Bone has a remarkable capacity to heal, far exceeding that of all other connective tissues. Unfortunately, hyaline articular cartilage has none (see Ch. 6). The inherent determination of fractures to repair is viewed as a highly efficient, primitive response to injury. Moreover, many residual deformities from fractures can remodel with time and leave no trace of the original injury.

The repair process is different in cortical and cancellous bone and this is not at all surprising when one considers the functions and biology of the two tissues (summarized in Table 10.1; return to Ch. 5 to review this in more detail).

Table 10.1

Features of cortical and cancellous bone

	Cortical	Cancellous
Location	Diaphyseal (shaft) bone	Metaphyseal (marrow) bone(carpal, tarsal, vertebral and flat bones)
Function	Mechanical	Primarily metabolic
Cortex	Thick	Thin
Periosteum	Thick	Thin
Turnover	Slow	Rapid
Blood supply	Slow	Rich

In general terms, cancellous or trabecular bone is strong in compression and weak in tension. Most often its honeycomb, sponge-like structure fails in compression, for example a crush fracture of a vertebral body with excessive axial loading as sustained in a fall from a height. The tissue is compacted as the trabecular bone fails. Healing is directly between endosteal surfaces with no significant periosteal (indirect) contribution. The process is favoured by immobility (fixation) and the close apposition of the fracture surfaces. Hence, the very mechanism of injury produces circumstances conducive to healing. The rich blood supply is central to the reparative process. Non-union of a vertebral crush fracture is unknown.

Case 10.1 Trauma: 1

Case history

Max is a 20-year-old apprentice plumber. He is travelling home from visiting family in the country when he is involved in a high-speed motor vehicle accident. His car hits a tree near the outskirts of a large town. The police report suggests that high speed, alcohol and fatigue have all contributed to the accident. The ambulance report shows that he is conscious (Glasgow Coma Score 14), was restrained and has signs indicative of a right knee injury, an open fracture of the right femur and a fracture of the right ankle. He has no other injuries.

He is brought to the Emergency Department where life-threatening injury is excluded and resuscitation commences along the ATLS guidelines discussed above. The Glasgow Coma Score (GCS) is a uniform system for quantifying the extent of neurological injury. It is particularly important for monitoring change in levels of consciousness and neurological deterioration because of increasing intracranial pressure after head injury. Such a cause of change after injury might be an expanding extradural haematoma, for example. The GCS allows accurate monitoring of this change and, because of standardization, interobserver error and vagaries of description are minimized. Three main areas are assessed and the sums of the sections are combined to give a score. A GCS of 8 or below has become the generally accepted definition of coma. In this case, our patient has a GCS of 14, indicating that he is not in a coma and has a mild head injury only.

Radiographs are taken after the patient has been stabilized. These show a fracture-dislocation of the right femur through the diaphysis (Fig. 10.1), a fracture of the right ankle (Fig. 10.2) and a swollen right knee without any obvious bony injury.

Fig. 10.1 Radiograph (AP view) showing a comminuted fracture of the shaft of the femur with a large butterfly segment.

Fig. 10.2 Radiograph (AP view) showing fracture/dislocation of the ankle.

According to the mechanism of certain cancellous bone fractures, there may be a gap at the fracture site. A displaced metaphyseal fracture is an example of this. Endosteal bone does not proliferate to fill the defect and delayed or non-union occurs. This is the rationale of open reduction and rigid internal fixation of widely displaced metaphyseal fractures in adults. Fracture fragments are reduced and rigidly internally fixed.

Diaphysial bone is fractured by either direct or indirect (transmitted) violence. Obviously, the forces involved are very variable. So too are the resultant fracture patterns, which vary according to the way in which the forces are applied.

The reparative tissue has two components—one from the periosteum and the other produced by osteoinduction of primitive local mesenchymal stem cells. It is a highly efficient healing mechanism (Fig. 10.3).

Fig. 10.3 Histology of healing fracture of diaphyseal bone: (A) early woven bone from organization of haematoma, which is well advanced (B); (C) note that the periosteum has been lifted by trauma.

The periosteal component

When the periosteum is lifted from the underlying cortical bone, whether it be by trauma, tumour or pus, it responds by laying down bone. This is an activation of the normal process of bone formation. This component is always most in evidence in a fracture on the side with the least tissue disruption. It does not entail endochondral ossification and results from activity of osteoblasts in the inner cambium (Latin: bark) layer of the periosteum. Periosteal new bone formation is stimulated by movement and is abolished by rigid internal fixation. In osteosarcoma (a primary tumour of bone), the periosteum is lifted by the tumour and new bone may form under the elevated periosteum giving rise to the radiological sign called Codman’s triangle.

