H Hothi1, J Henckel1, P Shearing2, T Holme1, A Cerquiglini1, A Di Laura1, A Atrey3, J Skinner1, A Hart1. 1. Institute of Orthopaedics and Musculoskeletal Science, University College London and the Royal National Orthopaedic Hospital, Stanmore, UK. 2. University College London, Torrington Place, London, UK. 3. University of Toronto and St Michael's Hospital, Toronto, Canada.
There are over 80 000 total hip arthroplasties performed every
year in the United Kingdom,[1] 300
000 in the United States[2] and
almost one million worldwide.[3] Expenditure
on orthopaedic implants place a large financial burden on health-care
budgets. Several implants have a demonstrated durability and good
function in patients over many years.[4,5] As
manufacturers’ patents expire it is understandable that cheaper
generic copies will be considered.[3] This has occurred in the pharmaceutical
industry for many years and has resulted in some cheaper drugs being
available.[6]Although there are approximately 260 proximal femoral stem designs,[1] many have similar
design features including collarless polished tapers, blade designs
and hydroxyapatite coated stems. These stems have undergone clinical
evaluation through the standard process such as Conformité Européene
marking, clinical trials and benchmarking to a known standard such
as that performed by the Orthopaedic Data Evaluation Panel (ODEP).[7]The Exeter (Stryker, Kalamazoo, Michigan) femoral stem is an
ODEP 10A* rated implant, which in its first format was introduced
in 1970, and now has several imitations including designs from Asian
manufactures. More recently, a new company (Orthimo AG, Zug, Switzerland) has
introduced the OptiStem XTR for which equivalence is claimed with
the established Exeter design,[4] with
an expectation of similar clinical outcomes.There is currently no independent method of determining design
equivalence between generic and branded orthopaedic implants. Retrieval
analysis of failed implants has shown that variations in surface
topography and geometry can play a significant role in the rate
of mechanical wear and corrosion of the components,[8,9] and indeed, a recent study has shown
that variations within tolerances of the same design can result
in considerable differences in material loss.[10]In this study, we used peer-reviewed methods to compare independently
the surface topography and geometry of the generic OptiStem XTR
femoral stem with the original Exeter stem.
Materials and Methods
We acquired ten boxed, as manufactured components consisting
of the generic OptiStem XTR model (n = 5) and branded Exeter (n
= 5) femoral stems. The five OptiStem XTR implants were all the
same size (44), number 1 range and were donated by the manufacturer from a random selection.
The Exeter stems were all size 44, number 1 and were purchased from
the hospital stock of one of the authors and selected at random
from different batches.The identifying laser markings on the stem shaft and top of the
trunnion (Fig. 1) were masked to anonymise the implants. Blinded
analysis of the ten stems was then performed by two examiners (HH
and ADL) independently. Figure 2 summarises the different parameters
that were investigated.Images of the laser markings
on the femoral shaft and trunnion of the new (a) Exeter and (b)
OptiStem XTR branded stems. All markings were masked (c) in order
to blind both examiners to the brand of stem being analysed.Summary of the stem design parameters
that were investigated: (a) trunnion diameter; (b) thread height;
(c) thread spacing; (d) trunnion roughness; (e) cone angle; (f)
stem mass and volume; (g) CCD angle; (h) femoral offset; (i) stem
length; (j) neck length; (k) shaft width in an anteroposterior view
at 2 mm, 50 mm and 100 mm from the stem tip; (l) width in lateral
view at 2 mm, 50 mm and 100 mm from the stem tip; and (m) surface
roughness at 2 mm, 50 mm and 100 mm from the step tip.
Mass measurement
The mass of each stem was measured using Mettler PC 4400 (Mettler
Toledo, Leicester, United Kingdom) digital scales. A total of three
separate readings were taken by both examiners for each stem; the
scales were zeroed before each measurement.
