Digital tracking algorithm reveals the influence of structural irregularities on joint movements in the human cervical spine.

BACKGROUND: Disc height loss and osteophytes change the local mechanical environment in the spine; while previous research has examined kinematic dysfunction under degenerative change, none has looked at the influence of disc height loss and osteophytes throughout movement. METHODS: Twenty patients with pain related to the head, neck or shoulders were imaged via videofluoroscopy as they underwent sagittal-plane flexion and extension. A clinician graded disc height loss and osteophytes as "severe/moderate", "mild", or "none". A novel tracking algorithm quantified motions of each vertebra. This information was used to calculate intervertebral angular and shear displacements. The digital algorithm made it practical to track individual vertebrae in multiple patients through hundreds of images without bias. FINDINGS: Cases without height loss/osteophytes had a consistent increase in intervertebral angular displacement from C2/C3 to C5/C6, like that of healthy individuals, and mild height losses did not produce aberrations that were systematic or necessarily discernable. However, joints with moderate to severe disc height loss and osteophytes exhibited reduced range of motion compared to adjacent unaffected joints in that patient and corresponding joints in patients without structural irregularities. INTERPRETATION: Digitally-obtained motion histories of individual joints allowed anatomical joint changes to be linked with changes in joint movement patterns. Specifically, disc height loss and osteophytes were found to influence cervical spine movement in the sagittal plane, reducing angular motions at affected joints by approximately 10% between those with and without height loss and osteophytes. Further, these joint changes were associated with perturbed intervertebral angular and shear movements.

A videofluoroscopy-based tracking algorithm for quantifying the time course of human intervertebral displacements.

The motions of individual intervertebral joints can affect spine motion, injury risk, deterioration, pain, treatment strategies, and clinical outcomes. Since standard kinematic methods do not provide precise time-course details about individual vertebrae and intervertebral motions, information that could be useful for scientific advancement and clinical assessment, we developed an iterative template matching algorithm to obtain this data from videofluoroscopy images. To assess the bias of our approach, vertebrae in an intact porcine spine were tracked and compared to the motions of high-contrast markers. To estimate precision under clinical conditions, motions of three human cervical spines were tracked independently ten times and vertebral and intervertebral motions associated with individual trials were compared to corresponding averages. Both tests produced errors in intervertebral angular and shear displacements no greater than 0.4° and 0.055 mm, respectively. When applied to two patient cases, aberrant intervertebral motions in the cervical spine were typically found to correlate with patient-specific anatomical features such as disc height loss and osteophytes. The case studies suggest that intervertebral kinematic time-course data could have value in clinical assessments, lead to broader understanding of how specific anatomical features influence joint motions, and in due course inform clinical treatments.

Evaluation of an injectable hydrogel and polymethyl methacrylate in restoring mechanics to compressively fractured spine motion segments.

BACKGROUND CONTEXT: Compressive fracture can produce profound changes to the mechanical profile of a spine segment. Minimally invasive repair has the potential to restore both function and structural integrity to an injured spine. Use of both hydrogels to address changes to the disc, combined with polymethyl methacrylate (PMMA) to address changes to the vertebral body, has the potential to facilitate repair. PURPOSE: The purpose of this investigation was to determine if the combined use of hydrogel injection and PMMA could restore the mechanical profile of an axially injured spinal motion segment. STUDY DESIGN: This is a basic science study evaluating a combination of hydrogel injection and vertebroplasty on restoring mechanics to compressively injured porcine spine motion segments. METHODS: Fourteen porcine spine motion segments were subject to axial compression until fracture using a dynamic servohydraulic testing apparatus. Rotational and compressive stiffness was measured for each specimen under the following conditions: initial undamaged, fractured, fatigue loading under compression, hydrogel injection, PMMA injection, and fatigue loading under compression. Group 1 received hydrogel injection followed by PMMA injection, whereas Group 2 received PMMA injection followed by hydrogel injection. This study was funded under a Natural Sciences and Engineering Research Council of Canada discovery grant. RESULTS: PMMA injection was found to alter the compressive stiffness properties of axially injured spine motion segments, restoring values from Groups 1 and 2 to 89.3%±29.3% and 81%±27.9% of initial values respectively. Hydrogel injection was found to alter the rotational stiffness properties, restoring specimens in Groups 1 and 2 to 151.5%±81% and 177.2%±54.9% of initial values respectively. Prolonged restoration of function was not possible, however, after further fatigue loading. CONCLUSIONS: Using this repair technique, replication of the mechanism of injury appears to cause a rapid deterioration in function of the motion segments. Containment of the hydrogel appears to be an issue with large breaches in the end plate, as it is posited to migrate into the cancellous bone of the vertebral body. Future work should attempt to evaluate methods in fully sealing the disc space.

Disc height loss and restoration via injectable hydrogel influences adjacent segment mechanics in-vitro.

