Foot Orthotics with Morton's Extension doesn't Affects the Forces inside First Metatarsophalangeal Joint in Osteoarthritis Pathology.

Rigid Morton’s extension (ME) are a kind of orthotics that have been used as conservative treatments of hallux rigidus (HR) named osteoarthritis, but only their effects on rst metatarsophalangeal joint (MPJ) mobility and position in healthy subjects have been studied, but not on its applied forces neither in HR subjects. This study sought to understand how ME orthotics with three different thicknesses could inuence the kinetic rst MPJ, measured dorsally using the Jack maneuver and comparing subjects with normal rst MPJ mobility versus those with HR. We aimed to clarify whether tension values were different between healthy and HR subjects. Methods


Introduction
Osteoarthritis of the rst metatarsophalangeal joint (MPJ) is a pathological condition referred to as Cotterill,3 hallux exus, hallux equinus, or hallux rigidus (HR). 37 HR is the most common presentation of osteoarthritis in the foot, with an incidence of ~2.5% in people older than 50 years of age. 8 The main symptoms are pain with an active or passive load under manual dorsal and plantar mobilization of the rst MPJ or during the heel off-phase of the gait cycle or pain related to impingement of the medial branch of the super cial peroneal nerve from the dorsal osteophyte, as well as cartilage destruction and restricted joint mobility. 2 HR could disturb the normal gait cycle and thus affect other structures of the body, such as the lower back and hip. 7 If this pathological status is not addressed, surgery will eventually be required to improve the symptoms and restore mobility. 5 Although most literature reviews have shown that non-surgery interventions cannot stop the degeneration progress of HR in the rst MPJ, 11 non-surgical management of symptomatic HR has been suggested as an earlystage (0-2) palliative solution 41 . Non-steroidal anti-in ammatory drugs, ultrasound therapy, shoe modi cations, hallux strapping, and rigid insoles have been identi ed as the best options to reduce clinical pain 4,15,41,42 , with a 60% success rate 16 . These rigid insoles [with a modi cation on the rst ray, which was also described as Morton's Extension (ME) 7,16 ] have been used in orthopaedics to treat restrictive pathologies like symptomatic HR. MEs are rectangular pieces of semi-rigid material (of varying thicknesses) that are placed under the insoles around the rst MPJ. Morton 13 was the rst author to argue for rst-ray alteration as an aetiology of overload disease under the second metatarsal bone, but it was Ebisui 13 and Kelso 21 who detected the relationship between the rst-ray dorsi exed position in the sagittal plane and the rst MPJ's restrictive dorsi exion motion and Dananberg 6 who related its biomechanical consequences.
The Windlass mechanism has been described as a spring system formed by a cable that is attached to a fat plantar pad and calcaneus bone on one end and the proximal phalanx of the hallux base and the rst metatarsal head on the other. This cable is the plantar aponeurosis and -under normal conditions -stabilises the medial arch of the foot during the gait cycle. The Windlass mechanism also rises and shortens the medial arch through the rst MPJ's dorsi exion during the heel off-phase of the gait cycle 18 . When the rst MPJ's mobility is restricted by soft tissue structures or bone alterations 26,35 , this Windlass mechanism is altered, thereby affecting the normal propulsion of the body. One of these bone alterations is metatarsus primus elevatus 24,31 , where the rst metatarsal bone takes an elevated position in the sagittal plane relative to the second metatarsal bone and to the oor. In this way, simulated restriction of the rst MPJ's dorsi exion with a 4-or 8-mm acrylic platform under the rst ray (e.g., a ME) was already demonstrated, using a classical goniometer, in healthy participants 36 . However, it remains unclear if the rst metatarsal bone's action, which leads to ME-induced metatarsus primus elevatus, would have the same reducing effects in subjects with the rst MPJ restriction pathology. Also, mobility assessment of the rst MPJ is not the only approach to assess the biomechanical function of the foot; kinetic parameters can also be useful 22 . Given this, the Jack's test describes a passive, static, weight-bearing resting position (WRP) to assess the mobility of the rst MPJ, thereby simulating the push-off phase of the gait cycle executing a simulated Windlass mechanism 18 , pulling the hallux in the dorsal direction passively with the subject in a WRP until the movement stops 10,19 . However, the pull tension necessary to perform this test under different ME speci cations has not been studied. A lot of pathologies of the locomotive system don't show any mobility and/or visual restrictions, cause biomechanical forces moments don't always have kinematics behaviour but also kinetics effects 9 . By these reasons authors think that tension values can represent better than mobility values what occurs inside the joint.
Therefore, the purpose of this research was to know the effects of three different ME insoles on the pulled tension values that were required to perform simulated dorsi exion of the rst MPJ, executing validated 38 Jack's test, in subjects with normal and restricted ranges of motion of the rst MPJ (i.e., HR). Also, this study sought to compare the tension values of healthy and HR subjects during the Jack's test without any orthotic element. Knowing these force-inside-joint alterations, the orthotics could be recommended to avoid overload inside the joint. The hypothesis was that there was difference in tension values between subjects with HR and those with normal dorsi exion mobility of the rst MPJ during the Jack's test with or without any of the ME insoles.

