PFFD and Orthotic/Prosthetic Intervention Monitored with Accelerometers

James A. Adderson, CP(c) Donald A. MacLeod, MSc Catherine L. McPhail, BA/BPHE Lome Leahey, FRCS(c)


Introduction

Determining the optimal orthotic / prosthetic intervention to enhance the functional ability of a child with proximal femoral focal deficiency is challenging. The determinates of gait account for the smooth sinusoidal curve of the displacement of the total body center of mass during normal walking. As the center of mass displacement increases the energy required to ambulate increases. This displacement of the center of mass can be influenced by differences in anatomy neuromotor control, and the body's ability to accommodate for the differences. Severe shortening of a lower limb often requires treatment to enhance ambulation. Non-surgical alternatives include a platform type orthosis or an extension prosthesis1. The treatment considered is dependent upon published evidence and the philosophy and experience available within an interdisciplinary team2, 3. The subject evaluated was a three year Old male with unilateral PFFD and ipsilateral absence of the fibula. The treatment options evaluated included: (1) no treatment, (2) equalizing leg length with a shoe lift (10 cm), (3) equalizing leg length with a shoe lift using a plastic AFO to stabilize the ankle, and (4) an extension prosthesis with the foot in plantar-flexion. Norlin et al (1981) showed that children under the age of ten must be compared to children of the same age because of the increase in velocity, stride length, and temporal phases with age4. Data collected for this subject is compared with five age-sex matched controls.

Procedure

The subject wore a lightweight belt that contained a triaxial packet of accelerometers and data logging computer. The accelerometer packet was located at the approximate center of mass in the lumbosacral region. After each walking trial, with the individual treatments, the data collected was downloaded and stored for analysis. The signal processing was completed using GAITVIEW™ software. A fast fourier transform determined the frequency content for the forward, lateral, and vertical directions while walking. A power spectral density analysis determined the relative power of each frequency component. A comparison of the harmonic value with the half-harmonic value evaluated the asymmetry in each plane. The signal analysis depicted the approximate center of mass movement in terms of its frequency, vertical and forward asymmetry, and lateral/ vertical, forward/vertical relative power.

Discussion

The accelerometer technique employed to monitor the various treatments has a clinical usefulness. Figure 1. provides visual data that displays the effect of treatment on the approximate center of mass.

The mechanical actions required during walking result in forward, lateral and vertical oscillations of the center of mass, which have a consistent phasic relationship5. By comparing the relative signal power of the harmonic and the half-harmonic, for the x (forward), y (lateral), and z (vertical) axis, the asymmetry could be evaluated for each plane (Figure 2. ).

Figure 3. presents the data for frequency, asymmetry, and power ratios for the four treatment options studied. The extension prosthesis closely resembled the control values for frequency, vertical and forward asymmetry, and power ratios when compared to the other three treatments during ambulating. Without treatment the frequency (correlates with velocity) was similar to the normal values. The lateral displacement is greater without treatment and is reflected in the lateral/ vertical power ratio. It was noted that without treatment the subject lowered his center of mass by walking with the contralateral limb flexed at the knee. The lift and lift with an AFO improves positioning of the knee and ankle. The power ratios are similar to the control group with this leg length equalization. However, the decreased frequency and increased forward and vertical asymmetry result in a slow asymmetric gait.

Conclusion

Monitoring the center of mass with accelerometers is clinically useful in determining the intervention that quantitatively allows for the most normal gait pattern. In this three-year-old individual with PFFD and fibular hemimelia, the extension prosthesis with a plantar-flexed ankle resembles normal gait. The extension prosthesis was the subjective choice of treatment by this patient.

Please address correspondence to:

James Adderson, C.P.
Queen Elizabeth II Health Science Center,
Halifax, Nova Scotia,
Canada

Queen Elizabeth II Health Sciences Centre, Halifax, Nova Scotia, Canada

IWK Grace Health Center for Children, Women & Families, Halifax, Nova Scotia, Canada