Biomechanical response to acupuncture needling in humans.

During acupuncture treatments, acupuncture needles are manipulated to elicit the characteristic "de qi" reaction widely viewed as essential to acupuncture's therapeutic effect. De qi has a biomechanical component, "needle grasp," which we have quantified by measuring the force necessary to pull an acupuncture needle out of the skin (pullout force) in 60 human subjects. We hypothesized that pullout force is greater with both bidirectional needle rotation (BI) and unidirectional rotation (UNI) than no rotation (NO). Acupuncture needles were inserted, manipulated, and pulled out by using a computer-controlled acupuncture needling instrument at eight acupuncture points and eight control points. We found 167 and 52% increases in mean pullout force with UNI and BI, respectively, compared with NO (repeated-measures ANOVA, P < 0.001). Pullout force was on average 18% greater at acupuncture points than at control points (P < 0.001). Needle grasp is therefore a measurable biomechanical phenomenon associated with acupuncture needle manipulation.

Internal deformations of intact and denucleated human lumbar discs subjected to compression, flexion, and extension loads.

Three rows of six evenly spaced 0.5 mm metal beads were implanted midsagittally into the discs of ten L4-5 human lumbar motion segments. The intradiscal bead displacements in response to compression, flexion, and extension loads were obtained by digitizing the bead positions from sagittal plane radiographs taken before and during the load application. Each disc was denucleated and the loading process was repeated. For the intact discs, in compression, the intradiscal bead displacements were predominantly anterior. In flexion, the beads in the center of the disc moved posteriorly whereas the beads closer to the periphery of the disc moved anteriorly. In extension, the central beads moved anteriorly and the beads closer to the periphery of the disc moved posteriorly. After denucleation, the bead displacements for compression and flexion implied an inward bulging of the inner wall of the annulus, despite outward bulging of the disc surface. We hypothesize that the inward bulging causes radial tensile stresses within the disc, leading to disruption of adjacent layers of annulus.

Thoracic spine centers of rotation in the sagittal plane.

The purpose of this study was to determine in vitro the centers of rotation of thoracic functional spinal units in the sagittal plane. The center of rotation is a convenient concept and part of a precise method of documenting the kinematics of a joint moving in a plane. Fresh cadaver functional spinal units from the thoracic region were utilized. Six load types were used that produced motions only in the sagittal plane, namely anterior and posterior shear forces, flexion and extension moments, and compression and distraction forces. The resulting motion with three degrees of freedom was measured with dial gauges. Statistical methods were used to analyze data from the viewpoint of vertebral level, load magnitude, and load type. Only the load type was found to be significantly related to the location of the centers of rotation. Although there was significant variability in the centers of rotation, there were definite locations related to each load type. The average center of rotation was 15-45 mm directly below the geometric center of the moving vertebra. The results of the present study may be helpful in the clinical interpretation of spinal kinematic studies.

Pin force measurement in a halo-vest orthosis, in vivo.

The halo-vest is an orthosis commonly used to immobilize and protect the cervical spine. The primary complications associated with the halo-vest have been attributed to cranial pin loosening. However, the pin force history during day-to-day halo-vest wear has not previously been reported. This paper presents a new technique developed to monitor cranial pin forces in a halo-vest orthosis, in vivo. A strain gaged, open-ring halo was used to measure the compressive and shear forces produced at the posterior pin tips. The strain gages measured the bending moments produced by these forces without compromising the structural integrity of the halo-vest system. The prototype halo measured the compressive and shear force components with a resolution of +/- 15 and +/- 10 N, respectively. To test the feasibility and durability of the device, it was applied to one patient requiring treatment with a halo-vest orthosis. At the time of halo-vest application, the mean compressive force in the two posterior pins was 368 N. Over the 3 month treatment period, the compressive forces decreased by a mean of 88%. The shear forces were relatively insignificant. Using this technology future work will be aimed at determining the causes of pin loosening, optimizing vest and pin designs, and investigating the safety of more rapid rehabilitation.

Cervical spine motion in the sagittal plane: kinematic and geometric parameters.

The kinematic function of the cervical spine has been examined previously by means of cineradiography or a sequence of lateral X-rays, usually of a flexion-extension range of motion. Interpretation of these studies, however, presents difficulties. One of the major problems is how to extract information from the X-ray images which is not only explicit, quantitative and accurate, but which is also diagnostically useful. Another problem is that as one increases the number of steps of motion between full flexion and full extension to obtain a more detailed examination, one also increases the amount of radiation exposure and the bulk of the data. Reported here is a technique which uses one lateral-view X-ray for each of five neck positions: full flexion, full extension, and three intermediate positions. From each set of X-rays, various parameters are derived to describe two types of data: kinematic (angles and centers of rotation), and geometric (pattern of curvature). This technique has been shown capable of identifying functional abnormalities in patients with neck pain who have no structural abnormalities detectable by X-ray. Further experience will be useful in better defining various types of functional abnormalities.