Methods
Study designA case-control study was carried out between January 2021 and March 2021, following the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) requirements 44 . This research was approved by the Institutional Review Board of a private hospital a liated with the authors in October 2020; the device used in the present research is non-dangerous and non-invasive. All legal permissions were obtained. Informed consent and data protection act forms were signed by each participant. The standards of the Helsinki declaration regarding human experimentation were respected. ParticipantsThe research case group associated with HR consisted of participants who met the following inclusion criteria: 1) restricted rst MPJ-assisted dorsal mobility, according to a validated active range of motion with the subject in a non-weight-bearing test, below 75°4 0 ; 2) restricted non-weight-bearing-assisted plantar exion of the rst MPJ under 35°4 0 ; 3) pain during active and passive plantar exion and dorsi exion of the rst MPJ 28 ; 4) no trauma or injury in the lower limbs and feet; 5) normal range of motion in the subtalar joint (30°), midtarsal joint (15° along the longitudinal axis), and ankle joint (at least 20° of dorsi exion with the knee fully extended); and 6) age between 30 and 60 years old. Subjects were excluded if they were under the effects of any drugs or had any hypermobility condition (e.g., ligamentous hyperlaxity). The control group consisted of healthy, age-matched subjects.

Measurement procedures, instruments, and variables
To set the rst metatarsal bone in the dorsi exion position, at insoles with ME thicknesses of 2, 4, and 8 mm 36 were made, in 45° shore-A hardness, of ethylene-vinyl acetate (EVA) (Fig. 1), adjusted to the size of the subjects' feet and incorporated randomly to the right foot for each measurement and for each subject. The ME was a rectangular piece of EVA that was also placed inside of the insoles under the area of the rst MPJ. Three measurements were made for each condition to determine consistency. To avoid any imbalance, the same at insole in the contralateral foot was placed.
To assess the effects of the four WRPs and three ME thicknesses on the rst MPJ, a digital algometer (FPX® 25, Wagner Instruments®, Greenwich CT, USA) with a rigid strip anchored to the iron hook's extremity was used. This device had a 10 × 0.01 kgf (kilogram-force) capacity/graduation and an accuracy of 0.3% of the full scale. Previous studies have reported good reliability and validity for this device (intra-rater reliability: 0.895, 95%CI = 0.846-0.928; SEM = 2.36, MDC = 6.55) 20 . In the static WRP, the proximal phalanx of the hallux was pulled to its maximal dorsal position until foot showed supination movement on rearfoot Helbing´s sign 30,33 , by an experienced clinician (RS-G), transmitting the tension needed to perform the Jack´s test 19 through the rigid strip anchored to the algometer (Fig. 2).

Sample size
The sample size was calculated using software from the Epidemiology Unit of Biostatistics (www. sterra.com) to detect differences in the kgf applied to the rst MPJ during the Jack's test between the case and control groups and between the different MEs. Previous measures in healthy subjects have shown that the mean strength with the 8-mm insole was 3.2 ± 0.7 kgf (mean ± SD) (personal observations). In another similar study, ten healthy subjects were recruited 36 . According to these data, we needed to include at least 46 subjects (23 in the control group and 23 in the case group) to detect a difference in the mean strength of 0.7 kgf using Student's t-test for independent samples with 80% power, in a bilateral contrast, and α = 0.05. Taking into account that some subjects could be lost to follow-up, we established a nal sample of 60 subjects (30 per group).

