and Claudio Chillemi2
(1)
Sapienza University of Rome, Latina, Italy
(2)
Latina, Italy
Parameniscal Cyst
This is a pseudocyst rising from a meniscal tear wherein the synovial fluid is entrapped. It is important to recognize a communication between the cyst and meniscal tear. This sign has a PPV of more than 87 %, with the exception of the anterior horn of the LM, where the PPV is near 65 %. Differential diagnosis includes the articular ganglia, in which there isn’t a communication with a meniscal tear (Fig. 1).
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Figure 1
Parameniscal cysts: sagittal consecutive PD FS images (a, b) show medial meniscal oblique tear (arrowhead) with cystic fluid collection on the posterior margin of the meniscus (arrows on b); note that the posterior horn of the medial meniscus must be normally attached to the posterior capsule and a connection between tear a cyst may be seen on (a)
Parrot-Beak Tear
A parrot-beak tear is a progression of a radial and longitudinal meniscal tear with displacement of the free edge. The tear is correlated with knee mechanical locking or catching. In case of a meniscal body abnormally small on sagittal images, it should suspect a parrot-beak tear, and a careful search for a displaced fragment on axial planes must be performed (Fig. 2).
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Figure 2
Displaced parrot-beak lesion: Axial TSE T2-weighted (a) and coronal PD FS (b) show a tear that combines features of a radial (perpendicular to C-shaped fibers) and longitudinal tear (parallel to C-shaped fibers). The result is the displacement of a meniscal portion similar to a parrot beak. Articular fluid into the lesion must be interpreted as instability feature of the tear (arrows in a and b). On coronal plane the meniscus seems absent. Abundant distension of the gastrocnemius-semimembranosus pseudo-bursa is also evident on axial plane
Patella Alta
In this condition, the patella is too high above the trochlear groove, due to long patellar tendon.
In patients with high-riding patella, the degree of flexion results too high for patellar engaging in the trochlea. When the knee presents a genu varum position, patella alta manifests itself because the extensor mechanism becomes the hypotenuse of a triangle, exposing the patella to subluxation. Patella alta may be measured on lateral radiographs or sagittal MR images with extended knee using the following:
Patellar Tendon/Patella or Insall-Salvati Ratio
This is the ratio between the lateral height of the patella and the length of the patellar tendon (from the lower apex of the patella to the tibial tuberosity); a ratio >1.50 indicates a patella alta, while a ratio < 0.70 indicates a low patella (Fig. 3).
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Figure 3
Insall-Salvati ratio: on the basis of measures on the image, the knee has an Insall-Salvati ratio of 4.1/5.7 = 0.71, compliant with “normal patella”
Patellar Delayed Ossification
This is a component of a syndrome known as nail-patella syndrome. This is an autosomal dominant ectodermal disorder affecting several organs with patellar hypoplasia, fingernail dysplasia, posterior iliac exostoses, and dysplasia of the radial heads. The condition should be suspected when the patient does not extend the knee completely and radiographs show a small and luxated patella or even a patellar aplasia.
Patellar Fractures
Patellar fractures account for 1 % of total knee fractures and are classified according to bone exposure presence and fracture trace characteristic. Both aspects influence their treatment and management. The main types of patella fractures include vertical, marginal, transverse, and osteochondral fractures. The most common fracture is the transversal one. Separate diagnostic, imaging, and management processes and considerations are applicable to each type.
Radiography: Radiograms are the most useful imaging modality in the majority of cases, with complications of a patellar fracture frequently identified radiographically. Anteroposterior (AP), lateral, and tangential or Merchant views should be featured in a radiographic examination for patellar fractures. In evaluating the trabecular pattern of the patella, comminution, and separation of fracture fragments, lateral views may be useful. In the assessment of vertical fractures and differentiating a fracture from a partitioned patella, tangential views are useful. Patella fractures may be obscured by AP radiographs. Radiology reports should note any distance over 3 mm between fragments of fractured patellar, as this may indicate increased incidence of posttraumatic degenerative arthritis and malunion. The identification of an osteochondral fracture is also crucial, as a loose body may occur if a fragment that contains cartilage, subchondral bone, and trabecular bone is displaced. In transverse fracture, it is important to evaluate the fragment displacement. Osteochondral fragments are best demonstrated on the lateral view; usually it is possible to identify the presence of an effusion and a high-riding patella. Radiographs are less useful than other modalities (e.g., MRI) in characterizing any cartilaginous injury associated with an osteochondral patellar fracture and can define fractures not otherwise detectable by radiographic examination. Sleeve fractures are located in the coronal plane of the patella, and are difficult to diagnose with radiographs. Differentiating between acute fractures from a partitioned patella may pose challenges on radiographs. The features of bipartite patella include even, corticated, opposing margins, best seen in the tangential projection. Comparison of two knees may be useful.
CT: CT is useful particularly when a suspected fracture cannot be seen on radiographs and for osteochondral injuries. If the CT scan results are normal, then a fracture can be excluded. The use of CT scanning can prevent delays in treatment and identify the location of fragments and the position of any intra-articular loose bodies. CT scanning has limited usefulness in evaluating soft tissue and bone marrow injury, and an MRI should be considered. CT scans may be better than MRIs in identifying loose bodies. CT scans are obtained with the patient in the supine position and with 15° of external rotation of feet pressed against a footrest at 90°. A complete exam comprises three scans with the knee resting, extended quadriceps contracted, and with a flexion of the knee of 15° with a relaxed leg. Multiplanar reconstruction should be done to better localize the displaced fragments. CT cannot determine the age of fractures because it may remain evident for up to 24 months.Stay updated, free articles. Join our Telegram channel
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