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Accessory ossicle or intraepiphyseal fracture of lateral malleolus: are we familiar with these?
  1. V Mandalia1,
  2. V Shivshanker2
  1. 1Department of Trauma and Orthopaedics, North Hampshire Hospital, Basingstoke, Hampshire, UK
  2. 2Department of Orthopaedic Surgery, Colchester General Hospital, Colchester, UK
  1. Correspondence to:
 Mr V Mandalia
 33 Cranham Road, Bristol BS10 5EF, UK; vmandaliayahoo.com

Abstract

A case of intraepiphyseal injury (type 7) to the lateral malleolus in a 11 year old child is described. This rare injury cannot be classified by commonly used Salter Harris classification for epiphyseal injury. Although less common, accessory ossicle of the malleoli is an important differential diagnosis for such injury. Details of type 7 intraepiphyseal injuries and accessory ossicle are described.

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An 11 year old child presented to an accident and emergency department after a fall from a bicycle, sustaining injury to the ankle. There was no history of previous injury to this ankle joint. The patient had pain over the lateral aspect of the ankle and clinical examination revealed swelling and tenderness over the lateral malleolus without any abnormality over the medial side of the joint. The lateral ligament complex, medial malleolus, ankle joint, and rest of the foot were normal on clinical and radiological examination. Radiographs showed intrapeiphyseal injury of the distal fibular epiphysis (fig 1). The ankle was splinted in a below knee cast and the patient was permitted partial weight bearing. Three months after the injury there was no tenderness over the lateral malleolus and radiographs showed union across the fracture site (fig 2).

Figure 1

 Radiograph showing intrapeiphyseal injury of the distal fibular epiphysis.

Figure 2

 Radiograph three months after the injury.

DISCUSSION

The most widely used classification for epiphyseal injuries is the one proposed by Salter and Harris in 1963. Although it is comparatively concise and of clinical importance, certain types of epiphyseal and physeal injuries cannot be readily classified with this system. John Ogden devised a more inclusive classification scheme in 1981, where he described up to nine different types of injury to the growth mechanism of the immature skeleton.1

Type 7 epiphyseal injury

Distal fibular epiphyseal injury is commonly either Salter-Harris type I or type II injury. Type 7 (intrapeiphyseal) injuries are less common, occurring as a result of supination inversion injury to the ankle joint.1 Although this injury is described in the literature, no major series of epiphyseal injuries specifically mention it.

Type 7 epiphyseal injuries, as described by Ogden,1 are intraepiphyseal injuries and represent propagation of the fracture from the articular surface through the epiphyseal cartilage into the secondary ossification centre. Unlike other types of epiphyseal injuries they do not involve primary physis at all. These types of injuries are common at malleoli; and within the distal humerus or distal femur as an osteochondral fracture.

Type 7 injury at the ankle is usually treated by closed reduction as weight bearing main articular surface is not usually interrupted. A transversely oriented fracture is usually stable but a more obliquely oriented fracture requires stabilisation. As this injury is rare, there are no long term studies available but theoretically there should not be major growth disturbance or angular deformity in the long term as the physis is not injured.

Accessory ossicle

An accessory epiphyseal ossification centre or accessory ossicle may develop in either malleolus.2 Whether these accessory ossification centres represent a variation of ossification or a response to repetitive occult microtrauma is conjectural. They are not anatomically separate entities from the main ossification centre, even though they appear to be radiologically. Accessory ossicles of the malleoli are common in skeletally immature individuals; the lateral ossicle has been termed the “os subfibulare” and the medial “os subtibiale”. They usually appear between the ages of 7 and 10 years and eventually fuse with the secondary ossification centre of the malleolus at skeletal maturation. In one study of normal children aged between 6 and 12 years, accessory centre of ossification was found in the medial malleolus in 20% and in the lateral malleolus in 1%.3 John Ogden in his study of 103 patients with malleolar injury and ossification variation found that bilateral involvement (involvement of either medial or lateral malleoli on both the ankle) and involvement of both lateral and medial malleoli on one side can occur.2 As the patient group in his study was obviously a highly selective population, statistics of incidence of the different variation cannot be derived. Such accessory centres of ossification rarely persist beyond skeletal maturation. A centre remaining unfused in adult life could cause confusion if found in an ankle that had recently been injured.

