What is a prosthetic knee revision?
The revision of a knee prosthesis previously implanted, is a surgery aimed at replacing one or more prosthetic components. There are many causes that can make this procedure necessary, some related to the attrition of the implant by time and use, others related to specific complications. The common goal, regardless of the reason for the review, is to restore a well-functioning joint.
When is the surgery needed?
Prosthetic revision is suitable in several circumstances:
joint insert wear: if the tibial insert wears out (to be expected being made up of a plastic material), the knee can lose stability and joint function can deteriorate accordingly.
aseptic mobilization of prosthetic components: the release of debris (especially polyethylene particles) can induce a reaction by the surrounding bone favoring the absorption and subsequent displacement of the components that should be "fixed", such as the femoral or tibial component;
Periprosthetic infection: infections almost always require surgical treatment, because prosthetic surfaces represent a fertile and protected soil for germs, which can hardly be eradicated with antibiotic therapies only; if the infection is not promptly treated, it may remain in the neckline of the prosthesis, which in this case is called septic mobilization;
degeneration of residual joint compartments: when a total prosthesis does not involve the kneecap, it is possible that over the years this develops painful degenerative alterations that require replacement; the same thing can happen to the other unreplaced parts such as the femoral-tibial compartment when only one of the two has been replaced (single-compartmental prosthesis);
stiffness: the stiff knee, when it is acknowledged after 6-8 weeks of the prosthetic surgery, is no longer treatable through simple mobilization under anesthesia, and often requires a prosthetic overhaul.
instability: when a prosthesis is unstable, the use of a prosthetic model with a greater degree of constraint can effectively solve the problem.
Periprosthetic fractures: occasionally a fracture affecting the bone surrounding the prosthesis requires the replacement of the implant to allow a healthy bone anchoring.
In the example below you can see the final result of the prosthetic replacement in case of severe septic mobilization associated with instability, where the long stems serve to ensure fixation despite poor local bone quality, the tibial shirt fills a large bone defect and the central "pin" makes stable a joint in which all ligaments have been severely damaged by the previous infection :
Insert wear
The joint insert, made of polyethylene, can wear out after many years of intense use, especially in heavy and continually active subjects. The replacement can extend the life of the implant thus overcoming possible small instabilities in the extremely near future, but above all preventing the onset of aseptic mobilization of the components. It is therefore a low-invasive surgical intervention capable not only of treating the current problem, but also of preventing far more serious consequences.
Aseptic mobilisation
The polyethylene debris released after many years of intense use can result in an irritating reaction in the surrounding bone (periprosthetic osteolysis), and in the more severe cases total mobilization. While the former is essentially asymptomatic, the second is painful being remarkably like the osteoarthritis pain the patient felt before his knee implant.
Depending on the severity of the bone defect, "customised" implants can be used, that is, suitable to find the necessary fixation even when faced with serious bone defects.
Periprosthetic infection and septic mobilisation
Periprosthetic infections, when chronic, have little chance of healing with conservative interventions such as local surgical clean up (which are often decisive when identified early). In fact, many bacteria are capable, if they have the time, to build a protected environment close to the prosthetic surfaces (the so-called "biofilm"), which makes them essentially can’t be eradicated except with the removal of the prosthesis itself.
In these cases, the only possibility of treatment is the removal of the infected prosthesis and the replacement of a new prosthetic element. Sometimes this is performed in a single surgical time (one-stage septic revision), but more often it is performed in two stages (two-stage septic revision). In the second case (safer although obviously less comfortable for the patient) the removal and the implant are carried out with at a 2-4 month interval during which the joint environment houses a specific spacer, with the dual task of cleaning up the tissues and maintain the space intended for the final prosthesis.
Joint degeneration of residual compartments
Single-compartmental prostheses, by definition, replace only one joint compartment, most often the medial compartment. This means that it is theoretically possible that, with the passing of time, one of the two residual compartments will undergo an osteoarthritis process that requires prosthetic replacement. In these cases the most validated solution is the conversion to total prosthesis, although in very selected cases you can resort to "integrative" solutions (for example, adding a lateral single-compartmental prosthesis or a femoro-patel-knee prosthesis to the pre-existing medial prosthesis).
Total prostheses can also require an "integrative" surgical action in case the kneecap, initially preserved by the surgeon, over time becomes painful. Deferred kneecap prosthesis is one of the most common knee replacement interventions.
Stiffness
Limitation of the articulation of the prosthetic knee can depend on multiple factors (inadequate regimen of post-operative kinesitherapy, excessive tension of soft tissues, "individual" predisposition to develop stiffness and ade abstentions, occult infections...). It should be remembered that the prosthetic knee almost never reaches the movement excursion of a healthy knee, because the prosthetic design of most total prostheses provides an ideal range between 0 and 130 degrees. A stiffness is relevant when it prevents the knee from full extension, because it causes a functional overload of the quadriceps muscle and therefore a pathological fatigue of the prosthetic limb, or when it does not allow a minimum flexion of 90 degrees needed to carry out most of the normal everyday movements (such as sitting). Relative flexory limitations (between 90 and 120 degrees) can hinder the descent of the stairs and reduce the functional performance of the most active subjects, but they are rarely so significant as to induce the patient to request therapeutic intervention. Rigidities, when recognized early (within 6 weeks of surgery), can benefit from a mobilisation of the knee under anesthesia; when diagnosed later it will always need a surgical intervention (arthroscopic or more often open surgery). Open-air implants usually involve replacing the components (or, more rarely, the insert alone) to reduce the size of the prosthesis and the surrounding soft tissues responsible for stiffness. Rigid knee revision surgery is particularly delicate and at risk of recurrence, so it must always be followed by particularly strict rehab protocols.
INSTABILITY
Instability is often an under-estimated among patients dissatisfied with their artificial articulation. Symptoms range from the feeling of subsidence to real painful manifestations, which occur only when the limb is loaded. In addition to these subjective disorders that the patient reports, there are objective problems related to early wear. Sometimes the knee is unstable from the start, other times the problem arises later, for example when the posterior cruciate ligament degenerates years after implantation of a total "cruciate-retaining" prosthesis (i.e. designed to preserve and exploit the function of this ligament). When muscle strengthening is not enough, prosthetic revision solves instability by adopting a prosthetic model with a greater degree of constraint. In this way the dysfunction of the surrounding soft tissues (ligaments, joint capsule, tendons) is resolved by the mechanical characteristics of the implant.
What does the revision involve?
The knee revision surgery is usually carried out through the same access route used in the previous intervention, appropriately extended in the most complex cases. The postoperative course is very variable: if the replacement of the insert and the secondary prosthesis of the kneecap have a generally faster course than the first implant, femoral and tibia re-implants may require longer recovery times, especially when the intervention is technically complex and you have to deal with extensive bone defects.