Weight-Bearing Computed Tomography of the Foot and Ankle in the Pediatric Population

Cone-Beam CT of the Extremities in Clinical Practice

Cone-beam CT (CBCT) is a promising tool with increasing applications in musculoskeletal imaging due to its ability to provide thin-section CT images of the appendicular skeleton and introduce weight bearing, which accounts for loading forces that typically interact with and affect this anatomy. CBCT devices include an x-ray source directly opposite a digital silicon detector panel that performs a single rotation around an object of interest, obtaining thin-section images. Currently, the majority of research has been focused on the utility of CBCT with foot and ankle pathologic abnormalities, due to the complex architectural arrangement of the tarsal bones and weight-bearing nature of the lower extremities. Associated software can provide a variety of options for image reconstruction, including metal artifact reduction, three-dimensional biometric measurements, and digitally reconstructed radiographs. Advancements in this technology have allowed imaging of the knee, hip, hand, and elbow. As more data are published, it is becoming evident that CBCT provides many additional benefits, including fast imaging time, low radiation dose, lower cost, and small equipment footprint. These benefits allow placement of CBCT units outside of the traditional radiology department, including the orthopedic clinic setting. These technologic developments have motivated clinicians to define the scope of CBCT for diagnostics, surgical planning, and longitudinal imaging. As efforts are made to create standardized protocol and measurements, the current understanding and surgical approach for various orthopedic pathologic conditions will continue to shift, with the hope of improving outcomes. ©RSNA, 2024 Test Your Knowledge questions for this article are available in the supplemental material.

Quantitative Skeletal Imaging and Image-Based Modeling in Pediatric Orthopaedics

PURPOSE OF REVIEW

Musculoskeletal imaging serves a critical role in clinical care and orthopaedic research. Image-based modeling is also gaining traction as a useful tool in understanding skeletal morphology and mechanics. However, there are fewer studies on advanced imaging and modeling in pediatric populations. The purpose of this review is to provide an overview of recent literature on skeletal imaging modalities and modeling techniques with a special emphasis on current and future uses in pediatric research and clinical care.

RECENT FINDINGS

While many principles of imaging and 3D modeling are relevant across the lifespan, there are special considerations for pediatric musculoskeletal imaging and fewer studies of 3D skeletal modeling in pediatric populations. Improved understanding of bone morphology and growth during childhood in healthy and pathologic patients may provide new insight into the pathophysiology of pediatric-onset skeletal diseases and the biomechanics of bone development. Clinical translation of 3D modeling tools developed in orthopaedic research is limited by the requirement for manual image segmentation and the resources needed for segmentation, modeling, and analysis. This paper highlights the current and future uses of common musculoskeletal imaging modalities and 3D modeling techniques in pediatric orthopaedic clinical care and research.

Classification of the Os Calcis Subtalar Morphology in Symptomatic Flexible Pediatric Pes Planus Deformity Using Weightbearing CT and Distance Mapping

BACKGROUND

The etiology of symptomatic pediatric pes planus (PP) deformity is unclear. Reduced os calcis subtalar joint (OCST) anterior facet morphology has been suggested to result in less support to the talar head and a higher propensity to develop PP deformity. Weightbearing computed tomography (WBCT) and distance mapping (DM) offer new opportunities to investigate PP deformity in general and the OCST specifically. The purpose of this study is to investigate the OCST morphology using DM and to classify PP subtalar subtypes with DM using Bruckner's A-D classification system.

METHODS

Forty feet in 25 patients in a national referral center were evaluated for symptomatic PP deformity that failed nonoperative treatment. A WBCT scan was performed as part of the preoperative evaluation. Visualization of the distance distribution between the articulating surfaces of the subtalar joint was based on a DM technique. Intra- and interobserver agreement of the subtalar morphology was assessed using Bruckner's classification system.

RESULTS

The mean age was 10.7 ± 1.4 years. The following mean ± SD and median ± ranges were semiautomatically measured for this group: Meary angle -21 ± 8, calcaneal inclination 15 ± 4 degrees, talar coverage angle 39 (range 32.6-49) degrees, and hindfoot moment-arm 16 ± 5 mm. Classifying subtalar morphology using DM yielded an excellent intra- and interobserver agreement. The individual percentages of each individual subtype were calculated: type A 5%, type B 48%, type C 4%, and type D in 44%.

CONCLUSION

This study demonstrated excellent intraobserver and interobserver agreement in classifying the OCST using DM. A higher prevalence of types B and D was observed compared to types A and C in this PP cohort.

LEVEL OF EVIDENCE

Level III, retrospective study.

Biomechanical Implications of Congenital Conditions of the Foot/Ankle

Segmental foot and ankle models are often used as part of instrumented gait analysis when planning interventions for complex congenital foot conditions. More than 40 models have been used for clinical analysis, and it is important to understand the technical differences among models. These models have been used to improve clinical planning of pediatric foot conditions including clubfoot, planovalgus, and equinovarus. They have also been used to identify clinically relevant subgroups among pediatric populations, quantify postoperative outcomes, and explain variability in healthy populations.

Assessment of foot deformities in individuals with cerebral palsy using weight-bearing CT

OBJECTIVE

The aims of this study were to visualize and quantify relative bone positions in the feet of individuals with cerebral palsy (CP) with a foot deformity and compare bone positions with those of typically developed (TD) controls.

MATERIALS AND METHODS

Weight-bearing CT images of 14 individuals with CP scheduled for tendon transfer and/or bony surgery and of 20 TD controls were acquired on a Planmed Verity WBCT scanner. Centroids of the navicular and calcaneus with respect to the talus were used to quantify foot deformities. All taluses were aligned and the size and dimensions of the individuals' talus were scaled to correct for differences in bone sizes. In order to visualize and quantify variations in relative bone positions, 95% CI ellipsoids and standard deviations in its principle X-, Y-, and Z-directions were determined.

RESULTS

In individuals with CP (age 11-17), a large variation in centroid positions was observed compared to data of TD controls. Radiuses of the ellipsoids, representing the standard deviations of the 95% CI in the principle X-, Y-, and Z-directions, were larger in individuals with CP compared to TD controls for both the calcaneus (3.16 vs 1.86 mm, 4.26 vs 2.60 mm, 9.19 vs 3.60 mm) and navicular (4.63 vs 1.55 mm, 5.18 vs 2.10 mm, 16.07 vs 4.16 mm).

CONCLUSION

By determining centroids of the calcaneus and navicular with respect to the talus on WBCT images, normal and abnormal relative bone positions can be visualized and quantified in individuals with CP with various foot deformities.