Joannès Dimnet

Spondylolisthesis, Pelvic Incidence, and Spinopelvic Balance : A Correlation Study

Study Design. A retrospective study of the sagittal alignment in developmental spondylolisthesis. Objectives. To investigate the role of pelvic anatomy and its effect on the global balance of the trunk in developmental spondylolisthesis. Summary of Background Data. Pelvic incidence (PI) is a fundamental anatomic parameter that is specific and constant for each individual, and independent of the three-dimensional orientation of the pelvis. Recent studies have suggested an association between a high PI and patients with isthmic spondylolisthesis. Methods. The lateral standing radiographs of the spine and pelvis of 214 subjects with developmental L5–S1 spondylolisthesis were analyzed with a dedicated software allowing the calculation of the following parameters: pelvic incidence (PI), sacral slope (SS), pelvic tilt (PT), lumbar lordosis (LL), thoracic kyphosis (TK), and grade of spondylolisthesis. All measurements were done by the same individual and compared to those of a cohort of 160 normal subjects. Student’s tests were used to compare the parameters between the curve types and Pearson’s correlation coefficients were used to investigate the association between all parameters (&agr; = 0.01). Results. PI, SS, PT, and LL are significantly greater (P < 0.01) in subjects with spondylolisthesis, while TK is significantly decreased. PI has a direct linear correlation (0.41–0.65) with SS, PT, and LL. Furthermore, the differences between the two populations increase in a direct linear fashion as the severity of the spondylolisthesis increases. Conclusions. Since PI is a constant anatomic pelvic variable specific to each individual and strongly determines SS, PT, and LL, which are position-dependent variables, this study suggests that pelvic anatomy has a direct influence on the development of a spondylolisthesis. Study participants with an increased pelvic incidence appear to be at higher risk of presenting a spondylolisthesis, and an increased PI may be an important factor predisposing to progression in developmental spondylolisthesis.

Potential excursion and relative tension of muscles in the shoulder girdle: Relevance to tendon transfers

Muscles used for transfer ought to have adequate structural properties. The purpose of this study was to provide a database of potential excursion (muscle excursion without reference to connective tissue restraints) and relative tension (muscle physiologic cross-sectional area in percentage among a group) in shoulder girdle muscles. Thirteen muscles in 13 human cadavers aged 17 to 89 years at death were studied. Potential excursion ranged from 6.7 cm (supraspinatus) to 33. 9 cm (latissimus dorsi). Relative tension ranged from 1.7% (levator scapulae) to 20.9% (deltoid). Significant discrepancies were found between the properties of some of the muscles used as transfers around the shoulder and the properties of the muscles for which they are commonly used as substitutes. Despite the limitations of cadaveric studies and the fact that many other factors are involved in muscle transfers, this database of structural properties of shoulder girdle muscles may help when planning tendon transfers around the shoulder.

Biomechanical model predicting values of muscle forces in the shoulder girdle during arm elevation

A three-dimensional (3D) geometric model for predicting muscle forces in the shoulder complex is proposed. The model was applied throughout the range of arm elevation in the scapular plan. In vitro testing has been performed on 13 cadaveric shoulders. The objectives were to determine homogeneous values of physiological parameters of shoulder muscles and to locate sites of muscular attachment to any bone of the shoulder complex. Muscular fiber lengths, lengths of contractile element (CE), and muscle volumes were measured, corresponding physiological cross-sectional area (PCSA) were calculated, and force/length muscle relations were found. An in vivo biplanar radiography was performed on five volunteers. The photogrammetric reconstruction of bone axes and landmarks were coupled with a geometric modeling of bones and muscle sites of attachment. Muscular paths were drawn and changes in lengths during movement have been estimated. Directions of muscle forces are the same as that of muscular path at the point of attachment to bone. Magnitudes of muscular forces were found from muscle lengths coupled with force/length relations. Passive forces were directly determined contrary to active muscle forces. A resulting active muscle force is calculated from balancing weight and passive forces at each articular center. Active muscle forces were calculated by distributing the resulting force among active muscles based on the muscular PCSA values.

