Engineering a Clinical Force Measuring Walker for Patients Recovering from Median Sternotomy

: Patients often need the use of their arms to assist with functional activities, but after bone disruption, pushing is frequently limited to <10 lb (4.5 kg). No method exists to measure arm weight bearing objectively in clinical settings. This project aimed to design, construct, and test a walker for patients who need to limit arm force to prevent excessive bone stress during post-fracture (iatro-genic or traumatic) ossification. First, a qualitative study was conducted to obtain critiques of a Clinical Force Measuring (CFM) walker prototype from rehabilitation professionals. Key statements and phrases were coded that allowed “themes” to emerge from transcribed interviews, which guided device revisions. Next, a second CFM Walker prototype was designed based on the qualitative data and device criteria/constraints and finally tested. The result was fabrication of a new lightweight, streamlined, and cost-effective prototype walker with a simple visual display and auditory cue with upper limit alarms. Key features included attachments for medical equipment and thin film force-sensing resistors integrated into the walker handles that progressively activated 3 LEDs and a buzzer when UEWB force exceeded programmed thresholds. The innovative CFM Walker will help patients with restricted UEWB, especially elderly adults, recover safer and faster in the future.


Introduction
Patients recovering from bone disruption due to trauma or surgery need to limit use of their upper extremities during bone healing often to < 10 lb (4.5 kg) of pushing, pulling, or lifting [1][2][3]. This restriction is thought to minimize shear force and movement between the bone halves to protect callus formation and osteogenesis [1][2][3][4][5]. Common patient diagnoses that require post-fracture (iatrogenic or traumatic) bone ossification include cardiac surgery via median sternotomy, total shoulder arthroplasty, and upper extremity bone fractures [1][2][3]5]. For example, median sternotomy is commonly performed to access the heart during a variety of different surgeries such as coronary artery bypass, heart valve replacement, heart transplantation, and thoracic trauma repairs. The procedure entails sawing longitudinally from the sternal notch to the xiphoid process, separating the sternum with retractors, and wiring the sternal halves together after surgery completion [6,7]. Complications can occur when the bone halves do not heal correctly, including deep wound infection (osteomyelitis), bony nonunion/instability, and or bone dehiscence [1][2][3][4].
Restricting arm use often limits patient functional independence, contributing to longer hospital stays and greater need for care after hospitalization. It is difficult to function independently when upper body daily activities are limited, especially for older adults. Restricting arm use is particularly problematic for patients who need assistance sitting down or standing up from a chair and or need to use a walker for ambulation. Loss of functional independence can contribute to a greater need for assistance and rehabilitation after hospital discharge [8][9][10][11]. Therefore, appropriate arm use is essential for timely return to function.
Little is known about how much upper extremity weight bearing (UEWB) force occurs when older patients attempt to use < 10 lb; therefore, their ability to safely resume Walker v2.0 is illustrated in Figure 1, beginning with the laboratory instrumentation and then 2 CFM Walker prototypes.

