Background: Ultrasound is an inexpensive method for quantifying plantar fascia thickness, especially in those with plantar fasciitis. Ultrasound has also been used to assess the effectiveness of various treatments for plantar fasciitis by comparing plantar fascia thickness before and after an intervention period. While a plantar fascia thickness over 4 mm via ultrasound has been proposed to be consistent with plantar fasciitis, some researchers believe the 4 mm plantar fascia thickness level to be a dubious guideline for diagnosing plantar fasciitis due to the lack of standardization of the measurement process for plantar fascia thickness. In particular, no universal guidelines exist on the positioning of the metatarsophalangeal (MTP) joints during the procedure and the literature also has inconsistent protocols. The purpose of this study is to investigate and compare the influence of MTP joint extension on plantar fascia thickness in healthy participants and those with unilateral plantar fasciitis. Methods: The plantar fascia thickness of forty participants (20 with unilateral plantar fasciitis and 20 control) was measured via ultrasound three times at three different MTP joint positions: 1) at rest, 2) 30° of extension from the plantar surface, and 3) maximal extension possible. Results: The plantar fascia became significantly thinner as MTP joint extension increased in both the plantar fasciitis group (p < 0.001) and the control group (p < 0.001). In the plantar fasciitis group, the involved plantar fascia was 1.2 to 1.3 mm thicker (p < 0.001) than the uninvolved side depending on the MTP joint position. In the control group, the difference in plantar fascia thickness between the two sides was less than 0.1 mm (p < 0.92) at any MTP joint position. Conclusions: MTP joint position can influence the ultrasound measurement of plantar fascia thickness. It is recommended that plantar fascia thickness measurements be performed with the toes at rest. If MTP joints must be extended, then the toes should be extended maximally and then noted to ensure subsequent ultrasound procedures are repeated. Standardizing the position of the MTP joints is not only important for attaining the most accurate thickness measurement of the plantar fascia, but is also important to researchers who use plantar fascia thickness to determine the effectiveness of various plantar fasciitis interventions. Keywords: Fasciitis, Fasciosis, Fasciopathy, Windlass, Toe dorsiflexion, Ultrasonography, Treatment * Correspondence: email@example.com School of Allied Health Professions, Loma Linda University, Loma Linda, CA 92350, USA Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Granado et al. Journal of Foot and Ankle Research (2018) 11:20 Page 2 of 7 Background The purpose of this study was to investigate the influ- The plantar fascia is a flat band of connective tissue ence of active MTP joint extension on plantar fascia residing in the sole of the foot with attachments from thickness. Since the ultrasound procedure is often the medial tubercle of the calcaneus to the proximal performed in those with plantar fasciitis, a comparison phalanges . If excessive and repetitive tensile forces between plantar fasciae in those with unilateral plantar are imposed onto the plantar fascia, presumed develop- fasciitis with healthy control participants was also con- ment of microtrauma results in a condition known as ducted in the study. More specifically, the objectives of plantar fasciitis [2, 3]. In the United States, approxi- this study were: 1) examine the changes in plantar fascia mately one million outpatient visits for plantar fasciitis thickness by MTP joint extension position and side (i.e., were made annually during 1995–2000 . It was also involved versus uninvolved) in the plantar fasciitis group; estimated that in 2007, the cost to treat plantar fasciitis 2) examine changes in plantar fascia thickness by MTP was between 192 and 376 million US dollars . Despite joint extension position and side (right versus left) in the the pervasiveness of this condition, no gold standard ex- control group; and 3) compare changes in plantar fascia ists for diagnosing plantar fasciitis, although the diagno- thickness by MTP joint extension position in the unin- sis is often made through a clinical history and physical volved side of the plantar fasciitis group with the control examination backed by imaging [6, 7]. group. We hypothesized that the thickness of the plantar Ultrasound is a widely used tool especially in conjunc- fascia would decrease in both groups studied as MTP tion with plantar fasciitis because it provides an inexpen- joint extension is increased. sive, and noninvasive method for quantifying the plantar fascia