#Circumscribed Abscess of Bone—­“Brodie’s Abscess."#—­The most important form of relapsing osteomyelitis is the circumscribed abscess of bone first described by Benjamin Brodie.  It is usually met with in young adults, but we have met with it in patients over fifty.  Several years may intervene between the original attack of osteomyelitis and the onset of symptoms of abscess.

Morbid Anatomy.[7]—­The abscess is nearly always situated in the central axis of the bone in the region of the ossifying junction, although cases are occasionally met with in which it lies nearer the middle of the shaft.  In exceptional cases there is more than one abscess (Fig. 120).  The tibia is the bone most commonly affected, but the lower end of the femur, or either end of the humerus, may be the seat of the abscess.  In the quiescent stage the lesion is represented by a small cavity in the bone, filled with clear serum, and lined by a fibrous membrane which is engaged in forming bone.  Around the cavity the bone is sclerosed, and the medullary canal is obliterated.  When the infection becomes active, the contents of the cavity are transformed into a greenish-yellow pus from which the staphylococcus can be isolated, and the cavity is lined by a thin film of granulation tissue which erodes the surrounding bone and so causes the abscess to increase in size.  If the erosion proceeds uniformly, the cavity is spherical or oval; if it is more active at some points than others, diverticula or tunnels are formed, and one of these may finally erupt through the shell of the bone or into an adjacent joint.  Small irregular sequestra are occasionally found within the abscess cavity.  In long-standing cases it is common to find extensive obliteration of the medullary canal, and a considerable increase in the girth of the bone.

[7] Alexis Thomson, Edin.  Med.  Journ., 1906.

[Illustration:  FIG. 120.—­Segment of Tibia resected for Brodie’s Abscess.  The specimen shows two separate abscesses in the centre of the shaft, the lower one quiescent, the upper one active and increasing in size.]

The size of the abscess ranges from that of a cherry to that of a walnut, but specimens in museums show that, if left to Nature, the abscess may attain much greater dimensions.

The affected bone is not only thicker and heavier than normal, but may also be curved or otherwise deformed as a result of the original attack of osteomyelitis.

The clinical features are almost exclusively local.  Pain, due to tension within the abscess, is the dominant symptom.  At first it is vague and difficult to localise, later it is referred to the interior of the bone, and is described as “boring.”  It is aggravated by use of the limb, and there are often, especially during the night, exacerbations in which the pain becomes excruciating.  In the early stages there are periods of days or weeks during which the symptoms abate, but as the abscess increases these