Safety assessment of jumps in ski racing.

The influence of important parameters on the flight trajectory for jumps in downhill World Cup races was investigated. To quantify the impact injury risk at landing, the parameter equivalent landing height (ELH) was introduced, which considered a variable slope inclination during the landing movement. Altogether, 145 runs at four different jumps in World Cup races and trainings were recorded and analyzed. A simulation model was developed to predict the flight phase of the skier. Drag and lift areas were selected by parameter identification to fit the simulation trajectory to the two-dimensional data from the video analysis. The maximum values of the ELH which can be absorbed with muscle force was taken from the study of Minetti et al. for elite female and male ski racers. A sensitivity analysis based on the four jumps showed that ELH is mainly influenced by takeoff angle, takeoff speed, and the steepness of the landing surface. With the help of the developed simulation software, it should be possible to predict the ELH for jumps in advance. In case of an excessive ELH, improvements can be made by changing the takeoff inclination or the approach speed.

Modeling the ski-snow contact in skiing turns using a hypoplastic vs an elastic force-penetration relation.

A ski-snow interaction model is presented. The force between ski and snow is decomposed into a penetration force normal to the snow surface, a shear force parallel to it, and friction. The purpose of this study was to investigate the benefits of a hypoplastic vs an elastic contact for penetration in the simulation of skiing turns. To reduce the number of influencing factors, a sledge equipped with skis was considered. A forward dynamic simulation model for the sledge was implemented. For the evaluation of both contact models, the deviation between simulated trajectories and experimental track data was computed for turns of 67 and 42 m. Maximum deviations for these turns were 0.44 and 0.14 m for the hypoplastic contact, and 0.6 and 7.5 m for the elastic contact, respectively. In the hypoplastic contact, the penetration depth of the ski's afterbody maintained nearly the same value as the part under maximum load, whereas it decreased in the elastic contact. Because the shear force is proportional to the penetration depth, the hypoplastic contact resulted in a higher shearing resistance. By replacing the sledge with a skier model, one may investigate more complex skier actions, skiing performance, or accident-prone skiing maneuvers.

Calculation of the contact pressure between ski and snow during a carved turn in Alpine skiing.

The macroscopic contact area between ski and snow and the contact pressure are crucial influencing factors for carved turns in Alpine skiing. In the present paper, a simulation model is developed to quantify these factors. The ski is modelled as an Euler-Bernoulli beam with variable cross section, camber, bending and torsional stiffness using measured data from skis. The reaction forces of the snow are decomposed in penetration and shear forces. For the penetration forces a hypoplastic constitutive law is applied incorporating elastic and plastic deformation of the snow at the contact area. For the shear forces metal cutting theory is used. Ski deformation, contact area and contact pressure are computed based on quasi-static equilibrium between forces exerted by the skier and snow reaction forces. Parameter studies are performed to investigate the influence of edging and distributing the load between the inner and outer ski. Higher edging angles as well as loading both skis affected the contact pressure positively by increasing the resistance against shearing. The results of our study agree well with measurement data taken from literature. Based on the results, the importance of actions of the skier during carved turns is concluded.

[Effect of risk factors and therapy on intermediate-term hip deformity in Perthes disease]. / Der Einfluss von Risikofaktoren und der Therapie auf die mittelfristige Hüftdeformität beim M. Perthes.

UNLABELLED: The prognostication of risk factors for the outcome of LCPD was examined. The radiographic results of conservative treatment with Thomas splint and operative treatment with intertrochanteric derotational varus osteotomy (DVO) were compared using the Index of Deformity (D). METHOD: Radiographs of 153 patients with unilateral LCPD were evaluated retrospectively 1.5-3 years and 3-7 years after diagnosis using the Index of Deformity and Index of Direction. We investigated the radiographic changes in relation to acetabular or metaphyseal reaction, Waldenström classification or limited range of motion at diagnosis and in dependence of therapy. RESULTS: There was a significant better acetabulum-head-index of deformity after DVO. Children with metaphyseal or acetabular reaction had bigger deformities. Patients with metaphyseal reaction had smaller deformities after DVO compared with conservative treatment. Free range of motion (ROM) was prerequisite for good results after DVO. Waldenström classification at diagnosis did not have any influence on morphologic changes after 3-7 years. CONCLUSION: DVO leads to a better containment than conservative treatment with Thomas splint, metaphyseal reaction should be an indication for operative treatment. Prerequisite for DVO is a free ROM. Because Waldenström class at diagnosis does not influence the outcome, free ROM before starting the treatment is more important than an immediate start of therapy.

[A 5-year study of arterial occlusive disease]. / Petogodisnje pracenje toka arterijske okluzivne bolesti.

The difference of the frequency of occurrence of risk factors has been tested by the method of the comparison of bound pairs of the group of 46 patients with arterial occlusive disease (AOD) - group A, and of the group of 54 subjects without AOD - group B. The patients with AOD included more diabetics, smokers, physically inactive persons and those suffering from hyperlipemia (P less than 0.05). The difference of number of adipose and hypertonic persons belonging to those two groups has not been statistically significant. The results of the measures to change risk profile of the group A have been observed. On the completion of the period of five years (1982-1987), increased weight has been normalized with 26.5% of the persons with AOD, normal blood pressure has been attained with 43.5% of patients, normolipemia with 8.7% and blood sugar has been checked once or twice a year for all patients. After that period of five years only 13% of the patients with AOD, have been smoking. During the mentioned period, only one patient with AOD has his leg amputated. Between 1982 and 1987, 21 persons died in the group A, respectively 45.7% of the subjects, while in the same period, 14 persons died in the group B, respectively 25.9% of them. The difference of the mortality between groups A and B has been statistically significant (P less than 0.05).

[Talus injuries]. / Talusverletzungen.

Due to function and anatomy the talus plays a special part among the tarsal bones. The extended joint surfaces cause a high rate of arthrosis in fractures, the difficult blood supply leads very often to a disturbance of the supply of the talus in the case of vulneration. Fractures are divided in central and peripheral forms which could be caused either by simple or compound fracture mechanism. The computed tomography offers precious informations in the diagnosis of talus vulnerations. Besides the conservative therapy of peripheral fractures there is the operative treatment of central fracture forms. Infection, arthrosis and necrosis of the talus are to be mentioned especially as postoperative complications.

New target device for ligament grafts in the knee joint.

New instruments for cruciate ligament grafts through a rectilinear drill hole are presented. They are applicable in order to simplify the procedure performed under X-ray view or arthroscopy in the open joint as well as in the close joint. The precise fixation of the grafts at the anatomical ligament end points is the essential advantage of this method.