Table 1 lists the common causes of limb length inequality encountered in pediatric practice

Table 1 : Causes of limb length discrepancy

1. Congenital
2. Infection
3. Post traumatic
4. Neoplasms
5. Irradiation
6. Skeletal Dysplasias
7. Miscellaneous

Congenital causes are the best known reasons for a limb length discrepancy. They are the most difficult to treat also. Examples are congenital short femur and the spectrum of proximal focal femoral deficiencies which may exhibit a shortening in the range of 6% to 90% of the femoral length, partial or total absence of the tibia and fibula etc. Other congenital causes of discrepancy would be conditions like hemihypertrophy giving rise to a lengthening and the constriction band syndrome.

Infection can cause shortening in a variety of ways e.g. by causing joint dislocation or by damaging the growth plate. By asymmetric damage to the growth plate, angular and complex deformities can be produced along with shortening. Depending on the growth contribution of a growth plate, the age of the patient at the time of physeal damage and the extent of the growth plate damage whether partial or total, minimal or large amount of shortening can be produced. An example is complete damage to the lower femoral epiphysis inhibiting 70% of the femoral growth or 40% of the total lower limb length. Roughly an inhibition of 1 cm would take place per year of remaining growth. However, this is true only for the latter years of growth. A third way by which infection can cause limb length discrepancy is by causing a pathological fracture which mal-unites.

Posttraumatic malunions would cause an immediate discrepancy of limb length. However damage to growth following trauma would cause an insidious increase in the discrepancy. Interestingly, accurate reduction of fractures e.g. of the femur or tibia may give rise to a lengthening because of a growth stimulation.

Neoplasms and irradiation also act by causing growth plate damage. Some skeletal dysplasias e.g. Ollier's or hereditary multiple exostosis cause shortening of a limb by affecting the growth plate.

Miscellaneous causes of limb shortening would include paralytic causes e.g. poliomyelitis which causes an inhibition of growth. Reduced mass and activity coupled with an abnormal vasomotor control may be responsible for this. Perthes disease and slipped capital femoral epiphysis cause shortening by early fusion across the growth plate and by causing varus of the upper femur. Rarely shortening can be iatrogenic following surgery in the vicinity of a growth plate or following varus osteotomies of the upper femur which shorten the limb.