Mechanical etiology of femoroacetabular
impingement (FAI) in ice hockey goaltenders
Whiteside D., Deneweth J., Bedi A., Pomeroy S., Murray M., Bancroft R., Zernicke R., Goulet G.
University of Michigan, Ann Arbor, USA
Abstract- The prevalence of femoroacetabular impingement (FAI) in ice hockey goaltenders is
anecdotally attributed to their unorthodox mechanics, namely repeated execution of the butterfly save.
Due to the difficulties of undertaking biomechanical analyses on the ice, empirical support for this
assertion has not been forthcoming. Consequently, this study employed a novel inertial-based system to
measure hip kinematics in seven elite goaltenders as they performed standard butterfly (BF) and
stopping (ST) movements. Although the butterfly involved significantly greater peak femoral axial
acceleration (BF: 79.06 m·s-2; ST: 35.35 m·s-2), stopping involved significantly more hip flexion (ST:
41.61°; BF: 29.24°) and internal rotation (ST: 22.23°; BF: 9.21°). Given that combined flexion and
internal rotation is the most common impingement mechanism, stopping appears more noxious than
the butterfly from an FAI perspective. Further, stopping is not unique to goaltenders, suggesting that
its link to FAI in ice hockey athletes warrants closer scrutiny.
Keywords- hip; injury; biomechanics; butterfly
1. INTRODUCTION
Hip injuries are a common cause of pain in ice hockey athletes and can lead to loss of playing time and longterm joint damage [1,2]. In particular, intra-articular injuries, such as femoroacetabular impingement (FAI),
impose extended recovery periods and typically require surgical intervention more frequently than other hip
injuries [3]. Equally concerning is the proposition that FAI is a precursor to early onset hip osteoarthritis [4].
The morphologic etiology of FAI is well understood, with the condition having been linked to structural
abnormalities in the femur and/or pelvis. During movement, this irregular architecture causes the femoral neck
to impinge on the acetabular rim in symptomatic patients. This impingement is more common in athletic
populations who repeatedly attain large ranges of motion during dynamic movements that can place stress on
the intermediary cartilage and lead to degeneration of the labrum. In this way, the severity of FAI is also
influenced by mechanics, whereby more extreme postures (most commonly combined flexion and internal
rotation [5,6]) can lead to more acute impingement. It follows that the mechanics involved in the anomalous
goaltender-specific butterfly save (BF: Fig. 1) are frequently implicated in FAI [7,8]. However, this theory
lacks empirical endorsement as unique constraints (environmental and equipment) have precluded traditional
biomechanical analyses in ice hockey. Consequently, although the structural FAI mechanism is well
understood and the prevalence of FAI in ice hockey is high [9], the specific link between the two remains
anecdotal. To address this issue, the current study utilized a full-body inertial-based motion capture system to
measure hip joint kinematics in two standard goaltender maneuvers and interpreted these mechanics from an
FAI perspective.
Figure 1. The butterfly (BF) and stopping (ST) movements
2.
MATERIAL AND METHODS
Subjects
Seven NCAA Division I and professional goaltenders (age: 22.7 ± 2.2 yr; height: 184.0 ± 7.0 cm; mass: 88.0
± 7.9 kg) were recruited to participate in the study. All athletes were healthy and undertaking a full-time
practice schedule at the time of testing. The Institutional Review Board sanctioned the protocol and
population prior to data collection.
Measurement System
Kinematic data were measured using a 17-segment based inertial measurement (IMU) system (MVN
BIOMECH, Xsens Technologies, Enschede, Netherlands) operating at 120 Hz. The system was affixed to each
athlete using a suitably sized Lycra bodysuit that effectively fixed each IMU (mass: 30 g; dimensions: 3.8 ×
5.3 × 2.1 cm) to a body segment. The x-axes of the upper leg sensors were aligned with the long axes of the
repsective femurs. Nine anthropometric measures were used to scale a subject-specific anatomical model,
wherein reference frames represented segments. Rotations of the femoral segment, relative to the pelvis
(described using the ZXY decomposition) were used to describe hip joint angles, in accordance with
recommended ISB protocols [10]. Positive angles denoted flexion, adduction, and internal rotation in the
respective planes. Note that commercial IMU systems use a static posture to calibrate the neutral posture (i.e.,
“zero angle”) at each joint. As such, the joint angles, here, are more accurately ‘excursions from neutral’ in the
respective planes, as they do not represent the absolute relative alignments of two segmental coordinate
systems.
Protocols
All testing sessions were undertaken at the goaltenders’ respective team practice facilities. Prior to the
commencement of the protocol, an ice resurfacer (Frank J. Zamboni & Company, Paramount, CA) cleaned
the surface of the ice. The athletes were initially attired in the Lycra body suit upon arrival to the practice
facility. To minimize the effect of soft tissue artifact, the lower limb sensors were fixed to each segment using
elastic adhesive tape (Sher-Light™, Covidien, Mansfield, MA) before the previously noted body
measurements were taken. Over the top of the suit, the goaltenders dressed in their standard game day attire
and padding, including leg pads, blocker, trapper, and mask. Both skates were also fitted with a sensor as part
of the IMU array.
