Existence of a neuropathic pain component in patients with osteoarthritis of the knee.

Abstract

Pain from osteoarthritis (OA) is generally classified as nociceptive (inflammatory). Animal models of knee OA have shown that sensory nerve fibers innervating the knee are significantly damaged with destruction of subchondral bone junction, and induce neuropathic pain (NP). Our objective was to examine NP in the knees of OA patients using painDETECT (an NP questionnaire) and to evaluate the relationship between NP, pain intensity, and stage of OA.
Ninety-two knee OA patients were evaluated in this study. Pain scores using Visual Analogue Scales (VAS), Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), painDETECT, duration of symptoms, severity of OA using the Kellgren-Lawrence (KL) system, and amount of joint fluid were evaluated and compared using a Spearman's correlation coefficient by rank test.
Our study identified at least 5.4% of our knee OA patients as likely to have NP and 15.2% as possibly having NP. The painDETECT score was significantly correlated with the VAS and WOMAC pain severity. Compared with the painDETECT score, there was a tendency for positive correlation with the KL grade, and tendency for negative correlation with the existence and amount of joint fluid, but these correlations were not significant.
PainDETECT scores classified 5.4% of pain from knee OA as NP. NP tended to be seen in patients with less joint fluid and increased KL grade, both of which corresponded to late stages of OA. It is important to consider the existence of NP in the treatment of knee OA pain.

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Yonsei Med J http://www.eymj.org Volume 53 Number 4 July 2012
801
Original Article
http://dx.doi.org/10.3349/ymj.2012.53.4.801
pISSN: 0513-5796, eISSN: 1976-2437
Yonsei Med J 53(4):801-805, 2012
Existence of a Neuropathic Pain Component in Patients
with Osteoarthritis of the Knee
Seiji Ohtori, Sumihisa Orita, Masaomi Yamashita, Tetsuhiro Ishikawa, Toshinori Ito,
Tomonori Shigemura, Hideki Nishiyama, Shin Konno, Hideyuki Ohta, Masashi Takaso, Gen Inoue,
Yawara Eguchi, Nobuyasu Ochiai, Shunji Kishida, Kazuki Kuniyoshi, Yasuchika Aoki, Gen Arai,
Masayuki Miyagi, Hiroto Kamoda, Miyako Suzkuki, Junichi Nakamura, Takeo Furuya,
Gou Kubota, Yoshihiro Sakuma, Yasuhiro Oikawa, Masahiko Suzuki, Takahisa Sasho,
Koichi Nakagawa, Tomoaki Toyone, and Kazuhisa Takahashi
Department of Orthopaedic Surgery, Graduate School of Medicine, Chiba University, Chiba, Japan.
Received: August 24, 2011
Revised: October 10, 2011
Accepted: October 11, 2011
Corresponding author: Dr. Seiji Ohtori,
Department of Orthopaedic Surgery,
Graduate School of Medicine,
Chiba University, 1-8-1 Inohana,
Chuo-ku, Chiba 260-8670, Japan.
Tel: 81-43-226-2117, Fax: 81-43-226-2116
E-mail: sohtori@faculty.chiba-u.jp
∙ The authors have no financial conflicts of
interest.
© Copyright:
Yonsei University College of Medicine 2012
This is an Open Access article distributed under the
terms of the Creative Commons Attribution Non-
Commercial License (http://creativecommons.org/
licenses/by-nc/3.0) which permits unrestricted non-
commercial use, distribution, and reproduction in any
medium, provided the original work is properly cited.
Purpose: Pain from osteoarthritis (OA) is generally classified as nociceptive (in-
flammatory). Animal models of knee OA have shown that sensory nerve fibers in-
nervating the knee are significantly damaged with destruction of subchondral bone
junction, and induce neuropathic pain (NP). Our objective was to examine NP in
the knees of OA patients using painDETECT (an NP questionnaire) and to evaluate
the relationship between NP, pain intensity, and stage of OA. Materials and Meth-
ods: Ninety-two knee OA patients were evaluated in this study. Pain scores using
Visual Analogue Scales (VAS), Western Ontario and McMaster Universities Osteo-
arthritis Index (WOMAC), painDETECT, duration of symptoms, severity of OA
using the Kellgren-Lawrence (KL) system, and amount of joint fluid were evaluat-
ed and compared using a Spearman’s correlation coefficient by rank test. Results:
Our study identified at least 5.4% of our knee OA patients as likely to have NP and
15.2% as possibly having NP. The painDETECT score was significantly correlated
with the VAS and WOMAC pain severity. Compared with the painDETECT score,
there was a tendency for positive correlation with the KL grade, and tendency for
negative correlation with the existence and amount of joint fluid, but these correla-
tions were not significant. Conclusion: PainDETECT scores classified 5.4% of
pain from knee OA as NP. NP tended to be seen in patients with less joint fluid and
increased KL grade, both of which corresponded to late stages of OA. It is impor-
tant to consider the existence of NP in the treatment of knee OA pain.
Key Words: Knee, osteoarthritis, neuropathic pain, nociceptive pain
INTRODUCTION
Knee osteoarthritis (OA) is a common and progressive joint disease. With an esti-
mated incidence rate of 240 per 100000 person-years, it is a major public health
problem in the US and often results in early retirement and joint replacement.1

