Spondylolysis and Spondylolisthesis
11 Spondylolysis and Spondylolisthesis
Spondylolisthesis was first described in 1782 by the Belgium obstetrician Herbiniaux, who reported severe obstetric difficulties due to an anteriorly displaced fifth lumbar vertebra. 1 Kilian considered the condition as a slow subluxation of the lumbosacral facet joints and accordingly coined the word spondylolisthesis. 2 In one of the early mechanical studies performed by Robert in 1855, he found that spondylolisthesis could not be produced unless there was a breach in the neural arch. 3 Lambl then recognized the cardinal importance of a neural arch defect (spondylolysis). 4 In 1882, Neugebauer 5 provided a clear description of how spondylolisthesis could occur due to L5/S1 facet joint aplasia with elongation of the pars interarticularis without a specific defect. This work, a century after Herbiniaux, was based upon many cases of obstetric problems that had occurred in Freiburg, Strasburg, Berlin, and Paris. Neugebauer’s thoughts were that the etiology would be either congenital or acquired, with the former being failure of the normal development of the L5 articular processes with attenuation of the pars interarticularis and progressive rounding of the upper border of the sacrum—the classic features of dysplastic spondylolisthesis. He felt that the acquired form was due to a fracture of the posterolateral elements of L5. In 1931 Meyer-Burgdorff reported that there was always a fatigue fracture of the isthmus and this was due to lumbar spine hyperlordosis. 6
The congenital theory had its proponents in Putti, 7 Le Double 8 and Willis 9 while Brailsford 9 and Brocher 10 proposed a dysplastic theory due to disturbance of the ossification of the vertebral arch during childhood very similar to developmental hip dysplasia. In 1963 Newman and Stone published their classification following a 15-year study of more than 300 cases and reported these five types: dysplastic, isthmic, degenerative, traumatic, and pathological. 11 Taillard gathered all these categories together into a final common pathway of rupture of the articular bolt safety mechanism. 13 Then in 1975 the International Society for the Study of the Lumbar Spine (ISSLS) put forward a classification based upon that of Newman and Stone, referred to as the Wiltse classification, which is still in use today 14 (▶ Table 11.1).
There have been other proposed classifications 15 but one of the most popular is that of Marchetti and Bartolozzi (M-B) in 1997. 16 However, like many classifications, such as idiopathic scoliosis (see ▶ 4), there are problems and, indeed, flaws. Before considering the M-B classification it is necessary to go back to the Wiltse classification drawn up by access only to plain films and oblique views whereas nowadays we have both three-dimensional (3D) computed tomography (CT) and 3D magnetic resonance imaging (MRI) available, which we have used extensively, making the geometry of spondylolisthesis more readily visualized.
11.2 Etiology and Radiology
This was the type reported by Herbiniaux 1 and Neugebauer 5 that so troubled obstetricians because only with this category does the L5 vertebra slip forward so far that it reduces the sagittal diameter of the pelvis to block the passage of the fetus.
This used to be referred to as congenital spondylolisthesis and although the basic pathology is congenital the deformity of spondylolisthesis may never occur or at least develops during subsequent growth. Hence the word dysplastic is deemed more appropriate and refers to congenital anomalies of the lumbosacral joint and, in particular, the L5/S1 facet joints (▶ Fig. 11.1). There is also an obligatory spina bifida occulta of L5 or S1.
Fig. 11.1 Parasagittal CT demonstrating elongation of the pars (arrows) and hypoplasia of the facet joint (large arrow).
There is then secondary deformation of the L5 vertebra being much more lordotic and the top of the sacrum becoming progressively more rounded. Finally, slippage can be so severe that the L5 vertebral body lies in front of the upper sacrum (spondyloptosis). Meanwhile the anteroposterior (AP) view shows a wide open sacrum and superimposition of the L5 vertebral body over the sacrum with the appearance of the upside-down Napoleon’s hat (▶ Fig. 11.2). When slippage begins the whole of the L5 vertebra is intact but progressive slippage does not occur unless the pars elongates and eventually develops a secondary spondylolysis. The top back corner of the body of the sacrum can appear to significantly indent into the local nerve roots (▶ Fig. 11.3) although clinical cauda equina problems are rare.
