Validation of the Foot and Ankle Outcome Score for Use in Infracalcaneal Heel Pain

Abstract

There is growing interest in adopting validated and reliable patient-reported outcome measures following surgery. While the Foot and Ankle Outcome Score (FAOS) has previously been validated for use in multiple foot/ankle conditions, it has not yet been validated in patients with infracalcaneal heel pain. In this study we aimed to validate the FAOS by looking at four psychometric properties of the survey: construct validity, content validity, reliability, and responsiveness, using patients in our practice with a clinical diagnosis of plantar fasciitis. A total of 150 patients (mean age 49.7 +/− 12.1 years [36 men and 114 women]) were included in one or more of the four components of this study. All FAOS subscales demonstrated adequate construct validity when compared with the physical health component of the SF-12, and 2 out of 5 subscales demonstrated moderate correlation with the mental health component of SF-12 (all Spearman rho >0.3, and p values <0.05). Most FAOS subscales demonstrated content validity and were found to contain relevant questions from the patient’s perspective. All five subscales demonstrated good test-retest reliability with intraclass correlation coefficients ≥ 0.827. Finally, four out of the 5 subscales (all but other symptoms) were responsive to change at a mean follow up of 12.2 months after surgery (p<0.05). We conclude that the FAOS is a responsive, reliable, and valid instrument for use in infracalcaneal heel pain. We believe that due to its ease of use and broad applicability, the FAOS could be more widely adopted in foot/ankle practices as patient-centered healthcare delivery and research becomes increasingly prioritized in the US and abroad.

Introduction:

Plantar fasciitis one of the most common conditions encountered by foot and ankle surgeons and accounts for over one million outpatient visits annually (1). It is estimated that approximately 10% of the population in the United States (US) will develop plantar fasciitis in their lifetime, and more than 2 million Americans experience symptoms of infracalcaneal heel pain at any one time (1–3). Each year, approximately 11–15% of professional healthcare visits to foot and ankle specialists are attributed to heel pain (4). Therefore, it is not surprising that the cost of diagnosing and treating plantar fasciitis creates a considerable economic strain on the US healthcare system (5). Despite its common and costly occurrence, patient outcomes following heel pain treatment continue to be inconsistently reported in outpatient medical records and across studies in the peer reviewed foot and ankle literature (6, 7) .

The use of validated, patient reported outcome measures (PROMs) is rapidly becoming standard practice within the surgical community both before and after surgery, and foot and ankle surgery is no exception (8). The use of PROMs better ensures that the patient remains at the center of care, and that healthcare providers address and prioritize outcomes that matter to patients. Despite being the most widely used outcome score for heel pain patients (and for all other foot/ankle ailments), the American Orthopaedic Foot and Ankle Society (AOFAS) Hindfoot scoring scale has been abandoned by its originators (the AOFAS) and is no longer recommended for use (9). Although some authors maintain there is still value in the subjective component of the AOFAS scoring scale (10), many have criticized its inability to capture pure patient reported outcome data and have raised serious concerns regarding its construct validity (11–13). In contrast, the Foot and Ankle Outcome Score (FAOS) is a readily available PROM that has been previously validated for use in multiple foot and ankle ailments, including hallux valgus, adult flatfoot, lateral ankle instability, ankle osteoarthritis, and hallux rigidus (14–18). It is an entirely self-administered, 42 item questionnaire that can be completed in less than 10 minutes.

Given its ease of administration, already broad applicability, and validation for use in multiple foot and ankle disorders, we wanted to potentially expand its use to include also infracalcaneal heel pain. The purpose of this study then was to validate the FAOS for use in patients with infracalcaneal heel pain by studying its psychometric properties, including construct validity, content validity, reliability and responsiveness in an urban-based foot/ankle specialty clinic with patients clinically diagnosed with plantar fasciitis.

