Transtibial Versus Anteromedial Portal ACL Reconstruction: Can A Hybrid Approach Achieve The Best Of Both?

OBJECTIVES: A number of studies suggest improved biomechanical and clinical results when ACL grafts are placed anatomically. Worldwide, the most common transtibial (TT) technique has been shown less anatomic than an anteromedial portal (AM) technique, though is much more familiar and less technically challenging. A hybrid transtibial approach (HTT) - using AM portal guidance of a flexible transtibial guide wire without knee hyperflexion - may offer anatomic graft placement while maintaining the relative ease of a transtibial technique. Our objective is to evaluate the anatomic and biomechanical performance of HTT, when compared to TT and AM approaches. METHODS: A total of 36 paired, fresh-frozen human knees were used. 24 knees (12 pairs) - each using a standard tibial tunnel - underwent all three techniques (TT, AM, HTT) for femoral tunnel placement, with direct measurement of femoral insertional overlap and femoral tunnel length for each technique. The remaining 12 knees (6 pairs) were used to evaluate graft kinematics and tunnel orientation. Among these knees, 3 size-matched groups (four specimens each) were assigned to each of TT, AM and HTT techniques. Specimens were tested in quad-load induced extension from 90o to 10o of flexion with a distal weight used to simulate half-shank, ankle and foot passive load. Bony kinematics were assessed using a trakSTAR motion tracking system with three sensors installed in each femur and tibia. After femoral tunnel preparation, two sensors were installed in each tunnel to relate tunnel orientation and tunnel inlet location within the joint to the sensors used for motion tracking for the femur and tibia of each specimen. Bone-patellar-bone autografts were harvested from each specimen and used for the assigned reconstructive technique. Analysis included determination of ACL graft length changes, ACL-to-femoral tunnel angle and ACL-to-tibial tunnel angle at five flex/ext angles. Data was analyzed for three flex/ext tests on each specimen. RESULTS: Femoral tunnel length averaged 42.6 mm ±2.8 mm for TT, 38.5 ±2.0 mm for HTT and 31.6 ±1.6 mm for AM, with HTT tunnels being comparable to TT (p=0.12) but significantly longer than AM (p=0.0005). Percentage insertional overlap of the femoral tunnel aperture averaged 37.0% ±28.6% for TT, 93.9% ±5.6% for HTT, and 79.7% ±7.7% for AM, with HTT overlap significantly greater than both TT (p=0.007) and AM (p=0.001). Mean ACL percent increase in length at 10o of flexion was significantly greater in HTT constructs as compared with TT constructs (30.1% vs 8.5%). HTT constructs also exhibited a significantly lower mean ACL-to-femoral tunnel angle vs TT constructs at 10o flexion (150.6o vs 160.8o). AM constructs demonstrated significantly lower mean ACL-to-femoral tunnel angles vs TT constructs at 30o, 50o and 70o of flexion. All other comparisons between the three construct types at the reported flexion angles were statistically similar. CONCLUSION: Our findings suggest highly anatomic femoral tunnel apertures are achieved with this hybrid technique, yet with the advantage of long femoral tunnels and minimal tunnel angulation comparable to a traditional transtibial technique. Further, grafts placed with this technique demonstrated more normal, anisometric behavior. The HTT technique may offer a more anatomic alternative to an AM portal approach, while maintaining the technical ease and familiarity of a traditional transtibial technique.