Spine J 2004 (Sep); 4 (5): 574–583 ~ FULL TEXT
Mitchell Haas, DC, Elyse Groupp, PhD, Dale F. Kraemer, PhD
Center for Outcome Studies,
Western States Chiropractic College,
2900 NE 132nd Avenue, Portland, OR 97230, USA.
BACKGROUND CONTEXT: There have been no trials of optimal chiropractic care in terms of number of office visits for spinal manipulation and other therapeutic modalities.
PURPOSE: To conduct a pilot study to make preliminary identification of the effects of number of chiropractic treatment visits for manipulation with and without physical modalities (PM) on chronic low back pain and disability.
STUDY DESIGN/SETTING: Randomized controlled trial with a balanced 4x2 factorial design. Conducted in the faculty practice of a chiropractic college outpatient clinic.
PATIENT SAMPLE: Seventy-two patients with chronic, nonspecific low back pain of mechanical origin.
MAIN OUTCOME MEASURES: Von Korff pain and disability (100-point) scales.
METHODS: Patients were randomly allocated to visits (1, 2, 3 or 4 visits/week for 3 weeks) and to treatment regimen (spinal manipulation only or spinal manipulation with physical modalities). All patients received high-velocity low-amplitude spinal manipulation. Half received one or two of the following PM at each visit: soft tissue therapy, hot packs, electrotherapy or ultrasound.
RESULTS: Pain intensity: At 4 weeks, there was a substantial linear effect of visits favoring a larger number of visits: 5.7 points per 3 visits (SE=2.3, p=.014). There was no effect of treatment regimen. At 12 weeks, the data suggested the potential for a similar effect of visits on patients receiving both manipulation and PM. Functional disability: At 4 weeks, a visits effect was noted (p=.018); the slope for group means was approximately 5 points per 3 visits. There were no group differences at 12 weeks.
CONCLUSIONS: There was a positive, clinically important effect of the number of chiropractic treatments for chronic low back pain on pain intensity and disability at 4 weeks. Relief was substantial for patients receiving care 3 to 4 times per week for 3 weeks.
From the FULL TEXT Article:
Mechanical low back pain (LBP) of musculoskeletal
origin is both a prevalent and costly health problem. Studies
indicate that 65% to 80% of adults will experience at least
one episode during their lifetime, and the 1-year period
prevalence ranges from 15% to 45%.  Annually, LBP
accounts for up to 40% of all lost workdays and has been
estimated to cost $49 billion within the industrial sector. [2, 3] LBP accounts for a significant number of physician
visits each year, second only to upper respiratory ailments. 
Recently, several large studies have been conducted that
call into question the characterization of back pain as a selflimiting condition. Many studies used “return-to-work” or
“discontinued care seeking” as an index of recovery. However,
a different picture of back pain emerged when pain
and functional disability were the primary outcomes assessed.
One population-based study showed that only 21%
of patients were pain free at 3 months and 25% were pain
free 12 months after the index visit.  The majority of
patients reported significant pain and disability at 12 months.
In fact, a practice-based observational study found that both
acute and chronic LBP cohorts were still experiencing substantial
pain and disability 3 and 4 years after presentation . Other studies have corroborated these findings and have
led to the more accurate characterization of low back pain as
a chronic condition. [7, 8] The essential distinction between
acute and chronic pain has been described by Waddell  as
“not necessarily the duration of pain, but the persistence of
chronic pain beyond expected recovery times and the intractable
nature of chronic pain.”
According to a national health-care-usage survey, chronic
LBP was one of the most frequent reasons people sought
alternative therapy.  This reflects an increasing demand
for complementary and alternative medicine (CAM) in general
and an increasing belief that CAM therapy is more
helpful than conventional medicine for treatment of back
pain. [11, 12] Of the estimated 42% of the US population
who used CAM therapies in 1997, almost one-third sought
chiropractic treatment.  Up to 40% of patients with
LBP chose chiropractic care to address their back problems. 
Manual therapy of the spine is practiced by a variety of
health-care providers: chiropractors, osteopaths and physical
therapists. The primary objective of spinal manipulation
in the treatment of back pain is the alleviation of pain, muscle
spasm and functional impairment.  The therapeutic manipulation consists of controlled directional, high-velocity,
low-amplitude thrust. 
