Abstract
Background
Achieving precise three-dimensional reconstruction during foot and ankle corrective surgeries is challenging. Intraoperative plantar pressure measurement (IPPM) can help assess foot morphology and detect abnormal pressure distributions. We developed an IPPM system using a navigation to align the floor reaction force with the femoral head’s center. This study evaluates its accuracy in anesthetized patients.
Patients and methods
Fifteen patients undergoing lower limb surgery were enrolled. Plantar pressure was measured intraoperatively in the supine position and compared with preoperative standing measurements. Similarity was evaluated using normalized cross-correlation (NCC) and peak pressure site agreement across 12 predefined foot regions.
Results
The IPPM device replicated standing plantar pressure, with a mean NCC of 0.92 ± 0.1 and a peak pressure site agreement rate of 76.7 ± 37.2 %.
Conclusions
The newly developed intraoperative plantar pressure-measuring device accurately replicates standing plantar pressure in anesthetized patients, offering potential for improving intraoperative assessments and corrective procedures.
1
Introduction
The human feet possess a unique three-dimensional morphology, which has evolved to facilitate the acquisition of erect bipedal walking . This intricate three-dimensional structure of the human feet can be compromised in conditions such as hallux valgus and progressive collapsing foot deformity, leading to pain, deformity, and decreased physical function . In cases where conservative treatments are ineffective for these foot deformities, corrective surgeries are performed. However, relying solely on two-dimensional information such as X-ray fluoroscopy and external appearance during surgery struggles to achieve accurate three-dimensional reconstruction and can sometimes lead to complications.
Transfer metatarsalgia after hallux valgus surgery serves as a representative example of these complications, occurring with a frequency of up to 27 % . Excessive shortening of the first metatarsal has been recognized as one of the mechanical factors contributing to transfer metatarsalgia; however, the occurrence of this complication cannot always be predicted solely from the radiographic length of the first metatarsal. Depending on the osteotomy method and the surgeon’s technique, three-dimensional changes such as plantarflexion or dorsiflexion of the first ray may occur, leading to alterations in load transfer that cannot be predicted solely from the two-dimensional radiographic length of the bone. Plantar pressure measurement is a valuable tool for assessing foot morphology and detecting abnormal distributions; thus it can be helpful to identify and monitor the possible complications after foot deformity reconstruction if it can be measured during surgery.
A few reports have documented the development of devices aiming to replicate standing plantar pressure measurements during surgical procedures performed in the supine position , . However, all of these devices have utilized a technique that applies pressure from the plantar or knee side, primarily in the longitudinal direction, without consideration for the physiological weight-bearing axis. As a result, they have demonstrated issues with reproducibility in accurately replicating the plantar pressure during standing, raising concerns about their reliability and effectiveness in clinical applications. In order to overcome these drawbacks, we developed a novel device based on the concept of replicating the plantar pressure during standing by navigating the ground reaction force generated on the plantar surface towards the center of the femoral head (CFH) , .
The system consists of force sensors to identify the pushing direction, a pressure sensor to measure plantar pressure distribution, and an optical navigation system to detect the CFH and navigate the pushing direction toward the CFH. The accuracy of this novel device, i.e., reproducibility of plantar pressure in the standing position, has already been demonstrated and reported in experiments conducted on awake healthy individuals . However, verification of whether similar accuracy can be achieved in patients under anesthesia has not been performed yet.
The objective of this study is to verify whether this novel device can accurately reproduce the standing plantar pressure in patients in a supine position under anesthesia.
2
Patients and methods
2.1
Study design and participants
This was a single-center prospective observational study conducted at the University of Tokyo Hospital. The study was approved by the institutional ethics committee (approval No. 2021125NI), and all participants provided written informed consent. Patients aged 20–85 years scheduled for lower limb surgery under general anesthesia were eligible for inclusion. Exclusion criteria included: (1) previous lower limb surgery, (2) marked deformity of the lower limb, (3) inability to obtain preoperative standing plantar pressure due to fracture or other reasons, (4) American Society of Anesthesiologists (ASA) class 3 or higher, and (5) anticipated surgery duration exceeding 3 h.
2.2
Concept overview of the intraoperative plantar pressure measurement (IPPM) device
The device used in the present study was developed as an application of the concept described in previous studies , , enabling intraoperative assessment of plantar pressure distribution in anesthetized patients in a supine position, which approximates that of the standing posture. The system consists of a pressure plate incorporating a 6-axis force sensor (FFS080YA501U6, Leptrino Co., Saku, Japan) and a pressure distribution sensor with a 16 × 40 array of 8 × 8 mm elements (LL Sensor, Xiroku Co., Tsukuba, Japan), mounted on a carbon fiber–reinforced plastic plate (335 × 160 × 5 mm). The device is connected to an optical tracking system (Polaris Spectra, Northern Digital Inc., Canada) and a computer workstation for processing and visualization ( Figs. 1 and 2 ). The optical system identifies the CFH using the pivot algorithm described by Hozeiter , and both the ground reaction force vector and the plantar pressure distribution are displayed in real time on a navigation monitor. The display of the standardized plantar pressure center utilized the average values obtained from a cohort of 32 healthy adults, serving as a reference for intraoperative alignment. The operator adjusts the device so that reaction force vector passes near the CFH and the point of force application aligns with the standardized plantar pressure center. The measurement process automatically terminates when both alignment errors fall within predefined thresholds (20 mm for CFH and 10 mm for pressure-center deviation), and the image with the lowest combined error is recorded as the optimal plantar pressure distribution.
