Spondylolisthesis, when caused by separation of the pars interarticularis, is a disease which affects only man. It is never present at birth and seldom below the age of four. It is usually present by the age of six if it is ever going to be; but the incidence does increase to adulthood in most races, and to age forty in Eskimos according to Stewart's studies. The incidence varies in different races. In the white race, the incidence is well over twice as high in men as in women.

Increase in slipping may occur up to adulthood or after surgical intervention at any age, but the period between ages ten to fifteen is the period of most rapid slipping. In my opinion solid dorsal fusion will stop further slipping. Fractures of the pars interarticularis caused by severe trauma usually heal, but not invariably.

At the age when the lesion appears, the spine of the affected individual is surprisingly normal appearing. The trapezoid shape of the body of the fifth lumbar vertebra and the dome-shaped sacrum are secondary changes. Spina bifida is thirteen times as common in association with defects of the pars interarticularis in the lower lumbar area as in normal spines, and the severe grades of idiopathic scoliosis are nearly four times as common.

Several vertebrae in the same patient may show defects of the pars interarticularis, and different segments in identical twins may show the defect. There is a five-fold increase in the incidence of defects of the pars interarticularis in the near relatives of people with spondylolisthesis.

The author believes the defect in the pars is due to two factors:

1. An hereditary defect or dysplasia probably in the cartilage model of the arch of the affected vertebra and usually of several vertebrae in this same individual.

2. The particular strain upon the pars interarticularis in the lower lumbar spine is due to the erect stance and to the lumbar curve.

As a result of the physical force of strain or tension, combined with the factor of dysplasia, the reparative process which bone normally is undergoing all the time progresses more toward bone resorption than toward bone formation.

In some, the factor of dysplasia is so pronounced on one side that a defect will occur unilaterally where strain must be relatively slight. In others, defects occur in the upper lumbar spine where the element of strain must also be small. In those cases which separate in spite of relatively little strain the hereditary weakness in the pars interarticularis must be especially pronounced.