Updated: 4/17/2017

Orthopaedic Implants

Topic
Review Topic
0
0
Questions
39
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Evidence
30
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0
Videos
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https://upload.orthobullets.com/topic/9063/images/Rigidity_moved.jpg
https://upload.orthobullets.com/topic/9063/images/compression (aaos.org).jpg
https://upload.orthobullets.com/topic/9063/images/distal-femoral-plate(boneandspine.com).jpg
https://upload.orthobullets.com/topic/9063/images/bridge plate (jbjs).jpg
https://upload.orthobullets.com/topic/9063/images/cortical-screws (google).jpg
Introduction
  • Characteristics of orthopaedic implants depends on
    • material properties
    • structural properties
Screws
  • Definitions
    • pitch
      • distance between threads
    • lead
      • distance advanced with one revolution
    • screw working distance (length)
      • defined as the length of bone traversed by the screw
    • outer diameter
    • root (inner) diameter
    • bending strength is proportionate to inner (minor) diameter^3  
    • pullout strength  is proportionate to outer (major) diameter^2  
      • maximized by
        • large outer diameter difference
        • fine pitch
      • pedicle screw pullout most affected by quality of bone (degree of osteoporosis)
  • Types of screws
    • cortical screws
    • cancellous screws
    • locking screws
Plate Properties
  • Overview & definitions
    • a load-bearing device that is most effective when placed on the tension side
    • plate working distance 
      • the length between the 2 screws closest to the fracture on each end of the fracture.  
      • decreasing the working distance increases the stiffness of the fixation construct
  • Structural properties
    • bending rigidity proportional to thickness to the 3rd power
    • titanium has Young's modulus of elasticity that most closely approximates cortical bone 
  • Biomechanics
    • absolute stability
      • constructs heal with primary (Haversian) healing
      • must eliminate micromotion with lag screw fixation
      • must be low strain at fracture site with high fixation stiffness
    • relative stability
      • constructs heal with enchondral healing
      • strain rates must be <10%, or fibrous union will predominate
Plate Variations
  • Concave plates
    • placing a concave bend on a plate is useful in transverse fractures to ensure compressive forces occur on both the far and near cortices of the fracture
  • Compression plates
    • compression plates work by placing a cortical screw eccentrically into an oval hole in the plate
  • Locking plates
    • advantages of locking plates
      • locked plate/screw constructs compared to non-locked plate/screw constructs result in less angulation in comminuted metaphyseal fractures
    • indications for locking plate technology  
      • indirect fracture reduction
      • diaphyseal/metaphyseal fractures in osteoporotic bone
      • bridging severely comminuted fractures
      • plating of fractures where anatomical constraints prevent plating on the tension side of the bone (e.g. short segment fixation).
    • locking plate screw biomechanics
      • bicortical locking screws have significantly more resistance to all applied forces, with resistance to torsion increased the most (versus unicortical)
      • unicortical locking screws have less torsion fixation strength than non-locking bicortical constructs
    • percutaneous locking plates
      • application has less soft-tissue stripping but higher chance malunion
    • hybrid locked plates
      • utilize locking and nonlocking screws in order to assist with fracture reduction (nonlocking screws) as well as provide a fixed angle construct (locking screws).
    • locking plate construct stability increases with:
      • bicortical locking screws
      • increased number of screws
      • screw divergence from screw hole < 5 degrees
      • longer plate
  • Bridging plates
    • provides relative stability, relative length and alignment
    • preserves the blood supply to the fracture fragments as the fracture site is undisturbed during the operative procedure
      • this theoretically improves secondary bone healing
    • allows some motion at fracture site; relative stability leads to callus formation 
Intramedullary nails
  • Overview
    • a load-sharing device
  • Structural Properties
    • stiffness
      • torsional rigidity
        • defined as amount of torque needed to produce torsional (rotational) deformation
        • proportional to the radius to the 4th power
        • depends on
          • shear modulus
          • polar moment of inertia
        • increased by reaming
        • decreased by slotting of nail
      • bending rigidity
        • proportional to the radius to the 4th power for a solid nail 
          • area moment of inertia of a solid cylinder {\displaystyle I_x = \frac{\pi}{4} r^4}
        • proportional to the radius to the 3rd power (approximately) for a hollow nail
          • area moment of inertia of a hollow cylinder {\displaystyle I_x = \frac{\pi}{4} \left({r_2}^4-{r_1}^4\right)} 
            • where r1 is inner radius and r2 is outer radius
          • for thin cylinders, {\displaystyle r\equiv r_{1}\approx r_{2}}{\displaystyle  r \equiv r_1 \approx r_2}and {\displaystyle r_{2}\equiv r_{1}+t}{\displaystyle r_2 \equiv r_1+t}. and {\displaystyle I_{x}=I_{y}\approx \pi {r}^{3}{t}}
        • depends on
          • material properties
            • Young modulus of elasticity of material
          • structural properties
            • area moment of inertia
            • length
  • Radius of curvature
    • intramedullary nail radius of curvature is greater (straighter) than the radius of curvature of the femur
  • Interlocking options
    • dynamic locking-->axially and rotationally stable fractures
    • static locking-->axially and rotationally unstable fractures
    • secondary dynamization for nonunion
      • remove proximal interlocking screw or move proximal interlocking screw from the static to dynamic slot
External fixators
  • Factors that increase stability of conventional external fixators
    • contact of ends of fracture 
    • larger diameter pins (most important) 
    • additional pins
    • decreased bone to rod distance
    • pins in different planes
    • increasing size or stacking rods
    • rods in different planes
    • increased spacing between pins
  • Factors that increase stability of circular (Ilizarov) external fixators
    • larger diameter wires
    • decreased ring diameter
    • olive wires
    • extra wires
    • wires cross perpendicular to each other
    • increased wire tension
    • placement of two central rings close to fracture
    • increased number of rings
Total Hip Implants
  • Structural Properties
    • rigidity depends on length and radius of femoral stem
  • Biomechanics
    • place femoral component in neutral or slight valgus to reduce moment arm and stress on cement
    • increasing femoral offset does the following
      • advantages
        • moves abductor moment away from center of rotation
        • increase abductor moment arm
        • reduces abductor force required for normal gait
      • disadvantages
        • increased strain on implant
        • increases strain on medial cement mantle
 

