David D. Aronsson

Slipped capital femoral epiphysis.

Slipped capital femoral epiphysis is a well known disorder of the hip in adolescents that is characterized by displacement of the capital femoral epiphysis from the metaphysis through the physis. The term slipped capital femoral epiphysis is a misnomer because the epiphysis is held in the acetabulum by the ligamentum teres, and thus it is actually the metaphysis that moves upward and outward while the epiphysis remains in the acetabulum. In most patients, there is an apparent varus relationship between the head and the neck, but occasionally the slip is into a valgus position, with the epiphysis displaced superiorly in relation to the neck106,109. In the vast majority of cases, the etiology is unknown. Although the condition may be associated with a known endocrine disorder71,77,129, with renal failure osteodystrophy74, or with previous radiation therapy75,77, this Instructional Course Lecture deals only with idiopathic slipped capital femoral epiphysis. Multiple theories have been proposed for the etiology of idiopathic slipped capital femoral epiphysis, and it is likely a result of both biomechanical and biochemical factors128. The combination of these factors results in a weakened physis with subsequent failure. Mechanical factors95 associated with the disorder are obesity62,72, increased femoral retroversion36,37,95, and increased physeal obliquity83. The vast majority of children with a slipped capital femoral epiphysis are obese, which increases the shear stress across the physis. Obesity is also associated with femoral retroversion, with anteversion averaging 10.6 degrees in adolescents with normal weight but only 0.40 degree in obese adolescents36. This femoral retroversion increases the stress across the physis95. Children with a slipped capital femoral epiphysis also have a more …

Compression-induced changes in intervertebral disc properties in a rat tail model.

STUDY DESIGN An Ilizarov-type apparatus was applied to the tails of rats to assess the influence of immobilization, chronically applied compression, and sham intervention on intervertebral discs of mature rats. OBJECTIVES To test the hypothesis that chronically applied compressive forces and immobilization cause changes in the biomechanical behavior and biochemical composition of rat tail intervertebral discs. SUMMARY OF BACKGROUND DATA Mechanical factors are associated with degenerative disc disease and low back pain, yet there have been few controlled studies in which the effects of compressive forces on the structure and function of the disc have been isolated. METHODS The tails of 16 Sprague-Dawley rats were instrumented with an Ilizarov-type apparatus. Animals were separated into sham, immobilization, and compression groups based on the mechanical conditions imposed. In vivo biomechanical measurements of disc thickness, angular laxity, and axial and angular compliance were made at 14-day intervals during the course of the 56-day experiment, after which discs were harvested for measurement of water, proteoglycan, and collagen contents. RESULTS Application of pins and rings alone (sham group) resulted in relatively small changes of in vivo biomechanical behavior. Immobilization resulted in decreased disc thickness, axial compliance, and angular laxity. Chronically applied compression had effects similar to those of immobilization alone but induced those changes earlier and in larger magnitudes. Application of external compressive forces also caused an increase in proteoglycan content of the intervertebral discs. CONCLUSIONS The well-controlled loading environment applied to the discs in this model provides a means of isolating the influence of joint-loading conditions on the response of the intervertebral disc. Results indicate that chronically applied compressive forces, in the absence of any disease process, caused changes in mechanical properties and composition of tail discs. These changes have similarities and differences in comparison with human spinal disc degeneration.

Mechanical modulation of vertebral body growth. Implications for scoliosis progression.

Study Design. The authors developed a rat‐tail model to investigate the hypothesis that vertebral wedging during growth in progressive spinal deformities results from asymmetric loading in a “vicious cycle.” Objectives. To document growth curves with axial compression or distraction applied to tail vertebrae to determine whether compression load slows growth and distraction accelerates it. Summary of Background Data. Progression of skeletal deformity during growth is believed to be governed by the Hueter‐Volkmann law, but there is conflicting evidence to support this idea. Methods. Twenty‐eight 6‐week‐old Sprague‐Dawley rats were assigned to one of three groups: compression loading, distraction loading, or sham (apparatus applied without loading). Under general anesthesia, two 0.7‐mm diameter stainless steel percutaneous pins were used to transfix each of two vertebrae. The pins were glued to 25‐mm diameter external ring fixators. Springs (load rate, 35 g/mm) were installed on three stainless steel threaded rods that were passed through holes in each ring and compressed with nuts to apply compression or distraction forces between 25‐75% of bodyweight. Vertebral growth rates in μm/day were measured by digitizing the length of the vertebrae images in radiographs taken 0, 1, 3, 5, 7, and 9 weeks later. Results. The loaded vertebrae grew at 68% of control rate for compressed vertebrae and at 114% for distracted vertebrae. (Differences statistically significant, P < 0.01 by analysis of variance.) For the compressed vertebrae, the pinned vertebrae, which were loaded at one of their two growth cartilages, grew at a reduced rate (85%), although this effect was not apparent for the distraction animals. Conclusions. The findings confirm that vertebral growth is modulated by loading, according to the Hueter‐Volkmann principle. The quantification of this relationship will permit more rational design of conservative treatment of spinal deformity during the adolescent growth spurt.

