David L. Glos

Transverse process hooks at upper instrumented vertebra provide more gradual motion transition than pedicle screws.

Study Design. Biomechanical study in a porcine model. Objective. To determine whether transverse process hooks (TPHs) placed at the proximal end of a long posterior spinal fusion construct provide a more gradual transition to normal motion of the adjacent cephalad motion segment compared with an all pedicle screw (APS) construct. Summary of Background Data. Proximal junctional kyphosis after instrumentation with long posterior spinal constructs has been increasingly associated with incidence of adjacent segment pathologies. Clinical studies have suggested that proximal anchor type may affect the incidence of proximal junctional kyphosis. Methods. Biomechanical tests were conducted on porcine thoracic spines before and after implantation of a long spinal fusion construct. In all specimens, dual long rods (Co-Cr) were implanted posteriorly using pedicle screws at T7–T15. Upper instrumented vertebra, T6, received either TPHs (n = 7) or pedicle screws (APSs) (n = 6). Each specimen was tested in flexion-extension then lateral bending. Moments were applied, and vertebral displacements were recorded. Range of motion (ROM) and stiffness (K) were determined for each motion segment. Differences between TPH and APS at the transition were determined using t tests. Results. In flexion-extension, ROM at the most proximal instrumented motion segment was 9% of control for APS versus 21% of control for TPH. Difference between APS and TPH at UIV was 0.5° (P < 0.008). Stiffness of TPH at T6–T7 was significantly lower than APS in FE (P < 0.003). For APS, the greatest mean ROM occurred at the first uninstrumented segment, whereas TPH maintained the pattern of monotonic increases in mean ROM from distal to proximal. Conclusion. TPHs at the upper instrumented vertebra provided a more gradual transition to normal motion compared with pedicle screws in long posterior spinal fusion constructs. TPH at the upper instrumented vertebra may be postulated to decrease the incidence of postoperative proximal junctional kyphosis compared with APS. Level of Evidence: N/A

Spinal Hemiepiphysiodesis Decreases the Size of Vertebral Growth Plate Hypertrophic Zone and Cells

BACKGROUND Hemiepiphysiodesis is a potential method to treat idiopathic juvenile scoliosis early. The purpose of the present study was to investigate a mechanism of curve creation in the pig thoracic model of spinal hemiepiphysiodesis by determining whether the structure of the vertebral growth plate varied with distance from the stapled, concave side of the spine. The hypotheses were that the heights of the hypertrophic zone, hypertrophic cells, and disc would be decreased on the treated side of the treated level as compared with both an unstapled control level and the side opposite the staple. METHODS Custom spine staples were implanted into six midthoracic vertebrae in each of five skeletally immature pigs. After eight weeks, the spines were harvested and histological sections were prepared. Hypertrophic zone height, hypertrophic cell height and width, and disc height were measured at discrete coronal plane locations at stapled and unstapled thoracic levels. Differences between stapled and unstapled levels and locations were compared with use of mixed linear modeling for repeated measures, followed by regression models to determine growth plate intercept and slope across the plane by thoracic level. RESULTS Zone height, cell height, and cell width were lowest on the stapled side of the stapled level, with significant differences in the overall statistical model (p < 0.02). Disc heights were significantly reduced (p < 0.0001) at the stapled levels across the coronal plane. CONCLUSIONS Unilateral control of intervertebral joint motion decreased growth plate height, cell size, and disc height.

