Hard Implants. Lecture #13

Transcription

1 Hard Implants Lecture #13 Refs: 1. Park, J. B. and Bronzino, J. D., eds., Biomaterials: Principles and Applications. CRC Press, Boca Raton, FL, Park, J. B., Biomaterials Science and Engineering. Plenum Press, New York, 1984.

2 Long Bone Repair Functions of skeleton Bone Provide frame to support organ systems Determine direction and range of body movements Provides anchoring point for most muscles and ligaments Act as levers when muscles contract joints function as pivots Only tissue able to undergo spontaneous regeneration and remodeling of its macroand micro-structure Osteogenic (bone forming) process Osteoclastic (bone removing) process Respond to both static and dynamic loads

3 Bone Healing Incomplete fracture cracks Micromotion between fracture fragments Heal with fracture line callus only primary healing Callus unorganized network of woven bone formed following fracture to achieve early stability of the fracture Complete fracture Macromotion, unstable Heal with voluminous callus stemming from sides of the bone secondary healing

4 Goals of Fracture Treatment Obtain rapid healing Restoring function Preserving cosmesis without general or local complication Non-surgical treatments Casting plastic or resin Bracing Surgical treatments External fracture site unopened Pins supported by external bars Internal fracture site opened Pins Nails Plates Wire Screws

5 Externally Fixed Fracture Tibial fracture fixed with 4 pins and an external bar

6 Internally Fixed Fracture Total hip replacement with double bone plate, screws, and surgical wire (arrows).

7 Spine Plates and screws for spinal fixation

8 Surgical Wires Reattach long fragments of bone Provide additional stability in long oblique or spiral fractures where other fixation devices are used Must, eventually, be twisted or knotted Strength reduced >25% due to stress concentrations Corrosion Annealed (heat treated) to make more ductile Braided wire More flexible for same diameter Bone ingrowth in grooves between strands Difficult to remove More difficult to pull through

9 Pins Straight wires Steinmann pins Kirchner wire ( Ø < 2.38 mm) Hold bones together either permanently or provisionally Guide large screws Holding power comes from elastic deformation of surrounding bone Mostly 316L stainless but also polylactic and polyglycolic acid for minimally loaded fractures

10 Pins Types of pin tip: (a) trochar end, (b) diamond end

11 Screws Most widely used device for fixation of bone fragments Two types Cortical bone screws fine threads Cancellous screws large threads for greater bone-to-thread contact V-threads or buttress threads Self-tapping or non-self tapping Holding power Size of pilot hole Depth of engagement Outside diameter Bone quality

12 Screws Screw pull-out strength Time after in vivo insertion Bone ingrowth into screw threads and/or resorption of surrounding bone Immediately surrounding bone undergoes necrosis but will revascularize if screw is firmly fixed Bone screws: (a) self-tapping, V-threaded, (b) non-selftapping, buttress threaded

13 Fixation of Cancellous Bone

14 Plates Facilitate fixation of bone fragments Rigid produce primary bone healing Flexible facilitate physiological loading Stiff plates can shield underlying bone from loading necessary for complete healing Closely applied plates can deter bone revascularization Current thinking more flexible plates w/small contact area Allow micromotion and revascularization Increase healing rate Decrease loss of bone mass in shielded region Decrease incidence of re-fracture following plate removal

15 Bone Plates Bone plates: (a) dynamic compression, (b) hybrid compression, (c) reconstruction, (d) buttress, (e) L-shaped buttress, (f) nail, (g) dynamic compression hip screw

16 Dynamic Compression Hip Screw

17 Intramedullary Devices IM Nails Internal struts to stabilize long bone fractures Fixation of femoral neck Better equipped to resist multidirectional bending than paltes Designed with high torsional rigidity resistance to twisting Large contact area with internal cortex to better transmit torsional load Addition of screws or nails Increases torsional stability Prevents shortening of bone

18 Intramedullary Devices Intramedullary devices: (a) Gross-Kempf; Uniflex, (c) Kuntscher, (d) Samson, (e) Harris, (f) Brooker-Wills distal locking pin, and Enders pin

19 Biomaterials for Fixation Material Properties Application Stainless steel Ti alloy Co-Cr alloys Low cost, easy fabrication High cost Low density and modulus Excellent bony contact High cost High density and modulus Difficult fabrication Surgical wire Pin, plate, screw IM nail Surgical wire Plate, screws, IM nails Surgical wire IM nails Polylactic acid Resorbable Pin, screw Polyglycolic acid Resorbable Pin, screw Nylon Non-resorbable Cerclage band

