Design of a new total knee replacement
Submitting InstitutionUniversity of Southampton
Unit of AssessmentGeneral Engineering
Summary Impact TypeTechnological
Research Subject Area(s)
Mathematical Sciences: Statistics
Engineering: Biomedical Engineering
Medical and Health Sciences: Clinical Sciences
Summary of the impact
Each year an estimated 1,324,000 artificial knee joints (total knee
replacements — TKR) are implanted worldwide; an estimated third of these
utilise an implant manufactured by DePuy International. Underlying
computer-based research performed by the Bioengineering Sciences Research
Group has played a central role during the development of a new design of
TKR for DePuy. The design programme, the biggest in DePuy's history, had a
budget in excess of US$10 million and aimed to replace the existing TKR
system, which had annual sales of approximately US$100 million.
Between 2007-2010, DePuy adopted the computational techniques developed
by the group as screening tools to (i) assess polyethylene wear and (ii)
account for the effects of surgical variability during the early design
phases. DePuy states "This research allowed us to choose the most
robust solution when proceeding to mechanical testing and saved years in
the design cycle. Patients also benefit from increased confidence in an
implant that is able to withstand the rigors of use".
In a total knee replacement (TKR), the damaged surfaces of the knee joint
are replaced by a metal component articulating against a polyethylene
component. It is the wear of the polyethylene component that constitutes
one of the main factors limiting the longevity of total knee (and hip)
replacements as it can lead to loosening, necessitating reimplantation, or
"revision". According to the National Joint Register for England and
Wales, 6.2% of all knee replacements were revision procedures in 2010.
Pre-clinical experimental wear testing of TKR components is an invaluable
tool for evaluating new implant designs and materials. However, this can
be a lengthy and expensive process, and hence parametric studies
evaluating the effects of geometric, loading, or alignment perturbations
may at times be cost-prohibitive. Also, experimental wear testing can only
be performed late in the design process, when physical parts exist.
Experimental studies in the academic literature had suggested that multi
directional movements, or "cross shear" of the metal on the polyethylene
component may lead to increased wear. Therefore, in an effort to provide
efficient implant wear evaluation, particularly in the early design phases
when only a CAD geometry exists, the Bioengineering Group developed
numerical techniques to simulate wear of total knee replacements that
incorporated cross shear; at the time there were only one or two groups in
the world attempting to computationally simulate this. In order to achieve
this, it was first necessary to be able model the variation in movement
and contact pressure distribution at the knee during typical daily
Initial work [3.2] implemented a simple Archard's wear law (with wear
simply being a function of contact pressure and sliding distance) and
achieved reasonable corroboration with experimental data for a specific
implant design. However, it was found that when this applied to a wide
range of implant designs and movement inputs, the law was found to be
implant specific, so the predictive power was low [3.3]. This led to the
development of a 2nd generation of wear algorithms, which also included a
cross-shear term. In a corroboration study of multiple implant designs,
subjected to a range of kinematic inputs, the 2nd generation wear
algorithms had a significant predictive capability for all implants [3.3].
These 2nd generation wear algorithms were subsequently used in the design
screening phase of the new DePuy total knee replacement.
In parallel, research was being performed to develop efficient methods
for assessing the impact of patient and surgical variability on the
performance of total knee replacement. The vast majority of published
research in this field performs either deterministic analyses or simple
one variable parametric studies. However, the in vivo kinematics, contact
pressure distribution and subsequent wear of total knee replacement are
highly variable and are likely to be a result of a complex interaction of
patient and surgery related parameters. To address these issues, we
applied probabilistic modelling techniques (Monte Carlo analysis and
response surface methods) in combination with computationally efficient
models (multibody dynamic analysis rather than the traditional finite
element modelling techniques) to explore the combined impact of patient
and surgical variability [3.4, 3.5]. Analysis of such a wide patient
cohort, together with their associated pathologies, had not been attempted
before; thus the implant was designed to cater for extremes of patient
size/weight. Again, these techniques were implemented by DePuy Inc to
assess the robustness of the new implant design.
This research was carried out between 2000 to 2010 and involved Professor
Mark Taylor (Head of group throughout the research period — left December
2011) and Prof Martin Browne (Current Head of Group) in supervisory roles
and Professor David Barrett (consultant orthopaedic registrar throughout
the research period) as clinical advisor. A team of PhD researchers were
responsible for several key areas of investigation: Ms Anne Celine Godest
(PhD student 2000-2003) developed the underlying force driven finite
element model that enabled knee kinematics and stresses to be reproduced.
Ms Lucy Knight (PhD student 2003-2007)  developed the preliminary (1st
generation) and cross shear (2nd generation) models. Mr Anthony Strickland
(PhD student 2006- 2009 and post-doctoral researcher 2009-2010) developed
the probabilistic techniques that were applied to the cross shear models
to understand the effect of surgical variability [3.6].
References to the research
(the best 3 are starred)
3.1 Godest, A.C., Beaugonin, M., Haug, E., Taylor, M. and Gregson, P. J.,
Simulation of a knee joint replacement during a gait cycle using explicit
finite element analysis. Journal of Biomechanics, 2002. 35(2): p. 267-276.
3.2* Knight, L.A., Pal, S., Coleman, J. C., Bronson, F., Haider, H.,
Levine, D. L., Taylor, M. and Rullkoetter, P. J., Comparison of long-term
numerical and experimental total knee replacement wear during simulated
gait loading. Journal of Biomechanics, 2007. 39: p. 1550-1558.
