Joint cartilage thickness and automated determination of bone age and bone health in juvenile idiopathic arthritis

Juvenile Idiopathic Arthritis (JIA) is the most common rheumatic disease in childhood. It can result in musculoskeletal pain, joint stiffness and swelling and decreased range of motion. If left untreated JIA will lead to disability of the affected joints. If kept untreated, chronically inflamed synovium and involvement of associated muscle and soft tissues, eventually leading to degeneration of the osteocartilaginous structures, are the primary cause for functional disability in JIA [1]. In pediatric rheumatology it is of great importance to follow osteocartilaginous degeneration [2]. The imaging modalities frequently used to visualize this process are conventional radiography, magnetic resonance imaging (MRI), and within the last decade ultrasonography (US). US is of particular benefit in the assessment of early signs of arthritis, such as joint effusion and synovial thickening [3]. Unlike with the MRI, without using any contrast, hyperemia in the synovial microcirculation as part of the inflammatory process can be detected using the color Doppler or the power Doppler technique [4]. US is relatively inexpensive and does not require any radiation. US is well tolerated, even in the very young patients, can be used to view different joints in one session, and can be viewed at the bedside or in the clinic room.

High frequency US can be used to easily view the joint cartilage as an anechoic structure, as cartilage has a high water content. In previous studies, we validated the assessment of cartilage thickness by US in a pediatric setting [5, 6]. We found a low intra- and inter-observer variability and a good level of agreement, with no significant systematic joint size-related differences in cartilage thickness measurements between MRI and US [7, 8]. Based on a large cohort of healthy children we established age- and sex-related normal reference values for cartilage thickness of the knee, ankle, wrist, second metacarpophalangeal (MCP) and second proximal interphalangeal (PIP) joints [5]. In a subsequent study we measured cartilage thickness of those joints in JIA patients and found reduced cartilage thickness in children with JIA compared to healthy controls [2, 8]. Children with polyarticular or systemic JIA had thinner cartilage than children with oligoarticular JIA [2].

Continuous exposure to inflammatory cytokines, together with glucocorticoid therapy, affects bone formation [9‐11]. This, combined with decreased physical activity and pubertal delay puts JIA patients at increased risk of impaired growth and reduced bone mineral density (BMD) [12]. The assessment of bone age is usually made using the Greulich and Pyle atlas [13]. Dual-energy X-ray densitometry (DXA) is the most commonly used method to assess BMD [14]. Recently, BoneXpert was developed bringing back the use of radiogrammetry, one of the oldest methods to assess BMD. This new digital X-ray radiogrammetry (DXR) method combines the assessment of bone-age with radiogrammetric assessment of (cortical) BMD) of the second to the fourth metacarpal joints [15, 16]. BoneXpert expresses the cortical BMD as a Bone Health Index (BHI), in which cortical BMD is corrected for size. The BoneXpert method makes use of conventional radiography of the hand, thereby making it attractive due to relatively low costs, and lower effective radiation dose compared to other effective methods [16]. The application has been tested in healthy pediatric populations and in the JIA population and showed to be a reliable method for assessing bone age and cortical BMD [17‐19]. BoneXpert method is an easy-to-use method provided that radiographs are of reasonable quality and the patients’ bone age lies within the age ranges of the program [17, 18]. The JIA population, derived from a biologic registry (Dutch) ABC register showed a delayed bone maturation and lower cortical BMD than healthy patients [19].

In the literature no reports are recorded to look at the relationship between cartilage thickness and bone health in JIA.

Therefore, the aim of our study was to evaluate bone maturation and bone density by using BoneXpert in a cross-sectional JIA population and correlate these findings to the cartilage thickness assessed by US.

This study was performed within a larger study on the use of US in JIA. In this study 155 patients with JIA aged between 5 and 15 years were invited to participate, 52 patients declined, 4 had unclassified arthritis, and 5 canceled their scheduled appointment due to illness. As a result, 94 patients were included in the initial study. In 27 patients digital radiogrammetry of the hand was not performed. Therefore, a total of 67 patients were evaluated by US and BoneXpert and included in the current study. The study included 16 boys and 51 girls, with a median age at investigation of 10.9 years (range 7.8–14.4 years), and a median disease duration of 41 months. Patient demographics, including distribution of JIA subcategories, are described in Table 1.

