Computed Tomography of the Knee Joint: Indications and Significance

Three-Dimensional Modelling and Printing in the Complex Knee

There is a rapidly growing interest in three-dimensional (3D) modelling and printing in orthopaedic surgery. Advancements in imaging technology with the advent of 3D modelling and 3D printing have revolutionised the medical field and provided physicians with powerful new tools to treat complex injuries and deformities. The knee, being regarded as one of the most complex joints in the body, has benefitted from this advancement with patient-specific models and custom instruments providing the treating clinician with detailed anatomical roadmaps specific to their patient.

Computed tomography (CT) is routinely used to produce 3D images by combining or stacking a series of 2D slices one on top of the other. Early techniques using this method produced images that estimated the bone edge, resulting in rough outlines that did not accurately depict the bone surface. Novel work has advanced 3D surface reconstruction, with imaging software being capable of applying algorithms with edge sharpening and smoothening capabilities. The result is high-quality images that more precisely represent the external surface geometry of bone. 3D shape mapping has provided the clinician with improved spatial orientation of conditions such as bone defects or loss, bony dysplasia and complex fractures.

3D printing describes a variety of techniques used for making physical objects from CT or magnetic resonance imaging (MRI) obtained graphical data. Through an additive process, successive layers of material such as metal or plastic are laid down until the object is complete. This revolutionary technology has now become far more accessible and more cost effective such that it has taken on a mainstream role in many areas of medicine. As technology advances with high-end 3D printing applications becoming available, direct 3D printing of functioning biocompatible tissue will be performed, a process known as bioprinting . These techniques are still in the early stages of development and at the time of this writing are not yet available. 3D printing is especially useful in complex cases where surgeons benefit from so-called virtual operations simulating the planned surgery. Fig. 4.1 highlights a case where 3D CT modelling was performed for a femoral condylar defect after a failed osteochondritis dissecans fixation. A virtual operation was performed using 3D CT images for planning bone cuts for osteochondral allograft implantation and fixation. Based on the 3D CT modelling, a set of surgery and patient specific tools were printed and used in the operation.

Computed Tomography in Trauma

Advances in computerised tomography have reduced data acquisition and 3D reconstruction times, allowing for faster and more accurate diagnosis and surgical planning in complex knee trauma. Multidetector CT (MDCT) is increasingly used as a first-line diagnostic modality instead of a plain film trauma series in advanced trauma life support in many centres. MDCT is able to image acutely unwell patients quickly and in nonanatomical positions where necessary to provide excellent bony and soft tissue detail. Plain film radiography has been reported to underestimate the extent of the fracture lines and subsequently risks missing fractures. Studies of tibial plateau fractures have shown surgical plans based on plain film radiography are modified in 6% to 60% of cases after CT and 21% of cases after MRI. Wicky et al. compared the diagnostic efficiency of plain film radiography and spiral CT with 3D reconstructions of 42 tibial plateau fractures. They evaluated the proposed surgical plan devised from different imaging methods for each fracture and found 3D CT was more precise and more closely matched the surgical report and postoperative radiograph. Other authors reported similar results, such as Manjula and Venkataratnam, who reported 3D CT was superior to both plain film and multiplanar 2D CT in demonstrating the spatial relationships of fracture fragments. Current evidence supports the use of fine MDCT with 3D reconstruction capabilities in complex knee trauma to facilitate accurate assessment of fracture patterns, areas of depression and displacement to assist surgical planning ( Fig. 4.2 ).

Computed Tomography Assessment of Bone Healing and Union

CT provides superior assessment of nonunion and visualisation of fracture lines compared with plain film radiography. The rapid data acquisition and high spatial resolution has seen CT used as the preferred investigation over radiography and MRI. Petfield et al. reported that the sensitivity of CT in detecting tibia fracture nonunions approached 100% and the specificity was 62%. A comparative study by Warwick et al. found MRI correlated well in the identification of spinal column fracture nonunions but that CT should be the preferred investigation for problematic or inconclusive cases. Although newer techniques in MRI sequencing have seen its use widen to include identification of occult fractures, most centres will still reserve CT for the identification of nonunion in difficult cases.

