External fixation Weerawat Frame (bar/ring) + pins/wires/screws Neutralize deforming forces Provide rigid stability to maintain the fracture Temporary reduction at length to avoid collapse of fracture Frame biomechanics Most important factors: the strength, competency of pin-bone interface 1. Pin geometry and thread design Screw diameter stiffness (strength) of the frame, risk of stress Fx at entry portal Screw hole >30% diameter 45% reduction of torsional strength of bone In adult, pin diameter 6mm. is maximum Small pitch height, low pitch angle dense cortical bone Large pitch height, high pitch angle cancellous bone Conical pins (taper thread) optimize screw-bone contact A. The use of tapered pins facilitates subsequent pin removal. B. Self-drilling pins with drill-type pin tip. C. Pins with a larger thread diameter suitable for cancellous bone insertion. D. Pins with small pitch angle and narrow thread diameter are applied in cortical bone. E. Hydroxyapatite-coated pins improve the pin–bone interface by encouraging direct bone apposition and ingrowth FIGURE 7-9 Various thread designs are used for specific purposes. 2. 3. 4. The pin-bone interface, biomaterials Titanium & titanium alloys (vs stainless steel): lower pin-bone interface stress than, lower rate of pin sepsis HA-coated taper pin: increase strength of fixation, lower rate of pin tract infection (cancellous > cortical) Pin insertion techniques Oversize pins >0.4mm. mismatch stress fracture Predrilling (sharp drill): reduce thermal necrosis, bone damage Self-tapping: increase purchase bone Self-drilling: increase heat, microfracture at both cortices, decrease pull out strength but clinically does not appear to be any increase incidence of pin tract infection or other pin associated complications Pin bone stress Large pin fixation Monolateral frames types Separate bars, attachable pin-bar clamps, bar-tobar clamps, separate Schanz pins Relative flexible Compression technique: arthrodesis, osteotomy, nonunion repair Distraction technique: deformity correction, intercalary bone transport, limb lengthening Simple monolateral fixators Simple four-pin frame concept - Allowing individual pins to be placed at difference angles and obliquities Simplicity Uniplanar, minimizing the transfixion of soft tissues Shear force can inhibit Fx healing and bone formation FIGURE 7-13 A simple four-pin external frame is the basic building block for more complex frame configurations. Increase stability (rigidity) of simple frame EF Increase number and size of pins, bars, side (monolateral, bilateral), planes Increase maximize pin separation distance, interconnecting bar distance Increase connection between planes, delta frame 90o of two plane (but some parts of anatomy can’t do that) Minimize distance between bone and connecting bar The weakest part of system is junction between fixator body (bar) and clamp or Schanz pin and clamp (the device should be routinely tightened during Rx) FIGURE 7-12 Factors affecting the stability of monolateral external fixation include pin distance from fracture site, pin separation, bone–bar distance, connecting bar size and composition, pin diameter, pin number, and pin–bone interface. A, pin to center of rotation; B, pin separation; C, bone–bar distance. Monotube fixators Fixed location for pin mounted in pin clusters thus the ability to vary pin location substantially less than simple monolateral fixators Can’t minimize bone-frame distance (large body configuration) Higher bending stiffness (large body configuration) Maximize rigidity in the plane of the pins and minimal at right angles to this plane Lower holding strength of pin clamp Steerage pin placement Pins placed perpendicular to long axis of bone fails to oppose the shear force vector Pins placed parallel to the fracture line (oblique Fx) and thus indirect opposition to shear force vector, the shear force actively converted into compression (dynamic stabilization of Fx edges) For Fx obliquities <30o: standard mode of fixation can be used For Fx obliquities 30o-60o: steerage pin concept can used For Fx obliquities >60o: shear is dominating (extreme) force; even with steerage pins Small wire fixation Multiple plane of fixation Tethering soft tissues (use small wire to reduce this) Eliminates the harmful effects of cantilever loading and shear forces Accentuating axial micromotion and dynamization A typical four-ring frame consists of eight crossed wires, two wires at each level, and four rings with supporting struts connecting two rings on either side of the fracture Circular fixators are less rigid than all other monolateral type fixators in all modes of loading, particularly in axial compression However, this may prove to be clinically beneficial to allow for axial micromotion and facilitate secondary bone healing Ilizarov fixator ทนต่อ axial motion (axial compression) ดีกว่า, แต่ทน shear force ได ้ใกล ้เคียง เมือ ่ เทียบกับ fixators แบบอืน ่ ๆ The overall stiffness and shear rigidity of the Ilizarov external fixator are similar to those of the half-pin fixators in bending and torsion Increase stability (rigidity) of ring EF 1. Increase wire diameter 2. Increase wire tension 3. Increase pin crossing angle to approach 90o 4. Decrease ring size (distance of ring to bone) 5. Increase number of wires 6. Use olive wires/ drop wires 7. Close ring position to either side of fracture FIGURE 7-6 Ilizarov's circular fixator using small (pathology) site tensioned wires attached to individual rings. 