Bone and Joint

Bone remodelling and fracture healing

Cutting cone and remodelling cycle, osteoclast-osteoblast coupling and stages of fracture healing

14 questions 2 source pages 1 images

Images appear with the first question taken from each source page — tap a question to open it.

Q1What is the sequence of cellular events in a cutting cone?📷▸

Osteoclasts at the front to remove bone
Followed by a layer of osteoblasts depositing successive centripetal layers of lamellae and osteoid
Tunnel size narrows to a haversian canal

Q2What are the labelled components of a cutting cone?▸

Q3How does the cutting cone tunnel change as it advances?▸

Q4From which direction do osteoblasts deposit new bone in a cutting cone?▸

Q5Which cells line the haversian canal after the cutting cone has passed?▸

Q6What is Perren's strain theory?▸

The degree of interfragmentary strain governs the cellular response and therefore the tissue that forms
<2% = primary bone healing
2-10% = secondary bone healing
>10% = nonunion / fibrous union

Q7Describe primary bone healing.▸

It is actually bone remodelling - haversian cutting cones cross the fracture site and directly lay down osteons
It is a process of intramembranous ossification where there is no intermediate step of cartilage formation, therefore no callus formation
Contact healing: direct bone apposition with gap <0.01mm
Gap healing: gap <1mm; vessels grow into the gap and osteoblasts lay lamellar or woven bone
Followed by haviersian remodelling by cutting cones (osteoclast reams, vessel grows in, trailing osteoblasts lay new bone)

Q8Describe the stages of secondary bone healing.▸

Hematoma and inflammation (<1wk): haematoma brings in haemtopoetic cells. Inflammatory mediators lead to granulation and angiogenesis; osteoclasts and macrophages remove debris
Soft callus (1 weeks to 1 month): endochondral ossification at intermediate strain/low O2; intramembranous ossification at low strain/high O2
Hard callus (1-4 months): soft calcified chondral callus remodels into hard mineralised osteoid callus
Remodelling: Wolff's law and piezoelectric charges dictate distribution of remodelling

Q9What adjuncts aid fracture healing?▸

Low intensity pulsed USG increases nanomotion across the fracture site, increasing healing and callus strength
Bone stimulator (electromagnetic stimulation) increases BMP concentration and osteoblastic differentiation

Q10Differentiate bone modelling from remodelling.▸

Modelling: change of shape or growth by independent, simultaneous actions of osteoblast and osteoclast
Remodelling: replacement of the same amount of matrix, sequential osteoblast then osteoclast within the same BMU
Cancellous remodelling: osteoclastic resorption + osteoblastic deposition of lamellar bone
Cortical remodelling: cutting cone

Q11What is the role of fibrinolysis in fracture healing?▸

Plays a key transition step from haematoma formation to angiogenesis and fracture healing
Cited from NEJM 2015 O'Keiffe

Q12Describe enchondral ossification during soft callus formation.▸

Occurs in areas of intermediate strain and low O2 tension
Stem cells differentiate into chondrocytes, laying down type I collagen and fibrocartilaginous matrix
Followed by vascularisation and osteoblasts laying down bone on the cartilaginous scaffold
Forms the interfragmentary callus and external callus

Q13Where does intramembranous ossification occur in secondary bone healing?▸

At sites of low strain and high O2 tension (when fracture strain decreases)
Osteoblasts from the cambrium layer of periosteum directly lay down woven bone
Woven bone is later remodelled to lamellar bone
Forms the medullary callus and periosteal callus

Q14What is the bone healing nonunion (BHN) model?▸

Described by Elliot, JBJS 2016
Unifies Perren's strain theory, Wolff's law and Frost's concept of mechanostat
Tissue around a fracture is called a bone healing unit, producing a physiological response to its biological and mechanical environment
The lecturer suggests drawing the graph

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