INTRODUCTION
Spine injuries related to recreational participation in athletic activities have been
well described in the literature [1], [2], [3], [4]. Common spinal injuries that occur include muscle strains, muscle spasms, disc herniations,
as well as vertebral body compression and avulsion fractures [1]. Spinal injuries have been described in football, hockey, wrestling, diving, skiing,
snowboarding, rugby, cheerleading, baseball, and golf [1], [4], [5]. The literature, however, is typically focused on a young patient population often
with an emphasis on catastrophic injuries that have a clearly identifiable traumatic
mechanism.
This article reports spinal injuries occurring in two older patients during a recreational
bobsled run. In addition, the variables that contribute to the increased propensity
of fractures in the aging spine are discussed. After reviewing the forces imparted
on the spine and the increased chances of spine fracture in the elderly, proposed
safety guidelines are recommended for recreational bobsledders. To our knowledge,
this is the first discussion in the literature to date on bobsled-related fractures
of the spine.
CASE REPORTS
The two cases presented involve a retired 57-year-old man (patient 1) and his 54-year-old
wife (patient 2). Neither patient had any history of fractures, medical comorbidities,
osteoporosis, or nicotine use. They are presented together due to the similarity between
the two cases. Both patients have consented to the publication of their case histories.
While on vacation, the couple elected to ride a local bobsled track previously used
in international competitions. They had an experienced driver navigate the bobsled
down a 4380 foot track. During the run, they each simultaneously described an immediate
sharp, stabbing pain in their back while in the middle of a specific turn. There was
no rollover, ejections, collisions, or any unusual occurrences during the ride. The
bobsled was reportedly under control at all times. After the run had been completed,
the two patients were brought to our institution for evaluation of severe back pain.
Neither patient had experienced numbness, tingling or loss of bowel or bladder function.
Both demonstrated strong and symmetric motor examinations with no sensory deficits,
normal rectal tone, and no pathological reflexes. Patient 1 demonstrated paraspinal
tenderness at the thoracolumbar junction and patient 2 had midline tenderness but
no palpable gap or step-offs at the thoracolumbar junction.
X-rays and computed tomographic (CT) images revealed that patient 1 had an isolated
T12 compression fracture ([Fig 1]) and patient 2 had an isolated T12 burst fracture ([Fig 2]). The burst fracture was associated with less than 10% canal compromise, no sagittal
or coronal deformity, and an intact posterior ligamentous complex. Injuries were classified
according to the Thoracolumbar Injury Classification and Severity Score (TLICS) [6], [7]. The TLICS scores for patient 1 and patient 2 was 1 and 2, respectively. They were
both treated nonoperatively with the use of a thoracolumbar orthosis (TLSO).
After passing a TLSO brace-trial, upright x-rays were obtained verifying stable alignment
of the spinal fractures. Both patients were discharged and returned home to England
within 2 weeks of their injuries. After 6 weeks of bracing, x-rays were obtained by
their primary care provider demonstrating stable alignment. The orthoses were discontinued
and a physiotherapy program was initiated. The patients had also independently sought
chiropractic care after that time. Through direct communications with the patients,
24-month follow-up was obtained. Their pain scores improved steadily, with a current
Visual Analog Scale (VAS) rating at 2/10 for both patients. They reported no functional
limitations from their injuries and a return to normal daily and recreational activities.
Osteoporosis evaluation was requested but deemed unnecessary by their primary care
provider due to their age and low number of risk factors.
Fig 1a–b
Sagittal (a) and axial (b) CT images of Patient 1 demonstrate a compression fracture
of T12 with anterior column.
Fig 2a–b
Sagittal (a) and axial (b) CT images of Patient 2 demonstrate a burst fracture of
T12 with <10% canal stenosis, normal spinal alignment, and no posterior column involvement.
DISCUSSION
Bobsledding began in the late 19th century in Switzerland and is now recognized as
an international sport. The particular track that the two patients rode is 4380 feet
in length, displays a vertical drop of 340 feet, and has a slope of 7.8%. The sled
itself weighs approximately 600 lbs and will reach maximum speeds of 80 miles per
hour. Injuries associated with bobsledding have not been previously described in detail.
Given the nature of bobsledding, it would be assumed that injuries occur during overturns,
collisions, or ejections. In reviewing these cases, it is unique that both middle-aged
individuals suffered thoracic fractures without any overt traumatic event. The determinants
of fracture, therefore, include the forces imparted by the bobsled ride as well as
the mechanical and biological status of the patient’s bone (eg, osteopenia, osteoporosis).
