Exogenous Collagen Crosslinking of the Intervertebral Disc Restores Joint Stability After Lumbar Posterior Decompression Surgery

Defining the Neutral Zone of sheep intervertebral joints during dynamic motions: an in vitro study

PurposeDogs are often used as an animal model in spinal research, but consideration should be given to the breed used as chondrodystrophic (CD) dog breeds always develop IVD degeneration at an early age, whereas non-chondrodystrophic (NCD) dog breeds may develop IVD degeneration, but only later in life. The aim of this study was to provide a mechanical characterization of the NCD [non-degenerated intervertebral discs (IVDs), rich in notochordal cells] and CD (degenerated IVDs, rich in chondrocyte-like cells) canine spine before and after decompressive surgery (nucleotomy).MethodsThe biomechanical properties of multisegmented lumbar spine specimens (T13–L5 and L5–Cd1) from 2-year-old NCD dogs (healthy) and CD dogs (early degeneration) were investigated in flexion/extension (FE), lateral bending (LB), and axial rotation (AR), in the native state and after nucleotomy of L2–L3 or dorsal laminectomy and nucleotomy of L7–S1. The range of motion (ROM), neutral zone (NZ), and NZ stiffness (NZS) of L1–L2, L2–L3, L6–L7, and L7–S1 were calculated.ResultsIn native spines in both dog groups, the greatest mobility in FE was found at L7–S1, and the greatest mobility in LB at L2–L3. Surgery significantly increased the ROM and NZ, and significantly decreased the NZS in FE, LB, and AR in both breed groups. However, surgery at L2–L3 resulted in a significantly larger increase in NZ and decrease in NZS in the CD spines compared with the NCD spines, whereas surgery at L7–S1 induced a significantly larger increase in ROM and decrease in NZS in the NCD spines compared with the CD spines.ConclusionsSpinal biomechanics significantly differ between NCD and CD dogs and researchers should consider this aspect when using the dog as a model for spinal research.

In bilateral cervical facet dislocation, biomechanical stabilities between anterior locking screw/plate fixation after anterior cervical discectomy and fusion (ACDFP) and posterior transpedicular screw/rod fixation after anterior cervical discectomy and fusion (ACDFTP) have not been compared using the human cadaver, although ACDFP has been performed frequently. In this study the stability of ACDFP, a posterior wiring procedure after ACDFP (ACDFPW), and ACDFTP for treatment of bilateral cervical facet dislocation were compared. Spines (C3-T1) from 10 human cadavers were tested in the intact state, and then after ACDFP, ACDFPW, and ACDFTP were performed. Intervertebral motion was measured using a video-based motion capture system. The range of motion (ROM) and neutral zone (NZ) were compared for each loading mode to a maximum of 2 Nm. The ROM for spines treated with ACDFP was below that of the intact spine in all loading modes, with statistical significance in flexion and extension, but NZs were decreased in flexion and extension and slightly increased in bending and axial rotation; none of these showed statistical significance. The ACDFPW produced statistically significant additional stability in axial rotation ROM and in flexion NZ than ACDFP. The ACDFTP provided better stability than ACDFP in bending and axial rotation, and better stability than ACDFPW in bending for both ROM and NZ. There was no significant difference in extension with either ROM or NZ for the three fixation methods. The spines treated with ACDFTP demonstrated the most effective stabilization, followed by those treated with ACDFPW, and then ACDFP. The spines receiving ACDFP also revealed a higher stability than the intact spine in most loading modes; thus ACDFP can also provide a relatively effective stabilization in bilateral cervical facet dislocation, but with the aid of a brace.