with accuracy levels comparable to magnetic res- Methods onance imaging (MRI) [8, 9]. Several studies agree that a Participants plantar fascia thickness over 4 mm via ultrasound is All participants signed an informed consent, and the consistent with plantar fasciitis [10–14]. However, some study was approved by the Loma Linda University researchers have argued that a lack of standardization in Human Research Participant Protection (HRPP) Program/ the measurement process for plantar fascia thickness Institutional Review Board (Approval No. 5150186).Partic- makes it challenging to properly validate the 4 mm ipants with plantar fasciitis were required to exhibit the reference guideline [7, 15]. One critical component for classic symptoms of plantar fasciitis (i.e., plantar heel ten- standardizing measurement procedures is a thorough derness, morning pain with the first few steps out of bed) characterization of the relationship between plantar and have had symptoms persisting longer than 6 weeks to fascia thickness and toe position. ensure participants were not in an acute phase of the con- Several studies that have demonstrated an increase in dition. Individuals were excluded from the study if they plantar fascia tension [16–18] and a rise in the medial lon- had any neurologic, systemic inflammatory, metabolic, gitudinal arch [17, 19] as a result of metatarsophalangeal connective tissue, or inner-ear disorders. Those with severe (MTP) joint extension. In addition, Garcia et al. who toe deformities, trauma/surgery to the lumbar spine or found that MTP joint extension results in an increase in lower extremities, an antalgic gait pattern, a cortisone plantar soft tissue stiffness along with a concomitant injection over the preceding three months, or recent decrease in overall plantar soft tissue thickness. Cumula- consumption of balance-altering medication were also ex- tively, these studies strongly suggest that MTP joint exten- cluded from the study. sion could influence measures of plantar fascia thickness. Yet, guidelines for positioning the MTP joint during ultra- sound measurements of the plantar fascia have not been Measurement of plantar fascia thickness developed. For example, the European Society of Muscu- Sagittal thickness of the plantar fascia was measured loskeletal Radiology has produced its procedural recom- with a 13–6 MHz linear array transducer (Sonosite mendations for how the plantar fascia thickness should be M-Turbo Ultrasound System, Bothell, WA, USA) and measured , but a recommendation for how the toes acoustic coupling gel applied onto the plantar surface of should be positioned during the examination is noticeably the heel. Participants were positioned in prone with the absent. While many ultrasound studies measuring plantar examined foot over the edge of the examination table fascia thickness either leave the toes in a resting position and the ankle in neutral. The transducer was positioned or do not even indicate the position of the toes dur- over the plantar surface of the heel approximately ing the examination, other authors have advocated ex- 0.5 cm medial to the midline longitudinal axis of the tending the toes to improve the border definition of foot in order to visualize a longitudinal view of the the plantar fascia during the procedure [11, 15, 22–24]. plantar fascia. The thickness of the plantar fascia was However, it is unclear if doing so alters the acquired ultra- then measured at the anterior margin of the calcaneus sound measurements. (Fig. 1). Granado et al. Journal of Foot and Ankle Research (2018) 11:20 Page 3 of 7 Fig. 1 Longitudinal sonogram of the plantar fascia with the thickness being measured at the anterior margin of the calcaneus The ultrasound measurement was performed with the joints were extended to 30° as measurement was not toes in three different MTP joint positions: 1) at rest, 2) necessary in the at rest or max extension positions. 30° of active extension from the plantar surface, and 3) Measurement of first MTP joint extension was per- maximal extension actively possible by the participant formed on the medial aspect of the foot with the prox- (Fig. 2). All of the toes were extended passively together imal arm of the goniometer parallel with the plantar to the desired position by the examiner whereby the par- surface of the foot and the distal arm aligned with the ticipant was then asked to actively hold the position midline of the proximal phalanx of the first toe. The while the examiner continued to monitor for any move- traditional method for measuring extension at the first ment. Goniometry was only performed when the MTP MTP joint involves aligning the proximal arm of the Fig. 2 An illustration of the three metatarsophalangeal (MTP) joint extension positions employed during the ultrasound measurement for plantar fascia thickness: (a) plantar surface of the foot; (b) at rest; (c) 30° relative to the plantar surface of the foot; (d) max extension possible Granado et al. Journal of Foot and Ankle Research (2018) 11:20 Page 4 of 7 goniometer with first metatarsal, either dorsally or over test and box plots. A 2 × 3 repeated factorial analysis of the medial surface of the foot rather than the plantar variance (ANOVA) was conducted to examine the effect surface of the foot . A modification was employed in of side (involved vs. uninvolved) and MTP joint exten- an effort to make comparisons between feet from differ- sion position (at rest vs. 30° vs. max) on plantar fascia ent individuals more reliable as outlined by Allen and thickness (mm) in the plantar fasciitis and control Gross . In the foot-flat position, the MTP joints are groups. To compare changes in plantar fascia thickness typically extended 20° from the midline of the metatar- by MTP joint extension position in the uninvolved side sals . However, it was felt that different foot types between the plantar fasciitis group and control group, could result in the first metatarsal having a variable pos- 2 × 3 mixed factorial ANOVA was used. The level of ition. The plantar surface of the foot being fixed would significance was set at p ≤ 0.05. not suffer from this inconsistency and would allow for a “standard” position in between the two other MTP joint Results positions (i.e., at rest and at max extension). When at The study involved 40 healthy participants (20 with uni- rest and at max extension, a specific joint angle was not lateral plantar fasciitis and 20 control) between the ages necessary, which was why goniometry was not per- of 18 and 65 years, all of whom had a body mass index formed. Ultrasound measurements conducted with the (BMI) below 35 kg/m . The mean ± SD age of the partic- MTP joints at rest were intended to place the least ipants was 44.8 ± 12.2 years and BMI 26.8 ± 4.5 kg/m . amount of tension onto the plantar fascia, whereas max- The majority were also females (n = 26, 65%, see Table 1). imal MTP joint extension was necessary to apply the In the plantar fasciitis group when analyzing the unin- most amount of tension. A standard MTP joint position volved and involved sides by MTP joint extension pos- during the initial and final conditions of ultrasound ition, the results from the 2 × 3 repeated ANOVA measurements would had most likely resulted in relative showed that there was a significant difference in mean ± inaccurate tissue tension in some participants (i.e., pres- SD plantar fascia thickness (mm) among the different ence of unwanted plantar fascia tension at the initial positions, at rest vs 30° vs max, (4.6 ± 0.13 vs. 4.3 ± 0.13 position or inadequate plantar fascia tension when the vs. 4.2 ± 0.13, F = 62.2, η = 0.77, p < 0.001), as well as MTP joints were extended maximally). 2,38 between involved and uninvolved side (see Table 2.) The ultrasound examinations were performed by one However, there was no significant interaction between licensed physical therapist with sixteen years of clinical side and position (F = 0.90, p = 0.41). Bonferroni post experience and who had completed several continuing 2,38 hoc comparisons showed that mean plantar fascia thick- education radiology courses. However, the examiner had ness differed significantly between at rest and 30°, at rest no previous experience in using ultrasound for imaging and max, and 30° and max (p < 0.001). purposes. Crofts et al.  demonstrated that relatively In the control group when comparing both sides by new ultrasound examiners with minimal, but structured MTP joint position, there was a significant difference in training could still acquire reliable ultrasound data. Thus mean ± SD plantar fascia thickness among the three dif- prior to the study, the examiner met with a certified ferent positions (3.4 ± 0.01 vs. 3.2 ± 0.01 vs. 3.0 ± 0.01, ultrasound technician for four instructional sessions over F = 56.1, η = 0.75, p < 0.001; see Table 2) and Bonfer- the course of one month to become familiar with the 2,38 roni post hoc comparisons showed that mean plantar equipment and technique. During the study, the exam- fascia thickness differed significantly between at rest and iner was blind to which foot was afflicted with plantar 30°, at rest and max, and 30° and max (p < 0.001). fasciitis when measuring plantar fascia thickness in However, there was no significant difference in mean ± SD participants. As well, the side first to be examined was plantar fascia thickness between right and left side randomly selected in each participant with a flip of a (3.2 ± 0.04 