Stem volume
Micro-CT scans of each component were performed using an XTH-225-ST
(Nikon Metrology NV, Derby, United Kingdom) scanner. A beam energy
and current of 200 Kv and 55 μA respectively were used and the scan
resolution was 95 μm. The raw scan data were imported into a 3D
image processing package (Simpleware, Exeter, United Kingdom) and
a greyscale threshold of 32 300 to 64 500 Hounsfield units was applied
to isolate the stems. A 3D render of each stem was generated from which
a measure of the volume of the component was obtained.
Stem dimensions
Each stem was placed on a flat surface within an imaging stand
with a ruler (millimetres scale) aligned parallel to the stem shaft.
A scale digitised image of each component was taken and imported
into an open source image processing tool (ImageJ, Bethesda, Maryland).
Using this we measured: the Caput-Collum-Diaphyseal (CCD) angle,
defined as the angle between the longitudinal axes of the femoral
neck and shaft; the femoral offset, defined as perpendicular distance
between the long axis of the stem and the centre of the trunnion;
the stem length, defined as the distance between the stem tip and shoulder;
and the neck length, defined as the distance between the centre
of the trunnion and point of intersection of the longitudinal axes
of the neck and shaft. We used digital callipers to measure the
width of the stems in their anteroposterior and lateral profiles
at 2 mm, 50 mm and 100 mm from the stem tip (Fig. 2).
Surface topography
A Contour GT-K 3D optical profilometer (Bruker, Coventry, United
Kingdom) was used to visualise the surface topography of the stem
trunnions and to determine the height, the spacing and the roughness
of the machined threads on their surfaces. A total of six measurement
scans were taken along the trunnion surfaces, three each on opposing
sides with the stem laid flat. The scan area was 1.256 mm × 0.942
mm using a 5× objective lens and 1× multiplier.A backscan of 500 μm and length of 400 μm was employed with a
threshold of 1%. The median of the distance between the peaks and
troughs of the raw plots generated from the scans was used to determine
thread height; the median of the distance between neighbouring peaks
was used to determine thread spacing.Following this, six additional scans (three on the front and
back) were taken along the longitudinal axis of the shaft of each
stem at 2 mm, 50 mm and 100 mm from the stem tip. A Gaussian regression
filter was applied to the raw data and a measure of μm Ra roughness
determined.
Trunnion analysis: roundness measuring
machine
A Talyrond 365 (Taylor Hobson, Leicester, United Kingdom) roundness
measuring machine was used to analyse the geometry and surface roughness
of the trunnions. The trunnion was first centralised and levelled
using on-board software. Following this a 5-micron diamond tipped
stylus on a 90° cone was used to take a series of 180 vertical traces along
the trunnion surface, capturing over one million data points.The raw data were imported into a software package (Tribosol;
Pontefract, United Kingdom) that is used for analysis of the geometry
of the taper and trunnion of hip components; we have previously
published our methods for this.[11] We
used the scans to determine the cone angle of each trunnion and
the changes in radius between the top and base of the trunnion.
We identified a common value for the radius which was measured at
the base of each of the ten trunnions; this radius was used to normalise
the base radius values between the stems. A measure of the radius
at a vertical distance of 7 mm from the same base radius was then
performed.The raw measurement data were then imported into Talymap 7 (Taylor
Hobson) to determine the surface roughness of the trunnions. From
the 180 vertical traces that were taken, we selected four traces
at a position of 0°, 90°,
180° and 270° from the starting point of the first scan trace. From
these we extracted a measure of the roughness parameter (μm Ra).
Statistical analysis
We performed the Student’s t-test to determine
if there were any significant differences between the two stem groups
in relation to the parameters investigated in this study. This analysis
was performed using the statistical software package Prism (GraphPad,
La Jolla, California) and throughout, a p-value < 0.05 was considered
statistically significant.We determined the strength of agreement in the data points generated
by the two independent examiners for the thread spacing, thread
depth, mass, trunnion roughness and stem shaft roughness parameters
by calculating the intraclass correlation coefficient with 95% confidence intervals
(CI); a coefficient close to 0 indicates poor agreement and close
to 1 indicates good agreement. Paired t-tests were
used to assess whether there were any significant differences in
the measures of the cone angle, trunnion radius, CCD angle, femoral
offset, stem length, neck length and stem shaft width determined
by both examiners.