3.3* Strickland, M.A. and M. Taylor, In-silico Wear Prediction for Knee
Replacements — Methodology and Corroboration. Journal of Biomechanics,
2009. 42: p. 1469-1474.
3.4 Strickland, M.A., M. Browne, and M. Taylor, Could passive knee laxity
be related to active gait mechanics? An exploratory computational
biomechanical study using probabilistic methods. Computer Methods in
Biomechanics and Biomedical Engineering, 2009. 12(6): p. 709 - 720.
3.5* Strickland, M.A., Dressler, M. R., Render, T., Browne, M. and
Taylor, M., Targeted computational probabilistic corroboration of
experimental knee, Medical Engineering and Physics, 2011. 33(3): p.
3.6 This work was supported by an EPSRC PhD studentship (Strickland),
Arthritis Research Campaign grant (Reference: T0527) and DePuy
International, and resulted in PhD theses (Knight, 2009; Strickland 2009).
Total funds provided by DePuy as part of a University Technology
Partnership since 1999 ~ £1.9M
Details of the impact
The UK hip and knee registry details the number and types of knee
implants being utilised in knee operations in the UK; the data reflects
the general trends observed worldwide [5.1]. The 2011 register shows that
81,979 knee replacements were implanted in 2010 in the UK alone; the
predecessor DePuy knee design, the PFC sigma, was noted as the market
leader between 2003-2010, being used in 36% of these procedures [5.2].
Historically, the development of new orthopaedic devices has relied on
accumulated knowledge from clinical practice and limited empirical testing
(either numerical or experimental). Pre-clinical evaluation has typically
occurred late in the process when design is close to being fixed and when
physical parts existed. The advances in numerical modelling conducted at
Southampton (to assess polyethylene wear and account for the influence of
surgical variability) has meant that screening of potential designs can be
performed much earlier in the design phase before physical prototypes
Since 2000, DePuy have been involved in a technology partnership with the
University of Southampton, with the objective of developing numerical
tools to help assess the performance of total joint replacements. Based on
the maturity of the developed techniques, in 2007 DePuy took the decision
to integrate Southampton's tools into the design process of a new total
knee joint replacement [5.3].
This work was carried out solely within the Bioengineering Group at
Southampton. The design phase was split into two distinct parts, an
initial screening of potential designs followed by refining of candidate
designs. In the initial phase, between 2008-2009, the wear simulation tool
was used to assess DePuy's existing products as well as those of
competitors. These data acted as a bench mark and informed the direction
of design process for DePuy.
A range of parameters thought to control both the kinematics and wear of
the new design were then investigated; this was the first time that DePuy
had exploited numerical modelling to this extent. As a result of using
this tool, a much larger number of design variants could be explored, and
a greater understanding of the effect of design changes on the performance
of the implant was realised. In addition, a significant reduction of time
in the design cycle was achieved.
In the second phase, between 2009-2010, Southampton's probabilistic
modelling techniques were used to assess the robustness of existing
designs and potential new designs to surgical variability.
The research was used by DePuy to show that that the final design was
more robust to surgical variability than their existing designs and those
of their competitors [5.3].
The information gained was also used to inform the design of the
instrumentation required to implant the new design. Knee development team
leader at DePuy US states "The techniques that were employed in
collaboration with Southampton allowed us to first of all evaluate
analytically our component design against various options in manners
that we would never have been able to mechanically test in any
reasonable amount of time. For example, the wear modeller was used to
assess a variety of testing conditions to challenge the robustness of
the design variants and allow us to choose the most robust solution to
progress to full mechanical testing. To have performed the same work
mechanically would have added years to the development program."
There have been pre-launch implantations of the new device which was
launched to a wider audience of users in Q2 of 2013. DePuy states: "The
new knee will be DePuy's flagship brand for the future and as such is a
critical component of our business. More than half of the DePuy Joint
Reconstruction sales are from total knees and so clearly it is an
important part of our business since the new knee will represent more
than 80% of that within 5 years' time". [5.3]
The new knee implant is central to DePuy's business in the UK and abroad.
DePuy states: "The techniques developed at Southampton for analysis
were central in the design evaluation process allowing us to distinguish
between designs and be selective about features to ensure a good robust
design solution. The product will ultimately become a major part of our
sales generating multiple millions of dollars of revenue and returning
even greater value to the patients who receive the product since they
will have the confidence in the design and its robustness to withstand
the rigors of use. The company has invested 10's of millions of dollars
in developing the new product and will invest 10 times that again in
deploying the product over the next several years and then we will
invest millions again in further developing and completing the product
line so that our customers will have a full line solution to all the
challenges they face in surgery for total knees. The product we hope
will return us to the lead position in the marketplace ahead of our main
Sources to corroborate the impact
5.1 A summary of the number of total knee replacement procedures carried
out worldwide (split between regions and countries including the UK) is
presented in: Steven M. Kurtz, Kevin L. Ong, Edmund Lau, Marcel Widmer,
Milka Maravic, Enrique Gómez-Barrena, Maria de Fátima de Pina, Valerio
Manno, Marina Torre, William L. Walter, Richard de Steiger, Rudolph G. T.
Geesink, Mikko Peltola and Christoph Röder, International survey of
primary and revision total knee replacement, International Orthopaedics
(SICOT) (2011) 35:1783-1789.
5.2 Knee replacement numbers by implant type in England and Wales can be
verified in the 8th National Joint Register at www.njrcentre.org.uk
5.3 Knee R&D team member, DePuy Orthopaedics Inc, Warsaw, Indiania,
USA This contact was a leading figure in the project management and
development of the new knee replacement design with DePuy Inc.