Methods:

Participants:

Institutional Review Board (IRB) approval was obtained from the primary author’s university. All prospectively enrolled subjects provided informed consent and HIPAA authorization. Retrospective data was obtained after IRB-approved waiver of research consent and HIPAA authorization. Eligible patients were identified through the Institute’s database by searching for diagnosis and procedure codes associated with plantar fasciitis and plantar fascial surgery, respectively. Patients identified with diagnosis codes specific for plantar fasciitis seen within our practice between June 2019 and August 2019, were sent a HIPAA-compliant Qualtrics Survey via email after their 1^st office visit and asked to participate. The initial survey involved the FAOS, which was to be answered regarding their heel pain, and an additional SF-12 general health questionnaire. The Qualtrics Survey also had additional items so that participants could rate the relevancy of each of the 42 items on the FAOS. Data from this initial email announcement (n=58) were used in the construct validity and content validity portions of the study.

Additionally, patients with plantar fasciitis in the practice who had, in the opinion of the primary investigator, plateaued in their treatment response (i.e., same or similar numeric pain rating scores on two or more consecutive office visits) during the study time frame were asked to participate in the reliability portion of the study. Patients were asked via email to complete the FAOS with respect to their heel pain on two separate occasions, approximately 4 weeks apart (n=26). Some of these patients (n=8) also participated in the construct and content validity portion of the study.

Finally, patients within our practice are routinely asked to provide FAOS data as standard of care both preoperatively and at 10 months to 1 year postoperatively. Data from patients who had previously undergone surgery for plantar fasciitis from June 2017 to June 2018 were abstracted from the records from an additional 74 patients to assess responsiveness of the FAOS.

Foot and Ankle Outcome Score (FAOS):

The FAOS is region-specific outcome instrument compromised of 42 questions organized within 5 subscales: pain, other symptoms, activities of daily living (ADL), sports and recreational activities and foot and ankle-related quality of life (QoL). Patients respond to each question on a scale from 0 to 4, and a score for each subscale is calculated according to a unique formula designed for this particular instrument. The subscale scores range from 0 (extreme symptoms) to 100 (no symptoms).

12-Item Short Form Health Survey (SF-12):

The SF-12 is a validated and well known general health outcome instrument. This survey is self-administered and consists of 12 items and 8 domains: Physical Functioning, Role-Physical, Bodily Pain, General Health, Vitality, Social Functioning, Role- Emotional, and mental health. This is further summarized into two scores: Physical Health Component Score and Mental Health Component Score. The SF-12 served as the comparative tool in this study as it has served as the gold standard for patient-reported health outcome instruments and has been used widely in foot and ankle-related studies.

Procedures:

The FAOS was assessed for validation by looking at four psychometric properties of the survey: construct validity, content validity, reliability, and responsiveness.

Construct Validity:

Construct validity assesses the extent to which an instrument measures what it is designed to measure. This is accomplished by comparing one instrument against a previously validated instrument. The Short Form Health Survey (SF-12) was used as a reference as it has already been validated for use foot and ankle disorders. Fifty-eight patients completed both the FAOS and SF-12 for inclusion in this component of the study. A Spearman’s correlation coefficient (rho) was used to compare the FAOS against the SF-12. A correlation coefficient between 0.3 and 0.7 was considered a moderate correlation, while a value below 0.3 suggested that the two instruments were poorly related to one another (13, 19). We hypothesized that all 5 FAOS subscales would have moderate correlation with the SF-12 Physical Health Score, and that all 5 FAOS subscales would have poor correlation with the SF-12 Mental Health Components consistent with other FAOS Validation studies (14–18).

Content Validity:

Content validity measures the patient’s own perception of the relevance of each FAOS question as it pertains to their diagnosis of plantar fasciitis. The same 58 patients as above completed the “Relevance Survey” which asked them to rate each FAOS question on a Likert scale of 1–3—with 1 being “of no importance”, 2 being “of somewhat importance”, 3 being “very important”. Within each subscale the patient responses were then summed and a mean score was created. If an item had a mean score of 2 or greater it was considered to have acceptable content validity (14, 17).