A mechanical model for the human intervertebral disc.

Stress relaxation experiments were performed on specimens from a human intervertebral disc. Specimens were made from the nucleus pulposus and from the external lamellae of the anulus fibrosus in two different orientations. Tests were run with varying moisture content so as to develop a relaxation master curve. A model was developed based on the experimental data. It was found that the short term master curve for the lamellae of the anulus and nucleus are similar, whereas the long term rubbery plateau is different between the lamellae and the nucleus. It was also established that the master curves for different lamellae and the nucleus were shifted relative to each other in the time domain due to changes in water content. The average relaxation modulus of the whole disc was obtained by averaging the properties between the anulus and nucleus. This model was then used for studies of Schmorl's nodes, of degenerated discs and for circumstances in which hydration is considered to be important.

The design and use of a microcomputerized real-time muscle fatigue monitor based on the medial frequency shift in the electromyographic signal.

We have designed a real-time microcomputerized muscle fatigue monitor based on the median frequency shift of the electromyographic signal, computed via the fast Fourier transform. For the ten subjects performing an isotonic and isometric trunk extension task on two separate days, preliminary results suggest a repeatable linear decrease in median frequency as a function of time.

Biomechanics of thoracolumbar spinal fixation. A review.

Extensive development of spinal instrumentation has occurred recently, benefitted by improved biomechanical knowledge. Reviewed here are various devices and the major biomechanical issues relevant to them. The devices are categorized by site of attachment. The major emphasis is on the most recently developed category: devices attached by transpedicular screws. Aspects of this last category reviewed here include screw design, screw placement, longitudinal linking devices (rods, plates), and transverse connectors (cross-linking). Emphasis is placed not only on current knowledge, but also on unresolved issues.

A new halo-vest: rationale, design and biomechanical comparison to standard halo-vest designs.

The traditional halo-vest rigidly grips the cranium, but not the torso. Unexpectedly large motion and forces in the cervical spine have been shown by others to be present during halo-vest wear. In an effort to reduce these motions and forces, an experimental vest has been designed. Motion of the vest on the thorax has been measured on four normal volunteers, for each of nine load types, for each of seven commercially available vests as well as the experimental vest. Despite its lighter weight and less cumbersome structure, the experimental vest has the lowest mobility score of all the vests tested.

Clinical, radiographic, and kinematic results from an adjustable four-pad halovest.

STUDY DESIGN: Charts and radiographs of all patients treated with this halovest at one university hospital were reviewed retrospectively. OBJECTIVES: To describe the outcomes from an adjustable four-pad halovest and to compare them with those from standard halovests, as previously published. SUMMARY OF BACKGROUND DATA: With standard halovests, there can be cervical motion up to 70% of normal values, substantial loads between the halo and vest, and complications of pin loosening, pin infections, and scapular pressure sores. The four-pad vest reduces halovest loads and vest-torso motions. METHODS: The four-pad vest has four independently adjustable pads that completely avoid contact with the scapula, clavicle, and abdomen. Clinical records were analyzed to determine the incidences of halo pin loosening, pressure sores, injury or surgical site nonunion, and loss of cervical alignment. Lateral radiographs were taken with the patient in the upright and supine positions at various times to determine intervertebral rotations (flexion-extension). RESULTS: The clinical results with the four-pad vest were at least as good as those for standard vests. Scapular pressure sores were prevented completely by the absence of vest-scapula contact. Kyphosis did not increase significantly with time. The mean segmental rotations were all 3 degrees or less and showed a smoothly decreasing pattern from C1-C2 to C6-C7. The value at Oc-C1 was opposite to that at C1-C2 and is the subject of further analysis. CONCLUSIONS: The rotations occurring with the four-pad vest are less than or equal to those occurring with standard vests, for overall cervical rotation and for individual motion segment rotations. This is consistent with the smaller halovest forces seen with this vest. Prospective, comparative testing will assess the clinical significance of these findings.

Repair of failed transpedicle screw fixation. A biomechanical study comparing polymethylmethacrylate, milled bone, and matchstick bone reconstruction.

In a random, controlled laboratory study, pedicle screws placed in human cadaveric vertebrae were axially loaded to failure. Three repair methods were tested. Use of low pressure polymethylmethacrylate yielded 149% of the original pullout strength, milled bone yielded 70% of the initial pullout strength, and matchstick bone yielded 56% of the initial pullout strength. Two incidents of cortical penetration during reinsertion in the matchstick group occurred. The results were statistically analyzed.

