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Biomechanics of common implants

some biomechanics explanation on implants especially lag screw and tension band principle - done for basic ortho skills course

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Biomechanics of common implants

  3. 3. LAG SCREW CONCEPT  Aim – to achieve compression across fracture  2 types  Cortical screw through gliding hole  Cancellous bone screw
  4. 4. LAG SCREW CONCEPT  Prerequisites: Screw must be placed perpendicular to fracture site
  5. 5. LAG SCREW CONCEPT  Prerequisites: NOT in comminuted fractures With cortical screws, requires over drilling
  6. 6. LAG SCREW CONCEPT  Cortical screw through gliding hole: Drill near cortex with drill bit = outer screw diameter Drill far cortex with drill bit = inner screw diameter
  7. 7. LAG SCREW CONCEPT  Cortical screw through gliding hole: Countersink for what? To create a platform on the near cortex for the screw head – ensures full contact
  8. 8. LAG SCREW CONCEPT  Cortical screw through gliding hole: Improper countersink – eccentric loading when compress, might displace #
  9. 9. LAG SCREW CONCEPT  Cortical screw through gliding hole: If countersink too deep – remove almost all cortical bone on near cortex, screw may break through and end up in medullary canal
  10. 10. LAG SCREW CONCEPT  Cortical screw through gliding hole: Tap the far cortex, must push it thru the near cortex – will form false track Final screw placement
  11. 11. LAG SCREW CONCEPT  Cancellous bone screw: Drill near cortex with 3.2mm drill bit for 6.5 screw, do not perforate far cortex Measure and tap with 6.5mm cancellous bone tap
  12. 12. LAG SCREW CONCEPT  Cancellous bone screw: Place screw, being cancellous bone, doesn’t require gliding hole and countersink, but requires washer as cortex is thin
  13. 13. TENSION BAND PRINCIPLE  Concept introduced by Pauwels (an engineer) An eccentrically loaded bone will always have a tension side and compression side
  14. 14. TENSION BAND PRINCIPLE A tension band converts a tension force into a compressive force across the whole width of bone
  15. 15. TENSION BAND PRINCIPLE Even when put on tension side, an intact buttress still needed in compression side for the tension band to work
  16. 16. TENSION BAND PRINCIPLE Dynamic tension band – produces increased compression with motion
  17. 17. TENSION BAND PRINCIPLE Static tension band – already producing compression at moment of reduction – remains constant
  18. 18. TENSION BAND PRINCIPLE Placement of wires ideally 5mm deep to the anterior surface, will be close to joint– but will not impair tension band principle if not.
  19. 19. TENSION BAND PRINCIPLE  Which one? More stable against torsional forces Less likely to cut thru the medial and lateral retinacula
  20. 20. TENSION BAND PRINCIPLE  What causes this? How to prevent it?
  21. 21. TENSION BAND PRINCIPLE Know any other techniques?