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Knee Replacement

13

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

Made UKRs

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.

New developments

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

manufacturer.

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

components.

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

or

[email protected]