Implantable MEMS compressive stress sensors: Design, fabrication and calibration with application to the disc annulus

Physiological stresses are fundamental to biomechanical testing, mechanobiological analyses, implant design, and tissue engineering. The purpose of this study was to design, fabricate, and evaluate compressive stress sensors packaged for extended, in vivo implantation in the annulus of the intervertebral disc. A commercial microelectromechanical systems (MEMS) pressure sensor die was selected as the active element for a custom stress sensor. The sensor die was modified and packaged to protect the electrical system from the biochemical and biomechanical environment. Completed sensors were calibrated under hydrostatic pressure and solid contact compression. Calibrations were performed before and after 8 weeks of in vivo implantation in a porcine disc. For the two reported sensors, stress and voltage were linearly correlated over a range of 0-1.8 MPa with less than 5% change in sensitivity. Sensitivity to solid contact stress was within 10% of that from hydrostatic pressure. In contrast to most previous studies, in which disc pressure was measured in the fluidic nucleus pulposus, these sensors may be used to measure in vivo dynamic compressive stresses in the annulus at magnitudes typical of the musculoskeletal system in a large animal over a relatively long post-operative time.

Biomechanics of spinal hemiepiphysiodesis for fusionless scoliosis treatment using titanium implant.

Study Design. In vitro study of the effect of hemiepiphyseal implant on biomechanical properties of porcine thoracic motion segments. Objective. Determine whether implantation of a titanium clip-screw construct alters spine biomechanical properties. Summary of Background Data. Growth modification is under investigation as a treatment of early adolescent idiopathic scoliosis. Biomechanical property changes due to device implantation are essential to characterize immediate postoperative treatment effects. Methods. In vitro biomechanical tests were conducted on 18 thoracic functional spinal units. Specimens were tested before and after implantation of a clip-screw construct in lateral bending, flexion-extension, or axial rotation (n = 6 per loading direction). Pure moments were applied, and range of motion, stiffness, and neutral zone were measured. Axial translations were determined bilaterally. Results. Implantation of the clip-screw construct decreased range of motion in lateral bending by 19% (P < 0.0003), flexion-extension by 11% (P < 0.04), and axial rotation by 8%. Mean stiffness in lateral bending toward and away from the treated side increased by 20% (P < 0.007) and 33%, respectively. In flexion and extension, mean stiffness increased by 10% and 16%, respectively. Treatment decreased the neutral zone in lateral bending toward and away from the instrumented side by 30% (P < 0.0003) and 47% (P < 0.02), respectively. In flexion and extension, neutral zone decreased by 20% (P < 0.04) and 26% (P < 0.007), respectively. In axial rotation toward and away from the treated side, mean neutral zone decreased by 22% (P < 0.04) and 7%, respectively. Range of axial translation decreased on the ipsilateral side by 49% (P < 0.001) and increased on the contralateral side by 17%. Conclusion. Implantation of a titanium clip-screw construct decreased range of motion by less than one-fifth, increased stiffness by one-third or less, and decreased the neutral zone by less than one-half. Range of axial translation decreased on the instrumented side and increased contralaterally. This study suggests that most of the flexibility of the spine is preserved in the immediate postoperative period after implantation of the spinal hemiepiphyseal construct.

In vivo dynamic compressive stresses in the disc annulus: a pilot study of bilateral differences due to hemiepiphyseal implant in a quadruped model.

Study Design. In vivo biomechanical study in quadruped model. Objective. To develop in vivo model capable of determining physiological compressive stresses bilaterally in the intervertebral disc annulus and preliminarily assess effects of a hemiepiphyseal implant. Summary of Background Data. Spine growth modification alters stress distributions in vertebral growth plates and discs. Quantification of stresses is required to help assess implant efficacy and disc health. More generally, despite widespread and necessary use of animals in preclinical studies of spine instrumentation, limited quantitative information is available on mechanobiological conditions in quadruped spines for comparisons with those of humans. Methods. Skeletally immature domestic pigs were instrumented with an implant and 4 stress sensors. Sensors were inserted into left and right sides of the annulus at 2 thoracic levels. A titanium staple-screw construct was implanted at 1 level. Signals were acquired intraoperatively, postoperatively during normal activities, and biweekly with the animal under anesthesia, for up to 8 weeks. Results. Stresses varied by sensor location relative to implant, postoperative time, activity, and animal. Intraoperatively, the mean peak stress due to staple insertion was 1.6 MPa at the sensor nearest the staple. Mean stress at the end of surgery was 0.23 MPa. Mean stress standing the first day was 0.38 MPa. Dynamic stresses were recorded at all locations, including the location nearest the staple. Highest mean stresses were those nearest the implant. With the animal under anesthesia, the dynamic stress range in the resting prone position was 0.1 MPa, whereas this range was 0.9 MPa when the spine was manually flexed. Conclusion. Compressive stresses were dynamic at both control and stapled levels, which indicated that the disc was not immobilized by the implant. These pilot results suggested that mean disc compression was increased within the first postoperative week. Stresses ranged up to levels measured in humans.

