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Knowledge of the normal movements of whole lumbar spine and lumbosacral joint is important for evaluating clinical pathologic conditions that may potentially produce unstable situations in these regions. At present there are few studies that report systemic three-dimensional movement analysis of these regions. The purpose of this in vitro study was to quantitatively determine three-dimensional movements of the whole lumbar spine and lumbosacral joint. Ten fresh human cadaveric spine specimens including from L1 to sacrum (six specimens) and ilium (four specimens) were studied. Pure moments of a maximum of 10 N-m were applied incrementally. Parameters of neutral zone, elastic zone, and range of motion for rotations as well as for translations were measured. Neutral zones for flexion-extension, right/left axial torque, and right-left lateral bending were, respectively: 1.6 degrees, 0.9 degrees, and 1.4 degrees (L1-2); 1.0 degrees, 0.8 degrees, and 2.0 degrees (L2-3); 1.4 degrees, 0.7 degrees, and 1.4 degrees (L3-4); 1.8 degrees, 0.4 degrees, and 1.6 degrees (L4-5); 3.0 degrees, 0.4 degrees, and 1.8 degrees (L5-S1). Ranges of motion for flexion, extension, axial torque (one side), and lateral bending (one side) were, respectively: 5.8 degrees, 4.3 degrees, 2.3 degrees, and 4.9 degrees (L1-2); 6.5 degrees, 4.3 degrees, 2.6 degrees, and 7.0 degrees (L2-3); 7.5 degrees, 3.7 degrees, 2.6 degrees, and 5.7 degrees (L3-4); 8.9 degrees, 5.8 degrees, 2.2 degrees, and 5.7 degrees (L4-5); 10.0 degrees, 7.8 degrees, 1.4 degrees, and 5.5 degrees (L5-S1). Neutral zone values were small except for flexion at L5-S1.(ABSTRACT TRUNCATED AT 250 WORDS)

The purpose of this presentation is to provide basic biomechanical information concerning the spine, its components, and the spinal cord. It is shown that this information is helpful in understanding the fundamental functions of the spinal column. The experimentally determined physical properties of the vertebra, various spinal ligaments, the disc, and the spinal cord under many different loading conditions are described. The role of the special characteristics of the spine ligaments in allowing physiological motions of the spine, preventing excessive motions between vertebrae, and protecting the spinal cord during trauma are discussed. Movements of the spinal cord within the spinal canal and associated changes of its section during physiological movements of the spine are also described. The kinematics of the various regions of the spine are discussed and their clinical significance is presented. The problems of spinal trauma and is associated spinal instability are analyzed. Guidelines are recommended to assess spinal stability. The proper application of such guidelines will provide the basis for sound clinical judgments.

The present study was designed to analyze the usefulness of plain radiographs in evaluating bone healing. Rabbit tibiae were osteotomized, externally fixed, and allowed to heal for 3-8 weeks. Bones were harvested, x-rayed, and tested to failure in a dynamic torsion tester. AP and lateral radiographs of 10 rabbit tibia pairs and 10 individual rabbit tibiae were selected randomly for use in a questionnaire, given to 93 physicians who routinely assess fracture healing to evaluate clinicians' ability to assess bone strength. The results indicated that clinicians can differentiate the relative strength of bones by comparing two sets of radiographs. However, the strength determination from a single set of radiographs of a fracture is unreliable, the tendency being to evaluate the fracture to be weaker than it actually is.

This epidemiologic study examined the relationship between postural discrepancies in the posterior plane and the subsequent development of back and neck pain. The study population comprised women who were members of the graduating classes of 1957, 1958, and 1959 of an eastern U.S. women's college. Information on postural asymmetry was gathered from two sources: (1) measurements made from posture pictures taken early in the freshman year of college and obtained from each woman in the study, and (2) subjective evaluations made by the physical education department faculty at the time of examination. Information on the development of back and neck disorders and associated risk factors during the subsequent 25 years was obtained by a postal questionnaire. Three parameters of postural asymmetry were examined from the posture picture measurements: elevation of one shoulder, elevation of one hip, and deviation of the spine from the midline of the body. None of these parameters nor the physical education department evaluations was associated with a subsequent report of low-back pain, mid-back pain, or neck pain.

