Patents of Systems and Methods Using Non-Ionizing Radiation for Measuring Rearfoot Deformations: A Review

Abstract

Individuals’ lifestyles are affected by valgus and varus deformities in the rearfoot, causing pain in the joints and plantar surface due to the misalignment between the tibial and calcaneus. In orthopedics, medical professionals measure this misalignment by using X-ray systems and goniometers. The X-ray emits ionizing radiation that can cause damage through cumulative exposure over a lifetime, whereas the goniometer will produce measurement errors. This patent review conducted a technological search of systems and methods across various databases using inclusion and exclusion criteria. These thirty-five obtained patents provide valuable information about mechanical, electronic, and mechatronic technologies and non-ionizing radiation to evaluate valgus and varus deformities. The patents are classified into stationary mechanisms, stationary electronic devices, dynamic mechanisms, dynamic electronic devices, stationary mechatronic devices, and dynamic mechatronic devices. They are further categorized based on their measurement methods as either visual or automatic. Additionally, the patents are grouped by usage mode into sitting, standing, and walking. This patent review aims to provide medical professionals with little-known techniques for measuring and evaluating the rearfoot alignment.

1. Introduction

Foot deformities are prevalent issues within the population, often leading to pain and significantly impacting individuals’ life quality. In the fields of orthopedics and podiatry, the foot is anatomically divided into three sections such as the rearfoot, midfoot, and forefoot [1,2]. In these anatomic sections, the load distribution is measured to identify areas with excessive load on the surface plantar. Rearfoot tilt is one of the leading factors, which causes overload in different zones and injuries in persons with diabetic feet. This is caused by misalignment between the tibia and calcaneus, impacting ankle stability and increasing the risk of developing an ankle sprain [3]. This misalignment can result in two specific deformities commonly known in orthopedics as varus and valgus. These deformities can be caused by conditions such as cerebral palsy, overweight, or inappropriate positioning during birth [4,5,6]. The varus and valgus assessment consists of measuring the angle $\left(\theta \right)$ formed between the midline of the tibia and the midline of the calcaneus. According to research [7,8], considering the calcaneus midline and the right leg as references, a rearfoot valgus is defined by an angle, denoted as $\theta$, within the range of $\theta \ge 4^{\circ }$ measured clockwise as shown in Figure 1a. In a rearfoot varus the angle $\theta$ within the range of $\theta \ge 4^{\circ }$ measured counterclockwise, as shown in Figure 1c. On the other hand, a normal/rectus rearfoot is considered when the angle $\theta$ is within a range between $0^{\circ }\le \theta \le 4^{\circ }$ clockwise or counterclockwise, as shown in Figure 1b. In orthopedics, the varus and valgus measurements are performed by using X-rays and instruments such as the goniometer to classify the rearfoot [9,10,11].

X-rays produce radiographs of the bone structure to identify anthropometric points and measure angles with a reliable degree of precision. These X-ray systems involve ionizing radiation, which has the potential to damage tissues and increase the risk of diseases such as cancer, cataracts, and skin burns due to accumulated exposure over a lifetime [12,13,14]. Medical professionals also use instruments like the goniometer, which enables the measurement of anthropometric lines marked through a palpation technique, allowing the orthopedist to quantify the tilt of the calcaneus without ionizing radiation. This instrument demonstrates reduced precision given its reliance on the medical professional’s expertise [15,16,17].

