BIODYNAMICS OF HIP FRACTURES IN ELDERLY FEMALES

L. Cabell, R. Shapiro, D. Pienkowski, R. Stine
Biodynamics Laboratory, University of Kentucky,
Lexington, KY 40506-0070
Division of Orthopaedic Surgery,
University of Kentucky, Lexington, KY 40536-0284

Presented at the 20th Annual Meeting of the American Society of Biomechanics
Atlanta, Georgia. October 17-19, 1996

INTRODUCTION

Falling is a common method of fracturing the hip, however, biomechanical and bone material studies have been unable to completely explain the mechanism of hip fracture and its relationship to falling in elderly females (Cummings, et al, 1985). It may be theorized that age related biodynamic changes of the lower extremity causing muscle generated overloading of the proximal femur could predispose the hip to fracture as a result of a single event. This study investigated the differences in joint angular displacement, moment, powers and muscle activity as a function of subjects' age. Age related differences may suggest the factors contributing significantly to hip fracture. Our preliminary data showed statistically significant differences in some of the biodynamic variables and also demonstrated some other interesting trends between the young and elderly women.

REVIEW AND THEORY

America's 250,000 hip fractures in post-menopausal women highly correlates to 50,000 deaths and cost approximately $7 billion each year (Phillips et al., 1988). Hayes et al. (1993) claimed that the mechanics of the fall rather than osteoporosis may well dominate the occurrence of the hip fractures. Other studies suggested that abnormal muscle coordination, or other internally generated mechanical forces, can overload the osteoporotic femur and cause fracture from falling (Klyver et al., 1995) or prior to falling (Yang et al, 1995). Repetitive excessive loading due to abnormal muscle activation patterns can, over a period of time, predispose the proximal femur to a spontaneous or low energy fracture from cumulative unrepaired fatigue damage (Cummings et al., 1985). In a review of studies of elderly gait, Winter (1991) reported the elderly have reduced mechanical power absorption at the knee and less hamstrings activity than a group of younger subjects. However, he did not characterize his subjects by gender.

The purpose of this study was to determine whether biomechanical and neuromuscular changes, associated with aging can change lower extremity loading patterns in a manner which predisposes elderly females to spontaneous, low energy hip fracture.

PROCEDURES

Six healthy female subjects volunteered to participate in this investigation and were divided into two groups:

Young (age 23.7 ( 0.6 yr., height 1.75 ( 0.08 m, mass 58.2 ( 7.3 kg, n = 3).

Elderly (age 66.4 ( 8.2 yr., height 1.57 ( 0.04 m, mass 51.8 ( 4.7 kg, n = 3).

Each subject was instrumented with Ag/Ag-Cl surface electrodes on the right side only. Muscles instrumented were: Gluteus medius, Adductor longus, Tensor fascia latae, Sartorius, Rectus femoris, Vastus lateralis, Lateral hamstrings, and Medial hamstrings. Data from EMG, synchronized with motion and force platform data provided information regarding muscle activation patterns for 3 sec. Retroreflective markers were placed on selected anatomical sites as described in the Simplified Helen Hayes marker set (Motion Analysis, Inc.) which enabled 3-D reconstruction of the lower extremity and pelvis from video data. Video data were collected from five video cameras at a sampling frequency of 60 Hz. After 3-D digitization, the data were low-pass filtered (6 Hz, Butterworth). Each instrumented subject was asked to walk at a self-selected pace over a piezoelectric force platform (Kistler Instrument, Inc.) while the ground reaction forces and EMG were collected at a nominal rate of 1000 Hz. The EMG signals were filtered with a high-pass filter at 40 Hz. A minimum of six trials was collected for each subject.

Kinematic and kinetic variables including joint moments and powers were calculated with the OrthoTrak II (Motion Analysis, Inc., Santa Rosa, CA) software package.

A three factorial repeated measures ANOVA was performed on the SAS statistical package to determine the statistical significance at 0.05 alpha level.

RESULTS

The results showed the statistically significant differences(p<.05) between the two groups for the hip angle in all three planes at heel strike (HS), midstance (MS) and toe off (TO). The joint moment profiles were consistent for both groups during the stance phase, and were not statistically significant (p>.05) for the flexion/extension, abduction/adduction, and internal/external rotations (see Figure 1, 2, 3).

Figure 1: Net hip flexion/extension moments (using clinical angles, extension is negative)

Figure 2: Net hip abduction/adduction moments (using clinical angles, abduction is positive)

Figure 3: Net hip internal/external rotation moments (external rotation is negative)

The joint mechanical powers (generated and absorbed) did not present the significant statistical difference (p>.05) between the groups. The EMG profiles demonstrated the higher activity level in Sartorius at HS and Medial hamstrings at TO in the older group while the Adductor longus showed grater activity at TO in the younger group.

DISCUSSION

As expected, the results demonstrated decrease range of motion in the hip region in the elderly, mainly in flexion/extension of the joint. As seen in Figure 1. the hip extensor moments were dominant during the first half of the stance phase which generated the hip power to control the upper body, and the hip flexor moments were dominant during the second phase. These findings were similar to Winter's report (1991). The amplitude of the hip abductor moment was slightly larger in the young population (Fig.2) supported by larger muscle hip power absorption during the HS and better control during the MS. The larger external rotation moment observed in the elderly group at HS (Fig.3) may be related to the increased activity observed in the Sartorius. The elderly subjects also appeared to produce a much greater internal rotation moment at MS. This difference was masked by the small subject size and large variability in the young group. However, these differences suggest an altered hip loading strategy in the elderly subjects. As a result of the small subject population and the high activity level of the subjects it was not surprising to find very little difference in the biodynamic variables between the two age groups. From these preliminary results some potential differences supporting the altered loading hypothesis appeared to exist. Further investigation with a larger population divided by age and level of daily activity is needed for a larger population and the comparison between the active and sedentary subgroups is recommended.

REFERENCES

Cummings, SR et al., Epidemiology Reviews, 7, 178-208, 1985.

Hayes, WC et al. Calcif Tiss Intl, 52, 192-198, 1993.

Klyver, H et al., Clinical Biomechanics, 10, 268-270, 1995.

Phillips, S et al., Bone, 9, 271-279, 1988.

Motion Analysis, Inc. Manual. Santa Rosa, CA, 1990.

Winter, D., Biomechanics and Motor Control of Human Gait, University of Waterloo Press, 1991.

Yang, KH et al., Transactions of the Orthopaedic Research Society, 41, 238-239, 1995