If the Cap Fits

5 July 2009 (Last Updated July 5th, 2009 18:30)

Customised implants are set to positively affect the future of treatments for osteoarthritis. Dr Terry Clyburn of the University of Texas Health Science Centre and Daniel Steines of ConforMIS examine the trends towards knee replacements tailored to each patient's specifications.

If the Cap Fits

Personalised medicine is one of the fastest-growing trends in the healthcare industry. While the pharmaceutical sector has dominated this development, medical device manufacturers are also recognising the needs of different patient groups and customising their products accordingly. Manufacturers of orthopaedic implants have recently taken a step in the direction of personalisation by developing implants designed to fit people of different genders or races.

More recent breakthroughs include using imaging studies, such as MRI and CT scans, to customise implants to each individual's anatomy. The field of knee replacement for patients with osteoarthritis has been leading this move toward patient-specific approaches to orthopaedic implants.

Osteoarthritis is the most common type of arthritis; an estimated eight million people in the UK live with the condition1. Each year about two million people in the UK see their GP about osteoarthritis. The condition is most common in people over 40, but sometimes occurs in younger people, with women more at risk than men. Many people with osteoarthritis will require a knee replacement at some stage in their disease.

About 70,000 knee replacement procedures were carried out in England and Wales alone in 20082, making this one of the most common surgical operations in the UK. Due to the overall ageing of the population as well as increasing obesity, we are likely to see an increase in the number of people requiring this procedure. Advances in implant design will go a long way in improving patient outcome by providing surgeons with options that can be tailored to each individual.

"Due to the one-size fits all approach, compromises may have to made in fit and preservation of a patient's natural kinematics."

The need for new options

Traditionally, orthopaedic implants have been mass-manufactured and are available in a limited range of sizes and shapes. Fitting the implant to the patient requires extensive bone resection to alter the joint to fit the shape of the implant. This involves complex and invasive surgery and results in healthy bone being sacrificed to conform to the implant shape.

Due to the one-size-fits-all approach, compromises may have to be made in the fit and in the preservation of the patient's natural kinematics. This can result in painful overhang or impingement, rotational misalignment or undercoverage that leads to subsidence and loosening of the implant. All of these potential problems may lead to a need for revision of the original surgery.

There are also implications for the hospital in the use of standardised implants. Traditional implant systems come with a large inventory of metal surgical cutting blocks, spacers and instruments. Hospitals have to inventory, manage and sterilise multiple cases of these heavy instruments after each procedure.

Although traditional implants are effective, advances in technology are providing personalised solutions to some of the challenges faced by patients and physicians.

Customised implants through new technology

A new technology, developed by ConforMIS, called iFit™ Technology uses a Computer-Aided-Design (CAD) programme to convert CT and MRI scans into a 3D model of the patient's knee. From these virtual models, the company designs implants made specifically for the patient. Design expertise and decision rules gathered from orthopaedic surgeons and multi-year cadaveric and patient studies have been embedded in the software. This technology has enabled the development of a comprehensive line of minimally traumatic, bone and cartilage-preserving knee implants and instrumentation designed to address all stages of osteoarthritis.

Known as 'image-to-implant', the process begins with a CT or MRI scan of the knee in a patient with osteoarthritis. This scan determines the extent of the osteoarthritis and aids in selection of the implant. A standardised protocol, which often includes a partial scan of the hip and ankle to establish alignment, is provided to the imaging centre to ensure the data needed to design the implant are captured properly. The protocol is compatible with the vast majority of scanners in use today.

The data are then interpreted through the CAD programme and provide information to generate implants that conform to the healthy bone or cartilage. The implant is made to fit the patient rather than the reverse. By designing devices that conform to a patient's unique anatomy, as shown in Figure 1, the implants allow the surgeon to resurface rather than replace the joint, providing far more tissue preservation, a reduction in surgical trauma and a simplified technique. Typically, it takes four to six weeks to make the patient-specific implant and custom instrumentation from the design file.

iFit technology for all patients

iFit technology is used to develop personalised implants for the complete range of patients with osteoarthritis, including patients with early, moderate or severe disease; young and old patients requiring total knee replacement (TKR); patients with isolated osteoarthritis of the medial or lateral tibiofemoral compartmentof the knee; and patients with advanced osteoarthritis affecting one of the tibiofemoral compartments along with the patellofemoral compartment.

