Andrew Mahar

Displaced pediatric supracondylar humerus fractures: biomechanical analysis of percutaneous pinning techniques.

Supracondylar humerus fractures are a common childhood occurrence. Displaced fractures are typically treated with closed reduction and percutaneous pinning. Controversy continues over the appropriateness of various pinning techniques. The most common include crossed or lateral pins. A biomechanical comparison of crossed pins, “parallel” lateral pins, and “divergent” lateral pins was performed using a pediatric synthetic bone model. Mechanical testing of each pin configuration was performed in extension, varus, valgus, internal rotation, and external rotation. The divergent configuration provided statistically greater stability than parallel pins under varus and valgus loading. Divergent pins had similar stability compared with crossed pins in extension, varus, and valgus testing. In axial rotation testing, crossed pins were more stable. If the surgeon feels confident in the ability of lateral pins to provide satisfactory fracture stability, divergent lateral pins provide greater stability than parallel lateral pins while avoiding ulnar nerve injury (associated with crossed pins).

Short-segment fixation of lumbar burst fractures using pedicle fixation at the level of the fracture.

Study Design. Cadaveric biomechanical study and retrospective chart review. Objective. Biomechanical comparison of segmental versus nonsegmental fixation of lumbar burst fractures and clinical analysis of short-term radiographic outcomes. Summary of Background Data. Traditional short nonsegmental posterior fixation of thoracolumbar burst fractures suffers from high rates of failure. Construct stability may be improved by inserting additional screws at the fracture level. Methods. Six intact human spines (L1–L3) were biomechanically tested in flexion-extension, lateral bending, and axial torsion. The inferior half of the L2 vertebral bodies and L2–L3 discs were resected to mimic an unstable L2 burst fracture with loss of anterior column support. Pedicle screws were inserted in L1 and L3 for the control group (nonsegmental fixation). Screws were inserted at all levels for the experimental group (segmental fixation). Construct stiffness and L1–L2 disc pressure were analyzed. Twelve patients with thoracolumbar burst fractures treated with this type of segmental fixation were reviewed. Results. Construct stiffness during axial torsion was significantly higher for segmental constructs compared with nonsegmental constructs (P < 0.02) with no differences between flexion-extension and lateral bending. Disc pressure fluctuations during flexion-extension were significantly higher for segmental compared with nonsegmental constructs (P < 0.02), with no differences in lateral bending and torsion. Mean preoperative kyphotic deformity was 9° and improved by 15° after surgery. Follow-up films on 9 patients showed 5° of kyphosis correction loss. Mean anterior vertebral body height was 58% of normal before surgery. After surgery height was 89% of normal and at final follow-up, 78%. Conclusions. Segmental fixation of burst fractures with screws at the level of the fracture offers improved biomechanical stability. Theoretically, segmental fixation provides for additional fixation points that may aid in fracture reduction and kyphosis correction. This specific parameter is not evaluated in this study but will be an important outcome measure for a planned randomized controlled trial.

Biomechanical evaluation of clavicle fracture plating techniques: does a locking plate provide improved stability?

Objectives: To evaluate the biomechanical properties of both plate location (superior versus anterior-inferior) and plate type Small Fragment Contourable Dual Compression Plate (CDCP) versus 3.5 mm Universal Locking System Contourable Dual Compression Plate (Locked CDCP) in a synthetic midshaft transverse clavicle fracture model. Methods: Twenty-four pre-osteomized synthetic clavicles were repaired with either CDCP or locked CDCP technology 3.5 mm plates in either the superior or anterior-inferior position to form 4 groups of 6 clavicles. These were subsequently tested to evaluate torsional and axial construct stiffness, as well as bending load to failure, bending failure stiffness, and method of failure. Results: In axial compression, locked CDCP constructs were significantly more stiff than CDCP constructs (p < 0.001), but no statistically significant effect of plate location was observed. Torsional tests demonstrated a significant 2-way interaction favoring locked CDCP plates in the superior position and standard CDCP plates in the anterior-inferior position (p < 0.001). Bending failure testing revealed that the superior plate location had higher load to failure and bending failure stiffness than the anterior-inferior location (p < 0.0001). In addition, the superior locked CDCP plates demonstrated significantly greater bending failure stiffness than superior CDCP plates (p < 0.0001). Conclusions: Biomechanically, repairing a midshaft clavicle fracture with a superior plate was more favorable compared to anterior-inferior plating in terms of both load to failure and bending failure stiffness. Furthermore, superior locked CDCP plates show improved bending failure stiffness over superior CDCP plates.

