Failure of the enlarged and deformed anterolateral portion of the femoral head to roll into the acetabulum during abduction alters hip joint mechanics. The resultant hinge abduction is associated with pain, and the patient often has restricted movement. A valgus osteotomy removes the deformed portion of the femoral head away from the weight-bearing area and ensures there is pain-free congruent range of movement around the weight-bearing position. The concomitant improvement in lever arm function and leg length results in a better gait pattern. In immature patients, abolition of hinge abduction allows the lateral acetabular ossification center to grow more normally.
Hinge abduction in Perthes disease was first described in the 1980s. The investigators described an abnormal movement of the hip during abduction associated with significant restriction of movement and pain. The abnormal movement cannot be detected clinically (although the restriction can). Arthrography under general anesthesia demonstrates an increase in medial pooling with abduction, suggesting an impingement of the lateral (or anterolateral) part of the enlarged and deformed cartilaginous femoral head on the lateral portion of the acetabulum. Some investigators have tried to quantify the widening of the medial joint space on plain films, whilst others have used different imaging techniques to describe the same phenomenon. Nevertheless, failure of the lateral (or anterolateral) part of the femoral head to roll into the acetabulum changes the biomechanics of the hip joint such that it then works like a hinge in abduction. Hinge abduction is accepted as a poor prognostic factor, and rapid progression to secondary osteoarthritis can be expected.
Aim of treatment
In cases of hinge abduction, the goal of the valgus osteotomy is not to reconstruct an anatomic hip joint, but to improve joint biomechanics by restoring the rolling motion of the femoral head into the acetabulum and joint congruity in the weight-bearing position. It does so by effectively moving the area of femoral head deformity away from the acetabulum and its labrum, hence off-loading it and making the spherical medial part of the femoral head load bearing. The procedure will never restore a full range of motion to the hip joint but it will reorient the arc of congruent joint movement, allowing abduction without hinging within the physiologic range. The valgus osteotomy also has the capacity to lengthen the leg and to lower the greater trochanter, thus improving the articulotrochanteric distance. This process improves the abductor lever arm and hence its efficiency. Thus, a valgus proximal femoral osteotomy relieves groin pain, and provides a better and more efficient functional range of motion for the patient. If designed appropriately, the procedure can also reduce the leg length difference, and all of these factors improve gait pattern and the long-term remodeling of the hip joint.
Timing of surgery
The timing of the surgery is crucial. By opting for the valgus osteotomy, the surgeon accepts that he or she is “salvaging” a poor mechanical situation and that a containment procedure would be ineffective. It is essential to the success of this procedure that the quality of the bone in the superomedial region of the femoral head is good enough to withstand the forces associated with weight bearing once it is redirected into a weight-bearing position.
The femoral head deformity associated with hinge abduction occurs almost exclusively in cases of major involvement of the femoral epiphysis. The Perthes disease should be in the healing stage before this osteotomy is considered. This situation can be judged on plain radiographs if there is significant ossification in the healing phase, but in the late fragmentation phase a magnetic resonance imaging (MRI) scan with intravenous gadolinium injection gives a better assessment of the revascularization of the femoral head. As a valgus osteotomy is a salvage procedure, there is little to be gained from performing the surgery too early.
Timing of surgery
The timing of the surgery is crucial. By opting for the valgus osteotomy, the surgeon accepts that he or she is “salvaging” a poor mechanical situation and that a containment procedure would be ineffective. It is essential to the success of this procedure that the quality of the bone in the superomedial region of the femoral head is good enough to withstand the forces associated with weight bearing once it is redirected into a weight-bearing position.
The femoral head deformity associated with hinge abduction occurs almost exclusively in cases of major involvement of the femoral epiphysis. The Perthes disease should be in the healing stage before this osteotomy is considered. This situation can be judged on plain radiographs if there is significant ossification in the healing phase, but in the late fragmentation phase a magnetic resonance imaging (MRI) scan with intravenous gadolinium injection gives a better assessment of the revascularization of the femoral head. As a valgus osteotomy is a salvage procedure, there is little to be gained from performing the surgery too early.
Preoperative assessment
Symptoms
The patient often presents with a relatively sudden deterioration in range of movement associated with significant pain, due to the abnormal joint biomechanics rather than the Perthes process itself.
Signs
The limp is confirmed on walking and a Trendelenburg sign is obvious. Measurement of the leg length discrepancy with blocks estimates the shortening of the involved hip and highlights the pelvic obliquity: care must be taken if there is a fixed adduction deformity. The reduction of hip movement, especially abduction, may be due to pain, muscle spasm, or femoral head deformity: limited abduction does not necessarily indicate hinge abduction. Further investigation is required to confirm the diagnosis.
Investigation
Despite the comments of some investigators, hinge abduction is a dynamic process. Thus preoperatively, it needs dynamic assessment: the static investigations of MRI and computed tomography scanning have a limited role. The most valuable investigation is a clinical examination combined with arthrography, performed under general anesthesia when all muscle spasm due to pain is absent. Leg length difference and fixed deformities must be documented: each may exacerbate the problems of hinge abduction and influence the choice of treatment. Following introduction of 2 to 3 mL of contrast material (Omnipaque 240/300; GE Healthcare, Amersham, UK) into the joint, an assessment of overall size and shape of the femoral head is made. Then the quality of femoroacetabular movement is assessed: as the leg is abducted, the sign of hinge abduction is noted with medial pooling of the dye and deformation of the lateral labrum ( Fig. 1 ). Such unstable movement may also be demonstrated with internal/external rotation of the femoral head in extension. The dynamic arthrogram also allows identification of the “position of best fit,” around which there is a cone of movement where there is congruity between the femoral head and the acetabulum. This position is usually in 15°–25° of adduction with sometimes a few degrees of flexion.
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