Lower Limb Alignment and Leg Length Discrepancy

Figure 13.1

Bryant’s triangle (a) helps differentiate between supra- and infratrochanteric shortening of the femur. A line is drawn from the ASIS (A) to the tip of the greater trochanter (B). Horizontal and vertical lines are drawn as demonstrated, intersecting at point (C), forming the triangle. The distance between points B&C correlate to the femoral neck, and comparison with the contralateral side may indicate shortening. Nelaton’s line runs from the ASIS to the ischial tuberosity. The greater trochanter should just touch the line. Proximal migration of the trochanter would suggest supratrochanteric shortening. Finally, Shoemaker’s line (b) also runs from the greater trochanter to the ASIS and beyond towards the abdomen. Both Shoemaker’s lines should intersect at or above the umbilicus in the midline. If they cross below, supratrochanteric shortening should be assumed

Finally the patient’s gait must be observed. If there is a significant discrepancy, they will have a short leg gait.

Epidemiology

LLD’s are relatively common, with a reported prevalence of 90% in normal adults with a mean inequality of 5.2 mm. LLD greater than 20 mm affects 1 in 1,000 people.

Aetiology

The aetiology of a pathological leg length discrepancy can be congenital acquired, and may shorten the limb or lengthen it. In cases of congenital defects, LLD can be calculated and predicted more easily as growth remains proportional. This is not the case in acquired disorders.

Limb Alignment

Limb deformities can happen in all planes. Normally up until the age of two there is a prevalence for physiological genu varum (bowlegs), between the ages of three and six children develop genu valgum (knock knees). This is physiological up to the age of eight or ten and persists to a mild degree in many adults (see Chap. 1) (Fig. 13.2). The onset or persistence of angular deformities after the age of 6 should be viewed with suspicion. Asymmetrical deformities and those associated with pain should be investigated.

Causes of a Shortened Limb

A summary of the different aetiologies of malalignment is summarised in Table 13.1.

Table 13.1

Aetiology of lower limb malalignment

Cause	Genu valgum	Genu varum
Congenital	Fibular hemimelia Skeletal dysplasia	Tibial hemimelia Skeletal dysplasia
Trauma	Partial physeal arrest	Partial physeal arrest
Arthritis	Juvenile RA
Infection	Partial growth arrest	Partial growth arrest
Metabolic	Rickets	Rickets
Others		Blount’s disease

Adapted from Childrens orthopaedics and fractures by Benson et al., 2009, Springer

Congenital

The majority of congenital limb deficiencies occur sporadically. Three main forms of congenital shortening of the lower limb exist:

Congenital femoral deficiency.
- This includes idiopathic coxa vara (see Chap. 9), congenital short femur and proximal femoral focal deficiency (PFFD).
- Of the latter two, congenital short femur is the milder form with an average growth retardation of about 10%. PFFD is a developmental defect of the proximal femur and ranges from hypoplasia of the proximal femur to complete absence of the proximal end. In truth, they are not separate disorders but a continuum of a spectrum of disorders.
- The cause of this is unknown and it can be associated with other anomalies in the limbs and face.
- In 1969 Aitken developed a classification (A–D), which describes the appearance of the acetabulum with reference to the femoral head and the increasing coxa vara that increases with severity.
Congenital fibular deficiency (Fig. 13.3).
- It is the most common of the lower limb congenital deficiencies.
- It similarly includes a spectrum of deformities from mild shortening of the tibia and fibula to complete absence with tibial shortening and bowing along with foot deformity.
- Clinically the patient may have anteriomedial bowing of the tibia and a dimple over the skin at the apex of the tibial bow. The foot is normally in an equinovalgus position.
- It is associated with congenital femoral deficiency, cruciate ligament deficiency, tarsal coalition and absent lateral rays.
Congenital tibial deficiency.
- This is the rarest of the congenital abnormalities described, and may also be seen at varying degrees of severity.
- The typical appearance is that of an anterolateral bowed tibia. The knee is normally in varus with a prominent fibula head. The foot is in equinovarus and it may show medial duplication/polydactyly.
Other congenital causes to be aware of include developmental dysplasia of the hip (Figs. 13.4), Ollier’s disease and CTEV.