Other Anatomic Disorders of the Spine

Classification

Historically, spondylolisthesis cases have been divided into congenital and acquired types. Recognition of cases in which the pars interarticularis is fractured (termed spondylolysis ), with slippage occurring through the fracture, or cases in which the pars is not fractured but elongated and the abnormal lumbosacral anatomy allows forward slippage became the basis for the well-known Wiltse classification. ^, Five types, based on the radiographic findings and age at onset, have been described ( Table 11.1 ) and Marchetti and Barolozzi amplified this classification. The remaining discussion in this chapter will be limited essentially to types I and II.

Most recently, the Spinal Deformity Study Group (SDSG) has developed a classification incorporating and emphasizing sagittal spinopelvic balance, in part because of difficulty in reproducibly classifying the degree of dysplasia. ^, Six types of spondylolisthesis are now recognized, based on the grade of slip and spinopelvic alignment, using measurements of pelvic incidence (PI), sacral slope (SS), pelvic tilt (PT), and the C7 sagittal plumbline (parameters described later, under “Radiographic Findings”). Three types of low-grade spondylolisthesis (Meyerding grades 0–2, slip <50%) are described, based on PI: low (type 1), normal (type 2), and high (type 3). High-grade spondylolisthesis (Meyerding grade ≥3, slip ≥50%) includes type 4 (balanced sacropelvis), type 5 (retroverted sacropelvis with balanced spine), and type 6 (retroverted sacropelvis with unbalanced spine; Fig. 11.1 ). Currently, the system has been validated as having modest to good interobserver and intraobserver reliability and ultimately strives to guide surgical treatment.

Pathoanatomy

Dysplastic Spondylolisthesis (Wiltse Type I)

The dysplastic form of spondylolisthesis occurs only at the L5–S1 level and is caused by primary congenital dysplasia of the L5–S1 facet joints ( Fig. 11.2 ). Its congenital nature is supported by frequent association with spina bifida occulta of L5 and the sacrum. The anatomic incompetence of this facet joint allows the slipping (listhesis) to begin. Without lysis or elongation of the pars, such forward vertebral slipping beyond 25% would almost certainly produce a neurologic deficit as the posterior neural arch impinges on the dura (see Fig. 11.2 ). Otherwise, the slip may remain asymptomatic until early adolescence and the prepubertal growth spurt, when leg pain and hamstring spasm result in gait disturbances, with or without back pain. ^{, , , ,} Dysplastic spondylolisthesis is more common in females and has an increased incidence in first-degree relatives of patients, suggesting a genetic cause that results in the congenitally incompetent facet.

A pars defect (lysis) along with a congenitally abnormal L5–S1 articulation is to be differentiated from the Wiltse type II (acquired) slip, in which the L5–S1 articulation is anatomically normal but a stress fracture occurs and the slip displaces through the stress fracture. In the presence of an isthmic defect or pars elongation in a dysplastic slip, the displacement can become severe, but without neurologic sequelae, because the dural sac is not impinged by the posterior elements.

An abnormal sacral dome and endplate may also predispose the patient to translation with kyphosis. Biomechanical weakness and “epiphyseal” disruption to the sacral endplate, similar to the proximal medial tibial epiphysis of infantile Blount disease or the physeal disruption of slipped capital femoral epiphysis, have been described. ^a

a References , , , , , , , , .

