Orthopaedic Simulation of a Morton’s Extension to Test the Effect on Plantar Pressures of Each Metatarsal Head in Patients without Deformity: A Pre-Post-Test Study

Background: the area beneath the metatarsal heads is a common location of foot pain, which is often associated with high plantar pressures. The aim of this study was to determine the effect of the application of a Morton’s extension on the pressure in the metatarsal bones of the foot using a pressure platform. Methods: twenty-five subjects without musculoskeletal pathology were selected for this study, and an experiment was conducted with them as the subjects, before and after application of a Morton’s extension. The foot regions were divided into the forefoot (transversely subdivided into six areas corresponding to the first, second, third, fourth, and fifth metatarsal heads, and the hallux), midfoot, and rearfoot, and then the maximum and average pressures exerted at each region were measured before and after placing a Morton’s extension. Main findings: we found a pressure reduction, with a p-value less than (p < 0.05), in the head of the second and third metatarsals in statics and dynamics. Conclusions: we can conclude that the Morton’s extension produces a variation in plantar pressures on the lesser metatarsals. The application of a Morton’s extension may be beneficial for the management of forefoot pathology. This study will help clinicians consider various tools to treat forefoot disorders. NCT05879094 (ClinicalTrial.gov (accessed on 18 May 2023)).


Introduction
Central metatarsalgia, one of the most prevalent issues in orthopaedic clinical practice, refers to a painful condition localised in the plantar forefoot region between the second and fourth metatarsal heads and/or the metatarsophalangeal joint [1]. Approximately 10% of the general population has experienced some form of pain in the metatarsal region during their lifetime, with most cases occurring in females [2]. This condition should not be considered a distinct diagnosis; it is a symptom with several potential contributing factors [2]. It can arise from various underlying disorders but is most commonly linked to mechanical alterations of the forefoot, such as claw toe, insufficiency of the great toe, or hallux valgus [3].
Additional primary causes of metatarsalgia include a disproportionate length of the second or third metatarsal, congenital deformities of the metatarsal heads, tightness in the gastrocnemius muscles or triceps, fixed equinus of the foot, pes cavus, and any hindfoot abnormality that leads to excessive loading of the forefoot [4].
(measured in kPa) following stretching, decreasing from 106.24 ± 20.89 to 87.45 ± 22.28 (p = 0.004) [21,22]. Therefore, employing the sample size calculation for the related samples difference test (same group before and after an intervention), with a two-tailed hypothesis at a statistical confidence level of 95%, an alpha error probability of 0.05, and a power of 0.80, a sample size of 18 subjects was determined.
Considering the potential for attrition, 25 subjects were enlisted for the study. The sample was assembled using a consecutive sampling method, employing a successive and non-randomised simple approach.
Participation selection and inclusion criteria were as follows: (1) no history of trauma to the foot; (2) the presence of at least 10 • of dorsiflexion at the ankle with the knee fully dorsiflexed; (3) unrestricted motion of the functional subtalar joint of 30 • ; (4) unrestricted motion along the longitudinal axis of the midtarsal joint of 15 • ; (5) unrestricted non-weightbearing motion of the first ray of at least 8 mm; (6) greater than 50 • of dorsiflexion of the hallux to the first metatarsal bisection during non-weight-bearing; (7) age greater than 18 years and younger than 60 years; (8) at the time of data collection, no lower limb dysfunction or chronic injury; and (9) no evidence of a non-fixed deformity at the first MTP and first metatarsal cuneiform joints.

Ethical Considerations
This research received approval from the ethics committee of CEIC Hospital Clínico San Carlos (code: 20/235-E). Prior to the commencement of the study, all participants provided their informed consent by signing the relevant form. The principles outlined in the Helsinki Declaration and all human experimentation regulations were duly adhered to [23].

