Musculo-skeletal adaptation and altered loading environments: An amputee model

Abstract

Amputation of the lower limb may result in musculo-skeletal changes similar to those that occur following space flight, immobilisation and prolonged bed rest. The similarities desist when one considers the invasive nature of amputation surgery and the impact that partial loss of a limb has on the loading characteristics of the affected leg. The aim of this study was to determine the musculo-skeletal changes that occur following trans-femoral and trans-tibial amputation, and to compare differences in the musculo-skeletal characteristics of these groups, which may occur as a function of the modified loading environment. Unique to this investigation was the study of a new trans-femoral amputee, which was incorporated to investigate the time course of any changes in muscle and bone atrophy and decreases in muscle strength in the early post-operative period. This study was also designed to provide a comparison with longer-term amputees and examine relationships between muscle morphology and strength and identified changes in gait behaviour by reference to normal gait patterns. Eight unilateral trans-femoral and 8 trans-tibial amputees (mean age 35.2yrs. ± 9.8 and 35.3yrs. ± 8.9 respectively) were subjects in the study. There were 7 males and 1 female in each of the amputee groups. A control group of similar number was used, with subjects matched on age, weight, height and gender. In the first phase of the investigation dual energy x-ray absorptiometry and magnetic resonance imaging was used to measure bone mineral density (B:MD) of the lumbar spine (L2-IA) and femoral neck (FN) and to calculate the volume and cross-sectional area (CSA) of selected muscles. Strength evaluation was assessed by measurement of maximal isometric hip torque using a Kin-Com dynamometer. Gait analyses were undertaken to determine differences in the angular kinematics of the residual and sound limb together with an electromyographic (EMG) assessment of the onset and offset of the activity of 4 hip muscles of the residual and sound thigh, which was synchronised with the kinematic measures. Differences in ground reaction force (GRF) between the residual and sound limb of each group were also examined. A single case study involving a 19-year-old motor accident victim who sustained a traumatic trans-femoral amputation of his right leg was conducted to determine the structural and functional changes over a 9-month period. Structural and functional evaluations were repeated every 3 months, beginning at 4 months post amputation, using similar methodologies and procedures described for the longer-term amputees. In the longer-term amputees the volume and CSA of the residual musculature of the trans-femoral group was significantly lower by comparison with the sound limb and no difference was found between the residual and sound musculature of the trans-tibial group. Mean torque of the residual hip was lower than that of the sound hip of the trans-femoral and trans-tibial group but the difference was not significant. Bone mineral density of the residual FN was significantly lower than that of the sound FN of the trans-femoral group but there was no difference in the trans-tibial group. No differences were found between the two experimental groups and controls at the L2-1A site. Muscle volume was significantly correlated with BMD of the L2- 1A vertebrae in the residual and sound limb of the trans-tibial group. Trans-femoral and trans-tibial amputees had a significantly slower walking velocity than that of the control group. Cadence of the trans-femoral group was significantly lower than both trans-tibial and the control groups. Stride length was not significantly different between the trans-femoral and control group but was significantly lower in the trans-tibial group. Significant differences were found between the trans-femoral and control group in the range of ankle and knee motion of the sound limb and between the sound and residual ankle, knee and hip joints of the trans-femoral and trans-tibial group. Mean GRF was lower in the residual limb compared to the sound limb for both groups, although the differences were not significant. The activity of rectus femoris, biceps femoris and adductor longus in the sound limb of the trans-femoral group were generally active for a longer duration compared to the controls and the duration of activity of these same muscles differed between the residual and sound limb musculature of the trans-tibial group. In the case study subject, BMD in the residual FN was 38.4 per cent lower than the sound FN at 4 months, decreasing to 42.1 per cent at the end of the 9-month evaluation. At this time point bone loss of the case study subject was greater than the average difference between the residual and sound limb of the longer-term amputees suggesting some recovery of bone mass may be possible. Rectus femoris and biceps femoris showed greater atrophy than the intact muscles, psoas major, adductor longus and the gluteals. At 7-months post amputation, hip torque of the residual limb in all planes of movement was lower by comparison with the sound limb. There was considerable intra-group variability in the data, which reflected the heterogeneity of the groups with respect to surgical fixation procedures, types of prosthesis used and their different physical activity levels. It was shown that longerterm trans-femoral amputees experienced considerable muscle and bone atrophy of their residual limb, which was greater than that experienced by the trans-tibial group. Although loading was not measured directly the difference between the two groups of amputees perhaps reflected their altered loading environment. Isometric hip torque was not different between the residual and sound limb of the trans-femoral and trans-tibial group, an unexpected result in the trans-femoral group considering the muscle atrophy present. The morphological changes combined with the prosthetic components were likely responsible for differences in amputee gait function. In the more recent amputee, muscle and bone atrophy was most rapid in the first 4 months but the volume and CSA of rectus femoris and biceps femoris continued to decrease up to 13 months post amputation. Isometric torque of the hip flexors and extensors decreased between 7 and 10 months and stabilised by 13 months post amputation while there was no change in the torque of the hip abductors and adductors from the initial measure. The potential for recovery of BMD, muscle size and muscle strength must be considered and may be applied to the design of more effective prostheses and rehabilitation strategies aimed at improving functional outcomes