Periprosthetic osteolysis following total knee replacement is a well recognized intermediate to long term complication. Over the last four decades, the prevalence of osteolysis following total knee replacement has increased. Development of periprosthetic osteolysis after knee replacment surgery is related to three factors, generation of wear debris, access of that debris to bone, and the biologic reaction to the wear debris. Although more common in association with loose components, osteolysis can occur with stable cementless implants and less commonly stable cemented implants.
Polyethylene particles have been isolated from tissue around failed total knee replacements. When compared to total hip replacements, polyethylene wear particles from knee replacesments are larger. However, the majority of particles are still less than one micron in size and are biologically active. Several important factors impact polyethylene wear. The polyethylene itself is one of the most important variables. Research over the past decade has demonstrated the importance of manufacturing technique, sterilization methods, packaging and shelf life on wear performance. It is now known that polyethylene sterilized with gamma radiation and stored in oxygen with a long shelf life is associated with higher prevalence of osteolysis. Oxidized polyethylene has a lower resistance to wear thus increasing the particle load.
Modularity has been associated with a higher prevalence of osteolysis most likely because it can result in a higher particle load from backside wear. One study compared the prevalence of osteolysis with all polyethylene tibial components and modular tibial components. At comparable follow-up, none of the patients with all polyethylene tibial components developed osteolysis. In contrast, 18% of the patients with modular tibial base-plates developed peri-prosthetic osteolysis. Although there was a bias in favor of the all polyethylene components since they had been implanted in lower demand patients, this study suggests that backside wear is a clinically important source of biologically important wear particles. The stability of the tibial insert locking mechanisms impacts backside wear. Excessive insert motion can accelerate backside wear and result in the generation of both polyethylene and metallic particles. Knee implant design has attempted to improve the effectiveness of tibial locking mechanisms to decrease backside motion and thus wear debris generation. Mobile bearing designs address this issue by using polished cobalt chrome tibial base plates to minimize the debris generation between the insert and base plate. It is unclear whether the total particle load is less with a mobile bearing design compared to a modular fixed bearing design with a well-designed insert lock detail.
Technical issues that effect knee alignment and ligament balancing can impact wear. Perfect alignment is difficult to achieve. Slight malalignment may not represent a functional problem for the patient, but can result in increased stresses in the polyethylene and potentially accelerate wear. Ligament balancing and implant design act in concert to dictate knee stability. A tight flexion gap most commonly associated with under release of the posterior cruciate in cruciate retaining knee designs, can lead to accelerated polyethylene wear posteromedially. Patients with a loose flexion gap and minimally conforming implants are prone to increase anterior-posterior translation of the femur on the tibia during gait, so called “paradoxical motion.” Translation of the femur on the tibia increases shear stresses in the polyethylene and may accelerate wear.
Finally, the patients who undergo total knee replacement have changed dramatically over that time period. Many total knee replacement patients today expect to return to active lives. Higher activity creates greater wear volume that in turn increases the likelihood of osteolysis. A study from Charlotte, North Caroline documented the multi-factorial nature of osteolysis. Of the 1287 Press-Fit Condylar knees with more than five year follow-up, 8.3% had a wear related failure. Cox hazard analysis demonstrated five factors that correlated with a wear related failure. These included patient age, patient gender, polyethylene shelf life, polyethylene finishing method and polyethylene sheet vendor. This study emphasized the fact that relatively small changes in polyethylene manufacturing can have a significant effect on wear. It should also be noted that these inserts were gamma sterilized in air and the results cannot be generalized to implants sterilized by other methods.
Access to the implant bone interface and peri-prosthetic bone is affected by implant design and surgical technique. In general, access to bone is more of an issue with cementless components compared to cemented components. Wear debris can gain access to periprosthetic bone through screw holes in the tibial baseplate and regions of the implant bone interface that lack bone ingrowth. Incomplete porous coatings also provide an access channel for wear debris. So called “hybrid cemented technique” for tibial implantation may increase the risk of wear debris access to the proximal tibia. In one study, hybrid cementing was associated with a high rate of tibial osteolysis and loosening.
