Definition

Spondylolysis refers to a bone defect in the pars interarticularis. Pars interarticularis translates to “bridge between the joints” and as such is the isthmus or bone bridge between the inferior and superior articular surfaces of the neural arch of a single vertebra (Figs. 49.1 and 49.2). When bilateral spondylolysis is present, the posterior aspect of the neural arch, including the inferior articular surfaces, is no longer connected by bone to the rest of the vertebra.

Spondylolysis is an acquired condition; it has never been found at birth [1]. Spondylolysis is most commonly acquired early in life [2,3] and is identified by lumbar radiographs in 4.4% of children 5 to 7 years of age [2,4]. By 18 years of age, 6% of the population has spondylolysis [2], with few additional cases thought to occur thereafter. The prevalence remains steady at approximately 6% in radiographic screening of adult spines [5,6]. Community population prevalence ranges from 5.7% to 11.5% when screening is performed by computed tomography (CT) scan [6,7].

Spondylolysis most commonly occurs at the L5 vertebrae, where about 90% of the cases are found. It is found with decreasing frequency at progressively higher lumbar levels [2,8,9]. It is more common in males than in females with roughly a 2:1 ratio (7.7%-9% vs 3.1%-4.6%) [6–8,10], can be unilateral (less common) or bilateral (more common), and has a suspected genetic predisposition [2,10,11].

The most likely cause of spondylolysis is a stress fracture of the pars that persists as a nonunion [12]. This is consistent with the higher incidence of spondylolysis suspected in adolescents and young adults who aggressively participate in sports requiring repetitive flexion-extension movements, such as gymnastics, throwing sports, football, wrestling, rugby, judo, dance, and swimming breast and butterfly strokes. Specifically among gymnasts, those with spondylolysis tend to be heavier, older, or training with more intensity. The incidence of spondylolysis is as high as 30% in professional soccer and baseball players in Japan [5]. Further supporting an acquired stress fracture as the cause is the lack of spondylolysis in the lumbar spines of individuals who have never walked [13].

Spondylolisthesis refers to displacement of a vertebral body in relation to the one below it. The most common type of spondylolisthesis, and the alignment abnormality that is implied when the term is used in this chapter, is an anterior displacement, also called anterolisthesis. Spondylolisthesis can also occur in a posterior direction, called retrolisthesis, or laterally, called laterolisthesis. Spondylolisthesis is an abnormal finding. Whenever spondylolisthesis is present, it is pathognomonic of structural and functional failure of the neural arch and facet joints, which are responsible for maintaining normal vertebral alignment.

Spondylolisthesis is classified by etiology and grade. There are five etiologic types (Table 49.1):

Dysplastic spondylolisthesis results from congenital dysplasia of one or more facet joints.

Isthmic or spondylolytic spondylolisthesis results from bilateral pars defects (bilateral spondylolysis).

Degenerative spondylolisthesis results from degeneration of the facet joints and intervertebral discs (most common at L4-L5 and with advancing age).

Traumatic spondylolisthesis results from fractures of posterior elements other than the pars, such as facet joints, laminae, or pedicles.

Pathologic spondylolisthesis results from pathologic changes in posterior elements due to malignant neoplasm, infection, or primary bone disease.

Isthmic spondylolisthesis is male predominant, whereas degenerative spondylolisthesis is more common in females [7]. This chapter is limited to discussion of spondylolytic (isthmic) spondylolisthesis.

The grade of spondylolisthesis is rated by the percentage of slippage of the posterior corner of the vertebral body above over the superior surface of the vertebral body below. At least 5% slippage must be present for a diagnosis of spondylolisthesis to be conferred. Slippage can be further categorized into five grades: grade I indicates slippage from 5% to 25%; grade II is 26% to 50%; grade III is 51% to 75%; grade IV is more than 75% [14]; and grade V is complete dislocation of adjacent vertebrae, also called spondyloptosis. Most cases (60%-75%) are classified as grade I; 20% to 38% are classified as grade II; and less than 2% of all cases are graded III, IV, and V [8,15]. Slip of 15% or more is associated with increased risk of radicular pain or weakness [16] and is always associated with moderate to severe degeneration of the lumbosacral disc [10].