BACKGROUND: Height loss can have a profound influence on the local mechanical environment of the disc. While disc height loss is incorporated into scales of degeneration, its direct influence on spine kinematics is unclear. Further, there is a need for minimally invasive techniques to restore disc height; injectable hydrogels are a potential solution. Tandem investigation of disc height loss and subsequent restoration will enhance understanding of spine dysfunction and aberrant movement. METHODS: Twenty porcine spine specimens with two functional segments were tested in repeated flexion and extension. Relative angular displacement of each segment was measured with full specimen disc height, disc height loss in one of the segments (superior or inferior), and disc height restoration via hydrogel injection. FINDINGS: Disc height loss decreased the range of motion at the affected segment and increased the range of motion at the adjacent segment. Relative angular displacement decreased at the affected segment by 13.8% (SD=5.3%) and 4.5% (SD=2.1%) for specimens with height loss in the superior and inferior discs respectively. Hydrogel injection was able to restore segmental kinematics to the pre-injury state, with 12.7% (SD=5.5%) and 6.4% (SD=4.2%) of motion regained at the affected segment for superior and inferior disc height loss specimens respectively. INTERPRETATION: Acute disc height loss reduces motion at an affected segment, while increasing motion at an adjacent segment in-vitro; relative motion appears to be governed by local stiffness. Injectable hydrogels show promise in their ability to restore kinematics to segments with disc height loss.

Closure of the annulus fibrosus of the intervertebral disc using a novel suture application device-in vivo porcine and ex vivo biomechanical evaluation.

BACKGROUND CONTEXT: Defects in the annulus fibrosus (AF) remain a challenge in the surgical treatment of lumbar disc herniations with persistent defects, allowing potential re herniation of nucleus pulposus (NP) tissue. A cervical porcine model was chosen to simulate human lumbar intervertebral disc (IVD). PURPOSE: The aim of this study was to determine the technical feasibility of closure of the AF of the IVD using a novel minimally invasive Kerrison-shaped suture application device. STUDY DESIGN: Ex vivo biomechanical and in vivo porcine device evaluations were performed. METHODS: Ex vivo biomechanical evaluation: 15 porcine spinal units were explanted and subjected to mock discectomy. The annular defect was closed using 2-0 non-absorbable (ultra-high molecular-weight polyethylene, UHMWPE) suture and Dines knot. The knot was backed up with two, three, or four throws. The spinal unit was subject to 4000 cycles of flexion/extension with 1500 N of axial load, and assessed for knot slippage. In vivo porcine device evaluation: three pigs (53-57 kg) were anesthetized and underwent a ventral surgical approach to the cervical spine. The AF of two discs was incised, and simulated partial NP discectomy was performed. The defect was closed at one level using the AnchorKnot device to apply the suture with a Dines knot and four throws. The pigs were observed for 4 weeks before euthanasia, allowing 7T magnetic resonance imaging (MRI) and histological evaluation. RESULTS: A Dines knot with four throws experienced no slippage after 4000 cycles. This configuration was tested in vivo. Clinically, the neurological examination in treated pigs was normal following surgery. Histological and MRI assessment confirmed sustained defect closure at 4 weeks. There was no reaction to the suture material and no NP extrusion at any of the sutured levels. CONCLUSIONS: This study demonstrates that it is technically feasible to perform AF defect closure in a porcine model. This novel device achieved AF defect closure that was maintained through 4 weeks in vivo.

Annulus Fibrosus Can Strip Hyaline Cartilage End Plate from Subchondral Bone: A Study of the Intervertebral Disk in Tension.

Study Design Biomechanical study on cadaveric spines. Objective Spinal bending causes the annulus to pull vertically (axially) on the end plate, but failure mechanisms in response to this type of loading are poorly understood. Therefore, the objective of this study was to identify the weak point of the intervertebral disk in tension. Methods Cadaveric motion segments (aged 79 to 88 years) were dissected to create midsagittal blocks of tissue, with ∼10 mm of bone superior and inferior to the disk. From these blocks, 14 bone-disk-bone slices (average 4.8 mm thick) were cut in the frontal plane. Each slice was gripped by its bony ends and stretched to failure at 1 mm/s. Mode of failure was recorded using a digital camera. Results Of the 14 slices, 10 failed by the hyaline cartilage being peeled off the subchondral bone, with the failure starting opposite the lateral annulus and proceeding medially. Two slices failed by rupturing of the trabecular bone, and a further two failed in the annulus. Conclusions The hyaline cartilage-bone junction is the disk's weak link in tension. These findings provide a plausible mechanism for the appearance of bone and cartilage fragments in herniated material. Stripping cartilage from the bony end plate would result in the herniated mass containing relatively stiff cartilage that does not easily resorb.

Head posture influences low back muscle endurance tests in 11-year-old children.