So attired, the goaltenders made their way on to the ice, where they were instructed to perform a self-selected
warm-up routine that involved skating and stretching. Upon verbal confirmation of their preparedness, the
athletes assumed a neutral posture for the purpose of calibrating the IMU system. They then commenced the
testing protocol, which involved three standard goaltender tasks (Fig. 2): 1) BF with recovery to standing; 2)
BF into sliding save; and 3) Long rebound sequence comprising a BF and skating. In the first task, the
goaltender commenced at the post, skated to the top of the crease, performed a BF and immediately recovered
to a standing position. They used each leg to recover from the BF three times, for a total of six BF
observations in this task. The second task required goaltenders to start from a standing position, drop into a
BF and then slide laterally along the ice utilizing a single leg push. They were asked to slide three times in
each direction, again for a total of six BF observations. In the third task, the goaltenders commenced at the
post, skated to the top of the crease and simulated a BF, before pivoting and skating to the opposite post to
which they had started. The goaltender started three times at each post, performing a total of six long rebound
sequences. From this task, six BFs were recorded, as well as twelve stopping movements (ST) (six ST at the
top of the crease and six ST at the post). Cumulatively, 18 BF movements (all the three tasks) and 12 ST
movements (from the long rebound sequence) were recorded. Players were instructed to simulate each task as
they would expect to perform them in a game, and the goaltender held his hockey stick throughout all tasks.
Kinematics of interest
Peak positive axial acceleration of the femur (PPA) was used to quantify the magnitude of the femoral shock
during each task. In the BF, there was a bilateral asymmetry in the timing of PPA, denoting how one knee
usually dropped to the ice slightly before the other. Consequently, although bilateral PPAs were recorded in
the BF, only the higher value was analyzed as this represented the femur that impacted the ice first and
absorbed the most shock. This was not an issue in the ST, since players only used one skate to stop, that being
the side that was analyzed. Because the magnitude of angular displacement at the hip influences the extent of
impingement, the tri-planar hip angles at the instant of PPA were measured.
Statistics
Descriptive statistics provided some of the first empirical observations of hip kinematics during goaltending
maneuvers. In addition, four paired t-tests were used to compare the four kinematic variables of interest (PPA
and tri-planar hip angles) between the BF and ST movements. A p-value of 0.05 was used to determined
significance.
Figure 2. The three movement tasks comprising butterfly (1c; 2b; 3d) and stopping maneuvers (3c; 3h).
3. RESULTS
The magnitude of PPA was significantly higher in the BF (79.06 m·s-2) compared with ST (35.25 m·s-2) (p =
0.01). However, the ST movement involved significantly more flexion (BF: 19.67°; ST: 40.72°; p < 0.001)
and internal rotation (BF: 10.16°; ST: 28.05°; p=0.015) than the BF. Frontal plane hip posture was not
significantly different between the two movements (BF: -9.91°; ST: –10.93°).
4. DISCUSSION
With goaltenders appearing to repeatedly drop to the ice in unconventional hip postures to execute the butterfly
save, there has been a logical assumption that this motion places excessive stress on the hip joints. While this
assertion may be true, the evidence presented in this study suggests that the mechanics involved in the stopping
maneuver are more closely related to FAI.
It is difficult to argue that the loading patterns involved in the butterfly save conform to the evolutionary
development of the hip joint. Given that the femoral shock measured in the butterfly movement was ~5%
higher than that measured in the tibias of runners with tibial stress fractures [11], practitioners’ inclination to
attribute hip pain to the butterfly save may hold some credence. However, the impingement mechanism is
purportedly related to extreme hip postures rather than repeated high-impact activities. Therefore, to suitably
appraise how these movements are related to the development of FAI, it is necessary to supplement the
impact/loading measures with hip joint rotations.
Although the peak femoral shock was ~4.5 g higher in the butterfly save, there was a distinct difference in the
hip postures across the two tasks at the time this shock occurred. More specifically, as the femoral shock
peaked, the magnitudes of flexion and internal rotation at the hip were significantly greater during the stopping
maneuver. This is critical as combined flexion and internal rotation is associated with the preponderance of FAI
cases, wherein the femoral neck impinges on anterior acetabulum and labrum [12]. The comparatively modest
levels of flexion and internal rotation recorded in the butterfly save also presents the possibility that labral
impingement may be negligible in this movement. Rather, the combined femoral accelerations and hip joint
mechanics point to compression of the femoral head into the acetabular fossa, proposing an alternative
explanation for hip pain related to butterfly-style goaltending and direction for future work. Notwithstanding
this possibility, even if the butterfly save does elicit anterior FAI, the disparate hip postures are
overwhelmingly indicative of more severe impingement during stopping.
In conclusion, the butterfly save is frequently cited as a primary cause of FAI in ice hockey goaltenders.
Based on the results in this study, it is evident that the butterfly save involves compressive femoral forces that
may be considered extreme and could be associated with hip pain. However, it is equally clear that the
mechanics of the stopping movement are much more provocative of FAI than the butterfly save. Moreover,
unlike the butterfly save, the stopping maneuver is not restricted to goaltenders, providing a more potent
explanation for the high prevalence of FAI in ice hockey athletes. It would appear that, in future discussions
and explorations of FAI in ice hockey, the stopping maneuver deserves at least as much attention as the
butterfly save.
5. ACKNOWLEDGMENTS
The authors wish to thank L. J. Scarpace (University of Michigan) and John Bernal (Grand Rapids Griffins) for
their assistance in this study.
6.
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
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