Abstract
The energy expended during walking by sixteen children who had proximal femoral focal deficiency was measured to compare the outcomes after Syme amputation (seven subjects) with those after Van Nes rotational osteotomy (nine subjects). Multivariate regression analysis revealed that the subjects who had had a Van Nes procedure had a mean oxygen cost (energy per unit of body mass expended per distance walked) that was 0.12 milliliter per kilogram of body mass per meter lower than that of the subjects who had had a Syme amputation (p = 0.001). The subjects who had had a Van Nes procedure tended to walk faster (p = 0.07). A significant decrease in the oxygen cost as a function of increasing age was observed for both groups (p < 0.0001, r2 = 0.79). We believe that the reduced energy expenditure associated with the Van Nes rotational osteotomy is one of several factors to consider when deciding which operation should be done for children who have proximal femoral focal deficiency.
Proximal femoral focal deficiency is a congenital limb deficiency involving the proximal end of the femur and the hip joint. Associated clinical problems may include instability of the hip, abduction contractures, malrotation of the femur, weak proximal muscles, formation of a pseudarthrosis, hypoplasia of the lateral femoral condyle, and paraxial fibular hemimelia with related deformities. Although some patients choose to be managed with a modified above-the-knee prosthesis with the socket fitted around the foot, operative intervention is usually recommended. The current options include a knee arthrodesis combined with either a Syme or a Boyd amputation, with subsequent fitting of an above-the-knee prosthesis, or a Van Nes rotational osteotomy for preservation of the ankle joint as a substitute for the knee joint, with subsequent fitting of a below-the-knee prosthesis.
It is often presumed that an individual who has a Van Nes rotational osteotomy for proximal femoral focal deficiency walks more like a person who has had a below-the-knee amputation and has a more normal gait pattern, whereas an individual who has had a Syme amputation for proximal femoral focal deficiency walks more like a person who has had an above-the-knee amputation. The use of the Van Nes procedure is controversial, however, because of the poor cosmetic appearance (the foot faces backward), the potential need for additional procedures to correct subsequent derotation during growth, the difficulty in obtaining a good prosthetic fit, and the lack of objective data to support the proposed mechanical advantage. Thus, the treatment of proximal femoral focal deficiency remains complex and difficult for the patient, the family, and the clinician4.
The assessment of energy expenditure is one method of quantifying function during walking. Results have been reported for individuals who have had an amputation of a lower limb5,6,12,15 and for those who have had a Van Nes rotational osteotomy after resection of a tumor2,7. Research has shown that adults who have had a below-the-knee amputation or a Van Nes rotational osteotomy after resection of a tumor expend less energy during walking than those who have had an above-the-knee amputation15; however, these results cannot be applied directly to children who have proximal femoral focal deficiency because of the other clinical problems associated with this condition, particularly pathological processes about the hip that may also affect function. Moreover, energy expenditure has not been compared between groups of children who have had different types of amputations.
We are unaware of any published reports that have quantified energy expenditure during walking by subjects who had proximal femoral focal deficiency. Thus, the purpose of the current study was to compare energy expenditure during walking by children who had had a Syme amputation for proximal femoral focal deficiency with that by children who had had a Van Nes rotational osteotomy for the same condition.
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Funds were received in total or partial support of the research or clinical study presented in this article. The funding sources were Grant 15956 from the Shriners Hospitals for Crippled Children and the Alberta Heritage Foundation for Medical Research (R. Z.).
†Department of Orthopaedic Surgery, University of California at Los Angeles School of Medicine, Los Angeles, California 90095-6902.
‡Department of Surgery, University of Calgary, 3330 Hospital Drive, N.W., Calgary, Alberta T2N 4N1, Canada.
§Child Amputee Prosthetics Project, Shriners Hospital, 3160 Geneva Street, Los Angeles, California 90020-1199.
Subjects
Subjects who had proximal femoral focal deficiency were recruited from the Child Amputee Prosthetic Projects at the University of California at Los Angeles Medical Center and the Shriners Hospital in Los Angeles. Subjects were included in the study if they had unilateral involvement, were between seven and eighteen years old, had had a minimum interval of two years since the latest operative procedure, were able to walk without assistive devices, and had a prosthesis that fit and functioned effectively as determined by an initial evaluation. Radiographs of the lower limb were evaluated, and the Aitken classification (class A, B, C, or D) was determined for each patient. Written informed consent was obtained from all subjects and a parent or legal guardian before the patient participated in the study.