Objectives:The use of hip arthroscopy has increased over recent years to treat various forms of hip pathologies including femoroacetabular impingement. While a capsulotomy facilitates adequate visualization and access for diagnostic and interventional purposes, the current literature remains divided over the use of routine capsular closure to address the iatrogenic instability that may be induced by an excessive or unclosed capsulotomy. The purpose of this biomechanical study was to determine if hip capsulotomy creates significant alterations in rotational range of motion and stability measures. Additionally, does a capsular repair return these variables to normal?Methods:Eight fresh-frozen cadaveric hip specimens consisting of the hemipelvis, femur, and overlying soft tissues were used for this study. All specimens were screened by computed tomography (CT) to assess for the absence of significant bony pathology. Each hip was then tested under torsional loads of 6 N·m applied via a pulley system in conjunction with a vertical force exerted by a servohydraulic frame. Conditions tested were as follows: 1) neutral flexion with the capsule intact 2) neutral flexion with a 4 cm inter-portal capsulotomy 3) neutral flexion with a 6 cm inter-portal capsulotomy 4) neutral flexion with capsulotomy repair. Soft tissue was retained during all interventions. Range of motion (from external to internal rotation), hysteresis area, and neutral zones were the outcome measures for stability obtained in each condition.Results:While the data demonstrated a positive and statistically significant correlation between the sizes of the capsulotomy and measures of joint instability (range of motion (ROM), hysteresis area, and neutral zone), no significant differences were seen between the intact and repaired hips with regards to such measures (Table). Larger size capsulotomies were accompanied by significant increases in all three measures of joint instability while the repair effectively restored the capsule to conditions similar to its intact state.Conclusion:To the best of the authors’ knowledge, this study is the first to demonstrate the biomechanical consequences of sequentially increasing capsulotomy incisions and their repair on a hip joint that still contains its surrounding soft tissue. The results of this study not only confirm the notion of introducing iatrogenic joint instability with larger sized capsulotomies, but also propose the complete capsular closure as an appropriate solution.

Objectives To compare the biomechanical stability of two-level PLIF constructs with unilateral and bilateral pedicle screw fixations. Methods Six cadaveric lumbar segments were evaluated to assess biomechanical stability in response to pure moment loads applied in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Each specimen was tested in six sequential configurations: (1) intact baseline; (2) facetectomy; (3) unilateral pedicle screws (UPS); (4) bilateral pedicle screws (BPS); (5) unilateral pedicle screws and cage (UPSC); and (6) bilateral pedicle screws and cage (BPSC). Results Significant reductions in motion were observed when comparing all instrumented conditions to the intact and facetectomy stages of testing. No significant differences in motion between UPS, BPS, UPSC, or BPSC were observed in response to FE range of motion (ROM) or neutral zone (NZ). ROM was significantly higher in the UPS stage compared to BPS in response to LB and AT loading. ROM was significantly higher in UPSC compared to BPSC in response to LB loading only. Similarly, NZ was significantly higher in UPSC compared to BPSC in response to only LB loading. In response to AT loading, ROM was significantly higher during UPS than BPS or BPSC; however, no significant differences were noted between UPSC and BPSC with respect to AT ROM or NZ. Conclusion BPS fixation is biomechanically superior to UPS fixation in multilevel PLIF constructs. This was most pronounced during both LB loading. Interbody support did contribute significantly to immediate stability.

Biomaterials play an increasing role in intervertebral disc regeneration and require preclinical testing, typically performed using organ culture and invitro models. Native human discs are limited, and animal models often fail to mimic human disc degeneration. Thus, enzymes like chondroitinase ABC (chABC) and papain are used to simulate degenerative tissue changes and enable biomaterial injection. In previous work, we characterized the biomechanical and morphological effects of papain, which forms cavities in the disc. In contrast, chABC does not form cavities, but its biomechanical effects remain insufficiently characterized. This study aims to evaluate the macroscopic and biomechanical effects of chABC-specifically, range of motion (ROM), neutral zone (NZ), and disc height-in a bovine organ culture model, and assess the distribution of an injected hydrogel, comparing the results to published papain data. Four groups of fresh bovine tail segments were prepared (n ≥ 10) and three received injections of chABC, papain, or PBS, followed by 7 days of culture. For papain and PBS, published data were supplemented with new specimens. Complex simulated physiological loading was applied to diminish disc swelling. The maximum volume of a serum-albumin-hydrogel was injected into all four groups. ROM, NZ, and disc height were measured before and after enzyme treatment, loading, and injection. Post-injection, microCT scans visualized material distribution within the discs. ChABC increased ROM by up to 92.1%, NZ by up to 79.4%, and decreased disc height by 2.1 mm. Hydrogel injection decreased ROM and NZ but increased disc height in all groups while enzyme treatments allowed more hydrogel injection (0.6 mL for chABC). Exemplary scans showed cloud-like hydrogel spread for chABC and a round-shaped degradation defect for papain. The findings indicate that chABC better simulates disc degeneration, whereas papain better models nucleotomies, and both enzymes preserve annulus integrity-providing valuable models for biomechanical testing.

