Dual and single memory rod construct comparison in an animal study

Contactless treatment for scoliosis by electromagnetically controlled shape-memory alloy rods: a preliminary study in rabbits

PURPOSE

To evaluate the safety and efficacy of a system aiming to correct scoliosis called "electromagnetically controlled shape-memory alloy rods" (EC-SMAR) used in a rabbit model.

METHODS

We heat-treated shape-memory alloy (SMA) rods to achieve a transition temperature between 34 and 47 °C and a C-shape austenite phase. We then developed a water-cooled generator capable of generating an alternating magnetic field (100 kHz) for induction heating. We next studied the efficacy of this system in vitro and determined some parameters prior to proceeding with animal experiments. We then employed a rabbit model, in which we fixed a straight rod along the spinous processes intraoperatively, and conducted induction heating postoperatively every 4 days for 1 month, while performing periodic X-ray assessments.

RESULTS

Significant kyphotic deformations with Cobb angles of about 45° (p < 0.01) were created in five rabbits, and no complications occurred throughout the experiment. The rabbits are still very much alive and do not show any signs of discomfort.

CONCLUSIONS

This is the first system that can modulate spinal deformation in a gradual, contactless, noninvasive manner through electromagnetic induction heating applied to SMA alloy rods. Although this study dealt with healthy spines, it provides promising evidence that this device also has the capacity to correct human kyphosis and even scoliosis in the future. These slides can be retrieved under Electronic Supplementary Material.

Posterolateral lumbar fusion using Escherichia coli-derived rhBMP-2/hydroxyapatite in the mini pig

BACKGROUND CONTEXT

Hydroxyapatite (HA) is used as a bone graft extender for posterolateral spinal fusion in human. It is also useful as a recombinant human bone morphogenetic protein (rhBMP)-2 carrier because of its high affinity for rhBMP-2.

PURPOSE

To assess the osteoinductivity of Escherichia coli-derived rhBMP-2 (E-BMP-2) using HA granules as a carrier and to evaluate the bone-forming ability depending on the different dosages of E-BMP-2.

STUDY DESIGN

A mini-pig lumbar posterolateral fusion model using microcomputed tomography (μCT) scanning.

PATIENT SAMPLE

Thirty-one adult male mini pigs were randomized into a single control group (n=8) without E-BMP-2 and two experimental groups with two different doses of E-BMP-2 (1 mg per side, n=8 and 3 mg per side, n=15).

OUTCOME MEASURES

Outcome was measured by plain radiography, manual palpation, CT, three-dimensional μCT, and histologic examinations.

METHODS

Bilateral intertransverse process arthrodesis was performed, and E-BMP-2 (0, 1.0, 3.0 mg per side) was implanted into the intertransverse space using HA granules as a carrier.

RESULTS

Three mini pigs were removed because of death. Among 28 experimental subjects, 19 animals achieved solid bony union. The fusion rates were 37.5% for control group, 71.4% for 1 mg group, and 84.6% for 3 mg group. Fusion rates were significantly different among groups (p=.031). However, there was no statistically significant difference in fusion rates between 1 and 3 mg groups (p=.587). Thirty-eight intertransverse fusion masses of 19 subjects underwent μCT scanning. The bone volumes determined by μCT were 12,603±3,240 mm(3) for control group, 18,718±3,000 mm(3) for 1 mg group, and 26,768±7,256 mm(3) for 3 mg group, and the difference between groups was statistically significant (p<.001).

CONCLUSIONS

This study shows that E-BMP-2 has osteoinductive activity in dose-dependent fashion, and porous HA granule is suitable for E-BMP-2 carrier in a porcine posterolateral fusion model. These preliminary findings suggest that E-BMP-2-adsorbed porous HA granules could be a novel effective bone graft substitute.

Extraspinal ossifications after implantation of vertical expandable prosthetic titanium ribs (VEPTRs)

INTRODUCTION

Though developed for thoracic insufficiency syndrome, the spinal growth-stimulating potential and the ease of placement of vertical expandable titanium ribs (VEPTRs) has resulted in their widespread use for early-onset spine deformity. Observation of implant-related ossifications warrants further assessment, since they may be detrimental to the function-preserving non-fusion strategy.

PATIENTS AND METHODS

Radiographs (obtained pre and post index procedure, and at 4-year follow-up) and the records of 65 VEPTR patients from four paediatric spine centres were analysed. Ossifications were classified as type I (at anchor points), type II (along the central part) or type III (re-ossification after thoracostomy).

RESULTS

The average age at the index procedure was 6.5 years (min 1, max 13.7). The most prevalent spine problem was congenital scoliosis (37) with rib fusions (34), followed by neuromuscular and syndromic deformities (13 and 8, respectively). Idiopathic and secondary scoliosis (e.g. after thoracotomy) were less frequent (3 and 4, respectively). Forty-two of the 65 (65 %) patients showed ossifications, half of which were around the anchors. Forty-five percent (15/33) without pre-existing rib fusions developed a type II ossification along the implant. Re-ossifications of thoracostomies were less frequent (5/34, 15 %). The occurrence of ossifications was not associated with patient-specific factors.