The efficacy of spinal manipulation for treatment of LBP
has been assessed in eight systematic reviews of randomized
trials published before 1997. [17–24] Of these, four found inconclusive
evidence for the efficacy of manipulation for
chronic LBP, although none found evidence of inefficacy
or an advantage for standard medical care. [17, 18, 20, 21]
Later reviews within this group found moderate to strong
evidence that manipulation was better than placebo, general
medical practice, massage, bed rest and analgesics for
chronic LBP. [22–24] A more recent trial showed a shortterm
advantage of osteopathic manipulation over chemonucleolysis
for disc herniation.  Other recent trials did
not exclusively investigate chronic LBP. [26–31]
What is evident from the randomized trials on spinal
manipulation is that the number of treatments and duration of
care is diverse and not evidence based. Currently, treatment
protocols are arbitrarily selected based on opinion or clinical
experience for randomized trials and clinical practice. 
The appropriate dosage of treatment as well as the duration
needed to achieve optimal pain relief for different subgroups
of low back pain has not been investigated. Our study
was therefore designed to make a preliminary assessment
of the dose-response relationship between the number of
chiropractic visits for treatment with spinal manipulation
and the pain relief in patients with chronic LBP.
This study was a prospective, randomized controlled trial
using a 4×2 balanced factorial design. The objectives were
to identify 1) the effect of the number of visits to a chiropractor
and 2) the effect of adding physical modalities (PM) to
a regimen of spinal manipulation on chronic low back pain
and disability. The study was conducted between February
and October 2002 at the Western States Chiropractic College
(WSCC) Outpatient Clinic, faculty practice. Neither participants
nor providers were blinded. Before randomization,
allocation was concealed from study personnel and participants
in opaque, sealed envelopes in the possession of a
project manager. The study analyst randomized the 72 participants
using a computer-generated, equal allocation algorithm
to one of four dosage groups and to one of two levels
of treatment intensity (spinal manipulation only or combination
of manipulation and PM). This permitted estimation of
visit trends with 18 participants per group and the estimation
of the effect of including PM in a regimen of manipulation
with 36 participants per treatment level. Each participant
was treated for 3 weeks. Visit Group 1 received a total of
3 office visits (1/week); Group 2, 6 visits (2/week); Group 3,
9 visits (3/week) and Group IV, 12 visits (4/week). All
patients received spinal manipulation at each visit. Half of
the patients within each visit group received one or two PM
per visit, while the other half received none. The primary
outcome was self-reported pain. The principal independent
variable of interest was the frequency of visits to a chiropractor.
Follow-up time points were 4 and 12 weeks after randomization.
Study guarantees of participant rights and safety
were approved by the Western States Chiropractic Institutional
Review Board (FWA 851). Data were secured at the
WSCC Center for Outcome Studies. 
Participants were recruited through advertisements in
local newspapers. Project managers conducted an initial eligibility
screening by telephone. At the first baseline visit,
all participants provided informed consent. They completed
the first baseline questionnaire and received the screening
physical examination. Eligible persons were invited to the
second baseline visit. Participants completed a second
baseline questionnaire, were randomized and received their
first treatment from the therapist. Follow-up at 4 and 12
weeks after randomization was by mailed questionnaire. For
ethical reasons, participants were permitted to seek care
for LBP outside the study protocol. Study chiropractors
were not permitted to recommend additional care upon completion
of study treatment.
Potential participants were eligible if they had a current
episode of chronic LBP. Low back was defined as the area
below the twelfth rib and above the gluteal fold.  Chronic
was defined as an episode of pain of at least 3 months’
duration. [20, 35] Additional inclusion criteria were age 18
years and older and English literacy.
People were excluded if they had received chiropractic
care in the 3 months before their baseline visit. They were
also ineligible if they had contraindications to spinal manipulation  or complicating conditions that could be related
to clinical outcomes. These included malignancy or history
of cancer, spinal infection, vertebral tumors or fracture,
lumbar instability, radiculopathy, cauda equina syndrome,
blood dyscrasia, pregnancy, severe trauma within the last 3
months, low back surgery within the previous 12 months,
referred LBP of organic origin or required referral for advanced
imaging (eg, magnetic resonance imaging and computed
tomography). Additional exclusion criteria were
involvement in litigation for a health problem, or suspicion
of noncompliance, indicated by a failure to attend a baseline
visit before randomization.
