Medial Portal Drilling: Effects on the Femoral Tunnel Aperture Morphology During Anterior Cruciate Ligament Reconstruction Daniel Hensler, MD, Zachary M. Working, BSEng, Kenneth D. Illingworth, MD, Eric D. Thorhauer, BS, Scott Tashman, PhD, and Freddie H. Fu, MD, DSc(Hon), DPs(Hon) Investigation performed at the Department of Orthopaedic Surgery, University of Pittsburgh, Pittsburgh, Pennsylvania Background: A goal of anatomic anterior cruciate ligament (ACL) reconstruction should be to create a femoral tunnel aperture that resembles the native attachment site in terms of size and orientation. Aperture morphology varies as a function of the drill-bit diameter, the angle in the horizontal plane at which the drilled tunnel intersects the lateral notch wall (transverse drill angle), and the angle of knee flexion in the vertical plane during drilling. Methods: A literature search was conducted to determine population-based dimensions of the femoral ACL footprint. The tunnel aperture length, width, and area associated with the use of different drill-bit diameters and transverse drill angles were calculated. The effect of the knee flexion angle on the orientation (anteroposterior and proximodistal dimension) and size of the femoral tunnel aperture relative to the native femoral insertion of the ACL were calculated with use of geometric mathematical models. Results: The literature search revealed an average femoral insertion site size of 8.9 mm for width, 16.3 mm for length, and 136.0 mm 2 for area. The use of a 9-mm drill bit at a transverse drill angle of 40° resulted in a tunnel aperture area of 99.0 mm 2 and a tunnel aperture length of 14.0 mm. Decreasing the transverse drill angle from 60° to 20° led to an increase of 152.9% in length and of 153.1% in tunnel aperture area. When a 9-mm drill bit and a transverse drill angle of 40° were used, the aperture seemed to best match the native ACL footprint when drilling was performed at a knee flexion angle of 102°; deviations from this angle in either direction resulted in increasing tunnel area mismatch compared with the baseline aperture. Increasing the knee flexion angle to 130° decreased the proximodistal dimension of the aperture by 2.78 mm and increased the anteroposterior distance by 0.65 mm, creating a mismatched area of 13.5%. Conclusions: The drill-bit diameter, transverse drill angle, and knee flexion angle can all affect femoral tunnel aperture morphology in medial portal drilling during ACL reconstruction. The relationship between drilling orientation and aperture morphology is critical knowledge for surgeons performing ACL reconstruction. Clinical Relevance: This study can help the surgeon to understand how drilling parameters affect the morphology of the femoral tunnel aperture during ACL reconstruction. R upture of the anterior cruciate ligament (ACL) is a com- mon ligamentous injury. The primary aim of ACL re- construction is to restore the function of the ACL and the native kinematics of the knee 1 . Despite the widespread use and excellent short-term success of traditional arthroscopic ACL re- construction with use of a transtibial technique, the development of late osteoarthritis of the knee has been reported 2,3 . Concerns exist that these findings are the consequence of surgically altered knee kinematics applied cyclically over time to mechanically sen- sitive articular cartilage chondrocytes. In the last decade, there has been an increasing interest in and focus on anatomic ACL re- construction to more accurately restore the anatomy and bio- mechanics of the pre-injury ACL 4,5 through precise restoration of the osseous attachment sites and native tension patterns of the native ACL. This approach allows surgeons to individualize ACL surgery relative to the specific anatomy of every patient 6,7 . Although Disclosure: None of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of any aspect of this work. One or more of the authors, or his or her institution, has had a financial relationship, in the thirty-six months prior to submission of this work, with an entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. No author has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article. 2063 COPYRIGHT Ó 2011 BY THE J OURNAL OF BONE AND J OINT SURGERY,I NCORPORATED J Bone Joint Surg Am. 2011;93:2063-71 d http://dx.doi.org/10.2106/JBJS.J.01705