George T. Rab

The geometry of slipped capital femoral epiphysis : Implications for movement, impingement, and corrective osteotomy

Metaphyseal impingement limits motion in high-grade slipped capital femoral epiphysis (SCFE). A three-dimensional volume/surface computer model was used to study the geometry of impingement, which may take the form of impaction, which causes levering or requires compensatory alteration in motion, or inclusion that occurs after remodeling and may lead to acetabular cartilage damage. The majority of deformities seen clinically can be reproduced with posterior epiphyseal displacement in the plane of the physis. By using the 3-D movements of normal walking, this model predicts little anterior metaphyseal impingement in the normal hip. As posterior slip angle increases to 25 degrees , minor impingement can be eliminated with as little as 20 degrees of external rotation. High-grade posterior slips (75 degrees ) require external rotation of 50-60 degrees during walking to minimize impaction. Sitting increases impingement for all slip geometries, requiring proportionately greater external rotation. As remodeling restores a more normal arc of motion, an increasing proportion of the femoral head is composed of the remodeled, included metaphyseal prominence. This study explores the potential role of contact between the acetabulum and the metaphysis in the production of abnormal range of motion after SCFE, and simulation estimates the correction needed by osteotomy to allow normal walking and sitting. The inclusion of significant metaphyseal surfaces in the remodeled hip may be one factor in subsequent degenerative changes associated with SCFE.

Hamstrings in cerebral palsy crouch gait.

Summary: After observing patients with increased anterior pelvic tilt following medial hamstring lengthening in cerebral palsy crouch gait, we became concerned that the hamstrings may be functionally important hip extensors. To evaluate this, we studied the three-dimensional motion of the hip and knee, calculated hamstring muscle length, and evaluated dynamic electromyography (EMG) of the medial hamstrings in 16 patients with diplegic cerebral palsy and crouch gait to determine if the hamstrings were extending the hip. Twelve of 16 patients exhibited marked prolongation of electrical activity in the medial hamstrings, and in eight of these 12, the hamstrings were contracting concentrically, thus aiding in hip extension during gait. Hamstrings may be important hip extensors in some cerebral palsy patients with crouch gait; however, other deformities contributing to crouch (such as hip flexion contracture) need to be considered before isolated hamstring lengthening is performed in these patients.

Oblique tibial osteotomy for Blount's disease (tibia vara)

Nine oblique proximal tibial osteotomies were performed on six children with genu varum, all but one of whom had Blounts disease. The osteotomy was a single-plane cut allowing simultaneous correction of varus and internal rotation and permitting postoperative cast wedging if necessary to improve position. All osteotomies healed within 10 weeks. The major complications were three episodes of weakness of the extensor hallucis longus, two of which resolved in 4 days and one of which resolved only partially. We were able to combine the osteotomy with other procedures, such as epiphyseal bar resection. All nine osteotomies achieved the desired correction of deformity.

Kinematic assessment of the upper extremity in brachial plexus birth palsy

Children with brachial plexus birth palsy (BPBP) may have shoulder external rotation and abduction weakness that can restrict activities of daily living (ADLs). Static range of motion measurements may not measure ADL restrictions. Motion analysis has been used to quantify gait limitations and measure changes associated with treatment. The purpose of this study was to determine whether upper extremity motion analysis (UEMA) can measure the differences in shoulder motion during ADLs between children with BPBP and normal children. Following a previously described UEMA protocol, 55 children with BPBP and 51 normal children (control group) were studied. Kinematic data of selected ADLs were collected before surgery. UEMA was used to measure statistically significant differences between children with BPBP and control subjects for all planes of shoulder motion in all activities tested. The authors conclude that UEMA can discriminate between children with BPBP and control subjects during selected ADLs, and suggest that UEMA can also be used to measure the effects of surgical interventions in children with BPBP.

Open fractures of the tibia and femur in children

Forty-seven children with 32 open tibial fractures, 11 open femoral fractures, and six closed tibial fractures with compartment syndrome, which were surgically opened, were assessed for the time to union and incidence of nonunion and infection. Treatment varied with fracture severity and patient age. Tibial external fixators were supplemented by short-leg casts, and fixators were removed at an average of 7 weeks. No child required further surgery to achieve union. There was one (2%) delayed union and no nonunions. Three fractures (6%) developed infections. No pin-tract infections occurred in tibial fractures treated with external fixation with casting. The relation between age and time to union was statistically significant. In open fractures without segmental bone loss or soft-tissue loss requiring major reconstruction, bony healing can be expected within 6 months.

A technique for determining femoral head containment during gait.

Summary Kinematic gait analysis was carried out on 42 hips, including normal joints and joints affected with Legg-Calvé-Perthes disease. Data were analyzed by a computer algorithm that calculated three-dimensional femoral head contact with the acetabulum (containment). Patients were studied in and out of various abduction orthoses. The three-dimensional containment of the hip is reproducibly altered by various devices; specifically, the Atlanta brace increases posterior coverage and slightly increases lateral coverage, and Petrie casts improve lateral and anterior coverage at the expense of posterior containment. This analysis allows modeling of osteotomies and enhances clinical assessment of actual changes in containment with bracing.

Three-dimensional finite element analysis of Legg-Calve-Perthes disease.

We studied the theoretical stress distribution in the proximal femoral epiphysis by means of a three-dimensional finite element model that allowed simulation of the central epiphyseal necrosis found in Legg-Calve-Perthes disease. The model was altered to study effects of age, extent of necrosis, portion of gait cycle, and effects of osteotomy on containment. It was found that small infarcts were less likely to exhibit collapse because of stress shielding, which decreased when the infarct was extensive or the area was not contained. There was little difference in stresses regardless of age or portion of the gait cycle. Femoral and pelvic osteotomies did little to modify mechanical stresses, and specifically failed to stimulate stress shielding in the case of extensive necrosis. This study suggests that basic mechanical behavior of the femoral epiphysis is similar in younger and older children, and good results in the former may be due to remodelling and low body weight. Although containment may have a place in less extensive lesions, there is no obvious mechanical support for the practice of performing osteotomies in the face of extensive epiphyseal necrosis.

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.

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

PurposeOblique 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.MethodsRetrospective review of surgical experience.ResultsThe 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.ConclusionsImprovements in the technical execution of oblique proximal tibial osteotomy enhances the correction and predictability of the procedure.