Chapter 3
Question 2
Which relevant patient characteristics are associated with an increased
likelihood of focal narrowing of the spinal canal?
Certain patients may be more likely to develop spinal stenosis. In this section, we examine the available evidence linking patient characteristics to developing a symptomatic narrowing of the spinal canal.
As with the preceding question, we consider only studies that reported some measurement of spinal diameter and that reported data on whether patients had or did not have a particular symptom. In some cases, this limits our treatment of the data to case-control studies, because they were the only studies to report symptomology. Due to a relative lack of information, parts of our answer to this question take the form of a systematic narrative review. While this review reflects the results of and critiques currently available studies, another purpose of it is to illustrate where there is a need for data.
Relationship Between Nonstenotic Canal Diameter and Development of Focal Spinal Stenosis
The results of the preceding analyses suggest that some relationship exists between symptoms and spinal canal size. Therefore, we conducted a meta-analysis to explore the relationship between focal spinal canal narrowing and symptomatic spinal stenosis. We located four studies that compared measurements of spinal canals among patients with stenosis to those of control subjects. They are listed in Table 11. One of these studies (Schonstrom, Bolender, and Spengler, 1985) provided only one measure in common for control and stenotic patients. This measure, mean transverse area of the dural sac, was calculated from the transverse area at the dural sac at four levels of the lumbar spine. However, none of the measurements at any level included data from all 13 control subjects. At two of the levels, data from only five subjects were used. Nearly half (27 of 52) of the measurements that could have been used to calculate mean transverse area were omitted. No data were provided on the number of levels measured for the stenosis patients. In addition, 7 of the 24 patients said to have stenosis in this study actually had protruding disks. Because of these issues of poor design and reporting, this study was excluded from the meta-analysis. For the remaining studies, we chose data for incorporation into the meta-analysis in the same manner as described in the section entitled "Relationship Between Spinal Canal Diameter and the Presence of Back Pain, Claudication, or Radicular Pain/Sciatica." Numbers used to calculate effect sizes appear in Table 11 in bold type.
The 1992 study by Yoshida (Yoshida, Shima, Taniguchi et al., 1992) incorporated two potential confounding factors that may influence interpretation of the results. First, there was an age difference between patient groups. Patients with stenosis had a mean age of 63.8 years. They were compared to a control group with a mean age of 20.2 years. Because the spinal canal tends to narrow with age (Twomey and Taylor, 1988), the younger controls may have larger canals than an age-matched control group might have had. This would have the effect of introducing bias in favor of finding a significant difference between groups. Also, the control patients all had herniated disks. Patients with disk problems may tend to have smaller spinal canals than do healthy controls (Heliovaara, Vanharanta, Korpi et al., 1986; Ramani, 1974; Ramani, 1976 ; Winston, Rumbaugh, and Colucci, 1984). If this is the case, this would bias the analysis against finding a significant difference between groups. The effect size from the study by Yoshida (−1.271) is larger (albeit not significantly so) than those from two other studies (Kim and Lee, 1995;
The 1995 study by Kim and Lee had two groups of patients with spondylolisthesis. Because isthmic spondylolisthesis is outside the scope of this assessment, the spinal measurements of patients with degenerative spondylolisthesis were used (Kim and Lee, 1995).
Prasartritha et al. (1997) did not provide a measure of dispersion among measurements of sagittal diameter (Prasartritha, Suntisathaporn, Vathana et al., 1997). Because of this, an exact effect size cannot be calculated. However, the report stated that the mean canal size was significantly different (p <0.05) between groups by t-test. Because the number of patients in each group was known, we were able to calculate the value of Student's t, assuming a p-value of 0.049. This translates to an effect size of at least −0.587. If the actual p-value was smaller (as it may well be), Student's t, and therefore Hedges' d, would be larger. The overall effect size calculated here is therefore a minimum value. How much further it may actually be from zero cannot be determined from the data provided.
Effect sizes (Hedges' d) were calculated as described in Chapter 2 of this evidence report. Effect sizes for each study, the summary statistic, and the 95 percent confidence intervals around them can be found in Table 12, and are illustrated graphically with a Forrest plot in Figure 15.