Traditionally, the preferred surgical procedure for patients with osteoarthritis requiring knee replacement has been TKR. However, unicompartmental knee replacement (UKR) has gained popularity in the UK due to the long-term survival of the implant, as well as the better kinematics and joint function3. While in the past it has been suggested that UKR comes with a higher failure rate, recent research demonstrates that UKR is as effective as TKR and shows no greater tendency to failure such as loosening of the implant4. Results showed that 15 years after follow-up, the success rate for patients undergoing TKR or UKR was comparable. However, patients in the UKR group maintained a better range of function.

With the emergence of personalised implants for UKR procedures, such as those developed through iFit technology, these findings are encouraging and should support an uptake in patients for which the procedure will provide the best outcome.

Illustrating the technology

The following example illustrates how an implant is designed for the medial or lateral compartment of the knee, for example, a UKR or unicondylar implant. Once the CT scan is completed, including a partial scan of the hip and ankle, the imaging centre uploads the images to a secure web server; which is then imported into the proprietary software. Within the software, the first step is to derive the outer contour of the bone, which is done via proprietary algorithms.

A surface model is captured by the CAD programme and shows the correct spatial orientation of the knee. The type and extent of deformity present in the patient can then be determined. The iFit process interactively defines the extent of misalignment present in the knee and builds it into the implant and instrumentation, so correction to alignment is designed into the implant.

"iFit technology is used to develop personalised implants for the complete range of patients with osteoarthritis."

Using the patient-specific bone geometry to drive the implant geometry, the process results in a patient-specific, uni-compartmental femoral component known as iUni™ (see Figure 2) in the CAD system. The patient's bone defines the sagittal geometry of the femoral component. A minimal posterior bone cut for the implant is incorporated into the design based on the patient's specific posterior condylar geometry and orientation. On the tibial plateau, the patient's specific bone profile of their tibia, as pictured in Figure 3, defines the geometry of the tibial implant. The modular tibial plateau and tibial inserts are designed on a minimal bone cut and provide a smooth articulating surface for the femoral component.

Importantly, because the implant is designed for this particular patient, the implant matches the patient's native posterior slope and provides for complete cortical rim coverage, a result that can not be achieved consistently with off-the-shelf implants. The placement of the fixation features for that patient is based on design principles for unicondylar implants1.

Finally, iFit technology also allows for the creation of disposable, patient-specific instrumentation and cutting jigs from the same scan data. The individualised jigs, or iJigs™, are exactly matched to the implant and to the patient's anatomy, reducing the number of steps required to size and place the cutting guides as well as the actual number of cuts.

iJig instrumentation ensures bone preparations are exactly matched to the position, size and shape of the implants determined digitally using the scan data. These operation tools cannot be used with any other patient, and are custom made to work with the unique custom prosthesis. As a result, iJigs provide precision comparable to expensive surgical navigation systems or surgical robots via pre-operative navigation. The disposable iJig set comes pre-sterilised in one small container for one-time use, allowing for efficient management, safety, and no storage or sterilisation costs. For the hospital, this is not just a cost saving compared to other advanced technologies designed to improve surgical results, but also a more efficient alternative compared with traditional implants.

Personalised implants allow for an anatomic fit that makes maximum use of the healthy tissue available and full usage of the hard cortical shell to bear the majority of the load when the patient is in motion. It also facilitates a more precise recreation of the patient's normal anatomy. Although it is early days for this technology in the UK, in the coming months we expect to see more patients undergoing this revolutionary procedure.

References

1. http://hcd2.bupa.co.uk/fact_sheets/html/osteoarthritis.html
2. National Joint Registry, Stats online:
http://www.njrcentre.org.uk/njrcentre/Healthcareproviders/
Accessingthedata/StatsOnline/NJRStatsOnline/tabid/179/Default.aspx
3. Newman, J et al. 'Unicompartmental or total knee replacement. The 15-year results of a prospective randomized controlled trial'. Journal of Bone and Joint Surgery [Br]; 2009;91-B:52-57.
4. Ibid.