Having a knee replacement is a tricky procedure with lots of factors used to guide the surgeon into getting it in the right position.
The femur needs to be central at the end of the bone, it also needs to have the correct joint line angle so that the leg is straight and the rotation must be right so that the foot faces straight forward. In addition to this the implant needs to be neutral i.t.o. flexion & extension so that the knee recovers full range of movement.
It is also important that the right amount of bone is removed so that the joint line is at the same place it was before and then the knee won't be too tight - causing stiffness, or too loose - causing instability.
The Tibia also has similar potential pitfalls as the rotation and angle are as important.
The added problem with getting the tibial slope slightly wrong is that if the starting point isn't perfect then the deformity is rotational as well as angular
Unfortunately, these factors are highly variable and there is a high chance that these components can be put in a slightly wrong position - even by the most experienced surgeons. A survey done on surgeons doing more than 400 Knee replacements a year, which is a huge amount, shows that 20% are being done with the components in the incorrect position.
In an effort to address this Knee surgeons have progressed to using computer aided guides in theatre and this was very effective in reducing the errors. Unfortunately, there is still a degree of surgical error, although it is much less. There are also the added problems of spending much longer in theatre and increased blood loss as the femoral canal is still breached.
The obvious next step was to see if this accuracy could be replicated in pre-op planning - so that the theatre time could be reduced and certain companies developed the technique of patient matched cutting blocks. These use scans taken a month before the procedure to replicate the patient's knee and make model cutting blocks, placing the new knee in exactly the right position. This has reduced the surgical error from 20% down to less than 2%. There is the added bonus of saving theatre time as well as the necessity of breaching the femur, thereby reducing blood loss.
It would seem unwise to take the chance of having a Knee replacement without this new technology.
I use Smith & Nephew Visionaire cutting blocks as they are the forerunners in developing this technology. More than 75 000 knees have been replaced in this way with excellent results.