HAPPi Kneecaps! A double-blind, randomised, parallel group superiority trial investigating the effects of sHoe inserts for adolescents with patellofemoral PaIn: phase II feasibility study

Patellofemoral pain (PFP) is a common musculoskeletal condition affecting 30% of adolescents [1]. Pain is often aggravated by activities involving weightbearing on a flexed knee; for example, running, jumping, ascending and descending stairs, and squatting. Pain and disability associated with PFP in adolescents can affect participation in physical activity and sport, general and mental health, and quality of life [2‐6]. PFP is persistent, even in adolescents, with 21% of 12- to 35-year-olds still reporting pain 6 years after an initial appointment with a general practitioner [7].

There are few evidence-informed recommendations to treat PFP in adolescents and thus, interventions developed for use in adults tend to be used in adolescents, often with poorer outcomes [8, 9]. Key differences between adolescents and adults may underpin worse outcomes in adolescents. Specifically, adolescents with greater hip abduction strength have an increased risk of PFP, while there is no association in adults [10]. Adults tend to present with reduced quadriceps strength [10], while adolescents demonstrate no quadriceps deficit until 16–18 years of age [4]. The presence of bilateral symptoms also differs between groups, with 55–60% of adults reporting bilateral symptoms [11, 12] compared to 70–79% of adolescents [13]. Poorer adherence to recommended treatments, particularly exercise therapy, also likely contributes to worse outcomes in adolescents [9]. Only two randomised controlled trials (RCTs) have investigated treatments specifically for adolescents with PFP [13, 14]. Their findings indicate that exercise therapy improves outcomes in this group, with further improvements arising when soft foot orthoses were prescribed [13, 14].

Foot orthoses are inserts worn in shoes that are contoured to the shape of the foot, and were recommended to treat PFP in the 2018 consensus statement [8]. Our previous work demonstrated the effectiveness of foot orthoses in adults with PFP [11, 15]. Compared to participants allocated to flat insoles or wait and see, adults with PFP who received prefabricated foot orthoses demonstrated greater global improvement at 6 weeks [11, 15], and a faster improvement in symptom severity [11]. This is important because duration and severity of symptoms predict poor long-term prognosis for PFP [16]. Although important preliminary information about the effects of foot orthoses in 10 female adolescents with PFP was provided in a pilot study [14], a modern approach to foot orthoses prescription with a larger, more diverse adolescent population is required. To inform the conduct of a full-scale RCT evaluating foot orthoses for adolescents with PFP, a feasibility trial is needed to determine whether contemporary clinical trial methods and foot orthoses prescription are feasible in this population.

The primary objective of this study was to determine the feasibility of conducting a full-scale RCT evaluating the effects of contoured, prefabricated foot orthoses on knee pain severity and patient-perceived global change in adolescents with PFP, compared to flat insoles. The secondary objective was to describe outcomes on a range of patient-reported outcome measures.

Between August 2019 and March 2020, 260 volunteers responded to advertisements and completed online screening (Fig. 1). An additional 19 volunteers were recruited from a pre-existing database. 149 volunteers were potentially eligible after online screening. Of these, 97 did not respond to email or telephone communications and 6 declined to participate in the physical screening. 45 consented to physical screening, from which 36 were eligible and enrolled in the study. The most common reasons for ineligibility via online screening were current foot orthoses use (n = 70, 53%), previous lower limb injury or surgery (n = 29, 22%) and other sources of anterior knee pain (n = 18, 14%). The most common reason for exclusion after physical screening was the presence of other sources of anterior knee pain (Osgood Schlatter’s Disease, n = 6; patellar tendinopathy, n = 2). In March 2020, recruitment was ceased at 36 participants due to COVID-19 restrictions.

Of the 36 participants randomised, 17 participants were allocated to receive contoured foot orthoses and 19 participants were allocated to flat insoles. 13 (76%) participants from the contoured foot orthoses group and 12 (63%) participants from the flat insole group attended appointments with their physiotherapist and were fitted with their allocated intervention. The mean number of appointments attended was 2.5 (SD 0.6) (contoured foot orthoses group 2.6 (0.7); flat insole group 2.4 (0.5)), and the mean number of inserts fitted was 2.6 (0.8) (contoured foot orthoses group 2.7 (1); flat insole group 2.6 (0.7)).  The remaining 11 participants did not attend physiotherapy appointments, and did not receive their allocated intervention. 3-month outcome measures were completed by 15 (88%) participants from the contoured foot orthoses group and 11 (58%) participants from the flat insole group (72% of total cohort). Participant characteristics are presented in Table 1.

