The Redesign of a Ski Plate to Reduce Knee Injuries University of Wisconsin – Madison Biomedical Engineering Department Biomedical Engineering 400 October 19, 2001 Team Members Nikhil Bagadia Jason Berta James Burke David Manthei Client Ray Vanderby, Jr. PhD Advisor Mitchell Tyler, MS, PE, Scientist1. INTRODUCTION In the United States alone, more than 20,000 knee related injuries occur each year while downhill skiing [Vermont Safety Research, 1999]. Knee injuries constitute between 25-30 percent of the total injuries incurred, making it the largest skiing injury category. Of the knee injuries reported, nine out of ten involve the ligamentous structures within the knee [Hull, 1997]. Ligament injuries are extremely painful and often require surgery and several months of rehabilitation, with no guarantee for a full recovery. The public’s push for safety in recreational activities has led the ski binding industry to address this problem through new technology and the adaptation of previous binding design. 1.1 TYPES OF SKI-RELATED KNEE INJURIES Today, due to the increased rigidity within current ski boot designs (and several other factors), the rate at which knee injuries occur has tripled since 1980 and has remained constant ever since [Ettlinger, 1995]. Two methods have been developed to combat this trend, one focusing on hardware (bindings and boots) and the other focusing on software (education and testing). The Vermont Safety Research group (VSR), a team of scientists that have been investigating ski-related injuries and their causes for nearly thirty years, believes that two injury scenarios occur most commonly while skiing, the Phantom Foot ACL and the Boot Induced ACL [VSR, 1998]. These two injuries have since been widely accepted as two leading scenarios that lead to serious knee injury. 1.1.1 THE PHANTOM FOOT ACL The Phantom Foot ACL was identified and developed by VSR from an analysis of over 14,000 skiing injuries and numerous videotapes of actual anterior cruciate ligament (ACL) injury footage [VSR, 1998]. This injury situation was named the “Phantom Foot” because it requires the tail of the downhill ski to act as a lever which points in a direction opposite that of the human foot. After a twenty-six year study, the authors of VSR explain the injury mechanism behind the Phantom Foot ACL, “Phantom Foot injuries can occur when the tail of the downhill ski, in combination with the stiff back of a modern ski boot, acts as a lever to apply a unique combination of twisting and bending loads to the knee” [VSR, 1998]. The loads transmitted to the skier’s knee, subsequently causing the ACL injury during a Phantom Foot injury. When these loads are greater than the ligament strength, and when the ski binding fails to release, knee injury is imminent. The basic skier profile that creates these unique loads applied to the knee occurs when the skier is involved in a rearward-twisting fall. The three most common skiing situations which cause this type of fall are believed to occur while: attempting to get up when moving after a fall, attempting to recover from an off balance position, and attempting to sit down after losing control. 1.1.2 THE BOOT INDUCES ACL The Boot Induced ACL injury was also identified by VSR but differs vastly from the Phantom Foot ACL. The Boot Induced injury occurs during a hard landing by skiers off-balance to the rear and only accounts for about 5 percent of the injuries studied. The authors of VSR describe it more specifically as, “when the skier lands, the tail of the uphill ski hits first…the pressure of the boot against the back of the leg increases, while at the same time, the muscles of the skier’s leg automatically contract to hold the leg in a fully extended position. By the time the portion of the ski under the boot heel hits the snow, there is no laxity left in the system to absorb the jarring impact and the back of the boot is able to drive the tibia out from under the femur, thereby tearing the ACL” [VSR, 1998]. A schematic of the forces generated during a Boot Induced ACL injury can be seen in Figure 1 (a). The Boot Induced injury occurs less often because it only occurs while jumping, but causes equally devastating effects. Although only two scenarios were discussed, several other injury situations are believed to exist. The forward twisting fall is believed to be another major cause of knee injury. The mechanisms behind the forward twisting fall are similar to those of a rearward-twisting fall, except the knee is hyper extended and coupled with a twisting load. When the combination of loads experienced by the knee is greater than the knee’s ligaments, serious knee injury can result. The scenarios discussed all produce combinations of forces that multi-release bindings supposedly accommodate for and release at a significantly high level. However, the frequency at which knee injuries occur remains consistently high, leading researchers to develop alternative methods to address the issue. 2. SOFTWARE METHOD “Tips for Knee Friendly Skiing”, produced by Vermont Safety Research, discusses a program dedicated to reduce the risk of serious knee injuries among downhill skiers. The program discussed was based on a twenty-six year study conducted by: Carl Ettlinger, Dr. Robert Johnson, and Dr. Jasper Shealy. The program was designed after analyzing serious ACL injuries incurred during alpine skiing [Vermont Safety Research, 1999]. The VSR program includes a pamphlet and an accompanying 19-minute video entitled “ACL Awareness” which shows actual footage of ACL injuries. The pamphlet and video educate the skier about possible injury situations and techniques to avoid sustaining an injury. A total of 4700 ski area employees at 25 different ski areas tested the experimental program in 1993. The results found that ski areas using the program reduced serious ACL injuries sustained by employees by more than half [VSR, 1999]. The pamphlet was available in all major ski stores, while the video was only available through mail order (cost $33). Although the VSR pamphlet and accompanying video show positive results, one major flaw exists. The pamphlet was designed to educate the recreational skier about ACL injuries but the study was only conducted on ski area employees. These employees are much more experienced and knowledgeable about skiing then the average recreational