Daniel M. Skrzypiec

Internal stress distribution in cervical intervertebral discs: the influence of an artificial cervical joint and simulated anterior interbody fusion.

There is concern that cervical interbody fusion can result in accelerated degenerative changes occurring at adjacent spinal levels. The cervical spine clearly evolved to be mobile. It would seem to be desirable for spinal surgeons to have an alternative to fusion, and spinal arthroplasty is an appealing concept. The Bristol Disc is a mechanical device comprising two articulating components that result in motion with 6 df. It has been shown to have favorable kinematics when compared with intact and fused cadaveric spines. The current study attempts to record changes in the distribution of stresses within cervical intervertebral discs adjacent to the artificial disc or a simulated fusion. The technique used to measure intradiscal stress distributions is based on earlier work by McNally and Adams on lumbar intervertebral discs. The study generated stress profiles through cervical intervertebral discs statically loaded in four different postures in addition to recording changes in intradiscal pressure within both the nucleus and the annulus during flexion. Similar stress profiles were recorded from intact specimens and those with the artificial joint inserted. The artificial joint resulted in reduced stresses in the annulus compared with spines with a simulated fusion. The study demonstrates how different testing conditions can result in researchers being confronted with paradoxical data, and the simulation of muscle forces is recommended.

Strength of the Cervical Spine in Compression and Bending

Study Design. Cadaveric motion segment experiment. Objectives. To compare the strength in bending and compression of the human cervical spine and to investigate which structures resist bending the most. Summary of Background Data. The strength of the cervical spine when subjected to physiologically reasonable complex loading is unknown, as is the role of individual structures in resisting bending. Methods. A total of 22 human cervical motion segments, 64 to 89 years of age, were subjected to complex loading in bending and compression. Resistance to flexion and to extension was measured in consecutive tests. Sagittal-plane movements were recorded at 50 Hz using an optical two-dimensional “MacReflex” system. Experiments were repeated 1) after surgical removal of the spinous process, 2) after removal of both apophyseal joints, and 3) after the disc-vertebral body unit had been compressed to failure. Results were analyzed using t tests, analysis of variance, and linear regression. Results were compared with published data for the lumbar spine. Results. The elastic limit in flexion was reached at 8.5° (SD, 1.7°) with a bending moment of 6.7 Nm (SD, 1.7 Nm). In extension, values were 9.5° (SD, 1.6°) and 8.4 Nm (3.5 Nm), respectively. Spinous processes (and associated ligaments) provided 48% (SD, 17%) of the resistance to flexion. Apophyseal joints provided 47% (SD, 16%) of the resistance to extension. In compression, the disc-vertebral body units reached the elastic limit at 1.23 kN (SD, 0.46 Nm) and their ultimate compressive strength was 2.40 kN (SD, 0.96 kN). Strength was greater in male specimens, depended on spinal level and tended to decrease with age. Conclusions. The cervical spine has approximately 20% of the bending strength of the lumbar spine but 45% of its compressive strength. This suggests that the neck is relatively vulnerable in bending.

When are intervertebral discs stronger than their adjacent vertebrae

Study Design. Mechanical testing of cadaveric tissues. Objective. To compare the strength of discs and vertebrae from the same spines in order to determine which are more vulnerable to injury, and to determine how their relative vulnerability depends on age and gender. Summary of Background Data. Vertebrae can strengthen and weaken according to mechanical demands, but the avascular intervertebral discs may be unable to “keep up.” Little is known about the relative strength of discs and vertebrae. Methods. Forty-seven thoracolumbar motion segments were obtained from 30 cadavers 48 to 91 years of age. Each was compressed until a vertebra fractured, and vertebral yield compressive stress (force per unit area) was calculated. Adjacent undamaged intervertebral discs were removed, and circumferential slices, 2.2 mm thick, were cut from the inner, middle, and outer regions of the anterolateral anulus. Slices were stretched to failure to determine their ultimate tensile stress. Results. Yield compressive stress of male and female vertebrae decreased by 69% and 75%, respectively, in the age range of 48 to 91 years (P < 0.001). In contrast, the ultimate tensile stress of the adjacent anulus did not fall significantly with age, except in the outer region of male discs, where it fell by 66% (P < 0.01). Disc strength was proportional to vertebral strength, but only for the outer anulus, and in male spines (r2 = 24%, P = 0.019, n = 22). Conclusion. The outer anulus can adapt to mechanical demands because it is the most metabolically active region of the disc. Disc and bone properties are better matched in male spines because male vertebrae are less affected by variable hormonal changes. The low adaptive potential of intervertebral discs makes them relatively weak in the strengthening spines of young men but relatively strong in the weakening spines of elderly women.

