aligning low friction joints has been developed to reduce the incidence of pin loosening.     

Experimental analysis of effects of pin pretensioning on external fixator rigidity

The effects of same-fragment pin pretensioning on the rigidity of a simple fixator system were investigated. An external fixator device constructed to apply same-fragment pin pretensioning of a defined amount was applied to a bone model made of pedilen cylinders. System rigidity was tested for varying degrees of pin pretensioning in axial compression and AP bending. Pin pretensioning, bringing transfixing pins together, tended to weaken the system in both modalities tested. Tensioning by bringing the pins apart increased the rigidity of the system in AP bending and axial compression.     

Injectable calcium phosphate bone cement Norian SRS for the treatment of intra-articular compression fractures of the distal radius in osteoporotic women

BACKGROUND: Distal radius fracture often presents a metaphyseal void which is more extended in elderly, osteoporotic patients. Bone graft and bone substitutes are reported to be beneficial in maintaining metaphyseal reduction. METHODS: We performed a prospective study on 52 menopausal, osteoporotic women with unstable intra-articular distal radius fractures to compare the outcome of percutaneous pinning and immobilisation in a cast for 6 weeks with that using injectable calcium phosphate bone cement (Norian Skeletal Repair System, SRS) to supplement pin and screw fixation and immobilisation in a cast for 3 weeks. All patients were reviewed 2 years (range 21-29 months) after surgery. RESULTS: Patients treated with SRS had better functional outcome, restoration of movement and grip strength ( p<0.001). In this group there was 1 mm loss of radial length, 3 degrees loss of radial inclination and 7 degrees loss of palmar tilt. In the control group radial length decreased 3 mm, radial inclination decreased 11 degrees and palmar tilt 12 degrees. Loss of reduction was significantly higher in the control group ( p<0.001). CONCLUSION: We conclude that the use of Norian SRS to supplement pin and screw fixation is effective in maintaining the reduction of unstable intra-articular distal radius fractures in osteoporotic patients and provides a better clinical outcome than percutaneous pinning.     

Experimental analysis of effects of pin pretensioning on external fixator rigidity

Summary The effects of same-fragment pin pretensioning on the rigidity of a simple fixator system were investigated. An external fixator device constructed to apply same-fragment pin pretensioning of a defined amount was applied to a bone model made of pedilen cylinders. System rigidity was tested for varying degrees of pin pretensioning in axial compression and AP bending. Pin pretensioning, bringing transfixing pins together, tended to weaken the system in both modalities tested. Tensioning by bringing the pins apart increased the rigidity of the system in AP bending and axial compression.