A Novel Vector-Based Computer Tomography Alignment Measurement Protocol for Total Knee Arthroplasty.

INTRODUCTION: Component position and overall limb alignment following total knee arthroplasty (TKA) have been shown to influence implant survivorships and clinical outcomes. While most surgeons utilize standard x-ray imaging for preoperative joint assessments, computer tomography scans (CT), coupled with automated digital analyses have been shown to provide additional surgical and clinical benefits. However, to date, a postoperative CT measurement protocol has not been reported for robotic-arm assisted TKA (RATKA). Therefore, the purpose of this paper was to assess the validity of a novel, vector-based CT alignment measurement protocol. Specifically, we compared: 1) final versus planned component alignment and placement; 2) inter-observer reliability; and 3) intra-observer reliability. MATERIALS AND METHODS: The CT-based technique utilized mathematical models to calculate prosthetic alignments from anatomical landmarks. To assess the models, 30 CT scans from multiple centers were collected on RATKA patients at six weeks postoperatively and analyzed using the proposed technique. Results were compared to the surgeons' preoperative plans for accuracy. Analyses were performed on the same protocol to determine inter-observer reliability. These analyses were repeated 30 days later to assess for intra-observer variability. RESULTS: The mean measurement errors compared between final versus planned component positions and alignments were: 0.79±1.48o varus in overall limb alignment (p=0.004); 0.34±1.20o varus (p=0.121); and 0.35±1.15o varus (p=0.17) for femoral and tibial varus/valgus alignment; 0.71±1.77o flexion (p=0.18) and 0.38±1.88o posterior (p=0.41) for femoral flexion and tibial slope. There was strong reproducibility between observers. Correlation analyses showed low variabilities, with slopes between 0.8 to 1.0 and all R>0.8. CONCLUSION: As robotic technologies become widely available in orthopaedic surgery, it is critical to have tools, such as CT protocols, which can quantitatively verify operative decisions concerning limb alignment and component placement. This study described a novel, vector-based, CT alignment measurement protocol for RATKA which has not previously been defined. The method demonstrated excellent accuracy to plan and low intra- and inter-observer variability. This is a valuable analysis tool for RATKA studies where component accuracy is assessed using postoperative CT images.

CT of the hip prosthesis: appearance of components, fixation, and complications.

Multidetector computed tomography (CT) is an excellent way to supplement the radiographic evaluation of problematic hip prostheses. Multidetector CT is well suited for assessing periprosthetic bone, determining precise acetabular cup position, and evaluating periprosthetic fluid collections or ossified masses. Metal implants pose a number of challenges in the performance and interpretation of CT examinations. However, metal artifacts can be minimized by decreasing the detector collimation and pitch, increasing the kilovolt peak and milliampere-seconds, and using appropriate reconstruction algorithms and section thickness. Image interpretation requires a basic understanding of hip reconstruction and hip implants, as well as use of a systematic method of analysis that incorporates prior radiographic findings and CT findings. Radiologists must be familiar with the normal and abnormal CT appearances of hip prostheses and be able to recognize common complications on CT scans.

Total hip arthroplasty in patients with bone deficiency of the acetabulum.

Total hip replacement (THR) requires revision in only a minority of cases (approximately 17% of prosthetic hips fail), but when THR failures occur there may be significant acetabular bone deficiency. There is a variety of surgical hardware and strategies available to address this problem. The causes of primary THR revision include aseptic loosening or particle disease, infection, recurrent dislocation, implant failure, periprosthetic fracture, and leg length discrepancy. Almost all patients who need THR revision undergo a standard radiographic evaluation of the pelvis and hip. In general, CT is an excellent tool for evaluating loosening of the prosthesis caused by either mechanical reasons or infection, and MR imaging is best suited for evaluating the soft tissues surrounding the prosthesis. Nuclear medicine studies are performed when results of CT and MR imaging are inconclusive. When patients are evaluated for revision THR, radiologists must check for acetabular cup loosening, the amount and type of bone stock loss, the amount of component migration, and the presence or absence of liner wear. Before revision hardware is placed, bone stock loss must be repaired, either by using bone grafting or by placing accessory acetabular hardware such as cups, rings, or cages. The long-term success of revision acetabular surgery varies; there is acetabular cup presence at 5 years after surgery in 60%-94% of cases. Complications include postoperative infections, repeat liner wear, bone graft failure, periprosthetic or prosthetic fractures, dislocation, vascular injury, and nerve injury.

Routine isotropic computed tomography scanning of chest: value of coronal and sagittal reformations.

OBJECTIVE: We sought to evaluate the usefulness of coronal and sagittal reformations from isotropic chest computed tomography (CT) examinations. METHODS: A total of 30 chest CT examinations were reconstructed into 2 sets of axial source images: 0.9-mm slice width with 0.45-mm reconstruction interval (isotropic) and 4-mm slices with 3-mm reconstruction interval. The isotropic dataset was reformatted into coronal and sagittal stacks with 4-mm slices. Three readers reviewed the image sets with 4-mm slice widths. Coronal and sagittal reformations were compared at the same sitting to axial images for depiction of anatomy and disease in the aorta, pulmonary arteries, hilar regions, mediastinum, lung parenchyma, pleura, diaphragm, thoracic spine, ribs, and trachea. A 5-point scale was used to determine whether nonaxial reformations showed anatomy and disease significantly better, somewhat better, same, somewhat worse or significantly worse than equivalent thickness axial source images. A 3-point scale was used to score if nonaxial image sets showed no, some, or significant additional information compared with the axial plane regarding the main diagnosis. RESULTS: There was better visualization of the hilar regions, diaphragm, spine, and trachea on the coronal reformations compared with source axial images (P < 0.05). Sagittal reformations scored better than axial source images for aorta, pleura, diaphragm, spine, and ribs (P < 0.05). The coronal and sagittal series showed significant additional information in 11% and 9% of patients, respectively. CONCLUSION: Radiologists should consider the use of one or both of coronal and sagittal planes in addition to the axial series in routine interpretation of chest CT.

