Principles of External Fixation

1. Principles of External Fixation Roman Hayda, MD Original Authors: Alvin Ong, MD & Roman Hayda, MD; March 2004; New Author: Roman Hayda, MD; Revised November, 2008

2. Overview • Indications • Advantages and disadvantages • Mechanics • Biology • Complications

3. Definitive fx care: Open fractures Peri-articular fractures Pediatric fractures Temporary fx care “Damage control” Long bone fracture temporization Pelvic ring injury Periarticular fractures Pilon fracture Malunion/nonunion Arthrodesis Osteomyelitis Limb deformity/length inequality Congenital Acquired Indications

4. Advantages • Minimally invasive • Flexibility (build to fit) • Quick application • Useful both as a temporizing or definitive stabilization device • Reconstructive and salvage applications Complex 3-C humerus fx

5. Disadvantages • Mechanical • Distraction of fracture site • Inadequate immobilization • Pin-bone interface failure • Weight/bulk • Refracture (pediatric femur) • Biologic • Infection (pin track) • May preclude conversion to IM nailing or internal fixation • Neurovascular injury • Tethering of muscle • Soft tissue contracture May result in malunion/nonunion, loss of function

6. Components of the Ex-fix • Pins • Clamps • Connecting rods

7. Pins • Principle: The pin is the critical link between the bone and the frame • Pin diameter • Bending stiffness proportional to r4 • 5mm pin 144% stiffer than 4mm pin • Pin insertion technique respecting bone and soft tissue < 1/3 dia

8. Pins • Various diameters, lengths, and designs • 2.5 mm pin • 4 mm short thread pin • 5 mm predrilled pin • 6 mm tapered or conical pin • 5 mm self-drilling and self tapping pin • 5 mm centrally threaded pin • Materials • Stainless steel • Titanium • More biocompatible • Less stiff

9. Pin Geometry ‘Blunt’ pins - Straight - Conical Self Drilling and Tapping

10. Pin coatings • Recent development of various coatings(Chlorohexidine, Silver, Hydroxyapatite) • Improve fixation to bone • Decrease infection • Moroni, JOT, ’02 • Animal study, HA pin 13X higher extraction torque vs stainless and titanium and equal to insertion torque • Moroni, JBJS A, ’05 • 0/50 pts pin infection in tx of pertrochanteric fx

11. Pin Insertion Technique • Incise skin • Spread soft tissues to bone • Use sharp drill with sleeve • Irrigate while drilling • Place appropriate pin using sleeve Avoid soft tissue damage and bone thermal necrosis

12. Pin insertion • Self drilling pin considerations • Short drill flutes • thermal necrosis • stripping of near cortex with far cortex contact • Quick insertion • Useful for short term applications vs.

13. Pin Length • Half Pins • single point of entry • Engage two cortices • Transfixation Pins • Bilateral, uniplanar fixation • lower stresses at pin bone interface • Limited anatomic sites (nv injury) • Traveling traction Courtesy Matthew Camuso

14. Pin Diameter Guidelines • Femur – 5 or 6 mm • Tibia – 5 or 6 mm • Humerus – 5 mm • Forearm – 4 mm • Hand, Foot – 3 mm < 1/3 dia Slide courtesy Matthew Camuso

15. Clamps • Two general varieties: • Single pin to bar clamps • Multiple pin to bar clamps • Features: • Multi-planar adjustability • Open vs closed end • Principles • Must securely hold the frame to the pin • Clamps placed closer to bone increases the stiffness of the entire fixator construct

16. Connecting Rods and/or Frames • Options: • materials: • Steel • Aluminum • Carbon fiber • Design • Simple rod • Articulated • Telescoping • Principle • increased diameter = increased stiffness and strength • Stacked (2 parallel bars) = increased stiffness

17. Bars • Stainless vs Carbon Fiber • Radiolucency • ↑ diameter = ↑ stiffness • Carbon 15% stiffer vs stainless steel in loading to failure • frames with carbon fiber are only 85% as stiff ? ? ? ?Weak link is clamp to carbon bar? Added bar stiffness ≠ increased frame stiffness Kowalski M, et al, Comparative Biomechanical Evaluation of Different External Fixator Sidebars: Stainless-Steel Tubes versus Carbon Fiber Bars, JOT 10(7): 470-475, 1996

18. Ring Fixators • Components: • Tensioned thin wires • olive or straight • Wire and half pin clamps • Rings • Rods • Motors and hinges (not pictured)

19. Ring Fixators • Principles: • Multiple tensioned thin wires (90-130 kg) • Place wires as close to 90o to each other • Half pins also effective • Use full rings (more difficult to deform) • Can maintain purchase in metaphyseal bone • Allows dynamic axial loading • May allow joint motion

20. Multiplanar Adjustable Ring Fixators • Application with wire or half pins • Adjustable with 6 degrees of freedom • Deformity correction • acute • chronic

21. Type 3A open tibia fracture with bone loss

22. Following frame adjustment and bone grafting

23. Frame Types • Uniplanar • Unilateral • Bilateral • Pin transfixes extremity • Biplanar • Unilateral • Bilateral • Circular (Ring Fixator) • May use Half-pins and/or transfixion wires • Hybrid • Combines rings with planar frames Unilateral uniplanar Unilateral biplanar

24. Hybrid Fixators • Combines the advantages of ring fixators in periarticular areas with simplicity of planar half pin fixators in diaphyseal bone From Rockwood and Green’s, 5th Ed

25. Biomechanical ComparisonHybrid vs Ring Frames • Ring frames resist axial and bending deformation better than any hybrid modification • Adding 2nd proximal ring and anterior half pin improves stability of hybrid frame Clinical application: Use full ring fixatorfor fx with bone defects or expected long frame time Pugh et al, JOT, ‘99 Yilmaz et al, Clin Biomech, 2003 Roberts et al, JOT, 2003

