Assessment of Knee Laxity After Retrograde Intramedullary Nailing for Periprosthetic Distal Femur Fractures: A...

Periprosthetic distal femur fractures have grown in volume because the rate of primary total knee arthroplasty (TKA) has increased. These injuries are increasingly treated with intramedullary nailing (IMN), but knee instability after retrograde IMN has not been well studied. A cadaveric study was performed to investigate TKA balance after retrograde femoral IMN. The main hypothesis was that retrograde femoral IMN preparation would lead to increased flexion space laxity.

A medial parapatellar approach and measured resection TKA technique were performed in 8 cadaveric knees. A tensioner was used to measure the flexion and extension spaces in millimeters and the anterior-to-posterior (AP) translation of the knee at various times. Flexion and extension gaps were measured at a constant 50 Newton-meters both with and without a femoral trial component in place. A tibial trial component was not placed to have a flat surface to reference. Owing to the size of the tensioner, the extension gap could only be measured without a femoral component in place. All knees were then closed, underwent opening reaming for a retrograde femoral nail (rIMN) under fluoroscopy, and then reopened to assess flexion and extension spaces and AP translation. A tibial component was intentionally not implanted to isolate the effect of femoral TKA preparation on rIMN placement and its subsequent impact on the flexion gap and AP stability of the knee. Differences were measured between the pre- and post-rIMN gaps and calculated as the post-rIMN gap minus the pre-rIMN gap. Simple descriptive statistics were performed, including mean and SD. Differences between the pre- and post-IMN measurements were assessed with paired students t tests.

Without a femoral component and compared with the pre-IMN space, the post-IMN flexion space was greater by 1.5 ± 1.2 mm ( P = 0.01), the post-IMN extension space was not different at 0 ± 0.53 mm ( P = 0.60), and the post-IMN AP translation was greater by 3.13 ± 2.3 mm ( P = 0.01). With a femoral component and compared with the pre-IMN space, the post-IMN flexion space was not different at 0.13 ± 0.64 mm ( P = 0.60).

After TKA femoral preparation, retrograde IMN did not significantly affect the flexion gap when a femoral component was in place. Without a femoral component in place, the extension gap was unchanged after IMN placement. Although the flexion gap was statistically greater after IMN placement when measured without a femoral component, the clinical relevance is questionable given this difference was, on average, only 1.5 mm greater and a femoral component was not present. Similarly, the differences in AP translation observed without a femoral component were attenuated once the component was in place, which may have been further attenuated if a tibial component was able to be placed. These findings suggest limited clinical impact of retrograde IMN of the knee in the context of modern TKA femoral preparation.