8. Center bone in middle of ring Wires Basic component: smooth 1.5, 1.8, 2mm. Strength & stiffness: increase ~ diameter, tension Beaded (olive) wires: compress to near cortex while far side tensioning, perform interfragmentary compression, stabilization (that smooth wire can’t achieve) Wire tension Limb lengthening: high tension up to 50kg Weight bearing and limb loaded: high tension up to 130kg Additional tension ทีเ่ พิม ่ ขึน ้ อีกจากการดึงขาหรือลงน้ าหนักจะทาให ้ wire breakage ได ้ ดังนัน ้ initial tension ต ้องคานึงจุดนีด ้ ้วย Ring diameter - Increase diameter Increase bone-ring distance decrease stability Decrease diameter greater effect than increase wire tension Wire orientation Wires placed parallel to each other, and parallel to the applied forces, provide little resistance to deformation The most stable configuration occurs when two wires intersect as close to 90 degrees as possible Changing pin orientation to a less acute angle decreases the stiffness in AP bending but has a lesser effect on lateral bending, torsion, and axial compression Clinically, a wire divergence angle of at least 60 degrees should be attempt Limb positioning in the ring The location of the tibial bone in the limb is actually eccentric to the external soft tissue envelope compared with the humerus or the femur To place a frame on a tibia with the center/center orientation, a very large ring would be needed; this would vastly increase the ring–bone distance and further decrease the frame stiffness Circular fixators are less rigid than all other monolateral type fixators in all modes of loading, particularly in axial compression Wire connecting bolts Mechanical slippage between the wire and the fixation bolt is the primary reason for loss of wire tension and thus frame instability When clamping a wire to the frame, the wire tension is reduced by approximately 7% To prevent yield of the clamp wire system, the fixator should be assembled with sufficient wires to ensure that the load transmitted to each wire by the patient does not exceed 15 Newtons Placing at least two tensioned wires onto each ring present in the frame construct Stiffness of a tensioned wire frame is more dependent on bone preload than on wire number, wire type, or frame design. Preload stiffness can be increased simply by compressing the rings together and achieving bone-on-bone contact Hybrid fixators - - Take advantage of tensioned wires' ability to stabilize complex periarticular fractures Should include a ring incorporating multiple levels of fixation in the periarticular fragment. Accomplished with at least three tensioned wires If possible, additional level of periarticular fixation is advantageous using adjunctive half-pins, in addition to the tensioned wires Multiple connecting bars or a full circular frame is preferred with a minimum of four half-pins attached to the shaft component Biology of EF and distraction histogenesis - External bridging callus Endochondral (indirect) bone formation Reduce micromotion by increase frame stiffness reduce healing rate Large motion (large gap, less rigid) increase blood vessels and fibrocartilage decrease remodeling and bone formation (hypertrophic nonunion) Dynamization - Convert static fixator and allows the passage of forces across the Fx site to occur Restore cortical contact and produce stable Fx pattern with inherent mechanical support Active dynamization of the fracture occurs with weight bearing or with loading when there is progressive closure of the fracture gap Limited Open Reduction/Internal Fixation with External Fixation - Very useful in metaphyseal bone and has been shown to work well in periarticular fractures - Abandoned in diaphyseal regions because of the increased incidence of pseudoarthrosis Distraction osteogenesis - Mechanical induction of new bone occurs between bony surfaces that are gradually pulled apart (the formation of a physis-like structure) With stable fixation direct intermembranous ossification Distraction osteogenesis also provides a significant neovascularization effect Distraction rate of 0.5 mm/day or less premature consolidation of the lengthening bone Distraction rate of 2 mm/day or greater undesirable changes within the distracted tissues Distraction rate of 1 mm/day เหมาะสมทีส ่ ด ุ (Ilizarov แนะนาให ้แบ่งเป็ น 4ช่วงในการยืดต่อวัน) Motion present at the fracture site bone resorption, local blood supply is traumatized by the moving bone ends atrophic fibrous nonunion Circular frames ลด abnormal force ใน compression mode ลดการทาลาย neovascular region --> ทาให ้มี endochondral remodeling Metaplasia and differentiation responses: bone > muscle > ligament > tendons Neurovascular structures จะค่อยๆเจริญขึน ้ แบบช ้าๆ, แต่จะไม่ทนต่อ acute distraction forces Contemporary EF applications - Temporary spanning fixation for complex articular injuries Valuable in polytrauma patient when rapid stabilization is necessary For periarticular fractures, the decision to convert to definitive stabilization is usually based on the adequacy of soft tissues. Latency period of at least 10 to 14 days is generally required to allow the soft tissues to recover to the extent where contemporary internal fixation techniques can be undertaken safely With long bone fractures (esp tibia), definitive conversion to IM nailing has within the first 2 to 3 weeks of frame to avoid colonization of the medullary canal by the external fixator pins Anterior pelvic external fixator constructs may be difficult in an obese patient. C-clamp used to provide posterior stability temporarily Pelvic frames are most useful in fractures that are vertically stable Definitive Fx management - Choice of EF depends on location and complexity of the fracture, type of wound The less stable the fracture