Figure 1. Spondylolisthesis at L5-S1.
When there is a defect in the pars interarticularis with no forward displacement, the term spondylolysis applies (Fig. 2). Newman and Stone28 described five different types of spondylolysis and spondylolisthesis, and this classification was further refined by Wiltse et al.55 Of the five types ([1] dysplastic, [2] isthmic, [3] degenerative, [4] traumatic, and [5] pathologic), it is the isthmic variety that is of importance in the athlete. Isthmic spondylolisthesis furthermore has been divided into three subtypes: lytic, pars elongation, and acute fractures. Although the pathology may vary from fibrous scar to fibrocartilage or hyaline cartilage,15,34 the common denominator in isthmic lesions is lysis at the pars. This is most commonly found at the L5 segment, followed by L4.3 More than one level may be involved,32 and it may be unilateral.31
Figure 2. Spondylolysis with bilateral pars defects.