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of NP in the knees of OA patients using painDETECT, and
to evaluate the relationship between NP and stage of OA.
MATERIALS AND METHODS
　　　
The ethics committee of our institution approved the protocol
for the human procedures used in this study. Furthermore, the
protocol and publication of the study were approved by our
institutional review board and informed consent was ob-
tained from each of the participants.
The patients who participated in this study were selected
from outpatients who attended our hospital for knee pain.
These 92 patients were selected from 112 knee pain pa-
tients who matched the following criteria. All patients had
knee pain for more than 1 month. Inclusion criteria were
the observation of the existence of OA of the knee joint on
examination of an anterior-posterior X-ray image. Patients
who had history of knee surgery, infection, and rheumatoid
arthritis were excluded from this study. Ultimately, 20 pa-
tients were excluded, and 92 patients having OA of the
knee joint were evaluated. The patients completed a self-
administered questionnaire on sociodemographic factors
(age and gender) and duration of knee symptoms. The
background of the subjects is summarized in Table 1.
Pain scores
Various screening tools have been developed to identify
NP, including LANSS, DN4, NPQ, painDETECT, and ID
Pain.9-13 These tools have been developed for evaluation of
NP such as nerve injury, radicular pain, and diabetic neurop-
athy. Hochman, et al.15 selected a list of NP descriptors from
these 5 validated questionnaires and used them for knee OA
pain. Five items were commonly identified to distinguish
nociceptive pain from NP in people with other chronic pain
conditions. They reported that 34% of knee OA patients de-
scribed their pain using quality descriptors suggestive of
NP, and concluded that the 5 items are reliable indicators.15
Bennett, et al.14 reviewed the strengths and weakness of
these 5 validated questionnaires and reported that painDE-
TECT did not require a clinical examination, and showed a
slightly higher sensitivity and specificity in comparison
with the other tools. Thus, in the current study, we selected
painDETECT for this advantage.
All patients completed the Visual Analogue Scale (VAS),
Western Ontario and McMaster Universities Osteoarthritis In-
dex (WOMAC), and painDETECT screening.9,16 WOMAC
Chronic pain is classified into nociceptive (inflammatory)
pain and neuropathic pain (NP). Nociceptive pain, such as
that in rheumatoid arthritis, results from continuously stim-
ulating nociceptors through chronic inflammation. NP is
generated from damage or dysfunction to the nervous sys-
tem. Major causes of NP include diabetes, lumbar or cervi-
cal radiculopathies, and spinal cord injury.2,3 Knee OA is
generally classified as nociceptive (inflammatory) pain.
It has been reported that intra-articular injection of mono-
iodoacetate into the femorotibial joint space in mice and
rats induces a linear pathology with similarities to OA, and
this model is now widely used for investigating the patho-
genesis of knee pain.4 In the early phase of rat OA, the level
of inflammatory neuropeptides in sensory nerves innervat-
ing the knee increases.5 On the other hand, as subchondral
bone is densely innervated, subchondral bone pathology
may cause neuropathy after destruction of chondral struc-
ture in the late phases of OA. In the late phase of OA in rat
models, markers of nerve injury in sensory nerves innervat-
ing the knee are significantly increased compared with con-
trols.6 Understanding whether nerve damage or neuropathy
underpins the lack of pharmacological sensitivity to cele-
coxib or diclofenac, but not morphine observed at later time
points would clarify the origin of pain.7,8 These observa-
tions suggest that the pain of knee OA is both nociceptive
pain and NP. However, the origin of pain is generally un-
known in clinical cases.
For measurement of NP in patients with other chronic
pain conditions the PainDETECT score, the Leeds Assess-
ment of Neuropathic Symptoms and Signs pain scale with a
self-report version (LANSS), the Douleur Neuropathique
with 4 questions (DN4), ID Pain, and the Neuropathic Pain
Questionnaire (NPQ) have been developed.9-13 Compared
with expert physician diagnosis of NP versus non-NP, the
sensitivity, specificity, and positive predictive values of these
measures ranges from 66.6% to 85%, 74% to 90%, and
71.4% to 86%, respectively.14 Only one report has described
the relationship between OA pain and NP in patients with
OA of the knee. Hochman, et al.15 previously reported that
34% of patients with OA of the knee had pain with an NP
component; however, the relationships among NP, pain in-
tensity, and stage of OA have not been examined.
Our hypothesis is that there is an NP component in knee
OA pain, and that the NP is particularly seen in late phases
of OA related to the severity of pain, severity of OA using
radiographic evaluation, and reduced amounts of joint fluid.
The purpose of our current study was to examine the extent