Under standardized operative techniques, the tibial shafts of 24 adult female rabbits were osteotomized. With a specially constructed spring calibrated clamp, a 20N compressive load was applied to both tibias. During the subsequent healing time, 7d per week, 4hr per day, a cyclical compression loading of 20 additional N at 55 cycles per min was applied to one of the paired tibias. The animals were suspended during the entire experiment so that external forces from activity were eliminated. X-rays of the healing bones were taken in two planes at weekly intervals. Animals were sacrificed at periods of three, four, six, seven and eight weeks and the bones tested in a dynamic torsion testing machine. Torque, angle, energy absorption to failure and stiffness were measured from the torque-angle curves. These four quantitated variables of bone “strength” were then studied as a function of time for the two loading patterns. In these experiments, the “strength” of the healing bone was not preferentially enhanced by either constant compression, or cyclic compression superimposed on constant compression.

Currently, three criteria are accepted as indications for prophylactic internal fixation of metastatic disease in long bone, including lesions (a) destroying 50% or more of the cortex; (b) 2.5 cm or greater in diameter; or (c) with pain unrelieved by radiation therapy. Using an oblong defect configuration in which one half of the cross-sectional area was destroyed, canine femora were torsion-tested at high speed to determine (a) the actual strength reduction incurred by a lesion destroying 50% of the cortical circumference, and (b) the effects and benefits of internal fixation using polymethylmethacrylate and/or a six-hole compression plate on such a defect. The femurs with a 50% circumferential cortical defect demonstrated only 12.7 +/- 3.8% of intact strength. Defects treated with a combination of plating (all screws bicortical) and polymethylmethacrylate (torque to failure 4.39 +/- 0.90 times greater than the defect alone or TE/TD = 4.39 +/- 0.90) were statistically stronger than defects treated with polymethylmethacrylate alone (TE/TD = 2.48 +/- 0.66; p less than 0.025) or by plating alone (TE/TD = 2.61 +/- 0.91; p less than 0.025), but torque-to-failure was only increased to approximately 56% of an intact bone. Plated intact bones (TE/TD = 5.33 +/- 0.41) were significantly weaker than intact bones (TE/TD = 8.50 +/- 2.52; p less than 0.001). Our results substantiate the need for using polymethylmethacrylate and internal fixation in combination when prophylactically fixing pathologic lesions of this proportion.

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.

Spinal injuries due to high-speed trauma are significant problems. The method of treatment depends on the stability determination of the injured spine. Young pig spines were injured at high speed to produce clinically relevant fractures and dislocations. The injuries were produced by dropping a mass onto the superior vertebra and causing three major types of trauma: flexion-compression, extension-compression, and pure compression. The multidirectional instability of each spine was measured before and after trauma by applying pure moments to the three vertebrae segments. Lateral radiographs were taken of each intact and injured spine. Flexion trauma produced the greatest instabilities in flexion and extension, while extension trauma produced the greatest instabilities in axial rotation and lateral bending. Lateral radiographs were found to be inaccurate predictors of spinal instability.

The authors performed an in vivo experiment in a canine model to study the natural history of spinal stability as a function of healing time post-injury. Three injuries, in addition to sham, were investigated: 1) transection of supra- and interspinous ligaments at C4-C5; 2) laminectomy at C4; and 3) bilateral facetectomy at C4-C5. Standardized functional flexion/extension stereo-radiographs of the cervical spine were obtained before injury and at 1, 3, 6, 9, 12, and 24 weeks postinjury. The authors found decreased ranges of motion (ROM) at the C4-C5 level for all injuries, including the sham. The decreases, in general, seemed to be in direct proportion to the severity of the injury. When the ROM were normalized to that of the sham at each time point, the relative ROM at C4-C5 increased for Injury 1, remained the same for Injury 2, and decreased for Injury 3. The relative ROM remained the same or increased at C3-C4, and increased at C5-6 for all injuries. It should be emphasized that care should be taken in extrapolating any data from this animal study to the human cervical spine.

