It is of paramount importance to distinguish hips that may or may not be suitable for containment in the transitional stage (fragmentation to early reossification stage), because long-term clinical and radiological outcomes may be influenced by the choice of treatment. This article summarizes the definition of hinge abduction, pathoanatomy of hinge segment, assessment of hinge abduction, determination of reducible and irreducible hinge abduction, and treatment strategies for containable and uncontainable hips in the transitional stage.
Some children with severe Legg-Calvé-Perthes disease (LCPD; Catterall 3 or 4, Herring B/C or C), may present late in the course of the disease, that is, in late fragmentation or reossification stage, with an already collapsed and deformed femoral head due to undue force passing through the anterosuperolateral aspect of the femoral head. Abnormal hinge movement of the hip joint, so-called hinge abduction, is the result of extrusion of the epiphyseal segment of the femoral head. If hinge abduction is fixed, abnormal hinge movement results in progressive subluxation, collapse of lateral pillar, and widening of the femoral head, preventing it sliding into the acetabular socket completely. The resulting alteration in the center of rotation presents with restricted hip motion, which is often associated with antalgic gait, short-limb gait, Trendelenburg gait, Duchenne gait, and out-toeing or in-toeing gaits. The authors reported that out-toeing and in-toeing gaits are apparently caused by the compensatory rotation of the proximal femur to avoid impingement by placing the femoral hump to the relatively deficient anterolateral part of the hip joint.
It is of paramount importance to distinguish hips that may or may not be suitable for containment in the transitional stage, because long-term clinical and radiological outcomes may be influenced by the choice of treatment. This article summarizes the definition of hinge abduction, pathoanatomy of hinge segment, assessment of hinge abduction, determination of reducible and irreducible hinge abduction, and treatment strategies for containable and uncontainable hips in the transitional stage (fragmentation to early reossification stage).
Definition of hinge abduction
Hinge abduction was first described by Grossbard, who described abnormal hinge movement in 4 patients with residual deformity in LCPD. The concept of hinge abduction was further elucidated by Catterall, who demonstrated the value of intraoperative arthrography in determining the presence of abnormal hinge movement. These authors defined, using intraoperative dynamic arthrography, that hinge abduction is a phenomenon of impingement of the outer part of the femoral head onto the lateral lip of the acetabulum, typically showing widening of medial joint space by levering the inferomedial portion of the femoral head laterally away from the teardrop in the acetabular floor in abduction on the anteroposterior (AP) view. Nakamura and colleagues further refined the definition of hinge abduction to include the observance of an increasing subluxation index (percentage ratio of the medial joint space to the acetabular width) with abduction or a positive impingement sign. They defined the latter as the relationship of the tangential point of the most superior part of epiphysis to the lateral edge of the bony acetabular rim in maximum abduction.
Pathoanatomy of hinge segment and assessment of hinge abduction
The collapse and subluxation of the femoral head appear to be influenced differently by area of necrosis, mechanical properties of the necrotic segment, and direction of hip loading force in severely involved LCPD. Rab and colleagues demonstrated in a theoretical study on subluxation using a 3-dimensional rigid body spring method hip model that the direction of subluxation was sensitive to the direction of hip loading force, which might be modified by the presence of pain and contractures. Three-dimensional pathoanatomy of the hinge segment may differ in each patient, and thereby the pattern of abnormal hinge movement can vary according to the size, location, and configuration of the impinging hump. Pécasse and colleagues claimed that deformation of the femoral head is usually multidirectional, flat, or broad in the frontal pane and phalloid in the axial direction. The authors have observed that the main hump on the femoral head can locate from anterior to lateral aspect of the femoral head. In this context, the authors think that hinge abduction is a complex manifestation of the hinge movement occurring in the continuum between lateral and anterior impingement.
The earliest clinical sign of abnormal hinge movement during follow-up, in general, is a sudden deterioration in range of motion of the hip, particularly abduction. When diagnosis of hinge abduction is suspected, the nature of the abnormal hinge movement should be carefully evaluated. It is essential to examine the hip to see whether it is caused either by contractures of adductor muscles or by true hinge abduction due to impingement of the deformed head against acetabular rim.
Three-dimensional understanding of hinge movement is important in planning the surgical method, because the pattern of abnormal hinge movement can vary according to the spatial features of the impinging hump of the femoral head. Preoperatively, the authors usually take plain AP radiographs with the hip in neutral, abduction, adduction, and frog-leg lateral position in addition to false profile view to determine subluxation and congruity. The authors found that 3-dimensional computed tomography (CT) examination with multiplanar reformation images is useful to assess the spatial features of the hinge segment of the femoral head. Magnetic resonance (MR) and ultrasound images also provide valuable information about spatial features of osteocartilaginous hinge segment. Intraoperatively, dynamic arthrography under general anesthesia is very helpful to determine the position of stability and congruity between the femoral head and the acetabulum. Lateral impingement can be detected on the anteroposterior arthrograms of the hip by moving the lower limb in adduction/abduction in combination with the internal rotation/external rotation position. In contrast, anterior impingement can be checked in the true lateral arthrograms of the hip by moving the hip into the flexion/extension position. The congruent position of the superior portion of the head is also confirmed by taking the craniocaudal projection of the image with the hip in extension.
