This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The ability to determine accurately the length of the lower extremities is essential to diagnosis and important in treatment. It is a difficult thing to do and requires knowledge, care, and practice. It may be accepted as a fact that the limbs are normally equal in length. It is true that in rare cases there may be a slight inequality, but an amount of inequality readily detected by measurement will usually produce an unevenness in the gait, a slight limp.
Fig. 498. - Measurements of the lower limbs, viewed from the front, a, left anterior superior spine; b, right anterior superior spine; c, left trochanter; d, right trochanter; e, left internal malleolus; f, right internal malleolus; g, umbilicus; h, lower end of median line.
Fig. B. - The limbs in this figure are of equal length but the pelvis is tilted. The pelvis a-b is tilted up on the left and down on the right. Apparent shortening of the left leg is seen by comparing g-e with g-f. Actual measurement shows a-e to be a trifle longer than b-f and a-c longer than b-d.
Fig. C. - One leg shorter than the other, but the pelvis is in the correct position. The actual shortening found by comparing a-e with b-f corresponds with the apparent shortening found by comparing g-e with g-f.
Fig. D. - Legs unequal, pelvis tilted down on the side of the short leg. The apparent lengths g-e and g-f, taken from the umbilicus g, show the legs apparently equal, but the distance b-f is longer than a-e and the absolute or actual amount of shortening is only to be found by levelling the pelvis as in Fig. C, when the apparent and actual amount of shortening will be found to agree.
To measure accurately, bony landmarks are preferable to the soft parts, such as the umbilicus; these bony points must be carefully identified, they must be in their normal position, and the tape-measure must be accurately applied. Measurements are usually taken either from the umbilicus or anterior superior spines to the internal malleoli. The latter is the more accurate and shows the actual 32 shortening, while the former shows the apparent shortening. To identify the tip of the internal malleolus is usually easy enough, but the anterior superior spine is not so evident. The anterior portion of the crest of the ilium should be followed forward until its anterior superior spine can be distinctly felt. In applying the tape it is better not to rest it on the superficial surface of the spine nearest the skin but rather on its inferior surface nearest the feet. It should be placed below the spine and then pushed firmly upward and backward against its lower surface. The superficial surface of the anterior superior spine is often so rounded or flat as to make it an uncertain point to measure from. To put the parts in their normal position it is necessary to see that a line joining the two anterior superior spines is at a right angle with the long axis of the body, otherwise the tilting of the pelvis will vitiate the results. Fig. 498, A, front view, shows the normal relation; g is the umbilicus; g-h, the median line; a, left anterior spine; b, right anterior spine; c, left trochanter; d, right trochanter; e, left internal malleolus; f, right internal malleolus. The line a b is to be at right angles to g-h. Then a-e = b-f and g-e = g-f.
Fig. 498, B shows the effect of tilting of the pelvis, the legs being of equal length. a-b instead of being at right angles to g-h is inclined upward on the left side and down on the right. Apparent shortening is seen by comparing g-f with g-e. Actual measurement shows a-e to be a trifle longer than b-f. This is accounted for by the tilting causing b-d to approach each other while a-c have separated.
If one hip is ankylosed its femur should be moved laterally until the line joining the two anterior superior spines is at right angles to the median line of the body; the opposite limb is then to be abducted to a similar degree and the measurements of the two limbs can then be compared.
When the legs are unequal and the pelvis is in a correct position, the apparent and actual measurements agree (Fig. 498, C).
When the legs are unequal the pelvis is tilted down on the side of the short leg (Fig. 498, D). Apparent length taken from the umbilicus shows the legs equal, but the distance b-f will be found to be longer than a-e. This will not give accurately the actual amount of shortening because of the tilting of the pelvis. It can only be determined by levelling the pelvis so as to make the distances a-c and b-d equal.
The length of the extremity below the neck of the femur can be determined by feeling for the tip of the greater trochanter on its upper posterior border and measuring to the external malleolus and comparing with the opposite side.