Ch 45 + 48 Delayed union, nonunion, malunion and bone graft Flashcards
- Osteosynthesis
healing of bone through surgical intervention with an implantable device
complex but robust process whereby the event of fracture initiates a cascade of biologic events directed at reestablishing the mechanical function of the bone
method of bone healing, primary or secondary (direct or indirect), is influenced by intrinsic factors of the individual patient and fracture and by extrinsic factors of methods of fracture repair and ancillary treatments
Fracture biology
determined by a number of factors
1.specific bone that is fractured, 2.mechanical environment of the fractured bone
3.biologic health of the fracture and patient
4.whether or not the fracture is treated with surgical intervention
5.manner in which the fracture is surgically stabilized
strain is used to describe the effects of loading on a fracture gap
strain potential; that is, cells or other material occupying the fracture gap would be potentially strained in a manner that is proportional to the change in the size of the gap
effects of change in the fracture gap during loading is that the ability to heal and recover from a fracture can be compromised.
- smaller fracture gaps experience greater strain potential than do larger fracture gaps under similar loading conditions
- formation of various tissue types within the fracture gap is dictated by the degree of interfragmentary strain (motion). In high strain environments, only flexible tissues such as granulation tissue can survive > can withstand almost 100% deformation.
- fibrocartilage is capable of accommodating 10% to 15% deformation.
- Bone tissue can only tolerate 2% deformation. Because survival of osteoblasts and osteocytes requires a low strain environment, fractures with small interfragmentary gaps often result in delayed healing because these small gaps lead to high strain during loading compared to comminuted fractures with larger interfragmentary gaps
- goal of fracture fixation should not be to simply decrease fracture gaps but:
- > (1) eliminate interfragmentary strain through anatomic reconstruction, compression of bone ends with rigid fixation, and absolute stability (eliminate fracture gap and interfragmentary strain)
- > (2) maintain a low strain environment through bridging techniques and implants that allow relative stability
non-union stratagies
- Mechanical strategies aim to realign fracture fragments and attenuate motion at the fracture site to allow healing to occur.
- Biologic strategies aim to preserve or enhance natural healing processes, which then will ultimately reestablish mechanical function.
What are the four tenets of bone healing and regeneration?
Mechanics
Scaffold
Growth factors
Cells
Inadequate Mechanical Environment
geometric and forces
- Geometric configuration centers on alignment of the fracture or reduction of fracture segments. The ultimate goal in fracture repair is that a healed and remodeled bone should take on a form and function of the bone as close as possible to that before injury.
- Too great a gap may overwhelm the healing mechanism.
- function without interference from adjacent soft tissues
Naturally occurring bone healing, as evidenced by development of callus in secondary bone formation, is strongly influenced by motion
- Appropriate motion is an essential part of bone healing because motion triggers both proliferation and differentiation of stem cells.
- - During weight bearing, most forces across the diaphysis are axial, healing tissues are compressed, and, as with a balloon, tension is created on the outside of the fracture callus > tensile strain is the most favorable mechanical environment for bone formation
- Closer to the center of the callus, stem cells are subjected more frequently to compressive forces; therefore, chondrogenesis is favored.
- Shear forces are least tolerated, and even modest amounts can be damaging to cells of a fracture callus. This is true for oblique fractures and axial load and also for transverse fractures in rotational loading
What is the general rule regarding the limits of a viable fracture gap
Should avoid fracture gaps which are approaching the size of the diameter of the bone
In what way is motion as essential part of bone healing?
Motion triggers proliferation and differentiation of stem cells
What is the most favourable mechanical environment for bone formation?
Low to moderate tensile strain and hydrostatic tensile stress
The amount of strain shown to enhance new bone formation in axial load is as high as 36% in some small gap models, and strains of 7% demonstrate far less bone healing
Define stress protection
Inhibition of healing of a fracturedue to too little strain being imparted onto the healing callus
not have adequate mechanical signaling to propagate and differentiate into functional bone
What is generally considered to be adequate strain for fracture healing?
- with regard to simple axial loading and fractures with a few to several millimeters of gap or comminution, strains of 5% to 10% are generally considered adequate
- Limiting strain to less than 1% to 2% should be avoided. Ideally, some strain, or motion, should be seen at low loads, then progressively less at higher loads.
Inadequate Biologic Environment
growth factors and cells
bone morphogenetic proteins > BMP-2 known to be essential
Growth factors> intercellular matrix. Then, Hemorrhage and platelet degranulation
- The principal cell type is the mesenchymal stem cell
- growth factors are active in a very specific temporal orchestration, with the concentration of each waxing and waning in order
- Considerable redundancy
- should concentrate on preserving the local fracture environment and promoting a positive cellular environment»_space; MIO that preserve soft tissue viability and local vascularity
- intrinsic factors (4)
Diaphyseal cortical bone
decreased/compromised vascularity of the periosteum
sparse soft tissue attachments
aged patient
all are negative intrinsic factors for growth factor activity
extrinsic factors
imposed by surgical decisions or techniques.