Statistical methods
To validate the reliability across the measurement trials, the intra-class correlation coe cients (ICCs) 25 were evaluated according to the speci cations of Landis and Koch: coe cients less than 0.20 represent a slight agreement, between 0.20 and 0.40 fair reliability, between 0.41 and 0.60 moderate reliability, between 0.61 and 0.80 substantial reliability, and between 0.81 and 1.00 almost perfect reliability. Coe cients of 0.90 or larger re ect su cient reliability given that reliability coe cients exceeding 0.90 increase the likelihood that a measure is also reasonably valid 32 .
All the continuous data were studied for normality using the Kolmogorov-Smirnov test; normal distributions were noted for p-values > 0.05. Independent Student's t-tests were used to determine if there were signi cant differences between the case and control groups under the WRP and three continuous variables used in the study. Similarly, ANOVA was used to test if there were signi cant differences in the applied tension values between the different conditions. Tukey's test was used for post-hoc comparisons. The Spearman rank correlation coe cient was used to determine the correlation between the thickness of the ME insoles and the effect on the pulled applied tension. We present each descriptive summary as mean ± SD. For all the analyses, we considered pvalues < 0.05 (within a 95% con dence interval) as statistically signi cant. We analysed the data using SPSS software, version 19.0 (SPSS Science, Chicago, IL, USA).

Results
A total of 58 subjects (34 females and 24 males) participated in the study; 28 subjects were recruited to the control group and 30 subjects were included in the case group (Fig. 3).
Representation of participants' recruitment IMPJ = rst metatarsophalangeal joint; n = population The sociodemographic characteristics of the case and control groups are shown in Table 1. The homogeneity of the four measured physical characteristics [weight, height, foot size, and body mass index (BMI)] guaranteed the applicability of the results to the sample. The distribution was normal (p > 0.05).
The reliability of the variables followed perfect ICC criteria and ranged from 0.963 to 0.989 (Table 2). According to our obtained values (Table 2), the control group required almost 1 kgf less effort than the case group to move the MPJ dorsally under the 4-mm ME [4.122 ± 0.162 kgf in the case group vs. 3.325 ± 0.139 kgf in the control group under WRP (p < 0.001); 4.211 ± 0.116 kgf in the case group vs. 3.538 ± 0.123 kgf in the control group under a 4mm ME (p < 0.001)]. The differences were smaller for the 2-mm ME: 4.139 ± 0.142 kgf in the case group vs. 3.421 ± 0.133 kgf in the control group (p < 0.001) (Fig. 4). Nevertheless, in the case group, the WRP and the different ME insoles had similar pulled tension values, which ranged from 4.122 ± 0.16 kgf in the WRP to 4.211 ± 0.116 kgf in the 4-mm ME condition (not statistically signi cantly different, p > 0.05); the differences were smaller with the 2and 8-mm MEs (4.139 ± 0.142 kgf with a 2-mm ME and 4.179 ± 0.126 kgf with an 8-mm ME) ( Table 2). For the controls, the WRP and different ME insole conditions showed similar pulled tension values, which ranged from 3.325 ± 0.139 kgf in the WRP to 3.538 ± 0.123 kgf with the 4-mm-thick ME; the 8-mm-thick ME (3.465 ± 0.134 kgf) and 2-mm-thick ME (3.421 ± 0.133kgf) values were quite similar (Table 2) (p > 0.05). These data are shown in Figure 4, where it is possible to see the differences in tension values between the groups.
Spearman's Rho correlations between the ME thickness and the amount of pulled joint tension were not statistically signi cant for either group (case, p = 0.715; control, p = 0.481) (Fig. 5).