Based on a review of previous studies and our opinion, biomechanical considerations suggest the following guidelines for the surgical management of CSM. It is not recommended that the dura mater, the pia mater, and the dentate ligaments be transected in the surgical treatment of CSM. Anterior decompression and fusion, preferably with the Smith-Robinson technique, is recommended for patients with anterior impingement of the spinal cord at one or two levels in the absence of a narrow spinal canal. This procedure is also advantageous when there is significant radiculopathy associated with the level(s) of pathology. Posterior decompression is recommended when there are three or more levels involved, and particularly when there is developmental stenosis of the canal, ie, a DAD below 13 mm and a SAD below 11 mm. Laminectomy and laminoplasty for CSM may not be any different as regards surgical outcome. One well-controlled study showed only one difference, a decrease in the ability of the laminoplasty patients to extend the neck. If there is evidence of instability or a potential for it, posterior decompression procedures should be accompanied by a facet fusion, or in the case of laminoplasty, some fusion modification such as that described by Itoh and Tsuji. There may also be circumstances in which significant multilevel anterior spur formation and compression in association with a stenotic canal should be treated with anterior and posterior surgery with appropriate attention to maintaining adequate stability. The advantages and disadvantages of these various surgical procedures and their relative appropriateness in various clinical situations will be gradually clarified through well-designed and executed laboratory and clinical investigations.

The compressive material properties of human lumbar vertebral trabecular bone were examined and compared to in vivo and in vitro measures of bone density and to the degree of macroscopic disc degeneration. No significant differences in the material properties were found for segment level (L1-L4) or for degeneration grade (I-IV), although trabecular bone specimens from segments with slightly degenerated disks (I-II) were stronger (24%) and stiffer (16%) on the average than specimens from segments with severely degenerated discs. Significant positive logarithmic regressions were obtained, however, between the in vivo and in vitro measures of trabecular bone density and trabecular bone mechanical properties. The results indicate that in vivo mineral measurements can accurately estimate the compressive material properties of lumbar vertebral trabeculae.

Mathematical models of the human spine structure are useful in predicting biomechanical behaviour of the spine where actual experiments may never be performed. These models, to be valid, should be based upon experimentally determined mechanical properties of the spine. Three dimensional flexibility and stiffness properties, including the coupling effects, were determined for all levels of the human thoracic spine. Fresh cadaver spines were used in a 100% humid atmosphere at 22°C to preserve physiological environment. Bottom vertebra of a two vertebrae construct was fixed. Twelve forces and moments, one at a time, were applied to the top vertebra. Vertebral displacement was accurately measured in three dimensional space. Load displacement curves for the main as well as the coupled motions were plotted. Flexibility and stiffness matrices were calculated for the center of the vertebral body. Values for the matrix coefficients, the load-displacement diagrams and the variation of the mechanical properties with the spine level are presented.

Ten fresh cadaveric whole cervical spine specimens (occiput to C7) were studied using well established techniques to document the movements in flexion, extension, left and right lateral bending and left and right axial rotation. Pure moments of maximum 1.5 newton meters were applied incrementally and three dimensional movements of the bones were recorded using stereophotogrammetry. Each moment was applied individually and in a three load/unload cycles. The motion measurements were made on the third load cycle. Parameters of neutral zone, elastic zone and range of motion were computed. Neutral zones for flexion/extension, right/left lateral bending and right/left axial rotation were respectively: 1.1, 1.5 and 1.6 (occiput-C1); and 3.2, 1.2 and 29.6 degrees (C1-C2). Ranges of motion for flexion, extension, lateral bending (one side) and axial rotation (one side) were respectively: 3.5, 21.0, 5.5 and 7.2 degrees (occiput-Cl joint) and 11.5, 10.9, 6.7 and 38.9 degrees (CI-CZ joint). The highest intervertebral motion in the spine was the axial rotation at the Cl-C2 joint, neutral zone constituting 75% of this motion.

The compressive mechanical properties of human lumbar vertebral trabeculae were examined on the basis of anatomic origin, bone density, and intervertebral disc properties. Trabecular bone compressive strength and stiffness increased with increasing bone density, the latter proportional to strength and stiffness to the one-half power. Regional variations within each segment were found, the most prevalent differences occurring in regions of bone overlying the disc nucleus in comparison with bone overlying the disc anulus. For normal discs, the ratio of strength of bone overlying the disc nucleus to bone overlying the disc anulus was 1.25, decreasing to 1.0 for moderately degenerated discs. These results suggest that an interdependency of trabecular bone properties and intervertebral disc properties may exist.