Fig. 11.2 AP radiograph of the lumbosacral region in a case of spondyloptosis giving the appearance of the upside-down Napoleon’s hat (arrows). Note also the spina bifida occulta (large arrow).
Fig. 11.3 Sagittal T2-weighted MRI of the lumbosacral region in a severe spondylolisthesis showing the upper back corner of the sacrum indenting the lower lumbar and sacral nerve roots (arrows).
Originally it was thought that the secondary lysis in association with dysplastic spondylolisthesis might be difficult to distinguish radiographically from a straightforward lytic (stress fracture) spondylolisthesis but this is not so as the elongated attenuated dysplastic pars is readily recognizable (▶ Fig. 11.1) along with the other typical features (the lordotic L5 body, the rounding of the sacrum, and the spina bifida occulta). Dysplastic spondylolisthesis is also much less common. The more downward the L5 vertebral body on the sacrum the more it allows the body weight above to exert a more shearing effect, favoring progressive kyphosis. 17, 18 Recently attention has been focused on what happens below the spine in the pelvis.
This was subdivided into the three categories 14 lytic, elongated but intact pars, and acute fracture.
This is by far the most prevalent variety (▶ Fig. 11.4) and cases have been, rarely, described in infants younger than a year of age. 3 By the time children get to the age of 7, the prevalence rate rises appreciably in the general population. However, certain sports during adolescence are considered to be an important contributing factor, 19 such as ballet dancing, gymnastics, trampolining, and fast bowling/pitching.
Fig. 11.4 Lateral radiograph of the lumbosacral region showing a grade 1 lytic spondylolisthesis. Note the defect of the pars (arrow). The L5 spinous process is no longer connected to the L5 vertebral body and is out of alignment with the L4 spinous process (lines).
Jackson et al felt that the prevalence of pars defects was only about 2 to 3% in the general population but in gymnasts that rate was four times higher. 19 They reported on 14-year-old female volunteers and with X-rays that included oblique projections they showed that 11% had pars defects and 6% had a grade 1 spondylolisthesis (▶ Fig. 11.5). Lytic spondylolisthesis can occur at multiple levels but nearly always at the L5/S1 level. Interestingly, in Jackson’s series, 38% had a spina bifida occulta at the bottom of the spine. Jackson emphasized that these girls were doing very hard gymnastic training with repetitive flexion-extension cycles of the lumbar spine. Just under 10% had positive isotope bone scans but no lysis on plain films suggesting a stress reaction. They also observed that many athletes were noted to have defects that healed as did Taillard. 12
Fig. 11.5 (a) This is an oblique tomogram of the lower lumbar spine which shows the posterior facet joints (arrows) and the bone in between, the pars interarticularis. These oblique views are very helpful in identifying lyses (arrowhead). (b) The oblique lumbar spine appearance has been likened to that of a Scottie dog by La Chapelle. 20 If the Scottie dog has a collar (arrowhead) this is a spondylolysis. This oblique view is particularly helpful in identifying such a lesion.
(Reproduced with the permission and copyright of The Medico-Legal Back: An Illustrated Guide, 2003, Cambridge Uniiversity Press. Dickson RA and Butt WP, fig 5.2.)
From around the world came reports on spondylolyses, in Japan in young athletes, 21 and in U.S. college football linemen. 22 The higher prevalence may be because now the search for lyses in young promising athletes includes the use of modern imaging techniques including MRI (▶ Fig. 11.6) and 3D CT and 3D MRI. In the UK it seems that every professional soccer player with a back problem, or indeed professional cricketer, comes to the clinical consultation along with their medical file that always includes an obligatory MRI scan. It would appear that some young athletes are denied a promising future purely on the basis of an MRI scan showing a lysis on one or both sides. Indeed, lyses and other defects are so common that the radiological reporting of them on plain films has already been cautioned against, 23 although seemingly not heeded.
Fig. 11.6 Sagittal T2-weighted fat-saturated MRI. Unilateral spondylolysis. There is bone edematous change (arrows) on either side of a pars defect (arrowhead).