On the other hand, there are other systems, such as 3D scanners based on laser or photometry, which capture the three-dimensional shape of the foot to analyze biomechanical behavior during walking or standing [9,10]. However, these systems are not specifically designed for measuring calcaneus tilt. They require capturing a 3D point cloud and applying 3D reconstruction methods to create a digital model and perform measurements. This procedure increases the time required for measurements. Engineering advancements exist designed to measure the tilt of the rearfoot specifically; nonetheless, these innovations remain underexploited within the field of orthopedics due to limited familiarity. These are mechanisms and electronic devices that employ non-ionizing radiation to measure the tilt of the calcaneus. They can be stationary or worn on the human body, enabling the evaluation of rearfoot valgus and varus during walking and standing. The mechanisms generally consist of an indicator that reflects degrees, allowing for visual measurement of calcaneus tilt. In contrast, electronic devices are worn on the body and can automatically measure calcaneus tilt. Moreover, electronic devices can be stationary systems, i.e., systems based on cameras to detect reference marks previously placed by medical professionals. They typically employ algorithms for detecting specific wavelengths and locating the centroid coordinates of the marks to achieve better accuracy in calculating the angle formed by the marks [18,19]. Although the mechanisms and electronic devices presented above are non-ionizing radiation systems, they are not widely known by medical professionals because they are world patents primarily led by the United States, followed by China and Russia. These systems measure the tilt of the calcaneus providing an appropriate treatment to prevent other complications such as sprains, joint pain, or plantar surface issues [20,21]. Additionally, the measurement system enables the identification of other types of pathologies, such as the association between varus rearfoot and arch height, and the relationship between valgus rearfoot and decreased arch height. These analyses can be used to evaluate conditions such as cavus or flat feet, which are deformities in the midfoot and cause impairments that affect the lifestyle [22,23,24].

This worldwide patent review provides a concise summary and classification of mechanisms, electronic devices, and mechatronic devices for non-ionizing radiation, which can be stationaries or dynamics to measure the tilt calcaneus and evaluate the valgus and varus rearfoot deformities. The systems are categorized into stationary mechanisms, stationary electronic devices, dynamic mechanisms, dynamic electronic devices, stationary mechatronic devices, and dynamic mechatronic devices. This categorization aims to provide medical professionals with information about the features of systems used to evaluate valgus and varus rearfoot.

2. Methodology

The methodology of this patent review involves conducting a targeted search of systems and methods used for measuring the tilt of the rearfoot, utilizing databases such as SCOPUS [25], Google Patents [26], SIGA [27], WIPO [28], and Spacenet [29]. This comprehensive review was conducted in February 2023 by seven experts with doctoral degrees and extensive experience in research related to metrology and measurement systems. The seven author collaborate in the exhaustive revision; however, the first author was responsible in the final decision to include the patents of this manuscript.

2.1. Classification

The International Patent Classification of the World Intellectual Property Organization (WIPO) was used for a better search of patents related to rearfoot tilt measurement systems. The classification provided by WIPO is standardized for all countries, allowing the determination of the field and subfields that best describe the invention, as well as its application area. In this case, the A61B 5/107 classification was used for patent search. This classification is shown in Figure 2.

2.2. Keywords

The advanced search tool was used in the databases applying keywords and boolean operators to narrow down the technological search: (Rearfoot OR Measurements) AND (Valgus OR Varus) AND (Hindfoot) AND (Systems OR Methods) AND (lower limb) AND (alignment).

2.3. Inclusion Criteria

The inclusion criteria encompass patents filed from the year 2000 onward, focusing on systems and methods specifically related to rearfoot alignment measurements. Patents in various statuses such as active, granted, abandoned, or expired are eligible for inclusion. The selection process is guided by the keywords outlined in Section 2.2, ensuring a targeted and systematic approach to identifying relevant patents. This approach aims to capture a comprehensive range of innovations and developments within the specified timeframe and subject area.

2.4. Exclusion Criteria

As previously mentioned in the preceding section, certain patents have been excluded from this review because they focus on systems related to hallux valgus and exclude the rearfoot deformities. Additionally, patents involving ionizing radiation systems, patents issued before 2000 are excluded from this review. In

Table 1. Excluded patents based on exclusion criteria.

Item	Title	Number	Country	Cite
A	US4062355	Device for use in evaluating the lower leg and foot	1977	[30]
B	US5470310	Modular night splint	1995	[31]
C	US6093163	Device for hallux valgus	2000	[32]
D	US10782181B2	Radiograph stand with weigh scale	2018	[33]
E	US4911177	Dynamic sagittal knee test apparatus	1990	[34]
F	CN116439687A	Hallux valgus intelligent rehabilitation recognition system and angle measurer	2023	[35]
G	JP2017192614A	Measuring instrument of hallux valgus angle	2016	[36]
H	JP2006006394A	Hallux valgus angle measuring tool, and hang tag and storage box having the function	2006	[37]

, patents A, B, and E are excluded, as their publication year is before 2000. The patents C, F, G and H are excluded as they employ systems for measuring hallux valgus deformity, which involves the deviation of the big toe towards the outside and the inner side of the first metatarsal. Finally, patent D applies X-rays to generate radiographs, which are produced using ionizing radiation. Using keywords in the six consulted databases, 130 patents were obtained. The exhaustive review and application of exclusion criteria narrowed the search to 35 patents, as illustrated in

Table 2. Obtained patents applying the methodology (Part A).

