2017 ISAKOS Biennial Congress ePoster #1022

 

Influence of Bone Mineral Density and Insertion Torque on ACL Soft Tissue Graft Fixation with Interference Screws

Elmar Herbst, MD, PhD, Münster GERMANY
Chris Como, BS, Pittsburgh UNITED STATES
Thomas R. Pfeiffer, MD, Cologne, NRW GERMANY
Jan-Hendrik Naendrup, BS, Köln GERMANY
Freddie H. Fu, MD, Pittsburgh, PA UNITED STATES
Richard Debski, PhD, Pittsburgh, PA UNITED STATES
Volker Musahl, MD, Pittsburgh, PA UNITED STATES

Orthopaedics Robotics Laboratory, Department of Orthopaedic Surgery, University of Pittsburgh, Pittsburgh, Pennsylvania, UNITED STATES

FDA Status Not Applicable

Summary

The results of this study demonstrate the strong correlation between bone mineral density and interference screw insertion torque with load-to-failure and stiffness of ACL hamstring tendon graft fixation, indicating the use of an oversized interference screw in the case of a decreased bone mineral density.

Abstract

Introduction

When performing an anterior cruciate ligament (ACL) reconstruction on middle-aged and older patients, surgeons must consider a possibly decreased bone mineral density (BMD). Particularly when using interference screws, graft fixation strength depends on BMD and insertion torque of the interference screw. However, the influence of different screw sizes has not been examined. Therefore, the purpose of this study was to investigate the influence of BMD and insertion torque on load-to-failure and stiffness of two interference screws for ACL soft tissue graft fixation. It was hypothesized that oversizing the interference screw by one millimeter relative to the bone tunnel diameter will result in a higher load-to-failure and stiffness compared to an interference screw matching the bone tunnel diameter.

Methods

Fourteen fresh-frozen human cadaveric knees (mean age 52.6 +/- 12.1 yrs.) were randomly assigned to one of the following screw conditions: 1) sized interference screw matching the bone tunnel and graft diameter, 2) one millimeter oversized interference screw compared to the bone tunnel and graft diameter. All interference screws had a length of twenty-five millimeters. To determine the BMD, dual X-ray absorptiometry (DXA) scans of the lateral femoral condyle were performed. A bone socket within the native ACL footprint was drilled according to the graft size to a depth of twenty-five millimeters, and the quadrupled autologous hamstring tendon grafts were inserted. A torque screwdriver (0.2 N-m – 4.5 N-m) recorded the insertion torque. The longitudinal axis of the femoral bone socket was aligned to the applied force of the materials testing machine (Instron® 5965 Dual Column Testing System). Specimens were preloaded with 10 N, followed by 20 cycles of preconditioning between 10 and 50 N and cyclic creep between 10 and 250 N.
A correlation analysis for each group was performed with the Pearson correlation coefficient to evaluate the correlation of both insertion torque and BMD with the maximum load-to-failure. The significance level was set at p < 0.05.

Results

When using the sized screw, five specimens failed by tendon pull out compared to six in the oversized screw condition. A significant correlation between BMD and load-to-failure (R = 0.770, p < 0.05) was found for the sized screw condition, such that higher BMD corresponded to higher load-to-failure. A strong, but not significant correlation was found for the oversized screw (R = 0.682, NS). Insertion torque correlated strongly with load-to-failure and stiffness for both the sized (R = 0.811, p < 0.05; R = 0.654, NS) and the oversized screw (R = 0.522, NS; R = 0.519, NS).

Discussion

The most important finding of this study is that both BMD and insertion torque strongly correlated with load-to-failure and stiffness of ACL soft tissue graft fixation. However, generally surgeons rely on subjective feel during insertion of interference screws rather than quantifying the insertion torque. Using a torque screwdriver to determine insertion torque may help to provide an objective method for surgeons to determine proper fixation and to interpret the graft fixation intraoperatively to assure adequate fixation of ACL grafts.