Jackson et al 19 and Wynne Davies 24 also confirmed the very familial nature of spondylolysis in the general population. Interestingly it has been reported that the prevalence rate of neural arch defects in Alaskan natives is of the order of 50%. 25 With modern imaging techniques maybe all Alaskans have a spondylolysis! A study of 485 skeletons from South Africa demonstrated a 3.5% prevalence rate for spondylolysis but with no difference between races and genders. 26 A study of 400 schoolchildren demonstrated that in those with spondylolyses almost a third of parents had the same lesion. 27 One important etiological factor in the development of a spondylolysis is the orientation of the lumbar posterior facet joints with defects being much higher in those with coronally orientated facet joints (▶ Fig. 11.7). 28 Perhaps that is what is passed on in families and then the development of lyses modulated by physical activity.
Fig. 11.7 Axial CT section through the L4/5 facet joints in a case of a left-sided L5 lysis. The left facet joint is more coronally orientated (arrow).
Various authorities have felt that either repetitive extension or flexion or both are the main cause of these fatigue fractures. 29– 33 Farfan believed that the pars defect was due originally to a single and not exceptional overload with repeated microfractures producing the lysis. 29 Others supported Farfan’s mechanical theory. 30– 32 Pfeil showed that it was the upright posture that was the cause of the lysis in age studies. 33
Severe slippage does not occur with lytic spondylolistheses seldom, if ever, beyond 50% and usually not more than 30%, principally because there is movement only in the sagittal plane in contrast to dysplastic spondylolisthesis which is effectively a progressive lumbosacral kyphosis.
Until maturity, which may, as far as the spine is concerned, be in the early 20s, 34 the lysis is filled with a zone of endochondral ossification 12 and therefore it is quite probable that some spondylolyses heal and then develop in new cases keeping the overall prevalence pool at a relatively constant figure. 12, 19 By contrast, lyses persisting in the more mature are established nonunions filled with hypertrophic callus and debris (▶ Fig. 11.8).
Fig. 11.8 (a,b) Axial CT scan through spondylolyses in a young adult showing the typical appearance of hypertrophic nonunions. The hypertrophy affects both sides of the lysis (arrows). (a) On the right side the pars is thickened due to reactive new bone formation (arrowhead).
In the Wiltse classification 13 subtype (b) was elongation of the pars without separation but this does not occur with the so-called isthmic spondylolisthesis. Rather, elongation of the pars is part and parcel of the progressive development of slippage in the dysplastic form of spondylolisthesis (▶ Fig. 11.1). Elongation of the pars should therefore not belong to the isthmic variety but should be included as part of the radiological appearance of dysplastic spondylolisthesis.
Finally, subtype (c) of the old classification was an acute pars fracture and it has been shown quite categorically that the pars only cannot be broken by one discrete injury 31 nor do we ever see it clinically other than being part and parcel of more severe traumatic damage; it does not occur in isolation. Therefore, the acute pars fracture category (c) should belong to one of the classifications of thoracolumbar fractures and dislocations and not spondylolisthesis.
In contrast to the lysis that develops secondarily to stretching/thinning/attenuation of the dysplastic type, the lysis of isthmic spondylolisthesis is the essential component of slippage. As there is therefore only one type of isthmic spondylolisthesis, the primary lytic variety, then it could be renamed primary lytic spondylolisthesis.
The degenerative variety of spondylolisthesis is due to primary generalized osteoarthritis (OA) affecting the synovial posterior facet joints and this is particularly prevalent in females who much more commonly have the gene for OA (▶ Fig. 11.9). Indeed, when we were residents we were taught that if we had a youngish, under 60, male, with OA hip then when we looked at the AP pelvis X-rays we must also check the status of the sacroiliac joints looking for inflammatory/erosive disease, so much less common in males is the genetic expression.
Fig. 11.9 Lateral radiograph of a 60-year-old woman with significant degenerative spinal disease. There is a degenerative spondylolisthesis at the usual L4/5 level. The L4 spinous process is connected to the L4 vertebral body and therefore moves forward out of alignment with the L5 spinous process (lines).