Assessment and intervention
Four chiropractors with 2 to 22 years of practice experience
served as the study therapists. They conducted a screening
physical examination, including history, palpation,
orthopedic/neurological examination and X-ray (if indicated). 
All participants received the principal therapy: high-velocity,
low-amplitude spinal manipulation, as described by
Bergmann et al.  For the participants randomized to
receive a combination of manipulation and PM, the treating
chiropractor also administered one or two PM at each visit
from among heat/ice, ultrasound, electrotherapy and manual
soft tissue therapy (massage  and/or trigger point therapy ). These modalities are commonly used by chiropractors.  Specific manipulation and PM were determined at each
visit by the therapist through ongoing evaluation of the
participants.  The study therapists did not include any
further intervention, such as exercise regimens and lifestyle
Study outcomes and baseline variables
Low back pain and functional disability were evaluated
using the Modified Von Korff (MVK) Scales of Underwood et
al.  This instrument consists of six 11–point numerical
rating scales (NRS). The MVK Pain Scale, the primary
outcome, is the average of three NRS assessing back pain
today, worst back pain in last 4 weeks and average back
pain in the last 4 weeks. We modified the instrument to
evaluate LBP over the previous week, rather than 4 weeks.
The MVK Disability Scale, a secondary outcome, is the
average score of three NRS measuring interference with 1)
daily activities, 2) social and recreational activities and 3) the ability to work outside or around the house. The two MVK
scales are scored from 0 to 100 with a lower score more
favorable. The MVK scales have been shown to be reliable,
valid and responsive instruments for measuring pain and
disability.  We assumed without further validation that
the shorter recall period used in our study would not compromise
instrument accuracy. For the purpose of interpreting
the data, a 10–point difference between groups, 20% to 25%
of the baseline score, was designated a priori as clinically
important. This magnitude was used in an earlier study. 
Additional therapy sought outside the study protocol from
chiropractors, medical doctors, physical therapists, acupuncturists and massage therapists was also recorded.
Baseline variables included sociodemographics, general
health status and the baseline scores of study outcomes.
The general health status measures were the energy/fatigue,
emotional well-being, self-rated health and depression
screen indexes of the Medical Outcomes Study 36–Item
Short-Form Health Survey (SF–36).  The SF–36 has
demonstrated content validity, construct validity, reliability,
generalizability, acceptability and practicality. [43–45]
All participants with follow-up data were included in
the analysis in their assigned treatment group regardless of
adherence to treatment schedule or of care received outside
the study protocol (intention-to-treat analysis). Separate,
preplanned analyses were conducted for the 4- and 12-week
follow-up data. For pain and disability, linear least-squares
regression models were fit to the data. Independent variables
included visits, visits squared and visits cubed (for
assessing dose-response trends); treatment regimen (for assessing
the inclusion of PM); second baseline value of the
dependent variable and all two-way and three-way interactions
among visits, treatment regimen and baseline. Variable
selection methods were applied to determine parsimonious
models. Square-root transformations were applied to normalize
the disability data, which were skewed to the right but
did not fit a log-normal distribution. Models were identified
using maximum R-squared variable selection and backward
elimination of variables not meeting a 5% statistical
significance criterion. Hierarchical models were fit. That is,
if an interaction of two terms was statistically significant,
then the two terms were also included in the model even
if these terms were not individually significant. Adjusted
least-square mean outcomes were estimated as the predicted
mean using the final regression models with
baseline set to the grand mean baseline value for the data.
We set the sample size a priori based on available resources.
Post hoc power calculations were performed on 4-
week pain and disability outcomes. Power analyses were
based on partial correlations of the dependent variable with
visits after adjusting for the baseline of the variable using
a multiple linear regression model. Power for the observed
partial correlations was 72% for pain and 91% for disability.
Significance of statistical tests were set a priori at the
.025 level to adjust for two primary outcomes (pain at 4 and 12 weeks). All analysis was performed on SAS Version 8.  Post hoc power calculations were performed using
PASS 2000. 
