This is because, as in the elbow, there is a sign to look for which will tell the reporting radiographer that there is an intra-articular fracture present, even if one is not visible. That sign is the lipohaemarthrosis which is a joint effusion with a fat/blood fluid level. Image 1 shows a lipohaemarthrosis, and it occurs when fat and blood have escaped from a fractured bone and leaked into the supra patella bursa. The bursa appears expanded and has a darker upper half and a lighter lower half. This is because the fat is less dense than blood and therefore floats on top of the blood and does not absorb as much of the x-ray beam. Joint effusions may occur without a fracture present but fat will not be included in the effusion. In this case the appearance will be similar to image 1 without the darker area within the bursa see image 2. Of course the knee may be fractured without an apparent effusion, do not be fooled into thinking no effusion, no fracture.

Image 2 shows an ordinary joint effusion, the appearances are similar to that of image 1 but without the fat/blood fluid level as this effusion does not contain any fat. Compare images 1 and 2 with image 3 which is a normal knee. It should be possible to see that in images 1 and 2 the patella tendon has been pushed anteriorly and the supra patella bursa has ballooned and is lifting the patella away from the femur. Image 4 shows a lateral knee with soft tissue swelling anterior to the patella. The patella has not been lifted forward and the bursa is not expanded. This should not be confused with a joint effusion.

Image 5 shows a fracture of the medial femoral condyle. The fracture can be seen centrally between the condyles, and the medial condyle is displaced medially and can be seen protruding past the medial border of the femur. This particular image may be mistaken for a longitudinal fracture of the patella by an inexperienced person. However, checking the cortex around all the visible bone, as should be done with all radiographs, the steps in the bone at the fracture sites can be seen. Image 6 again shows a longitudinal fracture but this time the fracture line is clearly seen beyond the patella and indeed splits into two. The lateral condyle in this case is separated and the fracture extends proximally up the shaft.

In both of the above cases the fractures may not be visualized if there were any rotation of the femur. This is where the lipohaemarthrosis comes into play. If there is a lipohaemarthrosis on the lateral but no fracture is seen on a slightly rotated A.P., thought may be given to producing a better A.P.

Image 7 is another good example of recognising the importance of the lipohaemarthrosis. This is a horizontal beam lateral, and there is a lipohaemarthrosis, the image has been turned and cropped to show more detail. At first glance there is little to see but remember there must be an intra-articular fracture for there to be a lipohaemarthrosis. Look carefully along the articular surface of the lateral femoral condyle and it should be possible to see an irregularity. If not, click on the image and it will be pointed out. Now look at image 8, the A.P. view. It should be possible in this view to see a small intra-articular bony fragment lying laterally in the joint space. Again, any difficulty, click on the image to be shown. Image 9 is a slight oblique view and shows the irregularity quite well, click on it for help. This is a small flake fracture from the lateral femoral condyle which could easily be missed if it were not for the lipohaemarthrosis.

Image 10 shows a very small fracture of the lateral tibial plateau but looking at image 11, even this was enough to cause a lipohaemarthrosis. Image 12 is similar in appearance to Image 10 but is an avulsion fracture of the joint capsule from the lateral tibial condyle known as a Segond fracture. Although this could be an isolated injury this type of fracture is indicative of an anterior cruciate ligament injury.

Images 13, 14 and 15 show the appearance of the depressed tibial plateau fracture. Image 13 is a fairly obvious fracture, image 14 less obvious, and image 15 even less visible but they all follow the same rules. The plateau has been pushed downwards with widening of the knee joint on the one side, and in the case of image 13, the fragment has been forced laterally. Occasionally the only way to diagnose a tibial plateau fracture is from the widening of the joint space and a sclerotic (lighter) band across the injured side. Image 16 is the lateral of image 15 and was taken in the horizontal position and does show a lipohaemarthrosis, which again shows the importance of the horizontal ray lateral in trauma. This fracture, which is a Salter-Harris type 4, could have been missed if not for the lipohaemarthrosis.

Stress fractures of the tibia are not confined to paediatrics. Image 17 shows a very missable fracture given away by a slight sclerotic band. Click on the image to be shown where the fracture is. Now scroll down to the lateral view, image 18, where you can see a similar appearance but with the addition of the give away break in the posterior cortex.

Image 19 shows a transverse fracture of the patella. A skyline view should not be done here as it would pull the fragments apart and the fracture would not be visible on it anyway. Image 20 shows a fine lucency running across the patella. As this patient had a large joint effusion this is likely to be an undisplaced fracture. Be careful however, these lucencies can be misleading and may only be due to trabecula patterns or projectional anomalies. Always highlight them in the report and let the doctor make the decision. They should always treat the patient not the radiograph.

It is not very often that a lipohaemarthrosis occurs with a fracture of the patella, as the fat content of the patella is not as great as the larger bones of the knee. Image 23 shows a fractured patella with a lipohaemarthrosis. There was no other fracture in this knee and the lipohaemarthrosis was caused by the fact that the patella was in several parts allowing blood and fat to escape into the bursa.

Image 24 shows a Pellegrini-Steida lesion. This is where there is continual minor trauma to the collateral ligaments resulting in calcification. Probably one for the radiologist just in case it is some other soft tissue pathology.

Image 25 shows a fibrous cortical lesion. These are quite harmless as a rule but again this is one for the radiologist. Having said that image 26 is also a fibrous cortical lesion but this time it has a subtle fracture through it. In this case it may be prudent to issue a report highlighting the fact that there is a fracture but also stating that a radiologist's opinion should be sought as to the nature of the pathology.

Images 27 to 31 show normal growth variants often mistaken for fractured patellae. Image 27 is a tripartite patella. There is another version of the tripartite where there is one fragment on both the medial and lateral sides of the patella. Images 28, 29 and 30 are bipartite where there is only one fragment which can be on either side. Image 31 shows a small un-united secondary ossification centre which has persisted into adulthood.

Image 32 shows the normal appearance of the paediatric / adolescent tibial tubercle which can often be mistaken for a fracture. The appearance of the developing tibial tubercle varies a lot and can be fragmented. It is also important to remember that this is also the site for "Osgood Schlater's" disease. If there is any doubt, pass it on to the radiologist.

Image 33 shows a large area of ossification within the patella tendon. It lies above the tibial tuberosity and is therefore not associated with it. It is probably due to old trauma or tendinosis.

Images 34 to 36 show a solitary osseous loose body. It is lying superior and slightly laterally to the patella and is probably in the supra patella bursa, although further imaging such as MRI is needed to give a definitive diagnosis.

Image 37 shows an accessory ossicle adjacent to the lateral tibial plateau. Although this is most likely to be a normal growth variant due to its appearance, it is possible that this is an un-united Segond fracture, see image 12.

Images 40 and 41 show an irregular ossification of the medial femoral condyle. The fragments are well corticated and contained within the radiolucent cartilaginous area of the "normal" condyle, not to be mistaken for loose osseous bodies where the condyles would almost certainly be normally ossified and the loose bodies extra to that. Note also the flattening of the tibial plateau. Image 40 is an intercondylar or tunnel view, these fragments did not show on the A.P. but were initially noted on the lateral view, image 41 where they appear at the posterior aspect and could be mistaken for a fracture. Note also the normal sesamoid bone lying posterior to the condyles which can also be seen in image 40 as a more opaque area of the lateral condyle, this sesamoid is known as the fabella.