Chronic plantar heel pain (CPHP) is a generalised term used to describe a range of undifferentiated conditions affecting the plantar heel. Clinical features are typically described as chronic pain beneath the heel, made worse by weight-bearing after prolonged periods of rest 1 . Plantar fasciitis is reported as the most common cause of CPHP 2 and the terms are frequently used interchangeably in the literature 3 . CPHP is also associated with inflammatory conditions such as spondyloarthritis 4 , though the majority of cases are unrelated to systemic disease 5 .

The epidemiology of CPHP in the general population is currently uncertain. An Australian population-based study involving 3,206 randomly selected participants has reported a heel pain prevalence of 3.6% 6 . American studies estimate that 7% of older adults report tenderness beneath the heel 7 , and that 1 million physician consultations per year are for the diagnosis and treatment of plantar fasciitis 8 . Plantar fasciitis is also estimated to account for approximately 8% of all running-related injuries 9 10 .

Diagnostic imaging has been used by many researchers and practitioners to investigate the involvement of specific anatomical structures in CPHP. Imaging types used include ultrasonography and magnetic resonance imaging (MRI) for investigation of soft tissue structures (e.g. the plantar fascia) and plain film x-rays for bone abnormalities (e.g. heel spur). These observations help to explain the underlying pathology of the disorder, and are of benefit in forming an accurate diagnosis and targeted treatment plan. Additionally, these studies provide objective criteria by which to measure the effect of current and future treatments.

At the time of writing, one published article had attempted to critically review diagnostic imaging studies for plantar fasciitis 11 . However, this review had a broad scope including both assessment and treatment studies, and did not include all available diagnostic imaging research. Furthermore, the article presented only a limited overview of study findings and did not investigate other potential causes of CPHP (e.g. heel spur). Therefore, the objective of this systematic review was to investigate all diagnostic imaging features associated with CPHP and evaluate study findings by meta-analysis where appropriate.

A total 764 citations were identified by the database search process (Additional file 3), of which 23 studies were included in the review (Additional file 4). The exclusion grounds for articles rejected after full-text assessment are available in Additional file 5.

Quality index scores ranged from 29 to 80% (mean = 55%) demonstrating moderate overall methodological quality (Additional file 6). The majority of studies provided inadequate descriptions of control group source populations and characteristics, and only eight studies 12 13 20 21 22 23 24 25 applied a blinding technique to the image assessor. Additionally, 11 studies 12 13 22 23 24 26 27 28 29 30 31 included data from both feet of control participants, and 11 studies 12 13 21 22 24 26 27 28 29 31 32 included data from both feet of participants with bilateral heel pain. As statistical tests assume that each data point represents a truly independent observation, inclusion of both feet may result in an artificially inflated sample size and decreased data variability, thereby increasing the risk of Type I error 33 . Despite these limitations, most studies reported predetermined outcome variables, and clearly described imaging equipment settings and measurement techniques.

The objective of this systematic review was to investigate the diagnostic imaging features associated with CPHP and evaluate study findings by meta-analysis where appropriate. The majority of studies provided inadequate descriptions of control group characteristics and only a small proportion applied a blinding technique to the image observer. Furthermore, by including data from both feet of participants, the sample sizes of approximately half the studies were inappropriately inflated. While the clinical signs and symptoms of CPHP participants were similarly described across the studies, various diagnostic terms have been used, including plantar fasciitis, painful heel syndrome and inferior calcaneal spur syndrome.

The studies included in this review describe a fusiform thickening of the plantar fascia close to the calcaneal enthesis, with associated fluid collection and increased vascularity. These findings suggest that many patients with chronic pain beneath the heel are likely to have plantar fasciitis, and that changes in the thickness of the plantar fascia may be particularly useful in diagnosing the condition. For example, plantar fascia thickness values greater than 4.0 mm have previously been used to form a case definition in plantar fasciitis research 23 . This reference value is supported by the data analysis of this review, though the threshold value of 4.0 mm relates only to measurement by ultrasonography.

Plantar fascia thickness values have also been used to measure the effect of treatments. For example, corticosteroid injection has been shown to significantly reduce plantar fascia thickness as early as two weeks 42 and one month 20 following treatment. Additionally, one of these studies 20 reports a statistically significant correlation between decreased plantar fascia thickness and improvement in symptoms (Pearson r = 0.61, P < 0.001). The intra-rater reliability of measuring plantar fascia thickness by ultrasonography has been reported to be very good, with the 95% limits of agreement ranging from -0.7 mm to 0.5 mm 43 . However, the reliability of this technique has not been examined in detail.

