Documentation:FIB book/Lumbar Spine and Back Protectors

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Introduction

Skiing and snowboarding are increasingly popular winter sports. It is estimated that there were 10.7 million unique participants in the 2021/2022 season in the United States alone, setting an all-time record[1]. As participation in these sports increases, so has the incidence of injury, particularly due to high speeds, obstacles, and the technical demands of both sports[2]. While literature may disagree on specific injury rates, there is consensus that spinal injuries (SPI) represent a significant portion of all severe snow sport injuries [3]. Despite the relatively high incidence of SPIs, back protectors are the only widely available product aimed at preventing SPIs. However, there is significant controversy in academia over the effectiveness of back protectors and the lack of sport – specific regulatory testing. The goal of this page is to outline the epidemiology of SPIs in alpine sports, regulatory standards for back protectors, and the biomechanical effect of back protectors in addressing known injury mechanisms.

Epidemiology of Lumbar Spine Injuries in Skiing and Snowboarding

Figure 1: Dainese back protector commonly used by motorcyclists and alpine ski racers.

There is large body of epidemiological research identifying trends in snow sport injuries. However, this research is limited by a lack of consistency in the reported metrics, such as fracture mechanism, which creates challenges in identifying injury patterns[3]. As such, the rate of spinal injuries (SPI) from snow sports is contested, and is reported to account for 1%-14% of all snow sport injuries[3]. However, it is generally agreed that SPIs represent up to 29% of all severe snow sport injuries (categorized as Injury Severity Score > 15) [3].

The lumbar spine has consistently shown to be the most common location in spinal fractures for both skiers and snowboarders[2]. Approximately 80% of lumbar fractures are compression fractures, while other fracture types, such as spinous process fractures, are considered rare[2]. Two accepted injury mechanisms commonly reported are compression fractures and flexion-extension injuries, as discussed below[2]. Falls are considered the most common cause of lumbar spine injuries among all classes of skiers and snowboarders[2].

Sex has been hypothesized as a predictor of injury likelihood. It has been observed that while male and female skiers have a similar incidence of acute SPIs, male snowboarders have disproportionate levels of SPIs compared to female snowboarders[3]. It is hypothesized that men are more likely to take more risks while snowboarding such as riding faster or attempting jumps, and therefore may be more susceptible to falls[3].

The main piece of safety equipment available to skiers and snowboarders aimed at preventing lumbar spine injuries are back protectors. Surveys at European resorts suggest that 40% - 50% of skiers and snowboarders wear back protectors, where 76% of survey respondents felt that back protectors could protect against spinal fractures [4]. This is despite a lack of regulation on the performance of back protectors in addressing the spinal fracture mechanisms seen in snow sports.

Regulations

Current back protectors provide external protection against injuries in the spinal column and back. Protectors vary by both the area of the back that they cover and the level of protection they provide. The level of protection is defined by the maximum allowable impact force transmitted to the body. Protection zones include: full back, central back, and lower back, with full back protectors being the most used. Despite the increased use of back protectors in recent years[5], no current standards or regulations exist specifically for snow sports. Instead, the BS EN 1621 [6] standard for motorcycle back protectors is used to test and regulate currently available snow sport products.

Standards: BS EN 1621-2:2014

The BS EN 1621 describes the requirements and testing methods for back protectors. Back protector dimensions are determined by the weight-to-shoulder length of the largest target user, though specific dimensions may vary between the type of back protector[6]. Impact testing is used to test the force transmitted through the protector.

A falling weight and rectangular bar impactor are used to impact the back protector. When released, the load is guided vertically downwards and the rectangular shape of the impactor replicates the blunt impact experienced when hitting the ground. The protector is placed on top an anvil that is mounted with a piezoelectric load cell, which records the forces transmitted through the protector. The recorded forces correlate to those that would be absorbed by the body during real impact. The bar impactor must create an impact with at least 9 cm of its length contacting the protector and within 1 cm of the protector border. These conditions improve repeatability and ensure worst case scenarios are evaluated. In addition, the protector should be tested as one piece[6].