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Questions (39)
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(SAE13BS.53) What method of spinal fixation requires the largest force to disrupt the bone-implant interface? Review Topic

QID: 8290
1

Sublaminar cables

4%

(6/154)

2

Laminar hooks

3%

(4/154)

3

Pedicle hooks

2%

(3/154)

4

Pedicle screws

91%

(140/154)

L 1

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(OBQ13.117) A 35-year-old patient is involved in a motor vehicle accident and sustains multiple fractures including a closed comminuted proximal meta-diaphyseal tibia fracture. The surgeon is considering bridge plating the fracture using a minimally invasive approach. Which of the following is true regarding bridge plating? Review Topic

QID: 4752
1

A locked plate construct (locked screws) or hybrid construct (locked and non-locked screws) is necessary.

25%

(731/2961)

2

Periosteal stripping is performed through two incisions proximal and distal to the fracture.

3%

(85/2961)

3

Bridge plating is performed following direct reduction of the fracture.

3%

(84/2961)

4

AO Type A diaphyseal fractures are best treated with this technique.

3%

(81/2961)

5

Bridge plating with a long working length creates a flexible, axially stable construct.

66%

(1965/2961)

L 3

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(SBQ12TR.28) After application of a unilateral tibial external fixator, it is observed that the frame does not provide sufficient rigidity across the fracture site. Altering the external fixator in which of the following ways will have the greatest impact on frame stiffness? Review Topic

QID: 3943
1

Increasing the distance between pins in each fragment

7%

(267/3629)

2

Increasing the pin diameter

53%

(1925/3629)

3

Reducing the distance between bone and connecting bar

28%

(1004/3629)

4

Increasing the connecting bar diameter

2%

(62/3629)

5

Adding one stacked connecting bar

10%

(353/3629)

L 4

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(OBQ11.7) Which of the following techniques increases strength and stability to an external fixation construct? Review Topic

QID: 3430
1

Unicortical pin fixation

0%

(4/2864)

2

Decreasing total pin separation distance

3%

(91/2864)

3

Increased working distance from the pin to fracture site

2%

(58/2864)

4

Decreasing the distance between the bone and the construct

94%

(2689/2864)

5

Using smaller diameter pins

0%

(5/2864)

L 1

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(OBQ11.59) A 27-year-old male undergoes intramedullary nailing of a midshaft tibia fracture with static locking proximally and distally. There is minimal healing noted 3 months postoperatively and the decision is made to dynamize the nail. For intramedullary nail dynamization, an interlocking screw should be placed in which of the holes shown in Figure A? Review Topic

