The musculoskeletal system

11 The musculoskeletal system






DEVELOPMENTAL DYSPLASIA OF THE HIP


The term developmental dysplasia of the hip, previously known as congenital dislocation of the hip, refers to a broad spectrum of disorders of hip development. It is now the preferred term to describe the abnormal relationship of the femoral head to the acetabulum, and it includes the varied appearance of the inadequately formed acetabulum together with complete dislocation of the femoral head as well as partial dislocation or subluxation.


The reliance on physical examination and newborn screening programs for the diagnosis has been disappointing, as dislocated hips are still diagnosed in later infancy and childhood. Late diagnosis of dislocation refers to the diagnosis being made after 3 months of age and implies a failure of neonatal detection.


Since the early description in the late 1970s of the use of ultrasound in the diagnosis of hip abnormalities, ultrasound has increasingly been used as a tool to try and improve patient outcome and to improve diagnosis of this late presentation. The ability to see the cartilaginous acetabulum and femoral head on ultrasound has made it an exceedingly attractive choice of modality for examining the infant hip. Radiography at this early age is not useful for this condition, as the femoral head and a large component of the acetabular roof has not ossified. The unossified head and labrum can be clearly seen on sonography and, in addition, a dynamic assessment can be made where the head is manipulated in the acetabulum while the sonographer watches. Also the hip can be examined in a number of scan planes, which aids in the diagnosis. Arthrography was previously used to demonstrate abnormalities such as an inverted labrum and is occasionally still used.13


Currently there is no one universally accepted technique of performing an ultrasound examination of the infant hip, in addition to which there are two widely differing viewpoints in approaches to hip ultrasound. The European approach is based on the work of Graf, an Austrian orthopedic surgeon. His technique is based on a coronal scan plane and on the cartilaginous component of the acetabular roof. The primary American approach is related to the femoral head movement and displacement from the acetabulum. There is no universal policy for whole population screening for developmental dysplasia and very view centers are sufficiently resourced to do so. National screening programs are present in Austria, Switzerland and Germany, where Graf established his technique. Infant screening, including the use of ultrasound, has still not been able to identify all children who are at risk of congenital displacement of the hip.


The results of ultrasound screening have consistently shown that the non-surgical treatment rate has increased. In a Norwegian study this treatment rate was almost double in the ultrasound-screened group (34 versus 18 per 1000 infants screened) and it is assumed that many children in the ultrasound group who subsequently received treatment actually had false-positive ultrasound tests. Some would argue that this is a bad thing, as there is a risk of developing vascular necrosis of the femoral head from treatment alone.


On the other hand the surgical treatment rate did not decrease significantly in newborns screened with ultrasonography compared with those screened by physical examination alone.4




Clinical tests


There are two clinical tests which help to identify unstable hips in newborn infants (Fig. 11.1):




These examinations should be performed by an appropriately trained health professional. There is evidence that those who specialize in hip abnormalities have a better outcome with fewer late presenters. However there are those who argue that some infants are normal at birth (and therefore screening) and then go on to develop dislocation.


There are a number of risk factors associated with developmental dysplasia of the hip:



All those infants with displaced or unstable hips identified at screening and with a positive screening test should be seen by an orthopedic surgeon experienced in the field. It is very important to identify unstable hips as soon as possible, as the tissues are still supple and the chance of successful treatment is better at an earlier age.


Ideally the orthopedic surgeon will perform a joint clinic with the sonographer or have access to a hip ultrasound service. This combined approach is undoubtedly the most successful and rewarding for the sonographer and will ultimately yield the best results. Also in this milieu of increasing litigation the security of operating as a team is beneficial to all, including the patient.


Infants with displaced or unstable hips should be examined by 2 weeks of age and for those at risk the examination is best performed when an infant is 4–6 weeks old. By this time, most immature hips have stabilized.


It is very important that the parents are kept informed and made aware that any screening program may not identify all affected children.


There are other classical signs of congenital hip displacement which may be present at birth and which become increasingly apparent after the first 6 weeks as the legs extend and the head of the femur displaces upwards. Parents should be made aware of



Once the child is walking, other suspicious features are a limp or anxieties about the child’s walk, discrepancy in leg length and abnormalities of lower limb posture (Fig. 11.2).




TECHNIQUE OF HIP SCANNING


There is no universally accepted standard technique for ultrasound examination of the infant hip in the world today, and there are at least three described methods practiced and in use:



Undoubtedly the best results are obtained with the sonographer working in close collaboration and conjunction with an orthopedic surgeon experienced in DDH so that consistent reliable results are produced and terminology understood by all, with improved outcomes for the infant.


The infant must be relaxed for the examination so that adequate assessment of the hip can be made and so that stress maneuvers are accurate. A screaming kicking infant cannot be examined and it is essential that they are given time to feed and calm down. A soother dipped in 5% dextrose water is invaluable.


Curvilinear probes can be used, but it is now widely accepted that high-frequency linear probes are undoubtedly the best and do not distort the image for angle measurements if these are to be taken. For a young infant a 15L8 can be used and a lower frequency will be needed in an older infant. Once the femoral head starts ossifying and produces a densely acoustic shadow, sonography is less reliable, but this is generally later and after 3–6 months. When the ossific nucleus enlarges, the acetabular floor is obscured by its acoustic shadow, but by this time plain radiographs are reliable.



The Graf technique


The Graf technique requires the sonographer to be able to identify important landmarks in the coronal plane and from these landmarks measure the alpha and beta angles. This technique requires some training and practice and the sonographer should ensure that they are adequately trained by an experienced practitioner in this method if it is to be used.5


It is recommended that a positioning device is used to immobilize the patient. The infants are placed on their side and the transducer is placed over the greater trochanter (Fig. 11.3). The hip should be in slight flexion. When the trochanter, femoral neck and acetabulum lie in the same plane, this is optimal. The transducer is positioned so that it lies exactly in the frontal plane parallel to the body’s long axis and should not be tilted. The standard sonographic plane should be easily achieved in this position.



The following main reference points need to be identified:



Once this standard plane has been achieved, the reference lines are drawn (Figs. 11.4 and 11.9).




The alpha angle is described as the bony roof angle and is the most important to measure. The beta angle is the cartilaginous roof angle and is not universally used (Table 11.1).



Table 11.1 is an adaptation of Graf’s classification of hip types. Types III (subluxation) and Type IV (dislocation) are probably the easiest to identify. Graf Type III involves persistent lateral displacement of the femoral head from the acetabular floor with deviation of the labrum. Type IV hips are complete dislocations, usually posterior and towards the head, with the femoral metaphysis obscuring the acetabular floor (Fig. 11.5).



From a sonographer’s point of view the standard image plane cannot be achieved when the head is dislocated, neither can accurate angles be measured. Types III and IV are very rarely detected outside specialist centers.



The Graf classification is complex, and training is needed in order to be competent in the technique.

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Dec 21, 2015 | Posted by in PEDIATRIC IMAGING | Comments Off on The musculoskeletal system

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