Five impact tests should be conducted consisting of ambient, wet, and hot/cold temperature impacts. These tests require conditioning the protector in various temperatures and environments before testing to evaluate their response in these situations. For low temperatures, impact conditioning occurs at approximately -10 degrees Celsius, which is comparable to conditions observed during snow sports.[6] The point of impact is randomly distributed across the protector for three tests and focused on known areas of weakness for two tests. Following impact, the protector must not be fragmented, but minor cracks and material loss is allowable.

To achieve a pass, the force transmitted through the protector and recorded by the load cell should observe the conditions described in the following table. The Mean Value variable represents the average force transmission value across the five impact tests.

Performance Level Requirements [6]
Mean Value Single Strike
Level 1 ≤ 18kN ≤ 24kN
Level 2 ≤ 9kN ≤ 12kN

Regulatory Limitations

Although motorcycle and snow sport back protectors shield wearers from impacts, the use of motorcycle regulations neglects the different conditions and injury types between the two activities.

Participants' speed, positioning, and external environment in snow sports vary greatly from those experienced in motorcycle impacts. Regulatory testing conditions involve a directly constrained impact test, whereas snow sports involve rotational and complex directional movements that are not tested for. Although temperature conditioning is used during regulatory testing, ambient testing is prioritized. Increased testing at low temperatures could evaluate how the protectors function at low temperatures over longer periods of time. Low temperatures can affect the dampening properties of the material as the material becomes harder and stiffer. which can increase the force transmission to the wearer. The force requirements and experimentation methods record overall force transmission, while concentrated regions of force transmission are not recorded. Subsequently, biological impact to regions on the back cannot be determined. Although regional back protectors exist for the central and lumbar regions, there is no regulatory testing method to isolate the effect of impact on these areas.

Previous studies completed by Michel et al.[5] have found that current protectors are mostly sufficient in dampening impact forces to the back region. However, the requirements may not be sufficient in assessing them as safety devices in snow sports. Regulations specific to snow sport back protectors and improved testing methods should be developed, and should consider the specific injury mechanisms involved in snow sports.

Injury Mechanism

Backward Fall: Flexion-Extension

Skiers and snowboarders are highly susceptible to SPIs. The backward fall was found to be the most common mechanism amongst snowboarders to result in SPIs.[7] For this reason, the “opposite-edge phenomenon”, where the back-edge of the snowboard catches snow and projects the rider backwards, is the subject of multiple studies. Richards et al.[8] simulated this accident scenario with Hybrid III dummies to study head injuries, and a follow-up study was conducted to further analyze the use of helmets.

Figure 2: Kinematics of the 50th percentile male snowboarder backward fall with an initial velocity of 15 km/h (A), 35 km/h (B), and 45 km/h (C)

To address the need for a similar investigation for SPIs, Wei et al.[7] used the human facet model (HFM) by MADYMO to assess risk for SPI based on the range of motion (ROM) experienced in a backward fall setting. This model is detailed in each functional spine unit (FSU), which allowed researchers to determine the risk for each of these units separately. The validated model was then run with different parameters, such as surface incline, initial velocity, body posture, anthropology, and angle of approach (angle between the snowboard and the normal to the incline). 324 experiments were run and the flexion-extension motion for each FSU was recorded. The authors were able to determine an injury risk by comparing the simulated ROM for each FSU with ROM threshold values derived from literature. The obtained threshold values were higher in the lumbar spine compared to the thoracic spine and ranged from 15.6° in L1-L2 to 24.1° in L5-Sacrum. Correspondingly, the thresholds were exceeded less in the lumbar region. Fall kinematics changed significantly with different velocities corresponding to beginner, intermediate, and good snowboarders (15, 35, and 45 km/h, respectively). For the lowest velocity, the entire spine was in flexion. In this scenario, the lower back was the first to impact the snow. In higher velocities, the model’s cervical and upper thoracic spine were in extension, while the lower thoracic and lumbar spine stayed in flexion. This resulted in a head-first impact for the 35 and 45 km/h simulation, as seen in Figure 2.