In addition to fascia thickening, areas of hypoechogenicity within the proximal plantar fascia have also been commonly reported and are strongly associated with CPHP. Sonographic studies have attributed this feature to the presence of underlying reparative processes, with associated fibre deterioration and tissue oedema 29 36 . Evidence from histopathological studies in plantar fasciitis support this view, with increased mucoid ground substance, collagen degeneration and angiofibroblastic hyperplasia the most commonly reported features 44 . However, markers of persistent inflammation such as lymphocyte and macrophage infiltration have been less frequently reported in the condition 44 . This suggests that plantar fasciitis may follow a similar pathological pathway to that of tendinopathy, where tissue changes are thought to proceed from an early reactive phase to progressive degeneration 44 45 . Imaging studies in tendinopathy provide evidence of similarity between these conditions, in which tendon appears thickened with focal areas of hypoechogenicity and increased vascularity 45 46 . Furthermore, tendon has been found to respond to corticosteroid injection in a similar way to the plantar fascia, with one study reporting a significant reduction in tendon diameter as early as one week following treatment 47 . Future longitudinal research investigating the imaging features and histology of plantar fasciits would be of great value, as direct evidence for pathological change over time is currently lacking. This concept has particular relevance to the management of CPHP, as future interventions may be selected according to condition chronicity.

The role of subcalcaneal spur in the pathogenesis of CPHP has been questioned in musculoskeletal medicine for several decades 39 44 . The basis of this uncertainty is the reportedly high prevalence of subcalcaneal spur in the asymptomatic population 38 , leading to an emerging view that the finding has limited diagnostic value 2 . However, comparisons to asymptomatic control groups in the statistical analysis of this review (odds ratio) demonstrate a strong association between CPHP and the presence of subcalcaneal spur. Inconsistencies in the association between spur formation and heel pain have not been adequately investigated, but possible explanations include variations in spur length (i.e. longer spurs may be more symptomatic) 39 and concurrent fat pad abnormalities 21 24 .

As the majority of studies investigating the presence of subcalcaneal spur used plain film x-ray, the precise relationships between spur formation and surrounding soft tissue were not reported. Nonetheless, one study described the location of spurs as being closely associated with the abductor hallucis and flexor digitorum brevis origins 21 . This finding is consistent with evidence from cadaveric research, in which subcalcaneal spurs are reported to most commonly occur immediately deep to the plantar fascia enthesis 48 .

The formation of subcalcaneal spur has traditionally been attributed to repetitive longitudinal traction of the plantar fascia 39 , with subsequent inflammation and reactive ossification 49 . However, recent histological and clinical studies suggest that vertical compressive forces may play a more important role 48 49 . Histological evidence shows that: spur formation can occur in loose connective tissue, surrounding fibrocartilage may not be aligned with the direction of traction, and spur trabeculae commonly forms perpendicular to its long axis 48 . Additionally, clinical studies have shown that spur development is unrelated to medial arch height 49 and can occur after surgical release of the plantar fascia 50 .

In the clinical management of CPHP, diagnostic imaging can provide objective information by which to either confirm or question the diagnosis of plantar fasciitis. This information can be particularly useful in cases that do not respond to first-line interventions, or when considering more invasive treatments (e.g. corticosteroid injection). The presence of a subcalcaneal spur in patients with CPHP is also likely to be an important finding, though a causal relationship has not been established. Further research involving the use of MRI and histological techniques is required to better define the role of spur formation and related bony abnormalities in the development of CPHP.

While this review was designed to be as comprehensive as possible, it is feasible that some studies that may have been suitable were not identified. In addition, as this review only included studies in which comparisons were made to asymptomatic control groups, many case-series studies and individual case reports have not been included. Therefore, the findings of this review are not exhaustive and do not describe all imaging features associated with CPHP.

This systematic review has identified 23 studies investigating the diagnostic imaging appearance of the plantar fascia and inferior calcaneum in people with CPHP. Analysis of these studies found that people with CPHP are likely to have a thickened plantar fascia with associated fluid collection, and that thickness values >4.0 mm are diagnostic of plantar fasciitis. Additionally, subcalcaneal spur formation is strongly associated with pain beneath the heel.

HBM and KBL are Editor-in-Chief and Deputy Editor-in-Chief, respectively, of the Journal of Foot and Ankle Research. It is journal policy that editors are removed from the peer review and editorial decision making processes for papers they have co-authored.

AMM led and designed the review, carried out searches and eligibility checks, extracted study data and performed the quality assessment, evaluated the appropriateness of pooling data, performed the meta-analyses, interpreted the findings and drafted the manuscript. KBL assisted in designing the review, evaluated the appropriateness of pooling data, assisted in the interpretation of findings and commented on the draft manuscript. JTB extracted study data, performed the quality assessment and commented on the draft manuscript. HBM assisted in the interpretation of findings and commented on the draft manuscript. ARB assisted in the review process and commented on the draft manuscript.

All authors read and approved the final manuscript.