Pressure sensors for in vivo measurements on spinal growth plates

A new transducer system has been developed for in vivo measurement of pressure on spinal growth plates. A model has been previously proposed for correcting spinal deformities with a staple-like implant, and this sensor will be used in vivo to quantify the pressure that alters growth in this new model of deformity and treatment. The pressure sensor die was 0.65 mm/sup 3/ and the overall packaged sensor size was 1.5 mm/spl times/4 mm/spl times/0.9 mm. The completed sensors were fully characterized by hydrostatic and compression tests, and performed linearly. The ruggedness and small package size will allow the sensor to be placed bilaterally in the annulus of porcine spinal intervertebral discs to record pressure for up to two weeks.

Flexible growing rods: a pilot study to determine if polymer rod constructs may provide stability to skeletally immature spines

BackgroundSurgical treatments for early onset scoliosis (EOS), including growing rod constructs, involve many complications. Some are due to biomechanical factors. A construct that is more flexible than current instrumentation systems may reduce complications. The purpose of this preliminary study was to determine spine range of motion (ROM) after implantation of simulated growing rod constructs with a range of clinically relevant structural properties. The hypothesis was that ROM of spines instrumented with polyetheretherketone (PEEK) rods would be greater than metal rods and lower than noninstrumented controls. Further, adjacent segment motion was expected to be lower with polymer rods compared to conventional systems.MethodsBiomechanical tests were conducted on 6 skeletally immature porcine thoracic spines (domestic swine, 35-40 kg). Spines were harvested after death from swine that had been utilized for other studies (IACUC approved) which had not involved the spine. Paired pedicle screws were used as anchors at proximal and distal levels. Specimens were tested under the following conditions: control, then dual rods of PEEK (6.25 mm), titanium (4 mm), and CoCr (5 mm) alloy. Lateral bending (LB) and flexion-extension (FE) moments of ±5 Nm were applied. Vertebral rotations were measured using video. Differences were determined by two-tailed t-tests and Bonferroni correction with four primary comparisons: PEEK vs control and PEEK vs CoCr, in LB and FE (α=0.05/4).ResultsIn LB, ROM of specimens with PEEK rods was lower than control at each instrumented level. ROM was greater for PEEK rods than both Ti and CoCr at every instrumented level. Mean ROM at proximal and distal noninstrumented levels was lower for PEEK than for Ti and CoCr. In FE, mean ROM at proximal and distal noninstrumented levels was lower for PEEK than for metal. Combining treated levels, in LB, ROM for PEEK rods was 35% of control (p<0.0001) and 270% of CoCr rods (p<0.01). In FE, ROM with PEEK was 27% of control (p<0.001) and 180% of CoCr (p<0.01).ConclusionsPEEK rods decreased flexibility versus noninstumented controls, and increased flexibility versus metal rods. Smaller increases in ROM at proximal and distal adjacent motion segments occurred with PEEK compared to metal rods, which may help decrease junctional kyphosis. Flexible growing rods may eventually help improve treatment options for young patients with severe deformity.

Flexible growing rods: polymer rods provide stability to skeletally immature spines

Objective Surgical treatments for early onset scoliosis (EOS) typically require multiple operations and many complications. A more flexible growing rod construct might result in a more flexible spine with fewer complications. Polymer rods (polyetheretherketone, PEEK) are relatively flexible in bending, and so might allow for greater range of motion (ROM) during treatment. The purpose of this study was to determine changes in ROM of the spine after implantation of simulated growing rod constructs with a range of clinically relevant structural properties. The hypothesis was that ROM of spines instrumented with PEEK rods would be both much greater than metal rods and significantly lower than uninstrumented controls.

