David Boone, CP, MPH, PhD

Almost every facet of lower limb prostheses construction has changed in the past decade. Fit and form have evolved to incorporate new materials into novel socket contours. Gel liners now cushion the skin, and new methods of suspension like electric vacuum pumps have improved the connection of the prosthesis to the body. Dynamic and lighter weight components have put a noticeable “spring” into the step of most users. Recent developments in computer-controlled components are technologically impressive, providing active motion and digital control to the knee and ankle joints. With all of the new technology being built into modern prostheses, it is not uncommon for the most advanced limbs to now cost as much as a new automobile.

With so much technological advancement, it is surprising that the techniques and instruments used by the prosthetist to optimize the critical aspect of alignment of prostheses have not changed much since the 1960s. But a gait alignment technology (ComPAS™) offering computerized, real-time gait analysis with socket reaction force measurements has recently been introduced to the industry. This device adds a multiaxial force sensor to almost any lower limb prosthesis and smart software that helps the prosthetist to see and to interpret even the smallest deviations in the dynamic balance of a prosthesis. It provides the kinetic information that one might seek in a gait laboratory, but the measurements can be made without specialized laboratories or technicians and the information is relayed instantaneously and directly to the prosthetist.


Prosthetists have long recognized that a successfully fit prosthesis results in part from successful alignment of the socket or body interface, relative to the supporting foot. It’s a matter of dynamic balance, and good alignment leads to smooth and comfortable walking, while poor alignment results in obvious gait impairment and even discomfort and tissue breakdown. It is simple in concept, but in practice it can be a difficult and frustrating practice. Even very small changes in the way in which the weight-bearing load is transferred between the prosthesis and the ground through alignment can significantly alter the long-term outcomes of the user. Because alignment affects the transmission of force to the residual limb, a prosthetist needs to understand the relationship of these forces to prosthetic alignment to fully understand how alignment is changing the function and fit of the prosthesis.

Prosthetic alignment makes up a good portion of the training of new prosthetists. It is a difficult skill to teach and to learn, perhaps taking months or years before a new prosthetist is very proficient. Alignment adjustments are traditionally based on observational gait analysis. Observations are interpreted by the prosthetist, and manual adjustments are then made to the alignment relationships of the prosthetic socket to the foot. The exact changes made at this point are based on what the prosthetist’s experience indicates are effective relationships between alignment and gait. This process is repeated iteratively until the prosthetist, and presumably the patient, agree that there is no longer a detectable or significant “improvement” to be gained through further dynamic alignment adjustments.

Yet the procedure of observational gait analysis is inconsistent because it relies on three variable factors: the prosthetist’s split second perceptions, the prosthetist’s experience, and the ability of the patient to contribute in a positive way to the process through accurate interpretation and communication of their own gait.


The newly developed gait alignment device directly addresses each of the main sources of variability in the alignment process without precluding or hindering the application of clinical skills by the prosthetist or input by the patient. Currently, there is no clinical instrument that can provide this information directly to the prosthetist in the clinic. This computerized measurement device is a breakthrough in easy-to-use, sophisticated instrumentation for clinical prosthetists. It is a system of electronic hardware and computer software that brings the promise and power of a sensitive gait laboratory to everyday prosthesis fitting. Designed from the start to fit in with standard prosthetics practice, application of the gait alignment device is as easy as assembling any standard modular prosthesis system and it is compatible with virtually all modular prostheses regardless of manufacturer.

Using this technology, the prosthetist receives consistent measurement and automated analysis of gait via user-friendly indicators. The system conveys the relationship of the patient’s gait to probable misalignment, and the primary forces influencing the patient’s perceptions of socket interface stresses and balance. The system continuously measures the dynamic forces and balance while the client is walking in virtually any chosen setting. The system software automatically extracts and interprets the relevant information from a series of steps and instantly provides prosthesis-specific gait analysis. Feedback to the prosthetist is presented in concise and easy to understand language. Instructions for adjusting the modular system alignment screws are simple and explicit. Specially designed graphs provide the prosthetist with relevant information in a manner that makes sense at a glance and with a minimum of training. The graphs represent “socket reactions” or how the socket is pushing back against the limb during the gait cycle. These socket reactions relate exactly to what the amputee feels and the prosthetist sees when dynamically aligning the prosthesis. Using this technology, the prosthetist gains input that is even more sensitive than the trained eye.

Prosthesis users may adjust their gait to maintain fairly consistent kinematics despite externally imposed changes in prosthetic alignment. Kinematics can be maintained by small adjustments to key moment arms, by changes in muscular force, and (in prosthetic instances) also by variations in socket pressures such that the net result is a consistent movement pattern. While such changes may be strongly felt by the subjects (due to substantial differences in socket reaction), they are extremely difficult to measure in the gait laboratory because the kinematic and kinetic differences approach the noise level of the measurement systems. The difficulty for gait laboratory measurements to reliably distinguish alignment deviations recently raised the question of whether computer-aided alignment is feasible since the “outcomes” of different alignments appear poorly distinguished. Similarly, gait laboratory analyses have often failed to detect expected changes in gait with different prosthetic components. By relying on direct measurements of the forces of walking with a prosthesis, the gait alignment technology detects differences between alignment conditions that may not be observable in movement patterns exhibited by the patient during a traditional observational gait analysis process.

Read “Empowering Prosthetics” by Mark McGuire, CPo, in the December 2008 issue of Rehab Management.

After an adjustment to the prosthesis, the prosthetist will find the most informative indicators of the adjustment from the first few steps taken before the patient adjusts to the change. The gait alignment technology analyzes the patient’s gait within the first four steps taken. The patient’s initial gait compensation may be abandoned over time, but it can be days or weeks before the influence of the physical change of the prosthesis might be manifested in the patient’s clinical outcomes. One important use of the gait alignment technology is to identify and document needed changes to alignment during the life span of the prosthesis. The final alignment of the prosthesis is recorded electronically for the clinical record of the patient.


There are many benefits from using instruments for prosthesis alignment. The information provided by gait alignment technology lets the prosthetist know very quickly what is happening biomechanically, removing guesswork and assumptions. Time is saved during initial alignment and later through decreased need for adjustment. Prosthesis users experience the results throughout the alignment process, and gain confidence from the increased information at the prosthetist’s fingertips. Most importantly, prosthetists may avoid the serious long-term consequences from misalignment of lower extremity prostheses.

David Boone, CP, MPH, PhD, is the cofounder and chief technology officer of OrthoCare Innovations, Seattle. For more information, contact .


Computerized Prosthesis Alignment System: ComPAS™ development was supported by SBIR Fast-Track Grant number HD047119 National Center for Medical Rehabilitation Research, and the National Center on Minority Health and Health Disparities of the National Institutes of Health.