Reliability:

The test-retest reliability measures the consistency of an instrument between two separate administrations. If no treatment or intervention occurs in between the two administrations, the patient should report comparable or similar responses. Twenty-six patients that had plateaued in their treatment response during the study time frame completed the FAOS on two separate occasions approximately 4 weeks apart. Intraclass correlation coefficients (ICC) was calculated with a value of 0.7 or greater considered acceptable reliability.

Responsiveness:

Responsiveness refers to the ability of an instrument to detect a change in a patient’s status following intervention (e.g., surgery). Seventy four patients who underwent surgery for plantar fasciitis were included in this component. Patients completed their postoperative FAOS surveys at a mean 12.2 months following surgery. Analyses of the FAOS scores were carried out using a paired t-test. Significance was set at P <0.05. Responsiveness was also evaluated by calculating the effect size (ES) for each FAOS subscale.

Results:

A total of 150 patients were included in one or more of the four components of this study: construct validity, content validity, reliability and responsiveness. The mean age of the population was 49.7 +/− 12.1 years (36 men and 114 women).

Construct Validity:

All FAOS subscales demonstrated adequate construct validity when compared with the physical health component of the SF-12, and 2 out of 5 subscales demonstrated moderate correlation with the mental health component of SF-12 (Table 1).

Table 1.

Construct validity between SF-12 and FAOS (n=58)^*.

SF-12 subdomain	Stat	FAOS Pain	FAOS Symptoms	FAOS ADLs	FAOS Sport/rec	FAOS QoL
Physical health component	Correlation coefficient (rho)	0.3187	0.3701	0.3908	0.5079	0.4483
	P Value	0.0392	0.0118	0.0075	0.0003	0.0018
Mental health component	Correlation coefficient (rho)	0.3495	0.2676	0.3676	0.2353	0.2709
	P Value	0.0180	0.0754	0.0124	0.1202	0.0717

^*The strength of the correlation is given by Spearman’s rho. Values greater than 0.3 suggest moderate or better correlation. Seven of the 10 comparisons including and all in the physical health component of the SF-12 showed at least moderate correlation with the FAOS.

ADL, activities of daily living; QoL, quality of life.

Content Validity:

Overall, most FAOS subscales were found to contain relevant questions from the patient’s perspective (Table 2). The subscales of: pain (2.10 +/− 0.56), other symptoms (2.06 +/− 0.45), and QoL (2.29 +/− 0.62) appeared most relevant.

Table 2.

Mean relevance scores for each for the FAOS domains (n=58)^*.

FAOS Subscale	Mean +/− sd
Pain	2.10 +/− 0.56
Symptoms	2.06 +/− 0.45
Daily Activities	1.82 +/− 0.57
Sports/Recreation	1.87 +/− 0.63
Quality of Life	2.29 +/− 0.62

^*A mean score of 2 or greater was considered acceptable content validity. Three out of the 5 subscales demonstrated satisfactory content validity.

Test-Retest Reliability:

Intraclass correlation coefficients calculated between the first and second administration of the FAOS survey can be found in Table 3. All subjects surveyed related no change in their infracalcaneal symptoms. All five subscales demonstrated good test-retest reliability with ICCs > 0.7.

Table 3.

Reliability of FAOS subscales (n=26)^*.

FAOS Subscale	Mean +/− sd First FAOS	Mean +/− sd Second FAOS	ICC (95% CI)
Pain	60.6 +/− 22.8	58.2 +/− 19.2	0.926 (0.802, 0.972)
Symptoms	61.1 +/− 20.4	57.6 +/− 20.7	0.929 (0.810, 0.973)
Daily Activities	71.9 +/− 22.8	68.7 +/− 23.3	0.892 (0.712, 0.960)
Sports/Recreation	62.8 +/− 29.7	69.4 +/− 27.3	0.896 (0.723, 0.961)
Quality of Life	46.1 +/− 23.0	40.4 +/− 20.5	0.827 (0.536, 0.935)

^*All five subscales demonstrated good test-retest reliability with intraclass correlation coefficients (ICCs) > 0.7.

