The aetiology of lower extremity bowing in infants and children:
Varieties of conditions results in leg bowing in the paediatric population and vary according to the patient's age.
Trauma,
neoplastic and infective (Osteomyelitis and Syphilis) causes need to be considered in all age groups.
Furthermore,
radiologists should always be alert of the possibility of non-accidental trauma (NAI) as a differential diagnosis.
Physiological leg bowing:
Genu varum (angle out): The most common cause for lower extremity bowing is physiological.
The cause of this is due to the folded leg position in utero.
The bowing of leg is symmetrical and is not progressive.
Usually by eighteen months to three years of age,
the alignment of leg corrects and straightens out [1].
Genu valgum (angle in): To a certain degree,
majority of children become knocked kneed between 3 to 5 years of age.
This usually resolves by seven years. Knock knees are usually associated with leg bowing (Fig.
1).
The causes of pathological bowing are detailed as below
(Entries are not exhaustive):
1.
Trauma
|
Torus/ Greenstick fracture ,Plastic deformation,
Malunion,
NAI
|
2.
Infective
|
Osteomyelitis,
Syphilis
|
3.
Tumours
|
Primary osseous tumours: Osteosarcoma,
Ewing’s,
haemangioma
Metastasis:lymphoma,neuroblastoma
Soft tissue tumours
|
4.
Metabolic
|
Rickets,
Hypophosphatasia,
Scurvy
|
5.
Genetic/Hereditary
|
Neurofibromatosis type 1,
Achondroplasia, Metaphyseal dysplasia,
Thanatophoric dysplasia,
Campomelic dysplasia
|
6.
Congenital or Developmental bone disorders
|
Fibrous dysplasia (McCune Albright syndrome),Skeletal dysplasia,
Fibular hemimelia
|
7.
Neuromuscular/ connective tissue disorders
|
Osteogenesis imperfecta,
Cerebral palsy,
Ehlers-Danlos syndrome,
Marfan’s syndrome
|
8.
Inflammatory
|
Infantile cortical hyperostosis or Caffey's disease
|
9.
Miscellaneous
|
Blount's disease,
Lead intoxication,
Fluoride intoxication
|
Red Flag signs indicating specialist referral [2,
3]:
1.
Limping or unwell (tachycardia,
fever,
malaise) child with functional disability of more than 4 weeks,
complaining of night pain or early morning stiffness.
2.
History of weight loss,
bruising,
constant pain unrelated to movement should alert the possibility of malignancy.
3.
Persistence of bow legs in children older than three years and significant knock knees beyond age seven.
4.
Unilateral or an excessive asymmetrical deformity
5.
Progressive deformity.
6.
Bowing as a result of trauma or associated skeletal deformities.
7.
In a case of genu valgum if the intermalleolar separation is more than 8 cm.
During supine position this is the distance between malleoli with knees together.
8.
Intercondylar distance > 6 cm in a case of genu varum.
Clinically this is the distance between knees with the malleoli touching.
Bowing of legs in the setting of trauma:
The epiphyseal growth plates are responsible for the longitudinal growth of a bone.
Injury to it may result into asymmetrical growth and bowing.
Torus or buckle fracture (caused by compressive force),
greenstick fracture (force applied to the side) and plastic deformation are the common form of injuries.
Targeted X-rays in at least two planes are necessary for the diagnosis.
Majority of these fractures are successfully treated in a cast (Fig.
2).
Infection and lower extremity bowing:
Osteomyelitis: Possibility of infection arises in the presence of one or more of the clinical or laboratory signs; e.g.
if the child is unwell,
tachycardic,
swelling and tenderness over the affected area with raised inflammatory markers. In paediatric population tibia is the second most commonly affected bone (24%) after femur (27%) [4].
If not adequately treated acute osteomyelitis may develop into chronic osteomyelitis and permanent deformity of extremity [5].
