The "N+10 Rule" to Avoid Graft-Tunnel Mismatch in Bone-Patellar Tendon-Bone ACL Reconstruction Using Independent Femoral Tunnel Drilling

Abstract

Background: Graft-tunnel mismatch (GTM) is a common problem in anterior cruciate ligament (ACL) reconstruction (ACLR) using bone-patellar tendon-bone (BPTB) grafts.

Hypothesis: Application of the "N+10 rule" in endoscopic ACLR with BPTB grafts will result in acceptable tibial tunnel length (TTL), minimizing GTM.

Study design: Controlled laboratory study.

Methods: Endoscopic BPTB ACLR was conducted on the paired knees of 10 cadaveric specimens using 2 independent femoral tunnel drilling techniques-accessory anteromedial portal and flexible reamer. The graft bone blocks were trimmed to 10 × 20 mm, and the intertendinous distance (represented by "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 number of degrees 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 GTM threshold of ±7.5 mm was set based on prior studies.

Results: The mean BPTB ACL intertendinous distance was 47.5 ± 5.5 mm. The mean measured intra-articular distance was 27.2 ± 3 mm. Using the N+10 rule, the mean total (flexion plus extension) GTM was 4.3 ± 3.2 mm (GTM in flexion, 4.9 ± 3.6 mm; GTM in extension, 3.8 ± 3.5 mm). In 18 of 20 (90%) cadaveric knees, the mean total GTM fell within the ±7.5-mm threshold. When comparing the actual measured TTL to the calculated TTL, there was a mean difference of 5.4 ± 3.9 mm. When comparing femoral tunnel drilling techniques, the total GTM for the accessory anteromedial portal technique was 2.1 ± 3.7 mm, while the total GTM for the flexible reamer technique was 3.6 ± 5.4 mm (P = .5).

Conclusion: The N+10 rule resulted in an acceptable mean GTM in both flexion and extension. The mean difference between the measured versus calculated TTL using the N+10 rule was also acceptable.

Clinical relevance: The N+10 rule is a simple and effective intraoperative strategy for achieving desired TTL regardless of patient-specific factors to avoid excessive GTM in endoscopic BPTB ACLR using independent femoral tunnel drilling.

Keywords: anterior cruciate ligament; graft-tunnel mismatch; reconstruction; tibial tunnel length.

Figure 1.

An intraoperative photo of a bone–patellar tendon–bone anterior cruciate ligament autograft showing the intertendinous distance (“N”; 55 mm) between the bone plugs.

Figure 2.

An intraoperative photo of the tibial ACL guide (Arthrex) being set using the N+10 rule. Given the intertendinous distance of 55 mm (as measured in Figure 1), the guide angle for drilling was set to 65°. The inset shows that the approximate drilled tunnel length was 50 mm.

Figure 3.

An intraoperative photo of the ACL tibial tunnel guide (Arthrex) positioned on the anatomic tibial footprint. The guide is set to 65° (“N” = 55 mm, as shown in Figure 1). Using this angle for the guide, the TTL is estimated to be 50 mm (inset). Notice how the guide is positioned parallel to the tibial articular surface for accurate distances. ACL, anterior cruciate ligament; TTL, tibial tunnel length.

Figure 4.

A derived formula for calculating the predicted TTL using the IAD and the known measured value of the intertendinous distance (N) and the set lengths of the bone blocks (20 mm). IAD, intra-articular distance; TTL, tibial tunnel length.