DPM: Direct Prosthetic Measurement

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By Edward S. Neumann, PhD, PE, CP, FAAOP
prosthetic foot/ankle graphic

DPM Defined

Direct prosthetic measurement (DPM) is the direct measurement of the forces and moments relevant to prosthetic fitting in a clinic.1,2 It is not based on indirect gait laboratory measurements, which rely on inverse dynamics for an estimate of the forces and moments, and should be simple enough for a practitioner to undertake in a clinical setting; it measures directly the forces and moments experienced by an individual as he or she dons and uses a prosthesis for activities of daily living. DPM may be accomplished by a load cell, sensors, or similar devices placed under or within the socket that measure the causes of intrasocket pressures and the propulsive force that the prosthesis user is able to generate.

Many of the current microprocessor knees incorporate some aspect of DPM to measure moment at the knee, though the actual values of the moment may not be made explicit to the practitioner. But the moment must be sensed by the component for it to function correctly, and the practitioner is expected to calibrate the component to the needs of the individual based on processed DPMs.

To be useful to clinicians, as well as reimbursement entities, DPM should combine the universal principles of science with the unique aspects of the individual's rehabilitation needs. It should also be inexpensive and easily used. The science behind DPM needs to be as good as or better than that produced by conventional gait lab studies if DPM is to improve rehabilitation and/or reduce its costs. Continued development of smallscale DPM instrumentation that fit components and residual limbs of different lengths and the development of the science that underlies DPM are important. Practitioners should have gained the requisite understanding of forces and the moments produced by them in physics courses or other courses taken and required for certification; no or very minimal new knowledge should be necessary for certified practitioners to apply DPM concepts. DPM does require thinking in 3D-a thought process that should come to practitioners easily-as forces and moments are measured in the sagittal, frontal, and transverse planes, which are already expressed as 3D representations.

Rationale for Using a DPM Approach to Prosthesis Design

Although a practitioner may guess intuitively at many of the causes of intrasocket pressure and propulsion, the O&P world awaits measurement of them and the development of their scientific basis. For many years, socket designs have advanced and new designs have been conceived that reduce pressures and improve comfort for individuals. No area of prosthesis design has better recognized the individual needs of each person with an amputation than socket design. Gel liners and negative pressure sockets are examples of this evolution in socket design, often undertaken without the benefit of clinical measurement. But outcomes assessment and outcomes measurement are needed for nearly all the prosthesis users being seen by practitioners in clinical settings. Reimbursement entities expect outcomes to be measured.

The application of the results of traditional laboratory experiments may be difficult for practitioners to apply in a clinical setting and their relevance to the rehabilitation needs of the individual patients they fit may not be easily observed. Not every person with an amputation responds to rehabilitation protocols the way that is expected and the way practitioners would prefer. Each person and his or her rehabilitation are, in many ways, unique, and this uniqueness must be recognized for the rehabilitation process to be successful.

DPM techniques may be highly relevant to the unique needs of the individual, but must be based on science, clinic-friendly, and easily applied to all individuals with amputations. The science behind DPM techniques is being researched, along with an improved understanding of the psychological needs of practitioners for design and outcomes measurement. But additional research needs to be undertaken for DPM to become prevalent among clinicians and used routinely by them, and O&P research funding entities need to recognize this. Due to the recent evolution in microprocessors, DPM can be embedded into future components, possibly in a miniaturized form.

DPM may change the way funding entities view rehabilitation. DPM research may demonstrate people with amputations progressing at individual rates through a series of increased residual limb loadings, with each stage increasing the load and the range that can be tolerated by the residual limb. For example, among individuals with transtibial amputations, studies under way show that loading of the heel and/or the forefoot of a prosthesis may relate to adaptation, with new prosthesis users possibly keeping loads close to the pylon initially and then further away from the pylon as rehabilitation progresses. Any loading of a transtibial prosthesis will require acceptance of increased pressure distally on the residual limb and lead to increased loading proximally as the load goes further out onto the toe of the prosthetic foot, where force reaches its maximum as rollover followed by propulsion occurs.3 Foot design may be important.4 Research shows that for people with transtibial amputations and a traditional socket design, heel loading creates pressure on the distal tibia, and toe loading creates pressure on the patella tendon region.5 It may be that people with more recent amputations are reluctant to load these regions of the residual limb, which may influence rehabilitation progress. DPM seeks to determine how prosthesis design relates to gait and residual limb impact, and whether residual limb loading progresses with training and experience. Similar research has yet to be carried out with people who have transfemoral amputations using conventional socket designs, though DPM has been used extensively with individuals who have transfemoral amputations and have undergone osseointegration.6-12

In engineering, and in many other science-based fields, measurement is essential to understanding and design. The application of abductive logic, which offers the potential to tie together the science of prosthetics with the unique characteristics of the individual, may be highly relevant to prosthetics design and fitting.13 Measurement and a scientific approach to prosthesis design may facilitate improved systems for classifying rehabilitation patients, or perhaps revolutionizing existing classification systems entirely. Also, utilization of the foot and ankle mechanisms currently available need to be better understood. DPM may help with understanding rehabilitation and the role played by foot and ankle mechanisms in socket pressure and gait. In addition, 3D printing techniques may help to design feet and sockets that best meet the rehabilitation needs of individuals, and do so more inexpensively than currently used methods and components.

