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It is well known that meta-computation can be used to guide other computations (at the object-level), and thereby reduce the costs of these computations. However, the question arises to what extent the cost of meta-computation offsetts the gains made by object-level savings. In this paper we discuss a set of equations that model this trade-off between object-savings and meta-costs. The model shows that there are a number of important limitations on the usefulness of meta-computation, and we investigate the parameters that determine these limitations.

The outcomes of bipolar hemiarthoplasty for osteonecrosis of femoral head have been disappointing due to cartilage degeneration and osteolysis. We investigated the cartilage degeneration, joint motion, and factors associated with osteolysis.We retrospectively reviewed 134 bipolar hemiarthroplasties. The wear rate of acetabular cartilage was calculated. The ranges of motion of outer bearing and inner bearing were determined, and the ratio (O/I ratio) was calculated.The mean degeneration rate of acetabular cartilage was 0.34 ± 0.35 mm/year. We could observe that the outer bearing motion was dominant, but decreased over time. In addition, the degeneration rate of cartilage and the decline rate of outer bearing motion of the osteolysis group were significantly higher than those of the control group.Close observation is needed in cases of high degeneration rate of cartilage and rapid decline of outer bearing motion due to possibility of osteolysis.