This tends to occur at the L4/5 level and sometimes the level above, the L3/4 level. The L5/S1 level is seldom if ever involved. Yes, the L5/S1 level is commonly affected by degenerative disk disease but this is a completely different pathology to primary degenerative osteoarthritis of the posterior facet joints that is the cause of degenerative spondylolisthesis.
Degenerative spondylolisthesis was called pseudospondylolisthesis by Junghans, 35 but the preferred term is degenerative spondylolisthesis as proposed by Newman. 36 Rosenberg made a detailed study of this condition in 200 patients and 20 skeletons. 37 He found it to be four times more common in females and six to nine times more frequently encountered at the L4/5 level. The condition is not usually encountered under the age of 50 years and the degree of slip never exceeds 30%.
The destruction of the articular cartilage of these facet joints is thought to be the principal factor in rendering them incompetent mechanically and allowing slippage to occur although Farfan believed that the altered geometry was due to multiple small compression fractures of the inferior articular processes. 38 Harms recently studied the geometry of the degenerative spondylolisthetic level and compared 23 patients with degenerative spondylolisthesis with 40 age- and sex-matched controls. 39 Both CTs and MRIs were assessed. One of the more important findings was that the patients’ facet joints were aligned in a more sagittal direction thus favoring slippage while the inclination of the vertebral end plates was more horizontal in the normal controls (▶ Fig. 11.10).
Fig. 11.10 Facet joint angulation is more sagittally orientated in degenerative spondylolisthesis.
The same group looked at facet joint remodeling and came to the conclusion that the sagittal alignment of the facet joints was more likely due to secondary remodeling rather than a preexisting morphology. 40 We in Leeds have also noted a much more sagittal orientation of the facet joints in degenerative spondylolisthesis in contrast to the more coronal orientation in lytic spondylolisthesis.
Quite obviously the incompetence of the facet joints due to the degenerative process may not be equal on both sides and so the slippage is often rotational rather than strictly in the transverse plane producing a local degenerative scoliosis. Therefore the condition of degenerative spondylolisthesis should be thought of in all three dimensions often including a significant scoliotic component.
The traumatic subcategory refers to an acute injury that fractures in some part of the bony hook other than the pars and therefore really should not be part of the classification of spondylolisthesis at all; rather, it should belong to the classification of thoracolumbar spinal fractures and fracture-dislocations as with so-called acute pars fractures from the isthmic category.
Finally, pathological spondylolisthesis refers to any pathological process whereby the pedicle, pars, or articular processes are weakened allowing forward thrust of the body weight above to produce slippage. These can be regarded as being generalized such as in Paget’s disease (▶ Fig. 11.11), arthrogryposis, Albers-Schönberg, syphilis, and osteogenesis imperfecta 11– 13 or can be localized whereby the segments adjacent to a very long fusion, as is occasionally seen with a fusion for idiopathic scoliosis years later, produce a spondylolysis and subsequent slippage (spondylolisthesis acquisita). 41 As these pathological processes increase in their own severity so images reveal the spondylolisthesis progressing pari passu with the underlying pathological process.
Fig. 11.11 Classically, Paget’s disease produces its vertebra magna and a small amount of pathological spondylolisthesis.
Therefore we propose that the Wiltse classification be reduced to four simple categories (▶ Table 11.2).
11.3 The Terminology and Measurement of Spondylolisthesis
This has been usefully summarized in a review article by Wiltse and Winter. 42
Anterior displacement has also been called anterior translation, slip, and olisthesis. The degree of this can be measured as recommended by Taillard, 43 expressing the degree of slip as a percentage of the AP diameter of the top of the first sacral vertebra (A/A’ × 100) (▶ Fig. 11.12). Meyerding graded slip by simply dividing the top of S1 into quarters and noting how far L5 slips by, say, 0, 1, 2, etc. 44 (▶ Fig. 11.13).