When data from all groups were combined, the test of heterogeneity was not statistically significant (Q = 0.002, p = 0.964, standardized residuals = �0.045), and the summary statistic, −0.597, was statistically significant (p = 0.002).
While the difference between means is statistically significant, there is 61.97 percent overlap between groups, as illustrated in Figure 16. The correlation between canal size and the presence and absence of stenosis is significant but low (r = −0.286). The binomial effect size display shown in Table 13 suggests that if the results of this meta-analysis can be generalized to the population at large, a person with stenosis is approximately 1.78 times more likely than a healthy person with no stenosis to have a small spinal canal. Although people with symptomatic stenosis tend to have smaller spinal canals than people without stenosis, the presence of a small canal is not necessarily predictive of stenosis.
Relationship Between Nonstenotic Canal Cross-Sectional Area and Development of Focal Spinal Stenosis
Besides canal diameter, other aspects of the spinal canal may make an individual patient more prone to develop spinal stenosis. A well-documented anatomical variation known as the trefoil-shaped canal may predispose the patient to problems associated with lateral stenosis (Epstein, Epstein, and Lavine, 1962; Schatzker and Pennal, 1968). This variation is characterized by a smaller sagittal diameter and deeper lateral recesses. The association between trefoil canal and spinal stenosis may be merely coincidental (Eisenstein, 1980). A study of trefoil canal in South African skeletons found that while stenosis and trefoil canal may coexist in the same skeleton, they are often present at different vertebrae. Other attempts have been made to identify spinal characteristics that are predictive of future stenosis (MacGibbon and Farfan, 1979). No information is available quantifying the accuracy of these predictions.
Relationship Between Patient Age and Development of Focal Spinal Stenosis
Disk degeneration, spinal instability, and hypertrophy of the facet joints accompany the aging process (Prescher, 1998; Twomey and Taylor, 1988). Degeneration normally begins in the second decade (Mirkovic, Garfin, Rydevik et al., 1992), and its prevalence increases with age (Powell, Wilson, Szypryt et al., 1986). By the age of 40 years, 80 percent of male and 65 percent of female disks are moderately degenerated (Mirkovic, Garfin, Rydevik et al., 1992). Increasing age also correlates with increasing spinal osteoarthritis (Magora and Schwartz, 1978). While increased disk degeneration and spinal osteoarthritis implies increased stenosis, this has not been specifically demonstrated.
A 1988 study of cadavers found a decline in lumbar spinal canal anterior-posterior diameter with age that is significant in males (t = 3.95, df = 4, p <0.01) but not in females (Twomey and Taylor, 1988). This study looked at lumbar spines of men and women age 25 to35 and > 65. Our meta-analyses indicate an association between canal size and back pain, claudication, and stenosis. A narrower spinal canal may render the patient more vulnerable to developing symptomatic stenosis in cases of minor additional trauma or pathology (Clark, 1969; Twomey and Taylor, 1988). The narrowing of the canal with age may therefore contribute to the development of symptomatic stenosis.
There is a correlation between presence of calcium deposits and decreased elastic/collagenous fiber ratio in the ligamentum flavum and age among patients with degenerative stenosis (Schrader, Grob, Rahn et al., 1999). Patients under the age of 60 had 0.032 percent calcification, while patients between the ages of 70 and 75 had 0.336 percent calcification. This correlation was statistically significant (p <0.05) by the Mann-Whitney-Wilcoxon test. Few patients without degenerative stenosis had these changes. No correlation was reported between degree of change and degree of stenosis or symptoms.
