What do we actually know about a common cause of plantar heel pain? A scoping review of heel fat pad syndrome

We followed the Preferred Reporting Items for Systematic Reviews and Meta-analyses extension for Scoping Reviews checklist (PRISMA-ScR) [12]. This checklist contains 20 essential items plus 2 optional items. The final study protocol was prospectively registered on the Open Science Framework (https://​osf.​io/​nfyjx/​).

Types of studies eligible for review included: (1) systematic reviews, observational cohort or cross-sectional studies, retrospective electronic medical record reviews that reported prevalence and etiology of HFPS; (2) cross-sectional diagnostic accuracy studies (e.g., psychometric information on sensitivity, specificity, and positive and negative likelihood ratios) and systematic reviews that reported HFPS diagnostic criteria; (3) randomized controlled trials (RCTs), non-RCTs, quasi-experimental studies (without control group), pre vs. post intervention studies, pilot-feasibility studies, and case series/reports that reported the effect of non-pharmacological treatments.

Included participants were men and women of any age with heel fat pad pain as well as those with heel fat pad pain plus diagnoses of diabetes mellitus or rheumatoid arthritis. Exclusion criteria were cadaveric studies; animal studies; participants with heel fat pad pain plus diagnoses of acromegaly, fracture, wound, pressure sore; falanga victims; participants with a clinical diagnosis of plantar fasciopathy (defined as localized tenderness at the medial calcaneal tuberosity and morning first-step heel pain that abates after a brief period of walking); participants with unspecified plantar heel pain without differentiating between plantar fasciopathy and HFPS; biomechanical studies involving healthy participants without heel fat pad pain. We included any non-pharmacological and non-surgical interventions for HFPS, such as exercise therapy (aerobic, neuromuscular, and strength/resistance), stretching/flexibility program, movement training, manual therapy, foot orthotics, shoe insert, footwear modification/recommendation, taping, electrotherapeutics, therapeutic ultrasound, therapeutic laser, shockwave therapy, and cryotherapy. There were no restrictions on the recruitment setting (e.g., clinic, hospital, gym, school, home) in which the interventions were performed.

Using MEDLINE, Scopus, Cinahl, EMBASE, Cochrane Library, SPORTDiscus and PEDro, we conducted a comprehensive search for the prevalence, etiology, diagnostic criteria, and intervention related to HFPS on May 7^th, 2021. The search was later updated on April 4^th, 2022. To identify potential publications from registered trials, we searched ClinicalTrials.gov and the World Health Organization's International Clinical Trials Registry Platform (ICTRP) for pertinent registrations. Finally, abstracts, conference proceedings, and bibliographies in relevant systematic reviews (if available) were hand searched for possible inclusions. There was no restriction on the year of publication. However, only publications in English and in Human were included.

We searched individual text words in the title and abstract supplemented with Medical Subject Headings (MeSH) terms. Key terms for inclusion were’plantar fat pad’ OR’heel fat pad’ OR’plantar heel fat pad’ OR 'heel pad’ OR’plantar heel pad’ OR’fat pad atrophy’ OR’heel pad atrophy’ OR’heel contusion’ OR ‘heel bruise’ OR ‘rearfoot fat pad’ OR ‘rearfoot fat pad atrophy’ OR ‘rearfoot pad atrophy’ OR ‘rearfoot bruise’. Key terms for exclusion were ‘infrapatellar fat pad’ OR ‘patellofemoral pain’. Electronic search results were uploaded to Covidence for study selection and data extraction.

Using the Covidence platform, citations were de-duplicated; titles/abstracts were screened by two reviewers who independently determined eligibility for formal full-text reviews. Disagreements were resolved by discussion between the two reviewers or adjudication by a third reviewer. If the title/abstract did not provide sufficient information to determine eligibility, the full-text article was retrieved. Each full-text candidate article was assessed by two reviewers who independently determined the final inclusion for data extraction. Disagreements were resolved by discussion between the two reviewers or adjudication by a third reviewer.