Fig. 11.12 The extent of anterior displacement, or slip, is expressed as a percentage obtained by dividing A, the amount of displacement (determined by the relationship of the posterior part of the cortex of the fifth lumbar vertebra to the posterior part of the cortex of the first sacral vertebra), by A, the maximum anteroposterior diameter of the first sacral vertebra, and multiplying by 100. The smaller drawing shows how to determine the posteroinferior tip of the body of the firth lumbar vertebra, which is often indistinct due to either hypoplasia or spur formation in this area (x on this drawing). Line a is drawn parallel to the front of the body of the fifth lumbar vertebra. Line b is drawn perpendicular to line a, to the posterosuperior tip of the body of the fifth lumbar vertebra (a point that usually is easily located). Line c is drawn parallel to line b and is exactly the same length as line b. The point at which line c intersects the inferior border of the body of the firth lumbar vertebra is point x. Point x is the relative constant used in measuring the percentage of slip.
(Reproduced with permission from L Wiltse, R Winter. Terminology and measurement of spondylolisthesis. Wolters Kluwer Health, Inc. 1983.)
Fig. 11.13 Meyerding grading.
Sacral inclination is also known as sacral tilt and refers to the angular relationship in the sagittal plane between the sacrum and the vertical plane. ▶ Fig. 11.14
The most important angular relationship in dysplastic spondylolisthesis is the angle of sagittal rotation also called sagittal roll, slip angle, or lumbosacral kyphosis. Dysplastic spondylolisthesis is of course a real lumbosacral kyphosis and is measured by the angle subtended by a line along the anterior border of L5 and a line along the posterior border of the first sacral vertebra (▶ Fig. 11.15).
Fig. 11.14 Sacral incliniation, g, is determined by drawing a line along the posterior border of the first sacral vertebra and measuring the angle created by this line intersecting a true vertical line.
(Reproduced with permission from L Wiltse, R Winter. Terminology and measurement of spondylolisthesis. Wolters Kluwer Health, Inc. 1983.)
As the fifth lumbar vertebra rotates round on the top of the sacrum there is rounding of the upper border of the sacrum and this can also be measured. ▶ Fig. 11.16
Then the fifth lumbar vertebra becomes progressively more lordotically shaped and wedging of the L5 vertebra can also be measured. ▶ Fig. 11.17
The angle of the lumbar lordosis can be measured by the angle subtended by the upper borders of the first and fifth lumbar vertebrae. ▶ Fig. 11.18
The sacro-horizontal angle, also called the sacral angle or Ferguson’s angle, is the angular relationship between the upper border of the sacrum and a horizontal line. ▶ Fig. 11.19
The lumbosacral joint angle is the angle subtended by lines along the lower border of L5 and the upper border of S1. ▶ Fig. 11.20
These measurements are all relevant to spondylolisthesis, anterior displacement being particularly relevant to isthmic spondylolisthesis and several of the others specifically to the dysplastic variety of which the sagittal roll angle is perhaps the most important, at least of those above the pelvis.
11.4 The Marchetti and Bartolozzi (M-B) Classification
This was originally published in 1982 in Italian but was revised in 1994 to introduce further elaboration of the developmental forms such as high and low dysplastic and in the acquired group the post-surgery, degenerative, and pathologic categories or subgroups (▶ Table 11.3). 15
The difference between the use of the words congenital and dysplastic was then pointed out. They considered all developmental forms as essentially alike—that is, defects such as lysis, elongation, and those involving the bony hook, the L5 vertebra, and the upper border of the sacrum, along with spondyloptosis being determined by the same congenital cause—the distinguishing features of these forms being the degree to which the congenital defect has determined morphological alterations. Thus, their classification (M-B) proposed that the dysplastic and isthmic forms of spondylolisthesis in the original Wiltse classification be classified in a single developmental category, “disagreeing with the definition of isthmic spondylolisthesis by Wiltse as a separate entity,” arguing that some forms must be classed as dysplastic. They were then able to argue that the category of developmental forms included the majority of cases of spondylolisthesis referred to them, somewhat surprisingly.