Our findings indicate that a full-scale RCT of contoured foot orthoses versus flat insoles for adolescents with PFP would not be feasible with the protocol used in this study, based on our a priori criteria to inform feasibility. Although our observed recruitment rate (1.2 participants per week) exceeded the pre-specified criterion of 1 participant per week, parameters for minimum adherence with shoe insert wear and drop-out rate were not met. Of the 15 participants (42%) who provided adherence data, only 7 (47%) met minimum adherence of 2 h per day, 5 days per week for at least 7 out of 12 weeks. At 3 months, 28% of participants had dropped out (10/36), of which 4 were related to the COVID-19 pandemic. There were only minor, transient adverses event reported, and minimal co-intervention use. Participants perceived both interventions to be credible, and results of the Credibility and Expectancy Questionnaire demonstrated success of blinding. The method of patient-reported outcome collection was feasible and acceptable in this group.

Our recruitment strategy involved recruiting adolescent volunteers from the community via advertisements, as well as contacting adolescents with PFP from a pre-existing database. Combined, this strategy led to a recruitment rate of 1.2 participants per week over 30 weeks. However, we acknowledge that a future full-scale RCT may not have access to such a database, and thus is likely to rely solely on recruitment from the community. If the 6 participants who were recruited from the pre-existing database are not considered, our recruitment rate for the remaining 30 participants was 1 participant per week. This also meets our a priori  criterion [17]. A full-scale trial in this population may need to run across multiple sites or use a broader recruitment strategy (e.g., through schools) to ensure efficient recruitment of the target sample size. Of the advertising methods used, social media was most successful, accounting for 90% of participants recruited from advertising (27/30). Future studies recruiting adolescents with PFP should consider using social media as their primary advertising method.

Our observed percentage of total volunteers entering the study (12.9%) is in line with previous PFP RCTs. Rathleff et al. [13] reported a 16.7% inclusion rate of adolescents with knee pain at screening, who were recruited through schools. Collins et al. [11] included 11.7% of adult volunteers with PFP, who were recruited through community advertising. The most common reasons for excluding volunteers were that they were already wearing foot orthoses (n = 70), had sustained a previous lower limb injury (n = 29), or had anterior knee pain from another source (e.g. patellar tendinopathy, Osgood-Schlatter disease) (n = 18). Interestingly, 97 out of 260 (37%) adolescents who responded to advertisements and completed the initial online screening form did not respond to follow-up email or telephone communications. Therefore, it is not possible to know how many of these volunteers were potentially eligible for physical screening. Our observed rate of non-response is substantially higher than the proportion of adults with PFP who did not respond to follow-up communications after volunteering for a previous study (182/1530, 11.9%) [11]. Although the reasons for this are unclear, adolescents are likely to have different considerations and preferences for communicating or engaging with researchers and barriers to participation, which need to be explored further.

We evaluated two aspects of adherence – logbook completion and time wearing the allocated inserts. 42% (15/36) of participants returned their logbooks, suggesting that data from the logbooks may not be representative of the whole group. Because the logbooks were the primary source of data relating to insert wear time, this also suggests that our insert adherence data may not be representative. 63% (5/8) of participants in the contoured foot orthoses group reported wear time that met our predefined criteria for minimum adherence for more than half of the 3-month study period, compared to 29% (2/7) of participants from the flat insole group, although average wear time was similar for both groups (4.5 hours per day, or 32 hours per week). This is lower than a previous study in older adults with patellofemoral osteoarthritis (mean age 60 years), where logbooks were completed by 69% of participants and average insert wear time was 37 hours per week [35]. Our findings are likely to overestimate insert wear, as participants who completed and returned their logbooks may be more likely to be adherent with other aspects of the study, such as wearing their allocated insert. Further research is needed to determine the most effective methods of facilitating and monitoring adherence in this population. For example, app- or web-based methods may be more acceptable to adolescents with PFP due to the high use of electronic devices in this demographic [36]. Notwithstanding this, the low adherence rates that we observed may indicate that shoe inserts alone are not a viable intervention in this population. This requires further exploration to determine the barriers and facilitators to wearing shoe inserts, as well as footwear preferences of adolescents with PFP. For example, logbook data suggests that our participants spent a substantial amount of time wearing flip-flops or similar footwear, which coincided with days of low shoe insert wear time. Alternatively, there may be other factors that affected adherence with wearing the shoe inserts, such as the COVID-19 pandemic. Participants were in lockdown (confined to their homes) during the study period and were undertaking home schooling. This likely resulted in participants substantially reducing their shoe wear time, compared to normal school attendance and sport participation.