Asymmetrical flexible tethering of spine growth in an immature bovine model.

Study Design. A 12-week bovine survival study of tethering anterior spine growth that included untethered control subjects. Objective. To determine the effects that a flexible cable attached to the anterolateral aspect of the thoracic spine has on spine growth in rapidly growing calves. Summary of Background Data. The search for a way to correct scoliosis without the use of an arthrodesis continues in an attempt to maintain normal spine mobility. Experience in the hemiepiphyseal stapling of long bones has provided a background rationale for attempting growth modulation in the spine. It is postulated that a mechanical tether to the anterior and lateral growth of the spine in a growing child with scoliosis may allow spontaneous correction of sagittal and coronal plane deformity obviating the need for an arthrodesis. Methods. Eight calves (age, 3–4 weeks; weight, 47 ± 4.6 kg) underwent right-side thoracotomies exposing the thoracic spine. Laterally directed anterior vertebral body screws were placed into each body and two vertebrae, either T6 and T7 or T8 and T9, were tethered with a stainless steel cable. After 12 weeks, radiographs were obtained to evaluate the degree of deformity that had developed. In addition, biomechanical testing to determine the range of motion in the tethered and untethered segments was performed. Results. The calves increased their weight 153% during the 12-week postoperative period. The radiographic analysis demonstrated scoliosis of 11.6° ± 4.8° in the tethered levels, as compared with 0.3° ± 1.7° in the control segments (P < 0.0001). Similarly, kyphosis developed in the tethered segments (5.1° ± 5.8°), as compared with −1.8° ± 3.1° at the control levels (P = 0.01). There was a significant wedging of the disc in the tethered (6.8° ± 1.6°) as compared with the untethered (0.7° ± 2°) segments (P < 0.0001). There was a trend toward lower height of the vertebrae on the tethered right side, as compared with the left side in the tethered segments (P = 0.075), whereas no side-to-side difference was noted in the control subjects (P = 0.48). Biomechanical analysis showed that the tether did not affect the range of motion in axial rotation or flexion–extension. However, the tether did restrict lateral bending as compared with that of the control subjects. When the tether was cut, the range of motion returned to levels matching that of the untethered control subjects. Conclusions. Anterolateral tethering of the spine creates kyphosis and scoliosis in this rapidly growing bovine model. The spinal tether limited motion primarily in lateral flexion. However, total lateral bending motion returned to levels comparable with control motion segments after removal of the tether. This method of spine growth modulation may provide a possible treatment for the correction of spine deformities without arthrodesis in patients who are skeletally immature. The exact mechanisms of growth modulation and the effects of tethering on disc function and integrity are unknown and deserve further study.

The mechanical properties of the ligamentum teres: a pilot study to assess its potential for improving stability in children's hip surgery.

The anatomic and histological characteristics of the ligamentum teres and its vascular contributions to the femoral head have been well described. The function of the ligamentum teres remains poorly understood. Although excision is the current standard in treating complete developmental hip dysplasia, we developed an interest in maintaining, shortening, and reattaching the ligament to assure early postoperative stability in developmental hip dysplasia. To analyze its potential for providing hip joint stability, we investigated the biomechanical properties of the ligamentum teres in an in vitro porcine model. Six immature porcine hips were dissected, with the proximal femur and acetabular anatomy kept intact, isolating the ligamentum teres. Specimens were loaded in tension using custom fixation rigs at 0.5 mm/s in line with the fibers. Data for displacement and force were collected and sampled at 10 Hz for duration of each test. The ligamentum teres failed in a stepwise fashion. The mean ultimate load to failure was 882 ± 168 N. Mean stiffness and failure stress were calculated as 86 ± 25 N/mm and 10 ± 2 MPa, respectively. The biomechanical function of the ligamentum teres is not inconsequential. We found the ultimate load of the ligamentum teres in the porcine model to be similar to those reported for the human anterior cruciate ligament. The strength of the ligamentum teres may confirm its potential for providing early stability in childhood hip reconstructions. In the setting of dysplasia, the preservation and the transfer of the ligamentum teres to augment stability should be considered as an adjunct to open reduction.