Item	Title	No. Patent	Country	Publication Year	Cite
1	Indication device	WO2019201403	France	2019	[38]
2	Measurement system or varus and valgus angles in feet	US8914988	United States	2014	[39]
3	Foot morphometric measuring device	US20160073931	United States	2016	[40]
4	Malalignment syndrome diagnosis apparatus based on plantar pressure and body movement and method thereof	KR101902551B1	South Korea	2018	[41]
5	Orthopedic director of hoof varus foot osteotomy	CN110215270B	China	2020	[42]
6	Method and system for collection of foot geometry data	US20170079559	United States	2017	[43]
7	System and method for foot classification	US7789840	United States	2010	[44]
8	Indicator for measuring foot movement relative to adjacent bodily structure	DK201670105	Denmark	2016	[45]
9	Device for Measurement of Ankle Joint and Foot Sizes	RU02325108	Russia	2006	[46]
10	Device for a foot	US7100296	United States	2004	[47]
11	Foot measuring device	CN214856709	China	2021	[48]
12	Gait training exercise and analysis systems for body support systems with adjustable user body weight force axis	US10376734B1	United States	2019	[49]
13	Foot tilt angle measuring method, method of selecting shoe or insole for shoe method of manufacturing shoe or insole for shoe, and foot unit tilt angle measuring device	US7325323	United States	2008	[50]

,

Table 3. Obtained patents applying the methodology (Part B).

Item	Title	No. Patent	Country	Publication Year	Cite
14	Correcting foot alignment	US7069665	United States	2006	[51]
15	System and method for foot assessment	US7582064	United States	2008	[52]
16	Footwear customization system and process	US6170177B1	United States	2001	[53]
17	Front and rear foot plane angle measuring instrument	CN215305878	China	2021	[54]
18	Foot varus angle detection modeling method and system	CN113658707	China	2021	[55]
19	System for foot assessment including a device and method	US20040193075	United States	2004	[56]
20	Method and apparatus for manufacturing custom orthotic footbeds	US7392559	United States	2006	[57]
21	Foot problem detection device for detecting inward turning, outward turning and gait of heel, insole and shoe	CN209862489	China	2019	[58]
22	Apparatus and method for imaging feet	US8567081B2	United States	2012	[59]
23	Method for diagnosing human feet condition	RU2814368C1	Russia	2024	[60]
24	Gait analysis system and method	CN106805980A	China	2017	[61]
25	Portable foot varus and valgus detection device	CN217659860U	China	2022	[62]
26	Orthopedic leg alignment system and method	AU2017228417B2	Australia	2022	[63]
27	Sensorized knee arthroplasty utilizing an intraoperative sensor system	US20240041619A1	France	2024	[64]
28	Device and method for knee ligament strain measurement	US7976482B2	United States	2009	[65]

and

Table 4. Obtained patents applying the methodology (Part C).

Item	Title	No. Patent	Country	Publication Year	Cite
29	Portable multifunctional ruler capable of measuring various limb related parameters and measuring method	CN112674757A	China	2021	[66]
30	System and Method For Determining Tibial Rotation	US20080208081A1	United States	2008	[67]
31	Device for measuring lateral stability of knee joint during flexion and extension	CN219183755U	China	2022	[68]
32	Smart knee fixture and system for measuring knee balancing	US20150328032A1	United States	2015	[69]
33	Device and method for hip-knee-ankle angle verification and femoral mechanical axis digitization	US9554745B2	United States	2017	[70]
34	Apparatus and method for joint characterization and treatments	US11000382B1	United States	2021	[71]
35	Device for checking stability and activity of subtalar joint	CN116421149A	China	2023	[72]

. Patents were excluded because they lacked relevant information; some did not focus on measuring the tilt of the rearfoot, while others were duplicates. The methodology used is shown in Figure 3.