Collection and Analysis of Data
Data on energy expenditure were obtained in the Pulmonary Function Laboratory at the University of California at Los Angeles Medical Center with use of standard indirect calorimetry measurement. A standard metabolic cart interfacing with computer software (Energy Expenditure Unit 2900; Sensormedics, Yorba Linda, California) and a standard treadmill (Quinton Instruments, Seattle, Washington) were used. Before testing was performed, the subjects fasted for a minimum of four hours. Each patient was then fitted with a Hans Rudolph low-resistance, non-rebreathing valve for collection of expired air. An index-finger pulse oximeter was used in order to monitor the heart rate.
Initially, each patient sat for ten minutes to adapt to the breathing apparatus and the oximeter. The patient next walked at a slow speed (twenty-two meters per minute) for ten minutes to practice walking on a motorized treadmill. A physical therapist was present to provide standby assistance and cuing as required for safety and achievement of a comfortable gait pattern. After a fifteen-minute rest period, walking was resumed and the speed of the treadmill was adjusted to the level at which the patient thought that he or she would be most comfortable walking for ten minutes. This was achieved by increasing and decreasing the speed according to the patient's feedback. After the patient was walking with a consistent gait pattern and with constant heart and respiratory rates (steady state), data were collected during a ten-minute period.
Gas volumes were corrected to standard temperature, pressure, and saturation units. The heart rate and the preferred walking speed were recorded. The outcome measures calculated on the basis of the data on oxygen consumption included the oxygen uptake rate (milliliters per kilogram of body mass per minute) and the oxygen cost (milliliters per kilogram of body mass per meter). The oxygen uptake rate is the amount of energy per unit of body mass that is expended per minute. The oxygen cost is the amount of energy per unit of body mass that is expended per distance walked, and it is derived by dividing the oxygen uptake rate by the walking speed.
All experimental procedures were approved by the University of California at Los Angeles Human Subject Protection Committee and the Review Board for Human Subjects, Shriners Hospital, Tampa, Florida.
Statistical Comparisons
To test the statistical difference between the outcomes after Van Nes rotational osteotomy and Syme amputation, both the subject's age and the Aitken classification of the proximal femoral focal deficiency were considered. A multivariate linear regression analysis was performed, with the independent variables being the type of operation (a categorical variable), the classification1 of the proximal femoral focal deficiency (a categorical variable), and the subject's age (a continuous variable). The primary focus of the analysis was to relate the subject's age and the type of operation to the oxygen cost during walking. Dependent variables (the speed of the treadmill, the oxygen uptake rate, and the heart rate) were also evaluated.
Subjects
The mean ages of the subjects who had had a Syme amputation and those who had had a Van Nes procedure were similar at the time of the study (Table I). All subjects who had had a Van Nes procedure wore an exoskeletal prosthesis with a corset-style attachment at the thigh, a hinged knee, and a socket for the foot. All subjects who had had a Syme amputation had a constant-friction, single-axis prosthetic knee joint. Younger and more active subjects who had had a Syme amputation typically had an exoskeletal prosthesis with a polyester shank segment for durability, while an endoskeletal prosthesis with a soft foam cover was preferred by older subjects and by girls for cosmetic reasons. None of the subjects used an energy-storing prosthetic foot.
All classifications1 of proximal femoral focal deficiency (classes A through D) were represented in each group. At the time of the data collection, three subjects who had had a Van Nes procedure had some degree of derotation of the distal part of the limb. These subjects used the subtalar joint rather than the ankle joint to actuate the prosthesis.
Energy Expenditure
A significant decrease in oxygen cost was associated with increasing age for the series as a whole (p = 0.0001). The mean oxygen cost (and standard deviation) was 0.12 ± 0.03 milliliter per kilogram of body mass per meter lower for the subjects who had had a Van Nes procedure than for those who had had a Syme amputation at similar ages (Table I and Fig. 1); this difference was significant (p = 0.001). The r2 value for the regression equation was 0.79. The distance of each data point from the best-fit line (residual error) was fairly small, indicating that this regression was a good fit with the data.
The subjects who had had a Van Nes procedure and who used the subtalar joint rather than the ankle joint to actuate the prosthesis were among the younger subjects; therefore, it was important to determine the effect of this group on the regression analysis. The analysis was repeated after these subjects had been excluded, but the over-all results were unchanged. The oxygen cost continued to decrease as a function of increasing age (p = 0.001) and was lower in the subjects who had had a Van Nes procedure (p = 0.003).