Results
Inter-examiner reliability
Statistical analysis of the raw data generated by both examiners
revealed very good agreement, with an interclass correlation coefficient
of 0.965 (95% CI 0.928 to 0.986), 0.942 (95% CI 0.868 to 0.984),
0.999 (95% CI 0.999 to 1.000), 0.995 (95% CI 0.981 to 0.999) and
0.782 (95% CI 0.653 to 0.982) for the thread spacing, thread depth, mass,
trunnion roughness and stem shaft roughness parameters respectively.
There were no significant differences between examiners for measurements
of cone angle (p = 0.343), trunnion radius (p = 0.726), CCD angle
(p = 0.793), femoral offset (p = 0.962), stem length (p = 0.253),
neck length (p = 0.189) and stem shaft width (p = 0.596).
Mass and volume measurement
Figure 3 plots the mass of the ten stems with three repeat measurements
for each. The median mass of the OptiStems was 160.50 g (interquartile range
(IQR) 160.27 to 160.93), whilst for the Exeter stems it was 165.77
g (IQR 162.25 to 165.91) (p < 0.001).Dot plot presenting the distribution
of the mass measurements of the ten stems (measurements taken three
times for each stem). The OptiStems (n = 5) had a lower overall
mass than the Exeter stems (n = 5); there was no difference in the volumes
of the two designs (t-test, p = 0.643).The median volume measured for the OptiStem and Exeter stems
was 22.00 cm3 (IQR 19.92 to 22.71) and 21.90 cm3 (IQR
21.47 to 22.23) respectively (p = 0.643).The median CCD angles of the OptiStem and Exeter components were
125.41° (IQR 124.16° to 125.73°) and 125.55° (IQR 124.17° to 125.67°)
respectively (p = 0.788).The median measured femoral offset of the OptiStem and Exeter components were
44.29 mm (IQR 44.14 to 44.41) and 44.256 mm (IQR 44.19 to 44.45)
respectively (p = 0.993).The median stem length for the OptiStem and Exeter stems was
149.48 mm (IQR 148.67 to 151.07) and 149.32 mm (IQR 148.93 to 150.45)
respectively (p = 0.808).The median neck length for the OptiStem and Exeter components
was 53.21 mm (IQR 52.88 to 53.76) and 52.89 mm (IQR 52.22 to 53.60)
respectively (p = 0.344).Table I summarises the median measurements of the stem shaft
width in the anteroposterior and lateral profiles at 2 mm, 50 mm
and 100 mm from the stem tip.Summary of the median (interquartile
range) measurements of the stem shaft width in the anteroposterior
(AP) and lateral profiles at 2 mm, 50 mm and 100 mm from the stem
tip. Comparison using t-testingFigure 4 presents typical examples of the measurement scans captured
for the ten stem trunnions. The median thread spacing for the OptiStem
was 102.03 μm (IQR 102.02 to 103.99) and 38.21 μm (IQR 25.51 to 47.09)
for the Exeter design (p < 0.001).Examples of the imaging scans generated
by the optical profilometer for the ten stem trunnions. The OptiStem
trunnions (top row) have a visibly more threaded surface topography
than the Exeter trunnions; the depth is greater and the peaks are
spaced a greater distance apart.The median thread height for the OptiStem was 3.5 μm (IQR 3.4
to 3.5) and 1.4 μm (IQR 1.2 to 1.6) for the Exeter implants (p <
0.001).The median stem shaft roughness for the OptiStem and Exeter designs
was 0.022 μm Ra (IQR 0.021 to 0.034) and 0.024 μm Ra (IQR 0.018
to 0.035) respectively (p = 0.536).
Cone angle
The median measured cone angle of the OptiStem trunnions was
5.7° (IQR 5.69° to 5.72°), whilst for the Exeter devices it was
5.64° (IQR 5.63° to 5.64°) (p = 0.007).