3. Results

Among the 35 patents obtained, the distribution by country is as follows: the United States accounts for 51.4%, followed by China with 28.6%, Russia and France with 5.7%, and South Korea, Denmark and Australia each representing 2.9%. In the patents are specified: title, patent number, publication year, and references. Additionally, this review provides a concise overview of the most relevant patents categorized by year and classification into Stationary Mechanisms, Stationary Electronic Devices, Dynamic Mechanisms, Dynamic Electronic Devices, Stationary Mechatronic Devices, and Dynamic Mechatronic Devices.

3.1. Brief Description of the Most Relevant Patents by Year

A brief description is provided for the most relevant patents by year, with a particular focus on those addressing the direct measurement of rearfoot valgus and varus angles in subjects. This description explains the operation of the different systems and methods applied for measuring the tilt of the rearfoot and identifies the valgus and varus.

• Footwear customization system and process
  (US6170177B1), United States, 2001
  This system employs positioned goniometers at the rearfoot and forefoot to measure alignment and evaluates the valgus or varus of the rearfoot and forefoot using encoders that capture the rotation of these segments. The collected data using a computer program is then used to create a personalized midsole to correct the alignment of the rearfoot or forefoot.
• Device for a foot
  (US7100296), United States, 2004
  This invention is a mechatronic device used to invert or evert the foot for foot casting purposes. The mechanical elements facilitate the support and rotation of the rearfoot to measure valgus and varus using a potentiometer and strain gauges.
• Correcting foot alignment
  (US7069665), United States, 2006
  The invention comprises a subtalar joint goniometer instrument composed of two mechanical elements that measure the angular alignment of the rearfoot in a standing position. These measurements are compared with a database that correlates degrees of various corrective pads to select a corrective alignment insole.
• Foot tilt angle measuring method, method of selecting shoe or insole for shoe method of manufacturing shoe or insole for shoe, and foot unit tilt angle measuring device
  (US7325323B2), United States, 2008
  This invention comprises a method for measuring rearfoot tilt angle utilizing a 3D model. The 3D model is used to generate a two-dimensional cross-sectional view of the foot, including the heel section. The central line of this two-dimensional view is identified, and the inward/outward tilt angle of the rearfoot is determined from the angle of this central line.
• System and method for foot classification
  (US7789840), United States, 2010
  This invention comprises a visual method of classifying a foot into a specific foot type selected from a group of twenty-four foot types based on characteristics such as rearfoot position, forefoot position, calcaneus alignment relative to the floor, talar stability, and tibial and femoral limb rotation. This visual characterization is performed using cameras to measure the rearfoot tilt in each frame or image using a goniometer.
• Apparatus and method for imaging feet
  (US8567081B2), United States, 2012
  The invention consists of an optical imaging section that measures the foot’s plantar surface contours, an alignment section that positions the foot relative to the imaging section, and a mechanism for applying a dorsally directed force to the foot’s lateral forefoot area to lock the midtarsal joint. The apparatus could assist in evaluating valgus and varus by specific foot alignment and the use of directional forces to lock the midtarsal joint.
• Measurement system for varus/valgus angles in feet
  (US8914988), United States, 2014
  This system functions when the user’s foot is kept in the neutral position of the subtalar joint. This action causes a platform component to pivot in relation to a matching depression on a base section, altering the alignment of the platform’s top surface compared to that of the base. The measurements can be visually assessed using graduated marks that indicate the angles corresponding to varus and/or valgus.
• Indicator for measuring foot movement relative to adjacent bodily structure