A total of 680 joints were assessed by US for cartilage thickness. Decreased cartilage thickness compared to healthy age-matched controls [5‐7] < −1 SD) was present in 27% of the examined joints (181 joints). The most common joint with decreased cartilage thickness was the second PIP joint, followed by the wrist. Decreased cartilage thickness was uncommon in the ankle joint (Table 2). If controlling for age and gender a significant decreased cartilage thickness was seen in the knees, wrists and second PIP joints, but not in the ankles or second MCP joints (Table 2). We found a highly significant correlation between the cartilage thickness measured in either the right- or left knee, right or left ankle, right or left wrist and right or left finger joints (r = 0.561–0.884, p < 0.001).

The automated bone age, using either the Greulich-Pyle or Tanner-Whitehouse method, correlated closely (r = 0.992, p < 0.001) and showed significantly lower bone age in JIA patients compared to healthy controls, when controlled for age and gender (Table 3). Delayed bone maturation was found in 22 (33%) patients (bone age < −1SD) of which 10 patients had a bone age < −2SD. BHI was decreased in children with JIA (Table 3). Thirty-four (50.7%) of the patients showed a decreased BHI below −1 SD of which 9/66 had a BHI < −2SD. Fourteen patients demonstrated a combination of a decreased BHI and a decreased bone age.

The decreased bone age was significantly correlated with a longer disease duration. This was not seen for the BHI. Both the bone age and BHI were not correlated to JADAS 10 (r = 0.056, p = 0.67 and r = 0.082, p = 0.52, respectively.

Decreased cartilage thickness measured by US in the knee, wrist and second MCPs was significantly negatively correlated with standardized bone age but this was not found for the ankle and 2nd PIP joints (Table 4). There is no difference in cartilage thickness and BHI, regardless whether BHI was < − 1 SD or < − 2 SD. Thus, no correlation was observed between cartilage thickness and BHI (table 4). Children with a bone age < −1 SD (n = 22) showed a significantly increased cartilage thickness in the right knee, both wrists, and both second MCPs, compared to children with a bone age > −1 SD (not shown). Decreased bone health and bone age were more pronounced in Tanner stage 1–3 (Table 5).

This is the first paper to investigate the relation between cartilage thickness measured by US and automated bone maturation and bone health index. JIA patients show a thinner cartilage than healthy controls. One in two patients showed a decreased BHI (<= −1SD), indicating the heavy burden of JIA on the bone health of these patients. Notably they have a lower cartilage thickness in their second MCP joint, this is one of the MCP joints that are used to to calculate the BHI. Interestingly, there is no correlation between cartilage thickness and BHI. This might be explained by the 3rd and 4th metacarpal joint, which are taken into account while calculating the BHI, but were not assessed by US.

Previous studies have shown that patients with JIA are known to have a delayed bone maturation [9‐12]. This was, however, not studied in relation to the cartilage thickness. Surprisingly, in the knee, wrist and MCP there is a significant correlation between a thicker cartilage and a delayed bone maturation, which is in contrast with one would have expected. However, in JIA, enhanced focal bone maturation is a well-known phenomenon as a result of joint inflammation, which may lead to accelerated thinning of the cartilage. One in five JIA patients showed both a decreased bone age and a decreased BHI.

The BoneXpert method used to measure bone maturation and BHI is an easy to use, high precision method. This study is one of the first research studies to demonstrate the use of the BoneXpert method in JIA. Nusman et al. reported on the feasibility of this method in JIA and the small difference between left- and right-hand radiographs [19].

The ongoing delayed bone maturation and decreased BHI in JIA patients with relatively mild disease, and no correlation with JADAS 10 score, shows the need for ongoing bone health strategies in JIA. Early on, corticosteroids were blamed for the bone health issues in JIA, however with the new treatments available and the continuing decreased usage of corticosteroids, bone health in JIA is still under threat.

This study is limited by the very specific population studied within an US cohort as previously described [2, 8]. These factors introduce a selection bias, which limits the generalizability to the full JIA population. However, patients included in this cohort were recruited consecutively in clinic and the subgroup evaluated in this study is a representation of that group. This group, however is a group of patients with longer standing diseases and may not represent the new JIA patient. Another limitation is that degradation of the cartilage may not appear uniformly. The degradation might affect the joint more severely on either side of the scanned area. Unlike the MRI, US cannot visualize the entire cartilage surface, which was a limitation of the study.

This first study on cartilage thickness, bone maturation and bone health shows the ongoing delayed bone maturation and decreased BHI in JIA patients. The delayed bone age is not related to a thinner cartilage, but a thicker cartilage assessed by US. Rheumatologist need to be aware of the ongoing issue with bone health in JIA patients, even in the biologic era.