Can compressive stress be measured experimentally within the annulus fibrosus of degenerated intervertebral discs

Abstract The aims were to assess the ability of a pressure transducer to measure compressive stress within the annulus fibrosus of degenerated intervertebral discs. Measurements could help to explain the mechanisms of disc failure and low back pain. The methods used were as follows. Thirteen full-depth cores of annulus, 7 mm in diameter, were removed from the middle and outer annuli of two severely degenerated human discs and constrained within a metal cylinder. Then static compressive forces were applied by a planeended metal indenter of diameter 6.8 mm, while a strain-gauged pressure transducer, side mounted in a needle of diameter 0.9 mm and calibrated in saline, was pulled through the issue. The transducer output was converted into stress, and the average measured stress was compared with the nominal applied stress. Measurements were repeated at up to 21 load levels, with the transducer oriented vertically and horizontally. The results showed that the measured and applied stress were linearly related (average r2=0.98) with a mean gradient (calibration factor) of 0.98 (vertical stress) and 0.92 (horizontal stress). Gradients ranged between 1.28 and 0.73. Damaged transducers grossly under-recorded ‘stress’ even though their output remained proportional to applied load. It was concluded that pressure transducers can measure compressive stress inside a degenerated human annulus. The tissue is sufficiently deformable to allow efficient coupling of stress between the matrix and transducer membrane. Damage to the transducer can give misleading results.

Evaluation of fracture topography and bone quality in periprosthetic femoral fractures: A preliminary radiographic study of consecutive clinical data

The unique configuration of periprosthetic femoral fractures (PFFs) is a major determinant of the subsequent management. The aim of this preliminary study was to investigate potential relationships between fracture angle (FA), fracture level (FL) and bone quality of Vancouver type B PFF. The FA, FL and the canal thickness ratio (CTR) were quantified for 27 patient X-rays. The CTR is an indicator of the underlying bone quality. Relationships between these factors were studied for the whole X-ray set, for a subgroup involving fracture above the tip of the stem and for subgroups with stable and unstable implants. When considering all cases, no significant correlation was found between the FA and any other measurement. Considering only cases with unstable implants, a statistically significant correlation was found between the FA and the FL (R(2)=0.489, p=0.002). No correlation was found between FA and any other measurement for stable implants suggesting that FA could be considered as an independent factor when classifying B1 fractures. Considering all cases, a weak correlation was found between CTR and FL (R(2)=0.152, p=0.044) suggesting that fractures below the tip of the stem may indicate a lower bone quality. This preliminary study suggests that the effect of FA on the optimal management of Vancouver type B1 fractures could be considered, independent of the quality of the bone or fracture position. Furthermore, fractures around or below the tip of the stem may suggest a poor bone quality. Larger number of patients is required to confirm these initial findings.

