Albert G. Veldhuizen

Cytotoxic, allergic and genotoxic activity of a nickel-titanium alloy

The nearly equiatomic nickel-titanium (NiTi) alloy is known for its shape memory properties. These properties can be put to excellent use in various biomedical applications, such as wires for orthodontic tooth alignment and osteosynthesis staples. The aim of this study was to evaluate the short-term biological safety of the NiTi alloy. We carried out an end-point dilution minimal essential medium (MEM) extract cytotoxicity test, a guinea-pig sensitization test and two genotoxicity tests: the Salmonella reverse mutation test and the chromosomal aberration test. The NiTi alloy showed no cytotoxic, allergic or genotoxic activity, similar to the clinical reference control material AISI 316 LVM stainless steel. This promising biological behaviour was most likely due to a minimal release of ions and in that way a reflection of the good corrosion resistance of the NiTi alloy. Given these very good results, together with the good tissue compatibility as shown in several implantation studies in the literature, the NiTi alloy can be regarded as a biologically safe implant material with many promising clinical applications.

Percutaneous vertebroplasty as a treatment for osteoporotic vertebral compression fractures: a systematic review

Percutaneous vertebroplasty has been performed for more than ten years to treat painful osteoporotic vertebral compression fractures. Clinical results have been encouraging, but little is known about the efficacy and safety of this minimally invasive procedure. We therefore performed a systematic review to assess the efficacy and safety of percutaneous vertebroplasty in osteoporotic vertebral compression fractures. A search was conducted using Medline, Embase and The Cochrane Controlled Trials Register. The search yielded fifteen studies, eleven prospective, three retrospective and one controlled trial. Totally 1,136 interventions were performed on 793 patients. Mean pain scores, measured using a 0 to 10 VAS score, improved significantly from 7.8 to 3.1 (−60.3%) immediately after percutaneous vertebroplasty. The short-term complication rate varied between 0.4 and 75.6%. Leakage of cement outside the vertebral body was markedly common, ranging from 3.3 to 75.6%. Although the majority was asymptomatic, a few devastating clinical adverse effects were reported (mean 2.4%). Although percutaneous vertebroplasty is a widely accepted treatment for osteoporotic vertebral fractures, we revealed only a single controlled trial. We conclude that there are insufficient data available to reliably assess efficacy of percutaneous vertebroplasty. The procedure has a low rate of clinical complications, but potential complications can be devastating. In the future, assessing the efficacy of percutaneous vertebroplasty requires controlled trials with long-term follow-up.

Comparative anatomical dimensions of the complete human and porcine spine

New spinal implants and surgical procedures are often tested pre-clinically on human cadaver spines. However, the availability of fresh frozen human cadaver material is very limited and alternative animal spines are more easily available in all desired age groups, and have more uniform geometrical and biomechanical properties. The porcine spine is said to be the most representative model for the human spine but a complete anatomical comparison is lacking. The goal of this descriptive study was to compare the anatomical dimensions of the cervical, thoracic, and lumbar vertebrae of the human and porcine spine in order to determine whether the porcine spine can be a representative model for the human spine. CT scans were made of 6 human and 6 porcine spines, and 16 anatomical dimensions were measured per individual vertebrae. Comparisons were made for the absolute values of the dimensions, for the patterns of the dimensions within four spinal regions, and normalised values of the dimensions within each individual vertebra. Similarities were found in vertebral body height, shape of the end-plates, shape of the spinal canal, and pedicle size. Furthermore, regional trends were comparable for all dimensions, except for spinal canal depth and spinous processus angle. The size of the end-plates increased more caudally in the human spine. Relating the dimensions to the size of the vertebral body, similarities were found in the size of the spinal canal, the transverse processus length, and size of the pedicles. Taking scaling differences into account, it is believed that the porcine spine can be a representative anatomical model for the human spine in specific research questions.

The aetiology of idiopathic scoliosis: biomechanical and neuromuscular factors

Abstract The aetiology of adolescent idiopathic scoliosis (AIS) remains an enigma. In the literature there are two opinions: one believes a deviating growth pattern is responsible for the condition – patients with AIS tend to be growing faster/be taller – while the other opinion assumes that the growth pattern is normal, but its presence is necessary to allow the development of the scoliosis. We discuss the two stage hypothesis: the natural history of AIS involves an initial stage in which a small curve develops due to a small defect in the neuromuscular control system and a second stage during adolescent growth in which the scoliotic curve is exacerbated by biomechanical factors.

