New Weight-Bearing Computed Tomography Studies Delineate Current Limitations and Future Possibilities

Editor’s note: This article is the second in a two-part series on weight-bearing computed tomography (WBCT). Part one, which offered an introduction to WBCT, was published in the August/September issue.

Interest in the utility of WBCT in different orthopaedic specialties has been on the rise. This technology allows surgeons to study a joint when loaded. Thus, the definition of certain joint positions and morphology during weight bearing in symptomatic and asymptomatic individuals became possible. The newly emerging measurements and parameters from WBCT could lead to better understanding of disease pathophysiology and modifications of current classifications, which would help guide appropriate surgical indications, patient selection, and prognostic expectations. This article presents an overview of the results from WBCT studies in foot and ankle surgery, knee surgery, and hand surgery. It delineates the current limitations and future horizons.

Foot and ankle surgery
Researchers have utilized WBCT to identify normal and pathological anatomy of the foot and ankle. Colin et al studied the morphology of the posterior facet of the subtalar joint in patients with no hindfoot pathology. The posterior facet was concave in 88 percent and flat in 12 percent of the 59 studied patients. It had a valgus orientation in 90 percent and varus orientation in 10 percent on the coronal plane. However, intraindividual differences were seen based on how posterior the subtalar vertical angle was measured.

Lepojärvi et al investigated the anatomy and rotation of the distal tibiofibular joint. WBCT images were obtained from 32 asymptomatic patients in three different ankle positions: neutral, external rotation, and internal rotation. In the neutral position, the fibula was anterior in the tibial incisura in 88 percent of the measurements. Patel et al used WBCT images in asymptomatic individuals to define normal range of the syndesmosis. In 200 studied ankles (50 men and 50 women), the upper limit for lateral fibula translation in non-injured individuals was 5.27 mm. Older age was associated with reduction in lateral translation, and fibulae in men were significantly more laterally translated.

Collan et al found significant differences between WBCT and non-WBCT measurements of first metatarsal alignment in hallux valgus patients. Najefi et al investigated the proportion of hallux valgus patients with increased first metatarsal pronation. Out of 102 feet, they found 32 feet (31.4 percent) with abnormal metatarsal pronation. The increased intermetatarsal angle was the most significant radiographic predictor of increased pronation. The sesamoid position was abnormal in 74.5 percent of patients. That position was more likely to change with an increased hallux valgus angle.

Assessment of the complex mechanical environment in acquired pes planovalgus deformity has benefited from WBCT technology. For example, Cody et al investigated talar and subtalar joint anatomy in adult patients with stage II pes planovalgus deformity and found that patients with flatfoot deformity had more innate valgus talar anatomy and more valgus alignment in the subtalar joint. This information could be used to identify patients at risk of progressive collapse of their flatfoot deformity.

atient-specific instru-mentation (PSI) in total ankle arthroplasty continues to gain popularity. The cutting guides are based on the patient’s anatomy. Preoperative WBCT images allow PSI models to accurately account for ankle deformity. Total ankle arthroplasty implants frequently use an extramedullary alignment guide, with a proximal pin at the tibial tubercle. Deformity of the knee or tibia or the presence of a total knee arthroplasty may complicate this process. The use of PSI eliminates the need for a proximal pin.

Wiley et al prospectively followed 20 patients who were surgically treated for tibial intra-articular pilon fractures. WBCT obtained 6 months postoperatively showed 21 percent reduction of the ankle joint space in the injured ankle. The middle lateral and middle central regions had the highest decrease in space. These data could ultimately be used to better classify post-traumatic ankle arthritis based on zones of joint space narrowing, correlate these findings with clinical outcome and prognosis, and assess the effectiveness of treatment options to decrease risk of arthritis.

Knee surgery
More recently, WBCT models have been extended to scan the knees and pelvis. Thawait et al compared WBCT and non-WBCT scans of 17 osteoarthritic knees and 18 asymptomatic knees, finding a significant reduction in joint space width (JSW) and increase in meniscal extrusion on WBCT compared with non-WBCT images. WBCT findings could enhance the current understanding of knee osteoarthritis progression. Kothari et al used three-dimensional (3D) images obtained via WBCT to demonstrate a significant association between a tibiofemoral JSW of <2 mm and physical function (score used to assess difficulty in daily physical activities) in 528 knees.></2>

Marzo et al showed significantly worse values in patellar tilt angle and tibial tubercle-trochlear groove offset (TTTG) in WBCT compared with non-WBCT in patients with acute or chronic patellar instability. Similarly, Hirschmann et al found that a non-WBCT scan may overestimate the TTTG offset as compared to WBCT scan due to the supine position and relaxed muscles. Further research to study threshold values in healthy individuals, versus individuals with patellofemoral disease, is warranted to better understand knee biomechanics and establish accurate surgical planning.

WBCT is one of the applications of cone-beam CT (CBCT). This technology uses a greater number of detectors as compared to multidetector CT scan and allows image acquisition without moving the patient. Nardi et al evaluated 20 symptomatic total knee arthroplasties using CBCT and found it to be an effective tool for reproducible measurements of patellar tilt angle and rotational alignment of the femoral and tibial components without significant impairment from the metal artifacts.

Hand surgery
Athletes and others who place high loads on the upper extremities can benefit from this imaging technology. Buckwalter et al used WBCT to study ulnar variance in 10 gymnasts. Patients completed two 45-second scans in the WBCT machine while doing a handstand on flat platform and parallettes. A conventional non-WBCT scan was also obtained. Significant differences in ulnar variance were seen in weight-bearing versus non–weight-bearing imaging. The results also varied based on the load and position of the wrist and the dominant versus non-dominant hand.

Current limitations of WBCT
WBCT offers significant advantages compared to conventional CT. However, limited contrast resolution hinders visualization of soft tissue compared with a multidetector CT scan. Upon comparing time spent (positioning the patient and imaging) for image acquisition using three different modalities, Richter et al found average results as follows:

• radiographs of both feet (bilateral dorsoplantar, lateral, and Saltzman hindfoot views): 902 ± 70 seconds
• conventional CT scans of both feet and ankles: 415 ± 46 seconds
• WBCT scan: 270 ± 44 seconds

Although WBCT average scanning time was shorter, there is still a risk of motion artifacts.

The application of this technology continues to be in an early clinical phase, with no standardized protocols in place to guide limb positioning for variable pathologies, imaging planes, and 3D reconstruction. Flat detectors used in CBCT have higher electronic noise compared with multidetector CT scan detectors, which represents a challenge to maintain image quality at a low radiation dose and visualize soft tissue.

Emerging developments
Photon-counting CT (PCCT), first approved in 2021, is a non-WBCT method that allows better assessment of small cortical and trabecular architecture, with half the radiation dose and higher image sharpness. Implementation of PCCT technology in WBCT scans would increase the spatial and contrast resolution of WBCT images. The prototype for an upright CBCT of the spine already exists, and the results showed potential applications in studying spine stability and quantifying neuroforaminal stenosis in a standing position. Incorporating other tomographic technology, algorithms, and machine learning into CT scanning could enhance our understanding and treatment strategies of different musculoskeletal pathologies.

Naji S. Madi, MD, is an assistant professor of foot and ankle surgery in the Department of Orthopaedic Surgery at West Virginia University in Morgantown, West Virginia.

References

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