- Open reduction and internal fixation invade the fracture environment
What is generally considered to be adequate strain for fracture healing?
5-10%
How to mesenchymal stem cells respond to tension and compression?
When under tension, stem cells with differentiate into an osteoblastic lineage
When under compression, stem cells differentiate into a chondroblastic lineage
Where are growth factors which are essential for fracture healing derived from?
Initially derived from the intercellular matrix
Haemorrhage and platelet degranulation further deliver growth factors and cytokines
greatest cellular activity
cellular activity is least
thick periosteum,
metaphyseal bone,
young patients,
high vascularity
modest hydrostatic tension
diaphyseal bone with limited soft tissue attachments,
limited medullary space,
motion that exceeds the mechanical limit of the early fracture callus
hostile environment for stem cell activity is transverse or short oblique fractures of the radius and ulna
Periosteum has low cellular activity. The medullary space and corresponding medullary vascular resources are limited, sometimes severely The requirements for an adequate mechanical environment, limiting the strain to very tight tolerances, are very high because the bone and the fracture gap are very small.
What layer of the periosteum is an important source of mesenchymal stem cells?
Cambium layer
delayed union
prolongation in time for fracture healing.
Retrospectively, a fracture that required a longer period of time to heal than expected was delayed
Mechanical Causes of Delayed Union (2)
- excessive fracture gaps
- motion at the fracture site
- Larger fracture gaps require longer healing times > larger demand for new bone formation (more fertile biologic environment)
- Larger motion results in larger callus, and larger calluses require more time to form.
- if motion at the fracture site exceeds the strain limits > viable nonunion may result.
Biologic Causes of Delayed Union
- intrinsic, extrinsic, or both.
- consequences of inadequate cellular activity.
- high-energy fracturing> periosteum is damaged, compromising source of MSC
- Soft tissue damage > limiting adequate vascular supply.
- growth factors may lie fallow without prerequisite viable cells
- limited temporal period of activity and a lack of responding cells
Treatment of delayed union
preemptive > recognizing adverse healing
minimal biologic activity:
- techniques that will encourage primary bone healing
- encourage a more aggressive cellular response with BMP, graft
fracture gaps > treated with natural or synthetic scaffolds.
Mechanical stiffness of devices should be adequate
postoperative
- serial physical examinations and radiographs
- 3 to 4 weeks, when initial bony activity can be appreciated radiographically
- Pain on palpation/lameness suggest insufficient stability
- If no healing is evident at the early postoperative examination, then the biologic environment should be enhanced
Nonunion
A fracture that fails to progress to osteosynthesis regardless of healing time is a nonunion
failure of an adequate mechanical environment, biologic environment, or both
Viable Nonunions
hypertrophic
- have considerable callus > “elephant’s foot” on either side of the fracture line
- due to excessive motion, or lack of an adequate mechanical environment
- Motion incites a cellular response > exceed the tolerable strain > fibrous tissue rather than bone or cartilage
TX
fibrous tissue at the area of the fracture line should be removed
rigid fixation with dynamic compression
- restore medullary blood flow > drill hole is made from the residual fracture line
- Blaeser 2003: Treatment of biologically inactive nonunions by a limited en bloc ostectomy and compression plate fixation: A review of 17 cases
- Oligotrophic nonunion is a viable nonunion
without radiographic evidence of biologic activity.
- difficult to differentiate from biologically inactive (nonviable) nonunion.
- excessive motion AND lack of cellular activity
- common technical complication = loose implants
TX
removal of loose implants, elimination of interfragmentary motion
reinstallation of a biologically active environment > autogenous bone graft, demineralized bone matrix,
Nonviable Nonunions
biologically inactive so that osteosynthesis cannot occur even with adequate fixation.
A section of the healing fracture area is devoid of viable bone.
4 x nonviable?
- dystrophic > nonviable bone on one or both sides of the fracture. Compromise to the vasculature of the bone has occurred, so there is no living bone tissue on the ends.
- necrotic > implies an infected section of bone, specifically a sequestrum. This bone, being biologically dead, prevents healing.
- defect > gap at the fracture site. This is usually due to a gap too large for normal biologic healing processes to occur; the gap is filled with fibrous tissue rather than living bone
- atrophic > a result of the above; it occurs when dead bone of the fracture area is removed by the host without a healing or restorative process (ends resorb)
fracture was treated by opening the medullary cavity, performing osteostixis and generous autogenous cancellous bone grafting, and replacing the type II external skeletal fixator with a less stiff type I external skeletal fixator.
Delayed union, non-union and mal-union in 442 dogs
Marshall 2022
Retrospective study
Delayed union occurred in 13.9%
non-union in 4.6%
malunion in 0.7%
Risk factors:
age
comminuted fracture
treatment with bone graft
surgical site infection
major implant failure
prognosis for radial fractures in toy breeds
appears better than historically believed.
3x previous studies nonunion rate: 3.4%- 8.8
radius and ulna most common
non-unions that did eventually heal after revision surgery
took a total median time of 208 days