Procedure
A foot pressure measurement device (FOOT TECHNOLOGY, S.L. Barcelona, España, www.medicimage.info (accessed on 12 February 2020) was employed to assess pressure distribution across the sole. The device possessed the following specifications: an active area of 400 × 400 mm, dimensions of 475 × 450 × 25 mm, thickness of 4 mm, pressure range from 10 kPa to 1200 kPa, and a frequency of 300 MHz. This instrument was equipped with 6024 sensors.
Static foot pressure was assessed by positioning the tips of both feet in alignment with the vertical and horizontal lines of the foot pressure measuring plate. Participants were instructed to face forward, avoid wearing shoes, and assume a double-leg stance on the sensor plate for 25 s [24]. This process was repeated for three measurement trials [25].
The walking measurement procedure adhered to the approach advocated by Palisanol et al. [26]. Participants commenced from a designated starting position, marked where their fifth step engaged the sensor plate. Walking was performed barefoot at a natural, comfortable pace. Each trial required the entire foot length to be captured on the sensor plate to ensure accurate data collection. After two preliminary trials for each lower limb to familiarise participants, three valid trials were subsequently recorded for each leg alternatingly [25].
Adhering to a consistent protocol administered by the same specialist across all study participants, a 5-mm EVA Morton's extension was uniformly affixed from the first metatarsal head to the proximal phalanx of the hallux [15]. A paper bandage secured this component in place [27].
For pressure analysis, the foot regions were segmented into distinct areas. The forefoot was transversely subdivided into six regions corresponding to the first, second, third, fourth, and fifth metatarsal heads, and the hallux. Additionally, the midfoot and rearfoot were considered ( Figure 1). Subsequently, measurements were taken for the maximum, average, and minimum pressures exerted on each region. Furthermore, the static foot pressure distribution ratio was calculated.
Adhering to a consistent protocol administered by the same specialist across all study participants, a 5-mm EVA Morton's extension was uniformly affixed from the first metatarsal head to the proximal phalanx of the hallux [15]. A paper bandage secured this component in place [27].
For pressure analysis, the foot regions were segmented into distinct areas. The forefoot was transversely subdivided into six regions corresponding to the first, second, third, fourth, and fifth metatarsal heads, and the hallux. Additionally, the midfoot and rearfoot were considered ( Figure 1). Subsequently, measurements were taken for the maximum, average, and minimum pressures exerted on each region. Furthermore, the static foot pressure distribution ratio was calculated.  (8) hallux. The circle with the cross indicates the area of greatest pressure of the plantar footprint. From lowest to May, the pressures are coloured in dark blue, light blue, green, yellow, and the maximum pressure is coloured in red.

Statistical Analysis
All the data underwent normality testing utilising the Kolmogorov-Smirnov test. The data are deemed to follow a normal distribution when p > 0.05. Descriptive statistical analysis presented results as mean ± SD and a 95% confidence interval. To compare demographic variables between male and female participants, an independent t-test was employed. Additionally, paired t-tests were executed to evaluate potential systematic differences in gait parameters.
For each intrasession test, the intraclass correlation coefficient (ICC) was evaluated using the classification proposed by Landis and Koch. ICC values ranging between 0.20 and 0.40 were deemed to indicate reasonable reliability. Scores falling between 0.40 and 0.60 denoted moderate reliability, while those in the range of 0.60 to 0.80 represented considerable reliability. The highest category encompassed scores between 0.80 and 1.00, characterised as nearly perfect. Some authors suggest that an ICC value of at least 0.75 is necessary to establish reliability [28]. According to the recommendations of Portney and Watkins, clinical measurements with reliability coefficients surpassing 0.90 enhance the likelihood of the measurement being valid [29].
Coefficients of variation (CV) were computed to compare parameters directly in absolute terms. The CV was utilised to describe the connection between the mean magnitude and each studied variable's variability.
The standard error of measurement (SEM) was used for each variable studied, and, for its best interpretation, it was expressed as a percentage of the mean (SEM%), (138) as  (8) hallux. The circle with the cross indicates the area of the greatest pressure of the plantar footprint. From lowest to May, the pressures are coloured in dark blue, light blue, green, yellow, and the maximum pressure is coloured in red.