They usually are asymptomatic. However, they may be injured. As these accessory centres are most often found in the evaluation of acute trauma, they may be mistaken for fractures. Smooth appearance of the accessory centre should help in differentiation.

An acutely symptomatic patient with an accessory ossification centre should be considered and treated as having a fracture. The most probable explanation of symptoms is that the accessory ossification centre is disrupted through the cartilaginous continuity, resulting in a fracture or pseudoarthrosis. The diagnosis of such injury by conventional radiography is difficult and a bone scan may help to diagnose such injury.

If an accessory centre is associated with ankle injury, it is appropriate to immobilise the ankle for three to four weeks and then to encourage mobilisation and weight bearing. Excision of the fragment should be reserved for the very few patients with recurrent symptoms over a prolonged period and with tenderness at the site of the accessory centre.

In our case it was a fracture and not accessory ossicle because (1) after the traumatic event, there was pain and tenderness over the distal fibular epiphysis of the previously asymptomatic ankle joint; (2) there was sharp fracture line in contrast with a smooth appearance seen in case of accessory ossicle; (3) on follow up, there were signs of fracture healing both clinically and radiologically.

Although it is easy to use the Salter-Harris classification for common epiphyseal injury, we need to be aware of more detailed classification as described by Ogden, to identify and understand less common epiphyseal injury.

JOHN OGDEN CLASSIFICATION OF INJURY TO GROWTH MECHANISM

Classification of injury to the growth mechanism as proposed by John Ogden is described.

The classification proposed by John Ogden is one concerned with injury to the growth mechanism rather than only physeal or epiphyseal injury. It is based partially on the Salter-Harris system. It is a more detailed scheme that permits further understanding of the injury to the growth mechanism as a whole.

Type 1

The epiphysis and most of the cellular regions of the physis separate from the metaphysis. The plane of cleavage is essentially through the zones of hypertrophic and degenerating cartilage cell columns, leaving more important resting and dividing cell layers of the germinal region undamaged and contiguous with the epiphysis.

Type 1 injuries are more common in neonates and infants with limited development of the secondary ossification centre. It may occur as abnormal fractures complicating underlying diseases such as rickets and osteomyelitis.

Because of the thick periosteum, displacement of the fracture is minimal making radiological diagnosis difficult. Slight widening of the physis may be the only sign. Complete displacement of the epiphysis although rare, can occur.

Type 1B

This occurs in children with systemic disorders affecting endochondral ossification patterns in metaphysis.

In contrast with type 1A where the fracture is primarily through the zone of hypertrophic cartilage, type 1B fractures may occur in the zone of degenerating cartilage and primary spongiosa. The initial fractures tend to be microscopic trabecular injuries.

Generally subsequent growth is normal in types 1A and 1B, because the essential germinal elements, the resting and dividing cellular layers of the growth plate and blood supply, are usually undisturbed.

Type 1C

It is a comparatively infrequent injury where there is an associated injury to a germinal portion of the physis. Localised area of growth region of the physis is subjected to a crushing injury. Birth or early infancy seems to be the most likely time of such injuries. Eventually an osseous bridge forms but only after the secondary ossification centre has developed and expanded to reach the damaged region.

Type 1 injuries are less likely to occur in children beyond the first two years, when the failure pattern is more likely to create a metaphyseal fragment (type 2 injury).

Type 2

It is the commonest of all the growth plate injuries. It frequently occurs in young children (from 3 to 7 years). The fracture line propagates through the hypertrophic zone of physis and then into the metaphysis. The metaphyseal fragment, which is generally triangular and may be extremely small, is diagnostic of this type of injury.

The periosteum is usually intact on the compression side with the metaphyseal fragment, whereas the opposite tension side is associated with disruptive periosteal damage as it is stripped and torn from the metaphysis. The intact periosteum on the compression side may be used as a hinge to assist in stable reduction. The periosteum may invaginate into the gap between epiphysis and metaphysis. Displacement is quite variable and may be extremely pronounced.

Type 2B

It entails further propagation of the fracture forces on the tensile side to create a free metaphyseal fragment. This free metaphyseal fragment makes reduction much more difficult and may necessitate open reduction to stabilise the comminuted fragments.

Type 2C

There is a thin layer of metaphysis along with, or instead of, the larger triangular fragment. This osseous layer traverses most of the metaphysis. It is more common in slower growing regions, such as phalanges, which normally have increased transverse trabeculation in the juxtaphyseal metaphysis.

