Key words
ischium - psoriatic arthritis - rheumatoid arthritis - enthesitis
Schlüsselwörter
Enthesitis - Sitzbein - Psoriasis-Arthritis - Rheumatoide Arthritis
Background
An enthesis is a location where the tendon, ligament, or joint capsule inserts into
the bone. Inflammation of the entheses (enthesitis) is a unique feature of
spondylarthritis (SpA) and is important in the identification, diagnosis, and
treatment of psoriatic arthritis (PsA) [1].
Apart from physical examination, imaging modalities that can assist in enthesitis
are conventional radiography, ultrasonography (USG), and magnetic resonance imaging
(MRI) in clinical practice [2]
[3]. USG and MRI are shown to be effective in
detecting entheseal active inflammation, erosions, and adjacent bone marrow edema;
however, one is highly operator-dependent, and the other is a costly method.
Anteroposterior pelvis radiograph (APPR) is the first-step method in cases where SpA
is suspected. Many entheseal sites at the pelvis and hip region can be evaluated
with APPR; however, little is known about radiographic changes of enthesopathy (RCE)
in the pelvis. In our clinical practice, we have frequently observed RCE in ischiums
of patients with PsA. We aimed to investigate the prevalence of ischial RCE in
patients with PsA, compared to patients with rheumatoid arthritis (RA).
Methods
The files of PsA, and RA patients, who were followed up in Rheumatology Outpatient
Clinic, between November 2014 and December 2018, were retrospectively reviewed. PsA
and RA patients were classified using the CASPAR and 2010 American College of
Rheumatology/European League Against Rheumatism criteria [4]
[5].
Available APPRs and lateral foot radiographs of subjects with RA, with matching age
and gender, were obtained. In our clinic, radiography is not ordered routinly in
patients with PsA who do not have axial complaints. Those radiographs were taken in
all patients with complaints of low back pain and heel pain.
The entheseal sites checked were: os ischium, Achilles, and the inferior calcaneal
attachment, both the left and right sides. Abnormalities such as cortical erosions,
enthesophytes (irregular bony proliferation) were recorded. Suspected cortical
irregularities were not recorded. Images were evaluated by two blinded
rheumatologists. This study was approved by the university ethics committee.
Data analysis was conducted using the statistical software package IBM SPSS
Statistics Version 22.0. Pearson’s chi-squared test with bootstrap sampling,
stratified according to age and gender, was used for comparisons of categorical
variables. Continuous variables were presented as median [Interquartile Range]
because of discrepancies with the normal distribution. The Kruskal-Wallis test was
used for continuous variables, and when significance was found, pairwise comparisons
were made. Spearman’s Rho test was used for correlation between two ordinal
variables. An A P value with type 1 error probability, less than .05, was accepted
as statistically significant in all analyses.
Results
84 patients were enrolled in the study: 38 with PsA (30 women and 8 men), and 46 with
RA (41 women and 5 men). [Table 1] shows
their demographic and clinical characteristics.([Table 2])
Table 1 Demographic and clinical characteristics of the study
population.
|
Patient groups
|
|
|
Variables
|
PsA (n:38)
|
RA (n:46)
|
p-value
|
|
Median age (IQR)
|
50 [19] # a
|
54 [21]a,b
|
0.009*
|
|
Females, n (%)
|
30 (78.9)
|
41 (89.1)
|
0.02
|
|
Median disease duration, years (IQR)
|
4.5 (3)
|
5 (7)
|
0.03
|
|
BMI (kg/m
2
) Median
[IQR]
|
29.14 (6.74)
|
26.29 (7.83)
|
0.173*
|
PsA: psoriatic arthritis; RA: rheumatoid arthritis; IQR: inter-quartile
range; *Kruskal-Wallis test; # Pairwise Comparison: Different
letters indicate groups that make a difference.