QID: 3482
FIGURES:
1

A only

73%

(1223/1668)

2

C only

1%

(10/1668)

3

B only

22%

(366/1668)

4

A and C

2%

(26/1668)

5

C and B

2%

(31/1668)

L 2

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(OBQ10.226) A locked plate used in a bridge plate fashion is biomechanically most similar to which of the following fixation methods? Review Topic

QID: 3325
1

Lag screw

0%

(6/1586)

2

Lag screw plus non-locked neutralization plate

2%

(27/1586)

3

External fixator without compression

92%

(1458/1586)

4

Lag screw plus locked neutralization plate

1%

(19/1586)

5

External fixator used in compression mode

4%

(67/1586)

L 1

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(OBQ10.234) Which of the following defines the working distance of a plate in a plate/screw fracture fixation construct? Review Topic

QID: 3333
1

The length of the interfragmentary lag screw

0%

(8/2465)

2

The length between the 2 screws closest to the fracture on each end of the fracture

70%

(1716/2465)

3

The distance from the bone to the plate

3%

(68/2465)

4

The length from the screw closest to the fracture to the screw furthest from the fracture on the same end of the plate

8%

(201/2465)

5

The length between the 2 screws furthest from the fracture on each end of the plate

18%

(450/2465)

L 3

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(OBQ10.169) A 24-year-old female presents with a transverse midshaft humerus fracture. Which of the following implants would create the most compression on both the far and near cortices? Review Topic

QID: 3262
1

Compression plate with concave bend (ends bowed towards bone)

78%

(1577/2026)

2

Large fragment locking plate with 3 bicortical locking screws proximal and distal to the fracture

3%

(61/2026)

3

Intramedullary nail

2%

(47/2026)

4

Compression plate with convex bend (ends bowed away from the bone)

16%

(321/2026)

5

Sarmiento style fracture brace

0%

(9/2026)

L 2

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(OBQ10.183) Which of the following is true regarding rigid locking plate constructs in fracture fixation? Review Topic

QID: 3276
1

Locking plates always enhance fracture healing more than non-locking plating

1%

(19/1575)

2

Locking plates reduce interfragmentary strain more than non-locking plating

75%

(1176/1575)

3

Locking plates are best utilized in diaphyseal fractures

2%

(39/1575)

4

Locking plates are contraindicated in patients with osteoporosis

1%

(8/1575)

5

Fractures treated with anatomic reduction and locked plate fixation demonstrate more strain than fractures treated with intramedullary fixation

21%

(325/1575)

L 2

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(OBQ10.33) A surgeon chooses a periarticular locking plate with unicortical proximal locking screws for an extra-articular distal femur fracture as seen in Figure A. Compared to an identical construct with bicortical unlocked proximal screw fixation, the periarticular locking plate with unicortical locking screws has which biomechanical properties? Review Topic

QID: 3121
FIGURES:
1

Greater torsional and axial fixation strength

14%

(463/3386)

2

Less torsional but greater axial fixation strength

46%

(1571/3386)

3

Equal torsional and axial fixation strength

4%

(131/3386)

4

Greater torsional but less axial fixation strength

18%

(618/3386)

5

Less torsional and axial fixation strength

17%

(586/3386)

L 4

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(OBQ10.60) Which of the following scenarios of treatment of a humerus fracture best achieves low strain at the fracture site and high stiffness of the treatment construct? Review Topic

QID: 3148
1

Functional bracing of a transverse midshaft fracture

1%

(5/644)

2

Comminuted midshaft fracture with locked bridge plating

19%

(120/644)

3

Short oblique fracture with interfragmentary lag screw and locked neutralization plate

76%

(490/644)

4

Uniplane external fixation of a spiral open fracture

0%

(3/644)

5

Oblique fracture with intramedullary nail fixation

3%

(21/644)

L 2

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(OBQ09.169) The greatest biomechanical difference between unicortical and bicortical locking screws is seen when what force is applied? Review Topic

QID: 2982
1

Compression

6%

(34/582)

2

Torsion

72%

(419/582)

3

Distraction

4%

(24/582)

4

Bending on the side of the plate

7%

(43/582)

5

Bending on the surface perpendicular to the plate

10%

(60/582)

L 2

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(OBQ09.109) Which of the following fracture patterns (Figures A through E) has the least amount of evidence-based support for use of locking or hybrid plating techniques? Review Topic