This type of primary sacral deformity ( Fig. 11.3 ) in susceptible individuals may produce a growth disturbance contributing to the onset of the deformity that becomes important during the prepubertal growth spurt. ^, Furthermore, such weakness of the sacral endplate may become critical when stressed by shear forces in the upright posture in patients with increased PI, SS, and lumbar lordosis (see later discussion under “Radiographic Findings”). ^, A higher PI may predispose for slipping (see Fig. 11.3B ) through a combination of shearing postural forces imposed on a congenitally abnormal sacral dome, the preexisting conditions for the high dysplastic grade of Marchetti and Bartolozzi. ^{, ,} An interesting report has noted that the rounded sacrum deformity in young children can reverse when the child’s activities are limited. This observed remodeling supports the theory of primary sacral dome dysplasia as a cause of spondylolisthesis. ^,

Isthmic Spondylolisthesis (Wiltse Type II)

The isthmic type of spondylolisthesis (termed acquired traumatic in the Marchetti-Bartolozzi classification) is a more common and benign form that rarely produces significant neurologic findings or gait disturbance ( Fig. 11.4 ). Pars stress fracture, or spondylolysis, is fairly common, reported to be 4.4% at 6 years of age and 6% at 18 years. A study of lumbar spine computed tomography (CT) scans in asymptomatic patients has noted rates in the general population of 5.7% and 3.1% of spondylolysis and spondylolisthesis, respectively. A more recent report demonstrates that a unilateral spondylolysis was significantly associated with a spinal malformation in patients with a normal PI while a larger number of patients who had bilateral spondylolysis had high PI. Spondylolysis is most prevalent at L5, accounting for 87% of all stress fractures, followed by lysis at L4 (10%) and L3 (3%). A recent study demonstrated age differences in with the prevalence of spondylolysis of 1% in children under age 3, 3.7% in children under age 6 and 4.7% among all children. Both familial and racial predispositions toward isthmic spondylolisthesis have been observed. ^{, , ,} The incidence of isthmic spondylolisthesis seems to stabilize in adulthood, during which the degenerative type of spondylolisthesis predominates.

Acquired pars defects appear to have mechanical causes. The pars region is the weakest area of the neural arch and is susceptible to fatigue fracture. Histologic analyses of fetal vertebrae demonstrate trabecular and cortical irregularity of the lumbosacral pars interarticularis which may act as a stress riser in the lower lumbar vertebrae. Another cause may be limited excursion of the lumbosacral facets as seen in the spondylolytic specimens from the Hamann-Todd collection in which the pars of L5 is subject to increased contact stress during normal extension movement producing a fatigue fracture. This supports the proposed “nutcracker” mechanism of spondylolysis and is seen in patients with a relatively low PI and SS (see Fig. 11.4B and later, “Radiographic Findings”), who consequently have a relatively horizontal L5 disk.

Pars defects have not been observed in newborns or nonambulatory patients, thus supporting the Marchetti-Bartolozzi contention that this form of spondylolisthesis is more accurately termed acquired. Pars lysis or elongation does not occur in primates that do not have an upright bipedal gait. The presence of lumbar lordosis, which is unique to humans, is thought to be necessary for spondylolisthesis to occur. Both flexion and extension forces have been implicated in the production of these stress fractures. ^{, ,} The increased incidence of isthmic spondylolytic defects in athletes who perform repetitive lumbar hyperextension (e.g., gymnasts, football linemen, cricket bowlers) confirms this mechanism. ^{, , ,} Spondylolisthesis occurs more frequently if L5 has short transverse processes and is high riding, relative to the iliac crest, creating hypermobility of this segment and allowing for forward translation. ^{, ,}

An acquired isthmic spondylolysis in a juvenile patient can progress to a listhesis as a result of shear forces during upright gait, as described for patients with the shear mechanism of spondylolysis. An anterior force on L5 is produced and increases as the spine is flexed, especially in patients with an increased PI and SS (see Fig. 11.3B ). The posterior muscle attachments that act on the laminae and spinous process hold this part of the neural arch in place and thus tend to separate or distract the spondylolysis further. With a strong anterior deflection force, a slip between the sacral apophysis and endplate may also occur and allow anterior translation and rotation of the slipping vertebra.