Table 2 Frequency of radiographic enthesopathic changes in
study groups.
|
Patient groups
|
|
|
Enthesopathic changes
|
PsA (n:38)
|
RA (n:46)
|
p value*
|
|
Ischium, n (%)
|
19 (50.0) a
|
13 (28.3) b
|
0.04
|
|
Achilles, n (%)
|
16 (42.1) # a
|
13 (28.3) b
|
0.33
|
|
Plantar, n (%)
|
22 (57.9%)
|
15 (32.6)
|
0.06
|
* Chi-Square Test results with bootstrap sampling, stratified
according to age and gender.; # Different letters indicate groups that make
a difference.
The frequency of RCE in ischial regions was found to be statistically significantly
higher in PsA compared to RA patients (50%, 28.3%, respectively,
p=0.04). Frequency of plantar and Achilles RCE were also higher in PsA
patients than in RA patients.
The relationship between the presence of ischial RCE and sacroiliac grades was
analyzed using Spearman's Rho test, a nonparametric correlation analysis. A
positive and statistically significant correlation was found between the frequency
of ischium enthesopathies and the grade of sacroiliitis (r=0.345;
p<0.05).
Discussion
Enthesitis is one of the first symptoms of PsA and is the most pathognomonic finding
that distinguishes PsA from rheumatoid arthritis [6]
[7]. Although it has been known
for years that enthesitis is an important process in the pathogenesis of PsA, there
is still more unknown data regarding enthesitis in PsA, than known data. In short,
there is simply not enough data in the literature regarding ischial enthesopathy in
PsA. In this study, half of the PsA patients were found to have RCE in ischium.
While the ischia are in a sitting position, the plantar regions become the standing
load-bearing areas. Like the Köbner phenomenon that occurs in the skin, in
the case of psoriasis, changes in areas subject to stress, such as the enthesis
areas in the ischium and plantar fascia, would be an expected finding [8].
The enthesopathy changes most typically visible on a radiograph as enthesitis are
new
bone formation and bone irregularities. In the CASPAR study, which contains the
highest number of patients in the literature on this subject, RCE in PsA patients
were compared with AS, RA, and other rheumatic diseases [2]. A significant difference was found between
the groups due to enthesopathies in the pelvic region, with the most prominent being
found in AS patients. Unlike our study, no difference was found between PsA and RA
patients in terms of enthesopathic erosion or new bone formation in the pelvic
region in the CASPAR study. The difference between the CASPAR study and the current
study may be due to the design of the studies. The CASPAR study was prospective,
while the current study was designed retrospectively. It should be noted that the
X-rays in the current study were only taken in patients with low back pain and heel
pain.
The clinical evaluation of enthesitis is made by determining the amount of tenderness
felt in the affected areas during physical examination. In a study involving
approximately 800 PsA patients, enthesitis was defined as “clinical
tenderness.” Achilles, plantar and lateral epicondyle enthesitis were
detected in 35% of these patients [9].
In fact, the incidence of enthesitis can be expected to be higher when considering
enthesitis in non-palpable areas. However, detection of enthesopathic changes in
areas where palpation cannot be performed is only possible with imaging methods. In
this study, the frequency in the ischial region is as common as the RCE seen in the
Achilles in patients with PsA. In ischium enthesopathy, pain may develop, especially
when sitting on a hard surface or during the movement from sitting to a standing
position, which is one of the functions of the muscles that are attached to the
ischia.
The weakness of this study is the use of conventional radiography, which shows the
damage, but not the inflammation, present in enthesis. It is shown that the use of
radiography for diagnosing enthesitis in early PsA is limited [10]. In cases where sacroiliac MRI is required
in psoriatic patients, an evaluation of the ischium regions may help to investigate
the enthesopathic changes that develop in these regions in the early stages of the
disease.
In conclusion, this study has shown that enthesopathic changes can often develop in
the ischium of PsA patients with symptoms in the corresponding region.
Author Contributions
AEY conceived of the presented idea. AEY and BB developed the theory and performed
the computations. BB and GSD evaluated the radiographs in a blinded way. AA
organized the radiographs and evaluated the patient files. MAO performed the
statistical analysis. BB wrote the research article. AEY and MAO revised the
article. All authors discussed the results and contributed to the final
manuscript.