Qualitative Research Study (Part 1)
First, a prototype was fabricated using biomedical device engineering and critical care equipment principles [27,28]. The CFM Walker v1.0 (see Figure 2) retained the externally mounted force transducers wirelessly connected to tablets. The tablets were housed in waterproof cases that could be disinfected and mounted directly to the walker with multi-planar adjustable mounting arms. A plate was attached below the left lateral support to suspend a chest tube reservoir tank with placement below the tube exit site to maintain gravity assist drainage of pleural secretions. Chest tubes exit the left lower chest wall after cardiac surgery and must have a water seal to maintain negative pressure within the pleural space. Walker legs were color-coded to facilitate adjustment for multiple patient use and included interchangeable front standard and 5" single plane wheeled legs. A portable oxygen tank mounting bracket was positioned on the lower front horizontal walker support and centered for optimal walker stability and symmetrical drag when using a front wheeled walker configuration. Ergonomic soft handle grips were added to improve patient ability to grip and patient comfort. A hook that rotated 90 degrees to suspend a urinary collection bag was mounted below the right lower lateral support to facilitate gravity assist drainage. The swivel hook allowed a urinary collection bag with a parallel or perpendicular oriented hook to hang parallel from the walker to keep it from obstructing a patient's gait. In addition, an S-shaped hook was attached directly to the right lower lateral support to tether a Foley Catheter (aka urinary catheter) in front of the patient's leg, so the tubing would not obstruct gait.
This part of the study used a qualitative description methodology (phenomenology). Qualitative inquiry is appropriate when seeking to describe a topic in depth through insights from participants [29,30]. By using an interview process, participants' perspectives were explored using open and probing questions. A purposeful sampling strategy was employed to achieve sufficient variability and understanding of the concepts.
Study participants were rehabilitation professionals with experience working in critical care and or with post-surgical patients. Criteria for selection of subjects included: 1) between the ages of 25-60 years, 2) rehabilitation professional (physical therapist, registered nurse, exercise physiologist, etc.), and 3) minimum of 6 months working in a hospital with critical care patients. This study was approved by the Eastern Washington University Institutional Review Board for Human Subjects (Protocol #HS-5953), and all subjects signed an informed consent prior to participation.
Data collection involved asking the study participants a number of open-ended questions to garner feedback on the instrumented walker prototype. Subject interviews were conducted via Zoom virtual meeting application to adhere with COVID-19 social distancing guidelines. The slides with component photographs and specific questions used during interviews are included in Appendix 1. The interviews were video-recorded so that the answers and comments could be transcribed for data analysis. The subject interviews began with an introduction and narrated video clips of the walker prototype from multiple angles. The interview questions were sequentially shown on a slide with close-up photographs of the specific walker parts/features from different angles.
Data analysis involved reviewing the transcribed interviews and identifying key statements and phrases that were significant within each category. Statements then were sorted into groups of similar statements that emerged as meaningful units or ''themes.'' Rich descriptions of participant perceptions corresponding with each theme were generated using their exact words and phrases. An iterative process of data analysis was used until saturation was achieved within each theme. Qualitative data were analyzed and triangulated before themes were named. Figure 3 shows the flow of steps used to design the final CFM walker prototype. Using data from the qualitative study and device testing, extensive revisions were made to the first walker prototype. The essential features that the CFM Walker needed to have were defined based on engineering design criteria and constraints, qualitative data, and published information [27,28]. Next, testing procedures for each of the essential design elements were developed. Table 1 outlines the design elements, criteria, constraints, and testing protocols used to test both CFM Walker prototypes.

Qualitative Research Study (Part 1)
A detailed parts list of the components used to fabricate the CFM Walker v1.0 is included in Appendix 2. The total cost for the components of this prototype was $1,423. The bulk of this cost was for the force transducers and tablet displays. The total cost did not include the actual medical devices (portable oxygen tank, chest tube, chest tube reservoir, urinary collection bag, and Foley / urinary catheter). Table 2 outlines the main themes for each walker components that emerged with data analysis. There were 5 overarching ideas that developed at the completion of data analysis. 1) The subjects (rehabilitation professionals) overwhelmingly expressed that a force measuring walker would be very useful with a variety of patients, particularly those recovering from open heart surgery. 2) Integrating the force measuring mechanism into the walker handles / structure would be optimal for arm biomechanics and the width of the device (to be maneuverable in narrow spaces). 3) Simplifying the force output display and adding upper limit visual and auditory alarms would be easier for patients and rehabilitation professionals to know when UEWB exceeds 10 lb. 4) The optimal leg combination was unanimously front 5" single plane wheels and back standard legs. 5) Subjects also identified revisions to improve the medical equipment attachments for the oxygen tank, Foley Catheter, and oximeter. Direct quotes were extracted from the transcribed data to provide rich descriptions of the subjects '

Engineering Design and Testing (Part 2)
The components of the CFM Walker v2.0 are shown in Figure 4. Thin-film force resistors (1.8 x 1.8 cm) were placed under the original walker handgrips (Figure 5.A). Khodadadi et al found that force transducers incorporated into walker handles had easier installation and less error than those installed on circular vertical walker legs [25]. These were connected to the Arduino System with male-to-female breadboard jumper wires. A visual display with 3 different colored LEDs was designed to simplify the force feedback interface. The LEDs were triggered as follows: the green LED was always on, the yellow LED was activated when force was greater than or equal to 7 lb, and the red LED was activated when force was greater than or equal to 10 lb. An auditory alarm that triggered when force exceeds 10 lb was also included. The system was programmed to trigger the red LED and buzzer when the left or right force transducer measured greater than the