We set an a priori drop-out rate of ≤20% to inform feasibility [17]. Overall, 28% (10/36) of participants were lost to follow up at 3 months. 31% (11/36) of participants did not attend any physiotherapy appointments, and therefore did not receive their allocated intervention. It is important to explore the reasons why these adolescents dropped out or did not attend physiotherapy. One consideration is the impact of the COVID-19 pandemic. Four participants were due to schedule their initial physiotherapy appointments when COVID-19 restrictions were coming into effect in Queensland. If these participants are not considered, then the total loss to follow-up is 17% (6/36), which is within our pre-specified feasibility criterion, and the number of participants not attending any physiotherapy appointments is 7 (19%). However, it is likely that there were other barriers to our participants attending physiotherapy appointments, such as relying on parents for transport and the geographic location of the clinics involved in our trial. These need to be addressed if a full-scale RCT is to be feasible.

There were minimal differences between groups on the Credibility and Expectancy Questionnaire. At baseline, prior to randomization, median scores for the two groups were identical or within 1 response category. After receiving the allocated intervention, the group who received flat insoles demonstrated lower scores on all items of the Credibility and Expectancy Questionnaire, although median scores for the two groups were within 1.5 categories. The similarity in scores between groups after receiving the intervention, and between baseline and follow up for item 6 (By the end of your therapy, how much improvement in your symptoms do you really feel will occur?) indicates success of blinding. No other studies have assessed credibility and expectancy outcomes in adolescents with PFP. However, success of blinding was demonstrated in a study investigating lower limb injury risk in military recruits randomized to receive contoured prefabricated foot orthoses or flat insoles [37]. This suggests that flat insoles are an appropriate comparator for foot orthoses, including for adolescents with PFP.

Our findings also indicate that adolescents with PFP have minimal adverse events associated with wearing shoe inserts. Six participants reported rubbing, blisters and foot pain associated with both types of shoe inserts, most often in the first 2 weeks of wear. However, these were minor, typically resolved in 1-2 days, and did not preclude ongoing insert wear. Similar to previous studies involving shoe inserts for knee pain [11, 15, 38], our findings indicate that contoured foot orthoses, as well as flat shoe inserts, are tolerated well by adolescents with PFP.

There are two key limitations of this study. Firstly, there was a higher drop-out rate observed in the flat insole group at 3 months (42% in flat insole vs. 12% in foot orthoses group). Half of the participants lost to follow-up in the flat insole group were due to the COVID-19 pandemic (4/19, 21%), prior to receiving their allocated inserts. We observed similar outcomes on the Credibility and Expectancy Questionnaire after participants had received their allocated shoe inserts, suggesting that there may be other factors affecting retention in this group. Secondly, only 42% of participants returned the logbook, resulting in incomplete data for adherence with the shoe inserts, use of co-interventions and pain relief, activities undertaken, and adverse reactions. Thus, it is not clear whether these data are representative of the entire cohort.

Based on a priori criteria for feasibility, our findings indicate that a full-scale RCT comparing contoured foot orthoses to flat insoles in adolescents with PFP would not be feasible using the current protocol, due to low rates of retention and adherence to the interventions. We recommend that feasibility issues are addressed prior to conducting a full-scale RCT, with protocol modifications to facilitate participant retention, logbook completion, and shoe insert wear.