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Background: High friction moments in hip implants contribute to the aseptic loosening of cementless cups, of which there are approximately 100,000 cases per year; sustained joint loading may cause such high moments. The most "critical" physical activities associated with sustained joint loading were identified in this study.Methods: Friction moments in the cup were telemetrically measured about 33,000 times in the endoprostheses of 9 subjects during >1,400 different activities. The highest moments were compared with the cup's fixation stability limit of approximately 4 Nm.Results: A total of 124 different activities caused friction moments meeting or exceeding the critical limit, with the highest value of 11.5 Nm. Most involved sustained high contact forces before or during the activity. The highest peak moments (6.3 to 11.5 Nm) occurred when moving the contralateral leg during 1-legged stance, during breaststroke swimming, muscle stretching, 2-legged stance with muscle contraction, and during static 1-legged stance. The median moments were highest (3.4 to 3.9 Nm) for unstable 1-legged stance, whole-body vibration training, 2-legged stance with an unexpected push at the upper body, 1-legged stance while exercising the contralateral leg, and running after 2-legged stance.Conclusions: Frequent unloading plus simultaneous movement of the joint are required to maintain good joint lubrication and keep the friction moments low. Frequent, sustained high loads before or during an activity may cause or contribute to aseptic cup loosening. During the first months after hip arthroplasty, such activities should be avoided or reduced as much as possible. This especially applies during postoperative physiotherapy. Whether these guidelines also apply for subjects with knee implants or arthrotic hip or knee joints requires additional investigation.Clinical Relevance: The risk of aseptic cup loosening may be reduced by avoiding sustained loading of hip implants without periodic joint movement. [ABSTRACT FROM AUTHOR]

Background: Malalignment after total knee replacement could cause overloading of the implant bearing as well as of the bone itself, leading to osteolysis and early loosening. To quantify the stresses the implant has to withstand and to define a safe zone of limb alignment, the total contact forces as well as their mediolateral distribution have to be determined. Analytical gait data and mathematical models have been used for this purpose. We performed this study to determine in vivo loads of five patients after implantation of an instrumented tibial baseplate.Methods: Five patients with osteoarthritis of the knee received total knee replacement. The tibial component was instrumented with strain gauges for the measurement of three forces and three moments. The signals from the gauges were transferred telemetrically to an external receiver. At twelve months after surgery, postoperative measurements were obtained with the patients walking at a self-selected comfortable speed across a level walkway. Peak axial and medial forces of fifteen to twenty gait cycles were averaged and reported as a percent of individual body weight.Results: During the stance phase of the gait cycle, two maxima of the axial force occurred. Typical values were 215% of body weight at the first peak and 266% of body weight at the second peak. The medial load share was typically 73% at the first axial force peak and 65% at the second axial force peak. Analysis of inter-individual variations revealed a linear correlation with limb alignment. A deviation of 1° varus from neutral alignment increased the medial load share by 5%.Conclusions: Consistent with the results of previous studies, we found that the force transferred by the medial compartment was usually greater than that transferred by the lateral compartment. Concerning the design of total knee replacements, an asymmetric tibial component with a larger medial contact area could possibly reduce peak contact stress on the bone and improve fixation of the implant. Mediolateral load distribution was quantified and correlated with limb alignment, thereby permitting the effects of malalignment after total knee replacement to be estimated. [ABSTRACT FROM AUTHOR]

The article investigates the effect of different arm positions and the shape of the thoracic spine on spinal loads for sitting and standing. Result of the study indicates a relationship between sit-to-stand load ratio and kyphosis angles. It concludes that spinal load differences depend on arm position and individual spinal shape.

This Editorial on Julius Wolff corresponds to a translation of the historic text, Zur Lehre von der Fracturenheilung, available at DOI 10.1007/s11999-010-1240-9 ; and a translation and abridgement of the historic text, Ueber die Innere Architectur der Knochen und ihre Bedeutung für die Frage vom Knochenwachstum, available at DOI 10.1007/s11999-010-1239-2 . (Supplemental materials are available with the online version of CORR.) An accompanying biographical sketch is available at DOI 10.1007/s11999-010-1258-z . [ABSTRACT FROM AUTHOR]

Study Design: The mechanical behavior of the lumbar spine after insertion of a ProDisc prosthesis was studied using 3-dimensional nonlinear finite element models., Objective: To determine how the mechanical behavior of the lumbar spine is affected by the implant position and height, as well as by removing different portions of the natural disc and resuturing the anterior longitudinal ligament (ALL)., Summary of Background Data: Little is known about how the affected and adjacent levels of the spine are influenced by the implant height and position or by the surgical procedure., Methods: The artificial disc ProDisc was integrated in a validated 3-dimensional, nonlinear, finite element model of the lumbar spine. The model was loaded with the upper body weight and muscle forces to simulate standing, 30 degrees flexion, 15 degrees extension, and 6 degrees axial rotation. The disc position was varied by up to 2 mm in both an anterior and posterior direction. Three different disc heights were investigated, as well as the influence of removing different portions of the natural disc and resuturing the ALL., Results: Implant position strongly influences intersegmental rotation for the loading cases of standing and flexion. A disc height 2 mm in excess of the normal disc space increases intersegmental rotation at implant level during standing and extension. The values for intersegmental rotation are closer to those for the intact spine when lateral portions of the anulus are not removed. Resuturing the ALL has a strong effect on the loading cases of extension and standing., Conclusions: When implanting an artificial disc, great care should be taken in choosing the optimal height and correct position for the implant. Lateral portions of the anulus should be preserved whenever possible. A perfect reconstruction of the ALL would help to restore the biomechanics to normal.