vs. 3.2 ± 0.03, F = 0.01, p = 0.92), and no coin. Ultrasound measurements were performed three 1,19 significant interaction between position and side times at each MTP joint position. Measuring plantar (F = 0.24, p =0.79). fascia thickness has been found to be more reliable when 2,38 the mean of three ultrasound measurements was used Table 1 Mean (SD) of general characteristics by group at baseline rather than a single measurement [22, 24, 28]. Plantar fasciitis group Control group (n = 20) (n = 20) Data analysis Female, n (%) 13 (65%) 13 (65%) Data was analyzed using SPSS Statistics Software version 24.0 (IBM Corp, Armonk, NY). Mean ± standard Age 47 (11.9) 43 (12.6) deviation (SD) was computed for quantitative variables BMI 28.3 (4.3) 25.3 (4.3) and frequencies (%) for categorical variables. Normality Abbreviations: SD standard deviation, BMI body mass index of continuous variables was assessed using Shapiro-Wilk Units: Age, years; BMI, kg/m Granado et al. Journal of Foot and Ankle Research (2018) 11:20 Page 5 of 7 Table 2 Mean (SD) plantar fascia thickness (mm) at each MTP joint position by group type Plantar fasciitis group (n = 20) Control group (n = 20) a 2 b 2 MTP Joint Extension Position Involved Uninvolved Difference (95% CI) p-value (η ) Right Left Difference (95% CI) p-value (η ) At rest 5.2 (1.1) 3.9 (0.7) 1.3 (0.8–1.8) < 0.001 (0.60) 3.4 (0.5) 3.4 (0.4) < 0.1 (0.1–0.2) 0.92 (0.00) 30° 4.9 (1.0) 3.7 (0.6) 1.2 (0.8–1.7) 3.2 (0.4) 3.2 (0.4) < 0.1 (0.08–0.1) Max 4.8 (1.0) 3.6 (0.7) 1.2 (0.7–1.8) 3.0 (0.4) 3.0 (0.4) < 0.1 (0.1–0.2) c 2 p-value (η ) < 0.001 (0.77) < 0.001 (0.75) Abbreviations: SD standard deviation, MTP metatarsophalangeal, CI confidence interval, η effect size Involved vs uninvolved Right vs left MTP joint extension position When comparing the uninvolved side from the plantar definition via ultrasound [11, 15, 22–24]. Since the fasciitis group with the average of two sides from the methodology lacks standardization, it is the recom- control group, a significant difference was found in mended that the toes be at rest during the ultrasound mean ± SD plantar fascia thickness between the two study measurement for best reproducibility and only at max groups (3.7 ± 0.04 vs. 3.2 ± 0.04, F =9.85, η = 0.21, MTP joint extension when improved visibility of the 1,38 p = 0.003). A significant difference in mean ± SD plantar fascia border is necessary. The position of the plantar fascia thickness among the three different posi- MTP joints during the procedure should always be re- tions (3.7 ± 0.06 vs. 3.5 ± 0.06 vs. 3.3 ± 0.06, F = 60.4, corded to guarantee consistent protocols are followed in 2,76 η = 0.61, p < 0.001) was also found with post hoc com- subsequent ultrasound measurements. For example, re- parisons showing mean plantar fascia thickness differed searchers studying the efficacy of a particular treatment significantly between at rest and 30°, at rest and max, intervention for plantar fasciitis would want to ensure and 30° and max (p < 0.001). However, there was no that any change in plantar fascia thickness is due to the significant interaction between position and group intervention and not because of inconsistent toe posi- (F = 0.69, p = 0.50). tioning during the ultrasound procedure. 2,76 The average plantar fascia thickness in healthy partici- Discussion pants that has been reported in the literature is between In 1993, the first report of ultrasound being used to 2.6 and 3.9 mm [8, 10, 13, 22, 31–34]. This relatively measure plantar fascia thickness was published . large range in normative values is most likely due to the Since then, ultrasound has become an important tool for discrepancies within the current methodology and par- not only visualizing the plantar fascia, but is also used to ticipant variation . For instance, Bisi-Balogun et al. assess the effectiveness of various treatments for plantar  measured the thickness along different locations of fasciitis [14, 29, 30]. However, the lack of standardization the plantar fascia and found that the mean and ± SD for measuring plantar fascia thickness with ultrasound could vary between 2.26 ± 0.4 mm to 3.06 ± 0.6 mm. may make the process more challenging by affecting the This is important because the location along the plantar accuracy of results. The main concept to ascertain from fascia where its thickness is measured has always lacked this study is that as MTP joints are actively extended, consistency in the literature [7, 15, 24, 32]. Further com- the plantar fascia decreases in thickness when observed plicating the matter is that different segments of the during ultrasound. The current practice for measuring plantar fascia are susceptible to further thickness varia- plantar fascia thickness via