MRI of the hip allows for the assessment of articular cartilage, ligaments, the presence of fluid as well as a cartilage and a specialised form of cartilage known as the labrum.
The initial radiological assessment of hip pain is with a standard X-ray. MRI of the hip may be performed where the X-ray is diagnostic, or where detected, to assess the severity of a condition.
Slipped upper capital femoral epiphysis (SUCFE)
In the dramatic case below, one can see the femoral head (F) slipping off the shaft of the femur at the level of the physeal plate (arrows). This gives an appearance of the femoral head akin to a scoop of ice-cream slipping off its cone, which is seen in both imaging planes. This diagnosis is known as slipped upper capital femoral epiphysis (SUCFE) and usually occurs in the early teenage years.
The condition occurs due to repetitive stress, with the physeal plate being a region of inherent weakness. The physeal plate is a critical structure within developing bones, as it is here where immature bone cells (and thus weaker) develop, with their formation allowing the bone to grow in length to its mature adult state. Any disease process that disrupts the physeal plate can result in its premature closer and thus a shorter bone and possible limb discrepancy.
Delayed diagnosis may result in healing of the femoral head in a less than ideal position and thus loss in incongruity of the joint.
Figure 1A. MRI of a child’s hip demonstrates femoral head (F) slipping off the shaft of the femur at the level of the physeal plate (arrows). This gives an appearance of the femoral head akin to a scoop of ice-cream slipping off its cone, which is seen in both imaging planes. This diagnosis is known as slipped upper capital femoral epiphysis (SUCFE)
Cam type femoroacetabular impingement (FAI)
In the same case below, imaging through the axial plane shows the lack of offset of the femoral head that manifests as a small “bump” (arrow) at the femoral head-neck junction. This bump is also known as a cam lesion (due to the similarity of it appearing and functioning like a cam shaft) or Ganz lesion. This bump, is typically not an issue in the resting position, however in various positions, particularly extreme ranges of motion, can collide with the acetabulum, to result in the condition of cam type femoroacetabular impingement (FAI).
Figure 1B. Imaging through the axial plane shows the lack of offset of the femoral head that manifests as a small “bump” (arrow) at the femoral head-neck junction.
Compare the shape and position of the femoral head with a normal example below. Note the normal contour of the head and how it is seated clear of the neck, providing adequate offset and thus preventing the head-neck junction (arrow) from colliding with the acetabulum during activity.
Figure 2. MRI of a child’s hip demonstrating normal contour of the femoral head.
Along a similar theme is the below case, which demonstrates a very subtle cam lesion (arrows). It is felt that the below imaging appearances, given the similarity with the case above, are likely caused by a very subtle slip of the femoral head (however not to the same extent) which is subclinical; that is to say, not noticed by the patient, nor treating doctor due to the relative lack of symptoms. The two diagnoses are thus felt to represent a spectrum of the same process.
Cam-type FAI is typically diagnosed in young males, often during their teenage years who place high demands on their hip with athletic activity, such as football. If present on both sides, the decreased mobility of each hip results in increased forces transmitted through the pelvis, specifically the pubic symphysis and thus is seen as an important contributing factor to the development of osteitis pubis (pubic symphysitis).
Interruption of the blood supply of the femoral head can occur at a critical stage during its development, which may be compounded by the repetitive microtrauma of activities of daily living. As the blood supply is interrupted, bone cells die (avascular necrosis), with growth of the head occurring in an asymmetric manner and where healing is limited. The progression of this condition ultimately is known as osteochondrosis and in the hip is known by the eponymous name of Perthes’ disease (and in the case of the capitellum of the elbow, is known as Panner’s disease). In its most severe form the femoral head collapses to result in a completely abnormal contour and incongruity of the joint (arrows).
Perthes’ disease occurs in a slightly younger age group than those patients presenting with SUCFE; typically in the eight to ten year old range. Once again, compare the contour of the femoral head in the case below with the normal case shown previously (Figure 2).
Children may often present with sudden pain and stiffness of their hip, for which the causes are numerous, including the cases already discussed.
Another cause is infection of the joint, known as septic arthritis as seen in the case below. The first manifestation of this condition is increased fluid in the joint known as an effusion (arrows). Other causes for an effusion include reactive/toxic synovitis as well as inflammatory arthritis such that due to juvenile chronic arthritis (JCA).
Septic arthritis caused by bacteria is typically a surgical emergency, as the joint provides the bacteria with easy access to the blood stream and can therefore rapidly lead to systemic septic shock and circulatory collapse. Further to this, chemicals released by the immune system in an attempt to destroy the bacteria within the joint unfortunately come into contact with the cartilage, where it is extremely toxic to the cartilage cells.
Septic arthritis can therefore result in rapid cartilage loss, which when occurs, is irreversible and thus predispose to the development of the early onset of osteoarthritis.
Assessment of non-bony structures
MRI is also used to assess non-bony (non-osseous) structures. In this case, the below young child presented with pain following a twisting injury, where a labral tear is present (arrow), peeled off its acetabular base (A) as is the joint capsule (double arrows). The diagnosis in this case was an episode of hip subluxation (partial instability), however without dislocation (complete instability).