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73 (79.3%) as unlikely NP.
Table 3 evaluates KL grade, BOP, and amount of joint
fluid of knee. There was no patient with KL0 and all pa-
tients were distributed from KL1 to KL4. The proportion of
patients with BOP was 33% of all patients and average
amount of joint fluid was 9.3±2.5 mL.
Table 4 shows the correlation between the painDETECT
consists of subsections for pain, stiffness, and physical func-
tion. PainDETECT is a patient-based questionnaire consist-
ing of seven weighted sensory descriptor items and two
items relating to the spatial (radiating) and temporal charac-
teristics of the patient’s pain pattern. The painDETECT
score was distributed from 0 to 38. Patients were divided
into three groups: neuropathic pain is likely (score ≥19),
possible (score ≥13 to ≤18), and unlikely (score ≤12). VAS
was evaluated at 3 time points. VAS at entry point, of se-
verest pain during 4 weeks, and of average pain during 4
weeks were evaluated for all patients.
Radiographic evaluation
Anterior-posterior view of X-ray examination was per-
formed in all patients. We used the Kellgren-Lawrence (KL)
severity classification system, which is a validated method
of classifying individual joints into one of five grades, with
0 representing normal and 4 being the most severe radio-
graphic disease.17 Evaluation was blinded and performed
by 5 observers. If at least 3 of the observers concurred, the
grading score was used to define KL grade.
Evaluation of knee joint fluid
Ballottment of the patella (BOP) was examined for all pa-
tients. If the BOP sign was positive, needle puncture into the
knee was performed in all positive patients and the amount
of joint fluid was evaluated.
Statistical Analysis
Data were compared using a Spearman’s correlation coeffi-
cient by rank test. p<0.05 was considered statistically sig-
nificant.
RESULTS