CONCLUSIONS

Implant-related ossifications around VEPTR are common. In contrast to harmless bone formation around anchors, ossifications around the telescopic part and the rod section are troublesome in view of their possible negative impact on chest cage compliance and spinal mobility. This potential side effect needs to be considered during implant selection, particularly in patients with originally normal thoracic and spinal anatomy.

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.

Rods in spinal surgery: a review of the literature

BACKGROUND CONTEXT

Spinal instrumentation has been used for more than five decades. Since the introduction of the Harrington rod in 1962, new rod materials and concepts have been developed. Rigid rod fixation has achieved higher fusion rates than previous methods. Recently, semirigid rod fixation devices have been used for both dynamic stabilization and fusion fixation. Memory rods, which have an interesting ability to return to their pre-bent shape when the temperature increases, are expected to be used for scoliosis correction.

PURPOSE

To review the previous literature regarding biofunctionality and biocompatibility of rods in spinal surgery.

CONCLUSION

The properties of each type of rod need to be taken into consideration when performing spinal instrumentation surgery.

A new lumbar posterior fixation system, the memory metal spinal system: an in-vitro mechanical evaluation

BACKGROUND

Spinal systems that are currently available for correction of spinal deformities or degeneration such as lumbar spondylolisthesis or degenerative disc disease use components manufactured from stainless steel or titanium and typically comprise two spinal rods with associated connection devices (for example: DePuy Spines Titanium Moss Miami Spinal System). The Memory Metal Spinal System of this study consists of a single square spinal rod made of a nickel titanium alloy (Nitinol) used in conjunction with connecting transverse bridges and pedicle screws made of Ti-alloy. Nitinol is best known for its shape memory effect, but is also characterized by its higher flexibility when compared to either stainless steel or titanium. A higher fusion rate with less degeneration of adjacent segments may result because of the elastic properties of the memory metal. In addition, the use of a single, unilateral rod may be of great value for a TLIF procedure. Our objective is to evaluate the mechanical properties of the new Memory Metal Spinal System compared to the Titanium Moss Miami Spinal System.

METHODS

An in-vitro mechanical evaluation of the lumbar Memory Metal Spinal System was conducted. The test protocol followed ASTM Standard F1717-96, "Standard Test Methods for Static and Fatigue for Spinal Implant Constructs in a Corpectomy Model." 1. Static axial testing in a load to failure mode in compression bending, 2. Static testing in a load to failure mode in torsion, 3. Cyclical testing to estimate the maximum run out load value at 5.0 x 10^6 cycles.

RESULTS

In the biomechanical testing for static axial compression bending there was no statistical difference between the 2% yield strength and the stiffness of the two types of spinal constructs. In axial compression bending fatigue testing, the Memory Metal Spinal System construct showed a 50% increase in fatigue life compared to the Titanium Moss Miami Spinal System. In static torsional testing the Memory Metal Spinal System constructs showed an average 220% increase in torsional yield strength, and an average 30% increase in torsional stiffness.

CONCLUSIONS

The in-vitro mechanical evaluation of the lumbar Memory Metal Spinal System showed good results when compared to a currently available spinal implant system. Throughout testing, the Memory Metal Spinal System showed no failures in static and dynamic fatigue.

Impact of unilateral corrective tethering on the histology of the growth plate in an established porcine model for thoracic scoliosis

STUDY DESIGN

Histological growth plate analysis. OBJECTIVE.: To evaluate the histological effects on vertebral growth plates following corrective mechanical tethering in the porcine scoliosis model.

SUMMARY OF BACKGROUND DATA

Theoretically, growth modulation allows progressive vertebral correction in the setting of scoliosis (Hueter-Volkmann principle).

METHODS

This IACUC-approved study divided 9 immature Yorkshire pigs into 2 groups: deforming tether release (TR, n = 4) group and anterior corrective (AC, n = 5) tether group. Once 50° coronal Cobb was demonstrated, TR had release of the deforming tether, whereas AC had release of the deforming tether and additional placement of a corrective tether. After 20 weeks of observation, pigs were killed, spines were removed, and apical samples were prepared for histological study. Growth plate analysis included the following histological parameters: proliferative zone height, hypertrophic zone height, and cell heights within the hypertrophic zone. Student t test was used to evaluate differences within and between groups.

RESULTS

No significant differences were found within the release group on the concave versus convex side in terms of proliferative zone height, hypertrophic zone height, and cell heights in the hypertrophic zone. In the anterior correction group, the proliferative zone height was significantly smaller on the concave side than on the convex side (P < 0.01); no significant differences were found in AC on the concave versus convex side in terms of hypertrophic zone height and cell heights in the hypertrophic zone. No significant differences were found in any parameters between TR and AC on either the concave or the convex side.

CONCLUSION

No significant decrease in any of the measured parameters was observed in the anterior correction group compared with the tether release group. These histological findings are consistent with preservation of growth potential.