In vitro biomechanical characteristics of the spine: a comparison between human and porcine spinal segments.

Study Design. An in vitro study on human and porcine multilevel spinal segments. Objective. To compare human and porcine thoracolumbar spinal segments with respect to their biomechanical characteristics and the effects of creep, recovery, and removal of ligaments and posterior parts on the biomechanical characteristics. Summary of Background Data. Availability of human cadaver spines for in vitro testing of new spinal implants and surgical procedures is limited. Therefore, it is important to search for animal models with representative biomechanical characteristics. Methods. A total of 6 human and 6 porcine cadaver spines were dissected in multilevel spinal segments. Pure moments were applied to each segment in flexion/extension, lateral bending, and axial rotation. Creep tests were performed for 30 minutes in 4 creep directions, followed by cyclic tests, a recovery period of 30 minutes, and a series of cyclic tests after removal of ligaments and posterior parts. The range of motion, neutral zone (NZ), and neutral zone stiffness (NZStiff) were calculated from the acquired load-displacement data and results were compared between human and porcine segments. Results. The porcine segments generally had significantly higher absolute values for range of motion and NZ and significantly lower absolute values for NZStiff than the human segments in all directions. The effects of creep and recovery were quite similar in the higher and midthoracic regions of the spine. The influence of removal of ligaments was the same in human and porcine segments. After removal of posterior parts, the lower thoracic porcine spine behaved quite similar to the lumbar human spine. Conclusion. This study showed that the porcine spine can be a good biomechanical model for the human spine in specific situations. The question if the porcine spine can be used to predict the behavior of a human spine depends mainly on the application and the research question.

Requirements for an artificial intervertebral disc.

Intervertebral disc degeneration is an important social and economic problem. Presently available artificial intervertebral discs (AIDs) are insufficient and the main surgical intervention is still spinal fusion. The objective of the present study is to present a list of requirements for the development of an AID which could replace the human lumbar intervertebral disc and restore its function. The list addresses geometry, stiffness, range of motion, strength, facet joint function, center of rotation, fixation, failsafety and implantation technique. Date are obtained from the literature, quantified where possible and checked for consistency. Existing AIDs are evaluated according to the presented list of requirements. Endplate size is a weak point in existing AIDs. These should be large and fit vertebral bodies to prevent migration. Disc height and wedge angle should be restored, unless this would overstretch ligaments. Finally, stiffness and range of motion in all directions should equal those of the healthy disc, except for the axial rotation to relieve the facet joints.

A biomechanical analysis of the vertebral and rib deformities in structural scoliosis

Abstract Although the structural changes occurring in the scoliotic spine have been reported as early as the 19th century, the descriptions and biomechanical explanations have not always been complete and consistent. In this study, three-dimensionally rendered CT images of two human skeletons with a scoliotic deformity and two patients with serious scoliosis were used to describe the intrinsic vertebral and rib deformities. The pattern of structural deformities was found to be consistent. Apart from the wedge deformation of the apical vertebrae, a rotation deformity was found in the transversal plane between the vertebral body and the posterior complex: the vertebral body was maximally rotated towards the convexity of the scoliotic curve, whereas the tip of the spinous process was pointed to posterior. The rib deformities at the convex side of the scoliotic curve showed an increased angulation of the rib at the posterior angle, whereas the rib curve on the concave side was flattened. The observed vertebral deformities suggest that these are caused by bone remodelling processes due to forces in the anterior spinal column, which drive the apical vertebral body out of the midline, whereas forces of the musculo-ligamentous structures at the posterior side of the spinal column attempt to minimize the deviations and rotations of the vertebrae. The demonstrated rib deformities suggest an adaptation to forces imposed by the scoliotic spine.