Multichannel CT: evaluating the spine in postoperative patients with orthopedic hardware.

Evaluating the spine in patients with metal orthopedic hardware is challenging. Although the effectiveness of conventional computed tomography (CT) can be limited by severe beam-hardening artifacts, the evolution of multichannel CT in recent years has made available new techniques that can help minimize these artifacts. Multichannel CT allows faster scanning times, resulting in reduced motion artifacts; thinner sections, with which it is possible to create a scanned volume of isotropic voxels with equivalent image resolution in all planes; and the generation of a higher x-ray tube current, which may result in better penetration of metal hardware and reduction of artifacts. Although 140 kVp and high milliamperage-second exposure are recommended for imaging patients with hardware, caution should always be exercised, particularly in children, young adults, and patients undergoing multiple examinations. The acquisition of multiplanar reformatted images in the axial, sagittal, coronal, and oblique planes and of three-dimensional volume-rendered images optimizes image interpretation. Wide window settings are best for reviewing images when hardware is present. The integrity of hardware is best assessed with multiplanar average intensity projection. Soft-tissue structures are best visualized by interactively varying the window width and level settings. Implementation of these techniques can yield diagnostic-quality images and aid in patient treatment.

CT with 3D rendering of the tendons of the foot and ankle: technique, normal anatomy, and disease.

Three-dimensional rendering of computed tomographic data with volume rendering (VR), shaded surface display (SSD), and maximum intensity projection has been performed for over 20 years. In the foot and ankle, no one image reformatting technique is satisfactory for displaying every anatomic relationship or disease process. Two-dimensional multiplanar reformatted (MPR) images are the basic images used for diagnosis. MPR images are especially useful for identifying small fractures. VR is useful for demonstrating the relationships between ankle tendons and the underlying osseous structures, and SSD is useful when fractures extend to the articular cortex and a disarticulated view is desired. Three-dimensional images are helpful in patients with congenital deformities, arthritis, and trauma.

Normal tibiofibular relationships at the syndesmosis on axial CT imaging.

OBJECTIVES: Computed tomography (CT) is reported to be superior to plain radiography for imaging the syndesmosis, but CT criteria differentiating normal from abnormal tibiofibular relationships do not exist. The purpose of this study was to define normal tibiofibular relationships at the syndesmosis on axial CT imaging and to report the reliability of these measurements. METHODS: Thirty healthy volunteers underwent CT evaluation of bilateral ankles. Axial CT measurements consisted of tibiofibular clear space, tibiofibular overlap, anterior tibiofibular interval, and fibular rotation (θ(fib)). To assess reliability, 3 investigators independently made each CT measurement on 2 separate occasions. RESULTS: Sixty ankles were included for analysis. CT measurements demonstrated excellent intrarater and interrater reliability. There was significant anatomic variability between individuals. Specifically, statistically significant gender differences were discovered in CT measurements of tibiofibular overlap and anterior tibiofibular interval. Variance between ankles of each subject was calculated. In an uninjured population, tibiofibular intervals do not vary by more than 2.3 mm, and the rotation of the fibula does not vary by more than 6.5° between ankles of the same person. CONCLUSIONS: Measurements of tibiofibular relationships made on axial CT images are reliable. Because of significant anatomic variation between individuals, using a patient's contralateral ankle for comparison provides a precise definition of normal tibiofibular relationships. These criteria allow for the detection of subtle variations in the tibiofibular relationships indicating instability and provide a tool for postoperatively assessing the reduction of the injured syndesmosis.

Multichannel CT Imaging of Orthopedic Hardware and Implants.

The introduction of multichannel CT scanners provides both radiologists and surgeons with a new tool to image patients with orthopedic hardware. The key parameters that have made it possible to image the implants and the surrounding bone with multichannel CT are the higher available technical factors (kVp and mAs) coupled with the ability to acquire thin slices over a large scan region. These properties make it possible to produce high-quality multiplanar reformations that facilitate visualization of the orthopedic device and the surrounding bone. An important consideration for multichannel CT imaging of hardware is the reduction of cone beam artifacts caused by the geometry of multichannel CT scanners. This artifact is reduced by using a narrower x-ray beam collimation and a low pitch setting. This article discusses CT scan parameters and image postprocessing used at our institution and illustrates common clinical problems encountered when imaging implanted orthopedic devices. These include fracture healing, loosening of joint prostheses, evaluation of particle disease, and the use of CT for preoperative planning in revision arthroplasty.

Three-dimensional volume rendering of the tendons of the ankle and foot.

Three-dimensional (3D) rendering is the process of creating two-dimensional (2D) images that convey the 3D relationships of an object or objects. In the past 10 years, the use of volume-rendering (VR) images has become a commonly used method of 3D display. The improved data sets that result from spiral and multidetector computed tomography, improved software, and more powerful workstations significantly improve radiologists' ability to create these images. The process by which these images are obtained is described. It is possible to simultaneously demonstrate the tendons and bones of the ankle and hindfoot using VR because of significant attenuation differences among the soft tissue, tendons, and bone: fat approximately -100, muscle approximately 30, tendon approximately 90, and bone approximately 250 to 500 Hounsfield units. These 3D images can be helpful in the care of patients with ankle/foot deformities resulting from trauma, idiopathic disorders, and arthritis. These images present a global image that provides improved understanding of the relationships of the bones and adjacent tendons.