In children and adolescents with bilateral spondylolysis, spondylolisthesis is already present in 50% to 75% at the time of initial diagnosis of the spondylolysis [10,11,17,18]. The incidence of spondylolisthesis increases with age. After diagnosis, concern about the progression of spondylolisthesis is common but prognostic factors are lacking, making it difficult to predict which patients are at risk for progression [10,18]. However, participation in competitive sports has not been found to influence the progression of spondylolisthesis [10]. In addition, unilateral spondylolysis almost never progresses to spondylolisthesis [10]. Whereas spondylolysis is male predominant [7], spondylolisthesis and slippage are more common in females when bilateral spondylolisthesis is present on CT [19]. Typically, progression occurs before and during the early teenage years [2], and only minor progression occurs after skeletal maturity [10]. Advancing age may lead to slight additional progression of spondylolytic spondylolisthesis, which is usually attributed to progressive degeneration of the disc and facet joints [20]. There is a positive correlation between the percentage of slip and the degree of degenerative change but no correlation with disc herniation [10].

Symptoms

Most people with spondylolysis and spondylolisthesis are asymptomatic. Less than 5% of children diagnosed with spondylolysis or spondylolisthesis have back pain before the age of 18 years [10]. Whereas the incidence of back pain increases with age, the incidence of back pain in those with spondylolysis or spondylolisthesis is similar to that of the general population [2,8,10,16]. In addition, the reverse is true: those with and without back pain have nearly an identical incidence of spondylolysis [21]. Furthermore, the degree of spondylolytic spondylolisthesis is not associated with the prevalence of back pain [10], and no study has linked progression of spondylolisthesis with pain symptoms. Disability because of back pain is no more prevalent in the population with spondylolysis and spondylolisthesis than in the general public [10,17]. Ultimately, in patients with established spondylolysis, with or without spondylolisthesis, it is difficult to attribute pain symptoms to these abnormalities.

One exception is the child or adolescent who initially presents with acute back pain, in whom reactive changes to the bone marrow from spondylolysis or spondylolisthesis likely contribute to the patient’s symptoms. For these patients and others with back pain from associated spondylolysis or spondylolisthesis, no distinct pain characteristics have been found to distinguish their pain from that experienced by others with common degenerative back disorders [2,17]. The back pain can range from mild to severe and is frequently described as a dull, aching pain in the back, buttocks, and posterior thigh [2].

Spondylolysis with spondylolisthesis combined with disc degeneration may result in significant narrowing of the neuroforamina at the affected level. This can cause compression or irritation of the exiting spinal nerve, resulting in radiating pain and neurologic sequelae in the lower limb, often in dermatomal or myotomal distribution. Because spondylolytic spondylolisthesis most commonly involves the L5-S1 level, the L5 nerve is most often affected by this problem.

Physical Examination

The physical examination in spondylolysis and spondylolisthesis has few specific or sensitive findings. Painful trunk range of motion is often noted with children and adolescents with symptoms from acute spondylolysis. It is suspected that pain with trunk extension may be common in acute spondylolysis as this motion shifts load to the posterior vertebral elements and thus through the region of the pars [22]. Indeed, limited range of motion for trunk extension has been observed [15,23]. However, the precision of painful trunk extension has not been determined, and these findings are common to other spinal disorders. Palpation of the back may reveal local tenderness at the lumbosacral junction, the level at which spondylolysis is most common [15].

Detection of spondylolisthesis on physical examination is difficult except in the rare cases of grade III or greater slips. Here, a “step-off” of the spinous processes can be seen or palpated at the level of the spondylolisthesis. In grade I and II spondylolisthesis, the step-off is much more difficult to detect and has never been shown to be a reliable finding. Neurologic deficits and positive results of straight-leg raising tests are rarely found in cases of spondylolytic spondylolisthesis, including cases with sciatica [15]. When neurologic deficits are noted, they usually involve the L5 roots, which can become irritated within their neuroforamina, presenting as an L5 radiculopathy (weakness of the extensor hallucis longus and hip abductors as well as sensory loss on the dorsum of the great toe) [15,24,25].

Diagnostic Studies