Development of prosthetic limbs for individuals with transfemoral amputation (TFA) has advanced considerably, with the introduction of new materials, improved socket design, and increasing sophistication of prosthetic joints including feedback control systems . However, despite current developments, approximately one-quarter of individuals with TFA remain dissatisfied with their prosthesis . Socket discomfort, difficulty of attachment, control of the prosthetic limb, and increased energy expenditure during daily functional activities are factors contributing to the dissatisfaction. One of the main challenges in improving functional outcome lies in achieving a successful interface between the extrinsic components (prosthesis and attachment) and the intrinsic component (residual limb) at both the mechanical and control levels. A recent innovation that establishes a structural interface is the osseointegrated fixation (OF). Although this was originally conceived in the 1880s as described in Murphy's comprehensive account of early attachment of prostheses to the skeleton , the first successful implant was not realized until 1990 . In Europe, three types of systems are currently under development: the Intraosseous Transcutaneous Amputation Prosthesis [5-6], the Endo-Exo Femur Prosthesis [7-8] and the Osseointegrated Prosthesis for Rehabilitation of Amputees (OPRA) [4,9-10]. The OPRA has the longest history, with several published patient outcomes, and is the system that the individuals in this study had implanted [4,9-12]. This procedure is still experimental, with fewer than 150 reported cases globally [7,9]. Decreased energy expenditure, increased functional outcome, increased range of hip movement, improved seating comfort, and ease of attachment of the prosthetic limb in subjects fitted with an OF are some of the benefits that have been reported [9-13].
Future progress lies in further synthesis of the prosthesis with the residual limb, both biomechanically and through a computer-controlled system. A direct biomechanical interface is achieved through connections from muscles or tendons to an extrinsic prosthetic component, a concept presented nearly a century ago by Biesalski and more recently by Weir et al. [14-15]. The OF increases the feasibility of a continuous interface between musculotendinous tissue and the prosthesis through its internal interaction with human tissue. Microprocessor-controlled systems involve transmitting signals from sensors intrinsic to the prosthesis (e.g., accelerometers, load cells, and gyrometers) or physiological sensors (signals emanating from muscles and nerves) to a microcomputer that modifies movement of the prosthetic device [1,16]. The advantage of incorporating sensors detecting signals from nerves or muscles is that they may convey information regarding the subject's movement control strategy. Electrodes measuring surface electromyography (sEMG) represent the least invasive type of natural sensor for motor control. sEMG sensors, or myoprocessors, have been successfully incorporated in the upper limb, although not as yet in the lower limb during locomotion [17-20]. One reason why sEMG has not been successfully employed in lower-limb prostheses during locomotion is that the muscle contraction is fluctuating in intensity and duration, producing a nonstationary stochastic signal .
Greater incorporation of the residual limb with the prosthetic component requires understanding of morphology, physiology, and biomechanics of the restructured limb, in particular the altered function of muscles. A TFA produces structural remodeling and altered functional role of the muscles of the residual limb. For example, rectus femoris (RF) and biceps femoris (BF) are transformed into uniarticular muscles, no longer having an action on the knee joint. These two muscles together with adductor magnus (AM) lose their fixed insertion points and therefore lack a strong anchor point on which to exert a force. …