Three-dimensional weightbearing computed tomography (CT) can be a powerful diagnostic tool. This author provides a guide to reading weightbearing CT images, how they can reveal pathology and how they can lead to more effective treatment.
What is three-dimensional cone beam imaging? Unlike conventional computed tomography (CT), which has a fan-shaped X-ray beam, modalities such as the pedCAT (Curvebeam) have a cone-shaped X-ray beam. This allows clinicians to obtain an image of a volume of tissue in one circumferential pass instead of having to take multiple slices with multiple exposures.
This cone beam, as a result, reduces radiation exposure to the patient. There have been questions about the amount of radiation exposure patients are receiving from the PedCAT machine. From my research, I have found that the radiation dose for scanning one foot is equivalent to performing dorsoplantar, oblique and lateral X-rays of one foot. In other words, it is equivalent to three exposures of the foot. For a bilateral scan, it is equivalent to taking three exposures of each foot. If we practice radiation safety, as we already do, we will not be bringing harm to our patients.
Remember the four radiation protection measures: time, distance, collimation and shielding. We want to keep the amount of time our patients are exposed to an absolute minimum. We want healthcare professionals and others not having the exam to stay at a safe distance. We always want to use the smallest exposure field possible (so only do bilateral scans if it is medically necessary). Lastly, we want to ensure appropriate shielding to the patient and the healthcare worker. Remember these radiation protection tips and we can keep patients and health professionals safe from overexposure to radiation.
For those pathologies in which plain film radiography is the initial imaging method of choice (e.g. fractures, bone tumors and tarsal coalitions), CT is the advanced imaging method of choice when more information is necessary (e.g. intra-articular fractures, occult fractures and small bone tumors).
What podiatrists need to remember is that X-ray and CT are very similar. We read CT images the same way we read plain radiographs. Based on the patient history, one is looking for malalignment, abnormal bone radiolucencies or radiopacities, any abnormal cortical disruption or production, and any abnormal soft tissue radiolucencies, radiopacities or foreign bodies. Therefore, if evaluating a CT scan of a non-union, one would look for the same features visible on a plain radiograph. Six to nine months after the injury or surgery, one would see no bridging of bone and fracture ends that have become rounded and sclerotic.
One of the main benefits of weightbearing CT is the ability to perform actual weightbearing examinations. The doctor is then able to evaluate the 2D images as well as the 3D images.
Consider a CT image of a woman standing in a shoe. We can remove the view of the outer portion of the shoe and evaluate how the foot is fitting inside of the shoe. We can then remove the view of the soft tissues to evaluate the alignment of the bones in the shoe (see photo 2). We would also be able to evaluate the fit of an orthotic or prosthetic device inside of the shoe.
One can also view the 3D image and the accompanying 2D images in the sagittal, axial (transverse) and coronal (frontal) planes. In the sagittal slice, we can appreciate the alignment of the long axis of the foot inside the shoe. In the axial (transverse) slice at the plantar aspect of the foot, we can see how the toes are pushed together in the tight-fitting toe box. This would be a valuable tool in showing a patient how the foot fits in a particular shoe, especially if the shoe is causing problems.
Another great feature of weightbearing CT is the ability to perform bilateral scans. Photo 4 shows a bilateral scan rotated in an oblique fashion. We also have the accompanying 2D sagittal, axial (transverse), and coronal (frontal) images. One can also view the same patient with the view of the the soft tissue structures removed. This is an excellent tool to show a patient exactly where a bunion is underneath the soft tissue. This will help the patient understand when the doctor is explaining the surgery.
In another view, we can evaluate the placement of the hardware on the sagittal slice. This is a great image if we were evaluating this patient postoperatively for aseptic loosening of the hardware or radiographic signs of osteomyelitis related to the hardware. Remember that the radiographic appearance of aseptic loosening will have a funnel-like radiolucent configuration around the hardware. Osteomyelitis will have a more circular appearance around the hardware.
Consider a 3D CT view of a patient with a collapsed arch due to Charcot neuroarthropathy. We are scanning the patient because of a pressure ulcer on the plantar aspect of the foot. In the right image in photo 7, the image has been rotated to show the patient’s plantar pressure ulcer (red arrow) with the soft tissue structures still in place. Once, the surgeon has removed the view of the soft tissue structures, we can see exactly which bone and which area of the bone are causing the ulcer. This valuable information can aid significantly in the proper offloading of the problem area.
Several authors have investigated the utility of weightbearing CT scans for various foot and ankle conditions.
Yoshioka and colleagues studied 10 patients with posterior tibial tendon dysfunction and 10 control patients, using weightbearing and non-weightbearing CT.1 The authors noted that the study clarified part of the clinical condition of the forefoot in flatfoot deformity, saying this may be applicable in basic research of the staging advancement and substage classification of flatfoot.
Krähenbühl and coworkers used weightbearing CT to determine the subtalar vertical angle in a study of 40 patients with osteoarthritis and 20 control patients.2 The study authors found that measuring the subtalar vertical angle was a reliable and consistent method to assess the varus/valgus configuration of the posterior facet of the subtalar joint.