QID: 2922
FIGURES:
1

Figure A

5%

(35/714)

2

Figure B

4%

(28/714)

3

Figure C

23%

(163/714)

4

Figure D

7%

(50/714)

5

Figure E

61%

(432/714)

L 2

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(OBQ09.177) Which of the following Figures shows a fixation construct achieving absolute stability? Review Topic

QID: 2990
FIGURES:
1

Figure A

1%

(9/1197)

2

Figure B

5%

(54/1197)

3

Figure C

5%

(60/1197)

4

Figure D

89%

(1060/1197)

5

Figure E

1%

(10/1197)

L 1

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(OBQ08.98) A 25-year-old male sustained the fracture seen in Figure A and undergoes open reduction internal fixation of the injury. What type of plating technique is used for the ulna? Review Topic

QID: 484
FIGURES:
1

Neutralization

1%

(19/1639)

2

Locking

1%

(16/1639)

3

Compression

2%

(35/1639)

4

Bridging

95%

(1564/1639)

5

Antiglide

0%

(1/1639)

L 1

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(OBQ08.126) Figure A is a radiograph taken after an open reduction and internal fixation of a periprosthetic distal femur fracture. With this type of hybrid locked plate fixation, what is the difference between screw A and screw B? Review Topic

QID: 512
FIGURES:
1

Screw A can assist in fracture reduction while screw B provides a fixed angle support

20%

(124/617)

2

Screw A provides improved axial stiffness while screw B provides a fixed angle support

11%

(65/617)

3

Screw A can be used to reduce the plate to bone while screw B can be used to lag fracture fragments together

2%

(15/617)

4

Screw A provides a fixed angle support while screw B can be used to reduce the plate to the bone

63%

(391/617)

5

Screw A can be used to lag fracture fragments together and screw B increases the plate bone frictional stability

3%

(19/617)

L 2

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(OBQ08.69) An adolescent patient is treated with a 6mm solid intramedullary nail. Compared to a 12mm solid nail of the same material, the 6mm nail has: Review Topic

QID: 455
1

1/2 the torsional rigidity

4%

(20/514)

2

1/4 the torsional rigidity

14%

(71/514)

3

1/16 the torsional rigidity

62%

(319/514)

4

1/8 the torsional rigidity

16%

(84/514)

5

the same torsional rigidity

4%

(18/514)

L 3

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(OBQ07.188) The resistance to pullout of a screw in osteoporotic bone is increased by all of the following EXCEPT? Review Topic

QID: 849
1

Placement parallel to the trabecular pattern

41%

(222/543)

2

Purchase in cortical bone

1%

(5/543)

3

Use of a fixed angle (locking screw construct)

3%

(14/543)

4

Tapping prior to screw placement

48%

(261/543)

5

Augmentation with polymethylmethacrylate

7%

(38/543)

L 4

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(OBQ07.13) Locking plate technology has relative indications for use in all of the following, EXCEPT: Review Topic

QID: 674
1

As a bridge for severely comminuted fractures

3%

(17/492)

2

Osteoporotic metaphyseal fractures

2%

(10/492)

3

Short fracture segments

18%

(88/492)

4

Oligotrophic diaphyseal nonunions

59%

(288/492)

5

Indirect fracture reduction techniques

18%

(87/492)

L 3

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(OBQ06.172) How does a dynamic compression plate achieve compression at the fracture of a long bone? Review Topic

QID: 358
1

Eccentric placement of a cortical screw into a hole in the plate

92%

(525/568)

2

Placing a cortical screw in lag fashion by overdrilling the near cortex

2%

(12/568)

3

Locking of the head of the screw into a threaded hole in the plate

1%

(8/568)

4

Concentric placement of a cortical screw into the center of the hole in the plate

3%

(16/568)

5

The plate allows secondary healing of bone and does not acheive compression at the fracture site

1%

(4/568)

L 1

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(OBQ06.264) Assuming all other variables are the same, which of the following increases fixation construct stiffness in a locking plate model? Review Topic

QID: 275
1

Unicortical locking screws compared to bicortical locking screws

2%

(9/591)

2

Angular cross-threading screws into a plate

6%

(34/591)

3

Overdrilling the near cortex for the screw holes

5%

(28/591)

4

Far cortical locking screws

17%

(98/591)

5

Increased number of screws in the plate

71%

(419/591)