Slipped capital femoral epiphysis: current concepts.

&NA; Slipped capital femoral epiphysis is a common hip disorder in adolescents, with an incidence of 0.2 (Japan) to 10 (United States) per 100,000. The etiology is unknown, but biomechanical and biochemical factors play an important role. Symptoms at presentation include pain in the groin, thigh, or knee. Ambulatory patients also may present with a limp. Nonambulatory patients present with excruciating pain. The slipped capital femoral epiphysis is classified as stable when the patient can walk and unstable when the patient cannot walk, even with the aid of crutches. Because the epiphysis slips posteriorly, it is best seen on lateral radiographs. The treatment of choice for stable slipped capital femoral epiphysis is single‐screw fixation in situ. This method has a high probability of long‐term success, with minimal risk of complications. In the patient with unstable slipped capital femoral epiphysis, urgent hip joint aspiration followed by closed reduction and single‐ or doublescrew fixation provides the best environment for a satisfactory result, while minimizing the risk of complications.

Progression of vertebral wedging in an asymmetrically loaded rat tail model

Study Design. A rat tail model was used to test the hypothesis that angulation and asymmetric axial compressive loading would lead to vertebral wedging because of asymmetric longitudinal growth in the physes. Objectives. To study the effect of angulation and asymmetric loading on the progression of spinal curvature in a rat tail model. Summary of Background Data. Large idiopathic scoliotic curves in children with significant growth remaining are the curves most likely to progress. The mechanism of progression of skeletal deformities is thought to be controlled by the Hueter-Volkmann law, whereby additional axial compression decelerates growth, and reduced axial compression accelerates growth. It has been hypothesized that spinal curvature leads to asymmetric loading transversely along the vertebral growth plate, causing progressive vertebral wedging by means of a vicious cycle. Methods. Two 32-mm diameter external ring fixators were glued to 0.7-mm pins that had been inserted percutaneously through the eighth and 10th caudal vertebra of 10 6-week-old Sprague-Dawley rats. Calibrated springs and 15° wedges, mounted on stainless steel threaded rods passing through holes distributed around the rings, imposed a 30° Cobb angle and axially compressed the instrumented vertebrae. Fluorochrome labels and radiographs were used to document the progression of vertebral wedging. Results. The wedging initially was entirely in the intervertebral discs, but by 6 weeks the wedging of the discs and vertebrae were approximately equal. Fluorochrome labeling confirmed that the vertebral wedging resulted from asymmetric growth in the physes. Conclusions. This study shows that vertebrae, when asymmetrically loaded, become wedged. This is consistent with the concept of mechanically provoked progression of scoliotic deformities according to the Hueter-Volkmann law.