Radiographs have a limited role in demonstrating early changes. However,
after four to six weeks these may show ill-defined lucencies with associated periosteal reaction.
MRI is the investigation of choice in demonstrating early altered bone marrow signal and inflammatory changes in the soft tissue.
It can also define the extent of the disease.
Bone scans may be the second-line investigation.
Syphilis: This is a rare cause for limb bowing in children.
Long bones are commonly affected in congenital or tertiary syphilis.
Recurrent ulceration in limbs with anterior bowing and classic thickening of tibia (sabre tibia) are usually present [6].
MRI is extremely helpful in showing osseous involvement.
Tumours causing leg bowing in children:
Depending on the location of the tumours in long bones these may cause bowing of the limbs.
The commonest paediatric primary osseous tumours are osteosarcoma andEwing’s sarcoma.
The child most commonly presents with painful limb prior to development of soft tissue swelling and mass lesion [7].
Pathological fractures are present in nearly 15% of cases [8].
Conventional radiographs may show an aggressive looking lesion eroding through the cortex and extending into the soft tissue,
significant periosteal reaction with associated soft tissue swelling (Fig.
3).
MRI defines extent of local disease and also for assessing bone and surrounding soft tissues changes.
CT is commonly used for staging purpose in malignant bone tumours while valuable in showing periosteal reaction inEwing's sarcoma [8].
Osteosarcomas affect metaphysis whileEwing’s sarcoma usually occurs in the diaphysis.
Metabolic disorders affecting architecture of the long bones:
Rickets: Abnormal vitamin D metabolism results in deficient bone mineralization.
The affected children may presents with hypocalcaemia seizures,
delayed gross motor development or tetany [9].
The classic radiographic features are osteopenic bones,
thinning of long bones cortices,
cupping and fraying of the metaphysis,
widened growth plate with frayed physeal borders and enlarged epiphysis (Fig.
4).
Associated insufficiency fractures/looser zones may also be seen.
Hypophosphataemic rickets: This condition is a dominant X-linked disorder secondary to phosphate-wasting.
Anterolateral bowing of the femur with coxa and tibia vara is the commonest deformity (Fig.
5).
Windswept lower limb deformity which is angular valgus deformity in one knee joint with varus deformity in the other knee is also a common finding [10].
Scurvy: The underlying cause of this disease is vitamin C deficiency.
Children are usually very lethargic,
complaining of bone pain and they are more prone to bleeding tendencies.
The important radiographic features include: generalised osteopenia,
periosteal reaction as a result of sub periosteal haemorrhage,
metaphyseal spurs and cupping,
dense zone of provisional calcification (Frankel's line) and bowing of long bones (Fig 6).
A round,
radiopaque shadow may also be seen surrounding the epiphyseal centre of ossification which is a typical finding called Wimberger's sign.
Connective tissue disorders:
Osteogenesis Imperfecta (OI): OI is an inherited connective tissue disorder.
The common finding is generalised osteoporosis and is more prone to multiple fractures resulting in callus formation and pseudoarthrosis.
There are several types of OI.
Type 1 is the most common and mildest form (Fig.
7).
The lower limb imaging findings are osteopenia,
bowing of long bones and diaphyseal fractures.
Type 2 is a perinatal lethal form (Fig.
8).
The condition is characterized by increased bone fragility,
low bone density and susceptibility to bone fractures.
Children with type 2 OI may present with multiple rib and long bone fractures.
Marked deformities of the long bones,
skull,
scapulae and pelvic bones are also seen.
The severe form is type 3 and the features are short stature,
spinal deformities,
limb bowing and multiple fractures (Fig.
9,
10) [11].
These severe forms have anterolateral bowing of proximal femur (shepherds' crook deformity),
multiple wormian bones and blue sclera.
Developmental bone disorders affecting lower extremity:
Fibrous dysplasia: This causes expansion of the bones due to replacement of medullary cavity with fibrous tissue.