Poor low back muscle endurance has been shown to be a predictor of chronic low back pain. While posture is a modulator of low back muscle endurance, it is unclear whether the phenomenon is neural or mechanical. This study examined low back muscle endurance with changing head and neck posture in a sample of 117 children using the Biering-Sørensen test. Each subject performed the test in a neutral posture followed by randomly selected flexed and extended head and neck positions. Head posture was found to significantly influence low back muscle endurance within subjects (p < .001), with extension yielding the highest endurance scores (boys = 186.6 ± 66.2 s; girls = 192.1 ± 59 s), followed by a neutral posture (boys = 171.3 ± 56.5 s; girls = 181.7 ± 57.3 s), and flexion (boys = 146.2 ± 63.8 s; girls = 159.8 ± 49.3 s). Given the minimal influence of changing moment from head and neck posture, it appears other mechanisms influence endurance score.

Significance of cartilage endplate within herniated disc tissue.

PURPOSE: Disc herniations sometimes contain hyaline cartilage fragments, but their origins and significance are uncertain. METHODS: Herniations were removed surgically from 21 patients (aged 35-74 years) whose main symptom was sciatica (10 patients) or back pain (11 patients). Frozen sections, 5 µm thick, were examined histologically, and antibodies were used to label the matrix-degrading enzyme MMP 1, pro-inflammatory mediator TNFα, and cell proliferation marker Ki-67. Proportions of each tissue type were quantified by image analysis. Cartilage and bone components of the endplate were examined in 7-µm frozen sections from 16 cadaveric spines, aged 61-98 years. RESULTS: Cartilage fragments were found in 10/21 herniations. They averaged 5.0 mm in length, comprised 25 % of the herniation area, and two had some bone attached. Hyaline cartilage was more common in herniations from patients with sciatica (7/10) than with back pain (3/11, P = 0.050), and the area (%) of the herniation occupied by the cartilage was greater in sciatica patients (P < 0.05). Cartilage fragments showed little evidence of swelling, proteoglycan loss or inflammatory cell invasion, although cell clustering was common, and TNFα was sometimes expressed. Each cartilage fragment showed at least one straight edge, as if it had been peeled off the bony endplate, and this mechanism of failure was demonstrated in preliminary mechanical experiments. CONCLUSION: Disc herniations often include hyaline cartilage pulled from the vertebral endplates. Cartilage fragments show little swelling or proteoglycan loss, and may be slow to resorb, increasing the risk of persisting sciatica. Loss of cartilage will increase endplate permeability, facilitating endplate inflammation and disc infection.

The use of a novel injectable hydrogel nucleus pulposus replacement in restoring the mechanical properties of cyclically fatigued porcine intervertebral discs.

Repeated flexion and extension of an intervertebral disc has been shown to affect the angular stiffness of spinal motion segments and is a barometer of the mechanical integrity of the disc. A degenerated disc that loses height causes higher levels of stress on the annulus and facet joints which may increase its level of degeneration; restoring disc height may therefore help to slow this degenerative cascade. Previous research has indicated that nucleus implants have the potential to improve the mechanical characteristics of a disc and an implant that is custom-fit to the intervertebral disc yields the best results with respect to decreasing annular degeneration. Two groups of porcine spinal motion segments were exposed to repeated flexion and extension. One group was then injected with a novel hydrogel while the other group was used as a control. Both groups were then exposed to another round of cyclic flexion and extension to examine the effect that the hydrogel had on restoring the original mechanics to the motion segments. Angular stiffness was restored to the group which received the hydrogel injection in addition to a significant improvement in specimen height. No significant changes were seen in the group which did not receive an injection. It would appear that use of the novel injectable hydrogel is able to restore angular stiffness to cyclically fatigued spinal motion segments. It is also important to note that continued repetition of the event causing specimen fatigue after performing hydrogel injection will result in an eventual return to the same fatigued state.

Extent of nucleus pulposus migration in the annulus of porcine intervertebral discs exposed to cyclic flexion only versus cyclic flexion and extension.

BACKGROUND: Repeated flexion of an intervertebral disc has been identified as a mechanism to produce posterior herniations. Repeated extension under certain conditions has also been shown to cause the nucleus of partially herniated discs to reverse and migrate anteriorly. While research shows that the nucleus pulposus migrates anteriorly in extension and infiltrates the annulus posteriorly in flexion, it is not known if a cycle of flexion followed by a cycle of extension produces more or less annular damage compared to pure flexion alone. METHODS: Two groups of porcine spinal motion segments were exposed to either repeated flexion with extension or just repeated flexion. Digitized photographs of dissected specimens enhanced with a radio-opaque blue dye enabled the quantification of the area of annulus infiltrated with nucleus pulposus. FINDINGS: Specimens exposed to both flexion and extension showed significantly more annular damage and axial creep compared to those exposed to flexion alone. INTERPRETATION: It would appear that while flexion alone can still cause nucleus pulposus to track through the annulus of an intervertebral disc, the effects are compounded when it is followed by a subsequent cycle of extension. Thus, movements which require both repetitive flexion and extension, have the potential to produce more annular damage than those which require merely flexion.