3D imaging in orthopaedic surgery
The benefits of MRI go beyond surface imaging, says Tim Spalding
Fluoroscopy provides a surgeon with the ability to image a structure in multiple planes, at the surgeon’s choosing. CT scans allow the production of three-dimensional (3D) images of the bony structures, allowing accurate visualisation – including removing parts that obstruct such a view. Soft tissues and articular cartilage need MRI for imaging, and accurate mapping of the joint surface requires 3D projection of the data. The additional benefit of MRI is that, more than surface imaging, MRI allows quantification of the underlying bone structure.
MRI images of the knee have been used for development of patient-specific instrumentation for knee arthroplasty, designing accurate cutting guides to lay on to exposed surfaces and facilitating saw cuts for definitive arthroplasty implants. The gain is minimising exposure and avoiding the need for drilling of intramedullary rods for alignment. The surgeon can view the 3D model and fine-tune the design to choose the best coverage and resection.
Can this technology help surgeons further in tailoring treatment strategies to individual knees?
Avenues now exist or are in development for imaging to allow improved cutting guides and surface implants.
The gap in treatment is the management of articular cartilage defects in patients where biological options are either too expensive (as in many countries) or are likely to have a low success rate – such as in the biologically older patient, probably meaning over age 45. Knee arthroplasty, relining an isolated compartment or relining the whole knee, remains a significant step and requires a certain level of intrusive symptoms to justify.
Designing a custom-fit implant to patch up defects in the articular surface may fill that gap in treatment and this requires both accurate modelling from 3D MRI imagery and the production of an accurate guide to direct the surgical instruments. The goal is a flush-fit exact match implant that is stable and well tolerated by the patient. Implants have been in use for several years now but 3D MRI-directed design intuitively should lead to better matching and positioning.
And what of the subchondral bone, where much focus has been placed as the cause of pain? MRI-directed design can tackle this area by altering the thickness required for reconstruction, ensuring adequate preparation to a stable base and optimal fixation.
So, where is this going? 3D templating with MRI can now accurately predict the size of implants to be used in knee arthroplasty, better than 2D modelling, and might now be directing the surgeon to the correct implant. Template-directed instrumentation (TDI) is being voiced, especially for the less-experienced surgeon. This is an interesting area of travel, but high preoperative planning expenses will need to be addressed to optimise this innovation and the benefit in the knee with large deformity needs study.
3D imagery in neurosurgery has allowed significant advances in stereotactic and functional neurosurgery, including virtual reality surgery, resulting in improved costs and time saving. Invasiveness and risks are reduced by such innovations. The key is the imaging, allowing the surgeon to manipulate the process to enable a given specific impression to be generated as required. Guesswork can be minimised and maybe this allows for improved results.
With such advances the door is open to new innovations and lateral thinking in diagnostics, interpretation and intervention. Meniscus imaging for reconstructive techniques and articular cartilage defect imaging for planning are examples. Planning is key, with the ability to detect surrounding suboptimal damaged cartilage allowing prediction for adequate clearance and preparation for biological techniques. And with mapping post-intervention, the surgeon can quantify the outcome – measuring 3D volumetric change. Maybe with the meniscus the functional motion can be evaluated.
Around the corner there is a paradigm shift in where imaging will lead the surgeon and the radiologist – not just for the knee but for all joints.
Episurf has innovated this technology combining the 3D MRI mapping for planning with the development of accurate 3D-printed milling cutting guides, allowing the accurate implantation of their custom surface implant Episealer. The result is an affordable customised new surface designed for a patient, implanted with short surgery time and minimal exposure.
Mr Tim Spalding is a consultant orthopaedic surgeon at the University Hospitals Coventry and Warwickshire NHS Trust