Clinical Features

The age at onset is probably the most important determinant of symptoms and the need for treatment. The more severe dysplastic types usually present at an earlier age because of greater instability of the lumbosacral junction, producing neurologic symptoms. Children may not have pain but demonstrate deformity and gait abnormalities from lumbosacral instability as the only neurologic signs. ^,

Spondylolysis is generally manifested as low back pain only. This is not surprising considering that the lysis produced by a stress fracture in, for example, an adolescent gymnast, produces no slippage and hence no neurologic risk. Pain may radiate to the buttocks and posterior of the thighs from mechanical instability, and it is usually aggravated by flexion and extension activities. Mechanical low back pain symptoms are always a cause for suspicion in a child or adolescent and mandate radiography, especially in a susceptible athlete or dancer.

Physical examination may actually be normal in patients with spondylolysis or a low-grade (<50%) listhesis. A listhesis may present a palpable step-off at the lumbosacral junction, and back pain may be elicited by standing hyperextension stress. As the degree of listhesis increases, more obvious physical findings can be appreciated, including a postural disturbance of flexed hips and knees combined with sacral prominence posteriorly and hyperlordosis proximal to the slip. Hamstring tightness, thought to be caused by lumbosacral instability and subsequent rotation of the sacrum into a more vertical position producing a retroverted pelvis, rather than being caused by actual nerve root or dural impingement, may result in a distorted gait as a result of the individual’s inability to take a normal stride. The individual may also have to walk on tiptoes because of the flexed hip and knee positions ( Fig. 11.5 ). Straight-leg raising with the patient supine on an examining table may be dramatically limited. Because of the vertical position of the sacrum and pelvis, a backward PT and an abdominal crease may be obvious cosmetic complaints, with the prominence of the sacrum posteriorly producing the so-called heart-shaped buttocks. An olisthetic scoliosis producing marked spinal decompensation with hyperlordosis can be observed (see Fig. 11.5 ) and is often rigid, having been produced by the irritative phenomenon of the slip and is a measure of how severe the spondylolisthetic crisis has become.

Nerve root weakness, especially of L5 but also S1 including a motor weakness or the loss of an ankle jerk reflex, may be present. Bowel and bladder function must be evaluated by history, including, for example, questions about incontinence and decreased frequency of urination. Sacral sensation and rectal tone are important evaluations of cauda equina function, especially postoperatively. Patients undergoing surgical treatment should have these functions evaluated preoperatively. ^, If the patient reports infrequent urination, bladder capacity should be determined by cystometrography and urologic consultation.

Radiographic Findings

The diagnosis of spondylolysis or spondylolisthesis is made from a standing lateral radiograph of the lumbosacral junction, and the hips and knees should be extended to limit any compensatory postures to allow for an accurate radiographic evaluation ( Fig. 11.6 ; see Fig. 11.2 ). A standing radiograph is emphasized to measure displacement and the true angulation of the lumbosacral junction, if present. The femoral heads should be visible on the lateral radiograph to measure PI and tilt. ^, In general, the degree of translational displacement, lumbosacral kyphosis, as measured by the slip angle (see Fig. 11.7C ), and the spinopelvic balance, together with the clinical findings, usually determine the course of treatment.

A unilateral pars defect may be difficult to see on a single lateral plain film. In this case, oblique radiographs may be obtained to visualize the so-called Scotty dog head and neck outline (the articular processes, superior and inferior, and the pars in between) at levels adjacent to the level of interest (see Fig. 11.6B ). Depending on the clinical scenario including the duration of symptoms, our preferred imaging modality is a CT scan of L4 and L5, the two most common levels to see a spondylolysis. This allows us to determine the presence of a lysis, and whether it is unilateral or bilateral and its chronicity. A defect that is wide and has a smooth edge suggests a long-standing chronic lesion, whereas an irregular edge suggests a more acute process (see Fig. 11.4D and E ). In the more acute fractures, an attempt to heal the lesion by external immobilization is justified. CT scans with three-dimensional reconstruction can be useful in the preoperative evaluation of patients with severe dysplastic spondylolisthesis to characterize the pathologic anatomy more precisely.

