Where patients have a painful arthritic knee with other conditions near or involving the knee joint, such as an old fracture of the lower femur or upper tibia which healed in a deformed or malunited position, or previous surgery with internal fixation devices (plates and screws), pronounced deformity due to significant bone stock loss and/or a grossly unstable knee due to stretching or tearing of ligaments – then a standard primary knee replacement may not suffice and a larger, complex primary knee replacement may be necessary.
In this setting, a variety of additional measures, including the use of more complex implants with longer stems, bone replacing metallic augments, bone grafting and larger plastic liners are often needed to restore the length and correct alignment of the lower limb and replace the missing bone stock, and adequately re-tension the collateral ligaments and other surrounding soft tissue structures. Where increasing degrees of deformity are corrected or previous trauma or surgery have resulted in attenuation of the collateral ligaments, if the degree of laxity is moderate, but there is adequate soft tissue such that, it’s likely with time and soft tissue healing, that the knee will develop some biological stability, then temporary balancing using a very stable tightly fitting or “constrained” plastic liner between the femoral and tibial components with a closely fitting “Cam-Post” articulation is appropriate. This complex primary total knee replacement, even though additional implants have been used to build up the tibial and or femoral bone stock and to restore the joint surface and alignment, the knee replacement itself, still has much the same articulating surface as the normal or standard total knee replacement and as such, usually, a similar functional result can be achieved, with good alignment, range of motion (knee bending) and mobility. Occasionally, where intramedullary stems are used inside the femoral and/or tibial canals, patients will experience a mild aching sensation or deep bony discomfort, where the rigid rod transfers load from body weight and activity to the surrounding femur and tibia. Usually over time, the surrounding bone will increase thickness and strengthen and the aching sensation will diminish or disappear. Also, as there is a larger volume of metallic implants used, there is a greater likelihood that patients will experience an internal aching sensation with changes in atmospheric pressure with different weather conditions, due to the body’s natural internal ”piezoelectric” charges and current along the long bones. These are normal phenomena following this type of surgery and can affect different persons differently, to a greater or lesser extent than others.
When significant bone stock loss from the shaft of the femur or tibia is present, such as after significant trauma, previous surgery or having to remove a bony or soft tissue tumour or address a non-correctable bony deformity, then a larger “endoprosthesis” may be needed to replace large segments of bone. These implants can be quite large and allow for reattachment of soft tissue structures such as the collateral ligaments and the patella tendons and require considerable preoperative planning and intra-operative attention to detail to try to correctly restore lower limb leg length and rotation of the femur and tibia to allow for adequate functioning of the extensor mechanism and lower limb nerves and blood vessels. Finally, where a combination of bony and soft tissue deficits exist, and it is not possible to achieve stability of the knee through the above described measures, it may be necessary to use a “rotating hinge” type knee replacement, where the mechanical device has a hinge that both flexes and extends and also rotates. These larger implants usually require a longer incision, a wider surgical approach and longer operative and recovery times. If consideration of an endoprosthesis or rotating hinge type implant is necessary, many of the routine anatomical landmarks will often be absent, making it more difficult and less reliable to achieve the usually desirable range of motion and level of function in the knee, with a necessary shift in the overall goals of surgery away from a high level of function to achieving a lower limb with approximately equal leg length with improved pain management and improved ability to stand and mobilise. Careful consideration is usually given to all patients being considered for knee replacements to help best identify in advance, who is likely to require such implants, but occasionally, unexpected intra-operative findings modify the selection of implants from the standard to a more constrained implant. It is still usually possible to achieve a good alignment, acceptable range of motion and stable, pain free or pain improved knee with these more complex knee replacements.