Intermediary callus and primary healing
To achieve true (primary) bone healing, the formation of an intermediary callus is necessary. It forms directly between the contacting bone fragments.
The main and absolute prerequisite for its appearance is maximum immobilization (rigid fixation) of the bone fragments, while the diastasis (gap) between them must not exceed 0.1 mm.
The mechanism of development of this type of callus is as follows:
- The space between tightly opposed fragments is gradually invaded by a dense vascular network.
- Following the blood vessels, cells from the Haversian systems of the bone migrate into this zone, forming fully functional bone tissue.
This type of union is optimal as it ensures structural restoration in the shortest possible time.
Secondary healing and its components
If mobility persists between the fragments or a significant diastasis is present, direct vascular ingrowth becomes impossible. Under such conditions, secondary healing develops, mediated by the formation of three other types of callus:
- Periosteal (external) callus. Located on the outer surface of the bone. It is formed through the active division of cells in the cambial layer of the periosteum (periosteum). This zone has an excellent blood supply, which provides it with outstanding regenerative capabilities. When union occurs with fragment displacement, the periosteum plays the leading role.
- Endosteal (internal) callus. Located on the side of the medullary canal. Its formation involves endosteal cells and bone marrow elements.
- Paraosteal callus. Arises from the soft tissues surrounding the fracture site. Externally, it resembles a bridge or a peculiar "arch" between the fragments. The prominence of this callus is directly proportional to the severity of trauma to the adjacent soft tissues.
Clinical significance of the paraosteal callus: even if it reaches impressive dimensions, this "bridge" cannot independently guarantee the strength of the union. If union is absent within the fracture zone itself (due to intermediary, periosteal, or endosteal calluses), the paraosteal bridge easily breaks even under relatively low functional loads.
Dynamics of normal bone union
If ideal repositioning (alignment) of the fragments is achieved during treatment, the process of normal union proceeds in a clear sequence of stages:
- Primary fixation. Periosteal and endosteal calluses form. Their main biomechanical function at this stage is to securely fix the fragments and ensure their immobility.
- Foundation formation. Under conditions of achieved immobility, an intermediary callus forms between the ends of the fragments.
- Reduction of temporary structures. Having fulfilled their fixing function, the periosteal and endosteal calluses gradually regress (reabsorb).
- Final transformation. The intermediary callus remodels, fully adopting the morphological structure of normal uninjured bone.
Features depending on bone type:
- In injuries to the diaphysis (the central tubular parts of the bone), the callus formation process necessarily includes an intermediate stage of cartilage tissue formation.
- Healing of cancellous bone (including fractures in metaepiphyseal zones) proceeds via a different scenario and occurs without a cartilaginous tissue stage.
Briefly: answers to topical questions
What clinical and radiologic signs are used to classify bone fractures?
Bone fractures are classified by localization, fracture plane pattern, degree of fragment displacement, mechanism of soft tissue injury, and according to the AO/ASIF Universal Fracture Classification.
- By localization — proximal, diaphyseal, and distal fractures; lower leg bone injuries are also subdivided into isolated fibular or tibial fractures and fractures of both bones of the leg.
- By fracture plane pattern — transverse, oblique, spiral, comminuted, multi-comminuted, marginal, avulsion, and perforating.
- By degree of fragment displacement — undisplaced or displaced by no more than half the width of the bone; displaced by more than half the width while maintaining fragment contact; with complete transverse displacement.
- By mechanism of soft tissue injury — primarily open and secondarily open fractures.
- According to the AO/ASIF Universal Fracture Classification: for proximal and distal fractures, type A represents extra-articular, type B partial articular (single bone), and type C complete articular (both bones); for diaphyseal fractures, type A represents simple, type B wedge, and type C complex fractures.
What types of bone fragment displacement exist in fractures?
Displacement of bone fragments by mechanism of origin is divided into primary (caused by the traumatic force) and secondary (caused by muscle pull). The following types are distinguished:
- By width — from no displacement (grade 0) to complete displacement across the entire width of the bone (grade III).
- Angulated — deformation of the segment axis.
- Rotational — rotation of fragments around the longitudinal axis (most characteristic of spiral fractures).
- By length — includes displacement with fragment overriding, distraction (formation of a diastasis), and impaction (driving fragments into each other in cancellous bone).
Within what timeframes do the various stages of callus formation occur?
The process of callus formation triggers immediately after trauma, although the timeframes for individual phases are cited in literature as follows.
- Initial phase — formation of mesenchymal tissue: one source indicates it begins on days 2–3 post-fracture and lasts 10–14 days.
- Second, osteogenic phase: its onset in the source is associated with days 10–14, when calcium supplements are prescribed starting from the second stage of bone regeneration.
The timeframes for the remaining phases are not specified in the cited sources; the duration of the process depends on the type and localization of the injury, as well as local and systemic factors.
What factors slow down the process of callus formation and fracture consolidation?
Slowing of callus formation and fracture consolidation is caused by a number of systemic and local factors, as well as management errors.
- Systemic factors — advanced age, endocrine and metabolic disorders, diabetes mellitus, thyrotoxicosis, collagenoses, tuberculosis, hormone therapy, and comorbid conditions.
- Local factors — fracture localization in an area with poor blood supply, massive destruction of bone and surrounding soft tissues, vascular and peripheral nerve injuries, fragment displacement, interposition of soft tissues or small bone fragments, and wound infection.
- Management and compliance errors — lack of adequate repositioning, unstable osteosynthesis, inadequate or frequently interrupted immobilization, prematurely early functional loading, and patient non-compliance with prescribed regimens.
PEIP — topography of calluses: Periosteal (outside), Endosteal (inside), Intermediary (between fragments), Paraosteal (in adjacent soft tissues).
Class and exam questions
Under what circumstances does an intermediate callus form?
It forms only with maximum fragment immobilization and a gap between them of no more than 0.1 mm. The space is populated by vessels and cells from the Haversian systems.
Why doesn't a parosteal callus provide reliable fixation?
It forms from adjacent soft tissues as a "bridge". Despite potentially large dimensions, without union directly at the fracture site, it is extremely fragile and breaks under minor loads.
How does the healing of diaphyseal fractures differ from metaphyseal and epiphyseal fractures?
In diaphyseal fractures, the callus formation process necessarily passes through a cartilaginous tissue stage. Healing of cancellous bone (metaepiphyseal zones) occurs without a cartilaginous stage.
What else to study on this topic
- Cellular mechanisms of vascular ingrowth during the formation of the intermediate callus
- Histological remodeling of the intermediate callus into normal bone tissue
- The effect of fragment mobility on the inhibition of vascular ingrowth
- Features of the regenerative potential of the cambial layer of the periosteum
- Topographical anatomy of bone elements (periosteum, endosteum, Haversian systems)
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