Disk herniation in association with spondylolisthesis has been reported in as many as 25% of patients at the next level above the slip and in 15% at the level of the listhesis itself. Magnetic resonance imaging (MRI) may therefore clarify the clinical picture in a patient with L5 radicular symptoms and minimal listhesis or in a patient with radicular symptoms not correlating with the level of the slippage. MRI is also useful to rule out other causes of lumbosacral dysfunction, such as tumor or infection, and may demonstrate a slipped vertebral apophysis, if present.

The radiographic parameters of the deformity define the severity and need for treatment, with increasing severity of slip parameters correlated to worsening quality of life questionnaire scores. Anterior translation, the listhesis of L5 on S1, is measured by the method of Taillard ( Fig. 11.7A ) or described by the classic Meyerding grades (see Fig. 11.7B ). ^, The lumbosacral kyphosis, or sagittal rotation, is measured by the slip angle (see Fig. 11.7C ), for which several methods of measurement have been described. We prefer to construct the slip angle from the tangential line of the upper endplate of L5 as it is consistently present and undeformed and thus is better suited for this measurement. ^, The body of L5 may become trapezoidal, a response to impression by the dome of the sacrum, and such distortion indicates a more severe deformity.

Because of the lumbosacral kyphosis, a standing or even supine anteroposterior (AP) radiograph of the sacrum will be difficult to interpret because of the superimposition of L5 and the sacrum (Napoleon’s hat sign; see Fig. 11.2B ). To eliminate this superimposition, evaluate for spina bifida occulta, and most importantly, evaluate the quality of the postoperative fusion mass accurately, a Ferguson view of the sacrum is recommended. The Ferguson view is an outlet view of the pelvis and thus shows the L5 and S1 bodies in their true AP position.

As noted earlier (see “Classification”), the importance of sagittal spinopelvic balance in determining pathogenesis, prognosis, and treatment guidelines has been recently emphasized and confirmed. ^{, ,} PI, PT, and SS are descriptive radiographic angles used to describe the sagittal relationships ( Fig. 11.8 ). PI, the angle created by a line from the midpoint of the sacral endplate to the center of rotation of the femoral heads and a line perpendicular to the sacral endplate drawn from its midpoint (see Fig. 11.8A ), is a fundamental anatomic relationship specific and constant for an individual subject. It describes pelvic morphology and ultimately lumbar lordosis and thoracic kyphosis in the upright position. PI remains constant during childhood and then increases in adolescence until reaching a maximal value, which is constant in adulthood. The degree of PI is unaffected by posture; it is constant whether supine, sitting, or standing. PT is the angle between a line joining the midpoint of the sacral endplate to the center of the femoral heads and the vertical line, whereas SS is the angle between the tangent to the sacral endplate and the horizontal (see Fig. 11.8B and C ). These two angles, measured with respect to the horizontal and vertical axes, describe the orientation of the pelvis in the sagittal plane and vary according to a person’s posture. PI is the arithmetic sum of SS and PT.

Studies of normal subjects have documented a link among PI, SS, and lumbar lordosis and thoracic kyphosis, with the latter spinal parameters adjusting to keep the head and trunk balanced over the femoral heads. ^, Thus as PI increases, SS and lumbar lordosis increase to maintain sagittal balance. Patients with spondylolisthesis have a greater PI than controls, suggesting that an increased PI can predispose to spondylolisthesis. ^b

b References , , , , , .

Typically, normal subjects have a PI of approximately 50 degrees, whereas patients with spondylolisthesis have a PI of 70 to 79 degrees, with PI increasing as slip severity worsens. ^{, ,} Unfortunately, the actual prediction of progression of listhesis, and thus the use of an abnormal PI as an indication for surgery, has not been proven. In contrast, slip percentage, Meyerding grade, and slip angle have been shown to be predictive of progression.