Pathoanatomy of hinge segment and assessment of hinge abduction
The collapse and subluxation of the femoral head appear to be influenced differently by area of necrosis, mechanical properties of the necrotic segment, and direction of hip loading force in severely involved LCPD. Rab and colleagues demonstrated in a theoretical study on subluxation using a 3-dimensional rigid body spring method hip model that the direction of subluxation was sensitive to the direction of hip loading force, which might be modified by the presence of pain and contractures. Three-dimensional pathoanatomy of the hinge segment may differ in each patient, and thereby the pattern of abnormal hinge movement can vary according to the size, location, and configuration of the impinging hump. Pécasse and colleagues claimed that deformation of the femoral head is usually multidirectional, flat, or broad in the frontal pane and phalloid in the axial direction. The authors have observed that the main hump on the femoral head can locate from anterior to lateral aspect of the femoral head. In this context, the authors think that hinge abduction is a complex manifestation of the hinge movement occurring in the continuum between lateral and anterior impingement.
The earliest clinical sign of abnormal hinge movement during follow-up, in general, is a sudden deterioration in range of motion of the hip, particularly abduction. When diagnosis of hinge abduction is suspected, the nature of the abnormal hinge movement should be carefully evaluated. It is essential to examine the hip to see whether it is caused either by contractures of adductor muscles or by true hinge abduction due to impingement of the deformed head against acetabular rim.
Three-dimensional understanding of hinge movement is important in planning the surgical method, because the pattern of abnormal hinge movement can vary according to the spatial features of the impinging hump of the femoral head. Preoperatively, the authors usually take plain AP radiographs with the hip in neutral, abduction, adduction, and frog-leg lateral position in addition to false profile view to determine subluxation and congruity. The authors found that 3-dimensional computed tomography (CT) examination with multiplanar reformation images is useful to assess the spatial features of the hinge segment of the femoral head. Magnetic resonance (MR) and ultrasound images also provide valuable information about spatial features of osteocartilaginous hinge segment. Intraoperatively, dynamic arthrography under general anesthesia is very helpful to determine the position of stability and congruity between the femoral head and the acetabulum. Lateral impingement can be detected on the anteroposterior arthrograms of the hip by moving the lower limb in adduction/abduction in combination with the internal rotation/external rotation position. In contrast, anterior impingement can be checked in the true lateral arthrograms of the hip by moving the hip into the flexion/extension position. The congruent position of the superior portion of the head is also confirmed by taking the craniocaudal projection of the image with the hip in extension.
Determination of reducible and irreducible hinge abduction
With hinge abduction, the center of movement of the femoral head is located on the lateral edge of acetabulum, and the labrum is deformed upwards in attempted abduction. Reducible hinge abduction can be demonstrated by the position of the femoral head that would center within the acetabulum in abduction without imposing undue pressure on the lateral edge of the acetabulum. The authors think that Hilgenreiner–labral angle and epiphyseal slip-in index measured in abduction on anteroposterior view of the arthrograms provide an objective measure of the degree of reducibility of flattened and extruded femoral head underneath the acetabulum ( Fig. 1 ). Hilgenreiner–labral angle is defined as the angle between the line parallel to the Hilgenreiner line and the line connecting the lateral acetabular margin and the labral tip in abduction between 30° and 45°. Epiphyseal slip-in index is calculated as the horizontal distance from lateral margin of the bony acetabular rim to the tip (tangential point) of the epiphysis slipping into the acetabulum in 40° of abduction relative to the horizontal distance between the lateral margin of the bony acetabular rim and the tip of the tear drop (acetabular depth). When there is irreducible hinge abduction, neither Hilgenreiner–labral angle nor epiphyseal slip-in index would substantially be improved in abduction.
Treatment strategy for the hips with reducible hinge abduction
In the transitional stage, if the epiphyseal collapse is not advanced, and the extruded epiphyseal segment is relatively small and soft enough to slip easily underneath the labrum on attempted abduction without imposing undue pressure on the lateral edge of the acetabulum, the hip can be deemed appropriate for containment. The authors’ current treatment approach for hinge abduction when detected during the transitional stage is as follows. The patient is placed in bed rest for abduction traction for approximately 1 week, and then the patient is taken to the operating theater for arthrographic assessment under general anesthesia. If the hip has reducible hinge abduction, the authors prefer to perform soft tissue release via medial approach that includes adductor tenotomy and psoas tenotomy, with or without medial joint capsular release sequentially to achieve abduction beyond 45°. The patient is then placed in Petrie cast for 3 to 6 weeks to maintain the femoral head in the acetabulum. At the end of this period, appropriate surgical containment (eg, femoral varus osteotomy, double-level osteotomy [ Fig. 2 ], triple innominate osteotomy, and shelf acetabuloplasty [ Fig. 3 ]), is undertaken, taking into consideration of the age, femoral neck-shaft angle, and the extent of uncoverage of the femoral head. After bony healing, the patient continues a program of intensive physical and hydrotherapy with protective partial weight bearing and night-time abduction splintage for several months.