Treatment of the unstable (acute) slipped capital femoral epiphysis

Slipped capital femoral epiphysis, the most common hip disorder in adolescence, traditionally has been classified according to symptom duration. An acute slip is 1 in which there are symptoms for < 3 weeks; for a chronic slip, there are symptoms for > 3 weeks. An acute-on-chronic slip is characterized by a combination of both with a recent exacerbation of symptoms. This classification system is misleading because it does not consider stability. A stable slipped capital femoral epiphysis has a good prognosis, but an unstable slip has a guarded prognosis. The priorities in treating an unstable (acute) slip are (1) to avoid avascular necrosis, (2) to avoid chondrolysis, (3) to prevent further slip, and (4) to correct deformity. The last priority, correcting the deformity, is associated with a high incidence of complications including avascular necrosis and chondrolysis, so manipulative reduction under anesthesia or an acute corrective osteotomy is not recommended. To address these priorities in treatment, the authors recommend preoperative bed rest to decrease the synovitis and intraarticular effusion. Operative stabilization is done in an elective fashion once the synovitis has subsided. The technique includes careful patient positioning on the fracture table, which may cause an incidental reduction, but no attempt is made to do a manipulative reduction. The technique is dependent on radiographic control. The femoral head and neck must be well visualized on the anteroposterior and lateral intensifier images before the operation is started. The slipped capital femoral epiphysis is stabilized with a single central screw, and nonweightbearing ambulation with crutches is recommended until a satisfactory painless range of motion has returned.

Enlargement of growth plate chondrocytes modulated by sustained mechanical loading

Background: Mechanical compression and distraction forces are known to modulate growth in vertebral growth plates, and they have been implicated in the progression of scoliosis. This study was performed to test the hypothesis that growth differences produced by sustained compression or distraction loading of vertebrae are associated with alterations in the amount of increase in the height of growth plate chondrocytes in the growth direction. Methods: Compression or distraction force of nominally 60% of body weight was maintained for four weeks on a caudad vertebra of growing rats by an external apparatus attached, by means of transcutaneous pins, to the two vertebrae cephalad and caudad to it. Growth of the loaded and control vertebrae was measured radiographically. After four weeks, the animals were killed and histological sections of the loaded and control vertebrae were prepared to measure the height of the hypertrophic zone (average separation between zonal boundaries), the mean height of hypertrophic chondrocytes, and the amount of increase in cell height in the growth direction. Results: Over the four weeks of the experiment, the growth rates of the compressed and distracted vertebrae averaged 52% and 113% of the control rates, respectively. The reduction in the growth rate of the compressed vertebrae was significant (p = 0.002). In the compressed vertebrae, the height of the hypertrophic zone, the mean chondrocyte height, and the amount of increase in cell height averaged 87%, 85%, and 78% of the control values, respectively, and all were significantly less than the corresponding control values. In the distracted vertebrae, these measurements did not differ significantly from the control values. The height of the hypertrophic zone and the mean chondrocyte height correlated with the growth rate (r 2 = 0.29 [p = 0.03] and r 2 = 0.23 [p = 0.06], respectively), when each variable was expressed as a proportion of the control value. The percentage changes in the measurements of the chondrocytic dimensions relative to the control values were smaller than the percentage changes in the growth rates, a finding that suggested that the rate of chondrocytic proliferation was also modulated by the mechanical loading. Conclusions: Mechanical loading of tail vertebrae in rats modulated their growth rate, which correlated with changes in the height of hypertrophic chondrocytes. The effects of compression were greater than those of distraction. Clinical Relevance: Information about the growth rate and chondrocytic response to mechanical loads in rat vertebrae undergoing mechanically modulated growth will be helpful in determining how human vertebral growth might respond to altered loading states during progression or treatment of scoliosis and other growth-related angular skeletal deformities.

Disc and vertebral wedging in patients with progressive scoliosis.

A retrospective longitudinal radiographic study of patients with progressive scoliosis was conducted to determine the relative amount of wedging between vertebrae and discs as a function of progression of the scoliosis curve, cause of the scoliosis, and anatomic curve region. Posteroanterior radiographs of 27 patients with idiopathic scoliosis and of 17 patients with scoliosis associated with cerebral palsy were studied. The amount of wedging of vertebrae and discs at the curve apex was measured by the Cobb method and expressed as a proportion of the curves Cobb angle. On average, the relative amount of vertebral and disc wedging did not differ significantly between initial and follow-up radiographs made after progression of the scoliosis. In both groups of patients, the mean vertebral wedging was more than the disc wedging in the thoracic region; the converse was found in curves in the lumbar and thoracolumbar regions. The patients with scoliosis associated with cerebral palsy had curves that were longer and more commonly in the thoracolumbar and lumbar regions. The relative wedging did not change significantly with curve progression and did not appear to differ by diagnosis. In the management of scoliosis, including small curves, it should be recognized that both the vertebrae and discs have a wedging deformity.