Type 2D

In this type of injury, the area of the metaphysis is driven into a segment of the growth plate, causing compression damage to a localised area. Because of localised damage to the growth plate, subsequent growth may be eccentric and lead to angular deformation.

Type 3A

This is an intra-articular fracture primarily involving the epiphysis, with the fracture extending from the articular surface through the epiphysis and then turning about 90° to extend along the growth plate toward the peripheral margins.

Type 3B

When there is a thin layer of metaphyseal bone with the epiphseal fragment, it is known as type 3B injury. This injury is common when the growth plate is undergoing the final phases of physiological epiphyseodesis. This is commonly seen in lateral humeral condyle and distal tibial epiphysis (Tillaux fracture).

Open reduction is frequently indicated to obtain accurate anatomical restoration. Prognosis is good provided there is no impairment of the circulation to the separated fragment. As these fractures occur as skeletal maturity is approaching, the chance for eventual growth disturbance is minimised by the lesser amount of anticipated longitudinal growth.

Type 3C

It includes injuries involving epiphyses that have developed major contour changes, such as the ischial tuberosity, in which epiphyseal fracture propagation may not necessarily involve a joint surface.

Type 4

It extends from the articular surface, across the epiphysis and physis, and subsequently through a significant segment of the metaphysis. It is important to achieve anatomical reduction to restore both a smooth articular surface as well as normal cytoarchitectural relations of the growth plate to minimise the potential risk of subsequent osseous bridging and localised premature growth arrest.

Despite this, growth damage and premature epiphysiodesis may still occur, most probably because of microscopic, compression type injury to regions of the growth plate.

Type 4B

There is additional propagation of the fracture line through remaining portions of the physis to create an additional free epiphyseal fragment. This tendency of fragmentation is more common when the patient is approaching skeletal maturity.

Type 4C

As in type 3C, type 4C fracture involves cartilaginous regions through which the fracture may propagate. Type C injury involves metaphysis, physis, and epiphysis.

Type 4D

It is a type of injury where there are multiple metaphyseaal-physeal-epiphyseal fragments. There is increased risk of traumatically induced, localised epiphyseodesis.

Type 5

This is a compression injury through certain segments of the epiphysis and physis, crushing germinal regions as well as adjacent hypertrophic regions. It is a comparatively infrequent injury and is virtually unrecognisable by standard diagnostic techniques. This injury may be misdiagnosed as a sprain and the patient may later present with angular deformity. Other causes of this type of injury are electrical injury, radiation, and frostbite.

Type 6

This type of injury involves the peripheral region of the growth plate, the zone of Ranvier. More commonly it results from a localised contusion or avulsion of that specific portion of the growth mechanism. Peripheral osseous bridge formation commonly occurs, leading to peripherally localised epiphyseodesis and subsequent angular deformity. This type of injury is thought to be the possible underlying cause of solitary osteochondroma.

Type 7

These injuries are completely intraepiphyseal and represent propagation of the fracture from the articular surface through the epiphyseal cartilage into the secondary ossification centre. They do not involve physis. These types of injuries are common at malleoli, and within the the distal humerus or distal femur as an osteochondral fracture.

Type 7A

The fracture passes through both the epiphyseal cartilage and bone of the secondary ossification centre.

Type 7B

It represents propagation of the fracture through the cartilaginous portions, with involvement of some of the preossifying regions of the expanding secondary ossification centre. These types of fractures may also involve non-articular regions such as the tibial tuberosity, the greater trochanter, and the fifth metatarsal at its proximal base.

Type 8

These are injuries to the metaphyseal growth and remodelling mechanisms and represent transient phenomena primarily related to vascularity. With these injuries, the metaphyseal circulation involved in primary spongiosa formation from the cartilage cell columns is temporarily disrupted, leading to failure of normal osseous remodelling and subsequent, transiently increased osseous density as the area is revascularised.

Type 9

This is a selective injury to the diaphyseal growth mechanism. Any direct injury to the periosteum may affect the ability of the bone to remodel and increase cortical volume circumferentially.

A highly osteogenic periosteal sleeve is one of the important mechanisms for longitudinal as well as appositional bone growth. Any type of insult with this mechanism may result in impaired growth of the bone.

REFERENCES

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