Mechanics of hip dysplasia reductions in infants using the Pavlik harness: a physics-based computational model.

Biomechanical factors influencing the reduction of dislocated hips with the Pavlik harness in patients of Developmental Dysplasia of the Hip (DDH) were studied using a three-dimensional computer model simulating hip reduction dynamics in (1) subluxated and (2) fully dislocated hip joints. Five hip adductor muscles were identified as key mediators of DDH prognosis, and the non-dimensional force contribution of each in the direction necessary to achieve concentric hip reductions was determined. Results point to the adductor muscles as mediators of subluxated hip reductions, as their mechanical action is a function of the degree of hip dislocation. For subluxated hips in abduction and flexion, the Pectineus, Adductor Brevis, Adductor Longus, and proximal Adductor Magnus contribute positively to reduction, while the rest of the Adductor Magnus contributes negatively. In full dislocations all muscles contribute detrimentally to reduction, elucidating the need for traction to reduce Graf IV type dislocations. Reduction of dysplastic hips was found to occur in two distinct phases: (a) release phase and (b) reduction phase.

Oblique tibial osteotomy revisited.

PURPOSE: Oblique proximal tibial osteotomy is a useful option for correcting deformity associated with Blount's disease (tibia vara). Safe, adequate correction depends on technical issues that have evolved since the original description of the procedure. METHODS: Retrospective review of surgical experience. RESULTS: The refinement of osteotomy plane orientation, based on the distal rather than the proximal tibia, reduces the likelihood of procurvatum after surgery. The stability of the osteotomy is enhanced by an improved screw fixation technique. The risk of compartment syndrome is low if prophylactic partial fasciotomy is performed concurrently. Avoidance of spinal or regional block anesthesia minimizes the possibility of failure to detect post-operative compartment syndrome. CONCLUSIONS: Improvements in the technical execution of oblique proximal tibial osteotomy enhances the correction and predictability of the procedure.

Pathological fracture after migration of cement used to treat distal femur physeal arrest.

Treatment of physeal arrest with bar removal and placement of interposition materials in young patients has been shown to restore physeal growth. Among the various materials that have been used to prevent early reformation of the physeal bar (fat, silastic, cartilage), Peterson recommended the use of Cranioplast, as this material can prevent bar reformation, and it is radiolucent because it does not contain barium. Peterson suggested steps be taken to prevent migration of the Cranioplast, as migration of the interposition material might allow for reformation of the physeal bar. Although Peterson had not observed cement migration leading to pathologic fracture, he felt that this was a potential concern. We describe the case of cement migration from the epiphysis into the diaphysis, leading to a pathological femur fracture. Despite migration of the Cranioplast, the physeal bridge did not reform, and the patient had nearly normal growth 7 years after the initial physeal arrest procedure.

Shoulder motion description: the ISB and Globe methods are identical.

BACKGROUND: Three-dimensional shoulder position may be described by rotation sequences such as the proposed ISB standard. Alternative techniques to describe position (the Globe method) seek to simplify this description by eliminating rotation sequences and substituting unambiguous measurements. METHODS: Both methods (ISB and Globe) were applied to an analysis of shoulder positions, and an overall comparison was performed. FINDINGS: The ISB and Globe methods are numerically identical and interchangeable. INTERPRETATION: While all analytic methods are mathematically equivalent, investigators have sought simpler and more easily-applied ways of describing shoulder position that would be accurate, easily understood by clinicians, and unambiguous. This study demonstrates that the ISB rotation sequence and Globe descriptive method are numerically the same.

Lateral acetabular rotation improves anterior hip subluxation.

Surgical reorientation of the acetabulum is used to improve stability of subluxated or dysplastic hips, but the specific mechanical consequences of reorientation have not been quantified. I used a rigid body spring model of the human hip to study the effects of different acetabular positions on hip stability during single-limb stance. The model predicted subluxation direction and magnitude, and the effective joint contact area, as functions of acetabular position. Frontal plane acetabular orientation varied from 20 degrees medial rotation to 50 degrees lateral rotation, corresponding to center-edge angles from 0 degrees to 70 degrees. Sagittal acetabular orientation varied from 45 degrees anterior rotation to 15 degrees posterior rotation. Center-edge angles less than 20 degrees produced progressive anterolateral subluxation, with dislocation occurring when center-edge angles were less than 0 degrees. Lateral subluxation disappeared when cen-ter-edge angles were 30 degrees or greater. Anteroposterior subluxation was controlled by anterior or posterior rotation of the acetabulum in the presence of low center-edge angles, but there was no specific position of stability that effectively stabilized the femoral head. Anterior subluxation also was controlled by lateral rotation of the acetabulum. Joint contact area increased 1% for every 3 degrees lateral acetabular rotation. The anterolateral subluxation associated with hip dysplasia can be controlled by acetabular reorientation. Joint contact area will increase, thereby reducing peak joint pressure. Anterior and lateral subluxation can be improved by lateral rotation alone, which may reduce the severity of anterior femoroacetabular impingement after periacetabular osteotomy.