Results of the study concluded that there was not a higher risk for SPIs in the lower thoracic and lumbar spine for low velocity falls. However, a smaller increase in injury risk in these areas was observed with increasing velocity. This indicates that the initial impact on the lower spine directly increases injury risk connected to the flexion-extension ROM for these FSUs. Therefore, the developed model suggests that a direct fall on the spine leads to a greater chance in spinal injury than an energy-matched indirect impact. This allows for the assumption that wearing protective gear on the back can decrease the risk for spinal injury in a backward fall.

Figure 3: Example of lumbar compression fracture after fall from height

Straight Fall: Compression

A study conducted in Vail with 119 patients found that the majority of skiers and snowboarders with fractures in the thoracic and lumbar spine suffered a compression fracture, with no significant difference observed between skiers and snowboarders[9]. However, to date, no sport-specific study has been conducted analyzing compression fractures in the lumbar spine.

Fractures in the lumbar spine result from hyperflexion and under axial loading. [10] Axial loading is a common occurrence in skiing and snowboarding due to jumping and straight falls. In a compression fracture, the vertebral body can fail on the anterior aspect, while the posterior aspect longitudinal ligament remain intact. This fracture mechanism causes a stable wedge fracture that usually does not impact the spinal cord. If posterior and anterior vertebral portions are affected, the compression resulted in a burst fracture. These can be unstable and injure the spinal column.[10]

Cadavers were tested upon to determine the dynamic fracture tolerance for the vertebrae of the lumbar spine in compressive loading scenarios. Stemper et al. [11] analyzed the lumbar spine as a whole column as well as isolated vertebral bodies obtained from post-mortem human subjects (PMHSs). The PMHS specimen were set up in a drop tower and the produced accelerations were measured. Compression fractures were evaluated with pre- and post-test X-rays and CT scans. The researchers determined that the rates of compression in their set-up and correlated it to the observed fractures. Higher accelerations of the vertebral column resulted in greater compression rates. They applied the obtained compression rates to isolated vertebral bodies from younger PMHS donors (23 subjects, mean age: 40.7 years, range: 18-55 years), which showed greater ultimate force, and thus risk for fracture, with increasing rate of compression. This confirms the intuitive idea that increasing accelerations on the lumbar spine results in an increased risk for compression fractures.

Failure Conditions for Lumbar Spine

Based on the most recent study examining the failure force for the lumbar spine, the average axial force and momentum causing failure are 3290 N and 51 Nm, respectively. Data has been obtained by examining 40 test cadaver specimens with a specifically designed test fixture that can simultaneously apply compression and flexion on the tested specimen. [12]

Biomechanical Effects of Back Protectors

Literary and biomechanical studies were conducted to evaluate effectiveness of back protectors at mitigating risk to SPIs.

Epidemiological Analysis of Back Protector Effectiveness

An epidemiological study was conducted in Marseille, France, in 2016 to analyze the effectiveness of back protectors in trauma patients of two-wheeled motor vehicle (TWMV) accidents [13]. 43% of trauma patients were wearing back protectors, which is consistent with a 2006 Australian study concerning the total proportion of vehicle riders wearing back protectors[14]. This correlation allows for the assumption that no riders in TWMV accidents were well-enough protected by back protectors to not be admitted to the hospital and, therefore, omitted from the study.

In the study, a Fisher test (p < 0.1) was conducted to assess the association between the following qualitative variables: sex, age, level of experience, cylinder size, type of road, protection used, type of accident, estimated speed prior to the accident, and death; and the following qualitative variables: thoracolumbar lesion and use of a back protector. The test “did not show any significant difference in the number of riders who sustained a thoracolumbar spinal injury based on the use or non-use of back protection”[13]. Thoracolumbar injuries are primarily caused by axial compression, which suggests that back protectors provide insignificant protection against axial forces. Straight compressive falls are a leading cause of fractures in the thoracic and lumbar spine[9] among snowboarders and skiers, which outlines a need potentially not being address by back protectors.