Pull-off strength of gardner-Wells tongs from cadaveric crania.

Failure of attachment ("pull-off") of Gardner-Wells tongs from the cranium occasionally occurs, and may cause problems, especially in cases of significant cervical spinal instability. The optimal method for Gardner-Wells tongs attachment is not well defined: the effect on pull-off strength of stem protrusion of the spring-loaded pin, and user accuracy in setting stem protrusion do not appear to be documented in the medical literature. The authors measured pull-off strength from five fresh cadaveric crania, for each of five stem protrusion settings. In a related experiment, staff surgeons were asked to follow the standard instructions for applying clinically available Gardner-Wells tongs (smooth, unmarked indicator stems on the spring-loaded pins), after which the actual stem protrusion lengths were recorded. When the indicator stem protruded 1.0 mm (manufacturer's recommendation), the mean pull-off strength (+/- standard deviation [SD] was 137 +/- 34 pounds (610 +/- 151 N). Even with stem protrusions as low as 0.25 mm, the lowest pull-off strength was 60 pounds. Failures of fixation occurring with traction loads of 35 to 50 pounds are almost surely associated with stem protrusions of less than 0.25 mm. Even without an indicator line on the stem at 1 mm, all of the 13 orthopaedists tested produced an actual stem protrusion of 0.37 mm or more. Secure attachment of larger cranial traction loads requires careful attention to pin tightening. Proper location on the skull and the risk of penetration through the inner table must also be kept in mind.

Water content in human intervertebral discs. Part I. Measurement by magnetic resonance imaging.

Tension-relaxation experiments were performed on human disc lamellae specimens. The water content was found to affect the viscoelastic behavior and a master relaxation curve was constructed from the experimental data. The water content of disc phantoms is measured by magnetic resonance imaging (MRI) techniques. MRI was used to compare the discs of patients of different ages. The possibility of obtaining cross-sectional water distribution in human intervertebral disc material using MRI techniques and its relation to the disc's mechanical properties was explored, with the goal of constructing a realistic mathematical model of the disc which takes into account the water content of the disc.

Water content in human intervertebral discs. Part II. Viscoelastic behavior.

Water content of intervertebral discs is a significant aspect of both viscoelastic behavior and age-related degenerative changes. Using water content as a dependent variable, stress-relaxation was measured using standardized anulus fibrosus specimens strained at various levels of strain. Synthesis of experimental data into a master relaxation curve allows prediction of specimen response over time intervals not readily accessible experimentally. A quantitative understanding of the role of water content may have important clinical application, since magnetic resonance imaging is a tool which should allow water content determination in vivo.

Placement of transpedicular vertebral screws close to anterior vertebral cortex. Description of methods.

Strengthening of the screw-vertebra interface has been shown to occur with implantation of longer transpedicular screws, the tips of which are placed closer to the anterior cortex of the vertebral body. Such implantation probably results in increased risk for anterior cortex penetration and associated vascular or pulmonary damage. Typically recommended lateral and posterior-anterior radiography is shown here to provide potentially misleading visualization during implantation. To reduce the risk of anterior cortical penetration, presented here are 1) a radiographic method ("near approach view") that avoids this problem and allows direct visualization of the relationship between drill bit or screw tip and anterior vertebral cortex, and 2) a surgical technique ("mallet method") that provides both an audible and a palpable change when the drill bit contacts the anterior cortex.

Pin loosening in a halo-vest orthosis: a biomechanical study.

STUDY DESIGN: The cranial pin force history of a halo-vest orthosis was measured using an instrumented halo in a clinical study with three patients. Pin force values at the time of halo-vest application and at subsequent clinical visits during the halo-vest wear period were compared. OBJECTIVES: To document the pin force reduction in the cranial pins of a halo-vest orthosis in vivo. SUMMARY OF BACKGROUND DATA: The halo-vest is an orthosis commonly used to immobilize and protect the cervical spine. An important problem with halo-vest use is pin loosening. There have been no previous reports of pin force history in vivo. METHODS: A custom-built strain-gauged, open-ring halo was used to measure the compressive force and superiorly-inferiorly directed shear forces produced at the tips of the two posterior pins. The instrumented halo was applied to three patients with cervical spine fractures. Pin force measurements were recorded at the time of halo application and at subsequent follow-up visits during the entire treatment period. RESULTS: A mean compressive force of 343 +/- 64.6 N was produced at the pin tips during halo application with the patient in a supine position. On average, the compressive forces decreased by 83% (P = 0.002) during the typical halo-vest wear period. The compressive forces were substantially greater than the shear forces, which averaged only -11+/-30.2 N at the time of halo application and which did not change significantly with time. CONCLUSIONS: The study confirmed the hypothesized decrease in the compressive pin forces with time. All patients had developed at least some clinical symptoms of pin loosening at the time of halo-vest removal.