Table 1 shows demographic characteristics of patients.
Ninety-two patients (71 female, 21 male) with a mean age
of 70.3 years were included in the study during the entire
period. Average duration of symptoms was 37.5 months.
The patients suffered from knee pain originating from OA
for at least 1 month. Pain at entry point using VAS was 5.5
and total WOMAC score was 57.1.
Table 2 shows the painDETECT score and indicates like-
ly NP (score ≥19), possible NP (score ≥13 to ≤18), and un-
likely NP (score ≤12). Within this study population, 5 (5.4%)
were classified as likely NP, 14 (15.2%) as possible NP, and
Table 1. Demographic Characteristics
Number of patients 92
Sex
Male: 21
Female: 71
Age (mean±SD yrs) (range)
70.3±8.0
(53-81)
Symptom duration (mean±SD months)
(range)
Pain scores
Visual Analogue Scale (VAS)
At entry point (mean±SD)
Severest pain during 4 wks
(mean±SD)
37.5±8.0
(1-180)
5.5±2.0
6.1±2.6
Average pain during 4 wks
(mean±SD)
WOMAC scores
Pain (mean±SD)
Stiffness (mean±SD)
Physical function (mean±SD)
Total (mean±SD)
WOMAC, western ontario and McMaster universities osteoarthritis index.
5.1±2.1
12.3±3.6
4.7±2.3
40.3±11.4
57.1±15.3
Table 2. PainDETECT Score
Score
Number of patients (%)
(total; 92 patients)
73 (79.3)
14 (15.2)
5 (5.4)
0-12
13-18
19-38
Neuropathic pain is likely (score ≥19), possible (score ≥13 to ≤18), and
unlikely (score ≤12).
Table 3. X-ray Evaluation, BOP, and Amount of Joint Fluid
KL grade
Number of patients (%)
(total; 92 patients)
0
24 (26)
25 (27)
33 (36)
10 (11)
Number of patients (%)
30 (33)
62 (67)
0
1
2
3
4
BOP
+
-
Amount of joint fluid (mean ± SD mL) (range: mL)

BOP, ballottment of the patella; KL, Kellgren-Lawrence.
9.3±2.5 (1-40)

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Some authors have reported that joint pain arises mainly
from free nerve endings that exist in the capsule or in the
synovium,19-21 whereas others have reported innervation of
the osteochondral junction in human knee OA samples and
indicated a possible contribution of the subchondral area to
OA knee pain.22,23 Regarding the mechanism of NP in knee
OA, it is highly possible that NP occurs in association with
damage to nerves innervating subchondral bone because of
its weight-bearing surface in late stage OA. In early phases,
synovitis produces joint fluid and stimulates free nerve end-
ings in synovium. On the other hand, a decrease in the
amount of joint fluid and destruction of the osteochondral
junction may induce injury to the nerve endings in this area
in late phases of OA. The current study showed the possi-
bility of NP pathogenesis in OA pain; however, this was not
clear because the correlations were not significant. There
was a tendency for positive correlation between painDE-
TECT score and the KL grade, and tendency for negative
correlation between the painDETECT score and the amount
of joint fluid. Further study using a larger population is
probably needed to clarify this.
Another mechanism has been postulated for NP in OA.
Approximately 60-80% of OA patients achieve pain relief
after local anesthetic treatment or surgical replacement of
the affected joint, indicating peripheral mechanisms driving
the pain, although central mechanisms are also thought to
play a role in some patients and dysfunction of diffuse nox-
ious inhibitory controls has been described.24-26 These cen-
tral mechanisms may overlap with those in other pain states
such as neuropathy.
There are several limitations of this study. The reliability
of painDETECT for NP in knee OA has not been fully eval-
uated, therefore, further study is needed to clarify this.
In conclusion, the PainDETECT scores for pain from
knee OA revealed that 5.4% are likely NP, and 15.2% are
possible NP. NP tended to be seen in patients with less joint
fluid and a KL grade that correspond to late stages of OA. It
is important to be aware of consider the existence of NP in
the treatment of knee OA pain.
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OA has a high prevalence in the elderly population and is a
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rsp value
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0.28
0.29
0.035
<0.001
0.066
0.039
0.746
0.504
0.653
0.222
0.152

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