26. MRI Compatability • Issues: • Safety • Magnetic field displacing ferromagnetic object • Potential missile • Heat generation by induced fields • Image quality • Image distortion

27. MRI Compatibility • Stainless steel components (pins, clamps, rings) most at risk for attraction and heating • Titanium (pins), aluminum (rods, clamps, rings) and carbon fiber (rods, rings) demonstrate minimal heating and attraction • Almost all are safe if the components are not directly within the scanner (subject to local policy) • Consider use of MRI “safe” ex fix when area interest is spanned by the frame and use titanium pins Kumar, JOR, 2006 Davison, JOT, 2004 Cannada, CORR, 1995

28. Frame Types • Standard frame • Joint spanning frame: • Nonarticulated • Articulated frame • Distraction or Correction frame

29. Standard Frame • Standard Frame Design • Diaphyseal region • Allows adjacent joint motion • Stable

30. Joint Spanning Frame • Joint Spanning Frame • Indications: • Peri-articular fx • Definitive fixation through ligamentotaxis • Temporizing • Place pins away from possible ORIF incision sites • Arthrodesis • Stabilization of limb with severe ligamentous or vascular injury: Damage control

31. Articulated Frame • Articulating Frame • Limited indications • Intra- and peri-articular fractures or ligamentous injury • Most commonly used in the ankle, elbow and knee • Allows joint motion • Requires precise placement of hinge in the axis of joint motion (Figure from: Rockwood and Green, Fractures in Adults, 4th ed, Lippincott-Raven, 1996)

32. Correction of Deformity or Defects • May use unilateral or ring frames • Simple deformities may use simple frames • Complex deformities require more complex frames • All require careful planning

33. 3B tibia with segmental bone loss, 3A plateau, temporary spanning ex fix

34. Convert to circular frame, orif plateau • Corticotomy and distraction

35. Consolidation *note: docking site bone grafted

36. Healed

37. EXTERNAL FIXATIONBiomechanics Leave the Eiffel tower in Paris! • Understand fixator mechanics • do not over or underbuild frame!

38. Fixator Mechanics: Pin Factors • Larger pin diameter • Increased pin spread • on the same side of the fracture • Increased number of pins (both in and out of plane of construct)

39. Fixator Mechanics: Pin Factors • Oblique fxs subject to shear • Use oblique pin to counter these effects Metcalfe, et al, JBJS B, 2005 Lowenberg, et al, CORR, 2008

40. Fixator Mechanics: Rod Factors • Frames placed in the same plane as the applied load • Decreased distance from bars to bone • Stacking of bars  

41. Frame Mechanics: Biplanar Construct • Linkage between frames in perpendicular planes (DELTA) • Controls each plane of deformation

42. Frame Mechanics: Ring Fixators  • Spread wires to as close to 90o as anatomically possible • Use at least 2 planes of wires/half pins in each major bone segment

43. Modes of Fixation • Compression • Sufficient bone stock • Enhances stability • Intimate contact of bony ends • Typically used in arthrodesis or to complete union of a fracture • Neutralization • Comminution or bone loss present • Maintains length and alignment • Resists external deforming forces • Distraction • Reduction through ligamentotaxis • Temporizing device • Distraction osteogenesis

44. Biology • Fracture healing by stable yet less rigid systems • Dynamization • Micromotion • micromotion = callus formation (Figures from: Rockwood and Green, Fractures in Adults, 4th ed, Lippincott-Raven, 1996) Kenwright, CORR, 1998 Larsson, CORR, 2001

45. Biology • Dynamization = load-sharing construct that promote micromotion at the fracture site • Controlled load-sharing helps to "work harden" the fracture callus and accelerate remodeling (Figures from: Rockwood and Green, Fractures in Adults, 4th ed, Lippincott-Raven, 1996) • Kenwright and Richardson, JBJS-B, ‘91 • Quicker union less refracture • Marsh and Nepola, ’91 • 96% union at 24.6 wks

46. Anatomic Considerations • Fundamental knowledge of the anatomy is critical • Avoidance of major nerves,vessels and organs (pelvis) is mandatory • Avoid joints and joint capsules • Proximal tibial pins should be placed 14 mm distal to articular surface to avoid capsular reflection • Minimize muscle/tendon impalement (especially those with large excursions)

47. Lower Extremity “safe” sites 14 mm • Avoid • Nerves • Vessels • Joint capsules • Minimize • Muscle transfixion

48. Upper Extremity “Safe” Sites • Humerus: narrow lanes • Proximal: axillary n • Mid: radial nerve • Distal: radial, median and ulnar n • Dissect to bone, Use sleeves • Ulna: safe subcutaneous border, avoid overpenetration • Radius: narrow lanes • Proximal: avoid because radial n and PIN, thick muscle sleeve • Mid and distal: use dissection to avoid sup. radial n.

49. Damage Control and Temporary Frames • Initial frame application rapid • Enough to stabilize but is not definitive frame! • Be aware of definitive fixation options • Avoid pins in surgical approach sites • Depending on clinical situation may consider minimal fixation of articular surface at initial surgery

50. Conversion to Internal Fixation • Generally safe within 2-3 wks • Bhandari, JOT, 2005 • Meta analysis: 6 femur, 9 tibia, all but one retrospective • Infection in tibia and femur <4% • Rods or plates appropriate • Use with caution with signs of pin irritation • Consider staged procedure • Remove and curette sites • Return following healing for definitive fixation • Extreme caution with established pin track infection • Maurer, ’89 • 77% deep infection with h/o pin infection