Two reviewers independently extracted data using customized standardized data extraction forms. Extracted data from the pair of reviewers were compared for consistency. All inconsistencies were resolved by discussion between the two data extractors or adjudication by a third person. Study characteristics (e.g., design, setting, geographic location, year of data collection, publication year, and sample size) and participant characteristics (e.g., age, sex, BMI, disease duration) were summarized. Results were synthesized in three domains of (1) prevalence; (2) etiology and diagnostic criteria; and (3) non-pharmacological and non-surgical interventions. Because a scoping review aims to provide an overview by mapping the available evidence rather than a synthesized answer/result to a question, critical appraisal or risk of bias assessment is generally not recommended in scoping reviews [13, 14].

Two studies reported the prevalence of HFPS [4, 9]. The Feet First Study [4] interviewed and physically examined 784 multiethnic, community-dwelling older adults, aged 65 or older, at their homes in Springfield, Massachusetts, USA in 2001–2002. Approximately 4.2% [mean standard error (MSE), 2.2%] had pain and tenderness on the heel fat pad; and 6.9% (MSE, 1.1%) had pain and tenderness on the plantar fascia. The prevalence of HFPS did not differ between sexes, but was significantly higher in Hispanic/Latino than in White or African/Black Americans.

To estimate the proportion of various pathologies among those with the general diagnosis of plantar heel pain, medical records of 250 patients with plantar heel pain at the Foot Clinic of Rehabilitation Medicine in South Korea in 2008 were retrospectively analyzed [9]. HFPS was diagnosed by the following criteria: less than 3 mm heel fat pad thickness assessed by ultrasound, pain at heel center or margin, worsening pain when barefoot or after a long period of standing. Plantar fasciopathy was diagnosed by tenderness on the medial calcaneal tuberosity and an ultrasonic hypoechoic fusiform-shaped swelling ≥ 4 mm thickness at the origin of plantar fascia. In this sample, 53% had plantar fasciopathy; 15% had HFPS, 10% had pes cavus, 9% had HFPS plus plantar faciopathy, 5% had pes planus, 4% had plantar fibromatosis, 2% had plantar fascia rupture, 2% had neuropathy or small shoe syndrome. Both studies suggested that HFPS may be the second leading cause of plantar heel pain, second to plantar fasciopathy.

Four studies described diagnostic criteria or morphological/biomechanical features of HFPS [9, 15‐17]. Logistic regression models identified factors associated with the diagnosis of HFPS (vs. plantar fasciopathy) in 250 patients with plantar heel pain [9]. Worsening pain during prolonged standing, pain at night, or bilateral pain greatly increased the likelihood of HFPS (odds ratios ranging from 20.9 to 25.0), while morning first-step pain and tenderness on the medial calcaneal tuberosity substantially decreased the likelihood of HFPS (odds ratios ranging from 0.04 to 0.07).

Comparing patients with (n = 185, age = 47.0 ± 14.7 years, BMI = 27.3 ± 6.6 kg/m², 48.6% women) vs. without (n = 190, age = 42.0 ± 13.0 years, BMI = 25.6 ± 4.7 kg/m², 45.8% women) HFPS seen in a podiatry care center, Lopez-Lopez and colleagues [16] found that ultrasound-measured unloaded heel fat pad thickness was significantly lower in those with HFPS (7.23 ± 1.39 vs. 10.36 ± 1.78 mm, p = 0.001, Cohen’s d = 1.959). The reduced thickness was more pronounced in women than men. Among women, the thickness discrepancy was 7.09 ± 1.44 (with HFPS) vs. 10.13 ± 1.68 mm (without HFPS). Among men, the thickness discrepancy was 7.37 ± 1.33 vs. 8.58 ± 3.43 mm. Applying the area under the Receiver Operating Curve (ROC) for identifying optimal thickness cutpoint for predicting negative HFPS, they determined a threshold of ≥ 8.77 mm for no heel pain, with a sensitivity value of 85.5% and a specificity value of 82.2%.

