Patellar length affects tendon contact after posterior-stabilized TKA

By Liam Davenport, MedWire Reporter

In patients undergoing posterior-stabilized (PS) total knee arthroplasty (TKA), patellar ligament length has the greatest effect on the extent of tendo-femoral contact, say US researchers who have developed a model to aid clinical decision-making.

Up to 14% of patients who have PS TKA subsequently experience painful patellar crepitus, which is defined as a palpable grinding sensation in the region of the distal quadriceps tendon. Recent studies have suggested that prosthetic design, component alignment, alteration of the joint line, patellar height, and patellar tracking, as well as the dimensions of the femoral component intercondylar notch, are associated with the complication.

Paul Rullkoetter, from the University of Denver in Colorado, and colleagues examined the influence of these factors on the potential for tendon to notch contact during simulated deep knee-bend activity using a specimen-specific finite element (FE) model of a PS-implanted knee.

Specifically, the team performed discrete perturbations in component placement and patellar alignment between 10° and 120° femoral flexion. In addition, three crepitus-control patient pairs matched for age, gender, and body mass index were represented in the model to assess quadriceps tendon contact, and to determine the impact of surgical interventions to improve any deleterious contact.

The team reports in the Journal of Orthopaedic Research that tendo-femoral contact was improved by increasing patellar length, flexing the femoral component, and lowering the joint line. This moved the contact away from the anterior edge of the notch and reduced the cumulative contact area within 2 mm of the intercondylar notch.

The greatest impact on tendo-femoral contact was achieved by perturbing the patellar tendon length by 6.5 mm, or 1 standard deviation (SD), and 13 mm, or 2 SD. Increasing patellar tendon length by 2 SD held the tendon further away from the notch, while decreasing tendon length by 2 SD caused contact between the quadriceps tendon and the intercondylar notch.

In the patient pairs, flexing the femoral component and lowering the joint line decreased the cumulative tendo-femoral contact, and combining these two interventions led to the greatest improvement in contact.

The team concludes: "Elucidating the parameters affecting tendency for quadriceps tendon contact with the intercondylar notch will aid in optimizing clinical outcome for patients with a current PS TKA.

"Additionally, the FE model representation of the mechanical simulator will be useful in understanding the relationship of intercondylar notch dimensions and relative position during the design phase of PS TKA."

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