Responsiveness:

Mean preoperative scores were compared to mean postoperative scores. Four out of the 5 subscales (all but other symptoms) were responsive to change after surgery (p<0.05, Table.4). Similarly, all subscales except other symptoms demonstrated acceptable effect sizes (>0.5). Based on effect sizes, pain and ADL subscales were the most responsive subscales to treatment.

Table 4.

Responsiveness of FAOS subscales (n=74)^*.

FAOS Subscale	Preoperative Score +/− sd	Postoperative Score +/− sd	P Value	ES
Pain	58.1 +/− 20.2	78.3 +/− 13.4	0.0003	1.178
Symptoms	66.9 +/− 14.5	70.8 +/− 16.6	0.2979	0.250
Daily Activities	68.0 +/− 18.7	85.9 +/− 9.0	0.0004	1.220
Sports/Recreation	60.9 +/− 29.5	82.2 +/− 17.8	0.0245	0.874
Quality of Life	33.2 +/− 26.8	57.8 +/− 25.4	0.0176	0.942

^*Responsiveness reflects whether the instrument changes as the condition changes. All FAOS subscales were responsive to change after plantar fascial surgery.

ES, effect size.

Discussion:

There has been a recent call within the foot/ankle community to begin adopting one or two validated and reliable PROMs during foot and ankle evaluations and after surgery (6, 7). The FAOS has proven to be a reliable and validated measure for multiple foot and ankle pathologies (14–18). Our study has now demonstrated acceptable construct and content validity, reliability, and responsiveness of the FAOS for use in what may be the most common foot/ankle ailment of all - infracalcaneal heel pain. We believe that due to its ease of use and broad applicability, the FAOS should be incorporated more widely into foot and ankle specialist practices in the US and abroad, especially as we increasingly prioritize patient-centered healthcare delivery and research.

Construct validity shows the ability of an instrument to measure what it was intended to measure. In comparing the FAOS to the SF-12, all of the FAOS subscales demonstrated good overall correlation with the physical health component. There was poor correlation with the mental health component with 2 out of the 5 subscales demonstrating moderate correlation which is consistent with previous studies. Perhaps the weakest performing area of the FAOS in heel pain was in its content validity (i.e., relevance) from the patient’s perspective. Although we found it to be relevant in 3 out of 5 subscales, both measures of patient function (ADLs and sports/rec.) only demonstrated a trend towards significance. As the FAOS was initially developed in the context of lateral ankle instability (where both ADLs and sports/rec. were deemed to be relevant by patients), our findings may call into question whether these two subscales can adequately play a role in assessing patient outcome in the context of heel pain. We also found acceptable test-retest reliability across all 5 subscales of the FAOS. Our findings of an ICC of 0.827 or greater for all domains was as good or better than previous validation studies. Finally, the FAOS was found to be responsive to the effects of treatment for infracalcaneal heel pain. This suggests the FAOS should be rather useful in the evaluation of effects of plantar fascia surgery as 4 out of 5 subscales demonstrated rather large effect sizes (all around 1).

This study, of course, is not without its limitations. Foremost, our findings are based on a single population of patients from one foot/ankle specialty practice. Furthermore, we looked only at patients with a clinical diagnosis of plantar fasciitis, so it will be hard to know how generalizable these findings are to patients presenting with other causes of infracalcaneal heel pain (e.g., bursitis, neoplasm, etc.). Also we used the SF-12 survey to assess for construct validity in our study, and recognize that perhaps more precise estimates of correlation would have been obtained with the use of the longer, more comprehensive SF-36 survey. Finally, our length of follow up (mean 12.2 months) was short of the 2 year follow up period that is typically recommended for most orthopedic research.

In conclusion, we found the FAOS to be a responsive, reliable, and valid instrument for use in infracalcaneal heel pain. We believe that due to its ease of use and broad applicability, the FAOS should be more widely adopted in foot/ankle practices as patient-centered healthcare delivery and research becomes increasingly prioritized in the US and abroad.