The aim of treatment of Perthes in the early part of the disease is to prevent the femoral head from getting deformed by muscular forces and weight-bearing stresses transmitted across the acetabular margin. To achieve this, femoral head extrusion must be preempted in children who are older than 8 years at onset of the disease by ensuring containment as soon as the disease is diagnosed. In children younger than 8 years in whom femoral head extrusion occurs, containment must be obtained by the early stage of fragmentation.
Why treat Perthes?
Perthes disease is a self-limiting disease in children; the interruption of the blood supply to the femoral epiphysis is temporary and complete revascularization of the epiphysis is the norm if the child is younger than 12 years at onset of the disease. No treatment is needed to facilitate the process of revascularization. As the blood supply to the epiphysis is reestablished, the necrotic bone is completely replaced by healthy new bone by a process of “creeping substitution” ( Fig. 1 ). This raises the question “why do we need to treat children with Perthes disease?”
Treatment is needed in some children in whom the femoral head may become deformed while revascularization proceeds over a period of 2 to 4 years. Treatment of these susceptible children should ideally be directed to attempting to prevent the femoral head from becoming deformed.
To plan an appropriate strategy for preventive treatment it is imperative that one understands what causes the femoral head to become deformed and when during the evolution of the disease irreversible deformation of the femoral head occurs.
How does femoral head deformation occur?
Following the vascular insult part or all of the femoral epiphysis becomes necrotic; this triggers a synovitis, articular cartilage hypertrophy, and hypertrophy of the ligamentum teres. These soft tissue changes compounded by muscle spasm cause the femoral head to extrude beyond the acetabular margin. Weight-bearing stresses and forces of muscular contraction are transmitted across the acetabular margin onto the extruded part of the femoral head. While normal healthy bone can quite effectively withstand these physiologic stresses, bone of the avascular epiphysis is not capable of withstanding them. If more than 20% of the width of the epiphysis extrudes outside the acetabular margin, there is a very high risk of irreversible femoral head deformation ( Fig. 2 ). Extrusion appears to be the most important factor that predisposes to femoral head deformation.
How does femoral head deformation occur?
Following the vascular insult part or all of the femoral epiphysis becomes necrotic; this triggers a synovitis, articular cartilage hypertrophy, and hypertrophy of the ligamentum teres. These soft tissue changes compounded by muscle spasm cause the femoral head to extrude beyond the acetabular margin. Weight-bearing stresses and forces of muscular contraction are transmitted across the acetabular margin onto the extruded part of the femoral head. While normal healthy bone can quite effectively withstand these physiologic stresses, bone of the avascular epiphysis is not capable of withstanding them. If more than 20% of the width of the epiphysis extrudes outside the acetabular margin, there is a very high risk of irreversible femoral head deformation ( Fig. 2 ). Extrusion appears to be the most important factor that predisposes to femoral head deformation.
When does femoral head deformation occur?
The disease was first divided by Waldenstrom into two stages, based on plain radiographic appearance: an initial evolutionary stage and a later stage of healing. Subsequently investigators divided the disease into 4 stages: the stage of avascular necrosis, the stage of fragmentation, the stage of reconstitution or regeneration, and the healed stage. More recently the first 3 stages were further divided into early and late stages ( Fig. 3 ), thus enabling clinicians to define the timing of events leading to femoral head deformation more clearly.
It was noted that in untreated children femoral head extrusion increased modestly through the initial stages of the disease, but abruptly increased as they reached the late stage of fragmentation, often exceeding 20% ( Fig. 4 ). This finding clearly suggests that the predilection for deformation increases once the disease evolves to this stage.
The epiphysis is most vulnerable to deformation during the late stage of fragmentation and in the early part of the stage of reconstitution. The reason for the vulnerability varies in these two stages. In the late stage of fragmentation the necrotic bone is being actively resorbed and the dead trabeculae are thus weakened and prone to collapse. On the other hand, in the early stage of reconstitution viable trabeculae of the woven bone that is newly laid down on the periphery of the epiphysis are also prone to collapse, because the bony trabeculae are initially laid down haphazardly and not in the direction that enables them to resist stress. Until the woven bone is replaced by mature lamellar bone (with trabeculae aligned so as to resist compressive and tensile forces), this propensity for deformation will persist ( Fig. 5 ).
The impression that femoral head deformation occurs either at the late stage of fragmentation or soon thereafter is supported by the observation that the metaphysis widens quite abruptly at this stage. Marked widening of the metaphysis suggests that the femoral head is flattened and that coxa magna is going to develop, as there is a strong correlation between metaphyseal width during the course of Perthes disease and the size of the femoral head at skeletal maturity.
The timeline of treatment of Perthes disease
Because it is clear that femoral head deformation tends to occur around the late stage of fragmentation, this stage divides the cycle of evolution of the disease into two distinct parts; the stages preceding this point form the early part of the disease cycle (see Fig. 3 ).
More importantly, this line defines when treatment aimed at preventing femoral head deformation is likely to succeed. Any preventive intervention is justified only from the onset of the disease to the early stage of fragmentation. Treatment instituted at the late stage of fragmentation or thereafter is either remedial in nature or frankly salvage in nature ( Fig. 6 ).
What are the treatment options?
There are 3 theoretical options for treatment directed toward preventing femoral head deformation.
- 1.
The first approach attempts to prevent the femoral head from bearing forces across the acetabular margin by either preventing or reversing extrusion of the femoral head.
- 2.
The second attempts to minimize stress on the femoral head by avoiding bearing weight on the limb.
- 3.
The third, relatively novel approach, which is still experimental, attempts to prevent the bone from becoming weak by reducing the osteoclastic resorption of the necrotic bone (see article by Little and Kim elsewhere in this issue).
The first of these options is what is most widely practiced currently, and is based on the principle of “containment.”
Containment
Containment is the term used to describe intervention that ensures that the anterolateral part of the femoral epiphysis is positioned within the acetabulum, thereby protecting the epiphysis from being subjected to deforming stresses.
Containment can be achieved by two different methods. The first involves positioning the femur either in abduction and internal rotation or in abduction and flexion ( Fig. 7 ), which can be done by casting, bracing, or surgery on the femur. Alternatively, containment can be achieved by an osteotomy of the pelvis that reorients the acetabulum such that it covers the anterolateral part of the femoral epiphysis (eg, Salter osteotomy, triple innominate osteotomy) or by creating a bony shelf over the extruded part of the epiphysis.
