[Computed tomographic analysis of humeral retrotorsion and glenoid retroversion]. / Analyse tomodensitométrique de la rétrotorsion humérale et de la rétroversion glénoïdienne.

PURPOSE OF THE STUDY: Analysis of the anatomic relations of the humeral head and the glenoid cavity is particularly important for clinical study of shoulder arthroplasty and glenohumeral instability. Analysis of humeral retroversion and glenoid retroversion is quite difficult and data in the literature are scarse. We conducted a computed tomography (CT) analysis of stable shoulders to detail retroversion of the entire height of the glenoid cavity and to measure humeral retrotorsion using two comparative methods. We also compared glenoid retroversion and humeral retrotorsion observed in individual subjects. MATERIAL AND METHODS: This prospective study used a standardized CT analysis method. Both shoulders of 30 persons free of glenohumeral instability were studied. Two methods, described by Dähnert and Bernageau, were used to analyze humeral retrotorsion. The Benageau method was used to analyze glenoid retroversion. RESULTS: According to the Dähnert method, humeral retrotorsion was 10 degrees +/- 13 degrees; it was 24 degrees +/- 13 degrees with the Bernageau method; data dispersion was 60 degrees and 65 degrees respectively. According to the Dähnert method, retrotorsion was more pronounced on the dominant side compared with the non-dominant side. There was a significant correlation between retrotorsion values for the two sides. For 95% of the shoulders, glenoid retorversion decreased progressively from the superior part of the glenoid cavity (12.8 degrees +/- 6.4 degrees ) to the lower part (3.1 degrees +/- 4.4 degrees ). Glenoid retroversion was greater on the dominant side. For 21 of the 30 persons (70%), there was a significant correlation between retroversion for the two sides. Correlation coefficients between glenoid retroversion and humeral retrotorsion were negative. Greater humeral retrotorsion was thus related with less pronounced glenoid retroversion and vice versa. DISCUSSION: This study allowed quantification of glenoid retroversion and humeral retrotorsion. There is a spiral twist in the joint surface of the glenoid cavity with progressive decrease in glenoid retorversion from the upper to the lower part of the cavity for 95% of the shoulders. To our knowledge, this spiral twist in the glenoid cavity is not taken into consideration in any of the currently available implants. The correlation for both parameters between the right and left side is probably determined genetically. The influence of dominance could be explained by adaptation to more or less pronounced stress. The negative correlation between humeral retrotorsion and glenoid retroversion would improve glenohumeral stability. A comparative study with unstable shoulders would be required to verify this hypothesis. The validity of the Dähnert method for assessing humeral retorversion is, in our opinion, insufficiently established. The Bernageau method, which provides a direct measurement, appears to be preferable despite the difficulty in identifying anatomic landmarks.

Effects of freezing/thawing on the biomechanical properties of human tendons.

Numerous biomechanical studies using osteoarticular complex need frozen cadaveric specimens. Some of these studies deal with the resistance of the tendinous structures, for example the resistance of some autografts, such as the patellar ligament and the semitendinosus and gracilis tendons for reconstruction of the anterior cruciate ligament. The aim of this study was the in-vitro evaluation of the mechanical modifications induced by freezing/thawing on human tendons. The long head of the biceps brachii tendon was used as the reference. Eight pairs of tendons of the long head of the biceps brachii were taken from eight fresh cadavers. After drawing lots, one was tested immediately, the other was deep-frozen and then thawed. With an Instron material-testing machine, we performed a relaxation test and a uniaxial tensile test, to estimate the ultimate tensile failure and the elastic modulus of each pair of tendons. Freezing had no influence on the tendinous relaxation, but altered significantly the ultimate tensile failure and Young's modulus of the tendons.

Development of a static simulator of the mandible.

Our aim was to build and assess a static mandibular simulator capable of reproducing various masticatory exercises as closely as possible to physiological situations. Unlike existing devices that reproduce a small number of exercises and of muscular bundles, our simulator is able to reproduce all types of effort in the sagittal plane and integrates eleven muscular groups considered by the majority of authors as being the most important in mastication. The value of our simulator arises from its ability to replicate the external forces (including muscular traction, intra-articular reaction and resultant forces applied to the mandible) as determined in vivo during previous studies. The importance of such a versatile simulator, in addition to enabling study of the mechanical behaviour of the mandible, is that it greatly enhances evaluation of a possible mathematical model by means of finite element analysis. Stages of the development and evaluation of this device are described.

[Experimental comparative study of 3 systems of osteosynthesis for proximal humeral fractures. Preliminary study of the mechanical properties of conserved trabecular bone]. / Etude comparative expérimentale de trois systèmes d'ostéosynthèse des fractures de l'humérus proximal. Etude préliminaire sur les propriétés mécaniques de l'os spongieux conservé.

PURPOSE OF THE STUDY: We have compared three systems for osteosynthesis of proximal humeral fractures on a four-fragment fracture model: Plate with tuberosity screwing (P), Kirschner wires with bone sutures (K) and an original implant, the "bilboquet" (Stryker) (B), composed of a hook for the head supported on a diaphyseal stem. MATERIAL AND METHODS: Because of the unavailability of fresh human material, we have turned to embalmed bones after having shown, on cancellous femoral head bone, the little influence of our method Vs freezing conservation. Settings were tested in compression. We measured their mechanical resistance judged on the ultimate strength and on the slope (displacement/load), to this limit. We similarly analyzed by video recording displacement of the different fragments according to four components (head slipping, its lateral displacement, its see-saw and the separation of tuberosities), to show the weak points of the three systems. RESULTS AND DISCUSSION: K was more deformable (p < 0.001) that P and B (average slopes in mm/100N to 0.67 for K, 0.42 for P and 0.43 for B). The ultimate strength differed but not significantly (average values in Newton to 1223 for K, 1601 for P and 1721 for B). We also showed for the three techniques, the significant correlation between slopes and bony densities. Video analysis showed that P opposed lateral displacement of the head and separation of tuberosities better, the resistance to slipping and see-saw seems to be improvable by greater divergence of screws or a modification of the plate (mini canon). B opposed slipping and see-saw better but its weak point was the risk of articular break-in of the hook, by defect of resistance to lateral displacement, that for us could be limited by a less peripheral position of teeth or a modification of their form. K is indeed the least effective of the four components but we think that it would be improvable by using, for example, a centro-medullary technique, allowing an internal cortical support.

Determination of the external forces applied to the mandible during various static chewing tasks.

The purpose of this study is to determine the external forces that are brought to bear on the mandible during 14 masticatory tasks, exerted in three defined mandibular positions. A static two-dimensional mandibular model is presented, taking into account projections in the sagittal plane of the forces exerted by six muscle groups and the joint reaction force. In calculating these external forces, two working hypotheses are adopted: the existence of a linear relationship between cross-sectional surface of the muscle and its maximum force of contraction, and the existence of a linear relationship between the electrical signal emitted by a muscle and the force the muscle then develops. Our first results are provided and compared with data from the literature. Individual variations recorded in the measurement of the different parameters involved in the equilibrium equations are such that they must be taken into account in the calculation of the forces. Moreover, the functional value of the muscles appears to be closely dependent on the mandibular position with which they are associated, which is not brought out in earlier studies. Finally, intra-joint action proves to be intense, although its direction varies according to the mandibular position and the type of exercise performed.