This article represents a systematic effort to answer the question, What are archaeology’s most important scientific challenges? Starting with a crowd-sourced query directed broadly to the professional community of archaeologists, the authors augmented, prioritized, and refined the responses during a two-day workshop focused specifically on this question. The resulting 25 “grand challenges” focus on dynamic cultural processes and the operation of coupled human and natural systems. We organize these challenges into five topics: (1) emergence, communities, and complexity; (2) resilience, persistence, transformation, and collapse; (3) movement, mobility, and migration; (4) cognition, behavior, and identity; and (5) human-environment interactions. A discussion and a brief list of references accompany each question. An important goal in identifying these challenges is to inform decisions on infrastructure investments for archaeology. Our premise is that the highest priority investments should enable us to address the most important questions. Addressing many of these challenges will require both sophisticated modeling and large-scale synthetic research that are only now becoming possible. Although new archaeological fieldwork will be essential, the greatest pay off will derive from investments that provide sophisticated research access to the explosion in systematically collected archaeological data that has occurred over the last several decades.

Commercially pure titanium type T5 and titanium type TAl alloy have been used at Stanmore in major bone and joint replacement prostheses since 1963 and 1971 respectively. Because laboratory studies indicated cast CoCrMo alloy to be a superior bearing material both titanium materials have been joined to cast CoCrMo alloy bearing elements to form composite prostheses with no apparent adverse clinical effects.

Laboratory studies and clinical experience have shown the inadequacy of the fatigue properties of titanium type T5 and cast CoCrMo alloy for highly stressed intramedullary stems. Nevertheless, although titanium type TAl alloy has superior fatigue properties highly stressed components of this material are not immune to fatigue failure.

Wear of U.H.M.W.P.E. appears to be greater when articulated with titanium type TAI alloy than it is when articulated with cast CoCrMo alloy and Al2O3 counterfaces. In addition, capsular tissue which rubs on titanium type TAl alloy can become heavily pigmented. However, to date no clinical complications appear to have resulted from either of these shortcomings. Whether a surface treatment for titanium type TAl alloy such as ion implantation will both improve its wear characteristics with U.H.M.W.P.E. and reduce the amount of metallic material liberated into the soft tissues has yet to be established.