Demographic predictors of severity of stable slipped capital femoral epiphyses

BACKGROUND The outcome of stable slipped capital femoral epiphysis is directly related to the severity of the slip. If it is assumed that the slip will be less severe if it is diagnosed early, then early diagnosis should improve the prognosis. It was our purpose to determine demographic predictors of the severity of a slipped capital femoral epiphysis. METHODS A retrospective study of 243 children with a total of 328 stable slipped capital femoral epiphyses was performed. Gender, race, age, and symptom duration were noted. Slip severity was classified as mild (<30 degrees ), moderate (30 degrees to 50 degrees ), or severe (>50 degrees ). Statistical analyses included bivariate, multivariate, linear correlation, and logistic regression techniques. RESULTS There were 159 boys and eighty-four girls; 149 children had unilateral and ninety-four had bilateral slipped capital femoral epiphysis. Of the bilateral slips, forty-two were simultaneous and fifty-two were sequential. The mean age (and standard deviation) was 12.6 +/- 1.8 years, the mean duration of the symptoms was 5.2 +/- 7.4 months, and the mean slip angle was 29 degrees +/- 20 degrees . There were 199 mild, sixty-eight moderate, and forty-five severe slips. The mean duration of symptoms was 3.5 +/- 5.0 months for the mild slips, 7.7 +/- 9.0 months for the moderate slips, and 8.8 +/- 10.6 months for the severe slips (p < 0.0001). Older children had more severe slips: the average age was 12.3 +/- 1.8 years for the children with a mild slip, 13.0 +/- 1.6 years for those with a moderate slip, and 13.8 +/- 1.8 years for those with a severe slip (p < 0.0001). Multivariate analyses demonstrated that, among the factors studied, only the age of the patient and the duration of the symptoms were associated with the slip severity. Symptom duration and patient age were used as predictors of slip severity in a logistic regression analysis, with > or =30 degrees and <30 degrees used as the categories for slip severity, older than 12.5 years old compared with 12.5 years old or younger used as the categories for age, and more than 2.0 months compared with 2.0 months or less used as the categories for symptom duration. This model predicted the probability of a slip with confidence (p < 0.0001). The odds ratios (with 95% confidence intervals) for age and symptom duration were 2.0 (1.15 to 3.53) and 4.1 (2.34 to 7.12), respectively. Thus, a child with a stable slipped capital femoral epiphysis is 2.0 times more likely to have a moderate or severe slip if he or she is older than 12.5 years of age at the time of the diagnosis and 4.1 times more likely to have a moderate or severe slip if the duration of symptoms was longer than two months. CONCLUSIONS The only two known significant predictors of the severity of a slipped capital femoral epiphysis are age at diagnosis and symptom duration. For any individual child, slip severity and symptom duration are unique; in a large population, there is a general correlation between slip severity and increases in patient age and increases in the duration of symptoms.

Surgical correction of vertebral axial rotation in adolescent idiopathic scoliosis : Prediction by lateral bending films

The pre- and postoperative radiographs of 45 patients with scoliosis were compared with the preoperative lateral bending radiographs. The purpose was to compare correction of Cobb angle and apical vertebral rotation between preoperative lateral bending and operative spinal instrumentation. Twenty-one patients had Harrington instrumentation, 12 had Drummond/Wisconsin spinous process segmental instrumentation, and 12 had Texas Scottish Rite Hospital instrumentation. From the pre- and postoperative radiographs, each vertebra was marked and digitized for computerized measurements of Cobb angle and apical vertebral rotation. Correction of Cobb angle on the lateral bending radiograph averaged 22 +/- 10 degrees, which was less than that achieved at operation (Harrington 23 +/- 7 degrees, Drummond/Wisconsin 29 +/- 10 degrees, and Texas Scottish Rite Hospital 36 +/- 6 degrees; p < 0.01, paired t test). In contrast, correction of apical vertebral rotation on the lateral bending radiograph averaged 4 +/- 8 degrees, which was not significantly different from that achieved at operation (Harrington 1 +/- 8 degrees, Drummond/Wisconsin 1 +/- 7 degrees, and Texas Scottish Rite Hospital 4 +/- 8 degrees). Spinal instrumentation markedly corrected the Cobb angle but minimally corrected apical vertebral rotation. In contrast, preoperative lateral bending produced a similar proportional correction of both.