Walking speed was related to age (p = 0.0001), with younger subjects preferring to walk more slowly. There was a suggestion of a relationship between the walking speed and the type of operative procedure (p = 0.07), with subjects who had had a Syme amputation preferring to walk more slowly (Fig. 2).
The oxygen uptake rate was related to age (p = 0.002), with older subjects having higher rates, but not to the type of operative procedure (p = 0.15) (Fig. 3). The heart rate was not related to age (p = 0.41) or to the type of procedure (p = 0.24). The sample size was too small for us to evaluate the relationship between each classification of proximal femoral focal deficiency and the oxygen cost.
In summary, when the subjects walked at their preferred speed, the energy expended for a given distance was significantly greater for those who had had a Syme amputation than for those who had had a Van Nes rotational osteotomy. In addition, the advantage in metabolic cost associated with the Van Nes procedure was apparently maintained even when postoperative derotation had occurred and the patient used the subtalar joint rather than the ankle joint to actuate the knee of the prosthetic limb.
Our data support previous findings of a higher oxygen cost for walking for younger subjects10,12-14. Waters et al.14 reported significant differences in oxygen cost among children (six to twelve years old), teenagers (thirteen to nineteen years old), and adults (twenty to fifty-nine years old) (p = 0.05), with younger children having the highest and adults, the lowest values (Table II). Those authors13 also demonstrated that oxygen cost decreases linearly for subjects between the ages of six and twenty years. We found that oxygen cost decreased linearly for subjects who had proximal femoral focal deficiency and were between the ages of seven and seventeen years. The mean oxygen cost for our subjects between the ages of thirteen and seventeen years was 51 per cent higher for those who had had a Van Nes procedure and 100 per cent higher for those who had had a Syme amputation as compared with the mean for the unaffected teenagers that was reported by Waters et al.14 (Tables I and II). The mean oxygen cost for our subjects between the ages of six and twelve was 83 per cent higher for those who had had a Van Nes procedure and 136 per cent higher for those who had had a Syme amputation as compared with the mean for the unaffected children in the study of Waters et al.14.
The walking speed of our subjects increased as a function of increasing age, as mentioned previously (Tables I and II). Subjects between the ages of six and twelve years old who had had a Van Nes rotational osteotomy walked at higher speeds than those who had had a Syme amputation, but both groups of subjects walked at speeds that were less than half that of unaffected children14 in this age-group. In the age-group of thirteen to nineteen years, subjects who had had a Van Nes procedure and those who had had a Syme amputation walked at speeds that were 78 and 64 per cent that of unaffected teenagers14, respectively. In the present study, the oxygen uptake rate increased rather than decreased as a function of increasing age (Fig. 3), as the younger children chose to walk at much slower speeds (Fig. 2) and therefore required fewer breaths per minute. This may have been due to their having less confidence when walking on the treadmill. The oxygen uptake rate was higher in the subjects who had had a Syme amputation. Both groups had higher oxygen uptake rates than those reported for unaffected subjects of similar ages (Tables I and II). The heart rate while walking at the preferred speed was higher for all of our subjects between the ages of thirteen and nineteen years than for unaffected subjects14 in a similar age-group. Subjects between the ages of six and twelve years who had had a Syme amputation had a mean heart rate that was similar to that for unaffected children14; the heart rate was lowest in the subjects who had had a Van Nes procedure. There was, however, intersubject variation; for example, one patient (Case 8) walked at a speed that was greater than the mean reported for unaffected teenagers14. The associated oxygen uptake rate and the heart rate were also higher than those for unaffected subjects14 in a similar age-group.
Energy expenditure by subjects who have proximal femoral focal deficiency has not been assessed, to our knowledge, so only limited direct comparisons between our results and those of others are possible. Waters et al.15 showed that the preferred walking speed of adults who had had a traumatic or operative amputation decreased as the level of amputation became progressively more proximal (from distal to the knee to disarticulation at the knee to proximal to the knee to disarticulation at the hip to hemipelvectomy). Those authors15 found that individuals who had had a more proximal amputation purposefully adapted to their inefficient gait by walking at lower speeds. At each more proximal level, the amount of energy expended per meter (oxygen cost) was greater but the amount of energy expended per minute (oxygen uptake rate) was not. Thus, subjects who had had a more proximal level of amputation used the same number of calories but walked shorter distances in the same amount of time.
