Patient Specific Technology

Why Better Technology Matters

Technological advances are striving to improve upon current results. In the past, the focus has been more on establishing materials to avoid wear characteristics of implants, and in ensuring that the implants can be adequately secured to the bone for both hip and knee replacements. This has since become much less of a problem, with the advent of advanced coatings, where bone can grow into the implant, as well as the latest generation cementing techniques.

In general, Dr Liew will perform a total hip replacement without cement, to avoid the risks associated with fat embolisms during cementation. For total knee replacements, the tibia is cemented while the femur is usually uncemented.

The Three Methods of Hip and Knee Replacement

There are three main ways to plan a total hip and knee replacement:

• Conventional: Using alignment guides and rods during surgery to judge the alignment of cuts. These have been shown to be adequate in establishing the centre canal of bones, and providing a good guide for where the implants should sit. They have been around since the start of hip and knee replacements, and various tools have come in to help increase the accuracy of this technique over time.
• Navigation: Using a computer to judge where cuts are being made prior to insertion of the prosthesis. 5mm pins are drilled into the bone on both sides of the joint - on the pelvis and femur for a hip replacement, and in the femur and tibia for a knee replacement. This enables a navigational sensor, or robot, to know where the joint is. A sensor is then placed on various parts of the joint before starting in order to accurately map the joint before the operation begins.
• Patient Specific Instrumentation: Using a pre-operative scan to create specific custom moulds used as cutting guides during surgery. This enables pre-operative planning that allows identification of abnormal anatomy, variants, or any anatomical issues that need thought and planning before execution. It is Dr Liew’s technique for both hip and knee replacements.

Various methods aim to improve alignment and push outcomes. Original 3D jigs many years ago were not accurate enough - the scanning and production technology were not adequate to ensure 100% accuracy to a patient’s anatomy, and were often abandoned. Over the last 10 years, accuracy of 3D jigs has been 100% with no failures of the jig position to the bone in Dr Liew’s own experience.

How Patient Specific Instrumentation Works

Patient Specific Instrumentation requires a pre-operative investigation - typically a 3D CT scan - to recreate the exact anatomy of the hip or knee in a 3D environment. The 3D image is used to create specialised cutting blocks which are then used to make accurate bone cuts during surgery. This scan takes only a minute to perform.

The side benefit is that it does not require damage to surrounding structures: no pins are inserted into the bone as in navigation and robotics, and no rods are inserted into the medullary shaft as in conventional techniques.

Pre-Navigation - Planning Before the Operating Theatre

Dr Liew performs all of his total hip and knee replacements using patient specific technology. He believes this technique allows an element of “Pre-Navigation” - the bone cuts can be assessed, changed, and planned prior to surgery rather than making those decisions during the procedure.

With the approach to the hip and knee, combined with pre-operative planning, and the fact that each hip and knee prosthesis is placed to match the pre-arthritic state, planning is essential to ensuring the best possible outcome.

This requires a 3D environment to visualise what the new replacement will look like in the patient’s bone before any incision is made. This can reduce operative time, which in turn reduces the time the wound is open and potentially reduces infection risk.

Who is Suitable?

Not all patients are suitable for patient specific instrumentation, but it can actually be easier to use patient specific instruments in some complex cases - for example, when there are existing implants in the femur or tibia, or when someone has had pelvic realignment surgery. If a patient has significant metalwork, the scanning can be difficult if the edges of the bone cannot be adequately scanned. This can occur when patients have plates and screws from previous fractures. Patients with uni-compartmental knee replacements can still have a scan when a revision to a total knee replacement is planned.

Many surgeons use differing methods, and most will be enthusiastic about their own method. Dr Liew has performed all of his total hip replacements with the direct anterior approach and patient specific technology, and all of his knee replacements with kinematic alignment and patient specific technology for many years. He continues to refine the technique as improvements become available. As a member of the hip and knee Medacta international advisory boards, he is often the first to see the outcome of international research where tried and tested improvements can be used.

To discuss whether you are suitable for patient specific technology, speak with your GP for a referral to see Dr Liew.

The Pre-Operative Templating Advantage

Templating is the step where the operation is planned, in full, before entering the theatre. From the pre-operative 3D CT scan, a model of the patient's own bone is reconstructed, and the entire reconstruction is designed within it: the implant type and exact size, the position and orientation of each component, and how the joint is restored to its pre-arthritic state.

For a total hip replacement, this means planning the position of the socket and stem, and reconstructing leg length and offset to match the other side — two of the measurements patients notice most after surgery. For a total knee replacement, it means planning the depth and angle of each bone cut so the implant restores the natural joint line, in keeping with the kinematic alignment philosophy.

The advantage is that these decisions are made — and checked — in advance, against a model of the individual patient rather than an average. The operation then becomes the execution of a verified plan, rather than a series of judgements made in the moment. Because the difficult decisions are resolved beforehand, operative time can be reduced, which in turn reduces the time the wound is open.

Measuring Every Cut and Component During Surgery

A plan is only as good as the ability to deliver it. This is where patient-specific technology and intra-operative measurement work together. The custom cutting guides are manufactured to seat on each patient's unique bone surface in only one position, so each bone resection is made exactly where it was planned — not estimated from generic alignment rods.

During hip replacement, Dr Liew operates on a specialised traction table with a radiolucent carbon-fibre extension. This allows real-time X-ray imaging to be taken during the procedure, so component position, leg length and offset can be measured against the pre-operative plan and adjusted before the wound is closed — rather than discovered afterwards on the recovery-room X-ray. The detail of how this imaging works is covered in enhanced accuracy during hip replacement surgery, and the equipment itself in the technology I use now.

For knee replacement, each bone cut and the resulting soft-tissue balance through the full range of motion is checked against the kinematic plan, so the knee is balanced as it moves rather than only in a single position. Throughout, single-use instrumentation is opened fresh for each patient, so every instrument performs to the same specification each time. The result is that component position is confirmed by measurement during the operation, not left to be assessed only once surgery is complete.

Frequently Asked Questions