This presentation started with the assumption that our patient needs a cervical spine operation. Attention has been focused on the questions of which operation is needed and why. Most operations substitute one abnormal situation for another. The second abnormal situation must be beneficial to the patient. It was suggested that we think about the concept of immediate postoperative stability and the methods of achieving it when needed. When decompressions are indicated, we should determine as precisely as possible where the pressure is and what surgical procedure is the best to relieve it. We have suggested that allographs may be as good or in some ways better than autographs. The validity of the statement that bone grafts under tension will be absorbed has been questioned. We have suggested that the maturation of a bone graft can be staged and the patient managed accordingly. The discussion included an analysis of the advantages and disadvantages of some of the surgical constructions that are employed into the occipital-atalantoaxial complex. The importance of preserving motion in this joint as much as is compatible with a solution of the clinical problem was indicated. We suggested the use of the halo applied preoperatively and maintained through surgery in order to have safe, thorough control for the highly unstable situations in the upper cervical spine. In the lower cervical spine the selection of procedures that do not add to already existing instability was emphasized. Opinion was submitted about the relative immediate postoperative stability of the various anterior and posterior procedures in the lower cervical spine. We submitted some ideas about the principles of the use of methylmethacrylate in cervical spine surgery. Specific details of the surgical procedures were discussed when needed and illustrative case reports were included, when they were thought to be helpful.

Instability is a mechanical entity, and an unstable structure is one that is not in a optimal state of equilibrium. In the spine, stability is affected by restraining structures that, if damaged or lax, will lend to altered equilibrium and thus instability. Instability is defined as loss of stiffness.

A system is suggested for the definition of position and motion of the human body parts. Suitable system locations and orientations are described and examples of the use of the system are demonstrated and its advantages pointed out.

Temporal changes in the physical properties of healing fractures in rabbits were studied. The mechanical environment at the fracture site was measured and monitored during healing. Animals were sacrificed after 3 to 8 weeks. The results of healing were quantified by whole bone dynamic torsional strength tests. Torque-angle curves were recorded by computer. At maximum torque four parameters were calculated: torque, angle, energy absorbed and stiffness. Torque and stiffness increased while energy remained constant and angle decreased with time. However, values calculated by a constant deformation criteria showed the three strength parameters to increase with time. The rate of increase was highest for stiffness followed by torque and energy.

There is evidence to suggest that abnormal coupling patterns in the lumbar spine may be an indicator of low-back problems. To quantify the normal coupling patterns, fresh cadaveric human lumbar spine specimens (L1-S1) were used. A pure axial torque or lateral bending moment of 10 N-m (in five equal steps) was applied to the specimen, in five spinal postures, and three-dimensional motions were measured at the five vertebral levels. The results indicated that the coupling patterns changed significantly with the intervertebral level. For example, in neutral posture, the left axial torque produced coupled lateral bending, which varied from approximately 2 degrees right lateral bending at L1-2, to approximately 0 degrees at L3-4, and to approximately 2.5 degrees left lateral bending at L5-S1. Additionally, there was coupled flexion of approximately 1 degrees to 2 degrees at all levels. Application of left lateral bending moment resulted in approximately 1.7 degrees of coupled right axial rotation at all levels, except at L1-L2, where it was 0 degrees. Additionally, there was coupled flexion of 0.7 degrees to 2 degrees at all levels. For example, at the L2-3 level, the left axial torque produced coupled right lateral bending that ranged from approximately 0.5 degrees at full extension to approximately 2.5 degrees at full flexion. There was also accompanying coupled flexion of approximately 0.4 degrees to 1.7 degrees. Application of left lateral bending moment at the L2-3 level produced axial rotation of approximately 2.5 degrees, which did not vary with the posture, while the other coupled motion varied from approximately 1.7 degrees flexion at full extension posture to approximately 0.8 degrees extension at full flexion posture.