Geng and colleagues studied weightbearing and non-weightbearing CT scans of 10 patients with hallux valgus and 10 control patients, reconstructing 3D models for the first metatarsal and the medial cuneiform.3 Researchers noted the study furthers an understanding of the physiological and pathological mobility of the first metatarsocuneiform joint.
As good as 3D CT images are, we should not underestimate the value of 2D images, namely the accompanying sagittal, axial (transverse), and coronal (frontal) images. For example, a coronal (frontal) image can reveal exactly which bone is the cause of a plantar wound.
In consideration of the close proximity of the bone to a wound, of course one should consider the possibility of osteomyelitis, which is often associated with ulcers. Again, one would be looking for the same findings on the CT that we would see on a plain radiograph: osteopenia; small, ill-defined radiolucencies in the medullary bone or cortex; new periosteal bone formation; cortical breakthrough; and/or gas in the soft tissue.
Consider a 3D CT image of a foot in a patient suspected of having post-traumatic degenerative joint disease (aka osteoarthritis). After removal of the view of the soft tissue structures, we can appreciate the narrowing of the ankle joint space. Again, if we are worried about degenerative joint disease, we are looking for the same findings that we would see on a plain film radiograph. Remember that the hallmark of this condition is the osteophyte. In addition, we would be looking for subchondral sclerosis and non-uniform joint space narrowing. The 3D image shows the accompanying sagittal, axial (transverse) and coronal (frontal) images. On the sagittal image, we can appreciate the bone-on-bone apposition in the anterior portion of the ankle joint.
In addition, in the coronal (frontal) image, we can further appreciate the joint space narrowing and bone-on-bone apposition. One can see all three radiographic features of degenerative joint disease. On the sagittal image, we see osteophyte formation, subchondral sclerosis and non-uniform joint space narrowing. Again, the cartilage has deteriorated to the point that there is bone-on-bone apposition. In the coronal (frontal) image, we also see the non-uniform joint space narrowing.
Photo 13 shows another example of a foot with degenerative joint disease at the first metatarsocuneiform joint. In the non-weightbearing images, we can see the radiographic changes of the condition: osteophyte formation, subchondral sclerosis and non-uniform joint space narrowing. However, on the weightbearing image, we can also now appreciate the bone-on-bone apposition.
In photo 14, we see a foot with a fracture of the fibular sesamoid, which was not evident on plain film radiographs. There is the 3D image with the soft tissue structures removed and the accompanying 2D sagittal, axial (transverse) and coronal (frontal) images. Though one can appreciate the fracture on the 3D image, we should also appreciate the value of the 2D images. In the sagittal slice, there is a jagged radiolucency in the fibular sesamoid. There are no sclerotic margins in the fracture line, indicating that this is an acute fracture, just as we would see in a plain film radiograph. In the axial (transverse) and coronal (frontal images), we can also see the radiolucent fracture line, devoid of sclerotic margins, which is indicative of an acute fracture. These same views would also be important later in the patient’s treatment to evaluate fracture healing.
Consider a perfect example of the benefit of weightbearing CT. On another lateral radiograph, an abnormality is not visible. However, in the 2D sagittal slice, we can see an obvious fracture in the anterior process of the calcaneus. This shows how CT is an important “next step” or adjunct to plain film radiography. As per imaging protocol, one should take plain film radiographs first. However, CT is the advanced imaging method of choice for bone trauma or bone pathology of any kind.
Weightbearing CT is the next generation of imaging for podiatric medicine. It is a safe imaging modality with low radiation exposure. It can provide superior images in comparison to conventional CT. I would like to remind podiatric physicians that we are the undisputed champions when it comes to the diagnosis and treatment of foot pathology. We are the best physicians to use this cutting-edge technology.
Weightbearing CT is a powerful tool that can enhance biomechanical evaluation, preoperative planning, postoperative evaluation, wound management and sports medicine as well as the treatment of arthritic conditions (especially degenerative joint disease) and trauma (especially when looking for occult or hairline fractures). This technology is an asset to podiatric medicine now and will continue to be so in the future.
Dr. Armstrong is the Dean of the School of Podiatric Medicine at Barry University. He is a Diplomate of the American Board of Podiatric Medicine and a Dilpomate of the American Board of Wound Management.
1. Yoshioka N, Ikoma K, Kido M, et al. Weight-bearing three-dimensional computed tomography analysis of the forefoot in patients with flatfoot deformity. J Orthop Sci. 2016; 21(2):154–8.
2. Krähenbühl N, Tschuck M, Bolliger L, et al. Orientation of the subtalar joint: measurement and reliability using weightbearing CT scans. Foot Ankle Int. 2016; 37(1):109–14.
3. Geng X, Wang C, Ma X, et al. Mobility of the first metatarsal-cuneiform joint in patients with and without hallux valgus: in vivo three-dimensional analysis using computerized tomography scan. J Orthop Surg Res. 2015; 10:140.
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Editor’s note: For related articles, see “Can Weightbearing CT Advance Our Planning For Forefoot Reconstructive Surgery?” in the July 2016 issue of Podiatry Today or the October 2013 online-exclusive case study “How Weightbearing CT Can Be A Valuable Diagnostic Tool” at http://tinyurl.com/kshu6c7
For other related articles, visit the archives at www.podiatrytoday.com