Statistical Analysis
All the data underwent normality testing utilising the Kolmogorov-Smirnov test. The data are deemed to follow a normal distribution when p > 0.05. Descriptive statistical analysis presented results as mean ± SD and a 95% confidence interval. To compare demographic variables between male and female participants, an independent t-test was employed. Additionally, paired t-tests were executed to evaluate potential systematic differences in gait parameters.
For each intrasession test, the intraclass correlation coefficient (ICC) was evaluated using the classification proposed by Landis and Koch. ICC values ranging between 0.20 and 0.40 were deemed to indicate reasonable reliability. Scores falling between 0.40 and 0.60 denoted moderate reliability, while those in the range of 0.60 to 0.80 represented considerable reliability. The highest category encompassed scores between 0.80 and 1.00, characterised as nearly perfect. Some authors suggest that an ICC value of at least 0.75 is necessary to establish reliability [28]. According to the recommendations of Portney and Watkins, clinical measurements with reliability coefficients surpassing 0.90 enhance the likelihood of the measurement being valid [29].
Coefficients of variation (CV) were computed to compare parameters directly in absolute terms. The CV was utilised to describe the connection between the mean magnitude and each studied variable's variability.
The standard error of measurement (SEM) was used for each variable studied, and, for its best interpretation, it was expressed as a percentage of the mean (SEM%), (138) as follows: SEM is derived from the ICC and DS: SEM = DS × sqrt (1 − ICC), and SEM % = SEM/mean × 100%.
The minimum detectable change (MDC) was also determined. MDC refers to the extent of variation in the value of each scale below which a change can be interpreted as originating from the inherent variability of the assessment method itself, rather than an actual alteration in the patient's clinical condition. The MDC was computed using a standardised mean (95% MDC), (139), following this formula: MDC is derived from SEM, where MDC = 1.96 × SEM × sqrt (MDC%). Statistical significance was established for p-values < 0.05. Finally, normality values were defined for the sample under study, encompassing all the variables obtained. These values were derived from VN = Mean ± 1.96 × SD.
All statistical analyses were conducted using SPSS 17.0 (SPSS Inc., Chicago, IL, USA). The measurement data collected before and after the implementation of the Morton's extension were processed, considering both static and dynamic aspects within a repeatedmeasures design. Statistical significance was established at a p value of <0.05, and a confidence interval of 95% was utilised.

Results
The study included a total of 25 participants, comprising 9 men and 16 women. The participants' ages ranged from 18 to 60 years, with a mean age of 40.36 years and a standard deviation of 15.41 years. The participants had a mean body mass index (BMI) of 24.67 kg/m 2 , with a standard deviation of 5.59 kg/m 2 . Comparing between men and women, there were no significant differences in age and BMI. However, significant differences were observed in weight and height between the two groups. A detailed overview of the participants' demographic and anthropometric characteristics is displayed in Table 1. Abbreviations: BMI, body mass index; Kg, kilograms; CM, centimetres; SD, standard deviation; CI, confidence interval. The student's t-test for independent samples was applied. In all analyses, p < 0.05 (with a 95% confidence interval) was considered statistically significant. Table 2 shows the results of the intrasession reliability analysis for the right foot and left foot of the variables studied in static and dynamic modes without a Morton's extension and with a Morton's extension. The results show a statistically significant relationship, better in static variables than in dynamic variables. Scheme 1 shows the comparative histogram of the maximum plantar pressures obtained at each metatarsal head. The light grey colour represents the plantar pressures with a Morton's extension and dark grey without a Morton's extension. In this variable, the second-to-fourth metatarsal heads in both feet are lower with a Morton's extension. However, in the first metatarsal head, the maximum pressure without a Morton's extension is higher, with a very observable difference graphically, much more than in the rest of the variables. Significantly, it can be observed, a significant reduction in plantar pressures (p < 0.05) in the head of the second metatarsal right, with a mean of 125. 35     A representative image of a plantar pressures platform of one participant before and after orthopaedic simulation is shown in Figure 2.
A representative image of a plantar pressures platform of one participant before and after orthopaedic simulation is shown in Figure 2