Two forms are known,
they are monostotic and polyostotic (Fig. 11).
Polyostotic fibrous dysplasia occurs in McCune- Albright syndrome (Fig.12,
13).
The radiographic manifestations are well defined,
expansile,
mixed sclerotic or lucent bone lesions.
There is characteristic ground glass opacity.
The lower extremity findings of severe polyostoic form are shepherd's crook deformity of femur neck (coxa vara),
protrusio acetabuli and severe bowing of long bones [12].
Leg bowing in genetic disorders:
Neurofibromatosis Type 1 (Von Recklinghausen disease): This is an autosomal dominant disorder,
however; 50% of cases may arise as spontaneous mutations.
Children usually present in infancy with unilateral anterolateral bowing of the lower leg but a child may be born with fracture or pseudoarthrosis [13,
14] (Fig.
14).
Tibia is the commonest bone affected in the long bones.
It can result in non union with the development of a pseudoartherosis.
On conventional radiographs usually the tibial bowing appears typically near the junction of the middle and distal thirds of the tibia.
Often the distal fragment is pointed with cupped proximal end [14].
Other feature may include a thin or absent fibula.
MRI is very useful for preoperative planning [14].
Thanatophoric Dysplasia: The commonest congenital lethal skeletal dysplasia.
It occurs sporadically with an unknown aetiology.
Ultrasound is capable of diagnosing the condition second trimester onwards [15] (Fig.
15).
Suspicious features include a dwarf foetus with shortened femur and bowing of limbs (Fig.16).
Other findings include narrow conical thorax,
short neck,
protuberant abdomen and clover-leaf deformity of the skull.
Intrauterine death is very common.
Achondroplasia: There is abnormal enchondral bone formation with typical proximal limb shortening.
The iliac wings are squared with narrow sciatic notches.
Flat acetabular roofs are present.
There are shortened and thickened long bones with metaphyseal flaring.
The distal femur has abnormal shape of growth plate.
The ankle varus deformity can result in relative shortening of tibia in comparison to fibula [16] (Fig.
17).
Campomelic dysplasia: Rare genetic disorder with mutation of chromosome 17.
The radiographic findings are dysplastic pelvic bones,
bowing of femur and tibia.
Other skeletal manifestations are hypoplastic scapula and spinal anomalies.
The limb bowing is diagnosed with antenatal ultrasound.
More than 95% are lethal in neonatal period,
death due for respiratory failure [17].
Ischio- pubic- patella syndrome is a characteristic imaging finding of this condition [18].
Metaphyseal dysplasia (Pyle's disease): Rare genetic disorder with autosomal recessive inheritance.
The radiographic findings are metaphyseal widening and cortical thinning of long bones resulting in Erlenmeyer flask deformity of the distal femur and proximal tibia [19] (Fig.18).
Other findings are mild hyperostotic cranium.
Inflammatory conditions resulting in leg bowing:
Caffey's disease (Infantile cortical hyperostosis): The affected infant usually presents with proliferative bone changes,
soft tissue swelling and irritability.
Plain films help in confirmation of the presence of skeletal manifestations and soft tissue swelling.
The condition is usually unilateral which typically only affects the diaphysis of the long bones. A thickened,
laminated sub periosteal new bone formation occurs with cortical hyperostosis.
Miscellaneous:
Blount’s disease: Aetiology is unknown.
This occurs in young children and adolescents and presents as growth disturbance of the medial tibial growth plate resulting in angulation of the bone.
Commonly bilateral and is asymmetric.
There are three types: infantile,
juvenile and adolescent forms.
Findings include tibia varus deformity with fragmented medial tibial epiphyses and beaking of the proximal tibial metaphyses (Fig.19).
The metaphyseal- diaphysis angle is > 11 degrees (the angle is formed between the lines drawn perpendicular to long axis of tibia and through the tibial metaphyseal beak [20].