Theoretical study of subluxation in early Legg-Calvé-Perthes disease.

The relationship between mechanical subluxation and femoral head necrosis geometry in Legg-Calvé-Perthes disease (LCP) was investigated with a three-dimensional rigid body-spring method hip model. Femoral head models with progressively larger regions of necrosis, corresponding to the four Catterall grades, were placed in a spherical acetabular model and studied in static single-limb stance configuration, with variable mechanical rigidity of the necrotic segment. The degree of subluxation was dependent on the geometric region of involvement, mechanical properties of the segment, and direction of loading force. In general, femoral head subluxation was always in the direction of the necrosis, modified by the anatomic and force environment. In the neutral position, the Catterall I models exhibited minimal subluxation. The Catterall II model subluxated anteriorly as collapse occurred, and the Catterall III model subluxated anteriorly and superiorly. Lateral subluxation could be produced by changing the loading force to a more vertical orientation. The Catterall IV model collapsed directly along the line of force application rather than subluxating. Reorientation of models with minor necrosis could improve stability, but reorientation of models with extensive necrosis had minimal effect on subluxation behavior. Higher-grade LCP involvement may lead to early subluxation, particularly anteriorly, which is difficult to visualize radiographically. The appearance of lateral subluxation may signal a change in the mechanical environment of the hip from clinical progression of the disease. Femoral head reorientation (osteotomy) may improve femoral head stability when necrosis is limited but is unlikely to reduce subluxation or collapse when extensive necrosis is present.

Three-dimensional muscle-tendon geometry after rectus femoris tendon transfer.

BACKGROUND: Rectus femoris tendon transfer is performed in patients with cerebral palsy to improve knee flexion during walking. This procedure involves detachment of the muscle from its insertion into the quadriceps tendon and reattachment to one of the knee flexor muscles. The purpose of the present study was to evaluate the muscle-tendon geometry and to assess the formation of scar tissue between the rectus femoris and adjacent structures. METHODS: Magnetic resonance images of the lower extremities were acquired from five subjects after bilateral rectus femoris tendon transfer. A three-dimensional computer model of the musculoskeletal geometry of each of the ten limbs was created from these images. RESULTS: The three-dimensional paths of the rectus femoris muscles after transfer demonstrated that the muscle does not follow a straight course from its origin to its new insertion. The typical muscle-tendon path included an angular deviation; this deviation was sharp (>35 degrees ) in seven extremities. In addition, scar tissue between the transferred rectus femoris and the underlying muscles was visible on the magnetic resonance images. CONCLUSIONS: The angular deviations in the rectus femoris muscle-tendon path and the presence of scar tissue between the rectus femoris and the underlying muscles suggest that the beneficial effects of rectus femoris tendon transfer are derived from reducing the effects of the rectus femoris muscle as a knee extensor rather than from converting the muscle to a knee flexor. These findings clarify our understanding of the mechanism by which rectus femoris tendon transfer improves knee flexion.

Alterations in functional movement after axillary burn scar contracture: a motion analysis study.

Children with axillary burns often develop scar contractures that restrict shoulder movement. Objective data on functional movement patterns after contracture formation is sparse. The purpose of this study was to determine how axillary contractures affect shoulder movement during activities of daily living (ADLs). This was a prospective study of children with axillary contractures scheduled for surgical release. Three-dimensional upper extremity kinematic analysis was used to assess shoulder, elbow, and trunk motion during two ADLs: high reach and hand to back pocket. Results were compared with a pool of 49 normal age-matched controls. Eleven children with axillary contractures were compared with controls. During high reach, significant decreases in shoulder flexion, shoulder internal rotation, arm pronation, and trunk extension occurred. Elbow flexion increased significantly. In the hand to back pocket task, shoulder extension and elbow flexion decreased and shoulder abduction increased. Axillary contractures result in quantifiable movement changes during ADLs. Aggressive rehabilitation is required to prevent contracture formation. Three-dimensional motion analysis is a unique tool for the quantification of functional limitations and provides an objective method to evaluate treatment efficacy in patients with axillary contractures.