Another study, however, found no relationship between heel fat pad properties (heel pad thickness, compressibility, and pressure distribution) and the presence of HFPS [15]. Heel pad properties were compared between patients with HFPS (n = 59, age = 43.9 ± 10.5 years, BMI = 29.0 ± 5.5 kg/m², 66.1% women) vs. younger healthy medical students with a lower BMI and lower proportion of women (n = 47, age = 23.5 ± 2.4 years, BMI = 22.5 ± 3.1 kg/m², 44.7% women). Heel pad thickness in loaded and unloaded conditions were measured by radiographs. Unloaded heel pad thickness was 20.45 ± 2.89 mm (HFPS) vs. 19.55 ± 2.52 (healthy); loaded heel pad thickness was 14.02 ± 3.38 mm (HFPS) vs. 11.81 ± 2.84 (healthy). Heel pad compressibility, defined as the ratio of loaded to unloaded thickness, did not differ between groups (0.69 ± 0.14 in HFPS vs. 0.60 ± 0.11 in healthy). Peak barefoot heel plantar pressures during normal-speed walking were quantified using a pressure-recording platform embedded in the walkway. No between-group differences in peak pressure were observed (28.40 ± 6.96 N/cm² vs. 31.70 ± 6.36).

In a case series of 9 patients with HFPS (age = 31 ± 8.5 years, sex and BMI unreported), ultrasound and/or MRI detected pathological heel fat pad morphologies were qualitatively described, including atrophy, fibrosis, edema, and defects in the fat pad septa with fluid in the surrounding tissues [17]. Quantitatively by ultrasound, the unloaded and loaded heel fat pad thickness was 19.8 ± 2.9 mm and 12.3 ± 2.9 mm, respectively; the compressibility index was 0.60 ± 0.09.

Our systematic search only identified two interventional studies that examined the effect of non-pharmacological, non-surgical management. One was a single case report and the other was a quasi-experimental intervention. A 33-year-old man with bilateral HFPS were treated with silicone gel heel cups [18]. Right heel pad stiffness measured by shear wave elastography decreased from 65.5 kPa to 51.2 (32-day follow-up) and to 40.7 (102-day follow-up). Similar stiffness reduction was observed in the left heel pad. Objective heel pad stiffness improvement was accompanied by subjective pain relief from 10 to 3 (32-day follow-up) and to 1 (102-day follow-up) on a 0–10 visual analogue scale.

Chae and colleagues [19] examined the immediate effect of low-dye taping (LDT) and that of low-dye + figure-of-8 taping (LDT +) on heel pain and peak ambulatory hindfoot plantar pressure in 19 participants with HFPS (n = 32 feet, age = 51.5 ± 14.1 years, BMI = 22.6 ± 2.6 kg/m², 73.7% women). It was unclear if participants were randomized into each taping group or if a cross-over design was used. No control group was included in this study design. Both types of taping significantly reduced pain [6.5 ± 1.7 (barefoot without taping), 4.2 ± 1.1 (LDT), and 3.5 ± 1.3 (LDT +) on a 0–10 visual analogue scale] and peak hindfoot pressure during walking [29.3 ± 11.9 (barefoot without taping), 26.3 ± 8.8 (LDT), 23.2 ± 7.0 (LDT +)]. There was no association between change of pain and change of peak pressure.

There was one prospective population-based cohort study (The Feet First Study) [4] published nearly 20 years ago. It showed that HFPS is a common foot problem in US community dwelling older adults with a 4.2% prevalence rate, second to plantar fasciopathy (6.9% prevalence rate). This finding was echoed by a retrospective record review of patients diagnosed with plantar heel pain in South Korea [9], in which 53% had plantar fasciopathy and 15% had HFPS. Given that HFPS is a distinct pathology and may be the second most frequent cause of plantar heel pain, it is unfortunate that many investigators combined participants with HFPS and plantar fasciopathy in their study sample. This oversight may have contributed to a lack of evidence-based treatment options for this condition. To our knowledge, this is the first systematic overview of research literature in HFPS. Study findings underscore the severely limited high-quality original research for HFPS. The current conservative treatment recommendations of this condition are mostly anecdotal. Future clinical trials with robust study designs are greatly needed.

