Excessive compression of the human tibio-femoral joint causes ACL rupture

Abstract

The knee is one of the most frequently injured joints in the human body. A recent study suggests that axial compressive loads on the knee may play a role in injury to the anterior cruciate ligament (ACL) for the flexed knee, because of an approximate 10° posterior tilt in the tibial plateau (J. Orthop. Res. 16 (1998) 122–127). The hypothesis of the current study was that excessive axial compressive loads in the human tibio-femoral (TF) joint would cause relative displacement and rotation of the tibia with respect to the femur, and result in isolated injury to the ACL when the knee is flexed to 60°, 90° or 120°. Sixteen isolated knees from eleven fresh cadaver donors (74.3±10.5 yr) were exposed to repetitive TF compressive loads increasing in intensity until catastrophic injury. ACL rupture was documented in 14/16 cases. The maximum TF joint compressive force for ACL failure was 5.1±2.1 kN for all flexion angles combined. For the 90° flexed knee, the injury occurred with a relative anterior displacement of 5.4±3.8 mm, a lateral displacement of 4.1±1.4 mm, and a 7.8±7.0° internal rotation of the tibia with respect to the femur.

Introduction

The knee is one of the most frequently injured joints in the human body. Epidemiological studies have shown there are approximately 80,000 anterior cruciate ligament (ACL) tears in the United States each year with a total cost nearly 1 billion dollars (Griffin et al., 2000). This study also reports that 70% of ACL tears are due to “non-contact” types of injury. In a study of high school soccer, volleyball and basketball players for one season, six of eight serious ACL injuries were “non-contact” (Hewett et al., 1999). Many studies have investigated the loading mechanisms that cause injury to the ACL. Boden et al. (2000) suggest that ACL injuries frequently occur in landing from a jump on one or both legs. In jump landings the knee may be flexed 60–80° (Hewett et al., 1996). ACL injury is also common in skiing with 25–30% of all ski-related knee injuries involving the ACL (Speer et al., 1995). These injuries are mainly associated with twisting or a hard landing from a jump with a flexed knee (Ettlinger et al., 1995).

The ACL functions as the primary restraint to limit anterior tibial displacements for both 30° and 90° of knee flexion (Butler et al., 1980; Fukubayashi et al., 1982; Torzilli et al., 1994). It provides approximately 85% of the total ligamentous restraining force during anterior tibial displacement. Fleming et al. (2001) confirm this notion by showing that normal, in vivo weight bearing in the knee induces tensile strain in the ACL due to anterior position shift of the tibia relative to the femur. This study also supports earlier investigations showing that tensile forces develop in the ACL under physiological levels of tibio-femoral (TF) joint compressive loading (Li et al., 1998; Markolf et al., 1981). Torzilli et al. (1994) show that physiological levels of compressive loading on the human TF joint generates anterior tibial translation with respect to the femur in the ACL-deficient knee for all flexion angles greater than 15°. This response is primarily thought to be due to the inherent posterior tilt of the tibial plateau of 10–15° (Li et al., 1998) (Fig. 1). This study further suggests “that excessive compressive loads caused by impact loads along the tibial shaft (e.g., load from a jump landing) may contribute to injury of the ACL, especially when the knee is flexed”.

The hypothesis of the current study was that the primary injury outcome for a flexed, isolated human knee joint under excessive TF compression would be ACL rupture. This study was designed to document the compressive loads required to cause an ACL rupture and the associated TF joint displacements and rotations when the knee is at 90° of flexion. Additional data showing the same injury in knees flexed to 60° and 120° will also be documented in the study.