Meanwhile the spinal surgical world has always been quite comfortable with the dysplastic (▶ Fig. 11.1) and lytic (▶ Fig. 11.4) varieties of spondylolisthesis coming to the clinic being discrete entities, with the latter being far more prevalent than the former and the former being much more important than the latter. The problem arises when M-B tried to use the suffix “-lysis” for the stress fractures of the old isthmic form of spondylolisthesis as well as the secondary lysis that occurs with attenuation of the pars in the dysplastic form. The word lysis comes from the Greek word lusis meaning a loosening and in medical parlance a loosening, decomposition, or breaking down, clearly a precise event occurring over time and so secondary in that regard. Perhaps therefore we should reserve the phrase secondary lysis as pertaining to secondary disruption of the attenuated pars in dysplastic spondylolisthesis and reserve the term primary lysis for that which occurs with the isthmic form of our revised Wiltse/ISSLS classification (▶ Table 11.2). Then the tendency for confusion would be minimized and so presumably would be the desirability to lump all “lyses” into one category, regardless of origin. Notwithstanding, Marchetti and Bartolozzi considered that some forms of isthmic spondylolisthesis must be classed as dysplastic for reasons that are not abundantly clear from their descriptions.
Then in the dysplastic category they further subdivided lesions as high dysplastic or low dysplastic according to how much dysplasia is present. In the high dysplastic group they state that there are most invariably localized kyphosis and angulation of the axis of the two vertebrae and we would quite agree and would add that these are the chief criteria for differentiating dysplastic from isthmic spondylolisthesis. The high dysplastic would have further local problems such as the upper S1 body rounded. These vertebral body changes in L5 and the top of the sacrum are secondary mechanical growth alterations and not as a result of the primary dysplasia. If a secondary lysis develops in the attenuated pars of L5 then slippage can freely progress, again an attribute typical of dysplastic spondylolisthesis. Meanwhile M-B describe low dysplastic spondylolisthesis as demonstrating relatively intact rectangular forms of L5 or L4 (if the spondylolisthesis is at that level), good preservation of the upper end plate of S1 or L5, and a parallel alignment of adjacent vertebral end plates with no sacral verticalization or compensatory hyperlordosis, all the attributes of a typical isthmic spondylolisthesis. M-B state that “forms with lysis and very rarely with elongation are so similar that they can be described here as one item.” Furthermore, in terms of progression, low dysplastic forms usually do not worsen, any slippage being invariably slow and displacement being translational rather than tilting, changes that are clearly not dysplastic, and so it is difficult to see why there should be high and low dysplastic forms when the latter appear to have no dysplastic elements at all.
Meanwhile for the past 10 to 20 years increasing emphasis has been placed upon the shape of the pelvis and spondylolisthesis. 45– 50 In 1997 Schwab et al looked at X-rays of children/adolescents with spondylolisthesis with particular reference to sagittal plane pelvic rotation with the degree of slip over time. 45 They sought to determine whether the degree of standing sagittal offset of L5 with respect to the acetabulum correlated with slip progression and symptoms. A total of 52 patients with dysplastic spondylolisthesis were looked at, with serial standing lateral radiographs including the hips and lumbar spine, to measure the sagittal pelvic tilt index (SPTI) as the ratio of the relative distances from the center of S2 to the projection of L5 in the center of the femoral heads horizontal (▶ Fig. 11.21). The lower the SPTI the more vertical the sacrum and more anteriorly displaced the hip joint with more progression and imbalance. A total of 32 patients had spondylolisthesis and 20 spondylolysis. Slip progression occurred in 13 patients with spondylolisthesis along with a decrease in the SPTI. These patients had significant symptoms whereas those patients with a stable SPTI did not progress and remained symptom free. They therefore postulated that an abnormal SPTI was etiological in dysplastic spondylolisthesis and led to progressive displacement. In the normal and balanced patient L5 is centered above the hip joint whereas with an unbalanced patient the more anteriorly translated L5 facilitates a lumbosacral kyphosis to develop a compensatory lumbar lordosis above.