ultrasound does not involve a tions due to gender and body weight characteristics . standardized position for the toes. Thus, the MTP joint This has even prompted some authors to suggest com- position can vary depending on the preference of the paring the thickness of symptomatic plantar fasciae with examiner causing the thickness measurements to poten- contralateral asymptomatic feet in those with unilateral tially vary as well. Ultimately, there is some evidence to plantar fasciitis rather than compare to a standardized suggest a need to re-examine how plantar fascia thick- threshold of 4 mm . McMillan et al.  conducted a ness is measured. Based upon the results from this systematic review and meta-analysis and found that study, the MTP joint position should be standardized participants with chronic plantar fasciitis had a plantar during the ultrasound procedure, either at rest or at max fascia thickness that was about 2.2 mm more than the extension to ensure ease of reproducibility. While keep- corresponding control participants. In our study, the ing the toes at rest is most likely the easiest position to plantar fascia thickness of the involved side in the plan- replicate and should be the established position when tar fascia group was significantly higher when compared measuring plantar fascia thickness, some clinicians pre- to the uninvolved side with a difference of 1.2 to 1.3 mm fer to extend the toes in order to improve the border depending on the MTP joint position. One possible Granado et al. Journal of Foot and Ankle Research (2018) 11:20 Page 6 of 7 reason for the slightly lower difference in contrast to the cases so it is reasonable to see a slightly thicker asymp- McMillan et al. study could be because the MTP joint tomatic plantar fascia in those with unilateral plantar extension position was carefully controlled in our study fasciitis as a potential harbinger [36, 37]. and not in the other studies analyzed in the McMillan A limitation of this study was that the examiner was et al. meta-analysis. not blind to the position of the MTP joints during the When analyzing the plantar fascia thickness of those ultrasound procedure. Controlling for this potential bias in the control group, the two sides were not significantly would be a welcome addition in future studies. As well, different (Table 2). But when plantar fascia thickness of it would be of strong interest to assess how MTP joint the control group was compared to the uninvolved side extension would affect plantar fascia thickness on MRI. in the plantar fasciitis group, the uninvolved plantar Observing a similar relationship on MRI to the ultra- fascia was still significantly thicker (see Table 2). The lit- sound results in this study would further highlight the erature has been inconclusive on the difference between need for MTP joint position to be standardized during asymptomatic plantar fasciae in those with unilateral the ultrasound procedure. plantar fasciitis and healthy individuals. Some studies have reported that the asymptomatic plantar fasciae Conclusions in those with unilateral plantar fasciitis were thicker Based upon the findings from this study, the amount of [11, 35], whereas other studies have reported no signifi- MTP joint extension can strongly influence the ultra- cant difference between the two groups [10, 13]. In this sound measurement of plantar fascia thickness and study, the asymptomatic plantar fasciae were thicker than should be taken into account during the procedure. It is the controls at every MTP joint extension position (Fig. 3). recommended that plantar fascia thickness measure- The former may be an indication of either an unhealthy ments be performed with the toes at rest. If MTP joints compensatory response during gait or an inherent bio- must be extended, then the toes should be extended mechanical flaw that predisposed these individuals with maximally and then noted to ensure subsequent unilateral plantar fasciitis to have increased plantar fascia ultrasound procedures are repeated. Standardizing the thickness on the asymptomatic side. Bilateral plantar fasci- position of the MTP joints is not only important for itis has been reported to be present in 13 to 30% of the attaining the most accurate thickness measurement of Fig. 3 Mean ± SD of plantar fascia thickness (mm) by MTP joint extension position, side, and group. Abbreviations: SD, standard deviation; MTP, metatarsophalangeal Granado et al. Journal of Foot and Ankle Research (2018) 11:20 Page 7 of 7 the plantar fascia, but is also imperative to researchers 13. Akfirat M, Sen C, Günes T. Ultrasonographic appearance of the plantar fasciitis. 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Published: May 29, 2018