The concept of spinopelvic balance has been further refined, describing the pelvis as balanced or unbalanced (retroverted), with the spine unbalanced or balanced ( Fig. 11.9 ; see Fig. 11.1 ). The ultimate goal is to differentiate which patients might benefit from surgical reduction of a spondylolisthesis versus those for whom it is unnecessary (see later, “Surgical Methods: High-Grade Spondylolisthesis”). The balanced pelvis is one in which compensatory increased lumbar lordosis and decreased thoracic kyphosis of the spine are adequate to maintain an adequate C7 plumbline or normal sagittal balance. In the unbalanced, or retroverted, pelvis, there is such a high PI because of increased PT (visualized as an anterior position of the femoral heads relative to the sacrum) that the spine cannot accommodate the associated high L5 incidence angle, leading to positive forward balance (see Fig. 11.1 ). ^, This positive balance, indicating an unbalanced spine, occurs when the C7 plumbline falls anterior to the femoral heads on the standing lateral radiograph. The spine is balanced when the plumbline falls on or posterior to the femoral heads.

Measurement of L5 incidence (see Fig. 11.8D ) has been shown to correlate with the outcome of spondylolisthesis treatment and was found to improve along with the slip angle in patients who had subjective improvement postoperatively. ^, PI, PT, and SS may be affected minimally by surgical treatment. Thus although pelvic morphology is not altered by clinically successful surgery, spinal balance measures (slip angle, L5 incidence, lumbar lordosis) seem to improve and correlate with outcome.

Prognosis and Natural History

Once the diagnosis of spondylolisthesis or spondylolysis has been made, the risk of progression is the main determinant of treatment. Although younger patients might seem to have a greater risk of deformity progression, long-term studies (>45 years) have not found predictive value in the age, percentage of slippage, lumbar index (lordosis), or slip angle at initial evaluation. ^{, ,} Only Seitsalo and co-workers have found prognostic value in the initial percentage of slippage. The adolescent growth spurt continues to be a period of interest because significant progression of translation or kyphotic deformity is uncommon after maturity in a patient with mild deformity (<50% slip). ^{, ,}

PI is now regarded as a key prognostic parameter. Because PI is often abnormally high in high-grade listhesis, one may conclude that the risk of progression in low-grade listhesis (<50% slip) in patients with a low or normal PI (e.g., patients with the nutcracker mechanism; see Fig. 11.4 ) is low compared with the shear mechanism in patients in whom the PI is higher. ^, Similarly, patients with a retroverted pelvis and increased PT are extremely likely to progress, even with a balanced spine, because of the biomechanical forces that promote continued listhesis and lumbosacral kyphosis (see Fig. 11.3 ).

The natural history of untreated spondylolisthesis may leave patients with some impairment, influencing job choice, avoidance of heavy lifting, and choice of recreational activities. In lower grade slips (<30%), progression is unexpected unless the patient is immature, and even then progression occurs rarely in isthmic (acquired) defects. ^{, ,} Conservative treatment of low-grade slips has been reported to yield the same results as operative treatment, and in general, the progression of listhesis is mild and has no correlation with later symptoms on long-term follow-up, which have been attributed primarily to disk degeneration. ^{, ,} Thus prophylactic treatment of lower grade spondylolisthesis to avoid later progression of slippage does not appear to be justified, especially when a low incidence of progression is expected, and the spinopelvic parameters (low or normal PI) are favorable. ^{, ,} Although patients with spondylolisthesis may have disk degeneration and other low back impairment as adults, ^{, ,} these problems do not appear to be caused by progression of the deformity, which in some series is not altered by the presence of fusion in situ. Patients with long follow-up were often treated in an era when the fusion techniques were questionable, thus raising doubts about the outcome data. Finally, the controversy surrounding the need for more aggressive treatment of higher grade spondylolisthesis stems from favorable or unfavorable interpretation of long-term natural history studies and the results of in situ posterior fusion. ^c

c References , , , , , .

Treatment