The Wiltse classification categorizes spondylolisthesis based on etiology. The five main categories are dysplastic, isthmic, degenerative, traumatic, and pathological [13, 14]. Isthmic is then subdivided into lytic, elongated, and acute fracture. This system is very widely used as a first step in describing the pathology exhibited on X-ray. Unfortunately, outside of placing the defect into a category, it does not assist in formulating a management plan. Also, as with many of the other earlier classification systems, the overall sagittal balance of the patient is not considered. Despite these shortcomings, the Wiltse classification system has remained one of the more important methods of categorizing spondylolisthesis. Table 7.2 summarizes this system.

Type 1, Dysplastic, describes cases where there is a congenital defect of the upper sacrum or the arch of L5 creating a situation where the forward thrust of the spinal column cannot be opposed and the slip progresses. The pars is usually normal but, if not, it is not the main pathology. There is a high association with spina bifida occulta (94 %) [14].

Type 2, Isthmic, involves a primary defect in the pars articularis from a failed neural arch. Approximately one-third are associated with spina bifida occulta and can be seen on 5–20 % of spine X-rays [15]. The three subcategories are lytic, elongated, and acute fracture. Lytic involves the fatigue fracture of the pars. This type is commonly found in the pediatric population, especially in athletes. This injury may reflect repetitive trauma or worsening of a pre-existing condition. The elongation subtype is similar to the lytic subtype but the pars is left intact. It is thought to be the result of multiple rounds of microfractures and healing, resulting in an elongated pars and slippage of vertebral body. The acute fracture subtype is caused by trauma and involves fracture of the pars and its resulting incompetence.

Type 3 is Degenerative. Longstanding intersegmental instability causes remodeling of the facets. This particular subtype occurs most commonly at L4–5, especially when L5 is sacralized. This subtype usually occurs in patients over the age of 50, and the slippage is usually less than 30 %. The pars remains intact [16].

Type 4 is Traumatic. An acute fracture of any part of the posterior element that keeps the spine from slipping forward will compromise its ability to maintain proper alignment.

Type 5 is Pathological. Bone disease, local or generalized, may prevent the posterior elements from opposing the forward slip of the spine. For example, Albers-Schönberg disease can cause pars fractures [13]. Other diseases such as arthrogryposis [16] and Paget’s disease can cause systemic bone disease and result in spondylolisthesis.

The Marchetti and Bartolozzi classification [17, 18] is one of the most commonly followed classification systems for spondylolisthesis. This is an etiology based system. The original classification proposed in 1982 classified spondylolisthesis into two broad categories: Developmental and Acquired (Table 7.3). The acquired type includes iatrogenic, traumatic, and pathologic subdivisions. This system was updated and modified by Marchetti and Bartolozzi in 1994 and sub-classified the developmental type into high and low dysplastic subtypes. The acquired type was expanded to include a traumatic subtype, and the term “postsurgical” was used to replace the iatrogenic subtype [18].

Herman and Pizzutillo [24] proposed a new classification system for spondylolysis and spondylolisthesis specifically for the adolescent and pediatric populations. This classification system is based on clinical presentation and spinal morphology (Table 7.4) and combines the basic classification elements of the Wiltse classification [13] and the Marchetti and Bartolozzi classification [18]. This classification system aims to provide treatment and management guidelines, including nonoperative care, which are relevant to children and adolescents.

This system classifies spondylolisthesis in pediatric patients into 4 main types: dysplastic, developmental, traumatic, and pathologic. Traumatic is subcategorized into acute and chronic groups. The chronic subgroup is further subcategorized into stress reaction, stress fracture, and spondylolytic defect (pars nonunion).

Type I, dysplastic, is similar to the dysplastic class of Wiltse and includes all congenital or developmental posterior elements defects except for pars. Progressive deformity and variable neurologic involvement, including radiculopathy and bladder/bowel changes, are usually seen in this class [24]. Close follow-up is recommended for children in this class, and operative intervention is indicated in cases of neurological presentation or progressive deformity irrespective of the severity.

Type II, developmental, includes spondylolysis and spondylolisthesis secondary to an incidentally diagnosed pars defect. These patients develop a defect secondary to genetic predisposition in most of the cases; sports activity or athletic overuse is generally not a causative factor [24, 25]. Progression is uncommon in this class, and conservative treatment and follow-up is recommended.

Type III includes traumatic defect of pars interarticularis and is subdivided as acute traumatic (high energy trauma) and chronic, or slowly evolving pars defect. Treatment recommendations for this type include conservative management in the initial stages, but most patients with type IIIB high grade spondylolisthesis require surgical intervention. Failure to respond to conservative treatment in 3 months is a surgical indication regardless of the subtype and grade of translation [24].

The Type IV (pathologic) variant includes defect of pars, lamina, and pedicle secondary to a pathologic process like tumors, infection, or osteogenesis imperfecta. Treatment is individualized in this category but includes surgical intervention in most cases.

While this system is valuable in that it differentiates traumatic and developmental isthmic spondylolisthesis, it does not consider patients with pars/isthmic defects or the degree of dysplasia. Mainly, this classification is used to guide non-surgical management.

In 2006, Mac-Thiong and Labelle proposed a new classification system designed to compensate for the shortcomings of the previous systems [21] (Table 7.5). The criticism of the Marchetti and Bartolozzi system is that while differentiation between high and low dysplastic subtypes is important, there were no criteria set forth to help categorize into these groups. Also, Herman’s system does not consider the degree of dysplasia or include patients with pars defects or elongation. In particular, none of these systems are able to guide surgical management of this complex disease process. Therefore, Mac-Thiong and Labelle developed a new system with specific goals in mind. First, this system was meant to guide surgical treatment. Second, it was to be used to grade the severity of spondylolisthesis. Third, the criteria for defining high and low dysplastic spondylolisthesis were delineated. Finally, the concept of sagittal spinopelvic balance was incorporated to help guide surgical planning. This system was developed by identifying 92 articles that discussed classification, risk factors for progression, spinopelvic balance, sacropelvic morphology, dysplastic changes, and surgical management of spondylolisthesis.