Radius
We identified a common radius of 6.1 mm at the base of each of
the ten trunnions. The median trunnion radius at a vertical distance
of 7 mm from the base was 5.752 mm (IQR 5.750 to 5.752) for the
OptiStem and 5.755 mm (IQR 5.750 to 5.752) for all the Exeter components
(p = 0.007).
Trunnion roughness parameter
Figure 5 presents a measure of the roughness parameter, taken
from four traces on each trunnion. The median value for the OptiStem
trunnions was 0.924 μm Ra (IQR 0.919 to 0.931) and 0.274 μm Ra (IQR
0.248 to 0.287) for the Exeter implants (p < 0.001). Figure 6
presents a typical trace plot taken from the two stem designs.Dot plot showing the distribution of
the surface roughness measured from four different scan traces on
each trunnion. The OptiStem trunnions had a median μm Ra that was over
three times greater than the Exeter trunnions.Example of a typical scan trace
taken using the diamond probe on the roundness measuring machine from
(a) the OptiStem trunnion and (b) the Exeter trunnion.
Discussion
This study is the first independent investigation of the equivalence
of a generic orthopaedic implant to its branded design. We compared
five generic OptiStem femoral stems with five branded Exeter stems
of supposedly matching sizes and found that the OptiStems were lighter,
had a rougher trunnion surface with a greater spacing and depth of
the machined threads, had greater trunnion cone angles and a smaller
radius at the top of the trunnion. There was no difference in stem
volume, CCD angle, offset, neck length, stem length, shaft width
or roughness of the polished stem shaft.This preliminary investigation found that whilst there were similarities
between the two designs, the generic OptiStem is different to the
branded Exeter design.The impact of generic drugs has driven down healthcare costs
in the pharmaceutical industry.[6] The
desire to extend generic technology to orthopaedic implants is understandable.
However, there are important differences between a generic drug
and a generic implant. A drug has a unique chemical formula, chemical
speciation and physical form. These parameters are measurable with
mass spectrometers and radiographic analysis.[12] An implant has
several physical and chemical parameters that can also be measured
but there is considerable variation in material composition, material
structure, and manufacturing process. We know that small changes
have had a dramatic effect on the outcome.[13]Several retrieval and laboratory based studies have demonstrated
the importance of surface roughness as a contributing factor for
mechanical wear and corrosion at the head-stem junction; a rougher
surface leads to greater material loss.[9,10,14] The roughness
parameter is calculated as the mean of the microscopic peaks and
valleys of the surface. We found that the median μm Ra of OptiStem
trunnions was more than three times greater than the Exeter trunnions.
There is concern therefore that the OptiStem components are at greater
risk of material loss at the taper junction when paired with cobalt
chrome modular heads. This is due to a reduced contact area between
trunnion peaks and the corresponding head taper surface, leading
to increased localised contact stresses and more prominent channels
for fluid ingress to occur between the valleys of the trunnion thread
and the taper surface, which could potentially lead to greater corrosion.
The additional concern is that
the OptiStem is designed to look like the Exeter, so an Exeter head
may be inadvertently used with an OptiStem in cases such as acetabular
component revisions.Our finding that the OptiStem implants were lighter than Exeter
stems despite there being no difference in their volumes is of interest
as this suggests that the material density of the OptiStem components
may also be lower. It is not clear what the clinical significance
of this finding may be. However, this is an indicator of possible
differences in the manufacturing process between the two stem designs. Indeed,
we noted a smaller range in the maximum and minimum measured values
for the different parameters for the OptiStem compared with the
Exeter components. This suggests a consistent manufacturing process
for the OptiStems, but the difference in density between the two
designs may be associated with differences in the microstructure
of the alloys such as grain size or orientation. This may affect
the mechanical properties of the component.Optimal fit between a stem trunnion and head taper can be achieved
if the angle of the trunnion and taper components are the same;
a perfect fit creates a seal preventing fluid ingress from occurring.