Osteoinduction

The fracture haematoma provides the tissue scaffold for the transformation into the definitive reparative tissue. This is a complex process that until recently was not well understood. Local, primitive mesenchymal cells are transformed into osteoblasts under the influence of bone morphogenetic proteins (BMPs) of humoral and platelet origins, and other undefined cytokines. This transformation sees a spectrum of cellular events in the resultant callus. The histological picture is far from homogeneous. Areas where an endochondral sequence is proceeding are seen immediately adjacent to foci of ossification without an antecedent cartilage phase as well as areas of intermediate cellular appearance. The overall picture is one of intense cellular activity. A biopsy of callus, viewed out of the context of trauma, could easily be misinterpreted as a neoplastic process.

The haematoma is rapidly invaded by blood vessels (angiogenesis) and the subsequent callus acquires its own circulation. This is essential for normal healing. It has been postulated that the local release of vascular stimulating factors is central to this event, but the control mechanisms have not yet been identified. Further, it follows that successful vascularization will depend upon the integrity of the encompassing soft tissues. Hence, a variable degree of impairment of repair, even to the point of non-union, is not an unexpected sequel in those injuries where the surrounding soft tissues are severely crushed and traumatized.

Fracture callus is best viewed as temporary tissue. It is gradually formed into a three-dimensional mesh of relatively disorganized woven bone, which under the influence of physical forces, and especially muscle activity, is gradually transformed into highly organized lamellar bone with a cortex with central remodelling and re-establishment of the medullary canal.

Ligaments, tendons and joint capsules are designed to transmit tensile forces and are thus extremely strong in tension. They are far stronger in tension than is cancellous bone. When there is a traumatic angular deformation of a joint, the ligament may be injured (partially or completely ruptured) or it may be torn off (avulsed) from its metaphyseal attachment, taking with it a piece of bone which may be small (often erroneously called a ‘chip’) or quite large, according to varying circumstances. Such injuries are called avulsion fractures and in many instances, the fragment will not unite with its bed because of the displacement and the inability of the underlying cancellous bone to bridge the gap.

Functional anatomy of the knee joint

The key anatomical structures of the knee relevant to osteoarthritis were reviewed in Chapter 6. Here, we will review the anatomy of the knee joint with an emphasis on its functional anatomy relevant to musculoskeletal injury.

Case 10.1 Trauma: 2

Case note: Potential damage to the knee joint

Max’s left knee is observed to be swollen with obliteration of the normal contours. The swelling extends above the patella. It is important to examine the knee joint for evidence of ligamentous damage, as it is known that a significant percentage of patients with a fracture of the shaft of the femur have concomitant knee ligament injury. This is not surprising given the forces that must be applied to the limb to produce a diaphysial fracture.

General joint morphology

Inasmuch as major morphological changes were required for hominids to walk upright and to stand erect with minimal effort, these are unique to the human knee and set it apart from the knee joints of all other creatures.

In the erect position, the line of centre of gravity passes behind the hip joint and in front of the knee and ankle joints. In full extension, the hip and knee joints are said to be ‘locked’. Extension of the former is restrained by the substantial iliofemoral ligament, the strongest ligament in the body. Stability of the locked knee is dependent upon the femoral condyles, which are flattened in an anteroposterior direction (a human trait), and the collateral ligaments, which become taut in extension, as does the anterior cruciate ligament (ACL). Further, the upper surface of the tibia slopes backwards, resisting hyperextension.

The locking of the hip and knee joints allows humans to stand without activity of the respective extensor mechanisms. The reader can readily confirm this. As the hip and knee flex, as they do when we walk or run, these muscle groups come into action to stabilize the joints. Standing is a very efficient mechanism. On the other hand, the ankle joint cannot be locked and the plantigrade position is maintained by the tonic activity of the calf muscles, which force the foot against the ground. The reader will recall that humans get sore and stiff in the calves on prolonged standing, but not so in the buttocks and thighs. Next, stand with the hips and knees flexed at about 20°. You will soon appreciate that it takes considerable muscle effort to maintain this position. Hence, a flexion contracture of the knee joint constitutes a significant disability, particularly if the joint is painful owing to an arthropathy. A flexion contracture of up to 30° in the hip joint can be accommodated by increasing lumbar lordosis.

Because of the widened human pelvis, the femur makes a coronal plane angle of 6–10° with the tibia in the fully extended position. This is relatively larger in the female because of the wider pelvis. This configuration results in a tendency for the patella to move laterally when the quadriceps muscle contracts (Fig. 10.4). This is resisted by the elevated lateral femoral condyle, which is well seen in profile. The upper surfaces of the tibial plateaus are increased in an anteroposterior direction to accommodate the flattened femoral condyles (those of the chimpanzee are much rounder). Lastly, the ACL, which becomes taut in extension, helps guide the femur medially on the tibia in the last 15° of extension—the screw-home movement. This crucial mechanism results in grooving of the anterolateral surface of the intercondylar notch and the smooth depression can be easily seen and felt in well-preserved bones.