L 4

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(OBQ06.191) A long oblique diaphyseal fracture is internally fixed with 2 lag screws. There is 2 mm of residual fracture fragment gap following screw fixation. This construct has which of the following compared to a comminuted diaphyseal fracture internally fixed with a long bridge plating technique? Review Topic

QID: 377
1

Greater interfragmental strain

58%

(339/588)

2

Greater ductility

2%

(10/588)

3

Greater primary Haversian remodeling

34%

(197/588)

4

Greater union rate

3%

(17/588)

5

Greater callus volume formation

4%

(21/588)

L 3

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(OBQ06.258) The distance of bone traversed by a screw is defined as which of the following terms? Review Topic

QID: 269
1

Pitch

15%

(74/509)

2

Working length

67%

(339/509)

3

Screw lead

15%

(76/509)

4

Core diameter

1%

(7/509)

5

Area moment of inertia

2%

(11/509)

L 3

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(OBQ06.244) Limited contact dynamic compression (LCDC) plates have what advantage over standard dynamic compression plates? Review Topic

QID: 255
1

Less implant-bone and fracture gap micromotion

3%

(17/556)

2

More fracture site compression

1%

(4/556)

3

Less implant-bone contact induced osteopenia

93%

(517/556)

4

Stiffer fracture fixation construct

0%

(2/556)

5

More stress shielding

2%

(13/556)

L 1

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(OBQ06.185) All of the following are indications for locked plating technology EXCEPT: Review Topic

QID: 371
1

Periarticular fracture with metaphyseal comminution

1%

(6/536)

2

Fracture in osteoporotic bone

4%

(21/536)

3

Bridge plating for severely comminuted fractures

5%

(27/536)

4

Compression plating of transverse fracture

85%

(456/536)

5

Plating of fractures where anatomical constraints prevent plating on the tension side of the bone

5%

(26/536)

L 1

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(OBQ06.237) Which statement is true regarding standard open plating techniques compared to minimally invasive submuscular plating techniques? Review Topic

QID: 248
1

standard plating results in greater compromise to both medullary and periosteal bloodflow

21%

(99/473)

2

standard plating results in greater compromise to periosteal bloodflow only

67%

(315/473)

3

standard plating results in greater compromise to medullary bloodflow only

2%

(10/473)

4

standard plating results in less compromise to both medullary and periosteal bloodflow

3%

(15/473)

5

there is no difference between the two techniques with respect to periosteal and medullary bloodflow

7%

(33/473)

L 5

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(OBQ05.141) A patient sustains the fracture shown in Figures A and B. Compared with open reduction and internal fixation with a conventional, non-locking condylar buttress plate, what potential complication is more likely to occur if this fracture is treated with closed reduction and minimally-invasive, locking screw-only fixation with a distal femur locking plate?
Review Topic

QID: 1027
FIGURES:
1

Non-anatomic reduction of the articular surface

80%

(1357/1688)

2

Soft tissue stripping

1%

(20/1688)

3

Shortening

8%

(130/1688)

4

Absence of visible callus

9%

(160/1688)

5

Devascularization of fracture fragments

1%

(12/1688)

L 2

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(OBQ05.275) You are planning an intramedullary nail to treat a geriatric patient with a peritrochanteric femur fracture. Which of the following preoperative considerations is correct regarding your implant? Review Topic

QID: 1161
1

The radius of curvature of an intramedullary nail is generally greater than the radius of curvature of the femur

66%

(702/1070)

2

Closed section nails have less stiffness than slotted nails

3%

(36/1070)

3

The medial/lateral nail starting point relative to the greater trochanter does not affect varus/valgus position in the fracture

2%

(26/1070)

4

The bending stiffness of your nail is proportional to the second power of the radius

23%

(250/1070)

5

Intramedullary nails allow for mostly direct intramembranous bone healing

5%

(52/1070)

L 3

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(OBQ05.114) Which of the following is the most accurate definition of stress shielding? Review Topic

QID: 1000
1

The decrease in physiologic stress in bone due to a stiffer structure sharing load

93%

(529/570)

2

Electrochemical potential created between two metals in physical contact and immersed in a conductive medium

1%

(8/570)

3

Degradation from exposure to a harsh environment

0%

(2/570)

4

Physical movement of two plates against each other leading to mechanical wear and material transfer at the surface

5%

(27/570)

5

Bone death secondary to compromise in blood supply

0%

(1/570)

L 1

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