Chapter 45: Delayed Unions, Non-unions and Malunions

Question 1

_______________ deposit bone matrix in the form of thin layers known as ________________?

Answer 1

Osteoblasts, lamellae

Question 2

In the process of deposition of the matrix, osteoblasts become encased in small hollows with the matrix called?

Answer 2

Lacunae

Question 3

T/F: lamellae in trabecular bone do not form Haversian systems?

Answer 3

true

Question 4

What are 2 types of viable non-unions? why do they occur?

Answer 4

Hypertrophic: motion
Oligotrophic: loose implants –> poor vascularity –> lack of cellular activity

Question 5

What are 3 types of nonviable non-unions? why do they occur?

Answer 5

Dystrophic: compromise of vasculature
Necrotic: infected
Atrophic: dead bone removed without replacement (defect: big gap)

Question 6

List in order of tolerance to strain from least to most tolerant:
(Cartilage, bone, fibrous tissue)
What implication does this have for healing?

Answer 6

Bone is least tolerant < cartilage <fibrous tissue

If too much strain you only make fibrous tissue, no bone

Question 7

Strain is = % deformation (decrease in width of fracture gap / total width of gap)
Do small fracture gaps have more or less strain?

Answer 7

More

Question 8

What are some intrinsic and extrinsic factors that affect healing of bone?

Answer 8

Intrinsic: Diaphyseal cortical bone, decreased vascularity of the periosteum, sparse soft tissue attachments, older patients
Extrinsic: ORIF with disruption of the fracture environment

Question 9

What is the greatest source of stem cells for fracture healing?

Answer 9

Cambium layer of periosteum ( best in young dogs)

Question 10

What are two mechanical causes of delayed union?

Answer 10

Excessive fracture gaps
Motion at the fracture site
- more motion = more callus
- motion exceeding strain limits = viable nonunion

Question 11

What are two biologic causes of delayed union?

Answer 11

Intrinsic /extrinsic factors
High energy fractures (periosteal disruption)

Question 12

How do you treat delayed union?

Answer 12

Preemptively! Fix it at time of first repair if you can.
- no large fracture gaps
-encourage cellular response with BMP, cancellous bone, demineralized bone matrix
- make more stable
- recheck at risk fx early to look for signs of delay (pain implies motion!)

Question 13

Define nonunion:

Answer 13

Failure to progress to osteosynthesis

Question 14

What are two types of viable non-unions and how do you treat them?

Answer 14

Hypertrophic - lots of callus from excess motion
-> Treat with removal of fibrous tissue and rigid fixation with a DCP

Oligotrophic - no evidence of biologic activity (no resorbtion, no growth) - due to lack of cellular activity / often loose implants
-> treat with removal of those loose implants, elimination of any interfragmentary motion, and biologically active components (BMP, matrix, graft etc.)

Question 15

What are the four subclassifications of nonviable nonunion?

Answer 15

Dystrophic - nonviable on one or both sides (vascular compromise)
Necrotic - infected section of bone (sequestrum) prevents healing
Defect - the gap is too large for normal biological healing
Atrophic - dead bone at the fracture site has been removed by host and now there is too big a gap

Question 16

How do you treat a nonviable nonunion?

Answer 16

Remove all the nonviable bone and promote regeneration of new bone in proper geometric shape
- cancellous graft
- growth factors / BMP
- rigid fixation in anatomic alignment
OR you can amputate

Question 17

What is a malunion?
Which plane is more tolerant of malalignment?
How do you treat?

Answer 17

Failure of mechanical reestablishment of form and function, but healing is still occurring
cranial/caudal plane is more tolerant as joint angles can compensate - rotational and mediolateral alignment is important to reestablish
Treatment: return bone to normal shape - might need CT and 3D recon

Question 18

What are adjunct/other treatments for nonunions and delayed unions?

Answer 18

1. Extracorporeal shockwave
   Bone absorbs sound energy -> direct stimulation of bone activity (in active bones - hypertrophic not atrophics)
2. Pulsed electromagnetic field -> induced activity through creation of electric field in conductive medium (bone) (for delayed and non-unions, osteomyelitis, failed arthrodeses)

Question 19

Local deformation expressed as units of length per length (expressed as a percentage%) is known as:

Answer 19

Strain

Question 20

Local force expressed in units of force per unit area is known as:

Answer 20

Stress

Question 21

True or false? A longer femur has less strain over a 1mm fracture gap?

Answer 21

True.

Question 22

In order for primary bone healing to occur it requires <_____% interfragmentary strain.

Answer 22

2%
But if you have a gap - it will not heal this way and you should be aware you could stress-protect with too little strain.