Lumbar segmental instability: Points to ponder

Instability of the cervical spine following whiplash trauma has been demonstrated in a number of studies. We hypothesized that, in patients with whiplash-associated disorder, rotation of the head would be accompanied by an earlier onset of neck muscle activity to compensate for intrinsic instability. The aim of the study was to examine the range of motion (RoM) of the cervical spine and the onset and activity of the sternocleidomastoid (SCM) muscles during axial rotation, in healthy control subjects and in patients with chronic whiplash-associated disorder. Forty-eight control subjects (42% male) and 46 patients (33% male) with chronic whiplash-associated disorder (symptoms lasting longer than 3 months) were examined. Cervical axial RoM differed significantly (P = 0.0001) between the groups, with the whiplash patients showing lower values (83 degrees +/- 30 degrees) than the healthy controls (137 degrees +/- 19 degrees). The whiplash patient group showed no evidence of the predicted earlier activation of SCM muscles. Many patients never reached the point in the RoM where SCM muscle activity rises steeply, as it does in the healthy controls (the 'elastic zone'), and their movements remained mostly within the region of low muscle activity (the 'neutral zone'). The whiplash patients appeared either unable or unwilling to drive the cervical spine into this region of high muscle activity, possibly because they were restricted by existing pain or fear of pain.

Cervical spine instability following axial compression injury: A biomechanical study

PurposeUnilateral hemilaminectomy with bilateral decompression (BDZ) was proposed as an alternative decompressive procedure in cervical spondylotic myelopathy (CSM). Despite promising clinical results, the destabilizing effect is yet unknown. We therefore performed a biomechanical study to investigate whether lateral mass screw fixation should follow BDZ.MethodsSix human C2–C7 cervical specimens were tested under various conditions: native, unilateral hemilaminectomy with bilateral decompression without/with fixation (BDZ/BDF), unilateral hemilaminectomy with bilateral decompression and unilateral foraminotomy without/with fixation (UFZ/UFF), unilateral hemilaminectomy with bilateral decompression and bilateral foraminotomy without/with fixation (BFZ/BFF), and laminectomy without/with fixation (LAZ/LAF). Instrumention was applied from C3–C6. For each condition, the three-dimensional kinematics of the cervical specimen were measured in three main loading directions with an ultrasonic motion analysis system. ANOVA was used to determine differences between the specific segment conditions to assess the parameter’s range of motion (ROM) and neutral zone (NZ).ResultsFor flexion–extension, lateral bending and axial rotation, ROM of BDZ, UFZ, BFZ and LAZ remained at the level of the native condition (p > 0.74), whereas fixation reduced ROM significantly (p < 0.01). Between BDF, UFF, BFF and LAF, no significant differences in reduction in ROM were seen (p > 0.49). Results for NZ were equivalent to ROM in flexion–extension and lateral bending. For axial rotation, NZ remained almost constant on the native level for all tested conditions.ConclusionBilateral decompression via a hemilaminectomy, even if combined with foraminotomy, could be a less invasive treatment option for multilevel CSM in patients with lordotic cervical alignment and absence of segmental instability.

Spinal kinematics after the implementation of rigid spinal instrumentation or the achievement of a solid fusion was studied using a sheep posterolateral spinal arthrodesis model. To investigate the effects of rigid spinal instrumentation or solid fusion on spinal kinematic parameters. Numerous studies have attempted to define spinal instability in terms of kinematics. Recent in vitro studies have documented the neutral zone, or a measure of spinal laxity, as more sensitive to spinal instability than the range of motion. Seven skeletally mature sheep underwent a single-level posterolateral lumbar arthrodesis using autologous bone graft augmented with transpedicular screw fixation. The animals were killed 4 months after surgery. The identical surgical procedures were performed in seven sheep cadaveric spines, which served as acute postoperative controls. Each functional spinal unit was tested biomechanically before and after hardware removal. The experimental control groups consisted of destabilized spines and spines that underwent transpedicular screw fixation alone, whereas the fusion groups included spines that underwent posterolateral fusion alone or posterolateral fusion with instrumentation. Rigid instrumentation and solid fusion significantly decreased the neutral zone and range of motion in all testing modes. In axial rotation and lateral bending, solid fusion reduced the range of motion significantly more than transpedicular screw fixation alone. However, in all testing modes, the neutral zones showed no statistical difference between transpedicular screw fixation alone and fusion groups. The range of motion was an equivalent or better indicator of fixation or fusion stability compared with the neutral zone. Moreover, the immediate postoperative fixation stability, even if using transpedicular screw fixation, was less than the stability present after a solid fusion.