Vertebral fracture risk
In a finite element model, Imai et al [8] found vertebral compression yield strength to be 2154 N (± 685 N). This value is
similar to values found in other biomechanical studies and can be considered the failure
load in an aging, osteopenic, or osteoporotic spine [9], [10]. Silva [9] defined fracture risk based on applied load and load to failure by the equation:
Factor of Risk (Φ) = Applied Load / Fracture Load
Fracture is a mechanical event that results when the load that is applied to a bone
exceeds its ability to bear the load [11]. If the factor of risk is less than 1, the bone is not predicted to fracture; however,
if Φ is greater than 1, then fracture can be expected [9].
In the clinical cases presented, variables that affect the applied load and fracture
load can be analyzed. In the vertebral column, the strongest factors that influence
applied load involve the position of the spine and the addition of weight away from
the body. Mathematical models developed to predict compressive loads across the thoracolumbar
spine demonstrate that loads markedly increase with forward flexion and increasing
weight held at a distance from the body’s center of mass [9], [12]. The example by Bouxsein et al [13] shows that the force at L2 is 0.5 times the body weight while standing and increases
to 1.5 × body weight when the trunk is flexed forward to only 30° with arms outstretched.
With the addition of weight, force magnitudes increase dramatically. Force magnitudes
seen across a spine associated with lifting weights of 15–30 kg are in the 1000 N–2000
N range for a person of average height and weight [9]. In light of the suggested 2154 N failure load of the elderly vertebrae, one can
recognize the increased risk of spine fracture in the aging spine [8]. During a high-speed turn while bobsledding, an additional centripetal force is
applied to the bodies of the passengers. The centripetal load is related to the mass
and speed of the bobsled as well as the radius of curvature of the turn. This is defined
by the equation:
Force = Mass × Velocity2/Radius
In the described setting of recreational bobsledding, the applied centripetal force
may greatly overshadow body weight forces by orders of magnitude.
Load to fracture is determined by the geometry of the bone, the material properties
of the bone, and the loading modality [9]. The geometric changes associated with aging vertebrae include an increase in width
at both the endosteal and periosteal surfaces. This expansion results in an increased
total area and provides partial protection against fracture [9], [14]. This has been more clearly demonstrated in men than in women and may provide an
explanation as to greater severity of injury in the female patient (T12 burst fracture)
than the male patient (T12 compression) despite an identical mechanism of injury [9]. Another factor is the material property of the bone. This consists of both cortical
and trabecular bone. The most significant change in cortical bone with age is an increase
in porosity [9], [15]. In addition, the toughness of cortical bone declines by 10% per decade [9]. The age-related trabecular bony changes include, most notably, a decrease in density
[9]. The compression strength of vertebral trabecular bone decreases by approximately
70% from 25 to 75 years of age [16], [17]. The combination of weakened cortical and trabecular bone contribute to age-related
changes in vertebral mechanical properties [16]. The final participating variable is the mode of loading. The loading mode is defined
by the direction, type, and speed of load applied [9]. The loading mode in this case is likely a combination of axial compression, forward
flexion, and torsional loading.
When considering these particular factors, spontaneous nonimpact-derived spinal fracture
on a high-speed bobsled run is not surprising. Documentation provided by the engineering
firm involved with design and analysis of the track reveals “g forces” in excess of
4 occurring five times during the ride with a peak horizontal “g force” of 4.67. Although
called a “g force,” this is in reality a measurement of acceleration defined as a
multiple of the acceleration of gravity (9.8 m/sec2). In a simplified model, the forces experience by the rider (assumed to be an average
of 70 kg) would be up to 3204 N.
Force = Mass (kg) × Acceleration (m/sec2)
Force = 70 × 4.67 × 9.8 m/sec2 = 3204 N
The subjects in the bobsled were also flexed forward and had an additional weight
of a protective helmet. The actual forces experienced by the rider’s spine may, therefore,
be greater than this estimated value. The patients, in their sixth decade of life,
may additionally have age-related bone changes, increasing cortical bone porosity,
and decreasing trabecular bone density that may contribute to a weakened load to failure.
Combined with large centripetal forces experienced through the bobsled turns, riders
are clearly at risk for fracture.
CONCLUSION
These cases highlight a need for possible safety guidelines or recommendations for
recreational bobsledding. Consumers, the managing team at a bobsled track, and the
ski patrol or EMT services should be aware of the increasing risk of spine fractures
associated with age. It may also be necessary to caution individuals with a diagnosis
of metabolic bone disease, renal disease, known ankylosing spinal disorder, and osteoporosis
or osteopenia from participating in recreational bobsledding. Additionally, slower
velocities during bobsled turns may reduce the risk of spinal fracture and could be
considered as an option. While this may diminish some of the subjective “thrill” of
the bobsled run, it may make for a safer experience. Beyond the specifics of bobsledding,
these cases highlight fracture risks for older individuals participating in recreational
sports that might otherwise be thought of as benign.