One of the difficulties in assessing bone healing is the accurate visualisation of the bony interface after either fracture fixation or osteotomy. This is especially so when healing is occurring by absolute stability where limited callus is produced. Other difficulties include the presence of metal artefact from metallic implants, impeding the value and diagnostic accuracy of CT by distorting the visualisation of bone, bone–metal interfaces and soft tissue structures. The artefacts may be present in different degrees of severity because of a variety of metals, shapes and sizes being used. Historically, metal artefact distorted the quality of CT images, rendering them difficult to interpret and of poor diagnostic value. Today, modern scanners with metal artefact reduction (MARS) capability provide images to clinicians with better resolution and less distortion.

MARS sequences are based on reduction of all its primary causes, such as beam hardening, scatter, photon starvation, noise, edge effects and the combined effect. Strategies include modifying standard acquisition and reconstruction, modifying projection data and/or image data, and applying dual-energy CT (DECT). Using modern software, postprocessing techniques such as MARS allow images previously uninterpretable to be reformatted and interpreted with high diagnostic accuracy ( Fig. 4.3 ).

Computed Tomography and Soft Tissue Injuries

Soft tissue injuries are commonly found in intraarticular fractures around the knee. The true incidence is unknown and highly variable but has been quoted to be between 52% to 99%. ^, Because CT, along with radiography, is a first-line investigative tool in the assessment of the severely injured knee, it may be used to identify soft tissue injuries acutely. Although most centres will rely on MRI, some authors have demonstrated the value of CT in the diagnosis of intraarticular ligament tears and avulsions in tibial plateau fractures. ^, They found that a smooth ligament contour without obscuration by the adjacent soft tissues was a reliable CT indicator for excluding ligament injury. Previously the thickness of the CT slices limited its accuracy, but with the advent of fine-slice CT the identification of soft tissue injuries has improved such that it may provide valuable information for the treating clinician. Mui et al. found only 2% of ligaments deemed intact on careful CT evaluation had partial or complete tears on MRI. The authors also attempted to determine whether fracture gap or articular depression on CT could be used to predict meniscal injury. Although more severe fracture gaps and depressed joint surfaces were associated with meniscal injury, no threshold could be judged and many were missed when no meniscal injury was predicted based on undisplaced fractures.

Most centres will favour MRI over CT arthrograms(CTas) in the assessment of intraarticular pathological conditions. Evidence suggests MRI provides higher interreader agreement and accuracy in assessing acute meniscal tears and chondral lesions ( Fig. 4.4 ). Traditionally, however, CTas were highly reliable and were commonly requested in patients presenting with injured knees to exclude suspected joint pathological conditions. During the 1970s and 1980s, CTa was the gold standard for the diagnosis of meniscal tears, bucket handle tears and meniscocapsular separation, with reliability reported to be 83% to 94%. With continuous rotation scanning, spiral acquisitions provide high-quality 2D multiplanar reconstructions with thin slices. Coronal, sagittal and even axial slices can detect tears that are not visible on MRI, and meniscocapsular separations based on contrast enhancement between meniscal wall and capsule. Today, some centres still prefer CTa to MRI in assessing meniscal healing after repair. Pujol et al. used CTa in assessing meniscal healing rates 6 months after repair, noting that CTa was the preferred modality because MRI was thought to be unsuitable and unreliable as a result of persistent nonspecific hypersignal within the tear ( Fig. 4.5 ). Nevertheless, contemporary approaches to suspected intraarticular pathological conditions generally favour MRI, CTa being used when MRI is contraindicated.

In assessing osteoarthritis, CTa remains useful in assessing cartilage thickness thanks to its spatial resolution and high contrast between low-attenuating cartilage and high-attenuating deep subchondral bone, with contrast material filling the gap in between. Omoumi et al. found CTa to be more accurate than MRI in evaluating cartilage thickness even on cartilage-sensitive sequences ( Fig. 4.6 ). CTa has been used in the predictive measurement of knee cartilage defects with printed 3D models, which are then used to build custom implants. Michalik et al. compared CTa with MRI in predicting the size of chondral defects using 3D modelling and a process of segmentation ( Fig. 4.7 ). CTa images were smoother and more vivid than the MRI processed images. They concluded that MRI underestimated the defect on average by 12% and CTa overestimated the defects by 3%. The popularity of CTa has nevertheless declined as MRI, which has the advantage of being noninvasive and without ionising radiation, has become more readily accessible.