gms | German Medical Science

Objectives: The use of metaphyseal cones is a suitable procedure to manage severe bone loss in patients requiring revision total knee replacement, as the metaphysis of these patients could present important defects and the bone is weak and might be easily damaged during the surgery.

Nowadays, the cones available in the market are rigid and, unless custom-specific designed, are unable to correctly adapt to the shape of the patient's bone. Therefore, flexible metaphyseal cones have been recently introduced to reduce potential bone trauma during implantation surgery as they theoretically improve the interaction cone-bone, reducing bone stress and relative risk of fracture.

Preliminary clinical study on the use of such devices have shown promising results but, however, an in-vitro biomechanical study is still required to evaluate and quantify the effects of flexible cones during their surgical implantation and their performance during common daily activities.

Methods: This study is performed using finite element analysis, based on an already validated and published knee finite element model.

For the bone, the geometry of a Sawbones femur was selected; the geometries of the orthopedic devices used were then provided by the industrial manufacturer.

To take into account different implant design geometries, two versions were considered for this study: a 2-zones and a 3-zones cone. The 2-zone cone is interacting medio-laterally with the bone, while the 3-zones also have a potential interaction on the anterior side of the bone too.

To perform the comparison, each cone design was modeled both as flexible and as rigid; the rigidity of the cones was achieved by fixing the two posterior metal flanges to avoid any relative movements. With this approach, both rigid and flexible cones are characterized by the same external and internal geometry and therefore the bone-cone interaction is the same, with flexibility being the only difference among designs. The cones were implanted following surgical guidelines provided by the manufacturer and coupled with a hinged TKA. Three activities were then simulated: surgical impaction, walking and chair-rise.

Results and conclusion: As flexible cones are able to deform their geometries, they showed, compared to rigid cone, lower average and max von Mises stress and, therefore, lower risk of bone damage intra-operatively.

In detail, the elastic 2-zones cone reduced the average stress of around 50% anteriorly and 40% posteriorly; the elastic 3-zones cone reduced the stress of 6% and 36%.

Moreover, the bone stress resulted to be more homogenously distributed all over the cortical bone, with lower bone peaks.

Considering instead the daily activities, results showed no major differences between rigid and flexible cones in terms of stress distribution, stress magnitude and implant micromotions.

Thus, the performances of the flexible cone are the same of the rigid cone when common activities are performed and therefore, despite the flexibility, the implant stability does not change.