The staff conducted 201 phone screens (Figure 1). Of these,
110 were ineligible or not interested. The study chiropractors
conducted 91 screening physical examinations; 10 persons
did not meet the study eligibility criteria. Of the 81 eligible
persons, 72 agreed to enrollment in the study and were
randomized. Only one participant was not compliant with
treatment, stopping after 5 of 9 visits because of time and
distance to the study site (moved). Three additional patients
missed one or two visits in the groups assigned 9 or 12
visits. The response rate to mailed questionnaires was excellent:
96% at 4 weeks and 93% at 12 weeks. Those not responding were well distributed across visit groups. Of a possible 144 questionnaires, 136 were returned by the participants. No adverse events were reported.
Spinal manipulation was performed at almost every visit
for all patients. Of the participants assigned PM, 75% received
soft tissue therapy (massage/trigger point), 92% hot/
cold, 44% electrotherapy and 22% ultrasound at least once.
The mean number of PM (one or two PM required at each
visit) ranged from 1.6 in Group 3 to 1.9 in Group 1. Visual
inspection of the data revealed no pattern of use across the
four visit groups.
Baseline characteristics are presented in Table 1. The
mean age was 48 years (SD = 14). Approximately half of
the participants were women and 84% white non-Hispanic.
The general health status indexes revealed that participants
were in good health but showed signs of fatigue and 43% indicated the possibility of depression. Overall pain and disability were moderate: mean pain index of 49.3 (SD = 22.5, 95% confidence interval [CI] = 44.0 to 54.5) and mean disability index of 38.9 (SD = 26.1, 95% CI = 32.8
to 45.1). There was some disparity in pain and disability
Study flow diagram.
SMT=spinal manipulative therapy.
Baseline characteristics: mean (SD) or percentage for the four dose groups
Observed mean outcomes and SDs are shown in Table 2. The four regression models for pain and disability outcomes are found in Table 3, and group means and 95%
confidence intervals, adjusted for baseline differences between
groups, are presented in Table 4. Adjusted group
means are also charted in Figures 2 and 3 for clarity.
At the 4-week follow-up, there was a substantial linear
effect of patient visits favoring a larger number of treatments
with spinal manipulation (B =–5.7, SE = 2.3, p=.014;
Table 3). That is, there was a mean of 5.7 points of improvement per additional 3 treatments (one visit per week). Compared with the group receiving 3 treatments, participants
assigned 9 treatments had an 11.4–point advantage and those
assigned 12 treatments showed a 17.1–point advantage in
mean pain score improvement (Fig. 2, Table 4). There was
no statistically significant or clinically important effect of
including/excluding PM at each visit in the regimen (p>.5).
At the 12-week follow-up, the best model selected by
the maximum R2 criterion included baseline pain (B = 0.4, CI = 0.2 to 0.6, p = .0008) and visits×PM interaction
(B = –5.4, CI = –8.7 to –2.1, p = .003). Thus, the hierarchical
model that included baseline pain, linear term in visits,
treatment regimen and visits×treatment regimen interaction
was fit. The regression coefficients and p values are summarized
in Table 3, and adjusted means are summarized in
Table 4. Although the individual regression coefficients for visits, treatment regimen and the interaction of these two
were not individually statistically significant (p>.15), the
overall test that all three of these regression coefficients
equal zero approached significance (p = .029). There was a
linear visits effect in those patients who received PM and
manipulation: 6.1 points of improvement in pain per additional
visit per week. In those who did not receive PM
(manipulation only), pain actually increased by a trivial 0.5
points per additional visit per week. However, Table 2 shows
that those receiving 3 visits per week (manipulation only)
had the smallest observed mean, whereas those receiving 4
per week had the largest observed means. This variation was
not consistent with a linear term in dose.
(SD) pain intensity
Regression models for
pain and disability intensity
Adjusted means for pain
intensity and functional
At 4 weeks, we found a visit effect (p = .018) and no
effect of treatment regimen (p>.50; Table 3). The regression
coefficients for disability in Table 3 are for root-transformed
data. Figure 3 shows the linear effect of visits for the group
means. The mean group differences were similar in magnitude
to those found for pain at 4 weeks (Figure 2, Table 4).