Failure Mode of Suture Anchors as a Function of Insertion Depth

Background Surgeons can control not only the angle but also the depth of suture anchor placement during arthroscopic rotator cuff repair, although the tendency may be to place suture anchors on the deep side to avoid damage from prominent anchor eyelets. However, little information is available regarding possible effects of suture anchor depth on construct failure mechanisms. Hypothesis Anchor depth affects the mode of suture failure with physiologically relevant cyclic loads. Study Design Controlled laboratory study. Methods Metallic screw-in suture anchors loaded with No. 2 braided polyester sutures were inserted into the bovine infra-spinatus footprint with the eyelet proud, standard, or deep. Sutures were hand tied to create a closed loop. Constructs were cyclically loaded from 10 to 90 N and, if still intact at 500 cycles, taken to ultimate failure (maximum load). Results When clinical failure was defined as greater than 3-mm construct elongation, anchors placed with the eyelet deep experienced statistically earlier clinical failure via cutting of the suture through the bone (P <. 02). However, anchors placed at this level did not experience catastrophic failure during cyclic loading. The standard and proud anchors experienced 3 mm of elongation at a greater number of cycles, but the suture material degraded at the anchor eyelet, and a majority of these constructs broke during cyclic physiologic loading. At failure testing, the deep anchors had a significantly increased failure load (164 N) compared to standard (133 N) (P <. 04) and proud (113 N) anchors (P <. 005). Conclusion Varying the depth of suture anchor insertion changes the mechanical properties and mode of failure of suture anchor constructs. Clinical Relevance Surgeons should be aware of the effects of suture anchor depth and abrasive eyelet wear on construct failure during arthroscopic rotator cuff repair.

Standing lateral radiographic positioning does not represent customary standing balance.

Study Design. Normal cohort evaluation of the accuracy of existing methods for radiographic measurement of sagittal spinal balance. Objectives. To examine the validity and reliability of sagittal vertical axis measurements during a variety of standing positions commonly used while obtaining lateral thoracolumbar spine radiographs. Summary of the Background Data. The sagittal vertical axis is a widely accepted radiographic measurement of global sagittal alignment of the spine. However, the sagittal vertical axis has not been measured in normal subjects while in functional positions because the arms must be elevated during acquisition of a lateral spinal radiograph. The purpose of this study was to quantify differences in sagittal vertical axis measurements between repeated functional positions and radiographic positions to identify the magnitude of the potential error in measuring the sagittal vertical axis radiographically. Methods. Reflective markers were attached to 15 healthy female adolescents, overlying the spinous processes of C7 and S1. Marker positions were recorded by a motion capture system during simultaneous acquisition of a lateral radiograph. Sagittal vertical axis calculation, using marker data, was matched to sagittal vertical axis measurement obtained by radiograph using anthropometric corrections to limit radiation to a single exposure. Four standing positions with varying shoulder and knee flexion as well as overground walking were examined. The mean sagittal vertical axis for each standing position and during gait was compared using a repeated measures analysis of variance. Intratrial and intertrial repeatability of sagittal vertical axis measurements was also determined. Results. The sagittal vertical axis was positive (C7 anterior to S1) for the functional positions (relaxed standing: 0.9 ± 2.0 cm, and throughout gait: 4.5 ± 2.0 cm), whereas shoulder flexion resulted in a negative sagittal vertical axis (-4.6 ± 3.2) and posterior rotation of the pelvis. Adding knee flexion resulted in a slight relative shift in the sagittal vertical axis anteriorly. No differences were observed in intertrial and intratrial reliability for relaxed standing and standing with shoulder flexion alone. Increased variability was observed between repeated trials involving knee flexion. Conclusions. Measurement of the sagittal vertical axis on radiographs from commonly utilized standing positions (shoulders flexed) results in an sagittal vertical axis that is at least 3 to 4 cm more posterior than a sagittal vertical axis observed during a functional position. Subject repositioning resulted in an intertrial variability of at least 0.8 cm in sagittal vertical axis, while variation as the subject held each standing posture had little contribution to overall error of measurement. Of the analyzed positions, shoulder flexion (45°) alone was the best position for a lateral radiograph due to minimal compromise to repeatability of sagittal vertical axis measurement. However, none of the radiographic positions reproduced the spinal balance of the subject’s functional standing posture.

Spinal growth modulation with an anterolateral flexible tether in an immature bovine model: disc health and motion preservation.