This in vitro study provides original data regarding the three-dimensional movements of the intervertebral foramen as the spine undergoes normal physiologic motions. Three-dimensional flexibility characteristics of fresh cadaver functional spinal units were determined. The same spinal units were fixed and thinly sectioned to yield quantitated anatomy of the soft and hard tissues of the specimen in three-dimensions. The flexibility and anatomic data were stored in a computer. A mathematical model was utilized to combine the two data sets, resulting in a precise three-dimensional description of the movements of the intervertebral foramen. Changes in foramen height, width, and area were obtained as functions of major spinal motions. Preliminary results indicate differences in intervertebral foramen kinematics of nondegenerated and degenerated spines. Such changes may affect the nerve root adversely and hence participate in pain production.

An apparatus is described that produces experimental contusion of the spinal cord and completely defines the mechanical parameters of the trauma. The data provided includes a continuous record of the force, displacement, velocity of deformation, and acceleration of deformation of the spinal cord and the underlying thorax.

Allogeneic bone from Sprague-Dawley rat femurs was subjected to levels of freezing and/or irradiation that are known to have produced changes in the associated immune responses to these grafts. This bone was transplanted into an experimentally created gap in the femur of Lewis rats. The subsequent healing of the transplants in the Lewis rats was studied at 2, 4, 8, and 16 weeks after transplantation using torsion testing to failure. There was no clear advantageous biologic response in the union of the grafted material accompanying the alterations in immunologic response as measured by biomechanical testing of the proximal osteosynthesis site in torsion. The torsional strength of all of these groups remained lower than that of intact bone. Furthermore, none of the frozen and/or irradiated allografts exceeded the strength of the fresh allograft at a statistically significant level.

Intradiscal pressure and volume measurements were made in 84 fresh cadaveric lumbar spine disc spaces. The nucleus was injected with a roentgenographic contrast agent under fluoroscopic examination. The intrinsic pressure, the pressure at which the agent entered the disc, and the maximum pressure that the disc could hold were measured. The discs were graded for degeneration. The intrinsic and maximum pressures were found to be inversely related to disc degeneration grade, and directly related to each other. Relatively greater degeneration was found at lower levels of the lumbar spine as compared to the upper levels. The intrinsic disc pressure may prove to be a useful clinical tool in the evaluation of spinal integrity.

A system is suggested for the definition of position and motion of the human body parts. The suitable system locations and orientations have been described. Some examples of the use of the system have been demonstrated and its advantages have been pointed out.

Does infant malnutrition produce smaller adult spinal canals? Lumbar and thoracic vertebrae (n X 1073), from a prehistoric American Indian population (15-55 yrs of age), were measured for anteroposterior (AP) and transverse (TR) vertebral canal sizes, nerve root tunnel (intervertebral foramen) widths (NRT), vertebral heights (VH), vertebral osteophytosis (VO), and tibial lengths. They underwent a dietary change from hunting-gathering, protein rich (PR), to maize agriculture, protein deficient (PD), between 950 and 1300 A.D. Multivariate analyses controlled for age, sex, culture, NRT, VH, VO, and wedging. Canal size was significantly smaller in the PD. AP diameters were generally and highly correlated with NRT, and thus both spinal stenosis and sciatica may have a developmental basis. Canal size was independent of statural components. Consequently, canal size is a most powerful tool in assessing the presence infant malnutrition. Moreover, perhaps the association between canal size and low-back pain (LBP) found in living populations has been underestimated, and this component of LBP is preventable.

Nineteen upper cervical spine specimens were dissected to examine the macroscopic and functional anatomy of alar ligaments. They are on both sides, symmetrically placed, approximately 10-13 mm long and elliptical in cross-section 3 X 6 mm in diameter. The fiber orientation is dependent on the height of dens axis, mostly in the cranial caudal direction. In 12 specimens there was a ligamentous connection between dens and lateral mass of the atlas as a part of the alar ligament. In 2 specimens anterior atlanto-dental ligament was identified. The computerized tomographic (CT) images can clearly show alar ligaments in axial, coronal, and sagittal planes. The ligaments limit the axial rotation in the occipito-atlanto-axial complex (to the right by left alar and vice versa) as well as in side bending. The ligament is most stretched, and consequently most vulnerable, when the head is rotated and in addition flexed. This mechanism, common in whiplash injuries, could lead to irreversible overstretching or rupture of the ligaments especially as the ligaments consist of mainly collagen fibers.