Discussion
Rigid MEs have been used as a conservative treatment for the rst stages of HR 41 and their effects over the rst ray have been studied with respect to the ME position 36 , but not their forces applied or developed with it. This research presents, with a similar piece to ME, highly valid data over an excellent homogeneous sample and reaches useful conclusions that are relevant to clinical practice.
First MPJ is a rolling joint regulated by rotational equilibrium theory described before 23 in which kinetic and kinematics forces are present in the different phases of the human gait. Taking into account that this study is not about the rst MPJ's mobility or position but instead about pulled, our results do not agree with those of some other studies, in which the authors argued that the dorsal rst MPJ's mobility was in uenced by the position of the rst ray 13,21,36 .
The hypothesis proposed at the beginning of the present study could not be con rmed according to our results because there was no statistically signi cant difference between the applied pulled tension in the Jack's test for the case or control groups, regardless of the ME thickness. Also, the differences that were detected were very small. In our study, the pulling force applied on the rst MPJ during the measurements did not show any proportional correlation with the ME thickness, as opposed to the results of Roukis et al. 36 , who showed a 19.3% incremental restriction on the rst MPJ's mobility in proportion with 4-mm rst-ray simulated dorsi exion. Our surprising results could mean that following reach the joint stop movement through ME´s thickness, the pulling force needed to perform the Jack´s test was the same, regardless of external ME´s restrictions. The total amount of dorsal mobility of the rst ray in WRP reported by previous studies was set at 4.9mm 17 , which is higher than that achieved in the ankle dorsi exion position; this is due to the increased tension on the plantar aponeurosis related to the Windlass mechanism 12,18 as it has been shown previously by the intrinsic correlation between rst MPJ and the triceps surae 27,39 . The ME thicknesses used in our experiment were 2, 4, and 8 mm; in our results, the 4-mm ME orthotic produced the greatest tension effort on the rst MPJ, which could be compared with the mobility results of Grady 17 , but without any statistical signi cance. This is likely due to "arti cial dorsal-opening" of the rst MPJ through the ME´s effects under the rst ray, and therefore this orthotic solution could be used to avoid pain inside the joint, pushing away the phalanx and metatarsal dorsal surfaces during the push-off phase, but this ME had no effects on the kinetic data, as shown in our research, because the exerted tension was enough to produce the needed dorsi exion of the rst MPJ. Further dynamic research is now needed to clarify if the present data could be applied to functional gait and if our kinetics results would be similar to the results of examining the kinematics variables under similar conditions. Nevertheless, according to our data, we could hypothesize that the case group had more di culty achieving peak mobility in the Jack's test, as shown by the greater force values applied, than in the control group, regardless of the ME's thickness. As expected, this is in accordance with the eld's current knowledge about the mobility of the rst MPJ 6,14,36 . Grebing et al. 17 detected a decrease of the rst-ray simulated dorsi exion when comparing healthy versus rst MPJ arthrodesis subjects, which explains the increase we observed in pulled force in the HR group, compared to the healthy control group.
There are controlling orthoses for hyper-pronated feet 29 , and these have been shown to restore the mobility of rst MPJs with restricted dynamic mobility (named functional hallux limitus) at the 5-month follow-up. It is also possible to improve this mobility in real time using cut-out orthoses 1 ; nevertheless, the objective of the present research was to assess the tension values of the ME on a totally restricted rst MPJ, not just dynamic-functional restriction. Moreover, Reina et al. 34 showed no statistical difference in the X-Ray IMA and HAV-angle values between custom-made foot orthoses and no orthoses in subjects with HAV, indicating that kinematics data are not always related to kinetics values, which is in line with our results.

Limitations
The present device had a 10 × 0.01 kgf capacity/graduation and an accuracy of 0.3% of the full scale; furthermore, the small effect size throughout the results showed between the WRP and MEs inside each control and case group are in line with another comparative kinetic and kinematics study with small effect sizes between the case and control groups 43 . Therefore, the reported values should be considered with caution. This is a novel force-kinetic study related to pulled tension and did not focus on the rst MPJ's mobility or position; therefore, further investigations are needed to be able to make comparisons with these results. Also, further dynamics measurements will be required to verify the ME effects discovered in the present simulated research. In addition, future research with X-Ray assessments to correlate the elevation of rst metatarsal bone with ME with ME and how it changes forces of dorsi exion, could be interesting. There is no reliable method for determining the nal position of the proximal phalanx of the hallux during the Jack manoeuvre. Future research will be needed to clarify this issue and improve the Windlass mechanism test.

Conclusion
The orthopaedic use of rigid ME as a palliative treatment for HR has been studied regarding mobility, but not force-kinetic effects. In the present study, we showed that with the use of similar MEs, the tension values detected during the simulated toe-off phase of the gait cycle (i.e., the Jack's test) in healthy individuals and subjects with HR had no correlation with the ME's thickness. Although we were able to con rm that performing the Jack's test in individuals with HR required higher kgf tension values than in healthy individuals, our data showed that the prescription of ME orthoses didn't affect to tension forces inside the rst MPJ and its prescription can be made carefree of joint damage.   Regarding to subjects data or something relative to the their privacy, is not applicable on the present manuscritp Availability of data and material All sources data and worn material to develop the present research are available under editors and reviewers requirements. Please contact author for data requests.

Competing interests
At the end of the text, under a subheading "Con ict of interest statement" all authors must disclose any nancial and personal relationships with other people or organisations that could inappropriately in uence (bias) their work. Examples of potential con icts of interest include employment, consultancies, stock ownership, honoraria, paid expert testimony, patent applications/registrations, and grants or other funding. The authors declare that they have no competing interests Funding All sources of funding should be declared as an acknowledgement at the end of the text. Authors should declare the role of study sponsors, if any, in the study design, in the collection, analysis and interpretation of data; in the writing of the manuscript; and in the decision to submit the manuscript for publication. If the study sponsors had no such involvement, the authors should state this. The authors declare that they have no funding source. Extensions insoles thickness between cases and control groups.   Difference in pulled joint tension applied (kgf) between the cases group and the control group. Mean + SD data between cases (red lines) and controls (blue lines) groups. It is showed the clear difference in highest´s values of cases (hallux rigidus) group