Study Design. An immature bovine model was used to evaluate multilevel anterolateral flexible tethering in a growing spine. Objective. To evaluate radiographic, biochemical, histologic, and biomechanical results of tethered spinal growth. Summary of Background Data. An anterolateral flexible tether has been shown to create a kyphotic and scoliotic spinal deformity in calves. Subsequent disc health and spinal motion has not been analyzed. Methods. Four consecutive thoracic vertebral bodies (T6–T9) were instrumented anteriorly in 36 1-month-old calves. Seventeen animals (Tether Group) were instrumented with a vertebral staple-two screw construct connected by 2 flexible stainless steel cables. Nineteen animals (Control Group) were instrumented with 1 vertebral body screw with no connecting cable. After a 6-month survival period, the spines were harvest en-bloc and underwent radiographic, computed tomography, biochemical, histologic, and biomechanical analysis. Results. On average, 37.6° ± 10.6° of coronal and 18.0° ± 9.9° of sagittal deformity was created in the Tether Group, with significant vertebral wedging toward the tether (P < 0.001). Disc thickness decreased significantly in the Tether Group (P < 0.001), however, disc wedging was not observed. There was no change in gross morphologic disc health or disc water content (P = 0.73). However, proteoglycan synthesis was significantly greater in the tethered discs compared with controls (P < 0.001), and collagen type distribution was different with a trend toward increased type II collagen present on the tethered side of the disc (P = 0.09). Tethers significantly increased spinal stiffness in lateral bending and in flexion/extension (P < 0.05) without affecting torsional stiffness, however, after tether removal range of motion returned to control values. Conclusion. Tethering resulted in vertebral wedging while maintaining spinal flexibility. Although changes in proteoglycan synthesis, collagen type distribution, and disc thickness were observed, the tethered discs had similar water content to control discs and did not demonstrate gross morphologic signs of degeneration. Growth modulation is an attractive treatment option for growing patients with scoliosis, avoiding multilevel fusions or brace wear. Strategies for fusionless scoliosis correction should preserve disc health, as adolescent patients will rely on these discs for decades after treatment.

Reconstruction plates for stabilization of mid-shaft clavicle fractures: Differences between nonlocked and locked plates in two different positions

Reconstruction plates permit contouring to the irregular anatomic shape of the clavicle. This study evaluated the biomechanical stability of locking and nonlocking clavicle reconstruction plates for treating midshaft, transverse fractures, comparing anterior-inferior to superior plate position. Twenty-four synthetic clavicles with mid-shaft fractures were repaired with either a locking or nonlocking clavicle reconstruction plate in either the anterior-inferior or superior plate position (n = 6/group). Repaired constructs were tested in axial compression, axial torsion, and cantilever bending failure. In compression, anterior-inferior plates were significantly stiffer than superior plates and locked plates stiffer than nonlocked. In torsion, anterior-inferior plates were stiffer, with a significant interaction term that favored anterior-inferior locked and superior nonlocked plates. In cantilever bending, superior plates had a significantly higher bending failure load and stiffness. Anterior-inferior plates failed at a significantly lower load ( approximately 40 N or approximately 4 kg), which could potentially occur in the postoperative period.

Spinal growth modulation with use of a tether in an immature porcine model.

BACKGROUND Spinal growth modulation by tethering the anterolateral aspect of the spine, as previously demonstrated in a nonscoliotic calf model, may be a viable fusionless treatment method for idiopathic scoliosis. The purpose of the present study was to evaluate the radiographic, histologic, and biomechanical results after six and twelve months of spinal growth modulation in a porcine model with a growth rate similar to that of adolescent patients. METHODS Twelve seven-month-old mini-pigs underwent instrumentation with a vertebral staple-screw construct connected by a polyethylene tether over four consecutive thoracic vertebrae. The spines were harvested after six (n = 6) or twelve months (n = 6) of growth. Monthly radiographs, computed tomography and magnetic resonance imaging scans (made after the spines were harvested), histologic findings, and biomechanical findings were evaluated. Analysis of variance was used to compare preoperative, six-month postoperative, and twelve-month postoperative data. RESULTS Radiographs demonstrated 14 degrees +/- 4 degrees of coronal deformity after six months and 30 degrees +/- 13 degrees after twelve months of growth. Coronal vertebral wedging was observed in all four tethered vertebrae and progressed throughout each animals survival period. Disc wedging was also created; however, in contrast to the findings associated with vertebral wedging, the tethered side was taller than the untethered side. Magnetic resonance images revealed no evidence of disc degeneration; however, the nucleus pulposus had shifted toward the side of the tethering. Midcoronal undecalcified histologic sections showed intact bone-screw interfaces with no evidence of implant failure or loosening. With the tether cut, stiffness decreased and range of motion increased in lateral bending away from the tether at both time-points (p < 0.05). CONCLUSIONS In this porcine model, mechanical tethering during growth altered spinal morphology in the coronal and sagittal planes, leading to vertebral and disc wedging proportional to the duration of tethering. The resulting concave thickening of the disc in response to the tether was not anticipated and may suggest a capacity for the nucleus pulposus to respond to the compressive loads created by growth against the tether.