The adverse effects of excessive tourniquet pressure on underlying muscle were studied using a cat model. A direct effect of the magnitude of tourniquet pressure on the degree of muscle disability was observed. The effect of direct pressure on the muscle may contribute to the postoperative weakness seen in the quadriceps muscle. Our data, in conjunction with the results of other studies that correlate certain tourniquet times and pressures with potential adverse effects, suggest that the selection of an appropriate tourniquet pressure and duration will minimize the development of clinically sig nificant complications.

Early growth, Vertebral neural canals, Adult health ABSTRACT Previous studies in animal populations have shown that stunted neural and thymolymphatic growth early in development may result in permanently impaired neural and immune function, decreased body growth, vertebral wedging, and decreased life-span. In the human adult, small verte- bral neural canal (VNC) diameters may reflect early stunted neural and immune development and impaired function that leads to decreased health (inferred by greater vertebral wedging) and life-span in the adult. VNC, which complete their growth by early childhood (age 4), are markers of early devel- opment in adults. On the other hand, features following general body growth, such as height, weight (represented here by vertebral body height) continues to grow until young adulthood. They are less reliable, because they readily experience catch-up growth (even in chronically stressed populations) and, unlike VNC, may mask poor early growth. To test associations between early growth and adult health and life-span in humans, we measured 2,060 VNC, vertebral heights, vertebral wedging, nerve-root tunnel lengths, severity of vertebral osteophytosis, and ages at death in 90 adult (aged 15-55 years) prehistoric skeletons (950-1300 A.D.). Tibia1 lengths were also measured in a subsample (n = 30). Multivariate, bivariate, and nonparametric analyses showed that small VNC are significantly associated with greater vertebral wedging and decreased life-span (P < 0.05-0.00001). VNC are independent of vertebral body heights and tibia1 lengths (general body growth). VNC, but not statural components, are useful in predicting adult health, presumably be- cause they reflect neural and immune development and do not readily experi- ence catch-up growth. Thus, longitudinal retrospective measures of early growth and adult health were systematically linked within individuals regard- less of confounding factors operating over the 350-year time period. Since this research was completed, this model has repeatedly been independently con- firmed in four living urban industrial populations. Longitudinal retrospective analysis was employed together with direct measures of VNC, neural and immune function. Together these results suggested that it may be essential to improve growth prior to early childhood in order to maximize adult health and life-span.

An in vitro study was conducted to establish the thresholds of thoracic spine stability. Flexion or extension producing horizontal forces of 43 percent body weight were applied to fresh two-vertebrae spine specimens. Spine components were transected in two different sequences until failure. Load-displacement curves were measured. Intact spine exhibited average inter-vertebral horizontal translatory displacement of 1.0 mm (s.d. = 0.4) and sagittal plane rotation of 1.4 degrees (s.d. = 0.8). Just prior to failure these average displacements increased to 2.4 mm (s.d. = 1.4) and 4.1 degrees (s.d. = 1.7). A horizontal displacement of 2.5 mm (on lateral X-ray) or 5 degrees of angulation of one vertebra with respect to the other may indicate an unstable spine. This information, together with other clinical indicators of spine instability discussed here, will be helpful in the clinical judgment regarding an injured spine.

A method is presented for constructing discrete-parameter type three-dimensional mathematical models and governing equations of motion of the spine structure. The anatomic structure is represented by any combination of rigid bodies, springs, and dashpots in space. These are positioned, orientated, and connected in a manner to represent the true mechanical function of the structure. The rigid bodies are of any shape and have 6 degrees-of-freedom, allowing three-dimensional motion. The springs and dashpots may have up to twenty-one stiffness and damping coefficients respectively to precisely represent the three-dimensional coupled behavior. The method is straightforward and simple to apply. The governing equations are in the matrix form and are easily generated and solved by computer techniques.

