New perspectives on bioengineering of joint tissues: joint adaptation creates a moving target for engineering replacement tissues.

The current paradigm in tissue engineering is that "full regeneration" or "total replacement" of normal tissue is required in order to restore joint function. However, there is considerable evidence that suggests that targets other than "normality" may actually be required for tissue substitutes. Sometimes "less than normal" tissue properties of substitutes may be required following an injury, and sometimes "more than normal" may be required (following tissue degradation, damage, and failure). Diarthrodial joints function as "organs" in a physiological sense and normal individual joint tissues work together to share the mechanical requirements demanded by internal and external forces. Each tissue has some genetic and biological ability to adapt and/or remodel, to accommodate to the changing biomechanical needs invoked by injury and each tissue changes with age. This dynamic genetic and environmentally driven situation affecting the (uninjured) tissues in both injured and uninjured joints suggests that there is a "moving target" for bioengineered replacement tissues. After degeneration, damage, and failure of adaptation of other joint components, the mechanical requirements of replacement tissues likely increases dramatically beyond those of their normal counterparts. These concepts have important implications to designs of tissue bioengineering experiments and to their mechanical targets.