Although polyethylene wear is the driving force in the development of periprosthetic osteolysis after total knee replacement, because of the complex geometry of knee implants, measurement of polyethylene wear in knee is difficult. As a result, the first radiographic sign of significant wear may in fact be osteolysis. The complex geometry of knee implants and the distal femur and proximal tibia can make recognition and quantitation of osteolysis difficult. Peri-prosthetic after total knee replacement occurs in the cancellous bone of the distal femur and proximal tibia. Not only do the implants obscure the bone, but since cancellous bone is less radiodense bone loss is less obvious. The posterior aspect of the femoral condyles and the medial femoral condyle under the medial collateral ligament are areas that appear to be prone to the development of osteolysis especially with cementless femoral components. Oblique radiographs are sometimes helpful in evaluating the posterior femoral condyles. Radiographs typically underestimate osteolysis. Both CT and MRI have be used to more accurately quantitate lesion extent.
Little data exists to help the surgeon guide management of distal femoral or proximal tibial osteolysis at revision surgery. Although not always the case, osteolysis in association with cemented components is usually associated with a loose implant. The decision to revise is based on the degree of bone loss and the patient’s symptoms. In contrast, severe osteolysis can develop after cementless knee replacement in association with osseo-integrated cementless components. Clinically, patients can remain asymptomatic despite extensive bone loss. Often however, osteolysis of the knee is associated with complaints of swelling or late instability. Wear particles can result in synovitis that results in an effusion. The synovitic process can effect knee stability especially with cruciate retaining implants as it can result in damage to the posterior cruciate ligament.
When there is osteolysis and the implants are wellfixed, the decision to operate is based on the degree of bone loss and patient symptoms. The decision to recommend revision surgery is more difficult in patients who are asymptomatic. In addition to the degree of bone loss, other factors to take into account include patient age and activity level, patient comorbities and the risk for the development of a pathologic fracture. No objective data exists to guide optimal timing for surgical intervention.
When the implants are stable, the surgeon has a choice to graft the osteolytic lesion and exchange the tibial insert or revise the component. Again, there is very little data available to direct treatment. Insert exchange with impaction grafting of lytic lesions can be successful provided the implant is otherwise well aligned and the knee can be made ligamentously stable with a new insert. It is important to remember that the posterior cruciate ligament can be damaged as part of the synovitic process. As a result, a standard tibial insert may not be sufficient to provide stability. Although most knee systems offer a more constrained tibial insert for their cruciate retaining designs, the actual impact these more constrained inserts have on articular stability varies significantly between designs. Knees the had early wear related failure likely have technical or implant related factors that contributed to the failure process. In such cases, revision surgery should be considered. In contrast, patients who functioned well for many years and then had a wear related failure are reasonable candidates for an insert exchange provided the implants are well fixed.
Management of bone defects is performed using a combination of allograft bone chips and structural grafts as well as metal augments. In general defects at the level of the distal femur or proximal tibia can be managed with metal augments. Larger defects usually require grafting. Contained defects can be treated with impaction grafting of allograft bone chips. This can be performed in the presence of well fixed components. Commercially available bone substitute putties may be helpful in containing the intra-articular communication with the defect once it has been packed with bone chips. Non-contained defects are managed with structural allografts. Femoral heads usually suffice for management of these larger defects, although a distal femoral or proximal tibial allograft may be necessary in some cases. Tibial and femoral extension stems should be used when grafting has been performed to help stress protect the graft. Less commonly, patients with severe bone loss and associated collateral ligament loss will require a hinge prosthesis. In these cases, the bone defects can usually be managed with the implant and not require grafting.
We have recently reviewed our experience with management of large osteolytic defects at the time of revision knee replacement. Twenty-eight knees underwent revision TKA requiring surgical management of major osteolytic defects. Three groups of osteolytic defects were identified based upon the degree of implant stability and the magnitude of bone loss. Outcome measures included the KSCRS, visual analog pain score, and radiographs. At a mean follow-up of 48 months, the average knee pain scores, range of motion, and KSCRS improved (p<.05). Eighty-six percent demonstrated clinical and functional improvement and were satisfied with the outcome. Radiographs for 24 revision TKA’s demonstrated component stability and incorporation of both cancellous and structural allografts. Revision TKA for major osteolytic defects may be effectively performed using a variety of bone grafting techniques. Both morselized and structural bone grafting, in combination with stemmed components was successful in managing revision TKA in the setting of major osteolysis. Significant improvement in clinical and radiographic outcomes may be anticipated using these surgical techniques.
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