Analysis of Back Protector Effectiveness

A 2010 Swiss study investigated the performance of 12 commercially available back protectors against EN1621 and EN1077[4] regulations, which refer to back protectors for motorcycles and standards for snow sport helmets, respectively. Back protection standards for motorcycles mainly involve blunt impact forces, as described above. Safety standards for snow sport helmets involve requirements and test methods for construction including field of vision, shock absorbing properties, resistance to penetration, retention system properties, marking and information. The requirement that can best be applied to back protectors concerns the penetration forces in helmets caused by sharp obstacles like stones and ice.

The regulated blunt impact forces in EN1621 are designed to simulate impacts with the curbside or crash barrier. The study assumes these impact mechanisms are comparable to snowboarding impacts that might occur in fun parks, such as collisions with the edge of a halfpipe or ski jump[4]. It also assumes the regulated penetration forces in EN1077 are equally applicable to the back as to the skull. However, an admitted limitation of this study is that it does not test for the protection against hyperextension or torsion.

Of the 12 protectors, 10 passed the EN1621 blunt impact test and 6 passed the EN1077 penetration test. Hard-shell protectors provided the most protection against penetration, while a variety of different protectors provided protection against blunt impact forces. Therefore, the majority of back protectors on the market have the ability to protect wearers from blunt impacts caused by collisions, motorcycle riders and snowboarders alike. However, only half of the back protectors on the market have the at least the ability to protect the wearer against penetrations as their helmets. Protection against flexion-extension of the spine was not considered in the study.

Relating Biomechanical Analysis of Backward Falls to Back Protectors

A 2018 study simulated a snowboarding accident using a human facet model and a Hybrid III ATD[7]. The snowboarder was subjected to a backwards fall at varying speeds. It showed that “the first impacted body region did not necessarily have the highest injury risk during snowboarding backward falls”[7]. The first impact, being the blunt forces sustained and mitigated by a back protector, are not the forces that are most likely to cause injury. The flexion-extension range of motions are described to carry the highest risks of spinal injuries, even at low speeds. Therefore, “current evaluation standards for back protectors might not be enough for snowboarder SPI prevention”[7]. In addition to altering the parameters of the EN1621 blunt impact force test to reflect snowboarding impact values, additional flexion-extension parameters should be included.

Next Steps

EN1621-2 states that “axial force [...] or bending and twisting forces on the back [...]” are not protected by back protectors[4]. This theory is corroborated by epidemiological data[13] but lacks a detailed biomechanical investigation[4][7]. It has been demonstrated that the blunt impact loads expected to be mitigated by back protectors do not carry the highest risk of spinal injury[7], thereby outlining the need for further biomechanical studies into back protector effectiveness at preventing SPIs.

Future Studies

Though there are numerous studies about lumbar spine injuries, the vast majority of them are focused on examining the behavior and the fracture limits of the lumbar spine in compression and flexion due to axial loading and bending. Additionally, they study fractures caused by motor vehicle collisions or the effect of osteoporosis, which is not strictly relevant to the risks attained during snow sports, and for determining the effectiveness of back protectors in snow sports.

There are no studies examining the effects of lateral loading on the lumbar spine or the failure tolerance of the vertebras. Such studies might help to evaluate the effectiveness of back protectors as they are most suitable for preventing lateral damage caused by falling or collisions with objects.

Creating regulations specifically for snow sport back protectors would be useful, as the dynamics of motorbike crashes and sports crashes may differ significantly from those seen in snow sporting impacts. Further research may focus on examining the material behavior of back protectors under different loading and temperature conditions. One study addressing this concern has been published[15], though it only tested the material under two different temperature conditions (-5o C and 20o C). This is insufficient for replicating snow sport conditions, as back protectors can be used in lower temperatures.

Further research is also needed to create multibody and finite element models in which the behaviors of back protectors and the spines can be studied simultaneously.