  (DK201670105), Denmark, 2016
  This indicator device includes an extended element with a proximal end and a distal end. At the distal end, there are attachments for securing the extended element to a part of the foot. The proximal end of the extended element is designed to move freely. The indicator is placed on the calcaneus to estimate the rearfoot valgus and varus qualitatively.
• Method and system for collection of foot geometry data
  (US20170079559), United States, 2017
  This invention comprises a foot geometry data collection system captures geometric data of a subject’s foot revealing varus and valgus deviations throughout the range of dorsiflexion and abduction movements. The system places the foot in a suspended loading state, allowing the foot’s musculature and bones to interact to transmit forces to the ankle and knee, while ensuring the ankle and knee are forced to remain aligned in a neutral configuration to reveal any varus or valgus deviations.
• Malalignment syndrome diagnosis apparatus based on plantar pressure and body movement and method thereof
  (KR101902551B1), South Korea, 2018
  This system comprises an insole pressure plantar and a 3D sensor to analyze changes in each step during walking, along with a method to measure valgus and varus of the rearfoot based on pressure plantar information.
• Indication device
  (WO2019201403), France, 2019
  The indicator device consists of an elongated indicator pin attached to a base, which is designed to be positioned on the heel section of a shoe or other footwear. The indicator enables qualitative estimation of valgus or varus rearfoot depending on the tilt of the calcaneus.
• Orthopedic director of hoof varus foot osteotomy
  (CN110215270B), China, 2020
  The invention dicloses mechanical and structural components intended for guiding and executing orthopedic procedures, encompassing an ejection device, template position detection plates, measurement scales, and fixation elements for measuring the valgus and varus rearfoot.
• Foot varus angle detection modeling method and system
  (CN113658707), China, 2021
  This invention introduces an insole pressure detection system and a machine learning regression algorithm based on gait characteristics and foot varus angle labels. The system and method enable the identification of varus rearfoot.
• Portable foot varus and valgus detection device
  (CN217659860U), China, 2022
  This device measures foot varus and valgus by adjusting a cushion to fit the foot sole and rotating a handle to move a detection rod and measuring plate. A pointer on the measuring plate indicates the position of the detection rod relative to the foot arch on a scale. This allows for classifying foot problems like varus and valgus by reading a meter at the top of the device.
• Orthopedic leg alignment system and method
  (AU2017228417B2), Australia, 2022
  An orthopedic measurement system is introduced to evaluate leg alignment. This system uses a tri-axial gyroscope to monitor leg motion. The gyroscope is mounted on the tibia, either by embedding it within a tibial prosthetic component or integrating it into an insert attached to the tibia. By detecting angular velocities during specific leg movements, the gyroscope determines the leg’s orientation relative to its mechanical axis. The recorded data are sent to a connected computer, which analyzes the information to calculate the alignment of the leg with respect to its mechanical axis.
• Portable multifunctional ruler capable of measuring various limb related parameters and measuring method
  (CN112674757A), China, 2021
  The invention is a portable multifunctional ruler designed to measure various limb parameters, including joint angles, limb length and thickness, and lower limb force lines. It offers strong clinical utility and broad application potential.
• Device and method for hip-knee-ankle angle verification and femoral mechanical axis digitization
  (US9554745B2), United States, 2017
  The device for verifying the hip-knee-ankle angle features a mounting base with a flat surface to align with the resected distal femur. It includes a first inertial sensor linked to a computer-assisted surgery (CAS) system to map the femur’s mechanical axis. A pivotable visual alignment guide on the base adjusts to align with the tibia’s mechanical axis.

