THE EFFECT OF AXIAL LOAD IN THE TIBIA ON THE RESPONSE OF THE 90º FLEXED KNEE TO BLUNT IMPACTS WITH A STIFF, DEFORMABLE INTERFACE

2004 STAPP Car Crash Conference

Meyer EG, Sinnott MT, Haut RC

Lower extremity injuries are a frequent outcome of automotive accidents. Knee injuries constitute approximately 10% of injuries recorded each year in NASS (Atkinson and Atkinson, 2001). While the lower extremity injury criterion is based on fracture of bone, most injuries are of less severity. Recent studies suggest microscopic, occult fractures may occur in the knee for impacts with rigid and stiff, deformable interfaces due to excessive levels of patello-femoral contact pressures (Atkinson and Haut, 2001). One method of reducing these contact pressures for a 90º flexed knee is to provide a parallel pathway for knee impact loads into the tibial tuberosity (Hering and Patrick, 1977). Yet, blunt loads onto the tibial tuberosity can cause posterior drawer motion of the tibia, leading to injury or rupture of the posterior cruciate ligament (PCL) (Viano et al., 1978). These data were used to develop a shear block sensor in the Hybrid III dummy knee. Recently our laboratory has shown that axial loads in the tibia, which are measured during blunt loading on the knee in typical automobile crashes (Bedewi and Diggs, 1999), can induce anterior drawer motion of the tibia and possibly help unload the PCL (Jayaraman et al., 2001). The purpose of this study was to explore this aspect in combined joint loading experiments for blunt impacts to the knee with a stiff, deformable interface.

Thirteen isolated 90º flexed human knee preparations from 7 cadavers aged 69±15.6 years have been tested to date in a biaxial servo hydraulic testing machine. The knees were isolated, and the femur and tibia/fibula bones were potted in epoxy. The long axis of the tibia was attached to a horizontal actuator, while the axis of the femur was positioned directly below a vertical actuator. A 15 cm by 15 cm deformable interface (3.3 MPa crush strength Hexcel) was attached to the vertical actuator and a 10 Hz load was applied directly on the patella and tibial tuberosity of the knee. Both with and without axial loads in the tibia, loads were applied at increasing intensity until gross failure of the joint. Pressure sensitive film measured the distribution of the patellofemoral joint loads, the Hexcel interface was analyzed for determination of load distribution between the patella and tibia, and posterior drawer displacement of the tibia was recorded.

Eight of 13 knees suffered PCL tears or avulsions at 12.9±2.7 kN and 17±3 mm of posterior tibial drawer relative to the femur. Knees that failed with no load applied along the tibial axis were more compliant (1.5 mm of tibial drawer/kN load on the anterior knee) and carried less of the knee load on the tibial tuberosity (14±7%), than knees that failed with a 5 kN tibial load (1.15 mm/kN and 20±7%). In subfailure experiments there was a significant correlation between posterior tibial drawer and the applied loads on the knee and tibia. Axial compressive loads in the tibia also reduced the contact pressures and contact forces in the patellofemoral joint.

This study showed that tibiofemoral compressive loads (axial loads in the tibia) that occur during automotive crashes with a 90º flexed knee can help stiffen the knee to prevent PCL injury during contact with the instrument panel. Furthermore, axial tibial loads can help reduce contact pressures in the patello-femoral joint that may cause chronic disease following a subfracture accident.

Orthopaedic Biomechanics Laboratories,

College of Osteopathic Medicine,

Michigan State University,

East Lansing, Michigan 48824

Please address correspondence to:

Roger C. Haut, Ph.D.,

Orthopaedic Biomechanics Laboratory,

College of Osteopathic Medicine,

A414 East Fee Hall,

Michigan State University,

East Lansing, MI 48824,

Tel: (517)355-0320,

Fax: (517)353-0789,

E-mail: haut@msu.edu