References

  1. ↑ "Estimated Snowsports Participants: U.S. Visitors at U.S. Resorts, 1996/96 - 2021/2022" (PDF). National Ski Areas Association. 2022.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 Bigdon, S.F.; Gewiess, J.; Hoppe, S.; Exadaktylos, A.K.; Benneker, L.M.; Fairhurst, P.G.; Albers, C.E. (2019). "Spinal injury in alpine winter sports: a review". Scandinavian Journal of Trauma, Resuscitation and Emergency Medicine. 27. doi:10.1186/s13049-019-0645-z – via BiomedCentral.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Huffman, W.H.; Jia, L.; Pirruccio, K.; Li, X.; Hecht, A.C.; Parisien, R.L. (2022). "Acute Vertebral Fractures in Skiing and Snowboarding". The Orthopaedic Journal of Sports Medicine. 10 – via Sage.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 Schmitt, K; Liechti, B; Michel, F; Stämpfli, R; Bruhwiler, P (2010). "Are current back protectors suitable to prevent spinal injury in recreational snowboarders?" (PDF). British Journal of Sports Medicine. 44: 822–826.
  5. ↑ 5.0 5.1 Michel, F.I.; Schmitt, K.; Liechti, B.; Stämpfli, R.; Brühwiler, P. (2010). "Functionality of back protectors in snow sports concerning safety requirements". Procedia Engineering. 2: 2869–2874. doi:10.1016/j.proeng.2010.04.080.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 Motorcyclists' protective clothing against mechanical impact - Part 2: Motorcyclists' back protectors - Requirements and test methods (2014). BS EN 1621-2, British Standards Institution.
  7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 7.6 Wei, W.; Evin, M.; Bailly, N.; Llari, M.; Laporte, J.; Arnoux, P. (2018). "Spinal injury analysis for typical snowboarding backward falls". Scandinavian Journal of Medicine & Science in Sports. 29. doi:10.1111/sms.13342.
  8. ↑ Richards, D.; Carhart, M.; Scher, I.; Thomas, R.; Hurlen, N. (2009). "Head Kinematics During Experimental Snowboard Falls: Implications for Snow Helmet Standards". Skiing Trauma and Safety. 17: 101-101.7. doi:10.1520/STP47472S.
  9. ↑ 9.0 9.1 Gertzbein, S.; Khoury, D.; Bullington, A.; St. John, T.; Larson, A. (2012). "Thoracic and lumbar fractures associated with skiing and snowboarding injuries according to the AO comprehensive". American Journal of Sports Medicine. 40: 1750–1754. doi:10.1177/0363546512449814.
  10. ↑ 10.0 10.1 Brandser, E.A.; El-Khoury, G.Y. (1997). "Thoracic and lumbar spine trauma". Imaging of Orthopedic Trauma. 35: 533–557.
  11. ↑ Stemper, B.; Yoganadan, N.; Baisden, J.; Umale, S.; Shah, A.; Shender, B.; Paskoff, G. (2015). "Rate-dependent fracture characteristics of lumbar vertebral bodies". Journal of the Mechanical Behavior of Biomedical Materials. 41: 271–279. doi:10.1016/j.jmbbm.2014.07.035.
  12. ↑ Tushak, S.K., Paul Donlon, J., Gepner, B.D., Chebbi, A., Pipkorn, B., Hallman, J.J., Forman, J.L. and Kerrigan, J.R. (2022). "Failure tolerance of the human lumbar spine in dynamic combined compression and flexion loading". Journal of Biomechanics, [online] 135, p.111051. doi:10.1016/j.jbiomech.2022.111051. ‌. line feed character in |journal= at position 85 (help)CS1 maint: multiple names: authors list (link)
  13. ↑ 13.0 13.1 13.2 Afquir S., Melot A., Ndiaye A., Hammad E. (2020). "Descriptive analysis of the effect of back protector on the prevention of vertebral and thoracolumbar injuries in serious motorcycle accident". Accident Analysis & Prevention.CS1 maint: multiple names: authors list (link)
  14. ↑ de Rome, L. (2006). "Planning for motorcycle safety: measures of success". Australasian Road Safety Research Policing Education Conference.
  15. ↑ Signetti, S., Nicotra, M., Colonna, M. and Pugno, N.M. (2019). "Modeling and simulation of the impact behavior of soft polymeric-foam-based back protectors for winter sports". Journal of Science and Medicine in Sport. 22.CS1 maint: multiple names: authors list (link)


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