Scoliosis vertebral growth plate histomorphometry: Comparisons to controls, growth rates, and compressive stresses: SCOLIOSIS VERTEBRAL GROWTH PLATE HISTOMORPHOMETRY

Scoliosis progression in skeletally immature patients depends on remaining growth. Relationships between vertebral growth plate histomorphometry, growth rates, and mechanical stresses have been reported in several animal studies. Hypertrophic zone heights and chondrocyte heights have been used to assess treatments that aim to modulate growth. The purpose of this study was to determine whether human vertebral physeal hypertrophic zone and cell heights differed between two groups: Severe scoliosis and autopsy controls. Severity was defined at time of surgical planning by curve magnitude and curve stiffness. Physeal samples were obtained from the convex side apex, and from the concave side when feasible. Histologic sections were prepared, and digital images were used to measure hypertrophic zone height, cell height, and cell width. Thirteen spinal deformity patients were included, mean curve magnitude 67° (±23). Etiologies were juvenile and adolescent idiopathic, congenital, neurofibromatosis, neuromuscular, and Marfan syndrome. Five age‐matched autopsy specimens without scoliosis served as controls. Results were presented by etiology, then all convex scoliosis specimens were combined and compared to controls. Zone heights for scoliosis, convex side, and controls were 152 µm (±34) and 180 µm (±42) (p = 0.21), cell heights 8.5 µm (±1.1) and 12.8 µm (±1.2) (p < 0.0005), and cell widths 14.9 µm (±1.5) and 15.0 µm (±2.5), respectively. Human values were compared to published animal models and to a quantitative theory of a stress ̶ growth curve. This quantification of vertebral physeal structures in scoliosis may be expected to help assess theories of progression and potential treatments using growth modulation.

Repetitive Stresses Generate Osteochondral Lesions in Skeletally Immature Rabbits

Background: The origin of juvenile osteochondritis dissecans (OCD) is unknown. Existing experimental animal models of OCD most frequently involve surgically created lesions but do not examine repetitive stress as a possible cause of OCD. Hypothesis: Repetitive stresses can cause OCD-like lesions in immature animals. Study Design: Controlled laboratory study. Methods: Six juvenile rabbits were subjected to repetitive loading forces of approximately 160% body weight to the right hindlimb during five 45-minute sessions per week for 5 weeks. The contralateral limb was the unloaded control. After 5 weeks, rabbits were euthanized and examined with radiographs, micro–computed tomography, and gross and histopathologic analysis. Results: All 6 rabbits developed osteochondral lesions in loaded limbs on the medial and lateral femoral condyles, while contralateral unloaded limbs did not demonstrate lesions. Loaded limbs developed relative osteopenia in the femoral epiphysis and tibial metaphysis with associated decreased trabecular density. Loaded limbs also demonstrated increased femoral subchondral bone thickness near the lesions. Lesions did not have grossly apparent extensive articular cartilage damage; however, cartilage thickness increased on histology with reduced ossification. Loaded knees demonstrated abundant chondrocyte cloning, limited cartilage fissuring, and a focal loss of cellularity at the articular surface. Conclusion: Low-grade lesions in human OCD have little gross articular cartilage involvement despite substantial changes to the subchondral bone as shown on magnetic resonance imaging and radiographs. Histopathology findings in this study included cartilage thickening and chondrocyte cloning resembling those of recently published human OCD biopsy studies. Our animal model supports the hypothesis that repetitive stress to immature knees may contribute to the development of human OCD. This model may be useful in understanding the pathophysiology and healing of human OCD. Clinical Relevance: Repetitive physiologic stress generated changes to the subchondral bone in immature animals without causing extensive articular damage. The similarities of these lesions in gross and histologic appearance with human OCD support repetitive stress as the likely the cause for human OCD.