The sport of motocross entails off-road motorcycle racing and is associated with a high incidence of traumatic injury. While prophylactic knee braces are routinely worn, there has been anecdotal concern that brace use is linked to femoral shaft fractures. While this risk remains unreported in the medical literature, preventing this complication has played a role in new commercial knee brace designs. We present two cases in which two motocross riders sustained transverse femoral shaft fractures at the proximal portion of each respective knee brace. The fracture locations measured on anterior-posterior radiograph were 22 and 21.1 cm proximal to the center of the knee, which is also the recommended proximal extent of motocross knee braces. We propose that the rigid knee brace protects the ligamentous knee structures but may focus undue force on the proximal aspect of the brace. New knee brace designs have incorporated features to dissipate the potentially injurious force to prevent femur fracture. While knee braces undoubtedly help prevent ligamentous knee injury, these cases question the safety of standard brace design and highlight the need for further brace development to better protect the patient’s bony structures, in addition to the knee joint.
Motocross is an off-road racing sport in which two-wheeled vehicles are ridden over various terrains and jumps. These vehicles can reach high speeds (up to 60 mph in 5 seconds) [
The American Motorcyclist Association (AMA) attempts to mitigate these risks by administering a set of rules annually. The rules dictate mandatory protective equipment that includes helmet with full face coverage, goggles/face shield, long-sleeve jerseys, protective pants, and boots that protect the ankle and foot [
Knee brace use has been associated with diminished incidence of motocross-related soft tissue knee injury [
In this report, we discuss two cases in which motocross riders sustained transverse femoral shaft fractures during competition. Both riders were wearing a knee brace at the time of their injury, and their fractures were found to correlate with the top of their respective braces. We aim to describe the fracture pattern and highlight future work needed to prevent knee brace-associated femur fractures without compromising ligamentous knee protection.
A 17-year-old male professional motocross athlete with a history of left tibial spine avulsion fracture and resultant chronic knee flexion contracture presented to the emergency department (ED) status-post motocross injury with isolated left thigh pain. The patient had been wearing a hard-shell, hinged, knee brace measuring approximately 43 cm in length. He reported riding over a jump of approximately 10 feet when his left leg slipped off, pinning and hinging his leg over his knee brace. He was found to have a closed and neurovascularly intact transverse femoral shaft fracture without ecchymosis, skin changes, or open wounds. The deformity measured approximately 26 cm from the tibial tuberosity on clinical exam, and the fracture was 22 cm proximal to the center of the knee as measured on anterior-posterior (AP) radiograph (Figure
(a) AP radiograph of the left femur and (b) lateral radiograph of the left femur obtained upon patient presentation to the Emergency Department. (c) Postoperative AP radiograph of the left femur demonstrating fracture line 22.0 cm proximal to the center of the knee. (d) Postoperative lateral radiograph of the left femur demonstrating fracture line 26.5 cm proximal to the tibial tuberosity. (e) Postoperative lateral radiograph of the left proximal femur.
A 17-year-old male professional motocross athlete with a history of pediatric left tibial shaft fractures (treated nonoperatively and complicated by painless varus malunion) presented to the ED after crashing his dirt bike. He had been wearing a hard-shell, hinged, knee brace measuring approximately 42 cm in length when he fell on his left side and hyperextended his left leg over the top of his knee brace. The patient complained of isolated left thigh pain. Evaluation of the patient revealed a closed, neurovascularly intact transverse femoral shaft fracture without ecchymosis, skin changes, or open wounds. The deformity was approximately 27.0 cm from the tibial tuberosity on clinical exam and measured 21.1 cm proximal to the center of the knee on AP radiograph (Figure
(a) AP radiograph of the left femur and (b) lateral radiograph of the left femur obtained upon patient presentation to the Emergency Department. (c) Postoperative AP radiograph of the left femur demonstrating fracture line 21.1 cm proximal to the center of the knee. (d) Postoperative lateral radiograph of the left femur demonstrating fracture line 27.0 cm proximal to the tibial tuberosity. (e) Postoperative radiograph of the left proximal femur.
We describe two cases in which motocross athletes suffered transverse femoral shaft fractures at the proximal edge of their prophylactic knee braces. The fracture locations suggest that while traditional knee braces protect the knee, they may be associated with femur fracture. The patients’ knee braces measured approximately 43 and 42 cm in length. The fractures were located approximately 26 and 27.0 cm from the tibial tuberosity and on AP radiograph measured 22 cm and 21.1 cm proximal to the center of the knee. The average market knee brace company recommends that the edges of the knee brace extend “8 inches,” or 20.3 cm above and below the center of the knee to achieve proper fit. Thus, the location of these fractures correlate with half the length of each rider’s respective knee braces and the manufacturer’s recommended proximal knee brace extent [
The idea that prophylactic knee bracing may have led to femoral shaft fracture is further supported by the mechanism of injury in our cases. The riders described similar mechanisms where their leg became pinned after slipping off the bike. In both of these injuries, the femur was hyperextended in a 3-point bending nature over the anterior edge of the hard-plastic knee brace. This explains the transverse fracture pattern seen in both cases, as opposed to a spiral fracture pattern often caused by rotational injury (Figure
A representative photo of a motocross knee brace. The braces worn by our patients terminated just distal to the site of fracture.
Given the clinical history and radiographic findings, we propose that the rigid design of the knee brace protects the knee but may transfer the force proximally, creating a stress riser for cantilever bending. This bending ultimately leads to femoral shaft fracture (Figure
Representative sketch of proposed mechanism of injury. (a) As the leg is subjected to an extension moment, the knee brace extends to its terminal extension point. At this point, further extension force is exerted primarily at the proximal and distal ends of the brace. This force is then absorbed, obligatorily, by the femur at the proximal end of the brace. When this force exceeds the femur’s ability to withstand this bending force, it (b) fractures.
The idea of protective gear causing a specific pattern of injury is further historically supported by a series of specific injury patterns that have resulted from adjustments in skiing equipment. The aforementioned boot top fracture findings led to an alteration of ski boots’ safety bindings, and the ski boot was made more flexible in order to reduce the incidence of boot top fractures [
Although there is a possibility that knee braces predispose motocross athletes to femur fracture, we do not recommend that these athletes discontinue knee brace wear as they have proven effective in protecting against ligamentous knee injury. The incidence of knee ligamentous injury is far greater than that of femur fracture in this cohort, and the brace therefore has a favorable risk-benefit ratio [
Despite the paucity of evidence identifying this problem in the medical literature, motocross industry has reacted to anecdotal evidence, and adjustments to knee brace design have already begun. The primary purpose of the motocross knee brace is to prevent knee hyperextension. Consequently, most effective braces are made of a rigid carbon fiber frame. Based on Kennedy et al.’s biomechanical study results [
In conclusion, this series is the first to describe a possible association between the current gold standard prophylactic knee brace design and transverse femur fractures. Although we do not advise against the use of prophylactic knee bracing, this study warrants further evaluation to determine the true incidence of femur fracture related to prophylactic knee bracing. A biomechanical study may lend further support for our proposed mechanism of this fracture pattern and empirically aid the design of safer braces.
The authors declare that there is no conflict of interest regarding the publication of this article.
The authors thank Mabry Johnson for her contributions in medical illustration.