Effects of disc injury on mechanical behavior of the human spine.

The effects of injury to the intervertebral disc were investigated using three-dimensional flexibility and creep measurements of functional spinal units from fresh cadaver lumbar spines. The techniques utilized were accurate and the data had a high degree of reproducibility. An injury to the annulus and a removal of the nucleus significantly altered the mechanical properties of the spinal unit. Not only were the main motions affected but also the coupled motions. Sagittal plane symmetry was disturbed, resulting in asymmetric facet joint movements. These effects of injury could be measured because of the three-dimensionality of the experiments. Previous studies, utilizing only axial compression loading, claimed to observe no changes due to the disc injuries and are, therefore, in conflict with the present findings.

Effect of test environment on intervertebral disc hydration.

STUDY DESIGN: Water content of fresh human lumbar intervertebral discs (with adjacent endplates) was assessed in three studies: 1) after each of seven specimen preparation steps. 2) during exposure to either saline spray or a saline bath, and 3) during exposure to a saline bath and 445 N axial compression, either without or with previous exposure to the bath and no compression ("free swelling"). OBJECTIVE: To assess the effect on disc hydration of various aspects of specimen preparation and testing environments. SUMMARY OF BACKGROUND DATA: Water content is an important determinant of disc behavior. Specimen preparation method and testing environments may be important determinants of water content, yet no work appears to have been reported specifically on this topic. METHODS: Endplate-disc-endplate specimens were prepared from refrigerated cadavers within 24 hours of death by transverse sectioning of adjacent vertebral bodies. Water content change was determined by specimen weight change across each time interval of interest. RESULTS: Specimen preparation (including multiple freeze-thaw cycles) produced no water content change. Saline spray and plastic film wrap resulted in no change, but saline bath exposure resulted in a 24% increase over 7 hours, 44% of which occurred in the first 0.5 hour. A subsequent 7 hours of 445 N compression reduced the overall increase to 10%. This was not significantly different from the 8% increase that resulted from initial exposure to saline bath and compression. CONCLUSIONS: Specimen preparation as typically performed and specimen exposure to saline spray and plastic film wrap do not result in hydration change. Exposure to saline bath results in substantial swelling, which can either be reversed or prevented by axial compression in the physiologic range. Whether discs exposed to saline spray and wrap without compression and those exposed to saline bath with compression behave the same and which of these more closely mimics the in vivo condition are important issues for the experimentalist to test.

A poroelastic-swelling finite element model with application to the intervertebral disc.

The swelling process that occurs in soft tissue is incorporated into a poroelastic finite element model. The model is applied to a spinal segment consisting of two vertebrae and a single intervertebral disc. The theory is an extension of the poroelastic theory developed by Biot and the model is an adaptation of an axisymmetric poroelastic finite element model of the intervertebral disc by Simon. The model is completely three-dimensional although the results presented here assume symmetry about the sagittal plane. The theory is presented in two stages. First the development of the poroelastic theory. Following this, the effects of swelling caused by osmotic pressure are developed and expressed as a modification of the constitutive law and initial stresses. In the case of the disc, this pressure is produced mainly by the fixed negative charges on the proteoglycans within the disc. In this development we assume that the number of fixed charges remains constant over time and that the distribution of mobile ions has reached equilibrium. The variations over time in osmotic pressure, and thus in swelling effects are therefore only dependent on the initial state and the change in water content. Variations of the swelling effects caused by changes in mobile ion concentrations will be the subject of a future paper. The results reported in this article illustrate the dramatic effect of swelling on the load carrying mechanisms in the disc. The authors believe it is likely that this will have important useful implications for our understanding not only of normal disc function, but also of abnormal function, such as disc degeneration, herniation, and others.

Internal displacement distribution from in vitro loading of human thoracic and lumbar spinal motion segments: experimental results and theoretical predictions.

Small metal markers are implanted within intervertebral discs and their displacements in response to a complex load (flexion, compression, and anterior shear) are measured radiographically. These are contrasted to the displacements predicted by a finite element model (FEM) that uses 20 constant-strain triangular elements and is based upon a linear elastic isotropic material. The central portion of the disc (nucleus pulposus) moves posteriorly and oppositely to the FEM prediction. The anterior and posterior portions of the disc agree more closely with the FEM than the central portions of the disc. In general, the FEM predicts much more accurately the up-down displacement components than it does the anterior-posterior components. The measured displacements provide a new class of information concerning the function of the interior of the disc, and also provide a new basis for validation of FEMs that attempt to mimic real intervertebral disc behavior. Implications for understanding of disc function and pathology are discussed.