Scoliosis correction with shape-memory metal: results of an experimental study

Abstract. The biocompatibility and functionality of a new scoliosis correction device, based on the properties of the shape-memory metal nickel-titanium alloy, were studied. With this device, the shape recovery forces of a shape-memory metal rod are used to achieve a gradual three-dimensional scoliosis correction. In the experimental study the action of the new device was inverted: the device was used to induce a scoliotic curve instead of correcting one. Surgical procedures were performed in six pigs. An originally curved squared rod, in the cold condition, was straightened and fixed to the spine with pedicle screws. Peroperatively, the memory effect of the rod was activated by heating the rod to 50°C by a low-voltage, high-frequency current. After 3 and after 6 months the animals were sacrificed. The first radiographs, obtained immediately after surgery, showed in all animals an induced curve of about 40° Cobb angle – the original curve of the rod. This curve remained constant during the follow-up. The postoperative serum nickel measurements were around the detection limit, and were not significantly higher compared to the preoperative nickel concentration. Macroscopic inspection after 3 and 6 months showed that the device was almost overgrown with newly formed bone. Corrosion and fretting processes were not observed. Histologic examination of the sections of the surrounding tissues and sections of the lung, liver, spleen and kidney showed no evidence of a foreign body response. In view of the initiation of the scoliotic deformation, it is expected that the shape-memory metal based scoliosis correction device also has the capacity to correct a scoliotic curve. Moreover, it is expected that the new device will show good biocompatibility in clinical application. Extensive fatigue testing of the whole system should be performed before clinical trials are initiated.

Biomechanical Characteristics of Different Regions of the Human Spine An In Vitro Study on Multilevel Spinal Segments

Study Design. An in vitro study on human multilevel spinal segments. Objective. To determine the differences in biomechanical characteristics between 4 separate regions of the human spine and to provide quantitative information is derived on the range of motion (ROM), neutral zone (NZ), neutral zone stiffness (NZstiff), and flexibility (FLEX). Summary of Background Data. Limited literature is available about the biomechanical behavior of different regions of the human spine, in particular with multilevel segments. Test setup en protocols were different between studies and therefore outcomes of separate regions are hardly comparable. Methods. A total of 24 spinal segments of 6 human cadaveric spines were prepared for biomechanical testing. Each specimen contained 4 vertebrae and 3 intervertebral discs: T1–T4, T5–T8, T9–T12, and L1–L4. Pure moments were applied to a maximum of 4 Nm in flexion/extension, lateral bending, and axial rotation. Displacement of individual motion segments was measured using a 3-dimensional movement registration system. ROM, NZ, NZstiff, and FLEX of the spinal regions were calculated from the acquired load-displacement data. Results. In axial direction, ROM and NZ decreased and NZ stiffness increased from high to low vertebral levels. For flexion/extension and lateral flexion highest ROM and NZ and lowest NZ stiffness values were found at the T1–T4 and L1–L4 regions. NZ magnitudes and NZ stiffnesses were negatively correlated (P < 0.05). Flexibility of the spinal regions was variable; no significant differences were found between the 4 spinal regions. Conclusion. This study showed the differences in ROM, NZ, and NZ stiffness between thoracolumbar regions of the human spine in axial rotation, flexion/extension, and lateral bending. Separate multilevel spinal segments were tested in 1 study, and therefore characteristics of different regions are truly comparable.

Curve progression and spinal growth in brace treated idiopathic scoliosis

The risk of progression of idiopathic scoliosis is correlated primarily to factors that predict potential remaining skeletal growth. The aim of the current study was to evaluate spinal growth, measured as the length of the scoliotic spine on serial longitudinal radiographs, and its relationship to progression of the scoliotic curve. The retrospective study was based on measurements made on standing anteroposterior radiographs of 60 patients with adolescent idiopathic scoliosis. In all patients, a Boston brace was prescribed during the followup period. Despite brace treatment, a significantly greater average progression rate of the scoliotic curve was seen in periods of rapid to moderate growth (≥ 10 mm per year) compared with periods of small or no growth (< 10 mm per year). The difference in progression rates concerned the increase of the Cobb angle and the increase of lateral deviation and axial rotation. These findings indicate the length of the spine measured on subsequent radiographs is an excellent parameter to determine spinal growth and thus an excellent predictor of scoliosis progression. With the presented growth charts, which were derived from the measured individual growth velocity values of the patients in the study, it is possible to predict future spinal growth at different chronologic ages.