The multidirectional stability potential of anterior, posterior, and combined instrumentations applied at L1-L3 was studied after L2 corpectomy and replacement with a carbon-fiber implant. To evaluate the biomechanical characteristics of short-segment anterior, posterior, and combined instrumentations in lumbar spine tumor vertebral body replacement surgery. The biomechanical properties of many different spinal instrumentations have been studied in various spinal injury models. Only a few studies, however, investigate the stabilization methods in spinal tumor vertebral body replacement surgery. Eight fresh frozen human cadaveric thoracolumbar spine specimens (T12-L4) were prepared for biomechanical testing. Pure moments (2.5 Nm, 5 Nm, and 7.5 Nm) of flexion-extension, left-right axial torsion, and left-right lateral bending were applied to the top vertebra in a flexibility machine, and the motions of the L1 vertebra with respect to L3 were recorded with an optoelectronic motion measurement system after reconditioning. The L2 vertebral body was resected and replaced by a carbon-fiber cage. Different fixation methods were applied to the L1 and L3 vertebrae. One anterior, two posterior, and two combined instrumentations were tested. Load-displacement curves were recorded and neutral zone and range of motion parameters were determined. The anterior instrumentation provided less potential stability than the posterior and combined instrumentations in all motion directions. The anterior instrumentation, after vertebral body replacement, showed greater motion than the intact spine, especially in axial torsion (range of motion, 10.3 degrees vs 5.5 degrees; neutral zone, 2.9 degrees vs. 0.7 degrees; P < 0.05). Posterior instrumentation provided greater rigidity than the anterior instrumentation, especially in flexion-extension (range of motion, 2.1 degrees vs. 12.6 degrees; neutral zone, 0.6 degrees vs. 6.1 degrees; P < 0.05). The combined instrumentation provided superior rigidity in all directions compared with all other instrumentations. Posterior and combined instrumentations provided greater rigidity than anterior instrumentation. Anterior instrumentation should not be used alone in vertebral body replacement.

In vitro study to characterize the flexibility of a new total posterior-element system when instrumented to L4-L5 segments. The goal of this in vitro study was to investigate whether an optimized version of the TOPS implant (Impliant Ltd., Ramat Poleg, Israel) is capable to restore the physiologic motion characteristic of a spinal segment after facetectomy. The TOPS implant is designed to replace the posterior elements of a functional spinal unit, to provide flexible restabilization and spinal alignment, while maintaining the intervertebral disc. The implant is composed of bilateral pedicle screws, connected with 2 crossbars in the transversal plane. The crossbars are joined together by an elastic element capable of transmitting tensile and compressive loads, as well as shear forces. Six human cadaver specimens (L3-S1) (median age 61 years: minimum 47 and maximum 74 years) were used for this in vitro experiment. The specimens were loaded with pure moments of +/-7.5 Nm in flexion/extension, lateral bending, and axial rotation. The following states were investigated: (1) intact; (2) after bilateral laminectomy, including facetectomy of the lower facet joints, of the upper vertebra L4; and (3) after device implantation. The range of motion (ROM), neutral zone, and intradiscal pressure were determined from a third cycle. In a second step, the ROM in axial rotation was determined as a function of different flexion/extension postures. In the neutral position, the laminectomy and facetectomy increased the median values of the ROM in flexion plus extension, lateral bending right plus left, and significantly in axial rotation left plus right from: 8.2 degrees, 7.6 degrees, 3.6 degrees to 12.1 degrees, 8.5 degrees, and 8.5 degrees (Wilcoxon signed rank test; P < 0.05). After fixation of the implant, the ROM was again reduced to 6.8 degrees, 7.8 degrees, and 3.8 degrees. In a flexed posture, the ROM in axial rotation was slightly increased compared to the neutral position. With increasing extension, the axial rotation decreased linearly from 3.7 degrees in neutral position to 2.3 degrees in 4 degrees extension in the segment L4-L5. The characteristic of the intradiscal pressure versus load with the implant was similar to that of the intact specimen. The TOPS implant almost ideally restored the ROM in lateral bending and axial rotation compared to that of the intact specimen. In the sagittal plane, 85% of the intact ROM could be obtained. The ROM in axial rotation as a function of flexion and extension angle also mimics the biomechanical behavior of the posterior complex of a lumbar spine. This relationship between ROM and posture emphasizes the importance of a proper implantation.

