Introduction
Component alignment and ligament balancing are critical factors in achieving a successful functional outcome following TKA1. Malalignment is an important cause of early failure. This can cause pain, instability, reduced range of movement, excessive polyethylene wear, and subsequent implant loosening. Conventional instrumentation uses anatomical bony landmarks. Reference errors with these landmarks can occur because these are either invisible (e.g., femoral head), virtual (e.g., mechanical axis) and difficult in the presence of associated deformities. Computer-assisted surgery (CAS) technology allows intra-operative quantitative measurements of axes. This information assists the surgeon in achieving component alignment and improved balancing of the knee, thereby avoiding the “outliers” in the alignment of the mechanical axis.
CAS: The Technology
Overall there are two mainstream technologies in current use: image based and imageless CAS. The former usually uses fluoroscopic assistance and has the ability to create a spatial link between the image and anatomical landmarks, the defined virtual points, planes, and axes. This enables the surgeon to visualise the fluoroscopic image of the implants intra-operatively and has control of every step in the procedure. Thus, with an image-based system, the surgeon can define the landmarks kinematically as well as visually. However, this has certain drawbacks. The fluoroscope is bulky, needs larger operating field & there is a potential radiation hazard. The imageless CAS is cheaper, less bulky, and easier to use.
In CT-based systems, scans are obtained pre-op and, intra-operatively the surgical field is registered and defined using either a surface-based or point-based system. Surface-based registration can provide a virtual 3D image and also provides assessment of bone density. This technique uses more radiation and can be costly.
The alternative is an image-free navigation. In this technique, the key anatomical references points (centre of the hip and ankle) are digitised by the surgeon. Accuracy is user dependent & complete 3D images cannot be obtained.
'Bone Morphing' is a newer technique and it provides complete 3D images. The computer computes the points digitised on the articular surfaces of the tibia and/or femur. Here the registration is done intra-operatively between the anatomical data and the statistical model. It cannot provide information on bone-density.
Virtual fluoroscopy is a system which does not require a registration procedure. The principle is to navigate on calibrated fluoroscopy images. After two to three images are acquired, the C-arm is removed. The images are 2-dimensional and expose the staff to radiation.
For the femoral component, the CAS technology provides the surgeon with precise information on flexion/extension and varus/valgus. For the tibial component, it provides more precise information regarding the orientation of tibial slope and varus/valgus positioning. This can assist in achieving more precise alignment and flexion/extension balancing of the knee.
Computer Assisted TKR Vs Conventional TKR
Different authors have published their experiences with these systems. A recent meta-analysis has shown more accurate AP & lateral alignment of the tibial and femoral components with fewer outliers outside the range of 3