Fig. 11.15 Sagittal rotation is the term used to express the angular relationship between the fifth lumbar and first sacral vertebrae. It is determined by extending a line along the anterior border of the body of the fifth lumbar vertebra until it intersects a line drawn along the posterior border of the body of the first sacral vertebra. The drawing on the right shows an alternative method of measuring sagittal rotation, to be used when the degree of olisthesis is small and lines a and b do not intersect. A third line, c, is added perpendicular to line a. Lines c and b intersect to form the angle of sagittal rotation. (Reproduced with permission from L Wiltse, R Winter. Terminology and measurement of spondylolisthesis. Wolters Kluwer Health, Inc. 1983.)
Then in 2002 Rajnics et al looked at 48 patients with isthmic spondylolisthesis and 30 healthy volunteers using digitized standing lateral spinal radiographs. 46 The lateral X-rays included both femoral heads. They measured the SFAC (the sacrofemoral anatomic constant or incidence) (▶ Fig. 11.22), the sacral slope (SS; already called the sacrohorizontal angle 41, and L1 to L5 lordosis ▶ Fig. 11.18. These three measures were all greater in patients with isthmic spondylolisthesis than in the healthy volunteers. They postulated that the horizontally positioned sacrum and hyperlordosis caused the shearing component of gravity to be greater than the compressive force (▶ Fig. 11.23) and thus causes fracture of the part of the vertebra. The two acetabula were located well anterior to the lumbosacral junction giving rise to instability. The SFAC is a hereditary factor that correlates well with the degree of slipping.
Fig. 11.16 Rounding of the top of the centrum of the first sacral vertebra is expressed as the relationship between lines a and b, drawn as shown. The result, when multiplied by 100, gives the percentage of rounding of the first sacral vertebra.
(Reproduced with permission from L Wiltse, R Winter. Terminology and measurement of spondylolisthesis. Wolters Kluwer Health, Inc. 1983.)
Fig. 11.17 Wedging of the olisthetic vertebra is expressed as a percentage determined by dividing line a by line b, drawn as shown, and multiplying by 100.
In the same year Marty et al studied the relationship between the sacrum and the angle of incidence 17 and compared these parameters in three populations—young adults (44), infants before walking (32), and patients with spondylolisthesis (39). They argued that the incidence was strongly correlated with the sacral slip and lumbar lordosis and so ensured individuals an economical standing position. The angle of incidence had also been shown to depend partly on the sagittal anatomy of the sacrum which was established in childhood while learning to stand and walk. Hence the purpose of the study was to define the relationship between the sacrum and angle of incidence and to compare these parameters in the above three populations. All underwent sagittal standing spine radiography. A close relationship existed between the angle of incidence and the slip of spondylolisthesis and all parameters in young infants were less than adults. They concluded that the sagittal anatomy of the sacrum played a key role in spinal sagittal balance. The sacrum in the spondylolisthesis group differed from normal with a greater angle of incidence and sacral slope that could predispose to vertebral slip.
Hubert Labelle seems to have done more work on spinopelvic relationships than practically anybody else and as usual he is a good read. In 2005 he and his team from Montreal viewed the radiological measurements of spinopelvic balance in relationship to dysplastic spondylolisthesis which he referred to as developmental, tacitly accepting the M-B classification. 47 Again, lateral standing radiographs were studied with dedicated software calculating pelvic incidence, sacral slope, pelvic tilt (▶ Fig. 11.24), L5 incidence angle (▶ Fig. 11.25), lumbosacral angle, lumbar lordosis, thoracic kyphosis, and grade of spondylolisthesis. They compared these measures in patients to an adult and children reference population. Pelvic incidence, sacral slope, pelvic tilt, and lumbar lordosis were all found to be significantly greater in subjects with dysplastic spondylolisthesis while thoracic kyphosis was significantly less. The difference between patient and reference populations increased linearly when the degree of spondylolisthesis increased. Better surgical outcomes occurred with an improvement in L5 incidence angle and lumbosacral angle while those who faired less well had a higher preoperative grade.
Fig. 11.18 The degree of lumbar lordosis is defined as angle c, as shown. With significant sagittal rotation of the fifth lumbar vertebra, there may be lordosis extending well up into the thoracic spine, in which case “total spinal lordosis” should be distinguished from “lumbar lordosis”.