3.2. Patent Categories

The patents include mechanisms, electronic devices, and mechatronic devices that can be stationary or dynamic for visually or automatically measuring the tilt of the calcaneus. Moreover, each patent presents inventions to perform measurements depending on the usage mode. These patents are classified based on technology type, mobility, measurement type, and usage type. The classification of technology type enables the identification of inventions based on mechanisms, electronic devices, or mechatronic devices. These inventions are classified based on mobility types, such as dynamic and stationary. A stationary system is fixed in place to perform measurements of rearfoot tilt, whereas a dynamic system remains in motion during the measurement, e.g., devices placed on the human body. In addition, the patents are classified by the measurement type as visual and automatic. Generally, the mechanisms are measured visually, while the electronic devices and mechatronic devices are measured automatically. Finally, the patents are classified based on the system’s usage mode, including sitting, standing, and walking.

Table 5. Classification of patents in accordance with technology, mobility, measurement, and usage mode.

Item Patent	Technology	Mobility	Measurement	Usage Mode
1	Mechanism	Dynamic	Visual	Standing and Walking
2	Mechanism	Stationary	Visual	Standing
3	Mechanism	Stationary	Visual	Standing
4	Electronic device	Stationary	Automatic	Walking
5	Mechanism	Stationary	Visual	Standing
6	Mechatronic device	Stationary	Automatic	Standing
7	Electronic device	Stationary	Automatic	Standing and Walking
8	Mechanism	Dynamic	Visual	Walking and Standing
9	Mechanism	Stationary	Visual	Standing
10	Mechatronic device	Stationary	Automatic	Standing
11	Mechanism	Stationary	Visual	Standing
12	Mechatronic device	Stationary	Automatic	Sitting, Standing and Walking
13	Electronic devices	Stationary	Automatic	Standing
14	Mechanism	Stationary	Visual	Standing
15	Electronic device	Dynamic	Automatic	Standing and Walking
16	Mechanism	Stationary	Visual	Standing
17	Mechanism	Stationary	Visual	Standing
18	Electronic device	Dynamic	Automatic	Standing
19	Mechanism	Dynamic	Visual	Standing
20	Electronic device	Stationary	Automatic	Sitting and Standing
21	Electronic device	Dynamic	Automatic	Standing and Walking
22	Electronic device	Stationary	Automatic	Standing
23	Electronic device	Stationary	Automatic	Standing
24	Electronic device	Dynamic	Automatic	Standing and Walking
25	Mechanism	Stationary	Visual	Standing
26	Mechatronic Device	Stationary	Automatic	Sitting, Standing and Walking
27	Electronic device	Dynamic	Automatic	Standing
28	Mechanism	Stationary	Visual	Standing
29	Mechanism	Stationary	Visual	Sitting and Standing
30	Electronic Device	Stationary	Automatic	Sitting and Standing
31	Mechanism	Stationary	Visual	Sitting and Standing
32	Mechatronic device	Dynamic	Automatic	Sitting, Standing or Walking
33	Mechatronic device	Stationary	Automatic	Sitting and Standing
34	Electronic device	Dynamic	Automatic	Sitting, Standing and Walking
35	Mechanism	Stationary	Visual	Sitting and Standing

displays the classification presented above for each patent, enabling the identification of valuable characteristics of these inventions.

According to the technology type, 45.7% of the obtained patents correspond to mechanisms, 37.1% are electronic devices, and 17.1% are mechatronic devices. In order to perform a technological analysis based on mobility type and technology, the patents are grouped into six categories: stationary mechanisms, stationary electronic devices, dynamic mechanisms, dynamic electronic devices, stationary mechatronic devices, and dynamic mechatronic devices. Figure 4 shows an example for each category based on mobility and technology.

The stationary mechanism category represents 37.1%, stationary electronic devices represent 20.0%, dynamic mechanism represents 8.6%, dynamic electronic devices represent 17.1%, stationary mechatronic devices 14.3%, and dynamic mechatronic devices 2.9%, as shown in Figure 5.

In addition, the categories of stationary electronic devices, dynamic electronic devices, stationary mechatronic devices and dynamic mechatronic devices are automatic systems, and represent 54.3% of the patents, whereas stationary mechanism and dynamic mechanism represent 45.7% of the patents. Finally, these categories are represented according to their usage mode, as shown in Figure 6.

4. Discussion

The United States leads in patent registrations across various disciplines; this review is no exception. In this particular analysis from 2000 to 2024, the United States accounts for 51.4% of the patent registrations, particularly focusing on inventions used to measure valgus and varus rearfoot alignments without ionizing radiation. According to the data in Table 5, the articulated mechanisms account for 45.7%, electronic devices 37.1%, and mechatronic devices 17.1% of patents registered. Articulated mechanisms have a majority of patent registration, slightly superior to electronic devices and superior to mechatronic devices. Additionally, the patents are classified depending on the mobility as stationary or dynamic to determine the usability and adaptability of this system for different analyses. This distinction helps medical professionals choose systems that best align with their specific assessment needs. On the other hand, inventions are classified according to the type of measurement, such as visual or automatic. Visual measurements increase errors because they depend on medical experience, while automatic measurements provide automated measurements and low error rates through proper calibration. The inventions are classified based on modes of usage (sitting, standing, or walking) to provide information about their features. The classified data shown in Table 5 is employed to create categories based on technology and mobility, as shown in Figure 5. The patent categories illustrated in Figure 5 reveal that the stationary mechanisms constitute the majority, at 37.1%. These systems consist of articulated elements designed to stabilize the foot and allow for visual measurement of calcaneus tilt using indicators and graduated measuring bases, facilitating evaluation of rearfoot valgus and varus. The stationary electronic devices category represents 20% and includes fixed 3D scanners and cameras used for measuring the calcaneus tilt. The stationary mechatronic device category represents the 14.3%, and includes a combination of fixed electronics and mechanisms components, which interact to provide a measurement of the calcaneus tilt. The dynamic mechanism category represents the 8.6% and consists of mechanical elements generally placed on the rearfoot to measure the tilt, using indicators and graduated measuring bases during different phases of the gait cycle. Finally, the Dynamic Electronic Device category represents the 17.1%. The inventions in this category involve systems based on landmarks and graduated measuring bases; moreover, plantar pressure insoles to measure the tilt of calcaneus.