Chest Tube & Reservoir
• Good location -low for drainage, same location often used clinically • Tube adequately protected (not touching ground, kinked, tangled) • Essential features: no gait obstruction, protected adequately, user-friendly • May not work for all reservoir types (with different shapes, handles, hooks) • Improvements: higher or with adjustable height, block swinging inward Walker Legs • Ideal combination: wheels only on front legs, commonly used clinically • Color-coding possibly helpful, not necessary (Healthcare professionals already familiar with this type of height adjustment mechanism) • Improvements: color-coding material needs to be nonporous for disinfecting, back leg "ski-type" gliders Oxygen Tank • Not a good location -tendency to tip forward, cause asymmetrical drag • Essential features: no gait obstruction, protected adequately, user-friendly • Would not fit most common portable oxygen tanks used in hospitals • Improvements: remove bracket, transport tank separately / with other device (IV pole), better to have another person to assist with oxygen tank if needed Force Transducers • Wider diameter grips better for patients • Transducer handles wider than normal -problematic for small patients, hard to fit through narrow spaces, change biomechanics of arm force • 2 sets of hand grips confusing for patients • Improvements: materials need to be nonporous for disinfecting, integrated transducers ideal to reduce width / weight and simplify build Display Screen Mounts • Location Issues: possibly cause tipping forward, obstruct patient view while walking, hard for provider to see if patient is large • Essential features: good adjustability with multi-angle articulation and wireless connection, intuitive, easy to use • Improvements: single unit instead of 2, reduce weight and size Display Interface • Good to have units in pounds for patient reference • Color-coded, graphical information helpful • Visual feedback display too complicated and small • Improvements: larger, simpler force output, upper limit signal warning lights (flashing lights, color LEDs) and auditory signal (buzzer) Urinary Collection Catheter & Bag • Good location -low for drainage, same location often used clinically • Catheter adequately protected (not touching ground, kinked, tangled) • Essential features: no gait obstruction, protected adequately, user-friendly • Improvements-block swinging inward, remove hook for catheter Overall Opinion • Force measuring walker with integrated handles clinically useful • Streamline attachments to reduce total weight and surface area • Some attachments helpful; remove oxygen tank, add oximeter • Simplify visual display and add auditory warning signal • Useful for a variety of patient populations (median sternotomy, arm fracture / surgery, critically ill…) preset 10 lb of force. The thin-film force resistors were calibrated with a force dynamometer in a 1-20 lb range. Force accuracy data are presented in Figure 6. An external power source was added to the system, and Figure 5.C illustrates the electrical schematic of the feedback system. The electrical components were housed in a clear acrylic waterproof case with exit holes for the 3 LEDs, speaker, and cord to the power source ( Figure 5.E). The electrical component housing and external power source were positioned on the front of the walker using a multi-planar clamp mount. This position allows the patient and healthcare professional to see the LEDs but does not obstruct their view when walking forward.
A bracket was fabricated to hold a standard handheld oximeter and was clamped to the right upper vertical walker support. This position allowed a healthcare provider to maintain line-of-sight for continuous oxygen saturation and heart rate monitoring. In this position, the oximeter probe could remain attached to the patient's finger while using the walker. The oximeter holder can also be placed on the left side or other horizontal or vertical walker support. The bracket was designed to make the digital display visible and allow easy oximeter placement and access to the power switch ( Figure 5.D). A swivel hook was mounted to the upper right horizontal support using a metal hose clamp to suspend a urinary collection bag (Figure 5.b). A 90 degree swivel hook was used so urinary collection bags with a parallel or perpendicular suspension hook could be attached to the walker and maintain bag orientation in the same plane as the right walker support frame. By moving this swivel hook to a higher location, the S-hook to tether the Foley Catheter was no longer needed. This higher attachment location ensured that the urinary collection bag  would not drag on the floor; it also could not swing inward, hitting the patient's foot because the lower right horizontal support blocked this motion. Even with this higher location, the urinary collection bag was still lower than the level of the bladder to maintain gravity assist drainage. The swivel hook was initially mounted using a Velcro strap, and replacing it with a hose clamp removed a porous component that would be difficult to disinfect.
A bracket to suspend the chest tube reservoir from the outside of the lower left horizontal walker support was fabricated (Figure 5.F). The bracket was 7.7 cm wide to keep the chest tube reservoir parallel to the walker side support and prevent front-to-back movement, which is important to preserve the water seal required to maintain negative pleural pressure and lung inflation. The bracket was 5.1 cm deep with a wedge-shaped spacer, so the chest tube reservoir handle would slide into the bracket and then be cradled tightly to prevent side-to-side movement. The CFM Walker v1.0 had a bracket mounted below the left lower horizontal support, which positioned the chest tube reservoir very close to the floor and front walker wheel. The new position not only moved the reservoir higher but to the outside rather than below it so any side-to-side movement would no longer interfere with a patient's gait.
Lastly, the walker's legs were modified slightly based on the qualitative data and engineering testing. The color-coding was removed because the rehabilitation professionals said it was not necessary (they are used to changing walker heights), and the tape was somewhat porous, so there were concerns about adequately disinfecting it. The study participants overwhelmingly preferred 5" single-plane wheels on the front with regular legs on the back of the walker. They stated that this configuration is what they use in clinical settings and prescribe for patients' home assistive devices. Walker drag was tested ( Table  3) using 4 types of front wheels (5" single-plane, 5" swivel, 3" single-plane, and 3" swivel). Results showed that with 5" single-plane front wheels walker horizontal push-pull resistance was significantly less than with the other wheels both with and without medical equipment attached on smooth, solid surface flooring (similar to that found in a hospital). Three types of "ski-like" gliders on the back walker legs were tested with the 5" singleplane wheels, and results suggested that none significantly reduced walker drag and in many cases increased it.