Examples of DPM Research

As stated previously, DPM methods have been used extensively in the research of patients with transfemoral amputations who have undergone osseointegration, especially to examine rehabilitation needs and to design femoral implants. DPM has also been used to determine moments resulting from alignment perturbations in individuals with knee disarticulations.14 It has been used with patients who have transtibial amputations to determine preferred alignments, and how patient propulsive forces and moments respond to different feet in different types of environments and for different activities.15-19 Sagittal plane forces directed along the pylon and the moment created by them appear to be relevant to gait, as do transverse and frontal plane forces and moments. DPM has been used in microprocessor knees to set knee parameters related to the moments generated as the prosthesis user walks. What remains to be determined through research is exactly how the forces and moments experienced and output by the residual limb via the socket are related to prosthesis use and gait.

Getting Started

There are numerous articles on DPM, as cited herein, which reveal capabilities and methods, and firms involved in DPM. Load cells that fit prostheses and measure propulsive forces and moments are available from JR3, Woodland, California; College Park Industries, Warren, Michigan (iPecs); and Orthocare Innovations, Mountlake Terrace, Washington (smart pyramid). Costs for some load cell systems may be considerably less than $10,000,and all measurements can be taken in clinical settings. Socket pressure measurement systems start with the systems produced by Tekscan, Boston.

Conclusions

DPM is relatively new, and research and development are required to make implementation in the clinic easy and relatively inexpensive. Educational needs for understanding and using DPM may not exceed current requirements to become certified and to enter clinical practice as a practitioner. Intrasocket pressure measurement and gait-related issues of prosthesis design require more study and the publication of results. DPM promises to revolutionize the way in which prostheses are designed, fabricated, and fitted. It may also change current reimbursement policies, and appears to fall within the scope of outcomes assessment.

Edward S. Neumann, PhD, PE, CP, FAAOP, is professor emeritus at the University of Nevada, Las Vegas, and can be reached at .

References

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  2. Dumas, R., L. Cheze, and L. Frossard. 2009. Loading applied on prosthetic knee of transfemoral amputee: Comparison of inverse dynamics and direct measurements. Gait & Posture 30 (4):560-2.
  3. Neumann, E. S., J. Brink, K. Yalamanchili, and J. S. Lee. 2012. Load cells enhance study of prosthetic foot rollover. Lower Extremity Review (August):29-36.
  4. Neumann, E. S., K. Yalamanchili, J. Brink, and J. Lee. 2012. Transducer-based comparisons of the prosthetic feet used by transtibial amputees for different walking activities: A pilot study. Prosthetics and Orthotics International. 36 (2):203-16.
  5. Neumann, E. S., J. Brink, K. Yalamanchili, and J. Lee. 2013. Regression estimates of pressure on transtibial residual limbs using load cell measurements of the forces and moments occurring at the base of the socket. Journal of Prosthetics and Orthotics 25 (1):1-12.
  6. Lee, W. C., L. A. Frossard, K. Hagberg, E. Häggström, D. L. Gow, S. Gray, and R. Brånemark. 2008. Magnitude and variability of loading on the osseointegrated implant of transfemoral amputees during walking. Medical Engineering & Physics 30 (7):825-33.
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  12. Frossard, L., N. Stevenson, J. Sullivan, M. Uden, and M. Pearcy. 2011. Categorization of activities of daily living of lower limb amputees during short-term use of a portable kinetic recording system: A preliminary study. Journal of Prosthetics and Orthotics 23 (1):2-11.
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  14. Kobayashi, T., M. S. Orendurff, and D. A. Boone. 2013. Effect of alignment changes on socket reaction moments during gait in transfemoral and knee-disarticulation prostheses: Case series. Journal of Biomechanics 46 (14):2539-45.
  15. Kobayashi, T., M. S. Orendurff, and D. A. Boone. 2014. Dynamic alignment of transtibial prostheses through visualization of socket reaction moments. Prosthetics and Orthotics International 39 (6):512-6.
  16. Kobayashi, T., M. S. Orendurff, M. Zhang, and D. A. Boone. 2012. Effect of transtibial prosthesis alignment changes on out-of-plane socket reaction moments during walking in amputees. Journal of Biomechanics 45 (15):2603-9.
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  19. Neumann, E. S., J. Brink, K. Yalamanchili, and J. Lee. 2012. Use of a load cell and force-moment analysis to examine transtibial prosthesis foot rollover kinetics for anterior-posterior alignment perturbations. Journal of Prosthetics and Orthotics. 24 (4):160-74.