The categories reflect the differences between various usage modes, as certain patents enable multiples (e.g., standing, sitting, and walking), as shown in Figure 6. In the stationary mechanism category, eight patents correspond to the standing mode usage because the mechanisms are fixed to perform measurements, which makes it difficult to perform measurements during walking. In the stationary electronic device category, four patents correspond to standing, one to sitting, and one to walking. This category includes inventions with sitting and walking modes; although these systems are stationary, they can perform measurements during walking using cameras or scanners. On the other hand, the dynamic mechanism category has three patents for standing usage mode and two for walking. Generally, inventions in this category are placed on the human body. Therefore, it is possible to perform visual measurements during walking and standing mode usage. Additionally, the dynamic electronic device category includes four patents in standing and three patents in walking; these devices are placed on the rearfoot for measuring the calcaneus angle in real-time or by means of pressure plantar. On the other hand, the category of stationary mechatronic devices includes six inventions for standing, one for sitting, and two for walking. The Stationary mechatronic devices are composed of mechanical and electronic components that collaborate to enhance accurate measurements in standing, sitting, and walking. Finally, inventions related to dynamic mechatronic devices were not found in this review.

4.1. Plantar Pressure Systems Used to Measure the Alignment Rearfoot

An interesting finding from this patent review is that the majority of patents directly measure the angle formed by the tibia and calcaneus utilizing mechanisms, electronic devices, or mechatronic devices. However, during the technological search within the dynamic electronic devices category, the patents developed by China (CN113658707, CN106805980A, CN209862489) were identified. These patents use plantar pressure systems to estimate the angle between the calcaneus and tibia. There is evidence from previous research [73] that relates plantar pressure to rearfoot alignment, indicating that higher plantar load on the medial foot is associated with valgus rearfoot in a static position. Another study relates the mediolateral shift of the center of pressure to rearfoot eversion. This study shows that the movement of the center of pressure along the mediolateral axis of the foot has a significant impact on rearfoot eversion [74].

4.2. Maintenance of the Systems

On the other hand, it is important to consider the maintenance and costs required for the optimal functioning of the presented systems. The mechanical patents require preventive maintenance to reduce measurement errors caused by the wear of mechanical parts. Maintenance involves lubricating moving elements monthly to minimize wear, as well as replacing parts annually to avoid measurement errors. However, these intervals can be adjusted depending on usage. In general, this system requires higher maintenance costs and can be operated in orthopedic clinics, provided that users are trained to avoid measurement errors. On the other hand, electronic devices require less maintenance than mechanical systems. Generally, this maintenance consists only of cleaning, making it cheaper. Mechatronic devices require a combination of preventive maintenance similar to that performed on mechanical and electronic systems. However, it is necessary to maintain synchronization between the mechanical parts and electronics to reduce measurement errors.

4.3. Application of Intelligent Algorithms in the Identified Inventions

Another aspect to consider is the generation of datasets provided by electronic and mechatronic devices, as they can be used for training intelligent algorithms. The patents CN106805980A, KR101902551B1, US7100296B2, CN113658707A, CN209862489, US20170079559 and US11399758B2 present electronic or mechatronic systems that provide real-time measurements under various conditions to capture datasets. These datasets can be used to train machine learning or deep learning algorithms to detect deformities in rearfoot alignment or other foot health conditions using input data such as plantar pressure or movement information. The wearable devices disclosed in patents CN106805980A, CN113658707A, and CN209862489 are based on plantar pressure and can be training with intelligent algorithms to estimate rearfoot alignment in real time over a prolonged period. This enables the provision of clinical value for preventive care or the design of plantar insoles to correct rearfoot alignment. Additionally, the dataset provided by this system can be used to analyze overpressure on plantar surfaces, contributing to ulcer prevention in persons with diabetic feet [75,76,77].