Damage or irritation to the heel fat pad could be caused by acute trauma (e.g., a single high-impact landing) or chronic overuse (e.g., repetitive or excessive jumping, running, or walking on hard surfaces). Fat pad atrophy are often associated with aging, rheumatological conditions, diabetes, or obesity. An accurate diagnosis of HFPS is critical for timely management. Based on Yi and colleagues’ work [9], we have summarized key differentiating pain characteristics and behaviors between HFPS and plantar fasciopathy [9] in a table of Fig. 2. Study findings [9] from the logistic regression models for identifying clinical characteristics associated with increased likelihood of HFPS or plantar fasciopathy could have been strengthened by further adjusting for potential confounders, such as age, sex, and BMI, and reporting 95% confidence interval of odds ratios.

In addition to clinical presentations, ultrasonography and magnetic resonance images (MRIs) have been used to corroborate and confirm the diagnosis of HFPS. Ultrasound quantified fat pad thickness ≤ 9 mm may be predicative of HFPS [16]. Contrarily, another study reported no thickness difference between HFPS and healthy controls [15]. An important caveat of this null finding—in this observational case–control study, the disparate age, BMI, and sex distribution between the case and control groups were unaccounted for in statistical analyses, potentially biasing study findings. The wide range (12–29 mm) of heel fat pad thickness in healthy adults [22‐24] may have complicated the effort to benchmark fat pad thickness as a diagnostic criterion. According to the Physical Stress Theory [25], thicker fat pad may be present in taller/heavier person or in endurance athletes who frequently load their heel. Healthy men had thicker fat pad than women [26, 27]. BMI or sex may need to be factored into fat pad thickness interpretation of these studies. Some studies [15, 17] computed fat pad compressibility index, operationalized as the ratio of loaded to unloaded thickness, to ascertain tissue stiffness. A lack of standardized compressive force applied by the ultrasound probe to simulate loaded condition precludes across-study comparisons. Although less economical, practical, and convenient that ultrasonography, MRI could provide more nuanced morphological assessment and qualitatively characterize heel fat pad atrophy, fibrosis, edema, and septal defects [17].

Viscoelastic heel cups and arch taping have been recommended for conservative management of HFPS [8, 10, 11]. Alarmingly, we could not find even a single RCT to substantiate the efficacy of these treatment strategies for HFPS. In contrast, we identified two recent systematic reviews and meta-analyses on the efficacy of foot orthoses for treating plantar fasciopathy [28, 29]. Low-dye arch taping has been shown to reduce the first-step pain in high-quality clinical trials and supported by clinical experts and patients as a reasonable first-line management for symptoms during weightbearing activities [30]. Our search yielded 2 interventional studies for HFPS [18, 19]: a single case and a quasi-experimental study. In a 33-year-old man with bilateral HFPS, heel pain decreased after 1-month and 3-month application of silicone gel heel cups [18]. Low-dye taping and low-dye plus figure-of-eight taping provided pain relief by 2 to 3 points on a 0–10 numeric pain rating scale when compared to barefoot walking [19]. In the absence of a control group, the change in self-reported pain observed in this study could be a result of regression-to-the-mean phenomenon [31] or placebo effects [32]. Our review brings to light the dire need for RCTs with scientific rigor to support evidence-based recommendations in conservative management of HFPS.

To our knowledge, this is the first scoping review of HFPS, comprising a comprehensive search of the prevalence, etiology and diagnostic criteria, and non-pharmacological, non-surgical management. This comprehensive and systemic scoping review closely followed the PRISMA-ScR guideline and checklist for conducting and reporting a clear, robust review. We broadly searched 9 databases and grey literatures and included all types of study designs and all years of publication. Despite our best effort and carefully defined inclusion/exclusion criteria, only 7 studies were included in the final data, which limits our ability to categorically synthesize evidence. Subpar scientific rigor in some of the included studies also dampens our confidence in part of the summarized evidence. We only included studies of people with confirmed HFPS, leaving out studies of heel fat pad biomechanics and morphologies in healthy individuals. Study participants with diagnosed plantar fasciopathy or plantar heel pain were excluded as well.