Relationship Between Patient Weight and Development of Focal Spinal Stenosis
A 1976 survey indicates that increased weight correlates with increased incidence of spinal osteoarthritis (Magora and Schwartz, 1978). Only 53 percent of slim patients (weight in kg less than height in cm minus 110) had spinal osteoarthritis, compared to 59 percent of average patients (weight in kg equal to height in cm minus 90-110), 69 percent of heavy patients (weight in kg equal to height in cm minus 80 to 90) and 94 percent of very heavy patients (weight in kg more than height in cm minus 80). Overweight patients may also be more likely to have disk degeneration than normal patients (Parkkola, Rytokoski, and Kormano, 1993). A 1991 survey stated that body mass index is a predictor of disk degeneration according to univariate and multivariate analyses, but did not provide a description of the statistical analysis utilized to reach this conclusion (Symmons, van Hemert, Vandenbroucke et al., 1991). The clinical significance of these findings is unclear. Disk degeneration is a common precursor to stenosis, but it occurs in all patients to some extent, and does not inevitably lead to stenosis. No direct evidence in support of a link between body weight and development of stenosis is available.
Relationship Between Osteoarthritis and Development of Focal Spinal Stenosis
Among patients with severe osteoarthritis of one or both hips, there may be an increased frequency of moderate or severe degenerative changes as noted on radiographs in the lumbar spine. Among patients aged 40 to 59, 36 percent of patients with osteoarthritis of the hips had such changes, compared to only 4 percent of control patients matched for age, sex, and occupation. Among patients aged 60 to 69, 53 percent of those with osteoarthritis had moderate or severe degenerative changes of the lumbar spine, compared to 27 percent of matched control patients (Brewerton,
1983). No mention was made of presence or absence of stenosis or back pain, claudication, or other symptoms. In the seven studies giving this information, 42 out of 903 (4.7 percent) patients with lumbar spinal stenosis had osteoarthritis or other hip disease (see Evidence Table 25). Underreporting of this important comorbidity restricts our ability to determine its prevalence or clinical relevance. Whether hip and spine problems are both parts of a larger disease or whether mechanical changes resulting from hip disease lead to spinal degeneration is not known.Relationship Between Diabetes and Development of Focal Spinal Stenosis
Although some skeletal defects may be more prevalent among patients with diabetes than in the general population, we were unable to locate any evidence on whether patients with diabetes are more prone to developing lumbar spinal stenosis. A cross-sectional epidemiological study published in 1994 found no significant difference in the prevalence of spondylolisthesis between diabetic and nondiabetic adults (Virta, Ronnemaa, and Laakso, 1994).
Relationship Between Type of Patient Employment and Development of Focal Spinal Stenosis
A 1993 report found no correlation (r= 0.07, n = 46) between work index (0-8 scale based on amount of lifting required in four categories of motion) and degree of spondylolisthesis (Virta and Ronnemaa, 1993). No numerical data were provided in support of this statement. In contrast, Lawrence found that coal miners and men doing outdoor or heavy manual work had earlier and greater degrees of disk degeneration than did business, professional or textile workers (Lawrence, 1969). Fifty percent of miners and 40 percent of outdoor workers aged 35-44 had disk degeneration of grade 2 or higher, compared to 14 percent of business and professional workers. Total incidence of disk degeneration for all age groups was significantly lower (p <0.01) in business, professional, and textile workers than in industrial, manual, trade, and outdoor workers or miners. The statistical test used was not stated. No such differences were found in women. Extent of degeneration as indicated by height of the intervertebral space (p = 0.003, n = 46) is associated with a greater degree of spondylolisthetic slip (Virta and Ronnemaa, 1993).
Summary
Patients with spinal stenosis tend to have narrower spines than asymptomatic patients. This suggests, although it does not prove, that patients with congenitally narrow spines may be more prone to developing focal stenosis. The shape of the patients' spinal canal may also contribute to the development of focal narrowing of the canal and symptomatic stenosis, but the evidence for this is weak. There is some evidence that disk degeneration, narrowing of the spinal canal, and degenerative changes in the spinal ligaments contributing to stenosis and instability increase with age. However, the strength of this relationship and the age at which stenosis is most likely to occur cannot be determined from the available information.
Heavier patients may be more likely to develop the degenerative changes leading to stenosis. Similarly, patients with osteoarthritis of the hips as well as patients who perform heavy labor tend to have more disk degeneration than other patients. While these data suggest a relationship between these characteristics and the development of stenosis, there is no direct evidence of a causal relationship. We located no evidence indicating a relationship between diabetes and spinal stenosis, and we located a single study giving evidence against the existence of such a link.
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