Femora! Injury Tolerance

Early studies o f the static strength o f the femur were conducted by Weber in 1859 and Messerer in 1880 and have been summarised by Melvin and Evans [104]. Weber performed three point bending tests with the force applied in 245N increments midway between the supports and transverse to the longitudinal axis o f the bone. The distance between the supports was 183mm in all cases. Data was obtained from four males and five females. The maximum load to fracture is summarised

below:-Subjects Peak Static Bending M oment and Torsional Fem oral M om ent (Messerer).

Messerer's femoral bending experiments were performed with a hydraulic testing machine having a load measurement resolution o f 10 to 50N. Three point bending tests were conducted with the support span o f two-thirds the length o f the femoral shaft, that is, 317mm in length. Loads were applied at the midspan. Bones from six males (ages 24 to 78 years) and six females (ages 20 to 82 years) were assessed. For lateral (left to right) loading, the average maximum bending moments at fracture are presented in Table 6.

Messerer also conducted static torsion tests on the femur. Bones from four males (ages 27 to 56 years) and seven females (ages 19 to 81 years) were evaluated. The average torsional moments are shown in Table 6.

He also noted that the upper and lower thirds o f the femur were o f lower torsional strength than the middle third. All the bones fractured with a spiral pattern at an angle of 45 degrees.

Static axial compression of the femur was conducted by Messerer with the ends o f the bones padded with felt to prevent local failure at the point o f force application. The average axial compressive failure force for shaft fractures was>

Subjects 1 Peak Static Axial Compressive Femoral

Load (kN)

Male 7.72

Female 7.11

Table 7. Peak Static Axial Compressive Fem oral Load (Messerer).

A more recent study o f the static bending strength o f the femur was conducted by Motoshima in 1960 and summarised by Yamada [98], Three point bending

tests were performed on the femurs o f 35 subjects. The ends o f the specimens were encastred in plaster or concrete with the force applied at midspan in the anterior-posterior direction using a 20mm diameter cylindrical loading head.

The average breaking loads and bending moments are summarised below for five age

groups:-Age G roups (Y ears)

Peak Static Bending M oment

(Nm)

Peak Static Load (kN)

20-39 234 2.72

40-49 213 2.47

50-59 203 2.35

60-69 201 2.33

70-89 184 2.14

Average 211 2.45

Table 8. Peak Static Bending M om ent and F ractu re Loads for W et Fem urs (Yamada).

Yamada indicates that the female femur has five-sixths o f the bending strength of the male femur.

There have been a number of studies o f the dynamic fracture tolerance of the femur, primarily due to research performed in the automobile industry. While acceptable criteria for predicting fracture have remained elusive, there appears to be universal agreement that the dynamic load carrying capacity o f the femur exceeds that under static loading. The load carrying capacity o f the femur under dynamic conditions was evaluated by Mather [105] using 32 pairs o f human femurs. For each pair, one was loaded statically while the other was loaded dynamically with a velocity o f 9.8m/s. Although the data was not able to compare the static and dynamic fracture forces an energy comparison was possible. The mean value o f dynamic energy was 1.7 times that o f the static energy.

The dynamic torsional loading o f the femur was also studied in connection with skiing accidents [106], Femurs were obtained from 65 autopsy individuals ranging from 27 to 92 years old. The ends o f the bones were embedded in gripping blocks and torsionally loaded to failure in less than

100ms. The mean value o f the peak torsional moment for males was>

Subjects Peak Dynamic Torsional Fem oral M om ent (Nm)

Male 204

Female 131

Table 9. Peak Dynamic Torsional Femoral M om ent (M artens).

Comparing the above data with results o f Messerer's static torsional tests shows that for males, the peak dynamic torsional moment is 17% greater that its static counterpart. However, for females the mean peak dynamic torsional results are lower than the static data by 4%. It is suggested that the differences are most likely due to the smaller sample size o f Messerer, that is, 7 versus 13 and the size of bones in certain subjects [104],

The first study o f femoral impact tolerance was conducted as part o f the automotive crash safety research [107], Ten unrestrained seated, embalmed male cadavers were tested on a sled facility. The knee target areas were covered with 37mm o f padding. The results o f the research indicated that for a moderately padded surface, an axial compressive force o f 6.2kN is a reasonably conservative value for the overall injury threshold level o f the patella-femur-pelvis complex. Further tests on two additional cadavers were reported by the authors. Loads o f 6.5, 7.6, 8.7, 8.8kN were sustained without fracture. It was suggested that loads of 8.7kN without fracture were not unreasonable.

Knees o f 26 fresh cadavers were impacted using impactor tests [108], There were 15 male and 11 females cadavers. The data indicates that the males ranged in age from 46 to 90 years old, had a body mass o f 39.6 to 88.5kg and a height o f 1.63 to 1,80m. The females ranged in age from 45 to 89 years old, 21.9 to 65.9kg and 1.50 to 1.66m. The impactor velocities varied from 3.8 to 23.2m/s using unpadded to padded impactor faces. These tests were also applied with variations in applied force, pulse amplitude and duration.

For the seven "rigid surface" impacts for two males and one female cadavers, femoral condyle fractures occurred in two tests. The peak forces associated with these tests were 18.0kN for a female and 19.6kN for a male. There were no femoral fractures in another five tests, where the peak force ranged from

16.2 to 22.7kN on four legs of male cadavers. For 28 lightly padded impacts to nine male and six female cadavers, femoral condylar and supracondylar fractures occurred in five tests, one undefined fracture occurred in a sixth test.

Three o f these were associated with males and had peak forces ranging from 13.6 to 28.5kN. A further three were associated with females whose fracture forces ranged from 13.3 to 19.6kN. For thick padded impacts, one female age 55 years sustained a femoral shaft fracture at 19.7kN. A 72 year old male cadaver sustained peak forces o f 15.7 and 13.7kN without fractures.

A series o f six unembalmed cadaver sled tests were performed with the objective o f achieving longer impact durations with both femurs being loaded simultaneously [109], Peak femoral fracture forces ranging from 10.2 to 23.0kN were sustained.

There have been several attempts to define a femoral injury criteria, using the available experimental data, that would enable a femoral fracture or no fracture to be predicted, based on an applied time history. Such a criterion would be useful in analysing crash test data from anthropomorphic test devices with femoral load cells. However, no test device has reproduced the human biomechanical response characteristics o f the knee-femur-pelvis region. A Femur Injury Criterion (FIC) was developed [110] which calculates the axial compressive force, F, to produce fracture as a function o f the primary load pulse duration,

T:-F(kN) = 23.14 - 0.7IT (ms), T < 20m s.

F(kN) = 8.90, T > 20ms.

A method is discussed for implementing this criterion for analysing complex wave shapes. Lowne [111] has analysed the data and concluded that the following criterion is appropriate for avoiding unacceptably high femoral compressive loads

:-♦ 12kN may not be exceeded.

♦ 1 OkN may not be exceeded except for durations o f less than 3ms.

♦ 7kN may not be exceeded except for durations o f less than 10ms.

The femur limit currently specified in FMVSS 208 and Aerospace Standard (AS) 8049 is a compressive load for each femur o f lOkN [112]. Previous FMVSS 208 specifications were 6.23kN and 7.55kN.

Tolerance limits to combined loading, for example simultaneous torsion and axial compression, do not appear to have been addressed to date.