20 Failure Modes of Fixation Devices Failure Mode Failure Location Reason for Failure Overload Fatigue Bone fracture site Implant screw hole Screw thread Bone fracture site Implant screw hole Screw thread Small size implant Unstable reduction Early weight bearing Small size implant Unstable reduction Early weight bearing Fracture non-union Corrosion Screw head-plate hole Bent area Different alloy implants (galvanic effect) Over-tightening screw Screw misalignment Over bent Loosening Screw Motion Wrong screw type Osteoporotic bone

21 Joint Replacement Estimated 16 million people in US affected with osteoarthritis Joint degeneration Last stage of destruction of articular cartilage angular Severe pain Loss of motion Angular deformity (occasional) Cartilage Congruent articulation between bones Transmit load across the joint Low friction movement between opposing joint surfaces Hip cartilage subjected to 8x body weight during fast walking 17 M load bearing cycles over 10-year period Low frictional coefficient 15x easier to move joint surfaces than ice skate moving over ice Total joint replacements are permanent Prosthetic failure and inability to replace leads to Severe shortening of extremity Instability or total joint rigidity

22 Types of Total Joint Replacements Joint Type Hip Knee Shoulder Ankle Elbow Wrist Finger Ball and socket Hinged, semiconstrained, surface replacement Unicompartment or bicompartment Ball and socket Surface replacement Hinged, semiconstrained, surface replacement Ball and socket, space filler Space filler

23 Biomaterials for Total Joint Replacements Material Application Properties Co-Cr alloy Ti alloy Stem, head (ball) Cup, porous coating Metal backing Stem, porous coating Metal backing Heavy, hard, stiff High wear resistance Low stiffness Low wear resistance Ti (pure) Porous coating Excellent osseointegration Tantalum Porous structure Excellent osseointegration Good mechanicaql strength Alumina Ball, cup Hard, brittle High wear resistance Zirconia Ball Heavy and high toughness High wear resistance UHMWPE Cup, knee bearing surface Low friction, wear debris Low creep resistance PMMA Bone cement Brittle, weak in tension Low fatigue strength

24 Design of Joint Replacement Based on kinematics and dynamic load transfer characteristic of joint Material properties, shape, and method of fixation Articulation surfaces should have minimal friction with least amount of wear products

25 Example: Load on Hip and Knee Joints

26 Fixation Methods Mechanical interlock Press-fitting Bone cement polymethylmethacrylate (PMMA) Threaded components Biological fixation Textured or porous surfaces allowing bone ingrowth Direct chemical bonding Coating with calcium hydroxyapatite Bioglas glass-ceramic

27 Example PMMA Bonded Hip Replacement Radiograph of bone cement fixed Charnley hip joint

28 Cerclage Bands

29 Porous Coatings Scanning electron micrographs of four different types of porous structures: (a) plasma sprayed coating (7x), (b) sintered wire mesh coating (7x), (c) sintered beads coating (20x), (d) Hedrocell porous tantalum (50x)

30 Hip Joint Replacement Femoral component (ball) Head, neck, shaft Ti or Co-Cr alloy Monolithic or modular Acetabular component (cup) Monolithic or modular Ultra-high-molecular-weight polyethylene (UHMWPE) Produces wear debris in use 1µm or smaller Can cause inflammation Interest in metal-metal and ceramic-ceramic

31 Hip System Modular hip system: head, femoral stem, porous coated proximal wedge, porous coated metal backing for cup, UHMWPE cup, and fixation screws

32 Knee Replacement Femoral, tibial, and/or patellar components More complicated geometry and biomechanics than hip Classified by how much they depend upon ligaments for stability Constrained Hinged articulation Fixed angle of rotation Indicated when all ligaments are absent Semiconstrained Control posterior displacement of the tibia on the femur Rely on ligaments to provide remaining constraint Nonconstrained Joint degradation with minimal or no ligament instability

33 Knee Systems Various types of knee joints: (a) metal hinged, (b) hinged with plastic liner, (c) intermedullary fixed semiconstrained, (d) surface replacement, uni-compartmental replacement, (f) bi-compartmental replacement

34 Limb Salvage Endoprothetic Replacement

35 Others Ankle, Shoulder, Elbow, and Finger Others: (a) ankle, (b) socket-ball shoulder, (c) hinged elbow joint, (d) encapsulated finger joint

36 Dental Implants Tooth replacement challenging Hostile environment Changing composition Changing ph Changing temperature Severe compressive stresses Self-tapping dental implants

37 Dental Implants Current dental implant designs