Torisonal, compression, and bending forces were applied to long oblique fractures of rabbit tibiae that were fixed with 2 screws in different positions. Torison, bending and compression tests were applied to the tibiae. Results for torsional load show that screws should be perpendicular to the fracture plane. For compressive load, screws should be fixed perpendicular to the long axis of the bone. When bending moments are applied in the same plane as the plane of the screws, there is no preferred position of the screws across the fracture. When bending moments are applied perpendicular to the plane of the screws, screws should be positioned perpendicular to the fracture plane.

The use of unicortical screws instead of bicortical screws in the extreme outer holes of dynamic compression plates (DCPs) has been recommended to minimize the stress riser effect at the end of the plates. The authors examined in vitro two groups of paired canine femurs after compression plates had been applied to the anterolateral diaphyseal surface bilaterally. Group I: intact paired femurs with bicortical peripheral screws in one DCP and unicortical peripheral screws in the other; Group II: osteotomized paired femurs again comparing unicortical and bicortical peripheral DCP screws. All specimens were torsion tested to failure and the torque, stiffness, energy, rotation, and failure fracture length were calculated. Unicortical screws did not enhance the torsional strength in either group, and significantly compromised stiffness, energy, and rotation in the osteotomized group. The only apparent benefit of unicortical peripheral screws in a DCP was a shorter, less comminuted fracture upon failure.

Cervical traction of one-third the person's weight was applied for 3 minutes to normal subjects. The mean and standard deviation of displacement and rotation between adjacent cervical vertebra was calculated in the sagittal plane. The average separation between adjacent vertebra was 0.7 mm with a standard deviation of 0.5 mm. The average angular motion between adjacent vertebra was 0.9 degrees with a standard deviation of 2.4 degrees.

Autogenous cancellous bone graft alone or in combination with electrical stimulation is commonly employed yet there exists no conclusive data that the strength of the healing defect is actually modified. The authors examined three groups of paired canine femurs that were torsion tested to failure. Group I (in vitro): an intact femur was compared to a standard defect; Group II (in vivo): a defect alone was compared to a defect plus graft killed at eight weeks; Group III (in vivo): a defect plus graft was compared to a defect graft plus Osteostim (BGS Medical Corp., Milwaukee, Wisconsin) implantable stimulator killed at eight weeks. In Group I the defect decreased the average strength 45% (p = 0.005); in Group II the grafted defect decreased the strength 20% (p = 0.121); in Group III the defect graft plus electrical stimulation increased strength 4% (p = 0.669). At eight weeks, bone grafting, alone or with electrical stimulation did not statistically increase the torsional strength of the healing bony defect.

Removal of the posterior elements will allow increased correction of axial deformity, in scoliosis. The clinician may take advantage of the creep and relaxation characteristic of the tissues to improve efficiency of correction. Axial loading has been shown theoretically to be more efficient for the more severe curves, (greater than 53 degrees) and transverse loading more efficient for the less severe curves (less than 53 degrees). Combined loading is always more efficient than either type alone. The Milwaukee brace can be just as effective as a cast in resisting deforming forces in scoliosis. Removal of axillary supports or thoracic pads or not wearing the brace when recumbent reduces the effectiveness of the Milwaukee brace. The strength of the thoracid lamina is a limiting factor in the amount of forces that may be applied to correct the deformity; 30 kilopond (65.8 lb) is the upper limit of this force. Coughing or buckling can apply dangerously high forces with the Harrington rod. Greater surgace contact of the hook to the lamina and small increments between notches on the rod may increase the tolerance limits of the system. Compression rods on the convex side probably add little or no correctional value. The Dwyer technique is biomechanically sound and effective and has the additional advantage of applying asymmetrical loads to the epiphyseal plates.

In a series of 25 subtrochanteric fractures transfixed with high-angle compression hip screws, there were no mechanical failures, delayed unions, or nonunions. Osseous union occurred in a mean time of 3.6 months. Malunion occurred in two instances, at least one of which was the result of technical error at the time of reduction. Rapid mobilization of patients with subtrochanteric fractures is an attainable goal. Early partial weight-bearing should be possible. Although no one internal fixation appliance may be optimal for all fractures in the subtrochanteric region, the high-angle compression hip screw is efficient for the fixation of a wide variety of fracture patterns that otherwise would have been difficult to manage.