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Identified 1 for report purposes.
01
Identified 2 for report purposes.
02
Identified 3 for report purposes.
03
The fatigue initiation region is approximately at 6 o'clock. This is in the inner vertical face as fitted.
04
Close up of the fracture face on the head end of Stem 1.
05
Close up of the fracture face on the stem end of Stem 1.
06
As Plate 04 but on stem 2.
07
Close up of the fracture face on the stem end of Stem 2.
08
Close up of the fracture face on the head end of Stem 2.
09
As Plate 04 but on stem 3.
10
Close up of the fracture face on the stem end of Stem 3.
11
Close up of the fracture face on the head end of Stem 3.
12
Fatigue striations that were typical of those present on undamaged and/or contaminated regions on the fracture faces on all three stems.
13
Polishing on the abutment face below the head on stem 1.
14
As Plate 14 but on stem 2.
15
As Plate 14 but on stem 3.
16
Angled view on the high stress low endurance fatigue initiation region on stem 1.
17
As Plate 17 but on stem 2.
18
As Plate 17 but on Stem 3.
19
Part of the separation edge near the initiation region on a micro section taken across a diameter and normal to the fracture face on stem 1.
20
As Plate 20 but remote from the initiation region.
21
Electrolytically etched in saturated aqueous oxalic acid. As Plate 20 but in etched condition.
22
Stem 2. The small open cracks normal to the surface are typical of very high stress low endurance fatigue separations in course grained cast materials in which the mechanical properties within the grains varies depending on the direction along which they are determined.
23
As Plate 20 but remote from the initiation region on Stem 2. Fatigue progression here is very fast due to very high stresses.
24
As Plate 20 but in the initiation region on Stem 3.
25
As Plate 20 but near the initiation on Stem 3.
26
As Plate 20 but remote from the initiation on Stem 3.
27
Typical dendritic structure.
28
Pieces of stem after bend testing.
29
Typical burnishing due to lateral movement against the bone cement.
30
As Plate 30.
31
As Plate 30.
32


Examination of three fractured C Fit femoral stems.

1. Introduction

1.1
A request was received for an examination to be carried out on the subject stems which had been recovered from the host patients after it had been determined that each had separated into two pieces whilst implanted.
1.2
It is understood that the subject stems had all been manufactured from castings in a cobalt-chromium-molybdenum alloy to British Standard Specification BS 7252 Part 4 1990 (ISO 5832/IV - 1978).
1.3
The stems had all fractured across a diameter at the junction between the cylindrical and approximately rectangular regions. These regions are required to be encased in bone cement during implantation. The cement must also fill the related end of the marrow channel in the femur so that the stems are fully supported in service from the annular shelves near their tops to their lower ends.
1.4
The examination requested was to determine the mechanism responsible for the separations.

2. Examination Results

2.1
The subject stems are shown in the as-received condition on Plates 01 to 03 inclusive. The stems had been identified by the manufacturers as shown in the captions to Plates 01 to 03. They were additionally identified 1 to 3 for report purposes.
2.2
Visual and macroscopic examination of the stems provided the following information:
a]
Stems 1 and 2 had been sectioned by slitting across a diameter approximately 10 mm below the fracture faces in the lower part of the stems as implanted (see Plates 01 and 02). This had presumably been done by a previous investigator.
b]
There was no evidence that any damage had been introduced during recovery of any of the pieces of stem.
c]
Each pair of fracture faces had been mechanically damaged by interfacial bruising and rubbing after separation (see Plates 04, 07 and 10).
d]
All the separations showed that the material was uniformly coarse grained (see Plates 05, 06, 08, 09, 11 and 12). This is normal for castings in material to BS 7252 Part 4.
e]
None of the fracture faces contained evidence that the material had been weakened locally by the presence of any form of nonmetallic inclusions (intergranular oxide films for example).
f]
No plastic deformation was associated with any of the fractures.
g]
All the fracture faces were typical of those produced in most coarse grained cast materials by a high stress, low endurance tension fatigue mechanisms caused by simple elastic bending (see supporting evidence on Plate 13). In all three cases, fatigue initiation had occurred in the inner straight edge of the stem which starts from just below the head (see Plates 17, 18 and 19). This is the position where the maximum cyclic tension stresses would occur from downward loading on the head if the rectangular section part of the stem had limited freedom within the femur during walking.
h]
The upper parts of each of the subject stems had been polished/burnished due to movement against the bone cement whilst implanted (see Plates 14 to 16 and 30 to 32 inclusive).
2.3
Examination of micro sections prepared on material removed by cutting the upper piece of each stem across a diameter passing through the fatigue initiation region and normal to the plane of separation confirmed that the material was free from non-metallic inclusions. The separations were all typical of those that result from high stress low endurance fatigue in most types of coarse grained cast material under adverse cyclic loading conditions. See Plates 20 to 28 inclusive.
2.4
The cylindrical lower pieces from each stem were subjected to bending tests. All deformed plastically without separation occurring (see Plate 29).
2.5
Vickers hardness tests carried out adjacent to the fracture faces on each of the prepared micro sections (Para 2.3) gave the following results:

Stem 1 301, 308 and 285 HV(20)
Stem 2 306, 308 and 274 HV(20)
Stem 3 310, 296 and 301 HV (20)

These results indicate that the tensile strength of the material from which the subject stems had been cast was approximately 920 MPa (minimum specification requirement 665 MPa).

3. Conclusions

3.1
It is considered that the separation in each of the subject stems resulted from a high stress low endurance fatigue mechanism initiated by surface tension stresses introduced by simple elastic bending of the stem whilst the patient in which it had been implanted was walking. It is also considered that the stems were free to bend because they had not been adequately supported with bone cement when implanted.
3.2
No evidence was found that the initiation and progression of the fatigue separations in the subject stems had been adversely influenced by the quality of the material from which they had been cast or by any faults introduced during manufacture.