4.4. Benefit and Limitations

The alternative technologies presented in this manuscript have their own advantages and limitations compared to the goniometer and X-ray measurements.

Table 6. Comparative analysis of alternative technologies, goniometer, and X-Ray systems.

Comparative Aspects	Alternative Technologies			Goniometer	X-Ray
	Mechanism	Electronic	Mechatronic
Precision	Poor precision	Good precision	Good precision	Poor precision	Good precision
Patient Safety	Require usage instructions	Safe devices	Require usage instructions	Safety devices	Safe, but can cause damages
Accessibility	They are mostly not portable	They are mostly portable	Not portable	Portable	Not portable

highlights comparative aspects of precision, patient safety, and accessibility.

The X-ray produces ionizing radiation ranging from 0.19 to 2.61 mSv (milisievert) per procedure without protection or using inadequate techniques. However, this value can decrease to 0.2–0.5 mSv per procedure with proper protection [78]. These values are below the FDA’s limit of 50 mSv per year for medical radiology. However, this exposure can be reduced to zero in evaluations of rearfoot and leg alignment by adopting these alternative technologies. On the other hand, professional radiologists are exposed to higher amounts of ionizing radiation; therefore, the use of alternative technologies that are free of ionizing radiation can significantly reduce the accumulation of radiation over the course of a year, as orthopedic professionals can perform the measurements. Furthermore, there is evidence of improper use of radiation procedures. For example, a publication [79] indicates that Thorotrast, a colloidal solution of thorium dioxide, was used as an X-ray contrast medium for arteriography from 1930 to 1950. Its half-life is $1.4\times {10}^{10}$ years, meaning that the administration of this contrast medium can lead to lifelong chronic $\alpha$ particle irradiation. Moreover, this publication presents a study conducted in Germany involving 2326 originally exposed subjects and 1890 non-exposed subjects. The study found that the median life expectancy of the exposed subjects was reduced by 14 years.

5. Conclusions

This patent review provides an overview of systems used to measure the tilt of the midline calcaneus relative to the midline tibia, characterizing valgus and varus deformities in the rearfoot. These systems employ mechanisms, electronic and mechatronic devices that can either be stationary for conducting measurements in a standing position or placed on the human body to perform measurements during the gait cycle. The technological search for patents demonstrates that there are more registers of mechanisms (45.7%) than electronic devices (37.1%) and mechatronic devices (17.1%). Moreover, 28.6% of the patents correspond to dynamic systems, while the 71.4% are stationary systems. On the other hand, it demonstrates the advancements in emerging technology development by countries like China, using a system based on plantar pressure for measuring varus and valgus of the rearfoot, through machine learning algorithms and mathematical methods. In conclusion, this worldwide patent review provides valuable information to orthopedists regarding systems for evaluating the condition of valgus and varus deformities in patients in standup or the gait cycle. Moreover, studies indicate that ionizing radiation reduces the median life expectancy to 14 years. Also, there is evidence that a colloidal solution of thorium dioxide was used as an X-ray contrast medium for arteriography from 1930 to 1950. Its half-life is $1.4\times {10}^{10}$ years, meaning that the administration of this contrast medium can lead to lifelong $\alpha$ particles irradiation, which can cause damage in cells and tissues, as well as cancer. The non-ionizing systems described in this patent manuscript eliminate the use of ionizing radiation, enabling the measurement of rearfoot alignment without exposure to radiation. Another significant aspect is that, although various systems can measure rearfoot tilt, only electronic and mechatronic devices enable automated rearfoot measurement. Another noteworthy finding in this review is the existence of plantar pressure systems that can be integrated with intelligent algorithms to estimate rearfoot alignment. However, these topics have been minimally explored in the literature.