*Significantly greater than push; †Significantly greater than 5" Wheel
A detailed parts list of the components used to fabricate the CFM Walker v2.0 is included in Appendix 2. The total cost for the components of this prototype was $238. The total cost did not include the actual medical devices (portable oxygen tank, chest tube, chest tube reservoir, urinary collection bag, and Foley / urinary catheter). By removing the externally mounted force transducers and creating a simple force feedback interface using an Arduino system, the cost ($1,171 savings) and weight (4.5 kg reduction) of the CFM Walker v2.0 were substantially reduced. The CFM Walker v2.0 met all criteria/constraints, and the testing results are shown in Table 4. In addition, technical diagrams of the CFM Walker v2.0 are shown in Appendix 3.

Discussion
The interview data obtained supported the overall clinical need for an assistive device with the ability to provide feedback regarding UEWB. This information was used to guide revisions of the CFM Walker v1.0 and engineering of the CFM Walker v2.0. Several specific alterations were made to the second walker prototype based on the qualitative data. Getting rid of the externally mounted force transducers and finding a force measuring mechanism that would be integrated and streamlined was a top priority. It was also necessary to design a force output display that was much simpler for patients to interpret and that had upper limit visual and auditory signals. Revisions to the medical equipment included removing the oxygen tank bracket and Foley Catheter S-hook. The only addition suggested was an attachment to hold a pulse oximeter. For the device to be easily disinfected, all components needed to be nonporous and water-resistant, so replacing some materials was needed.
Finally, follow-up qualitative interviews were conducted with the rehabilitation professionals. The CFM Walker v2.0 was taken to the study participants so they were able to use the walker and test the force measuring system and medical equipment attachments. They were provided a brief overview of the CFM Walker v1.0 revisions incorporated into the CFM Walker v2.0. Then they were asked these general questions -What is your opinion of the: • force transducers integrate into the walker handles?
• force feedback display and buzzer?
• electronic components housing and mounting arm?
• urinary collection bag swivel hook?
As described in PART 1, the interviews were again video recorded, transcribed, coded, and sorted into themes. The qualitative interviews were overwhelmingly positive, and the rehabilitation professionals indicated that the CFM Walker v2.0 could be used with hospitalized patients. They said the walker was lighter than expected. Study participants reaffirmed that a variety of patient populations could benefit from use of the CFM Walker v2.0. One study participant stated that an Intensive Care Unit telemetry monitor could be placed in the oximeter holder for patients using that technology in lieu of a handheld pulse oximeter. It was also noted that the oximeter holder could be mounted on the upper lateral walker supports, making it easier for tall rehabilitation professionals to see the display screen. Subjects recommended minor improvements to the electronic housing box and external power source to make it more streamlined.

Conclusions
In conclusion, previous research suggests that patients are not good at estimating arm force <10 lb and that feedback training is effective at reducing it [12][13][14]18,20,31]. Therefore using an instrumented walker and feedback training would be beneficial in clinical practice, especially with older patients. The qualitative data obtained from rehabilitation professionals (PART 1) indicated that a CFM walker with integrated handles would be clinically useful. Suggestions for the CFM Walker v1.0 led to modifications including, streamlining, modifying, removing, and adding components. Finally, engineering tests of the CFM Walker v2.0 demonstrated that it met essential criteria for making it feasible for patients who need to limit UEWB to prevent excessive bone stress during postfracture ossification. Table 5 compares the engineering features of each walker rendition. Ultimately the CFM Walker v2.0 could improve outcomes for patients recovering from heart surgery performed via median sternotomy and certain orthopedic conditions that are associated with upper body bone fracture (iatrogenic or traumatic) and subsequent osteogenesis.