pattern, the more complex a frame needs to be applied to control motion at the bone ends If possible, weight bearing should be a consideration If periarticular extension or involvement, the ability to bridge the joint provides stability for both hard and soft tissues Allow for multiple débridements and subsequent soft tissue reconstruction Pins are placed outside the zone of injury to avoid potential pin site contamination with the operative field Ring fixators have a definite advantage for extra-articular injuries in that they allow for immediate weight bearing and can gradually correct deformity and malalignment, as well as achieve active compression or distraction at the fracture site Monolateral applications - - - - Major indication is in the distal radius and in the tibial shaft, followed closely by temporary application of trauma frames for complex femoral and humeral shaft injuries Much less common for forearm injuries In distal radius Fx: o 2 pins in the metacarpals, 2 pins in the distal aspect of the radius proximal to the fracture line, wrist position can be adjusted into neutral or extension to help avoid finger stiffness and carpal tunnel syndrome without compromising fracture reduction o For unstable fractures, augmentation of the fixator construct with multiple dorsal and radial percutaneous pins corrects the dorsal tilt and maintains the reduction o External fixation devices function best when maintaining radial length alone. o This is best accomplished with frames that do not span the wrist joint but just cross the fracture, leaving the wrist free In acute femoral Fx o At least four pins placed along anterolateral aspect of femoral shaft o Independent pins placed out of plane increased stability over monotube or simple monolateral frames (straight line) In acute humeral shaft Fx o Pin tract sequelae and inhibit shoulder & elbow motion o Most frequent indication is stabilization of severely contaminated open Fx or gunshot wounds that occur in association with vascular disruption In tibial Fx o In general, the most proximal and distal pins are first inserted and the connecting rod is attached o Alternatively, the proximal 2 pins and distal 2 pins connected by solitary bars (use as reduction tools). Once reduction has been achieved, additional bar–to-bar construct is connected o In highly comminuted Fx, weight bearing is delayed until visible callus is achieved o Dynamization does seem to facilitate Fx healing if used within first 6-8 wk after Fx o If discrepancy of >1.5-2 cm, dynamization is not indicated Small wire EF Ideally suited to high energy Fx involve metaphyseal regions Olive wires con be used “tension-compression fixation”, similar to effect of small lag screws “Hybrid” = traditional monolateral diaphyseal bar attached to solitary circular periarticular ring, at least 3 divergent bars and 270o of separation 4-wire fixation construct = dual plating for bicondylar tibial plateau Fx Bone transport - Cancellous grafting placed directly into the defect through posterolateral approach Internal bone transport (primary method of bony reconstruction) indicated for defects greater than 4cm Transport is delayed for at least 3wk after free flap coverage If no flap, corticotomy and transport can be undertaken immediately at the time of wound closure Latency period 7-10days allowed before initiation of transport Distraction 0.25-0.5mm/day Transport in acute fracture proceeds at much slower rate, 0.5-0.75mm/day (standard rate of 1mm/day typical for standard limb lengthening Acute shortening (decrease transport distance) greater than 4cm is not recommended Docking site is impacted and gradually compressed 0.25mm every 48hr until docking site is radiographically healed Hexapod fixators Frame management Pin insertion technique - Incising skin directly at site of pin insertion Trocar and drill sleeve is advanced directly to bone Predrilling, appropriate depth of pin is advanced to achieve bicortical purchase - Correct pin site insertion technique removes most of factors that cause pin site infection and subsequent pin loosening Immediate postoperative compressive dressing should be applied, removed within 10d-2wk If pin drainage does develop, providing pin care 3 times/day should be undertaken Once healed, only showering is necessary Removal of serous crust, recommendations include using NSS as the cleansing agent in concert with dilute HO Avoid ointments for post cleansing care - ต ้องเห็น 3 ใน 4 neocotecies ใน regenerate zone, film AP + lat Consolidation time อย่างน ้อย 1.5-2 เท่า ของ total distraction time - Infection, loosening, metal fatigue failure Pin care Frame removal Pin complications Premature consolidation - - Most, incomplete osteotomy > premature healing of osteotomy site Most common in pediatric population Early phase: distract ต่อจนแยกกันใหม่ (audible ache, snap, pop + sudden pain & swelling) หลังจากนัน ้ ให ้หยุดดึงแล ้วเปลีย ่ นเป็ น compress จนหายปวด แล ้วค่อยดึงต่อ (ถ ้า premature consolidation fracture แล ้วยังดึงต่อไปเรือ ่ ยๆ จะเกิด rupture neovascular channels cyst incomplete regenerate formation and failure Most common causes of incomplete regenerate: disruption periosteum and soft tissue during corticotomy, too rapid distraction, frame instability Regenerate reFx or late deformity after removal of apparatus: premature frame removal before complete healing of regenerate or Fx Contractures - From excessive joint distraction, extended period of time Preventive measures: avoiding transfixion of tendons and maximizing muscle excursion before placing transfixion wires or half-pins Physical therapy, splinting is helpful
