Many design philosophies have been espoused for cementless fixation of total
hip arthroplasty. Several have enjoyed at least some degree of success. The
Fit and fill concept is vital for proper seating and initial stability
of the cementless femoral components (Poss et al.,
1988). However, with the unrivalled success of certain fit without
fill designs particularly flat tapered stems, complete fill of the femoral
canal is no longer considered a necessity for primary stability (Parvizi
et al., 2004). Likewise, porous coating using beads, fiber mesh,
or plasma spray was thought necessary to provide reliable long-term stabilization
in many early designs. Since then, surface roughening and hydroxyapatite coatings
have proven effective (Delaunay and Kapandji, 2001).
This study may suggest that the tendency of stress transmission differs depending
on the size, position and angle of the grooves that plays a vital role for long-term
stability of cementless femoral stems.
MATERIALS AND METHODS
To optimize the groove dimensions and inclination on the implant surface, a
static simulation on a human femur bone was performed. As regards the stress
to the transmission by a load at the proximal femur on the bone implant surroundings,
the geometry of grooves introduced on the implant surface were changed and this
influenced the stresses on the bone and bone-implant interfacial micro motions
as was investigated FEA.
The methodology followed was as under: First, a femoral implant with specific
groove geometry was modelled with three-dimensional Computer Aided Design (CAD)
Pro-e software. Next, from the patients medical images (DICOM data) the
bone contour was extracted with 3-D image processing software and a 3-D bone
model was prepared with 3-D CAD software. The 3-D model of the implant and bone
were properly matched and processed for virtual implantation. Using this bone-implant
model, the stress environment formed in the implant surroundings was investigated
using FEA software ANSYS. Table 1 shows material properties
for each element.
||Groove depth a, length b and inclination
θ on stem surface (b) boundary conditions on bone-implant
||Effect of groove length on stress development (keeping groove
depth 0.5 mm)
Value of the load was considered F = 2 kN, which is equivalent to load acting
on the joint of a man weighing 70 kg and walking with speed 1.1 m sec-1
toward the distal direction along the femoral axis (Duda
et al., 1997). As for the boundary condition, the coefficient of
friction was assumed to be one between the implant and the bone. Furthermore,
it was assumed that cancellous bone was induced inside all the grooves on the
implant surface. In the analysis model, the number of elements was 47,160 and
the number of nodal points was 582,081 as shown in Fig. 1a
RESULTS AND DISCUSSION
The stresses increase linearly with increase of groove length (Fig.
2). It has been observed that the variation of stress with the groove depth,
increases initially, upto a groove depth of 1 mm approximately, remains constant
upto 1.5 mm approx. and thereafter, slightly decreases (Fig. 3).
||Effect of groove depth on stress development (keeping groove
length 2 mm)
||Effect of groove inclination on stress development (keeping
groove depth 0.5 mm and length 2 mm)
Figure 4 shows the variation of stress with groove inclination
to the transverse plane. Upto an inclination of 80°, the stresses remain
approximately in the range of 48-58 MPa. Beyond this, there is a sharp increase
in the stress, reaching a value of approx. 70 MPa at 90° inclination. (when
the groove inclination is perpendicular to the transverse plane). The three
dimensional stress distributions on the femur using Von Mises criterion, under
loading are as shown in Fig. 5. Figure 5
shows contour plots of stresses when groove length was 5.7 mm (groove depth
The stresses tended to increase when the groove length increased, obviously
due to reduction in surface area. Furthermore, for different groove depths the
von Mises stresses somewhat remained constant between 1 to 1.5 mm. This is thus
found to be the optimized groove depth.
There are no reports on the effect of Groove Geometry on Cementless Femoral
Stem Component in hip arthroplasty the literature. However, Vidalain
(2011) has outlined briefly the advantage of full coating for the fixation
of a femoral stem with horizontal and vertical grooves in terms of enhance primary
mechanical stability through increased osseointegration without using optimized
||Contour plot of bone-implant system for groove length 5.7
Therefore, considering the optimum dimensions of the grooves one can enhance
long-term stability of cementless femoral stems under body physiological loading
conditions for specific stem design.
The implant-bone fit is critical to the success and longevity of hip implants.
Due to the presence of grooves on the surface of the stem, the maximum contact
on the internal cortical bone surface, especially proximally, produces higher
stress values and reduces micromotion and sinkage. Results showed that the presence
of grooves with optimum dimensions and inclination on the stem surface could
improve the implant-bone fit for cementless femoral stems.
We thank the Council of Scientific and Industrial Research (CSIR), NMITLI project
for a generous grant to undertake this study on selected medical implants, RP01841.