At 12 weeks, there was no effect of number of visits
or treatment regimen (p=.50). Hence, the adjusted mean
disability score for all groups was 19.9 (95% CI = 14.4 to
26.3; Table 4). A comparison to baseline (mean=38.9, 95%
CI = 32.8 to 45.1) showed a substantial and statistically significant improvement (p<.0001) in functional disability for
all study groups at this time point (Fig. 3, Table 4).
A secondary analysis was conducted to identify potential
confounding of results resulting from baseline differences
among the groups with respect to age, gender and income.
When potential confounding variables that contribute to
the overall R2 of the model were included in regression
models for the various outcomes, these variables did not
substantially change the associations described above (results
Fifteen of 72 participants sought health care outside the
study regimen for LBP by the 4–week follow-up and a total
of 30 sought outside care by the 12–week follow-up (Figure
4). Most outside care was sought by a few individuals: 5
persons had 15 of 32 outside visits made by 4 weeks and
6 persons had 60 of the total 117 outside visits made by 12
weeks. Exclusion of these 6 individuals from the analysis
would increase unadjusted group means less than 0.3 points.
The 32 outside visits by 4 weeks included 6 to chiropractors
and 5 to medical doctors. The remaining visits were
to physical therapists (12) and massage therapists (9). The
majority (19 of 32), including all of the chiropractic care, was
sought by participants in the lowest visit group (Group I).
Of the 117 total visits by 12 weeks, 26 were to chiropractors,
27 to medical doctors, 23 to physical therapists, 32 to
massage therapists and 8 to acupuncturists. Much of the care
was sought by members of Group I (48 of 117). Groups I
and II made 25 of 26 outside visits to chiropractors. Patients
allocated to receive study PM had 69 of 117 visits and those
without PM had 48 of 117 visits.
Adjusted means for the
4-week and 12-week pain intensity
Adjusted means for the
4-week and 12-week disability models
The total number of visits
made by the 72 participants
This was the first randomized trial to study the doseresponse
relationship between number of visits to a chiropractor
and clinical outcomes. The principal finding was a
relationship between pain outcomes and visits to a chiropractor
for chronic LBP at 4 weeks (1 week after completion
of treatment). Relief was substantial and the 23 of 100 to
28 of 100 points of pain relief achieved for participants
receiving 9 and 12 treatments is clinically important to the
patient, even by conservative interpretation of numeric rating
scales for pain.  The inclusion/exclusion of physical
modalities at each visit had no meaningful effect on pain
improvement at this time point.
Pain results at 12 weeks must be interpreted with caution.
The nonhierarchical analysis suggests a potential dose-response
for patients receiving both manipulation and PM at
each visit, and no such trend in patients receiving manipulation
only (Figure 2). This would mean that either PM or
a combination of manipulation and PM produces a sustainable
dose-response not found in patients receiving manipulation
alone. However, the statistical significance of the
variables depends on the model used. Also, inspection of
the data in Table 2 for patients without PM suggests that
groups receiving either three visits per week or four visits
per week could represent random outliers. Findings may
have been statistical artifacts of small group analysis and
the results spurious. In particular, the magnitude of the difference between PM and no PM groups for four visits per week
is difficult to explain. The effects of PM, of dose in those
not receiving PM and nonlinear effects involving dose need
further study because this pilot study was not adequately
statistically powered to address all these questions.
An effect of the inclusion of PM in the treatment regimen
on LBP intensity would be unexpected in light of two recently
published trials. Hurwitz et al.  demonstrated in
a large trial that the inclusion of passive PM (hot/cold,
ultrasound and electrotherapy) with manipulation was no
more effective than manipulation alone for a mix of acute
and chronic LBP. Hsieh et al.  obtained similar results
for active soft tissue therapy in subacute patients. Our
findings may have differed from those of the previous trials
because active modalities were included in the treatment of
chronic patients. Also, the number of treatments in the chronic
pain study of Hurwitz et al. may have been insufficient to reveal an important effect of PM (meanthree to four visits),
similar to the case of our three-visit group in Figure 2. Our
pilot raises the possibility that it may be premature to dismiss
the inclusion of PM in a regimen of spinal manipulation for
The dose-response for pain intensity in Figure 2 suggests
that more chiropractic treatments may be required to reach the
optimal benefit that would have been indicated by a saturation
of the dose-response curve. Recommendations on duration
and frequency of manipulation/chiropractic care for chronic
LBP vary widely and are based on clinical experience and
opinion.  A multidisciplinary RAND panel found that
evidence-based consensus was not possible but on average
expected the typical patient to show improvement in 4 to 6
weeks with three visits per week.  In contrast, an all chiropractic expert panel recommended 30 visits for the
average chronic patient over 14 weeks.  Our data support
the investigation of a larger number of visits in future trials.