The closest comparative data are from studies of function after Van Nes rotational osteotomy as a means of limb salvage after resection of a tumor2,3,7,8,11. However, children who have proximal femoral focal deficiency would be expected to have greater energy expenditure because of the associated abnormality of the hip joint. Energy expenditure during walking was assessed by Cammisa et al. for eight subjects who had had a Van Nes rotational osteotomy and for sixty-three subjects who had had an above-the-knee amputation because of a malignant tumor of the distal end of the femur (Table II). Although those authors believed that the subjects who had had a Van Nes procedure performed better (that is, were able to walk for longer without tiring), with the numbers available for study no significant difference in oxygen cost was found between the two groups. This is not surprising, as the group that had had a Van Nes procedure was composed almost entirely of children and the group that had had an above-the-knee amputation was composed primarily of adults. Cammisa et al. did not consider the effect of age on energy expenditure in their statistical analysis and, as discussed earlier, a significant difference between children and adults has been demonstrated14. The subjects who had had a Van Nes procedure in the study by Cammisa et al. had a mean oxygen cost of 0.26 ± 0.05 milliliter per kilogram of body mass per meter and a mean age of fifteen years; these values are consistent with our findings for subjects who had had a Van Nes procedure (Fig. 1). A more precise comparison cannot be made, as data for individual subjects were not reported by Cammisa et al.
McClenaghan et al. studied energy expenditure by eight subjects who had had a Van Nes rotationplasty and by four who had had an above-the-knee amputation for the treatment of a malignant tumor of the distal end of the femur or the proximal end of the tibia. Those authors reported that rotationplasty resulted in greater walking efficiency because of the lower mean oxygen cost (Table II), but they did not test these data statistically. Again, as in the study by Cammisa et al., the subjects who had had a Van Nes procedure were younger, but age was not considered in the analysis. When data for their individual subjects are compared with those for our subjects, the younger children in their study had lower oxygen costs, whereas the older children had values similar to those in our study.
With the number of subjects included in our study, we found no significant differences between those who had had a Syme amputation and those who had had a Van Nes procedure when oxygen uptake rate, heart rate, and walking speed were used as outcome measures. Similar to adults after an amputation, many of our subjects chose to conserve energy by walking more slowly than able-bodied children of similar ages, which reduced the amount of energy expended per minute. In contrast, one of our subjects (Case 8) preferred a high speed on the treadmill, which resulted in a higher oxygen uptake rate, but the associated oxygen cost was relatively low (Table I). Because oxygen cost is calculated by normalizing the oxygen uptake rate to the walking speed, this measure eliminates factors that may affect walking speed, such as motivation, experience, or comfort. Some subjects appeared to enjoy walking on the treadmill. Others needed more reassurance, disliked breathing through the mouthpiece, and were more cautious about increasing their walking speed.
Rose et al. reported that heart rate is an adequate measure of energy expenditure by children. Those investigators asked subjects to walk as fast as possible and found a positive relationship between the heart rate and the oxygen uptake rate in normal children and in those who had cerebral palsy. The only measure that was sensitive to the differences between these two groups of children was maximum speed on the treadmill. The calculation of oxygen cost appears to be the best method of quantifying energy expenditure when assessing the difference between groups of subjects, as the data are normalized to both weight and walking speed.
One criticism of the Van Nes rotational osteotomy has been that subsequent derotation during growth may necessitate operative intervention. While an additional operation is usually necessary to obtain optimum function biomechanically, we found that subjects who had derotation expended less energy during walking than those who had had a Syme amputation. The difference between the two study groups was unchanged when the three subjects who had had tibial derotation after a Van Nes procedure were excluded from the statistical analysis. We observed fewer compensatory maneuvers by the subjects who had had a Van Nes procedure, including those who had had derotation. In particular, there was a decreased prevalence of vaulting of the contralateral limb, implying that there was better flexion of the prosthetic knee during swing. The subjects who had had a Syme amputation had a gait that was more characteristic of that associated with an above-the-knee prosthesis—that is, there was considerable vaulting.
In summary, we found that the Van Nes rotational osteotomy resulted in less energy expenditure during walking by children who had proximal femoral focal deficiency. Although our study focused on young patients, minimization of energy expenditure may actually have greater functional implications when these subjects become older adults. While energy expenditure is an important factor when clinical decisions are made for children who have proximal femoral focal deficiency, psychosocial factors and the need for subsequent operations also must be considered for each patient and his or her family. Cosmetic appearance will undoubtedly continue to be a major factor in any decision.
NOTE: The authors thank Fred Dorey, Ph.D., for his statistical expertise.
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