Discussion
This study aimed to analyse the impact of the Morton's extension application on variations in plantar pressures in the forefoot. The main goal was to ascertain if applying a 5 mm Morton's extension could effectively decrease pressures on the lesser metatarsals.
The key finding revealed that the Morton's extension reduced peak pressure in the forefoot compared with the control condition.
The central finding of this study is that the Morton's extension exerts a direct influence on forefoot pressures, particularly in the hallux, first, second, and third metatarsals, compared with control pressures (without a Morton's extension). Our hypothesis is corroborated by previous research, where diverse therapeutic strategies such as applying a metatarsal bar while walking [27] and running [8], along with orthoses designed with a medial arch support [30], were shown to intervene directly in reducing the pressures under investigation. The Morton's extension diminishes the pressure beneath the second and third metatarsal heads by redistributing the pressure load to neighbouring tissues. This phenomenon potentially enhances sensory input from deep pressure receptors (largefibre afferents) in the adjacent tissue, suggesting a possible modulation of metatarsalgia according to the gate-control theory [31,32]. A study by Hodge et al. determined that only 32% of pain variance in metatarsalgia was attributed to average pressure beneath the metatarsal head.
Consequently, it was hypothesised that pain sensation might contribute to this observation [33]. Similarly, Postema et al. observed that subjects experiencing pain preferred custom-moulded insoles [34], which alleviated metatarsal head loading and harmonised plantar pressures [12,35]. Thus, insoles may mitigate forefoot issues [36]. We identified an increase in pressure at the head of the first metatarsal, both in static and dynamic conditions. Plantar pressures arise from the vertical force applied during walking, divided by the contact area. This assumes the sensors' spatial resolution was sufficient and that the plantar surface maintained full contact with the measurement platform [36]. Morton's various studies describe a connection between hypermobility of the first tarsometatarsal joint (TMTJ1) and foot disorders [37]. Consequently, the adjacent metatarsals and the longitudinal arch are affected due to this instability in the first ray. Therefore, we posit that stabilising the first metatarsophalangeal joint directly influences plantar pressures on the lesser metatarsals, reducing them.
In the existing literature, no prior evidence exists regarding using the Morton's extension to assess its impact on variations in plantar pressures. Consequently, comparing the outcomes of our study with those of other researchers proves challenging. Nonetheless, our methodology stands as a significant strength.
The Morton's extension material chosen for this study is among the most commonly employed in the design of plantar orthoses [38][39][40][41]. The measurements conducted have been repeated extensively to ensure the reliability of the analytical data. Specifically, each of the 25 study participants underwent 24 analyses. The selection algorithm for the eight designated areas for plantar pressure analysis was determined based on prior research [40]. Although previous studies did not subdivide the five metatarsal heads [41], Deschamps et al., in a reliability study concerning plantar pressure measurements, concluded that, while precision is imperative, reliability improves when conducted by a single observer [42]. In line with the findings of Van der Leeden et al., an average of three readings was also chosen for the present study, as they demonstrated that a minimum of three measurements yielded a consistent average [5].
Our findings highlight variations dependent on the specific foot under analysis. These outcomes align with those discovered by Yoon et al., who proposed significant disparities in forefoot pressure between the left and right feet when comparing pressures before and after the placement of a metatarsal bar [27]. This observation prompts us to treat the data distinctly for each foot. Another question that piqued our interest was whether the use of the EXM induced pressure variability in the midfoot and rearfoot. Earlier investigations indicate that employing a metatarsal bar increases the contact area of the forefoot [41,43]. These results correspond with findings in other studies where weight distribution is believed to reduce plantar pressures across the entire contact area [44]. In contrast to our results, where the application of the EXM did not yield significant changes in the contact area, we speculate that pressures in the midfoot and rearfoot remained unchanged.
Our findings have several implications; utilising real-time plantar pressure measurements before and after placing the Morton's extension can be a valuable tool for monitoring treatment in clinical settings. Nonetheless, this study does possess some limitations. The Morton's extension remained consistent in material, thickness, and hardness throughout the study. Body mass index was considered, as the sample primarily consisted of individuals categorised as normal weight or overweight, akin to other studies [21,45,46]. Nevertheless, further investigations are warranted, especially encompassing obesity, as effects may diverge. Similarly, studies differentiated by gender are also necessary, given the potential differences between men and women. Additionally, it is worth noting that the observed effects on plantar pressures could be better understood over time through a longitudinal study design. This study effectively accomplishes its research objectives by highlighting the immediate impacts of the orthopaedic Morton's extension application. However, forthcoming research is imperative to validate any potential long-term effects.

Conclusions
In summary, the Morton's extension induces alterations in plantar pressures. Specifically, in static conditions, it leads to heightened pressure in the first metatarsal and the hallux. Conversely, a noteworthy reduction in pressure is observed in the second, third, and fourth metatarsals. Notably, the fifth metatarsal, midfoot, and rearfoot remain unaffected.
In the dynamic aspect, we also observe elevated pressure in the first metatarsal and the hallux. Notably, there is a marked decrease in pressure across the second and third metatarsals. Conversely, pressures in the fourth and fifth metatarsals, and the midfoot and rearfoot remain unaltered, demonstrating no significant changes.
We propose that the application of the Morton's extension for individuals with metatarsalgia should be tailored and fine-tuned based on ongoing monitoring of plantar pressure measurements.
Tailored insoles incorporating the Morton's extension can potentially mitigate plantar pressure on the second and third metatarsal bones. By reducing metatarsal head loading and the balanced redistribution of plantar pressures, such insoles may effectively alleviate metatarsalgia within these metatarsal regions, both in static and dynamic conditions.
Within clinical practice, numerous patients express concerns about pain or deformities in the forefoot. Introducing adjustments to footwear and implementing insoles or orthoses can offer substantial benefits in addressing such forefoot issues. This study serves as a valuable resource for clinicians, equipping them to explore a range of strategies for managing forefoot disorders.