We found that the OptiStem trunnions had a greater cone angle than
those of the Exeter stems and consequently the radius of the OptiStem
trunnions was smaller. It is noted that the scale of the differences
between the two designs for these two parameters are small. The measured
values of the different parameters may be within the design tolerances
of the Exeter component, however, we do not have access to these
data to confirm or refute this. It is interesting that there is
no overlap between the measurements and therefore the fit between
the same femoral head and these two stems designs would be different.
It remains unclear what affect these parameters have on clinically relevant
levels of material loss at the taper junction.[15] We note however
that the manufacturer (Orthimo AG) supplies the OptiStem XTR with
a femoral head design referred to as the OptiHead XTR. This study
has not included analysis of this head design and we do not know
if the effect of trunnion differences in the OptiStem would be mitigated
by the design of the OptiHead, or if indeed they would be worsened.Our study demonstrates the importance of independent verification
of manufacturing finishes of orthopaedic implants. This is especially
pertinent for generic implants that claim design equivalence to
branded designs. There have been numerous examples in recent history
of seemingly small changes in implant design resulting in large
differences in clinical results.[16,17] Indeed, this was demonstrated
with the Exeter stem which was associated with an increased incidence
of loosening when a matt surface finish was used compared with the
polished stem design.[16] The
current study examined surface finish which confirmed that both
designs were highly polished with no significant difference in their
roughness. Another notable example is that of the 3M Capital hip
(3M Healthcare, Oakdale, Minnesota) which was marketed as being
a low-cost design emulating the established Charnley hip (DePuy Synthes,
Warsaw, Indiana). This was however quickly discontinued following
high incidences of early loosening[17] which were thought to be due, in
part, to the increased surface roughness of this design over the
Charnley.We acknowledge that there are other factors which may impact
the performance of the stem which must be investigated in future
studies. For example, it is important that the microstructural properties
of the alloy used are fully characterised and features such as grain
size and orientation are considered. Several other tests are possible,
such as an examination of the method of manufacturing the stem,
strength testing under load and testing the taper under load for
debris generation. Future work should also consider any instruments
that may be used with this generic design, which are known to play
a large role in the performance of implants.We found a difference in trunnion roughness, trunnion cone angle
and radius, and implant mass when comparing the generic and branded
stem designs. All implants require standard regulatory processes
to be followed. It does not appear feasible that generic implants
can be manufactured predictably to guarantee the same performance
as generic drugs.Take home message:- The design of the generic femoral stem in this study is not
the same as the branded stem on which it is based.- It does not appear feasible that generic orthopaedic implants
can be manufactured predictably to guarantee the same performance
as generic drugs.
Table I
Summary of the median (interquartile
range) measurements of the stem shaft width in the anteroposterior
(AP) and lateral profiles at 2 mm, 50 mm and 100 mm from the stem
tip. Comparison using t-testing
Authors: Robert K Whittaker; Harry S Hothi; Antti Eskelinen; Gordon W Blunn; John A Skinner; Alister J Hart Journal: J Orthop Res Date: 2016-10-25 Impact factor: 3.494
Authors: Sevi B Kocagöz; Richard J Underwood; Shiril Sivan; Jeremy L Gilbert; Daniel W Macdonald; Judd S Day; Steven M Kurtz Journal: Semin Arthroplasty Date: 2013-12-01
Authors: Anna Panagiotidou; Jay Meswania; Jia Hua; Sarah Muirhead-Allwood; Alister Hart; Gordon Blunn Journal: J Orthop Res Date: 2013-08-21 Impact factor: 3.494
Authors: Ashley K Matthies; Radu Racasan; Paul Bills; Liam Blunt; Suzie Cro; Anna Panagiotidou; Gordon Blunn; John Skinner; Alister J Hart Journal: J Orthop Res Date: 2013-08-05 Impact factor: 3.494
Authors: V C Panagiotopoulou; K Davda; H S Hothi; J Henckel; A Cerquiglini; W D Goodier; J Skinner; A Hart; P R Calder Journal: Bone Joint Res Date: 2018-08-04 Impact factor: 5.853