Disability outcomes further support a larger number of
visits in future trials (Figure 3). A linear dose-response was noted at 4 weeks for patients with and without PM. At 12
weeks, there was no evidence within this pilot study of any
treatment effects. The considerable improvement observed at
12 weeks was independent of the quantity of care received
in the range of visits under study. The data cannot distinguish
a plateau effect from the absence of a threshold effect in
terms of benefit accrued.
The majority of outside health care for LBP was limited to
only 6 of 72 individuals. Because the remaining participants
sought little care, it is unlikely that outside care impacted
study findings significantly. It is not surprising that participants in the lowest study visit group were most likely to
have additional care, but the motivation for care seeking is
not clear. Possible reasons certainly include pain exacerbation
between data collection points. However, expectation
of the quantity of care required for pain relief may have
played a role.
Clearly, the primary limitation of this study was the small
sample size used in this pilot. Point estimates of the mean
outcomes for the 8 groups could not be measured with
precision (large confidence intervals). The observed effects
may have resulted from sampling error causing unknown
imbalance in unidentified confounding variables. Effects
may have also depended on the root transformation used to
normalize the disability data. These data were positively
skewed, although the skewing was not as “severe” as for a
log-normal distribution (ie, requiring a natural logarithm
transform). Larger sample sizes may provide better assessment
of the overall distribution and extent of skewness, as
well as reveal additional significant higher-order interaction
effects, including deviations from linearity.
A further limitation of the present study was that we
did not control patient attention bias and expectation (more
treatment is better). One alternative design to address this
concern would require all participants to attend the same
number of study visits. They would be blinded to group
assignment by receiving sham therapy at nontreatment visits.
However, blinding to sham manipulation/PM presents a challenge
to investigators if the institutional review board requires
disclosure of sham procedures in the study design.
Patients aware of the existence of treatment alternatives
might contrast and discern real from sham. A second alternative
design might include assessment visits (rather than visits
for sham treatment) to control attention bias, and include
patient blinding to the mix of treatment and assessment visits
to control, at least in part, the effects of expectation. Provider blinding to the number of treatment visits would require a different design.
Comprehensive assessment for chiropractic management
of LBP requires systematic investigation of the effects on
clinical outcomes of number and frequency of patient visits,
intensity of therapeutic modality utilization (manipulation
and PM) and duration of patient care. [20, 24, 50] Our study was a first step, looking at relief up to 2 months after completion of care. To date, there have been no trials on concentration of visits or duration of care. Only one case series found that patients, with constant, severe chronic LBP that was unresponsive to conservative/operative care, showed marked improvement following 2 weeks of daily manipulation.  The effect of treatment parameters on the durability
of outcomes in the long term is also unknown.
There was a positive, clinically important effect of the
number of chiropractic treatments on chronic LBP at 4
weeks. Relief was substantial for patients receiving care
three to four times per week for 3 weeks. The sustainability
of relief patterns to 12 weeks was not clear. A concentrated
course of chiropractic care of up to 12 visits in 3 weeks
appears appropriate for the treatment of chronic LBP.
The study findings should be generalizable to the practice
of manual medicine providers who use high-velocity low-amplitude
spinal manipulation and PM for the treatment of
LBP. However, the effect of additional care, such as exercise
and different manipulation techniques, on the observed outcomes
is unknown. The data show room for considerable
improvement and suggest further study on regimens, including
a greater number of visits for care. The effect of duration
of care on outcomes and the durability of pain/disability
relief in the long term remain to be investigated.
The authors wish to acknowledge the following faculty
and staff of theWestern States Chiropractic College for their
roles in the study. The study chiropractors were David Corll,
DC; Elizabeth Dunlop, DC; David Peterson, DC; and Anita
Roberts, DC. Our project managers were Alisa Fairweather,
MPH, and Bonnie Ganger. Michael Attwood was responsible
for all database applications. He also assisted with descriptive
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