More than a million UK residents see their GPs about
osteoarthritis every year, a number that will grow significantly
with rising population age and obesity. As implant designs and
outcomes improve, more of these patients will receive partial or
total joint replacements. Indeed, over 75,000 knee replacements
were done in England andWales in 2008, making it the commonest
joint replacement.With better success rates in recent years, such
surgery is being offered to progressively younger patients.
Knee Replacement –
First Results of Custom
Problems with conventional designs
Increasing expectations of patients have led manufacturers
to respond with continual improvements in design.
However, the range of sizes and shapes remains limited
with conventional knee replacements and the surgeon
still has to cut the bone to fit the implant using standard
cutting jigs.This can result in under or oversized implants
that do not conform to the patient’s natural shape, causing
an imperfect fit and a variable degree of precision in
placing the implant.
Making the patient’s bone fit the implant also requires
more bone resection than may be really necessary.
Furthermore, conventional off-the-shelf implants require
large inventories of components and instruments.The cost
of managing, sterilising, and transporting these inventories
is significant and they take up considerable storage space.
As a result of these limitations, there have been a number
of attempts to improve on conventional knee designs by
the use of computer techniques and different groups have
produced contrasting innovations. In ‘computer-aided
surgery’, conventional knees are implanted to a higher
degree of precision using computer navigation.‘Robotic
surgery’ takes this further by using a robotic device to
make the bone cuts.
Both add precision to the procedure, but neither utilises
a custom implant to maximise the value of this precise,
data driven approach.
Recent advances include the ability to create custom
implants using Computer Aided Design (CAD) software to
convert CT and MRI scans into 3D models of the patient’s
knee. From these models, manufacturers create an implant
made specifically for the patient.This methodology
facilitates the production of minimally traumatic, bone and
cartilage-preserving knee implants and custom instruments
that address all types of osteoarthritis of the knee,
including single, double and tri-compartmental disease.
This ‘image-to-implant’ technology is a significant
improvement on existing computer-aided surgery and on
conventional knee designs. Implants that correspond to the
individual size and geometry of the patient enable the joint
to be resurfaced rather than replaced, with less bone loss,
less trauma and a simpler technique.
How a custom implant is created – case study
Let’s take the patient with medial compartment
osteoarthritis.The present trend is towards partial knee
replacement (UKR) due to the good long-term survival
and better kinematics of UKR compared with total knees
(TKR). UKRs are as effective as TKRs and have better
functional outcomes at 15 years. So let’s say we want to
produce a custom made UKR.What is the procedure?
At consultation, standard criteria are used to decide the
type of surgery, in this case medial UKR.The surgeon
arranges a CT of the knee and partial scans of the hip and
ankle.The imaging centre then transmits this data to the
Once the data is received, the software programme
reconstructs a virtual 3D image of the patient’s knee.
By reference to the hip and ankle data, the knee model
is then placed in correct spatial orientation and, using
CAD, a virtual implant is created allowing for appropriate
correction of leg alignment.
The patient’s bone morphology drives the implant
geometry resulting in patient-specific femoral (Figure 1)
and tibial components (Figure 2).The data then drives
computerised machinery that produces patient specific
The same technology creates a custom set of surgical jigs
(Figure 3) exactly matched to the implant and the patient.
This ensures that the bone cuts correspond precisely
to the geometry of the implant, giving an exact match
between patient morphology, implant geometry and
surgical instrumentation (see post-operative X-rays
Figures 4 and 5).
First UK cases performed
Everyone’s knees are different and while most patients
are suitable for conventional joint replacements, many are
poorly served by standard off-the-shelf implants. In the last
year, Mr Glyn Evans has performed the first such custom
made unicompartmental knee replacements in the UK and
is continuing to recruit cases suitable for this technique
exclusively at London Bridge Hospital.
Figures 1, 2 and 3
3D CAD design of
the patient’s knee.
Figures 4 and 5.
For information, contact Mr Glyn Evans’ secretary on
020 7407 3069