IN VIVO LOCATION OF JOINT CENTERS OF THE SHOULDER SYSTEM: GLENO-HUMERAL AND SCAPULO-THORACIC JOINTS BETWEEN TWO POSTURES DESCRIBING THE ARM ELEVATION IN THE PLANE OF SCAPULA USING TECHNIQUES BASED UPON BIPLANAR RADIOGRAPHY

In biomechanics, the knowledge of accurate location of a joint center is essential because equilibration of the external loads and muscular forces about the joint is performed about this specific point. This paper focuses on the location of centers of gleno-humeral joint and scapulo-thoracic joint in a subject moving their arm in the scapular plane with a magnitude of 120°. Biplanar radiography with successive exposures has been used locating anatomical axes of bones. Geometric models of bones were defined allowing access to bone morphology by superposing model projections onto X-ray imaged bone contours. Functional models were used so as to represent the behavior in motion of shoulder joints. These techniques allowed us to access to results describing the linear and angular relative displacements of the shoulder bones between two different postures. The gleno-humeral and scapulo-thoracic finite joint centers (FH and FS) are first defined through the location of the corresponding helical axis of motion (HAM) moving the joint from positions occupied in initial and final postures. The gleno-humeral and scapulo-thoracic mean joint centers (MH and MS) are then calculated using a new technique, which defines that each joint center has the point having the smallest migrations while moving continuously from initial to final postures. This allows for the analysis of the linear and angular clearances, which affect joint center migration. The whole continuous movement has been parsed into several steps to test the stability of the mean joint center throughout the motion.

The use of a photogrammetric method for the three-dimensional evaluation of spinal correction in scoliosis

PurposeClinical parameters, characterizing the spinal deformations due to scoliosis, are still directly measured on the spinal curve plane projections.MethodsA 3D spinal curve has been reconstructed from its two projections, using photogrammetric techniques. Each spinal curve is a compound of several plane regions, where it is purely flexed, and short zones of connection, where abduction and axial rotation components are concentrated. All spinal curves are represented as linear chains of regional planes articulated together. The regional plane is represented by a triangle, where one summit corresponds to the point of maximum offset. The set of weight forces, representing pelvis and spine, forms a bundle of vertical forces. The dispersion of the bundle illustrates the postural stability of patients.Results and ConclusionsThe first objective was to numerically describe the changes of the 3D spinal feature, due to the correcting treatment. Changes are calculated from the comparison between 3D radiologic situations, between before and after treatment. The second objective was to determine the direction of the external force, which would be the most efficient for correcting the patient set spine/rib cage. A mild mechanical analysis is proposed, for representing the transit of the external force, from rib cage to thoracic regional plane.

CONTROL OF ERRORS USING A SIMPLIFIED CLINICAL SETUP FOR MOTION ANALYSIS: ERRORS DUE TO CALIBRATION AND THREE-DIMENSIONAL RECONSTRUCTION

Different systems of motion analysis have been described. They usually associate several cameras with a force platform. They can analyze very sophisticated human movements. They are, however, expensive and require significant technical formation from users. A new system is proposed for simple and standard clinical applications. It uses only two cameras and a coupled force plate delivering only the vertical component of the patient weight and the location of his center of mass. It is inexpensive, simple to use and delivers accurate results. This is obtained through a strict experimental protocol, and a new method of data treatment which allows the control of errors at each step of the successive calculations. This paper describes the new system, the new calibration procedure and the control of errors.

A NEW TECHNIQUE ESTIMATING LOCATION OF MEAN JOINT CENTERS OF ROTATION WITH ASSOCIATED DISPERSIONS AND ASSESSING TO FUNCTIONAL COORDINATE SYSTEM OF MOVING LIMB SEGMENTS

Joint centers are obtained from data treatment of a set of markers placed on the skin of moving limb segments. Finite helical axis (FHA) parameters are calculated between time step increments. Artifacts associated with nonrigid body movements of markers entail ill-determination of FHA parameters. Mean centers of rotation may be calculated over the whole movement, when human articulations are likened to spherical joints. They are obtained using numerical technique, defining point with minimal amplitude, during joint movement. A new technique is presented. Hip, knee, and ankle mean centers of rotation are calculated. Their locations depend on the application of two constraints. The joint center must be located next to the estimated geometric joint center. The geometric joint center may migrate inside a cube of possible location. This cube of error is located with respect to the marker coordinate systems of the two limb segments adjacent to the joint. Its position depends on the joint and the patient height, and is obtained from a stereoradiographic study with specimen. The mean position of joint center and corresponding dispersion are obtained through a minimization procedure. The location of mean joint center is compared with the position of FHA calculated between different sequential steps: time sequential step, and rotation sequential step where a minimal rotation amplitude is imposed between two joint positions. Sticks are drawn connecting adjacent mean centers. The animation of stick diagrams allows clinical users to estimate the displacements of long bones (femur and tibia) from the whole data set.