A description of spinal fatigue strength

Understanding fatigue failure of the spine is important to establish dynamic loading limits for occupational health and safety. In this study experimental data were combined with published data to develop a description of the predictive parameters for spinal fatigue failure. 41 lumbar functional spinal units (FSUs) from cadaveric spines (age 49.0 ± 11.9 yr) where cyclically loaded. Three different levels of sinusoidal axial compression (0-3 kN, 0-2kN or 1-3kN) were applied for 300,000 cycles. Further, published data consisted of 70 thoracic and lumbar FSUs loaded in axial compression for 5000 cycles. Cyclic forces ranged from lower peaks (Fmin) of 0.7-1kN to upper peaks (Fmax) of 1.2-7.1 kN. Based on Wöhler analysis, a fatigue model was developed accounting for three parameters: I) specimen-specific scaling based on the endplate area, II) specimen-specific strength dependency on age or bone mineral density, III) load-specific correction factors based on Fmax and Fmin. The most predictive model was achieved for a combination of Fmax, endplate area and bone mineral density; this model explained 61% of variation (p<0.001). A model including Fmax, endplate area and age explained only 28% of variation (p<0.001). Inclusion of a load-specific correction factor did not significantly improve model prediction of fatigue failure. This analysis presents the basis for the prediction of specimen-specific fatigue failure of the lumbar spine, provided the endplate area and bone mineral density can be derived.

Failure of the human lumbar motion-segments resulting from anterior shear fatigue loading.

An in-vitro experiment was designed to investigate the mode of failure following shear fatigue loading of lumbar motion-segments. Human male lumbar motion-segments (age 32–42 years, n=6) were immersed in Ringer solution at 37°C and repeatedly loaded, using a modified materials testing machine. Fatigue loading consisted of a sinusoidal shear load from 0 N to 1,500 N (750 N±750 N) applied to the upper vertebra of the motion-segment, at a frequency of 5 Hz. During fatigue experiments, several failure events were observed in the dynamic creep curves. Post-test x-ray, CT and dissection revealed that all specimens had delamination of the intervertebral disc. Anterior shear fatigue predominantly resulted in fracture of the apophyseal processes of the upper vertebrae (n=4). Exposure to the anterior shear fatigue loading caused motion-segment instability and resulted in vertebral slip corresponding to grade I and ‘mild’ grade II spondylolisthesis, as observed clinically.

SHEAR LOAD SHARING IN THE HUMAN LUMBAR SPINE

There is growing concern that low back pain is related to shear load [Norman, 1998]. However, in contrast to axial compressive properties, little is known about the shear mechanical properties of spine. At shear failure in human spine, the disc supports 77% of the shear load [Cripton. 1995]. Evaluation of shear load sharing using porcine spine showed that in the initial overload phase the neural arch was not involved [Yingling, 1999]. We hypothesised that shear load sharing mechanisms in the human and porcine spine are different and that there is early involvement of the neural arch in the human spine.

Compressive load-sharing in the cervical spine

Background: Neck muscles stabilise the head, but muscle tension imposes high compressive forces on the cervical spine. Little is known about which structures resist these high forces. Purpose: To quantify compressive load-sharing within the cervical spine. Methods: Seventeen cervical “motion segments” from cadavers aged 54–92 yr (mean 72 yr), were subjected to 200 N compression while positioned in simulated flexed and extended postures. Up to 5 Nm of bending was applied in various planes. Vertebral movements were recorded at 50 Hz using an optical MacReflex system. Tangent stiffness was calculated in compression and in bending. Load-sharing was evaluated from compressive stress measurements obtained by pulling a pressure transducer through the intervertebral disc. All measurements were repeated after 2 hr of creep loading at 150 N, and following sequential removal of the spinous process, apophyseal joints and uncovertebral joints. Results: Most compression was resisted by the disc. However, creep increased compressive load-bearing by the neural arch, from 21% to 28% in flexed posture, and from 27% to 45% in extended posture, with most of this loading being resisted by the apophyseal joints. Uncovertebral joints resisted 10% of compression in extended posture, and 20% in flexed posture. Flexion and extension movements were resisted primarily by ligaments of the neural arch, and by the apophyseal joints, respectively, whereas lateral bending was resisted mostly by the apophyseal and uncovertebral joints. Conclusion: Cervical apophyseal joints play a major role in compressive load-bearing, and also offer strong resistance to backwards and lateral bending. Uncovertebral joints primarily resist lateral bending. Conflicts of Interest: None Source of Funding: Scholarship from the Greek Government