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Poster 256: The "N+10 Rule" to Avoid Graft Tunnel Mismatch in Bone-Patellar Tendon-Bone Anterior Cruciate Ligament Reconstruction Using Independent Femoral Tunnel Drilling

Objectives: To determine the amount of tibial graft tunnel mismatch during application of the “N+10” rule in arthroscopic assisted anterior cruciate ligament (ACL) reconstruction with bone-patellar tendon-bone (BTB) graft using independent femoral tunnel drilling. To compare actual (measured) tibial tunnel length and desired (calculated) tibial tunnel length when using the “N+10” rule for independent femoral tunnel drilling. Methods: Twenty paired knees from 10 fresh frozen cadaveric specimens (5 female and 5 male specimens) underwent arthroscopic assisted bone-patellar tendon-bone ACL reconstruction using independent femoral tunnel drilling with either a hyperflexion accessory anteromedial portal (n=10) or flexible reamer technique (n=10). The prepared patellar tendon graft bone blocks were trimmed to 10 x 20mm for all specimens and the intertendinous distance (N) between the bone blocks was measured. The “N+10 Rule” was used to set the angle of the ACL tibial tunnel guide to the appropriate degree setting for drilling. The amount of excursion or recession of the tibial bone plug in relation to the anterior tibial cortical aperture was measured in both flexion and extension. A +/-6mm threshold was used to assess this strategy based on prior biomechanical aperture fixation studies. As a secondary outcome, the intraarticular distance (IAD) and the actual tibial tunnel length (TTL) were measured and compared to the planned TTL based on the equation: IAD + TTL = N + 20 (size of bone plug in millimeters) Results: The average BTB ACL intertendinous distance (N) was 47.5mm (SD =5.5mm). The average measured IAD was 27.2mm (SD=3.0mm). Using the “N+10 Rule”, the average mismatch of the tibial tunnel was 4.9mm (SD=3.6mm) and 3.8mm (SD=3.5mm) in flexion and extension, respectively. Both values fall within the +/- 6mm threshold. When comparing the actual measured tibial tunnel length (TTL) to the desired TTL, there was an average difference of 5.4mm (SD=3.9mm) Conclusions: The “N+10” rule resulted in an acceptable average mismatch in both flexion and extension using the threshold of +/- 6mm for 20 cadaveric knees. Measured actual TTL compared to the desired calculated TTL were acceptable 61% of the time using the same threshold. The “N+10” is a simple and effective intraoperative strategy for achieving desired tibial tunnel length to avoid excessive graft tunnel mismatch. Figure 1: Derived formula for calculating ideal tibial tunnel length (TTL) using intraarticular distance (IAD), and the known values of the intertendinous distance (N) and lengths of the bone blocks (20mm). Figure 2: Intraoperative photo of BTB ACL graft measuring intertendinous distance between the bone plugs (N=55mm). Following the "N+10" rule Figure 3: Intraoperative photo of tibial ACL guide measurement of tibial tunnel length at 50mm with guide set to 65 degrees. Figure 4: Intraoperative photo of the ACL tibial tunnel guide positioned on the anatomic tibial footprint set to 65 degrees (N=55mm). Using this setting for the guide, the tibial tunnel measured 50mm as seen in the zoomed in photo, as per the formula.