A fabricated mesh cage and/or posterior occipitocervical instrumentation alone has been used for reconstruction of ventral defect of the upper cervical spine. However, using a trimmed mesh cage it was hard to achieve optimal clival screw purchase and it migrated or broke. A specific instrumentation at the craniovertebral junction (CVJ) should incorporate the morphology of the CVJ and biomechanical validation. The purpose of the present study was to develop an innovative clivus plate integrated with the clinical anatomy of CVJ and to evaluate the stability of the clivus plate fixation (CPF), stand-alone or combined with a posterior occipitocervical fixation (POCF). Dimensions relevant to the clivus plate were measured on 40 adult dry bones and CT images of 30 patients. The CPF was composed of the clivus plate and a titanium mesh cage. The clivus plate was anchored to the clivus, atlas and C3 body and connected to the mesh cage. Six fresh cadaveric head-neck specimens (Oc-C4) were used in this study (46 ± 15 years old, 2 F/4 M). A continuous pure moment of ±1.5 Nm was applied to the specimen in flexion, extension, lateral bending and axial rotation. The status of intact, CPF alone, and CPF plus POCF was tested on each specimen. The CPF was implanted to the specimen following resection of the C1 anterior arch, C2 vertebral body, C2-C3 disc and atlantoaxial ligaments. The POCF was applied with screws anchoring at the occiput, C1, C3 and C4. The range of motion (ROM) and neutral zone (NZ) from the occiput to C3 were calculated. The clivus plate was developed based on measurements of 40 adult dry bones and CT images of 30 patients. The plates were successfully applied to all specimens. No obvious loosening or mismatch was observed. The mean clival length and widest and narrowest diameter of the clivus were 26, 33 and 19 mm, respectively. The clivus screw length was 8 mm for the caudal holes and 10 mm for the cephalad hole. The CPF reduced ROMs to 3.9° in flexion, 2.8° in extension, 4.2° in lateral bending and 6.8° in axial rotation. The combined CPF and POCF constrained motion within 0.6° in all directions and more than the CPF (P < 0.05). NZs after the CPF were 1.0° in flexion-extension, 2.1° in lateral bending and 2.2° in axial rotation, respectively. NZs after the CPF plus POCF were within 0.2° in all directions and less than the CPF (P < 0.05). This study demonstrated screw purchase in the adult clivus and developed an innovative clivus plate fixation for reconstructing an extensive ventral defect in the upper cervical spine. The clivus plate fixation combined a posterior instrumentation ensuring reliable upper cervical stability.

Basic knowledge about the thoracic spinal flexibility is limited and to the authors’ knowledge, no in vitro studies have examined the flexibility of every thoracic spinal segment under standardized experimental conditions using pure moments. In our in vitro study, 68 human thoracic functional spinal units including the costovertebral joints (at least n = 6 functional spinal units per segment from T1-T2 to T11-T12) were loaded with pure moments of ±7.5 Nm in flexion/extension, lateral bending, and axial rotation in a custom-built spine tester to analyze range of motion (ROM) and neutral zone (NZ). ROM and NZ showed symmetric motion behavior in all loading planes. In each loading direction, the segment T1-T2 exhibited the highest ROM. In flexion/extension, the whole thoracic region, with exception of T1-T2 (14°), had an average ROM between 6° and 8°. In lateral bending, the upper thoracic region (T1-T7) was, with an average ROM between 10° and 12°, more flexible than the lower thoracic region (T7-T12) with an average ROM between 8° and 9°. In axial rotation, the thoracic region offered the highest overall flexibility with an average ROM between 10° and 12° in the upper and middle thoracic spine (T1-T10) and between 7° and 8° in the lower thoracic spine (T10-T12), while a trend of continuous decrease of ROM could be observed in the lower thoracic region (T7-T12). Comparing these ROM values with those in literature, they agree that ROM is lowest in flexion/extension and highest in axial rotation, as well as decreasing in the lower segments in axial rotation. Differences were found in flexion/extension and lateral bending in the lower segments, where, in contrast to the literature, no increase of the ROM from superior to inferior segments was found. The data of this in vitro study could be used for the validation of numerical models and the design of further in vitro studies of the thoracic spine without the rib cage, the verification of animal models, as well as the interpretation of already published human in vitro data.