(A) Illustration depicting the concept of the Intraoperative Plantar Pressure Measurement (IPPM) system. The system involves two distinct processes: first, aligning the floor reaction force with the center of the femoral head, and second, directing the force application point towards the standardized plantar pressure center. These independent procedures collectively ensure the accurate replication of the physiological weight-bearing axis during surgical interventions. (B) The photograph showcases the actual size and setup of the IPPM system. W: An optical marker attached to the patient in a supine position, X: IPPM device, Y: Navigation monitor, Z: Optical tracking sensor.
Photograph illustrating the practical application of the IPPM system during a measurement and configuration of the IPPM device.
2.3
Measurement procedure
On the day before or two days before surgery, preoperative standing plantar pressure measurements were conducted using the same sensors as those incorporated in the device. On the day of the surgery, after the completion of anesthesia induction and prior to surgical site skin preparation and draping, plantar pressure measurements using the IPPM device were performed in the operating room with the patient in a supine position. The time from calibrating the device, detecting the femoral head, processing the device against the foot, to completing the measurement was recorded. To prevent knee flexion during device application, an assistant manually stabilized the distal thigh just proximal to the patella by pressing down on the anterior surface using a handheld dynamometer (ErgoFET, Hoggan Scientific LLC, Salt Lake City, UT, USA), and the force required for this stabilization was measured.
2.4
Data analysis
The plantar pressure distribution measured by the IPPM device in a supine position was processed by template matching against the standing position plantar pressure distribution measured preoperatively, and the normalized cross-correlation (NCC) was measured . The determination of the first and second peak pressure sites within 12 regions of the foot (medial heel, lateral heel, mid foot, metatarsal 1–5, hallux, second toe, third toe, fourth and fifth toe) was conducted by an independent board-certified orthopedist with over 10 years of experience as a foot and ankle specialist from the plantar pressure measurements. The measurements were conducted blinded on a total of 30 images, comprising 15 measurements each in the standing and supine positions without access to subject information or measurement conditions. We evaluated the agreement rate as 100 % when the combination of peak pressure sites in the standing and supine positions matched, regardless of the order of the first and second sites. When one out of the two peak pressure sites, up to the second position, matched in both the standing and supine positions, the agreement rate was rated as 50 %. In cases where there was no matching peak pressure site between the standing and supine positions, up to the first and second positions, the agreement rate was rated as 0 %.
3
Results
The measurements were conducted on 16 cases in this experiment. One case was excluded from the analysis as the preoperative standing measurement showed biased loading on the heel without any load on the forefoot, which was deemed inappropriate. The patient backgrounds, anesthesia, and surgical procedures for the remaining 15 cases included in the analysis were presented in Table 1 . The patients’ characteristics included a mean age of 51.3 ± 16.5 years, with 9 out of 15 being female (60 %), a mean BMI of 23.0 ± 2.8 kg/m 2, and with 9 out of 15 cases (60 %) involving the right foot. General anesthesia was administered to all cases with the addition of popliteal sciatic nerve block in 11 cases, femoral nerve block in 1 case, and spinal anesthesia in 1 case. The surgical sites consisted of 7 cases of forefoot surgery, 7 cases of midfoot or hindfoot surgery, and 1 case of knee surgery.
Table 1
Demographic characteristics of study participants.
| No. |
Age
(years) |
Sex
(F or M) |
Height
(cm) |
Weight
(kg) |
BMI
(kg/m 2) |
Side
(R or L) |
Anesthesia | Pathology/Surgical procedure |
|---|---|---|---|---|---|---|---|---|
| 1 | 56 | F | 159.0 | 76.4 | 30.2 | R | G, F | Knee osteoarthritis/Total knee arthroplasty |
| 2 | 50 | F | 154.5 | 43.0 | 18.0 | L | G, P | Iatrogenic hallux varus/MTP joint arthrodesis |
| 3 | 46 | M | 169.6 | 74.4 | 25.9 | R | G, P | Plantar plate injury (II MTP)/Plantar plate reconstruction |
| 4 | 70 | F | 150.2 | 44.6 | 19.8 | L | G, P | Hallux valgus/Osteotomy |
| 5 | 43 | M | 172.8 | 68.2 | 22.8 | L | G | Soft tissue tumor of the sole/Excision |
| 6 | 48 | M | 155.0 | 57.8 | 24.1 | R | G, P | CAI/ATFL repair |
| 7 | 71 | F | 162.7 | 58.0 | 21.9 | R | G, P | Hallux valgus/Osteotomy |
| 8 | 73 | F | 143.4 | 49.0 | 23.8 | R | G | Hallux valgus/Osteotomy |
| 9 | 20 | M | 168.5 | 72.6 | 25.6 | L | G, P | CAI/ATFL repair |
| 10 | 73 | F | 148.0 | 49.2 | 22.5 | R | G, P | Hallux valgus/Osteotomy |
| 11 | 20 | F | 157.9 | 59.5 | 23.9 | L | G, P | CAI & Tarsal coalition/ATFL repair & coalition resection |
| 12 | 61 | F | 157.0 | 48.4 | 19.6 | R | G, P | Rheumatoid arthritis/MTP joint arthrodesis |
| 13 | 37 | M | 173.5 | 67.0 | 22.3 | R | G, P | Accessory navicular/Accessory navicular resection |
| 14 | 48 | M | 153.3 | 50.5 | 21.5 | R | G, P | Rheumatoid arthritis/Total ankle arthroplasty |
| 15 | 53 | F | 159.4 | 59.2 | 23.3 | L | G, S | Osteochondral lesion of the talus/Arthroscopy |
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